The selective ER downregulator, fulvestrant, is currently approved as a second line endocrine therapy after onset of resistance to prior antiestrogen therapy in postmenopausal breast cancer patients. Resistance to antihormonal therapies is common and, therefore, we anticipate that fulvestrant-resistance will occur as well. The current study was undertaken to investigate the underlying molecular changes after fulvestrant-resistance and find new therapeutic targets and agents for fulvestrant-resistant breast cancer cells. We developed a unique fulvestrant-resistant cell line (MCF-7/F), derived from MCF-7 estrogen receptor α (ERα)-positive human breast cancer cells, by culturing them in 1 µM fulvestrant containing medium for ≈18 months. MCF-7/F cells became irreversibly ERα negative as withdrawal of fulvestrant did not alter the ERα-negative phenotype, determined by real-time PCR, Western blot analysis, and ERE-luciferase transfection assays. MCF-7/F cells grew in a hormone-independent manner. Interestingly, MCF-7/F cells overexpressed both epidermal growth factor receptor (EGFR) and breast cancer resistant protein (BCRP). Gefitinib, a specific EGFR tyrosine kinase inhibitor, preferentially inhibited the growth of MCF-7/F cells relative to MCF-7 cells by inhibiting both MAPK44/42 and Akt phosphorylation. MCF-7/F cells became less sensitive to chemotherapeutic agents such as mitoxantrone. Moreover, fumitremorgin C, a specific BCRP inhibitor, significantly increased the efficacy of mitoxantrone in MCF-7/F cells. Gefitinib increased the inhibitory effect of mitoxantrone on cell growth. Similarly, fumitremorgin C increased the inhibitory effect of gefitinib on cell growth, suggesting that there is a bidirectional crosstalk between EGFR and BCRP. More importantly, these results provide a molecular basis for using gefitinib, BCRP inhibitors, and chemotherapeutic agents as combination therapy approaches in fulvestrant-resistant breast cancer.

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