Role of calcium-independent phospholipase A2β in human pancreatic islet β-cell apoptosis

Abstract

Death of β-cells due to apoptosis is an important contributor to β-cell dysfunction in both type 1 and type 2 diabetes mellitus. Previously, we described participation of the Group VIA Ca2+-independent phospholipase A2 (iPLA2β) in apoptosis of insulinoma cells due to ER stress. To examine whether islet β-cells are similarly susceptible to ER stress and undergo iPLA2β-mediated apoptosis, we assessed the ER stress response in human pancreatic islets. Here, we report that the iPLA2β protein is expressed predominantly in the β-cells of human islets and that thapsigargin-induced ER stress promotes β-cell apoptosis, as reflected by increases in activated caspase-3 in the β-cells. Furthermore, we demonstrate that ER stress is associated with increases in islet iPLA2β message, protein, and activity, iPLA2β-dependent induction of neutral sphingomyelinase and ceramide accumulation, and subsequent loss of mitochondrial membrane potential. We also observe that basal activated caspase-3 increases with age, raising the possibility that β-cells in older human subjects have a greater susceptibility to undergo apoptotic cell death. These findings reveal for the first time expression of iPLA2β protein in human islet β-cells and that induction of iPLA2β during ER stress contributes to human islet β-cell apoptosis. We hypothesize that modulation of iPLA2β activity might reduce β-cell apoptosis and this would be beneficial in delaying or preventing β-cell dysfunction associated with diabetes.