Prolonged Elimination of Negative Feedback Control Mechanisms Along the Insulin Signaling Pathway Impairs β-Cell Function In Vivo

Abstract

Cellular stress and proinflammatory cytokines induce phosphorylation of insulin receptor substrate (IRS) proteins at Ser sites that inhibit insulin and IGF-I signaling. We therefore examined the effects of mutation of five "inhibitory" Ser phosphorylation sites on IRS2 function in transgenic mice that overexpress, selectively in pancreatic β-cells, either wild-type (WT) or a mutated IRS2 protein (IRS2^5A). Islets size, number, and mRNA levels of catalase and superoxide dismutase were increased, whereas those of nitric oxide synthase were decreased, in 7- to 10-week-old IRS2^5A-β mice compared with IRS2^WT-β mice. However, glucose homeostasis and insulin secretion in IRS2^5A-β mice were impaired when compared with IRS2^WT-β mice or to nontransgenic mice. This was associated with reduced mRNA levels of Glut2 and islet β-cell transcription factors such as Nkx6.1 and MafA Similarly, components mediating the unfolded protein response were decreased in islets of IRS2^5A-β mice in accordance with their decreased insulin secretion. The beneficial effects of IRS2^5A on β-cell proliferation and β-cell transcription factors were evident only in 5- to 8-day-old mice. These findings suggest that elimination of inhibitory Ser phosphorylation sites of IRS2 exerts short-term beneficial effects in vivo; however, their sustained elimination leads to impaired β-cell function.