Abstract 4365: Estrogen receptor β agonists reverses letrozole therapy resistance of breast cancer through FOXO3 and FOXM1 mediated actions

Introduction: Breast cancer (BC) is the most common cancer in women worldwide and leading cause of cancer death. More than 70% of cancers are estrogen receptor alpha (ERα) positive. Despite effective therapeutic strategies for treating hormone receptor-positive (HR+) breast cancer, resistance to endocrine therapy that is either de novo or acquired still occurs. Understanding the molecular pathways that contribute to BC resistance will enable to develop new treatments for improving the efficacy of endocrine therapy. Studies have shown that estrogen receptor beta (ERβ) and its agonists inhibits the different types of cancers including BC. Herein, we investigated mechanism(s) for therapeutic efficacies of ERβ agonists in breast cancer. Methods: To investigate the mechanisms of ERβ agonists (S-Equol and LY500307), in the treatment of breast cancer, we used therapy sensitive (MCF7-Aro) and letrozole resistant (MCF7-Aro-LTLT) cells and ERβ knock down cell lines, generated by CRISPR-Cas9 technology. The wild type and ERβ knock down cells with or without treatment of ERβ agonists were analyzed, for cell proliferation, protein (Western), Gene expression array analysis, RNA-seq analysis, data-independent acquisition (DIA) mass spectrometry analyses and RNA (RT-qPCR) analysis. Xenograft and patient derived BC explants (PDEX) were used to test the antiproliferative activity of ERβ agonists. Results: Our results demonstrated that treatment with ERβ agonists, S-Equol and LY500307 inhibited the growth of both endocrine therapy sensitive and resistant breast cancer cells. Gene expression array analysis of LY500307 treated breast cancer cells showed the modulation of signaling molecules involved in cell death and cell cycle pathways. RNA-seq analysis showed that treatment with S-equol modulated the ERβ target genes involved in tumor progression and resistance to hormone therapies. Mass spectrometry based DIA analysis and Reactome pathway analysis revealed that S-equol treatment modulated the key proteins involved in DNA replication and cell cycle. Both ERβ agonists S-Equol and LY500307 inhibited the cell proliferation of parental cells whereas they could not elicits the inhibitory effect on the knock out cells. By using ERα positive PDX tumors, we have demonstrated that both the ERβ agonists inhibited the proliferation of human BC tumors. Our mechanistic studies confirmed that increased FOXO3 activation by S-equol in LTLT cells can reverse letrozole resistance. Our results showed that inhibiting FOXM1 by ERβ agonists significantly reduce the proliferation of letrozole-resistant cells. These changes were not observed in ERβ knockdown cells confirming the anti-proliferative role of ERβ in breast cancer progression. Conclusions: Collectively, our results suggest that ERβ agonists may offer a new targeted therapy for endocrine therapy resistant breast tumors. (Supported by BCRP 151884/DOD grant W81XWH-16-1-0294 and P30 CA-54174 NCI-Mays cancer center grant). Citation Format: Kumaraguruparan Ramasamy, Suryavathi Viswanadhapalli, M.A. Sureshkumar, Susan Weintraub, Ratna K. Vadlamudi, Rajeshwar R. Tekmal. Estrogen receptor β agonists reverses letrozole therapy resistance of breast cancer through FOXO3 and FOXM1 mediated actions [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4365.