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.