THE ACTIVATION STATE OF THE INTEGRIN ALPHA(IIB)BETA(3) AFFECTS OUTSIDE-IN SIGNALS LEADING TO CELL SPREADING AND FOCAL ADHESION KINASE PHOSPHORYLATION

Integrins bind extracellular matrix and transduce signals mediating cell adhesion spreading, and migration. It is unclear how these distinct responses follow hom a common event: integrin clustering. We examined the relationship between integrin-mediated signals and the integrin's activation state using a cell line expressing alpha(IIb)beta(3) (Clone B) and a panel of monoclonal antibodies against this integrin. Nonactivating antibodies used to cluster alpha(IIb)beta(3) stimulated focal adhesion kinase (FAK) phosphorylation, regardless of affinity, subunit specificity, or ligand-blocking phenotype. Coated on plastic, these antibodies supported cell adhesion, spreading, and FAK phosphorylation. In contrast, clustering of alpha(IIb)beta(3) induced with activating antibodies, or binding of soluble fibrinogen to antibody activated alpha(IIb)beta(3), did not induce FAR phosphorylation. Thus, clustering of alpha(IIb)beta(3) on Clone B does not necessarily result in FAM phosphorylation. Coated on plastic, activating antibodies supported cell adhesion, but not spreading or FAK phosphorylation. Therefore, it appears the resting, not the active form of alpha(IIb)beta(3), induces cell spreading and FAR phosphorylation in Clone B. These data indicate that ''inside-out'' signals may alter not only the binding specificity of an integrin, but the ''outside-in'' biochemical signals that integrin initiates as well. This activation state linked signaling represents a novel mechanism, which may explain how diverse cellular responses are induced by integrin-matrix interactions.