The evolving role of oestrogen receptor beta in clinical breast cancer

Interaction between Estrogen Receptors and p53: A Broader Role for Tamoxifen?

Tamoxifen is one of the most widely used anticancer drugs in the world. It is a safe drug with generally well-tolerated side effects and has been prescribed for the treatment of early-stage and advanced-stage or metastatic estrogen receptor α (ERα/ESR1)-positive breast cancer. Tamoxifen therapy also provides a 38% reduction of the risk of developing breast cancer in women at high risk. With the advent of newer medications targeting ERα-positive breast cancer, tamoxifen is now mainly used as adjuvant therapy for lower-risk premenopausal breast cancer and cancer prevention. It is widely accepted that tamoxifen as a selective estrogen receptor modulator exerts its therapeutic effect by competitively binding to ERα, leading to the recruitment of corepressors and inhibition of transcription of genes involved in the proliferation of breast cancer epithelium. As such, expression of ERα in breast tumors has been considered necessary for tumors to be responsive to tamoxifen therapy. However, ERα-independent effects of tamoxifen in various in vitro and in vivo contexts have been reported over the years. Importantly, the recent discovery that ERα and estrogen receptor β (ERβ/ESR2) can bind tumor suppressor protein p53 with functional consequences has provided new insights into the mechanisms underlying response to tamoxifen therapy and resistance. Furthermore, these findings have paved the way for broadening the use of tamoxifen by potentially repurposing it to treat triple negative (negative for ERα, human epidermal growth factor receptor 2, and progesterone receptor) breast cancer. Herein, we summarize these developments and discuss their mechanistic underpinnings and clinical implications.

Therapeutic Role of Tamoxifen for Triple-Negative Breast Cancer: Leveraging the Interaction Between ERβ and Mutant p53

The absence of effective therapeutic targets and aggressive nature of triple-negative breast cancer (TNBC) renders this disease subset difficult to treat. Although estrogen receptor beta (ERβ) is expressed in TNBC, studies on its functional role have yielded inconsistent results. However, recently, our preclinical studies, along with other observations, have shown the potential therapeutic utility of ERβ in the context of mutant p53 expression. The current case study examines the efficacy of the selective estrogen receptor modulator tamoxifen in p53-mutant TNBC with brain metastases. Significant increase in ERβ protein expression and anti-proliferative interaction between mutant p53 and ERβ were observed after cessation of tamoxifen therapy, with significant regression of brain metastases. This case study provides supporting evidence for the use of tamoxifen in p53-mutant, ERβ+TNBC, especially in the setting of brain metastasis.

ERα-targeted endocrine therapy, resistance and the role of GPER

Endocrine therapy is an effective option for the treatment of estrogen receptor alpha (ERα)-positive breast cancers. Unfortunately, a large fraction of women relapse with endocrine-resistant tumors. The presence of constitutively active ERα mutants, found in a subset of relapse tumors, is thought to be an important endocrine resistance mechanism and has prompted the search for more effective anti-hormone drugs that can effectively inhibit these mutant versions of the receptor. The G protein-coupled estrogen receptor (GPER) is also thought to contribute to the development of endocrine resistance, in part, due to its activation by clinically used selective estrogen receptor modulators and downregulators (SERMs/SERDs). Therefore, next-generation drugs should be screened for potential activity towards GPER. Here, we highlight the need for truly ERα-selective SERMs and SERDs that do not cross-react with GPER for the treatment of ERα-positive breast cancers.

TP53 Status as a Determinant of Pro- vs Anti-Tumorigenic Effects of Estrogen Receptor-Beta in Breast Cancer

BACKGROUND

Anti-tumorigenic vs pro-tumorigenic roles of estrogen receptor-beta (ESR2) in breast cancer remain unsettled. We investigated the potential of TP53 status to be a determinant of the bi-faceted role of ESR2 and associated therapeutic implications for triple negative breast cancer (TNBC).

METHODS

ESR2-TP53 interaction was analyzed with multiple assays including the in situ proximity ligation assay. Transcriptional effects on TP53-target genes and cell proliferation in response to knocking down or overexpressing ESR2 were determined. Patient survival according to ESR2 expression levels and TP53 mutation status was analyzed in the basal-like TNBC subgroup in the Molecular Taxonomy of Breast Cancer International Consortium (n = 308) and Roswell Park Comprehensive Cancer Center (n = 46) patient cohorts by univariate Cox regression and log-rank test. All statistical tests are two-sided.

RESULTS

ESR2 interaction with wild-type and mutant TP53 caused pro-proliferative and anti-proliferative effects, respectively. Depleting ESR2 in cells expressing wild-type TP53 resulted in increased expression of TP53-target genes CDKN1A (control group mean [SD] = 1 [0.13] vs ESR2 depletion group mean [SD] = 2.08 [0.24], P = .003) and BBC3 (control group mean [SD] = 1 [0.06] vs ESR2 depleted group mean [SD] = 1.92 [0.25], P = .003); however, expression of CDKN1A (control group mean [SD] = 1 [0.21] vs ESR2 depleted group mean [SD] = 0.56 [0.12], P = .02) and BBC3 (control group mean [SD] = 1 [0.03] vs ESR2 depleted group mean [SD] = 0.55 [0.09], P = .008) was decreased in cells expressing mutant TP53. Overexpressing ESR2 had opposite effects. Tamoxifen increased ESR2-mutant TP53 interaction, leading to reactivation of TP73 and apoptosis. High levels of ESR2 expression in mutant TP53-expressing basal-like tumors is associated with better prognosis (Molecular Taxonomy of Breast Cancer International Consortium cohort: log-rank P = .001; hazard ratio = 0.26, 95% confidence interval = 0.08 to 0.84, univariate Cox P = .02).

CONCLUSIONS

TP53 status is a determinant of the functional duality of ESR2. Our study suggests that ESR2-mutant TP53 combination prognosticates survival in TNBC revealing a novel strategy to stratify TNBC for therapeutic intervention potentially by repurposing tamoxifen.

Tamoxifen regulates cell fate through mitochondrial estrogen receptor beta in breast cancer

Tamoxifen has both cytostatic and cytotoxic properties for breast cancer. Tamoxifen engaged mitochondrial estrogen receptor beta (ERβ) as an antagonist in MCF-7 BK cells, increasing reactive oxygen species (ROS) concentrations from the mitochondria that were required for cytotoxicity. In part this derived from tamoxifen down-regulating manganese superoxide dismutase (MnSOD) activity through nitrosylating tyrosine 34, thereby increasing ROS. ROS activated protein kinase C delta and c-jun N-terminal kinases, resulting in the mitochondrial translocation of Bax and cytochrome C release. Interestingly, tamoxifen failed to cause high ROS levels or induce cell death in MCF7BK-TR cells due to stimulation of MnSOD activity through agonistic effects at mitochondrial ERβ. In several mouse xenograft models, lentiviral shRNA-induced knockdown of MnSOD caused tumors that grew in the presence of tamoxifen to undergo substantial apoptosis. Tumor MnSOD and mitochondrial ERβ are therefore targets for therapeutic intervention to reverse tamoxifen resistance and enhance a cell death response.

Inhibition of ERβ induces resistance to cisplatin by enhancing Rad51-mediated DNA repair in human medulloblastoma cell lines

Cisplatin is one of the most widely used and effective anticancer drugs against solid tumors including cerebellar tumor of the childhood, Medulloblastoma. However, cancer cells often develop resistance to cisplatin, which limits therapeutic effectiveness of this otherwise effective genotoxic drug. In this study, we demonstrate that human medulloblastoma cell lines develop acute resistance to cisplatin in the presence of estrogen receptor (ER) antagonist, ICI182,780. This unexpected finding involves a switch from the G2/M to G1 checkpoint accompanied by decrease in ATM/Chk2 and increase in ATR/Chk1 phosphorylation. We have previously reported that ERβ, which is highly expressed in medulloblastomas, translocates insulin receptor substrate 1 (IRS-1) to the nucleus, and that nuclear IRS-1 binds to Rad51 and attenuates homologous recombination directed DNA repair (HRR). Here, we demonstrate that in the presence of ICI182,780, cisplatin-treated medulloblastoma cells show recruitment of Rad51 to the sites of damaged DNA and increase in HRR activity. This enhanced DNA repair during the S phase preserved also clonogenic potential of medulloblastoma cells treated with cisplatin. In conclusion, inhibition of ERβ considered as a supplemental anticancer therapy, has been found to interfere with cisplatin–induced cytotoxicity in human medulloblastoma cell lines.

Oestrogen receptor β in adenoid cystic carcinoma of salivary glands

AIMS

This study aimed to describe the expression of oestrogen receptor (ER)α, ERβ and aromatase in salivary gland adenoid cystic carcinoma (ACC).

METHODS AND RESULTS

ERα, ERβ and aromatase expression was analysed by immunohistochemistry in tissue microarray blocks from 38 cases of ACC and seven normal salivary glands. The intracellular localization and amount of total protein expression were investigated by immunofluorescence and western blotting in an ACC cell line. Western blotting analysis showed overexpression of ERα, ERβ and aromatase in the ACC cell line; however, with immunofluorescence, only ERβ was shown to be expressed in the nucleus. Immunohistochemistry revealed positive nuclear expression of ERβ, positive cytoplasmic expression of aromatase and a lack of ERα expression as compared with normal salivary glands.

CONCLUSIONS

The nuclear expression of ERβ indicates that oestrogen may be active in ACC and possibly able to mediate E2-targeted gene transcription. This study strongly suggests that ERβ may be involved in tumour progression, playing a role in tumour development, and thus corroborating the indication for ER antagonists in the clinical control of ACC. This study opens a new perspective on the potential use of anti-oestrogens and aromatase inhibitors as therapeutic agents against ACC.

Potent genistein derivatives as inhibitors of estrogen receptor alpha-positive breast cancer

The estrogen receptor (ER) is a major target for the treatment of breast cancer cells. Genistein, a soy isoflavone, possesses a structure similar to estrogen and can both mimic and antagonize estrogen effects although at high concentrations it inhibits breast cancer cell proliferation. Hence, to enhance the anti-cancer activity of Genistein at lower concentrations, we have synthesized seven structurally modified derivatives of Genistein (MA-6, MA-8, MA-11, MA-19, MA-20, MA-21 and MA-22) based on the structural requirements for an optimal anti-cancer effect. Among those seven, three derivatives (MA-6, MA-8 and MA-19) showed high antiproliferative activity with IC(50) levels in the range of 1-2.5 μM, i.e., at much lower concentrations range than Genistein itself, in three ER-positive breast cancer cell lines (MCF-7, 21PT and T47D) studied. In our analysis, we noticed that at IC(50) concentrations, the MA-6, MA-8 and MA-19 Genistein derivatives induced apoptosis, inhibited ER-α messenger RNA expression and increased the ratio of ER-β to ER-α levels in a manner comparable to the parent compound Genistein. Of note, these three modified Genistein derivatives exerted their effects at concentrations 10-15 times lower than the parent compound, decreasing the likelihood of significant ER- α pathway activation, which has been a concern for Genistein. Hence these compounds might play a useful role in breast cancer chemoprevention.

Nuclear and cytoplasmic expressions of ERβ1 and ERβ2 are predictive of response to therapy and alters prognosis in familial breast cancers

Estrogen receptor (ER) α has been studied extensively in familial breast cancers but there are limited data on ERβ and its isoforms. This is an important issue since many BRCA1-associated tumours are "triple negative" and are resistant to conventional and targeted therapies. We performed an immunohistochemical study of pan-ERβ, ERβ1 and ERβ2 in a cohort of 123 familial breast carcinomas (35 BRCA1, 33 BRCA2 and 55 BRCAX) using a cut-off for positivity at 20% (Shaaban et al. in Clin Cancer Res 14:5228-5235, 2008). BRCA1 cancers were more likely to be nuclear ERα negative and nuclear pan-ERβ positive (21/32, 66%) when compared with BRCA2 (2/29, 7%) and BRCAX cancers (11/49, 22%) (both P < 0.001). For survival analysis, expression was also stratified using cut-offs defined by Bates et al. (Breast Cancer Res Treat 111:453-459, 2008) (score out of 7). Cytoplasmic ERβ2 expression correlated with shorter overall survival at 15 years regardless of cut-off used (both P < 0.046) At a cut-off score of 6 out of 7, cytoplasmic ERβ2 expression correlated with a poorer response to chemotherapy in both univariate (P = 0.011) and multivariate analyses including grade, lymph node status and chemotherapy as an interaction variable (P = 0.045, Hazard ratio 1.22, 95% CI 1.004-9.87). A similar trend was seen in a univariate analysis with a cut-off of 20% although this did not reach statistical significance (P = 0.057). Expression of nuclear ERβ1 was associated with a favourable response to endocrine therapy at 15 years regardless of cut-offs employed (both P < 0.025). However, this did not reach statistical significance in a multivariate analysis (P > 0.05). Since a significant proportion of ERα negative familial breast carcinomas are positive for nuclear ERβ1 and cytoplasmic ERβ2, the different ERβ isoforms and their intracellular location may need to be assessed, to identify patients that may benefit from hormonal and chemotherapy.

Breast and prostate cancer: more similar than different

Breast cancer and prostate cancer are the two most common invasive cancers in women and men, respectively. Although these cancers arise in organs that are different in terms of anatomy and physiological function both organs require gonadal steroids for their development, and tumours that arise from them are typically hormone-dependent and have remarkable underlying biological similarities. Many of the recent advances in understanding the pathophysiology of breast and prostate cancers have paved the way for new treatment strategies. In this Opinion article we discuss some key issues common to breast and prostate cancer and how new insights into these cancers could improve patient outcomes.