Acute Insulin Signaling in Pancreatic Beta-Cells Is Mediated by Multiple Raf-1 Dependent Pathways

Abstract

Insulin enhances the proliferation and survival of pancreatic β-cells, but its mechanisms remain unclear. We hypothesized that Raf-1, a kinase upstream of both ERK and Bad, might be a critical target of insulin in β-cells. To test this hypothesis, we treated human and mouse islets as well as MIN6 β-cells with multiple insulin concentrations and examined putative downstream targets using immunoblotting, immunoprecipitation, quantitative fluorescent imaging, and cell death assays. Low doses of insulin rapidly activated Raf-1 by dephosphorylating serine 259 and phosphorylating serine 338 in human islets, mouse islets, and MIN6 cells. The phosphorylation of ERK by insulin was eliminated by exposure to a Raf inhibitor (GW5074) or transfection with a dominant-negative Raf-1 mutant. Insulin also enhanced the interaction between mitochondrial Raf-1 and Bcl-2 agonist of cell death (Bad), promoting Bad inactivation via its phosphorylation on serine 112. Insulin-stimulated ERK phosphorylation was abrogated by calcium chelation, calcineurin and calmodulin-dependent protein kinase II inhibitors, and Ned-19, a nicotinic acid adenine dinucleotide phosphate receptor (NAADPR) antagonist. Blocking Raf-1 and Ca²⁺ signaling resulted in nonadditive β-cell death. Autocrine insulin signaling partly accounted for the effects of glucose on ERK phosphorylation. Our results demonstrate that Raf-1 is a critical target of insulin in primary β-cells. Activation of Raf-1 leads to both an ERK-dependent pathway that involves nicotinic acid adenine dinucleotide phosphate-sensitive Ca²⁺ stores and Ca²⁺-dependent phosphorylation events, and an ERK-independent pathway that involves Bad inactivation at the mitochondria. Together our findings identify a novel insulin signaling pathway in β-cells and shed light on insulin’s antiapoptotic and mitogenic mechanisms.