Abstract 1008: S-Equol inhibits breast cancer growth by regulating phosphorylation status of estrogen receptor β

Background: Breast cancer is the most common malignancy in females and second most common cause of cancer related mortality in women. Since 70% of all breast cancers are estrogen receptor-positive (ER+ve), endocrine therapy such as anti-estrogens or aromatase inhibitors, targeting the estrogen receptor (ER) pathway is the most common treatment used for ER+ve breast cancers. However, patients will develop de novo or acquired resistance to therapy, leads to tumor progression, and metastasis. It is well documented that ERβ functions as tumor suppressor in different cancers including breast cancer. We recently showed that phosphorylation status of ERβ is important for its antitumor activity. However, little is known about the role of ERβ phosphorylation status in hormone therapy and resistance; therefore, we investigated whether phosphorylation status of ERβ has role in overcoming hormone therapy resistance in ER+ve breast cancers. Experimental design: To elucidate the importance of phosphorylation status of ERβ, we used CRISPR-Cas9 system to knockout ERβ in MCF7-Aro (therapy-sensitive) and Letrozole resistant (MCF7aro-LTLT) cells. Several mutant clones were identified for both MCF7-Aro cells and LTLT cells and the depletion of ERβ protein in both cell clones was confirmed by immunoblotting. The parental and knockout cells with or without treatment of S-equol were analyzed for cell proliferation, protein (Western) and RNA (RT-qPCR) analysis. Results: First we analyzed the cell proliferation in parental (MCF7 Aro and LTLT) and ERβ knock out cells. The proliferation rate is increased in the ERβ knockout cells compared to the parental cells. Treatment with ERβ agonist S-Equol to the parental cells inhibited the cell proliferation whereas in the knock out cells, the effect of S-equol is compromised. RNA-seq analysis of S-equol treated parental cells showed the downregulation of ERβ target genes involved in tumor progression and resistance to hormone therapies. In contrast, compared to parental cells, ERβ knock out cells showed diverse effects to S-equol treatment. RT-qPCR analysis revealed that S-Equol could not modulate the ERβ-target genes in ERβ knock out cells compared to parental cells. Conclusions: Our findings provide evidence that phosphorylation status of ERβ is important for elucidating its antitumor activity in therapy-resistant cells. The differential effects of S-equol on parental and ERβ knockout cells suggest that the antiproliferative action of S-equol is partly mediated by ERβ. We believe that our ongoing studies may further validate the role of phosphorylation status of ERβ by using both ERβ agonists and phosphorylation-regulating compounds in both therapy sensitive and resistant cells. Citation Format: Kumaraguruparan Ramasamy, Cathy Samayoa, Shaorong Chen, Rong Li, Ratna K. Vadlamudi, Rajeshwar R. Tekmal. S-Equol inhibits breast cancer growth by regulating phosphorylation status of estrogen receptor β [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1008.