Studies of the role of group VI phospholipase A2 in fatty acid incorporation, phospholipid remodeling, lysophosphatidylcholine generation, and secretagogue-inducecd arachidonic acid release in pancreatic islets and insulinoma cells

An 84-kDa group VI phospholipase A(2) (iPLA(2)) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet beta-cells, A housekeeping role for iPLA(2) in generating lysophosphatidylcholine (LPC) accepters for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA(2) inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet beta-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA(2) in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA(2) with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [H-3]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and EEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [C-14]glucose incorporation into phospholipids. EEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA(2) did not alter the phospholipid head-group classes into which [H-3]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA(2) accelerated arachidonate incorporation into PC and that inhibition of islet iPLA(2) reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that EEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA(2) is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.