Search

Numerous studies have demonstrated that adiponectin plays a crucial role in the protection from diabetes and atherosclerosis. Recently, we have identified AdipoR1 and AdipoR2 as the major receptors for adiponectin, although the precise mechanism whereby adiponectin improves insulin sensitivity through these receptors has not been fully elucidated. In the present study, we have extensively analyzed the changes in the levels of proteins in liver, the major site of adiponeetin effects, by overexpressing each receptor. B6 male mice were treated with the adenovirus encoding LacZ, AdipoR1 or AdipoR2 (n=4, each group). The mice overexpressing AdipoR1 or AdipoR2 in the liver showed significantly lower insulin concentrations with comparable glucose levels compared to the control mice, indicating that these mice are more sensitive to insulin. The livers were removed 7 days after the adenovirus injection and were extracted followed by a proteomics analysis. Spots whose intensity was altered (either increased or decreased more than 1.5 fold) by AdipoR1 or AdipoR2 overexpression compared to LacZ control were applied to mass spectrometry. Total 84 proteins were altered in expression by overexpressing AdipoR1, while 146 proteins were modulated by overexpressing AdipoR2. Interestingly, most of the enzymes in the glycolytic pathway and the TCA cycle were up-regulated by AdipoR1 overexpression, whereas these trends were not observed by AdipoR2 overexpression. Moreover, AdipoR1 overexpression prominently increased catalase expression and decreased SOD1 and SOD2 expression, suggesting that the increased catalase suppresses oxidative stress, leading to reductions in SOD1 and SOD2. This up-regulation of catalase by AdipoR1 was also confirmed in the primary hepatocytes overespressing AdipoR1, suggesting that AdipoR1-mediated signal directly modulates catalase expression. The enhanced glucose catabolism increases NADH production necessarily accompanied by the increased electron leak, leading to oxidative stress. These data suggest the fine mechanism of the effects by adiponectin that adiponectin coordinately enhances glucose catabolism by up-regulating the regulatory enzymes expression and the protection machinery for the oxidative stress against the increased NADH production through AdipoR1. Thus, the subjects with hyperadiponectinemia have better glucose utilization without increasing oxidative stress, leading to protection of diabetes and atherosclerosis. [ABSTRACT FROM AUTHOR]

Recent studies using several knockout models have shown that autocrine insulin (and/or IGF-1) regulates growth and some functions of pancreatic β cells through activation of insulin receptor (IR)→insulin receptor substrate (IRS) proteins→Class IA phosphoinositide 3-kinase (PI3K). Class IA PI3K is composed of a catalytic subunit (p110) and a regulatory subunit (p85), which occurs in multiple isoforms. Among them, p85α and β are two major isoforms. P85α is derived from the Pik3rl gene that also encodes two shorter isoforms, p55α and p50α, while p85β is derived from the Pik3r2. To identify the role of Class IA P13K in β cells, we have generated β cell specific Pik3rl gene knockout (βPik3rlKO) mice using Cre-LoxP system, since the whole body Pik3rl KO mice die within 1 week after birth. To gain more insight in the PI3K pathway of β cells, we ablated the genes of two major regulatory subunits, p85α and p85β, of PI3K by crossing mice lacking Pik3rl in β cells with Pik3r2 null mice, for creating β cells-specific double knockout mice (βDKO mice). βPik3r1KO mice grew normally and were indistinguishable from control RIP-Cre transgenic mice. There was an 80% reduction of the regulatory subunits in isolated islets from βPik3rlKO mice, while p85β still remained. βPik3rlKO mice displayed modestly elevated fed glucose concentrations at 4 months of age and older, whereas fasting glucose levels were normal and they never developed diabetes during entire life. However, βPik3rlKO mice exhibited glucose intolerance and markedly suppressed glucose stimulated insulin secretion (GSIS) in vivo at 2 months and older. GSIS was also impaired using isolated islets from βPik3rlKO mice especially at lower concentrations of glucose, suggesting the existence of a defect in glucose sensing. Moreover, islet size of βPik3rlKO mouse was decreased by 30% compared to that of RIP-Cre mouse. Until a few months of age that we have observed for, βDKO mice also grew normally. However, βDKO mice showed more prominent glucose intolerance than βPik3rlKO mice due to a declined insulin secretion. Despite this, βDKO mice did not develop diabetes at this stage presumably because Pik3r2 null background is more sensitive to insulin than wild-type, as we have previously shown. These data suggest that Class IA PI3K is required for age-dependent islet growth and normal insulin secretion, and that decreased Class IA PI3K activity causes postprandial hyperglycemia. This may also provide a novel therapeutic approach for diabetes. [ABSTRACT FROM AUTHOR]

Adiponectin has been reported to suppress gluconeogenesis in liver through downregulating PEPCK and G6Pase, and also shown to be involved in lipid regulation; activating PPARαleads to an increase in fatty acid oxidation while suppression of SREBP1c decreases fatty acid synthesis in liver. These actions are supposed to contribute to the insulin sensitizing effects of adiponectin. The glucose lowering action of adiponectin has been believed to be mainly mediated by AMPK activation, which is known to suppress key gluconeognic enzymes in liver. However, induction of a dominant negative mutant of AMPK only partially attenuated the glucose lowering action of adiponectin, suggesting the existence of some other regulatory mechanisms. This prompted us to investigate the direct crosstalk between adiponectin signaling and insulin signaling as one of the candidate mechanisms. Here we show adiponectin increases hepatic IRS-2 protein, which is one of the main downstream molecules of insulin receptor and plays essential roles in glucose and lipid metabolism in liver. In the liver of db/db mice, the up-regulation of IRS-2 protein induced by adiponectin led to enhanced p85 recruitment to IRS-2 and Akt phosphorylation caused by insulin. Interestingly, the mRNA of IRS-2 showed a sharp and transient upregulation 2hrs after adiponectin administration. During this time course, plasma insulin concentration did not change, nor the CREB phosphorylation in the liver of adiponectin-treated mice. Moreover, the suppression of SREBP1c by adiponectin took place 4 to 8hrs after adiponectin administration. Thus, the up-regulation of IRS-2 expression by adiponectin is likely to be independent of insulin, CREB or SREBP1c action. Moreover, we have recently reported that the expression of adiponectin receptors is negatively regulated by insulin signaling via FOXO-1 exclusion from nucleus, and accordingly, fasting leads to up-regulation of AdipoRs. On the other hand, the serum concentration of adiponectin and the expression of adiponectin in adipose tissues are also elevated in the fasted state. Taken together, it is assumed that the adiponectin action is enhanced by fasting, while refeeding or hyperinsulinemic condition suppresses it. These data suggest that adiponectin signaling contributes to the upregulation of IRS-2 during fasting in liver, which may be one of the mechanisms whereby adiponectin exerts its insulin sensitizing effect in liver and contributes to the maintenance of systemic insulin sensitivity. [ABSTRACT FROM AUTHOR]