Molecular basis for the constitutive activity of estrogen-related receptor alpha-1

ERRα and ERRγ coordinate expression of genes associated with Alzheimer's disease, inhibiting DKK1 to suppress tau phosphorylation

Alzheimer's disease (AD) is a prevalent neurodegenerative disease characterized by cognitive decline and learning/memory impairment associated with neuronal cell loss. Estrogen-related receptor α (ERRα) and ERRγ, which are highly expressed in the brain, have emerged as potential AD regulators, with unelucidated underlying mechanisms. Here, we identified genome-wide binding sites for ERRα and ERRγ in human neuronal cells. They commonly target a subset of genes associated with neurodegenerative diseases, including AD. Notably, Dickkopf-1 (DKK1), a Wnt signaling pathway antagonist, was transcriptionally repressed by both ERRα and ERRγ in human neuronal cells and brain. ERRα and ERRγ repress RNA polymerase II (RNAP II) accessibility at the DKK1 promoter by modulating a specific active histone modification, histone H3 lysine acetylation (H3K9ac), with the potential contribution of their corepressor. This transcriptional repression maintains Wnt signaling activity, preventing tau phosphorylation and promoting a healthy neuronal state in the context of AD.

Estrogen-Related Receptor α: A Key Transcription Factor in the Regulation of Energy Metabolism at an Organismic Level and a Target of the ABA/LANCL Hormone Receptor System

The orphan nuclear receptor ERRα is the most extensively researched member of the estrogen-related receptor family and holds a pivotal role in various functions associated with energy metabolism, especially in tissues characterized by high energy requirements, such as the heart, skeletal muscle, adipose tissue, kidney, and brain. Abscisic acid (ABA), traditionally acknowledged as a plant stress hormone, is detected and actively functions in organisms beyond the land plant kingdom, encompassing cyanobacteria, fungi, algae, protozoan parasites, lower Metazoa, and mammals. Its ancient, cross-kingdom role enables ABA and its signaling pathway to regulate cell responses to environmental stimuli in various organisms, such as marine sponges, higher plants, and humans. Recent advancements in understanding the physiological function of ABA and its mammalian receptors in governing energy metabolism and mitochondrial function in myocytes, adipocytes, and neuronal cells suggest potential therapeutic applications for ABA in pre-diabetes, diabetes, and cardio-/neuroprotection. The ABA/LANCL1-2 hormone/receptor system emerges as a novel regulator of ERRα expression levels and transcriptional activity, mediated through the AMPK/SIRT1/PGC-1α axis. There exists a reciprocal feed-forward transcriptional relationship between the LANCL proteins and transcriptional coactivators ERRα/PGC-1α, which may be leveraged using natural or synthetic LANCL agonists to enhance mitochondrial function across various clinical contexts.

Novel Pan-ERR Agonists Ameliorate Heart Failure Through Enhancing Cardiac Fatty Acid Metabolism and Mitochondrial Function

BACKGROUND

Cardiac metabolic dysfunction is a hallmark of heart failure (HF). Estrogen-related receptors ERRα and ERRγ are essential regulators of cardiac metabolism. Therefore, activation of ERR could be a potential therapeutic intervention for HF. However, in vivo studies demonstrating the potential usefulness of ERR agonist for HF treatment are lacking, because compounds with pharmacokinetics appropriate for in vivo use have not been available.

METHODS

Using a structure-based design approach, we designed and synthesized 2 structurally distinct pan-ERR agonists, SLU-PP-332 and SLU-PP-915. We investigated the effect of ERR agonist on cardiac function in a pressure overload-induced HF model in vivo. We conducted comprehensive functional, multi-omics (RNA sequencing and metabolomics studies), and genetic dependency studies both in vivo and in vitro to dissect the molecular mechanism, ERR isoform dependency, and target specificity.

RESULTS

Both SLU-PP-332 and SLU-PP-915 significantly improved ejection fraction, ameliorated fibrosis, and increased survival associated with pressure overload-induced HF without affecting cardiac hypertrophy. A broad spectrum of metabolic genes was transcriptionally activated by ERR agonists, particularly genes involved in fatty acid metabolism and mitochondrial function. Metabolomics analysis showed substantial normalization of metabolic profiles in fatty acid/lipid and tricarboxylic acid/oxidative phosphorylation metabolites in the mouse heart with 6-week pressure overload. ERR agonists increase mitochondria oxidative capacity and fatty acid use in vitro and in vivo. Using both in vitro and in vivo genetic dependency experiments, we show that ERRγ is the main mediator of ERR agonism-induced transcriptional regulation and cardioprotection and definitively demonstrated target specificity. ERR agonism also led to downregulation of cell cycle and development pathways, which was partially mediated by E2F1 in cardiomyocytes.

CONCLUSIONS

ERR agonists maintain oxidative metabolism, which confers cardiac protection against pressure overload-induced HF in vivo. Our results provide direct pharmacologic evidence supporting the further development of ERR agonists as novel HF therapeutics.

Estrogen-Related Receptors Gene Expression and Copy Number Alteration Association With the Clinicopathologic Characteristics of Breast Cancer

Purpose:

It has been suggested that dysregulation of transcription factors expression or activity plays significant roles in breast cancer (BC) severity and poor prognosis. Therefore, our study aims to thoroughly evaluate the estrogen-related receptor isoforms (ESRRs) expression and copy number alteration (CNA) status and their association with clinicopathologic characteristics in BC.

Methods:

A METABRIC dataset consist of 2509 BC patients’ samples was obtained from the cBioPortal public domain. The gene expression, putative CNA, and relevant tumor information of ESRRs were retrieved. ESRRs messenger RNA (mRNA) expression in BC cell lines was obtained from the Cancer Cell Line Encyclopedia (CCLE). Association and correlation analysis of ESRRs expression with BC clinicopathologic characteristics and molecular subtype were performed. Kaplan–Meier survival analysis was conducted to evaluate the prognostic value of ESRRs expression on patient survival.

Results:

ESRRα expression correlated negatively with patients’ age and overall survival, whereas positively correlated with tumor size, the number of positive lymph nodes, and Nottingham prognostic index (NPI). Conversely, ESRRγ expression was positively correlated with patients’ age and negatively correlated with NPI. ESRRα and ESRRγ expression were significantly associated with tumor grade, expression of hormone receptors, human epidermal growth factor receptor 2 (HER2), and molecular subtype, whereas ESRRβ was only associated with tumor stage. A significant and distinct association of each of ESRRs CNA with various clinicopathologic and prognostic factors was also observed. Kaplan–Meier survival analysis demonstrated no significant difference for survival curves among BC patients with high or low expression of ESRRα, β, or γ. On stratification, high ESRRα expression significantly reduced survival among premenopausal patients, patients with grade I/II, and early-stage disease. In BC cell lines, only ESRRα expression was significantly higher in HER2-positive cells. No significant association was observed between ESRRβ expression and any of the clinicopathologic characteristics examined.

Conclusions:

In this clinical dataset, ESRRα and ESRRγ mRNA expression and CNA show a significant correlation and association with distinct clinicopathologic and prognostic parameters known to influence treatment outcomes; however, ESRRβ failed to show a robust role in BC pathogenesis. ESRRα and ESRRγ can be employed as therapeutic targets in BC-targeted therapy. However, the role of ESRRβ in BC pathogenesis remains unclear.

Binding of Androgen- and Estrogen-Like Flavonoids to Their Cognate (Non)Nuclear Receptors: A Comparison by Computational Prediction

Flavonoids are plant bioactives that are recognized as hormone-like polyphenols because of their similarity to the endogenous sex steroids 17β-estradiol and testosterone, and to their estrogen- and androgen-like activity. Most efforts to verify flavonoid binding to nuclear receptors (NRs) and explain their action have been focused on ERα, while less attention has been paid to other nuclear and non-nuclear membrane androgen and estrogen receptors. Here, we investigate six flavonoids (apigenin, genistein, luteolin, naringenin, quercetin, and resveratrol) that are widely present in fruits and vegetables, and often used as replacement therapy in menopause. We performed comparative computational docking simulations to predict their capability of binding nuclear receptors ERα, ERβ, ERRβ, ERRγ, androgen receptor (AR), and its variant ART877A and membrane receptors for androgens, i.e., ZIP9, GPRC6A, OXER1, TRPM8, and estrogens, i.e., G Protein-Coupled Estrogen Receptor (GPER). In agreement with data reported in literature, our results suggest that these flavonoids show a relevant degree of complementarity with both estrogen and androgen NR binding sites, likely triggering genomic-mediated effects. It is noteworthy that reliable protein-ligand complexes and estimated interaction energies were also obtained for some suggested estrogen and androgen membrane receptors, indicating that flavonoids could also exert non-genomic actions. Further investigations are needed to clarify flavonoid multiple genomic and non-genomic effects. Caution in their administration could be necessary, until the safe assumption of these natural molecules that are largely present in food is assured.

The emerging role of estrogen related receptorα in complications of non-small cell lung cancers

Approximately 85% of lung cancer cases are recognized as non-small cell lung cancer (NSCLC) with a perilous (13–17%) 5-year survival in Europe and the USA. Although tobacco smoking has consistently emerged as the leading cause of NSCLC complications, its consequences are distinctly manifest with respect to sex bias, due to differential gene and sex hormone expression. Estrogen related receptor α (ERRα), a member of the nuclear orphan receptor superfamily is normally expressed in the lungs, and activates various nuclear genes without binding to the ligands, such as estrogens. In NSCLC ERRα expression is significantly higher compared with healthy individuals. It is well established ERα and ERβ‚ have 93% and 60% identity in the DNA and ligand binding domains, respectively. ERα and ERRα have 69% (70% with ERRα-1) and 34% (35% with ERRα-1) identity, respectively; ERRα and ERRβ‚ have 92 and 61% identity, respectively. However, whether there is distinctive ERRα interaction with mammalian estrogens or concurrent involvement in non-ER signalling pathway activation is not known. Relevant to NSCLC, ERRα promotes proliferation, invasion and migration by silencing the tumor suppressor proteins p53 and pRB, and accelerates G₂-M transition during cell division. Epithelial to mesenchymal transition (EMT) and activation of Slug (an EMT associated transcription factor) are the prominent mechanisms by which ERRα activates NSCLC metastasis. Based on these observations, the present article focuses on the feasibility of antiERRα therapy alone and in combination with antiER as a therapeutic strategy for NSCLC complications.

Estrogen-related receptors: novel potential regulators of osteoarthritis pathogenesis

Osteoarthritis (OA) is a chronic inflammatory disease that is associated with articular cartilage destruction, subchondral bone alterations, synovitis, and even joint deformity and the loss of joint function. Although current basic research on the pathogenesis of OA has made remarkable progress, our understanding of this disease still needs to be further improved. Recent studies have shown that the estrogen-related receptor (ERR) family members ERRα and ERRγ may play significant roles in the pathogenesis of OA. In this review, we refer to the latest research on ERRs and the pathogenesis of OA, elucidate the structure and physiopathological functions of the ERR orphan nuclear receptor family, and systematically examine the relationship between ERRs and OA at the molecular level. Moreover, we also discuss and predict the capacity of ERRs as potential targets in the clinical treatment of OA.

Structural Insights into the Specificity of Ligand Binding and Coactivator Assembly by Estrogen-Related Receptor β

Estrogen-related receptor β (ERRβ) is a nuclear receptor critical for many biological processes. Despite the biological and pharmaceutical importance of ERRβ, deciphering the structure of ERRβ has been hampered by the difficulties in obtaining a pure and stable protein for structural studies. In fact, the ERRβ ligand-binding domain remains the last unsolved ERR structure and also one of only a few unknown nuclear receptor structures. Here, we report the identification of a critical single-residue mutation resulted in robust solubility and stability of an active ERRβ ligand-binding domain, thereby providing a protein tool enabling the first probe into the biochemical and structural studies of this important receptor. The crystal structure reveals key structural features that have enabled the integration of the molecular determinants of signals transduced across the ligand binding and coregulator recruitment by all three ERR subtypes, which also provides a framework for the rational design of selective and potent ligands for the treatment of various ERR-mediated diseases.

Estrogen receptor alpha (ERα)-mediated coregulator binding and gene expression discriminates the toxic ERα agonist diethylstilbestrol (DES) from the endogenous ERα agonist 17β-estradiol (E2)

Diethylstilbestrol (DES) is a synthetic estrogen and proven human teratogen and carcinogen reported to act via the estrogen receptor α (ERα). Since the endogenous ERα ligand 17β-estradiol (E2) does not show these adverse effects to a similar extent, we hypothesized that DES' interaction with the ERα differs from that of E2. The current study aimed to investigate possible differences between DES and E2 using in vitro assays that detect ERα-mediated effects, including ERα-mediated reporter gene expression, ERα-mediated breast cancer cell (T47D) proliferation and ERα-coregulator interactions and gene expression in T47D cells. Results obtained indicate that DES and E2 activate ERα-mediated reporter gene transcription and T47D cell proliferation in a similar way. However, significant differences between DES- and E2-induced binding of the ERα to 15 coregulator motifs and in transcriptomic signatures obtained in the T47D cells were observed. It is concluded that differences observed in binding of the ERα with several co-repressor motifs, in downregulation of genes involved in histone deacetylation and DNA methylation and in upregulation of CYP26A1 and CYP26B1 contribute to the differential effects reported for DES and E2.

Identification, characterization and mRNA transcript abundance profiles of estrogen related receptor (ERR) in Sepiella japonica imply its possible involvement in female reproduction

Estrogen related receptors (ERRs) are widely detected in vertebrates and apparently have functions in reproduction. The functions of ERRs in reproduction of invertebrates, especially in mollusk cephalopods, are largely unknown. In the present study, An homologue of vertebrate ERR gene was first cloned from female Sepiella japonica, an important Cephalopod species in coastal water of China. Results indicate the S. japonica ERR (sjERR) gene is comprised of 1513 nucleotides, containing a 1389 bp open reading frame, which encode for 463 amino acid (aa) residues. The deduced sjERR protein possessed six typical nuclear receptors (NR) domains (A-F), with a DNA-binding domain (DBD) and a highly conserved ligand-binding domain (LBD), compared to the other molluscan ERRs. Results from tissue analyses indicated that sjERR mRNA transcript abundance was in largest amounts in tissues of the brain, liver, ovary that are possibly involved in reproduction. The sjERR mRNA transcript abundance was temporally regulated during the different sexual maturation phases of female S. japonica and was affected by in vivo administrations of vertebrate steroid estradiol-17β (E2). An in vivo knockdown of sjERR gene expression resulted in a marked down-regulation in expression of genes involved in ovarian development, such as Vitellogenin, CDK1, and Cyclin B, indicating there is a possible involvement of sjERR in reproduction. Both fusion protein transient transfections and immunohistochemical analyses indicated a presence of sjERR in the nucleus, implying a possible mechanism of action of the sjERR in the nucleus through activation of specific gene transcriptions.

Computational insights into the interaction mechanisms of estrogen-related receptor alpha with endogenous ligand cholesterol

Estrogen-related receptor alpha (ERRα) has attracted increasing concerns. ERRα, orphan nuclear receptor, plays important roles in energy metabolism. Therefore, small molecule agonists of ERRα could be a potential therapeutic strategy in the treatment of metabolic diseases such as diabetes. Recently, Wei et al. identified cholesterol as the endogenous agonist of ERRα. However, the detailed molecular mechanism of cholesterol bound with ERRα remains ambiguous. Thus, in this study molecular docking and molecular dynamics (MD) simulations were performed to characterize how cholesterol affects the behavior of ERRα. Based on the results, we found that a proven residue Phe232 and others including Leu228, Glu235, Arg276, and Phe399 were key residues to ligand binding. A hydrogen-bonding interaction between cholesterol and Glu235 ensured the orientation of the ligand in the binding pocket, while hydrophobic interactions between cholesterol and the above-mentioned residues promoted the stability of ERRα-cholesterol complex. In the presence of the proliferator-activated receptor γ coactivator 1α (PGC-1α), the cholesterol-ERRα interaction became more stable. Interestingly, we observed that cholesterol facilitated the binding of ERRα with its coactivator PGC-1α via stabilizing the conformation of helix 12 and the interaction surface of ERRα/PGC-1α. Overall, these findings would be valuable for the future rational design of novel ERRα agonists.

A novel fatty acid-binding protein 5-estrogen-related receptor α signaling pathway promotes cell growth and energy metabolism in prostate cancer cells

Epidermal or cutaneous fatty acid-binding protein is an intracellular lipid-binding protein, also known as FABP5, and its expression level is closely related to cancer cell proliferation and metastatic activities in various types of carcinoma. However, the molecular mechanisms of FABP5 in cancer cell proliferation and its other functions have remained unclear. In the present study, we have clearly revealed that FABP5 activated expression of metabolic genes (ATP5B, LCHAD, ACO2, FH and MFN2) via a novel signaling pathway in an ERRα (estrogen-related receptor α)-dependent manner in prostate cancer cell lines. To clarify the novel function of FABP5, we examined the activation mechanisms of the ERRα target genes via FABP5. A direct protein-protein interaction between FABP5 and ERRα was demonstrated by immunoprecipitation and GST pull-down assays. We have clearly revealed that FABP5 interacted directly with transcriptional complex containing ERRα and its co-activator PGC-1β to increase expression of the ERRα target genes. In addition, we have shown that FABP5 knockdown induced high energy stress leading to induction of apoptosis and cell cycle arrest via AMPK-FOXO3A signaling pathway in prostate cancer cells, suggesting that FABP5 plays an important role in cellular energy status directing metabolic adaptation to support cellular proliferation and survival.

Establishment of primary mixed cell cultures from spontaneous canine mammary tumors: Characterization of classic and new cancer-associated molecules

There are many factors which make canine cancer like cancer in humans. The occurrence of spontaneous mammary tumors in pet dogs, tumor genetics, molecular targets and exposure to the same environmental risk factors are among these factors. Therefore, the study of canine cancer can provide useful information to the oncology field. This study aimed to establish and characterize a panel of primary mixed cell cultures obtained from spontaneous canine mammary tumors. Eight established cell cultures obtained from one normal mammary gland, one complex adenoma, one mixed adenoma, two complex carcinomas and two mixed carcinomas were analyzed. The gene expression levels of classic molecular cancer players such as fibroblast growth factor receptor (FGFR) 2, breast cancer (BRCA) 1, BRCA2 and estrogen receptor (ESR) 1 were evaluated. For the first time, three orphan nuclear receptors, estrogen-related receptors (ERRs) α, β and γ were studied in canine mammary cancer. The highest expression level of ERRα was observed in complex carcinoma-derived cell culture, while the highest levels of ERRβ and γ were observed in cells derived from a mixed carcinoma. Meanwhile, complex carcinomas presented the highest levels of expression of ESR1, BRCA1 and FGFR2 among all samples. BRCA2 was found exclusively in complex adenoma. The transcription factor GATA3 had its highest levels in mixed carcinoma samples and its lowest levels in complex adenoma. Proliferation assays were also performed to evaluate the mixed cell cultures response to ER ligands, genistein and DES, both in normoxia and hypoxic conditions. Our results demonstrate that morphological and functional studies of primary mixed cell cultures derived from spontaneous canine mammary tumors are possible and provide valuable tool for the study of various stages of mammary cancer development.

Estrogen-Related Receptors and the control of bone cell fate

Bone loss is naturally occurring in aging males and females and exacerbated in the latter after menopause, altogether leading to cumulative skeleton fragility and increased fracture risk. Two types of therapeutic strategies can be envisioned to counteract age- or menopause-associated bone loss, aiming at either reducing bone resorption exerted by osteoclasts or, alternatively, promoting bone formation by osteoblasts. We here summarize data suggesting that inhibition of the Estrogen-Related Receptors α and/or γ could promote bone formation and compensate for bone loss induced by ageing or estrogen-deficiency.

Gene expression underlying enhanced, steroid-dependent auditory sensitivity of hair cell epithelium in a vocal fish

BACKGROUND

Successful animal communication depends on a receiver's ability to detect a sender's signal. Exemplars of adaptive sender-receiver coupling include acoustic communication, often important in the context of seasonal reproduction. During the reproductive summer season, both male and female midshipman fish (Porichthys notatus) exhibit similar increases in the steroid-dependent frequency sensitivity of the saccule, the main auditory division of the inner ear. This form of auditory plasticity enhances detection of the higher frequency components of the multi-harmonic, long-duration advertisement calls produced repetitively by males during summer nights of peak vocal and spawning activity. The molecular basis of this seasonal auditory plasticity has not been fully resolved. Here, we utilize an unbiased transcriptomic RNA sequencing approach to identify differentially expressed transcripts within the saccule's hair cell epithelium of reproductive summer and non-reproductive winter fish.

RESULTS

We assembled 74,027 unique transcripts from our saccular epithelial sequence reads. Of these, 6.4 % and 3.0 % were upregulated in the reproductive and non-reproductive saccular epithelium, respectively. Gene ontology (GO) term enrichment analyses of the differentially expressed transcripts showed that the reproductive saccular epithelium was transcriptionally, translationally, and metabolically more active than the non-reproductive epithelium. Furthermore, the expression of a specific suite of candidate genes, including ion channels and components of steroid-signaling pathways, was upregulated in the reproductive compared to the non-reproductive saccular epithelium. We found reported auditory functions for 14 candidate genes upregulated in the reproductive midshipman saccular epithelium, 8 of which are enriched in mouse hair cells, validating their hair cell-specific functions across vertebrates.

CONCLUSIONS

We identified a suite of differentially expressed genes belonging to neurotransmission and steroid-signaling pathways, consistent with previous work showing the importance of these characters in regulating hair cell auditory sensitivity in midshipman fish and, more broadly, vertebrates. The results were also consistent with auditory hair cells being generally more physiologically active when animals are in a reproductive state, a time of enhanced sensory-motor coupling between the auditory periphery and the upper harmonics of vocalizations. Together with several new candidate genes, our results identify discrete patterns of gene expression linked to frequency- and steroid-dependent plasticity of hair cell auditory sensitivity.

Constitutive activities of estrogen-related receptors: Transcriptional regulation of metabolism by the ERR pathways in health and disease

The estrogen-related receptors (ERRs) comprise a small group of orphan nuclear receptor transcription factors. The ERRα and ERRγ isoforms play a central role in the regulation of metabolic genes and cellular energy metabolism. Although less is known about ERRβ, recent studies have revealed the importance of this isoform in the maintenance of embryonic stem cell pluripotency. Thus, ERRs are essential to many biological processes. The development of several ERR knockout and overexpression models and the application of advanced functional genomics have allowed rapid advancement of our understanding of the physiology regulated by ERR pathways. Moreover, it has enabled us to begin to delineate the distinct programs regulated by ERRα and ERRγ that have overlapping effects on metabolism and growth. The current review primarily focuses on the physiologic roles of ERR isoforms related to their metabolic regulation; therefore, the ERRα and ERRγ are discussed in the greatest detail. We emphasize findings from gain- and loss-of-function models developed to characterize ERR control of skeletal muscle, heart and musculoskeletal physiology. These models have revealed that coordinating metabolic capacity with energy demand is essential for seemingly disparate processes such as muscle differentiation and hypertrophy, innate immune function, thermogenesis, and bone remodeling. Furthermore, the models have revealed that ERRα- and ERRγ-deficiency in mice accelerates progression of pathologic processes and implicates ERRs as etiologic factors in disease. We highlight the human diseases in which ERRs and their downstream metabolic pathways are perturbed, including heart failure and diabetes. While no natural ligand has been identified for any of the ERR isoforms, the potential for using synthetic small molecules to modulate their activity has been demonstrated. Based on our current understanding of their transcriptional mechanisms and physiologic relevance, the ERRs have emerged as potential therapeutic targets for treatment of osteoporosis, muscle atrophy, insulin resistance and heart failure in humans.

Estrogen-related Receptor β Reduces the Subnuclear Mobility of Estrogen Receptor α and Suppresses Estrogen-dependent Cellular Function

Estrogen-related receptor (ERR) is a member of the nuclear receptor superfamily that has strong homology with estrogen receptor (ER) α. ERR has three subtypes (α, β, and γ) expressed in estrogen-sensitive organs, including ovary, breast, and brain. No endogenous ligands of ERRs have been identified, but these receptors share a common DNA element with ERα and control estrogen-mediated gene transcription. Recent evidence suggests a role of ERRs in estrogen-related pathophysiology, but the detailed mechanisms of ERR functions in estrogen-related tissues are unclear. Using live-cell imaging with fluorescent protein labeling, we found that only ERRβ among the ERRs exhibits a punctate intranuclear pattern overlapping with ERα following 17β-estradiol (E2)-stimulation. Fluorescence recovery after photobleaching showed significant reduction of the mobility of ligand-activated ERα with co-expression of ERRβ. Fluorescence resonance energy transfer revealed that ERRβ directly interacts with ERα. The N-terminal domain of ERRβ was identified as the region that interacts with ERα. We also found a correlation between punctate cluster formation of ERα and interaction between the receptors. Expression of ERRβ significantly repressed ERα-mediated transactivity, whereas that of other ERR subtypes had no effect on the transactivity of ERα. Consistent with this finding, E2-stimulated proliferation of MCF-7 breast carcinoma cells and bcl-2 expression was significantly inhibited by expression of ERRβ. These results provide strong evidence for a suppressive effect of ERRβ on estrogen signaling through reduction of the intranuclear mobility of ERα. The findings further suggest a unique inhibitory role for ERRβ in estrogen-dependent cellular function such as cancer cell proliferation.

Production and characterization of monoclonal antibodies to estrogen-related receptor alpha (ERRα) and use in immunoaffinity chromatography

Estrogen-related receptor alpha (ERRα) is an orphan nuclear receptor whose elevated expression is thought to contribute to breast, colon, and ovarian cancers. In order to investigate the role of ERRα in human disease, there is a need for immunological reagents suitable for detection and purification of ERRα. We expressed recombinant human ERRα in Escherichia coli, purified the protein, and used it to generate monoclonal antibodies (mAbs) to ERRα. Nine high-affinity mAbs were chosen for their abilities to detect overexpressed ERRα in enzyme-linked immunosorbent assays (ELISAs) and Western blots, after which isotyping and preliminary epitope mapping was performed. The mAbs were all IgG subtypes and reacted with several different regions of full-length ERRα. A majority of the mAbs were found to be useful for immunoprecipitation of ERRα, and several could detect DNA-bound ERRα in electrophoretic mobility supershift assays (EMSAs) and chromatin immunoprecipitation (ChIP). The suitability of mAbs to detect ERRα in immunofluorescence assays was assessed. One mAb in particular, 2ERR10, could specifically detect endogenous ERRα in mammary carcinoma cells. Finally, we performed assays to screen for mAbs that gently release ERRα in the presence of a low-molecular-weight polyhydroxylated compound (polyol) and nonchaotropic salt. Using gentle immunoaffinity chromatography, we were able to isolate ERRα from mammalian cells by eluting with a polyol-salt solution. Our characterization studies show that these monoclonal antibodies perform well in a variety of biochemical assays. We anticipate that these novel reagents will prove useful for the detection and purification of ERRα in research and clinical applications.

Negative regulation of ERRα by a novel nucleolar protein

The regulation of estrogen-related receptor (ERR) transcriptional activity is poorly understood. To explore the underlying mechanism, we sought to isolate ERRα-binding protein(s). In a yeast two-hybrid screen, we identified a novel protein that has been characterized as a retinoic acid resistance factor (RaRF) (manuscript in-preparation). A specific interaction between RaRF and ERRα was confirmed in a GST pull-down assay in vitro and immunoprecipitation (IP) in mammalian cells. Further yeast two-hybrid assays and IP analyses indicated that the C-terminus of ERRα is required for RaRF binding. Consistent with our interaction data, transfection of RaRF significantly reduced the ability of ERRα, but not ERRγ, to transactivate an ERR-responsive luciferase reporter. In contrast, down-regulation of RaRF using shRNA increased ERRα activity without affecting that of ERRγ. RaRF was subsequently shown to repress the expression of the ERR target gene pS2. Further fluorescence microscopy revealed that ERRα or ERRγ is normally expressed in the nucleoplasm, with ERRα, but not ERRγ, translocating to the nucleolus when RaRF is expressed. Taken together, our data suggest that RaRF sequesters ERRα in the nucleolus through a specific interaction, thereby inhibiting its transcriptional activity.

Estrogen-related receptor ERRα-mediated downregulation of human hydroxysteroid sulfotransferase (SULT2A1) in Hep G2 cells

Hydroxysteroid sulfotransferase SULT2A1 catalyzes the sulfation of hydroxysteroids and xenobiotics. It plays an important role in the detoxification of hydroxyl-containing xenobiotics and in the regulation of the biological activities of hydroxysteroids. ERRα is an orphan member of the nuclear receptor superfamily that is closely related to estrogen receptor alpha (ERα). Here we report that the mRNA expression of human SULT2A1 was suppressed by ERRα in Hep G2 cells. To investigate the mechanisms of this regulation, the effects of ERRα on human SULT2A1 promoter transcription in Hep G2 cells were investigated. Reporter luciferase assay results showed that ERRα significantly represses human SULT2A1 promoter transcription in Hep G2 cells. Deletion analysis indicated that human SULT2A1 promoter region between positions -188 and -130 is necessary for its repression by ERRα in Hep G2 cells. The 5' DNA -188 to -130 region of human SULT2A1 contains IR2 and DR4 hormone response elements and two putative ERRα response elements (ERREs) (ERRE188: GCAAGCTCA and ERRE155: ATAAGTTCA). Interestingly, ERRE188 overlaps with the IR2 element and ERRE155 overlaps with the DR4 element. Our further investigation demonstrated that ERRα represses human SULT2A1 promoter transcription by competing with other nuclear receptors for binding to IR2 or DR4 elements. The interaction of ERRE188 and ERRE155 elements with ERRα was confirmed by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analysis. Our results suggest that ERRα may play an important role in regulating the metabolism of drugs and xenobiotics and in regulating endogenous hydroxysteroid activities via the regulation of SULT2A1.

Protein evolution by molecular tinkering: diversification of the nuclear receptor superfamily from a ligand-dependent ancestor

Phylogenetic reconstruction of the structure and function of the ancestor of the nuclear receptor protein family reveals how functional diversity evolves by subtle tinkering with an ancestral template.

Understanding how protein structures and functions have diversified is a central goal in molecular evolution. Surveys of very divergent proteins from model organisms, however, are often insufficient to determine the features of ancestral proteins and to reveal the evolutionary events that yielded extant diversity. Here we combine genomic, biochemical, functional, structural, and phylogenetic analyses to reconstruct the early evolution of nuclear receptors (NRs), a diverse superfamily of transcriptional regulators that play key roles in animal development, physiology, and reproduction. By inferring the structure and functions of the ancestral NR, we show—contrary to current belief—that NRs evolved from a ligand-activated ancestral receptor that existed near the base of the Metazoa, with fatty acids as possible ancestral ligands. Evolutionary tinkering with this ancestral structure generated the extraordinary diversity of modern receptors: sensitivity to different ligands evolved because of subtle modifications of the internal cavity, and ligand-independent activation evolved repeatedly because of various mutations that stabilized the active conformation in the absence of ligand. Our findings illustrate how a mechanistic dissection of protein evolution in a phylogenetic context can reveal the deep homology that links apparently “novel” molecular functions to a common ancestral form.

Many protein families are so diverse that it is hard to determine their ancestral functions and to understand how their derived functions evolved. The existence of so many different functions within protein families often creates the impression that complex, novel functions must have evolved repeatedly and independently. Nuclear receptors (NRs) are a large family of related proteins that regulate key biological processes in animals by binding to specific DNA sequences and triggering expression of nearby target genes. Many NRs are activated by a specific hormone or other small molecule, but some do not require a ligand, and still others are incapable of activating gene expression and so act primarily as repressors of transcription. To understand how the functional diversity of NRs evolved, we reconstructed the structural and functional characteristics of the ancient protein from which the entire family evolved, using genomic, biochemical, functional, and structural analyses in a phylogenetic framework. We show, contrary to current belief, that the ancestral NR was a ligand-activated transcriptional activator that existed in the earliest period of animal evolution. Our analysis reveals how the extraordinary functional diversity of modern receptors was generated by subtle tinkering with this ancestral template—slightly reshaping the ligand cavity, stabilizing the protein's active conformation so it no longer required a ligand, or disabling the protein's capacity to activate transcription without affecting its other properties. We predict that, when sufficient data are gathered to allow detailed evolutionary reconstructions in other protein families, it will become apparent that most protein functional diversity evolved by tinkering with ancient functions; invoking the evolution of wholesale “novelty” will seldom be necessary.

Regulation of metabolism by nuclear hormone receptors

The worldwide epidemic of metabolic disease indicates that a better understanding of the pathways contributing to the pathogenesis of this constellation of diseases need to be determined. Nuclear hormone receptors comprise a superfamily of ligand-activated transcription factors that control development, differentiation, and metabolism. Over the last 15 years a growing number of nuclear receptors have been identified that coordinate genetic networks regulating lipid metabolism and energy utilization. Several of these receptors directly sample the levels of metabolic intermediates and use this information to regulate the synthesis, transport, and breakdown of the metabolite of interest. In contrast, other family members sense metabolic activity via the presence or absence of interacting proteins. The ability of these nuclear receptors to impact metabolism and inflammation will be discussed and the potential of each receptor subfamily to serve as drug targets for metabolic disease will be highlighted.

The significance of the expression of ERRalpha as a potential biomarker in breast cancer

It was shown the functional crosstalk between ERRalpha and ERalpha in breast cancer, however, the biological significance of estrogen-related receptor alpha (ERRalpha) remains largely unclear. Therefore, we examined the expression of ERRalpha in 39 primary human breast cancer tissues and 19 matched normal tissues using RT-PCR and immunohistochemistry in the context of the aromatase, ERalpha and proliferation markers (c-myc, Ki-67) expression. Compared to the normal breast tissue, breast cancer tissues showed a slightly higher expression level of ERRalpha mRNA (mean 46.2+/-S.D.42.0, 57.7+/-S.D.58.7, respectively). However, ERRalpha mRNA levels in breast cancer tissues showed greater diversity than in normal tissues. Immunohistochemical analysis of breast cancers revealed perinuclear and cytoplasmic localization of ERRalpha. Our study shows that there is no correlation between ERRalpha and ERalpha expression. We demonstrated a positive correlation between ERRalpha and c-myc at the transcriptional level and statistically significant positive correlation between aromatase and the ERRalpha at protein level. It seems that ERRalpha could play an important role in the alternative pathway to classical estrogen receptors-dependent pathway in cell signaling. Development and use of ERRs modulators might lead in the future to design new well-tolerated and individualized therapeutic agents.

Helix 3-helix 5 interactions in steroid hormone receptor function

Steroid hormones working through their receptors regulate a wide variety of physiologic processes necessary for normal homeostasis. Recent years have witnessed great advances in our understanding of how these hormones interact with their receptors, and have brought us closer to the era of directed drug design. We previously described a novel intramolecular interaction between helix 3 and helix 5 which is responsible for a Mendelian form of human hypertension. Further studies revealed that this interaction is highly conserved throughout the steroid hormone receptor family and functions as a key regulator of steroid hormone receptor sensitivity and specificity. Here, we review the contribution of helix 3-helix 5 interaction to steroid hormone receptor activity, with an eye towards how this knowledge may aid in the creation of novel therapeutic agonists and antagonists.

The NR3B subgroup: an ovERRview

Members of the NR3B group of the nuclear receptor superfamily, known as the estrogen-related receptors (ERRs), were the first orphan receptors to be identified two decades ago. Despite the fact that a natural ligand has yet to be associated with the ERRs, considerable knowledge about their mode of action and biological functions has emerged through extensive biochemical, genetic and functional genomics studies. This review describes our current understanding of how the ERRs work as transcription factors and as such, how they control diverse developmental and physiological programs.

Crystal structure of human estrogen-related receptor alpha in complex with a synthetic inverse agonist reveals its novel molecular mechanism

Inverse agonists of the constitutively active human estrogen-related receptor alpha (ERRalpha, NR3B1) are of potential interest for several disease indications (e.g. breast cancer, metabolic diseases, or osteoporosis). ERRalpha is constitutively active, because its ligand binding pocket (LBP) is practically filled with side chains (in particular with Phe(328), which is replaced by Ala in ERRbeta and ERRgamma). We present here the crystal structure of the ligand binding domain of ERRalpha (containing the mutation C325S) in complex with the inverse agonist cyclohexylmethyl-(1-p-tolyl-1H-indol-3-ylmethyl)-amine (compound 1a), to a resolution of 2.3A(.) The structure reveals the dramatic multiple conformational changes in the LBP, which create the necessary space for the ligand. As a consequence of the new side chain conformation of Phe(328) (on helix H3), Phe(510)(H12) has to move away, and thus the activation helix H12 is displaced from its agonist position. This is a novel mechanism of H12 inactivation, different from ERRgamma, estrogen receptor (ER) alpha, and ERbeta. H12 binds (with a surprising binding mode) in the coactivator groove of its ligand binding domain, at a similar place as a coactivator peptide. This is in contrast to ERRgamma but resembles the situation for ERalpha (raloxifene or 4-hydroxytamoxifen complexes). Our results explain the novel molecular mechanism of an inverse agonist for ERRalpha and provide the basis for rational drug design to obtain isotype-specific inverse agonists of this potential new drug target. Despite a practically filled LBP, the finding that a suitable ligand can induce an opening of the cavity also has broad implications for other orphan nuclear hormone receptors (e.g. the NGFI-B subfamily).

Estrogen-related receptor alpha1 transcriptional activities are regulated in part via the ErbB2/HER2 signaling pathway

We previously showed that (a) estrogen-related receptor alpha1 (ERRalpha1) down-modulates estrogen receptor (ER)-stimulated transcription in low ErbB2-expressing MCF-7 mammary carcinoma cells, and (b) ERRalpha and ErbB2 mRNA levels positively correlate in clinical breast tumors. We show here that ERRalpha1 represses ERalpha-mediated activation in MCF-7 cells because it failed to recruit the coactivator glucocorticoid receptor interacting protein 1 (GRIP1) when bound to an estrogen response element. In contrast, ERRalpha1 activated estrogen response element- and ERR response element-mediated transcription in ERalpha-positive, high ErbB2-expressing BT-474 mammary carcinoma cells, activation that was enhanced by overexpression of GRIP1. Likewise, regulation of the endogenous genes pS2, progesterone receptor, and ErbB2 by ERRalpha1 reflected the cell type-specific differences observed with our reporter plasmids. Importantly, overexpression of activated ErbB2 in MCF-7 cells led to transcriptional activation, rather than repression, by ERRalpha1. Two-dimensional PAGE of radiophosphate-labeled ERRalpha1 indicated that it was hyperphosphorylated in BT-474 relative to MCF-7 cells; incubation of these cells with anti-ErbB2 antibody led to reduction in the extent of ERRalpha1 phosphorylation. Additionally, mitogen-activated protein kinases (MAPK) and Akts, components of the ErbB2 pathway, phosphorylated ERRalpha1 in vitro. ERRalpha1-activated transcription in BT-474 cells was inhibited by disruption of ErbB2/epidermal growth factor receptor signaling with trastuzumab or gefitinib or inactivation of downstream components of this signaling, MAPK kinase/MAPK, and phosphatidylinositol-3-OH kinase/Akt, with U0126 or LY294002, respectively. Thus, ERRalpha1 activities are regulated, in part, via ErbB2 signaling, with ERRalpha1 likely positively feedback-regulating ErbB2 expression. Taken together, we conclude that ERRalpha1 phosphorylation status shows potential as a biomarker of clinical course and antihormonal- and ErbB2-based treatment options, with ERRalpha1 serving as a novel target for drug development.

Definition of the molecular basis for estrogen receptor-related receptor-alpha-cofactor interactions

Estrogen receptor-related receptor-alpha (ERRalpha) is an orphan nuclear receptor that does not appear to require a classical small molecule ligand to facilitate its interaction with coactivators and/or hormone response elements within target genes. Instead, the apo-receptor is capable of interacting in a constitutive manner with coactivators that stimulate transcription by acting as protein ligands. We have screened combinatorial phage libraries for peptides that selectively interact with ERRalpha to probe the architecture of the ERRalpha-coactivator pocket. In this manner, we have uncovered a fundamental difference in the mechanism by which this receptor interacts with peroxisome proliferator-activated receptor-gamma coactivator-1alpha, as compared with members of the steroid receptor coactivator subfamily of coactivators. Our findings suggest that it may be possible to develop ERRalpha ligands that exhibit different pharmacological activities as a consequence of their ability to differentially regulate coactivator recruitment. In addition, these findings have implications beyond ERRalpha because they suggest that subtle alterations in the structure of the activation function-2 pocket within any nuclear receptor may enable differential recruitment of coactivators, an observation of notable pharmaceutical importance.

Molecular cloning and functional study of rat estrogen receptor-related receptor gamma in rat prostatic cells

BACKGROUND

Based on high homology of ERRs with ERs, we hypothesize that ERRs might functionally cross talk with ERs or independently in prostatic cells.

METHODS

We examined the ERRgamma expressions in rat prostates and Nb rat prostate cancer model, and its growth regulation in stable transfectants of prostatic cells.

RESULTS

We cloned the ERRgamma cDNA from rat prostate by RACE-PCR. Its expression was confirmed by Northern and immunoblottings. Real-time RT-PCR showed that its expression in castrated prostates was androgen-dependent. ERRgamma was expressed in prostatic epithelial cells, but showed reduced expressions in neoplastic prostates. Transfections confirmed that ERRgamma was expressed in prostatic cells as nuclear protein and transcriptionally active without estradiol. Its overexpression in ERRgamma-stable transfectants of NbE-1 and MAT-Lu cells inhibited their in vitro proliferation, anchorage-independent growth in soft-agar and tumorigenicity in nude mice.

CONCLUSIONS

Our studies show that ERRgamma is functionally expressed in rat prostate and may play anti-proliferative actions in prostatic cells. Its co-expression with ERs suggests that besides ERs, ligand-independent ERRgamma is also involved in prostatic growth and functions.

Expression of estrogen receptor-related receptor isoforms and clinical significance in endometrial adenocarcinoma

Estrogen receptor-related receptors (ERRs) are members of orphan nuclear receptors closely related to the estrogen receptors (ERs). Researches showed that the ERRs bound to estrogen response elements and interfered in the ER signal pathway. Therefore, it might be associated with estrogen-dependent diseases. The purpose of this study was to explore whether ERRs were involved in the tumorigenesis of endometrial adenocarcinoma. We examined the expression of ERRs in endometrial adenocarcinoma and normal endometrium using semiquantitative reverse transcription-polymerase chain reaction and immunohistochemistry. Clinicopathologic features including FIGO stage, histologic grade, myometrial invasion, and nodal metastasis were reviewed. Results showed that messenger RNA (mRNA) levels of ERRalpha, ERRbeta, and ERRgamma were positively associated with the immunoreactivities (P= 0.009, P= 0.014, and P= 0.001, respectively). The expression rate and relative level of ERRalpha mRNA in ERalpha-positive endometrial adenocarcinomas were lower than in normal endometriums (P= 0.049 and P= 0.023), whereas the relative level of ERRgamma mRNA in ERalpha-positive endometrial adenocarcinomas was higher than in normal endometriums (P= 0.014). Expression of ERRalpha mRNA was positively correlated with FIGO stage (P= 0.019) and myometrial invasion (P= 0.043). A negative correlation was observed between expression of ERRgamma mRNA and nodal metastasis (P= 0.021). Results suggested that ERRalpha and ERRgamma might participate in the tumorigenesis of endometrial adenocarcinoma. ERRalpha and ERRgamma are promising to be new prognostic factors in endometrial adenocarcinoma.

Expression of estrogen receptor-related receptors, a subfamily of orphan nuclear receptors, as new tumor biomarkers in ovarian cancer cells

A subfamily of orphan receptors, estrogen receptor-related receptors (ERRs), has been demonstrated to modulate the transcription of some estrogen responsive genes via variant estrogen response elements (EREs). This study was conducted to determine whether human ERRalpha, ERRbeta, and ERRgamma might be involved in the tumorigenesis of ovarian cancer. RT-PCR was performed to analyze the expression of hERRalpha, hERRbeta, hERRbeta-2, and hERRgamma mRNA in five ovarian cancer cell lines as well as 33 samples of ovarian cancer and 12 samples of normal ovary. Serum CA-125 levels were also analyzed in all samples by ELISA. Progression-free survival and overall survival of patients with different expression of ERRs were analyzed by the Kaplan-Meier method. To analyze the subcellular localization of ERRalpha, a green fluorescent protein (GFP)-reporter plasmid of hERRalpha was constructed and transfected into the ovarian cancer cell line OVCAR-3. Expression of hERRalpha-GFP fusion protein was observed in the nucleus of OVCAR-3 ovarian cancer cell lines. We observed increased expression of hERRalpha mRNA (P = 0.020) and hERRgamma mRNA (P = 0.045) in ovarian cancers compared to normal ovaries. In contrast, hERRbeta was only observed in 9.1% of ovarian cancers. We found a positive correlation between the serum CA-125 levels and hERRalpha expression (P = 0.012), but not hERRbeta and hERRgamma expression. Survival analysis showed that the hERRalpha-positive group has a reduced overall survival (P = 0.015), and the ERRgamma-positive group has a longer progression-free survival (P = 0.020). In multivariate analysis, expression of hERRalpha was an independent prognostic factor for poor survival (relative risk, 3.032; 95% CI, 1.27-6.06). Based on our results, ERRs may play an important role in ovarian cancer. hERRalpha may represent a biomarker of poor prognosis, and hERRgamma may be a new therapeutic target in ovarian cancer.

Positive and negative transcriptional regulation of aromatase expression in human breast cancer tissue

By performing primer-specific RT-PCR analyses, three laboratories including ours have found that exons I.3 and PII are the two major exon Is present in aromatase mRNAs isolated from breast tumors. These results suggest that promoters I.3 and II are the major promoters directing aromatase expression in breast tumors. The characterization of transcription factors that interact with the two elements near promoters I.3 and II, i.e., S1 and CREaro, helps us better understand the mechanism of the switch of promoter usage between normal breast tissue and cancer tissue. The positions of the two regulatory regions were mapped by DNase I footprinting and DNA deletion analyses. We applied the yeast one-hybrid approach to screen a human breast tissue hybrid cDNA expression library for genes encoding the proteins binding to these regions. Our results suggest that in normal breast tissue, the function of promoters I.3 and II is suppressed through the binding of EAR-2, COUP-TFI, and RARgamma to S1, and through the binding of Snail/Slug proteins to their binding site that quenches the CREaro activity. In cancer tissue, the expression levels of EAR-2, COUP-TF1, EARgamma, Snail, and Slug decrease, and aromatase expression is then up-regulated through the binding of ERRalpha to S1 and the binding of CREB1 or related factors to CREaro. In a separate study, we found that estrogen could up-regulate aromatase expression in breast cancer cells by a non-genomic action of ERalpha via cross-talk with growth factor-mediated pathways. Our preliminary results suggest that protein kinase C delta participates in this ERalpha-growth factor mediated regulation. To further understand the regulatory mechanism, we have recently initiated an in vivo footprinting analysis of the -260/+76 bp region of promoter I.3. The experiments were conducted with both MCF-7 and MDA-MB-231 breast cancer cells. Our results revealed several footprinted sites. Five regions (sites 1-5) were then selected for functional analysis through DNA site-directed mutagenesis experiments. This analysis has also confirmed the promoter I.3 TATA site and Snail/Slug binding site. These mutants showed higher luciferase activity when compared to the wild-type, indicating that the proteins binding to these sites were acting as repressors. By reviewing findings from our laboratory and other laboratories, a detailed mechanism for the transcriptional regulation of aromatase expression in breast cancer tissue is summarized and discussed.

A critical role of helix 3-helix 5 interaction in steroid hormone receptor function

The ligand-binding domains of steroid hormone receptors possess a conserved structure with 12 alpha-helices surrounding a central hydrophobic core. On agonist binding, a repositioned helix 12 forms a pocket with helix 3 (H3) and helix 5 (H5), where transcriptional coactivators bind. The precise molecular interactions responsible for activation of these receptors remain to be elucidated. We previously identified a H3-H5 interaction that permits progesterone-mediated activation of a mutant mineralocorticoid receptor. We were intrigued to note that the potential for such interaction is widely conserved in the nuclear receptor family, indicating a possible functional significance. Here, we demonstrate via transcriptional activation studies in cell culture that alteration of residues involved in H3-H5 interaction consistently produces a gain of function in steroid hormone receptors. These data suggest that H3-H5 interaction may function as a molecular switch regulating the activity of nuclear receptors and suggest this site as a general target for pharmacologic intervention. Furthermore, they reveal a general mechanism for the creation of nuclear receptors bearing increased activity, providing a potentially powerful tool for the study of physiologic pathways in vivo.

Estrogen receptor mutations in human disease

As early as the 1800s, the actions of estrogen have been implicated in the development and progression of breast cancer. The estrogen receptor (ER) was identified in the late 1950s and purified a few years later. However, it was not until the 1980s that the first ER was molecularly cloned, and in the mid 1990s, a second ER was cloned. These two related receptors are now called ERalpha and ERbeta, respectively. Since their discovery, much research has focused on identifying alterations within the coding sequence of these receptors in clinical samples. As a result, a large number of naturally occurring splice variants of both ERalpha and ERbeta have been identified in normal epithelium and diseased or cancerous tissues. In contrast, only a few point mutations have been identified in human patient samples from a variety of disease states, including breast cancer, endometrial cancer, and psychiatric diseases. To elucidate the mechanism of action for these variant isoforms or mutant receptors, experimental mutagenesis has been used to analyze the function of distinct amino acid residues in the ERs. This review will focus on ERalpha and ERbeta alterations in breast cancer.

Effects of estradiol, benzophenone-2 and benzophenone-3 on the expression pattern of the estrogen receptors (ER) alpha and beta, the estrogen receptor-related receptor 1 (ERR1) and the aryl hydrocarbon receptor (AhR) in adult ovariectomized rats

The estrogen receptors (ERs) are members of a super family of ligand-activated transcription factors mediating estrogenic responses. A close functional kinship was found for the structurally related estrogen receptor-related receptor1 (ERR1), a constitutively active transcription factor. The aryl hydrocarbon receptor (AhR) mediates the toxic and estrogenic effects of a wide variety of environmental contaminants and industrial pollutants. Both the ERR1 and the AhR are known to modulate the ER's signalling pathways in multiple ways. Organic chemicals with a certain structural relationship to steroid hormones often induce a tissue- or cell-specific variety of responses distinct from estrogenic responses and this may involve ERR1 and AhR. The UV-screens benzophenone-2 and benzophenone-3 (BP2, BP3), structurally related to known steroid receptor ligands, are used in cosmetics and plastics to improve product stability and durability. Both BP2 and BP3 were shown to exert uterotrophic effects and BP2 was shown to bind to the estrogen receptors. Whether such effects are also exerted in other organs is unknown. Therefore, an approach to a multi-organic risk assessment for these substances was made by measuring the gene-expression of the four mentioned receptors in the pituitary, the uterus and the thyroid after a five-day treatment in comparison to estradiol. Though BP2 seems to exert an estrogen-like effect while BP3 does not, there are regulatory effects on receptor expression for both substances that indicate a kind of endocrine disruption that is not assessed by "classical" estrogenic markers.

Evidence for ligand-independent transcriptional activation of the human estrogen-related receptor alpha (ERRalpha): crystal structure of ERRalpha ligand binding domain in complex with peroxisome proliferator-activated receptor coactivator-1alpha

The crystal structure of the ligand binding domain (LBD) of the estrogen-related receptor alpha (ERRalpha, NR3B1) complexed with a coactivator peptide from peroxisome proliferator-activated receptor coactivator-1alpha (PGC-1alpha) reveals a transcriptionally active conformation in the absence of a ligand. This is the first x-ray structure of ERRalpha LBD, solved to a resolution of 2.5 A, and the first structure of a PGC-1alpha complex. The putative ligand binding pocket (LBP) of ERRalpha is almost completely occupied by side chains, in particular with the bulky side chain of Phe328 (corresponding to Ala272 in ERRgamma and Ala350 in estrogen receptor alpha). Therefore, a ligand of a size equivalent to more than approximately 4 carbon atoms could only bind in the LBP, if ERRalpha would undergo a major conformational change (in particular the ligand would displace H12 from its agonist position). The x-ray structure thus provides strong evidence for ligand-independent transcriptional activation by ERRalpha. The interactions of PGC-1alpha with ERRalpha also reveal for the first time the atomic details of how a coactivator peptide containing an inverted LXXLL motif (namely a LLXYL motif) binds to a LBD. In addition, we show that a PGC-1alpha peptide containing this nuclear box motif from the L3 site binds ERRalpha LBD with a higher affinity than a peptide containing a steroid receptor coactivator-1 motif and that the affinity is further enhanced when all three leucine-rich regions of PGC-1alpha are present.

Biochemical and biological characterization of a novel anti-aromatase coumarin derivative

Estrogen stimulates the proliferation of estrogen receptor (ER)-positive breast cancer cells. Aromatase is the enzyme responsible for the conversion of androgens into estrogens, and synthetic aromatase inhibitors such as letrozole, anastrozole, and exemestane have proven to be effective endocrine regimens for ER-positive breast cancer. In a recent study, we have found that 4-benzyl-3-(4'-chlorophenyl)-7-methoxycoumarin is a potent competitive inhibitor of aromatase with respect to the androgen substrate. Its K(i) value was determined to be 84 nm, significantly more potent than several known aromatase inhibitors. The specific interaction of this compound with aromatase was further demonstrated by the reduction of its binding by several mutations at the active site region of aromatase and evaluated by computer modeling analysis. The structure-activity studies have revealed that three functional groups (i.e. 3-(4'-chlorophenyl), 4-benzyl, and 7-methoxyl) of this coumarin are important in its inhibition of aromatase. In addition, through a matrigel thread three-dimensional cell culture, this compound was shown to behave like known aromatase inhibitors that suppress the proliferation of aromatase and estrogen receptor positive MCF-7aro breast cancer cells. This coumarin has been shown not to be cytotoxic at up to 40 mum. It was found not to be an inhibitor of steroid 5alpha-reductase that also utilizes androgen as the substrate and not to be a ligand of ERalpha, ERbeta, estrogen-related receptors, or androgen receptor. These results demonstrate that coumarins (a common type of phytochemical) or their derivatives can be potent inhibitors of aromatase and may be useful in suppressing aromataseand ER-positive breast tumors.

Structural Basis for the Deactivation of the Estrogen-related Receptor γ by Diethylstilbestrol or 4-Hydroxytamoxifen and Determinants of Selectivity

The estrogen-related receptor (ERR) gamma behaves as a constitutive activator of transcription. Although no natural ligand is known, ERRgamma is deactivated by the estrogen receptor (ER) agonist diethylstilbestrol and the selective ER modulator 4-hydroxytamoxifen but does not significantly respond to estradiol or raloxifene. Here we report the crystal structures of the ERRgamma ligand binding domain (LBD) complexed with diethylstilbestrol or 4-hydroxytamoxifen. Antagonist binding to ERRgamma results in a rotation of the side chain of Phe-435 that partially fills the cavity of the apoLBD. The new rotamer of Phe-435 displaces the "activation helix" (helix 12) from the agonist position observed in the absence of ligand. In contrast to the complexes of the ERalpha LBD with 4-hydroxytamoxifen or raloxifene, helix 12 of antagonist-bound ERRgamma does not occupy the coactivator groove but appears to be completely dissociated from the LBD body. Comparison of the ligand-bound LBDs of ERRgamma and ERalpha reveals small but significant differences in the architecture of the ligand binding pockets that result in a slightly shifted binding position of diethylstilbestrol and a small rotation of 4-hydroxytamoxifen in the cavity of ERRgamma relative to ERalpha. Our results provide detailed molecular insight into the conformational changes occurring upon binding of synthetic antagonists to the constitutive orphan receptor ERRgamma and reveal structural differences with ERs that explain why ERRgamma does not bind estradiol or raloxifene and will help to design new selective antagonists.

Regulation of PPARgamma coactivator 1alpha (PGC-1alpha) signaling by an estrogen-related receptor alpha (ERRalpha) ligand

Peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha) is a transcriptional coactivator that is a key component in the regulation of energy production and utilization in metabolic tissues. Recent work has identified PGC-1alpha as a strong coactivator of the orphan nuclear receptor estrogen-related receptor alpha (ERRalpha), implicating ERRalpha as a potential mediator of PGC-1alpha action. To understand the role of ERRalpha in PGC-1alpha signaling, a parallel approach of high-throughput screening and gene-expression analysis was used to identify ERRalpha small-molecule regulators and target genes. We report here the identification of a potent and selective ERRalpha inverse agonist that interferes effectively with PGC-1alpha/ERRalpha-dependent signaling. This inverse agonist inhibits the constitutive activity of ERRalpha in both biochemical and cell-based assays. Also, we demonstrate that monoamine oxidase B is an ERRalpha target gene whose expression is regulated by PGC-1alpha and ERRalpha and inhibited by the ERRalpha inverse agonist. The discovery of potent and selective ERRalpha modulators and their effect on PGC-1alpha signaling provides mechanistic insight into gene regulation by PGC-1alpha. These findings validate ERRalpha as a promising therapeutic target in the treatment of metabolic disorders, including diabetes and obesity.

A novel crosstalk mechanism between nuclear receptor-mediated and growth factor/Ras-mediated pathways through PNRC–Grb2 interaction

It has been demonstrated that proline-rich nuclear receptor coregulatory protein (PNRC) is a nuclear receptor coactivator that interacts with nuclear receptors through an SH3-binding motif located in its C-terminus. In the present report, a physical interaction between PNRC and Grb2 (an adapter protein involved in growth factor/Ras-mediated pathways) has been demonstrated using the GST pull-down assay, the yeast two-hybrid assay, as well as by coimmunoprecipitation. Cotransfection and fluorescence imaging have also confirmed the colocalization of PNRC and Grb2 in mammalian cells. Transient transfection experiments have demonstrated that, by interacting with each other, Grb2 decreases the coactivator activity of PNRC for nuclear receptors, and that PNRC suppresses Grb2-mediated Ras/MAP-kinase activation. Furthermore, it was discovered that HeLa cells overexpressing PNRC grew more slowly when compared to matched controls. Additionally, using a RT-PCR analysis of mRNA on six pairs of cancer/noncancer tissues, PNRC expression was found to be significantly lower in breast cancer tissue than in noncancer tissue. Based on these findings, we believe that PNRC and Grb2, by interacting with each other, can suppress nuclear receptor-mediated regulation and growth factor-mediated regulation in human breast tissue. This is a newly identified crosstalk mechanism for modulating these two important types of regulatory pathways.

A polymorphic autoregulatory hormone response element in the human estrogen-related receptor alpha (ERRalpha) promoter dictates peroxisome proliferator-activated receptor gamma coactivator-1alpha control of ERRalpha expression

The orphan nuclear estrogen-related receptor alpha (ERRalpha) and transcriptional cofactor peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) are involved in the regulation of energy metabolism. Recently, extensive cross-talk between PGC-1alpha and ERRalpha has been demonstrated. The presence of PGC-1alpha is associated with an elevated expression of ERRalpha, and the two proteins can influence the transcriptional activities of one another. Using a candidate gene approach to detect regulatory variants within genes encoding nuclear receptors, we have identified a 23-bp sequence (ESRRA23) containing two nuclear receptor recognition half-site motifs that is present in 1-4 copies within the promoter of the human ESRRA gene encoding ERRalpha. The ESRRA23 sequence contains a functional ERR response element that is specifically bound by ERRalpha, and chromatin immunoprecipitation shows that endogenous ERRalpha occupies its own promoter in vivo. Strikingly, introduction of PGC-1alpha in HeLa cells by transient transfection induces the activity of the ESRRA promoter in a manner that is dependent on the presence of the ESRRA23 element and on its dosage. Coexpression of ERRalpha and PGC-1alpha results in a synergistic activation of the ESRRA promoter. In experiments using ERRalpha null fibroblasts, the ability of PGC-1alpha to stimulate the ESRRA promoter is considerably reduced but can be restored by addition of ERRalpha. Taken together, these results demonstrate that an interdependent ERRalpha/PGC-1alpha-based transcriptional pathway targets the ESRRA23 element to dictate the level of ERRalpha expression. This study further suggests that this regulatory polymorphism may provide differential responses to ERRalpha/PGC-1alpha-mediated metabolic cues in the human population.

Discovery of novel splice forms and functional analysis of cancer-specific alternative splicing in human expressed sequences

We report here a genome-wide analysis of alternative splicing in 2 million human expressed sequence tags (ESTs), to identify splice forms that are up-regulated in tumors relative to normal tissues. We found strong evidence (P < 0.01) of cancer-specific splice variants in 316 human genes. In total, 78% of the cancer-specific splice forms we detected are confirmed by human-curated mRNA sequences, indicating that our results are not due to random mis-splicing in tumors; 73% of the genes showed the same cancer-specific splicing changes in tissue-matched tumor versus normal datasets, indicating that the vast majority of these changes are associated with tumorigenesis, not tissue specificity. We have confirmed our EST results in an independent set of experimental data provided by human-curated mRNAs (P-value 10(-5.7)). Moreover, the majority of the genes we detected have functions associated with cancer (P-value 0.0007), suggesting that their altered splicing may play a functional role in cancer. Analysis of the types of cancer-specific splicing shifts suggests that many of these shifts act by disrupting a tumor suppressor function. Sur prisingly, our data show that for a large number (190 in this study) of cancer-associated genes cloned originally from tumors, there exists a previously uncharacterized splice form of the gene that appears to be predominant in normal tissue.

Analysis of protein tyrosine kinase inhibitors in recombinant yeast lacking the ERG6 gene

Studies of small-molecule-protein interactions in yeast can be hindered by the limited permeability of yeast to small molecules. This diminished permeability is thought to be related to the unique sterol composition of fungal membranes, which are enriched in the steroid ergosterol. We report the construction of the novel Saccharomyces cerevisiae yeast strain DCY250, which is compatible with yeast two-hybrid-based systems and bears a targeted disruption of the ERG6 gene to ablate ergosterol biosynthesis and enhance permeability to small molecules. The small-molecule inhibitors of protein tyrosine kinases (PTKs) PP1, PP2, herbimycin A, and staurosporine were investigated with yeast tribrid systems that detect the activity of the PTKs v-Abl and v-Src. These tribrid systems function by expression of the PTK, a B42 activation domain fused to the phosphotyrosine-binding Grb2 SH2 domain, a DNA-bound LexA-GFP-(AAYANAA)(4) universal PTK substrate, and a lacZ reporter gene. Yeast genetic systems that lack functional ERG6 were found to be as much as 20-fold more sensitive to small-molecule inhibitors of PTKs than systems with ERG6, and these deficient systems may provide a useful platform for the discovery and analysis of small-molecule-protein interactions.

Peroxisome proliferator-activated receptor coactivator-1alpha (PGC-1alpha) coactivates the cardiac-enriched nuclear receptors estrogen-related receptor-alpha and -gamma. Identification of novel leucine-rich interaction motif within PGC-1alpha

The transcriptional coactivator PPARgamma coactivator-1alpha (PGC-1alpha) has been characterized as a broad regulator of cellular energy metabolism. Although PGC-1alpha functions through many transcription factors, the PGC-1alpha partners identified to date are unlikely to account for all of its biologic actions. The orphan nuclear receptor estrogen-related receptor alpha (ERRalpha) was identified in a yeast two-hybrid screen of a cardiac cDNA library as a novel PGC-1alpha-binding protein. ERRalpha was implicated previously in regulating the gene encoding medium-chain acyl-CoA dehydrogenase (MCAD), which catalyzes the initial step in mitochondrial fatty acid oxidation. The cardiac perinatal expression pattern of ERRalpha paralleled that of PGC-1alpha and MCAD. Adenoviral-mediated ERRalpha overexpression in primary neonatal cardiac mycoytes induced endogenous MCAD expression. Furthermore, PGC-1alpha enhanced the transactivation of reporter plasmids containing an estrogen response element or the MCAD gene promoter by ERRalpha and the related isoform ERRgamma. In vitro binding experiments demonstrated that ERRalpha interacts with PGC-1alpha via its activation function-2 homology region. Mutagenesis studies revealed that the LXXLL motif at amino acid position 142-146 of PGC-1alpha (L2), necessary for PGC-1alpha interactions with other nuclear receptors, is not required for the PGC-1alpha.ERRalpha interaction. Rather, ERRalpha binds PGC-1alpha primarily through a Leu-rich motif at amino acids 209-213 (Leu-3) and utilizes additional LXXLL-containing domains as accessory binding sites. Thus, the PGC-1alpha.ERRalpha interaction is distinct from that of other nuclear receptor PGC-1alpha partners, including PPARalpha, hepatocyte nuclear factor-4alpha, and estrogen receptor alpha. These results identify ERRalpha and ERRgamma as novel PGC-1alpha interacting proteins, implicate ERR isoforms in the regulation of mitochondrial energy metabolism, and suggest a potential mechanism whereby PGC-1alpha selectively binds transcription factor partners.

To ERR in the estrogen pathway

Estrogens control a variety of physiological and disease-linked processes, most notably reproduction, bone remodeling and breast cancer, and their effects are transduced through classic unclear receptors referred to as estrogen receptor-alpha (ER alpha) and ER beta. Recent results obtained using the estrogen-related receptors (ERR alpha, -beta and -gamma), a subfamily of orphan nuclear receptors closely related to the ERs, have shown that the ERRs share target genes, coregulatory proteins, ligands and sites of action with the ERs. In addition, the ERRs can actively influence the estrogenic response, suggesting that pharmacological modulation of ERR activity will be clinically useful to prevent and/or treat a variety of conditions related to women's health.

Structural and functional evidence for ligand-independent transcriptional activation by the estrogen-related receptor 3

The crystal structure of the ligand binding domain (LBD) of the estrogen-related receptor 3 (ERR3) complexed with a steroid receptor coactivator-1 (SRC-1) peptide reveals a transcriptionally active conformation in absence of any ligand. The structure explains why estradiol does not bind ERRs with significant affinity. Docking of the previously reported ERR antagonists, diethylstilbestrol and 4-hydroxytamoxifen, requires structural rearrangements enlarging the ligand binding pocket that can only be accommodated with an antagonist LBD conformation. Mutant receptors in which the ligand binding cavity is filled up by bulkier side chains still interact with SRC-1 in vitro and are transcriptionally active in vivo, but are no longer efficiently inactivated by diethylstilbestrol or 4-hydroxytamoxifen. These results provide structural and functional evidence for ligand-independent transcriptional activation by ERR3.