PLATELET-ACTIVATING-FACTOR MEDIATES PHOSPHATIDYLCHOLINE HYDROLYSIS BY PHOSPHOLIPASE-D IN HUMAN ENDOMETRIUM

Human preimplantation embryos and endometrium secrete platelet-activating factor (PAF). The mechanism of phosphatidylcholine (PC) degradation stimulated by PAF was investigated in endometrial explants prelabeled with [methyl-H-3]choline or preincubated with [H-3]butan-1-ol. Analysis of the water-soluble metabolites of PAF-induced PC hydrolysis in secretory endometrium demonstrated that the stimulated generation of [H-3]choline (H-3]Cho) precedes that of [H-3]choline phosphate ([H-3]ChoP) and [H-3]glycerophosphocholine (H-3])GPC). Within 30 sec there was a rapid rise in PAF-induced [H-3]Cho generation and by 2 min this had increased to 59.9% +/- 10.6% (p < 0.02), with no effect upon [H-3]ChoP and [H-3]GPC during this period. Both [H-3]GPC and [H-3]ChoP, however, were increased at a later time point. The slower [H-3]ChoP generation may suggest that PC-specific phospholipase C activation as well as delayed [H-3]GPC rise may be due to PC-specific phospholipase A2 and lysophospholipase activation. Phospholipase D activity was confirmed by the incorporation of high-specific-activity [H-3]butan-1-ol into [H-3]phosphatidylbutanol ([H-3]PBut). The rapid generation of [H-3]PBut, which paralleled the rise in intracellular [H-3]Cho, strongly suggests that PC breakdown is catalyzed by the phospholipase D pathway. It is proposed that PAF induces PC hydrolysis as a consequence of an early phospholipase D-catalyzed breakdown of PC in human secretory endometrium. This may be an alternative source for prostaglandin synthesis and an important pathway essential for long-term activation of local cellular events at the time of implantation.