Phosphatidic acid directly activates endothelial cell protein kinase C

The metabolism of phosphatidic acid (PA) yields diacylglycerol (DAG), a known activator of protein kinase C (PKC). To examine potential direct effects of PA on PKC activation, PKC purified from bovine pulmonary artery endothelial cells (BPAEC) was utilized in an in vitro assay examining γ-[³²P]ATP phosphorylation of H1 histone. In the presence of Ca²⁺ and phosphatidylserine (PS), DAG (80 μM) produced maximal PKC activity (6.4 pmol γ-[³²P]ATP incorporated/μg/min). Dioleoyl-PA (80 μM) and l-stearoyl,2-arachidonyl-PA (80 μM) activated PKC in a concentration-dependent manner (maximal activity of 2.01 ± 0.1 pmol/μg/min). Unlike unlabelled phorbol esters or DAG, dioleoyl-PA did not significantly alter the binding of [³H]-phorbol dibutyrate to PKC, suggesting that PA directly activates endothelial cell PKC in a manner distinct from DAG-mediated PKC activation.