Tyrosine phosphorylation and cytoskeletal reorganization in platelets are triggered by interaction of integrin receptors with their immobilized ligands.

Agonist-induced platelet activation causes fibrinogen binding to integrin alpha IIb beta 3 (glycoprotein (GP) IIb-IIIa) and platelet aggregation. This is associated with the phosphorylation of specific platelet proteins on tyrosine residues. Since fibrinogen immobilized on a solid matrix can bind platelet GP IIb-IIIa without the need for exogenous agonists, we examined whether platelet adhesion to a fibrinogen matrix induces tyrosine phosphorylation. Platelets adhered to fibrinogen in a GP IIb-IIIa-dependent manner and assumed a spread morphology. This change in cell shape was associated with tyrosine phosphorylation of multiple proteins, most prominently two unidentified proteins (101 and 105 kDa) and pp125FAK, a focal adhesion protein-tyrosine kinase. Pretreatment of platelets with prostaglandin I2 to increase cAMP, with cytochalasin D to inhibit actin polymerization, or with ADP scavengers to remove ADP did not affect initial adhesion, but inhibited both platelet spreading and tyrosine phosphorylation of pp125FAK and the 101- and 105-kDa proteins. This suggested that adhesion to fibrinogen caused cytoskeletal reorganization and the local release of ADP from platelet-dense granules, which potentiated the biochemical and morphological responses of the platelets to fibrinogen. Platelet adhesion to a collagen matrix also led to the induction of tyrosine phosphorylation of the 101- and 105-kDa proteins and pp125FAK. In this case, tyrosine phosphorylation was dependent on the interaction of collagen with integrin alpha 2 beta 1 (GP Ia-IIa), and it was independent of both GP IIb-IIIa and ADP. These results indicate that platelet adhesion to fibrinogen or collagen induces signal transduction that is initiated through integrins GP IIb-IIIa and alpha 2 beta 1, respectively. In both cases, tyrosine phosphorylation is accompanied by cytoskeletal reorganization and changes in cell shape. However, different regulatory components may be interposed between each of these integrins and the enzymes that control the level of protein tyrosine phosphorylation.