Evidence for proteolytic processing and stimulated organelle redistribution of iPLA2β

Over the past decade, important roles for the 84-88 kDa Group VIA Ca2+-independent phospholipase A2 (iPLA2β) in various organs have been described. We demonstrated that iPLA2β participates in insulin secretion, insulinoma cells and native pancreatic islets express full-length and truncated isoforms of iPLA2β, and certain stimuli promote perinuclear localization of iPLA2β. To gain a better understanding of its mobilization, iPLA2β was expressed in INS-1 cells as a fusion protein with EGFP, enabling detection of subcellular localization of iPLA2β by monitoring EGFP fluorescence. Cells stably-transfected with fusion protein expressed nearly 5-fold higher catalytic iPLA2β activity than control cells transfected with EGFP cDNA alone, indicating that co-expression of EGFP does not interfere with manifestation of iPLA2β activity. Dual fluorescence monitoring of EGFP and organelle Trackers combined with immunoblotting analyses revealed expression of truncated iPLA2β isoforms in separate subcellular organelles. Exposure to secretagogues and induction of ER stress are known to activate iPLA2β in β-cells and we find here that these stimuli promote differential localization of iPLA2β in subcellular organelles. Further, mass spectrometric analyses identified iPLA2β variants from which N-terminal residues were removed. Collectively, these findings provide evidence for endogenous proteolytic processing of iPLA2β and redistribution of iPLA2β variants in subcellular compartments. It might be proposed that in vivo processing of iPLA2β facilitates its participation in multiple biological processes.