Activation of cytosolic phospholipase A2 and fatty acid transacylase is essential but not sufficient for thrombin-induced smooth muscle cell proliferation.

Thrombin is a potent stimulant of smooth muscle cell (SMC) proliferation in inflammatory conditions, leading to pathological thickening of vascular walls in atherosclerosis and airway remodeling in asthma. Cell proliferation requires the formation and remodeling of cell membrane phospholipids (PLs), involving the activation of PL-metabolizing enzymes. Yet, the role of specific PL-metabolizing enzymes in SMC proliferation has hardly been studied. To bridge this gap, in the present study, we investigated the role of key enzymes involved in PL metabolism, the PL-hydrolyzing enzyme phospholipase A2 (PLA2) and the PL-synthesizing enzyme lysophosphatidic acid-fatty acid transacylase (LPAAT), in thrombin-induced proliferation of bovine aortic SMCs (BASMCs). Concomitantly with the induction of BASMC proliferation, thrombin activated cytosolic PLA2 (cPLA2-alpha), expressed by selective release of arachidonic acid and mRNA expression, as well as LPAAT, expressed by nonselective incorporation of fatty acid and mRNA expression. Specific inhibitors of these enzymes, arachidonyl-trifluoromethyl-ketone for cPLA2 and thimerosal for LPAAT, suppressed their activities, concomitantly with suppression of BASMC proliferation, suggesting a mandatory requirement for cPLA2 and LPAAT activation in thrombin-induced SMC proliferation. Thrombin acts through the protease-activated receptor (PAR-1), and, accordingly, we found that thrombin-induced BASMC proliferation was suppressed by the PAR-1 inhibitor SCH-79797. However, the PAR-1 inhibitor did not prevent thrombin-induced mRNA expression of cPLA2 and LPAAT, implying that the activation of cPLA2 and LPAAT is essential but not sufficient for thrombin-induced proliferation of BASMCs.