Protein-tyrosine kinase p72syk is activated by thromboxane A2 mimetic U44069 in platelets

We show that p72syk is rapidly activated following the stimulation of thromboxane A2 mimetics, U44069 and STA2 in porcine platelets. The activity of p72syk reached a maximum at 10 s and decreased to a basal level within 60 s after 1 microM U44069 stimulation. This activation was enhanced in a dose-dependent manner and completely canceled by the pretreatment of platelet suspension with ONO3708, a specific antagonist of thromboxane A2. Pretreatment of platelets with aspirin as well as apyrase did not affect the activation of p72syk. When both extra- and intra-cellular Ca2+ were depleted, the activation of p72syk was still persistent; in contrast, the deactivation process was completely abrogated even at 120 s after U44069 stimulation. These results suggest that p72syk is a responsible enzyme to the protein-tyrosine phosphorylation events, and that p72syk functions mainly before Ca2+ recruitment in thromboxane A2-stimulated platelets.