β-Cell Calcium-Independent Group VIA Phospholipase A2 (iPLA2β)

Abstract

Evidence that group VIA cytosolic calcium-independent phospholipase A2 (iPLA2β) participates in β-cell signal transduction includes the observations that inhibition of iPLA2β with the bromoenol lactone suicide substrate suppresses glucose-stimulated insulin secretion and that overexpression of iPLA2β amplifies insulin secretory responses in INS-1 insulinoma cells. Immunofluorescence analyses also reveal that iPLA2β accumulates in the perinuclear region of INS-1 cells stimulated with glucose and forskolin. To characterize this phenomenon further, iPLA2β was expressed as a fusion protein with enhanced green fluorescent protein (EGFP) in INS-1 cells so that movements of iPLA2β are reflected by changes in the subcellular distribution of green fluorescence. Stimulation of INS-1 cells overexpressing iPLA2β-EGFP induced greater insulin secretion and punctate accumulation of iPLA2β-EGFP fluorescence in the perinuclear region. To determine the identity of organelles with which iPLA2β might associate, colocalization of green fluorescence with fluorophores associated with specific trackers targeted to different subcellular organelles was examined. Such analyses reveal association of iPLA2β-EGFP fluorescence with the ER and Golgi compartments. Arachidonate-containing plasmenylethanolamine phospholipid species are abundant in β-cell endoplasmic reticulum (ER) and are excellent substrates for iPLA2β. Arachidonic acid produced by iPLA2β-catalyzed hydrolysis of their substrates induces release of Ca2+ from ER stores—an event thought to participate in glucose-stimulated insulin secretion.