Estrogen Receptor β as a Therapeutic Target in Breast Cancer Stem Cells

Abstract

Background: Breast cancer cells with tumor-initiating capabilities (BSCs) are considered to maintain tumor growth and govern metastasis. Hence, targeting BSCs will be crucial to achieve successful treatment of breast cancer. Methods: We characterized mammospheres derived from more than 40 cancer patients and two breast cancer cell lines for the expression of estrogen receptors (ERs) and stem cell markers. Mammosphere formation and proliferation assays were performed on cells from 19 cancer patients and five healthy individuals after incubation with ER-subtype selective ligands. Transcriptional analysis was performed to identify pathways activated in ERβ-stimulated mammospheres and verified using in vitro experiments. Xenograft models (n = 4 or 5 per group) were used to study the role of ERs during tumorigenesis. Results: We identified an absence of ERα but upregulation of ERβ in BSCs associated with phenotypic stem cell markers and responsible for the proliferative role of estrogens. Knockdown of ERβ caused a reduction of mammosphere formation in cell lines and in patient-derived cancer cells (40.7%, 26.8%, and 39.1%, respectively). Gene set enrichment analysis identified glycolysis-related pathways (false discovery rate < 0.001) upregulated in ERβ-activated mammospheres. We observed that tamoxifen or fulvestrant alone was insufficient to block proliferation of patient-derived BSCs while this could be accomplished by a selective inhibitor of ERβ (PHTPP; 53.7% in luminal and 45.5% in triple-negative breast cancers). Furthermore, PHTPP reduced tumor initiation in two patient-derived xenografts (75.9% and 59.1% reduction in tumor volume, respectively) and potentiated tamoxifen-mediated inhibition of tumor growth in MCF7 xenografts. Conclusion: We identify ERβ as a mediator of estrogen action in BSCs and a novel target for endocrine therapy.

Estrogen Receptor-β Modulation of the ERα-p53 Loop Regulating Gene Expression, Proliferation, and Apoptosis in Breast Cancer

“…However, the actions of ERβ may well show context and cell type dependent differences. In this regard, ERβ has been reported recently to be present in the majority of breast cancer stem cells, with ERβ expression declining as stem cells differentiate (60). In breast cancer stem cells that lack ERα but contain ERβ, E2 or the ERβ agonist DPN increased mammosphere formation and proliferation, and changed the breast stem cell metabolism.…”

“…In breast cancer stem cells that lack ERα but contain ERβ, E2 or the ERβ agonist DPN increased mammosphere formation and proliferation, and changed the breast stem cell metabolism. Further, an ERβ antagonist, PHTPP, when combined with tamoxifen, reduced tumor growth over that achieved by tamoxifen alone (60). Hence, ERβ may function as a proliferative signal in some contexts when ERα is absent (60).…”

“…Further, an ERβ antagonist, PHTPP, when combined with tamoxifen, reduced tumor growth over that achieved by tamoxifen alone (60). Hence, ERβ may function as a proliferative signal in some contexts when ERα is absent (60). …”

Comparison of the roles of estrogens and androgens in breast cancer and prostate cancer

“…Ma et al found that ERβ was upregulated and acted as an estrogen mediator in BC cells in the absence of ERα, thus facilitating MCF‐7 proliferation. Additionally, ERβ knockdown decreased mammospheres formation in MCF‐7 cell lines and patient‐derived cancer cells . A recent study showed that when ERα expression was lost or decreased by 4‐methyl‐2,4‐bis(4‐hydroxyphenyl)pent‐1‐ene, a recognized endocrine disruptor, E2‐stimulated cell proliferation was mediated by ERβ signaling pathways, such as glycolysis‐related pathways and cell cycle–dependent kinase (CDK) .…”

Mechanisms of resistance in estrogen receptor positive breast cancer: overcoming resistance to tamoxifen/aromatase inhibitors

“…Post-translational modifications also influence function, localization, and interaction with other regulators. In addition to ERα, there also exists transcription factor ERβ, encoded by ESR2 , as well as alternatively spliced and truncated variants of ESR1 /ERα [4]. The biology of ER is complex and how breast tumors can gain ER function then maintain this despite ER inhibition is not well understood.…”