ERα Target Genes Research Articles

Abstract 227: Estrogen receptor-α and tumor initiating cells are novel targets of diallyl trisulfide in breast cancer cells

Abstract Epidemiological studies suggest a protective role for Allium vegetables, such as garlic, against breast and other cancers. Diallyl trisulfide (DATS) is a promising oil-soluble anticancer metabolite generated after cutting or chewing of Allium vegetables. We have shown previously that DATS inhibits viability of human breast cancer cells (MCF-7 and MDA-MB-231) and a cell line derived from spontaneously developed tumor of a mouse mammary tumor virus-neu mouse by causing apoptosis induction. We also found that a non-tumorigenic normal human mammary epithelial cell line (MCF-10A) is resistant to cell death induction by DATS. Studies by others have demonstrated in vivo efficacy of DATS against MCF-7 xenografts in nude mice. The present study builds upon these observations and identifies novel targets of DATS in breast cancer cells. MCF-7 and T47D cells exposed to DATS exhibited downregulation of estrogen receptor-α (ER-α) protein, which peaked between 12 and 24 h post-treatment. Diallyl sulfide and diallyl disulfide analogs were relatively less effective in suppressing protein level of ER-α compared with DATS. The 17β-estradiol (E2)-induced expression of pS2, an ER-α target gene product, was also decreased in the presence of DATS. Downregulation of ER-α protein expression resulting from DATS treatment was accompanied by a decrease in nuclear level of ER-α, suppression of ER-α mRNA, and inhibition of ERE2e1b-luciferase reporter activity. Moreover, apoptotic cell death induced by DATS was partially but statistically significantly attenuated in the presence of E2 in MCF-7 cells as well as by overexpression of ER-α in MDA-MB-231 cells. DATS treatment inhibited breast cancer stem cell (bCSC) population as evidenced by flow cytometric analysis of CD44high/CD24low/epithelial specific antigen positive population, aldehyde dehydrogenase 1 activity, and mammosphere frequency in MCF-7 and SUM159 cells. Inhibition of bCSC by treatment with DATS was associated with downregulation of EZH2 and Kruppel-like factor 4 proteins in both MCF-7 and SUM159 cells. In conclusion, the present study identifies ER-α and bCSC as novel targets of DATS in mammary cancer cells. This study was supported by the grant RO1 CA113363-09 awarded by the National Cancer Institute. Citation Format: Eun-Ryeong Hahm, Su-Hyeong Kim, Shivendra V. Singh. Estrogen receptor-α and tumor initiating cells are novel targets of diallyl trisulfide in breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 227. doi:10.1158/1538-7445.AM2014-227

Abstract 4048: GFRα1 is required for GDNF-induced viability, migration, and signaling through RET in breast cancer cells

Abstract Hormone resistance is a major challenge in treating breast cancer patients. Although anti-estrogens, such as tamoxifen, are initially successful controlling disease in estrogen receptor alpha positive (ERα+) tumors, patients with metastatic disease and as many as 40 % of patients receiving adjuvant tamoxifen acquire resistance to treatment, relapse, and die. The receptor tyrosine kinase, RET, its co-receptor, GFRα1, and their ligand, GDNF, are proteins that are emerging as potential contributors to hormone resistance in breast cancer. RET and GFRα1 were recently shown to be overexpressed in a subset of ERα+ breast cancer tumors, and GFRα1 expression was associated with lymphovascular invasion. In vitro, silencing RET with siRNA increased MCF7 cells' sensitivity to tamoxifen and, in the absence of estrogens, treatment of MCF7 cells with GDNF induced upregulation of ERα target genes. Additionally, GDNF stimulation of MCF7 cells enhanced proliferation, survival, and cell scattering. Together, clinical and preclinical data suggest that this pathway plays an important role in driving breast cancer invasion, metastasis, and hormone resistance. However, the role of GFRα1 has not been examined in tumor cell invasion and metastasis. In this study we tested the functional consequences of silencing GFRα1 in ERα + MCF7 cells on tumor cell migration, invasion, and RET signaling. We showed that targeted reduction of GFRα1 significantly inhibited GDNF-induced migration in both a transwell (p=<0.001) and scratch wound assay (p=<0.001 at 24 hours). In addition, GDNF-induced increases in viability were significantly decreased when MCF7 cells were treated with neutralizing antibodies against GFRα1 (p=<0.001). Silencing GFRα1 in GDNF-treated MCF7 cells led to reduced invasion through collagen I, although this difference was not significant. Lastly, siRNA targeted reduction of GFRα1 in MCF7 cells inhibited GDNF-induced phosphorylation of RET. Together the data demonstrate that GFRα1 is important in migration and cell viability as well as RET signaling in breast cancer cells. Citation Format: Catherine A. Ferrante, Nikki DeAngelis, Raluca Verona. GFRα1 is required for GDNF-induced viability, migration, and signaling through RET in breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4048. doi:10.1158/1538-7445.AM2014-4048

The transcriptional co-repressor TLE3 suppresses basal signaling on a subset of estrogen receptor α target genes.

Chromatin constitutes a repressive barrier to the process of ligand-dependent transcriptional activity of nuclear receptors. Nucleosomes prevent the binding of estrogen receptor α (ERα) in absence of ligand and thus represent an important level of transcriptional regulation. Here, we show that in breast cancer MCF-7 cells, TLE3, a co-repressor of the Groucho/Grg/TLE family, interacts with FoxA1 and is detected at regulatory elements of ERα target genes in absence of estrogen. As a result, the chromatin is maintained in a basal state of acetylation, thus preventing ligand-independent activation of transcription. In absence of TLE3, the basal expression of ERα target genes induced by E2 is increased. At the TFF1 gene, the recruitment of TLE3 to the chromatin is FoxA1-dependent and prevents ERα and RNA polymerase II recruitment to TFF1 gene regulatory elements. Moreover, the interaction of TLE3 with HDAC2 results in the maintenance of acetylation at a basal level. We also provide evidence that TLE3 is recruited at several other regulatory elements of ERα target genes and is probably an important co-regulator of the E2 signaling pathway. In sum, our results describe a mechanism by which TLE3 affects ligand dependency in ERα-regulated gene expression via its binding restricting function and its role in gene regulation by histone acetylation.

Modification of ASC1 by UFM1 Is Crucial for ERα Transactivation and Breast Cancer Development

Biological roles for UFM1, a ubiquitin-like protein, are largely unknown, and therefore we screened for targets of ufmylation. Here we show that ufmylation of the nuclear receptor coactivator ASC1 is a key step for ERα transactivation in response to 17β-estradiol (E2). In the absence of E2, the UFM1-specific protease UfSP2 was bound to ASC1, which maintains ASC1 in a nonufmylated state. In the presence of E2, ERα bound ASC1 and displaced UfSP2, leading to ASC1 ufmylation. Polyufmylation of ASC1 enhanced association of p300, SRC1, and ASC1 at promoters of ERα target genes. ASC1 overexpression or UfSP2 knockdown promoted ERα-mediated tumor formation in vivo, which could be abrogated by treatment with the anti-breast cancer drug tamoxifen. In contrast, expression of ufmylation-deficient ASC1 mutant or knockdown of the UFM1-activating E1 enzyme UBA5 prevented tumor growth. These findings establish a role for ASC1 ufmylation in breast cancer development by promoting ERα transactivation.

Low levels of 3,3'-diindolylmethane activate estrogen receptor α and induce proliferation of breast cancer cells in the absence of estradiol.

Background3,3′-diindolylmethane (DIM) is an acid-catalyzed dimer of idole-3-carbinol (I3C), a phytochemical found in cruciferous vegetables that include broccoli, Brussels sprouts and cabbage. DIM is an aryl hydrocarbon receptor (AhR) ligand and a potential anticancer agent, namely for the treatment of breast cancer. It is also advertised as a compound that regulates sex hormone homeostasis.MethodsHere we make use of RNA expression assays coupled to Chromatin Immunoprecipitation (ChIP) in breast cancer cell lines to study the effect of DIM on estrogen signaling. We further make use of growth assays, as well as fluorescence-activated cell sorting (FACS) assays, to monitor cell growth.ResultsIn this study, we report that ‘physiologically obtainable’ concentrations of DIM (10 μM) activate the estrogen receptor α (ERα) signaling pathway in the human breast cancer cell lines MCF7 and T47D, in a 17β-estradiol (E2)-independent manner. Accordingly, we observe induction of ERα target genes such as GREB1 and TFF1, and an increase in cellular proliferation after treatment with 10 μM DIM in the absence of E2. By using an ERα specific inhibitor (ICI 182 780), we confirm that the transcriptional and proliferative effects of DIM treatment are mediated by ERα. We further show that the protein kinase A signaling pathway participates in DIM-mediated activation of ERα. In contrast, higher concentrations of DIM (e.g. 50 μM) have an opposite and expected effect on cells, which is to inhibit proliferation.ConclusionsWe document an unexpected effect of DIM on cell proliferation, which is to stimulate growth by inducing the ERα signaling pathway. Importantly, this proliferative effect of DIM happens with potentially physiological concentrations that can be provided by the diet or by taking caplet supplements.

Tristetraprolin Represses Estrogen Receptor α Transactivation in Breast Cancer Cells

Estrogen receptor α (ERα) mediates the effects of 17β-estradiol (E2) in normal mammary gland, and it is a key participant in breast cancer tumor development. ERα transactivation activity is mediated by the synergistic interaction of two domains designated AF1 and AF2. The function of AF2 is to recruit coactivator and corepressor proteins that allow ERα to oscillate between the roles of transcriptional activator and repressor. In contrast, the mechanism responsible for AF-1 transcriptional activity is not completely understood. In this study, we identified tristetraproline (TTP) as a novel ERα-associated protein. TTP expression in MCF7 cells repressed ERα transactivation and reduced MCF7 cell proliferation and the ability of the cells to form tumors in a mouse model. We show that TTP transcriptional activity is mediated through its recruitment to the promoter region of ERα target genes and its interaction with histone deacetylases, in particular with HDAC1. TTP expression attenuates the coactivating activity of SRC-1, suggesting that exchange between TTP and other coactivators may play an important role in fine-tuning ERα transactivation. These results indicate that TTP acts as a bona fide ERα corepressor and suggest that this protein may be a contributing factor in the development of E2-dependent tumors in breast cancer.

SIP1/NHERF2 enhances estrogen receptor alpha transactivation in breast cancer cells.

The estrogen receptor alpha (ERα) is a ligand-activated transcription factor that possesses two activating domains designated AF-1 and AF-2 that mediate its transcriptional activity. The role of AF-2 is to recruit coregulator protein complexes capable of modifying chromatin condensation status. In contrast, the mechanism responsible for the ligand-independent AF-1 activity and for its synergistic functional interaction with AF-2 is unclear. In this study, we have identified the protein Na+/H+ Exchanger RegulatoryFactor 2 (NHERF2) as an ERα-associated coactivator that interacts predominantly with the AF-1 domain of the nuclear receptor. Overexpression of NHERF2 in breast cancer MCF7 cells produced an increase in ERα transactivation. Interestingly, the presence of SRC-1 in NHERF2 stably overexpressing MCF7 cells produced a synergistic increase in ERα activity. We show further that NHERF2 interacts with ERα and SRC-1 in the promoter region of ERα target genes. The binding of NHERF2 to ERα in MCF7 cells increased cell proliferation and the ability of MCF7 cells to form tumors in a mouse model. We analyzed the expression of NHERF2 in breast cancer tumors finding a 2- to 17-fold increase in its mRNA levels in 50% of the tumor samples compared to normal breast tissue. These results indicate that NHERF2 is a coactivator of ERα that may participate in the development of estrogen-dependent breast cancer tumors.

Flightless I (Drosophila) homolog facilitates chromatin accessibility of the estrogen receptor α target genes in MCF-7 breast cancer cells

The coordinated activities of multiple protein complexes are essential to the remodeling of chromatin structure and for the recruitment of RNA polymerase II (Pol II) to the promoter in order to facilitate the initiation of transcription in nuclear receptor-mediated gene expression. Flightless I (Drosophila) homolog (FLII), a nuclear receptor coactivator, is associated with the SWI/SNF-chromatin remodeling complex during estrogen receptor (ER)α-mediated transcription. However, the function of FLII in estrogen-induced chromatin opening has not been fully explored. Here, we show that FLII plays a critical role in establishing active histone modification marks and generating the open chromatin structure of ERα target genes. We observed that the enhancer regions of ERα target genes are heavily occupied by FLII, and histone H3K4me3 and Pol II binding induced by estrogen are decreased in FLII-depleted MCF-7 cells. Furthermore, formaldehyde-assisted isolation of regulatory elements (FAIRE)-quantitative polymerase chain reaction (qPCR) experiments showed that depletion of FLII resulted in reduced chromatin accessibility of multiple ERα target genes. These data suggest FLII as a key regulator of ERα-mediated transcription through its role in regulating chromatin accessibility for the binding of RNA Polymerase II and possibly other transcriptional coactivators.

Diallyl trisulfide inhibits estrogen receptor-α activity in human breast cancer cells

Organosulfur compounds from garlic effectively inhibit growth of transplanted as well as spontaneous cancers in preclinical animal models without any adverse side effects. However, the mechanisms underlying anticancer effect of this class of compounds are not fully understood. This study reports, for the first time, that garlic organosulfide diallyl trisulfide (DATS) inhibits estrogen receptor-α (ER-α) activity in human breast cancer cells. Exposure of MCF-7 and T47D cells to DATS resulted in downregulation of ER-α protein, which peaked between 12- and 24-h post-treatment. DATS was relatively more effective in suppressing ER-α protein expression compared with its mono and disulfide analogs. The 17β-estradiol (E2)-induced expression of pS2 and cyclin D1, ER-α target gene products, was also decreased in the presence of DATS. Downregulation of ER-α protein expression resulting from DATS treatment was accompanied by a decrease in nuclear levels of ER-α protein, ER-α mRNA suppression, and inhibition of ERE2e1b-luciferase reporter activity. DATS-mediated inhibition of cell viability and apoptosis induction were not affected in the presence of E2. In agreement with these results, ectopic expression of ER-α in MDA-MB-231 cell line failed to confer any protection against cell proliferation inhibition or apoptosis induction resulting from DATS exposure. DATS treatment caused a decrease in protein levels of peptidyl-prolyl cis-trans isomerase (Pin1), and overexpression of Pin1 partially attenuated ER-α downregulation by DATS. DATS-induced apoptosis was modestly but significantly augmented by overexpression of Pin1. In conclusion, this study identifies ER-α as a novel target of DATS in mammary cancer cells.

A FluoPol-ABPP PAD2 High-Throughput Screen Identifies the First Calcium Site Inhibitor Targeting the PADs

The protein arginine deiminases (PADs) catalyze the post-translational hydrolysis of peptidyl-arginine to form peptidyl-citrulline in a process termed deimination or citrullination. PADs likely play a role in the progression of a range of disease states because dysregulated PAD activity is observed in a host of inflammatory diseases and cancer. For example, recent studies have shown that PAD2 activates ERα target gene expression in breast cancer cells by citrullinating histone H3 at ER target promoters. To date, all known PAD inhibitors bind directly to the enzyme active site. PADs, however, also require calcium ions to drive a conformational change between the inactive apo-state and the fully active calcium bound holoenzyme, suggesting that it would be possible to identify inhibitors that bind the apoenzyme and prevent this conformational change. As such, we set out to develop a screen that can identify PAD2 inhibitors that bind to either the apo or calcium bound form of PAD2. Herein, we provide definitive proof of concept for this approach and report the first PAD inhibitor, ruthenium red (Ki of 17 μM), to preferentially bind the apoenzyme.

Systems level-based RNAi screening by high content analysis identifies UBR5 as a regulator of estrogen receptor-α protein levels and activity.

Estrogen receptor-α (ERα) is a central transcription factor that regulates mammary gland physiology and a key driver in breast cancer. In the present study, we aimed to identify novel modulators of ERα-mediated transcriptional regulation via a custom-built siRNA library screen. This screen was directed against a variety of coregulators, transcription modifiers, signaling molecules and DNA damage response proteins. By utilizing a microscopy-based, multi-end point, estrogen responsive biosensor cell line platform, the primary screen identified a wide range of factors that altered ERα protein levels, chromatin remodeling and mRNA output. We then focused on UBR5, a ubiquitin ligase and known oncogene that modulates ERα protein levels and transcriptional output. Finally, we demonstrated that UBR5 also affects endogenous ERα target genes and E2-mediated cell proliferation in breast cancer cells. In conclusion, our multi-end point RNAi screen identified novel modulators of ERα levels and activity, and provided a robust systems level view of factors involved in mechanisms of nuclear receptor action and pathophysiology. Utilizing a high throughput RNAi screening approach we identified UBR5, a protein commonly amplified in breast cancer, as a novel regulator of ERα protein levels and transcriptional activity.

The atypical ubiquitin ligase RNF31 stabilizes estrogen receptor α and modulates estrogen-stimulated breast cancer cell proliferation

Estrogen receptor α (ERα) is initially expressed in the majority of breast cancers and promotes estrogen-dependent cancer progression by regulating the transcription of genes linked to cell proliferation. ERα status is of clinical importance, as ERα-positive breast cancers can be successfully treated by adjuvant therapy with antiestrogens or aromatase inhibitors. Complications arise from the frequent development of drug resistance that might be caused by multiple alterations, including components of ERα signaling, during tumor progression and metastasis. Therefore, insights into the molecular mechanisms that control ERα expression and stability are of utmost importance to improve breast cancer diagnostics and therapeutics. Here we report that the atypical E3 ubiquitin ligase RNF31 stabilizes ERα and facilitates ERα-stimulated proliferation in breast cancer cell lines. We show that depletion of RNF31 decreases the number of cells in the S phase and reduces the levels of ERα and its downstream target genes, including cyclin D1 and c-myc. Analysis of data from clinical samples confirms correlation between RNF31 expression and the expression of ERα target genes. Immunoprecipitation indicates that RNF31 associates with ERα and increases its stability and mono-ubiquitination, dependent on the ubiquitin ligase activity of RNF31. Our data suggest that association of RNF31 and ERα occurs mainly in the cytosol, consistent with the lack of RNF31 recruitment to ERα-occupied promoters. In conclusion, our study establishes a non-genomic mechanism by which RNF31 via stabilizing ERα levels controls the transcription of estrogen-dependent genes linked to breast cancer cell proliferation.

Identification of a Novel Compound That Suppresses Breast Cancer Invasiveness by Inhibiting Transforming Growth Factor-β Signaling via Estrogen Receptor α.

Breast cancer is the most frequently diagnosed cancer and the leading cause of death by cancer among females worldwide. An overwhelming majority of these deaths is because of metastasis. Estrogen stimulates and promotes growth of breast tumors, whereas transforming growth factor-beta (TGF-β) signaling promotes invasion and metastasis. We previously reported that estrogen and estrogen receptor alpha (ERα) suppressed breast cancer metastasis by inhibiting TGF-β signaling, whereas antiestrogens that suppress breast cancer growth, such as the selective ER modulator tamoxifen (TAM) or the pure antiestrogen fulvestrant (ICI 182,780), cannot suppress TGF-β signaling or breast cancer invasiveness. Therefore, we predicted that a compound that inhibits TGF-β signaling but does not facilitate ERα signaling would be ideal for suppressing breast cancer invasiveness and growth. In the present study, we identified an ideal candidate compound, N-23. Like estrogen, N-23 strongly decreased expression of TGF-β/Smad target gene plasminogen activator inhibitor-1 (PAI-1), but it did not increase the expression of ERα target gene pS2. While estrogen decreased the levels of phosphorylated Smad2 and Smad3, N-23 had no effect. In addition, TGF-β-dependent recruitment of Smad3 to the PAI-1 gene promoter was inhibited in the presence of estrogen or N-23. We also investigated the effects of N-23 on proliferation, migration, and invasion of breast cancer cells. In contrast to estrogen, N-23 inhibited the cellular proliferation of breast cancer cells. Moreover, we showed that N-23 suppressed the migration and invasion of breast cancer cells to the same extent as by estrogen. Taken together, our findings indicate that N-23 may be a candidate compound that is effective in inhibiting breast cancer progression.

Co-regulated gene expression by oestrogen receptor α and liver receptor homolog-1 is a feature of the oestrogen response in breast cancer cells

Oestrogen receptor α (ERα) is a nuclear receptor that is the driving transcription factor expressed in the majority of breast cancers. Recent studies have demonstrated that the liver receptor homolog-1 (LRH-1), another nuclear receptor, regulates breast cancer cell proliferation and promotes motility and invasion. To determine the mechanisms of LRH-1 action in breast cancer, we performed gene expression microarray analysis following RNA interference for LRH-1. Interestingly, gene ontology (GO) category enrichment analysis of LRH-1–regulated genes identified oestrogen-responsive genes as the most highly enriched GO categories. Remarkably, chromatin immunoprecipitation coupled to massively parallel sequencing (ChIP-seq) to identify genomic targets of LRH-1 showed LRH-1 binding at many ERα binding sites. Analysis of select binding sites confirmed regulation of ERα−regulated genes by LRH-1 through binding to oestrogen response elements, as exemplified by the TFF1/pS2 gene. Finally, LRH-1 overexpression stimulated ERα recruitment, while LRH-1 knockdown reduced ERα recruitment to ERα binding sites. Taken together, our findings establish a key role for LRH-1 in the regulation of ERα target genes in breast cancer cells and identify a mechanism in which co-operative binding of LRH-1 and ERα at oestrogen response elements controls the expression of oestrogen-responsive genes.

Interaction of Glucocorticoid Receptor (GR) with Estrogen Receptor (ER) α and Activator Protein 1 (AP1) in Dexamethasone-mediated Interference of ERα Activity

The role of glucocorticoids in the inhibition of estrogen (17-β-estradiol (E2))-regulated estrogen receptor (ER)-positive breast cancer cell proliferation is well established. We and others have seen that synthetic glucocorticoid dexamethasone (Dex) antagonizes E2-stimulated endogenous ERα target gene expression. However, how glucocorticoids negatively regulate the ERα signaling pathway is still poorly understood. ChIP studies using ERα- and glucocorticoid receptor (GR)-positive MCF-7 cells revealed that GR occupies several ERα-binding regions (EBRs) in cells treated with E2 and Dex simultaneously. Interestingly, there was little or no GR loading to these regions when cells were treated with E2 or Dex alone. The E2+Dex-dependent GR recruitment is associated with the displacement of ERα and steroid receptor coactivator-3 from the target EBRs leading to the repression of ERα-mediated transcriptional activation. The recruitment of GR to EBRs requires assistance from ERα and FOXA1 and is facilitated by AP1 binding within the EBRs. The GR binding to EBRs is mediated via direct protein-protein interaction between the GR DNA-binding domain and ERα. Limited mutational analyses indicate that arginine 488 located within the C-terminal zinc finger domain of the GR DNA-binding domain plays a critical role in stabilizing this interaction. Together, the results of this study unravel a novel mechanism involved in glucocorticoid inhibition of ERα transcriptional activity and E2-mediated cell proliferation and thus establish a foundation for future exploitation of the GR signaling pathway in the treatment of ER-positive breast cancer.

Downregulation of steroid receptor coactivator-2 modulates estrogen-responsive genes and stimulates proliferation of mcf-7 breast cancer cells.

The p160/Steroid Receptor Coactivators SRC-1, SRC-2/GRIP1, and SRC-3/AIB1 are important regulators of Estrogen Receptor alpha (ERα) activity. However, whereas the functions of SRC-1 and SRC-3 in breast tumourigenesis have been extensively studied, little is known about the role of SRC-2. Previously, we reported that activation of the cAMP-dependent protein kinase, PKA, facilitates ubiquitination and proteasomal degradation of SRC-2 which in turn leads to inhibition of SRC-2-coactivation of ERα and changed expression of the ERα target gene, pS2. Here we have characterized the global program of transcription in SRC-2-depleted MCF-7 breast cancer cells using short-hairpin RNA technology, and in MCF-7 cells exposed to PKA activating agents. In order to identify genes that may be regulated through PKA-induced downregulation of SRC-2, overlapping transcriptional targets in response to the respective treatments were characterized. Interestingly, we observed decreased expression of several breast cancer tumour suppressor genes (e.g., TAGLN, EGR1, BCL11b, CAV1) in response to both SRC-2 knockdown and PKA activation, whereas the expression of a number of other genes implicated in cancer progression (e.g., RET, BCAS1, TFF3, CXCR4, ADM) was increased. In line with this, knockdown of SRC-2 also stimulated proliferation of MCF-7 cells. Together, these results suggest that SRC-2 may have an antiproliferative function in breast cancer cells.

Cyclin D1 Determines Estrogen Signaling in the Mammary Gland In Vivo

The CCND1 gene, which is frequently overexpressed in cancers, encodes the regulatory subunit of a holoenzyme that phosphorylates the retinoblastoma protein. Although it is known that cyclin D1 regulates estrogen receptor (ER)α transactivation using heterologous reporter systems, the in vivo biological significance of cyclin D1 to estrogen-dependent signaling, and the molecular mechanisms by which cyclin D1 is involved, are yet to be elucidated. Herein, genome-wide expression profiling conducted of 17β-estradiol-treated castrated virgin mice deleted of the Ccnd1 gene demonstrated that cyclin D1 determines estrogen-dependent gene expression for 88% of estrogen-responsive genes in vivo. In addition, expression profiling of 17β-estradiol-stimulated cyclin D1 small interfering RNA treated MCF7 cells shows cyclin D1 is required for estrogen-mediated gene expression in vitro. Genome-wide chromatin immunoprecipitation-Seq analysis revealed a cyclin D1-DNA bound form associated with genes that were regulated by estrogen in a cyclin D1-dependent manner. The cyclin D1-dependent estrogen signaling pathways identified in vivo were highly enriched for extracellular membrane-associated growth factor receptors (epidermal growth factor receptor, ErbB3, and EphB3) and their ligands (amphiregulin, encoded by AREG gene), and matrix metalloproteinase. The AREG protein, a pivotal ligand for epidermal growth factor receptors to promote cellular proliferation, was induced by cyclin D1 via the AREG promoter. Chromatin immunoprecipitation analysis demonstrated the recruitment of cyclin D1 to the breast cancer 1 (Brca1)/ERα binding site of the Areg gene. Cyclin D1 genetic deletion demonstrated the in vivo requirement for cyclin D1 in assembling the estrogen-dependent amplified in breast cancer 1-associated multiprotein complex. The current studies define a requirement for cyclin D1 in estrogen-dependent signaling modules governing growth factor receptor and ligand expression in vivo and reveal a noncanonical function of cyclin D1 at ERα target gene promoters. Cyclin D1 mediates the convergence of ERα and growth factor signaling at a common cis-element of growth factor genes.

Transcription factor Ets1 cooperates with estrogen receptor α to stimulate estradiol-dependent growth in breast cancer cells and tumors.

The purpose of this study was to explore the role of transcription factor Ets1 in estrogen receptor α (ERα)-positive breast cancer progression. We expressed human Ets1 or empty vector in four human ERα-positive breast cancer cell lines and observed increased colony formation. Further examination of cellular responses in stable Ets1-expressing MCF7 clones displayed increased proliferation, migration, and invasion. Ets1-expressing MCF7 tumors grown in the mammary fat pads of nude mice exhibited increased rates of tumor growth (7.36±2.47 mm3/day) compared to control MCF7 tumors (2.52±1.70 mm3/day), but maintained their dependence on estradiol for tumor growth. Proliferation marker Ki-67 staining was not different between control and Ets1-expressing tumors, but Ets1-expressing tumors exhibited large necrotic centers and elevated apoptotic staining. Ets1 was shown to cooperate with ERα and the p160 nuclear receptor coactivator (NCOA/SRC) family to increase activation of a consensus estrogen response element luciferase reporter construct. Ets1-expressing MCF7 cells also exhibited elevated expression of the ERα target genes, progesterone receptor and trefoil factor 1. Using GST-pulldown assays, Ets1 formed stable complexes containing both ERα and p160 nuclear receptor coactivators. Taken together, these data suggest that the Ets1-dependent estradiol sensitization of breast cancer cells and tumors may be partially due to the ability of Ets1 to cooperate with ERα and nuclear receptor coactivators to stimulate transcriptional activity of estrogen-dependent genes.

Abstract 4759: HOXB7 functions as a co-activator of estrogen receptor in the development of tamoxifen resistance.

Abstract Background: Multiple factors including long term treatment with tamoxifen are involved in the development of selective estrogen receptor modulator (SERM)-resistance in ERα positive breast cancer. Increased expression of HER family members can also directly alter cellular responses to tamoxifen, but the mechanism underlying the increased expression of the HERs is not clear. In this report we show that HOXB7 is an upstream regulator of HER2 and ER expression. Results: Overexpression of HOXB7 results in upregulation of HER2 and ERα target genes, while knockdown of HOXB7 with siRNA in tamoxifen-resistant cell lines causes decrease of HER2 and ERα target genes expression and loss of tamoxifen resistance. To mediate upregulation of HER2 and ER-target genes occurring in tamoxifen resistance, the HOXB7-ERα complex recruits ERα coactivators and promotes HER2- and ER-target genes transcriptional activity through direct binding to the HER2 enhancer and regulatory regions of ER target genes. However, depletion of HOXB7 negatively affects the binding affinity of its cofactors to the HER2 enhancer element and ER-binding sites. Also, it is likely that miR196a controls HOXB7 expression; reducing miR196a levels in tamoxifen-resistant cells causes an increase of HOXB7 expression. Overexpression of miR196a sensitizes MCF-7 tamoxifen resistant cells to tamoxifen through downregulation of HER2 and ER-target genes while inhibition of miR196a by anti-miR196a inhibitors increases tamoxifen resistance through the upregulation of HER and ER target genes in MCF-7 cells. Further, miRNA 196a is regulated by c-MYC which undergoes stabilization through the EGFR-HER2 signaling pathway. Depletion of c-MYC by MYC shRNA enhances tamoxifen sensitivity by increasing the level of miR196a transcripts. Also, by c-MYC ChIP assay, we found that c-MYC inhibits miR196a expression through direct binding to its promoter region. Conclusions: Overexpression of HOXB7 is a key event in the initiation and maintenance of tamoxifen resistance. These studies suggest that HOXB7 acts as a key regulator orchestrating two major groups of target molecules, ERα and HER2, in the oncogene hierarchy. Blocking MYC expression or reexpression of miR196a in TAM-R cells may provide novel strategies for reversing SERM-resistance. Therefore, we have provided additional evidence that supports the thesis that functional antagonism of HOXB7 has the potential to circumvent tamoxifen resistance. Citation Format: Kideok Jin, Wei Wen Teo, Takahiro Yoshida, Sunju Park, Saraswati Sukumar. HOXB7 functions as a co-activator of estrogen receptor in the development of tamoxifen resistance. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4759. doi:10.1158/1538-7445.AM2013-4759

A TR-FRET-based functional assay for screening activators of CARM1.

Epigenetics is an emerging field that demands selective cell-permeable chemical probes to perturb, especially in vivo, the activity of specific enzymes involved in modulating the epigenetic codes. Coactivator-associated arginine methyltransferase 1 (CARM1) is a coactivator of estrogen receptor α (ERα), the main target in human breast cancer. We previously showed that twofold overexpression of CARM1 in MCF7 breast cancer cells increased the expression of ERα-target genes involved in differentiation and reduced cell proliferation, thus leading to the hypothesis that activating CARM1 by chemical activators might be therapeutically effective in breast cancer. Selective, potent, cell-permeable CARM1 activators will be essential to test this hypothesis. Here we report the development of a cell-based, time-resolved (TR) FRET assay that uses poly(A) binding protein 1 (PABP1) methylation to monitor cellular activity of CARM1. The LanthaScreen TR-FRET assay uses MCF7 cells expressing GFP-PABP1 fusion protein through BacMam gene delivery system, methyl-PABP1 specific antibody, and terbium-labeled secondary antibody. This assay has been validated as reflecting the expression and/or activity of CARM1 and optimized for high throughput screening to identify CARM1 allosteric activators. This TR-FRET platform serves as a generic tool for functional screening of cell-permeable, chemical modulators of CARM1 for elucidation of its in vivo functions.