Endothelial eicosanoid metabolism and signal transduction during exposure to oxygen radicals injury

Several physiological agonists that induce elevation of cytosolic free calcium (Ca2+)-levels act via receptor coupled G-proteins, involving activation of phospholipase C (PLC) and hydrolysis of phosphatidylinositol 4,5-bisphosphate. Activation of the inositol signal transduction pathway that precedes Ca2+ ion mobilization is a well accepted signaling pathway in endothelial cell eicosanoid synthesis. This study was designed to examine possible involvement of phosphoinositides in the effects of oxygen free radicals on Ca2+ liberation and eicosanoid synthesis in human umbilical venous endothelial cells (HUVEC). Hydrogen peroxide (H2O2) was chosen as oxygen radicals generating agent. Stimulation of HUVEC with H2O2 (0.1 mmol/l) led to significant rises in inositol phosphate and diacylglycerol (DAG) levels within 300 seconds and an inhibition of Ca2+ release from internal stores. Eicosanoid formation was detectable despite unchanged levels of cytosolic free Ca2+ and no detectable activation of membrane associated phospholipase A2 (PLA2). This suggests that eicosanoid formation may be mediated through the activation of a Ca2+ independent, cytosolic 40 kDa PLA2 isoenzyme and that DAG could serve as an alternative source for arachidonic acid and seems to sensitize a cytosolic PLA2.