Glucagon is the other major pancreatic islet hormone that is involved in the regulation of body fuel metabolism. Ingestion of protein appears to be the major physiological stimulus for secretion of glucagon. Glucagon's principal target tissue is the liver. Like insulin, glucagon is secreted first into the portal blood and is therefore anatomically well positioned to regulate hepatic metabolism.
Although the amino acids released by digestion of a protein meal appear to be the major glucagon secretagogue, glucagon's main actions on the liver appear to involve the regulation of carbohydrate and lipid metabolism. Glucagon is particularly important in stimulating glycogenolysis, gluconeogenesis, and ketogenesis. Glucagon does not act solely on the liver but also has glycogenolytic action on cardiac and skeletal muscle, lipolytic action on adipose tissue, and proteolytic actions on several tissues. However, these extrahepatic effects appear to be more prominent at pharmacological concentrations of glucagon. At more physiological concentrations, the liver is the major target tissue.
In many circumstances, glucagon's actions on liver antagonize those of insulin, and the mechanism of glucagon action is understood in considerable detail.
Pancreatic α cells secrete glucagon in response to ingested protein
Glucagon is a 31–amino-acid peptide (molecular weight, ~3500 Da) synthesized by α cells in the islets of Langerhans. In humans, the glucagon gene is located on chromosome 2. The initial gene product is the mRNA encoding preproglucagon. As is the case for insulin, a peptidase removes the signal sequence of preproglucagon during translation of the mRNA in the rough endoplasmic reticulum to yield proglucagon. Proteases in the α cells subsequently cleave the proglucagon (molecular weight, ~9000 Da) into the mature glucagon molecule and several biologically active peptides (Fig. 51-11). Neuroendocrine cells (i.e., L cells) within the gut process the proglucagon differently to yield not glucagon but GLP-1—a potent incretin—and other peptides.
FIGURE 51-11 Synthesis of the glucagon molecule. The proglucagon molecule includes amino-acid sequences that, depending on how the peptide chain is cleaved, can yield glucagon-related polypeptide (GRPP), glucagon, intervening peptide 1 (IP-1), GLP-1, IP-2, and GLP-2. Proteases in the pancreatic α cells cleave proglucagon at points that yield GRPP, glucagon, and a C-terminal fragment. Proteases in neuroendocrine cells in the intestine cleave proglucagon to yield glicentin, GLP-1, IP-2, and GLP-2.
Pancreatic α Cells
The mature glucagon molecule is the major secretory product of the α cell. As with insulin, the fully processed glucagon molecule is stored in secretory vesicles within the cell's cytosol. Although amino acids are the major secretagogues, the concentrations of amino acids required to provoke secretion of glucagon in vitro are higher than those generated in vivo. This observation suggests that other neural or humoral factors amplify the response in vivo, in a manner analogous to the effects of incretin on insulin secretion. However, the best studied incretin (GLP-1) inhibits glucagon secretion. Whereas both glucose and several amino acids stimulate insulin secretion by β cells, only amino acids stimulate glucagon secretion by α cells; glucose inhibits glucagon secretion. The signaling mechanism by which α cells recognize either amino acids or glucose is not known.
Glucagon, like the incretins, is a potent insulin secretagogue. However, because most of the α cells are located downstream from the β cells (recall that the circulation of blood proceeds from the β cells and then out past the α cells), it is unlikely that glucagon exerts an important paracrine effect on insulin secretion.
Intestinal L Cells
Proteases in neuroendocrine cells in the intestine process proglucagon differently than do α cells (see Fig. 51-11). L cells produce four peptide fragments: glicentin, GLP-1, intervening peptide 2 (IP-2), and GLP-2. Glicentin contains the amino-acid sequence of glucagon but does not bind to glucagon receptors. Both GLP-1 and GLP-2 are glucagon-like in that they cross-react with some antisera directed to glucagon, but GLP-1 and GLP-2 have very weak biological activity as glucagon analogs. However, GLP-1—released by the gut into the circulation in response to carbohydrate or protein ingestion—is one of the most potent incretins, stimulating insulin secretion. GLP-2 is not an incretin, and its biological actions are not known.
Glucagon, acting through cAMP, promotes the synthesis of glucose by the liver
Glucagon is an important regulator of hepatic glucose production and ketogenesis in the liver. As shown in Figure 51-12, glucagon binds to a receptor that activates the heterotrimeric G protein Gαs, which stimulates membrane-bound adenylyl cyclase (see p. 53). The cAMP formed by the cyclase in turn activates PKA, which phosphorylates numerous regulatory enzymes and other protein substrates, thus altering glucose and fat metabolism in the liver. Whereas insulin leads to the dephosphorylation of certain key enzymes (i.e., glycogen synthase, acetyl CoA carboxylase, phosphorylase), glucagon leads to their phosphorylation.
FIGURE 51-12 Glucagon signal transduction. Glucagon generally antagonizes the effects of insulin in the liver. Glucagon binds to a Gαs-coupled receptor, activating the adenylyl cyclase–cAMP–PKA cascade. Glucagon has three major effects on liver cells. First, glucagon promotes net glycogen breakdown. Glucagon inhibits glycogen synthesis by reducing the activity of glucokinase (1) and glycogen synthase (2). However, glucagon promotes glycogen breakdown by activating glycogen phosphorylase (3) and G6Pase (4). Second, glucagon promotes net gluconeogenesis. The hormone inhibits glycolysis and carbohydrate oxidation by reducing the activity of glucokinase (1), phosphofructokinase (5), and pyruvate kinase (6). Glucagon also stimulates gluconeogenesis by increasing the transcription of PEPCK (9), fructose-1,6-bisphosphatase (10), and G6Pase (4). Third, glucagon promotes the oxidation of fats. The hormone inhibits the activity of acetyl CoA carboxylase (11). Glucagon indirectly stimulates fat oxidation because the decreased levels of malonyl CoA relieve the inhibition of malonyl CoA on CAT (13). The numbering scheme for these reactions is the same as that in Figure 51-8.
A particularly clear example of the opposing actions of insulin and glucagon involves the activation of glycogenolysis (see p. 1182). PKA phosphorylates the enzyme phosphorylase kinase (see Fig. 58-9), thus increasing the activity of phosphorylase kinase and allowing it to increase the phosphorylation of its substrate, glycogen phosphorylase b. The addition of a single phosphate residue to phosphorylase b converts it to phosphorylase a. Liver phosphorylase b has little activity in breaking the one to four glycosidic linkages of glycogen, but phosphorylase a is very active. In addition to converting phosphorylase b to the active phosphorylase a form, PKA also phosphorylates a peptide called inhibitor 1 (see Fig. 3-7). In its phosphorylated form, inhibitor I decreases the activity of phosphoprotein phosphatase 1 (PP1), which otherwise would dephosphorylate both phosphorylase kinase and phosphorylase a (converting them to their inactive forms). PP1 also activates glycogen synthase. Thus, via inhibitor I, glucagon modulates several of the enzymes involved in hepatic glycogen metabolism to provoke net glycogen breakdown. As a result of similar actions on the pathways of gluconeogenesis and lipid oxidation, glucagon also stimulates these processes. Conversely, glucagon restrains glycogen synthesis, glycolysis, and lipid storage.
Glucagon also enhances gluconeogenesis by genomic effects, acting synergistically with glucocorticoids (see p. 1022). The genomic effects of glucagon occur as PKA phosphorylates the transcription factor cAMP response element–binding protein (CREB; see p. 89), which interacts with the cAMP response elements (CREs; see p. 89), increasing the expression of key gluconeogenic enzymes (e.g., G6Pase and PEPCK). Phosphorylated CREB also increases the expression of peroxisome proliferator–activated receptor-γ coactivator-1α (PGC-1α), which also enhances the expression of key gluconeogenic enzymes. Insulin restrains the transcription of these two enzymes in two ways, both via the PI3K/Akt pathway (see Fig. 51-6). First, insulin increases the release of the transcription-factor domain of sterol regulatory element–binding protein 1 (SREBP-1; see pp. 87–88), which antagonizes the transcription of mRNA encoding the two enzymes. Second, insulin increases the phosphorylation of the transcription factor FOXO1, thereby promoting its movement out of the nucleus and subsequent degradation; this action prevents FOXO1 from binding to the promoter regions of G6Pase and PEPCK.
These actions of glucagon can be integrated with our understanding of insulin's action on the liver in certain physiological circumstances. For example, after an overnight fast, when insulin concentrations are low, glucagon stimulates the liver to produce the glucose that is required by the brain and other tissues for their ongoing function. N51-9 With ingestion of a protein meal, absorbed amino acids provoke insulin secretion, which can inhibit hepatic glucose production and promote glucose storage by liver and muscle (see above). If the meal lacked carbohydrate, the secreted insulin could cause hypoglycemia. However, glucagon secreted in response to a protein meal balances insulin's action on the liver and thus maintains glucose production and avoids hypoglycemia.
Maintaining Plasma Glucose Levels during Starvation
Contributed by Fred Suchy
Hepatic gluconeogenesis is critical to maintaining normal plasma glucose levels during starvation. Glucagon and glucocorticoids positively regulate gluconeogenesis through synergistic signaling pathways.
Glucagon promotes the interaction of cAMP response element–binding protein (CREB; see p. 89) with CREB-binding protein (CBP; see p. 84) and CREB-regulated transcription coactivator 2 (CRTC2). Both CBP and CRTC2 facilitate the binding of CREB to cAMP response elements (CREs).
The hepatocyte nuclear factors forkhead O box (FOXO) and peroxisome proliferator–activated receptor-γ (PPARγ) coactivator 1α (PGC-1α) also acts synergistically to increase transcription of gluconeogenic genes.
The response to glucocorticoids is mediated by the glucocorticoid receptor, which binds to glucocorticoid response elements (GREs) in the promoters of gluconeogenic genes.
Sirtuin 1, an NAD-dependent deacetylase, is another energy sensor and modifier of the transcriptional activity of some of these transcription factors. For example, it deacetylates and affects the activity of PGC-1α. In contrast, insulin secreted postprandially represses transcription of gluconeogenic enzymes through activation of the Akt signaling pathway.
Glucagon promotes oxidation of fat in the liver, which can lead to ketogenesis
Glucagon plays a major regulatory role in hepatic lipid metabolism. As we saw in our discussion of insulin (see Fig. 51-8), the liver can esterify fatty acids with glycerol to form TAGs, which it can store or export as VLDL particles. Alternatively, the liver can partially oxidize fatty acids—and form ketone bodies (see p. 1185)—or fully oxidize them to CO2. Whereas fatty-acid esterification and storage occur in the liver cytosol, oxidation and ketogenesis occur within the mitochondria.
Glucagon stimulates fat oxidation indirectly by increasing the activity of CAT I (see pp. 1183–1185), which mediates the transfer of fatty acids across the outer mitochondrial membrane. Glucagon produces this stimulation by inhibiting acetyl CoA carboxylase, which generates malonyl CoA, the first committed intermediate in the synthesis of fatty acids by the liver. Malonyl CoA is also an inhibitor of the CAT system. By inhibiting acetyl CoA carboxylase, glucagon lowers the concentration of malonyl CoA, releases the inhibition of CAT I, and allows fatty acids to be transferred into the mitochondria. These fatty acids are oxidized to furnish ATP to the liver cell. If the rate of fatty-acid transport into the mitochondria exceeds the need of the liver to phosphorylate ADP, the fatty acids will be only partially oxidized; the result is the accumulation of the keto acids β-hydroxybutyric acid and acetoacetic acid, which are two of the three ketone bodies. These keto acids can exit the mitochondria and the liver to be used by other tissues as oxidative fuel.
During fasting, the decline in insulin and the increase in glucagon promote ketogenesis (see pp. 1185–1187); this process is of vital importance to the CNS, which can use keto acids but not fatty acids as fuel. In the adaptation to fasting, glucagon therefore plays the important role of stimulating the conversion of fatty acids to ketones and provides the brain with the fuel that is needed to allow continued function during a fast. We discuss fasting in more depth beginning on pages 1188–1192. N51-10
Contributed by Eugene Barrett
During fasting, falling insulin levels and rising glucagon levels promote the conversion of stored fat to ketone bodies and promote gluconeogenesis from muscle protein.
Insulin and glucagon are the principal regulators of body fuel metabolism. The integrated secretion and action of insulin and glucagon are vital to fuel homeostasis. In humans, after an overnight fast, plasma [insulin] is low. The low [insulin] and the availability of glucagon result in the production of glucose by the liver from the breakdown of endogenous glycogen as well as from the conversion of noncarbohydrate substrates into glucose (i.e., gluconeogenesis). These processes are highly regulated so that the rate at which glucose is produced by the liver matches the rate at which it is used by tissues, especially by the CNS. If the amount of glucose being provided by the liver is inadequate, plasma [glucose] will decline. The β cell, sensing this decrease, reduces the amount of insulin being secreted, whereas the α cell increases glucagon secretion. These two changes will increase glucose production by the liver and correct the plasma [glucose] toward normal. Conversely, if glucose is being overproduced and plasma [glucose] rises, insulin secretion increases and glucagon secretion is suppressed. Hepatic glucose production therefore declines and plasma [glucose] returns toward normal.
If fasting continues for several more days, insulin secretion continues to decline and glucagon secretion increases. The decline in insulin concentration allows an increased rate of proteolysis in muscle and mobilization of fatty acids from adipose tissue. The amino acids released by muscle serve as a substrate for hepatic gluconeogenesis. This response is particularly important after the first several days of fasting, when hepatic glycogen stores have been depleted and gluconeogenesis is the major pathway of hepatic glucose production. At the same time, the increased glucagon concentration stimulates ketogenesis in the liver and provides ketone bodies for the CNS. Ketones provide an alternative fuel source for the brain that allows the brain to decrease its use of glucose. Because most of the glucose comes from gluconeogenesis—and because the building blocks for gluconeogenesis come from accelerated proteolysis—the availability of ketone bodies allows the body to use the energy stored in fat and spare body protein. Because much of this body protein comes from the structural proteins in skeletal muscle and because catabolism of these proteins impairs muscle function (such as strength and mobility), it is a clear survival advantage to have the brain burn fat and not protein for fuel.
We discuss fasting beginning on pages 1188–1192.
In addition to its effects on hepatic glucose and lipid metabolism, glucagon also has the extrahepatic actions of accelerating lipolysis in adipose tissue and proteolysis in muscle. However, these effects are generally demonstrable only with high concentrations of glucagon, and although they may be important in certain pathological situations associated with greatly elevated glucagon concentrations (e.g., ketoacidosis or sepsis), they appear less important in the day-to-day actions of glucagon.