Site-directed estrogen receptor antibodies stabilize 4-hydroxytamoxifen ligand, but not estradiol, and indicate ligand-specific differences in the recognition of estrogen response element DNA in vitro

Three-dimensional models of non-steroidal ligands: a comparative molecular field analysis

The estrogen receptor, ER, is an important biological target whose inhibition is known to be therapeutically relevant in the treatment of postmenopausal osteoporosis. In the present study, two prediction methods (CoMFA and GRIND (Almond)) were used to describe the binding modes of a set of estrogen receptor ligands. The critical alignment step presented in CoMFA was solved by using the information of the molecular descriptors space generated by grid-independent descriptors (GRIND). Then, it was possible to build robust and high predictive models based on the alignment-independent model. Since the structure of estrogen receptor is solved, the results of the present 3D QSAR models, given by the PLS maps based on molecular interaction fields (MIF) were compared to ligand-binding ER domains and showed good agreement.

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.

Estrogen receptor interaction with estrogen response elements

The estrogen receptor (ER) is a ligand-activated enhancer protein that is a member of the steroid/nuclear receptor superfamily. Two genes encode mammalian ER: ERalpha and ERbeta. ER binds to specific DNA sequences called estrogen response elements (EREs) with high affinity and transactivates gene expression in response to estradiol (E(2)). The purpose of this review is to summarize how natural and synthetic variations in the ERE sequence impact the affinity of ER-ERE binding and E(2)-induced transcriptional activity. Surprisingly, although the consensus ERE sequence was delineated in 1989, there are only seven natural EREs for which both ERalpha binding affinity and transcriptional activation have been examined. Even less information is available regarding how variations in ERE sequence impact ERbeta binding and transcriptional activity. Review of data from our own laboratory and those in the literature indicate that ERalpha binding affinity does not relate linearly with E(2)-induced transcriptional activation. We suggest that the reasons for this discord include cellular amounts of coactivators and adaptor proteins that play roles both in ER binding and transcriptional activation; phosphorylation of ER and other proteins involved in transcriptional activation; and sequence-specific and protein-induced alterations in chromatin architecture.

Estrogen receptors alpha and beta exhibit different estradiol and estrogen response element binding in the presence of nonspecific DNA

Estrogen receptors (ER) alpha and beta bind estradiol (E(2)) and estrogen response element (ERE) DNA sequences with high affinities. The different migration of ER--ERE complexes in the presence or absence of nonspecific DNA suggests that DNA may affect ER conformation and function. We measured the rate of E(2)--ER association and specific ER--ERE binding capacity (ERE--SBC) in the presence or absence of nonspecific DNA. Whereas DNA did not alter the rate of E(2)--ER alpha association, both ERE-containing and plasmid DNA decreased the rate of E(2) association with ER beta. Poly(dI-dC) decreased ERE--SBC of ER alpha, but did not affect the ERE--SBC of ER beta. Salmon sperm genomic DNA decreased the ERE--SBC of ER alpha, but increased the ERE--SBC of ER beta. We speculate that interaction of ER with genomic DNA may contribute to ER activation and play a role in the observed differences in transcriptional activity of ER alpha and ER beta.

Differential effects of xenoestrogens on coactivator recruitment by estrogen receptor (ER) alpha and ERbeta

It has been proposed that tissue-specific estrogenic and/or antiestrogenic actions of certain xenoestrogens may be associated with alterations in the tertiary structure of estrogen receptor (ER) alpha and/or ERbeta following ligand binding; changes which are sensed by cellular factors (coactivators) required for normal gene expression. However, it is still unclear whether xenoestrogens affect the normal behavior of ERalpha and/or ERbeta subsequent to receptor binding. In view of the wide range of structural forms now recognized to mimic the actions of the natural estrogens, we have assessed the ability of ERalpha and ERbeta to recruit TIF2 and SRC-1a in the presence of 17beta-estradiol, genistein, diethylstilbestrol, 4-tert-octylphenol, 2',3',4', 5'-tetrachlorobiphenyl-ol, and bisphenol A. We show that ligand-dependent differences exist in the ability of ERalpha and ERbeta to bind coactivator proteins in vitro, despite the similarity in binding affinity of the various ligands for both ER subtypes. The enhanced ability of ERbeta (over ERalpha) to recruit coactivators in the presence of xenoestrogens was consistent with a greater ability of ERbeta to potentiate reporter gene activity in transiently transfected HeLa cells expressing SRC-1e and TIF2. We conclude that ligand-dependent differences in the ability of ERalpha and ERbeta to recruit coactivator proteins may contribute to the complex tissue-dependent agonistic/antagonistic responses observed with certain xenoestrogens.

Selectivity of antibodies to estrogen receptors alpha and beta (ERalpha and ERbeta) for detecting DNA-bound ERalpha and ERbeta in vitro

Antibodies are widely used to detect estrogen receptor (ER) in ER-DNA complexes in electrophoretic mobility shift assays (EMSA). We compared the specificity of antibodies raised to different regions of ERalpha or ERbeta for detecting recombinant human ERalpha (rhERalpha) and recombinant rat ERbeta (rrERbeta) when bound to a consensus estrogen response element (ERE). ERalpha-specific antibodies specifically slowed the migration of the ER-ERE complex by 32 to 84% and inhibited rhERalpha-ERE binding by 17 to 75%. None of antibodies to ERbeta supershifted rhERalpha-ERE complex. Some ERalpha-specific antibodies increased whereas some decreased rrERbeta-ERE binding. Anti-ERbeta antibodies supershifted different amounts of the rrERbeta-ERE complex. Our results indicate that supershift and inhibition of ER-ERE interaction with a specific antibody are equally reliable in the detection of rhERalpha and rrERbeta. ERalpha antibody Ab10, antisera G20 and AT3B, and ERbeta-antiserum Y19 offered the best discrimination between ERalpha and ERbeta. Comparison of the peptide sequences against which various antibodies were raised indicate directions for new ERalpha and ERbeta- specific antibody development. We conclude that a cognate ER antibody that retards the migration of the ER-ERE complex by at least 40% or inhibits ER-ERE interaction by at least 8% provides a reliable detection of a specific ER isoform in EMSA.

Role of estrogen receptor ligand and estrogen response element sequence on interaction with chicken ovalbumin upstream promoter transcription factor (COUP-TF)

Estrogen-responsive genes are regulated by altering the balance of estrogen receptor (ER) interaction with transcription activators and inhibitors. Here we examined the role of ER ligand on ER interaction with the Chicken Ovalbumin Upstream Promoter Transcription Factor (COUP-TF) orphan nuclear receptor. COUP-TF binding to half-site estrogen response elements (EREs) was increased by the addition of estradiol (E2) -liganded ER (E2-ER), but not by ER liganded with the antiestrogens 4-hydroxytamoxifen (4-OHT-ER) or tamoxifen aziridine (TAz-ER). ER did not bind to single half-sites. Conversely, COUP-TF enhanced the ERE binding of purified E2-ER, but did not affect TAz-ER-ERE binding. In contrast, only antiestrogens enhanced direct interaction between ER and COUP-TF as assessed by GST pull-down assays. Identical results were obtained using either purified bovine or recombinant human ERalpha. Co-immunoprecipitation assays showed that ER and COUP-TF interact in extracts from MCF-7 and ERalpha-transfected MDA-MB-231 cells. Here we document that ER ligand impacts COUP-TF-ER interaction. COUP-TF interaction is mediated by the DNA binding and ligand-binding domains of ER. We suggest that changes in ER conformation induced by DNA binding reduce ER-COUP-TF interaction. Transient transfection of human MCF-7 breast cancer cells with a COUP-TFI expression vector repressed E2-induced luciferase reporter gene expression from single or multiple tandem copies of a consensus ERE. COUP-TFI stimulated 4-OHT-induced luciferase activity from a minimal ERE. Alone, COUP-TFI increased transcription from ERE half-sites or a single ERE in a sequence-dependent manner. These data provide evidence that the ERE sequence and its immediate flanking regions influence whether COUP-TF enhances, inhibits, or has no effect on ER ligand-induced ERE reporter gene expression and that COUP-TFI activates gene transcription from ERE half-sites. We suggest that COUP-TFI plays a role in mitigating estrogen-responsive gene expression.

Estrogen receptor binding to estrogen response elements slows ligand dissociation and synergistically activates reporter gene expression

Estradiol (E2)-liganded estrogen receptor (ER) bound to three or four tandem copies of a consensus ERE (EREc38) in a cooperative manner. E2-ER binding to one or two EREs was non-cooperative. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), ER-ERE binding was not cooperative, regardless of the number of EREs. Here we evaluated how binding to EREc38 affects ER conformation in the ligand binding domain (LBD) as reflected in the dissociation kinetics of [3H]ligand from the ER. Binding of ER to EREc38 slowed the rate of dissociation of either E2 or 4-OHT, indicating that DNA allosterically modulates the LBD conformation creating a tighter fit between the ligand and the ER. Conformational differences in ER induced by E2 versus antiestrogen were not reflected in differences in E2 or 4-OHT dissociation parameters under these conditions. No difference in the association rate of E2- versus 4-OHT-liganded ER binding to EREc38 was detected in electrophoretic mobility shift assay (EMSA). Synergistic, E2-dependent activation of a reporter gene was detected from three and four, but not one or two, tandem copies of EREc38. These observations suggest that cooperative binding of E2-ER to multiple copies of EREc38 is likely responsible for transcriptional synergy and that cooperativity may not involve direct interaction between the LBDs of ERE-bound ER. Since the number of copies of EREc38 did not alter E2 dissociation kinetics, functional synergy must involve cellular factors in addition to the ER ligand.

Comparison of tamoxifen ligands on estrogen receptor interaction with estrogen response elements

The estrogen receptor (ER) is a ligand-activated transcription factor that binds to specific DNA sequences, estrogen response elements (EREs). Estradiol-liganded ER (E2-ER) binds cooperatively to stereoaligned EREs that are surrounded by naturally-occurring AT-rich sequences with a stoichiometry of one E2-ER dimer per ERE. When ER is bound by 4-hydroxytamoxifen (4-OHT), the active metabolite of the widely used therapeutic antiestrogen tamoxifen (TAM), the receptor binds to EREs with high affinity. However, one molecule of 4-OHT ligand dissociates from the ER dimer apparently during the process of binding to DNA, yielding a stoichiometry of one [3H]4-OHT molecule per ERE. To determine whether DNA-binding induced ligand dissociation is a general property of type I antiestrogens that are not covalently attached to the ER, we examined the interaction of ER liganded by tamoxifen (TAM) with EREs. We demonstrate that TAM-ER binds EREs with lower affinity than E2-ER, 4-OHT-ER, or ER liganded by the covalent antiestrogen tamoxifen aziridine. Unlike E2-ER, both TAM and 4-OHT-ER bind EREs non-cooperatively. Like 4-OHT, TAM appears to dissociate from the liganded ER as the receptor binds EREs. Additionally, partial proteolysis of ERE-bound ER by trypsin revealed different cleavage patterns for E2 versus 4-OHT and TAM. These findings indicate that the behavior of the ER liganded by TAM is generally similar to that of the antiestrogen 4-OHT.

Synthesis and pharmacology of conformationally restricted raloxifene analogues: highly potent selective estrogen receptor modulators

The 2-arylbenzothiophene raloxifene, 1, is a selective estrogen receptor modulator (SERM) which is currently under clinical evaluation for the prevention and treatment of postmenopausal osteoporosis. In vivo structure-activity relationships and molecular modeling studies have indicated that the orientation of the basic amine-containing side chain of 1, relative to the stilbene plane, is an important discriminating factor for the maintenance of tissue selectivity. We have constructed a series of analogues of 1 in which this side chain is held in an orientation which is orthogonal to the stilbene plane, similar to the low-energy conformation predicted for raloxifene. Herein, we report on the synthesis of these compounds and on their activity in a series of in vitro and in vivo biological assays reflective of the SERM profile. In particular, we describe their ability to (1) bind the estrogen receptor, (2) antagonize estrogen-stimulated proliferation of MCF-7 cells in vitro, (3) stimulate TGF-beta3 gene expression in cell culture, (4) inhibit the uterine effects of ethynyl estradiol in immature rats, and (5) potently reduce serum cholesterol and protect against osteopenia in ovariectomized (OVX) rats without estrogen-like stimulation of uterine tissue. These data demonstrate that one of these compounds, LY357489,4, is among the most potent SERMs described to date with in vivo efficacy on bone and cholesterol metabolism in OVX rats at doses as low as 0.01 mg/kg/d.

Chicken ovalbumin upstream promoter-transcription factor interacts with estrogen receptor, binds to estrogen response elements and half-sites, and inhibits estrogen-induced gene expression

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) was identified as a low abundance protein in bovine uterus that co-purified with estrogen receptor (ER) in a ligand-independent manner and was separated from the ER by its lower retention on estrogen response element (ERE)-Sepharose. In gel mobility shift assays, COUP-TF bound as an apparent dimer to ERE and ERE half-sites. COUP-TF bound to an ERE half-site with high affinity, Kd = 1.24 nM. In contrast, ER did not bind a single ERE half-site. None of the class II nuclear receptors analyzed, i.e. retinoic acid receptor, retinoid X receptor, thyroid receptor, peroxisome proliferator-activated receptor, or vitamin D receptor, were constituents of the COUP-TF.DNA binding complex detected in gel mobility shift assays. Direct interaction of COUP-TF with ER was indicated by GST "pull-down" and co-immunoprecipitation assays. The nature of the ER ligand influenced COUP-TF-ERE half-site binding. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), COUP-TF-half-site interaction decreased. Conversely, COUP-TF transcribed and translated in vitro enhanced the ERE binding of purified estradiol (E2)-liganded ER but not 4-OHT-liganded ER. Co-transfection of ER-expressing MCF-7 human breast cancer cells with an expression vector for COUP-TFI resulted in a dose-dependent inhibition of E2-induced expression of a luciferase reporter gene under the control of three tandem copies of EREc38. The ability of COUP-TF to bind specifically to EREs and half-sites, to interact with ER, and to inhibit E2-induced gene expression suggests COUP-TF regulates ER action by both direct DNA binding competition and through protein-protein interactions.

Molecular determinants of tissue selectivity in estrogen receptor modulators

Interaction of the estrogen receptor/ligand complex with a DNA estrogen response element is known to regulate gene transcription. In turn, specific conformations of the receptor-ligand complex have been postulated to influence unique subsets of estrogen-responsive genes resulting in differential modulation and, ultimately, tissue-selective outcomes. The estrogen receptor ligands raloxifene and tamoxifen have demonstrated such tissue-specific estrogen agonist/antagonist effects. Both agents antagonize the effects of estrogen on mammary tissue while mimicking the actions of estrogen on bone. However, tamoxifen induces significant stimulation of uterine tissue whereas raloxifene does not. We postulate that structural differences between raloxifene and tamoxifen may influence the conformations of their respective receptor/ligand complexes, thereby affecting which estrogen-responsive genes are modulated in various tissues. These structural differences are 4-fold: (A) the presence of phenolic hydroxyls, (B) different substituents on the basic amine, (C) incorporation of the stilbene moiety into a cyclic benzothiophene framework, and (D) the imposition of a carbonyl "hinge" between the basic amine-containing side chain and the olefin. A series of raloxifene analogs that separately exemplify each of these differences have been prepared and evaluated in a series of in vitro and in vivo assays. This strategy has resulted in the development of a pharmacophore model that attributes the differences in effects on the uterus between raloxifene and tamoxifen to a low-energy conformational preference imparting an orthogonal orientation of the basic side chain with respect to the stilbene plane. This three-dimensional array is dictated by a single carbon atom in the hinge region of raloxifene. These data indicate that differences in tissue selective actions among benzothiophene and triarylethylene estrogen receptor modulators can be ascribed to discrete ligand conformations.

hsp70 is not required for high affinity binding of purified calf uterine estrogen receptor to estrogen response element DNA in vitro

Bovine estrogen receptor (ER) was purified to near homogeneity by estrogen response element (ERE) affinity chromatography, and its ERE binding ability was measured in vitro. Highly purified ER bound EREs with reduced affinity compared to partially purified ER. Partially purified ER contained hsp70, but highly purified ER did not. We examined whether addition of purified recombinant human hsp70 or purified bovine hsp70 would restore the higher ERE binding affinity, stoichiometry, and ligand retention detected with partially purified receptor and how hsp70 affected the rate of ER-ERE association and dissociation. ER-ERE binding was not affected by antibodies to either constitutive or induced forms of hsp70, regardless of ER purity. Addition of purified hsp70, with or without ATP and Mg2+, did not affect the association or dissociation rates of highly purified liganded ER binding to ERE. hsp70 Did not alter the total amount of ER-ERE complex formed. Similarly, hsp70 did not affect the rate of [3H]estradiol (E2) or [3H]4-hydroxytamoxifen (4-OHT) ligand dissociation from ER in the presence or absence of EREs. These data contrast with a report showing that maximal ERE binding by highly purified recombinant human ER required hsp70. We conclude that ER, purified from a physiological source, i.e., calf uterus, does not require hsp70 for maximal ER-ERE binding in vitro. Additionally, once ER is activated and bound by ligand, the receptor assumes its proper tertiary structure, and hsp70 does not impact ER ligand binding domain conformation.

Binding of type II nuclear receptors and estrogen receptor to full and half-site estrogen response elements in vitro

The mechanism by which retinoids, thyroid hormone (T3) and estrogens modulate the growth of breast cancer cells is unclear. Since nuclear type II nuclear receptors, including retinoic acid receptor (RAR), retinoid X receptor (RXR) and thyroid hormone receptor (TR), bind direct repeats (DR) of the estrogen response elements (ERE) half-site (5'-AGGTCA-3'), we examined the ability of estrogen receptor (ER) versus type II nuclear receptors, i.e. RARalpha, beta and gamma, RXRbeta, TRalpha and TRbeta, to bind various EREs in vitro . ER bound a consensus ERE, containing a perfectly palindromic 17 bp inverted repeat (IR), as a homodimer. In contrast, ER did not bind to a single ERE half-site. Likewise, ER did not bind two tandem (38 bp apart) half-sites, but low ER binding was detected to three tandem copies of the same half-site. RARalpha,beta or gamma bound both ERE and half-site constructs as a homodimer. RXRbeta did not bind full or half-site EREs, nor did RXRbeta enhance RARalpha binding to a full ERE. However, RARalpha and RXRbeta bound a half-site ERE cooperatively forming a dimeric complex. The RARalpha-RXRbeta heterodimer bound the Xenopus vitellogenin B1 estrogen responsive unit, with two non-consensus EREs, with higher affinity than one or two copies of the full or half-site ERE. Both TRalpha and TRbeta bound the full and the half-site ERE as monomers and homodimers and cooperatively as heterodimers with RXRbeta. We suggest that the cellular concentrations of nuclear receptors and their ligands, and the nature of the ERE or half-site sequence and those of its flanking sequences determine the occupation of EREs in estrogen-regulated genes in vivo .

Structure-activity relationships of selective estrogen receptor modulators: modifications to the 2-arylbenzothiophene core of raloxifene

The 2-arylbenzothiophene raloxifene, 1, is a selective estrogen receptor modulator which is currently under clinical evaluation for the prevention and treatment of postmenopausal osteoporosis. A series of raloxifene analogs which contain modifications to the 2-arylbenzothiophene core have been prepared and evaluated for the ability to bind to the estrogen receptor and inhibit MCF-7 breast cancer cell proliferation in vitro. Their ability to function as tissue-selective estrogen agonists in vivo has been assayed in a short-term, ovariectomized (OVX) rat model with end points of serum cholesterol lowering, uterine weight gain, and uterine eosinophil peroxidase activity. These studies have demonstrated that (1) the 6-hydroxy and, to a lesser extent, the 4'-hydroxy substituents of raloxifene are important for receptor binding and in vitro activity, (2) small, highly electronegative 4'-substituents such as hydroxy, fluoro, and chloro are preferred both in vitro and in vivo, (3) increased steric bulk at the 4'-position leads to increased uterine stimulation in vivo, and (4) additional substitution of the 2-aryl moiety is tolerated while additional substitution at the 4-, 5-, or 7-position of the benzothiophene results in reduced biological activity. In addition, compounds in which the 2-aryl group is replaced by alkyl, cycloalkyl, and naphthyl substituents maintain a profile of in vitro and in vivo biological activity qualitatively similar to that of raloxifene. Several novel structural variants including 2-cyclohexyl, 2-naphthyl, and 6-carbomethoxy analogs also demonstrated efficacy in preventing bone loss in a chronic OVX rat model of postmenopausal osteopenia, at doses of 0.1-10 mg/kg.

Stability of the ligand-estrogen receptor interaction depends on estrogen response element flanking sequences and cellular factors

To determine whether accessory proteins mediate the ligand- and DNA sequence-dependent specificity of estrogen receptor (ER) interaction with DNA, the binding of partly purified vs highly purified bovine ER to various estrogen response elements (EREs) was measured in the presence of different ER ligands. Partly purified estradiol-liganded ER (E2-ER) binds cooperatively to stereoaligned tandem EREs flanked by naturally occurring AT-rich sequences, with a stoichiometry of one E2-ER dimer per ERE. In contrast, highly purified E2-ER binds with a 10-fold lower affinity and non-cooperatively to EREs flanked by the AT-rich region. Moreover, the binding stoichiometry of highly purified E2-ER was 0.5 E2-ER dimer, or one monomer per ERE, independent of the ERE flanking sequence. Interestingly, the binding of ER liganded with the antiestrogen 4-hydroxytamoxifen (4-OHT-ER) was non-cooperative with an apparent stoichiometry of 0.5 4-OHT-ER dimer per ERE, regardless of ER purity or ERE flanking sequence. We recently showed that when 4-OHT-ER binds DNA, one molecule of 4-OHT dissociates from the dimeric 4-OHT-ER-ERE complex, accounting for the reduced apparent binding stoichiometry. In contrast, ER covalently bound by tamoxifen aziridine (TAz) gave an ERE binding stoichiometry of one TAz-ER dimer per ERE, and TAz-ER binds cooperatively to multiple AT-rich EREs, regardless of the purity of the receptor. We have obtained evidence that purification of ER removes an accessory protein(s) that interacts with ER in a sequence- and/or DNA conformational-dependent manner, resulting in stabilization of E2, but not 4-OHT, in the ligand binding domain when the receptor binds to DNA. We postulate that retention of ligand by ER maintains the receptor in a conformation necessary to achieve high-affinity, cooperative ERE binding.