ADHESIVE LIGAND-BINDING TO INTEGRIN ALPHA(IIB)BETA(3) STIMULATES TYROSINE PHOSPHORYLATION OF NOVEL PROTEIN SUBSTRATES BEFORE PHOSPHORYLATION OF PP125(FAK)

Tyrosine phosphorylation of multiple platelet proteins is stimulated by thrombin and other agonists that cause platelet aggregation and secretion. The phosphorylation of a subset of these proteins, including a protein tyrosine kinase, pp125FAK, is dependent on the platelet aggregation that follows fibrinogen binding to integrin alpha(IIb)beta3. In this report, we examined whether fibrinogen binding, per se, triggers a process of tyrosine phosphorylation in the absence of exogenous agonists. Binding of soluble fibrinogen was induced with Fab fragments of an anti-beta3 antibody (anti-LIBS6) that directly exposes the fibrinogen binding site in alpha(IIb)beta3, Proteins of 50-68 kD and 140 kD became phosphorylated on tyrosine residues in a fibrinogen-dependent manner. This response did not require prostaglandin synthesis, an increase in cytosolic free calcium, platelet aggregation or granule secretion, nor was it associated with tyrosine phosphorylation of pp125FAK. Tyrosine phosphorylation of the 50-68-kD and 140-kD proteins was also observed when (a) fibrinogen binding was stimulated by agonists such as epinephrine, ADP, or thrombin instead of by anti-LIBS6; (b) fragment X, a dimeric plasmin-derived fragment of fibrinogen was used instead of fibrinogen; or (C) alpha(IIb)beta3 complexes were cross-linked by antibodies, even in the absence of fibrinogen. In contrast, no tyrosine phosphorylation was observed when the ligand consisted of monomeric cell recognition peptides derived from fibrinogen (RGDS or gamma400-411). Fibrinogen-dependent tyrosine phosphorylation was inhibited by cytochalasin D. These studies demonstrate that fibrinogen binding to alpha(IIb)beta3 initiates a process of tyrosine phosphorylation that precedes platelet aggregation and the phosphorylation of pp125FAK. This reaction may depend on the oligomerization of integrin receptors and on the state of actin polymerization, organizational processes that may juxtapose tyrosine kinases with their substrates.