Group VIA Phospholipase A2 (iPLA2β) Modulates Bcl-x 5′-Splice Site Selection and Suppresses Anti-apoptotic Bcl-x(L) in β-Cells

Diabetes is a consequence of reduced beta-cell function and mass, due to beta-cell apoptosis. Endoplasmic reticulum (ER) stress is induced during beta-cell apoptosis due to various stimuli, and our work indicates that group VIA phospholipase A(2)beta (iPLA(2)beta) participates in this process. Delineation of underlying mechanism(s) reveals that ER stress reduces the anti-apoptotic Bcl-x(L) protein in INS-1 cells. The Bcl-x pre-mRNA undergoes alternative pre-mRNA splicing to generate Bcl-x(L) or Bcl-x(S) mature mRNA. We show that both thapsigargin-induced and spontaneous ER stress are associated with reductions in the ratio of Bcl-x(L)/Bcl-x(S) mRNA in INS-1 and islet beta-cells. However, chemical inactivation or knockdown of iPLA(2)beta augments the Bcl-x(L)/Bcl-x(S) ratio. Furthermore, the ratio is lower in islets from islet-specific RIP-iPLA(2)beta transgenic mice, whereas islets from global iPLA(2)beta(-/-) mice exhibit the opposite phenotype. In view of our earlier reports that iPLA(2)beta induces ceramide accumulation through neutral sphingomyelinase 2 and that ceramides shift the Bcl-x 5'-splice site (5'SS) selection in favor of Bcl-x(S), we investigated the potential link between Bcl-x splicing and the iPLA(2)beta/ceramide axis. Exogenous C-6-ceramide did not alter Bcl-x 5'SS selection in INS-1 cells, and neutral sphingomyelinase 2 inactivation only partially prevented the ER stress-induced shift in Bcl-x splicing. In contrast, 5(S)-hydroxytetraenoic acid augmented the ratio of Bcl-x(L)/Bcl-x(S) by 15.5-fold. Taken together, these data indicate that beta-cell apoptosis is, in part, attributable to the modulation of 5'SS selection in the Bcl-x pre-mRNA by bioactive lipids modulated by iPLA(2)beta.