Persistent activation of platelet membrane phospholipase C by proteolytic action of trypsin and thrombin

Trypsin causes rapid activation of intact platelets that mimics many actions of thrombin, including the stimulation of phospholipase C (PLC). We have examined the effects of thrombin and trypsin on PLC in a platelet membrane preparation using exogenous 3H]-phosphatidylinositol 4,5-bisphosphate (PIP2) as substrate. Trypsin induced PIP2 breakdown, which was maximal at 20 micrograms/ml, but was reduced at higher concentrations. alpha- and gamma-Thrombins also stimulated PLC-induced hydrolysis of PIP2 in membranes. This effect was inhibited by leupeptin. Exogenous 3H]phosphatidylinositol 4-monophosphate (PIP) was hydrolyzed in response to both thrombin and trypsin in the same ratio as PIP2. Activation of membrane-bound PLC persisted after removal of thrombin and trypsin. The hydrolysis of 3H]phosphatidylinositol was not activated by alpha-thrombin and trypsin. We examined the question of whether calpain was involved in the observed PLC activation by thrombin and trypsin. Although dibucaine activated a Ca2(+)-dependent protease as judged by the hydrolysis of actin-binding protein and by the activation of phosphoprotein phosphatases, it failed to stimulate the generation of phosphatidic acid in 32P-prelabeled platelets. Moreover, when PLC was assayed in the membranes, the addition of Ca2(+)-activated neutral proteinases did not increase the rate of hydrolysis of either PIP or PIP2. Our results show that proteases such as trypsin and thrombin are able to stimulate membrane-bound PLC, but this activation does not seem to be related to calpain.