The confluence-dependent interaction of cytosolic phospholipase A2-alpha with annexin A1 regulates endothelial cell prostaglandin E2 generation

The regulated generation of prostaglandins from endothelial cells is critical to vascular function. Here we identify a novel mechanism for the regulation of endothelial cell prostaglandin generation. Cytosolic phospholipase A(2)-alpha (cPLA(2)alpha) cleaves phospholipids in a Ca(2+)-dependent manner to yield free arachidonic acid and lysophospholipid. Arachidonic acid is then converted into prostaglandins by the action of cyclooxygenase enzymes and downstream synthases. By previously undefined mechanisms, nonconfluent endothelial cells generate greater levels of prostaglandins than confluent cells. Here we demonstrate that Ca(2+)-independent association of cPLA(2)alpha with the Golgi apparatus of confluent endothelial cells correlates with decreased prostaglandin synthesis. Golgi association blocks arachidonic acid release and prevents functional coupling between cPLA(2)alpha and COX-mediated prostaglandin synthesis. When inactivated at the Golgi apparatus of confluent endothelial cells, cPLA(2)alpha is associated with the phospholipid-binding protein annexin A1. Furthermore, the siRNA-mediated knockdown of endogenous annexin A1 significantly reverses the inhibitory effect of confluence on endothelial cell prostaglandin generation. Thus the confluence-dependent interaction of cPLA(2)alpha and annexin A1 at the Golgi acts as a novel molecular switch controlling cPLA(2)alpha activity and endothelial cell prostaglandin generation.