The binding of receptor-recognized forms of a2-macroglobulin (a2M) to macrophage a2M signaling receptors increases inositol-I ,4,5-triphosphate synthesis and induces Ca2+ mobilization. In this report, we demonstrate that ligation of the macrophage a2M signaling receptor is also associated with synthesis of platelet activating factor (PAF) by both the de novo and remodeling pathways. Both a2M-methylamine and a cloned and expressed 20-kDa receptor binding fragment (RBF) from rat a,M+, stimulated macrophage synthesis of PAF from [3H]acetate, [3H]methylcholine, and l-O-[3H]alkyl lyso-PAF by two-to threefold. PAF levels reached a peak in 20 min after the cells were exposed to a,M-methylamine or RBF; they remained elevated for about 1 h after ligand addition to the cells. When [3H]methylcholine was the substrate, pertussis toxin did not block PAF synthesis, but the protein kinase C inhibitor staurosporin reduced synthesis by 65-70%. Cycloheximide completely abolished the increase in synthesis of PAF by macrophages exposed to a,M-methylamine. By contrast, when [3H]acetate was employed as a precursor, staurosporin or cycloheximide did not abolish the increase in PAF synthesis. These studies suggest that protein kinase C i s necessary for the induction of the de novo pathway by a2M-methylamine. Both a,M-methylamine and RBF stimulated the activity of lyso-PAF acetyltransferase by about fourfold. Both ligands also stimulated the activity of PAF acetylhydrolase by about six-to sevenfold, indicating that ligation of the azM signaling receptor also regulates the degradation of PAF. The ability of receptor-recognized forms of azM to regulate levels of PAF suggests that a,M-proteinase complexes not only regulate macrophage function by activating intracellular signaling but also may indirectly regulate the function of other cells that cannot bind a2M-proteinase complexes. D 1996 WiIey-Liss, Inc.