Inhibition of pp125(FAK) in cultured fibroblasts results in apoptosis

The tyrosine kinase called pp125(FAK) is believed to play an important role in integrin-mediated signal transduction. pp125(FAK) is associated both functionally and spatially with integrins, which are the cell surface receptors for extracellular matrix components. Although the precise function of pp125(FAK) is not known, two possibilities have been proposed: pp125(FAK) may regulate the assembly of focal adhesions in spreading or migrating cells, or pp125(FAK) may participate in a signal transduction cascade to inform the nucleus that the cell is anchored. To test these models in living cells, a peptide representing the focal adhesion kinase (FAK)-binding site of the beta(1) tail was coupled to carrier protein and injected into cultured cells to competitively inhibit the binding of pp125(FAK) to endogenous integrin, thus inhibiting activation of pp125(FAK) On a cell-by-cell basis. In addition, an antibody directed against an epitope adjacent to the focal adhesion targeting sequence on pp125(FAK) was microinjected, as an alternative means of inhibiting pp125(FAK) activation. It was observed that when rounded cells were injected with either the integrin peptide or the anti-FAK antibody, the cells rapidly began to apoptose, within 4 h after injection. These results indicate that pp125(FAK) may play a critical role in suppressing apoptosis in fibroblasts.