REGULATION OF THE PROTEIN-TYROSINE KINASE PP72(SYK) BY PLATELET AGONISTS AND THE INTEGRIN ALPHA(IIB)BETA(3)

Agonist stimulation of platelets induces multiple waves of tyrosine phosphorylation, several of which are dependent on the integrin alpha(IIb)beta(3). At least two classes of protein tyrosine kinases are activated during various stages of platelet activation, 1) Src family tyrosine kinases are activated during an early phase of platelet activation by an integrin-independent mechanism and 2) pp125(FAK) is activated during a late stage of platelet activation, and it is dependent on platelet aggregation mediated by fibrinogen binding to alpha(IIb)beta(3). In this report, we examined the mechanism of agonist-induced phosphorylation and activation of the tyrosine kinase pp72(syk), which is known to couple with immune response receptors in B cells and mast cells. pp72(syk) was found to be regulated by both agonist and integrin receptors in a pattern distinct from that of pp60(src) and pp125(FAK). Specifically, thrombin induced the tyrosine phosphorylation and activation of pp72(syk) independent of platelet aggregation. However, full activation of pp72(syk) required integrin engagement since treatment with antibodies that block fibrinogen binding to alpha(IIb)beta(3) reduced pp72(syk) phosphorylation by 40%. Furthermore, fibrinogen binding to alpha(IIb)beta(3) stimulated directly with an anti-beta(3) antibody activated pp72(syk) 3-fold and stimulated its tyrosine phosphorylation. Thus, pp72(syk) is the only platelet tyrosine kinase identified to date that can be directly activated through integrin ligation. In addition, we found that the activation of pp72(syk) is dependent upon the state of actin polymerization and that pp72(syk) redistributes to actin-rich cytoskeletal complexes in an aggregation-dependent manner. These results suggest a role for pp72(syk) in both early, integrin-independent tyrosine phosphorylation events as well as those dependent upon subsequent integrin engagement.