Donald P. McDonnell

The Estrogen Receptor β-Isoform (ERβ) of the Human Estrogen Receptor Modulates ERα Transcriptional Activity and Is a Key Regulator of the Cellular Response to Estrogens and Antiestrogens1

The human estrogen receptor alpha (ERalpha) and the recently identified ERbeta share a high degree of amino acid homology; however, there are significant differences in regions of these receptors that would be expected to influence transcriptional activity. Consequently, we compared the mechanism(s) by which these receptors regulate target gene transcription, and evaluated the cellular consequences of coexpression of both ER subtypes. Previously, it has been determined that ERalpha contains two distinct activation domains, ERalpha-AF-1 and ERalpha-AF-2, whose transcriptional activity is influenced by cell and promoter context. We determined that ERbeta, like ERalpha, contains a functional AF-2, however, the ERbeta-AF-2 domain functions independently within the receptor. Of additional significance was the finding that ERbeta does not contain a strong AF-1 within its amino-terminus but, rather, contains a repressor domain that when removed, increases the overall transcriptional activity of the receptor. The importance of these findings was revealed when it was determined that ERbeta functions as a transdominant inhibitor of ERalpha transcriptional activity at subsaturating hormone levels and that ERbeta decreases overall cellular sensitivity to estradiol. Additionally, the partial agonist activity of tamoxifen manifest through ERalpha in some contexts was completely abolished upon coexpression of ERbeta. In probing the mechanisms underlying ERbeta-mediated repression of ERalpha transcriptional activity we have determined that 1) ERalpha and ERbeta can form heterodimers within target cells; and 2) ERbeta interacts with target gene promoters in a ligand-independent manner. Cumulatively, these data indicate that one role of ERbeta is to modulate ERalpha transcriptional activity, and thus the relative expression level of the two isoforms will be a key determinant of cellular responses to agonists and antagonists.

BISPHENOL A INTERACTS WITH THE ESTROGEN RECEPTOR ALPHA IN A DISTINCT MANNER FROM ESTRADIOL

We investigated the interaction of bisphenol A (BPA, an estrogenic environmental contaminant used in the manufacture of plastics) with the estrogen receptor alpha (ERalpha) transfected into the human HepG2 hepatoma cell line and expanded the study in vivo to examine the effect of BPA on the immature rat uterus. Bisphenol A was 26-fold less potent in activating ER-WT and was a partial agonist with the ERalpha compared to E2. The use of ERalpha mutants in which the AF1 or AF2 regions were inactivated has permitted the classification of ER ligands into mechanistically distinct groups. The pattern of activity of BPA with the ERalpha mutants differed from the activity observed with weak estrogens (estrone and estriol), partial ERalpha agonists (raloxifene or 4-OH-tamoxifen), or a pure antagonist (ICI 182, 780). Intact immature female Sprague-Dawley rats were exposed to BPA alone or with E2 for 3 days. Unlike E2, BPA had no effect on uterine weight; however, like E2, both peroxidase activity and PR levels were elevated, though not to the level induced by E2. Following simultaneous administration, BPA antagonized the E2 stimulatory effects on both peroxidase activity and PR levels but did not inhibit E2-induced increases of uterine weight. These results demonstrate that BPA is not merely a weak estrogen mimic but exhibits a distinct mechanism of action at the ERalpha.

The Human Estrogen Receptor-α Is a Ubiquitinated Protein Whose Stability Is Affected Differentially by Agonists, Antagonists, and Selective Estrogen Receptor Modulators

The human estrogen receptor α-isoform (ERα) is a nuclear transcription factor that displays a complex pharmacology. In addition to classical agonists and antagonists, the transcriptional activity of ERα can be regulated by selective estrogen receptor modulators, a new class of drugs whose relative agonist/antagonist activity is determined by cell context. It has been demonstrated that the binding of different ligands to ERα results in the formation of unique ERα-ligand conformations. These conformations have been shown to influence ERα-cofactor binding and, therefore, have a profound impact on ERα pharmacology. In this study, we demonstrate that the nature of the bound ligand also influences the stability of ERα, revealing an additional mechanism by which the pharmacological activity of a compound is determined. Of note we found that although all ERα-ligand complexes can be ubiquitinated and degraded by the 26 S proteasome in vivo, the mechanisms by which they are targeted for proteolysis appear to be different. Specifically, for agonist-activated ERα, an inverse relationship between transcriptional activity and receptor stability was observed. This relationship does not extend to selective estrogen receptor modulators and pure antagonists. Instead, it appears that with these compounds, the determinant of receptor stability is the ligand-induced conformation of ERα. We conclude that the different conformational states adopted by ERα in the presence of different ligands influence transcriptional activity directly by regulating cofactor binding and indirectly by modulating receptor stability.

Dissection of the LXXLL Nuclear Receptor-Coactivator Interaction Motif Using Combinatorial Peptide Libraries: Discovery of Peptide Antagonists of Estrogen Receptors α and β

ABSTRACT Recruitment of transcriptional coactivators following ligand activation is a critical step in nuclear receptor-mediated target gene expression. Upon binding an agonist, the receptor undergoes a conformational change which facilitates the formation of a specific coactivator binding pocket within the carboxyl terminus of the receptor. This permits the α-helical LXXLL motif within some coactivators to interact with the nuclear receptors. Until recently, the LXXLL motif was thought to function solely as a docking module; however, it now appears that sequences flanking the core motif may play a role in determining receptor selectivity. To address this issue, we used a combinatorial phage display approach to evaluate the role of flanking sequences in influencing these interactions. We sampled more than 108 variations of the core LXXLL motif with estradiol-activated estrogen receptor alpha (ERα) as a target and found three different classes of peptides. All of these peptides interacted with ERα in an agonist-dependent manner and disrupted ERα-mediated transcriptional activity when introduced into target cells. Using a series of ERα-mutants, we found that these three classes of peptides showed different interaction patterns from each other, suggesting that not all LXXLL motifs are the same and that receptor binding selectivity can be achieved by altering sequences flanking the LXXLL core motif. Most notable in this regard was the discovery of a peptide which, when overexpressed in cells, selectively disrupted ERβ- but not ERα-mediated reporter gene expression. This novel ERβ-specific antagonist may be useful in identifying and characterizing the ERβ-regulated process in estradiol-responsive cells. In conclusion, using a combinatorial approach to define cofactor-receptor interactions, we have clearly been able to demonstrate that not all LXXLL motifs are functionally equivalent, a finding which suggests that it may be possible to target receptor-LXXLL interactions to develop receptor-specific antagonists.

Estrogen Levels in Childhood Determined by an Ultrasensitive Recombinant Cell Bioassay

We hypothesized that estradiol levels are higher in prepubertal girls than in prepubertal boys and that this greater secretion of estradiol might drive the more rapid epiphyseal development and earlier puberty in girls. Since previous estradiol assays have lacked adequate sensitivity to test the hypothesis of higher estradiol levels in girls, we developed a new ultrasensitive assay to measure estrogen levels. The assay uses a strain of Saccharomyces cerevisiae genetically engineered for extreme sensitivity to estrogen. Yeast were transformed with plasmids encoding the human estrogen receptor and an estrogen-responsive promoter fused to the structural gene for beta-galactosidase. Ether extracts of 0.8 ml of serum were incubated with yeast for 8 h and the beta-galactosidase response was used to determine estrogen bioactivity relative to estradiol standards prepared in charcoal-stripped plasma. The assay was highly specific for estradiol with < 3% cross-reactivity with estrone, estriol, or estradiol metabolites. The detection limit was < 0.02 pg/ml estradiol equivalents (100-fold lower than existing assays). Using this assay, we measured estrogen levels in 23 prepubertal boys (9.4 +/- 2.0 yr) and 21 prepubertal girls (7.7 +/- 1.9 [SD] yr). The estrogen level in girls, 0.6 +/- 0.6 pg/ml estradiol equivalents, was significantly greater than the level in boys, 0.08 +/- 0.2 pg/ml estradiol equivalents (P < 0.05). We conclude that the ultrasensitive recombinant cell bioassay for estrogen is approximately 100-fold more sensitive than previous estradiol assays, that estrogen levels are much lower prepubertally, in both sexes, than reported previously, and that prepubertal girls have 8-fold higher estrogen levels than prepubertal boys.

The A and B isoforms of the human progesterone receptor operate through distinct signaling pathways within target cells.

The biological response to progesterone is mediated by two distinct forms of the human progesterone receptor (hPR-A and hPR-B). In most cell contexts, hPR-B functions as a transcriptional activator of progesterone-responsive genes, whereas hPR-A functions as a transcriptional inhibitor of all steroid hormone receptors. We have created mutations within the carboxyl terminus of hPR which differentially effect the transcriptional activity of hPR-B in a cell- and promoter-specific manner. Analogous mutations, when introduced into hPR-A, have no effect on its ability to inhibit the transcriptional activity of other steroid hormone receptors. The observed differences in the structural requirements for hPR-B and hPR-A function suggest that transcriptional activation and repression by PR are mediated by two separate pathways within the cell. In support of this hypothesis, we have shown that hPR-A mediated repression of human estrogen receptor (hER) transcriptional activity is not dependent on hER expression level but depends largely on the absolute expression level of hPR-A. Thus, it appears that hPR-A inhibits hER transcriptional activity as a consequence of a noncompetitive interaction of hPR-A with either distinct cellular targets or different contact sites on the same target. We propose that hPR-A expression facilitates a ligand-dependent cross-talk among sex steroid receptor signaling pathways within the cell. It is likely, therefore, that alterations in the expression level of hPR-A or its cellular target can have profound effects on the physiological or pharmacological responses to sex steroid hormone receptor ligands.

Common Estrogen Receptor Polymorphism Augments Effects of Hormone Replacement Therapy on E-Selectin but Not C-Reactive Protein

Background—The estrogen receptor-&agr; (ER-&agr;) IVS1-401 polymorphism identifies a group of women (≈20%) who have augmented effects of hormone replacement therapy (HRT) on levels of HDL cholesterol. This study sought to determine if this augmentation extends to HRT regulation of E-selectin and C-reactive protein (CRP) and to explore possible mechanisms by which this polymorphism might influence estrogen action. Methods and Results—Serum levels of soluble E-selectin and CRP were measured at baseline and 1 year in 264 postmenopausal women randomized to treatment with oral conjugated equine estrogen (0.625 mg/d), estrogen plus progestin (medroxyprogesterone acetate 2.5 mg/d), or placebo. Women with the ER-&agr; IVS1-401 C/C genotype receiving HRT had nearly a 2-fold greater reduction in E-selectin compared with C/T or T/T women (P for interaction=0.02). In contrast, there was no augmentation of the HRT-associated increase in CRP among the C/C women compared with C/T or T/T women (P for interaction=0.54). Of luciferase reporter constructs containing sequences spanning the IVS1-401 T/C polymorphism, expression of the construct containing the C allele was enhanced >10-fold, with cotransfection of a constitutively expressed B-myb vector. In contrast, B-myb resulted in only a 2.5-fold increase in expression of the T allele construct. Conclusions—Women with the ER-&agr; IVS1-401 C/C genotype have greater reductions in E-selectin but no further increases in CRP with HRT. The C allele produces a functional binding site for the transcription factor B-myb. The impact of this polymorphism on ER-&agr; transcription and other estrogen-sensitive intermediate and clinical end points has not yet been established.

27-Hydroxycholesterol Links Hypercholesterolemia and Breast Cancer Pathophysiology

Cholesterol and Cancer Obesity and high cholesterol levels are associated with an increased risk of breast cancer in post-menopausal women. Nelson et al. (p. 1094) found that a specific metabolite of cholesterol, 27-hydroxycholesterol (27HC), promoted tumor growth and metastasis in mouse models of mammary cancer by serving as a partial agonist for the estrogen receptor and the liver X receptor. The most aggressive human breast cancers were found to express the highest level of the enzyme that converts cholesterol to 27HC, suggesting that 27HC produced within tumors (in addition to circulating 27HC) may contribute to tumorigenesis. The activity of a specific metabolite of cholesterol may help explain why obesity is a risk factor for breast cancer. Hypercholesterolemia is a risk factor for estrogen receptor (ER)–positive breast cancers and is associated with a decreased response of tumors to endocrine therapies. Here, we show that 27-hydroxycholesterol (27HC), a primary metabolite of cholesterol and an ER and liver X receptor (LXR) ligand, increases ER-dependent growth and LXR-dependent metastasis in mouse models of breast cancer. The effects of cholesterol on tumor pathology required its conversion to 27HC by the cytochrome P450 oxidase CYP27A1 and were attenuated by treatment with CYP27A1 inhibitors. In human breast cancer specimens, CYP27A1 expression levels correlated with tumor grade. In high-grade tumors, both tumor cells and tumor-associated macrophages exhibited high expression levels of the enzyme. Thus, lowering circulating cholesterol levels or interfering with its conversion to 27HC may be a useful strategy to prevent and/or treat breast cancer.

The Molecular Pharmacology of SERMs

Estrogen-containing medicines have been used successfully for the past 50 years for the treatment of conditions associated with menopause. Although initially considered a reproductive hormone, millions of years of clinical exposure to estrogen(s) have indicated that its influence extends to a variety of target tissues not generally considered to be involved in reproduction. Specifically, estrogen has positive actions in the skeleton, the cardiovascular system and possibly the central nervous system, activities that combine to have a positive impact on mortality and morbidity. However, despite the medical benefits afforded by estrogen replacement therapy, the number of women who initiate or remain on therapy for longer than one year is relatively small. This is due in part to the fear that estrogens increase the risk for breast cancer. Consequently, it was realized several years ago that novel estrogen receptor modulators were needed, which would retain the beneficial effects of estrogens in most target organs but be inactive in the breast. Although the perfect tissue-selective estrogen remains to be identified, progress in this direction has been made. In the past year, for example, we have seen selective estrogen receptor modulators (SERMs) enter into the clinic for the prevention of osteoporosis. Compounds of this class, which function as estrogens in the skeletal system but oppose estrogen action in the breast, are the first step in developing the perfect hormone replacement therapy. This review summarizes the complex pharmacology of the SERMs and illustrates how they differ mechanistically from estradiol, the physiological ligand of the estrogen receptor.

Human and rat peroxisome proliferator activated receptors (PPARs) demonstrate similar tissue distribution but different responsiveness to PPAR activators

We have isolated a human peroxisomal proliferator activated receptor (hPPAR) from a human liver cDNA library. Based on sequence analysis, we have determined that this cDNA encodes the human PPAR alpha. When assayed in a reconstituted hPPAR responsive transcription system in mammalian CV-1 cells, this receptor was shown to be transcriptionally activated by hypolipidemic agents like clofibric acid, and ETYA (5,8,11,14-eicosatetraynoic acid; a synthetic arachidonic acid homolog). When analyzed in CV-1 cells, the rat PPAR alpha was similarly transcriptionally regulated. However, when assayed in a human liver cell line (HepG2) we noticed that ETYA was a more efficient activator of hPPAR alpha than rPPAR alpha. Thus, factors other than the receptor are important in determining the cellular responsiveness to this class of compounds. Interestingly, WY-14,643, another peroxisome proliferator, was a much more potent activator of rPPAR alpha than human PPAR alpha when assayed in both cell lines. This may explain in part why certain fibrates are potent hepatocarcinogens in rodents. Northern analysis indicates that hPPAR alpha and rPPAR alpha are well expressed in heart, kidney and liver. We further demonstrate that hPPAR alpha and human retinoid X receptor alpha synergistically interact to bind and transactivate through a peroxisomal proliferator response element. Thus in a similar cell and promoter context the rat and human PPARs show a differential response to certain activators. Cumulatively these data suggest that differential ligand responsiveness does not provide a complete explanation for the different biological effects exhibited by hypolipidemic drugs when administered to humans and rats.