Benzopyrans are selective estrogen receptor beta agonists with novel activity in models of benign prostatic hyperplasia

Benzopyran selective estrogen receptor beta agonist-1 (SERBA-1) shows potent, selective binding and agonist function in estrogen receptor beta (ERbeta) in vitro assays. X-ray crystal structures of SERBA-1 in ERalpha and beta help explain observed beta-selectivity of this ligand. SERBA-1 in vivo demonstrates involution of the ventral prostate in CD-1 mice (ERbeta effect), while having no effect on gonadal hormone levels (ERalpha effect) at 10x the efficacious dose, consistent with in vitro properties of this molecule.

Subtle Side-Chain Modifications of the Hop Phytoestrogen 8-Prenylnaringenin Result in Distinct Agonist/Antagonist Activity Profiles for Estrogen Receptors α and β

In search of therapeutic agents for estrogen-related pathologies, phytoestrogens are being extensively explored. In contrast to naringenin, 8-prenylnaringenin is a potent hop-derived estrogenic compound, highlighting the importance of the prenyl group for hormonal activity. We investigated the effects of substituting the prenyl group at C(8) with alkyl chains of varying lengths and branching patterns on estrogen receptor (ER) subtype ERalpha- and ERbeta-binding affinities and transcriptional activities. In addition, features of the ligand-induced receptor conformations were explored using a set of specific ER-binding peptides. The new 8-alkylnaringenins were found to span an activity spectrum ranging from full agonism to partial agonism to antagonism. Most strikingly, 8-(2,2-dimethylpropyl)naringenin exhibited full agonist character on ERalpha, but pronounced antagonist character on ERbeta. Knowledge on how ER-subtype-selective activities can be designed provides valuable information for future drug or tool compound discovery.

Tetrahydrofluorenones with conformationally restricted side chains as selective estrogen receptor beta ligands

A series of 2-9a bridged tetrahydrofluorenone derivatives were prepared which exhibited significant binding affinity for ERbeta and were highly selective.

Bending of the estrogen response element by polyamines and estrogen receptors alpha and beta: a fluorescence resonance energy transfer study

Estrogenic regulation of gene expression is mediated by the binding of the hormone to its receptors (ERalpha and ERbeta) followed by their binding to estrogen response element (ERE). Previous studies showed that natural polyamines -- putrescine, spermidine, and spermine -- facilitated ERalpha.ERE recognition. We determined the effects of natural and synthetic polyamines on the bending of a 27-mer oligonucleotide (ODN) harboring the ERE (ERE-ODN), using fluorescence resonance energy transfer (FRET) technique. Complementary strands of the ERE-ODN were labeled with fluorescein and tetramethylrhodamine, as donor and acceptor, respectively. The ERE-ODN was intrinsically bent with an end-to-end distance of 76 +/- 2 Angstrom, compared to a theoretical value of 98 Angstrom. The end-to-end distance of the ERE-ODN was reduced to 64 Angstrom in the presence of 250 microM spermine. A control ODN with scrambled sequence did not show intrinsic bending or spermine-induced bending. Alkyl substitution at the pendant amino groups reduced the ability of spermine to bend the ERE-ODN. Both ERalpha and ERbeta decreased the end-to-end distance of the ERE-ODN, although ERalpha was more efficient than ERbeta in inducing ERE bending. Spermine-induced bending of the ERE-ODN was significantly increased by ERalpha. Fluorescence anisotropy measurement showed that the equilibrium association constant of ERalpha-ERE binding increased by 12-fold in the presence of 250 microM spermine compared to control. The free energy change (Delta G) of ERalpha.ERE complex formation was -13.1 kcal/mol at 22 degrees C in the presence of spermine. Our results suggest that polyamine-induced bending of the ERE might be a mechanism for enhancing ERalpha-ERE binding affinity and thereby fine-tuning the transcriptional response of estrogen-responsive genes.

Estrogen receptor (ER)-beta isoforms: a key to understanding ER-beta signaling

Estrogen receptor beta (ER-beta) regulates diverse physiological functions in the human body. Current studies are confined to ER-beta1, and the functional roles of isoforms 2, 4, and 5 remain unclear. Full-length ER-beta4 and -beta5 isoforms were obtained from a prostate cell line, and they exhibit differential expression in a wide variety of human tissues/cell lines. Through molecular modeling, we established that only ER-beta1 has a full-length helix 11 and a helix 12 that assumes an agonist-directed position. In ER-beta2, the shortened C terminus results in a disoriented helix 12 and marked shrinkage in the coactivator binding cleft. ER-beta4 and -beta5 completely lack helix 12. We further demonstrated that ER-beta1 is the only fully functional isoform, whereas ER-beta2, -beta4, and -beta5 do not form homodimers and have no innate activities of their own. However, the isoforms can heterodimerize with ER-beta1 and enhance its transactivation in a ligand-dependent manner. ER-beta1 tends to form heterodimers with other isoforms under the stimulation of estrogens but not phytoestrogens. Collectively, these data support the premise that (i) ER-beta1 is the obligatory partner of an ER-beta dimer, whereas the other isoforms function as variable dimer partners with enhancer activity, and (ii) a single functional helix 12 in a dimer is sufficient for gene transactivation. Thus, ER-beta behaves like a noncanonical type-I receptor, and its action may depend on differential amounts of ER-beta1 homo- and heterodimers formed upon stimulation by a specific ligand. Our findings have provided previously unrecognized directions for studying ER-beta signaling and design of ER-beta-based therapies.

Structure-function relationship of estrogen receptor alpha and beta: impact on human health

17Beta-estradiol (E2) controls many aspects of human physiology, including development, reproduction and homeostasis, through regulation of the transcriptional activity of its cognate receptors (ERs). The crystal structures of ERs with agonists and antagonists and the use of transgenic animals have revealed much about how hormone binding influences ER conformation(s) and how this conformation(s), in turn, influences the interaction of ERs with co-activators or co-repressors and hence determines ER binding to DNA and cellular outcomes. This information has helped to shed light on the connection between E2 and the development or progression of numerous diseases. Current therapeutic strategy in the treatment of E2-related pathologies relies on the modulation of ER trancriptional activity by anti-estrogens; however, data accumulated during the last five years reveal that ER activities are not only restricted to the nucleus. ERs are very mobile proteins continuously shuttling between protein targets located within various cellular compartments (e.g., membrane, nucleus). This allows E2 to generate different and synergic signal transduction pathways (i.e., non-genomic and genomic) which provide plasticity for cell response to E2. Understanding the structural basis and the molecular mechanisms by which ER transduce E2 signals in target cells will allow to create new pharmacologic therapies aimed at the treatment of a variety of human diseases affecting the cardiovascular system, the reproductive system, the skeletal system, the nervous system, the mammary gland, and many others.

Synthesis of Anthranylaldoxime Derivatives as Estrogen Receptor Ligands and Computational Prediction of Binding Modes

N-Me-anthranylaldoximes possess a hydrogen-bonded pseudocyclic A' ring in place of the typical phenolic A-ring that is characteristic of most estrogen receptor (ER) ligands. We have investigated the role played by substituents introduced into either one or both of the peripheral 3- and 4-phenyl rings in modulating ER binding affinity. An efficient synthetic strategy was employed for the preparation of differentially substituted 3- and 4-aryl derivatives that involved exploiting the different reactivity of bromo- versus chloro-aryl groups in palladium-catalyzed cross-couplings. The binding data showed that ERalpha affinity could be improved by a single p-OH group in the 4-phenyl ring, whereas the same substitution on the 3-phenyl ring caused a dramatic reduction of ERbeta affinity. The most ERalpha-selective compound was the one with two p-OH groups on both phenyl substituents. To rationalize these results, ligand docking followed by molecular mechanics Poisson-Boltzmann/surface area (MM-PBSA) studies were carried out. These analyses suggested a molecular basis for the interaction of these compounds with the ERs and enabled the development of models able to predict the mode of ligand binding.

A second binding site for hydroxytamoxifen within the coactivator-binding groove of estrogen receptor beta

Evidence is presented that the estrogen antagonist 4-hydroxytamoxifen (HT) can occupy not only the core binding pocket within the ligand-binding domain of estrogen receptor (ER) beta but also a second site on its surface. The crystal structure of the ligand-binding domain (LBD) associated with HT was determined to 2.2 A and revealed two molecules of HT bound to the protein. One was located in the consensus ligand-binding pocket, whereas the other bound to a site that overlaps with the hydrophobic groove of the coactivator recognition surface. Relative to the ERalpha-tamoxifen structure, helix 12 has been displaced from the coactivator recognition surface and occupies a unique position. Although it has been demonstrated that association of the antagonist with the core ligand-binding pocket is sufficient to induce an antagonist ligand-binding domain conformation, this structure suggests that small molecules may directly antagonize receptor-coactivator interactions. These results provide a direct demonstration of two binding sites for HT in ERbeta, as has been previously suggested for ERalpha by using biochemical methods, and represent a crystal structure of a small nonpeptide molecule occupying the coactivator recognition site.

Synthesis and characterization of 3-arylquinazolinone and 3-arylquinazolinethione derivatives as selective estrogen receptor beta modulators

On the basis of the stucture of genistein, a new series of 3-arylquinazolines was prepared and tested for their estrogen receptor (ER) alpha and beta affinities. 5,7-Dihydroxy-3-(4-hydroxyphenyl)-4(3H)-quinazolinone (1aa) acts as an agonist on both ER subtypes. It has 62-fold higher binding affinity [IC(50)(ERbeta) = 179 nM] and 38-fold higher functional potency in a transcription assay [EC(50)(ERbeta) = 76 nM] with ERbeta than with ERalpha, thus improving upon the selectivity of genistein. All of the analogues showed preferential binding affinity for ERbeta. Many are also more potent in activating transcription by ERbeta than by ERalpha. Transformation of the C=O functionality at position 4 into a C=S group provided 5,7-dihydroxy-3-(4-hydroxyphenyl)-4(3H)-quinazolinethione (1ba), which acts as an agonist on both ER subtypes but has 56-fold higher binding affinity for ERbeta over ERalpha [IC(50)(ERbeta) = 47 nM] and 215-fold higher potency in the transcription assay [EC(50)(ERbeta) = 13 nM]. These ERbeta-selective compounds may represent valuable tools in understanding the differences in structure and biological function of ERbeta and ERalpha.

The discovery of tetrahydrofluorenones as a new class of estrogen receptor beta-subtype selective ligands

Synthesis and derivatization of a series of substituted tetrahydrofluorenone analogs giving potent, ERbeta subtype selective ligands are described. Several analogs possessing ERbeta binding affinities comparable to 17beta-estradiol but with greater than 75-fold selectivity over ERalpha are reported.

Activation function 1 domain plays a negative role in dimerization of estrogen receptor beta

Transcriptional potential of estrogen receptor beta (ERbeta) depends on the ligand binding and subsequent dimerization of the receptor protein. In order to examine the role of N-terminally located activation function 1 (AF-1) protein domain in the dimerization process of ERbeta, we used yeast SOS-Recruitment System (SRS). Two variants of ERbeta protein were expressed in the yeast cells: full length receptor and a truncated form, lacking AF-1. We observed that upon 17beta-estradiol treatment only the shorter form of the receptor dimerized, whereas the full-length one did not. This result suggests an inhibitory function of AF-1 in dimer formation and supports previous studies showing that N-terminal domain of ERbeta suppresses transcriptional activity.

Identification of ligands with bicyclic scaffolds provides insights into mechanisms of estrogen receptor subtype selectivity

Estrogen receptors alpha (ERalpha) and beta (ERbeta) have distinct functions and differential expression in certain tissues. These differences have stimulated the search for subtype-selective ligands. Therapeutically, such ligands offer the potential to target specific tissues or pathways regulated by one receptor subtype without affecting the other. As reagents, they can be utilized to probe the physiological functions of the ER subtypes to provide information complementary to that obtained from knock-out animals. A fluorescence resonance energy transfer-based assay was used to screen a 10,000-compound chemical library for ER agonists. From the screen, we identified a family of ERbeta-selective agonists whose members contain bulky oxabicyclic scaffolds in place of the planar scaffolds common to most ER ligands. These agonists are 10-50-fold selective for ERbeta in competitive binding assays and up to 60-fold selective in transactivation assays. The weak uterotrophic activity of these ligands in immature rats and their ability to stimulate expression of an ERbeta regulated gene in human U2OS osteosarcoma cells provides more physiological evidence of their ERbeta-selective nature. To provide insight into the molecular mechanisms of their activity and selectivity, we determined the crystal structures of the ERalpha ligand-binding domain (LBD) and a peptide from the glucocorticoid receptor-interacting protein 1 (GRIP1) coactivator complexed with the ligands OBCP-3M, OBCP-2M, and OBCP-1M. These structures illustrate how the bicyclic scaffolds of these ligands are accommodated in the flexible ligand-binding pocket of ER. A comparison of these structures with existing ER structures suggests that the ERbeta selectivity of OBCP ligands can be attributed to a combination of their interactions with Met-336 in ERbeta and Met-421 in ERalpha. These bicyclic ligands show promise as lead compounds that can target ERbeta. In addition, our understanding of the molecular determinants of their subtype selectivity provides a useful starting point for developing other ER modulators belonging to this relatively new structural class.

Lessons learnt from structural studies of the oestrogen receptor

Over the last 10 years, structural studies of the ligand-binding domains of nuclear hormone receptors have provided a wealth of information on the nature of ligand-binding and its role in receptor activation. This review examines the insights that studies on oestrogen receptor subtypes ERalpha and ERbeta have provided in terms of (1) basis of receptor activation; (2) recruitment of coregulators; (3) hormone recognition; and (4) subtype-selective ligands. It also highlights the implications for the binding of endocrine disruptors.

A Western blot and molecular genetic investigation of the estrogen receptor beta in giant cell arteritis

OBJECTIVE

The epidemiology of giant cell arteritis (GCA) may indicate a pathogenetic relationship between GCA and female sex hormone metabolism; GCA is two to four times more common in women compared with men. Our previous analyses gave no support for the hypothesis that the pathogenesis of GCA should be related to somatic mutations in the estrogen receptor alpha (ERalpha) gene. The object of the present study was to investigate the size of the estrogen receptor beta (ERBeta), and the size and nucleotide sequence of the ERBeta gene in temporal arteries in GCA.

METHODS

The ERBeta protein was analyzed by Western blot technique and the ERBeta gene by RT-PCR and direct sequencing of the PCR product.

RESULTS

Western blot analysis revealed an ERBeta of normal size. There were no aberrations in size or nucleotide sequence in the ERBeta gene in the GCA patients.

CONCLUSION

The present observations gave no support for the hypothesis that somatic mutations in the ERBeta gene should be involved in the pathogenesis of GCA.

Targeted mutation of key residues at the start of helix 12 in the hERalpha ligand-binding domain identifies the role of hydrogen-bonding and hydrophobic interactions in the activity of the protein

Estradiol (E(2)) and tamoxifen exert their effects through two members of the nuclear receptor superfamily, estrogen receptor (ER)-alpha and -beta. We want to identify the key interactions linking ligand-binding and activity of the ERalpha. Asp-351 and Leu-536 participate in hydrogen bond (Asp-351) and hydrophobic (Leu-536) interactions at the start of helix 12 in the ligand-binding domain (LBD) of the ERalpha. Mutations at each position alter ER activity, but we do not know which is more important. We mutated these residues in combination and individually and assessed the activity of the mutated ERs in the absence and presence of E(2) and 4-OHT on an ERE-driven and an AP-1-driven promoter, as well as their ability to interact with coregulators. On an ERE-driven promoter, the residue at position 351 determined whether E(2) stimulated or reduced the activity of the ER, as well as the level of activity in the presence of 4-OHT. Surprisingly, mutation of both residues generally did not produce cumulative deleterious effects, and they exerted counterbalancing effects on the basal activity on both promoters. Our results identify the contributions of specific interactions to the activity of the hERalpha, and support the concept that this region couples ligand-binding with ER activity.

[Epididymis in an experimental model of DHT deficiency: immunolocalization of ERalpha and ERbeta in rat epididymal epithelial cells. In vivo and in vitro studies]

INTRODUCTION

The aim of this study was to determine the effect of reduced availability of dihydrotestosterone (DHT) on the expression of estrogen receptors alpha and beta (ERalpha and ERbeta) in the epididymis in vivo and in vitro. Expression of estrogen receptors (ERs) is interesting because of the fact that the male reproductive system is controlled not only by androgens but also, in a far-reaching and complex manner, by estrogens. Control by estrogens is exercised through activation of ERs widely distributed in the epididymal epithelium. Epididymal epithelial cells contain a 5alpha-reductase (5alpha-red) which catalyzes the irreversible conversion of testosterone (T) into the most potent and chief androgen of the epididymis, dihydrotestosterone, known to maintain and regulate the structure and functions of the epididymis. Two isoforms of the 5alpha-red were identified: type 1 (5alpha-redl) and type 2 (5alpha-red2). 5alpha-reductase type 2 is more widely expressed in the epididymis than 5alpha-redl. DHT deficit was produced by inhibition of 5alpha-red2 using finasteride (Proscar, MSD Sweden), a steroid inhibitor of this enzyme.

MATERIAL AND METHODS

The study was performed in the adult, male Wistar rats randomly divided into control (K) and study (Fin56) groups (5 animals in each). Animals in the study group received 5mg finasteride/kg b.w., orally during 56 days (duration of one spermatogenesis). Immunoexpression of ERs was also studied in epididymal epithelial cells cultured with or without finasteride.

RESULTS

It was shown that DHT deficiency, both in vivo and in vitro condition, modulated ERs expression in comparison to the epididymis from control rats and to epididymal cells cultured without finasteride. Distribution of ERalpha and ERbeta in epididymal cells changed (from nucleus to cytoplasm) and the level of ERs expression was markedly decreased.

CONCLUSION

The present findings show that the DHT deficiency caused by finasteride altered the expression of ERalpha and ERbeta in the epididymis and possibly may have destabilized the functioning of this organ.

Estrogen receptor β selective ligands: Discovery and SAR of novel heterocyclic ligands

A series of ligands with varying heterocyclic cores and substituents that display a range of selectivity's (up to >100x) for ER-beta over ER-alpha are reported.

Identification of the Structural Requirements of the Receptor-Binding Affinity of Diphenolic Azoles to Estrogen Receptors α and β by Three-Dimensional Quantitative Structure−Activity Relationship and Structure−Activity Relationship Analysis

Three-dimensional (3D) quantitative structure-activity relationship (QSAR) and structure-activity relationship (SAR) analyses were applied concurrently to a data set of highly selective estrogen receptor beta (ERbeta) agonists. The data set consisted of diphenolic azoles characterized by similar structural skeletons but with different binding modes to the estrogen receptor site. Models were developed separately with respect to the relative binding affinities (RBAs) to ERalpha and ERbeta. Steric and electrostatic fields were calculated for a training set of 72 compounds using comparative molecular field analysis (CoMFA). The model developed for ERalpha RBA yielded R2 of 0.91 and q(cv)2 of 0.60. The model developed for ERbeta RBA yielded R2 of 0.95 and q(cv)2 of 0.40. Both models were validated successfully using an external test set of 32 compounds. A new concept of test set evaluation based on the variability of the biological response due to the variability of the living organism has been introduced. The CoMFA analysis was supported by a SAR study. In addition to the most favorable steric and electrostatic regions identified by CoMFA, a number of structural descriptors were identified as being important for binding. These are the number of substituents attached to the main skeleton of each compound, the largest distance between the oxygen atoms of each molecule, and the angle defined by the planes that split the phenyl or the naphthyl and the benzisoxazole or the benzoxazole moiety in a morphometrically longitudinal way.

Conformational dynamics of estrogen receptors alpha and beta as revealed by intrinsic tryptophan fluorescence and circular dichroism

Estrogen receptors (ER alpha and ER beta) are ligand-activated nuclear receptors that mediate the action of estrogens. These receptors activate transcription by similar mechanism(s), although the overall amino acid sequence identity is only 47%. In order to compare the structural and conformational features of ER alpha and ER beta, we monitored their intrinsic tryptophan fluorescence during thermal unfolding. The 50% unfolding temperatures (T(M)) of ER alpha and ER beta were 39+/-1 and 40+/-2 degrees C, respectively. Estradiol had no significant effect on the T(M) of ER alpha or ER beta. In contrast, binding of the estrogen-response element increased the T(M) of ER alpha and ER beta by 10 degrees C. Thermal unfolding of estradiol-bound ER alpha and ligand-free ER beta showed two-step transitions, with the formation of intermediates that were stable between 36-48 and 34-42 degrees C, respectively. We confirmed the presence of intermediate states during thermal unfolding by circular dichroism spectroscopy. Atomic force microscopy showed that the ER beta intermediate consisted of discrete globular particles, whereas the ER alpha intermediate showed a speckled appearance, with sparse well-defined particles. Fluorescence-quenching studies showed the presence of two classes of tryptophan in unliganded ER alpha and ER beta. Binding of estradiol to ER beta exposed its tryptophans, whereas estradiol reduced the accessibility of the tryptophans of ER alpha. Our results illustrate the differential effects of ligands on the unfolding of ER alpha and ER beta, and identify partially unfolded intermediates. Differences in the conformational flexibility and stability of ER alpha and ER beta may represent functional differences of ligand-bound ERs in recruiting coactivator proteins and initiating transcription.

Use of binding energy in comparative molecular field analysis of isoform selective estrogen receptor ligands

A diverse set of 30 estrogen receptor ligands whose relative binding affinities (RBA) with respect to 17beta-estradiol were available in both isoforms of the nuclear estrogen receptor (ERalpha, ERbeta) were studied with a combination of comparative molecular field analysis (CoMFA) and binding energy calculations. The ligands were docked inside the ligand-binding domain (LBD) of both ERalpha and ERbeta utilizing the docking program Gold. The binding energy (DeltaE) and corresponding non-bonded interactions (NB) of the subsequent protein-ligand complexes were calculated in both the gas-phase and implicit aqueous solution using the generalized born surface area (GB/SA) model. A partial least-squares analysis of the calculated energies indicated that the NB(g) were sufficiently predictive in ERalpha, but performed poorly in ERbeta. Further analysis of the calculated energies by dissecting the ligands into two distinct classes, estrogen-like and heterocyclic, yielded more predictive models. In particular the DeltaE calculated in solution proved particularly predictive for the estrogen-like ligands in ERbeta. Finally the estrogen subtype selective nature RBA (ERalpha/ERbeta) of a test-set consisting of six of the original ligands was predicted. The combined CoMFA and non-bonded interaction energy model ranked correctly the ligands in order of increasing RBA (ERalpha/ERbeta), illustrating the utility of this method as a prescreening tool in the development of novel estrogen receptor subtype selective ligands.

Homology-modeled ligand-binding domains of zebrafish estrogen receptors alpha, beta1, and beta2: from in silico to in vivo studies of estrogen interactions in Danio rerio as a model system

Homology models were constructed for the ligand-binding domains of zebrafish estrogen receptors (zfERs) alpha, beta(1), and beta(2). Estradiol-binding sites are nearly identical in zfERs and their human homologs, suggesting that zebrafish will serve as a good model system for studying human ER-binding drugs. Conversely, studies of endocrine disruptor effects on zebrafish will benefit from the wealth of data available on xenoestrogen interactions with human ERs. Compounds flagged by the Interagency Coordinating Committee on the Validation of Alternative Methods for endocrine disruptor screening were docked into our zfER homology models. Ideally, these in silico docking studies would be complemented with in vivo binding studies. To this end, fluorescently tagged estradiol was docked into zfERalpha and found to bind in the same manner as in human ERalpha, with fluorescein preferentially occupying a region between helices 11 and 12. Fluorescently tagged estradiol was synthesized and was found to localize along the path of primordial germ cell migration in the developing zebrafish embryo 3 d after fertilization, consistent with previous reports of 1) a role for estradiol in sex determination, and 2) the first appearance of ERs 2 d after fertilization. These data provide a foundation for future in silico and in vivo binding studies of estrogen agonists and antagonists with zebrafish ERs.

Reconstitution of functional nuclear receptor proteins using high pressure refolding

Members of the nuclear receptor superfamily are ligand dependent transcription factors and many of the receptors are difficult to produce in their functional form. Here, we describe a method for obtaining functional nuclear receptor ligand binding domain proteins from bacterial expressed inclusion bodies by high hydrostatic pressure induced refolding. High pressure refolding successfully reconstituted activity from several insoluble nuclear receptor proteins and represents a valuable tool for both functional and structural investigation of proteins or fragments thereof that might otherwise remain insoluble.

Estrogen receptors beta4 and beta5 are full length functionally distinct ERbeta isoforms: cloning from human ovary and functional characterization

We describe here the cloning and functional characterization of two unique ER isoforms, ERbeta4 and ERbeta5. The full length ERbeta4 and ERbeta5 were identified by asymmetric PCR using human ovary cDNA, cloning, and sequence analyses. Both receptors share identical sequences with ERbeta1 from exon 1 to exon 7. In the place of exon 8, ERbeta4 has unique sequences arising from a region downstream of the ERbeta gene and upstream of the SYNE2 gene. ERbeta5 has sequences arising from retention of the 5' end of the intron between exon 7 and 8. Both receptors bind promoter sequences on DNA but do not bind estrogen. They translocate to the nucleus and exhibit three to four times higher estrogen-independent transcriptional activity than ERbeta1. When co-transfected with ERalpha, they predominantly form heterodimers and negatively regulate its transcriptional activity. Estrogen-independent transcriptional activity of ERbeta5, but not ERbeta4, was inhibited by ERalpha, demonstrating for the first time that ERalpha regulates ERbeta. Tissue-specific expression of ERbeta4 and ERbeta5, together with their ligand-independent transcriptional properties and ERalpha modulating activities, could have a number of implications in seemingly unlinked biological processes regulated by estrogen.

ERbeta ligands. 3. Exploiting two binding orientations of the 2-phenylnaphthalene scaffold to achieve ERbeta selectivity

The 2-phenylnaphthalene scaffold was explored as a simplified version of genistein in order to identify ER selective ligands. With the aid of docking studies, positions 1, 4, and 8 of the 2-phenylnaphthalene template were predicted to be the most potentially influential positions to enhance ER selectivity using two different binding orientations. Both orientations have the phenol moiety mimicking the A-ring of genistein. Several compounds predicted to adopt orientations similar to that of genistein when bound to ERbeta were observed to have slightly higher ER affinity and selectivity than genistein. The second orientation we exploited, which was different from that of genistein when bound to ERbeta, resulted in the discovery of several compounds that had superior ER selectivity and affinity versus genistein. X-ray structures of two ER selective compounds (i.e., 15 and 47) confirmed the alternate binding mode and suggested that substituents at positions 1 and 8 were responsible for inducing selectivity. One compound (i.e., 47, WAY-202196) was further examined and found to be effective in two models of inflammation, suggesting that targeting ER may be therapeutically useful in treating certain chronic inflammatory diseases.

Binding of estrogen receptor beta to estrogen response element in situ is independent of estradiol and impaired by its amino terminus

The functions of 17beta-estradiol (E2) are mediated by estrogen receptor (ER) alpha and beta. ERs display similar DNA- and ligand-binding properties in vitro. However, ERbeta shows lower transcriptional activity than ERalpha from the estrogen response element (ERE)-dependent signaling. We predicted that distinct amino termini contribute to differences in transcription efficacies of ERs by affecting in situ ER-ERE interactions. We used chromatin immunoprecipitation and a novel in situ ERE competition assay, which is based on the ability of ER to compete for ERE binding with a designer activator that constitutively induces transcription from an ERE-driven reporter construct. Interference of activator-mediated transcription by unliganded or liganded ERs was taken as an indication of ER-ERE interaction. Results revealed that ERs interacted with ERE similarly in the absence of E2. However, E2 enhanced the ERE binding of ERalpha but not that of ERbeta. The removal of the amino terminus increased the ERbeta-ERE interaction independent of E2. The ERbeta amino terminus also prevented E2-mediated enhancement of the chimeric ERalpha-ERE interaction. Thus, the amino terminus of ERbeta impairs the binding of ERbeta to ERE. The abrogation of ligand-dependent activation function 2 of the amino-terminally truncated ERbeta resulted in the manifestation of E2 effect on ERbeta-ERE interaction. This implies that E2-mediated enhancement of ERbeta-ERE interaction is masked by the activation function 2, whereas the intact amino terminus is a dominant region that decreases the binding of ERbeta to ERE. Thus, ERbeta-ERE interaction is independent of E2 and is impaired by its amino terminus. These findings provide an additional explanation for differences between ERalpha and ERbeta functions that could differentially affect the physiology and pathophysiology of E2 signaling.

Structure-based virtual screening for plant-based ERbeta-selective ligands as potential preventative therapy against age-related neurodegenerative diseases

ERbeta has been associated with estrogen-induced promotion of memory function and neuronal survival. Based on the optimized complex structure of human ERbeta LBD bound with genistein, computer-aided structure-based virtual screening against a natural source chemical database was conducted to determine the occurrence of plant-based ERbeta-selective ligands. Twelve representative hits derived from database screening were assessed for their binding profiles to both ERs, three of which displayed over 100-fold binding selectivity to ERbeta over ERalpha.

Understanding the selectivity of genistein for human estrogen receptor-beta using X-ray crystallography and computational methods

We present X-ray crystallographic and molecular modeling studies of estrogen receptors-alpha and -beta complexed with the estrogen receptor-beta-selective phytoestrogen genistein, and coactivator-derived NR box peptides containing an LXXLL motif. We demonstrate that the ligand binding mode is essentially identical when genistein is bound to both isoforms, despite the considerably weaker affinity of this ligand for estrogen receptor-alpha. In addition, we examine subtle differences between binding site residues, providing an explanation for why genistein is modestly selective for the beta isoform. To this end, we also present the results of quantum chemical studies and thermodynamic arguments that yield insight to the nature of the interactions leading to estrogen receptor-beta selectivity. The importance of our analysis to structure-based drug design is discussed.

Modification of the Estrogenic Properties of Diphenols by the Incorporation of Ferrocene. Generation of Antiproliferative Effects in Vitro

We report here the synthesis and the strong and unexpected antiproliferative effect of the organometallic diphenolic compound 1,1-bis(4'-hydroxyphenyl)-2-ferrocenyl-but-1-ene (4) on both hormone-dependent (MCF7) and -independent (MDA-MB231) breast cancer cells (IC(50) = 0.7 and 0.6 microM). Surprisingly, 6 [1,2-bis(4'-hydroxyphenyl)-2-ferrocenyl-but-1-ene], the regioisomer of 4, shows only a modest effect on these cell lines. This pertinent organometallic modification seems to trigger an intracellular oxidation of the structurally favorable compound 4, leading to the generation of a potent cytotoxic compound.

Mouse Estrogen Receptor β Isoforms Exhibit Differences in Ligand Selectivity and Coactivator Recruitment

Estrogens exert their physiological effects through two estrogen receptor (ER) subtypes, ERalpha and ERbeta. In mouse, the cloning of an alternative splice variant of the wild-type ERbeta (mERbeta1), mERbeta2, which contains an 18 amino acid insertion in the ligand binding domain, contributed an additional level of complexity to estrogen signaling. In this study we have assayed the interaction of several known ligands with mERbeta1 and mERbeta2. The binding affinity of estradiol was 14-fold higher for mERbeta1 than for mERbeta2. In contrast, raloxifene was dramatically (8-fold) mERbeta2 selective. The selectivity for mERbeta2 was abolished when the 2-arylbenzothiophene core of the raloxifene molecule was tested for binding affinity, demonstrating that the 3-aroyl side chain of raloxifene plays an important role in contributing to its mERbeta2 selectivity. The opposite isoform selectivity found for estradiol and raloxifene in our ligand binding assay was also reflected in the transactivation assay system. That is, mERbeta2 required 10-fold greater estradiol concentrations for maximal activation compared to mERbeta1, whereas raloxifene was more potent in antagonizing estradiol-induced gene expression via mERbeta2 than mERbeta1. The raloxifene core behaved as a pure agonist. Furthermore, mERbeta2 showed significantly decreased estradiol-induced maximal transcriptional activity as compared to mERbeta1. A pull-down assay indicated that the interactions of TIF2 and RAP250 with mERbeta2 were weaker than with mERbeta1. To assess TIF2 and RAP250 interactions with ERs more quantitatively, we examined the interaction of LXXLL containing peptides derived from TIF2 and RAP250 with mERbeta1 and mERbeta2 using surface plasmon resonance analysis. Our results indicate that mERbeta2 interacts with both coactivators with lower affinity, which may explain its reduced transcriptional activity. Taken together, these results suggest that ligand selectivity and coactivator recruitment of the ERbeta isoforms constitute additional levels of specificity that influence the transcriptional response in estrogen target cells.

Reflections on the discovery and significance of estrogen receptor beta

We have known for many years that estrogen is more than the female hormone. It is essential in the male gonads, and in both sexes, estrogen has functions in the skeleton and central nervous system, on behavior, and in the cardiovascular and immune systems. An important aspect of the discovery of estrogen receptor (ER) beta is that the diverse functions of estrogen can now be divided into those mediated by ERalpha and those mediated by ERbeta. Pharmacological exploitation of this division of the labors of estrogen is facilitated by the ligand-binding specificity and selective tissue distribution of the two ERs. Because the ligand binding domains of ERalpha and ERbeta are significantly different from each other, selective ligands can be (and have been) developed to target the estrogenic pathway that is malfunctioning, without interfering with the other estrogen-regulated pathways. Because of the absence of ERbeta from the adult pituitary and endometrium, ERbeta agonists can be used to target ERbeta with no risk of adverse effects from chemical castration and uterine cancer. Some of the diseases in which there is hope that ERbeta agonists will be of benefit are prostate cancer, autoimmune diseases, colon cancer, malignancies of the immune system, and neurodegeneration.

7-Substituted 2-phenyl-benzofurans as ER beta selective ligands

A series of 2-(4-hydroxy-phenyl)-benzofuran-5-ols with relatively lipophilic groups in the 7-position of the benzofuran was prepared and the affinity and selectivity for ER beta was measured. Many of the analogues were found to be potent and selective ER beta ligands. Additional modifications at the benzofuran 4-position as well as at the 3'-position of the 2-phenyl group were found to further increase selectivity. Such modifications led to compounds with <10 nM potency and >100-fold selectivity for ER beta.

QSAR of estrogen receptor modulators: exploring selectivity requirements for ERα versus ERβ binding of tetrahydroisoquinoline derivatives using E-state and physicochemical parameters

Considering importance of developing selective estrogen receptor modulators (SERMs), the present paper explores selectivity requirements of tetrahydroisoquinoline derivatives for binding with ER(alpha) versus ER(beta) receptors using E-state index and physicochemical parameters. The best model [n=21, Q(2)=0.512, R(a)(2)=0.613, R=0.819, F=11.6 (df 3,17)] for ER(alpha) binding data obtained from radioligand binding assay showed importance of C(1), C(15) and lipophilicity (logP) while the best model [n=21, Q(2)=0.768, R(a)(2)=0.796, R=0.904, F=40.1 (df 2,18)] for ER(beta) binding data showed importance of C(1) and molar refractivity (MR). While modeling ER(alpha)/ER(beta) selectivity [n=21, Q(2)=0.695, R(a)(2)=0.739, R=0.882, F=19.8 (df 3,17)], C(1), C(15) and molar refractivity were found to be significant contributors. The data obtained from cellular transcription assay were also modeled. In case of ER(alpha), the best equation involving E-state values of C(1) and C(14) and logP explained 62.1% of the variance while the best equation for ER(beta) involving E-state values of C(1) and C(15) and MR explained 64.6% of the variance of the response variable. In case of ER(alpha)/ER(beta) selectivity, the best equation involving E-state values of O(8), C(14) and N(27) showed 48.3% explained variance, which increased to 63.5% on deletion of single outlier. From the analysis it appears that the nitrogen atom of the aminoethoxyphenyl substituent and 6-hydroxy substituent of the tetrahydroisoquinoline nucleus play important roles for ER(alpha)/ER(beta) selectivity in addition to R(1) and R(2) substituents.

Modulation of estrogen signaling by the novel interaction of heat shock protein 27, a biomarker for atherosclerosis, and estrogen receptor beta: mechanistic insight into the vascular effects of estrogens

OBJECTIVE

We sought to discover proteins that associate with estrogen receptor beta (ERbeta) and modulate estrogen signaling.

METHODS AND RESULT

Using a yeast 2-hybrid screen, we identified heat shock protein 27 (HSP27) as an ERbeta-associated protein. HSP27 is a recently identified biomarker of atherosclerosis that is secreted at reduced levels from atherosclerotic compared with normal arteries. In vitro protein-binding assays confirmed the specific interaction of HSP27 with ERbeta and not ERalpha. HSP27 expression was absent in coronary arteries with complex atherosclerotic lesions. Interestingly, HSP27 expression was also absent in 60% of coronary arteries from young males and females (27+/-6.5 years) with normal histology or nonobstructive fatty streaks/atheromas. Moreover, the absence of HSP27 in these normal or minimally diseased arteries coincided with the loss of ERbeta expression. Only 35% of arteries showed coexpression of HSP27 and ERbeta. Relative to controls, estradiol-mediated transcription was reduced 20% with overexpression of HSP27 and increased 44% when HSP27 protein levels were reduced with HSP27 siRNA.

CONCLUSIONS

HSP27, an ERbeta-associated protein, shows attenuated expression with coronary atherosclerosis and modulates estrogen signaling.

Synthesis and activity of substituted 4-(indazol-3-yl)phenols as pathway-selective estrogen receptor ligands useful in the treatment of rheumatoid arthritis

Pathway-selective ligands for the estrogen receptor (ER) inhibit NF-kappaB-mediated inflammatory gene expression causing a reduction of cytokines, chemokines, adhesion molecules, and inflammatory enzymes. SAR development of a series of 4-(indazol-3-yl)phenols has led to the identification of WAY-169916 an orally active nonsteroidal ligand with the potential use in the treatment of rheumatoid arthritis without the classical proliferative effects associated with estrogens.

Structure-based design of estrogen receptor-beta selective ligands

We present the structure-based optimization of a series of estrogen receptor-beta (ERbeta) selective ligands. X-ray cocrystal structures of these ligands complexed to both ERalpha and ERbeta are described. We also discuss how molecular modeling was used to take advantage of subtle differences between the two binding cavities in order to optimize selectivity for ERbeta over ERalpha. Quantum chemical calculations are utilized to gain insight into the mechanism of selectivity enhancement. Despite only two relatively conservative residue substitutions in the ligand binding pocket, the most selective compounds have greater than 100-fold selectivity for ERbeta relative to ERalpha when measured using a competitive radioligand binding assay.

Estrogen receptor ligands. Part 4: The SAR of the syn-dihydrobenzoxathiin SERAMs

A series of estrogen receptor ligands based on a dihydrobenzoxathiin scaffold is described and evaluated for estrogen/anti-estrogen activity in both in vitro and in vivo models. The most active analogue, 22, was found to be 40-fold ERalpha selective in a competitive binding assay, and 22 demonstrated very potent in vivo antagonism of estradiol driven proliferation in an immature rat uterine weight gain assay.

A novel pesticide-induced conformational state of the oestrogen receptor ligand-binding domain, detected by conformation-specific peptide binding

The diverse effects of different natural and synthetic oestrogen receptor ligands depend on induction of different receptor conformations, allowing differential interactions with other transcription factors. Different conformations of the oestrogen receptor ligand binding domains can be monitored by conformation-specific binding to peptides selected from phage-displayed peptide libraries. We now report that a group of chlorinated pesticides, including 2,4-dichlorodiphenyl-dichloroethylene, induces a peptide recognition pattern different from those induced by any one of the classical oestrogen receptor ligands. The pesticide-complexed oestrogen receptors recognized peptides reacting with the receptors complexed both with the natural oestrogen 17beta-oestradiol and with the synthetic partial antagonist 4-hydroxy-tamoxifen, respectively, indicating that the pesticide-induced conformation shares features with both the 17beta-oestradiol- and the 4-hydroxy-tamoxifen-induced conformations. The substitution H524A in the ligand binding domain conferred the pesticide-specific peptide recognition pattern and transactivation activity to the oestradiol- and the 4-hydroxy-tamoxifen-complexed receptors, indicating that one important determinant of the pesticide-induced conformation is a lack of stabilisation of any one particular receptor conformation by ligand interaction with H524, which is known to interact with both oestradiol and 4-hydroxy-tamoxifen. Thus, peptide binding analyses of oestrogen receptor conformations induced by environmental endocrine disruptors can give novel information about molecular mechanisms of oestrogen action in general.

A proteomic microarray approach for exploring ligand-initiated nuclear hormone receptor pharmacology, receptor selectivity, and heterodimer functionality

Nuclear hormone receptors (NHRs) are major regulators of development and homeostasis in multiple organ systems. These proteins are ligand-modulated transcription factors that regulate gene expression in response to changes in circulating levels of their cognate hormones or hormone analogs. When NHRs bind ligands, they adopt distinct conformations that enable or disable the binding of coregulator proteins in a manner that reflects the agonist versus antagonist character of the ligand. Using the estrogen receptor ligand binding domain as a representative member of the NHR family, we show the development of functional protein microarrays and use them to explore coactivator recruitment and NHR homo- and heterodimer functionality. These NHR protein microarrays can be fabricated in either a forward mode (coactivator recruited to printed NHR) or a reversed mode (NHR recruited to printed coactivator). From these microarrays, we can predict the potency and pharmacological character of various NHR ligands through the nature of their coactivator recruitment. Additionally different coactivator proteins can be functionally classified and their affinity for NHRs can be quantified. NHR-selective antagonist ligands and small molecule coactivator mimics disrupt the coactivator-NHR complex. This novel proteomic approach was also used to assess coactivator recruitment to explore heterodimer functionality. Heterodimers of the estrogen receptor were found only to recruit coactivators when both monomers are bound with agonist ligands, an observation that provides an insight into the complex biology of hormones that act on tissues containing both NHR subtypes. We can extend this NHR proteomic approach to the analysis of multidomain full-length NHR constructs and can concurrently monitor the activation state of different classes of NHRs with a mixture of endogenous or synthetic ligands of varying NHR selectivity and pharmacology.

The expression of ER beta cx in human breast cancer and the relationship to endocrine therapy and survival

PURPOSE

Estrogen receptor (ER) alpha-positive breast cancer is often treated with endocrine therapy using either antiestrogens or aromatase inhibitors. However, 30% of patients who receive endocrine therapy will derive no benefit from such treatments and may indeed suffer adverse effects. Currently, there are no ways to predict response to such treatments. ER beta cx, a variant of ER beta, has a dominant-negative effect over ER alpha, and its expression thought to modulate response to endocrine treatment may represent a predictor of response to endocrine therapy.

EXPERIMENTAL DESIGN

We investigated the expression of the ER beta cx in 82 frozen breast samples (8 benign, 1 ductal carcinoma in situ, and 73 malignant) by Western blot analysis. The relationship between the expression of ER beta cx variants with prognosis and outcome of endocrine therapy was examined.

RESULTS

There was a statistically significant association between the presence of ER beta cx and the response to endocrine therapy (Fisher's exact test, P = 0.04). We also examined the influence of the ER beta cx status of a tumor on time to progression and death. There was a relationship between the presence of ER beta cx and survival, with patients whose tumors express ER beta cx having a longer survival rate (P = 0.05). Cell-type specificity of expression was assessed by immunohistochemistry on a selection of histological samples.

CONCLUSIONS

On the basis of this small group of patients, we conclude that the expression of ER beta cx correlated with favorable response to endocrine therapy. A larger study involving the staining of archival material is currently underway to confirm these preliminary results.

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.

Dissecting Physiological Roles of Estrogen Receptor α and β with Potent Selective Ligands from Structure-Based Design

The distinct roles of the two estrogen receptor (ER) isotypes, ERalpha and ERbeta, in mediating the physiological responses to estrogens are not completely understood. Although knockout animal experiments have been aiding to gain insight into estrogen signaling, additional information on the function of ERalpha and ERbeta will be provided by the application of isotype-selective ER agonists. Based on the crystal structure of the ERalpha ligand binding domain and a homology model of the ERbeta-ligand binding domain, we have designed steroidal ligands that exploit the differences in size and flexibility of the two ligand binding cavities. Compounds predicted to bind preferentially to either ERalpha or ERbeta were synthesized and tested in vitro using radio-ligand competition and transactivation assays. This approach directly led to highly ER isotype-selective (approximately 200-fold) and potent ligands. To unravel physiological roles of the two receptors, in vivo experiments with rats were conducted using the ERalpha- and ERbeta-selective agonists in comparison to 17beta-estradiol. The ERalpha agonist induced uterine growth, caused bone-protective effects, reduced LH and FSH plasma levels, and increased angiotensin I, whereas the ERbeta agonist did not at all or only at high doses lead to such effects, despite high plasma levels. It can thus be concluded that estrogen effects on the uterus, pituitary, bone, and liver are primarily mediated via ERalpha. Simultaneous administration of the ERalpha and ERbeta ligand did not lead to an attenuation of ERalpha-mediated effects on the uterus, pituitary, and liver parameters.

Identification of a functional variant of estrogen receptor beta in an African population

In this study, we identified five novel polymorphisms in the estrogen receptor beta (ERbeta) gene in an African population. Interestingly, two of these variants are expected to change the amino acid sequence of the ERbeta protein. These changes correspond to an isoleucine to valine substitution at amino acid position 3 (I3V) and a valine to glycine substitution at position 320 (V320G), respectively. The functional consequences of these amino acid substitutions were determined in different in vitro assays. The I3V mutation displayed no differences with regard to transcriptional activity in a reporter assay, as compared with the wild-type receptor. The V320G mutation, however, showed significantly decreased maximal transcriptional activity in a reporter assay, although its binding affinity for 17beta-estradiol was not affected. A pull-down assay indicated that the interaction of full-length TIF2 with hERbetaV320G was weaker than with hERbetawt. Moreover, surface plasmon resonance analysis revealed reduced interaction of the V320G ERbeta variant with the NR box I and II modules of TIF2. To our knowledge, this represents the first identification of a functional polymorphism in the ERbeta gene. This novel polymorphism provides a tool for human genetic studies of diseases in the African population.

Tetrahydroisoquinolines as subtype selective estrogen agonists/antagonists

Two series of 6-hydroxy and 7-hydroxy tetrahydroisoquinolines were prepared. Evaluating a range of C-1, C-4, and N-substituents led to the discovery of ER alpha and ER beta selective analogs.

Distinct effects of the antiestrogen Faslodex on the stability of estrogen receptors-alpha and -beta in the breast cancer cell line MCF-7

The effects of estrogen receptor (ER) ligands on the stability and transcriptional activity of ERbeta in the breast cancer cell lines MCF-7 and HeLa were examined. We found that ERbeta was degraded in the presence of 17beta-estradiol. Tamoxifen and Faslodex (ICI 182,780) prevented ERbeta receptor destabilization. In contrast to ERalpha, ERbeta degradation was not abolished by inhibitors of the proteasome-mediated protein degradation pathway. Furthermore, single point mutations in helix 12 of the receptor dramatically affected the stability and subsequent transcriptional activation of ERbeta.