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 endothelial cells in a protein kinase C (PKC)-dependent manner. Here we show that IFN-gamma induces a sixfold arachidonic acid (AA) release from cultured rat microvascular endothelial cell membranes compared with non-treated cells. Since this result suggests that AA could act as a second messenger for IFN-gamma, we analysed its capacity to directly activate PKC. We have previously shown that IFN-gamma induces a transient, multiphasic activation of PKC via the action of the phospholipase D (PLD) pathway. Here we show that AA is able to activate PKC. In an attempt to characterize the source of the liberated AA after IFN-gamma induction in endothelial cells we used a panel of enzyme inhibitors. The IFN-gamma-induced release of AA could not be modified by interfering either with the phospholipase A2 (PLA2) pathway using bromophenacyl bromide (BPB), or with the phospholipase C (PLC) pathway using neomycin. The phosphatidic acid phosphatase (PAPase) inhibitor propranolol, inhibiting the generation of diacylglycerol (DAG) and further AA from phosphatidic acid (PA), could totally down-regulate the IFN-gamma-induced release of AA. Since PA is produced solely by the action of PLD from phosphatidylcholine (PC) we conclude that the AA originated from the cell membrane-associated PC. In summary, we show here that IFN-gamma causes the liberation of cell membrane-associated, PC-linked AA. This AA could directly activate PKC in a similar multiphasic manner to IFN-gamma, suggesting that it is a true second messenger for IFN-gamma in cultured endothelial cells.