Insights into the Role of Estrogen Receptor β in Triple-Negative Breast Cancer

Abstract

Estrogen receptors (ERα and ERβ) are ligand-activated transcription factors that play different roles in gene regulation and show both overlapping and specific tissue distribution patterns. ERβ, contrary to the oncogenic ERα, has been shown to act as an oncosuppressor in several instances. However, while the tumor-promoting actions of ERα are well-known, the exact role of ERβ in carcinogenesis and tumor progression is not yet fully understood. Indeed, to date, highly variable and even opposite effects have been ascribed to ERβ in cancer, including for example both proliferative and growth-inhibitory actions. Recently ERβ has been proposed as a potential target for cancer therapy, since it is expressed in a variety of breast cancers (BCs), including triple-negative ones (TNBCs). Because of the dependence of TNBCs on active cellular signaling, numerous studies have attempted to unravel the mechanism(s) behind ERβ-regulated gene expression programs but the scenario has not been fully revealed. We comprehensively reviewed the current state of knowledge concerning ERβ role in TNBC biology, focusing on the different signaling pathways and cellular processes regulated by this transcription factor, as they could be useful in identifying new diagnostic and therapeutic approaches for TNBC.

Keywords: TNBC; cancer cell metabolism; estrogen receptor β; oncosuppressor.

Figure 1

Schematic representation of ERβ gene, protein isoforms (ERβ1–5), and most used antibody epitopes. For the gene, 0K and 0N represent two promoters at the 5′ end of the gene, exons 1–8 are represented by boxes, and the introns are represented by lines. CX represents a 3′ non-coding exon present in the long form of ERβ2 protein (ERβcx). Size (bp) of each exon is showed by numbers above boxes, arrows indicate the start (ATG) and the stop (TAG) codons, and dotted lines link gene regions with the encoded protein domains. For protein isoforms, from N-terminus to C-terminus, A/B: activation function 1 (AF1) domain, C: DNA-binding domain (DBD), D: hinge domain, E: ligand-binding domain (LBD) or activation function 2 (AF2) domain, F: C-terminal domain. Square brackets show regions targeted by antibodies PPZ0506, MC10, 14C8, PPG5/10, and PA1-313. Numbers indicate the amino acids of the protein.

Figure 2

Proposed mechanism of ERβ-mediated inhibition of metastatic phenotype via suppression of TGF-β signaling in TNBC. In cancer cells, TGF-β/SMAD pathway drives invasiveness, cell migration, and metastasis formation. Ligand-activated ERβ blocks these processes by binding EREs in the CST genes, enhancing cystatin gene expression; cystatins, in turn, block canonical TGFβ signaling directly interacting with the TGFβ receptor (TβR), reducing SMAD2 and SMAD3 phosphorylation.

Figure 3

Proposed mechanism of ERβ-mediated inhibition of EMT via EGFR degradation in TNBC. EGF (epidermal growth factor), through the interaction with its receptor (EGFR), promotes epithelial to mesenchymal transition (EMT) in BC cells. Generally, EGFR signaling leads to the phosphorylation and activation of down-stream factors, such as ERK1/2 that, in turn, down-regulates the miR-200b-200a-429. This miRNA family is known to target and inhibit the action of ZEB-1 and SIP-1- transcription factors that repress E-cadherin expression. E-cadherins regulate cellular adhesion and are generally lost in EMT. ERβ blocks this network through induction of EGFR degradation, leading to up-regulation of E-cadherin protein expression and consequent EMT repression.

Figure 4

Proposed mechanism of ERβ-mediated regulation of unfolded protein response (UPR) in TNBC. Endoplasmic reticulum (EnR) stress activates inositol-requiring enzyme 1α (IRE1α), IRE1α self-dimerizes and undergoes autophosphorylation, then IRE1α induces X-box-binding protein 1 (XBP1) mRNA splicing with the formation of spliced XBP1 (XBP1s) mRNA. XBP1s-encoded protein functions as a potent transcription factor that triggers UPR-involved gene expression, whose expression promotes cell survival and inhibits apoptosis. ERβ induces dissociation of heat shock protein 90 (HSP90) from IRE1α and increases the expression of Synoviolin 1 (SYVN1) that ubiquitinates IRE1α. Both processes are known to induce IRE1α degradation, leading to downregulation of pro-survival XBP1s, unfolded protein accumulation in EnR, and apoptosis.

Figure 5

Proposed mechanism of ERβ-mediated regulation of oxidative phosphorylation (OXPHOS) in TNBC. ERβ interacts with glucose-regulated protein 75 (GRP75) and undergoes translocation into the mitochondria with the aid of the translocase of the outer membrane (TOM) complex. In mitochondria, ERβ binds to mitochondrial DNA (mtDNA) in displacement loop (D-loop) region and drives the expression of genes encoding for the components of respiratory complexes I, III, IV, and V, responsible for OXPHOS. OXPHOS activation leads to an increase of mitochondrial Ca²⁺, reactive oxygen species (ROS), and ATP concentrations.

Figure 6

Proposed mechanism of ERβ-mediated regulation of cholesterol biosynthesis in TNBC. ERβ interacts with chromatin repressive complexes, e.g., polycomb repressive complexes 1 and 2 (PRC1/2), binds to ERE present in sterol regulatory element binding factor 1 (SREBF1) gene promoter and inhibits SREBF1 expression. SREBF1 gene encodes for sterol regulatory element binding protein 1 (SREBP1), which drives expression of cholesterol biosynthesis genes by binding to sterol regulatory elements (SREs) present in their promoters. Inhibition of SREBF1 transcription reduces expression of SREBP1-driven genes leading to the downregulation of cholesterol biosynthesis. Alternatively, ERβ by unknown mechanism induces expression of miR-181a-5p, which targets cholesterol biosynthesis genes and regulates their expression post-transcriptionally.