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 ph osphorylation, several of which are dependent on the integrin alpha(II b)beta(3). At least two classes of protein tyrosine kinases are activa ted during various stages of platelet activation, 1) Src family tyrosi ne kinases are activated during an early phase of platelet activation by an integrin-independent mechanism and 2) pp125(FAK) is activated du ring a late stage of platelet activation, and it is dependent on plate let aggregation mediated by fibrinogen binding to alpha(IIb)beta(3). I n this report, we examined the mechanism of agonist-induced phosphoryl ation and activation of the tyrosine kinase pp72(syk), which is known to couple with immune response receptors in B cells and mast cells. pp 72(syk) was found to be regulated by both agonist and integrin recepto rs in a pattern distinct from that of pp60(src) and pp125(FAK). Specif ically, thrombin induced the tyrosine phosphorylation and activation o f pp72(syk) independent of platelet aggregation. However, full activat ion of pp72(syk) required integrin engagement since treatment with ant ibodies that block fibrinogen binding to alpha(IIb)beta(3) reduced pp7 2(syk) phosphorylation by 40%. Furthermore, fibrinogen binding to alph a(IIb)beta(3) stimulated directly with an anti-beta(3) antibody activa ted pp72(syk) 3-fold and stimulated its tyrosine phosphorylation. Thus , pp72(syk) is the only platelet tyrosine kinase identified to date th at can be directly activated through integrin ligation. In addition, w e found that the activation of pp72(syk) is dependent upon the state o f actin polymerization and that pp72(syk) redistributes to actin-rich cytoskeletal complexes in an aggregation-dependent manner. These resul ts suggest a role for pp72(syk) in both early, integrin-independent ty rosine phosphorylation events as well as those dependent upon subseque nt integrin engagement.