Conditional ablation of Gsk-3β in islet beta cells results in expanded mass and resistance to fat feeding-induced diabetes in mice

Aims/hypothesis Glycogen synthase kinase 3 beta (GSK-3 beta) is an enzyme that is suppressed by insulin and when elevated results in insulin resistance in skeletal muscle and diabetes. Its role in beta cell development and function is little known. Because of the enzyme's anti-proliferative and proapoptotic properties, the hypothesis to be tested here was that beta cell specific deficiency of GSK-3 beta in mice would result in enhanced beta cell mass and function. Methods Mice with beta cell deficiency of GSK-3 beta (beta-Gsk-3 beta [also known as Gsk3b](-/-)) were generated by breeding Gsk-3 beta(flox/flox) mice with mice overexpressing the Cre recombinase gene under the control of the rat insulin 2 gene promoter (RIP-Cre mice), and glucose tolerance, insulin secretion, islet mass, proliferation and apoptosis were measured. Changes in islet proteins were investigated by western blotting. Results On a normal diet beta-Gsk-3 beta(-/-) mice were found to have mild improvement of glucose tolerance and glucose-induced insulin secretion, and increased beta cell mass accompanied by increased proliferation and decreased apoptosis. On a high-fat diet beta-Gsk-3 beta(-/-) mice exhibited improved glucose tolerance and expanded beta cell mass with increased proliferation relative to that in control mice, resisting fat-fed diabetes. Molecular mechanisms accounting for these phenotypic changes included increased levels of islet IRS1 and IRS2 proteins and phospho-Akt, suggesting enhanced signalling through the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, and increased islet levels of pancreas/duodenum homeobox protein 1 (PDX1). Inhibition of GSK3 in MIN6 cells in vitro led to increased IRS1 and IRS2 protein levels through inhibition of proteosomal degradation. Conclusions/interpretation These results are consistent with a mechanism whereby endogenous GSK-3 beta activity controls islet beta cell growth by feedback inhibition of the insulin receptor/P13K/Akt signalling pathway.