INTERFERON-GAMMA INDUCES A PHOSPHOLIPASE D-DEPENDENT RELEASE OF ARACHIDONIC-ACID FROM ENDOTHELIAL-CELL MEMBRANES - A MECHANISM FOR PROTEIN-KINASE-C ACTIVATION

Interferon-gamma (IFN-gamma) induces MHC class II expression on endoth elial cells in a protein kinase C (PKC)-dependent manner. Here we show that IFN-gamma induces a sixfold arachidonic acid (AA) release from c ultured rat microvascular endothelial cell membranes compared with non -treated cells. Since this result suggests that AA could act as a seco nd messenger for IFN-gamma, we analysed its capacity to directly activ ate PKC. We have previously shown that IFN-gamma induces a transient, multiphasic activation of PKC via the action of the phospholipase D (P LD) pathway. Here we show that AA is able to activate PKC. In an attem pt to characterize the source of the liberated AA after IFN-gamma indu ction in endothelial cells we used a panel of enzyme inhibitors. The I FN-gamma-induced release of AA could not be modified by interfering ei ther with the phospholipase A2 (PLA2) pathway using bromophenacyl brom ide (BPB), or with the phospholipase C (PLC) pathway using neomycin. T he phosphatidic acid phosphatase (PAPase) inhibitor propranolol, inhib iting the generation of diacylglycerol (DAG) and further AA from phosp hatidic acid (PA), could totally down-regulate the IFN-gamma-induced r elease of AA. Since PA is produced solely by the action of PLD from ph osphatidylcholine (PC) we conclude that the AA originated from the cel l membrane-associated PC. In summary, we show here that IFN-gamma caus es the liberation of cell membrane-associated, PC-linked AA. This AA c ould directly activate PKC in a similar multiphasic manner to IFN-gamm a, suggesting that it is a true second messenger for IFN-gamma in cult ured endothelial cells.