Phorbol 12-myristate 13-acetate, A23178 and l-adrenaline inhibit phospholipid methylation in human monocytes and lymphocytes. Inhibition is independent of oxyradical production and phospholipid hydrolysis

Abstract

The Ca ++ ionophore A23187 and phorbol 12-myristate 13-myristate 13-acetate (PMA) caused dose-dependent inhibition of phospholipid (PL) methylation in unfractionated mononuclear cells (MNC), monocytes, and lymphocytes as measured by incorporation of 3- H-methyl-groups from [ 3 H-methyl]- L-methionine into phosphatidylcholine (PC), dimethyl phosphatidylethanolamine (PE), and monomethyl PE. This inhibitory effect did not correlate with monocyte superoxide release and was unaltered by the presence of either catalase and superoxide dismutase the NADPH osidase inhibitor, diphenylene iodonium (DPI), indicating that oxyradical-mediated oxidation of methionine was not the major cause of inbition of PL methylation. Furthermore L-adrenaline, which elevates cAMP and does not stimulate superoxide release, also inhibited PL methylation. Inhibition by PMA was not due to reduction in intracellular levels of methionine or S-adenosyl methionine. A23187 caused reduction of S-adenosyl methionine levels only at 1 μM, and had no effect at lower concentrations. Inhibition of PL methylation was shown not to be due to phospholipase A 2-dependent hydrolysis of newly methylated PL. Attmepts to reverse the inhibitory effect of either A23187 or PMA with the putative protein kinase inhibitors W-7 and H-7 were inconclusive. The mechanism of inhibition of PL methylation by A23187 and PMA remains ubclear, but does not appear to be due to oxidation of methionine or hydrolysis of newly methylated PL.