Ca2+-independent arachidonic acid release by vascular endothelium requires protein synthesis de novo

Abstract

We investigated the mechanism by which the G-protein activators aluminium fluoride and vanadate stimulate arachidonic acid release in pig aortic endothelial cells. Our previous study demonstrated a novel Ca(2+)-independent pathway of phospholipase A2 (PLA2) activation stimulated by aluminium fluoride in this model. In the present study, we found that sodium metavanadate stimulated a rapid concentration-dependent release of [3H]arachidonic acid from prelabelled cells. A more than 3-fold enhancement of arachidonic acid release was achieved in cells treated with 1 mM vanadate for 20 min. Synthesis of prostaglandin products was similarly enhanced. The release of arachidonic acid was not dependent on the presence of extracellular Ca2+, but did require protein synthesis de novo. Both cycloheximide and actinomycin D completely blocked aluminium fluoride- and vanadate-stimulated arachidonic acid release. Because fluoride and vanadate are known protein tyrosine phosphatase inhibitors, it is possible that PLA2 activation occurred secondarily to changes in protein tyrosine phosphorylation. Both aluminium fluoride and vanadate stimulated the rapid phosphorylation of 58, 93 and 120 kDa tyrosine-containing protein substrates. However, in contrast with arachidonic acid release, this response was found to be sensitive to the presence of extracellular Ca2+ and insensitive to blockers of protein synthesis de novo. Furthermore H2O2 treatment resulted in rapid tyrosine phosphorylation of the same substrates without a concomitant increase in arachidonic acid release. These results suggest that the effects of aluminium fluoride and vanadate on PLA2 are not due to changes in protein tyrosine phosphorylation, but do require rapid protein synthesis de novo.