Regulation of platelet-activating factor (PAF) biosynthesis via coenzyme A-independent transacylase in the macrophage cell line IC-21 stimulated with lipopolysaccharide.

The regulation of PAF synthesis by the macrophage cell line IC-21 challenged with bacterial endotoxin was investigated. The LPS-induced increase in cellular PAF levels was rapid, sustained and attained maximal levels within 30 min following LPS stimulation. PAF accumulation was accompanied by the activation of the CoA-independent transacylase and acetyl-CoA: lyso-PAF acetyltransferase, whereas the release of free [3H]arachidonic acid in prelabeled cells reflecting the activation of phospholipase A2, occurred primarily within the initial 1-5 min of treatment with LPS. Cell lysates from LPS-stimulated macrophages exhibited a markedly increased enzymatic activity that was capable of both acylation of 1-[3H]alkyl-2-lyso-GPC (lyso-PAF) and deacylation of 1-[3H]alkyl-2-acyl-GPC generating [3H]lyso-PAF via CoA-independent transacylation of exogenous lysoplasmenylethanolamine compared with extracts from resting macrophages. Pretreatment of the cells with LPS for 5 and 30 min enhanced significantly the transfer of [14C]arachidonic acid from 1-[3H]alkyl-2-[14C]arachidonoyl-GPC into plasmenylethanolamine in prelabeled cell homogenates following the addition of exogenous lysoplasmenylethanolamine. Taken together, these data suggest that the CoA-independent transacylase, but not phospholipase A2, is a key enzyme responsible for the prolonged generation of lyso-PAF and that the increased capability of CoA-independent transacylation followed by CoA-dependent acetylation of lyso-PAF can sustain the biosynthesis of PAF in LPS-stimulated IC-21 macrophages.