Biosynthesis of platelet-activating factor in human polymorphonuclear leukocytes. Involvement of the...

The generation of platelet-activating factor (PAF) in response to complement-coated zymosan particles, ionophore A23187 and 12-O-tetradecanoylphorbol 13-acetate (TPA) was studied in human polymorphonuclear leukocytes (PMN). TPA was an active stimulator of PAF biosynthesis but showed a time course more protracted than that observed in response to other secretagogues. TPA was not found to activate acetyl-CoA:lyso-PAF acetyltransferase activity nor to significantly enhance the incorporation of either [3H] lyso-PAF or [3H]acetate into a lipid fraction co-migrating as PAF. To assess whether TPA could be a secretagogue that promotes the biosynthesis of PAF through the dithiothreitol-insensitive cholinephosphotransferase pathway, this enzymic activity was assayed in homogenates from PMN preincubated in the presence of several secretagogues. None of the agonists was found to enhance this enzyme activity. However, TPA did enhance the incorporation of [methyl-3H]choline and both alkyl- and acetyl-labeled 1-O-hexadecyl-2-acetyl-sn-glycerol into a lipid fraction co-migrating with PAF. This incorporation showed a time course parallel to that of the generation of PAF in response to TPA. Incubation of [methyl-3H]choline-labeled cells in the presence of 1-O-hexadecyl-2-acetyl-sn-glycerol caused an enhanced incorporation of the label into the fraction co-migrating as PAF, and this incorporation was synergistically enhanced by TPA. Pulse-chase experiments with choline showed that TPA caused an early accumulation of choline into phosphatidylcholine and PAF rather than into CDP-choline. The present data indicate that TPA is the only agonist that could initiate the biosynthesis of PAF in human PMN through the cholinephosphotransferase pathway and that this process of biosynthesis is regulated at an enzyme step other than dithiothreitol-insensitive cholinephosphotransferase.