LIGATION OF THE ALPHA(2)-MACROGLOBULIN SIGNALING RECEPTOR ON MACROPHAGES INDUCES SYNTHESIS OF PLATELET-ACTIVATING-FACTOR

The binding of receptor-recognized forms of alpha(2)-macroglobulin (al pha(2)M) to macrophage alpha(2)M signaling receptors increases inosito l-1,4,5-triphosphate synthesis and induces Ca2+ mobilization. In this report, we demonstrate that ligation of the macrophage alpha(2)M signa ling receptor is also associated with synthesis of platelet activating factor (PAF) by both the de novo and remodeling pathways. Both alpha 2M-methylamine and a cloned and expressed 20-kDa receptor binding frag ment (RBF) from rat alpha(1)M+, stimulated macrophage synthesis of PAF from [H-3]acetate, [H-3]methylcholine, and 1-O-[H-3]alkyl lyso-PAF by two- to threefold. PAF levels reached a peak in 20 min after the cell s were exposed to alpha(2)M-methylamine or RBF; they remained elevated for about 1 h after ligand addition to the cells. When [H-3]methylcho line was the substrate, pertussis toxin did not block PAF synthesis, b ut the protein kinase C inhibitor staurosporin reduced synthesis by 65 -70%. Cycloheximide completely abolished the increase in synthesis of PAF by macrophages exposed to alpha(2)M-methylamine. By contrast, when [H-3]acetate was employed as a precursor, staurosporin or cycloheximi de did not abolish the increase in PAF synthesis. These studies sugges t that protein kinase C is necessary for the induction of the de novo pathway by alpha(2)M-methylamine. Both alpha(2)M-methylamine and RBF s timulated 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 alpha(2)M sig naling receptor also regulates the degradation of PAF. The ability of receptor-recognized forms of alpha(2)M to regulate levels of PAF sugge sts that alpha(2)M-proteinase complexes not only regulate macrophage f unction by activating intracellular signaling but also may indirectly regulate the function of other cells that cannot bind alpha(2)M-protei nase complexes. (C) 1996 Wiley-Liss, Inc.