Mutagenesis of cysteines in the hormone binding domain of the human estrogen receptor. Alterations in binding and transcriptional activation by covalently and reversibly attaching ligands

Estrogen Receptor Alpha Mutations, Truncations, Heterodimers, and Therapies

Annual breast cancer (BCa) deaths have declined since its apex in 1989 concomitant with widespread adoption of hormone therapies that target estrogen receptor alpha (ERα), the prominent nuclear receptor expressed in ∼80% of BCa. However, up to ∼50% of patients who are ER+ with high-risk disease experience post endocrine therapy relapse and metastasis to distant organs. The vast majority of BCa mortality occurs in this setting, highlighting the inadequacy of current therapies. Genomic abnormalities to ESR1, the gene encoding ERα, emerge under prolonged selective pressure to enable endocrine therapy resistance. These genetic lesions include focal gene amplifications, hotspot missense mutations in the ligand binding domain, truncations, fusions, and complex interactions with other nuclear receptors. Tumor cells utilize aberrant ERα activity to proliferate, spread, and evade therapy in BCa as well as other cancers. Cutting edge studies on ERα structural and transcriptional relationships are being harnessed to produce new therapies that have shown benefits in patients with ESR1 hotspot mutations. In this review we discuss the history of ERα, current research unlocking unknown aspects of ERα signaling including the structural basis for receptor antagonism, and future directions of ESR1 investigation. In addition, we discuss the development of endocrine therapies from their inception to present day and survey new avenues of drug development to improve pharmaceutical profiles, targeting, and efficacy.

Optimization of small molecule degraders and antagonists for targeting estrogen receptor based on breast cancer: current status and future

The estrogen receptor (ER) is a classical receptor protein that plays a crucial role in mediating multiple signaling pathways in various target organs. It has been shown that ER-targeting therapies inhibit breast cancer cell proliferation, enhance neuronal protection, and promote osteoclast formation. Several drugs have been designed to specifically target ER in ER-positive (ER+) breast cancer, including selective estrogen receptor modulators (SERM) such as Tamoxifen. However, the emergence of drug resistance in ER+ breast cancer and the potential side effects on the endometrium which has high ER expression has posed significant challenges in clinical practice. Recently, novel ER-targeted drugs, namely, selective estrogen receptor degrader (SERD) and selective estrogen receptor covalent antagonist (SERCA) have shown promise in addressing these concerns. This paper provides a comprehensive review of the structural functions of ER and highlights recent advancements in SERD and SERCA-related small molecule drugs, especially focusing on their structural optimization strategies and future optimization directions. Additionally, the therapeutic potential and challenges of novel SERDs and SERCAs in breast cancer and other ER-related diseases have been discussed.

Bioinformatic Evaluation of Features on Cis-regulatory Elements at 6q25.1

Eukaryotic non-coding regulatory features contribute significantly to cellular plasticity which on aberration leads to cellular malignancy. Enhancers are cis-regulatory elements that contribute to the development of resistance to endocrine therapy in estrogen receptor (ER)-positive breast cancer leading to poor clinical outcome. ER is vital for therapeutic targets in ER-positive breast cancer. Here, we review and report the different regulatory features present on ER with the objective to delineate potential mechanisms which may contribute to development of resistance. The UCSC Genome Browser, data mining, and bioinformatics tools were used to review enhancers, transcription factors (TFs), histone marks, long non-coding RNAs (lncRNAs), and variants residing in the non-coding region of the ER gene. We report 7 enhancers, 3 of which were rich in TF-binding sites and histone marks in a cell line-specific manner. Furthermore, some enhancers contain estrogen resistance variants and sites for lncRNA. Our review speculates putative models suggesting potential aberrations in gene regulation and expression if these regulatory landscapes and assemblies are altered. This review gives an interesting perspective in designing integrated in vitro studies including non-coding elements to study development of endocrine resistance in ER-positive breast cancer.

Covalent ERα Antagonist H3B-6545 Demonstrates Encouraging Preclinical Activity in Therapy-Resistant Breast Cancer

Nearly 30% of patients with relapsed breast cancer present activating mutations in estrogen receptor alpha (ERα) that confer partial resistance to existing endocrine-based therapies. We previously reported the development of H3B-5942, a covalent ERα antagonist that engages cysteine-530 (C530) to achieve potency against both wild-type (ERαWT) and mutant ERα (ERαMUT). Anticipating that the emergence of C530 mutations could promote resistance to H3B-5942, we applied structure-based drug design to improve the potency of the core scaffold to further enhance the antagonistic activity in addition to covalent engagement. This effort led to the development of the clinical candidate H3B-6545, a covalent antagonist that is potent against both  ERαWT/MUT, and maintains potency even in the context of ERα C530 mutations. H3B-6545 demonstrates significant activity and superiority over standard-of-care fulvestrant across a panel of ERαWT and ERαMUT palbociclib sensitive and resistant models. In summary, the compelling preclinical activity of H3B-6545 supports its further development for the potential treatment of endocrine therapy-resistant ERα+ breast cancer harboring wild-type or mutant ESR1, as demonstrated by the ongoing clinical trials (NCT03250676, NCT04568902, NCT04288089).

SUMMARY

H3B-6545 is an ERα covalent antagonist that exhibits encouraging preclinical activity against CDK4/6i naïve and resistant ERαWT and ERαMUT tumors.

Segregation of nuclear and membrane-initiated actions of estrogen receptor using genetically modified animals and pharmacological tools

Estrogen receptor alpha (ERα) and beta (ERβ) are members of the nuclear receptor superfamily, playing widespread functions in reproductive and non-reproductive tissues. Beside the canonical function of ERs as nuclear receptors, in this review, we summarize our current understanding of extra-nuclear, membrane-initiated functions of ERs with a specific focus on ERα. Over the last decade, in vivo evidence has accumulated to demonstrate the physiological relevance of this ERα membrane-initiated-signaling from mouse models to selective pharmacological tools. Finally, we discuss the perspectives and future challenges opened by the integration of extra-nuclear ERα signaling in physiology and pathology of estrogens.

Design and synthesis of novel benzothiophene analogs as selective estrogen receptor covalent antagonists against breast cancer

Endocrine therapy (ET) has benefited patients with estrogen receptor alpha (ERα) positive breast cancer for decades. Selective estrogen receptor modulator (SERM) such as Tamoxifen represents the clinical standard of care (SoC). Despite the therapeutic importance of current SoC agents, 30-50% of prolonged treatment patients inevitably generated resistant tumor cells, usually eventually suffered tumor relapse and developed into metastatic breast cancer (MBC), which was the leading cause of female cancer-related mortality. Among these, most resistant tumors remained dependent on ERα signaling, which reignited the need for the next generation of ERα related agents. We hypothesized that selective estrogen receptor covalent antagonists targeting ERα would provide a therapeutic alternative. In the current work, series of novel benzothiophene hybrids bearing electrophile moieties were synthesized and biologically evaluated. The representative analogue 15c exhibited potent anti-proliferative effect in MCF-7 cell lines in vitro, and further mechanism studies confirmed the necessity of covalent bonding. More importantly, 15c could attenuate the expression of TFF-1, GREB-1 and downregulate the levels of cellular ERα protein.

Estrogen Receptor Covalent Antagonists: The Best Is Yet to Come

The development of tamoxifen and subsequent estrogen receptor alpha (ERα) antagonists represents a tremendous therapeutic breakthrough in the treatment of breast cancer. Despite the ability of ERα antagonists to increase survival rates, resistance to these therapies is an all-too-common occurrence. The majority of resistant tumors, including those with hotspot mutations in the ligand-binding domain of ERα, remain dependent on ERα signaling, indicating that either a more potent or novel class of antagonist could have clinical benefit. With this thought in mind, we developed a novel ERα antagonist that exhibits enhanced potency due to its ability to covalently target a unique cysteine in ER. This review describes the design of this antagonist, H3B-5942, and discusses opportunities for future improvements, which could reduce the risk of escape mutations to this therapeutic modality.

Discovery of Selective Estrogen Receptor Covalent Antagonists for the Treatment of ERαWT and ERαMUT Breast Cancer

Mutations in estrogen receptor alpha (ERα) that confer resistance to existing classes of endocrine therapies are detected in up to 30% of patients who have relapsed during endocrine treatments. Because a significant proportion of therapy-resistant breast cancer metastases continue to be dependent on ERα signaling, there remains a critical need to develop the next generation of ERα antagonists that can overcome aberrant ERα activity. Through our drug-discovery efforts, we identified H3B-5942, which covalently inactivates both wild-type and mutant ERα by targeting Cys530 and enforcing a unique antagonist conformation. H3B-5942 belongs to a class of ERα antagonists referred to as selective estrogen receptor covalent antagonists (SERCA). In vitro comparisons of H3B-5942 with standard-of-care (SoC) and experimental agents confirmed increased antagonist activity across a panel of ERα^WT and ERα^MUT cell lines. In vivo, H3B-5942 demonstrated significant single-agent antitumor activity in xenograft models representing ERα^WT and ERα^Y537S breast cancer that was superior to fulvestrant. Lastly, H3B-5942 potency can be further improved in combination with CDK4/6 or mTOR inhibitors in both ERα^WT and ERα^MUT cell lines and/or tumor models. In summary, H3B-5942 belongs to a class of orally available ERα covalent antagonists with an improved profile over SoCs.Significance: Nearly 30% of endocrine therapy-resistant breast cancer metastases harbor constitutively activating mutations in ERα. SERCA H3B-5942 engages C530 of both ERα^WT and ERα^MUT, promotes a unique antagonist conformation, and demonstrates improved in vitro and in vivo activity over SoC agents. Importantly, single-agent efficacy can be further enhanced by combining with CDK4/6 or mTOR inhibitors. Cancer Discov; 8(9); 1176-93. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 1047.

A cis-regulatory mutation in troponin-I of Drosophila reveals the importance of proper stoichiometry of structural proteins during muscle assembly

Rapid and high wing-beat frequencies achieved during insect flight are powered by the indirect flight muscles, the largest group of muscles present in the thorax. Any anomaly during the assembly and/or structural impairment of the indirect flight muscles gives rise to a flightless phenotype. Multiple mutagenesis screens in Drosophila melanogaster for defective flight behavior have led to the isolation and characterization of mutations that have been instrumental in the identification of many proteins and residues that are important for muscle assembly, function, and disease. In this article, we present a molecular-genetic characterization of a flightless mutation, flightless-H (fliH), originally designated as heldup-a (hdp-a). We show that fliH is a cis-regulatory mutation of the wings up A (wupA) gene, which codes for the troponin-I protein, one of the troponin complex proteins, involved in regulation of muscle contraction. The mutation leads to reduced levels of troponin-I transcript and protein. In addition to this, there is also coordinated reduction in transcript and protein levels of other structural protein isoforms that are part of the troponin complex. The altered transcript and protein stoichiometry ultimately culminates in unregulated acto-myosin interactions and a hypercontraction muscle phenotype. Our results shed new insights into the importance of maintaining the stoichiometry of structural proteins during muscle assembly for proper function with implications for the identification of mutations and disease phenotypes in other species, including humans.

Modulation of nuclear receptor function by cellular redox poise

Nuclear receptors (NRs) are ligand-responsive transcription factors involved in diverse cellular processes ranging from metabolism to circadian rhythms. This review focuses on NRs that contain redox-active thiol groups, a common feature within the superfamily. We will begin by describing NRs, how they regulate various cellular processes and how binding ligands, corepressors and/or coactivators modulate their activity. We will then describe the general area of redox regulation, especially as it pertains to thiol-disulfide interconversion and the cellular systems that respond to and govern this redox equilibrium. Lastly, we will discuss specific examples of NRs whose activities are regulated by redox-active thiols. Glucocorticoid, estrogen, and the heme-responsive receptor, Rev-erb, will be described in the most detail as they exhibit archetypal redox regulatory mechanisms.

DHEA metabolites activate estrogen receptors alpha and beta

Dehydroepiandrosterone (DHEA) levels were reported to associate with increased breast cancer risk in postmenopausal women, but some carcinogen-induced rat mammary tumor studies question this claim. The purpose of this study was to determine how DHEA and its metabolites affect estrogen receptors α or β (ERα or ERβ)-regulated gene transcription and cell proliferation. In transiently transfected HEK-293 cells, androstenediol, DHEA, and DHEA-S activated ERα. In ERβ transfected HepG2 cells, androstenedione, DHEA, androstenediol, and 7-oxo DHEA stimulated reporter activity. ER antagonists ICI 182,780 (fulvestrant) and 4-hydroxytamoxifen, general P450 inhibitor miconazole, and aromatase inhibitor exemestane inhibited activation by DHEA or metabolites in transfected cells. ERβ-selective antagonist R,R-THC (R,R-cis-diethyl tetrahydrochrysene) inhibited DHEA and DHEA metabolite transcriptional activity in ERβ-transfected cells. Expression of endogenous estrogen-regulated genes: pS2, progesterone receptor, cathepsin D1, and nuclear respiratory factor-1 was increased by DHEA and its metabolites in an ER-subtype, gene, and cell-specific manner. DHEA metabolites, but not DHEA, competed with 17β-estradiol for ERα and ERβ binding and stimulated MCF-7 cell proliferation, demonstrating that DHEA metabolites interact directly with ERα and ERβin vitro, modulating estrogen target genes in vivo.

The role of calcium in the activation of estrogen receptor-alpha

Environmental estrogen mimics, including metalloestrogens that can activate estrogen receptor-alpha (ERα), may contribute to breast cancer risk. However, the underlying mechanisms through which these molecular mimics activate the ERα are generally poorly understood. With concern to this important question, we investigated whether intracellular calcium may mediate the cross-talk between signaling pathways that activate ERα and the ligand-binding domain of ERα. MCF-7 cells treated with EGF, ATP, extracellular calcium, or caffeine to increase intracellular calcium triggered a rapid recruitment of ERα to estrogen-responsive promoters and stimulated expression of estrogen-responsive genes including pS2, complement C3, and progesterone receptor. Induction was blocked by an antiestrogen but also by the chelation of intracellular calcium. Treatment with extracellular calcium also increased the growth of MCF-7 cells through an ER-dependent mechanism. We found that EGF and extracellular calcium activated the C-terminus of ERα and the activation was blocked by the antiestrogen. Mechanistic investigations identified four potential sites on the solvent-accessible surface of the ERα ligand-binding domain as important for calcium activation of the receptor. Taken together, our results suggest that calcium mediates the cross-talk between ERα-activating signaling pathways and the ligand-binding domain of ERα providing a potential explanation for the ability of certain environmental metalloestrogens to activate the receptor.

Functional and structural evaluation of cysteine residues in the human arsenic (+3 oxidation state) methyltransferase (hAS3MT)

Arsenic (+3 oxidation state) methyltransferase (AS3MT) catalyzes the methylation of inorganic arsenic (iAs) and plays important role in the detoxication of this metalloid. There are fourteen cysteine residues in the human AS3MT (hAS3MT), among which twelve are absolutely conserved; Cys334 and Cys360 are unique; Cys368 and Cys369 are identified as a CysCys pair. The roles of several conserved cysteine residues in rat AS3MT and hAS3MT have been reported. Herein, the other conserved cysteine residues (Cys72, Cys271, Cys375) and the unique ones (Cys334, Cys360) were systematically replaced by serine using site-directed mutagenesis to study their functions. The mutants were investigated for enzymatic activity, kinetics, thermal stability and secondary structures. Present results indicate that C72S is completely inactive in methylation of iAs and has distinct changes in the secondary structures; Cys72 might form a critical intramolecular disulfide bond with Cys250; Cys271 and Cys375 do not affect the activity and structure of the hAS3MT. However, the mutations of Cys334 and Cys360 can decrease the enzymatic turnovers and change the conformation of the hAS3MT. The kinetic data show that Cys271, Cys334, Cys360 and Cys375 are not involved in the SAM binding. Additionally, all these cysteine residues except Cys375 affect the thermotropic properties of the hAS3MT.

Activation of estrogen receptor-alpha by the anion nitrite

In this study, the ability of nitrite and nitrate to mimic the effects of estradiol on growth and gene expression was measured in the human breast cancer cell line MCF-7. Similar to estradiol, treatment of MCF-7 cells with either 1 mumol/L nitrite or 1 mumol/L nitrate resulted in approximately 4-fold increase in cell growth and 2.3-fold to 3-fold increase in progesterone receptor (PgR), pS2, and cathepsin D mRNAs that were blocked by the antiestrogen ICI 182,780. The anions also recruited estrogen receptor-alpha (ERalpha) to the pS2 promoter and activated exogenously expressed ERalpha when tested in transient cotransfection assays. To determine whether nitrite or nitrate was the active anion, diphenyleneiodonium was used to inhibit oxidation/reduction reactions in the cell. The ability of diphenyleneiodonium to block the effects of nitrate, but not nitrite, on the induction of PgR mRNA and the activation of exogenously expressed ERalpha suggests that nitrite is the active anion. Concentrations of nitrite, as low as 100 nmol/L, induced a significant increase in PgR mRNA, suggesting that physiologically and environmentally relevant doses of the anion activate ERalpha. Nitrite activated the chimeric receptor Gal-ER containing the DNA-binding domain of GAL-4 and the ligand-binding domain of ERalpha and blocked the binding of estradiol to the receptor, suggesting that the anion activates ERalpha through the ligand-binding domain. Mutational analysis identified the amino acids Cys381, His516, Lys520, Lys529, Asn532, and His547 as important for nitrite activation of the receptor.

Effects of tobacco smoke condensate on estrogen receptor-alpha gene expression and activity

Metallo-estrogens are a new class of potent environmental estrogens. This study investigates whether tobacco smoke condensate (TSC), which contains metals and metalloids, elicits estrogen-like effects at environmentally relevant doses. Treatment of human breast cancer cells, MCF-7, with 40 microg/ml TSC resulted in a 2.5-fold stimulation of cell growth. TSC decreased the concentration of estrogen receptor (ER)-alpha protein and mRNA (63 and 62%, respectively), and increased the expression of the estrogen-regulated genes, progesterone receptor and pS2 (5- and 2-fold, respectively). In addition, TSC activated ER-alpha in COS-1 or CHO cells transiently transfected with wild-type ER-alpha and an ERE-CAT or an ERE-luciferase reporter gene (11- and 6-fold, respectively). TSC also activated a chimeric receptor (GAL-ER) containing the hormone binding domain of ER-alpha (3.5-fold). It blocked the binding of estradiol to the receptor without altering the affinity of estradiol (K(d) = 2.2-6.8 x 10(-10) m). Transfection assays with ER-alpha mutants identified C381, C447, H524, N532, E523, and D538 in the hormone binding domain as important for activation by TSC. In ovariectomized rats, low doses of TSC [10 or 20 mg/kg body weight (bw)] increased uterine wet weight (1.7- and 2.1-fold), and induced the expression of progesterone receptor and complement C3 in the uterus (2- and 26-fold) and mammary gland (4.4- and 15-fold). Both the in vitro and in vivo TSC effects were blocked by the antiestrogen ICI 182,780, suggesting the involvement of ER. Collectively, these results provide strong evidence that low doses of TSC, acting through the hormone binding domain, exert estrogen-like effects in cell culture and animals.

Estrogen receptor signaling pathways in human non-small cell lung cancer

Lung cancer is the most common cause of cancer mortality in male and female patients in the US. The etiology of non-small cell lung cancer (NSCLC) is not fully defined, but new data suggest that estrogens and growth factors promote tumor progression. In this work, we confirm that estrogen receptors (ER), both ERalpha and ERbeta, occur in significant proportions of archival NSCLC specimens from the clinic, with receptor expression in tumor cell nuclei and in extranuclear sites. Further, ERalpha in tumor nuclei was present in activated forms as assessed by detection of ER phosphorylation at serines-118 and -167, residues commonly modulated by growth factor receptor as well as steroid signaling. In experiments using small interfering RNA (siRNA) constructs, we find that suppressing expression of either ERalpha or ERbeta elicits a significant reduction in NSCLC cell proliferation in vitro. Estrogen signaling in NSCLC cells may also include steroid receptor coactivators (SRC), as SRC-3 and MNAR/PELP1 are both expressed in several lung cell lines, and both EGF and estradiol elicit serine phosphorylation of SRC-3 in vitro. EGFR and ER also cooperate in promoting early activation of p42/p44 MAP kinase in NSCLC cells. To assess new strategies to block NSCLC growth, we used Faslodex alone and with erlotinib, an EGFR kinase inhibitor. The drug tandem elicited enhanced blockade of the growth of NSCLC xenografts in vivo, and antitumor activity exceeded that of either agent given alone. The potential for use of antiestrogens alone and with growth factor receptor antagonists is now being pursued further in clinical trials.

Functional and structural comparisons of cysteine residues in the Val108 wild type and Met108 variant of human soluble catechol O-methyltransferase

Catechol O-methyltransferase (COMT) plays an important role in the inactivation of biologically active and toxic catechols. This enzyme is genetically polymorphic with a wild type and a variant form. Numerous epidemiological studies have shown that the variant form is associated with an increased risk of developing estrogen-associated cancers and a wide spectrum of mental disorders. There are seven cysteine residues in human S-COMT, all of which exist as free thiols and are susceptible to electrophilic attack and/or oxidative damage leading to enzyme inactivation. Here, the seven cysteine residues were systematically replaced by alanine residues by means of site-directed mutagenesis. The native forms and cysteine/alanine mutants were assayed for enzymatic activity, thermal stability, methylation regioselectivity, and reactivity of cysteine residues to thiol reagent. Our data showed that although there is only one encoding base difference between these two COMT forms, this difference might induce structural changes in the local area surrounding some cysteine residues, which might further contribute to the different roles they might play in enzymatic activity, and to the different susceptibility to enzyme inactivation.

The transcriptional activity of estrogen receptor-alpha is dependent on Ca2+/calmodulin

Estrogen binds to estrogen receptors in cells, thereby activating the receptors and eliciting biological effects. One of the best characterized effects of estrogen receptor-alpha (ERalpha) is transcriptional activation that regulates selected target genes in the nucleus. Work from several laboratories has documented a Ca2+-dependent interaction between ERalpha and calmodulin. In addition, we previously showed that antagonism of calmodulin function in cells prevented estradiol from inducing ERalpha transcriptional activity, suggesting that association of ERalpha with calmodulin participates in ERalpha function. In this study we adopted a multifaceted approach to directly address this hypothesis. The calmodulin binding domain on ERalpha was identified and several mutant ERalpha constructs unable to bind calmodulin were generated. Elimination of calmodulin binding prevented estradiol from stimulating ERalpha transcriptional activation. Essentially identical results were obtained when intracellular Ca2+ was chelated with the cell-permeable chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester (BAPTA-AM). Moreover, CaM1234, a calmodulin mutant that is unable to bind Ca2+, functioned as a dominant negative construct. Transfection of cells with CaM1234 reduced estradiol-stimulated ERalpha transcriptional activity. These data indicate that binding to calmodulin is required for normal transcriptional function of ERalpha.

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.

Effect of a short CAG (glutamine) repeat on human androgen receptor function

BACKGROUND

The human androgen receptor (AR) gene contains an uninterrupted CAG repeat that is polymorphic in length in the general population (range, 11-31 CAG's; median, 21). The CAG repeat encodes a glutamine repeat in the N-terminal transactivation domain of the AR protein. We previously reported that a 17-CAG AR gene was much more common in a cohort of men with prostate cancer (8.5%) than in the general European American population (1.3%). This suggested that a 17-CAG repeat may have pathophysiological consequences. The goal of the present study was to directly test the hypothesis that a 17-CAG repeat might uniquely affect androgen action in human prostate cancer cells.

METHODS

DU145 cells, lacking endogenous AR, were transiently transfected with an AR expression plasmid (with a CAG repeat ranging in length from 14 to 25) and an androgen-responsive reporter plasmid (PSA-luciferase).

RESULTS

We found a significant effect of CAG repeat length on AR protein levels per unit amount of DNA transfected (one-way ANOVA, P = 0.02), indicating the need to express transactivation data per unit amount of AR protein. CAG17 AR had 40% more transactivation activity per unit amount of AR protein than CAG21 AR (P < 0.01).

CONCLUSIONS

Thus, an AR with a 17-CAG repeat may mediate more efficacious growth stimulation of androgen-dependent prostate epithelial cells, and thereby increase the risk that prostate cancer cells develop more efficiently into a clinically significant cancer.

Identification of estrogen receptor beta expression in Chinese hamster ovary (CHO) cells and comparison of estrogen-responsive gene transcription in cells adapted to serum-free media

Most cultured cell lines require addition of serum to the medium to maintain their proliferative capacity. For studies examining the cellular effects of estrogens serum is charcoal-stripped to remove steroids. Nonetheless, addition of the selective estrogen receptor modulator (SERM) 4-hydroxytamoxifen (4-OHT) inhibits the basal transcriptional activity of estrogen receptors alpha or beta (ERalpha or ERbeta) in transfected cells. We tested the hypothesis that elimination of serum from the culture medium will block 4-OHT's repression of basal activity. Chinese hamster ovary (CHO-K1) cells adapted to serum-free medium exhibited estrogen responsiveness that was identical with that of the cells grown in serum-containing media. 4-OHT-suppressed basal transcription of an estrogen response element (ERE)-reporter in ERalpha-transfected cells even in the absence of serum, indicating that the 4-OHT suppressive activity is not mediated by blocking ER interaction with serum estrogens. We speculate that 4-OHT-ER recruits co-repressors to suppress basal transcription. We discovered that CHO-K1 cells express ERalpha and ERbeta mRNA. However only ERbeta protein was expressed and use of ERbeta-selective 2,3-bis(4-hydroxy-phenyl)propionitrile (DPN) and ERalpha-selective 4-propyl-1,3,5-tris(4-hydroxy-phenyl)pyrazole) (PPT) revealed that only ERbeta was transcriptionally active. In conclusion, growing CHO-K1 in serum-free medium does not impact the estrogen responsiveness and this cell line expresses functional ERbeta.

Estrogen-like activity of metals in MCF-7 breast cancer cells

The ability of metals to activate estrogen receptor-alpha (ERalpha) was measured in the human breast cancer cell line, MCF-7. Similar to estradiol, treatment of cells with the divalent metals copper, cobalt, nickel, lead, mercury, tin, and chromium or with the metal anion vanadate stimulated cell proliferation; by d 6, there was a 2- to 5-fold increase in cell number. The metals also decreased the concentration of ERalpha protein and mRNA by 40-60% and induced expression of the estrogen-regulated genes progesterone receptor and pS2 by1.6- to 4-fold. Furthermore, there was a 2- to 4-fold increase in chloramphenicol acetyltransferase activity after treatment with the metals in COS-1 cells transiently cotransfected with the wild-type receptor and an estrogen-responsive chloramphenicol acetyltransferase reporter gene. The ability of the metals to alter gene expression was blocked by an antiestrogen, suggesting that the activity of these compounds is mediated by ERalpha. In binding assays the metals blocked the binding of estradiol to the receptor without altering the apparent binding affinity of the hormone (K(d) = 10(-10) M). Scatchard analysis employing either recombinant ERalpha or extracts from MCF-7 cells demonstrated that (57)Co and (63)Ni bind to ERalpha with equilibrium dissociation constants of 3 and 9.5 x 10(-9) and 2 and 7 x 10(-9) M, respectively. The ability of the metals to activate a chimeric receptor containing the hormone-binding domain of ERalpha suggests that their effects are mediated through the hormone-binding domain. Mutational analysis identified amino acids C381, C447, E523, H524, N532, and D538 as potential interaction sites, suggesting that divalent metals and metal anions activate ERalpha through the formation of a complex within the hormone-binding domain of the receptor.

Regulation of nuclear receptor transcriptional activity by a novel DEAD box RNA helicase (DP97)

We have identified a novel DEAD box RNA helicase (97 kDa, DP97) from a breast cancer cDNA library that interacts in a hormone-dependent manner with nuclear receptors and represses their transcriptional activity. DP97 has RNA-dependent ATPase activity, and mapping studies localize the interacting regions of DP97 and nuclear receptors to the C-terminal region of DP97 and the hormone binding/activation function-2 region of estrogen receptors (ER), as well as several other nuclear receptors. Repression by DP97 maps to a small region (amino acids 589-631) that has homology to a repression domain in the corepressor protein NCoR2/SMRTe. This region of DP97 is necessary and sufficient for its intrinsic repression activity. The N-terminal helicase region of DP97 is, however, dispensable for its transcriptional repressor activity. The knockdown of endogenous cellular DP97 by antisense DP97 or RNA interference (siRNA for DP97) results in significant enhancement of the expression of estradiol-ER-stimulated genes and attenuation of the repression of genes inhibited by the estradiol-ER. This implies that endogenous DP97 normally dampens stimulation and intensifies repression of estradiol-ER-regulated genes. Our findings add to the growing evidence that RNA helicases can associate with nuclear receptors and function as coregulators to modulate receptor transcriptional activity.

Identification by MALDI-TOF mass spectrometry of 17 alpha-bromoacetamidopropylestradiol covalent attachment sites on estrogen receptor alpha

Mass spectrometry was used to identify the sites of covalent attachment of [(14)C]-17alpha-bromoacetamidopropylestradiol ([(14)C]17BAPE(2), an estradiol agonist) to the ligand-binding domain (LBD) of mouse estrogen receptor alpha (ERalpha). A glutathione S-transferase (GST)-LBD chimera protein was overexpressed in Escherichia coli, using a vector encoding GST fused with a C-terminal portion of mouse ERalpha (Ser(313)-Ile(599)), via a sequence enclosing a thrombin cleavage site (located 14 amino acids ahead of Ser313). [(14)C]17BAPE(2) covalent labeling experiments were carried out on the GST-LBD chimera immobilized on glutathione-Sepharose. After thrombin cleavage of the chimeric LBD, two major [(14)C]17BAPE(2)-labeled species of 34 ( approximately 75%) and 30 kDa ( approximately 25%) were detected by SDS-PAGE and autoradiography. Their identity was assessed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS): two main signals were consistent with the mass of the full-length (Ser(313)-Ile(599)) and truncated LBD (Ser(313)-Ala(573)), both comprising the extra 14 N-terminal amino acids and covalently bound [(14)C]17BAPE(2) (via HBr elimination). A purified (14)C-labeled LBD preparation was trypsinized to identify the covalent attachment sites of 17BAPE(2). HPLC of tryptic fragments only revealed two discrete and practically equivalent radioactive fractions. MALDI-TOF MS analysis of these two fractions showed only two signals which exactly matched the molecular masses of the [(14)C]17BAPE(2)-alkylated Cys(534)Lys(535) and Cys(421)-Arg(438) peptides, respectively. Hydrolysis of the second (14)C-labeled fraction by Staphylococcus aureus V8 Glu-C endoproteinase generated signals typical of alkylated the Cys(421)-Glu(423) tripeptide. We concluded that Cys421 and Cys534 were equivalent alternative covalent attachment sites of 17BAPE(2) on the LBD. These biochemical data were interpreted using the crystallographic structures of estradiol-LBD and raloxifene- or 4-hydroxytamoxifen-LBD complexes. The covalent attachment to Cys421, Cys534, or both could be interpreted according to the starting structure. Various hypotheses based on the biochemical results and molecular modeling simulations are discussed, with the likely involvement of dynamic interconversion between multiple conformational states of the LBD-17BAPE(2) complex.

Structure-function relationships in estrogen receptors and the characterization of novel selective estrogen receptor modulators with unique pharmacological profiles

This article summarizes recent research on the development of estrogen receptor alpha (ER alpha) and estrogen receptor beta (ER beta) subtype-selective ligands based on our understanding of structure-activity relationships in these two estrogen receptors and differences in their ligand binding domains and activation function domains. The use of these ligands should enable greater understanding of the unique biologies mediated by ER alpha versus ER beta and may, as well, provide selective estrogen receptor modulators having unique biological and pharmacological profiles optimal for prevention and treatment of breast cancer, for menopausal hormone replacement, for prevention of osteoporosis, and for potential cardiovascular benefit.

Reciprocal mutagenesis between human alpha(L349, M528) and rainbow trout (M317, I496) estrogen receptor residues demonstrates their importance in ligand binding and gene expression at different temperatures

Several fish proteins exhibit compromised function at temperatures outside of their normal physiological range. In this study, the effect of temperature on the ligand binding and the transactivation abilities of the rainbow trout estrogen receptor (rtER) and human estrogen receptor alpha (hER alpha) were examined. Saturation analysis and gene expression assays, using GST-ER and Gal4-ER fusion proteins consisting of the D, E and F domains of human (hER alpha def) and rainbow trout (rtERdef) receptors, show that GST-rtERdef E2 binding affinity and transactivation ability decrease with increasing temperature. A comparison of the amino acid sequence differences between their ligand binding pockets identified two conservative amino acid residue substitutions in rtER (M317, I496) and hER alpha (L349, M528). The effect of these substitutions on ligand binding and transactivation were examined by constructing reciprocal mutants, which effectively exchanged the binding pockets between rtER and hER alpha. The rtERdef M317L:I496M double mutant exhibited increased E2 binding affinity and transactivation ability at higher temperatures, and displayed hER alpha phenotypic behavior for the phytoestrogen, coumestrol. The hER alpha def L349M:M528I double mutant also exhibited a modest trend towards adopting the rtER phenotype. These studies demonstrate that conservative changes in residue hydrophobicity and volume can significantly affect ER ligand binding and transactivation ability in a temperature-dependent manner. The lack of a complete exchange of phenotypes between rtER and hER alpha indicates that factors outside of the ligand binding pocket are also involved.

Codon 325 sequence polymorphism of the estrogen receptor alpha gene and bone mineral density in postmenopausal women

Estrogen receptor alpha (ER alpha) encoding gene is one of the candidate genes to be involved in the development of osteoporosis. Until now correlation between three ER gene polymorphisms (identified with PvuII, XbaI and BstUI) and bone mineral density (BMD) have been investigated. The results of these studies are contradictory. Thus the aim of our work was to search for new, yet unknown, and probably more informative polymorphism(s) of the ER alpha gene. For detection of mutations the whole coding region of the ER alpha gene was screened systematically. In a group of 85 late postmenopausal women all of the eight exons were amplified by polymerase chain reaction (PCR) and fragments were further analyzed by single-stranded conformation polymorphism (SSCP) analysis. Mutations were confirmed by direct DNA sequencing. In the whole coding region of the ER alpha gene two silent mutations in codon 87 and 325, respectively, were found. The silent mutation in codon 85 of exon 1 (GCG-->GCC; A87A) was described previously, as BstUI polymorphism. On the other side, the silent mutation in codon 325 (CCC-->CCG; P325P), located in exon 4, has not been analyzed so far in correlation with BMD. According to the distribution of genotypes CC:CG:GG=49.4:41.2:9.4, we can affirm the existence of genetic polymorphism in codon 325 in our population of late postmenopausal women. The mean femoral neck BMD, but not the lumbar spine BMD, was significantly lower (P=0.029) in the homozygous GG-women with CCG/CCG codon 325 as compared to the homozygous CC-women with the normal codon CCC/CCC. Our results suggest that codon 325 sequence polymorphism of the ER alpha gene might be one of the factors associated with low femoral neck BMD.

Involvement of cyclic AMP response element binding protein (CREB) and estrogen receptor phosphorylation in the synergistic activation of the estrogen receptor by estradiol and protein kinase activators

Estrogen receptor (ER) and cAMP signaling pathways interact in a number of estrogen target tissues including mammary and uterine tissues. One aspect of this interaction is that estradiol and protein kinase A (PKA) activators can cooperate synergistically to activate ER-mediated transcription of both endogenous genes and reporter genes containing only estrogen response elements. The purpose of this study was to investigate the molecular mechanism of this interaction between signaling pathways. Site-directed mutagenesis of the potential PKA phosphorylation sites in the ER indicated that phosphorylation of these sites was not necessary for the observed transcriptional synergy. In transient transfection assays in two different cell lines using reporter constructs containing either cAMP response elements, estrogen response elements or both types of elements, with the addition or absence of cAMP response element binding protein (CREB) expression plasmid, we observed that only one of these cell lines exhibited estrogen/PKA transcriptional synergy. Experiments demonstrated that CREB itself was involved in the transcriptional synergy, and that transfection of CREB restored transcriptional synergy in the cell line in which it was lacking. A functional interaction between ER and CREB was also demonstrated using a mammalian cell protein interaction assay; a dominant negative mutant of CREB did not exhibit this interaction. Therefore, these data indicate that CREB protein is required for the transcriptional synergy between cAMP and estrogen signaling pathways. Furthermore, CREB cooperated with the ER on genes that did not contain cAMP response elements, but contained only estrogen response elements. We propose that activated CREB is recruited to estrogen responsive genes by an ER--coactivator complex containing proteins such as CREB binding protein (CBP) and that the interaction of CREB with ER may assist in stabilizing its interaction with CBP and in promoting estrogen-ER and PKA transcriptional synergy.

Molecular mechanisms of estrogen action: selective ligands and receptor pharmacology

Estrogens exert profound effects on the physiology of diverse target cells and these effects appear to be mediated by two estrogen receptor (ER) subtypes, ERalpha and ERbeta. We have investigated how ER ligands, ranging from pure agonists to antagonists, interact with ERalpha and ERbeta, and regulate their transcriptional activity on different genes. Mutational mapping-structure activity studies indicate that different residues of the ER ligand binding domain are involved in the recognition of structurally distinct estrogens and antiestrogens. We have identified from ligands of diverse structure, several particularly interesting ones that are high potency selective agonists via ERalpha and others that are full agonists through ERalpha while being full antagonists through ERbeta. Antiestrogens such as hydroxytamoxifen, which are mixed agonist/antagonists through ERalpha, are pure antagonists through ERbeta at estrogen response element-containing gene sites. Studies with ERalpha/beta chimeric proteins reveal that tamoxifen agonism requires the activation function 1 region of ERalpha. Through two-hybrid assays, we have isolated an ER-specific coregulator that potentiates antiestrogen antagonist effectiveness and represses ER transcriptional activity. We have also focused on understanding the distinct pharmacologies of antiestrogen- and estrogen-regulated genes. Although antiestrogens are thought to largely act by antagonizing the actions of estrogens, we have found among several new ER-regulated genes, quinone reductase (QR), a detoxifying phase II antioxidant enzyme, that has its activity up-regulated by antiestrogens in an ER-dependent manner in breast cancer cells. This response is antagonized by estrogens, thus showing 'reversed pharmacology'. Increased QR activity by antiestrogens requires a functional ER (ERalpha or ERbeta) and is, interestingly, mediated via the electrophile response element in the QR gene 5' regulatory region. The up-regulation of QR may contribute to the beneficial effects of tamoxifen, raloxifene, and other antiestrogens in breast cancer prevention and treatment. Estrogens rapidly up-regulate expression of several genes associated with cell cytoarchitectural changes including NHE-RF, the sodium hydrogen exchanger regulatory factor, also known as EBP50. NHE-RF/EBP50 is enriched in microvilli, and may serve as a scaffold adaptor protein in regulating early changes in cell architecture and signal transduction events induced by estrogen. Analyses of the regulatory regions of these primary response genes, and the antioxidant and other signaling pathways involved, are providing considerable insight into the mechanisms by which ligands, that function as selective estrogen receptor modulators or SERMs, exert their marked effects on the activities and properties of target cells. The intriguing biology of estrogens in its diverse target cells is thus determined by the structure of the ligand, the ER subtype involved, the nature of the hormone-responsive gene promoter, and the character and balance of coactivators and corepressors that modulate the cellular response to the ER-ligand complex. The continuing development of ligands that function as selective estrogens or antiestrogens for ERalpha or ERbeta should allow optimized tissue selectivity of these agents for menopausal hormone replacement therapy and the treatment and prevention of breast cancer.

Identification and characterization of a novel factor that regulates quinone reductase gene transcriptional activity

The regulation of the quinone reductase (QR) gene as well as other genes involved in detoxification is known to be mediated by an electrophile/antioxidant response element (EpRE/ARE). We have previously observed that QR is up-regulated by the antiestrogen trans-hydroxytamoxifen in breast cancer cells. QR gene regulation by the antiestrogen-occupied estrogen receptor (ER) is mediated by the EpRE-containing region of the human QR gene, and the ER is one of the complex of proteins that binds to the EpRE. In an effort to further understand the mechanism for ER regulation of QR gene we identified other protein factors that regulate QR gene transcriptional activity in breast cancer cells. One of these protein factors, hPMC2 (human homolog of Xenopus gene which prevents mitotic catastrophe), directly binds to the EpRE and interacts with the ER in yeast genetic screening and in vitro assays. Interestingly hPMC2 interacts more strongly to ER beta when compared with ER alpha. In transient transfection assays using reporter constructs containing the EpRE, hPMC2 alone can slightly activate reporter in ER-negative MDA-MB-231 breast cancer cells. The activation of QR gene activity by hPMC2 is enhanced in the presence of ER beta.

Effects of selenite on estrogen receptor-alpha expression and activity in MCF-7 breast cancer cells

To determine whether selenite has estrogen-like activities, the effects of this compound on estrogen receptor-alpha (ER-alpha) and other estrogen-regulated genes were measured in the human breast cancer cell line MCF-7. Treatment of cells with 1 uM of sodium selenite resulted in a 40% decrease in the amount of estrogen receptor-alpha and in a parallel decrease of 40% in ER-alpha mRNA. Progesterone receptor concentration increased 2.6-fold and pS2 mRNA increased 2.4-fold after selenite treatment. The induction of progesterone receptor and pS2 was blocked by the anti-estrogen ICI-182,780. In transient co-transfection experiments of Wild-type ER-alpha and an estrogen response element-reporter construct, selenite stimulated CAT activity. In binding assays, selenite blocked the binding of estradiol to ER-alpha (K(i) = 23 +/- 17 nM, n = 3) suggesting that this compound interacts with the hormone binding domain of the receptor. To determine whether interaction of selenite with the hormone binding domain results in receptor activation, COS-1 cells were transiently co-transfected with the chimeric receptors GAL-ER, which contains the hormone binding domain of ER-alpha and the DNA binding domain of the transcription factor GAL4, and a GAL4-responsive CAT reporter gene. Treatment of cells with estradiol or selenite resulted in a three- to five-fold increase in CAT activity. The effects of selenite on the chimeric receptor were blocked by the antiestrogen, suggesting that selenite activates ER-alpha through an interaction with the hormone binding domain of the receptor. Transfection assays with ER-alpha mutants identified C381, C447, H524, and N532 as interaction sites of selenite with the hormone binding domain.

Development of high throughput screening assays using fluorescence polarization: nuclear receptor-ligand-binding and kinase/phosphatase assays

Fluorescence polarization (FP) has been used to develop high throughput screening (HTS) assays for nuclear receptor-ligand displacement and kinase inhibition. FP is a solution-based, homogeneous technique requiring no immobilization or separation of reaction components. The FP-based estrogen receptor (ER) assay is based on the competition of fluorescein-labeled estradiol and estrogen-like compounds for binding to ER. These studies determined the Kd for this interaction to be 3 nM for ERalpha and 2 nM for ERbeta; IC50 values for 17beta-estradiol, tamoxifen, 4-OH-tamoxifen, and diethylstibestrol were determined to be 5.6, 189, 26, and 3.5 nM, respectively. In a screen of 50 lead compounds from a transcriptional activation screen, 21 compounds had IC50 values below 10 microM, with one having an almost 100-fold higher affinity for ERbeta over ERalpha. These data show that an FP-based competitive binding assay can be used to screen diverse compounds with a broad range of binding affinities for ERs. The FP-based protein-tyrosine kinase (PTK) assay uses fluorescein-labeled phosphopeptides bound to anti-phosphotyrosine antibodies. Phosphopeptides generated by a kinase compete for this binding. In c-Src kinase reactions, polarization decreased with time as reaction products displaced the fluorescein-labeled phosphopeptide from the anti-phosphotyrosine antibodies. The experimentally determined IC50 of AG 1478 was 400 pM, while Genistein did not inhibit the epidermal growth factor receptor at similar concentrations. Like the FP-based PTK assay, the protein kinase C (PKC) assay utilizes competition. PKC isoforms had different turnover rates for the peptide substrate. The IC50 for staurosporine was less than 10 nM for all PKC isoforms. Tyrosine phosphatase assays use direct binding rather than competition. Increasing concentrations of T-cell protein-tyrosine phosphatase (TC PTP) increased the rate of dephosphorylation. This change in polarization was dependent on TC PTP and was inhibited by 50 microM Na3VO4. The IC50 of Na3VO4 was 4 nM for TC PTP. These data demonstrate that a FP-based assay can detect kinase and phosphatase activity. Homogeneous, fluorescent techniques such as FP are now methods of choice for screening many types of drug targets. New HTS instrumentation and assay methods like these make FP a technology easily incorporated into HTS.

A functional significance for codon third bases

Most amino acids are specified by more than one trinucleotide codon. Here we show that amino acids of differing functional importance may be distinguished by the pattern of synonymous codon usage. GC-rich genes tend to be of a greater transcriptional (p<0.01) and mitogenic (p<0.0001) significance than AT-rich genes, consistent with GC-->AT mutational drift in methylated genomic regions. Third-base GC retention also identifies critical amino acids within individual proteins, as indicated by non-random patterns of codon variation between gene homologs and also by differential sequelae of site-directed mutagenesis. Sequence analysis of human receptor tyrosine kinase genes confirms that functionally important transmembrane hydrophobic amino acids are specified by codons containing GC third bases more often than are transmembrane neutral amino acids (chi(2)=134.2). Amino acids encoded by GC third bases thus appear more tightly linked to cell function and survival than are those encoded by AT third bases.

Steroidal affinity labels of the estrogen receptor alpha. 4. Electrophilic 11beta-aryl derivatives of estradiol

Ten electrophilic estradiol 11beta-aryl derivatives were synthesized, with three different types of 11beta-substituent: (i) pOO(CH(2))(2)X (compounds: 6, X = OSO(2)CH(3); 7, X = I; 13, X = NHCOCH(2)Cl; 15, X = N(CH(3))COCH(2)Br; and 16, X = N(CH(3))COCH(2)Cl); (ii) pOO(CH(2))(5)X (compounds: 17, X = I; 20, X = NHCOCH(2)Br; and 22, X = N(CH(3))COCH(2)Br); and (iii) pOC(triple bond)CCH(2)X (compounds: 27, X = NHCOCH(2)Cl; and 29, X = N(CH(3))COCH(2)Cl). The range of their apparent affinity constants for binding the lamb uterine estrogen receptor alpha (ERalpha) was 3-40% that of estradiol. Six electrophiles, chloroacetamides 13, 16, 27, and 29, iodide 17, and bromoacetamide 20 (whose arm linking the electrophilic carbon to the 11beta-phenyl group includes at least six bonds), were able to irreversibly inhibit the binding of [(3)H]estradiol to ER (25-60% decrease in binding sites), with the following compound effectiveness order: 17 < 13 < 16 approximately 20 approximately 27 approximately 29. Mesylate 6, iodide 7 (whose linking arm includes only three bonds), and bromoacetamides 15 and 22 (which differ from 16 by the Cl to Br change and from 20 by the NH to NCH(3) change, respectively) were much less effective (<10% decrease in binding sites, if any). The fact that the inactivation of estradiol-binding sites by the six electrophiles was totally prevented by estradiol indicated that they were ER affinity labeling agents. When ER was modified by methyl methanethiosulfonate, an SH-specific reagent, the different compounds led to very contrasting results in ER affinity labeling. With modified ER, iodide 17 and chloroacetamides 27 and 29 were practically inactive, chloroacetamides 13 and 16 and bromoacetamide 20 were still active but less effective than on the native ER, whereas tertiary bromoacetamides 15 and 22, found to be practically inactive on native ER, became the most effective electrophiles ( approximately 45% and approximately 65% binding sites inactivated, respectively). The results indicate that in the steroid-filled hormone-binding pocket: (i) nucleophilic residues are localized on the beta-side but relatively remote from the steroid nucleus (distance from C-11 > "seven bonds"); (ii) relatively discrete changes in the electrophilic functionality, such as Cl to Br or NH to NCH(3) of haloacetamido compounds, can markedly modify the positioning of the electrophilic center which could no longer react with the nucleophilic residues; and (iii) cysteine residues (probably homologues of human ERalpha cysteine 381 and/or cysteine 530) are, at least partly, the covalent attachment sites of the electrophiles. Moreover, modification of cysteine residues by methyl methanethiosulfonate changes the structure of the hormone-binding pocket, whose labeling by the various electrophiles is profoundly altered.

Temperature dependence of estrogen binding: importance of a subzone in the ligand binding domain of a novel piscine estrogen receptor

The full length estrogen receptor from Oreochromis aureus (OaER) was cloned and expressed in vitro and in vivo as a functional transcription factor. Amino acid residues involved in the thermal stability of the receptor are located at/near subzones beta1 and beta3, which are highly conserved in other non-piscine species but not in OaER. Hormone binding studies, however, indicate that OaER is thermally stable but exhibited a approximately 3-fold reduced affinity for estrogen at elevated temperatures. Transfection of OaER into various cell lines cultured at different temperatures displayed a significant estrogen dose-response shift compared with that of chicken ER (cER). At 37 degrees C, OaER requires approximately 80-fold more estrogen to achieve half-maximal stimulation of CAT. Lowering of the incubation temperature from 37 degrees C to 25 degrees C or 20 degrees C resulted in a 4-fold increase in its affinity for estrogen. The thermally deficient transactivation of OaER at temperatures above 25 degrees C was fully prevented by high levels of estrogen. Thus, compared to cER, the OaER exhibits reduced affinity for estrogen at elevated temperature as reflected in its deficient transactivation capability. Amino acid replacements of OaER beta3 subzones with corresponding amino acids from cER could partially rescue this temperature sensitivity. The three-dimensional structure of the OaER ligand binding domain (LBD) was modelled based on conformational similarity and sequence homology with human RXRalpha apo, RARgamma holo and ERalpha LBDs. Unliganded and 17beta-estradiol-liganded OaER LBD retained the overall folding pattern of the nuclear receptor LBDs. The residues at/near the subzone beta3 of the LBD constitute the central core of OaER structure. Thus, amino acid alteration at this region potentially alters the structure and consequently its temperature-dependent ligand binding properties.

Cysteine 530 of the human estrogen receptor alpha is the main covalent attachment site of 11beta-(aziridinylalkoxyphenyl)estradiols

The efficiency of 11beta-[p(aziridinylethoxy)phenyl]estradiol 1 and 11beta-[p(aziridinylpentoxy)phenyl]estradiol 2 affinity labeling of the estrogen receptor alpha (ERalpha) was evaluated on the basis of their capacity to inhibit [(3)H]estradiol binding to lamb and human ERalphas. Relative to RU 39 411 (11beta-[p(dimethylaminoethoxy)phenyl]estradiol), the most closely related and chemically inert analogue of 1, the two electrophiles irreversibly inhibited [(3)H]estradiol binding to the lamb ERalpha. The fact that the compound effects were prevented (i) when the ERalpha hormone-binding site was occupied by estradiol and (ii) when the ERalpha-containing extracts were pretreated with methyl methanethiosulfonate (an SH-specific reagent) suggested that the compounds specifically alkylated ERalpha at cysteine residues. Wild-type human ERalpha was alkylated as efficiently as lamb ER, whereas the quadruple cysteine --> alanine mutant, in which all cysteines of the hormone-binding domain (residues 381, 417, 447, and 530) were changed to alanines, showed no significant electrophile labeling. The single C530A mutant was much less sensitive to the action of the electrophiles than the three other single mutants (C381A, C417A, and C447A). Moreover, analysis of the three double mutants (C381A/C530A, C417A/C530A, and C447A/C530A) showed that only the C381A/C530A mutant was less susceptible to electrophile labeling than the single C530A mutant. We concluded that in the hormone-binding pocket C530 was the main covalent attachment site of aziridines 1 and 2, whereas C381 could be a secondary site. These results agreed with the crystal structure of the hormone-binding domain of the human ERalpha bound to estrogen or antiestrogen, since C381 and C530 appeared to be (i) located in structural elements involved in delineating the hormone-binding pocket and (ii) in spatial proximity to each other, which was closer in the crystal structure of the ER:antiestrogen complex than in that of the ER:estrogen complex. Since C530 and C381 were also the main and secondary covalent attachment sites of tamoxifen aziridine (a nonsteroidal affinity-labeling agent), we propose a selective mode of superimposition of tamoxifen-class antiestrogens with RU 39 411-class antiestrogens, which could account for the relative positioning of the two types of ligands in the ERalpha hormone-binding pocket.

Estrogen receptor variants ERdelta5 and ERdelta7 down-regulate wild-type estrogen receptor activity

The estrogen receptor (ER) plays a key role in mediating the effect of estrogens. It is the primary target for endocrine therapy for many diseases, including breast cancer. The ER contains six domains that are associated with distinct functions; the presence of all six domains is required for ligand-dependent receptor activity. ER variants, reported in breast tumors and other neoplasms, usually lack one or more domains or a part of a domain. Such deletions can have dramatic effects on ER activity, cellular response to hormone, and response to hormonal therapy. We used simple and rapid yeast systems to understand more clearly how ER variants alter the response of wild-type ER (wtER) to estrogen and antiestrogens. We co-expressed ER variant, ERdelta5 or ERdelta7, with wtER in yeast containing an ERE-LacZ reporter. We found that ERdelta5 and ERdelta7 decreased the response of wtER to 1 nM 17beta-estradiol by 41-43 and 24-34%, respectively. Alone, ERdelta5 displayed weak hormone-independent transcriptional activity that was not affected by tamoxifen or ICI 182,780. ERdelta7, in contrast, showed no constitutive activity and no response to ligands. To further understand whether ERdelta5 and ERdelta7 affect wtER activity by forming a variant:wtER heterodimer, we used the yeast two-hybrid system. The protein-protein interaction results showed that ERdelta5 and ERdelta7 could form neither homodimers with themselves nor heterodimers with wtER. This finding suggests that the influence of ERdelta5 and ERdelta7 on wtER is not mediated by suppressing wtER through heterodimerization.

Steroid hormones modulate H19 gene expression in both mammary gland and uterus

H19 is an imprinted and developmentally regulated gene whose product remains apparently untranslated. In a previous study on breast adenocarcinomas, we reported that overexpression of the H19 gene was significantly correlated with the presence of steroid receptors, suggesting the putative role of hormones in H19 transcription. To determine the mode of steroid action, we have detected levels of H19 RNA synthesis during mammary gland development by in situ hybridization (ISH): two peaks of H19 transcription occur during puberty and pregnancy. Furthermore, we demonstrated by ISH that in the uterus H19 RNA synthesis is high during estrus and metestrus phases. To test steroid control of H19 transcription, ovariectomized and adrenalectomized mice were supplemented, 1 week after surgery, with 17-beta-estradiol (E2, 20 microg/kg/day), progesterone (P, 1 mg/kg/day) or corticosterone (B, 0.3 mg/ kg/day) for 2 weeks. According to ISH data, E2 and to a lesser extent B stimulated H19 transcription in the uterus, whereas P inhibited it. To confirm the in vivo results, in vitro experiments were performed using cultures of MCF-7 cells (a hormone-sensitive mammary cell line). E2 stimulated the endogenous H19 gene of this cell line and tamoxifen inhibited this effect. Furthermore, we performed transient cotransfections in MCF-7, in HBL-100 (another hormone-sensitive mammary cell line) and in BT-20 (a hormone-insensitive mammary cell line) with various constructs of ERalpha (WT or mutated) and PR-A, in presence or absence of steroid hormones. We demonstrated that ERalpha up-regulated the H19 promoter in MCF-7 and in HBL-100, whereas PR-A did not have any effect per se. Moreover, in MCF-7, PR-A antagonized clearly the ERalpha-mediated promoter enhancement, but in HBL-100 this counteracting effect on the ERalpha up-regulation was not found. Interestingly, the same experiments performed in BT-20 cell line provided very similar results as those obtained in MCF-7 cells, with a clear down-regulation mediated by PR-A on the H19 promoter. All these in vitro data are in agreement with in vivo results. In addition, data obtained with ERalpha mutants indicate that H19 promoter activation is both ligand-dependent and ligand-independent. We have thus demonstrated that H19 gene expression is controlled by steroid hormones; furthermore, this gene is highly expressed in hormone-sensitive organs when the hormonal stimulation is accompanied with a morphological repair.

Induction of thioredoxin, a redox-active protein, by ovarian steroid hormones during growth and differentiation of endometrial stromal cells in vitro

Human thioredoxin (hTrx) is a cellular redox-active protein that catalyzes dithiol/disulfide exchange reactions, thus controlling multiple biological functions, including cell growth-promoting activity. Here we show that the expression of hTrx protein and messenger RNA was up-regulated by incubation with 17beta-estradiol (E2) in primary culture of stromal cells isolated from human endometrium. Maximal enhancement of hTrx protein and messenger RNA was observed after 6-12 h of incubation with 10-100 nM E2, and the enhancing effect was suppressed by tamoxifen, an estrogen antagonist. Release of hTrx into the culture medium was markedly augmented after 5-day exposure of E2 plus progesterone (P) accompanied by in vitro differentiation of endometrial stromal cells (decidualization). Immunocytochemical studies showed that hTrx was localized in the nucleus, nucleolus, and cytosol in the stromal cells. Strongly enhanced immunoreactivity for hTrx was observed in the E2-treated cells, whereas there was no apparent difference in the pattern of subcellular localization among the untreated and E2- and/or P-treated cells. Although 1-50 microg/ml recombinant hTrx alone did not promote endometrial stromal cell growth, epidermal growth factor-dependent mitogenesis was additively enhanced by hTrx. Our results indicate that hTrx modulates endometrial cell growth, acting as a comitogenic factor for epidermal growth factor, which is known to be a mediator of estrogen action. It is also suggested that hTrx is deeply involved in the hormonal control of the endometrium by E2 and P, playing a regulatory role in endometrial cell growth and differentiation.

Transcriptional regulation of the human quinone reductase gene by antiestrogen-liganded estrogen receptor-alpha and estrogen receptor-beta

We have previously reported that antiestrogens stimulate quinone reductase (NAD(P)H:(quinone-acceptor) oxidoreductase (QR or NQO1); EC 1.6.99.2) enzymatic activity, an action that may provide protective effects against the toxicity and mutagenicity caused by quinones. We have now investigated the transcriptional regulation of the QR gene by antiestrogens. In transfection experiments employing the 5'-flanking (863-base pair) region of the human QR gene promoter with its electrophile/antioxidant response element (EpRE/ARE) or deleted or mutated constructs, we observe that antiestrogens induced an increase in QR gene promoter reporter activity in estrogen receptor (ER) negative breast cancer and endometrial cancer cells transfected with ER, and this induction by antiestrogens was repressed by estradiol. The stimulation of QR transcriptional activity required the 31-base pair electrophile-responsive region from the human QR gene promoter and a functional ER. Intriguingly, antiestrogens were stronger activators of the QR EpRE via the ER subtype ERbeta than ERalpha. Oligonucleotide gel mobility and antibody shift assays reveal that the ER binds to the EpRE but is only a minor component of the proteins bound to the EpRE in ER-containing MCF-7 breast cancer cells. While binding of ERbeta to the estrogen response element was weaker when compared with ERalpha, ERbeta and ERalpha showed similar binding to the EpRE. Together these findings provide evidence that QR gene regulation by the antiestrogen-occupied ER is mediated by the EpRE-containing region of the human QR gene and indicate that the ER is one of the complex of proteins that binds to the EpRE. In addition, that ERbeta is a more potent activator at EpRE elements than is ERalpha suggests that the different levels of these two receptors in various estrogen target cells could impact importantly on the antioxidant potency of antiestrogens in different target cells. These findings have broad implications regarding the potential beneficial effects of antiestrogens since EpREs mediate the transcriptional induction of numerous genes, including QR, which encode chemoprotective detoxification enzymes.

Three-dimensional models of estrogen receptor ligand binding domain complexes, based on related crystal structures and mutational and structure-activity relationship data

On the basis of the recently determined crystal structures of the ligand binding domains (LBDs) of the retinoic acid nuclear receptors (NRs), we present a three-dimensional (3D) molecular model of the human estrogen receptor alpha (hERalpha) LBD. A literature search for mutants affecting the binding properties has been performed; 45 out of 48 published mutants can be explained satisfactorily on the basis of the model. Estradiol has been docked into the binding pocket to probe its interactions with the protein. Energy minimizations and molecular dynamics calculations were performed for various ligand orientations. To evaluate their quality, the different models were scored using known structure-activity relationship (SAR) data for selected close estradiol homologues. The two best models explain largely the binding affinities of more distantly related ligands.

Determinants of ligand specificity of estrogen receptor-alpha: estrogen versus androgen discrimination

We have been interested in understanding how the estrogen receptor (ER) binds estrogens and discriminates between different classes of steroids with closely related structures. Using insights from our prior studies on ER and from sequence comparisons of steroid receptors, we identified three residues in the hormone-binding domain of the human ER, Leu345, Thr347, and Glu353, that we considered were likely to be involved in steroid A-ring recognition and therefore estrogen versus androgen discrimination. We then tested the effect on ER activity of mutating these ER residues to the corresponding androgen receptor residues. Specifically, we examined the ability of the mutant receptors to bind and be activated by 17beta-estradiol and three different androgens. No change in receptor activity was observed with the T347N mutation, while the L345S mutation greatly reduced ER activity in response to all ligands. Interestingly, the E353Q substitution behaved as expected, causing a 9-fold reduction in the transactivation potency of estradiol and a concomitant 10-140-fold increase in the transactivation potency of different androgens. These reciprocal changes in the transcriptional effectiveness of estrogens and androgens correlated with a decreased affinity of the E353Q ER for estradiol binding and an increased affinity for androgen binding. Therefore, amino acid Glu353 appears to be playing a significant role in binding the A-ring phenolic group of estradiol and in receptor discrimination between estrogens and the most closely structurally related steroids, androgens. Based on this data and our earlier observations, we propose a model for the orientation of ligand within the binding pocket of ER in which the A-ring 3-phenol of estradiol is hydrogen bonded to Glu353 in helix-3 and the 17beta-hydroxyl of estradiol is hydrogen bonded to His524 in helix-11. Our findings with estrogen and androgen suggest that this orientation of the steroid in the ligand-binding pocket, with the steroid A-ring in contact with helix-3 and the D-ring in contact with helix-11 residues, is likely to be general for all the steroid hormone receptors.

Cancer-preventive selenocompounds induce a specific redox modification of cysteine-rich regions in Ca(2+)-dependent isoenzymes of protein kinase C

Since protein kinase C (PKC) serves as a receptor for phorbol ester type tumor promoters and oxidants and has unique redox-active cysteine-rich regions, we have determined whether various chemopreventive selenocompounds could affect this enzyme. At lower concentrations, selenite decreased the kinase activity (IC50 = 0.5 microM), while at higher concentrations it decreased phorbol ester binding. However, when the catalytic and regulatory domains of PKC were separated by proteolysis, the catalytic domain retained its sensitivity to selenite, while the regulatory domain lost its sensitivity. Cysteine residues were quantitated in PKC modified with selenite by using 5,5'-dithiobis(2-nitrobenzoic acid) and also by using 2-nitro-5-thiosulfobenzoic acid after sulfitolysis. At lower concentrations, selenite induced a modification of four cysteine residues resulting in the formation of two disulfides, while at higher concentrations it induced a modification of seven to eight cysteine residues resulting in the formation of three to four disulfides. Contrary to selenite, selenocystine and selenodiglutathione (GSSeSG) readily inactivated the kinase activity, but not the phorbol ester binding. These two agents induced a two-stage modification of PKC; a limited modification at low concentrations leads to a loss of affinity for ATP, while an excessive modification at high concentrations leads to a loss of Vmax. Selenocystine and GSSeSG were 100,000-fold more potent than GSSG in inactivating PKC. The isoenzymes alpha, beta, and gamma exhibited an identical susceptibility to these selenocompounds. These results suggested that the cysteine residues present within the catalytic domain of these isoenzymes, although apart in the sequence, may be clustered in the tertiary structure to react with selenite, as well as may be in close proximity to some of the cysteines in the regulatory domain. Selenite did not affect protein kinase A, whereas GSSeSG and selenocystine inactivated the catalytic subunit after dissociation from the regulatory subunit at concentrations 100- and 800-fold, respectively, higher than that required for PKC inactivation. All three selenocompounds did not affect the activities of phosphorylase kinase and protein phosphatase 2A. Taken together, these results suggest that the accessible redox-active cysteine residues present in the PKC catalytic domain can react with certain specificity with redox-active selenocompounds such as selenite, selenocystine, and GSSeSG relative to other protein kinases tested.

Determinants for the repression of estrogen receptor transcriptional activity by ligand-occupied progestin receptors

There is considerable evidence for cross-talk between the estrogen and progestin signaling pathways, including examples of repression or attenuation of estrogen-stimulated endpoints by progestin receptor (PR) agonists and antagonists. We have previously described an experimental system for examining aspects of this cross-talk, namely the repression of estrogen receptor (ER) transcriptional activity by liganded PR (Kraus, W. L., Weis, K. E., Katzenellenbogen, B. S., Mol. Cell. Biol. 15 (1995) 1847-1857). Under promoter and cell type conditions where liganded PR was not a good activator of transcription, PR isoforms were shown to act as potent ligand-dependent repressors of ER transcriptional activity. In the current study, we have identified multiple determinants of this repression by systematically manipulating potentially important variables in this system (e.g. PR A:PR B ratio, sequence of the response elements, receptor structure, and ligand type). Alterations in several of these parameters had profound effects on the ability of PR to repress the activity of ER. Decreases in the PR A:PR B ratio and changes in the sequence of the progestin response element in the reporter gene construct abolished the repressive action of agonist-occupied PR A on ER transcriptional activity. In addition, point or deletion mutations in the amino-terminal A/B region of ER, including a triple point mutation which eliminates phosphorylation sites previously shown to be important in the activity of the receptor, made the ER more sensitive to the repressive actions of liganded PR. The PR ligands that promoted the most potent repression of ER activity were those with 11beta phenyl substitutions, suggesting that the phenyl moiety in the 11beta position is the important structural feature leading to strong repression. Interestingly, changes in the structure of the ER ligand and the sequence of the estrogen response element did not influence the magnitude of repression by PR. The fact that alterations in these check points along the estrogen signaling pathway had little or no effect on the magnitude of repression suggests that liganded PR interferes with the ability of ER to interact productively with the transcriptional machinery; in other words, PR-mediated repression occurs downstream of the events leading to the ligand-dependent conversion of ER to a transcriptionally active form. Our results indicate that a number of parameters which are naturally varied in vivo, such as the sequence of PR DNA binding sites and the PR A:PR B ratio, can dramatically alter the repression of ER activity by liganded PR, and may explain the differential affects of progestin-occupied PR on the expression of different estrogen regulated genes.

Characterization of bacterially expressed rat estrogen receptor beta ligand binding domain by mass spectrometry: structural comparison with estrogen receptor alpha

Functional rat estrogen receptor beta ligand binding domain (rER beta LBD, aa 210-485) and human estrogen receptor alpha ligand binding domain (hER alpha LBD, aa 301-553) were expressed in Escherichia coli. Hormone binding assays revealed that both ER beta and ER alpha LBDs bound the natural ligand estradiol (E2) with similar affinity (Kd approximately 100 pM). Competitive binding experiments were carried out with ICI 164384, 4-hydroxytamoxifen, 16 alpha-bromo-estradiol, and genistein employing [3H]E2 as a tracer. No significant differences in responses of ER alpha and ER beta LBDs to ICI 164384 and 4-hydroxytamoxifen were observed, 16 alpha-Bromo-estradiol and genistein discriminated between the ER subtypes and acted as ER alpha and ER beta selective ligands, respectively. Final purification of recombinant proteins was achieved on an E2 affinity column, where they were subjected to in situ carboxymethylation. The partially carboxymethylated proteins actively bound E2. The carboxymethylated rER beta LBD had a molecular mass of 32251.6 Da, equivalent to the calculated mass with the addition of three carboxymethyl groups. No other proteins (of lower or higher molecular mass) were detected, so the LBD was considered structurally authentic and pure. By using a combination of intact protein mass spectrometric fragmentation and trypsin proteolysis (98% sequence coverage), it was established that rER beta cysteine-289 and -354 were not carboxymethylated on the affinity column, suggesting that they were shielded from alkylation in the E2-bound conformational state. Concurrent analysis of hER alpha LBD showed that under the same experimental conditions, the two equivalent ER alpha cysteines were not alkylated (alpha C381 and alpha C447). These data support close structural relationship between the E2-bound ER alpha LBD and ER beta LBD proteins.

Steroidal affinity labels of the estrogen receptor. 3. Estradiol 11 beta-n-alkyl derivatives bearing a terminal electrophilic group: antiestrogenic and cytotoxic properties

With the aim of developing a new series of steroidal affinity labels of the estrogen receptor, six electrophilic 11 beta-ethyl (C2), 11 beta-butyl (C4), or 11 beta-decyl (C10) derivatives of estradiol bearing an 11 beta-terminal electrophilic functionality, i.e. bromine (C4), (methylsulfonyl)oxy (C2 and C4), bromoacetamido (C2 and C4), and (p-tolylsulfonyl)oxy (C10), were synthesized. The range of their affinity constants for binding the estrogen receptor was 0.4-37% that of estradiol; the order of increasing affinity (i) relative to the 11 beta-alkyl arm was ethyl < butyl and (ii) relative to the electrophilic functionality was bromoacetamido < bromine < (methylsulfonyl)oxy. Regardless of the conditions used, including prolonged exposure of the receptor to various pH levels (7-9) and temperatures (0-25 degrees C), the extent of receptor affinity labeling by the 11 beta-ethyl and 11 beta-butyl compounds, if any, was under 10%. This was in sharp contrast to results obtained using 11 beta-((tosyloxy)decyl)estradiol which labeled from 60% to 90% of the receptor hormone-binding sites with an EC50 of approximately 10 nM. Estrogenic and antiestrogenic activities of the compounds were determined using the MVLN cell line, which was established from the estrogen-responsive mammary tumor MCF-7 cells by stable transfection of a recombinant estrogen-responsive luciferase gene. The two 11 beta-ethyl compounds were mainly estrogenic, whereas the three 11 beta-butyl and the 11 beta-decyl compounds essentially showed antiestrogenic activity. The fact that the chemical reactivities of 11 beta-ethyl and 11 beta-butyl compounds were not compromised by interaction with the estrogen receptor made the synthesized high-affinity compounds potential cytotoxic agents which might be able to exert either (i) a specific action on estrogen-regulated genes or (ii) a more general action in estrogen-target cells. Therefore the ability of the compounds (1) to irreversibly abolish estrogen-dependent expression of the luciferase gene and (2) to affect the proliferation of MVLN cells were determined. All electrophiles were able to irreversibly suppress expression of the luciferase gene; the antiestrogenic electrophiles were more potent than the estrogenic ones but less efficient than 4-hydroxytamoxifen, a classical and chemically inert triphenylethylene antiestrogen. Only the antiestrogenic electrophiles decreased cell proliferation; however, they were less potent than 4-hydroxytamoxifen. In conclusion, the synthesized electrophilic estradiol 11 beta-ethyl and 11 beta-butyl derivatives (i) were not efficient affinity labels of the estrogen receptor and (ii) did not display significant cytotoxicity in estrogen-sensitive mammary tumor cells. However, since these derivatives displayed high affinity for the estrogen receptor, they could be used to prepare potential cytotoxic agents which might be selective for tumors affecting estrogen-target tissues, by coupling them with a toxic moiety.

17 alpha (haloacetamidoalkyl) estradiols alkylate the human estrogen receptor at cysteine residues 417 and 530

Results obtained in a previous study suggested that cysteine residues in the estrogen receptor were covalent attachment sites for four 17 alpha-(haloacetamidoalkyl) estradiols (halo, bromo or iodo; alkyl, methyl, ethyl, or propyl). To identify the putative concerned cysteines, we expressed wild-type and various cysteine --> alanine mutants of the human estrogen receptor in COS cells and determined their ability to be alkylated by the four electrophiles. The quadruple mutant, in which all the cysteines (residues 381, 417, 447, and 530) of the hormone-binding site were changed to alanines, showed very little electrophile labeling, whereas the four single mutants (C381A, C417A, C447A, and C530A) were alkylated as efficiently as the wild-type receptor. These results (i) demonstrate that cysteine residues were covalent attachment sites of electrophiles and (ii) indicate that more than one cysteine residue could be alkylated. Analysis of three double mutants (C381A/C530A, C417A/C530A, and C447A/C530A) provided strong evidence that only C417 and C530 were sites for electrophile covalent attachment. Since C530 was also alkylated by tamoxifen aziridine, a nonsteroidal affinity-labeling agent, we propose a selective mode of superimposition of tamoxifen-class antiestrogens with estradiol, which could account for the relative positioning of the two types of ligands in the receptor hormone-binding pocket. According to the structure of the hormone-binding pocket of nuclear receptors, as inferred from crystallographic studies and general sequence alignment of hormone-binding domains, C417 and C530 appear to be (1) located at the extreme border or in structural elements involved in delineation of the hormone-binding pocket, (2) spatially in close proximity to each other, and (3) in positions highly homologous to those of glucocorticoid receptor sites alkylated by affinity- and photoaffinity-labeling agents, respectively.

The quinone reductase gene: a unique estrogen receptor-regulated gene that is activated by antiestrogens

Antiestrogens are thought to exert most of their beneficial effects in breast cancer by antagonizing the actions of estrogen. We report here that antiestrogens also stimulate the expression of quinone reductase (QR) [NAD(P)H:quinone oxidoreductase, EC 1.6.99.2], which may provide protective effects against the toxicity and mutagenicity caused by quinones. QR is up-regulated by low concentrations of antiestrogens (trans-hydroxytamoxifen, tamoxifen, and ICI182,780) in estrogen receptor (ER)-containing breast cancer cells, and this increase is suppressed by estrogen via an ER-dependent mechanism. Since regulation of the QR gene, as well as other genes involved in detoxification such as the glutathione S-transferase Ya subunit (GST Ya) gene, is known to be mediated by an electrophile/antioxidant response element (EpRE/ARE), we examined the effects of antiestrogens on a 41-bp electrophile responsive region derived from the GST Ya gene. Transfection of this EpRE-containing region into ER-negative breast cancer cells in the presence or absence of an expression vector for the human ER, as well as mutagenesis studies, revealed that the EpRE-containing construct was activated by antiestrogen to the same extent as by tert-butylhydroquinone (TBHQ), a known activator of EpREs; however, only the stimulation by antiestrogen, and not TBHQ, required ER and was repressed by estradiol, although activation by both inducers mapped to the same 10-bp EpRE consensus sequence. Thus, there appear to be two pathways for QR induction, one that is activated by electrophile inducers such as TBHQ and is ER independent, and a second that is antiestrogen regulated and ER dependent; both pathways act through the EpRE. The anticancer action of antiestrogens may thus derive not only from the already well-known repression of estrogen-stimulated activities but also from the activation of detoxifying enzymes, such as QR, that may contribute to the beneficial antioxidant activity of antiestrogens.