TYROSINE PHOSPHORYLATION AND CYTOSKELETAL REORGANIZATION IN PLATELETSARE TRIGGERED BY INTERACTION OF INTEGRIN RECEPTORS WITH THEIR IMMOBILIZED LIGANDS

Agonist-induced platelet activation causes fibrinogen binding to integ rin alpha(IIb)beta3 (glycoprotein (GP) IIb-IIIa) and platelet aggregat ion. 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 a gonists, 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 ch ange in cell shape was associated with tyrosine phosphorylation of mul tiple proteins, most prominently two unidentified proteins (101 and 10 5 kDa) and pp125FAK, a focal adhesion protein-tyrosine kinase. Pretrea tment of platelets with prostaglandin I2 to increase cAMP, with cytoch alasin D to inhibit actin polymerization, or with ADP scavengers to re move ADP did not affect initial adhesion, but inhibited both platelet spreading and tyrosine phosphorylation of pp125FAK and the 101- and 10 5-kDa proteins. This suggested that adhesion to fibrinogen caused cyto skeletal reorganization and the local release of ADP from platelet-den se granules, which potentiated the biochemical and morphological respo nses of the platelets to fibrinogen. Platelet adhesion to a collagen m atrix also led to the induction of tyrosine phosphorylation of the 101 - and 105-kDa proteins and pp125FAK. In this case, tyrosine phosphoryl ation was dependent on the interaction of collagen with integrin alpha 2beta1 (GP Ia-IIa), and it was independent of both GP IIb-IIIa and ADP . These results indicate that platelet adhesion to fibrinogen or colla gen induces signal transduction that is initiated through integrins GP IIb-IIIa and alpha2beta1, respectively. In both cases, tyrosine phosp horylation is accompanied by cytoskeletal reorganization and changes i n cell shape. However, different regulatory components may be interpos ed between each of these integrins and the enzymes that control the le vel of protein tyrosine phosphorylation.