Calcium-Independent PLA2Beta in Beta-Cell Apoptosis

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DESCRIPTION (provided by applicant): Endoplasmic reticulum (ER) stress leads to beta-cell apoptosis that is suppressed by inhibition of the Group VIA Ca2+-independent phospholipase A2 (iPLA2beta), and our hypothesis is that iPLA2beta participates in ER stress-induced beta-cell apoptosis. In both type 1 and type 2 diabetes mellitus (DM), beta-cell apoptosis contributes to the loss of beta-cells and decreases in beta-cell function. It is therefore important to understand the mechanisms underlying beta-cell apoptosis if this process is to be prevented or delayed. The secretory nature of beta-cells endows them with a highly developed ER making them susceptible to ER stress. This process is suggested to promote beta-cell apoptosis and, in part, contribute to the decreases in beta-cell mass and beta-cell dysfunction in T2DM. The iPLA2beta catalyzes hydrolysis of arachidonic acid from membrane phospholipids and has a signal transduction role in beta-cells. We observed that ER stress-induced beta-cell apoptosis is related to the levels of iPLA2beta expression, is associated with perinuclear accumulation of a caspase-3-cleaved iPLA2beta isoform and increased generation of arachidonic acid and ceramides, and is suppressed by iPLA2beta inhibition. ER stress-induced apoptosis of pancreatic islets is also attenuated by inactivation of iPLA2beta and iPLA2beta -KO islets are less susceptible to ER stress. We propose to further examine the role of iPLA2a in a-cell apoptosis using pancreatic islets (wild type and iPLA2beta -KO) and INS-1 cells (parental, iPLA2beta over expressing, and iPLA2beta -KO) through the following Aims: Aim 1 is to characterize participation of iPLA2beta in beta-cell apoptosis using stimuli that induce both a-cell apoptosis and ER stress. Aim 2 is to examine the roles and subcellular localization of iPLA2beta isoforms in beta-cell apoptosis by expressing truncated and mutated iPLA2beta isoforms in a-cells. Aim 3 is to identify beta-cell subcellular phospholipid substrates for iPLA2beta during ER stress using ESI/MS/MS protocols. Aim 4 is to examine involvement of iPLA2beta phosphorylation and iPLA2beta -derived arachidonic acid in beta-cell apoptosis using LC/ESI/MS/MS and biochemical analyses.