Dual targeting of EphA2 and ER restores Tamoxifen sensitivity in ER/EphA2-positive breast cancer

Overexpression and altered function of EphA2 receptor tyrosine kinase are critical in the progression of breast cancer and provide a target for breast cancer therapy. We have previously demonstrated that EphA2 overexpression decreases estrogen dependence and Tamoxifen sensitivity both in vitro and in vivo. EA5, a novel monoclonal antibody that mimicks the binding of ephrin A to EphA2, reverses the effect of EphA2 overexpression and restores Tamoxifen sensitivity in EphA2-transfected MCF-7 cells in vitro. To explore the role of EphA2 overexpression on ER-dependent mechanisms, we used two different ER+/EphA2-transfected cell line models (MCF-7^neo/MCF-7 ^EphA2 and T47D^neo/T47D^EphA2). EA5 inhibits primary tumor growth and restores Tamoxifen sensitivity in the MCF-7 ^EphA2 xenografts. Using the T47D^EphA2 in vitro model, we verified that EphA2 decreases ER activation in response to E2 stimulation consistent with our earlier results in MCF-7^EphA2 model. We found no direct interaction between ER and EphA2 and no difference in expression of canonical ER-dependent proteins or ER co-regulators. However, E2 stimulation phosphorylates FAK^Tyr925 only in ER+/EphA2+ cell lines. Treatment of T47D^EphA2 cells with EA5 and Tamoxifen leads to dephosphorylation of FAK^Tyr925 in response to E2. Our data demonstrate that dual targeting of EphA2 and ER is a promising approach for delaying resistance to Tamoxifen. The data support our hypothesis that EphA2 impacts ER function via a FAK dependent pathway.