Background: Resistance to tamoxifen therapy is one of the major barriers to the successful treatment of breast cancer, and ER-α expression is currently the main biomarker of response to tamoxifen treatment. Elucidating the regulation of ER-α expression may provide new therapeutic targets for overcoming tamoxifen resistance. Materials and Methods: Quantitative RT-PCR, immunoblotting, and immunofluorescence were used to determine 17β-estradiol-induced expression of αEF1 and ER-α at the mRNA and protein levels in the presence or absence of ICI 182,780, PI-103, PKI, or BAY 11-7082. Luciferase assay was used to determine αEF1-driven transcriptional activities of the wild-type and mutant human ER-α promoters. Quantitative CHIP assay was used to detect the direct association of αEF1 with the wild-type human ER-α proximal promoter. Immunohistochemistry was used to determine the expression of αEF1 and ER-α protein in breast cancer specimens. Results: We found that 17β-estradiol up-regulated αEF1 expression in a dose- and time-dependent manner in MCF-7 breast cancer cells. Down-regulation of the ER-α significantly abolished this effect, showing that 17β-estradiol-induced expression of αEF1 is ER-dependent. Blockade of the PI3K or NF-κB pathway resulted in an interference on the up-regulation of αEF1 by 17β-estradiol. Interestingly, ectopic expression of αEF1 exhibited a repression on ER-α expression in MCF-7 cells. Luciferase and CHIP assays further confirmed that αEF1 transcriptionally inhibited the promoter activity of human ER-α by binding to its E2-box (CACCTG). Importantly, up-regulation of αEF1 expression rendered MCF-7 cells less sensitive to tamoxifen treatment, whereas adding ER-α comprised the effect of αEF1 on the cell sensitivity to tamoxifen. Through the study of breast cancer specimens, we found a strong inverse correlation between αEF1 and ER-α protein expression, further supporting the contribution of estrogen/αEF1/ER cascade to tamoxifen resistance in breast cancer. Conclusions: Our data indicates a key role of αEF1 in developing tamoxifen resistance in breast cancer, by altering ER-α expression. Given that re-expressing ER-α in ER-α-negative breast cancer can restore tamoxifen sensitivity, restoring ER-α expression by inhibiting αEF1 provides a potential new strategy for overcoming endocrine resistance.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 284. doi:1538-7445.AM2012-284