Estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) belong to a superfamily of nuclear receptors called steroid hormone receptors, which, upon binding ligand, dimerize and translocate to the nucleus where they activate or repress the transcription of a large number of genes, thus modulating critical physiologic processes. ERβ has multiple isoforms that show differing association with prognosis. Expression levels of the full length ERβ1 isoform are often lower in aggressive cancers as compared to normal tissue. High ERβ1 expression is associated with improved overall survival in women with breast cancer. The promise of ERβ activation, as a potential targeted therapy, is based on concurrent activation of multiple tumor suppressor pathways with few side effects compared to chemotherapy. Thus, ERβ is a nuclear receptor with broad-spectrum tumor suppressor activity, which could serve as a potential treatment target in a variety of human cancers including breast cancer. Further development of highly selective agonists that lack ERα agonist activity, will be necessary to fully harness the potential of ERβ.

BackgroundAmong women, breast cancer is the leading cause of cancer-related death worldwide. Estrogen receptor α-positive (ERα+) breast cancer accounts for 70% of all breast cancer subtypes. Although ERα+ breast cancer initially responds to estrogen deprivation or blockade, the emergence of resistance compels the use of more aggressive therapies. While ERα is a driver in ERα+ breast cancer, ERβ plays an inhibitory role in several different cancer types. To date, the lack of highly selective ERβ agonists without ERα activity has limited the exploration of ERβ activation as a strategy for ERα+ breast cancer.MethodsWe measured the expression levels of ESR1 and ESR2 genes in immortalized mammary epithelial cells and different breast cancer cell lines. The viability of ERα+ breast cancer cell lines upon treatments with specific ERβ agonists, including OSU-ERb-12 and LY500307, was assessed. The specificity of the ERβ agonists, OSU-ERb-12 and LY500307, was confirmed by reporter assays. The effects of ERβ 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 ERα+ human breast cancer samples to determine the correlation between ESR2 expression and overall survival and that of putative ESR2-regulated genes.ResultsIn this study, we demonstrate the efficacy of highly selective ERβ agonists in ERα+ breast cancer cell lines and drug-resistant derivatives. ERβ agonists blocked cell proliferation, migration, and colony formation and induced apoptosis and S and/or G2/M cell-cycle arrest of ERα+ breast cancer cell lines. Also, increases in the expression of the key tumor suppressors FOXO1 and FOXO3a were noted. Importantly, the strong synergy between ERβ agonists and ERα antagonists suggested that the efficacy of ERβ agonists is maximized by combination with ERα blockade. Lastly, ESR2 (ERβ gene) expression was negatively correlated with ESR1 (ERα gene) and CCND1 RNA expression in human metastatic ERα+/HER2- breast cancer samples.ConclusionOur results demonstrate that highly selective ERβ agonists attenuate the viability of ERα+ breast cancer cell lines in vitro and suggest that this therapeutic strategy merits further evaluation for ERα+ breast cancer.

Background:Triple negative breast cancer (TNBC) represents about 10-15% of all breast cancers. Given the lack of targeted therapies, chemotherapy and immunotherapy are the only treatment options. The five-year survival rate of TNBC patients is approximately 15% less than other forms of breast cancer. Therefore, there is an urgent need to develop novel effective therapeutics for TNBC.Material and Methods:RNA isolated from human immortalized mammary epithelial MCF10A, and various TNBC cell lines was DNase treated and quantitative RT-PCR (qRT-PCR) performed using gene-specific primers spanning exon-exon junctions that include introns in the corresponding genomic sequence to avoid genomic DNA amplification. Gene expression was calculated by ΔΔCt method using GAPDH as an internal control. Immuno-blot analyses were performed on lysates prepared from cells in log phase of growth. Proteins were detected by Enhanced Chemi-Luminescence (ECL) method. Viability of cells treated with ERβ specific agonists was determined by using CellTiter-Glo® 2.0 assay. MB231 cells were treated with ERβ agonists following transfection with pcDNA3 (vector) or Flag-tagged ESR2 plasmids. Western blot analyses using specific antibodies and qRT-PCR were performed with the protein extracts and RNA isolated from the cells, respectively. Results:Our experiments demonstrate that ESR2 (ERβ) is differentially expressed and its level of expression is about 3 to 22-fold higher in TNBC cell lines tested than MCF10A. We have shown that viability of estrogen receptor (ER) positive breast cancer cell lines can be significantly inhibited by selective activation of ERβ. A highly selective ERβ agonist has been invented in the Drug Development Institute at the Ohio State University. Treatment of TNBC lines with these ERβ agonists exhibited a dramatic reduction of cell viability. While comparable IC50 values were observed with our in house and commercially available compounds, TNBC had 2-3-fold lesser IC50 values than ER-positive cell lines. Increased expression of FOXO3 and c-Myc1 and decreased expression of c-Myc2 isoform was noted in ERβ overexpressing MB231 cells upon treatment with ERβ agonists.Conclusion:Our results demonstrate that treatment with highly selective ERβ agonists could be an effective therapeutic strategy for TNBC patients. To improve and develop novel therapeutics for TNBC understanding the mechanism of action of these compounds will be crucial. Citation Format: Jharna Datta, Natalie Willingham, Jasmine Manouchehri, Joel David, Mirisha Sheth, Bhuvaneswari Ramaswamy, Ramesh Ganju, Mathew A Cherian. Estrogen receptor beta agonists for triple negative breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS18-39.

Background: Among women, breast cancer is the leading cause of cancer-related death worldwide. Estrogen receptor α positive (ERα+) breast cancer accounts for 70% of all breast cancer subtypes. Although ERα+ breast cancer initially responds to estrogen deprivation or blockade, resistance emergence compelling the use of more aggressive therapies. While ERα is a driver in ERα+ breast cancer, ERβ plays an inhibitory role in several different cancer types. To date, the lack of highly selective ERβ agonists without ERα activity has limited the exploration of ERβ activation as a strategy for ERα+ breast cancer. Methods: We measured expression levels of ESR1 and ESR2 genes in immortalized mammary epithelial cells and different breast cancer cell lines. The viability of ERα+ breast cancer cell lines upon treatments with specific ERβ agonists, including OSU-ERb-12 and LY500307 was assessed. The specificity of the ERβ agonists, OSU-ERb-12 and LY500307, was confirmed by reporter assays. The effects of the 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 ERα+ 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 ERβ agonists in ERα+ breast cancer cell lines and drug-resistant derivatives. ERβ agonists blocked cell proliferation, migration and colony formation; and induced apoptosis and S and/or G2/M cell cycle arrest of ERα+ breast cancer cell lines. Also, increases in the expression of the key tumor suppressors FOXO1 and FOXO3a were noted. Importantly, the strong synergy between ERβ agonists and ERα antagonists suggested that the efficacy of ERβ agonists is maximized by combination with ERα blockade. Lastly, ESR2 (ERβ gene) expression was negatively correlated with ESR1 (ERα gene) and CCND1 RNA expression in human metastatic ER+/HER2- breast cancer samples. Conclusion: Our results demonstrate that highly selective ERβ agonists attenuate the viability of ERα+ breast cancer cell lines in vitro and suggest that this therapeutic strategy merits further evaluation for ERα+ breast cancer.

Background: Estrogen receptor (ER) positive breast cancer accounts 80% of all breast cancer subtypes. ER exists as ERα and ERβ, which are encoded by ESR1 and ESR2 genes, respectively. ERα stimulates the growth of breast cancer cells, while ERβ suppresses cancer cells. We hypothesize that activation of the ERβ may inhibit the viability of cancer cells in ER positive breast cancer without any effect on ERα. Material and Methods: Quantitative RT-PCR was performed with the DNase-treated RNA isolated from various breast cancer cell lines using gene-specific primers spanning exon-exon junctions that include introns in the corresponding genomic sequence to avoid genomic DNA amplification. Gene expression was calculated by ΔΔCt method using GAPDH as an internal control. Immuno-blot analyses were performed on lysates prepared from cells in log phase of growth. Proteins were detected by Enhanced Chemi-Luminescence (ECL) method. Viability of cells treated with ERβ specific agonists was determined by using CellTiter-Glo® 2.0 assay. HEK 293T cells were co-transfected with ERE-luciferase, Renila luciferase, and ESR1/ESR2 plasmids. Following treatment with ERβ agonists or estradiol, Dual Luciferase activity was measured with the lysates prepared from the cells. For clinical correlation, an IRB-approved single institution retrospective analysis was performed for 37 patients with metastatic HER2-negative ER-positive breast cancer to determine the mRNA expression levels of the genes including CCND1, MYC, IGF-1, Bcl-2, MMP-1, FN1; IGFBP-4, E2F4, CXCL12, PGR, EBAG9, and TRIM25 and correlated with ERα and ERβ. Descriptive analysis of gene expression was done using Spearman correlation coefficients and Cox proportional hazards regression. Results: Our experiments show that ERβ is expressed in ER positive breast cancer cell lines and that drugs which selectively activate ERβ, without activating ERα, inhibit the viability of these cells. One such compound has been invented at our institution is a highly selective activator of ERβ. Clinically, we found that ESR2 is positively correlated with CXCL12 (rho = 0.54, p < 0.001) and IGFBP4 (rho = 0.58, p < 0.001), and negatively correlated with CCND1 (rho = -0.45, p = 0.005), ESR1 (rho = -0.35, rho = 0.033). We did not find a correlation between ESR2 and Overall Survival in this data set. Conclusion: ERβ agonist is highly selective to treat ER positive breast cancer. We are in the process of testing this compound in murine xenografts of human breast cancer cell lines. If the experiments planned in this project indicate that this is a viable strategy for breast cancer, we will plan to move forward to early trials in human patients in the future. Citation Format: Mathew Cherian, Jharna Datta, Mahmoud Kassem, Natalie Willingham, Jasmine Manouchehri, Joel David, Mirisha Sheth, Alexa Magner, Rahul Mal, Evan Morgan, Jeffrey VanDeusen, Sagar Sardesai, Nicole Williams, Daniel Stover, Maryam Lustberg, Robert Wesolowski, Bhuvaneswari Ramaswamy, Ramesh Ganju. Estrogen receptor β agonists: A novel therapeutic strategy for breast cancer [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 5158.