Participation in cellular prostaglandin synthesis of type-II phospholipase A2 secreted and anchored on cell-surface heparan sulfate proteoglycan.

Rat-liver-derived BRL-3A cells, which express both type-II phospholipase A2 (PLA2) and cytosolic PLA2 (cPLA2), generated prostaglandin E2 (PGE2) in the presence of fetal calf serum. When the cells were treated with tumor necrosis factor (TNF), PGE2 generation was greatly stimulated. The production of PGE2 observed in both cases was suppressed by a type-II PLA2-specific inhibitor, thielocin A1. Appreciable amounts of type-II PLA2 were released into the medium from the TNF-stimulated cells when heparin was added extracellularly. The release of type-II PLA2 from TNF-stimulated cells was also found in the presence of heparan sulfate or dextran sulfate, whereas other glycosaminoglycans showed no effects under the same conditions. These findings suggest that type-II PLA2 expressed in BRL-3A cells mostly associates with the cell surface by binding to cellular heparan sulfate proteoglycan. Removal of cell-surface-associated type-II PLA2, by either extracellular addition of heparin or by prior treatment of the BRL-3A cells with heparitinases, resulted in marked reduction of PGE2 synthesis in the cells. Exposure of BRL-3A cells to thrombin also induced the apparent secretion of type-II PLA2, and thrombin-stimulated PGE2 generation was suppressed by heparin effectively. Type-II PLA2 secreted and attached to heparan sulfate on the cell surface may therefore play an essential role in PGE2 synthesis by BRL-3A cells.