Phospholipase D activity in human platelets is inhibited by protein kinase A, involving inhibition of phospholipase D1 translocation

Phospholipase D hydrolyzes phosphatidylcholine to phosphatidic acid and choline. It is established that thrombin induces PLD activation in human platelets. We found recently that two isoforms of PLD, PLD1 and PLD2 are present in platelets and these become translocated to the plasma membrane area upon thrombin activation. Since cAMP is a negative regulator in platelets, we measured the effect of the platelet antagonists PGE1 and forskolin (both of which raise intracellular cAMP levels) on PLD activity in resting and thrombin-activated platelets. We found that both PGE1 and forskolin inhibited thrombin-induced PLD activation by 40-50%, but interestingly PGE1 caused a modest elevation of PLD activity in resting platelets. Further studies using inhibitors as well as specific activators of protein kinases A and G suggest a role for PKA in the inhibition of thrombin-activated PLD. We found that PGE1, forskolin and the PKA activator inhibited PLD1 translocation in thrombin-stimulated platelets, and that the PKG-activator had no effect. The present results suggest a role for PKA in the regulation of thrombin-induced PLD1 activity and translocation in platelets.