Activation of Phospholipase A2 and Acylation of Lysophospholipids: The Major Regulatiors for Platelet Activating Factor Production in Rat Neutrophils1

Rat inflammatory neutrophils induced arachidonic acid release and platelet-activating factor (PAF) production in response to opsonized zymosan (OPZ) dose-dependently. Phospholipase A2 activity also increased dose-dependently, paralleling the increases in arachidonic acid and PAF. The time courses of the activities of phospholipase A2 and acetyltransferase, and the amounts of free arachidonic acid, lyso-PAF, and PAF demonstrated that activation of the enzymes in the remodeling pathway could be required for PAF production in rat neutrophils, which agrees with the documented fact for macrophages. Phospholipase A2 could be a rate-limiting enzyme for PAF production, since an increased lyso-PAF amount or addition of exogenous lyso-PAF reflected the increase in PAF formation in the cells. This phospholipase A2 activity in rat neutrophils could be attributed to cytosolic type phospholipase A2, because the activity was mostly suppressed by a specific antibody to cytosolic phospholipase A2. As previously reported, pretreatment of neutrophils with the acyl-CoA synthetase inhibitor, triacsin C, or the acyltransferase inhibitor, merthiolate, enhanced PAF production as well as arachidonic acid release by the cells in response to OPZ. Triacsin C inhibited arachidonoyl-CoA production and merthiolate suppressed the transacylation of lyso-PAF to 1-alkyl-2-arachidonoyl-GPC. These results suggest that these inhibitors of acylation of lyso-PAF caused accumulation of lyso-PAF, which resulted in enhancement of PAF production when phospholipase A2 and acetyltransferase were activated by OPZ. Thus the activation of cytosolic phospholipase A2 and the acylation of lyso-PAF by such as arachidonic acid could be regulating factors for PAF production in stimulated rat neutrophils.