Dual inhibition of focal adhesion kinase and epidermal growth factor receptor pathways cooperatively induces death receptor-mediated apoptosis in human breast cancer cells

The focal adhesion kinase (FAK) and epidermal growth factor receptor (EGFR) are protein-tyrosine kinases that are overexpressed and activated in human breast cancer. To determine the role of EGFR and FAK survival signaling in breast cancer, EGFR was stably overexpressed in BT474 breast cancer cells, and each signaling pathway was specifically targeted for inhibition. FAK and EGFR constitutively co-immunoprecipitated in EGFR-overexpressing BT474 cells. In low EGFR-expressing BT474-pcDNA3 vector control cells, inhibition of FAK by the FAK C-terminal domain caused detachment and apoptosis via pathways involving activation of caspase-3 and -8, cleavage of poly(ADP-ribose) polymerase, and caspase-3-dependent degradation of AKT. This apoptosis could be rescued by the dominant-negative Fas-associated death domain, indicating involvement of the death receptor pathway. EGFR over-expression did not inhibit detachment induced by the FAK C-terminal domain, but did suppress apoptosis, activating AKT and ERK1/2 survival pathways and inhibiting cleavage of FAK, caspase-3 and -8, and poly(ADP-ribose) polymerase. Furthermore, this protective effect of EGFR signaling was reversed by EGFR kinase inhibition with AG1478. In addition, inhibition of FAK and EGFR in another breast cancer cell line (BT20) endogenously overexpressing these kinases also induced apoptosis via the same mechanism as in the EGFR-overexpressing BT474 cells. The results of this study indicate that dual inhibition of FAK and EGFR signaling pathways can cooperatively enhance apoptosis in breast cancers.