Genetic deficiency of glycogen synthase kinase-3beta corrects diabetes in mouse models of insulin resistance.

Despite treatment with agents that enhance beta-cell function and insulin action, reduction in beta-cell mass is relentless in patients with insulin resistance and type 2 diabetes mellitus. Insulin resistance is characterized by impaired signaling through the insulin/insulin receptor/insulin receptor substrate/ PI-3K/ Akt pathway, leading to elevation of negatively regulated substrates such as glycogen synthase kinase-3 beta ( Gsk-3 beta). When elevated, this enzyme has antiproliferative and proapoptotic properties. In these studies, we designed experiments to determine the contribution of Gsk-3b to regulation of beta-cell mass in two mouse models of insulin resistance. Mice lacking one allele of the insulin receptor ( lr(+/-)) exhibit insulin resistance and a doubling of beta-cell mass. Crossing these mice with those having haploinsufficiency for Gsk-3 beta ( Gsk-3 beta(+/-)) reduced insulin resistance by augmenting whole-body glucose disposal, and significantly reduced beta-cell mass. In the second model, mice missing two alleles of the insulin receptor substrate 2 ( lrs2(-/-)), like the lr(+/-) mice, are insulin resistant, but develop profound beta-cell loss, resulting in early diabetes. We found that islets from these mice had a 4-fold elevation of Gsk-3 beta activity associated with a marked reduction of beta- cell proliferation and increased apoptosis. lrs2(-/-) mice crossed with Gsk-3 beta(+/-) mice preserved b- cell mass by reversing the negative effects on proliferation and apoptosis, preventing onset of diabetes. Previous studies had shown that islets of lrs2(-/-) mice had increased cyclindependent kinase inhibitor p27(kip1) that was limiting for b- cell replication, and reduced Pdx1 levels associated with increased cell death. Preservation of b- cell mass in Gsk-3 beta(+/-) lrs2(-/-) mice was accompanied by suppressed p27(kip1) levels and increased Pdx1 levels. To separate peripheral versus beta-cell-specific effects of reduction of Gsk3 beta activity on preservation of beta-cell mass, mice homozygous for a floxed Gsk-3 beta allele ( Gsk-3(F/F)) were then crossed with rat insulin promoter-Cre ( RIP-Cre) mice to produce beta-cell-specific knockout of Gsk- 3 beta (beta Gsk-3 beta(-/-)). Like Gsk-3 beta(+/-) mice, beta Gsk-3 beta(-/-) mice also prevented the diabetes of the lrs2(-/-) mice. The results of these studies now define a new, negatively regulated substrate of the insulin signaling pathway specifically within beta-cells that when elevated, can impair replication and increase apoptosis, resulting in loss of beta-cells and diabetes. These results thus form the rationale for developing agents to inhibit this enzyme in obese insulin-resistant individuals to preserve beta-cells and prevent diabetes onset.