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Home / Resources / Clinical Gems / International Textbook of Diabetes Mellitus, 4th Ed., Excerpt #71: Regulation of Glucose Metabolism in Liver Part 7 of 11

International Textbook of Diabetes Mellitus, 4th Ed., Excerpt #71: Regulation of Glucose Metabolism in Liver Part 7 of 11

Fructose-1,6-bisphosphatase/phosphofructokinase

Allosteric regulation of hexose flux is another example where the regulation of gluconeogenesis occurs by interaction with glycolytic regulation. The conversion of F-1-P to F-1,6-P2 is irreversibly catalyzed by the gluconeogenic enzyme fructose-1,6- bisphosphatase, and the opposite glycolytic reaction is irreversibly catalyzed by phosphofructokinase. Like the intersection between PC-PEPCK and L-PK, fructose-1,6-bisphosphatase and PFK resemble a futile cycle but which is actually a finely tuned point of metabolic control [15]. Liver phosphofructokinase is allosterically inhibited by PEP and citrate, which are indicative of abundant energy and substrate for gluconeogenesis (Figure 13.6). More importantly, phosphofructokinase is regulated by F-2,6-P2, a product of F-1,6-P2 and the bifunctional enzyme. Fructose-1,6-bisphosphatase is also inhibited by F-2,6-P2, the product of the bifunctional enzyme during feeding [15]. However, like most other substrate cycles discussed in this chapter, complete deactivation of the opposing fluxes of fructose-1,6-bisphosphatase and the bifunctional enzyme is not complete, even during a substrate load [42]. The regulation of the relative activities of FBPase and PFK is addressed again in the discussion of hepatic glucose disposal later.

Hepatic energetics and glucose production

Hepatic gluconeogenesis uses more than 40% of the energy consumed by the human liver [36]. Gluconeogenesis consumes 4 moles of ATP, 2 moles of GTP, and 2 moles of NADH for every mole of glucose produced. This energy is supplied in large part by the oxidation of fatty acids, which is promoted in the fasted state via a fall in the concentration of malonyl CoA and activation of carnitine palmitoyltransferase 1 (CPT 1) [43]. The allosteric and substrate effectors mentioned throughout this section (NADH/NAD+, ATP/AMP, acetyl-CoA, citrate, etc.) are products of fatty acid oxidation. Consequently, fat oxidation is required for optimal gluconeogenesis in hepatocytes and perfused liver [44]. When defects in fat oxidation occur, either due to targeted ablation in mice or inborn errors in humans, impaired gluconeogenesis and hypoglycemia almost always emerge as complications. In contrast, exogenous lipid infusion induces gluconeogenesis in vivo [45]. The metabolic dependence of hepatic gluconeogenesis on fat oxidation is imparted by the allosteric and covalent activation of the enzymes of gluconeogenesis and glycolysis [44] (Figure 13.7). Recall that PC activity is allosterically activated by acetyl-CoA, a product of the β-oxidation of fatty acids. Condensation of acetyl-CoA with oxaloacetate is the first step of the tricarboxylic acid (TCA) cycle and leads to the generation of citrate and NADH, and ultimately to a rise in ATP/ADP through the actions of the respiratory chain. Together, citrate, NADH and ATP, are the same factors that suppress PFK, PK, and PDH in glycolysis. In addition, the high energy cofactors, ATP, GTP, and NADH, are consumed by the gluconeogenic enzymes, PC, PEPCK, phosphoglycerate kinase, and glyceraldehyde-3-phosphate dehydrogenase. During fasting, low insulin and high counterregulatory hormones promote adipose lipolysis, elevated circulating fatty acids, upregulation of hepatic lipid oxidation, upregulation of gluconeogenic genes, and an abundance of the cofactors required to stimulate gluconeogenesis. As fasting continues into starvation, gluconeogenic substrates become scarce, fatty acid oxidation increases and elevated mitochondrial NADH/NAD+ reduces the activity of the TCA cycle. Under these conditions acetyl-CoA is converted to ketones to supplement falling rates of hepatic glucose production.

Hepatic energetics as a therapeutic target for diabetes

Biguanides, such as metformin, have been used as antidiabetic agents for several decades and today metformin remains the first medicine prescribed to newly diagnosed type 2 diabetics. Metformin improves glycemia by suppressing hepatic gluconeogenesis, but the mechanism by which this occurs has only recently, and only partially, been uncovered. Rather than inhibiting PEPCK or other gluconeogenic enzymes directly, metformin appears to reduce the gluconeogenic potential of liver by impairing hepatic energy metabolism [46]. Metformin impairs ATP synthesis by inhibition of respiratory complex I [47] and may also reduce the hepatocellular redox state by suppressing NADH utilization in the respiratory chain. Suppressed energy charge has secondary effects that reduce gluconeogenic gene expression through AMPK signaling [48], though this does not appear to be necessary for many of the metabolic effects of metformin [46]. A primary action on gluconeogenesis by metformin may be more metabolic in nature. By limiting energy supply, metformin inhibits high rates of gluconeogenesis, consistent with the requisite role of hepatic energy production in support of gluconeogenesis described earlier. In contrast to many other therapeutic targets of hepatic glucose metabolism, metformin has relatively few adverse effects, with rare manifestations of lactic acidosis being the most common.

Substrate supply and hepatic glucose production

The availability of gluconeogenic substrate is an important determinant of hepatic glucose production [49]. Glucagon and other hormones act on peripheral tissues to increase substrate delivery to liver. These events are particularly relevant during fasting and diabetes when a rise in the glucagon:insulin ratio causes activation of hormone-sensitive lipase and other lipases, which catalyzes the hydrolysis of triglycerides to yield free fatty acids and glycerol. The free fatty acids are transported to liver and re-esterified or oxidized. Oxidation is increased during fasting and diabetes because lower glycolytic flux in these states decreases malonyl-CoA mediated inhibition of CPT1 transport of fatty acids into mitochondria and increases the rate of fatty acid oxidation [43]. Fatty acid oxidation provides NADH and ATP required for optimal gluconeogenic activity [44]. Glucagon and glucocorticoid action also increases the supply of gluconeogenic precursors by inducing skeletal muscle autophagy, protein synthesis, and amino acid supply, while insulin has the opposite effect [50]. In addition counterregulatory hormones and fatty acids activate fat oxidation in muscle and suppress PDH activity which in turn stimulates aerobic glycolysis in the periphery. The lactate formed under these conditions is used by liver for gluconeogenesis and forms the basis of the Cori cycle. This cycle is particularly important during exercise where lactate formation in muscle provides ATP and defers the oxidative burden to liver.

The relative importance of substrate supply and hormone action on hepatic glucose production has been studied using mouse genetics and sophisticated metabolic clamps. Chronic ablation of the liver insulin receptor results in hyperglycemia [51], but acute ablation does not affect glycemia [52,53]. Specific restoration of the liver insulin receptor in mice missing the receptor in peripheral tissue is also insufficient to normalize hepatic glucose production [54]. Thus, the peripheral effects of insulin are important determinants for hepatic glucose metabolism. In canine models, where hormone and substrate can be delivered directly to liver by infusion into the portal vein, insulin action acutely suppresses hepatic glucose production by reducing glycogenolysis and directing gluconeogenic G6P to glycogen synthesis [55]. Although hepatic insulin is sufficient to mediate changes in gluconeogenic gene expression, insulin per se, does not rapidly alter gluconeogenic flux [55]. Thus, insulin action in liver is effective at regulating glycogen flux in the course of minutes, but requires hours to establish the molecular conditions to increase hepatic gluconeogenesis. In contrast, experiments where insulin was delivered to peripheral tissue, gluconeogenesis was reduced mainly by insulin-mediated suppression of adipose lipolysis and reduced delivery of free fatty acids to liver [56]. Insulin’s effect on hepatic gluconeogenic gene expression and its ability to restrict substrate supply from the periphery work together to reduce gluconeogenesis.

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