Factors that influence the proportions of platelet-activating factor and 1-acyl-2-acetyl-sn-glycero-3-phosphocholine synthesized by the mast cell.

Recent studies have demonstrated that inflammatory cells can be divided into two groups depending on the type of 2-acetylated phospholipids [1-radyl-2-acetyl-sn-glycero-3-phosphocholine (GPC)] they produce: those that produce predominantly platelet-activating factor (PAF), and those that produce predominantly its 1-acyl analogue (1-acyl-2-acetyl-GPC; AAGPC) [Triggiani, Schleimer, Warner & Chilton (1991) J. Immunol. 147, 660-666]. The present study has examined the factors that regulate the production of these two molecules in mouse bone marrow-derived mast cells (BMMC). Initial experiments indicated that PAF and AAGPC were catabolized by BMMC in a differential manner via two pathways: the first, exclusive for AAGPC, involved a 1-acyl hydrolase that removed the long chain at the sn-1 position of the molecule, and the second, common to AAGPC and PAF, involved acetylhydrolase that removed the acetate at the sn-2 position of the two molecules. Experiments were next designed to identify conditions where the differential catabolism of AAGPC and PAF could be eliminated in order to uncover other factors that regulate the proportions of AAGPC and PAF produced. Phenylmethanesulphonyl fluoride (PMSF) completely blocked the 1-acylhydrolase activity while having little or no effect on the acetyl hydrolase activity, thereby eliminating the influence of the catabolic pathway unique to AAGPC. Moreover, PMSF did not alter the release of arachidonic acid from phospholipid subclasses. PMSF-treated BMMC produced larger quantities of AAGPC than of PAF. The AAGPC/PAF ratio detected in PMSF-treated BMMC was very similar to the ratio of arachidonate contained in and released from 1-acyl-/1-alkyl-linked phosphatidylcholine (PC). BMMC supplemented with arachidonic acid in culture for 3 days increased their total arachidonic acid content in PC as well as the ratio of 1-acyl-2-arachidonoyl-GPC to 1-alkyl-2-arachidonoyl-GPC. These changes resulted in parallel and significant increases in both the total amount of 1-radyl-2-acetyl-GPC and the AAGPC/PAF ration in BMMC. These data indicate that the AAGPC/PAF ratio produced by inflammatory cells is regulated by at least two factors: (1) differential catabolism of these two molecules, and (2) the distribution of arachidonate in 1-acyl- and 1-alkyl-2-arachidonyl-GPC. These observations support the concept of a common pathway for AAGPC and PAF biosynthesis in which the two precursor molecules are 1-acyl-2-arachidonoyl-GPC and 1-alkyl-2-arachidonoyl-GPC, respectively.