Interactions of thiophosphatidic acid with enzymes which metabolize phosphatidic acid. Inhibition of phosphatidic acid phosphatase and utilization by CDP-diacylglycerol synthase

Phospholipase activation and secretion: evidence that PLA2, PLC, and PLD are not essential to exocytosis

Numerous studies have identified phospholipase metabolites as membrane fusogens, and phospholipase D (PLD) (J.R. Coorssen and R.J. Haslam. FEBS Lett. 316: 170-174, 1993), C (PLC), and A2 (PLA2) activities correlate with secretion. Do these enzymes have essential or modulatory roles? This study confirms that secretion does not require Ca2+ or PLC (Coorssen et al. Cell Regul. 1: 1027-1041, 1990). Arachidonic acid (AA), phosphatidic acid (PA) and analogues, exogenous metabolites of PLA2 and PLD, were tested in electropermeabilized human platelets. AA potentiated guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)-induced secretion, and eicosanoids were not essential. Endogenous [3H]AA formation correlated with GTP gamma S-induced secretion, and phorbol 12-myristate 13-acetate (PMA) promoted these effects. Inhibitors were used to probe phospholipase influences on secretion. Only PLD inhibitors blocked secretion. However, PMA blocked inhibition of protein kinase C (PKC) and secretion by quercetin, suggesting that PA formed by PLD supports PKC activation and GTP gamma S-induced secretion. Thus PA analogues had no effect alone but enhanced GTP gamma S-induced PKC activity and secretion. Slower PLD activation compared with secretion also indicates a nonessential role. This is the first report of a Ca(2+)-independent PLA2 activity in human platelets, use of quercetin as a PLD inhibitor, and dissociation of PLA2, PLC, and PLD activities from secretion. No major phospholipase activities are essential to the final steps in exocytosis, but modulatory roles are indicated.

Evidence that activation of phospholipase D can mediate secretion from permeabilized platelets

Studies on electropermeabilized human platelets indicated that any two of three distinct factors must be present for marked secretion of dense or alpha-granule constituents to occur. These factors are Ca2+, activation of protein kinase C (PKC) and activation of an unidentified GTP-binding protein ('GE'). Thus, in the absence of Ca2+, phorbol ester and GTP[S] acted synergistically to promote secretion, whereas in the presence of Ca2+, either activation of PKC or addition of GTP[S] was sufficient. In all cases, secretion correlated with the activation of phospholipase D (PLD), as detected by the formation of [3H]phosphatidic acid (PA) in the absence of ethanol or of [3H]phosphatidylethanol (PEt) in the presence of ethanol. Secretion did not correlate with phospholipase C (PLC) activity or with the accumulation of 1,2-diacylglycerol (DAG), both of which required Ca2+ and were inhibited by phorbol ester. Ethanol partially inhibited secretion in the absence of Ca2+. BAPTA, a known inhibitor of Ca(2+)-independent secretion in permeabilized cells, caused parallel inhibitions of secretion and PLD activity. GTP[S] enhanced PKC activity, as indicated by pleckstrin phosphorylation, apparently by stimulating the formation of PA in the absence of Ca2+, as well as of DAG in the presence of Ca2+. PA and stable analogues, including PEt, stimulated the Ca(2+)-independent phosphorylation of pleckstrin and other proteins in platelet supernatant fraction. The results suggest that PA formed by activation of PLD may mediate secretion from permeabilized platelets by PKC-dependent and independent mechanisms. However, in intact platelets stimulated by thrombin, PLD accounted for only 10-20% of the total PA formed and can only play a major role in secretion if this PA fraction is distinct from that formed by the combined actions of PLC and DAG kinase.