Identification of genes involved in growth inhibition of breast cancer cells transduced with estrogen receptor

Estrogen Receptor Mediates the Radiosensitivity of Triple-Negative Breast Cancer Cells

Background

This study aimed to evaluate differences in the radiosensitivities of triple-negative breast cancer (TNBC) and luminal-type breast cancer cells and to investigate the effects of estrogen receptor (ER) expression on the biological behaviors of the cells.

Material/Methods

Colony-forming assays were performed to detect differences in radiosensitivities in breast cancer cell lines. Gene transfection technology was used to introduce the expression of ERα in TNBC cells to compare the difference in radiosensitivity between the TNBC cells and ERα transfected cells. CCK-8 assays were used to observe changes in the proliferation of TNBC cells after ERα transfection. Immunofluorescence was used to detect the number of γH2AX foci in nuclei. Flow cytometry was used to detect changes in cell cycle distribution and apoptosis. Western blotting was used to detect changes in autophagy-associated proteins.

Results

The radioresistance of the TNBC cell line MDA-MB-231 (231 cells) was greater than that of ERα-positive luminal-type breast cancer cell line MCF-7. Moreover, 231 cell proliferation and radioresistance decreased after ERα transfection. Interestingly, ERα-transfected 231 cells showed increased double-stranded breaks and delayed repair compared with 231 cells, and ERα-transfected 231 cells showed increased G₂/M phase arrest and apoptosis after irradiation compared with those in 231 cells. ERα transfection in 231 cells reduced autophagy-related protein expression, suggesting that autophagy activity decreased in 231 ER-positive cells after irradiation.

Conclusions

TNBC cells were more resistant to radiation than luminal-type breast cancer cells. ERα expression may have major roles in modulating breast cancer cell radiosensitivity.

Potential role of estrogen receptor beta as a tumor suppressor of epithelial ovarian cancer

Ovarian cancer is the gynecological cancer exhibiting the highest morbidity and improvement of treatments is still required. Previous studies have shown that Estrogen-receptor beta (ERβ) levels decreased along with ovarian carcinogenesis. Here, we present evidence that reintroduction of ERβ in BG-1 epithelial ovarian cancer cells, which express ERα, leads in vitro to a decrease of basal and estradiol-promoted cell proliferation. ERβ reduced the frequency of cells in S phase and increased the one of cells in G2/M phase. At the molecular level, we found that ERβ downregulated total retinoblastoma (Rb), phosphorylated Rb and phospho-AKT cellular content as well as cyclins D1 and A2. In addition, ERβ had a direct effect on ERα, by strongly inhibiting its expression and activity, which could explain part of the anti-proliferative action of ERβ. By developing a novel preclinical model of ovarian cancer based on a luminescent orthotopic xenograft in athymic Nude mice, we further revealed that ERβ expression reduces tumor growth and the presence of tumor cells in sites of metastasis, hence resulting in improved survival of mice. Altogether, these findings unveil a potential tumor-suppressor role of ERβ in ovarian carcinogenesis, which could be of potential clinical relevance for the selection of the most appropriate treatment for patients.

SAHA/Vorinostat induces the expression of the CD137 receptor/ligand system and enhances apoptosis mediated by soluble CD137 receptor in a human breast cancer cell line

HDAC inhibitors (HDACis) represent a class of anticancer agents including suberoylanilide hydroxamic acid (SAHA, Vorinostat), which has shown a strong antitumor effect, both in vitro and in vivo. Induction of apoptotic genes is an important pathway of SAHA cytotoxic mechanism of action and it has been largely described that SAHA induces sensitization of cell death receptor-resistant breast cancer cells to apoptosis. In this study, we investigated the activation of some apoptotic genes which could be responsible for the in vivo antitumor potency of SAHA in a model of human breast cancer. We found that the apoptotic gene pattern induced by SAHA in the MDA-MB-231 cell line involves the upregulation of some molecules belonging to the TNF superfamily. In particular, we demonstrated that the upregulation of the CD137 receptor/ligand system correlates with a synergistic cytotoxic effect when MDA-MB-231 cells are treated with the combination of SAHA and soluble CD137 receptor. To our knowledge, this is the first study to indicate that this member of the TNF superfamily, CD137, is modulated by SAHA treatment in breast cancer cells, suggesting that the combination of SAHA with this TNF-related receptor could be a new therapeutic approach for the treatment of tumors.

Tamoxifen-induced epigenetic silencing of oestrogen-regulated genes in anti-hormone resistant breast cancer

In the present study, we have taken the novel approach of using an in vitro model representative of tamoxifen-withdrawal subsequent to clinical relapse to achieve a greater understanding of the mechanisms that serve to maintain the resistant-cell phenotype, independent of any agonistic impact of tamoxifen, to identify potential novel therapeutic approaches for this disease state. Following tamoxifen withdrawal, tamoxifen-resistant MCF-7 cells conserved both drug resistance and an increased basal rate of proliferation in an oestrogen deprived environment, despite reduced epidermal growth-factor receptor expression and reduced sensitivity to gefitinib challenge. Although tamoxifen-withdrawn cells retained ER expression, a sub-set of ER-responsive genes, including pS2 and progesterone receptor (PgR), were down-regulated by promoter DNA methylation, as confirmed by clonal bisulphite sequencing experiments. Following promoter demethylation with 5-Azacytidine (5-Aza), the co-addition of oestradiol (E2) restored gene expression in these cells. In addition, 5-Aza/E2 co-treatment induced a significant anti-proliferative effect in the tamoxifen-withdrawn cells, in-contrast to either agent used alone. Microarray analysis was undertaken to identify genes specifically up regulated by this co-treatment. Several anti-proliferative gene candidates were identified and their promoters were confirmed as more heavily methylated in the tamoxifen resistant vs sensitive cells. One such gene candidate, growth differentiation factor 15 (GDF15), was carried forward for functional analysis. The addition of 5-Aza/E2 was sufficient to de-methylate and activate GDF15 expression in the tamoxifen resistant cell-lines, whilst in parallel, treatment with recombinant GDF15 protein decreased cell survival. These data provide evidence to support a novel concept that long-term tamoxifen exposure induces epigenetic silencing of a cohort of oestrogen-responsive genes whose function is associated with negative proliferation control. Furthermore, reactivation of such genes using epigenetic drugs could provide a potential therapeutic avenue for the management of tamoxifen-resistant breast cancer.

Estrogen receptors similarly mediate the effects of 17β-estradiol on cellular responses but differ in their potencies

17β-estradiol (E2), as the main circulating estrogen hormone, plays critical roles in the physiology and pathophysiology of various tissues. The E2 information is primarily conveyed by the transcription factors, estrogen receptors (ERs) α and β. ERs share similar structural and functional features. Experimental studies indicate that upon binding to E2, ERs directly or indirectly interact with DNA and regulate gene expressions with ERα being more potent transregulator than ERβ. However, studies also showed that ERβ induces alterations in phenotypic features of cancer cell lines independent of E2. These observations suggested that the manner in which the unliganded ERβ induces phenotypic alterations in cancer cell models differs from that of ERα. Studies demonstrated that while requiring E2 for function at low levels of synthesis, the unliganded ERα at augmented concentrations modulates gene expressions and cellular growth. We, therefore, anticipated that heightened levels of ERβ synthesis could similarly circumvent the dependency on E2 leading to gene transcriptions and cellular proliferation. To test this prediction, we used adenovirus-infected cancer cell lines in which ERs were shown to induce genomic and cellular responses. We found that while ERβ at low levels of synthesis was dependent upon E2 for function, the receptor at high levels regulated gene expression and cellular proliferation independent of E2. We then addressed whether ERs at comparable levels that require E2 for function differentially alter gene expressions and cellular responses. We found that ERs mediate the effects of E2 on gene expression, cellular proliferation, apoptosis, and motility with an overlapping pattern. However, ERα was more potent regulator than ERβ in inducing cellular responses. Our results suggest that differences in potencies to regulate the expression of genes are a critical feature of the ER subtypes in mediating E2 signaling in cancer cell lines.

Global analysis of estrogen receptor beta binding to breast cancer cell genome reveals an extensive interplay with estrogen receptor alpha for target gene regulation

Background

Estrogen receptors alpha (ERα) and beta (ERβ) are transcription factors (TFs) that mediate estrogen signaling and define the hormone-responsive phenotype of breast cancer (BC). The two receptors can be found co-expressed and play specific, often opposite, roles, with ERβ being able to modulate the effects of ERα on gene transcription and cell proliferation. ERβ is frequently lost in BC, where its presence generally correlates with a better prognosis of the disease. The identification of the genomic targets of ERβ in hormone-responsive BC cells is thus a critical step to elucidate the roles of this receptor in estrogen signaling and tumor cell biology.

Results

Expression of full-length ERβ in hormone-responsive, ERα-positive MCF-7 cells resulted in a marked reduction in cell proliferation in response to estrogen and marked effects on the cell transcriptome. By ChIP-Seq we identified 9702 ERβ and 6024 ERα binding sites in estrogen-stimulated cells, comprising sites occupied by either ERβ, ERα or both ER subtypes. A search for TF binding matrices revealed that the majority of the binding sites identified comprise one or more Estrogen Response Element and the remaining show binding matrixes for other TFs known to mediate ER interaction with chromatin by tethering, including AP2, E2F and SP1. Of 921 genes differentially regulated by estrogen in ERβ+ vs ERβ- cells, 424 showed one or more ERβ site within 10 kb. These putative primary ERβ target genes control cell proliferation, death, differentiation, motility and adhesion, signal transduction and transcription, key cellular processes that might explain the biological and clinical phenotype of tumors expressing this ER subtype. ERβ binding in close proximity of several miRNA genes and in the mitochondrial genome, suggests the possible involvement of this receptor in small non-coding RNA biogenesis and mitochondrial genome functions.

Conclusions

Results indicate that the vast majority of the genomic targets of ERβ can bind also ERα, suggesting that the overall action of ERβ on the genome of hormone-responsive BC cells depends mainly on the relative concentration of both ERs in the cell.

ERalpha as ligand-independent activator of CDH-1 regulates determination and maintenance of epithelial morphology in breast cancer cells

Estrogen receptor alpha (ERalpha) and E-cadherin are primary markers of luminal epithelial breast cancer cells with E-cadherin being a main caretaker of the epithelial phenotype. E-cadherin repression is needed for cancer cells to acquire motile and invasive properties, and it is known that in ER-positive breast cancer cells, estrogen down-regulate E-cadherin gene transcription. We report here that ERalpha is bound to the E-cadherin promoter in both the presence and the complete absence of estrogen, suggesting an unexpected role for unliganded ERalpha in E-cadherin transcription. Indeed, our data reveal that activation by unliganded ERalpha and repression by estrogen-activated ERalpha require direct binding to a half-estrogen response element within the E-cadherin promoter and exchange from associated coactivators to corepressors. Therefore, these results suggest a pivotal role for unliganded ERalpha in controlling a fundamental caretaker of the epithelial phenotype in breast cancer cells. Here, we show that ERalpha-positive breast cancer T47D cells transduced with the sfRON kinase undergo a full epithelial-mesenchymal conversion and lose E-cadherin and ERalpha expression. Our data show that, although the E-cadherin gene becomes hypermethylated and heterochromatic, kinase inhibitors can restore E-cadherin expression, together with an epithelial morphology in an ERalpha-dependent fashion. Similarly, transfection of ERalpha, in the absence of ligands, was sufficient to restore E-cadherin transcription in both sfRON-T47D and other ERalpha-, E-cadherin-negative cells. Therefore, our results suggest a novel role for the ERalpha that plays the dual role of ligand-independent activator and ligand-dependent repressor of E-cadherin in breast cancer cells.

Do Estrogen Receptor beta Polymorphisms Play A Role in the Pharmacogenetics of Estrogen Signaling?

Estrogen hormones play critical roles in the regulation of many tissue functions. The effects of estrogens are primarily mediated by the estrogen receptors (ER) alpha and beta. ERs are ligand-activated transcription factors that regulate a complex array of genomic events that orchestrate cellular growth, differentiation and death. Although many factors contribute to their etiology, estrogens are thought to be the primary agents for the development and/or progression of target tissue malignancies. Many of the current modalities for the treatment of estrogen target tissue malignancies are based on agents with diverse pharmacology that alter or prevent ER functions by acting as estrogen competitors. Although these compounds have been successfully used in clinical settings, the efficacy of treatment shows variability. An increasing body of evidence implicates ERalpha polymorphisms as one of the contributory factors for differential responses to estrogen competitors. This review aims to highlight the recent findings on polymorphisms of the lately identified ERbeta in order to provide a functional perspective with potential pharmacogenomic implications.

Gene expression profiling reveals that the regulation of estrogen-responsive element-independent genes by 17 beta-estradiol-estrogen receptor beta is uncoupled from the induction of phenotypic changes in cell models

Estrogen hormone 17beta-estradiol (E(2)) is involved in the physiology and pathology of many tissues. E(2) information is conveyed by the transcription factors estrogen receptors (ER) alpha and beta that mediate a complex array of nuclear and non-nuclear events. The interaction of ER with specific DNA sequences, estrogen-responsive elements (EREs), constitutes a critical nuclear signaling pathway. In addition, E(2)-ER regulates transcription through interactions with transfactors bound to their cognate regulatory elements on DNA, hence the ERE-independent signaling pathway. However, the relative importance of the ERE-independent pathway in E(2)-ERbeta signaling is unclear. To address this issue, we engineered an ERE-binding defective ERbeta mutant (ERbeta(EBD)) by changing critical residues in the DNA-binding domain required for ERE binding. Biochemical and functional studies revealed that ERbeta(EBD) signaled exclusively through the ERE-independent pathway. Using the adenovirus infected ER-negative cancer cell models, we found that although E(2)-ERbeta(EBD) regulated the expression of a number of genes identified by microarrays, it was ineffective in altering cellular proliferation, motility, and death in contrast to E(2)-ERbeta. Our results indicate that genomic responses from the ERE-independent pathway to E(2)-ERbeta are not sufficient to alter the cellular phenotype. These findings suggest that the ERE-dependent pathway is a required signaling route for E(2)-ERbeta to induce cellular responses.

Chemokines: novel targets for breast cancer metastasis

Recent studies have highlighted the possible involvement of chemokines and their receptors in breast cancer progression and metastasis. Chemokines and their receptors constitute a superfamily of signalling factors whose prognosis value in breast cancer progression remains unclear. We will examine here the expression pattern of chemokines and their receptors in mammary gland physiology and carcinogenesis. The nature of the cells producing chemokines or harboring chemokine receptors appears to be crucial in certain conditions for example, the infiltration of the primary tumor by leukocytes and angiogenesis. In addition, chemokines, their receptors and the interaction with glycosaminoglycan (GAGs) are key players in the homing of cancer cells to distant metastasis sites. Several lines of evidence, including in vitro and in vivo models, suggest that the mechanism of action of chemokines in cancer development involves the modulation of proliferation, apoptosis, invasion, leukocyte recruitment or angiogenesis. Furthermore, we will discuss the regulation of chemokine network in tumor neovascularity by decoy receptors. The reasons accounting for the deregulation of chemokines and chemokine receptors expression in breast cancer are certainly crucial for the comprehension of chemokine role in breast cancer and are in several cases linked to estrogen receptor status. The targeting of chemokines and chemokine receptors by antibodies, small molecule antagonists, viral chemokine binding proteins and heparins appears as promising tracks to develop therapeutic strategies. Thus there is significant interest in developing strategies to antagonize the chemokine function, and an opportunity to interfere with metastasis, the leading cause of death in most patients.

ERRgamma suppresses cell proliferation and tumor growth of androgen-sensitive and androgen-insensitive prostate cancer cells and its implication as a therapeutic target for prostate cancer

Estrogen receptor-related receptors (ERR) are orphan nuclear receptors, which are constitutively activated without estrogen binding. Recent evidence indicates that the ligand-independent ERRs may be involved in similar ER-mediated regulatory pathways and modulate estrogen responsiveness in certain target cells. We recently showed that an ERR subtype, ERRgamma, is coexpressed with ERbeta in normal human prostatic epithelial cells and exhibits reduced expression in many prostate cancer cell lines and clinical neoplastic prostate tissues. Based on this, we hypothesize that ERRgamma may have growth regulatory roles in prostate and prostate cancer. We showed in this study that ERRgamma was expressed in epithelial cell nuclei in fetal and pubertal human prostates, whereas its nuclear expression became reduced in advanced prostate cancer lesions. Stable ERRgamma expression by retroviral transduction suppressed significantly both in vitro cell growth and in vivo tumorigenicity of two prostate cancer cell lines, LNCaP and DU145, as evidenced by a cell-cycle arrest at G(1)-S transition and also induction of two cyclin-dependent kinase inhibitors p21(WAF1/CIP1) and p27(KIP1). We further showed by reporter assay that induction of p21 and p27 by ERRgamma was mediated through direct transactivation of their gene promoters. Moreover, we also showed that a selective ERRgamma-agonist, DY131, could potentiate the ERRgamma-induced growth inhibition in LNCaP-ERRgamma and DU145-ERRgamma cells in a dose-dependent manner compared with respective parental cells. Taken together, our results show that ERRgamma may perform an antiproliferative or tumor-suppressing function in prostate cancer cells. More importantly, our results suggest that ERRgamma could be a novel therapeutic target for prostate cancer treatment.

Estrogen-regulated gene networks in human breast cancer cells: involvement of E2F1 in the regulation of cell proliferation

Estrogens generally stimulate the proliferation of estrogen receptor (ER)-containing breast cancer cells, but they also suppress proliferation of some ER-positive breast tumors. Using a genome-wide analysis of gene expression in two ER-positive human breast cancer cell lines that differ in their proliferative response to estrogen, we sought to identify genes involved in estrogen-regulated cell proliferation. To this end, we compared the transcriptional profiles of MCF-7 and MDA-MB-231ER+ cells, which have directionally opposite 17beta-estradiol (E2)-dependent proliferation patterns, MCF-7 cells being stimulated and 231ER+ cells suppressed by E2. We identified a set of approximately 70 genes regulated by E2 in both cells, with most being regulated by hormone in an opposite fashion. Using a variety of bioinformatics approaches, we found the E2F binding site to be overrepresented in the potential regulatory regions of many cell cycle-related genes stimulated by estrogen in MCF-7 but inhibited by estrogen in 231ER+ cells. Biochemical analyses confirmed that E2F1 and E2F downstream target genes were increased in MCF-7 and decreased in 231ER+ cells upon estrogen treatment. Furthermore, RNA interference-mediated knockdown of E2F1 blocked estrogen regulation of E2F1 target genes and resulted in loss of estrogen regulation of proliferation. These results demonstrate that regulation by estrogen of E2F1, and subsequently its downstream target genes, is critical for hormone regulation of the proliferative program of these breast cancer cells, and that gene expression profiling combined with bioinformatic analyses of transcription factor binding site enrichment in regulated genes can identify key components associated with nuclear receptor hormonal regulation of important cellular functions.

Estrogen receptor beta, a possible tumor suppressor involved in ovarian carcinogenesis

Ovarian cancer is one of the leading causes of death from gynecological tumors in women. Several lines of evidence suggest that estrogens may play an important role in ovarian carcinogenesis, through their receptors, ERalpha and ERbeta. Interestingly, malignant ovarian tumors originating from epithelial surface constitute about 90% of ovarian cancers and expressed low levels of ERbeta, compared to normal tissues. In addition, restoration of ERbeta in ovarian cancer cells, leads to strong inhibition of their proliferation and invasion, while apoptosis is enhanced. In this manuscript, recent data suggesting a possible tumor-suppressor role for ERbeta in ovarian carcinogenesis are discussed.

Transcription factor decoy against promoter C of estrogen receptor α gene induces a functional ERα protein in breast cancer cells

This study addresses the hypothesis that transfection of oligonucleotide mimicking a negative regulatory sequence of promoter C of estrogen receptor alpha (ER alpha) gene is sufficient for its re-expression in ER-negative human cancer cell lines. Even if the negative transcription regulator subtracted by the transcription factor decoy is not yet been identified, we demonstrated that after this decoy treatment, the cells produced a functional ER alpha protein able to respond to 17-beta-estradiol and to transactivate a transfected estrogen response element (ERE)-regulated reporter gene. The effects of reactivated ER alpha protein and its estrogen dependence on endogenous target gene expression level, such as ER beta, have been also assessed. The proliferation of the cells transfected with low levels of decoy was significantly increased by estrogen and not by tamoxifen, suggesting that the levels of reactivated ER alpha in these decoy conditions confers a certain hormone sensitivity. On the contrary, high-level expression of ER alpha obtained at high doses of transfected decoy molecule produced a progressive decrease of cell proliferation. Since ER alpha is important in the transcription of different genes and its loss is involved in several pathological processes including neoplastic and chronic diseases, our findings may be of relevance for a possible new therapeutical approach of such diseases.

Expression of estrogen receptor beta in prostate carcinoma cells inhibits invasion and proliferation and triggers apoptosis

The involvement of estrogen receptor beta (ERbeta) in prostate carcinogenesis has been hypothesized. Several reports have shown that ERbeta expression was decreased when prostate cells undergo neoplastic transformation, suggesting that it could play a tumor-suppressor role. By restoring ERbeta expression in prostatic carcinoma cells by adenoviral delivery, we aimed to test this hypothesis. We observed that ERbeta strongly inhibited the invasiveness and the growth of these cells. In addition, ERbeta cells were undergoing apoptosis, as shown by quantification of Bax, poly(ADP-ribose) polymerase and caspase-3 expression. Our data suggest that ERbeta acts as a tumor-suppressor by its anti-proliferative, anti-invasive and pro-apoptotic properties.

Estrogen receptor beta inhibits 17beta-estradiol-stimulated proliferation of the breast cancer cell line T47D

Estrogen receptor (ER) beta counteracts the activity of ERalpha in many systems. In agreement with this, we show in this study that induced expression of ERbeta in the breast cancer cell line T47D reduces 17beta-estradiol-stimulated proliferation when expression of ERbeta mRNA equals that of ERalpha. Induction of ERbeta reduces growth of exponentially proliferating cells with a concomitant decrease in components of the cell cycle associated with proliferation, namely cyclin E, Cdc25A (a key regulator of Cdk2), p45(Skp2) (a key regulator of p27(Kip1) proteolysis), and an increase in the Cdk inhibitor p27(Kip1). We also observed a reduced Cdk2 activity. These findings suggest a possible role for ERbeta in breast cancer and imply that ERbeta-specific ligands may reduce proliferation of ER-positive breast cancer cells through actions on the G(1) phase cell-cycle machinery.

Elevated expression of the estrogen receptor prevents the down-regulation of p21Waf1/Cip1 in hormone dependent breast cancer cells

Expression of an estrogen receptor alpha (ER) transgene in hormone independent breast cancer and normal breast epithelial cells arrests cell cycling when estradiol is added. Although endogenously expressed ER does not typically affect estradiol-induced cell cycling of hormone dependent breast cancer cells, we observed that elevated expression of a green fluorescent protein fused to ER (GFP-ER) hindered entry of estrogen treated MCF-7 cells into S phase of the cell cycle. In analyses of key cell-cycle regulating proteins, we observed that GFP-ER expression had no affect on the protein levels of cyclin D1, cyclin E, or p27, a cyclin dependent kinase (Cdk) inhibitor. However, at 24 h, p21 (Waf1, Cip1; a Cdk2 inhibitor) protein remained elevated in the high GFP-ER expressing cells but not in non-GFP-ER expressing cells. Elevated expression of p21 inhibited Cdk2 activity, preventing cells from entering S phase. The results show that elevated levels of ER prevented the down-regulation of p21 protein expression, which is required for hormone responsive cells to enter S phase.