The inhibition of platelet-activating factor-induced platelet activation by oleic acid is associated with a decrease in polyphosphoinositide metabolism

In an earlier study (Miwa, M., Hill, C., Kumar, R., Sugatani, J., Olson, M. S., and Hanahan, D. J. (1987) J. Biol. Chem. 262, 527-530) it was shown that an inhibitor of platelet-activating factor (PAF), a powerful endogenous mediator of platelet aggregation, was present in freeze-clamped perfused livers. Subsequently, we determined that this substance was a mixture of unsaturated free fatty acids (FFA). Among these FFA, oleic acid between 10 and 100 microM was found to be a potent inhibitor of PAF-induced platelet aggregation and serotonin secretion. Consequently, in order to understand the molecular mechanism of oleic acid action, we investigated the effects of this FFA on several biochemical events associated with platelet aggregation induced by PAF. The effect of oleic acid and/or PAF on the level of 32P]phosphatidylinositol 4-phosphate (PIP) and 32P]phosphatidylinositol 4,5-bisphosphate (PIP2) was examined by using platelets labeled with 32P]phosphate. Oleic acid induced a dose-dependent decrease in the levels of 32P]PIP and 32P]PIP2; a maximal decrease in 32P]PIP and 32P]PIP2 of approximately 50 and 25%, respectively, was observed within seconds after the addition of 20 microM oleic acid and persisted for at least 15 min. Oleic acid did not induce the formation of 3H]inositol phosphates in platelets prelabeled with 3H]inositol, suggesting that the decrease in 32P]PIP and 32P]PIP2 was not due to a stimulation of phospholipase C. In contrast to oleic acid, PAF induced a dose-dependent increase in the 32P]PIP level, reaching a maximum of approximately 200% 3 min after the addition of 1 nM PAF to the platelets. This increase in 32P]PIP was accompanied by platelet aggregation and secretion, and a close correlation was established between the 32P]PIP level and the degree of aggregation. Oleic acid and PAF, when added together to the platelets, interacted by affecting the level of 32P]PIP and 32P]PIP2 in an opposite way since the decrease in the level of 32P]PIP and 32P] PIP2 induced by oleic acid was partially reversed by an excess of PAF. The decrease in the levels of 32P] PIP and 32P]PIP2 caused by oleic acid was associated with an inhibition of platelet aggregation induced by PAF. Interestingly, oleic acid did not block 3H]PAF binding to platelets but inhibited the PAF-induced phosphorylation of platelet proteins of 20 kDa and 40 kDa. These results suggest that inhibition of the PAF response by oleic acid may be at one of the steps in the signal transduction.(ABSTRACT TRUNCATED AT 400 WORDS)