Phosphatidylinositol turnover during stimulation of plasminogen activator inhibitor-1 secretion induced by oxidized low density lipoproteins in human endothelial cells.

In a previous study (Latron et al. 1991. Arterioscler. Thromb. 11: 1821-1829) we have shown that oxidized low density lipoproteins (ox-LDL) stimulated the synthesis and secretion of plasminogen activator inhibitor-1 (PAI-1) by human umbilical vein endothelial cells (HUVEC) in culture. The present study is intended to give insight into the intracellular process responsible for this stimulation. The HUVEC lipids were labeled for 16 h with [3H]arachidonate and incubated either with native LDL (n-LDL) or ox-LDL for various times (15, 30, 60 min). Compared with unstimulated cells (no LDL added), ox-LDL induced a significant increase in the intracellular level of unesterified [3H]arachidonate, concomitantly with a significant decrease of the phosphatidylinositol fraction. The most marked effect was observed at 30 min and was significantly much less with n-LDL. Phospholipase inhibitors (4-bromophenacylbromide and mepacrine) added to the culture medium completely prevented the ox-LDL-induced stimulation of phosphatidylinositol degradation, [3H]arachidonate release, and PAI-1 secretion. HUVEC possess both phospholipase C and A activities and a high lysophospholipase activity, the phospholipase A pathway being in vitro more sensitive to inhibition by 4-bromophenacylbromide than the phospholipase C pathway. These results suggest that the stimulation of PAI-1 secretion by ox-LDL is mediated by the hydrolysis of membrane phosphatidylinositol through the activation of phospholipase A.