Insulin is both a hormone regulating energy metabolism and a growth factor. We and others have shown that physiological doses of insulin initiate complex signals in primary human and mouse ␤-cells, but the functional significance of insulin's effects on this cell type remains unclear. In the present study, the role of insulin in ␤-cell apoptosis was examined. Exogenous insulin completely prevented apoptosis induced by serum withdrawal when given at picomolar or low nanomolar concentrations but not at higher concentrations, indicating that physiological concentrations of insulin are antiapoptotic and that insulin signaling is self-limiting in islets. Insulin treatment was associated with the nuclear localization of Pdx1 and the prosurvival effects of insulin were largely absent in islets 50% deficient in Pdx1, providing direct evidence that Pdx1 is a signaling target of insulin. Physiological levels of insulin did not increase Akt phosphorylation, and the protective effects of insulin were only partially altered in islets lacking 80% of normal Akt activity, suggesting the presence of additional insulin-regulated antiapoptotic pathways. Proteomic analysis of insulin-treated human islets revealed significant changes in multiple proteins. Bridge-1, a Pdx1-binding partner and regulator of ␤-cell survival, was increased significantly at low insulin doses. Together, these data suggest that insulin can act as a master regulator of islet survival by regulating Pdx1.diabetes ͉ pancreatic ␤-cells ͉ programmed cell death ͉ autocrine insulin feedback signaling ͉ maturity onset diabetes of the young

No abstract

No abstract

Cells are programmed to die when critical signaling and metabolic pathways are disrupted. Inhibiting the type 2 ryanodine receptor (RyR2) in human and mouse pancreatic ␤-cells markedly increased apoptosis. This mode of programmed cell death was not associated with robust caspase-3 activation prompting a search for an alternative mechanism. Increased calpain activity and calpain gene expression suggested a role for a calpain-dependent death pathway. Using a combination of pharmacological and genetic approaches, we demonstrated that the calpain-10 isoform mediated ryanodine-induced apoptosis. Apoptosis induced by the fatty acid palmitate and by low glucose also required calpain-10. Ryanodine-induced calpain activation and apoptosis were reversed by glucagon-like peptide or short-term exposure to high glucose. Thus RyR2 activity seems to play an essential role in ␤-cell survival in vitro by suppressing a death pathway mediated by calpain-10, a type 2 diabetes susceptibility gene with previously unknown function.The pancreatic ␤-cell plays a central role in the pathogenesis of diabetes mellitus. A reduction in ␤-cell mass mediated at least in part by an increase in apoptosis is characteristic of the diabetic state (1-3). It is becoming clear that several pathways can lead to ␤-cell apoptosis, including cytokine signaling, excessive Ca 2ϩ influx during chronic hyperglycemia, high levels of free fatty acids, hypoxia or hypoglycemia, endoplasmic reticulum (ER) 1 stress, and loss of growth factor signaling (1, 3-12). Whether various inducers of apoptosis employ distinct molecular mechanisms has not been systematically studied.Intracellular Ca 2ϩ stores play an important role in the regulation of apoptosis in many cell types (13,14). The present study was undertaken to test the hypothesis that alterations in specific intracellular Ca 2ϩ stores may induce apoptosis in pancreatic ␤-cells. There are at least three classes of intracellular Ca 2ϩ stores in ␤-cells, and these are sensitive, respectively, to inositol trisphosphate (IP 3 )/thapsigargin, nicotinic acid adenine dinucleotide phosphate, and cyclic ADP ribose/ryanodine (15-18). In many cell types, ryanodine receptor Ca 2ϩ channels (RyR) transmit Ca 2ϩ signals directly to closely associated mitochondria (19). In the MIN6 ␤-cell line, RyR were shown to regulate ATP production (20). Because of their role in regulating intracellular Ca 2ϩ and mitochondrial function, we focused specifically on RyR as likely mediators of ␤-cell apoptosis. Of the three RyR subtypes, two have been reported to be present in ␤-cells, RyR1 and RyR2. The latter is more abundant and can be distinguished from the former by its insensitivity to dantrolene (21,22). Ryanodine, a plant alkaloid, is the most specific probe for all RyR subtypes, and its activity is lost in RyR-deficient cells (23,24).In the present study, we examined the role of RyR in the survival of human and mouse pancreatic islets. We uncovered a novel apoptosis pathway that is initiated when Ca 2ϩ flux through RyR2 is blocked. The ...

No abstract