Potentiation of arachidonic acid release by phorbol myristate acetate in platelets is not due to inhibition of arachidonic acid uptake or incorporation into phospholipids

Abstract

Activators of protein kinase C, such as tumor-promoting phorbol esters (e.g., phorbol myristate acetate), mezerein, (−)-indolactam V and 1-oleoyl 2-acetoyl glycerol, potentiate arachidonic acid release caused by elevation of intracellular Ca 2+ with ionophores. This action of protein kinase C-activators required protein phosphorylation, and was attributed to enhanced hydrolysis of phospholipids by phospholipase A 2 (Halenda, et al. (1989) Biochemistry 28, 7356–7363). Recently Fuse et al. ((1989)J. Biol. Chem. 264, 3890–3895) reported that the apparent enhanced release of arachidonate was actually due to inhibition of the processes of re-uptake and re-esterification of released arachidonic acid. They attributed this to loss of arachidonyl-CoA synthetase and arachidonyl-CoA lysophosphatide acyltransferase activities, which were measured in membranes obtained from phorbol myristate acetate-treated platelets. In this paper, we show that phorbol myristate acetate, at concentrations that strongly potontiate arachidonic acid release, does not inhibit either arachidonic acid uptake into platelets or its incorporation into specific phospholipids. Furthermore, the fatty acid 8,11,14-eicosatrionoic acid, a competitive substrate for arachidonyl-CoA synthetase, totally blocks arachidonic acid uptake into platelets, but, unlike phorbol myristate acetate, does not potentiate arachidonic acid release by Ca 2+ ionophores. We conclude that the action of phorbol myristate acetate is to promote the process of arachidonic acid release by phospholipase A 2.