Phorbol myristate enhanced specific incorporation of arachidonic acid into phospholipids through lysophospholipid acyltransferase in cultured smooth muscle cells

Effects of n-3 polyunsaturated fatty acids on the reacylation of arachidonic acid in peritoneal macrophages

Mouse peritoneal macrophages were prelabelled with [3H]AA and activated ex vivo with phorbol myristate acetate (PMA) in the presence or absence of added n-3 polyunsaturated fatty acids (n-3 PUFA). The n-3 PUFA did not affect the release of [3H]AA from stimulated macrophages. Added AA, enhanced, in a dose-dependent manner, the amount of free [3H]AA in the medium after activating the cells with PMA. Both n-3 PUFA and AA were actively acylated into the lipids of PMA activated macrophages, but the n-3 PUFA did not compete with reacylation of AA. In unstimulated cells, in the absence of PMA, n-3 PUFA effectively competed with AA for acylation into membrane phospholipids. These studies suggest that distinct pools of AA exist in macrophages.

Role of protein kinase C in the regulation of prostaglandin synthesis in human endothelium

The present study specifically addresses the role of protein kinase C (PKC) activation in human endothelial cell Ca2+ mobilization, a response that is functionally coupled to the production of the potent arachidonate (AA) metabolite, prostacyclin (PGI2). Phorbol 12-myristate 13-acetate (PMA), alpha-thrombin, and sodium fluoride (NaF), a direct G-protein activator, produced a rapid and time-dependent translocation of PKC from the cytosol to the membrane. Activation of PKC by brief pretreatment of human umbilical vein endothelial cell (HUVEC) monolayers with PMA resulted in the inhibition of NaF-induced inositol phosphate increases and attenuation of both alpha-thrombin- and NaF-activated increases in intracellular Ca2+ (Ca2+i). Ca2+ mobilization induced by ionophore A23187 was not affected by PKC preactivation, suggesting PKC-dependent negative feedback inhibition of phosphatidylinositol (PI)-specific phospholipase C (PLC). Agonist-stimulated AA release and PGI2 synthesis in PMA-pretreated cultured human endothelial cells, however, was potentiated, and the enhanced PGI2 synthesis produced by A23187, NaF, and alpha-thrombin was dependent upon the dose of PMA. Treatment of HUVEC monolayers with an intracellular Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid-acetoxymethylester (BAPTA-AM), dramatically reduced alpha-thrombin-, NaF-, and A23187-induced PGI2 synthesis, demonstrating the importance of Ca2+i availability in PGI2 synthesis. BAPTA pretreatment did not inhibit PMA-induced PKC activation, and BAPTA-mediated inhibition of agonist-stimulated PGI2 synthesis was partially attenuated by prior PMA pretreatment. Staurosporine, a potent PKC inhibitor, at concentrations that inhibited PKC-induced phosphorylation of histone-1, augmented both alpha-thrombin- and NaF-induced production of inositol phosphates but markedly inhibited alpha-thrombin-, NaF-, and A23187-induced PGI2 synthesis. The downregulation of PKC activity by prolonged PMA treatment (18 h) produced similar inhibition of PGI2 synthesis by these agonists (approximately 50% inhibition). These studies indicate that the integrated phospholipase A2 and PLC activities are under complex regulation by factors that include both PKC activation and [Ca2+i]. PKC exerts dual effects on prostaglandin synthesis via negative regulation of Gp-coupled PI-specific PLC and positive feedback regulation of AA release and PGI2 synthesis. PKC is thus a critical determinant in the regulation of human endothelial cell prostaglandin synthesis by both receptor-mediated and G-protein-dependent cellular activation.

Protein kinase C promotes arachidonate mobilization through enhancement of CoA-independent transacylase activity in platelets

A role for protein kinase C in arachidonate mobilization was demonstrated. Treatment of rat platelets with phorbol myristate acetate (PMA) or the diacylglycerol 1-oleoyl-2-acetylglycerol increased the transfer rate of arachidonate (AA) from phosphatidylcholine to phosphatidylethanolamine and stimulated AA release. The transfer dose-dependently induced by PMA was inhibited by staurosporine. Ether phospholipids were the acceptors of AA in these stimulated transfer reactions. Membrane-bound protein kinase C activity was enhanced by PMA, and this increase was inhibited by staurosporine. AA transfer between phospholipids is due to the action of polyunsaturated-fatty-acid-specific transacylases. For this purpose, transacylase activities were assayed in cell-free systems from PMA-treated platelets. We observed that the CoA-independent transacylase activity was modulated in parallel to AA transfer as a function of PMA concentration. Taken together, the data show that protein kinase C activation might promote the mobilization of AA in platelets through the enhancement of CoA-independent transacylase activity.

Sodium fluoride unmasks the accumulation of lysophosphatidylcholine in intact pancreatic islet cells

When intact but dispersed neonatal islet cells, prelabelled with [14C-Me]choline, were stimulated with a calcium ionophore, ionomycin alone elicited only small rises in lysophosphatidylcholine; in contrast, pretreatment for 20 min with sodium fluoride (20mM) unmasked a consistent accumulation of lysophospholipid (to 155% of control at 1 min, 162% at 5 min and 212% at 10 min). Fluoride was shown to inhibit (by 40-50%) the reacylation of exogenous acyl- or alkyl-linked lysophosphatidylcholines by a delayed and indirect effect, whereas, in contrast, 12-O-tetradecanoylphorbol-13-acetate or dioctanoylglycerol actually augmented acylation. Thus, increased production of lysophosphatidylcholine in intact islets is obscured by rapid removal mechanisms, one of which might involve protein kinase C (or diglycerides directly). The use of sodium fluoride partially obviates this clearance, but this finding may necessitate a re-interpretation of claims that G protein agonists such as fluoride directly activate phospholipase A2 in some cells.