Pancreatic Islets and Insulinoma Cells Express a Novel Isoform of Group VIA Phospholipase A₂ (iPLA₂β) that Participates in Glucose-Stimulated Insulin Secretion and Is Not Produced by Alternate Splicing of the iPLA₂β Transcript

Many cells express a group VIA 84 kDa phospholipase A₂ (iPLA₂β) that is sensitive to inhibition by a bromoenol lactone (BEL) suicide substrate. Inhibition of iPLA₂β in pancreatic islets and insulinoma cells suppresses, and overexpression of iPLA ₂β in INS-1 insulinoma cells amplifies, glucose-stimulated insulin secretion, suggesting that iPLA₂β participates in secretion. Western blotting analyses reveal that glucose-responsive 832/13 INS-1 cells express essentially no 84 kDa iPLA₂β-immunoreactive protein but predominantly express a previously unrecognized immunoreactive iPLA₂β protein in the 70 kDa region that is not generated by a mechanism of alternate splicing of the iPLA₂β transcript. To determine if the 70 kDa-immunoreactive protein is a short isoform of iPLA ₂β, protein from the 70 kDa region was digested with trypsin and analyzed by mass spectrometry. Such analyses reveal several peptides with masses and amino acid sequences that exactly match iPLA₂β tryptic peptides. Peptide sequences identified in the 70 kDa tryptic digest include iPLA₂β residues 7-53, suggesting that the N-terminus is preserved. We also report here that the 832/13 INS-1 cells express iPLA ₂β catalytic activity and that BEL inhibits secretagogue-stimulated insulin secretion from these cells but not the incorporation of arachidonic acid into membrane PC pools of these cells. These observations suggest that the catalytic iPLA₂β activity expressed in 832/13 INS-1 cells is attributable to a short isoform of iPLA ₂β and that this isoform participates in insulin secretory but not in membrane phospholipid remodeling pathways. Further, the finding that pancreatic islets also express predominantly a 70 kDa iPLA₂β -immunoreactive protein suggests that a signal transduction role of iPLA ₂β in the native β-cell might be attributable to a 70 kDa isoform of iPLA₂β.