Genetic Deficiency of Glycogen Synthase Kinase-3β Corrects Diabetes in Mouse Models of Insulin Resistance

Abstract

Despite treatment with agents that enhance β-cell function and insulin action, reduction in β-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-3β (Gsk-3β). When elevated, this enzyme has antiproliferative and proapoptotic properties. In these studies, we designed experiments to determine the contribution of Gsk-3β to regulation of β-cell mass in two mouse models of insulin resistance. Mice lacking one allele of the insulin receptor (Ir+/−) exhibit insulin resistance and a doubling of β-cell mass. Crossing these mice with those having haploinsufficiency for Gsk-3β (Gsk-3β+/−) reduced insulin resistance by augmenting whole-body glucose disposal, and significantly reduced β-cell mass. In the second model, mice missing two alleles of the insulin receptor substrate 2 (Irs2−/−), like the Ir+/− mice, are insulin resistant, but develop profound β-cell loss, resulting in early diabetes. We found that islets from these mice had a 4-fold elevation of Gsk-3β activity associated with a marked reduction of β-cell proliferation and increased apoptosis. Irs2−/− mice crossed with Gsk-3β+/− mice preserved β-cell mass by reversing the negative effects on proliferation and apoptosis, preventing onset of diabetes. Previous studies had shown that islets of Irs2−/− mice had increased cyclin-dependent kinase inhibitor p27kip1 that was limiting for β-cell replication, and reduced Pdx1 levels associated with increased cell death. Preservation of β-cell mass in Gsk-3β+/−Irs2−/− mice was accompanied by suppressed p27kip1 levels and increased Pdx1 levels. To separate peripheral versus β-cell–specific effects of reduction of Gsk3β activity on preservation of β-cell mass, mice homozygous for a floxed Gsk-3β allele (Gsk-3F/F) were then crossed with rat insulin promoter-Cre (RIP-Cre) mice to produce β-cell–specific knockout of Gsk-3β (βGsk-3β−/−). Like Gsk-3β+/− mice, βGsk-3β−/− mice also prevented the diabetes of the Irs2−/− mice. The results of these studies now define a new, negatively regulated substrate of the insulin signaling pathway specifically within β-cells that when elevated, can impair replication and increase apoptosis, resulting in loss of β-cells and diabetes. These results thus form the rationale for developing agents to inhibit this enzyme in obese insulin-resistant individuals to preserve β-cells and prevent diabetes onset.

Methylglyoxal impairs insulin signalling and insulin action on glucose-induced insulin secretion in the pancreatic beta cell line INS-1E

“…Indeed, our data dovetail with a previous work showing that the overexpression of a dominant-negative Pkb (also known as Akt) in beta cells leads to impaired insulin secretion [45]. Recent facts have been reported showing that GSK-3β activation is linked to beta cell failure in diabetic mouse models, and is involved in negative regulation of beta cell proliferation and mass [46,47]. Moreover, GSK-3β activity is enhanced in insulin-resistant states, and in skeletal muscle from type 2 diabetic patients [48].…”

GSK-3 inactivation or depletion promotes β-cell replication via down regulation of the CDK inhibitor, p27 (Kip1)

“…Also, several studies using animal models of diabetes showed the importance of downregulation or loss of p27 in b-cells for accelerating replication of b-cells and thereby expansion of b-cell mass for correcting diabetes. [36][37][38] We have found robust interaction of p27 with GSK-3 in human islets and rodent b-cells (Fig. 2).…”

“…Studies using in-vitro and in-vivo models have demonstrated that GSK-3 inactivation/deficiency results in p27 instability/degradation. 31,37 Also, increased proliferation of both primary and transformed rodent b-cells following GSK-3 inactivation by small molecule inhibitors including BIO or 1-AKP have been reported. 39 A study demonstrated increased proliferation of highly differentiated primary rodent cardiomyocytes after BIO-mediated GSK-3 inhibition.…”

GSK-3α directly regulates β-adrenergic signaling and the response of the heart to hemodynamic stress in mice

“…Further complicating understanding, the strategy used in these models (mutation of the inhibitory serine 21 phosphorylation site of GSK-3α and serine 9 phosphorylation site of GSK-3β to alanine residues) blocks only 1 mechanism by which GSK-3α can be inhibited (i.e., PI3-kinase-mediated activation of Akt) and does not affect Wntmediated or p38-MAPK-mediated inhibition (15)(16)(17). Finally, to date, no studies have examined possible metabolic effects of GSK3s and how they might affect cardiac function, even though this is a critical factor in GSK-3-regulated disease processes, including glucose intolerance and diabetes (18,19).…”