Pancreatic deletion of insulin receptor substrate 2 reduces beta and alpha cell mass and impairs glucose homeostasis in mice.

AIMS/HYPOTHESIS: Insulin signalling pathways regulate pancreatic beta cell function. Conditional gene targeting using the Cre/loxP system has demonstrated that mice lacking insulin receptor substrate 2 (IRS2) in the beta cell have reduced beta cell mass. However, these studies have been complicated by hypothalamic deletion when the RIPCre (B6.Cg-tg(Ins2-cre)25Mgn/J) transgenic mouse (expressing Cre recombinase under the control of the rat insulin II promoter) is used to delete floxed alleles in insulin-expressing cells. These features have led to marked insulin resistance making the beta cell-autonomous role of IRS2 difficult to determine. To establish the effect of deleting Irs2 only in the pancreas, we generated PIrs2KO mice in which Cre recombinase expression was driven by the promoter of the pancreatic and duodenal homeobox factor 1 (Pdx1, also known as Ipf1) gene. MATERIALS AND METHODS: In vivo glucose homeostasis was examined in PIrs2KO mice using glucose tolerance and glucose-stimulated insulin secretion tests. Endocrine cell mass was determined by morphometric analysis. Islet function was examined in static cultures and by performing calcium imaging in Fluo3am-loaded beta cells. Islet gene expression was determined by RT-PCR. RESULTS: The PIrs2KO mice displayed glucose intolerance and impaired glucose-stimulated insulin secretion in vivo. Pancreatic insulin and glucagon content and beta and alpha cell mass were reduced. Glucose-stimulated insulin secretion and calcium mobilisation were attenuated in PIrs2KO islets. Expression of the Glut2 gene (also known as Slc2a2) was also reduced in PIrs2KO mice. CONCLUSIONS/INTERPRETATION: These studies suggest that IRS2-dependent signalling in pancreatic islets is required not only for the maintenance of normal beta and alpha cell mass but is also involved in the regulation of insulin secretion.

Liver-specific deletion of insulin receptor substrate 2 does not impair hepatic glucose and lipid metabolism in mice.

AIMS/HYPOTHESIS: Hepatic insulin resistance is thought to be a critical component in the pathogenesis of type 2 diabetes but the role of intrinsic insulin signalling pathways in the regulation of hepatic metabolism remains controversial. Global gene targeting in mice and in vitro studies have suggested that IRS2 mediates the physiological effects of insulin in the liver. Reduced hepatic production of IRS2 is found in many cases of insulin resistance. To investigate the role of IRS2 in regulating liver function in vivo, we generated mice that specifically lack Irs2 in the liver (LivIrs2KO). MATERIALS AND METHODS: Hepatic insulin signalling events were examined in LivIrs2KO mice by western blotting. Glucose homeostasis and insulin sensitivity were assessed by glucose tolerance tests and hyperinsulinaemic-euglycaemic clamp studies. The effects of high-fat feeding upon glucose homeostasis were also determined. Liver function tests were performed and expression of key metabolic genes in the liver was determined by RT-PCR. RESULTS: Proximal insulin signalling events and forkhead box O1 and A2 function were normal in the liver of LivIrs2KO mice, which displayed minimal abnormalities in glucose and lipid homeostasis, hepatic gene expression and liver function. In addition, hepatic lipid homeostasis and the metabolic response to a high-fat diet did not differ between LivIrs2KO and control mice. CONCLUSIONS/INTERPRETATION: Our findings suggest that liver IRS2 signalling, surprisingly, is not required for the long-term maintenance of glucose and lipid homeostasis, and that extra-hepatic IRS2-dependent mechanisms are involved in the regulation of these processes.