Mechanisms involved in the stimulation of prostacyclin synthesis by human lymphocytes in human umbilical vein endothelial cells.

1 Endothelial cells play an important role in the modulation of vascular tone because of their ability to produce vasoactive substances such as prostacyclin (PGI(2)). Cell-cell contact between human umbilical vein endothelial cells (HUVEC) and peripheral blood lymphocytes has been shown to stimulate endothelial PGI(2) synthesis by increasing free arachidonic acid availability through endothelial cytosolic phospholipase A2 (cPLA(2)) activation. In this study, we sought to determine whether phospholipase C (PLC) and D (PLD) activation also contributes, besides cPLA(2), to the lymphocyte-induced PGI(2) synthesis in HUVEC, and to delineate further the potential mechanisms of cPLA(2) activation triggered by the interaction of HUVEC with lymphocytes. 2 Pretreatment of endothelial cells with the PI-PLC inhibitor U-73122 before the coincubation with lymphocytes markedly inhibited the PGI(2) output whereas the diacylglycerol (DAG) lipase inhibitor RHC 80267 and ethanol had no effect. These results suggest that PLC may be involved through inositol trisphosphate generation and calcium mobilization, and that neither DAG nor phosphatidic acid (PtdOH) was used as sources of arachidonic acid. 3 The stimulated PGI(2) synthesis was protein kinase C (PKC)-independent but strongly inhibited by the mitogen-activated protein kinase kinase (MEK) inhibitors PD98059 and U-0126 and by the Src kinase inhibitor PP1. 4 Immunoblot experiments showed an increased phosphorylation of the extracellular signal-regulated kinases 1/2 (ERK1/2) upon lymphocyte addition till 4 h coincubation. Phosphorylation was markedly inhibited by U-0126 and PP1 addition. 5 Collectively, these results suggest that the signaling cascade triggered by lymphocytes in endothelial cells involves an Src kinase/ERK1/2 pathway leading to endothelial cPLA(2) activation.