Activity of Estrogen Receptor β Agonists in Therapy-Resistant Estrogen Receptor-Positive Breast Cancer

Background: Among women, breast cancer is the leading cause of cancer-related death worldwide. Estrogen receptor a -positive (ERa+) breast cancer accounts for 70% of all breast cancer subtypes. Although ER alpha+ breast cancer initially responds to estrogen deprivation or blockade, the emergence of resistance compels the use of more aggressive therapies. While ERa is a driver in ERa+ breast cancer, ER beta plays an inhibitory role in several different cancer types. To date, the lack of highly selective ER beta agonists without ERa activity has limited the exploration of ER beta activation as a strategy for ERa+ breast cancer. Methods: We measured the expression levels of ESR1 and ESR2 genes in immortalized mammary epithelial cells and diferent breast cancer cell lines. The viability of ER alpha+ breast cancer cell lines upon treatments with specific ER beta agonists, including OSU-ER beta -12 and LY500307, was assessed. The specificity of the ER beta agonists, OSU-ER beta -12 and LY500307, was confirmed by reporter assays. The effects of ER beta agonists on cell proliferation, cell cycle, apoptosis, colony formation, cell migration, and expression of tumor suppressor proteins were analyzed. The expression of ESR2 and genes containing ERE-AP1 composite response elements was examined in ER alpha+ human breast cancer samples to determine the correlation between ESR2 expression and overall survival and that of putative ESR2-regulated genes. Results: In this study, we demonstrate the efficacy of highly selective ER beta agonists in ER alpha+ breast cancer cell lines and drug-resistant derivatives. ER beta agonists blocked cell proliferation, migration, and colony formation and induced apoptosis and S and/or G2/M cell-cycle arrest of ER alpha+ breast cancer cell lines. Also, increases in the expression of the key tumor suppressors FOXO1 and FOXO3a were noted. Importantly, the strong synergybetween ER beta agonists and ERaantagonists suggested that the efficacy of ER beta agonistsis maximized by combination with ERa blockade. Lastly,ESR2(ERb gene) expressionwas negatively correlated withESR1(ERa gene) andCCND1RNA expression in humanmetastatic ER alpha+/HER2- breast cancer samples. Conclusion:Our results demonstrate that highly selective ERbagonists attenuate theviability of ER alpha+ breast cancer cell linesin vitroand suggest that this therapeutic strategymerits further evaluation for ER alpha+ breast cancer.