Extra- and intracellular metabolism of platelet-activating factor by cultured mesangial cells.

Abstract

Metabolism of platelet-activating factor (PAF) was examined in cultured mesangial cells from human and rat glomeruli. Human mesangial cells, similar to those from rat, generated PAF after A23187. Both human and rat mesangial cells rapidly hydrolyzed [3H]PAF to lyso-[3H]PAF, and reacylated it into 1-alkyl-2-acyl glycerophosphocholine. Extra- and intracellular metabolism of PAF was then analyzed separately. The majority of [3H]PAF metabolism occurred extracellularly and generated Lyso-[3H]PAF. Intracellularly generated lyso-PAF was rapidly converted to 1-alkyl-2-acyl glycerophosphocholine. Cells prelabeled with [3H]PAF released some [3H]PAF within minutes and then rapidly converted it to lyso-PAF extracellularly. Under control conditions no acetylhydrolase activity was released from cells into the buffer. Acetylhydrolase activity could, however, be released from cell surface into buffer by limited trypsinization, supporting its location on the outer cell membrane. The acetylhydrolase activity was different from phospholipase A2, since phosphatidylcholine was not a substrate for the enzyme. In summary our results show that both rat and human mesangial cells can generate and metabolize PAF. Acetylhydrolase for PAF is present intracellularly, but also and predominantly on the outer cell surface of cells. This ectoenzymatic acetylhydrolase activity may be important in the rapid inactivation of PAF presented to cells, thus protecting cells from deleterious effects of PAF.