The beta 1 subfamily of integrins is thought to play an important role in both the adhesion/migration and proliferation/differentiation of T cells. beta 1 integrins can provide T cell costimulation through interaction of very late antigen (VLA) 4 (VLA-4) (alpha 4 beta 1) and VLA-5 (alpha 5 beta 1) with the extracellular matrix protein fibronectin (FN), or by VLA-4 binding to its cell surface ligand, vascular cell adhesion molecule (VCAM) 1. The mechanism by which beta 1 integrin members transduce T cell-costimulatory signals is poorly understood. Studies in non-T cells have demonstrated regulation of the tyrosine focal adhesion kinase pp125FAK by beta 1 integrin engagement and, most recently, indicate a role for pp125FAK in linking integrin-mediated signal transduction to the Ras pathway (Schaller, M. D., and J. T. Parsons, 1994, Curr. Opin. Cell. Biol. 6: 705-710; Schlaepfer, D. D., S. K. Hanks, T. Hunter, and P. Van der Geer. 1994. Nature (Lond.), 372:786-790). Although pp125FAK kinase occurs in T cells, there are no reports on its regulation in this cell type. The studies described in this article characterize novel regulation of pp125FAK by the T cell receptor (TCR)-CD3 antigen complex and beta 1 integrins, and provide the first account, in any cell type, of integrin alpha 4 beta 1-mediated pp125FAK tyrosine phosphorylation. We demonstrate a rapid and sustained synergistic increase in tyrosine phosphorylation of human pp125FAK in Jurkat T cells after simultaneous (a) triggering of the TCR-CD3 complex, and (b) alpha 4 beta 1 and alpha 5 beta 1 integrin-mediated binding of these cells to immobilized FN or alpha 4 beta 1 integrin-mediated binding to immobilized VCAM-1. Studies with normal peripheral blood-derived CD4+ human T blasts confirm the synergistic action of a TCR-CD3 complex-mediated costimulus with a FN- or VCAM-1-dependent signal in the induction of T cell pp125FAK tyrosine phosphorylation. In vitro kinase assays performed on pp125FAK immunoprecipitates isolated from Jurkat cells and normal CD4+ T cells identified a coprecipitating 57-kD tyrosine-phosphorylated protein (pp57), distinct from pp59fyn or pp56lck. These results indicate, for the first time, the involvement of a specific kinase, pp125FAK, in alpha 4 beta 1- and alpha 5 beta 1-mediated T cell-costimulatory signaling pathways. In addition, the data demonstrate novel regulation of pp125FAK tyrosine phosphorylation by the TCR-CD3 complex.

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