Interferon‐γ Induces a Phospholipase D‐Dependent Release of Arachidonic Acid from Endothelial Cell Membranes: A Mechanism for Protein Kinase C Activation

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Abstract
Interferon‐γ (IFN‐γ) induces MHC class II expression on endothelial cells in a protein kinase C (PKC)‐dependent manner. Here we show that IFN‐γ 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‐γ, we analysed its capacity to directly activate PKC. We have previously shown that IFN‐γ 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‐γ induction in endothelial cells we used a panel of enzyme inhibitors. The IFN‐γ‐induced release of AA could not be modified by interfering either with the phospholipase A2 (PLA2) pathway using bromophcnacyl 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‐γ‐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‐γ causes the liberation of cell membrane‐associated, PC‐linked AA. This AA could directly activate PKC in a similar multiphasic manner to IFN‐γ, suggesting that it is a true second messenger for IFN‐γ in cultured endothelial cells.