Coregulators in nuclear estrogen receptor action: from concept to therapeutic targeting

Profile of bazedoxifene/conjugated estrogens for the treatment of estrogen deficiency symptoms and osteoporosis in women at risk of fracture

Decreasing levels of estrogens during menopause are associated with reduced bone density and an increased risk of osteoporosis. Many women also experience bothersome vasomotor and vaginal symptoms during the menopausal transition. Results of systematic reviews and meta-analyses of randomized controlled trials have shown that both systemic estrogen therapy or hormone therapy (estrogen combined with a progestin) are useful to prevent bone loss, and they are the most effective treatment for such climacteric symptoms as hot flushes, sweating, vaginal dryness, and dyspareunia. Unfortunately, estrogen therapy and hormone therapy increase the risk of endometrial and breast cancer, respectively. The selective estrogen receptor modulators (SERMs) result in positive estrogenic effects on bone, with no negative effects on the endometrium and breast but do not provide relief from postmenopausal symptoms. The combination of a SERM with estrogen as a tissue selective estrogen complex (TSEC) is a new strategy for the prevention of bone loss and the treatment of climacteric symptoms. This combination is particularly interesting from a clinical point of view, taking into account that estrogen alone did not increase breast cancer risk by the Women’s Health Initiative. TSEC is hypothesized to provide the benefits of estrogen-alone therapy, with an improved tolerability profile because the SERM component can make possible the elimination of progestin. The objective of this review was to critically evaluate the evidence from the reports published to date on the use of bazedoxifene (a third-generation SERM) in combination with conjugated estrogens in postmenopausal women. The conclusion is that effectively, the combination of bazedoxifene and conjugated estrogens may be a promising alternative to hormone therapy for the prevention of osteoporosis and the treatment of postmenopausal symptoms in non-hysterectomized postmenopausal women.

Estrogen receptor α/β-cofactor motif interactions; interplay of tyrosine 537/488 phosphorylation and LXXLL motifs

The Estrogen Receptors ERα and ERβ bind cofactor proteins via short LXXLL motifs. The exact regulation and selectivity of these interactions remains an open question and the role of post-translational modifications (PTMs) is virtually unexplored. Here, we designed an X(7)-LXXLL-X(7) T7 phage display library and screened this against four ER protein constructs: the 'naked' ERα and ERβ Ligand Binding Domains (LBDs) and the tyrosine phosphorylated ERα (pY537) and ERβ (pY488) LBDs. The site-selective tyrosine phosphorylated protein constructs were obtained via a protein semi-synthesis approach. Phage display screening yielded preferential sets of peptides. LXXLL peptides with a low pI/acidic C-terminus prefer binding to the naked ERβ over the phosphorylated ERβ analogue and ERα constructs. Peptides with a high pI/basic C-terminus show the opposite behaviour. These findings not only show regulation of the ERβ-cofactor interaction via tyrosine phosphorylation, but also suggest that ERβ and its tyrosine 488 phosphorylation play crucial roles in modulating interactions of coactivators to ERα since the natural Steroid Receptor Coactivators (SRCs) feature LXXLL motifs with acidic C-termini, while the repressor protein RIP140 features LXXLL motifs with basic C-termini. This insight provides explanation for ER transcriptional activity and can lead to more focussed targeting of the ER-coactivator interaction.

Endocrine-Disrupting Chemicals (EDCs): In Vitro Mechanism of Estrogenic Activation and Differential Effects on ER Target Genes

BACKGROUND

Endocrine-disrupting chemicals (EDCs) influence the activity of estrogen receptors (ERs) and alter the function of the endocrine system. However, the diversity of EDC effects and mechanisms of action are poorly understood.

OBJECTIVES

We examined the agonistic activity of EDCs through ERα and ERβ. We also investigated the effects of EDCs on ER-mediated target genes.

METHODS

HepG2 and HeLa cells were used to determine the agonistic activity of EDCs on ERα and ERβ via the luciferase reporter assay. Ishikawa cells stably expressing ERα were used to determine changes in endogenous ER target gene expression by EDCs.

RESULTS

Twelve EDCs were categorized into three groups on the basis of product class and similarity of chemical structure. As shown by luciferase reporter analysis, the EDCs act as ER agonists in a cell type- and promoter-specific manner. Bisphenol A, bisphenol AF, and 2-2-bis(p-hydroxyphenyl)-1,1,1-trichloroethane (group 1) strongly activated ERα estrogen responsive element (ERE)-mediated responses. Daidzein, genistein, kaempferol, and coumestrol (group 2) activated both ERα and ERβ ERE-mediated activities. Endosulfan and kepone (group 3) weakly activated ERα. Only a few EDCs significantly activated the "tethered" mechanism via ERα or ERβ. Results of real-time polymerase chain reaction indicated that bisphenol A and bisphenol AF consistently activated endogenous ER target genes, but the activities of other EDCs on changes of ER target gene expression were compound specific.

CONCLUSION

Although EDCs with similar chemical structures (in the same group) tended to have comparable ERα and ERβ ERE-mediated activities, similar chemical structure did not correlate with previously reported ligand binding affinities of the EDCs. Using ERα-stable cells, we observed that EDCs differentially induced activity of endogenous ER target genes.

The molecular mechanisms underlying the pharmacological actions of estrogens, SERMs and oxysterols: implications for the treatment and prevention of osteoporosis

Estrogen therapy and hormone therapy are effective options for the prevention and treatment of osteoporosis, although because of their significant side effect profile, long term use for these applications is not recommended. Whereas SERMs (Selective Estrogen Receptor Modulators) exhibit a more favorable side effect profile, the currently available medicines in this class are substantially less effective in bone than classical estrogens. However, the results of substantial efforts that have gone into defining the mechanisms that underlie the pharmacology of estrogens, antiestrogens and SERMs have informed the development of the next generation of SERMs and have led to the development of TSECs (Tissue Selective Estrogen Complexes), a new class of ER-modulator. Further, the recent determination that the oxysterol 27-hydroxycholesterol functions as an endogenous SERM has highlighted an unexpected link between hypercholesterolemia and bone biology and must be considered in any discussions of ER-pharmacology. This review considers the most recent progress in our understanding of ER pharmacology and how this has and will be translated into new medicines for the treatment and prevention of osteoporosis.

The underappreciated role of allostery in the cellular network

Allosteric propagation results in communication between distinct sites in the protein structure; it also encodes specific effects on cellular pathways, and in this way it shapes cellular response. One example of long-range effects is binding of morphogens to cell surface receptors, which initiates a cascade of protein interactions that leads to genome activation and specific cellular action. Allosteric propagation results from combinations of multiple factors, takes place through dynamic shifts of conformational ensembles, and affects the equilibria of macromolecular interactions. Here, we (a) emphasize the well-known yet still underappreciated role of allostery in conveying explicit signals across large multimolecular assemblies and distances to specify cellular action; (b) stress the need for quantitation of the allosteric effects; and finally, (c) propose that each specific combination of allosteric effectors along the pathway spells a distinct function. The challenges are colossal; the inspiring reward will be predicting function, misfunction, and outcomes of drug regimes.

β-arrestin-selective G protein-coupled receptor agonists engender unique biological efficacy in vivo

Biased G protein-coupled receptor agonists are orthosteric ligands that possess pathway-selective efficacy, activating or inhibiting only a subset of the signaling repertoire of their cognate receptors. In vitro, D-Trp(12),Tyr(34)-bPTH(7-34) [bPTH(7-34)], a biased agonist for the type 1 PTH receptor, antagonizes receptor-G protein coupling but activates arrestin-dependent signaling. In vivo, both bPTH(7-34) and the conventional agonist hPTH(1-34) stimulate anabolic bone formation. To understand how two PTH receptor ligands with markedly different in vitro efficacy could elicit similar in vivo responses, we analyzed transcriptional profiles from calvarial bone of mice treated for 8 wk with vehicle, bPTH(7-34) or hPTH(1-34). Treatment of wild-type mice with bPTH(7-34) primarily affected pathways that promote expansion of the osteoblast pool, notably cell cycle regulation, cell survival, and migration. These responses were absent in β-arrestin2-null mice, identifying them as downstream targets of β-arrestin2-mediated signaling. In contrast, hPTH(1-34) primarily affected pathways classically associated with enhanced bone formation, including collagen synthesis and matrix mineralization. hPTH(1-34) actions were less dependent on β-arrestin2, as might be expected of a ligand capable of G protein activation. In vitro, bPTH(7-34) slowed the rate of preosteoblast proliferation, enhanced osteoblast survival when exposed to an apoptotic stimulus, and stimulated cell migration in wild-type, but not β-arrestin2-null, calvarial osteoblasts. These results suggest that bPTH(7-34) and hPTH(1-34) affect bone mass in vivo through predominantly separate genomic mechanisms created by largely distinct receptor-signaling networks and demonstrate that functional selectivity can be exploited to change the quality of G protein-coupled receptor efficacy.

A broad view of scaffolding suggests that scaffolding proteins can actively control regulation and signaling of multienzyme complexes through allostery

Enzymes often work sequentially in pathways; and consecutive reaction steps are typically carried out by molecules associated in the same multienzyme complex. Localization confines the enzymes; anchors them; increases the effective concentration of substrates and products; and shortens pathway timescales; however, it does not explain enzyme coordination or pathway branching. Here, we distinguish between metabolic and signaling multienzyme complexes. We argue for a central role of scaffolding proteins in regulating multienzyme complexes signaling and suggest that metabolic multienzyme complexes are less dependent on scaffolding because they undergo conformational control through direct subunit-subunit contacts. In particular, we propose that scaffolding proteins have an essential function in controlling branching in signaling pathways. This new broadened definition of scaffolding proteins goes beyond cases such as the classic yeast mitogen-activated protein kinase Ste5 and encompasses proteins such as E3 ligases which lack active sites and work via allostery. With this definition, we classify the mechanisms of multienzyme complexes based on whether the substrates are transferred through the involvement of scaffolding proteins, and outline the functional merits to metabolic or signaling pathways. Overall, while co-localization topography helps multistep pathways non-specifically, allosteric regulation requires precise multienzyme organization and interactions and works via population shift, either through direct enzyme subunit-subunit interactions or through active involvement of scaffolding proteins. This article is part of a Special Issue entitled: The emerging dynamic view of proteins: Protein plasticity in allostery, evolution and self-assembly.

FOXP1 and estrogen signaling in breast cancer

Breast cancers are considered to be primarily regulated by estrogen signaling pathways because estrogen-dependent proliferation is observed in the majority of breast cancer cases. Thus, hormone therapy using antiestrogen drugs such as tamoxifen is effective for breast cancers expressing estrogen receptor α (ERα). However, acquired resistance during the endocrine therapy is a critical unresolved problem in breast cancer. Recently, a forkhead transcription factor FOXA1 has been reported to play an important role in the regulation of ERα-mediated transcription and proliferation of breast cancer. Interestingly, immunohistochemical analysis of breast cancer specimens has revealed that nuclear immunoreactivities of FOXP1 as well as those of FOXA1 are positively correlated with hormone receptor status, including ERα and progesterone receptor. In particular, the double-positive immunoreactivities of FOXP1 and FOXA1 are significantly associated with a favorable prognosis for survival of breast cancer patients receiving adjuvant tamoxifen therapy. The functions of FOXP1 and FOXA1 have been characterized in cultured cells; further, similar to FOXA1, FOXP1 is assumed to be a critical transcription factor for ERα signaling, and both forkhead transcription factors can serve as predictive factors for acquired endocrine resistance in breast cancer.

Exposure to bisphenol A results in a decline in mouse spermatogenesis

Bisphenol A (BPA), a chemical used in many consumer products, interferes with the endocrine system of mammals, including humans. The aim of the present study was to investigate the effect of BPA on spermatogenesis and semen quality. The objective of this study was to assess the effects of BPA on mouse spermatogenesis. CD1 mice were used in all experiments. Mice were treated with different doses of BPA (0, 20 and 40 μg kg–1 day–1from postnatal Day (PND) 3 to PND21, PND 35 or PND49. After 5 weeks BPA treatment, oestrogen receptor α expression was increased in mouse testis, whereas the meiotic progression of germ cells was slowed. Thus, both the quality and quantity of spermatozoa were decreased in 7-week-old mice. However, BPA had no effect on DNA methylation of imprinted genes such as Igf2, Igf2r, Peg3 and H19, in germ cells. In addition, exposure of male mice to BPA resulted in abnormal offspring that were smaller with a low-quality pelage when they were 35 days old. In conclusion, BPA hampers spermatogenesis and the subsequent development of offspring.

Differential regulation of neuregulin 1 expression by progesterone in astrocytes and neurons

Glial-neuronal interactions are crucial processes in neuromodulation and synaptic plasticity. The neuregulin 1 family of growth and differentiation factors have been implicated as bidirectional signaling molecules that are involved in mediating some of these interactions. We have shown previously that neuregulin 1 expression is regulated by the gonadal hormones progesterone and 17beta-estradiol in the CNS, which might represent a novel, indirect mechanism of the neuromodulatory actions of these gonadal hormones. In the present study, we sought to determine the effects of progesterone and 17beta-estradiol on neuregulin 1 expression in rat cortical astrocytes and neurons in vitro. We observed that progesterone increased the expression of neuregulin 1 mRNA and protein in a dose-dependent manner in cultured astrocytes, which was blocked by the progesterone receptor antagonist RU-486. In contrast, 17beta-estradiol did not increase either neuregulin 1 mRNA or protein in astrocytes. We observed no effect of either progesterone or 17beta-estradiol on neuregulin 1 mRNA and protein in rat cortical neurons in vitro. Finally, we observed that treatment of cortical neurons with recombinant NRG1-beta1 caused PSD-95 to localize in puncta similar to that observed following treatment with astrocyte-conditioned medium. These results demonstrate that progesterone regulates neuregulin 1 expression, principally in astrocytes. This might represent a novel mechanism of progesterone-mediated modulation of neurotransmission through the regulation of astrocyte-derived neuregulin 1.

Current concepts and significance of estrogen receptor β in prostate cancer

An increasing amount of evidence points at important roles for estrogen receptors in prostate carcinogenesis and progression. Of the two estrogen receptors, estrogen receptor β is the most prominent within the prostate gland. Although there is much yet to be known, the findings from the discovery of the receptor in 1996 until now point at a role of the receptor in maintaining differentiation and reducing cellular proliferation in the prostate. Moreover, estrogen receptor β is the main target for phytoestrogens, perhaps at least partially explaining the difference in incidence of prostate cancer in the Western world compared to Asia where the intake of soy-based, phytoestrogen-rich food is higher. The tumor suppressive capability of estrogen receptor β makes it a promising drug target for the treatment and prevention of prostate cancer. This review will focus on different aspects of estrogen receptor signaling and prostate cancer.

PELP1/MNAR suppression inhibits proliferation and metastasis of endometrial carcinoma cells

Proline-, glutamic acid- and leucine-rich protein-1/modulator of non-genomic activity of estrogen receptor (ER) (PELP1/MNAR) is a novel nuclear receptor (NR) co-activator that plays an essential role in the actions of ER. Emerging findings suggest that PELP1/MNAR is a novel proto-oncogene, whose expression is deregulated in several hormone-responsive cancers, including endometrial cancer. In this study, we demonstrate that PELP1/MNAR is widely expressed in endometrial carcinoma cell lines. To investigate its possible role in endometrial carcinoma progression, we adopted an RNA interference technology to downregulate PELP1/MNAR expression in Ishikawa endometrial carcinoma cells. PELP1/MNAR downregulation substantially reduced cell proliferation, and the cells in which PELP1/MNAR expression was knocked down also exhibited a decreased migration and invasion ability, as shown by Boyden chamber and invasion assays. The results showed that the expression of MMP-2 and MMP-9 was also decreased. These results suggest that PELP1/MNAR plays a role in endometrial cancer progression and metastasis, and that PELP1/MNAR may be a potential therapeutic target for endometrial cancer.

Genetic variants in ER cofactor genes and endometrial cancer risk

Given that the transcriptional regulatory activity of estrogen receptor (ER) is modulated by its biochemical cofactors, genetic variation within the ER cofactor genes may alter cellular response to estrogen exposure and consequently modify the risk for endometrial cancer. We genotyped 685 tagging SNPs within 60 ER cofactor genes in 564 endometrial cancer cases and 1,510 controls from Sweden, and tested their associations with the risk of endometrial cancer. We investigated the associations of individual SNPs by using a trend test as well as multiple SNPs within a gene or gene complex by using multi-variant association analysis. No significant association was observed for any individual SNPs or genes, but a marginal association of the cumulative genetic variation of the NCOA2 complex as a whole (NCOA2, CARM1, CREBBP, PRMT1 and EP300) with endometrial cancer risk was observed (P_adjusted = 0.033). However, the association failed to be replicated in an independent European dataset of 1265 cases and 5190 controls (P = 0.71). The results indicate that common genetic variants within ER cofactor genes are unlikely to play a significant role in endometrial cancer risk in European population.

Differential estrogenic actions of endocrine-disrupting chemicals bisphenol A, bisphenol AF, and zearalenone through estrogen receptor α and β in vitro

BACKGROUND

Endocrine-disrupting chemicals (EDCs) are widely found in the environment. Estrogen-like activity is attributed to EDCs, such as bisphenol A (BPA), bisphenol AF (BPAF), and zearalenone (Zea), but mechanisms of action and diversity of effects are poorly understood.

OBJECTIVES

We used in vitro models to evaluate the mechanistic actions of BPA, BPAF, and Zea on estrogen receptor (ER) α and ERβ.

METHODS

We used three human cell lines (Ishikawa, HeLa, and HepG2) representing three cell types to evaluate the estrogen promoter activity of BPA, BPAF, and Zea on ERα and ERβ. Ishikawa/ERα stable cells were used to determine changes in estrogen response element (ERE)-mediated target gene expression or rapid action-mediated effects.

RESULTS

The three EDCs showed strong estrogenic activity as agonists for ERα in a dose-dependent manner. At lower concentrations, BPA acted as an antagonist for ERα in Ishikawa cells and BPAF acted as an antagonist for ERβ in HeLa cells, whereas Zea was only a partial antagonist for ERα. ERE-mediated activation by BPA and BPAF was via the AF-2 function of ERα, but Zea activated via both the AF-1 and AF-2 functions. Endogenous ERα target genes and rapid signaling via the p44/42 MAPK pathway were activated by BPA, BPAF, and Zea.

CONCLUSION

BPA and BPAF can function as EDCs by acting as cell type-specific agonists (≥ 10 nM) or antagonists (≤ 10 nM) for ERα and ERβ. Zea had strong estrogenic activity and activated both the AF-1 and AF-2 functions of ERα. In addition, all three compounds induced the rapid action-mediated response for ERα.

Influence of berry polyphenols on receptor signaling and cell-death pathways: implications for breast cancer prevention

Breast cancer is the most commonly diagnosed cancer among women worldwide. Many women have become more aware of the benefits of increasing fruit consumption, as part of a healthy lifestyle, for the prevention of cancer. The mechanisms by which fruits, including berries, prevent breast cancer can be partially explained by exploring their interactions with pathways known to influence cell proliferation and evasion of cell-death. Two receptor pathways, estrogen receptor (ER) and tyrosine kinase receptors, especially the epidermal growth factor receptor (EGFR) family, are drivers of cell proliferation and play a significant role in the development of both primary and recurrent breast cancer. There is strong evidence to show that several phytochemicals present in berries such as cyanidin, delphinidin, quercetin, kaempferol, ellagic acid, resveratrol, and pterostilbene interact with and alter the effects of these pathways. Furthermore, they also induce cell death (apoptosis and autophagy) via their influence on kinase signaling. This review summarizes in vitro data regarding the interaction of berry polyphenols with the specific receptors and the mechanisms by which they induce cell death. This paper also presents in vivo data of primary breast cancer prevention by individual compounds and whole berries. Finally, a possible role for berries and berry compounds in the prevention of breast cancer and a perspective on the areas that require further research are presented.

Association of double-positive FOXA1 and FOXP1 immunoreactivities with favorable prognosis of tamoxifen-treated breast cancer patients

Breast cancer is primarily a hormone-dependent tumor that can be regulated by the status of the steroid hormones estrogen and progesterone. Forkhead box A1 (FOXA1) is a member of the forkhead box transcription factor family and functions as a pioneer factor of the estrogen receptor (ER) in breast cancer. In the present study, we demonstrate that FOXA1 mRNA was upregulated by estrogen and that estrogen receptor-α (ERα) recruitment to ER-binding sites in the vicinity of the FOXA1 gene was increased by estrogen in ERα-positive MCF-7 breast cancer cells. The estrogen-induced FOXA1 upregulation was repressed by 4-hydroxytamoxifen treatment. We also demonstrated that the proliferation and the migration of MCF-7 cells were decreased by FOXA1-specific small interfering RNA (siRNA; siFOXA1). Furthermore, siFOXA1 decreased the estrogen response element-driven transcription and the estrogen-dependent upregulation of ERα target genes in MCF-7 cells. Next, the immunohistochemical analyses of FOXA1 were performed using two groups of breast cancer specimens. The nuclear immunoreactivity of FOXA1 was detected in 80 (74%) of 108 human invasive breast cancers and was negatively correlated with tumor grade and positively correlated with hormone receptor status, including ERα and progesterone receptor, pathological tumor size, and immunoreactivity of FOXP1, another FOX family transcription factor. FOXA1 immunoreactivity was significantly elevated in the relapse-free breast cancer patients treated with tamoxifen. Notably, the double-positive immunoreactivities of FOXA1 and FOXP1 were significantly associated with a favorable prognosis for the relapse-free and overall survival of patients with tamoxifen-treated breast cancer, with lower P values compared with FOXA1 or FOXP1 immunoreactivity alone. These results suggest that FOXA1 plays an important role in the proliferation and migration of breast cancer cells by modulating estrogen signaling and that the double-positive immunoreactivities of FOXA1 and FOXP1 are associated with a favorable prognosis of tamoxifen-treated breast cancer.

Dissecting the role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in colon, breast, and lung carcinogenesis

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is a promising drug target since its agonists increase serum high-density lipoprotein; decrease low-density lipoprotein, triglycerides, and insulin associated with metabolic syndrome; improve insulin sensitivity; and decrease high fat diet-induced obesity. PPARβ/δ agonists also promote terminal differentiation and elicit anti-inflammatory activities in many cell types. However, it remains to be determined whether PPARβ/δ agonists can be developed as therapeutics because there are reports showing either pro- or anti-carcinogenic effects of PPARβ/δ in cancer models. This review examines studies reporting the role of PPARβ/δ in colon, breast, and lung cancers. The prevailing evidence would suggest that targeting PPARβ/δ is not only safe but could have anti-carcinogenic protective effects.

Med1 plays a critical role in the development of tamoxifen resistance

Understanding the molecular pathways that contribute to the development of tamoxifen resistance is a critical research priority as acquired tamoxifen resistance is the principal cause of poor prognosis and death of patients with originally good prognosis hormone-responsive breast tumors. In this report, we provide evidence that Med1, an important subunit of mediator coactivator complex, is spontaneously upregulated during acquired tamoxifen-resistance development potentiating agonist activities of tamoxifen. Phosphorylated Med1 and estrogen receptor (ER) are abundant in tamoxifen-resistant breast cancer cells due to persistent activation of extracellular signal-regulated kinases. Mechanistically, phosphorylated Med1 exhibits nuclear accumulation, increased interaction with ER and higher tamoxifen-induced recruitment to ER-responsive promoters, which is abrogated by inhibition of Med1 phosphorylation. Stable knockdown of Med1 in tamoxifen-resistant cells not only reverses tamoxifen resistance in vitro but also in vivo. Finally, higher expression levels of Med1 in the tumor significantly correlated with tamoxifen resistance in ER-positive breast cancer patients on adjuvant tamoxifen monotherapy. In silico analysis of breast cancer, utilizing published profiling studies showed that Med1 is overexpressed in aggressive subsets. These findings provide what we believe is the first evidence for a critical role for Med1 in tamoxifen resistance and identify this coactivator protein as an essential effector of the tamoxifen-induced breast cancer growth.

Role of estrogen receptor signaling in breast cancer metastasis

Metastatic breast cancer is a life-threatening stage of cancer and is the leading cause of death in advanced breast cancer patients. Estrogen signaling and the estrogen receptor (ER) are implicated in breast cancer progression, and the majority of the human breast cancers start out as estrogen dependent. Accumulating evidence suggests that ER signaling is complex, involving coregulatory proteins and extranuclear actions. ER-coregualtory proteins are tightly regulated under normal conditions with miss expression primarily reported in cancer. Deregulation of ER coregualtors or ER extranuclear signaling has potential to promote metastasis in ER-positive breast cancer cells. This review summarizes the emerging role of ER signaling in promoting metastasis of breast cancer cells, discusses the molecular mechanisms by which ER signaling contributes to metastasis, and explores possible therapeutic targets to block ER-driven metastasis.

Bisphenol A exposure modifies methylation of imprinted genes in mouse oocytes via the estrogen receptor signaling pathway

Bisphenol A (BPA), a synthetic additive used to harden polycarbonate plastics and epoxy resin, is ubiquitous in our everyday environment. Many studies have indicated detrimental effects of BPA on the mammalian reproductive abilities. This study is aimed to test the potential effects of BPA on methylation of imprinted genes during oocyte growth and meiotic maturation in CD-1 mice. Our results demonstrated that BPA exposure resulted in hypomethylation of imprinted gene Igf2r and Peg3 during oocyte growth, and enhanced estrogen receptor (ER) expression at the levels of mRNA and protein. The relationship between ER expression and imprinted gene hypomethylation was substantiated using an ER inhibitor, ICI182780. In addition, BPA promoted the primordial to primary follicle transition, thereby speeding up the depletion of the primordial follicle pool, and suppressed the meiotic maturation of oocytes because of abnormal spindle assembling in meiosis I. In conclusion, neonatal exposure to BPA inhibits methylation of imprinted genes during oogenesis via the ER signaling pathway in CD-1 mice.

Repressor of estrogen receptor activity (REA) is essential for mammary gland morphogenesis and functional activities: studies in conditional knockout mice

Estrogen receptor (ER) is a key regulator of mammary gland development and is also implicated in breast tumorigenesis. Because ER-mediated activities depend critically on coregulator partner proteins, we have investigated the consequences of reduction or loss of function of the coregulator repressor of ER activity (REA) by conditionally deleting one allele or both alleles of the REA gene at different stages of mammary gland development. Notably, we find that heterozygosity and nullizygosity for REA result in very different mammary phenotypes and that REA has essential roles in the distinct morphogenesis and functions of the mammary gland at different stages of development, pregnancy, and lactation. During puberty, mice homozygous null for REA in the mammary gland (REAf/f PRcre/+) showed severely impaired mammary ductal elongation and morphogenesis, whereas mice heterozygous for REA (REAf/+ PRcre/+) displayed accelerated mammary ductal elongation, increased numbers of terminal end buds, and up-regulation of amphiregulin, the major paracrine mediator of estrogen-induced ductal morphogenesis. During pregnancy and lactation, mice with homozygous REA gene deletion in mammary epithelium (REAf/f whey acidic protein-Cre) showed a loss of lobuloalveolar structures and increased apoptosis of mammary alveolar epithelium, leading to impaired milk production and significant reduction in growth of their offspring, whereas body weights of the offspring nursed by females heterozygous for REA were slightly greater than those of control mice. Our findings reveal that REA is essential for mammary gland development and has a gene dosage-dependent role in the regulation of stage-specific physiological functions of the mammary gland.

Cracking the estrogen receptor's posttranslational code in breast tumors

Estrogen signaling pathways, because of their central role in regulating the growth and survival of breast tumor cells, have been identified as suitable and efficient targets for cancer therapies. Agents blocking estrogen activity are already widely used clinically, and many new molecules have entered clinical trials, but intrinsic or acquired resistance to treatment limits their efficacy. The basic molecular studies underlying estrogen signaling have defined the critical role of estrogen receptors (ER) in many aspects of breast tumorigenesis. However, important knowledge gaps remain about the role of posttranslational modifications (PTM) of ER in initiation and progression of breast carcinogenesis. Whereas major attention has been focused on the phosphorylation of ER, many other PTM (such as acetylation, ubiquitination, sumoylation, methylation, and palmitoylation) have been identified as events modifying ER expression and stability, subcellular localization, and sensitivity to hormonal response. This article will provide an overview of the current and emerging knowledge on ER PTM, with a particular focus on their deregulation in breast cancer. We also discuss their clinical relevance and the functional relationship between PTM. A thorough understanding of the complete picture of these modifications in ER carcinogenesis might not only open new avenues for identifying new markers for prognosis or prediction of response to endocrine therapy but also could promote the development of novel therapeutic strategies.

17β-estradiol signaling and regulation of Sertoli cell function

In this review, we will present an overview of estrogen actions in the testis from immature and adult animals, with special emphasis on signaling mechanisms involved in the 17β-estradiol regulation of Sertoli cell function in immature rats. 17β-estradiol activates Sertoli cell proliferation in immature rats by a mechanism that involves the translocation of the estrogen receptors ESR1 and ESR2 to the plasma membrane, phosphorylation of epidermal growth factor receptor and activation of mitogen-activated protein kinase 3/1. Activation of the G protein-coupled estrogen receptor (GPER) also induces phosphorylation of mitogen-activated protein kinase 3/1 via epidermal growth factor receptor transactivation, which in turn increases expression of the antiapoptotic protein BCL2 and decreases the expression of proapoptotic protein BAX, indicating an antiapoptotic role of E2-GPER in immature rat Sertoli cells. In conclusion, ESRs and GPER can mediate rapid 17β-estradiol signaling in Sertoli cells, and modulate transcriptional events important for Sertoli cell function and maintenance of normal testis development and homeostasis. Our findings are important to clarify the role of estrogen in a critical period of testicular development and to direct further studies, which may contribute to better understand the causes of male infertility.

A polymorphism in the nuclear receptor coactivator 7 gene and breast cancer susceptibility

The nuclear receptor coactivator 7 (NCoA7) gene codes for an estrogen receptor-associated protein that plays a significant role in the cellular response to estrogens. Given that NCoA7 is expressed in the mammary gland, alterations in this gene may affect breast cancer risk. In this study, we compared the genotype and allele frequencies of the missense single nucleotide polymorphism (SNP) rs1567, located in the coding region of the NCoA7 gene and resulting in an amino acid exchange from asparagine to glutamine, in 305 women with sporadic breast cancer and 346 women without any malignancy. Statistical analysis of the observed frequencies did not reveal a significant difference between the cancer and control groups, nor did a comparison between histological breast cancer subgroups. In conclusion, the results of our phenotype-genotype association study indicate that NCoA7 SNP rs1567 does not affect breast cancer susceptibility.

Roscovitine confers tumor suppressive effect on therapy-resistant breast tumor cells

Introduction

Current clinical strategies for treating hormonal breast cancer involve the use of anti-estrogens that block estrogen receptor (ER)α functions and aromatase inhibitors that decrease local and systemic estrogen production. Both of these strategies improve outcomes for ERα-positive breast cancer patients, however, development of therapy resistance remains a major clinical problem. Divergent molecular pathways have been described for this resistant phenotype and interestingly, the majority of downstream events in these resistance pathways converge upon the modulation of cell cycle regulatory proteins including aberrant activation of cyclin dependent kinase 2 (CDK2). In this study, we examined whether the CDK inhibitor roscovitine confers a tumor suppressive effect on therapy-resistant breast epithelial cells.

Methods

Using various in vitro and in vivo assays, we tested the effect of roscovitine on three hormonal therapy-resistant model cells: (a) MCF-7-TamR (acquired tamoxifen resistance model); (b) MCF-7-LTLTca (acquired letrozole resistance model); and (c) MCF-7-HER2 that exhibit tamoxifen resistance (ER-growth factor signaling cross talk model).

Results

Hormonal therapy-resistant cells exhibited aberrant activation of the CDK2 pathway. Roscovitine at a dose of 20 μM significantly inhibited the cell proliferation rate and foci formation potential of all three therapy-resistant cells. The drug treatment substantially increased the proportion of cells in G2/M cell cycle phase with decreased CDK2 activity and promoted low cyclin D1 levels. Interestingly, roscovitine also preferentially down regulated the ERα isoform and ER-coregulators including AIB1 and PELP1. Results from xenograft studies further showed that roscovitine can attenuate growth of therapy-resistant tumors in vivo.

Conclusions

Roscovitine can reduce cell proliferation and survival of hormone therapy-resistant breast cancer cells. Our results support the emerging concept that inhibition of CDK2 activity has the potential to abrogate growth of hormonal therapy-resistant cells.

Mediator-dependent nuclear receptor function

As gene-specific transcription factors, nuclear receptors are broadly involved in many important biological processes. Their function on target genes requires the stepwise assembly of different coactivator complexes that facilitate chromatin remodeling and subsequent preinitiation complex (PIC) formation and function. Mediator has proved to be a crucial, and general, nuclear receptor-interacting coactivator, with demonstrated functions in transcription steps ranging from chromatin remodeling to subsequent PIC formation and function. Here we discuss our current understanding of (i) pathways involved in Mediator recruitment and function through nuclear receptor target gene enhancers and promoters, (ii) conditional requirements for the strong nuclear receptor-Mediator interactions mediated by NR AF2 domains and the MED1 LXXLL motifs, (iii) Mediator functions, through different nuclear receptor-interacting subunits, in different metabolic pathways, (iv) emerging functions of Mediator as a corepressor in addition to its major role as a coactivator and (v) mechanisms by which Mediator acts to transmit signals from enhancer-bound nuclear receptors to the general transcription machinery at core promoters to effect PIC formation and function. As a nuclear receptor coregulator with increasingly diverse functions, Mediator may thus modulate nuclear receptor signaling through several different mechanisms.

Prognostic value of novel biomarkers in astrocytic brain tumors: nuclear receptor co-regulators AIB1, TIF2, and PELP1 are associated with high tumor grade and worse patient prognosis

Estrogen receptors alpha (ERα) and beta (ERβ) and their co-regulatory proteins are key components of complex signaling networks that specifically regulate the growth and development of various tissues and tumors. Still, their protein expression profiles and possible role in the pathogenesis of astrocytic tumors remain largely unknown. The purpose of the present study is to evaluate the differential protein expression of ΕRα, ERβ, and their co-activators, AIB1, TIF2, and PELP1 in astrocytic tumors of World Health Organization (WHO) grade II-IV, using immunohistochemistry. Potential correlations with clinicopathological parameters and patient prognosis were also explored. ERα protein expression was undetectable while ERβ levels were significantly decreased with progression of tumor grade (P < 0.001). High expression of ERβ was an independent favorable prognostic factor on multivariate analysis (P = 0.003). Expression of AIB1, TIF2, and PELP1 was not correlated with ERβ expression and followed an opposite trend, with increasing levels in high-grade relative to low-grade tumors (P < 0.001). Univariate survival analysis revealed that high AIB1, TIF2, and PELP1 expression was associated with worse prognosis (P = 0.049, P = 0.033, and P = 0.020, respectively). ERβ and ER co-activators AIB1, TIF2, and PELP1 appear to play an important role in the pathogenesis and progression of astrocytic tumors and might have prognostic significance. The mechanisms underlying their involvement in astrocytic tumorigenesis, as well as their utility for prognostic and therapeutic purposes merit further investigation.

Crosstalk of TGF-β and estrogen receptor signaling in breast cancer

Estrogen receptor-α (ERα) and transforming growth factor (TGF)-β signaling pathways are major regulators during mammary gland development, function and tumorigenesis. Predominantly, they have opposing roles in proliferation and apoptosis. While ERα signaling supports growth and differentiation and is antiapoptotic, mammary gland epithelia cells are very sensitive to TGF-β-induced cell cycle arrest and apoptosis. Their regulatory pathways intersect, and ERα blocks TGF-β pathway by multiple means, including direct interactions of its signaling components, Smads. However, relatively little is known of the dysfunction of their interactions in cancer. A better understanding would help to develop new strategies for breast cancer treatment.

Suppression of estrogen receptor transcriptional activity by connective tissue growth factor

Secreted growth factors have been shown to stimulate the transcriptional activity of estrogen receptors (ER) that are responsible for many biological processes. However, whether these growth factors physically interact with ER remains unclear. Here, we show for the first time that connective tissue growth factor (CTGF) physically and functionally associates with ER. CTGF interacted with ER both in vitro and in vivo. CTGF interacted with ER DNA-binding domain. ER interaction region in CTGF was mapped to the thrombospondin type I repeat, a cell attachment motif. Overexpression of CTGF inhibited ER transcriptional activity as well as the expression of estrogen-responsive genes, including pS2 and cathepsin D. Reduction of endogenous CTGF with CTGF small interfering RNA enhanced ER transcriptional activity. The interaction between CTGF and ER is required for the repression of estrogen-responsive transcription by CTGF. Moreover, CTGF reduced ER protein expression, whereas the CTGF mutant that did not repress ER transcriptional activity also did not alter ER protein levels. The results suggested the transcriptional regulation of estrogen signaling through interaction between CTGF and ER, and thus may provide a novel mechanism by which cross-talk between secreted growth factor and ER signaling pathways occurs.

The molecular mechanisms underlying the pharmacological actions of ER modulators: implications for new drug discovery in breast cancer

Our understanding of the molecular mechanisms underlying the pharmacological actions of estrogen receptor (ER) ligands has evolved considerably in recent years. Much of this knowledge has come from a detailed dissection of the mechanism(s) of action of the Selective Estrogen Receptor Modulators (SERMs) tamoxifen and raloxifene, so called for their ability to function as ER agonists or antagonists depending on the tissue in which they operate. These mechanistic insights have had a significant impact on the discovery of second generation SERMs, some of which are in late stage clinical development for the treatment/prevention of breast cancer as well as other estrogenopathies. In addition to the SERMs, however, have emerged the Selective Estrogen Degraders (SERDs), which as their name suggests, interact with and facilitate ER turnover in cells. One drug of this class, fulvestrant, has been approved as a third line treatment for ER-positive metastatic breast cancer. Whereas the first generation SERMs/SERDs were discovered in a serendipitous manner, this review will highlight how our understanding of the molecular pharmacology of ER ligands has been utilized in the development of the next generation of SERMs/SERDs, some of which are likely to have a major impact on the pharmacotherapy of breast cancer.

Comparative evaluation of ERα and ERβ significance in breast cancer: state of the art

Over 30 years of clinical data have unequivocally established estrogen receptor (ER)-α as a critical clinical biomarker and valid therapeutic target to fight breast cancer. However, ERα remains imperfect with respect to both of these activities, mainly because the mechanisms by which estrogens mediate their activity are far more complex than originally anticipated. The cloning of a second estrogen receptor, ERβ, has led to a full re-evaluation of our original view of the action of estrogen in breast tissues. Important challenges remain with respect to the design, selection and normalization of the most appropriate methods for assaying the expression and functionality of both receptors. Solving these challenges remains a priority in order to decide upon specific endocrine therapies and save patients who are still dying from a potentially curable disease.

Estrogen receptor β: switching to a new partner and escaping from estrogen

Estrogen receptor (ER) β, the "second" ER, plays a gatekeeper role by inhibiting cell proliferation, promoting apoptosis, and impeding the progression of prostate cancer. Ironically, its presumed ligand, 17β-estradiol, promotes cancer development in experimental models. The mechanisms underlying the interplay between estrogens and ERβ in prostate cancer remain largely unclear. Research on a previously unknown tethering partner of ERβ, Krüppel-like zinc finger transcription factor 5 (KLF5), and its downstream gene target (FOXO1) helps to unlock this puzzle. 17β-Estradiol is not required to maintain the tumor-suppressive function of ERβ in the prostate, a tissue with limited estrogen availability; moreover, the presence of 17β-estradiol abrogates ERβ- and KLF5-mediated signaling and promotes cellular proliferation. Future research into ERβ will likely involve this estrogen independency and the preference for binding nonclassical DNA elements through tethering. The development of ERβ-based therapies may lead to improved drug efficacy.

Minireview: estrogen receptor-initiated mechanisms causal to mammalian reproductive behaviors

While estrogen-facilitated changes in gene expression constitute some of the best-analyzed biochemical phenomena in the regulation of transcription, there have been at least two aspects of this topic that have led to much experimental work about estrogen actions on brain and behavior. The first task has required parsing various behavioral and neurochemical functions according to whether they depend on estrogen receptor-α or estrogen receptor-β. The second task has been the formulation of how nuclear actions of estrogens comport with membrane-initiated actions. With respect to these issues, applications of molecular endocrine approaches to lordosis behavior came first. Currently, the last in the chain of reproductive behaviors, maternal behavior, and an entire range of neural and cognitive functions even more complex in their determinants, must be analyzed using current molecular techniques. This minireview of estrogen actions on the chain of female reproductive behaviors highlights challenging new questions about estrogen actions on cells in the brain, questions that have important practical applications far beyond traditionally studied sex behaviors.

Estrogen regulation of human with-no-lysine (K) kinase-4 gene expression involves AP-1 transcription factor

With-no-lysine (K) kinase-4 (WNK4) is a serine/threonine protein kinase and plays a crucial role in the regulation of blood pressure and electrolyte homeostasis. Whether or not estrogen, an important regulator of physiologic functions, modulates human WNK4 (hWNK4) gene expression is unknown. In the current study, real-time PCR assay showed that 17β-estradiol (E(2)) with estrogen receptor alpha (ERα) suppressed the level of hWNK4 mRNA in cultured human embryo kidney 293 cells (HEK293). Luciferase activity analysis of the truncated hWNK4 promoters indicated that a regulatory region from -216 to -202 is essential for the basal transcriptional activity and response to E(2)/ERα. Using an electrophoresis mobility shift assay and a chromatin immunoprecipitation assay, we identified an activator protein-1 (AP-1) element at position -215/-205, to which AP-1 binding was enhanced by E(2)/ERα. Accordingly, AP-1 protein was increased by E(2)/ERα using Western blot analysis. Moreover, re-chromatin-immunoprecipitation and co-immunoprecipitation assays showed a direct interaction between ERα and AP-1 in HEK293 cells. In summary, these data document that E(2)/ERα regulates hWNK4 expression partly through AP-1 binding to the hWNK4 promoter.

Genetic variation of ESR1 and its co-activator PPARGC1B is synergistic in augmenting the risk of estrogen receptor-positive breast cancer

Introduction

Given the role of estrogen in breast carcinogenesis and the modification of estrogen receptor (ER) activity by its biochemical cofactors, we hypothesize that genetic variation within ER cofactor genes alters cellular response to estrogen exposure and consequently modifies the risk for ER-positive breast cancer.

Methods

We genotyped 790 tagging SNPs within 60 ER cofactor genes in 1,257 cases and 1,464 controls from Sweden and in 2,215 cases and 1,265 controls from Finland, and tested their associations with either ER-positive or ER-negative breast cancer.

Results

Seven SNPs showed consistent association with ER-positive breast cancer in the two independent samples, and six of them were located within PPARGC1B, encoding an ER co-activator, with the strongest association at rs741581 (odds ratio = 1.41, P = 4.84 × 10^-5) that survived Bonferroni correction for multiple testing in the combined ER-positive breast cancer sample (P_corrected = 0.03). Moreover, we also observed significant synergistic interaction (P_interaction = 0.008) between the genetic polymorphisms within PPARGC1B and ESR1 in ER-positive breast cancer. By contrast, no consistent association was observed in ER-negative breast cancer. Furthermore, we found that administration of estrogen in the MCF-7 cell line induced PPARGC1B expression and enhanced occupancies of ER and RNA polymerase II within the region of SNP association, suggesting the upregulation of PPARGC1B expression by ESR1 activation.

Conclusions

Our study revealed that DNA polymorphisms of PPARGC1B, coding a bona fide ER co-activator, are associated with ER-positive breast cancer risk. The feed-forward transcriptional regulatory loop between PPARGC1B and ESR1 further augments their protein interaction, which provides a plausible mechanistic explanation for the synergistic genetic interaction between PPARGC1B and ESR1 in ER-positive breast cancer. Our study also highlights that biochemically and genomically informed candidate gene studies can enhance the discovery of interactive disease susceptibility genes.

RAC3 is a pro-migratory co-activator of ERα

ERα is a ligand-dependent nuclear receptor that is important in breast cancer genesis, behavior and response to hormone-based therapies. A T7 phage display screen against full-length human ERα, coupled with genome-wide exon arrays, was used to identify RAC3 as a putative ERα co-regulator. RAC3 is a rho family small GTPase that is associated with cytoskeletal rearrangement. We demonstrate a novel role for nuclear RAC3 as an ERα transcriptional activator, with prognostic implications for metastatic disease. Through in vitro and cell-based studies, RAC3 was shown to exist in a GTP-bound state and act as a ligand specific ERα co-activator of E2-induced transcription. Over expression of RAC3 induced pro-growth and pro-migratory genes that resulted in increased migration of ERα-positive breast cancer cells. Chemical inhibition and genetic knockdown of RAC3 antagonized E2-induced cell proliferation, cell migration, and ERα mediated gene expression, indicating that RAC3 is necessary for full ERα transcriptional activity. In agreement with the molecular and cellular data, RAC3 over expression in ERα-positive breast cancers correlated with a significant decrease in recurrence free survival and a significant increase in the odds ratio of metastasis. In conclusion, RAC3 is novel ERα co-activator that promotes cell migration and has prognostic value for ERα-positive breast cancer metastasis. RAC3 may also be a useful therapeutic target for ERα-positive breast cancers.

Significance of ER-Src axis in hormonal therapy resistance

The estrogen receptor (ER) is implicated in the progression of breast cancer. Despite positive effects of hormonal therapy, initial or acquired resistance to endocrine therapies frequently occurs. Recent studies suggested ERα-coregulator PELP1 and growth factor receptor ErbB2/HER2 play an essential role in hormonal therapy responsiveness. Src axis couples ERα with HER2 and PELP1, thus representing a new pathway for targeted therapy resistance. To establish the significance of ER-Src axis in PELP1 and HER2 mediated therapy resistance, we have generated model cells that stably express Src-shRNA under conditions of PELP1, HER2 deregulation. Depletion of Src using shRNA substantially reduced E2 mediated activation of Src and MAPK activation in resistant model cells. Pharmacological inhibition of Src using dasatinib, an orally available inhibitor substantially inhibited the growth of therapy resistant MCF7-PELP1, MCF7-HER2, and MCF7-Tam model cells in proliferation assays. In post-menopausal xenograft based studies, treatment with dasatinib significantly inhibited the growth of therapy resistant cells. IHC analysis revealed that the tumors were ERα positive, and dasatinib treated tumors exhibited alterations in Src and MAPK signaling pathways. Combinatorial therapy of tamoxifen with dasatinib showed better therapeutic effect compared to single agent therapy on the growth of therapy resistant PELP1 driven tumors. The results from our study showed that ER-Src axis play an important role in promoting hormonal resistance by proto-oncogenes such as HER2, PELP1, and blocking this axis prevents the development of hormonal independence in vivo. Since PELP1, HER2, and Src kinase are commonly deregulated in breast cancers, combination therapies using both endocrine agents and dasatinib may have better therapeutic effect by delaying the development of hormonal resistance.

Ligands specify estrogen receptor alpha nuclear localization and degradation

BACKGROUND

The estrogen receptor alpha (ERα) is found predominately in the nucleus, both in hormone stimulated and untreated cells. Intracellular distribution of the ERα changes in the presence of agonists but the impact of different antiestrogens on the fate of ERα is a matter of debate.

RESULTS

A MCF-7 cell line stably expressing GFP-tagged human ERα (SK19 cell line) was created to examine the localization of ligand-bound GFP-ERα. We combined digitonin-based cell fractionation analyses with fluorescence and immuno-electron microscopy to determine the intracellular distribution of ligand-bound ERα and/or GFP-ERα.Using fluorescence- and electron microscopy we demonstrate that both endogenous ERα and GFP-ERα form numerous nuclear focal accumulations upon addition of agonist, 17β-estradiol (E2), and pure antagonists (selective estrogen regulator disruptor; SERD), ICI 182,780 or RU58,668, while in the presence of partial antagonists (selective estrogen regulator modulator; SERM), 4-hydroxytamoxifen (OHT) or RU39,411, diffuse nuclear staining persisted.Digitonin based cell fractionation analyses confirmed that endogenous ERα and GFP-ERα predominantly reside in the nuclear fraction. Overall ERα protein levels were reduced after estradiol treatment. In the presence of SERMs ERα was stabilized in the nuclear soluble fraction, while in the presence of SERDs protein levels decreased drastically and the remaining ERα was largely found in a nuclear insoluble fraction. mRNA levels of ESR1 were reduced compared to untreated cells in the presence of all ligands tested, including E2. E2 and SERDs induced ERα degradation occurred in distinct nuclear foci composed of ERα and the proteasome providing a simple explanation for ERα sequestration in the nucleus.

CONCLUSIONS

Our results indicate that chemical structure of ligands directly affect the nuclear fate and protein turnover of the estrogen receptor alpha independently of their impact on transcription. These findings provide a molecular basis for the selection of antiestrogen compounds issue from pharmacological studies aimed at improving treatment of breast cancer.

Orphan nuclear receptors in breast cancer pathogenesis and therapeutic response

Nuclear receptors comprise a large family of highly conserved transcription factors that regulate many key processes in normal and neoplastic tissues. Most nuclear receptors share a common, highly conserved domain structure that includes a carboxy-terminal ligand-binding domain. However, a subgroup of this gene family is known as the orphan nuclear receptors because to date there are no known natural ligands that regulate their activity. Many of the 25 nuclear receptors classified as orphan play critical roles in embryonic development, metabolism, and the regulation of circadian rhythm. Here, we review the emerging role(s) of orphan nuclear receptors in breast cancer, with a particular focus on two of the estrogen-related receptors (ERRalpha and ERRgamma) and several others implicated in clinical outcome and response or resistance to cytotoxic or endocrine therapies, including the chicken ovalbumin upstream promoter transcription factors, nerve growth factor-induced B, DAX-1, liver receptor homolog-1, and retinoic acid-related orphan receptor alpha. We also propose that a clearer understanding of the function of orphan nuclear receptors in mammary gland development and normal mammary tissues could significantly improve our ability to diagnose, treat, and prevent breast cancer.

Dual targeting of EphA2 and ER restores tamoxifen sensitivity in ER/EphA2-positive breast cancer

Overexpression and altered function of EphA2 receptor tyrosine kinase are critical in the progression of breast cancer and provide a target for breast cancer therapy. We have previously demonstrated that EphA2 overexpression decreases estrogen dependence and Tamoxifen sensitivity both in vitro and in vivo. EA5, a novel monoclonal antibody that mimicks the binding of ephrin A to EphA2, reverses the effect of EphA2 overexpression and restores Tamoxifen sensitivity in EphA2-transfected MCF-7 cells in vitro. To explore the role of EphA2 overexpression on ER-dependent mechanisms, we used two different ER+/EphA2-transfected cell line models (MCF-7(neo)/MCF-7(EphA2) and T47D(neo)/T47D(EphA2)). EA5 inhibits primary tumor growth and restores Tamoxifen sensitivity in the MCF-7(EphA2) xenografts. Using the T47D(EphA2) in vitro model, we verified that EphA2 decreases ER activation in response to E2 stimulation consistent with our earlier results in MCF-7(EphA2) model. We found no direct interaction between ER and EphA2 and no difference in expression of canonical ER-dependent proteins or ER co-regulators. However, E2 stimulation phosphorylates FAK(Tyr925) only in ER+/EphA2+ cell lines. Treatment of T47D(EphA2) cells with EA5 and Tamoxifen leads to dephosphorylation of FAK(Tyr925) in response to E2. Our data demonstrate that dual targeting of EphA2 and ER is a promising approach for delaying resistance to Tamoxifen. The data support our hypothesis that EphA2 impacts ER function via a FAK dependent pathway.