Hormonal Regulation and Integration of Mammalian Metabolism

CHAPTER 23 HORMONAL REGULATION AND INTEGRATION OF MAMMALIAN METABOLISM multicellular organisms is cell differentiation and division of labor. Specialized functions of the tissues and organs require specialized fuels and patterns of metabolism. Hormonal and neuronal signals integrate and coordinate the metabolic activities of different tissues and optimize the allocation of fuels and precursors to each organ. Although our focus is on mammalian systems, mammals are hardly unique in possessing hormonal signaling systems. Insects and nematode worms have highly developed systems for hormonal regulation, with fundamental mechanisms similar to those in mammals. Plants, too, use hormonal signals to coordinate the activities of their differentiated, specialized tissues. In this chapter we will look at the specialized metabolism of several major organs and tissues and the integration of metabolism in mammals. We begin with an overview of the broad range of hormones and hormonal mechanisms, then turn to the tissue-specific functions regulated by these mechanisms. We discuss the distribution of nutrients to various organs, emphasizing the central role of the liver, and the metabolic cooperation among these organs. To illustrate the integrative role of hormones, we describe the interplay of insulin, glucagon, and epinephrine in coordinating fuel metabolism in muscle, liver, and adipose tissue. We also introduce other hormones, produced in adipose tissue, muscle, gut, and brain, that play key roles in coordinating metabolism and behavior. We discuss the long-term hormonal regulation of body mass and the role of obesity in the development of metabolic syndrome and type 2 diabetes. Finally, we discuss interventions used to manage diabetes. In this chapter, we illustrate the following principles: The tissues in a mammal are connected by a neurosecretory system that coordinates their activities. Mammals use chemically diverse hormones in a highly specific and multidirectional signaling system, connecting the tissues with each other and with the central nervous system. Among the tissues and organs of an animal, there is a striking division of labor. The specialized role of each organ is reflected in its metabolic activities and capabilities. The circulatory system connects all of the tissues by carrying hormonal signals and metabolites between and among them. Because the brain requires a continuous supply of glucose, maintaining an adequate concentration of glucose in the blood is a high priority in the activities of the other tissues. The liver integrates the use of fuels (glucose, fatty acids, and amino acids) by each tissue to keep the blood glucose level within the optimal range. Hormones carried in the blood (insulin, glucagon, epinephrine, cortisol) mediate this regulation. Maintaining an optimal body mass is an important priority in the adult mammal. Body mass is a function of dietary intake, physical activity, and the choice of metabolic fuel, all of which are subject to hormonal regulation. Hormonal signals between the brain, the adipose tissue, and the gastrointestinal tract help to set activity and feeding behavior. The metabolic activities of cells and organisms are complex and intertwined; perturbation at one point in the system has far-reaching consequences for health. When the normal energy-yielding metabolism of glucose and fatty acids is impeded by defective insulin signaling, the result is the disease diabetes. 23.1 Hormone Structure and Action Hormones are small molecules or proteins that are produced in one tissue, released into the bloodstream, and carried to other tissues, where they act through specific receptors to bring about changes in cellular activities. Hormones serve to coordinate the metabolic activities of several tissues or organs. Virtually every process in a complex organism is regulated by one or more hormones: maintenance of blood pressure, blood volume, and electrolyte balance; embryogenesis; sexual differentiation, development, and reproduction; hunger, eating behavior, digestion, and fuel allocation — to name but a few. The coordination of metabolism in mammals is achieved by the neuroendocrine system. Individual cells in one tissue sense a change in the organism’s circumstances and respond by secreting a chemical messenger that passes to another cell in the same or different tissue, where the messenger binds to a specific receptor molecule and triggers a change in this target cell. These chemical messengers may relay information over very short or very long distances. In neuronal signaling (Fig. 23-1a), the chemical messenger is a neurotransmitter (acetylcholine, for example) that may travel only a fraction of a micrometer across a synaptic cle to the next neuron in a network. In endocrine signaling (Fig. 23- 1b), the messenger is a hormone that is carried in the bloodstream to neighboring cells or to distant organs and tissues; it may travel a meter or more before encountering its target cell. Except for this anatomic difference, the neuronal and endocrine signaling mechanisms are remarkably similar, and the same molecule can sometimes act as both neurotransmitter and hormone. Epinephrine and norepinephrine, for example, serve as neurotransmitters at certain synapses of the brain and at neuromuscular junctions of smooth muscle and as hormones that regulate fuel metabolism in liver and muscle. The following discussion of cellular signaling emphasizes hormone action, drawing on discussions of fuel metabolism in earlier chapters, but most of the fundamental mechanisms described here also occur in neurotransmitter action. FIGURE 23-1 Signaling by the neuroendocrine system. (a) In neuronal signaling, electrical signals (nerve impulses) originate in the cell body of a neuron and travel very rapidly over long distances to the axon tip, where neurotransmitters are released and diffuse to the target cell. The target cell (another neuron, a myocyte, or a secretory cell) is only a fraction of a micrometer or a few micrometers away from the site of neurotransmitter release. (b) In endocrine signaling, hormones (such as insulin produced in pancreatic β cells) are secreted into the bloodstream, which carries them throughout the body to target tissues that may be a meter or more away from the secreting cell. Both neurotransmitters and hormones interact with specific receptors on or in their target cells, triggering responses. Hormones Act through Specific High- Affinity Cellular Receptors Hormones exert their effects through specific receptors in target cells. The high affinity of the hormone-receptor interaction allows cells to respond to very low concentrations of hormone. Recall from Chapter 12 (Fig. 12-2) that there are four general types of intracellular consequences of ligand-receptor interaction: (1) a second messenger, such as cAMP, cGMP, or inositol trisphosphate, generated inside the cell, acts as an allosteric regulator of one or more enzymes; (2) a receptor tyrosine kinase is activated by the extracellular hormone; (3) a change in membrane potential results from the opening or closing of a hormone-gated ion channel; and (4) a steroid or steroidlike molecule causes a change in the level of expression (transcription of DNA into mRNA) of one or more genes, mediated by a nuclear hormone receptor protein. It can be helpful to characterize cell surface hormone receptors as metabotropic, those that activate or inhibit an enzyme downstream from the receptor, or ionotropic, those that open or close an ion channel in the plasma membrane, resulting in a change in membrane potential (ΔVm) or in the concentration of an ion such as Ca2+ (Fig. 23-2). FIGURE 23-2 Two general mechanisms of hormone action. The peptide and amine hormones are faster-acting than steroid and thyroid hormones. Peptide hormones act from outside the cell, binding a plasma membrane hormone receptor. Steroid and thyroid hormones pass through the plasma membrane and enter the nucleus, where they regulate the expression of specific genes. A single hormone molecule, in forming a hormone-receptor complex, activates a catalyst that produces many molecules of second messenger, so the receptor serves as both signal transducer and signal amplifier. The signal may be further amplified by a signaling cascade, such as we saw in the regulation of glycogen synthesis and breakdown by epinephrine (see Fig. 12- 7). Signal amplification allows one epinephrine molecule to result in the production of many thousands or millions of molecules of glucose 1-phosphate from glycogen. Water-insoluble hormones, including the steroid, retinoid, and thyroid hormones, readily pass through the plasma membrane of their target cells to reach their receptor proteins in the nucleus (Fig. 23-2). The hormone-receptor complex itself carries the message: it interacts with DNA to alter the expression of specific genes, changing the enzyme content of the cell and thereby changing cellular metabolism (see Fig. 12-34). Hormones that act through plasma membrane receptors generally trigger very rapid physiological or biochemical responses. Just seconds aer the adrenal medulla secretes epinephrine into the bloodstream, skeletal muscle responds by accelerating the breakdown of glycogen. By contrast, the thyroid hormones and the sex (steroid) hormones promote maximal responses in their target tissues only aer hours or even days. These differences in response time correspond to different modes of action. In general, the fast-acting hormones lead to a change in the activity of one or more preexisting enzymes in the cell, by allosteric mechanisms or covalent modification. The slower-acting hormones generally alter gene expression, resulting in the synthesis of more (upregulation) or less (downregulation) of the regulated protein(s). Hormones Are Chemically Diverse Mammals have several classes of hormones, distinguishable by their diverse chemical structures and their modes of action (Table 23-1). Peptide, catecholamine, eicosanoid, and endocannabinoid hormones act from outside the target cell via cell surface receptors. Steroid, vitamin D, retinoid, and thyroid hormones enter the cell and act through nuclear receptors. Nitric oxide (a gas) also enters the cell, but activates a cytosolic enzyme, guanylyl cyclase. TABLE 23-1 Classes of Hormones Hormones can also be classified by the way they get from their point of release to their target tissue. Endocrine (from the Greek endon, “within,” and krinein, “to release”) hormones are released into the blood and carried to target cells throughout the body (insulin and glucagon are examples). Paracrine hormones are released into the extracellular space and diffuse to neighboring target cells (the eicosanoid hormones are of this type). Autocrine hormones affect the same cell that releases them, binding to receptors on the cell surface. The peptide hormones vary in size from 3 (thyrotropin-releasing hormone; Fig. 23-3) to more than 200 (human chorionic gonadotropin) amino acid residues. They include the pancreatic hormones insulin, glucagon, and somatostatin; the parathyroid hormone calcitonin; and all the hormones of the hypothalamus and pituitary. Peptide hormones (some of which are actually small proteins) are synthesized as proproteins (prohormones) that are activated upon release by proteolytic cleavage, much as we saw with zymogen activation of pancreatic enzymes (Fig. 6-42) and the blood-clotting cascade (Fig. 6-44). In some cases, a preproprotein is synthesized and processed to create more than one active peptide product. FIGURE 23-3 The structure of thyrotropin-releasing hormone (TRH). Purified (through heroic efforts) from extracts of hypothalamus, TRH proved to be a derivative of the tripeptide Glu–His–Pro. The side-chain carboxyl group of the amino-terminal Glu forms an amide (red bond) with the residue’s α -amino group, creating pyroglutamate, and the carboxyl group of the carboxyl-terminal Pro is converted to an amide (red — NH2). Such modifications are common among the small peptide hormones. In a typical protein of Mr~50,000, the charges on the amino- and carboxyl- terminal groups contribute relatively little to the overall charge on the molecule, but in a tripeptide these two charges dominate the properties of the molecule. Formation of the amide derivatives removes these charges. Insulin is a small protein (Mr 5,800) with two polypeptide chains, A and B, joined by two disulfide bonds. (The amino acid sequence of bovine insulin is shown in Figure 3-24.) It is synthesized in the pancreas as an inactive single-chain precursor, preproinsulin (Fig. 23-4), with an amino-terminal “signal sequence” that directs its passage into secretory vesicles. (Signal sequences are discussed in Chapter 27; see Fig. 27-38.) Proteolytic removal of the signal sequence and formation of three disulfide bonds produces proinsulin, which is stored in secretory granules in pancreatic β cells. When blood glucose is elevated sufficiently to trigger insulin secretion, proinsulin is converted to active insulin by specific proteases, which cleave two peptide bonds to form the mature insulin molecule and C peptide, which are released into the blood by exocytosis. The capillaries that serve peptide- producing endocrine glands are fenestrated (punctuated with tiny holes or “windows”), so the hormone molecules readily enter the bloodstream for transport to target cells elsewhere. Insulin, acting through its receptor tyrosine kinase (Fig. 12-21) has profound effects on both anabolic and catabolic processes in many tissues, which we explore in detail below. FIGURE 23-4 Insulin. Mature insulin is formed from its larger precursor preproinsulin by proteolytic processing. Removal of a 23 amino acid segment (the signal sequence) at the amino terminus of preproinsulin and formation of three disulfide bonds produce proinsulin. Further proteolytic cuts remove the C peptide from proinsulin to produce mature insulin, composed of A and B chains. Pro-opiomelanocortin (POMC) is a spectacular example of a proprotein that undergoes specific cleavage to produce several active hormones. The POMC gene encodes a large polypeptide that contains within it at least nine biologically active peptides (Fig. 23-5). The proprotein is processed differently in different tissues, depending on which proteases the cells express. The active products influence an astonishing number of physiological systems. FIGURE 23-5 Proteolytic processing of the pro-opiomelanocortin (POMC) precursor. The initial gene product of the POMC gene is a long polypeptide that undergoes cleavage by a series of specific proteases to produce ACTH (corticotropin), β - and γ -lipotropin, α -, β -, and γ -MSH (melanocyte- stimulating hormone, or melanocortin), CLIP (corticotropin-like intermediary peptide), β -endorphin, and Met-enkephalin. The points of cleavage are pairs of basic residues, Arg–Lys, Lys–Arg, or Lys–Lys. Some Hormones Are Released by a “Top-Down” Hierarchy of Neuronal and Hormonal Signals The changing levels of specific hormones regulate specific cellular processes, but what regulates the regulators — what sets the level of each hormone? The brief answer is that the central nervous system receives input from many internal and external sensors — signals about danger, hunger, dietary intake, blood composition, for example — and orchestrates the production of appropriate hormonal signals by the endocrine tissues. For a more complete answer, consider the path of cortisol release by the adrenal gland, triggered by a stress detected by the central nervous system. Figure 23-6 illustrates the “chain of command” in this top-down hormonal signaling hierarchy. The hypothalamus, a pea-size region of the brain (Fig. 23-7), is the coordination center of the endocrine system; it receives and integrates messages from the central nervous system. In response, the hypothalamus produces releasing factors, including corticotropin-releasing hormone (CRH), that pass directly to the nearby pituitary gland through blood vessels and neurons that connect the two glands. The pituitary gland secretes adrenocorticotropic hormone (also called corticotropin or ACTH), which travels through the blood to the adrenal cortex and triggers the release of cortisol. Cortisol, the ultimate hormone in this cascade, acts through its receptor in many types of target cells to alter their metabolism. In hepatocytes, one effect of cortisol is to increase the rate of gluconeogenesis.

FIGURE 23-6 Cascade of top-down hormone release following central nervous system input to the hypothalamus. Solid black arrows indicate hormone production and release; broken black arrows indicate the action of hormones on target tissues. In each endocrine tissue along the pathway, a stimulus from the level above is received, amplified, and transduced into release of the next hormone in the cascade. The cascade is sensitive to regulation at several levels through feedback inhibition (thin, dashed arrows) by the ultimate hormone (in this case, cortisol). The product therefore regulates its own production, as in feedback inhibition of biosynthetic pathways within a single cell.

FIGURE 23-7 Neuroendocrine origins of hormone signals. Location of the hypothalamus and pituitary gland and details of the hypothalamus- pituitary system. Signals from connecting neurons stimulate the hypothalamus to secrete releasing factors into a blood vessel that carries the hormones directly to a capillary network in the anterior pituitary. In response to each releasing factor, the anterior pituitary releases the appropriate hormone into the general circulation. Posterior pituitary hormones are synthesized in neurons arising in the hypothalamus; the hormones are transported along axons to nerve endings in the posterior pituitary and stored there until released into the blood in response to a neuronal signal. Hormonal cascades such as those responsible for the release of cortisol result in large amplifications of the initial signal and allow exquisite fine-tuning of the output of the ultimate hormone (Fig. 23-6). At each level in the cascade, a small signal elicits a larger response. For example, the initial electrical signal to the hypothalamus results in the release of a few nanograms of corticotropin-releasing hormone, which elicits the release of a few micrograms of corticotropin. Corticotropin acts on the adrenal cortex to cause the release of milligrams of cortisol, for an overall amplification of at least a millionfold. At each level of a hormonal cascade, feedback inhibition of earlier steps in the cascade is possible; an unnecessarily elevated level of the ultimate hormone or of an intermediate hormone inhibits the release of earlier hormones in the cascade. These feedback mechanisms accomplish the same end as those that limit the output of a biosynthetic pathway (compare Fig. 23-6 with Fig. 22-37): a product is synthesized (or released) only until the necessary concentration is reached. In the blood-clotting cascade (Fig. 6-44) we saw a similar pattern: a signal stimulates a cascade of protein activations; feedback mechanisms limit their action and the duration of the response. “Bottom-Up” Hormonal Systems Send Signals Back to the Brain and to Other Tissues In addition to the top-down hierarchy of hormonal signaling shown in Figure 23-6, some hormones are produced in the digestive tract, muscle, and adipose tissue and communicate the current metabolic state to the hypothalamus (Fig. 23-8). These signals are integrated in the hypothalamus, and an appropriate neuronal or hormonal response is elicited. FIGURE 23-8 Regulation of feeding behavior by two-way information flow between tissues and the hypothalamus. When food intake and energy production are adequate, peptide hormones released by the stomach, intestine, and adipose tissue feed back on the hypothalamus to signal satiety and reduce feeding behavior. Other tissue-specific peptide hormones signal inadequate supplies of stored triacylglycerols or low blood glucose levels. All of these signals impinge, directly or indirectly, on AMP- activated protein kinase (AMPK) in the hypothalamus, which integrates these signals and influences feeding behavior and energy-yielding metabolism in the tissues. Nerves carry electrical signals from the brain to the other tissues to complete the information circuit and achieve homeostasis (not shown). TRH, thyrotropin-releasing hormone; GLP-1, glucagon-like peptide-1; GIP, gastric inhibitory polypeptide. Adipokines, for example, are peptide hormones, produced in adipose tissue, that signal the adequacy of fat reserves. The adipokine leptin, released when adipose tissue is well-filled with triacylglycerols, acts in the brain to inhibit feeding behavior, whereas adiponectin signals depletion of fat reserves and stimulates feeding. Other tissues produce and release other hormones. Ghrelin is produced in the gastrointestinal tract when the stomach is empty and acts in the hypothalamus to stimulate feeding behavior; when the stomach fills, ghrelin release ceases. Incretins are peptide hormones produced in the gut aer ingestion of a meal; they increase secretion of insulin and decrease secretion of glucagon from the pancreas. Neuropeptide Y (NPY) is a hormone produced in the hypothalamus and in the adrenal glands. Its release promotes feeding and reduces energy expenditure for nonessential activities. Peptide YY (PYY3–36), produced in the intestine, signals satiety in the brain. Irisin is a peptide hormone produced in muscle as a result of exercise; it acts to convert white adipose tissue into beige adipose tissue, which dissipates energy as heat (see Section 23.2). We return to these hormones (summarized in Table 23-2) when we discuss the regulation of body mass in humans (Section 23.4). TABLE 23-2 Some Peptide Hormones That Act on Feeding Behavior and Fuel Selection in Mammals Hormone Production site(s) Target tissue(s) Action(s) Insulin Pancreatic β Muscle, Stimulates glucose uptake cells adipose, liver and synthesis of glycogen and fat Glucagon Pancreatic α cells Liver, adipose Stimulates gluconeogenesis and glucose release to blood Leptin Adipose tissue Hypothalamus Reduces hunger Adiponectin Adipose tissue Muscle, liver, others Stimulates catabolism and feeding behavior Ghrelin Stomach, intestine Brain Signals hunger Incretins: GLP-1, GIP Intestine Pancreas Stimulate insulin release NPY Hypothalamus, adrenals Brain, autonomic nervous system Stimulates feeding behavior PYY3–36 Intestine Brain Signals satiety Irisin Muscle (a er exercise) Adipose Turns white adipose tissue to beige SUMMARY 23.1 Hormone Structure and Action Hormones connect all of the organs in the body, carrying information and signals between the central nervous system and all of the tissues. Hormones are chemically diverse with a wide range of biological roles. Peptide hormones are synthesized as proteins on ribosomes, then cleaved proteolytically to form the active peptide(s). Hormones are regulated by a top-down hierarchy of interactions between the brain and endocrine glands: nerve impulses stimulate the hypothalamus to send specific hormones to the pituitary gland, thus stimulating (or inhibiting) the release of a second rank of hormones. The anterior pituitary hormones in turn stimulate other endocrine glands (adrenals, for example) to secrete their characteristic hormones, which in turn stimulate specific target tissues. Some hormones act in bottom-up signaling: adipose tissue, muscle, and the gastrointestinal tract release peptide hormones that act on other tissues or in the central nervous system. 23.2 Tissue-Specific Metabolism Each tissue of the human body has a specialized function, reflected in its anatomy and metabolic activity (Fig. 23-9). Skeletal muscle allows directed motion; adipose tissue stores and distributes energy in the form of fats, which serve as fuel throughout the body and as thermal insulation; in the brain, cells pump ions across their plasma membranes to produce electrical signals. The liver plays a central processing and distribution role in metabolism and furnishes all other organs and tissues with an appropriate mix of nutrients via the bloodstream. The functional centrality of the liver is indicated by the common reference to all other tissues and organs as “extrahepatic.” We therefore begin our discussion of the division of metabolic labor by considering the transformations of carbohydrates, amino acids, and fats in the mammalian liver. This is followed by brief descriptions of the primary metabolic functions of adipose tissue, muscle, brain, and the tissue that interconnects all others: the blood. FIGURE 23-9 Specialized metabolic functions of mammalian tissues. The Liver Processes and Distributes Nutrients During digestion in mammals, the three main classes of nutrients (carbohydrates, fats, and proteins) undergo enzymatic hydrolysis into their simple constituents (monosaccharides, fatty acids, and amino acids). This breakdown is necessary because the epithelial cells lining the intestinal lumen absorb only relatively small molecules. Many of the fatty acids and monoacylglycerols released by digestion of fats in the intestine are reassembled within these epithelial cells into triacylglycerols (TAGs). Aer being absorbed, most sugars and amino acids and some reconstituted TAGs pass from intestinal epithelial cells into blood capillaries and travel in the bloodstream to the liver; the remaining TAGs enter adipose tissue via the lymphatic system. The portal vein (Fig. 23-9) is a direct route from the digestive organs to the liver, and the liver therefore has first access to ingested nutrients. The liver has two main cell types. Kupffer cells are phagocytes, important in immune function. Hepatocytes, of primary interest here, transform dietary nutrients into the fuels and precursors required by other tissues and export them via the blood. The kinds and amounts of nutrients supplied to the liver are determined by the composition of the diet, the interval between meals, and several other factors. The demand of extrahepatic tissues for fuels and precursors varies from one organ to another, and it also varies with the level of activity and overall nutritional state of the individual. To meet these changing circumstances, the liver has remarkable metabolic flexibility. For example, when the diet is rich in protein, hepatocytes synthesize more of the enzymes needed for amino acid catabolism and gluconeogenesis. Within hours aer a shi to a high-carbohydrate diet, the levels of these enzymes begin to drop and the hepatocytes increase their synthesis of enzymes essential to carbohydrate metabolism and fat synthesis. Liver enzymes turn over (that is, are synthesized and degraded) at 5 to 10 times the rate of enzyme turnover in other tissues, such as muscle. Extrahepatic tissues also can adjust their metabolism to prevailing conditions, but none of these tissues is as adaptable as the liver, and none is so central to the organism’s overall metabolism. We turn now to a survey of the possible fates of sugars, amino acids, and lipids that enter the liver from the bloodstream. Carbohydrates The glucose transporter of hepatocytes (GLUT2) allows rapid, passive diffusion of glucose, so that the concentration of glucose in a hepatocyte is essentially the same as that in the blood. Glucose entering hepatocytes is phosphorylated by glucokinase (hexokinase IV) to yield glucose 6-phosphate. Glucokinase has a much higher Km for glucose (10 mM) than do the hexokinase isozymes in other cells (p. 539) and, unlike these other isozymes, it is not inhibited by its product, glucose 6-phosphate. The presence of glucokinase allows hepatocytes to continue phosphorylating glucose when the glucose concentration rises well above levels that would overwhelm other hexokinases. The high Km of glucokinase also ensures that the phosphorylation of glucose in hepatocytes is minimal when the glucose concentration is low, preventing the liver from consuming glucose as fuel via glycolysis. This spares glucose for other tissues. Fructose, galactose, and mannose, all absorbed from the small intestine, are also converted to glucose 6-phosphate by enzymatic pathways examined in Chapter 14 (see Fig. 14-9). Glucose 6- phosphate is at the crossroads of carbohydrate metabolism in the liver. It may take any of several major metabolic routes (Fig. 23- 10), depending on the current metabolic needs of the organism. By the action of various allosterically regulated enzymes, and through hormonal regulation of enzyme synthesis and activity, the liver directs the flow of glucose into one or more of these pathways. FIGURE 23-10 Metabolic pathways for glucose 6-phosphate in the liver. Here and in Figures 23-11 and 23-12, anabolic pathways are generally shown leading upward, catabolic pathways leading downward, and distribution to other organs leading horizontally. The numbered processes in each figure are described in the text. Glucose 6-phosphate is dephosphorylated by glucose 6- phosphatase to yield free glucose (see Fig. 15-6), which is exported to replenish blood glucose. Export is the predominant pathway when glucose 6-phosphate is in limited supply, because the blood glucose concentration must be kept sufficiently high (4 to 5 mM) to provide adequate energy for the brain and other tissues. Glucose 6-phosphate not immediately needed to form blood glucose is converted to liver glycogen, or has one of several other fates. Following glycolysis and the pyruvate dehydrogenase reaction, the acetyl-CoA so formed can be oxidized for ATP production by the citric acid cycle, with ensuing electron transfer and oxidative phosphorylation yielding ATP. (Normally, however, fatty acids are the preferred fuel for ATP production in hepatocytes.) Acetyl-CoA can also serve as the precursor of fatty acids, which are incorporated into TAGs and phospholipids, and of cholesterol. Much of the lipid synthesized in the liver is transported to other tissues by blood lipoproteins. Alternatively, glucose 6-phosphate can enter the pentose phosphate pathway, yielding both reducing power (NADPH), needed for the biosynthesis of fatty acids and cholesterol, and ribose 5-phosphate, a precursor for nucleotide biosynthesis. NADPH is also an essential cofactor in the detoxification and elimination of many drugs and other xenobiotics (compounds that don’t occur naturally but are the products of human activity, such as drugs, food additives, and preservatives) that are metabolized in the liver. Amino Acids Amino acids that enter the liver follow several important metabolic routes (Fig. 23-11). They are precursors for protein synthesis, a process discussed in Chapter 27. The liver constantly renews its own proteins, which have a relatively high turnover rate (average half-life of hours to days), and it is also the site of biosynthesis of most plasma proteins. Alternatively, amino acids pass in the bloodstream to other organs to be used in the synthesis of tissue proteins. Other amino acids are precursors in the biosynthesis of nucleotides, hormones, and other nitrogenous compounds in the liver and other tissues. FIGURE 23-11 Metabolism of amino acids in the liver. Amino acids not needed as biosynthetic precursors are transaminated or deaminated and degraded to yield pyruvate and citric acid cycle intermediates, with various fates; the ammonia released is converted to the excretory product urea. Pyruvate can be converted to glucose and glycogen via gluconeogenesis, or can be converted to acetyl-CoA, which has several possible fates: oxidation via the citric acid cycle and oxidative phosphorylation to produce ATP, or conversion to lipids for storage. Citric acid cycle intermediates can be siphoned off into glucose synthesis by gluconeogenesis. The liver also metabolizes amino acids that arrive intermittently from other tissues. The blood is adequately supplied with glucose just aer the digestion and absorption of dietary carbohydrate or, between meals, by the conversion of liver glycogen to blood glucose. During the interval between meals, especially if prolonged, some muscle protein is degraded to amino acids. These amino acids donate their amino groups (by transamination) to pyruvate, the product of glycolysis, to yield alanine, which is transported to the liver and deaminated. Hepatocytes convert the resulting pyruvate to blood glucose via gluconeogenesis , and the ammonia to urea for excretion . One benefit of this glucose-alanine cycle is the smoothing out of fluctuations in blood glucose between meals. The amino acid deficit incurred in muscles is made up aer the next meal by incoming dietary amino acids. Lipids The fatty acid components of lipids entering hepatocytes also have several different fates (Fig. 23-12). Some are converted to liver lipids. Under most circumstances, fatty acids are the primary oxidative fuel in the liver. Free fatty acids may be activated and oxidized to yield acetyl-CoA and NADH. The acetyl-CoA is further oxidized via the citric acid cycle, and oxidations in the cycle drive the synthesis of ATP by oxidative phosphorylation. Excess acetyl-CoA, not required by the liver, is converted to acetoacetate and β -hydroxybutyrate; these ketone bodies circulate in the blood to other tissues to be used as fuel for the citric acid cycle. Ketone bodies, unlike fatty acids, can cross the blood-brain barrier, providing the brain with a source of acetyl-CoA for energy-yielding oxidation. Ketone bodies can supply a significant fraction of the energy in some extrahepatic tissues — up to one-third in the heart and as much as 60% to 70% in the brain during prolonged fasting. Some of the acetyl-CoA derived from fatty acids (and from glucose) is used for the biosynthesis of cholesterol, which is required for membrane synthesis. Cholesterol is also the precursor of all steroid hormones and of the bile salts, which are essential for the digestion and absorption of lipids. FIGURE 23-12 Metabolism of fatty acids in the liver. The other two metabolic fates of lipids require specialized mechanisms for the transport of insoluble lipids in blood. Fatty acids are converted to the phospholipids and TAGs of plasma lipoproteins, which carry lipids to adipose tissue for storage. Some free fatty acids (FFAs) are bound to serum albumin and carried to the heart and skeletal muscles, which take up and oxidize FFAs as a major fuel. Serum albumin is the most abundant plasma protein; one molecule can carry up to seven FFA molecules (see Fig. 17-3). The liver thus serves as the body’s distribution center, exporting nutrients in the correct proportions to other organs, smoothing out fluctuations in metabolism caused by intermittent food intake, and processing excess amino groups into urea and other products to be disposed of by the kidneys. Certain nutrients are stored in the liver, including iron ions and vitamin A. The liver also detoxifies xenobiotics. Detoxification oen includes the cytochrome P-450–dependent hydroxylation of relatively insoluble organic compounds, making them sufficiently soluble for further breakdown and excretion (see Box 21-1). Adipose Tissues Store and Supply Fatty Acids There are two primary types of adipose tissue, white and brown (Fig. 23-13), with different roles, and we focus first on the more abundant of the two. White adipose tissue (WAT) is amorphous and widely distributed in the body: under the skin, around deep blood vessels, and in the abdominal cavity. The adipocytes of white adipose tissue are large (diameter 30 to 70 μ m), spherical cells, completely filled with a single large lipid droplet that constitutes about 65% of the cell mass and squeezes the mitochondria and nucleus into a thin layer against the plasma membrane (Fig. 23-13a, c). The lipid droplet contains TAGs and sterol esters and is coated with a monolayer of phospholipids, oriented with their polar head groups facing the cytosol. Specific proteins are associated with the surface of the droplets, including perilipin and the enzymes for synthesis and breakdown of TAGs (see Fig. 17-2). White adipose tissue typically makes up about 15% of the mass of a healthy young adult human. Adipocytes are metabolically active, responding quickly to hormonal stimuli in a metabolic interplay with the liver, skeletal muscles, and heart. FIGURE 23-13 White and brown adipose tissue. Schematic views of typical mouse adipocytes from (a) white adipose tissue (WAT) and (b) brown adipose tissue (BAT). White adipocytes are larger and contain a single huge lipid droplet, which squeezes the mitochondria and nucleus against the plasma membrane. In brown adipocytes, mitochondria are much more prominent, the nucleus is near the center of the cell, and multiple small fat droplets are present. Below are light micrographs of (c) adipocytes in white adipose tissue, stained to show nuclei, and (d) a region of mixed white and brown adipocytes, stained with an antibody specific to UCP1, the uncoupling protein responsible for thermogenesis. Adipocytes have an active glycolytic metabolism, oxidize pyruvate and fatty acids via the citric acid cycle, and carry out oxidative phosphorylation. During periods of high carbohydrate intake, adipose tissue can convert glucose (via pyruvate and acetyl-CoA) to fatty acids, convert the fatty acids to TAGs, and store the TAGs as large lipid droplets — although in humans, much of the fatty acid synthesis occurs in hepatocytes. Adipocytes store TAGs arriving from the liver (carried in the blood as very-low-density lipoproteins) and from the intestinal tract (carried in chylomicrons), particularly aer meals rich in fat. When the demand for fuel rises (between meals, for example), lipases in adipocytes hydrolyze stored TAGs to release FFAs, which can travel in the bloodstream to skeletal muscle, the heart, and, during starvation, the liver. The release of fatty acids from adipocytes is greatly accelerated by epinephrine, which stimulates the cAMP-dependent phosphorylation of perilipin and thus gives lipases that are specific for tri-, di-, and monoacylglycerols access to TAGs in lipid droplets (see Fig. 17-2). Hormone-sensitive lipase is also stimulated by phosphorylation. Insulin counterbalances this effect of epinephrine, decreasing the activity of the lipase. The breakdown and synthesis of TAGs in adipose tissue constitute a substrate cycle; up to 70% of the fatty acids released by the three lipases are reesterified in adipocytes, re-forming TAGs. Each cycle consumes ATP (used to activate the fatty acids as acyl-CoA esters), so the net effect of the substrate cycling is the breakdown of ATP and the accompanying release of heat. In adipose tissue, glycerol liberated by adipocyte lipases cannot be reused in the synthesis of TAGs, because adipocytes lack glycerol kinase. Instead, the glycerol phosphate required for TAG synthesis is made from pyruvate by glyceroneogenesis, requiring the action of the cytosolic PEP carboxykinase (see Fig. 21-22). In addition to its central function as a fuel depot, adipose tissue plays an important role as an endocrine organ, producing and releasing hormones that signal the state of energy reserves and coordinate metabolism of fats and carbohydrates throughout the body. We return to this function in Section 23.4 when we discuss the hormonal regulation of body mass. Brown and Beige Adipose Tissues Are Thermogenic In small vertebrates and hibernating animals, a significant proportion of the adipose tissue is brown adipose tissue (BAT), distinguished from white adipose tissue by its smaller (diameter 20 to 40 μ m), differently shaped (polygonal, not round) adipocytes (Fig. 23-13b, d). Like white adipocytes, brown adipocytes store TAGs, but in several smaller lipid droplets per cell rather than as a single central droplet. Brown adipocytes have more mitochondria and a richer supply of capillaries and innervation than white adipocytes; it is the cytochromes of mitochondria and the hemoglobin in capillaries that give brown adipose tissue its characteristic color. A unique feature of brown adipocytes is their production of uncoupling protein 1 (UCP1), also called thermogenin (see Fig. 19-36). This protein is responsible for one of the principal functions of brown adipose tissue: thermogenesis. In brown adipocytes, fatty acids stored in lipid droplets are released, enter mitochondria, and undergo complete conversion to CO2 by β oxidation and the citric acid cycle. The reduced FADH2 and NADH so generated pass their electrons through the respiratory chain to molecular oxygen. In white adipocytes, protons pumped out of the mitochondria during electron transfer reenter the matrix through ATP synthase, with the energy of electron transfer conserved in ATP synthesis. In brown adipocytes, UCP1 provides an alternative route for the reentry of protons that bypasses ATP synthase. The energy of the proton gradient is thus dissipated as heat, which can maintain the body (especially the nervous system and viscera) at its optimal temperature when the ambient temperature is relatively low. In the human fetus, differentiation of fibroblast preadipocytes into brown adipose tissue begins at the twentieth week of gestation, and at the time of birth, brown adipose tissue represents 1% to 5% of total body mass. The brown fat deposits are located where the heat generated by thermogenesis can ensure that vital tissues — blood vessels to the head, major abdominal blood vessels, and the viscera, including the pancreas, adrenal glands, and kidneys — are not chilled as the newborn enters a world of lower ambient temperature (Fig. 23-14a).

FIGURE 23-14 Brown adipose tissue in infants and adults. (a) At birth, human infants have brown fat distributed as shown here, to protect the spine, major blood vessels, and internal organs. (b) Positron emission tomography (PET) scanning can show metabolic activity in a living person, in real time. PET scans allow visualization of isotopically labeled glucose in precisely localized regions of the body. A positron-emitting glucose analog, 2-[18F]-fluoro-2-deoxy- -glucose (FDG), is injected into the bloodstream; a short time later, a PET scan shows how much of the glucose has been taken up by each part of the body — a measure of metabolic activity. On the le is a PET scan of a healthy 25- year-old man who fasted for 12 hours, then stayed for 1 hour in a cold (19°C) room, with his legs on ice to thoroughly chill him. At the end of the hour, he was injected with [18F]-FDG, then remained under cold conditions for another hour. Whole-body PET scans were then done at 24°C. For the control scan, the same man underwent the same PET scan protocol two weeks later, but this time following 2 hours at 27°C instead of chilling (right). Intense labeling of the brain and heart shows high rates of glucose uptake; labeling of the kidneys and bladder indicates clearance of FDG. In the scan a er chilling (le ), [18F]-FDG labels brown adipose tissue in the region above the collarbone and along the vertebrae. At birth, white adipose tissue development begins and brown adipose tissue begins to disappear. Young adult humans have much-diminished deposits of brown adipose tissue, ranging from 3% of all adipose tissue in males to 7% in females, making up less than 0.1% of body mass. However, adults have, distributed among their white adipose cells, significant numbers of adipocytes that can be converted by cold exposure or by β -adrenergic stimulation into cells very similar to brown adipocytes. These beige adipocytes have multiple lipid droplets, are richer in mitochondria than white adipocytes, and produce UCP1, so they function effectively as heat generators. Brown and beige adipocytes produce heat by oxidation of their own fatty acids, but they also take up and oxidize both fatty acids and glucose from the blood at rates out of proportion to their mass. In fact, the detection of brown adipose tissue by PET scanning depends on the adipocytes’ relatively high rate of glucose uptake and metabolism (Fig. 23-14b). In adaptation to warm or cold surroundings, and in the normal differentiation of white, brown, and beige adipose tissue, the nuclear transcription factor PPARγ (described later in the chapter) plays a central role. And as we noted above, the peptide hormone irisin, produced in muscle by exercise, triggers the development of beige adipose tissue that continues to burn fuel long aer the exercise ends. Muscles Use ATP for Mechanical Work Metabolism in skeletal muscle cells — myocytes — is specialized to generate ATP as the immediate source of energy for contraction. Moreover, skeletal muscle is adapted to do its mechanical work intermittently, on demand. Sometimes skeletal muscles must work at their maximum capacity for a short time, as in a 100 m sprint; at other times more prolonged work is required, as in running a marathon or in prolonged physical labor. There are two general classes of muscle tissue, which differ in physiological role and fuel utilization. Slow-twitch muscle, also called red muscle, provides relatively low tension but is highly resistant to fatigue. It produces ATP by the relatively slow but steady process of oxidative phosphorylation. Red muscle is very rich in mitochondria and is served by dense networks of blood vessels, which bring the oxygen essential to ATP production. Fast- twitch muscle, or white muscle, has fewer mitochondria than red muscle and is less well supplied with blood vessels, but it can develop greater tension and do so faster. White muscle is quicker to fatigue because, when active, it uses ATP faster than it can replace it. There is a genetic component to the proportion of red and white muscle in any individual, but with training, an athlete can improve the endurance of fast-twitch muscle. Skeletal muscle can use free fatty acids or glucose as fuel, depending on the degree of muscular activity (Fig. 23-15). In resting muscle, the primary fuels are FFAs from adipose tissue. These are oxidized and degraded to yield acetyl-CoA, which enters the citric acid cycle, ultimately yielding the energy for ATP synthesis by oxidative phosphorylation. Muscle in light activity uses blood glucose in addition to fatty acids. The glucose is phosphorylated, then degraded by glycolysis to pyruvate, which is converted to acetyl-CoA and oxidized via the citric acid cycle and oxidative phosphorylation. FIGURE 23-15 Energy sources for muscle contraction. Different fuels are used for ATP synthesis during bursts of heavy activity and during light activity or rest. Phosphocreatine can rapidly supply ATP. In maximally active fast-twitch muscles, the demand for ATP is so great that the blood flow cannot provide O2 and fuels fast enough to supply sufficient ATP by aerobic respiration alone. Under these conditions, stored muscle glycogen is broken down to lactate by fermentation (p. 525). Each glucose unit degraded yields three ATP, because phosphorolysis of glycogen produces glucose 6- phosphate (via glucose 1-phosphate), sparing the ATP normally consumed in the hexokinase reaction. Lactic acid fermentation thus responds more quickly than oxidative phosphorylation to an increased need for ATP, supplementing basal ATP production by aerobic oxidation of other fuels via the citric acid cycle and respiratory chain. The secretion of epinephrine, which stimulates both the release of glucose from liver glycogen and the breakdown of glycogen in muscle tissue, greatly enhances the use of blood glucose and muscle glycogen as fuels for muscular activity. (Epinephrine mediates the so-called fight-or-flight response, discussed more fully below.) The relatively small amount of glycogen (about 1% of the total weight of skeletal muscle) limits the glycolytic energy available during all-out exertion. Moreover, the accumulation of lactate and consequent decrease in pH in maximally active muscles reduces their efficiency. Skeletal muscle, however, contains another source of ATP, phosphocreatine (10 to 30 mM), which can rapidly regenerate ATP from ADP by the creatine kinase reaction: During periods of active contraction and glycolysis, this reaction proceeds predominantly in the direction of ATP synthesis; during recovery from exertion, the same enzyme resynthesizes phosphocreatine from creatine and ATP. Because of the relatively high levels of ATP and phosphocreatine in muscle, these compounds can be detected in intact muscle, in real time, by NMR spectroscopy (Fig. 23-16). Creatine serves to shuttle ATP equivalents from the mitochondrion to sites of ATP consumption and can be the limiting factor in the development of new muscle tissue (Box 23-1). FIGURE 23-16 Phosphocreatine buffers ATP concentration during exercise. A “stack plot” of magnetic resonance spectra (of 31P) shows inorganic phosphate (Pi), phosphocreatine (PCr), and ATP (each of its three phosphates giving a signal). The series of plots represents the passage of time, from a period of rest to one of exercise, and then of recovery. Notice that the ATP signal barely changes during exercise, kept high by continued respiration and by the reservoir of phosphocreatine, which diminishes during exercise. During recovery, when ATP production by catabolism is greater than ATP use by the (now resting) muscle, the phosphocreatine reservoir is refilled. [Data from M. L. Blei, K. E. Conley, and M. J. Kushmerick, J. Physiol. 465:203, 1993, Fig. 4.] BOX 23-1 Creatine and Creatine Kinase: Invaluable Diagnostic Aids and the Muscle Builder’s Friends Animal tissues that have a high and fluctuating need for ATP, primarily skeletal muscle, cardiac muscle, and brain, contain several isozymes of creatine kinase. A cytosolic isozyme (cCK) is present in regions of high ATP use (myofibrils and sarcoplasmic reticulum, for example). By converting ADP produced during periods of high ATP use back to ATP, cCK prevents the accumulation of ADP to concentrations that could inhibit ATP-using enzymes by mass action. Another isozyme of creatine kinase is located in regions where the inner and outer membranes of mitochondria come into contact. This mitochondrial isozyme (mCK) probably serves to shuttle ATP equivalents produced in mitochondria to cytosolic sites of ATP use (Fig. 1). The species that diffuses from the mitochondrion to ATP-consuming activities in the cytosol is therefore creatine phosphate, not ATP. The mCK isozyme co-localizes with the adenine nucleotide transporter (in the inner mitochondrial membrane) and porin (in the outer mitochondrial membrane), which suggests that these three components may function together to transport ATP equivalents formed in mitochondria into the cytosol. FIGURE 1 Mitochondrial creatine kinase (mCK) transfers a phosphoryl group from ATP to creatine (Cr) to form phosphocreatine (PCr), which diffuses to sites of ATP use; at these sites, cytosolic creatine kinase (cCK) passes the phosphoryl group into ATP. Cytosolic CK can also use ATP produced by glycolysis to synthesize PCr. During periods of little ATP demand, the pools of ATP and PCr are replenished in preparation for the next period of intense demand for ATP. In resting muscle, the concentration of PCr is three to five times that of ATP, buffering the cell against rapid depletion of ATP during short bursts of ATP demand. [Information from U. Schlattner et al., Biochim. Biophys. Acta 1762:164, 2006, Fig. 1.] In knockout mice lacking the mitochondrial isozyme, myocytes compensate by producing more mitochondria, closely associated with myofibrils and sarcoplasmic reticulum, allowing quick diffusion of mitochondrial ATP to the sites of ATP use. Nevertheless, these mice have a reduced capacity for running, indicating a defect in some aspect of energy-supplying metabolism. Creatine and phosphocreatine spontaneously break down to form creatinine (Fig. 2). To maintain high creatine levels, these losses have to be compensated for, either by dietary creatine, obtained primarily from meat (muscle) and dairy products, or by de novo synthesis from glycine, arginine, and methionine (see Fig. 22-28), which occurs primarily in liver and kidney. De novo synthesis of creatine is a major consumer of these amino acids, particularly in vegans, for whom this is the only source of creatine; plants do not contain creatine. Muscle tissue has a specific system to take up creatine (exported by liver or kidney) from the blood, against a substantial concentration gradient. Efficient uptake of dietary creatine requires continuous exercise; without exercise, creatine supplementation is of little value. FIGURE 2 Spontaneous (nonenzymatic) formation of creatinine from phosphocreatine or creatine consumes a few percent of the body’s total creatine per day, which must be replaced by biosynthesis or from the diet. Heart muscle contains a unique isozyme of creatine kinase (MB, for myocardial band), which is not normally found in the blood but appears there when released from heart muscle damaged by a heart attack. The blood level of MB begins to rise within 2 hours of the heart attack, typically peaks 12 to 36 hours a er the heart attack, and returns to normal levels in 3 to 5 days. Measurement of the MB isozyme in blood therefore confirms a diagnosis of heart attack and indicates approximately when it occurred. Children with inborn errors in the enzymes of creatine synthesis or uptake suffer severe intellectual disability and seizures. They have much-reduced levels of brain creatine as measured by NMR (see Fig. 23-16). Creatine supplementation raises their brain creatine and creatine phosphate concentrations and brings about partial improvement of the symptoms. In the healthy kidney, creatinine from creatine breakdown is efficiently cleared from the blood into the urine. When renal function is defective, creatinine levels in the blood rise above the normal range of 0.8 to 1.4 mg/dL. Elevated blood creatinine is associated with renal failure in diabetes and other conditions in which renal function is temporarily or permanently compromised. Renal clearance of creatinine varies slightly with age, ethnicity, and gender, so correcting the calculation for those factors yields a more sensitive measure of the extent of renal function, the glomerular filtration rate (GFR). Body builders who are adding muscle mass have a greater need for creatine and commonly take creatine supplements of up to 20 g per day for a few days, followed by lower maintenance doses. The combination of exercise and creatine supplementation increases muscle mass (Fig. 3) and improves performance in high-intensity, short-duration work. FIGURE 3 Many body builders take supplemental creatine to supply phosphocreatine in new muscle tissue. Aer a period of intense muscular activity, the individual continues breathing heavily for some time, using much of the extra O2 for oxidative phosphorylation in the liver. The ATP produced is used for gluconeogenesis (in the liver) from lactate that has been carried in the blood from the muscles. The glucose thus formed returns to the muscles to replenish their glycogen, completing the Cori cycle (Fig. 23-17; see also Box 15-1). FIGURE 23-17 Metabolic cooperation between skeletal muscle and the liver: the Cori cycle. Extremely active muscles use glycogen as their energy source, generating lactate by glycolysis. During recovery, some of the lactate is transported to the liver and converted to glucose by gluconeogenesis. The glucose is released to the blood and returned to the muscles to replenish their glycogen stores. The overall pathway, glucose → lactate → glucose, constitutes the Cori cycle. Actively contracting skeletal muscle generates heat as a byproduct of imperfect coupling of the chemical energy of ATP with the mechanical work of contraction. This heat production can be put to good use when ambient temperature is low: skeletal muscle carries out shivering thermogenesis, rapidly repeated muscle contraction that produces heat but little motion, helping to maintain the body at its preferred temperature of 37°C. Heart muscle differs from skeletal muscle in that it is continuously active in a regular rhythm of contraction and relaxation, and has a completely aerobic metabolism at all times. Mitochondria are much more abundant in heart muscle than in skeletal muscle, making up almost half the volume of the cells (Fig. 23-18). The heart uses mainly FFAs as a source of energy, but also some glucose and ketone bodies taken up from the blood; these fuels are oxidized aerobically to generate ATP. Like skeletal muscle, heart muscle does not store lipids or glycogen in large amounts. It does have small amounts of reserve energy in the form of phosphocreatine, enough for a few seconds of contraction. Because the heart is normally aerobic and obtains its energy from oxidative phosphorylation, the failure of O2 to reach part of the heart muscle when the blood vessels are blocked by lipid deposits (atherosclerosis) or blood clots (coronary thrombosis) can cause that region of the heart muscle to die. This is what happens in myocardial infarction (heart attack). FIGURE 23-18 Electron micrograph of heart muscle. In the profuse mitochondria of heart tissue, pyruvate (from glucose), fatty acids, and ketone bodies are oxidized to drive ATP synthesis. This steady aerobic metabolism allows the human heart to pump blood at a rate of nearly 6 L/min, or about 350 L/h — which amounts to 200× 106L of blood over 70 years. The Brain Uses Energy for Transmission of Electrical Impulses The metabolism of the brain is remarkable in several respects. The neurons of the adult mammalian brain normally use only glucose as fuel (Fig. 23-19). (Astrocytes, the other major cell type in the brain, can oxidize fatty acids.) The brain, which constitutes about 2% of total body mass, has a very active respiratory metabolism (Fig. 23-15); more than 90% of the ATP produced in the neurons comes from oxidative phosphorylation. The brain uses O2 at a fairly constant rate, accounting for almost 20% of the total O2 consumed by the body at rest. Because the brain contains very little glycogen, it is constantly dependent on incoming glucose in the blood. Should blood glucose fall significantly below a critical level for even a short time, severe and sometimes irreversible changes in brain function may result.

FIGURE 23-19 The fuels that supply ATP in the brain. The energy source used by the brain varies with nutritional state. The ketone body used during starvation is β -hydroxybutyrate. Electrogenic transport by the Na+K+ ATPase maintains the transmembrane potential essential to information transfer among neurons. Although the neurons of the brain cannot directly use free fatty acids or lipids from the blood as fuels, they can, when necessary, get up to 60% of their energy requirement from the oxidation of β -hydroxybutyrate (a ketone body), formed in the liver from fatty acids. The capacity of the brain to oxidize β - hydroxybutyrate via acetyl-CoA becomes important during prolonged fasting or starvation, aer liver glycogen has been depleted, because it allows the brain to use body fat as an energy source. This spares muscle proteins — until they become the brain’s ultimate source of glucose, via gluconeogenesis in the liver, during severe starvation. In neurons, energy is required to create and maintain an electrical potential across the plasma membrane. The membrane contains an electrogenic ATP-driven antiporter, the Na+K+ ATPase, which simultaneously pumps two K+ ions into and three Na+ ions out of the neuron (see Fig. 11-39). The resulting transmembrane potential changes transiently as an electrical signal, an action potential, sweeps from one end of a neuron to the other (see Fig. 12-33). Action potentials are the chief mechanism of information transfer in the nervous system, so depletion of ATP in neurons would have disastrous effects on all activities coordinated by neuronal signaling. Blood Carries Oxygen, Metabolites, and Hormones Blood mediates the metabolic interactions among all tissues. It transports nutrients from the small intestine to the liver and from the liver and adipose tissue to other organs; it also transports waste products from extrahepatic tissues to the liver for processing and to the kidneys for excretion. Oxygen moves in the bloodstream from the lungs to the tissues, and CO2 generated by tissue respiration returns via the bloodstream to the lungs for exhalation. Blood also carries hormonal signals from one tissue to another. In its role as signal carrier, the circulatory system resembles the nervous system: both regulate and integrate the activities of different organs. The average adult human has 5 to 6 L of blood. Almost half of this volume is occupied by three types of blood cells (Fig. 23-20): erythrocytes (red cells), filled with hemoglobin and specialized for carrying O2 and CO2; much smaller numbers of leukocytes (white cells) of several types (including lymphocytes, also found in lymphatic tissue), which are central to the immune system to defend against infections; and platelets (cell fragments), which help to mediate blood clotting. The liquid portion is the blood plasma, which is 90% water and 10% solutes. Dissolved or suspended in the plasma are many proteins, lipoproteins, nutrients, metabolites, waste products, inorganic ions, and hormones. More than 70% of the plasma solids are plasma proteins, primarily immunoglobulins (circulating antibodies), serum albumin, apolipoproteins (for lipid transport), transferrin (for iron transport), and blood-clotting proteins such as fibrinogen and prothrombin. FIGURE 23-20 The composition of blood (by weight). Whole blood can be separated into blood plasma and cells by centrifugation. About 10% of blood plasma is solutes, about 10% of these consisting of inorganic salts, 20% small organic molecules, and 70% plasma proteins. The major dissolved components are listed here. Blood contains many other substances, o en in trace amounts. These include other metabolites, enzymes, hormones, vitamins, trace elements, and bile pigments. Measurements of the concentrations of components in blood plasma are important in the diagnosis and treatment of many diseases. The ions and low molecular weight solutes in blood plasma are not fixed components; they are in constant flux between blood and various tissues. Dietary uptake of the inorganic ions that are the predominant electrolytes of blood and cytosol (Na+, K+, and Ca2+ ) is, in general, counterbalanced by their excretion in the urine. For many blood components, something near a dynamic steady state is achieved: the concentration of a component changes little, although a continuous flux occurs between the digestive tract, blood, and urine. The plasma levels of Na+, K+, and Ca2+ remain close to 140, 5, and 2.5 mM, respectively, with little change in response to dietary intake. Any significant departure from these values can result in serious illness or death. The kidneys play an especially important role in maintaining ion balance by selectively filtering waste products and excess ions out of the blood while preventing the loss of essential nutrients and ions. The human erythrocyte loses its nucleus and mitochondria during differentiation. It therefore relies on glycolysis alone for its supply of ATP. The lactate produced by glycolysis returns to the liver, where gluconeogenesis converts it to glucose, to be stored as glycogen or recirculated to peripheral tissues. The erythrocyte has constant access to glucose in the bloodstream. The concentration of glucose in plasma is subject to tight regulation. We have noted the constant requirement of the brain for glucose and the role of the liver in maintaining blood glucose in the normal range, 60 to 90 mg/100 mL of whole blood (~4.5 mM). (Because erythrocytes make up a significant fraction of blood volume, their removal by centrifugation leaves a supernatant fluid, the plasma, containing the “blood glucose” in a smaller volume. To convert blood glucose to plasma glucose concentration, multiply the blood glucose level by 1.14.) When blood glucose in a human drops to 70 mg/100 mL (the hypoglycemic condition), the person experiences discomfort and mental confusion (Fig. 23-21); further reductions lead to coma, convulsions, and, in extreme hypoglycemia, death. Maintaining the normal concentration of glucose in blood is therefore a high priority, and a variety of regulatory mechanisms have evolved to achieve that end. Among the most important regulators of blood glucose are the hormones insulin, glucagon, and epinephrine, as discussed in the next section.

FIGURE 23-21 Physiological effects of low blood glucose in humans. Blood glucose levels of 40 mg/100 mL and below constitute severe hypoglycemia. SUMMARY 23.2 Tissue-Specific Metabolism In mammals there is a division of metabolic labor among specialized tissues and organs. The liver is the central processing and distribution organ for nutrients. Glucose 6-phosphate may be used to synthesize glycogen or fatty acids, or it may be sent into the citric acid cycle to produce ATP, or into the pentose phosphate pathway to yield NADPH and pentoses. Amino acids are used to synthesize liver and plasma proteins, or their carbon skeletons are converted to glucose and glycogen by gluconeogenesis; the ammonia formed by deamination is converted to urea. Fatty acids in the liver may undergo β oxidation, or be converted to triglycerides, phospholipids or cholesterol, or be converted into ketone bodies. White adipose tissue stores large reserves of triacylglycerol molecules and releases them into the blood in response to epinephrine or glucagon. Brown and beige adipose tissue are specialized for thermogenesis, the result of fatty acid oxidation in uncoupled mitochondria. Skeletal muscle is specialized to produce and use ATP for mechanical work, oxidizing fatty acids and glucose during low to moderate muscular activity. During strenuous muscular activity, glycogen is the ultimate fuel, supplying ATP through glycolysis and fermentation to lactate. Phosphocreatine directly replenishes ATP during active contraction. Heart muscle obtains nearly all its ATP from oxidative phosphorylation, with fatty acids as the primary fuel. The neurons of the brain use only glucose and β - hydroxybutyrate as fuels, the latter being important during fasting or starvation. The brain uses most of its ATP for the active transport of Na+ and K+ to maintain the electrical potential across the neuronal membrane. The blood transfers nutrients, oxygen, waste products, and hormonal signals among tissues and organs. It is made up of cells (erythrocytes, leukocytes, and platelets) and electrolyte-rich water (plasma) containing many dissolved proteins. 23.3 Hormonal Regulation of Fuel Metabolism The minute-by-minute adjustments that keep the blood glucose level near 4.5 mM involve the combined actions of insulin, glucagon, epinephrine, and cortisol on metabolic processes in many body tissues, but especially in liver, muscle, and adipose tissue. Insulin signals to these tissues that blood glucose is higher than necessary; as a result, cells take up excess glucose from the blood and convert it to glycogen and triacylglycerols for storage. Glucagon signals that blood glucose is too low, and tissues respond by producing glucose through glycogen breakdown and (in the liver) gluconeogenesis, and by oxidizing fats to reduce the need for glucose. Epinephrine is released into the blood to prepare the muscles, lungs, and heart for a burst of activity. Cortisol mediates the body’s response to longer-term stresses. In this section, we discuss these hormonal regulations in the context of three normal metabolic states — well fed, fasted, and starving. Insulin Counters High Blood Glucose in the Well-Fed State Aer a high-carbohydrate meal, blood glucose rises several-fold. In response, insulin is released and acts through its plasma membrane receptors in muscle and liver (see Figs. 12-22, 12-23) to stimulate glucose uptake, phosphorylation, and oxidation via glycolysis and the citric acid cycle (Table 23-3). In response to insulin, GLUT4 glucose transporters sequestered in intracellular vesicles move to the plasma membrane, dramatically increasing the uptake of glucose from the blood (see Box 11-1). In the liver, insulin activates glycogen synthase and inactivates glycogen phosphorylase, so that much of the glucose 6-phosphate derived from blood glucose is channeled into glycogen. TABLE 23-3 Effects of Insulin on Blood Glucose: Uptake of Glucose by Cells and Storage as Triacylglycerols and Glycogen Metabolic effect Target enzyme ↑ Glucose uptake (muscle, adipose tissue) ↑ Glucose transporter (GLUT4) ↑ Glucose uptake (liver) ↑ Glucokinase (increased expression) ↑ Glycogen synthesis (liver, muscle) ↑ Glycogen synthase ↓ Glycogen breakdown (liver, muscle) ↓ Glycogen phosphorylase ↑ Glycolysis, acetyl-CoA production (liver, muscle) ↑ PFK-1 (by PFK-2) ↑ Pyruvate dehydrogenase complex ↑ Fatty acid synthesis (liver) ↑ Acetyl-CoA carboxylase ↑ Triacylglycerol synthesis (adipose tissue) ↑ Lipoprotein lipase Insulin also stimulates the storage of excess fuel as fat in adipose tissue (Fig. 23-22). Excess acetyl-CoA not needed for energy production via the citric acid cycle is used for fatty acid synthesis. These fatty acids are converted to TAGs in the liver and exported to adipose tissue as components of plasma lipoproteins (VLDL; see Fig. 21-40). In adipose tissue, TAGs are released from VLDL as fatty acids, which are taken up by adipocytes and reconverted to TAGs for storage in response to insulin stimulation. FIGURE 23-22 The well-fed state: the lipogenic liver. Immediately a er a calorie-rich meal, glucose, fatty acids, and amino acids enter the liver. Blue arrows follow the path of glucose; orange arrows follow the path of lipids. Insulin released in response to the high blood glucose concentration stimulates glucose uptake by the tissues. Some glucose is exported to the brain for its energy needs, and some goes to adipose and muscle tissue. In the liver, excess glucose is oxidized to acetyl-CoA, which is used to make triacylglycerols for export to adipose and muscle tissue. The NADPH necessary for lipid synthesis is obtained by oxidation of glucose in the pentose phosphate pathway. Excess amino acids are converted to pyruvate and acetyl-CoA, which are also used for lipid synthesis. Dietary fats move from the intestine as chylomicrons, via the lymphatic system, to the liver, muscle, and adipose tissues. In summary, an important effect of insulin is to bring about the conversion of excess blood glucose aer a meal to two storage forms: glycogen (in the liver and muscle) and TAGs (in adipose tissue). Pancreatic β Cells Secrete Insulin in Response to Changes in Blood Glucose The peptide hormones insulin, glucagon, and somatostatin are produced by clusters of specialized pancreatic cells, the islets of Langerhans (Fig. 23-23). Each cell type of the islets produces a single hormone: α cells produce glucagon; β cells, insulin; and δ cells, somatostatin. When glucose enters the bloodstream from the intestine aer a carbohydrate-rich meal, the resulting increase in blood glucose causes the pancreas to secrete insulin (and to decrease the secretion of glucagon). FIGURE 23-23 The endocrine system of the pancreas. The pancreas contains both exocrine cells (see Fig. 18-3b), which secrete digestive enzymes in the form of zymogens, and clusters of endocrine cells, the islets of Langerhans. The islets contain α , β , and δ cells (also known as A, B, and D cells, respectively), each cell type secreting a specific peptide hormone. As shown in Figure 23-24, when blood glucose rises, GLUT2 transporters carry glucose into the β cells, where it is immediately converted to glucose 6-phosphate by glucokinase and enters glycolysis. With the higher rate of glucose catabolism, [ATP] increases, causing ATP-gated K+ channels in the plasma membrane to close. Reduced efflux of K+ depolarizes the membrane. (Recall from Section 12.6 that exit of K+ through an open K+ channel hyperpolarizes the membrane; thus, closing the K+ channel effectively depolarizes the membrane.) Membrane depolarization opens voltage-gated Ca2+ channels, and the resulting increase in cytosolic [Ca2+] triggers the release of insulin by exocytosis. The brain integrates inputs on energy supply and demand, and signals from the parasympathetic and sympathetic nervous systems also affect (stimulate and inhibit, respectively) insulin release. A simple feedback loop limits hormone release: insulin lowers blood glucose by stimulating glucose uptake by the tissues; the reduced blood glucose is detected by the β cell as a diminished flux through the glucokinase reaction; this slows or stops the release of insulin. This feedback regulation holds blood glucose concentration nearly constant despite large fluctuations in dietary intake. FIGURE 23-24 Glucose regulation of insulin secretion by pancreatic β cells. When the blood glucose level is high, active metabolism of glucose in the β cell raises intracellular [ATP], closing K+ channels in the plasma membrane and thus depolarizing the membrane. In response to this membrane depolarization, voltage-gated Ca2+ channels open, allowing Ca2+ to flow into the cell. (Ca2+ is also released from the ER, in response to the initial elevation of [Ca2+] in the cytosol.) Cytosolic [Ca2+] is now high enough to trigger insulin release by exocytosis. The numbered processes are discussed in the text. The activity of ATP-gated K+ channels is central to the regulation of insulin secretion by β cells. The channels are octamers of four identical Kir6.2 subunits and four identical SUR1 subunits (Fig. 23-25a) and are constructed along the same lines as the K+ channels of bacteria and those of other eukaryotic cells (see Fig. 11-45). The four Kir6.2 subunits form a cone around the K+ channel and function as the selectivity filter and ATP-gating mechanism (Fig. 23-25b). When [ATP] rises, indicating increased blood glucose, the K+ channels close, thus depolarizing the plasma membrane and triggering insulin release as shown in Figure 23-24. The sulfonylurea drugs, oral medications used in the treatment of type 2 diabetes mellitus, bind to the SUR1 (sulfonylurea receptor) subunits of the K+ channels, closing the channels and stimulating insulin release.

FIGURE 23-25 ATP-gated K+ channels in β cells. (a) The ATP-gated channel, viewed in the plane of the membrane. The channel is formed by four identical Kir6.2 subunits, which are surrounded by four SUR1 (sulfonylurea receptor) subunits. The SUR1 subunits have binding sites for ADP and the drug diazoxide, both of which favor the open channel, and tolbutamide, a sulfonylurea drug that favors the closed channel. The Kir6.2 subunits constitute the channel, and they contain, on the cytosolic side, binding sites for ATP and phosphatidylinositol 4,5-bisphosphate (PIP2), which favor the closed and the open channel, respectively. (b) The structure of the Kir6.2 portion of the channel, viewed in the plane of the membrane. For clarity, only two transmembrane domains and two cytosolic domains are shown. Three K+ ions (green) are shown in the region of the selectivity filter. Mutation in certain amino acid residues (shown in red) leads to neonatal diabetes; mutation in others (shown in blue) leads to hyperinsulinism of infancy. This structure was obtained by mapping the known Kir6.2 sequence onto the crystal structures of a bacterial Kir channel (KirBac1.1) and the amino and carboxyl domains of another Kir protein, Kir3.1. [Data from (b) KirBac1.1: PDB ID 1P7B, A. Kuo et al., Science 300:1922, 2003; Kir3.1: PDB ID 1U4E, S. Pegan et al., Nature Neurosci. 8:279, 2005. Coordinates courtesy of Frances M. Ashcro , Oxford University, used with permission of S. Haider and M. S. P. Sansom to re-create a model published in J. F. Antcliff et al., EMBO J. 24:229, 2005.] Mutations in the ATP-gated K+ channels of β cells are, fortunately, rare. Mutations in Kir6.2 that result in constantly open K+ channels (red residues in Fig. 23-25b) lead to neonatal diabetes mellitus, with severe hyperglycemia that requires insulin therapy. Other mutations in Kir6.2 or SUR1 (blue residues in Fig. 23-25b) produce permanently closed K+ channels and continuous release of insulin. If untreated, individuals with these mutations develop a condition in which excessive insulin causes severe hypoglycemia (low blood glucose) leading to irreversible brain damage. One effective treatment is surgical removal of part of the pancreas to reduce insulin production. Glucagon Counters Low Blood Glucose Several hours aer the intake of dietary carbohydrate, blood glucose levels fall slightly because of the ongoing oxidation of glucose by the brain and other tissues. Lowered blood glucose triggers the pancreas to secrete glucagon and simultaneously decrease the release of insulin (Fig. 23-26). FIGURE 23-26 The fasting state: the glucogenic liver. A er some hours without a meal, the liver becomes the principal source of glucose for the brain. Liver glycogen is broken down to glucose 1-phosphate, and this is converted to glucose 6-phosphate, then to free glucose, which is released into the bloodstream. Amino acids from the degradation of proteins in liver and muscle, and glycerol from the breakdown of TAGs in adipose tissue, are used for gluconeogenesis. The liver uses fatty acids as its principal fuel, and excess acetyl-CoA is converted to ketone bodies for export to other tissues; the brain is especially dependent on this fuel when glucose is in short supply (see Fig. 23-19). Blue arrows follow the path of glucose; orange arrows, the path of lipids; and purple arrows, the path of amino acids. Glucagon causes an increase in blood glucose concentration in several ways (Table 23-4). Like epinephrine, it stimulates the net breakdown of liver glycogen by activating glycogen phosphorylase and inactivating glycogen synthase; both effects are the result of phosphorylation of the regulated enzymes, triggered by cAMP. Glucagon inhibits glucose breakdown by glycolysis in the liver and stimulates glucose synthesis by gluconeogenesis. Both effects result from lowering the concentration of fructose 2,6-bisphosphate, an allosteric inhibitor of the gluconeogenic enzyme fructose 1,6-bisphosphatase (FBPase-1) and an activator of the glycolytic enzyme phosphofructokinase-1. Recall that [fructose 2,6-bisphosphate] is ultimately controlled by a cAMP-dependent protein phosphorylation reaction (see Fig. 14-25). Glucagon also inhibits the glycolytic enzyme pyruvate kinase, by promoting its cAMP- dependent phosphorylation, thus blocking the conversion of phosphoenolpyruvate to pyruvate and preventing oxidation of pyruvate via the citric acid cycle (see Fig. 14-26). The resulting accumulation of phosphoenolpyruvate favors gluconeogenesis. This effect is augmented by glucagon’s stimulation of the synthesis of the gluconeogenic enzyme PEP carboxykinase. By stimulating glycogen breakdown, preventing glycolysis and promoting gluconeogenesis in hepatocytes, glucagon enables the liver to export glucose, restoring blood glucose to its normal level. TABLE 23-4 Effects of Glucagon on Blood Glucose: Production and Release of Glucose by the Liver Metabolic effect Effect on glucose metabolism Target enzyme ↑ Glycogen breakdown (liver) Glycogen → glucose ↑ Glycogen phosphorylase ↓ Glycogen synthesis (liver) Less glucose stored as glycogen ↓ Glycogen synthase ↓ Glycolysis (liver) Less glucose used as fuel in liver ↓ PFK-1 ↑ Gluconeogenesis (liver) ↑ FBPase-2 ↓ Pyruvate kinase ↑ PEP carboxykinase ↑ Fatty acid mobilization (adipose tissue) Less glucose used as fuel by liver, muscle ↑ Hormone- sensitive lipase ↑ PKA (perilipin– ) ↑ Ketogenesis Provides alternative to glucose as energy source for brain ↓ Acetyl-CoA carboxylase Although its primary target is the liver, glucagon (like epinephrine) also affects adipose tissue, activating TAG breakdown by causing cAMP-dependent phosphorylation of perilipin and hormone-sensitive lipase. The activated lipase liberates free fatty acids, which are exported to the liver and other tissues as fuel, sparing glucose for the brain. The net effect of glucagon is therefore to stimulate glucose synthesis and release by the liver and to mobilize fatty acids from adipose tissue, to be used instead of glucose by tissues other than the brain. All these effects of glucagon are mediated by cAMP-dependent protein phosphorylation. During Fasting and Starvation, Metabolism Shis to Provide Fuel for the Brain A healthy adult human has three types of fuel reserves: glycogen stored in the liver and, in smaller quantities, in muscles; large quantities of TAG in adipose tissues; and tissue proteins, which can be degraded when necessary to provide fuel (Table 23-5). TABLE 23-5 Available Metabolic Fuels in a Normal-Weight, 70 kg Man and in an Obese, 140 kg Man at the Beginning of a Fast Type of fuel Weight (kg) Caloric equivalent (thousands of kcal (kJ)) Estimated survival (months) Normal-weight, 70 kg man Triacylglycerols (adipose tissue) 15         140 (590) Proteins (mainly muscle) 6       24 (100) Glycogen (muscle, liver) 0.23         0.90 (3.8) Circulating fuels (glucose, fatty acids, triacylglycerols, etc.) 0.023           0.10 (0.42) Total 165 (690) 3 a Obese, 140 kg man Triacylglycerols (adipose tissue) 80         750 (3,100) Proteins (mainly muscle) 8       32 (130) Glycogen (muscle, liver) 0.23         0.92 (3.8) Circulating fuels 0.025         0.11 (0.46) Total 783 (3,200) 14 Survival time is calculated on the assumption of a basal energy expenditure of 1,800 kcal/day. Two hours aer a meal, the blood glucose level is diminished slightly, and tissues receive glucose released from liver glycogen. There is little or no synthesis of TAGs. By four hours aer a meal, blood glucose has fallen further, insulin secretion has slowed, and glucagon secretion has increased. These hormonal signals mobilize TAGs from adipose tissue, which now become the primary fuel for muscle and liver. Figure 23-27 shows the responses to prolonged fasting. To provide glucose for the brain, the liver degrades certain proteins — those most expendable in an organism not ingesting food. Their nonessential amino acids are transaminated or deaminated (Chapter 18), and the extra amino groups are converted to urea, which is exported via the bloodstream to the kidneys and excreted in the urine. a FIGURE 23-27 Fuel metabolism in the liver during prolonged fasting or in uncontrolled diabetes mellitus. The numbered steps are described in the text. A er depletion of stored carbohydrates (glycogen), gluconeogenesis in the liver becomes the main source of glucose for the brain (blue arrows). NH3 from amino acid deamination is converted into urea and excreted (green arrows). Glucogenic amino acids from protein breakdown (purple arrows) provide substrates for gluconeogenesis, and glucose is exported to the brain. Fatty acids from adipose tissue are imported into the liver and oxidized to acetyl- CoA (orange arrows), and acetyl-CoA is the starting material for ketone body formation in the liver and for export to the brain to serve as an energy source (red arrows). Excess ketone bodies are excreted in the urine. Also in the liver, and to some extent in the kidneys, the carbon skeletons of glucogenic amino acids are converted to pyruvate or intermediates of the citric acid cycle. These intermediates (as well as the glycerol derived from TAGs in adipose tissue) provide the starting materials for gluconeogenesis in the liver, yielding glucose for export to the brain. Fatty acids released from adipose tissue are oxidized to acetyl-CoA in the liver, but as oxaloacetate is depleted by the use of citric acid cycle intermediates for gluconeogenesis, entry of acetyl-CoA into the cycle is inhibited and acetyl-CoA accumulates. This favors the formation of acetoacetyl-CoA and ketone bodies. Aer a few days of fasting, the levels of ketone bodies in the blood rise (Fig. 23-28) as they are exported from the liver to the heart, skeletal muscle, and brain, which use these fuels instead of glucose (Fig. 23-27, ). When the concentration of ketone bodies in the blood exceeds the ability of the kidneys to reabsorb ketones , these compounds begin to appear in the urine. FIGURE 23-28 Plasma concentrations of fatty acids, glucose, and ketone bodies during six weeks of starvation. Despite the hormonal mechanisms for maintaining the glucose level in blood, glucose begins to diminish within 2 days of beginning a fast. The levels of ketone bodies, almost unmeasurable before the fast, rise dramatically a er 2 to 4 days of fasting, with β -hydroxybutyrate as the major contributor. These water-soluble ketones, acetoacetate and β -hydroxybutyrate, supplement glucose as an energy source for the brain during a long fast. Acetone, a minor ketone body, is not metabolized but is eliminated in the breath. A much smaller rise in blood fatty acids also occurs, but this does not contribute to energy metabolism in the brain, as fatty acids do not cross the blood-brain barrier. [Data from G. F. Cahill, Jr., Annu. Rev. Nutr. 26:1, 2006, Fig. 2.] Triacylglycerols stored in the adipose tissue of a normal-weight adult could provide enough fuel to maintain a basal rate of metabolism for about three months; a very obese adult has enough stored fuel to endure a fast of more than a year (Table 23- 5). When fat reserves are gone, the degradation of essential proteins begins; this leads to loss of heart and liver function and, in prolonged starvation, to death. Stored fat can provide adequate energy (calories) during a fast or a rigid diet, but vitamins and minerals must be provided, and sufficient dietary glucogenic amino acids are needed to replace those being used for gluconeogenesis. Rations for those on a weight-reduction diet are commonly fortified with vitamins, minerals, and amino acids or proteins. Epinephrine Signals Impending Activity When an animal is confronted with a stressful situation that requires increased activity — fighting or fleeing, in the extreme case — neuronal signals from the brain trigger the release of epinephrine and norepinephrine from the adrenal medulla. Both hormones dilate the respiratory passages to facilitate the uptake of O2, increase the rate and strength of the heartbeat, and raise the blood pressure, thereby promoting the flow of O2 and fuels to the tissues (Table 23-6). This is the “fight-or-flight” response. TABLE 23-6 Physiological and Metabolic Effects of Epinephrine: Preparation for Action Epinephrine acts primarily on muscle, adipose, and liver tissues. It activates glycogen phosphorylase and inactivates glycogen synthase by cAMP-dependent phosphorylation of the enzymes, thus stimulating the conversion of liver glycogen to blood glucose, the fuel for anaerobic muscular work. Epinephrine also promotes the anaerobic breakdown of muscle glycogen by lactic acid fermentation, stimulating glycolytic ATP formation. The stimulation of glycolysis is accomplished by raising the concentration of fructose 2,6-bisphosphate, a potent allosteric activator of the key glycolytic enzyme phosphofructokinase-1. Epinephrine also stimulates fat mobilization in adipose tissue, by activating hormone-sensitive lipase and moving aside perilipin (see Fig. 17-2). Finally, epinephrine stimulates glucagon secretion and inhibits insulin secretion, reinforcing its effect of mobilizing fuels and inhibiting fuel storage. Cortisol Signals Stress, Including Low Blood Glucose A variety of stressors (anxiety, fear, pain, hemorrhage, infection, low blood glucose, starvation) stimulate release of the glucocorticoid cortisol from the adrenal cortex (see Fig. 23-6). Cortisol acts on muscle, liver, and adipose tissue to supply the organism with fuel to withstand the stress. Cortisol is a relatively slow-acting hormone that alters metabolism by changing the kinds and amounts of certain enzymes synthesized in its target cells, rather than by regulating the activity of existing enzyme molecules. In adipose tissue, cortisol leads to an increased release of fatty acids from stored TAGs. The exported fatty acids serve as fuel for other tissues, and the glycerol is used for gluconeogenesis in the liver. Cortisol stimulates the breakdown of nonessential muscle proteins and the export of amino acids to the liver, where they serve as precursors for gluconeogenesis. In the liver, cortisol promotes gluconeogenesis by stimulating synthesis of PEP carboxykinase; glucagon has the same effect, whereas insulin has the opposite effect. Glucose produced in this way is stored in the liver as glycogen or exported immediately to tissues that need glucose for fuel. The net effect of these metabolic changes is to restore blood glucose to its normal level and to increase glycogen stores, ready to support the fight-or-flight response commonly associated with stress. The effects of cortisol therefore counterbalance those of insulin. During extended periods of stress, the continued release of cortisol loses its positive adaptive value and begins to cause damage to muscle and bone and to impair endocrine and immune function. Cushing disease is a medical condition in which a tumor on the pituitary gland causes the adrenal glands to overproduce cortisol. It is treated by surgery to remove the tumor, followed by chemotherapy to kill remaining tumor cells. Addison disease results from underproduction of cortisol, and is treated by administering hydrocortisone (the pharmaceutical name for cortisol). SUMMARY 23.3 Hormonal Regulation of Fuel Metabolism Fluctuations in blood glucose (normally 70 to 100 mg/100 mL, or about 4.5 mM) due to dietary intake or vigorous exercise are counterbalanced by a variety of hormonally triggered changes in metabolism in several organs. High blood glucose elicits the release of insulin, which speeds the uptake of glucose by tissues and favors the storage of fuels as glycogen and triacylglycerols while inhibiting fatty acid mobilization in adipose tissue. Low blood glucose triggers release of glucagon, which stimulates glucose release from liver glycogen and shis fuel metabolism in liver and muscle to fatty acid oxidation, sparing glucose for use by the brain. In prolonged fasting, TAGs become the principal fuel; the liver converts the fatty acids to ketone bodies for export to other tissues, including the brain. Epinephrine prepares the body for increased activity by mobilizing glucose from glycogen and other precursors, releasing the glucose into the blood. Cortisol, released in response to a variety of stressors (including low blood glucose), stimulates gluconeogenesis from amino acids and glycerol in the liver, thus raising blood glucose and counterbalancing the effects of insulin. 23.4 Obesity and the Regulation of Body Mass In the U.S. population, more than 40% of adults are obese, including more than 10% who are severely obese, as defined in terms of body mass index (BMI), calculated as (weightinkg)/(heightinm)2. A BMI below 25 is considered normal; an individual with a BMI of 25 to 30 is overweight; a BMI greater than 30 indicates obesity; a BMI greater than 40 indicates severe obesity. Obesity is life-threatening. It significantly increases the likelihood of developing type 2 diabetes, as well as heart attack, stroke, and cancers of the colon, breast, prostate, and endometrium. Consequently, there is great interest in understanding how body mass and the storage of fats in adipose tissue are regulated. To a first approximation, obesity is the result of taking in more calories in the diet than are expended by the body’s fuel- consuming activities. The body can deal with an excess of dietary calories in three ways: (1) convert excess fuel to fat and store it in adipose tissue, (2) burn excess fuel by extra exercise, and (3) “waste” fuel by diverting it to heat production (thermogenesis) by uncoupled mitochondria. In mammals, a complex set of hormonal and neuronal signals acts to keep fuel intake and energy expenditure in balance so as to hold the amount of adipose tissue at a suitable level. Dealing effectively with obesity requires understanding how the various checks and balances work under normal conditions and how these homeostatic mechanisms can fail. Adipose Tissue Has Important Endocrine Functions One early hypothesis to explain body-mass homeostasis, the “adiposity negative-feedback” model, postulated a mechanism that inhibits eating behavior and increases energy expenditure whenever body weight exceeds a certain value, called the set point; the inhibition is relieved when body weight drops below the set point (Fig. 23-29). This model predicts that a feedback signal originating in adipose tissue influences the brain centers that control eating behavior and metabolic and motor activity. The first such signal to be discovered was leptin, in 1994. Subsequent research revealed that adipose tissue is an important endocrine organ that produces peptide hormones, known as adipokines. Adipokines may act locally (autocrine and paracrine action) or systemically (endocrine action), carrying information about the adequacy of the energy reserves (TAGs) stored in adipose tissue to other tissues and to the brain. Normally, adipokines produce changes in fuel metabolism and feeding behavior that reestablish adequate fuel reserves and maintain body mass. When adipokines are over- or underproduced, the resulting dysregulation may result in life- threatening disease. FIGURE 23-29 Set-point model for maintaining constant mass. When the mass of adipose tissue increases (dashed outline), released leptin inhibits feeding and fat synthesis and stimulates oxidation of fatty acids. When the mass of adipose tissue decreases (solid outline), lowered leptin production favors greater food intake and less fatty acid oxidation. Leptin (Greek leptos, “thin”) is an adipokine (167 amino acid residues) that, on reaching the brain, acts on receptors in the hypothalamus to curtail appetite. Leptin was first identified as the product of a gene designated OB (obese) in laboratory mice. Mice with two defective copies of this gene (ob/ob genotype; lowercase letters signify a mutant form of the gene) show the behavior and physiology of animals in a constant state of starvation: their plasma cortisol levels are elevated; they exhibit unrestrained appetite, are unable to stay warm, grow abnormally large, and do not reproduce. As a consequence of unrestrained appetite, they become severely obese, weighing as much as three times more than normal mice (Fig. 23-30). They also have metabolic disturbances similar to those seen in diabetes, and they are insulin-resistant. When leptin is injected into ob/ob mice, they eat less, lose weight, and increase their locomotor activity and thermogenesis. FIGURE 23-30 Obesity caused by defective leptin production. Both of these mice, which are the same age, have defects in the OB gene. The mouse on the right was injected daily with purified leptin and weighs 35 g. The mouse on the le got no leptin and consequently ate more food and was less active; it weighs 67 g. A second mouse gene, designated DB (diabetic), also has a role in appetite regulation. Mice with two defective copies (db/db) are obese and diabetic. The DB gene encodes the leptin receptor. When the receptor is defective, the signaling function of leptin is lost. The leptin receptor is expressed primarily in regions of the brain known to regulate feeding behavior — neurons of the arcuate nucleus of the hypothalamus (Fig. 23-31a). Leptin carries the message that fat reserves are sufficient, and it promotes reduction of fuel intake and increase in expenditure of energy. Leptin-receptor interaction in the hypothalamus alters the release of neuronal signals to the region of the brain that affects appetite. Leptin also stimulates the sympathetic nervous system, increasing blood pressure, heart rate, and thermogenesis by uncoupling the mitochondria of brown adipocytes (Fig. 23-31b). Recall that the uncoupling protein UCP1 forms a channel in the inner mitochondrial membrane that allows protons to reenter the mitochondrial matrix without passing through the ATP synthase complex. This permits constant oxidation of fuel (fatty acids in a brown or beige adipocyte) without ATP synthesis, dissipating energy as heat and consuming dietary calories or stored fats in potentially large amounts.

FIGURE 23-31 Hypothalamic regulation of food intake and energy expenditure. (a) Role of the hypothalamus in its interaction with adipose tissue. The hypothalamus receives input (leptin) from adipose tissue and responds with neuronal signals to adipocytes. (b) This signal (norepinephrine) activates protein kinase A, which triggers mobilization of fatty acids from TAG and their uncoupled oxidation in mitochondria, generating heat but not ATP. DAG, diacylglycerol; MAG, monoacylglycerol. Leptin Stimulates Production of Anorexigenic Peptide Hormones Two types of neurons in the arcuate nucleus control fuel intake and metabolism (Fig. 23-32). The orexigenic (appetite- stimulating) neurons stimulate eating by producing and releasing neuropeptide Y (NPY), which causes the next neuron in the circuit to send the signal to the brain: Eat! The blood level of NPY rises during starvation and is elevated in mice with either two defective copies of the leptin gene (ob/ob) or two defective copies of the leptin receptor gene (db/db). The high NPY concentration presumably contributes to the obesity of these mice, which eat voraciously. FIGURE 23-32 Hormones that control eating. In the arcuate nucleus of the hypothalamus, two sets of neurosecretory cells receive hormonal input and relay neuronal signals to the cells of muscle, adipose tissue, and liver. Leptin and insulin are released from adipose tissue and the pancreas, respectively, in proportion to the mass of body fat. The two hormones act on anorexigenic neurosecretory cells to trigger release of α -MSH (melanocyte-stimulating hormone); α -MSH carries the signal to second- order neurons in the hypothalamus, which puts out the signals to eat less and metabolize more fuel. Leptin and insulin also act on orexigenic neurosecretory cells to inhibit the release of NPY, reducing the “eat more” signal sent to the tissues. As described later in the text, the gastric hormone ghrelin stimulates appetite by activating the NPY-expressing cells; PYY3–36, released from the colon, inhibits these neurons and decreases appetite. Each of the two types of neurosecretory cells inhibits hormone production by the other, so any stimulus that activates orexigenic cells inactivates anorexigenic cells, and vice versa. This strengthens the effect of stimulatory inputs. The anorexigenic (appetite-suppressing) neurons in the arcuate nucleus produce α -melanocyte–stimulating hormone (α -MSH; also known as melanocortin), formed from its polypeptide precursor pro-opiomelanocortin (POMC; Fig. 23-5). Release of α - MSH causes the next neuron in the circuit to send the signal to the brain: Stop eating! The amount of leptin released by adipose tissue depends on both the number and the size of adipocytes. When weight loss decreases the mass of lipid tissue, leptin levels in the blood decrease, the production of NPY increases, and the processes in adipose tissue shown in Figure 23-31 are reversed. Uncoupling is diminished, slowing thermogenesis and saving fuel, and fat mobilization slows in response to reduced signaling by cAMP. Consumption of more food, combined with more efficient utilization of fuel, results in replenishment of the fat reserve in adipose tissue, bringing the system back into balance. Leptin Triggers a Signaling Cascade That Regulates Gene Expression The leptin signal is transduced by a mechanism also used by receptors for some growth factors. The leptin receptor has a single transmembrane segment. When leptin binds to the extracellular domains of two monomers, they dimerize and undergo phosphorylation on several Tyr residues. This initiates a chain of events that ends in the nucleus with the increased synthesis of target genes including the gene for POMC, from which α -MSH is produced. The increased catabolism and thermogenesis triggered by leptin are due in part to increased synthesis of the mitochondria in brown and beige adipocytes. Leptin stimulates UCP1 synthesis by altering synaptic transmissions from neurons of the arcuate nucleus to adipose and other tissues via the sympathetic nervous system. The consequent increased release of norepinephrine in these tissues acts through β3-adrenergic receptors to stimulate transcription of the UCP1 gene. The resulting uncoupling of electron transfer from oxidative phosphorylation consumes fat and is thermogenic (Fig. 23-31). Might human obesity be the result of insufficient leptin production and therefore be treatable by the injection of leptin? Blood levels of leptin are, in fact, usually much higher in obese animals (including humans) than in animals of normal body mass (except, of course, in ob/ob mutants, which cannot make leptin). Some downstream element in the leptin response system must be defective in obese individuals, and the elevation in leptin is the result of an (unsuccessful) attempt to overcome the leptin resistance. In those very rare humans with extreme obesity who have a defective leptin gene (OB), leptin injection does result in dramatic weight loss. In the vast majority of obese individuals, however, the OB gene is intact. In clinical trials, the injection of leptin did not have the weight-reducing effect observed in obese ob/ob mice. Clearly, most cases of human obesity involve one or more factors in addition to leptin. Adiponectin Acts through AMPK to Increase Insulin Sensitivity Adiponectin is a peptide hormone produced almost exclusively in adipose tissue, an adipokine that sensitizes other organs to the effects of insulin. Adiponectin circulates in the blood and powerfully affects the metabolism of fatty acids and carbohydrates in liver and muscle. It increases the uptake of fatty acids from the blood by myocytes and the rate at which fatty acids undergo β oxidation in muscle. It also blocks fatty acid synthesis and gluconeogenesis in hepatocytes, and stimulates glucose uptake and catabolism in muscle and liver. These effects of adiponectin are indirect and not fully understood, but the AMP-activated protein kinase (AMPK) mediates many of them. Acting through its GPCR, adiponectin triggers phosphorylation and activation of AMPK. Recall that AMPK is activated by factors that signal the need to shi metabolism toward energy generation and away from energy- requiring biosynthesis (Fig. 23-33; see also p. 503). When activated, AMPK profoundly affects the metabolism of individual cells and, through its actions in the brain, the metabolism of the whole animal. FIGURE 23-33 The role of AMP-activated protein kinase (AMPK) in maintaining energy homeostasis. ADP produced by synthetic reactions is converted to ATP and AMP by adenylate kinase. AMP activates AMPK, which reciprocally regulates ATP-consuming and ATP-generating pathways by phosphorylating key enzymes (see Fig. 23-34). Conditions or agents that inhibit ATP production by catabolic reactions (such as hypoxia, lack of glucose, or metabolic poisons) raise [AMP], activate AMPK, and stimulate catabolism. Cellular or organismal activities that consume ATP (muscle contraction, growth) increase [AMP] and stimulate catabolic reactions to replenish ATP. When [ATP] is high, ATP prevents AMP binding to AMPK, thus lowering AMPK activity and slowing catabolism. AMPK Coordinates Catabolism and Anabolism in Response to Metabolic Stress AMPK has emerged as a central player in the coordination of metabolic pathways, organism activity, and feeding behavior (Fig. 23-34). This heterotrimeric, AMP-activated protein kinase monitors the energy and nutrient status in individual cells and shis metabolism toward energy generation when necessary to maintain metabolic homeostasis. Furthermore, by responding to a variety of hormone signals, AMPK in the hypothalamus acts to keep the whole organism in energetic balance (Fig. 23-8). FIGURE 23-34 Formation of adiponectin and its actions through AMPK. Extended fasting or starvation decreases triacylglycerol reserves in adipose tissue, which triggers adiponectin production and release from adipocytes. Adiponectin acts through its plasma membrane receptors in various cell types and organs to inhibit energy- consuming processes and stimulate energy-producing processes. It acts in the brain to stimulate feeding behavior and inhibit energy-consuming physical activity, and in brown fat to inhibit thermogenesis. Adiponectin exerts some of its metabolic effects by activating AMPK, which regulates (by phosphorylation) specific enzymes in key metabolic processes. PFK-2, phosphofructokinase-2; GLUT1 and GLUT4, glucose transporters; FAS I, fatty acid synthase I; ACC, acetyl-CoA carboxylase; HSL, hormone- sensitive lipase; HMGR, HMG-CoA reductase; GPAT, an acyl transferase; GS, glycogen synthase; eEF2, eukaryotic elongation factor 2 (required for protein synthesis; see Chapter 27); and mTORC1, mammalian target of rapamycin complex 1 (a protein kinase complex that regulates protein synthesis on the basis of nutrient availability). Exercise, through conversion of ATP to ADP and AMP, also stimulates AMPK. AMPK monitors the energy status of a cell through its response to increased [AMP]/[ATP]. Many of the energy-consuming reactions in cells convert ATP to ADP or AMP. Adenylate kinase catalyzes the reaction 2ADP → AM P+ ATP, so [AMP] is a sensitive measure of the cell’s energy status. AMPK is allosterically activated by AMP binding, and ATP prevents AMP binding, so the enzyme is activated when the cell is energetically depleted (high [AMP]) and inactivated when energy is plentiful (high [ATP], and high [ATP]/[AMP]). AMPK responds to the energetic needs of the whole organism through a second mode of regulation. The enzyme is activated 100-fold by phosphorylation of Thr172 by liver kinase B1 (LKB1), which is itself subject to regulation by upstream components, including adiponectin. When activated by phosphorylation and AMP binding, AMPK phosphorylates specific enzymes in metabolic pathways that are crucial to energy homeostasis (Fig. 23-34). When AMPK senses depletion of ATP in an individual cell, lipid synthesis is inhibited and use of lipid as fuel is stimulated. One enzyme regulated by AMPK in the liver and in white adipose tissue is acetyl-CoA carboxylase, which produces malonyl-CoA, the first intermediate committed to fatty acid synthesis. Malonyl- CoA is a powerful inhibitor of the enzyme carnitine acyltransferase 1, which starts the process of β oxidation by transporting fatty acids into the mitochondrion (see Fig. 17-6). By phosphorylating and inactivating acetyl-CoA carboxylase, AMPK inhibits fatty acid synthesis while relieving inhibition (by malonyl-CoA) of β oxidation (see Fig. 17-13). Cholesterol synthesis, a heavy energy consumer, is also inhibited by AMPK, which phosphorylates and inactivates HMG-CoA reductase, an enzyme central to sterol synthesis (see Fig. 21-34). Similarly, AMPK inhibits fatty acid synthase and acyl transferase, effectively blocking the synthesis of TAGs. In addition to its effects on lipid metabolism, AMPK inhibits the synthesis of glycogen and of protein (see Fig. 23-35). Inadequate supplies of oxygen (hypoxia) or blood glucose (hypoglycemia) are among the stressors that trigger AMPK activation. In the hypothalamus, AMPK is positioned to receive a variety of signals from throughout the body (Fig. 23-8). Ghrelin and adiponectin signal “empty stomach” and “fat tissue depleted,” and, like low blood glucose, they elicit hypothalamic signals, mediated by AMPK, that stimulate feeding and inhibit energy- requiring biosynthetic processes. Leptin, as we have seen, brings to the brain the signal “adipose tissue is full,” slowing catabolic processes and favoring growth and biosynthesis. At the cytoplasmic surface of lysosomes, AMPK interacts with a second central regulator of cellular activity, the protein kinase mTOR. This enzyme, at the center of an enormous complex, mTORC1, gauges whether sufficient nutrients and low molecular weight substrates are available to support cell growth and proliferation. Together, the two protein kinases, AMPK and mTOR, control major aspects of a cell’s activity and fate. The mTORC1 Pathway Coordinates Cell Growth with the Supply of Nutrients and Energy The highly conserved Ser/Thr kinase mTOR forms a complex, mTORC1, with a scaffold protein, raptor, and other regulatory proteins. mTORC1 is recruited to the cytosolic surface of the lysosome through raptor by another complex, Ragulator, and a number of associated Rag G proteins that sense amino acid sufficiency in the cell. The Ragulator-Rag complex is tethered to the lysosomal membrane by covalently linked lipids. When mTORC1 is docked to the Ragulator-Rag complex, it is in contact with another G protein, Rheb (Fig. 23-35). This massive complex integrates signals from inside and outside of the lysosome about the energy status of the cell, the availability of critical amino acids needed for protein synthesis, and the presence of growth factors. When these signals indicate that the cell has what it needs to grow, GTP-bound Rheb activates the protein kinase activity of mTOR, which then phosphorylates many different proteins required for transcription, increased ribosome synthesis, and expression of genes encoding the enzymes of lipid synthesis and mitochondrial proliferation (Fig. 23-36). Because the lysosome is where the cell recycles defective or unneeded components, or extracellular substances brought into the cell through phagocytosis, it is, in effect, the warehouse of parts for cellular construction. The mTORC1-Ragulator-Rag complex is the supply chain manager determining whether the assembly line can operate. FIGURE 23-35 The mTORC1-Ragulator-Rag complex on the lysosomal surface. This model integrates the structures of mTORC1 and Ragulator- Rag, separately determined by cryo-EM. Raptor acts as a scaffold protein organizing the assembly, which puts the mTOR protein kinase in contact with the G protein Rheb for activation. [Information from J. H. Park et al., Trends Biochem. Sci. 45:367, 2019, Fig. 1; K. B. Rogala et al., Science 366:468, 2019, Fig. 5A.] FIGURE 23-36 A summary of mTORC1 activation signals and the cellular processes that active mTORC1 stimulates. The Ser/Thr protein kinase of mTORC1 is activated by the G protein Rheb, reflecting the integration of many signals that indicate that the cell is prepared for growth. By phosphorylating key target proteins, mTORC1 activates energy (ATP and NADPH) production for biosynthesis and stimulates the synthesis of proteins and lipids, allowing cell growth and proliferation. Fasting results in inactivation of mTORC1 by AMPK, leading to increased breakdown of protein and glycogen in the liver and muscle and mobilization of TAGs in adipose tissue. Chronic activation of mTORC1 by overeating results in excess deposition of TAGs in adipose tissue, as well as in liver and muscle, which may contribute to insulin insensitivity and type 2 diabetes. Mutations that produce constantly activated mTORC1 are commonly associated with human cancers. Diet Regulates the Expression of Genes Central to Maintaining Body Mass Proteins in a family of ligand-activated transcription factors known as peroxisome proliferator-activated receptors (PPARs) respond to changes in dietary lipid by altering the expression of genes involved in fat and carbohydrate metabolism. (These transcription factors were first recognized for their roles in peroxisome synthesis — thus their names.) Their normal ligands are fatty acids or fatty acid derivatives. PPARγ , PPARα , and PPARδ act in the nucleus by forming heterodimers with another nuclear receptor, RXR, then binding to regulatory regions of DNA near the genes under their control and changing the rate of transcription of those genes (Fig. 23-37). FIGURE 23-37 Mode of action of PPARs. PPARs are transcription factors that, when bound to their cognate ligand, form heterodimers with the nuclear receptor RXR. The dimer binds specific regions of DNA known as response elements, stimulating transcription of genes in those regions. [Information from R. M. Evans et al., Nat. Med. 10:355, 2004, Fig. 3.] PPARγ turns on genes that act in the differentiation of fibroblasts into adipocytes and genes that encode proteins required for lipid synthesis and storage in adipocytes (Fig. 23-38). PPARγ is activated by the thiazolidinedione drugs that are used to treat type 2 diabetes (discussed below). FIGURE 23-38 Metabolic integration by PPARs. The three PPAR isoforms regulate lipid and glucose homeostasis through their coordinated effects on gene expression in liver, muscle, and adipose tissue. PPARα and PPARδ regulate lipid utilization; PPARγ regulates lipid storage and the insulin sensitivity of various tissues. PPARα is expressed in liver, kidney, heart, skeletal muscle, and brown adipose tissue. The ligands that activate this transcription factor include eicosanoids and free fatty acids. In hepatocytes, PPARα turns on the genes necessary for the uptake and β oxidation of fatty acids and for the formation of ketone bodies during fasting. PPARδ is a key regulator of fat oxidation, which responds to changes in dietary lipid. PPARδ acts in liver and muscle, stimulating the transcription of at least nine genes encoding proteins for β oxidation and for energy dissipation through uncoupling of mitochondria. By stimulating fatty acid breakdown in uncoupled mitochondria, PPARδ causes fat depletion, weight loss, and thermogenesis. Short-Term Eating Behavior Is Influenced by Ghrelin, PPY3–36, and Cannabinoids The peptide hormone ghrelin is produced in cells lining the stomach. It is a powerful appetite stimulant that works on a shorter time scale (between meals) than leptin and insulin. Ghrelin receptors are located in the hypothalamus, affecting appetite, as well as in heart muscle and adipose tissue. Ghrelin acts through a GPCR to generate the second messenger IP3, which mediates the hormone’s action. The concentration of ghrelin in the blood fluctuates strikingly throughout the day, peaking just before a meal and dropping sharply just aer a meal (Fig. 23-39). Injection of ghrelin into humans produces immediate sensations of intense hunger. Individuals with Prader- Willi syndrome, whose blood levels of ghrelin are exceptionally high, have an uncontrollable appetite, leading to extreme obesity that oen results in death before the age of 30. FIGURE 23-39 Variations in blood concentrations of glucose, ghrelin, and insulin relative to meal times. (a) Plasma levels of ghrelin rise sharply just before the usual time for meals (7 breakfast, 12 noon lunch, 5:30 dinner) and drop precipitously just a er meals, paralleling subjective feelings of hunger. (b) Plasma glucose rises sharply aer a meal, (c) followed immediately by a rise in insulin level in response to the increased blood glucose. [(a, c) Data from D. E. Cummings et al., Diabetes 50:1714, 2001, Fig. 1. (b) Data from M. D. Feher and C. J. Bailey, Br. J. Diabet. Vasc. Dis. 4:39, 2004.] PYY3–36 is a peptide hormone (34 amino acid residues) secreted by endocrine cells in the lining of the small intestine and colon in response to food entering from the stomach. The level of PYY3–36 in the blood rises aer a meal and remains high for some hours. The hormone is carried in the blood to the arcuate nucleus, where it acts on orexigenic neurons, inhibiting NPY release and reducing hunger (Fig. 23-32). Humans injected with PYY3–36 feel little hunger and eat less than normal amounts for about 12 hours. Endocannabinoids (Fig. 23-40) are eicosanoid lipid messengers that act through specific GPCRs in the brain and peripheral nervous system to increase appetite, heighten the sensory response to food (especially sweet and fatty foods), and elevate mood. When food enters the mouth, neuronal signals travel to the brain, and from there through the vagus nerve to the intestine, which then produces and releases endocannabinoids. The receptors for endocannabinoids control ion channels of sensory neurons, changing their membrane potentials and sending signals to the brain. Palatable food sensed in this way motivates further consumption of that food. The taste of fats (which are particularly high in caloric content) causes cannabinoid release, which effectively triggers further consumption. Well-conserved across vertebrate species, this system probably evolved to maximize the intake of food and to guard against starvation. In mammals, cannabinoid action stimulates an increase in fat mass and inhibits energy loss by motor activity or thermogenesis. Cannabinoid receptors also mediate the psychoactive effects of Δ9 -tetrahydrocannabinol (Fig. 23-40), the main active ingredient in marijuana, long known for its stimulating effect on appetite.

FIGURE 23-40 Cannabinoids. Two endocannabinoids produced by animals, and two products from the cannabis plant. Endocannabinoids carry retrograde signals: secreted by a postsynaptic cell, they diffuse across the synaptic cle and activate GPCR receptors in the presynaptic cell. The receptors are of two types: CB1 in the central nervous system, and CB2 in the peripheral nervous system. Δ9- Tetrahydrocannabinol (THC), the psychoactive compound in cannabis, is an agonist of both types of cannabinoid receptors in animals; cannabidiol (CBD) does not bind either. Microbial Symbionts in the Gut Influence Energy Metabolism and Adipogenesis An adult human is host to about 1014 microbial cells that inhabit the gut. These microbes function as a major endocrine organ, producing a variety of metabolites with profound effects on host metabolism, feeding behavior, and body mass. Lean and obese individuals harbor different combinations of microbial symbionts in the gut. Investigation of this observation led to the discovery that gut microbes release fermentation products — the short- chain fatty acids acetate, propionate, butyrate, and lactate — that enter the bloodstream and trigger metabolic changes in adipose tissue (Fig. 23-41). Propionate, for example, drives the expansion of white adipose tissue by acting on GPCRs in the plasma membranes of several cell types, including adipocytes. These receptors trigger differentiation of precursor cells (preadipocytes) into adipocytes and inhibit lipolysis in existing adipocytes, leading to an increase in white adipose tissue mass — that is, obesity. Gut microbes also convert primary bile acids, synthesized in the liver, into the secondary bile acids deoxycholate and lithocholate, which enter the bloodstream and act through GPCRs and steroid receptors to activate beige adipocytes to produce UCP1 and increase energy expenditure. FIGURE 23-41 Effects of gut microbe metabolism on health. The enormous number and diversity of microorganisms (the microbiota) in the colon generate metabolic products that may have significant effects on health, both positive and negative. For example, the metabolism of primary bile acids by microbiota produces secondary products that act through nuclear receptors to stimulate thermogenesis in host brown adipose tissue (BAT) and increase both energy consumption and insulin sensitivity, while reducing inflammation. Metabolism of undigested carbohydrates by microbiota produces short-chain fatty acids (SCFAs) that signal expansion of the host’s white adipose tissue (WAT), promoting obesity. SCFAs produced by microbiota are also a readily metabolizable energy source for the host. Microbial production of the SCFA propionate prevents lipogenesis in the liver and lowers blood cholesterol, both favorable to health. On the other hand, metabolic conversion of phosphatidylcholine and -carnitine to trimethylamine (TMA), and its further conversion in liver to trimethylamine- N-oxide (TMAO), results in receptor-mediated changes in cholesterol transport and macrophage activity. The combination of altered sterol transport and increased macrophage activity can lead to formation of atherosclerotic plaque (see Fig. 21-46). These findings raise the possibility of preventing obesity by altering the makeup of the microbial community in the gut. Weight reduction might be accomplished either by adding, directly to the gut, microbial species (probiotics) that disfavor adipogenesis, or by adding to the diet nutrients (prebiotics) that favor the dominance of probiotic microbes. For example, experiments in mice have shown that fructans, polymers of fructose that are indigestible by animals, favor a specific microbial community. When this combination of microorganisms is present, fat storage in white adipose tissue and liver decreases, and there is none of the decrease in insulin sensitivity that is associated with obesity and lipid deposition in the liver (see below). Investigators have transplanted fecal material from lean mice to fat ones, and found that a new collection of microbes became established in the gut of the recipient animals, and the animals lost weight. Endocrine cells in the lining of the intestinal tract secrete peptides — the anorexigenic PYY3–36 and GLP-1 and the orexigenic ghrelin — that modulate food intake and energy expenditure. Interaction with specific microbes in the gut, or with their fermentation products, may trigger release of these peptides. Understanding how diet and microbial symbionts interact to affect energy metabolism and adipogenesis is an important key to understanding the development of obesity, metabolic syndrome, and type 2 diabetes and is a major challenge for the future. The exquisitely interconnected system of neuroendocrine controls of food intake and metabolism presumably evolved to protect against starvation and to eliminate counterproductive accumulation of fat (extreme obesity). The difficulty most people face in trying to lose weight testifies to the remarkable effectiveness of these controls. SUMMARY 23.4 Obesity and the Regulation of Body Mass Adipose tissue produces leptin, a hormone that regulates feeding behavior and energy expenditure so as to maintain adequate reserves of fat. Leptin production and release increase with the number and size of adipocytes. Leptin acts on receptors in the brain, causing the release of anorexigenic (appetite-suppressing) peptides, including α -MSH, that act in the brain to inhibit eating. Adiponectin stimulates fatty acid uptake and oxidation and inhibits fatty acid synthesis. It also sensitizes muscle and liver to insulin. At least some of the actions of adiponectin are mediated by AMPK, which is also activated by metabolic stress (low [AMP]), and by exercise. The protein kinase complex mTORC1 gauges the supply of essential amino acids and other metabolites, triggering cell growth if all required nutrients are available. It thus complements AMPK in determining the energetic status of a cell. Expression of the enzymes of lipid synthesis is under tight and complex regulation. PPARs are transcription factors that determine the rate of synthesis of many enzymes involved in lipid metabolism and adipocyte differentiation. Ghrelin, a hormone produced in the stomach, acts on orexigenic (appetite-stimulating) neurons in the arcuate nucleus to produce hunger before a meal. PYY3–36, a peptide hormone of the intestine, acts at the same site to lessen hunger aer a meal. Endocannabinoids signal the availability of sweet or fatty food and stimulate its consumption. Microbial symbionts in the gut produce fermentation products and secondary bile acids, which influence release of gut hormones that regulate body mass. 23.5 Diabetes Mellitus Diabetes mellitus is a relatively common disease: about 9% of the U.S. population, and nearly 25% of the U.S. population over the age of 65, show some degree of abnormality in glucose metabolism that is indicative of diabetes or a tendency toward the condition. There are two major clinical classes of diabetes mellitus: type 1 diabetes, sometimes referred to as insulin- dependent diabetes mellitus (IDDM), and type 2 diabetes, or non- insulin-dependent diabetes mellitus (NIDDM), also called insulin- resistant diabetes. The discovery of insulin and its role in diabetes led to its development as a pharmaceutical, saving millions of lives (Box 23-2). BOX 23-2 MEDICINE The Arduous Path to Purified Insulin Millions of people with type 1 diabetes mellitus inject themselves daily with pure insulin to compensate for the lack of production of this critical hormone by their own pancreatic β cells. Insulin injection is not a cure for diabetes, but it allows people who otherwise would have died young to lead long and productive lives. The discovery of insulin, which began with an accidental observation, illustrates the combination of serendipity and careful experimentation that led to the discovery of many hormones. In 1889, Oskar Minkowski, a young assistant at the Medical College of Strasbourg, and Josef von Mering, at the Hoppe-Seyler Institute in Strasbourg, had a friendly disagreement about whether the pancreas, known to contain lipases, was important in fat digestion in dogs. To resolve the issue, they began an experiment on the digestion of fats. They surgically removed the pancreas from a dog, but before their experiment got any farther, Minkowski noticed that the dog was now producing far more urine than normal (a common symptom of untreated diabetes). Also, the dog’s urine had glucose levels far above normal (another symptom of diabetes). These findings suggested that lack of some pancreatic product caused diabetes. Minkowski tried unsuccessfully to prepare an extract of dog pancreas that would reverse the effect of removing the pancreas — that is, would lower the urinary or blood glucose levels. Despite considerable effort, no significant progress was made in the isolation or characterization of the “antidiabetic factor” until the summer of 1921, when Frederick G. Banting, a young scientist working in the laboratory of J. J. R. MacLeod at the University of Toronto, and a student assistant, Charles Best, took up the problem. By that time, several lines of evidence pointed to a group of specialized cells in the pancreas (the islets of Langerhans; see Fig. 23-23) as the source of the antidiabetic factor, which came to be called insulin (from Latin insula, “island”). Taking precautions to prevent proteolysis by the pancreatic proteases trypsin and chymotrypsin, Banting and Best (later aided by biochemist J. B. Collip) succeeded in December 1921 in preparing a purified pancreatic extract that cured the symptoms of experimentally induced diabetes in dogs. On January 25, 1922 (just one month later!), their insulin preparation was injected into Leonard Thompson, a 14-year-old boy severely ill with diabetes mellitus. Within days, the levels of ketone bodies and glucose in Thompson’s urine dropped dramatically; the extract saved his life and the lives of other seriously ill children who also received these early preparations (Fig. 1). In 1923, Banting and MacLeod won the Nobel Prize for their isolation of insulin. Banting immediately announced that he would share his prize with Best; MacLeod shared his with Collip. FIGURE 1 A child with type 1 diabetes, before (le ) and a er (right) three months of treatment with an early preparation of insulin. By 1923, pharmaceutical companies were supplying thousands of patients throughout the world with insulin extracted from porcine pancreas. With the development of genetic engineering techniques in the 1980s, it became possible to produce unlimited quantities of human insulin by inserting the cloned human gene for insulin into a microorganism, which was then cultured on an industrial scale. Many people with diabetes are now fitted with implanted insulin pumps, which release adjustable amounts of insulin on demand to meet changing needs at meal times and during exercise. There is a reasonable prospect that, in the future, transplanted pancreatic tissue will provide a source of insulin that responds as well as a normal pancreas, releasing insulin into the bloodstream only when blood glucose rises. Diabetes Mellitus Arises from Defects in Insulin Production or Action Type 1 diabetes usually begins early in life, and symptoms quickly become severe. This disease responds to insulin injection, because the metabolic defect stems from an autoimmune destruction of pancreatic β cells and a consequent inability to produce sufficient insulin. Type 1 diabetes requires both insulin therapy and careful, lifelong control of the balance between dietary intake, activity, and insulin dose. Characteristic symptoms of untreated type 1 (and type 2) diabetes are excessive thirst and frequent urination, leading to the intake of large volumes of water. These symptoms are due to excretion of large amounts of glucose in the urine. (“Diabetes mellitus” means “excessive excretion of sweet urine.”) Type 2 diabetes is slower to develop (typically but not always in obese adults), and the symptoms are milder and oen go unrecognized at first. This is really a group of diseases in which the regulatory activity of insulin is disordered: insulin is produced, but some feature of the insulin-response system is defective. Many individuals with this disorder are insulin- resistant. The connection between type 2 diabetes and obesity (discussed below) is an active area of research. The pathology of diabetes includes cardiovascular disease, renal failure, blindness, and neuropathy. In 2019, the global mortality from diabetes was estimated at 4.2 million and rising. It is essential to understand diabetes and its relationship to obesity and to find countermeasures that prevent or reverse the damage done by this disease. Individuals with either type of diabetes are unable to take up glucose efficiently from the blood. Recall that insulin triggers the movement of GLUT4 glucose transporters to the plasma membrane in muscle and adipose tissue (see Fig. 12-23 and Box 11-1). Biochemical measurements on blood and urine samples are essential in the diagnosis and treatment of diabetes. A sensitive diagnostic criterion is the level of HbA1c, a glucose derivative of hemoglobin, which forms in the blood and reflects the average blood glucose level (see Box 7-2). Another measurement to confirm the diagnosis of diabetes is the glucose-tolerance test. The individual fasts overnight, then drinks a test dose of 100 g of glucose dissolved in a glass of water. The blood glucose concentration is measured before the test dose and at 30 min intervals for several hours thereaer. A healthy individual assimilates the glucose readily, the blood glucose rising to no more than about 9 or 10 mM; little or no glucose appears in the urine. In diabetes, individuals assimilate the test dose of glucose poorly; their blood glucose level rises dramatically and returns to the fasting level very slowly. Because the blood glucose levels exceed the kidney threshold (about 10 mM), glucose also appears in the urine. Carboxylic Acids (Ketone Bodies) Accumulate in the Blood of Those with Untreated Diabetes With glucose unavailable to cells, fatty acids become the principal fuel, which leads to another characteristic metabolic change in diabetes: excessive but incomplete oxidation of fatty acids in the liver. The acetyl-CoA produced by β oxidation cannot be completely oxidized by the citric acid cycle, because the high [NADH]/[NAD+] ratio produced by β oxidation inhibits the cycle (recall that three steps of the cycle convert NAD+ to NADH). Accumulation of acetyl-CoA leads to overproduction of the ketone bodies β -hydroxybutyrate and acetoacetate, which cannot be used by extrahepatic tissues as fast as they are made in the liver (see Figs. 17-15, 17-16). In addition to β -hydroxybutyrate and acetoacetate, the blood of individuals with diabetes contains small amounts of acetone, which results from the spontaneous decarboxylation of acetoacetate:

The overproduction of ketone bodies, called ketosis, results in greatly increased concentrations of ketone bodies in the blood (ketonemia) and urine (ketonuria).The ketone bodies are carboxylic acids, which ionize, releasing protons. In uncontrolled diabetes, this acid production can overwhelm the capacity of the blood’s bicarbonate buffering system and produce a lowering of blood pH called acidosis or, in combination with ketosis, ketoacidosis, a potentially life-threatening condition. In Type 2 Diabetes the Tissues Become Insensitive to Insulin In the industrialized world, where the food supply is more than adequate, there is a growing epidemic of obesity and the type 2 diabetes associated with it. The hallmark of type 2 diabetes is the development of insulin resistance, a state in which more insulin is needed to bring about the biological effects produced by a lower concentration of insulin in the normal, healthy state. In the early stages of the disease, pancreatic β cells secrete enough insulin to overcome the lower insulin sensitivity of muscle and liver. But the β cells eventually fail, and the lack of insulin becomes apparent in the body’s inability to regulate blood glucose. The intermediate stage, preceding type 2 diabetes mellitus, is sometimes called metabolic syndrome. This is typified by obesity, especially in the abdomen; hypertension (high blood pressure); abnormal blood lipids (high TAG and LDL, low HDL); slightly high fasting blood glucose; and a reduced ability to clear glucose in the glucose-tolerance test. Individuals with metabolic syndrome oen also show changes in blood proteins associated with abnormal clotting (high fibrinogen concentration) or inflammation (high concentration of the C- reactive peptide, not to be confused with the C peptide generated during the proteolytic maturation of insulin). About 30% of the adult population in the United States has these indicators of metabolic syndrome. What predisposes individuals with metabolic syndrome to develop type 2 diabetes? According to the “lipid toxicity” hypothesis (Fig. 23-42), the action of PPARδ on adipocytes normally keeps the cells ready to synthesize and store triacylglycerols — the adipocytes are insulin-sensitive and produce leptin, which leads to their continued intracellular deposition of TAGs. However, excess caloric intake in obese individuals causes adipocytes to become filled with TAGs, leaving adipose tissue unable to meet any further demand for TAG storage. Lipid-filled adipose tissue releases protein factors, including MCP-1 (monocyte chemotaxis protein-1), that attract macrophages, which infiltrate the tissue and may eventually represent as much as 50% of the adipose tissue by mass. Macrophages trigger the inflammatory response mediated by TNFα release, which impairs TAG deposition in adipocytes and favors release of free fatty acids into the blood. These excess fatty acids enter liver and muscle cells, where they are converted to TAGs that accumulate as lipid droplets. This ectopic (Greek ektopos, “out of place”) deposition of TAGs inhibits GLUT4 transporter movement to the plasma membrane, which leads to insulin insensitivity in liver and muscle, the hallmark of type 2 diabetes. FIGURE 23-42 Overloading adipocytes with triacylglycerols triggers inflammation in fat tissue and ectopic lipid deposition and insulin resistance in muscle. In an individual of healthy body mass, dietary TAG uptake equals TAG oxidation for energy. In overweight individuals, excess caloric intake results in enlarged adipocytes, engorged with TAG and unable to store more. Enlarged adipocytes secrete MCP-1 (monocyte chemotaxis protein-1), attracting macrophages. Macrophages infiltrate the adipose tissue and produce TNFα (tumor necrosis factor α ), which triggers lipid breakdown and release of fatty acids into the blood. The fatty acids enter myocytes, where they accumulate in small lipid droplets. This ectopic lipid storage in muscle somehow causes insulin resistance, perhaps by triggering lipid-activated protein kinases that inactivate some element in the insulin-signaling pathway. GLUT4 glucose transporters leave the myocyte surface, preventing glucose entry into muscle; the myocyte has now become insulin- resistant. It cannot use blood glucose for its fuel, so fatty acids are mobilized from adipose tissue and become the primary fuel. The increased influx of fatty acids into muscle leads to further deposition of ectopic lipids. In some individuals, insulin resistance develops into type 2 diabetes. [Information from A. Guilherme et al., Mol. Cell Biol. 9:367, 2008, Fig. 1.] According to this hypothesis, excess stored fatty acids and TAGs are toxic to liver and muscle. Some individuals are less well equipped genetically to handle this burden of ectopic lipids and are more susceptible to the cellular damage that leads to development of type 2 diabetes. Insulin resistance probably involves impairment of several of the mechanisms by which insulin acts on metabolism, which include changes in protein levels and changes in the activities of signaling enzymes and transcription factors. For example, both adiponectin synthesis in adipocytes and adiponectin levels in the blood decrease with obesity and increase with weight loss. There are genetic factors that predispose individuals to type 2 diabetes. Although 80% of people with type 2 diabetes are obese, most obese individuals do not develop type 2 diabetes. Given the complexity of the regulatory mechanisms we have discussed in this chapter, it is not surprising that the genetics of diabetes is complex, involving interactions among variant genes and environmental factors, including diet and lifestyle. A growing number of genetic loci have been reliably linked to type 2 diabetes; variation in any of these “diabetogenes” alone would cause a relatively small increase in the likelihood of developing type 2 diabetes. Type 2 Diabetes Is Managed with Diet, Exercise, Medication, and Surgery Studies show that at least four factors improve the health of individuals with type 2 diabetes: dietary restriction, regular exercise, drugs that increase insulin sensitivity or insulin production, and surgery that reroutes food passage through the gastrointestinal tract. Dietary restriction (and accompanying weight loss) reduces the overall burden of handling fatty acids. The lipid composition of the diet influences, through PPARs and other transcription factors, the expression of genes that encode proteins involved in burning fat. Exercise contributes to weight loss directly by consuming calories. Exercise also increases release of irisin from muscle into the blood. Irisin increases the expression of UCP1 genes in white adipose tissue and also stimulates the development of beige adipocytes, so that even aer the exercise ends, energy continues to be used in thermogenesis. Exercise activates AMPK, as does adiponectin; AMPK shis metabolism toward fat oxidation and inhibits fat synthesis. Several classes of drugs are used in the management of type 2 diabetes (Table 23-7). Their targets include AMPK, the K+ channels in β cells, PPARs, and GLP receptors. TABLE 23-7 Treatments for Type 2 Diabetes Mellitus Intervention/treatment Direct target Effect of treatment Weight loss Adipose tissue; Reduces lipid burden; increases capacity for lipid storage in adipose tissue; reduction in TAG content restores insulin sensitivity Exercise AMPK, activated by increasing [AMP]/[ATP] Aids weight loss (see Fig. 23-34) Bariatric surgery Unknown Leads to weight loss, better control of blood glucose Sulfonylureas: glipizide (Glucotrol), glyburide (Micronase), glimepiride (Amaryl) Pancreatic β cells; K+ channels blocked Stimulates insulin secretion by pancreas (see Fig. 23-24) Biguanides: metformin (Glucophage) AMPK, activated Increases glucose uptake by muscle; decreases glucose production in liver Thiazolidinediones: rosiglitazone (Avandia), pioglitazone (Actos) PPARγ Stimulates expression of genes potentiating the action of insulin in liver, muscle, adipose tissue; increases glucose uptake; decreases glucose synthesis in liver GLP-1 modulators: exenatide (Byetta), sitagliptin (Januvia), dulaglutide (Trulicity) Glucagon- like peptide-1, dipeptide protease IV Enhances insulin secretion by pancreas In cases of extreme obesity, dramatic weight loss can be achieved by bariatric surgery, which reroutes food movement through the stomach and small intestine. In many cases, this procedure also moderates or even reverses type 2 diabetes. In Roux-en-Y gastric bypass (RYGBP, named for César Roux, the Swiss surgeon who developed the procedure), the stomach is reduced to a small pouch attached to the esophagus, and the middle region of the small intestine (the jejunum) is attached directly to the pouch. Food bypasses most of the stomach and the duodenum, and goes primarily to the “Roux limb” of the intestine. Stomach acid and digestive enzymes travel through bypassed portions of the gut to join the food in the common channel. People who undergo RYGBP surgery not only experience dramatic weight loss but also are less hungry. Remarkably, this surgery also reverses type 2 diabetes in many cases. The explanations for these effects are likely to lie in altered communication among the gut, the brain, and other organs. This may result from changes in the kind and amount of peptide hormones (such as GLP-1 and PYY3–36) secreted in the intestine that signal satiety and inhibit feeding behavior. The last word has not been written on this issue. SUMMARY 23.5 Diabetes Mellitus Metabolic syndrome, which includes obesity, hypertension, elevated blood lipids, and insulin resistance, is oen the prelude to type 2 diabetes. Uncontrolled diabetes is characterized by high glucose levels in the blood and urine and the production and excretion of ketone bodies. In diabetes, insulin is either not produced or not recognized by the tissues, and the uptake of blood glucose is defective. Lacking access to glucose, cells rely on fatty acid oxidation, which results in ketone body formation, producing ketoacidosis. The insulin resistance that characterizes type 2 diabetes may be a consequence of abnormal lipid storage in muscle and liver, in response to a lipid intake that cannot be accommodated by adipose tissue. Effective treatments for type 2 diabetes include exercise, appropriate diet, and drugs that increase insulin sensitivity or insulin production. Surgical alteration of the digestive tract leads to weight loss and oen reverses type 2 diabetes. Chapter Review KEY TERMS Terms in bold are defined in the glossary. hormone neuroendocrine system metabotropic ionotropic endocrine paracrine autocrine hypothalamus hepatocyte white adipose tissue (WAT) adipocyte brown adipose tissue (BAT) uncoupling protein 1 (UCP1) thermogenesis beige adipose tissue myocyte plasma proteins ATP-gated K+ channels sulfonylurea drugs glucagon cortisol body mass index (BMI) adipokines leptin arcuate nucleus orexigenic anorexigenic α -melanocyte–stimulating hormone (α -MSH) adiponectin AMP-activated protein kinase (AMPK) mTORC1 (mechanistic target of rapamycin complex 1) PPAR (peroxisome proliferator-activated receptor) ghrelin endocannabinoids probiotics prebiotics diabetes mellitus type 1 diabetes type 2 diabetes glucose-tolerance test ketosis acidosis ketoacidosis metabolic syndrome PROBLEMS 1. Peptide Hormone Activity Explain how two peptide hormones as structurally similar as oxytocin and vasopressin can have such different effects (see Fig. 23-8). 2. Metabolism of Glutamate in the Brain Brain tissue takes up glutamate from the blood, transforms it into glutamine, and releases the glutamine into the blood. What does this metabolic conversion accomplish? How does this conversion take place? The amount of glutamine produced in the brain can exceed the amount of glutamate entering from the blood. How does this extra glutamine arise? (Hint: You may want to review amino acid catabolism in Chapter 18; recall that NH+ 4 is very toxic to the brain.) 3. Proteins as Fuel during Fasting When muscle proteins undergo catabolism in skeletal muscle during a fast, what are the fates of the amino acids? 4. Absence of Glycerol Kinase in Adipose Tissue The biosynthesis of triacylglycerols requires glycerol 3-phosphate. Adipocytes, specialized for the synthesis and degradation of TAGs, cannot use glycerol directly because they lack glycerol kinase, which catalyzes the reaction Glycerol+ AT P →  glycerol 3-phosphate+ AD P How does adipose tissue obtain the glycerol 3-phosphate necessary for TAG synthesis? 5. Oxygen Consumption during Exercise A sedentary adult consumes about 0.05 L of O2 in 10 seconds. A sprinter running a 100 m race consumes about 1 L of O2 in 10 seconds. Aer finishing the race, the sprinter continues to breathe at an elevated (but declining) rate for some minutes, consuming an extra 4 L of O2 above the amount consumed by the sedentary individual. a. Why does the need for O2 increase dramatically during the sprint? b. Why does the demand for O2 remain high aer the sprinter finishes the race? 6. Thiamine Deficiency and Brain Function Individuals with thiamine deficiency show some characteristic neurological signs and symptoms, including loss of reflexes, anxiety, and mental confusion. Why might thiamine deficiency manifest as changes in brain function? 7. Potency of Hormones Under normal conditions, the human adrenal medulla secretes epinephrine (C9H13NO3) at a rate sufficient to maintain a concentration of 10−10M in circulating blood. To appreciate what that concentration means, calculate the volume of water that you would need to dissolve 1.0 g (about 1 teaspoon) of epinephrine to a concentration equal to that in blood. 8. Regulation of Hormone Levels in the Blood The half-life of most hormones in the blood is relatively short. For example, when researchers inject radioactively labeled insulin into an animal, half of the labeled hormone disappears from the blood within 30 min. a. What is the importance of the relatively rapid inactivation of circulating hormones? b. In what ways can the organism make rapid changes in the level of a circulating peptide hormone? 9. Water-Soluble versus Lipid-Soluble Hormones On the basis of their physical properties, hormones fall into one of two categories: those that are very soluble in water but relatively insoluble in lipids (e.g., epinephrine) and those that are relatively insoluble in water but highly soluble in lipids (e.g., steroid hormones). In their role as regulators of cellular activity, most water-soluble hormones do not enter their target cells. The lipid-soluble hormones, by contrast, do enter their target cells and ultimately act in the nucleus. What is the relationship between solubility, the location of receptors, and the mode of action of these two classes of hormones? 10. Metabolic Differences between Muscle and Liver in a “Fight-or-Flight” Situation When an animal confronts a “fight- or-flight” situation, the release of epinephrine promotes glycogen breakdown in the liver and skeletal muscle. The end product of glycogen breakdown in the liver is glucose; the end product in skeletal muscle is pyruvate. a. What is the reason for the different products of glycogen breakdown in the two tissues? b. What is the advantage of these specific glycogen breakdown routes to an animal that must fight or flee? 11. Excessive Amounts of Insulin Secretion: Hyperinsulinism Certain malignant tumors of the pancreas cause excessive production of insulin by β cells. Affected individuals exhibit shaking and trembling, weakness and fatigue, sweating, and hunger. a. What is the effect of hyperinsulinism on the metabolism of carbohydrates, amino acids, and lipids by the liver? b. What are the causes of the observed symptoms? Suggest why this condition, if prolonged, leads to brain damage. 12. Thermogenesis Caused by Thyroid Hormones Thyroid hormones are intimately involved in regulating the basal metabolic rate. Liver tissue of animals given excess thyroxine shows an increased rate of O2 consumption and increased heat output (thermogenesis), but the ATP concentration in the tissue is normal. Different explanations have been offered for the thermogenic effect of thyroxine. One is that excess thyroxine causes uncoupling of oxidative phosphorylation in mitochondria. How could such an effect account for the observations? Another explanation suggests that thermogenesis is due to an increased rate of ATP utilization by the thyroxine-stimulated tissue. Is this a reasonable explanation? Why or why not? 13. Function of Prohormones What are the possible advantages of synthesizing hormones as prohormones? 14. Sources of Glucose during Starvation The typical human adult uses about 160 g of glucose per day. Of this, the brain alone uses 120 g. The body’s available reserve of glucose (∼20 g of circulating glucose and ∼190 g of glycogen) is adequate for about one day. Aer the glucose reserve has been depleted during starvation, how does the body obtain more glucose? 15. Parabiotic ob/ob Mice By careful surgery, researchers can connect the circulatory systems of two mice so that the same blood circulates through both animals. In these parabiotic mice, products released into the blood by one animal reach the other animal via the shared circulation. Both animals are free to eat independently. Suppose a researcher parabiotically joins a mutant ob/ob mouse (both copies of the OB gene are defective) and a normal OB/OB mouse (both copies of the OB gene are functional). What would happen to the weight of each mouse in the parabiotic pair? 16. Calculation of Body Mass Index A biochemistry professor weighs 260 lb (118 kg) and is 5 feet 8 inches (173 cm) tall. What is his body mass index (BMI)? How much weight would he have to lose to bring his BMI down to 25 (normal)? 17. Insulin Secretion Predict the effects on insulin secretion by pancreatic β cells of exposure to the potassium ionophore valinomycin. Explain your prediction. 18. Effects of a Deleted Insulin Receptor A strain of mice specifically lacking the insulin receptor of liver is found to have mild fasting hyperglycemia (bloodglucose= 132mg/dL, vs. 101 mg/dL in controls) and a more striking hyperglycemia in the fed state (glucose= 363mg/dL, vs. 135 mg/dL in controls). The mice have higher than normal levels of glucose 6-phosphatase in the liver and elevated levels of insulin in the blood. Explain these observations. 19. Decisions on Drug Safety The drug rosiglitazone (Avandia) is effective in lowering blood glucose levels in patients with type 2 diabetes, but a few years aer rosiglitazone came into widespread use, it seemed that using the drug came with an increased risk of heart attack. In response, the U.S. Food and Drug Administration (FDA) severely restricted the conditions under which it could be prescribed. Two years later, aer additional studies had been completed, the FDA lied the restrictions, and today rosiglitazone is available by prescription in the United States, with no special limitations. Many other countries ban it completely. If it were your responsibility to decide whether this drug should remain on the market (labeled with suitable warnings about its side effects) or should be withdrawn from the market altogether, what factors would you weigh in making your decision? 20. Type 2 Diabetes Medication The drugs acarbose (Precose) and miglitol (Glyset), used in the treatment of type 2 diabetes mellitus, inhibit α -glucosidases in the brush border of the small intestine. These enzymes degrade oligosaccharides derived from glycogen or starch to monosaccharides. Suggest a possible mechanism for the salutary effect of these drugs for individuals with diabetes. What side effects, if any, would you expect from these drugs? Why? (Hint: Review lactose intolerance, p. 523.) DATA ANALYSIS PROBLEM 21. Cloning the Sulfonylurea Receptor of the Pancreatic β Cell Glyburide, a member of the sulfonylurea family of drugs, is used to treat type 2 diabetes. It binds to and closes the ATP- gated K+ channel shown in Figures 23-26 and 23-27. a. Given the mechanism shown in Figure 23-27, would treatment with glyburide result in increased or decreased insulin secretion by pancreatic β cells? Explain your reasoning. b. How does treatment with glyburide help reduce the symptoms of type 2 diabetes? c. Would you expect glyburide to be useful for treating type 1 diabetes? Explain your answer. Aguilar-Bryan and coauthors (1995) cloned the gene for the sulfonylurea receptor (SUR) portion of the ATP- gated K+ channel from hamsters. The research team went to great lengths to ensure that the gene they cloned was, in fact, the SUR-encoding gene. Here we explore how it is possible for researchers to demonstrate that they have cloned the gene of interest rather than another gene. The first step was to obtain pure SUR protein. As was already known, drugs such as glyburide bind SUR with very high affinity (Kd < 10nM ), and SUR has a molecular weight of 140 to 170 kDa. Aguilar-Bryan and coworkers made use of the high-affinity glyburide binding to tag the SUR protein with a radioactive label that would serve as a marker to purify the protein from a cell extract. First, they made a radiolabeled derivative of glyburide, using radioactive iodine (125I): d. In preliminary studies, the 125I-labeled glyburide derivative (hereaer, [125I]glyburide) was shown to have the same Kd and binding characteristics as unaltered glyburide. Why was it necessary to demonstrate this? (What alternative possibilities did it rule out?) Even though [125I]glyburide bound to SUR with high affinity, a significant amount of the labeled drug would probably dissociate from the SUR protein during purification. To prevent this, [125I]glyburide had to be covalently cross-linked to SUR. There are many methods for covalent cross-linking; Aguilar-Bryan and coworkers used UV light. When aromatic molecules are exposed to short-wave UV, they enter an excited state and readily form covalent bonds with nearby molecules. By cross-linking the radiolabeled glyburide to the SUR protein, the researchers could simply track the 125I radioactivity to follow SUR through the purification procedure. The research team treated hamster HIT cells (which express SUR) with [125I]glyburide and UV light, purified the 125I-labeled 140 kDa protein, and sequenced its 25 residue amino-terminal segment; they found the sequence PLAFCGTENHSAAYRVDQGVLNNGC. The investigators then generated antibodies that bound to two short peptides in this sequence, one binding to PLAFCGTE and the other to HSAAYRVDQGV, and showed that these antibodies bound the purified 125I- labeled 140 kDa protein. e. Why was it necessary to include this antibody-binding step? Next, the researchers designed PCR primers based on the sequences above, and then cloned a gene from a hamster cDNA library that encoded a protein with these sequences (see Chapter 9 on biotechnology methods). The cloned putative SUR cDNA hybridized to an mRNA of the appropriate length that was present in cells known to contain SUR. The putative SUR cDNA did not hybridize to any mRNA fraction of the mRNAs isolated from hepatocytes, which do not express SUR. f. Why was it necessary to include this putative SUR cDNA–mRNA hybridization step? Finally, the cloned gene was inserted into and expressed in COS cells, which do not normally express the SUR gene. The investigators mixed these cells with [125I]glyburide, with or without a large excess of unlabeled glyburide, exposed the cells to UV light, and measured the radioactivity of the 140 kDa protein produced. Their results are shown in the table. Experiment Cell type Added putative SUR cDNA? Added excess unlabeled glyburide? 125I label in 140 kDa protein 1 HIT No   No   + + + 2 HIT No   Yes − 3 COS No   No   − 4 COS Yes No   + + + 5 COS Yes Yes − g. Why was no 125I-labeled 140 kDa protein found in experiment 2? h. How would you use the information in the table to argue that the cDNA encoded SUR? i. What other information would you want to collect to be more confident that you had cloned the SUR gene? Reference Aguilar-Bryan, L., C.G. Nichols, S.W. Wechsler, J.P. Clement, IV, A.E. Boyd, III, G. González, H. Herrera-Sosa, K. Nguy, J. Bryan, and D.A. Nelson. 1995. Cloning of the β cell high-affinity sulfonylurea receptor: a regulator of insulin secretion. Science 268:423–426. PART I I I INFORMATION PATHWAYS PART OUTLINE 24 Genes and Chromosomes 25 DNA Metabolism 26 RNA Metabolism 27 Protein Metabolism 28 Regulation of Gene Expression The third and final part of this book explores the biochemical mechanisms underlying the apparently contradictory requirements for both genetic continuity and the evolution of living organisms. What is the molecular nature of genetic material? How is genetic information transmitted from one generation to the next with high fidelity? How do the rare changes in genetic material that are the raw material of evolution arise? How is genetic information ultimately expressed in the amino acid sequences of the astonishing variety of protein molecules in a living cell? Today’s understanding of information pathways has arisen from the convergence of genetics, physics, and chemistry in modern biochemistry. This convergence was epitomized by the discovery of the double-helical structure of DNA, postulated by James Watson and Francis Crick in 1953. Genetic theory contributed the concept of coding by genes. Physics permitted the determination of molecular structure by x-ray diffraction analysis. Chemistry revealed the composition of DNA. The profound impact of the Watson-Crick hypothesis arose from its ability to account for a wide range of observations derived from studies in these diverse disciplines. This discovery revolutionized our understanding of the structure of DNA and inevitably stimulated questions about its function. The double-helical structure itself clearly suggested how DNA might be copied so that the information it contains can be transmitted from one generation to the next. Clarification of how the information in DNA is converted into functional proteins came with the discoveries of messenger RNA and transfer RNA and with the deciphering of the genetic code. These and other major advances gave rise to the central dogma of molecular biology, comprising the three major processes in the cellular utilization of genetic information. The first is replication, the copying of parental DNA to form daughter DNA molecules with identical nucleotide sequences. The second is transcription, the process by which parts of the genetic message encoded in DNA are copied precisely into RNA. The third is translation, whereby the genetic message encoded in messenger RNA is translated on the ribosomes into a polypeptide with a particular sequence of amino acids. In bacteria, transcription and translation are tightly coupled such that ribosomes are in direct contact with RNA polymerases. The mRNA is translated as soon as it is synthesized. The overall complex has been given the name expressome. Part III explores these and related processes. In Chapter 24 we examine the structure, topology, and packaging of chromosomes and genes. The processes underlying the central dogma are elaborated in Chapters 25 through 27. Finally, in Chapter 28, we turn to regulation, examining how the expression of genetic information is controlled. A major theme running through these chapters is the added complexity inherent in the biosynthesis of macromolecules that contain information. Assembling nucleic acids and proteins with particular sequences of nucleotides and amino acids represents nothing less than preserving the faithful expression of the template upon which life itself is based. We might expect the formation of phosphodiester bonds in DNA or peptide bonds in proteins to be a trivial feat for cells, given the arsenal of enzymatic and chemical tools described in Part II. However, the framework of patterns and rules established in our examination of metabolic pathways thus far must be enlarged considerably to take into account molecular information. Bonds must be formed between particular subunits in informational biopolymers, avoiding either the occurrence or the persistence of sequence errors. This requirement has an enormous impact on the thermodynamics, chemistry, and enzymology of the biosynthetic processes. Formation of a peptide bond requires an energy input of only about 21 kJ per mole of bonds and can be catalyzed by relatively simple enzymes. But to synthesize a bond between two specific amino acids at a particular place in a polypeptide, the cell invests about 125 kJ/mol while making use of more than 200 enzymes, RNA molecules, and specialized proteins. The chemistry involved in peptide bond formation does not change because of this requirement, but additional processes are layered over the basic reaction to ensure that the peptide bond is formed between particular amino acids. Biological information is expensive. The dynamic interaction between nucleic acids and proteins is another central theme of Part III. Regulatory and catalytic RNA molecules are gradually taking a more prominent place in our understanding of these pathways (discussed in Chapters 26 through 28). However, most of the processes that make up the pathways of cellular information flow are catalyzed and regulated by proteins. An understanding of these enzymes and other proteins can have practical as well as intellectual rewards, because they form the basis of recombinant DNA technology (introduced in Chapter 9). Evolution again constitutes an overarching theme. Many of the processes outlined in Part III can be traced back billions of years, and a few can be traced to LUCA, the last universal common ancestor. The ribosome, most of the translational apparatus, and some parts of the transcriptional machinery are shared by every living organism on this planet. Genetic information is a kind of molecular clock that can help define ancestral relationships among species. Shared information pathways connect humans to every other species now living on Earth, and to all species that came before. Exploration of these pathways is allowing scientists to slowly open the curtain on the first act — the events that may have heralded the beginning of life on Earth.