Decreased IRS Signaling Impairs β-Cell Cycle Progression and Survival in Transgenic Mice Overexpressing S6K in β-Cells

OBJECTIVE The purpose of this study was to evaluate the role of the S6K arm of mammalian target of rapamycin complex 1 (mTORC1) signaling in regulation of beta-cell mass and function. Additionally, we aimed to delineate the importance of in vivo S6K activation in the regulation of insulin signaling and the extent to which alteration of insulin receptor substrate (IRS) signaling modulates beta-cell mass and function. RESEARCH DESIGN AND METHODS The current experiments describe the phenotype of transgenic mice overexpressing a constitutively active form of S6K under the control of the rat insulin promoter. RESULTS Activation of S6K signaling in these mice improved insulin secretion in the absence of changes in beta-cell mass. The lack of beta-cell mass expansion resulted from decreased G(1)-S progression and increased apoptosis. This phenotype was associated with increased p16 and p27 and decreased Cdk2 levels. The changes in cell cycle were accompanied by diminished survival signals because of impaired IRS/Akt signaling. CONCLUSIONS This work defines the importance of S6K in regulation of beta-cell cycle, cell size, function, and survival. These experiments also demonstrate that in vivo downregulation of IRS signaling by TORC1/S6K induces beta-cell insulin resistance, and that this mechanism could explain some of the abnormalities that ultimately result in beta-cell failure and diabetes in conditions of nutrient overload. Diabetes 59:2390-2399, 2010