Beta1 integrin-mediated activation of focal adhesion kinase and its association with Fyn and Zap-70 in human NK cells.

Adhesive interactions mediated by cell surface receptors have been shown to induce signal transduction pathways that regulate changes in cellular function. We have reported recently that fibronectin (FN) receptors, alpha4beta1 and alpha5beta1 integrins, on NK cells transduce transmembrane signals leading to tyrosine phosphorylation of 60-, 70-, and 120-kDa proteins. In the current study, we have identified a 120-kDa phosphoprotein as the focal adhesion kinase (p125FAK), a structurally unique nonreceptor protein tyrosine kinase that localizes to focal adhesions. Activity of p125FAK was induced by adhesion of NK cells to plastic-immobilized FN, by cross-linking of cell surface-bound FN or FN fragments, FN120 or FN40, with anti-FN mAb, or by cross-linking of alpha4beta1 or alpha5beta1 integrins with alpha-chain-specific Abs. We also observed that enhanced in vitro kinase activity was associated with immunoprecipitates of alpha4beta1 or alpha5beta1 integrins from lysates of FN-adherent NK cells as compared with BSA-treated NK cells. In addition to p125FAK activity, FN-induced kinase activity was also found to be mediated by Fyn, Lyn, and Zap-70, as assessed by in vitro phosphorylation of the immunoprecipitated kinases in the presence of [gamma-32P]ATP. Clustering of FN receptors on NK cells by agonists such as immobilized FN or alpha4- or alpha5-specific Abs also induced association of Fyn and Zap-70 with p125FAK. Our observations indicate that activation and phosphorylation of p125FAK as well as Zap-70 and certain kinases of the src family play an important role in formation of active signaling complexes in response to triggering via beta1 integrins on NK cells. These results also suggest the existence of cross-talk or points of convergence between the beta1 integrin-mediated and other receptor-signaling pathways.