Estrogen Signals through ERβ in Breast Cancer; What We Have Learned since the Discovery of the Receptor

Abstract

Estrogen receptor (ER) β (ERβ) is the second ER subtype that mediates the effects of estrogen in target tissues along with ERα that represents a validated biomarker and target for endocrine therapy in breast cancer. ERα was the only known ER subtype until 1996 when the discovery of ERβ opened a new chapter in endocrinology and prompted a thorough reevaluation of the estrogen signaling paradigm. Unlike the oncogenic ERα, ERβ has been proposed to function as a tumor suppressor in breast cancer, and extensive research is underway to uncover the full spectrum of ERβ activities and elucidate its mechanism of action. Recent studies have relied on new transgenic models to capture effects in normal and malignant breast that were not previously detected. They have also benefited from the development of highly specific synthetic ligands that are used to demonstrate distinct mechanisms of gene regulation in cancer. As a result, significant new information about the biology and clinical importance of ERβ is now available, which is the focus of discussion in the present article.

Keywords: ERβ; ERβ ligands; breast cancer; tumor microenvironment; tumor progression.

Figure 1.

Flow chart depicting effects of ERβ on EMT, cell migration and metastasis in breast cancer. Cystatins 1, 2, 4 and 5 are direct targets of ERβ in triple negative breast cancer (TNBC) cells. Expression of ERβ in TNBC cells followed by agonist activation inhibits metastasis in vivo by inducing the expression of cystatins that downregulate TGFβ signaling. Beclin-1, a key regulator of autophagy is upregulated by Claudin-6, a direct target of ERβ in breast cancer. Claudin-6 inhibits breast cancer cell migration and invasion. ERβ represses transcription of the activators of the cytoskeleton remodeler RhoC, ELMO1 and GRP141, by directly binding to their regulatory regions, thereby preventing RhoC activation and actin-based cell migration. Approximately 80% of TNBCs harbor oncogenic mutations of p53. ERβ directly interacts with mutant p53 and inhibits its prometastatic signaling. ERβ also inhibits epithelial–mesenchymal transition (EMT) by inducing EGFR degradation that results in upregulation of the epithelial markers miR-200a-b-429 and E-cadherin.

Figure 2.

Structural and functional domains of estrogen receptor β. Domains are shown for both the full length ERβ (also called ERβ1) and its isoforms ERβ2–5. Numbers indicate amino acid length of individual domains and the full length proteins. All ERβ isoforms are identical until the hinge region, where they begin diverging from the C-terminal of the ligand-binding domain (LBD). The ligand-independent transactivation function (AF-1) resides in the N-terminus of the receptor and serves as an interaction site for regulatory factors. The DNA-binding domain (DBD) recognizes estrogen response elements (ERE) in regulatory regions of target genes, whereas the hinge region harbors a nuclear localization signal (NLS). The LBD consists of the ligand-binding transactivation function (AF-2) and provides an interface for receptor dimerization and co-activator binding.

Figure 3.

Chemical structure of synthetic ligands of estrogen receptors tamoxifen, raloxifene and LY500307.

Figure 4.

Anti-tumor activities of the selective ERβ agonist LY500307. (A) Activating ERβ with LY500307 enables triple negative breast cancer (TNBC) and melanoma cells to secrete interleukin-1β (IL-1β), which stimulates the recruitment of anti-tumor neutrophils to the metastatic niche suppressing lung metastasis. (B) Similar to TNBC, ERβ and LY500307 prevent lung metastasis in inflammatory breast cancer (IBC) by inhibiting actin-based cell migration through the repression of the direct targets GPR141 and ELMO1 that activate the cytoskeleton remodeler RhoC. (C) Activation of ERβ with LY500307 also inhibits tumor growth and increases the survival of mice with highly aggressive glioblastoma (GBM) by reducing cell proliferation and inducing cell cycle arrest and apoptosis. (D) LY500307 greatly improves the therapeutic efficacy of immune checkpoint blockade (ICB) therapy with anti-PDL1 antibodies in TNBC and colorectal cancer. Activating ERβ in tumor cells with LY500307 prevents them from secreting CSF-1 in the tumor microenvironment, thus diminishing the recruitment of myeloid-derived suppressor cells (MDSCs), which, along with increased CD8⁺ T cells, leads to smaller tumors in mice. → and ⊥ represent positive and negative regulation, respectively and Red X indicates blockade.