Biosynthesis and release of platelet-activating factor from human monocytes.

The aim of the present study was to investigate the pathways of platelet-activating factor (PAF) production and release from human monocytes. For this purpose, both phagocytic stimuli and stimuli induced by soluble agents were used. The phagocytic stimuli exerted their effect in a receptor-specific mechanism related to surface Fc, C3b and C3d receptors. Stimuli induced by soluble agents, such as A23187 and pH 10.6, which do not require interaction with specific receptors, were also effective in inducing PAF release. In contrast, C5a, a soluble agent which induces a receptor-mediated release of PAF from neutrophils, failed to induce PAF release from monocytes. PAF release from monocytes could be dissociated from phagocytosis and from release of lysozyme. The PAF release required the presence of extracellular cations, the activation of membrane esterase and phospholipase A2 and the integrity of the microfilament system. Moreover, PAF release was modulated by lipoxygenase and intracellular cAMP levels. The relevance of an acetylation process in the biosynthesis of PAF was suggested by the increase of PAF yields in the presence of sodium acetate and by the incorporation of 14C-sodium acetate into molecules of active PAF.