Group VIA PLA 2 (iPLA 2β) is activated upstream of p38 mitogen-activated protein kinase (MAPK) in pancreatic islet β-cell signaling

Abstract

Group VIA phospholipase A 2 (iPLA 2β) in pancreatic islet β-cells participates in glucose-stimulated insulin secretion and sarco(endo)plasmic reticulum ATPase (SERCA) inhibitor-induced apoptosis, and both are attenuated by pharmacologic or genetic reductions in iPL A2β activity and amplified by iPLA 2β overexpression. While exploring signaling events that occur downstream of iPLA 2β activation, we found that p38 MAPK is activated by phosphorylation in INS-1 insulinoma cells and mouse pancreatic islets, that this increases with iPLA 2β expression level, and that it is stimulated by the iPLA 2β reaction product arachidonic acid. The insulin secretagogue D-glucose also stimulates β-cell p38 MAPK phosphorylation, and this is prevented by the iPLA 2β inhibitor bromoenol lactone. Insulin secretion induced by D-glucose and forskolin is amplified by overexpressing iPLA 2β in INS-1 cells and in mouse islets, and the p38 MAPK inhibitor PD169316 prevents both responses. The SERCA inhibitor thapsigargin also stimulates phosphorylation of both β-cell MAPK kinase isoforms and p38 MAPK, and bromoenol lactone prevents both events. Others have reported that iPLA 2β products activate Rho family G-proteins that promote MAPK kinase activation via a mechanism inhibited by Clostridium difficile toxin B, which we find to inhibit thapsigargin- induced β-cell p38 MAPK phosphorylation. Thapsigargin- induced β-cell apoptosis and ceramide generation are also prevented by the p38 MAPK inhibitor PD169316. These observations indicate that p38 MAPK is activated downstream of iPLA 2β in β-cells incubated with insulin secretagogues or thapsigargin, that this requires prior iPLA 2β activation, and that p38 MAPK is involved in the β-cell functional responses of insulin secretion and apoptosis in which iPLA 2β participates. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc.