Integrin alpha IIb beta 3-mediated pp125FAK phosphorylation and platelet spreading on fibrinogen are regulated by PI...

Activation of the focal adhesion kinase pp125FAK correlates with its phosphorylation on tyrosine residues and is mediated by multiple receptor-ligand pairs. In platelets, pp125FAK phosphorylation is triggered by alpha IIb beta 3 integrin or Fc gamma RII receptor interaction with immobilized fibrinogen and IgG, respectively. In this study we used platelets as a model system to explore the role of PI 3-kinase relative to pp125FAK phosphorylation. Treatment of the platelets with two PI 3-kinase inhibitors, wortmannin and LY294002, inhibited in a dose-dependent manner alpha IIb beta 3-mediated platelet spreading on fibrinogen having no effect on platelet spreading on IgG. Both inhibitors also completely abolished alpha IIb beta 3-mediated pp125FAK phosphorylation but not pp72syk phosphorylation. Furthermore, Fc gamma RII- and thrombin-induced pp125FAK phosphorylation were not affected by wortmannin and LY294002. Finally, the PI 3-kinase inhibitors' effect on alpha IIb beta 3-mediated spreading and pp125FAK phosphorylation was reversed by phorbol ester treatment. These results establish that the role of PI 3-kinase relative to pp125FAK phosphorylation in platelets is receptor type-specific yet essential for alpha IIb beta 3-mediated cell spreading and pp125FAK phosphorylation.