Dissociation of the alpha(IIb)beta(3)-Integrin by EGTA stimulates the tyrosine kinase pp72(syk) without inducing platelet activation

Incubation of human platelets with EG;TA under conditions that dissociate the alpha(IIb)beta(3)-integrin stimulated tyrosine phosphorylation of pp72(syk) (6.8-fold) and of pro teins of 62 (2.2-fold), 68 (2.5-fold) and 130 kDa (1.4-fold), Stimulation of tyrosine phosphorylation of pp72(syk) was associated with an increase of pp72(syk) kinase activity, In contrast to pp72(syk), tyrosine phosphorylation of the fo cal adhesion kinase pp125(FAK) was not stimulated by EGTA. Preincubation of platelets with the monoclonal antibody P2, which binds to the alpha(IIb)beta(3) complex and thus stabilizes it, strongly reduced the increase of tyrosine phosphorylation of pp72(syk), p62, and p68 induced by EGTA, The Y2/51 monoclonal antibody, which recog nizes only the beta(3) glycoprotein, did not inhibit the stimulation of protein tyrosine phosphorylation evoked by EGTA, Stimulation of tyrosine phosphorylation of pp72(syk), p62, p68, and p130 induced by EGTA was not observed in thrombasthenic platelets, which lack the alpha(IIb)beta(3)-integrin. The results indicate that the dissociation of the alpha(IIb)beta(3) complex in intact platelets activates pp72(syk). The mechanism of activation was found to be insensitive to inhibition by cAMP and cGMP and only partially dependent on cytosolic Ca2+, suggesting a close functional coupling of alpha(IIb)beta(3)-integrin and pp72(syk). Since platelets retain their discoid shape after EG;TA treatment, we further conclude that pp72(syk) stimulation alone is not sufficient for platelet activation.