Estrogen receptor binding to DNA is not required for its activity through the nonclassical AP1 pathway

Selective disruption of ER{alpha} DNA-binding activity alters uterine responsiveness to estradiol

In vitro models have been used to demonstrate that estrogen receptors (ERs) can regulate estrogen-responsive genes either by directly interacting with estrogen-responsive element (ERE) DNA motifs or by interacting with other transcription factors such as AP1. In this study, we evaluated estrogen (E(2))-dependent uterine gene profiles by microarray in the KIKO mouse, an in vivo knock-in mouse model that lacks the DNA-binding function of ERalpha and is consequently restricted to non-ERE-mediated responses. The 2- or 24-h E(2)-mediated uterine gene responses were distinct in wild-type (WT), KIKO, and alphaERKO genotypes, indicating that unique sets of genes are regulated by ERE and non-ERE pathways. After 2 h E(2) treatment, 38% of the WT transcripts were also regulated in the KIKO, demonstrating that the tethered mechanism does operate in this in vivo model. Surprisingly, 1438 E(2)-regulated transcripts were unique in the KIKO mouse and were not seen in either WT or alphaERKO. Pathway analyses revealed that some canonical pathways, such as the Jak/Stat pathway, were affected in a similar manner by E(2) in WT and KIKO. In other cases, however, the WT and KIKO differed. One example is the Wnt/beta-catenin pathway; this pathway was impacted, but different members of the pathway were regulated by E(2) or were regulated in a different manner, consistent with differences in biological responses. In summary, this study provides a comprehensive analysis of uterine genes regulated by E(2) via ERE and non-ERE pathways.

Regulation of Kiss1 and dynorphin gene expression in the murine brain by classical and nonclassical estrogen receptor pathways

Kisspeptin is a product of the Kiss1 gene and is expressed in the forebrain. Neurons that express Kiss1 play a crucial role in the regulation of pituitary luteinizing hormone secretion and reproduction. These neurons are the direct targets for the action of estradiol-17beta (E(2)), which acts via the estrogen receptor alpha isoform (ER alpha) to regulate Kiss1 expression. In the arcuate nucleus (Arc), where the dynorphin gene (Dyn) is expressed in Kiss1 neurons, E(2) inhibits the expression of Kiss1 mRNA. However, E(2) induces the expression of Kiss1 in the anteroventral periventricular nucleus (AVPV). The mechanism for differential regulation of Kiss1 in the Arc and AVPV by E(2) is unknown. ER alpha signals through multiple pathways, which can be categorized as either classical, involving the estrogen response element (ERE), or nonclassical, involving ERE-independent mechanisms. To elucidate the molecular basis for the action of E(2) on Kiss1 and Dyn expression, we studied the effects of E(2) on Kiss1 and Dyn mRNAs in the brains of mice bearing targeted alterations in the ER alpha signaling pathways. We found that stimulation of Kiss1 expression by E(2) in the AVPV and inhibition of Dyn in the Arc required an ERE-dependent pathway, whereas the inhibition of Kiss1 expression by E(2) in the Arc involved ERE-independent mechanisms. Thus, distinct ER alpha signaling pathways can differentially regulate the expression of identical genes across different brain regions, and E(2) can act within the same neuron through divergent ER alpha signaling pathways to regulate different neurotransmitter genes.

Membrane estradiol signaling in the brain

While the physiology of membrane-initiated estradiol signaling in the nervous system has remained elusive, a great deal of progress has been made toward understanding the activation of cell signaling. Membrane-initiated estradiol signaling activates G proteins and their downstream cascades, but the identity of membrane receptors and the proximal signaling mechanism(s) have been more difficult to elucidate. Mounting evidence suggests that classical intracellular estrogen receptor-alpha (ERalpha) and ERbeta are trafficked to the membrane to mediate estradiol cell signaling. Moreover, an interaction of membrane ERalpha and ERbeta with metabotropic glutamate receptors has been identified that explains the pleomorphic actions of membrane-initiated estradiol signaling. This review focuses on the mechanism of actions initiated by membrane estradiol receptors and discusses the role of scaffold proteins and signaling cascades involved in the regulation of nociception, sexual receptivity and the synthesis of neuroprogesterone, an important component in the central nervous system signaling.

The regulation of MS-KIF18A expression and cross talk with estrogen receptor

This study provides a novel view on the interactions between the MS-KIF18A, a kinesin protein, and estrogen receptor alpha (ERα) which were studied in vivo and in vitro. Additionally, the regulation of MS-KIF18A expression by estrogen was investigated at the gene and protein levels. An association between recombinant proteins; ERα and MS-KIF18A was demonstrated in vitro in a pull down assay. Such interactions were proven also for endogenous proteins in MBA-15 cells were detected prominently in the cytoplasm and are up-regulated by estrogen. Additionally, an association between these proteins and the transcription factor NF-κB was identified. MS-KIF18A mRNA expression was measured in vivo in relation to age and estrogen level in mice and rats models. A decrease in MS-KIF18A mRNA level was measured in old and in OVX-estrogen depleted rats as compared to young animals. The low MS-KIF18A mRNA expression in OVX rats was restored by estrogen treatment. We studied the regulation of MS-KIF18A transcription by estrogen using the luciferase reporter gene and chromatin immuno-percipitation (ChIP) assays. The luciferase reporter gene assay demonstrated an increase in MS-KIF18A promoter activity in response to 10⁻⁸ M estrogen and 10⁻⁷M ICI-182,780. Complimentary, the ChIP assay quantified the binding of ERα and pcJun to the MS-KIF18A promoter that was enhanced in cells treated by estrogen and ICI-182,780. In addition, cells treated by estrogen expressed higher levels of MS-KIF18A mRNA and protein and the protein turnover in MBA-15 cells was accelerated. Presented data demonstrated that ERα is a defined cargo of MS-KIF18A and added novel insight on the role of estrogen in regulation of MS-KIF18A expression both in vivo and in vitro.

Surface plasmon resonance study of cooperative interactions of estrogen receptor alpha and transcriptional factor Sp1 with composite DNA elements

We have applied surface plasmon resonance (SPR) spectroscopy to study the cooperative interactions of estrogen receptor alpha (ERalpha) and transcription factor Sp1 with a composite DNA element, containing an estrogen response element (ERE) half-site upstream of two adjacent Sp1 sites (+571 ERE/Sp1 composite site in promoter A of the human PR gene). Using nuclear extracts of MCF-7 breast cancer cells as sample, we have shown that Sp1 is associated with Sp1-binding sites only, whereas ERalpha can be recruited to DNA both through direct binding to the ERE half-site and/or through protein-protein interactions with DNA-bound Sp1. The ERE half-site and the proximal Sp1 site are only 4 bp apart, and our data suggests that one transcription factor bound to DNA constitutes a sterical hindrance of the accessibility of the binding site for the other transcription factor. Our data confirms previous observations that ERalpha increases the amount of Sp1 recruited to the composite binding site in a dose-dependent manner. Using recombinant proteins, we have unambiguously proved the formation of a ternary complex of ERalpha/Sp1-composite DNA, for which previously published electrophoretic mobility shift assay (EMSA) results are contradictive. With this study, we have demonstrated that the solid-liquid-phase SPR assay is a powerful alternative for studying multiprotein-DNA interactions and is superior to the EMSA experiments as it is capable of real-time measurements, can quantify the amount of protein bound, and can capture transient and weak binding interactions. The comprehensive characterization of the synergistic interactions between ERalpha-DNA, Sp1-DNA, and ERalpha-Sp1 contributes to the understanding of how ERalpha and Sp1 influence and activate gene transcription.

Breast cancer-derived M543V mutation in helix 12 of estrogen receptor alpha inverts response to estrogen and SERMs

We have isolated from human breast cancers several mutations in the Helix 12 component of activation function 2 (AF-2) in the estrogen receptor alpha (ERalpha). We used a novel approach to detect changes in the hormone-binding domain of ERalpha, based on the evidence that antiestrogens, such as 4-hydroxytamoxifen (ZOHT) and ICI 182,780, block the function of ERalpha by binding and folding the AF-2 transcriptional domain in a way that inhibits its association with coactivator proteins. We have identified a Helix 12 mutation, M543V, which leads to greater ERalpha transcription with ZOHT and other antiestrogens (including 1,1-dichloro-2,2,3-triarylcyclopropanes, DTACs) than with 17-beta estradiol (E2). We also found an independent mutation at the same position, M543I, which did not show this inverted ligand phenotype. In comparison to further Helix 12 mutations made in vitro, it appears that relative hydrophobicity of the amino acid side chains on the inner face of Helix 12 is key to maintaining the transcriptionally active, agonist conformation with bound E2. This active conformation can be induced, resulting in increased transcription, by adding excess p160 coactivator AIB1 in transcriptional assays with E2-bound receptors, while the ZOHT-bound receptors were not further activated by AIB1. Other experiments show that the cross talk between ERalpha and AP-1 protein from AP-1-binding sites is not dependent on Helix 12 integrity. We show that two alleles containing a proline substitution in Helix 12 that inactivate AF-2 function of ERalpha at EREs have little negative effect on function through AP-1 elements, supporting a prominent role for the N-terminal AF-1 of ERalpha in AP-1/ERalpha transcriptional cross talk.

Regulation of Msx2 Gene Expression by Steroid Hormones in Human Nonmalignant and Malignant Breast Cancer Explants Cultured in Vitro

Muscle segment homeobox genes, which regulate developmental programs and are expressed in embryonic and adult tissue, play a role in development of some malignancies. There are no reports on the expression of these families of genes in breast cancer tissue. The aim of this study was to compare expression of Msx2 gene in breast cancer of different genotypes as well as in surrounding nonmalignant tissues. Explants obtained during surgery were divided according to their sex steroid receptor status determined by immunocytochemistry. Four explants obtained from malignant and nonmalignant tissue of each individual patient were incubated in a control medium or with the addition of progesterone (10(-7) M) alone, estradiol 17 beta (10(-5) M) or both. The relative level of Msx2 transcripts was evaluated by a semiquantitative RT-PCR and cell proliferation by Alamar Blue test. Results of RT-PCR analysis showed that the relative expression of Msx2 gene depended on the presence of ER/PR receptors both in nonmalignant and malignant tissues Relative amount of Msx2 mRNA was the highest in surrounding nonmalignant ER+/PR- and ER-/PR+ tissue, whereas in ER-/PR- and ER+/PR+ tissue it was 1.4-1.6-fold lower. Tumorigenesis led to about a twofold decrease in the relative amount of Msx2 mRNA except for ER+/PR+ immunophenotype, where no changes were observed. Addition of estradiol or progesterone to the culture of ER-/PR- type tissue explants did not change significantly the relative amount of Msx2 gene mRNA. An opposite effect was observed in ER+/PR- type of tissue. Addition of estradiol alone, or estradiol and progesterone together to tissue culture explants decreased two to three fold the relative amount of Msx2 gene mRNA in both, malignant and surrounding tissues. Progesterone alone had no effect on Msx2 gene expression in this type of tissue. The most complicated regulation was observed in ER+/PR+ type of tissue. Culture of tissue explants supplemented with estradiol significantly increased the relative amount of Msx2 gene mRNA in the surrounding tissue. Progesterone enhanced the stimulatory effect of estradiol in surrounding tissues but not in the malignant tissue. Increased expression of Msx2 correlated with an increased proliferation in ER-/PR- and ER+/PR+ types, but not in ER+/PR- type of tissues. In conclusion, obtained results provide evidence that estrogen affects Msx2 gene expression. Significant changes in the relative amount of Msx2 gene mRNA and lack of canonical ERE element in 5'-upstream sequence of this gene suggest that regulation takes place indirectly probably by protein-protein interaction. The decrease in the relative amount of Msx2 gene mRNA in ER+/PR- type tumor suggests that progesterone also affects Msx2 gene expression by an indirect mechanism(s).

Hormone-activated estrogen receptors in annelid invertebrates: implications for evolution and endocrine disruption

As the primary mediators of estrogen signaling in vertebrates, estrogen receptors (ERs) play crucial roles in reproduction, development, and behavior. They are also the major mediators of endocrine disruption by xenobiotic pollutants that mimic or block estrogen action. ERs that are sensitive to estrogen and endocrine disrupters have long been thought to be restricted to vertebrates: although there is evidence for estrogen signaling in invertebrates, the only ERs studied to date, from mollusks and cephalochordates, have been insensitive to estrogen and therefore incapable of mediating estrogen signaling or disruption. To determine whether estrogen sensitivity is ancestral or a unique characteristic of vertebrate ERs, we isolated and characterized ERs from two annelids, Platynereis dumerilii and Capitella capitata, because annelids are the sister phylum to mollusks and have been shown to produce and respond to estrogens. Functional assays show that annelid ERs specifically activate transcription in response to low estrogen concentrations and bind estrogen with high affinity. Furthermore, numerous known endocrine-disrupting chemicals activate or antagonize the annelid ER. This is the first report of a hormone-activated invertebrate ER. Our results indicate that estrogen signaling via the ER is as ancient as the ancestral bilaterian animal and corroborate the estrogen sensitivity of the ancestral steroid receptor. They suggest that the taxonomic scope of endocrine disruption by xenoestrogens may be very broad and reveal how functional diversity evolved in a gene family central to animal endocrinology.

Genomic responses from the estrogen-responsive element-dependent signaling pathway mediated by estrogen receptor alpha are required to elicit cellular alterations

Estrogen (E2) signaling is conveyed by the transcription factors estrogen receptor (ER) alpha and beta. ERs modulate the expression of genes involved in cellular proliferation, motility, and death. The regulation of transcription by E2-ERalpha through binding to estrogen-responsive elements (EREs) in DNA constitutes the ERE-dependent signaling pathway. E2-ERalpha also modulates gene expression by interacting with transregulators bound to cognate DNA-regulatory elements, and this regulation is referred to as the ERE-independent signaling pathway. The relative importance of the ERE-independent pathway in E2-ERalpha signaling is unclear. To address this issue, we engineered an ERE-binding defective ERalpha mutant (ERalpha(EBD)) by changing residues in an alpha-helix of the protein involved in DNA binding to render the receptor functional only through the ERE-independent signaling pathway. Using recombinant adenovirus-infected ER-negative MDA-MB-231 cells derived from a breast adenocarcinoma, we found that E2-ERalpha(EBD) modulated the expression of a subset of ERalpha-responsive genes identified by microarrays and verified by quantitative PCR. However, E2-ERalpha(EBD) did not affect cell cycle progression, cellular growth, death, or motility in contrast to E2-ERalpha.ERalpha(EBD) in the presence of E2 was also ineffective in inducing phenotypic alterations in ER-negative U-2OS cells derived from an osteosarcoma. E2-ERalpha, on the other hand, effectively repressed growth in this cell line. Our findings suggest that genomic responses from the ERE-dependent signaling pathway are required for E2-ERalpha to induce alterations in cellular responses.

The silent estrogen receptor--can we make it speak?

One of the most common cancers in women world wide, breast cancer is classically an endocrine-dependent cancer. It has been known for over a century that development, progression and metastasis of breast cancer are strongly influenced by hormonal factors. Indeed about two-thirds of breast cancers express the estrogen receptor α (ERα) protein, a key predictor of prognosis and response to endocrine therapy. These cancers are frequently amenable to therapies that target estrogen signaling pathways, including selective estrogen receptor modulators like tamoxifen, selective estrogen receptor downregulators like fulvestrant; and agents that reduce estrogen ligand like aromatase inhibitors and ovarian suppression through luteinizing hormone-releasing hormone (LHRH) agonists. It is likely that these approaches, especially adjuvant tamoxifen, have contributed to the reduction in breast cancer mortality that has been observed in recent years. However, data from clinical studies have suggested that only about 60% of ERα-positive breast cancers respond to hormonal therapy. Further, those tumors that lack expression of ERα and the estrogen-regulated progesterone receptor (PgR) are unresponsive to hormone therapy. Thus the problem of acquired or de novo endocrine resistance is a substantial one. Recent molecular and biological advances have contributed to our understanding about potential underlying mechanisms. Here we will focus especially on silencing the expression of ERα as one such endocrine-resistance mechanism and how it might be exploited clinically.

A novel role of Shc adaptor proteins in steroid hormone-regulated cancers

Tyrosine phosphorylation plays a critical role in growth regulation, and its aberrant regulation can be involved in carcinogenesis. The association of Shc (Src homolog and collagen homolog) adaptor protein family members in tyrosine phosphorylation signaling pathway is well recognized. Shc adaptor proteins transmit activated tyrosine phosphorylation signaling that suggest their plausible role in growth regulation including carcinogenesis and metastasis. In parallel, by sharing a similar mechanism of carcinogenesis, the steroids are involved in the early stage of carcinogenesis as well as the regulation of cancer progression and metastatic processes. Recent evidence indicates a cross-talk between tyrosine phosphorylation signaling and steroid hormone action in epithelial cells, including prostate and breast cancer cells. Therefore, the members of Shc proteins may function as mediators between tyrosine phosphorylation and steroid signaling in steroid-regulated cell proliferation and carcinogenesis. In this communication, we discuss the novel roles of Shc proteins, specifically p52(Shc) and p66(Shc), in steroid hormone-regulated cancers and a novel molecular mechanism by which redox signaling induced by p66(Shc) mediates steroid action via a non-genomic pathway. The p66(Shc) protein may serve as an effective biomarker for predicting cancer prognosis as well as a useful target for treatment.

Kisspeptin/metastin: a key molecule controlling two modes of gonadotrophin-releasing hormone/luteinising hormone release in female rats

Kisspeptin (also known as metastin), a hypothalamic peptide, has attracted attention as a key molecule in the release of gonadotrophin-releasing hormone (GnRH) in various mammalian species, such as rodents, sheep and primates. Two populations of kisspeptin neurones in the brain may control two modes of GnRH release to time the onset of puberty and regulate oestrous cyclicity in rats and mice. One population of kisspeptin neurones, located in the anteroventral periventricular nucleus, appears to be responsible for the induction of the GnRH surge that leads to the luteinising hormone surge and ovulation. The other, located in the hypothalamic arcuate nucleus, appears to be involved in generating GnRH pulses, resulting in luteinising hormone pulses followed by follicular development and steroidogenesis in the ovary. The present review focuses on the physiological role of the two populations of kisspeptin neurones in controlling gonadal functions by generating the two modes of GnRH release in a female rat model.

p150/glued modifies nuclear estrogen receptor function

Estrogen modulates gene expression through interactions with estrogen receptors (ERs) that bind chromosomal target genes. Recent studies have suggested an interaction between the cytoskeletal system and estrogen signaling; these have implicated a role of cytoplasmic microtubules in scaffolding ERalpha and enhancing nongenomic function; in addition, other experiments demonstrate that dynein light chain 1 may chaperone ERalpha to the nucleus, indirectly increasing transcriptional potency. Actin/myosin and dynein light chain 1 are also required for estrogen-mediated chromosomal movement that is required for transcriptional up-regulation of ERalpha targets. We present evidence that the dynactin component, p150/glued, directly influences the potency of nuclear ER function. Increasing the stoichiometric ratio of p150/glued and ERalpha by overexpression enhances estrogen responses. ERalpha enhancement by p150/glued does not appear to be influenced by shifts in subcellular localization because microtubule disruption fails to increase nuclear ERalpha. Rather, we find that modest amounts of p150/glued reside in the nucleus of cells, suggesting that it plays a direct role in nuclear transcription. Notably, p150/glued is recruited to the pS2 promoter in the presence of hormone, and, in MCF-7 cells, knockdown of p150/glued levels reduces estrogen-dependent transcription. Our results suggest that p150/glued modulates estrogen sensitivity in cells through nuclear mechanisms.

Estrogen actions in the male reproductive system involve estrogen response element-independent pathways

The estrogen receptor-alpha (ERalpha) acts through multiple pathways, including estrogen response element (ERE)-dependent (classical) and ERE-independent (nonclassical) mechanisms. We previously created a mouse model harboring a two-amino-acid mutation of the DNA-binding domain (E207A, G208A) that precludes direct binding of ERalpha to an ERE. After crossing heterozygous mutant mice with an ERalpha knockout (ERKO) line, it was possible to assess the degree of physiological rescue by the isolated ERalpha nonclassical allele (-/AA; AA) when compared with ERKO mice (-/-) and to wild type (+/+; WT). In male ERKO mice up to 8 months of age, testosterone levels were high, although LH levels were similar to WT. Testosterone was normal in the AA mice, indicating that the AA allele rescues the enhanced testosterone biosynthesis in ERKO mice. Male ERKO mice exhibited distention of the seminiferous tubules as early as 2-3 months of age as a consequence of decreased water resorption in the efferent ducts. By 3-4 months of age, ERKO mice had impaired spermatogenesis in approximately 40% of their tubules, and sperm counts and motility declined in association with the histological changes. In the AA mice, histological defects were greatly reduced or absent, and sperm counts and motility were rescued. Levels of aquaporins 1 and 9, which contribute to water uptake in the efferent ducts, were reduced in ERKO mice and partially or fully rescued in AA mice, whereas another water transporter, sodium-hydrogen exchanger-3, was decreased in both ERKO and AA mice. We conclude that non-ERE-dependent estrogen pathways are sufficient to rescue the defective spermatogenesis observed in ERKO mice and play a prominent role in ERalpha action in the testis, including pathways that regulate water resorption and androgen biosynthesis.

Modulation of transcription parameters in glucocorticoid receptor-mediated repression

Glucocorticoid receptors (GRs) affect both gene induction and gene repression. The disparities of receptor binding to DNA and increased vs. decreased gene expression have suggested significant mechanistic differences between GR-mediated induction and repression. Numerous transcription factors are known to modulate three parameters of gene induction: the total activity (Vmax) and position of the dose-response curve with glucocorticoids (EC50) and the percent partial agonist activity with antiglucocorticoids. We have examined the effects on GR-mediated repression of five modulators (coactivators TIF2 [GRIP1, SRC-2] and SRC-1, corepressor SMRT, and comodulators STAMP and Ubc9), a glucocorticoid steroid (deacylcortivazol [DAC]) of very different structure, and an inhibitor of histone deacetylation (trichostatin A [TSA]). These factors interact with different domains of GR and thus are sensitive topological probes of GR action. These agents altered the Vmax, EC50, and percent partial agonist activity of endogenous and exogenous repressed genes similarly to that previously observed for GR-regulated gene induction. Collectively, these results suggest that GR-mediated induction and repression share many of the same molecular interactions and that the causes for different levels of gene transcription arise from more distal downstream steps.

Classical estrogen receptor alpha signaling mediates negative and positive feedback on gonadotropin-releasing hormone neuron firing

During the female reproductive cycle, the neuroendocrine action of estradiol switches from negative feedback to positive feedback to initiate the preovulatory GnRH and subsequent LH surges. Estrogen receptor-alpha (ERalpha) is required for both estradiol negative and positive feedback regulation of LH. ERalpha may signal through estrogen response elements (EREs) in DNA and/or via ERE-independent pathways. Previously, a knock-in mutant allele (ERalpha-/AA) that selectively restores ERE-independent signaling onto the ERalpha-/- background was shown to confer partial negative but not positive estradiol feedback on serum LH. The current study investigated the roles of the ERE-dependent and ERE-independent ERalpha pathways for estradiol feedback at the level of GnRH neuron firing activity. The above ERalpha genetic models were crossed with GnRH-green fluorescent protein mice to enable identification of GnRH neurons in brain slices. Targeted extracellular recordings were used to monitor GnRH neuron firing activity using an ovariectomized, estradiol-treated mouse model that exhibits diurnal switches between negative and positive feedback. In wild-type mice, GnRH neuron firing decreased in response to estradiol during negative feedback and increased during positive feedback. In contrast, both positive and negative responses to estradiol were absent in GnRH neurons from ERalpha-/- and ERalpha-/AA mice. ERE-dependent signaling is thus required to increase GnRH neuron firing to generate a GnRH/LH surge. Furthermore, ERE-dependent and -independent ERalpha signaling pathways both appear necessary to mediate estradiol negative feedback on serum LH levels, suggesting central and pituitary estradiol feedback may use different combinations of ERalpha signaling pathways.

Non-classical genomic estrogen receptor (ER)/specificity protein and ER/activating protein-1 signaling pathways

17beta-estradiol binds to the estrogen receptor (ER) to activate gene expression or repression and this involves both genomic (nuclear) and non-genomic (extranuclear) pathways. Genomic pathways include the classical interactions of ligand-bound ER dimers with estrogen-responsive elements in target gene promoters. ER-dependent activation of gene expression also involves DNA-bound ER that subsequently interacts with other DNA-bound transcriptions factors and direct ER-transcription factor (protein-protein) interactions where ER does not bind promoter DNA. Ligand-induced activation of ER/specificity protein (Sp) and ER/activating protein-1 [(AP-1); consisting of jun/fos] complexes are important pathways for modulating expression of a large number of genes. This review summarizes some of the characteristics of ER/Sp- and ER/AP-1-mediated transactivation, which are dependent on ligand structure, cell context, ER-subtype (ERalpha and ERbeta), and Sp protein (SP1, SP3, and SP4) and demonstrates that this non-classical genomic pathway is also functional in vivo.

Estrogen Receptors: Role in Breast Cancer

The estrogen receptor (ER) exists in two forms known as ERalpha and ERbeta. Currently, a clinical role has only been established for ERalpha. The primary use of ERalpha in breast cancer is for predicting likely response to hormone treatment. Patients with breast cancers expressing ERalpha are approximately seven to eight times more likely to benefit from endocrine therapy than ERalpha-negative patients. For the initial three to five years after primary diagnosis, ERalpha-positive patients generally have a better outcome than ERalpha-negative patients. Overall, however, the prognostic value of ERalpha is relatively weak and only of limited value in the clinically important subgroup of patients with lymph node-negative disease. Further work is required to establish if ERbeta has a clinical role in breast cancer.

Hormonal regulation of cardiac KCNE2 gene expression

The KCNE2 gene encodes a single transmembrane domain protein that modulates a variety of K+ channel functions in various tissues. Here we show that cardiac KCNE2 transcript levels are approximately 10-fold upregulated at the end of pregnancy. This upregulation was mimicked by 17-beta estradiol but not by 5alpha-dihydrotestosterone treatments in ovariectomized mice. To investigate the mechanism of KCNE2 transcriptional regulation by estrogen, we experimentally identified KCNE2 transcription start sites, delineated its gene structure and characterized its promoter region. Estrogen treatment stimulated KCNE2 promoter activity in a dose-dependent manner and ICI 182,780 blocked estrogen stimulation. A direct genomic mechanism was demonstrated by (i) the loss of estrogen responsiveness in the presence of a DNA-binding domain mutant estrogen receptor alpha or mutant KCNE2 ERE and (ii) binding of ERalpha to the KCNE2 ERE. These findings show that a genomic mechanism of estrogen action alters KCNE2 expression, which may have important physiological implications.

What goes on behind closed doors: physiological versus pharmacological steroid hormone actions

Steroid-hormone-activated receptor proteins are among the best-understood class of factors for altering gene transcription in cells. Steroid receptors are of major importance in maintaining normal human physiology by responding to circulating concentrations of steroid in the nM range. Nonetheless, most studies of steroid receptor action have been conducted using the supra-physiological conditions of saturating concentrations (> or =100 nM) of potent synthetic steroid agonists. Here we summarize the recent developments arising from experiments using two clinically relevant conditions: subsaturating concentrations of agonist (to mimic the circulating concentrations in mammals) and saturating concentrations of antagonists (which are employed in endocrine therapies to block the actions of endogenous steroids). These studies have revealed new facets of steroid hormone action that could not be uncovered by conventional experiments with saturating concentrations of agonist steroids, such as a plethora of factors/conditions for the differential control of gene expression by physiological levels of steroid, a rational approach for examining the gene-specific variations in partial agonist activity of antisteroids, and a dissociation of steroid potency and efficacy that implies the existence of separate, and possibly novel, mechanistic steps and cofactors.

New insights into the classical and non-classical actions of estrogen: evidence from estrogen receptor knock-out and knock-in mice

Estrogen receptor alpha (ERalpha) mediates estrogen (E2) actions in the brain and is critical for normal reproductive function and behavior. In the classical pathway, ERalpha binds to estrogen response elements (EREs) to regulate gene transcription. ERalpha can also participate in several non-classical pathways, including ERE-independent gene transcription via protein-protein interactions with transcription factors and rapid, non-genotropic pathways. To distinguish between ERE-dependent and ERE-independent mechanisms of E2 action in vivo, we have created ERalpha null mice that possess an ER knock-in mutation (E207A/G208A; "AA"), in which the mutant ERalpha cannot bind to DNA but retains activity in ERE-independent pathways (ERalpha(-/AA) mice). Understanding the molecular mechanisms of ERalpha action will be helpful in developing pharmacological therapies that differentiate between ERE-dependent and ERE-independent processes. This review focuses on how the ERalpha(-/AA) model has contributed to our knowledge of ERalpha signaling mechanisms in estrogen regulation of the reproductive axis and sexual behavior.

Estrogen receptor alpha signaling pathways differentially regulate gonadotropin subunit gene expression and serum follicle-stimulating hormone in the female mouse

Estrogen, acting via estrogen receptor (ER)alpha, regulates serum gonadotropin levels and pituitary gonadotropin subunit expression. However, the cellular pathways mediating this regulation are unknown. ERalpha signals through classical estrogen response element (ERE)-dependent genomic as well as nonclassical ERE-independent genomic and nongenomic pathways. Using targeted mutagenesis in mice to disrupt ERalpha DNA binding activity, we previously demonstrated that ERE-independent signaling is sufficient to suppress serum LH levels. In this study, we examined the relative roles of ERE-dependent and -independent estrogen signaling in estrogen regulation of LH, FSH, prolactin, and activin/inhibin subunit gene expression, pituitary LH and FSH protein content, and serum FSH levels. ERE-independent signaling was not sufficient for estrogen to induce pituitary prolactin mRNA or suppress pituitary LHbeta mRNA, LH content, or serum FSH in estrogen-treated ovariectomized mice. However, ERE-independent signaling was sufficient to reduce pituitary glycoprotein hormone alpha-subunit, FSHbeta, and activin-betaB mRNA expression. Together with previous serum LH results, these findings suggest ERE-independent ERalpha signaling suppresses serum LH via reduced secretion, not synthesis. Additionally, ERE-dependent and ERE-independent ERalpha pathways may distinctly regulate steps involved in the synthesis and secretion of FSH.

Human myeloblastic leukemia cells (HL-60) express a membrane receptor for estrogen that signals and modulates retinoic acid-induced cell differentiation

Estrogen receptors are historically perceived as nuclear ligand activated transcription factors. An estrogen receptor has now been found localized to the plasma membrane of human myeloblastic leukemia cells (HL-60). Its expression occurs throughout the cell cycle, progressively increasing as cells mature from G(1) to S to G(2)/M. To ascertain that the receptor functioned, the effect of ligands, including a non-internalizable estradiol-BSA conjugate and tamoxifen, an antagonist of nuclear estrogen receptor function, were tested. The ligands caused activation of the ERK MAPK pathway. They also modulated the effect of retinoic acid, an inducer of MAPK dependent terminal differentiation along the myeloid lineage in these cells. In particular the ligands inhibited retinoic acid-induced inducible oxidative metabolism, a functional marker of terminal myeloid cell differentiation. To a lesser degree they also diminished retinoic acid-induced earlier markers of cell differentiation, namely CD38 and CD11b. However, they did not regulate retinoic acid-induced G(0) cell cycle arrest. There is thus a membrane localized estrogen receptor in HL-60 myeloblastic leukemia cells that can cause ERK activation and modulates the response of these cells to retinoic acid, indicating crosstalk between the membrane estrogen and retinoic acid evoked pathways relevant to propulsion of cell differentiation.

The aryl hydrocarbon receptor complex and the control of gene expression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. The toxicity of the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin is almost exclusively mediated through this receptor. However, the key alterations in gene expression that mediate toxicity are poorly understood. It has been established through characterization of AhR-null mice that the AhR has a required physiological function, yet how endogenous mediators regulate this orphan receptor remains to be established. A picture as to how the AhR/ARNT heterodimer actually mediates gene transcription is starting to emerge. The AhR/ARNT complex can alter transcription both by binding to its cognate response element and through tethering to other transcription factors. In addition, many of the coregulatory proteins necessary for AhR-mediated transcription have been identified. Cross talk between the estrogen receptor and the AhR at the promoter of target genes appears to be an important mode of regulation. Inflammatory signaling pathways and the AhR also appear to be another important site of cross talk at the level of transcription. A major focus of this review is to highlight experimental efforts to characterize nonclassical mechanisms of AhR-mediated modulation of gene transcription.

Mutations of glucocorticoid receptor differentially affect AF2 domain activity in a steroid-selective manner to alter the potency and efficacy of gene induction and repression

The transcriptional activity of steroid hormones is intimately associated with their structure. Deacylcortivazol (DAC) contains several features that were predicted to make it an inactive glucocorticoid. Nevertheless, gene induction and repression by complexes of glucocorticoid receptor (GR) with DAC occur with potency (lower EC 50) greater than and efficacy (maximal activity, or A max) equal to those of the very active and smaller synthetic glucocorticoid dexamethasone (Dex). Guided by a recent X-ray structure of DAC bound to the GR ligand binding domain (LBD), we now report that several point mutants in the LBD have little effect on the binding of either agonist steroid. However, these same mutations dramatically alter the A max and/or EC 50 of exogenous and endogenous genes in a manner that depends on steroid structure. In some cases, Dex is no longer a full agonist. These properties appear to result from a preferential inactivation of the AF2 activation domain in the GR LBD of Dex-bound, but not DAC-bound, receptors. The Dex-bound receptors display normal binding to, but a greatly reduced response to, the coactivator TIF2, thus indicating a defect in the transmission efficiency of GR-steroid complex information to the coactivator TIF2. In addition, all GR mutants that are active in gene induction with either Dex or DAC have greatly reduced activity in gene repression. This contrasts with the reports of GR mutations preferentially suppressing GR-mediated induction. The properties of these GR mutants in gene induction support the hypothesis that the A max and EC 50 of GR-controlled gene expression can be independently modified, indicate that the receptor can be modified to favor activity with a specific agonist steroid, and suggest that new ligands with suitable substituents may be able to affect the same LBD conformational changes and thereby broaden the therapeutic applications of glucocorticoid steroids.

Agonist-like SERM effects on ERalpha-mediated repression of MMP1 promoter activity predict in vivo effects on bone and uterus

Estradiol receptors (ER), ERalpha and ERbeta, are ligand-dependent transcription factors that regulate gene expression. Human and murine genetics suggest that ERalpha is the key target for estradiol action on bone, uterus and breast. To date, the molecular mode of action of estradiol and selective estradiol receptor modulators (SERMs) on bone is not fully understood. This is exemplified by a lack of in vitro assays that reliably predict SERM agonist activities in vivo. We hypothesized that ligand-dependent ERalpha transrepression, via protein-protein interactions at AP1, may predict estrogenic effects on bone. We modeled this using the MMP1 promoter, which encodes an AP1 binding site. We show that ICI-182780, raloxifene, 4-hydroxytamoxifen and estradiol all exhibit differential agonistic activities on the MMP1 promoter by suppressing activity by 20-80%. Transrepression efficacy and potency correlated with both uterotrophic (R(2)=0.98) and osteoprotective (R(2)=0.80) potential in the ovariectomized rat. This identifies MMP1 promoter transrepression as an agonist activity commonly shared by AF2 agonists and "antagonists" alike. Mutation analysis showed that the repression by estradiol and SERMs required correct amino acid sequences in the AF-2 domain. For instance, L540Q AF2 mutation did not alter responses to raloxifene, although it greatly increased responses to ICI-182780 (threefold) and reduced estradiol's effect by 20%. Furthermore, all tested ligands repressed the MMP1 promoter through the L540Q mutant with identical efficacy. Together, these data suggest that estradiol and SERMs share common agonist transcriptional activity via protein-protein interactions at AP1.

The influence of sex hormones on coagulation and inflammation in the trauma patient

Recent clinical studies have shown a sex dimorphism of morbidity and mortality due to shock, trauma, and sepsis, with females tolerating these insults better than males. Experimental animal studies have suggested that sex hormones have a pivotal role in this dimorphism. In the present investigation, a prospective cohort study at a university level-1 trauma center was conducted to evaluate the association between sex hormones and alterations in coagulation and inflammation. Patients with an admission to the intensive care unit, injury severity score (ISS) greater than 4, and obtainable consent were included in the study. In addition to routine clinical laboratories and patient outcomes, plasma TNF-[alpha], IL-6, IL-8, estradiol, progesterone, and testosterone were measured. Sixty-two patients (71% men, 29% women) met criteria for entry. Mean age was 42 +/- 17 years, and mean ISS was 23 +/- 13, with no statistical difference in age or ISS between sexes. Estradiol levels were positively correlated with ISS (P < 0.05) and negatively correlated with TNF-[alpha] (P < 0.01). Initial estradiol levels were higher in patients who developed an infection (P < 0.05). Testosterone was negatively correlated with age (P < 0.01) and was higher in patients who developed acute respiratory distress syndrome (P < 0.05) and in patients who did not survive (P < 0.05). The estradiol-to-progesterone ratio (E2-Pr) was higher in the survivors (P < 0.05). The E2-Pr had positive correlations with fibrinogen levels, rate of fibrin deposition and cross-linking, and overall clot strength (P < 0.05). Estradiol-to-progesterone ratio was negatively correlated with partial thromboplastin times (P < 0.01). In men, the E2-Pr was also negatively correlated with the time to onset of clot formation (P = 0.03). Sex hormone levels (or their ratios) were not correlated to platelet count or international normalized ratios. These findings provide evidence that sex hormone levels in the early posttraumatic period are significantly associated with alterations in the hemostatic and inflammatory response to trauma.

Gene expression profiling reveals that the regulation of estrogen-responsive element-independent genes by 17 beta-estradiol-estrogen receptor beta is uncoupled from the induction of phenotypic changes in cell models

Estrogen hormone 17beta-estradiol (E(2)) is involved in the physiology and pathology of many tissues. E(2) information is conveyed by the transcription factors estrogen receptors (ER) alpha and beta that mediate a complex array of nuclear and non-nuclear events. The interaction of ER with specific DNA sequences, estrogen-responsive elements (EREs), constitutes a critical nuclear signaling pathway. In addition, E(2)-ER regulates transcription through interactions with transfactors bound to their cognate regulatory elements on DNA, hence the ERE-independent signaling pathway. However, the relative importance of the ERE-independent pathway in E(2)-ERbeta signaling is unclear. To address this issue, we engineered an ERE-binding defective ERbeta mutant (ERbeta(EBD)) by changing critical residues in the DNA-binding domain required for ERE binding. Biochemical and functional studies revealed that ERbeta(EBD) signaled exclusively through the ERE-independent pathway. Using the adenovirus infected ER-negative cancer cell models, we found that although E(2)-ERbeta(EBD) regulated the expression of a number of genes identified by microarrays, it was ineffective in altering cellular proliferation, motility, and death in contrast to E(2)-ERbeta. Our results indicate that genomic responses from the ERE-independent pathway to E(2)-ERbeta are not sufficient to alter the cellular phenotype. These findings suggest that the ERE-dependent pathway is a required signaling route for E(2)-ERbeta to induce cellular responses.

Estrogen stimulates transcription from the human prolactin distal promoter through AP1 and estrogen responsive elements in T47D human breast cancer cells

Human prolactin (hPRL) is a pleiotropic and versatile hormone that exercises more than 300 biological activities through binding to its cognate receptors. Recently, multiple studies have implicated hPRL in the development of human breast cancer. As a target of hPRL, both normal and neoplastic human breast cells also synthesize and secrete hPRL, which therefore establishes an autocrine/paracrine action loop in the mammary gland. In contrast to the extensive studies of regulation of hPRL expression in the pituitary gland, regulation of hPRL in mammary tissue and human breast cancer cells has not been extensively addressed. Extrapituitary PRL expression is primarily regulated by a distal promoter located 5.8 kb upstream to the pituitary promoter. As a result of alternative promoter usage, extrapituitary PRL is regulated by different signalling pathways and different hormones, cytokines or neuropeptides compared to regulation in the pituitary. Here, we present evidence that shows estrogen directly induces hPRL gene expression in T47D human breast cancer cells. We have identified a functional, non-canonical estrogen responsive element (ERE) and an AP1 site located in the hPRL distal promoter. Gel shift and chromatin immunoprecipitation assays demonstrated that both estrogen receptor (ER)alpha and ERbeta directly bind to the ERE. However, only ERalpha interacts with AP1 proteins that bind to the AP1 site in the hPRL distal promoter. Promoter-reporter gene studies demonstrate that both ERE and AP1 sites are required for full induction of the promoter activity by estradiol. Our studies suggest that the interactions between estrogens, ERs, the ERE and AP1 transcription factors in regulation of autocrine/paracrine PRL in the human breast may be critical for oncogenesis and may contribute to progression of breast cancer.

Loss of ERE binding activity by estrogen receptor-α alters basal and estrogen-stimulated bone-related gene expression by osteoblastic cells

Estrogen receptor (ER)-alpha can signal either via estrogen response element (ERE)-mediated pathways or via alternate pathways involving protein-protein or membrane signaling. We previously demonstrated that, as compared to wild type (WT) controls, mice expressing a mutant ER-alpha lacking the ability to bind EREs (non-classical estrogen receptor knock-in (NERKI)) display significant impairments in the skeletal response to estrogen. To elucidate the mechanism(s) underlying these in vivo deficits, we generated U2OS cells stably expressing either WT ER-alpha or the NERKI receptor. Compared to cells transfected with the control vector, stable expression of ER-alpha, even in the absence of E2, resulted in an increase in mRNA levels for alkaline phosphatase (AP, by 400%, P < 0.01) and a decrease in mRNA levels for insulin growth factor-I (IGF-I) (by 65%, P < 0.001), with no effects on collagen I (col I) or osteocalcin (OCN) mRNA levels. By contrast, stable expression of the NERKI receptor resulted in the suppression of mRNA levels for AP, col I, OCN, and IGF-I (by 62, 89, 60, and 70%, P < 0.001). While E2 increased mRNA levels of AP, OCN, col I, and IGF-I in ER-alpha cells, E2 effects in the NERKI cells on AP and OCN mRNA levels were attenuated, with a trend for E2 to inhibit col I mRNA levels. In addition, E2 had no effects on IGF-I mRNA levels in NERKI cells. Collectively, these findings indicate that ERE signaling plays a significant role in mediating effects of estrogen on osteoblastic differentiation markers and on IGF-I mRNA levels.

Estrogen response element-independent estrogen receptor (ER)-alpha signaling does not rescue sexual behavior but restores normal testosterone secretion in male ERalpha knockout mice

Estrogen receptor (ER)-alpha mediates estradiol (E(2)) actions in the male gonads and brain and is critical for normal male reproductive function. In the classical pathway, ERalpha binds to estrogen response elements (EREs) to regulate gene transcription. ERalpha can also regulate gene transcription independently of EREs via protein-protein interactions with transcription factors and additionally signal via rapid, nongenomic pathways originating at the cell membrane. This study assessed the degree to which ERE-independent ERalpha signaling can rescue the disrupted masculine sexual behaviors and elevated serum testosterone (T) levels that have been shown to result from ERalpha gene deletion. We utilized male ERalpha null mice that possess a ER knock-in mutation (E207A/G208A; AA), in which the mutant ERalpha is incapable of binding to DNA and can signal only through ERE-independent pathways (ERalpha(-/AA) mice). We found that sexual behavior, including mounting, is virtually absent in ERalpha(-/-) and ERalpha(-/AA) males, suggesting that ERE-independent signaling is insufficient to maintain any degree of normal sexual behavior in the absence of ERE binding. By contrast, ERE-independent signaling in the ERalpha(-/AA) mouse is sufficient to restore serum T levels to values observed in wild-type males. These data indicate that binding of ERs to EREs mediates most if not all of E(2)'s effects on male sexual behavior, whereas ERE-independent ERalpha signaling may mediate E(2)'s inhibitory effects on T production.

Estrogen receptor DNA binding is not required for estrogen-induced breast cell growth

In this study, we determined whether ER DNA binding is necessary for estrogen to stimulate the growth of breast cancer cells. To investigate the requirement of ER DNA binding we expressed either wild-type or a DNA-binding mutant ERalpha in a clone of the MCF-7 breast cancer cell line that no longer expressed endogenous ERalpha. Estrogen did not activate non-genomic kinase cascades in the parental MCF-7 cells or in cells expressing ERalpha mutant. In cells expressing the ERalpha mutant, estrogen did not induce ERE-dependent gene expression but did induce AP-1- and Sp1-dependent gene expression and the cell cycle regulatory genes cyclin D1 and c-myc. However, we demonstrated that estrogen still induced cell proliferation in MCF-7 cells expressing the ERalpha mutant. These results demonstrate that ER DNA binding is not absolutely required for estrogen to induce breast cancer cell growth.

A functional serine 118 phosphorylation site in estrogen receptor-alpha is required for down-regulation of gene expression by 17beta-estradiol and 4-hydroxytamoxifen

To evaluate the contribution of ERK1/2 phosphorylation of estrogen receptor (ER)-alpha to activation and repression of endogenous genes, we produced stably transfected lines of HeLa cells with functional ERK1/2 pathways that express similar levels of wild-type human ERalpha and ERalpha mutated to inactivate the well-known MAPK site at serine 118 (ERalphaS118A). We compared effects of the S118A mutation on 17beta-estradiol (E(2))-mediated transactivation, which is heavily dependent on activation function (AF) 2 of ERalpha and on 4-hydroxytamoxifen (OHT)-mediated transactivation, which is heavily dependent on AF1, which includes S118. To examine whether S118 was the key ERK/MAPK phosphorylation site in ERalpha action, we compared the effects of the S118A mutant and the ERK inhibitor U0126 on expression of endogenous genes. In several estrogen response element-containing genes, the S118A mutation strongly reduced induction by E(2), and U0126 did not further reduce expression. Expression of another group of estrogen response element-containing genes was largely unaffected by the S118A mutation. The S118A mutation had variable effects on genes induced by ER tethering or binding near specificity protein-1 and activator protein-1 sites. For five mRNAs whose expression is strongly down-regulated by E(2) and partially or completely down-regulated by OHT, the S118A mutation reduced or abolished down-regulation by E(2) and nearly abolished down-regulation by OHT. In contrast, for Sma and mothers against decapentaplegic-3-related, which is down-regulated by E(2) and not OHT, the S118A mutation had little effect. These data suggest that there may be distinct groups of genes down-regulated by ERalpha and suggest a novel role for ERK phosphorylation at serine 118 in AF1 in regulating expression of the set of genes down-regulated by OHT.

Tamoxifen Induction of CCAAT Enhancer-binding Protein α Is Required for Tamoxifen-induced Apoptosis

Low concentrations of tamoxifen or its active metabolite 4-hydroxytamoxifen (OHT) induce estrogen receptor alpha (ERalpha)-dependent apoptosis. To analyze the pathway of OHT-ERalpha-induced apoptosis, we developed stably transfected lines of HeLa cells expressing wild-type ER and an inactive mutant ERalpha unable to bind estrogen response elements. HeLa cells expressing the mutant ERalpha and HeLa cells expressing wild-type ERalpha in which the ER was knocked down with an ER-specific small interfering RNA were not killed by Tam or OHT, suggesting that estrogen response element-mediated transcription is required for Tam- and OHT-induced apoptosis. Microarray analysis to identify a gene(s) whose expression is important in OHT-ER-mediated apoptosis identified 19 mRNAs that OHT up-regulated by >1.6-fold and 15 down-regulated mRNAs. Gene function and the time course of induction by OHT-ERalpha led us to further investigate CCAAT enhancer-binding protein alpha (C/EBPalpha), which has roles in cell cycle progression and apoptosis, and p21. Quantitative reverse transcription-PCR, Western blot analysis, and RNA interference knockdown suggest that cell cycle arrest resulting from OHT-ERalpha induction of p21 may facilitate apoptosis. OHT-ERalpha, but not E2-ERalpha, induced C/EBPalpha mRNA and protein. RNA interference knockdown of C/EBPalpha nearly abolished OHT-ERalpha-induced apoptosis. We isolated stable cell lines that were resistant to OHT-induced apoptosis, contain full-length functional ERalpha, and undergo apoptosis in response to etoposide. In these OHT-resistant cell lines both before and after OHT treatment, C/EBPalpha levels are much lower than in OHT-sensitive cells. These studies establish a novel molecular site responsible for Tam- and OHT-ERalpha-induced apoptosis of cancer cells.

Evidence that estrogen receptor beta enhances MMP-13 promoter activity in HIG-82 cells and that this enhancement can be influenced by ligands and involves specific promoter sites

Degradation of articular cartilage is characteristic of osteoarthritis, and matrix metalloproteinase-13 (MMP-13) has been implicated in this condition. Estrogen receptors (ERs) are present in connective tissues, indicating these tissues' potential responsiveness to estrogen. We based this study on the hypothesis that estrogen receptor beta (ERbeta) can modulate MMP-13 promoter activity. Transfection of cells with ERbeta constructs led to the induction of the endogenous MMP-13 gene, as evidenced by increased mRNA levels. The results also indicated that MMP-13 promoter construct activity in the HIG-82 cell line significantly increased when ERbeta was present, and that estrogen downregulated this response in a dose-dependent manner. ERbeta was shown to enhance MMP-13 expression somewhat more strongly than ERalpha, and the impact of a number of selective ER modulators (tamoxifen, raloxifene, and ICI 182,780) on ERbeta enhancement of promoter activity was found to be significantly less than that of estrogen. Furthermore, transcription regulatory sites in the MMP-13 promoter, specifically AP-1 and PEA-3, were shown to act in conjunction to mediate ERbeta effects. Thus, ERbeta likely influences MMP-13 promoter expression in normal and disease processes.

Estrogen receptors inhibit Smad3 transcriptional activity through Ap-1 transcription factors

Breast tumorigenesis and breast cancer progression involves the deregulation or hyperactivation of intracellular signaling proteins that leads to uncontrolled cellular proliferation, invasion and metastasis. For example, the expression and cellular responses to estogen receptor (ER) and transforming growth factor beta (TGFbeta) signaling pathways change during breast tumorigenesis and breast cancer progression. While the expression and activity of ER and TGFbeta maybe significant in the development of breast cancer, alterations in the cross-talk between these pathways may be equally important. Autocrine and paracrine effects of TGFbeta on breast cancer cell growth have been known for some time, but only recently have direct interactions between ER and TGFbeta been described. The purpose of this article was to further characterize the cross-talk between ER and TGFbeta, by examining ER interaction with Smad3, a downstream mediator of TGFbeta signaling. Transient transfection of Cos1 cells with p3TP-lux, demonstrate that ERalpha and ERbeta(1) repress Smad3 transcriptional activity in an estradiol-dependent manner and that this effect is inhibited by antiestrogen treatment. The ERbeta variants, ERbeta(2) and ERbeta(5), did not have any effect on Smad3 transcriptional activity. Further experiments attempted to characterize the molecular mechanism by which activated ER inhibits Smad3 transcriptional activity. Results indicate that ligand-bound ER does not affect Smad3 protein expression levels and that ER does not form direct protein interactions with Smad3. Transient transfection of Cos1 cells with the Ap-1 transcription factor c-Jun but not c-Fos was able to rescue the inhibitory effect of estrogen on Smad3 transcriptional activity. Based on these results, a model is proposed whereby c-Jun is limiting in its ability to act as a Smad3 co-activator in the presence of E(2)-bound ER, possibly due to ER sequestering c-Jun away from the Smad3 responsive promoter.

Regulation of mouse Slo gene expression: multiple promoters, transcription start sites, and genomic action of estrogen

The large conductance, voltage- and Ca(2+)-activated K(+) channel plays key roles in diverse body functions influenced by estrogen, including smooth muscle and neural activities. In mouse (m), estrogen up-regulates the transcript levels of its pore-forming alpha-subunit (Slo, KCNMA1), yet the underlying genomic mechanism(s) is (are) unknown. We first mapped the promoters and regulatory motifs within the mSlo 5'-flanking sequence to subsequently identify genomic regions and mechanisms required for estrogen regulation. mSlo gene has at least two TATA-less promoters with distinct potencies that may direct mSlo transcription from multiple transcription start sites. These qualities mark mSlo as a prototype gene with promoter plasticity capable of generating multiple mRNAs and the potential to adapt to organismal needs. mSlo promoters contain multiple estrogen-responsive sequences, e.g. two quasi-perfect estrogen-responsive elements, ERE1 and ERE2, and Sp1 sites. Accordingly, mSlo promoter activity was highly enhanced by estrogen and blocked by estrogen antagonist ICI 182,780. When promoters are embedded in a 4.91-kb backbone, estrogen responsiveness involves a classical genomic mechanism, via ERE1 and ERE2, that may be complemented by Sp factors, particularly Sp1. Simultaneous but not individual ERE1 and ERE2 mutations caused significant loss of estrogen action. ERE2, which is closer to the proximal promoter, up-regulates this promoter via a classical genomic mechanism. ERE2 strategic position together with ERE1 and ERE2 independence and Sp contribution should ensure mSlo estrogen responsiveness. Thus, the mSlo gene seems to have uniquely evolved to warrant estrogen regulation. Estrogen-mediated mSlo genomic regulation has important implications on long term estrogenic effects affecting smooth muscle and neural functions.

Multiple transcription factor elements collaborate with estrogen receptor alpha to activate an inducible estrogen response element in the NKG2E gene

Estrogen receptors (ERs) regulate transcription by interacting with regulatory elements in target genes. However, known ER regulatory elements cannot explain the expression profiles of genes activated by estradiol (E2) and selective estrogen receptor modulators (SERMs). We previously showed that the killer cell lectin-like receptor (NKG2E) gene is regulated by E2, tamoxifen, and raloxifene. Here we used the NKG2E gene as a model to investigate the mechanism whereby target genes are regulated by E2 and SERMs with ERalpha. The ER regulatory element in the NKG2E promoter was mapped to the -1825 and -1686 region. Full activation of the NKG2E promoter required the collaboration between a transcription factor cluster containing c-jun, heat-shock factor 2, and CCAAT/enhancer-binding protein beta and a unique variant estrogen response element (ERE) that has only a two nucleotide spacer between half sites. The cluster elements and the variant ERE were inactive on their own, but the regulation by E2 and SERMs was restored when the c-jun, heat-shock factor-2, and CCAAT/enhancer-binding protein beta cluster was placed upstream of the variant ERE. The activation of the NKG2E gene by E2 and selective ER modulators was associated with the recruitment of the p160 coactivators glucocorticoid receptor-interacting protein 1 and amplified in breast cancer 1 but not steroid receptor coactivator 1. These studies identified one of the most complex ER regulatory units thus far reported and demonstrate that a cluster of flanking transcription factors collaborate with ER to induce a functional ERE in the NKG2E promoter.

Nonclassical estrogen receptor alpha signaling mediates negative feedback in the female mouse reproductive axis

Ovarian estrogen exerts both positive and negative feedback control over luteinizing hormone (LH) secretion during the ovulatory cycle. Estrogen receptor (ER) alpha but not ERbeta knockout mice lack estrogen feedback. Thus, estrogen feedback appears to be primarily mediated by ERalpha. However, it is now recognized that, in addition to binding to estrogen response elements (EREs) in DNA to alter target gene transcription, ERalpha signals through ERE-independent or nonclassical pathways, and the relative contributions of these pathways in conveying estrogen feedback remain unknown. Previously we created a knockin mouse model expressing a mutant form of ERalpha (AA) with ablated ERE-dependent but intact ERE-independent activity. Breeding this allele onto the ERalpha-null (-/-) background, we examine the ability of ERE-independent ERalpha signaling pathways to convey estrogen feedback regulation of the female hypothalamic-pituitary axis in vivo. ERalpha-/AA exhibited 69.9% lower serum LH levels compared with ERalpha-/- mice. Additionally, like wild type, ERalpha-/AA mice exhibited elevated LH after ovariectomy (OVX). Furthermore, the post-OVX rise in serum LH was significantly suppressed by estrogen treatment in OVX ERalpha-/AA mice. However, unlike wild type, both ERalpha-/AA and ERalpha-/- mice failed to exhibit estrous cyclicity, spontaneous ovulation, or an afternoon LH surge response to estrogen. These results indicate that ERE-independent ERalpha signaling is sufficient to convey a major portion of estrogen's negative feedback actions, whereas positive feedback and spontaneous ovulatory cyclicity require ERE-dependent ERalpha signaling.

Enhanced NF kappa B and AP-1 transcriptional activity associated with antiestrogen resistant breast cancer

Background

Signaling pathways that converge on two different transcription factor complexes, NFκB and AP-1, have been identified in estrogen receptor (ER)-positive breast cancers resistant to the antiestrogen, tamoxifen.

Methods

Two cell line models of tamoxifen-resistant ER-positive breast cancer, MCF7/HER2 and BT474, showing increased AP-1 and NFκB DNA-binding and transcriptional activities, were studied to compare tamoxifen effects on NFκB and AP-1 regulated reporter genes relative to tamoxifen-sensitive MCF7 cells. The model cell lines were treated with the IKK inhibitor parthenolide (PA) or the proteasome inhibitor bortezomib (PS341), alone and in combination with tamoxifen. Expression microarray data available from 54 UCSF node-negative ER-positive breast cancer cases with known clinical outcome were used to search for potential genes signifying upregulated NFκB and AP-1 transcriptional activity in association with tamoxifen resistance. The association of these genes with patient outcome was further evaluated using node-negative ER-positive breast cancer cases identified from three other published data sets (Rotterdam, n = 209; Amsterdam, n = 68; Basel, n = 108), each having different patient age and adjuvant tamoxifen treatment characteristics.

Results

Doses of parthenolide and bortezomib capable of sensitizing the two endocrine resistant breast cancer models to tamoxifen were capable of suppressing NFκB and AP-1 regulated gene expression in combination with tamoxifen and also increased ER recruitment of the transcriptional co-repressor, NCoR. Transcript profiles from the UCSF breast cancer cases revealed three NFκB and AP-1 upregulated genes – cyclin D1, uPA and VEGF – capable of dichotomizing node-negative ER-positive cases into early and late relapsing subsets despite adjuvant tamoxfien therapy and most prognostic for younger age cases. Across the four independent sets of node-negative ER-positive breast cancer cases (UCSF, Rotterdam, Amsterdam, Basel), high expression of all three NFκB and AP-1 upregulated genes was associated with earliest metastatic relapse.

Conclusion

Altogether, these findings implicate increased NFκB and AP-1 transcriptional responses with tamoxifen resistant breast cancer and early metastatic relapse, especially in younger patients. These findings also suggest that agents capable of preventing NFκB and AP-1 gene activation may prove useful in restoring the endocrine responsiveness of such high-risk ER-positive breast cancers.

Estrogen Receptor α and the Activating Protein-1 Complex Cooperate during Insulin-like Growth Factor-I-induced Transcriptional Activation of the pS2/TFF1 Gene

Insulin like growth factor I (IGF-I) displays estrogenic activity in breast cancer cells. This activity is strictly dependent on the presence of estrogen receptor alpha (ERalpha). However the precise molecular mechanisms involved in this process are still unclear. IGF-I treatment induces phosphorylation of the AF1 domain of ERalpha and activation of estrogen regulated genes. These genes are characterized by important differences in promoter architecture and response element composition. We show that promoter structure is crucial for IGF-I-induced transcription activation. We demonstrate that on a complex promoter such as the pS2/TFF1 promoter, which contains binding sites for ERalpha and for the activating protein-1 (AP1) complex, transcriptional activation by IGF-I requires both ERalpha and the AP1 complex. IGF-I is unable to stimulate transcription of an estrogen-regulated gene under the control of a minimal promoter containing only a binding site for ERalpha. We propose a molecular mechanism with stepwise assembly of the AP1 complex and ERalpha during transcription activation of pS2/TFF1 by IGF-I. IGF-I stimulation induces rapid phosphorylation and an increase in protein levels of the AP1 complex. Binding of the phosphorylated AP1 complex to the pS2/TFF1 promoter allows recruitment of the chromatin remodeling factor Brg1 followed by binding of ERalpha via its interaction with c-Jun.

Effects of loss of classical estrogen response element signaling on bone in male mice

The role of estrogen signaling in the male skeleton via estrogen receptor (ER)-alpha is now well established. ERalpha can elicit responses through either classical estrogen response elements (ERE) pathways or nonclassical, non-ERE pathways. In the present study, we examined the effects of either the attenuation or loss of classical ERalpha signaling on the murine male skeleton. To accomplish this, we crossed male mice heterozygous for a knock-in mutation [nonclassical ERalpha knock-in (NERKI)], which abolishes the ERE-mediated pathway with female heterozygous ERalpha knockout mice (ERalpha+/-) and studied the F1 generation ERalpha+/+, ERalpha+/-, ERalpha+/NERKI, and ERalpha-/NERKI male progeny longitudinally using bone density and histomorphometry. The only ERalpha allele present in ERalpha-/NERKI mice is incapable of classical ERE-mediated signaling, whereas the heterozygous ERalpha+/NERKI mice have both one intact ERalpha and one NERKI allele. As compared with ERalpha+/+ littermates (n=10/genotype), male ERalpha+/NERKI and ERalpha-/NERKI mice displayed axial and appendicular skeletal osteopenia at 6, 12, 20, and 25 wk of age, as demonstrated by significant reductions in total bone mineral density (BMD) at representative sites (areal BMD by dual-energy x-ray absorptiometry at the lumbar vertebrae and femur and volumetric BMD by peripheral quantitative computed tomography at the tibia; P<0.05-0.001 vs. ERalpha+/+). The observed osteopenia in these mice was evident in both trabecular and cortical bone compartments. However, these decreases were more severe in mice lacking classical ERalpha signaling (ERalpha-/NERKI mice), compared with mice in which one wild-type ERalpha allele was present (ERalpha+/NERKI mice). Collectively, these data demonstrate that classical ERalpha signaling is crucial for the development of the murine male skeleton.

Signaling by estrogens

By regulating activities and expression levels of key signaling molecules, estrogens control mechanisms that are responsible for crucial cellular functions. Ligand binding to estrogen receptor (ER) leads to conformational changes that regulate the receptor activity, its interaction with other proteins and DNA. In the cytoplasm, receptor interactions with kinases and scaffolding molecules regulate cell signaling cascades (extranuclear/nongenomic action). In the nucleus, estrogens control a repertoire of coregulators and other auxiliary proteins that are associated with ER, which in turn determines the nature of regulated genes and level of their expression (genomic action). The combination of genomic and nongenomic actions of estrogens ultimately confers the cell-type and tissue-type selectivity. Recent studies have revealed some important new insights into the molecular mechanisms underlying ER action, which may help to explain the functional basis of existing selective ER modulators (SERMs) and provide evidence into how ER might be selectively targeted to achieve specific therapeutic goals. In this review, we will summarize some new molecular details that relate to estrogen signaling. We will also discuss some new strategies that may potentially lead to the development of functionally selective ER modulators that can separate between the beneficial, prodifferentiative effects in bone, the cardiovascular system and the CNS as well as the "detrimental," proliferative effects in reproductive tissues and organs.

Potential Biological Functions Emerging from the Different Estrogen Receptors

Technological advances and new tools have brought about tremendous advances in elucidating the roles of estradiol and the estrogen receptors (ERs) in biological processes, especially within the female reproductive system. Development and analysis of multiple genetic models have provided insight into the particular functions of each of the ERs. This article reviews the insights into ER biology in female reproduction gained from the development and use of new types of experimental models.

Disruption of estrogen receptor DNA-binding domain and related intramolecular communication restores tamoxifen sensitivity in resistant breast cancer

A serious obstacle to successful treatment of estrogen receptor (ER)-positive human breast cancer is cell resistance to tamoxifen (TAM) therapy. Here we show that the electrophile disulfide benzamide (DIBA), an ER zinc finger inhibitor, blocks ligand-dependent and -independent cell growth of TAM-resistant breast cancer in vitro and in vivo. Such inhibition depends on targeting disruption of the ER DNA-binding domain and its communication with neighboring functional domains, facilitating ERalpha dissociation from its coactivator AIB1 and concomitant association with its corepressor NCoR bound to chromatin. DIBA does not affect phosphorylation of HER2, MAPK, AKT, and AIB1, suggesting that DIBA-modified ERalpha may induce a switch from agonistic to antagonistic effects of TAM on resistant breast cancer cells.

Functional analysis of p53 binding under differential stresses

Hypoxia and DNA damage stabilize the p53 protein, but the subsequent effect that each stress has on transcriptional regulation of known p53 target genes is variable. We have used chromatin immunoprecipitation followed by CpG island (CGI) microarray hybridization to identify promoters bound by p53 under both DNA-damaging and non-DNA-damaging conditions in HCT116 cells. Using gene-specific PCR analysis, we have verified an association with CGIs of the highest enrichment (> 2.5-fold) (REV3L, XPMC2H, HNRPUL1, TOR1AIP1, glutathione peroxidase 1, and SCFD2), with CGIs of intermediate enrichment (> 2.2-fold) (COX7A2L, SYVN1, and JAG2), and with CGIs of low enrichment (> 2.0-fold) (MYC and PCNA). We found little difference in promoter binding when p53 is stabilized by these two distinctly different stresses. However, expression of these genes varies a great deal: while a few genes exhibit classical induction with adriamycin, the majority of the genes are unchanged or are mildly repressed by either hypoxia or adriamycin. Further analysis using p53 mutated in the core DNA binding domain revealed that the interaction of p53 with CGIs may be occurring through both sequence-dependent and -independent mechanisms. Taken together, these experiments describe the identification of novel p53 target genes and the subsequent discovery of distinctly different expression phenomena for p53 target genes under different stress scenarios.