Regulation of the protein tyrosine kinase pp72syk 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) pp125FAK 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 pp72syk, which is known to couple with immune response receptors in B cells and mast cells. pp72syk was found to be regulated by both agonist and integrin receptors in a pattern distinct from that of pp60src and pp125FAK. Specifically, thrombin induced the tyrosine phosphorylation and activation of pp72syk independent of platelet aggregation. However, full activation of pp72syk required integrin engagement since treatment with antibodies that block fibrinogen binding to alpha IIb beta 3 reduced pp72syk phosphorylation by 40%. Furthermore, fibrinogen binding to alpha IIb beta 3 stimulated directly with an anti-beta 3 antibody activated pp72syk 3-fold and stimulated its tyrosine phosphorylation. Thus, pp72syk 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 pp72syk is dependent upon the state of actin polymerization and that pp72syk redistributes to actin-rich cytoskeletal complexes in an aggregation-dependent manner. These results suggest a role for pp72syk in both early, integrin-independent tyrosine phosphorylation events as well as those dependent upon subsequent integrin engagement.