Decreased responsiveness of basal gluconeogenesis to insulin action in hepatocytes isolated from genetically obese (fa/fa) Zucker rats

Diabetes in Zucker diabetic fatty rat

Male Zucker diabetic fatty fa/fa (ZDF) rats develop obesity and insulin resistance at a young age, and then with aging, progressively develop hyperglycemia. This hyperglycemia is associated with impaired pancreatic β-cell function, loss of pancreatic β-cell mass, and decreased responsiveness of liver and extrahepatic tissues to the actions of insulin and glucose. Of particular interest are the insights provided by studies of these animals into the mechanism behind the progressive impairment of carbohydrate metabolism. This feature among others, including the development of obesity- and hyperglycemia-related complications, is common between male ZDF rats and humans with type 2 diabetes associated with obesity. We discuss the diabetic features and complications found in ZDF rats and why these animals are widely used as a genetic model for obese type 2 diabetes.

Increasing fructose 2,6-bisphosphate overcomes hepatic insulin resistance of type 2 diabetes

Hepatic glucose production is increased as a metabolic consequence of insulin resistance in type 2 diabetes. Because fructose 2,6-bisphosphate is an important regulator of hepatic glucose production, we used adenovirus-mediated enzyme overexpression to increase hepatic fructose 2,6-bisphosphate to determine if the hyperglycemia in KK mice, polygenic models of type 2 diabetes, could be ameliorated by reduction of hepatic glucose production. Seven days after treatment with virus encoding a mutant 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase designed to increase fructose 2,6-bisphosphate levels, plasma glucose, lipids, and insulin were significantly reduced in KK/H1J and KK.Cg-A(y)/J mice. Moreover, high fructose 2,6-bisphosphate levels downregulated glucose-6-phosphatase and upregulated glucokinase gene expression, thereby reversing the insulin-resistant pattern of hepatic gene expression of these two key glucose-metabolic enzymes. The increased hepatic fructose 2,6-bisphosphate also reduced adiposity in both KK mice. These results clearly indicate that increasing hepatic fructose 2,6-bisphosphate overcomes the impairment of insulin in suppressing hepatic glucose production, and it provides a potential therapy for type 2 diabetes.

Involvement of both phosphatidylinositol 3-kinase and p44/p42 mitogen-activated protein kinase pathways in the short-term regulation of pyruvate kinase L by insulin

Pyruvate kinase L (PK-L) is a key regulatory enzyme of the hepatic glycolytic/gluconeogenic pathway that can be dephosphorylated and activated in response to insulin. However, the signaling cascades involved in this insulin effect have not been established. In this work we have investigated the potential involvement of phosphatidylinositol 3-kinase (PI 3-K) and p44/p42 mitogen-activated protein kinase (MAPK) pathways in the short-term modulation of PK-L by insulin in primary cultures of rat hepatocytes. Wortmannin, at a concentration of 100 nM, caused a marked inhibition of the PI 3-K/protein kinase B pathway, which became complete at 500 nM wortmannin. Likewise, wortmannin at 100 and 500 nM, elicited partial and total inhibitions of insulin-mediated activation of PK-L, respectively. However, this PI 3-K inhibitor also reduced insulin-mediated phosphorylation of p44/p42 MAPK in cultured rat hepatocytes, indicating that both the PI 3-K and MAPK pathways could be involved in PK-L activation by insulin. Three facts appear to reinforce this hypothesis: 1) the selective and complete inhibition of the PI 3-K/protein kinase B pathway by LY294002 (50 microM) was accompanied by a partial blockade of insulin-induced PK-L activation; 2) when signaling through the MAPK cascade was selectively suppressed by the presence of PD98059 (50 microM), a 50% reduction of insulin-induced activation of PK-L was observed; and 3) the effect of PD98059 (50 microM) on PK-L activation was reinforced by the additional presence of 100 nM wortmannin. We also observed that the blockade of p70 S6-kinase by rapamycin did not affect the activation of PK-L by insulin. From these findings it can be concluded that both PI 3-K and MAPK pathways, but not p70 S6-kinase, are involved in the short-term activation of PK-L by insulin in rat hepatocytes.

Modulation of gluconeogenesis by epinephrine in hepatocytes isolated from genetically obese (fa/fa) Zucker rats

The obese (fa/fa) Zucker rat shows an impaired sympathetic tone which is accompanied by an altered thermogenesis and changes in both lipid and carbohydrate metabolism. In this work, we have investigated the regulatory effects of epinephrine on the rate of gluconeogenesis from a mixture of [(14)C]lactate/pyruvate, in hepatocytes isolated from obese (fa/fa) rats and their lean (Fa/-) littermates. Epinephrine caused a dose-dependent stimulation of the rate of [(14)C]glucose formation in both obese and lean rat hepatocytes, the maximal rates being five- and twofold higher than the corresponding basal values (0.50 +/- 0.06 and 1.96 +/- 0.15 micromol of lactate converted to glucose/g of cell x 20 min, respectively). No significant differences were found between the calculated half-maximal effective concentrations (EC(50)) for epinephrine in obese and lean rat liver cells. The stimulation of gluconeogenesis by epinephrine was accompanied by a decrease in the cellular concentration of fructose 2,6-bisphosphate, and an inactivation of both pyruvate kinase and 6-phosphofructo 2-kinase, to similar extents in both types of hepatocytes. Epinephrine also significantly raised the hepatocyte content of cyclic AMP, with about a twofold increase at a saturating concentration of the catecholamine (1 microM), in both lean and obese rat liver cells. However, at suboptimal concentrations of epinephrine, the rise in cyclic AMP levels was significantly less marked in obese than in lean rat hepatocytes. Nevertheless, no significant differences were found in either the affinity or the number of beta-adrenergic receptors, in radioligand binding studies carried out in liver plasma membranes obtained from obese and lean Zucker rats. In conclusion, compared to the corresponding basal values, the response of gluconeogenesis from lactate to the stimulatory effect of epinephrine is higher in obese (fa/fa) than in lean (Fa/-) Zucker rat hepatocytes, with no significant differences in the calculated EC(50) values for this hormone. This occurs in spite of an apparent decreased sensitivity of the adenylate cyclase system to the stimulatory effect of epinephrine in obese rat liver cells.

Acute effect of leptin on hepatic glycogenolysis and gluconeogenesis in perfused rat liver

Leptin circulates in blood and is involved in body weight control primarily via hypothalamic receptors. To examine its direct metabolic action, effects of short-term portal leptin infusion: 1) on postprandial basal and epinephrine-stimulated glycogenolysis; and 2) on postabsorptive lactate-stimulated gluconeogenesis were studied in isolated perfused rat livers. Incremental epinephrine (150 pmol x min-1 x g-1 liver)-stimulated glucose release (in micromol/g liver within 30 minutes; control: 28.3 +/- 2.8) was suppressed (P <.05) by 44% (15.8 +/- 1.6), by 48% (14.6 +/- 4.1), and by 53% (13.3 +/- 2.1) during insulin (3 pmol x min-1 x g-1 liver), leptin (30 pmol x min-1 x g-1 liver), and simultaneous leptin + insulin infusion. Perfusate cyclic adenosine monophosphate increased approximately twofold during epinephrine stimulation in all groups. Neither leptin nor insulin affected hepatic lactate production, bile flow, or portal pressure in the fed state. In the postabsorptive state (20-hour fasting), rates of lactate (10 mmol/L)-dependent hepatic glucose release (in micromol. min-1 x g-1 liver; control: 0.12 +/- 0.01) were increased (P <.01) to 0.35 +/- 0.02 and to 0.24 +/- 0.01 by glucagon (3 pmol x min-1 x g-1 liver) and by leptin (15 pmol x min-1 x g-1 liver), respectively. In parallel, lactate uptake rates (in micromol x min-1 x g-1 liver) were higher in the presence of both glucagon (0.90 +/- 0. 03) and leptin (0.84 +/- 0.02) compared with control (0.68 +/- 0.04). In conclusion, leptin modulates hepatic glucose fluxes and may contribute to direct humoral regulation of liver glycogen stores in the fasted as well as in the fed state.

Perturbation of fuel homeostasis caused by overexpression of the glucose-6-phosphatase catalytic subunit in liver of normal rats

The terminal step in hepatic gluconeogenesis is catalyzed by glucose-6-phosphatase, an enzyme activity residing in the endoplasmic reticulum and consisting of a catalytic subunit (glucose-6-phosphatase (G6Pase)) and putative accessory transport proteins. We show that Zucker diabetic fatty rats (fa/fa), which are known to exhibit impaired suppression of hepatic glucose output, have 2.4-fold more glucose-6-phosphatase activity in liver than lean controls. To define the potential contribution of increased hepatic G6Pase to development of diabetes, we infused recombinant adenoviruses containing the G6Pase cDNA (AdCMV-G6Pase) or the beta-galactosidase gene into normal rats. Animals were studied by one of three protocols as follows: protocol 1, fed ad libitum for 7 days; protocol 2, fed ad libitum for 5 days, fasted overnight, and subjected to an oral glucose tolerance test; protocol 3, fed ad libitum for 4 days, fasted for 48 h, subjected to oral glucose tolerance test, and then allowed to refeed overnight. Hepatic glucose-6-phosphatase enzymatic activity was increased by 1.6-3-fold in microsomes isolated from AdCMV-G6Pase-treated animals in all three protocols, and the resultant metabolic profile was similar in each case. AdCMV-G6Pase-treated animals exhibited several of the abnormalities associated with early stage non-insulin-dependent diabetes mellitus, including glucose intolerance, hyperinsulinemia, decreased hepatic glycogen content, and increased peripheral (muscle) triglyceride stores. These animals also exhibited significant decreases in circulating free fatty acids and triglycerides, changes not normally associated with the disease. Our studies show that overexpression of G6Pase in liver is sufficient to perturb whole animal glucose and lipid homeostasis, possibly contributing to the development of metabolic abnormalities associated with diabetes.

Indirect effect of insulin to suppress endogenous glucose production is dominant, even with hyperglucagonemia

Suppression of endogenous glucose production (EGP) is one of insulin's primary metabolic effects and failure of this action is a major contributor to fasting hyperglycemia of type 2 diabetes mellitus. Classically, insulin was thought to suppress the liver directly, via hyperinsulinemia in the portal vein. Recently, however, we and others have demonstrated that at least part, and possibly most of insulin's action to suppress EGP is normally mediated via an extrahepatic (i.e., indirect) mechanism. We have suggested that this mechanism involves insulin suppression of adipocyte lipolysis, leading to lowered FFA and reduced EGP ("Single Gateway Hypothesis"). Previous studies of the indirect insulin effect from this laboratory were done under conditions of lowered portal glucagon. Because of the possibility that the direct (i.e., portal) effect of insulin may have been underestimated with hypoglucagonemia, these studies examined the relative importance of portal insulin, versus peripheral insulin (administered at one-half the dose to equalize peripheral insulin levels) at four rates of portal glucagon infusion: 0, 0.65 (under-), 1.5 (basal-), and 3.0 ng/kg per min (over-replacement). Portal versus peripheral insulin suppressed steady-state EGP to the same extent (52%), confirming that the primary effect of insulin to suppress EGP is via the peripheral mechanism. This conclusion was maintained regardless of portal glucagonemia, although there was some evidence for an increase in the direct insulin effect at hyperglucagonemia. The indirect effect of insulin is the primary mechanism of steady-state EGP suppression under normal conditions. The direct effect increases with hyperglucagonemia; however, the indirect effect remains predominant even under those conditions.

Modulation of epinephrine-stimulated gluconeogenesis by insulin in hepatocytes isolated from genetically obese (fa/fa) Zucker rats

Genetically obese (fa/fa) Zucker rats present an impaired response of hepatic glucose production to the inhibition by insulin. In this work, we have investigated the modulation by this hormone of epinephrine-stimulated gluconeogenesis, in hepatocytes isolated from obese (fa/fa) rats and their lean (Fa/-) littermates. Epinephrine (1 microM) caused a maximal stimulation of [14C]lactate conversion to [14C]glucose in hepatocytes isolated from either obese or lean animals. The stimulation of gluconeogenesis by epinephrine was accompanied by a significant reduction of fructose 2,6-bisphosphate levels, an inactivation of both pyruvate kinase and 6-phosphofructo 2-kinase, and by a 2-fold increase in the cellular concentrations of cAMP. The presence of insulin in the incubation medium antagonized, in a concentration-dependent manner, the effects of epinephrine. In hepatocytes isolated from lean rats, the reversion caused by insulin was complete, the concentration required for half-maximal insulin action ranging from 0.22 to 0.56 nM. In contrast, in obese rat hepatocytes, insulin only partially blocked epinephrine-mediated effects, and the sensitivity to insulin was 2- to 4-fold lower, as indicated by the corresponding half-maximal insulin action values. Furthermore, insulin (10 nM) almost completely blocked the increase in cAMP levels induced by epinephrine in lean rat hepatocytes, whereas it only provoked a small and nonsignificant reduction of epinephrine-stimulated levels of the cyclic nucleotide in hepatocytes obtained from obese rats.

Stimulation of glucose-6-phosphatase gene expression by glucose and fructose-2,6-bisphosphate

Glucose-6-phosphatase, a key enzyme in the homeostatic regulation of blood glucose concentration, catalyzes the terminal step in gluconeogenesis and glycogenolysis. Glucose, the product of the glucose-6-phosphatase reaction, dramatically increases the level of glucose-6-phosphatase mRNA transcripts in primary hepatocytes (20-fold), and the maximum response is obtained at a glucose concentration as low as 11 mM. Glucose specifically increases glucose-6-phosphatase mRNA and L-type pyruvate kinase mRNA. In the rat hepatoma-derived cell line, Fao, glucose increases the glucose-6-phosphatase mRNA only modestly (3-fold). In the presence of high glucose concentrations, overexpression of glucokinase in Fao cells via recombinant adenovirus vectors increases lactate production to the level found in primary hepatocytes and increases glucose-6-phosphatase gene expression by 21-fold. Similar overexpression of hexokinase I in Fao cells with high levels of glucose does not increase lactate production nor does it change the response of glucose-6-phosphatase mRNA to glucose. Glucokinase overexpression in Fao cells blunts the previously reported inhibitory effect of insulin on glucose-6-phosphatase gene expression in these cells. Raising the cellular concentration of fructose-2,6-bisphosphate, a potent effector of the direction of carbon flux through the gluconeogenic and glycolytic pathways, also stimulated glucose-6-phosphatase gene expression in Fao cells. Increasing the fructose-2,6-bisphosphate concentration over a 15-fold range (12 +/- 1 to 187 +/- 17 pmol/plate) via an adenoviral vector overexpression system, led to a 6-fold increase (0.32 +/- 0. 03 to 2.2 +/- 0.33 arbitrary units of mRNA) in glucose-6-phosphatase gene expression with a concomitant increase in glycolysis and a decrease in gluconeogenesis. Also, the effects of fructose-2, 6-bisphosphate concentrations on fructose-1,6-bisphosphatase gene expression were stimulatory, leading to a 5-6-fold increase in mRNA level over a 15-fold range in fructose-2,6-bisphosphate level. Liver pyruvate kinase and phosphoenolpyruvate carboxykinase mRNA were unchanged by the manipulation of fructose-2,6-bisphosphate level.

Abnormal regulation of hepatic glucocorticoid receptor mRNA and receptor protein distribution in the obese Zucker rat

This study examines the cellular distribution of glucocorticoid receptor (GR) protein and transcriptional activity of the GR gene in the liver of Zucker obese (fa/fa) rats. Immunoabsorption and Western blotting showed an increase in nuclear GR protein level but a decrease in cytosolic GR levels in the liver of 5-week old male obese rats (fa/fa) compared to their lean littermates (Fa/-). These changes were confirmed by receptor-ligand binding assays with [3H]-dexamethasone which showed a sixfold increase in average obese nuclear GR binding and a twofold reduction in cytosolic GR binding. HSP90, but not HSP70, levels were reduced in hepatic cytosol and increased in hepatic nuclei prepared from obese rats. Using Northern blot analysis of hepatic RNA, we demonstrated a twofold increase in hepatic mRNAs for GR, malic enzyme (ME), tyrosine aminotransferase (TAT), and glyceraldehyde 3-PO4-dehydrogenase in the obese rat. Increased transcription of GR and ME mRNAs in obese nuclei was indicated in nuclear run-on assays. These data suggest that there is increased nuclear localization of GR in the liver of obese rats and suggests that increased transcription of GR gene may contribute to this effect. The described changes may contribute to the abnormal regulation by glucocorticoids of some hepatic genes in the Zucker fa/fa rat.