The estrogen receptor enhances AP-1 activity by two distinct mechanisms with different requirements for receptor transactivation functions

Gender and the injured brain

Anesthesiologists are frequently confronted with patients who are at risk for neurological complications due to perioperative stroke or prior traumatic brain injury. In this review, we address the growing and fascinating body of data that suggests gender and sex steroids influence the pathophysiology of injury and outcome for these patients. Cerebral ischemia, traumatic brain injury, and epilepsy are reviewed in the context of potential sex differences in mechanisms and outcomes of brain injury and the role of estrogen, progesterone, and androgens in shaping these processes. Lastly, implications for current and future perioperative and intensive care are identified.

Targeting the activator protein 1 transcription factor for the prevention of estrogen receptor-negative mammary tumors

The oncogene erbB2 is overexpressed in 20% to 30% human breast cancers and is most commonly overexpressed in estrogen receptor (ER)-negative breast cancers. Transgenic mice expressing erbB2 develop ER-negative mammary tumors, mimicking human breast carcinogenesis. Previously, we have shown that activator protein 1 (AP-1) regulates proliferation of ER-negative breast cancer cells. We hypothesized that blockade of AP-1 in mouse mammary epithelial cells will suppress ER-negative tumorigenesis induced by erbB2. Trigenic erbB2 mice were generated by crossing a bigenic pUHD-Tam67/MMTV-rtTA mouse to a MMTV-erbB2 mouse. The resulting trigenic mice develop tumors and express a doxycycline-inducible c-Jun dominant negative mutant (Tam67) in the mammary glands. In vivo AP-1 blockade by Tam67 expression started delayed mammary tumor formation in MMTV-erbB2 mice by more than 11 weeks. By 52 weeks of age, 100% (18 of 18) of the untreated animals had developed mammary tumors, whereas 56% (9 of 16) of the doxycycline-treated trigenic mice developed tumors. In addition, the tumors that arose in the AP-1-blocked erbB2 mice failed to express Tam67. Twenty-five percent of the doxycycline-treated MMTV-erbB2 mice survived more than 72 weeks of age without developing mammary tumors. Examination of normal-appearing mammary glands from these mice showed that AP-1 blockade by Tam67 also significantly prevents the development of premalignant lesions in these glands. The expression of erbB2 either in normal mammary tissue or in mammary tumors was not altered. Our results show that blocking the AP-1 signaling in mammary cells suppresses erbB2-induced transformation, and show that the AP-1 transcription factor is a critical transducer of erbB2. These results provide a scientific rationale to develop targeted drugs that inhibit AP-1 to prevent the development of ER-negative breast cancer.

Estrogen receptor beta isoform-specific induction of transforming growth factor beta-inducible early gene-1 in human osteoblast cells: an essential role for the activation function 1 domain

The estrogen receptors (ER) alpha and beta are important ligand-mediated transcription factors known to play significant biological roles in numerous tissues including bone. Despite the high homology shared by these receptors, recent studies have suggested that their function is largely unique. Although these receptors have been studied in detail for more than a decade, little data exist concerning the mechanisms by which these two proteins regulate distinct sets of genes. Using the TGFbeta-inducible early gene-1 (TIEG) as a model, we demonstrate that TIEG is rapidly induced in response to estrogen in osteoblasts by ERbeta, but not ERalpha. We have identified the regulatory elements utilized by ERbeta and have demonstrated that ERbeta recruits steroid receptor coactivator (SRC)1 and SRC2 to this regulatory region. Additionally, deletion of the ERbeta-activation function 1 (AF1) domain drastically decreases the estrogen induction of TIEG. Through the use of chimeric receptors, we have demonstrated that the AF1 domain of ERbeta is responsible for recruiting SRC1 and SRC2 and inducing the expression of TIEG in osteoblasts. Finally, SRC1, but not SRC2, is essential for TIEG induction by ERbeta. Overall, these data demonstrate that the estrogen induction of TIEG is ERbeta specific and that the AF1 domain of ERbeta confers this specificity. Finally, a novel and important role for ERbeta's AF1 is implicated in the recruitment of specific coactivators, suggesting that the AF1 may play a significant role in conferring the differences in regulation of gene expression by these two receptors.

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.

Genomic analysis of estrogen cascade reveals histone variant H2A.Z associated with breast cancer progression

We demonstrate an integrated approach to the study of a transcriptional regulatory cascade involved in the progression of breast cancer and we identify a protein associated with disease progression. Using chromatin immunoprecipitation and genome tiling arrays, whole genome mapping of transcription factor-binding sites was combined with gene expression profiling to identify genes involved in the proliferative response to estrogen (E2). Using RNA interference, selected ERα and c-MYC gene targets were knocked down to identify mediators of E2-stimulated cell proliferation. Tissue microarray screening revealed that high expression of an epigenetic factor, the E2-inducible histone variant H2A.Z, is significantly associated with lymph node metastasis and decreased breast cancer survival. Detection of H2A.Z levels independently increased the prognostic power of biomarkers currently in clinical use. This integrated approach has accelerated the identification of a molecule linked to breast cancer progression, has implications for diagnostic and therapeutic interventions, and can be applied to a wide range of cancers.

Early life and adult exposure to isoflavones and breast cancer risk

Soy and red-clover isoflavones are commonly consumed within the diet or as a dietary supplement due to a range of presumed beneficial health benefits. These isoflavones are thought to protect against heart diseases as well as breast and other types of cancer. Isoflavones are structurally similar to estrogens and may act as estrogen agonists or antagonists by binding to estrogen receptors. Because of an increased use of isoflavones in processed foods and dietary supplements as well as the greater consumption of soy products, dietary intakes of isoflavones are increasing in children and adolescents in North America. Estrogens are a known component of numerous hormone related cancers including breast cancer. It is with these facts in mind that we review the existing epidemiological and experimental animal studies for a resolution to a proposed correlation between increased isoflavone consumption and breast cancer. There is conflicting evidence from epidemiological, intervention and experimental animal studies regarding the chemopreventing effects of soy isoflavones in breast cancer. Isoflavones are weak estrogens and their effect depends upon the dose, time of exposure and species involved. It would, therefore, not be safe to indisputably accept soy or red-clover as a source of isoflavone resource to prevent breast cancer.

Estrogen receptor beta in the brain: from form to function

Estrogens have numerous effects on the brain, both in adulthood and during development. These actions of estrogen are mediated by two distinct estrogen receptor (ER) systems, ER alpha (ERalpha) and ER beta (ERbeta). In brain, ERalpha plays a critical role in regulating reproductive neuroendocrine function and behavior, however, a definitive role for ERbeta in any neurobiological function has been slow in forthcoming. Clues to the function of ERbeta in the central nervous system can be gleaned from the neuroanatomical distribution of ERbeta and the phenotypes of neurons that express ERbeta. ERbeta immunoreactivity has been found in populations of GnRH, CRH, vasopressin, oxytocin and prolactin containing neurons in the hypothalamus. Utilizing subtype-selective estrogen receptor agonists can help determine the roles for ERbeta in non-reproductive behaviors in rat models. ERbeta-selective agonists exert potent anxiolytic activity when animals were tested in a number of behavioral paradigms. Consistent with this, ERbeta-selective agonists also inhibited the ACTH and corticosterone response to stress. In contrast, ERalpha selective agonists were found to be anxiogenic and correspondingly increased the hormonal stress response. Taken together, our studies implicate ERbeta as an important modulator of some non-reproductive neurobiological systems. The molecular and neuroanatomical targets of estrogen that are mediated by ERbeta remain to be determined. A number of splice variants of ERbeta mRNA have been reported in brain tissue. Imaging of eGFP labeled chimeric receptor proteins transfected into cell lines shows that ERbeta splice variation can alter trafficking patterns and function. The originally described ERbeta (herein termed ERbeta1) is characterized by possessing a high affinity for estradiol. Similar to ERalpha, it is localized in the nucleus and is trafficked to nuclear sites termed "hyperspeckles" following ligand binding. In contrast, ERbeta2 contains an 18 amino acid insert within the ligand-binding domain and as a result can be best described as a low affinity form of ERbeta. A delta3 (delta3) variant of ERbeta has a deletion of the 3rd exon (coding for the second half of the DNA-binding domain) and as a result does not bind an estrogen response element in DNA. delta3 variants are trafficked to a unique low abundance and larger nuclear site following ligand binding. A delta4 (delta4) variant lacks exon 4 and as a result is localized to the cytoplasm. The amount of individual splice variant mRNAs varies depending upon brain region. Examination of neuropeptide promoter regulation by ERbeta splice variants demonstrates that ERbeta functions as a constitutively active transcription factor. Moreover, it appears that splice variation of ERbeta alters its ability to regulate transcription in a promoter-dependent and ligand-dependent fashion.

Vascular endothelial growth factor receptor-2 expression is down-regulated by 17beta-estradiol in MCF-7 breast cancer cells by estrogen receptor alpha/Sp proteins

17beta-Estradiol (E2) induces and represses gene expression in breast cancer cells; however, the mechanisms of gene repression are not well understood. In this study, we show that E2 decreases vascular endothelial growth factor receptor 2 (VEGFR2) mRNA levels in MCF-7 cells, and this gene was used as a model for investigating pathways associated with E2-dependent gene repression. Deletion analysis of the VEGFR2 promoter indicates that the proximal GC-rich motifs at -58 and -44 are critical for the E2-dependent decreased response in MCF-7 cells. Mutation or deletion of these GC-rich elements results in loss of hormone responsiveness and shows that the -60 to -37 region of the VEGFR2 promoter is critical for both basal and hormone-dependent decreased VEGFR2 expression in MCF-7 cells. Western blot, immunofluorescent staining, RNA interference, and EMSAs support a role for Sp proteins in hormone-dependent down-regulation of VEGFR2 in MCF-7 cells, primarily through estrogen receptor (ER)alpha/Sp1 and ERalpha/Sp3 interactions with the VEGFR2 promoter. Using chromatin immuno-precipitation and transient transfection/RNA interference assays we show that the ERalpha/Sp protein-promoter interactions are accompanied by recruitment of the co-repressors SMRT (silencing mediator of retinoid and thyroid hormone receptor) and NCoR (nuclear receptor corepressor) to the promoter and that SMRT and NCoR knockdown reverse E2-mediated down-regulation of VEGFR2 expression in MCF-7 cells. This study illustrates that both SMRT and NCoR are involved in E2-dependent repression of VEGFR2 in MCF-7 cells.

Nervous system physiology regulated by membrane estrogen receptors

Our understanding of estrogen signaling in the nervous system has undergone a significant shift in recent years. For over three decades, the idea that all estradiol actions were explained by direct regulation of transcription held sway. Within the past decade, the idea that in addition to classical effects, membrane-initiated actions of estradiol are important has gained traction. While several novel putative membrane estrogen receptors (ERs) have been described, a large fraction of measured responses appear to be due to membrane-localized estrogen receptor-alpha (ER alpha) and estrogen receptor-beta (ER beta), the same proteins that regulate gene expression. These membrane-localized ERs participate in the regulation of the synthesis of neuroprogesterone, dorsal root ganglion (DRG) neuron excitation, and female sexual receptivity. This is achieved by the modulation of intracellular cell signaling pathways usually associated with the activation of G protein-coupled receptors (GPCRs). ER alpha and ER beta are themselves not GPCRs that directly activate G proteins to regulate physiological responses, but rather interact with traditional GPCRs to initiate cell signaling. This review presents results that support a direct protein-protein interaction between ER alpha and ER beta with metabotropic glutamate receptors (mGluRs), allowing estradiol to signal through mGluRs. This ER/mGluR hypothesis explains how estradiol can activate a wide-range of intracellular pathways and provides an underlying mechanism for the hitherto seemingly unrelated rapid membrane actions in the nervous system.

Transcriptional regulation of endothelial nitric oxide synthase expression in uterine artery endothelial cells by c-Jun/AP-1

Despite extensive studies have shown that increased endothelial nitric oxide synthase (NOS3) expression in the uterine artery endothelial cells (UAEC) plays a key role in uterine vasodilatation, the molecular mechanism controlling NOS3 expression in UAEC is unknown. According to the sheep NOS3 promoter sequence isolated in our laboratory, we hypothesize that the activator protein-1 (AP-1) site in the proximal sheep NOS3 promoter (TGAGTCA, -682 to -676) is important for NOS3 expression. We developed a c-Jun adenoviral expression system to overexpress c-Jun protein into UAEC to investigate the effects of c-Jun/AP-1 on NOS3 expression. Basal levels of c-Jun protein and mRNA were detected in UAEC. c-Jun protein was overexpressed in a concentration and time-dependent fashion in UAEC infected with sense c-Jun (S-c-Jun), but not sham and antisense c-Jun (A-c-Jun) adenoviruses. Infection with S-c-Jun adenovirus (25 MOI, multiplicity of infection) resulted in efficient c-Jun protein overexpression in UAEC up to 3 days. In S-c-Jun, but not sham and A-c-Jun adenovirus infected UAEC, NOS3 mRNA and protein levels were increased (P<0.05) compared to noninfected controls. Increased NOS3 expression was associated with increased total NOS activity. Transient transfections showed that c-Jun overexpression augmented the transactivation of the sheep NOS3 promoter-driven luciferase/reporter constructs with the AP-1 site but not of deletion constructs without the AP-1 site. When the AP-1 site was mutated, c-Jun failed to trans-activate the sheep NOS3 promoter. AP-1 DNA binding activity also increased in c-Jun overexpressed UAEC. Lastly, the pharmacological AP-1 activator phorbol myristate acetate increased AP-1 binding, trans-activated the wild-type but not the AP-1 mutant NOS3 promoter and dose-dependently stimulated UAEC NOS3 and c-Jun protein expression. Hence, our data show that c-Jun/AP-1 regulates NOS3 transcription involving the proximal AP-1 site in the 5'-regulatory region of the sheep NOS3 gene.

Agonist activity of the 3-hydroxy metabolites of tibolone through the oestrogen receptor in the mouse N20.1 oligodendrocyte cell line and normal human astrocytes

17beta-oestradiol (E(2)) may have a beneficial impact on the development of age-related diseases, in part through alpha and beta oestrogen receptors (ER) in glia. Tibolone, a synthetic steroid, could influence glial-mediated neuroprotection if agonist oestrogenic activity is demonstrable. We used the N20.1 mouse oligodendrocyte cell line as a glial cell model to evaluate the response of ERalpha and ERbeta through oestrogen-response element (ERE) and AP-1-driven reporters to E(2), 4-hydroxytamoxifen (4OHT) and to two tibolone metabolites, 3alpha-hydroxytibolone (3alpha-OH-Tib) and 3beta-hydroxytibolone (3beta-OH-Tib). In addition, we tested the activity of these same ligands through the endogenous ERalpha in human normal astrocytes. Because endogenous ER was not detected in the N20.1 cells, we tested the ability of exogenous ER to activate transcription in response to ligands (100 nM) using a transient cotransfection assay with an ERalpha expression vector. To test the antagonist activity of 3alpha-OH-Tib and 3beta-OH-Tib, we used them in combination with E(2) (10(-8) M), at concentrations of 10(-7) M and 10(-6) M. The human normal astrocytes were treated similarly, with the exception that no ER-encoding DNA was used. Specific ER ligand mediated activity was shown using the E(2) antagonist ICI 182 780 and the pSG5 empty vector. E(2), 3alpha-OH-Tib, and 3beta-OH-Tib stimulated ERalpha on an ERE-promoter at each concentration (P < 0.001) but not at an AP-1-driven promoter. 4OHT was an effective antagonist, but did not exhibit agonist activity on the ERE-driven promoter. 4OHT was an effective agonist through ERalpha on an AP-1-driven promoter. 3alpha-OH-Tib and 3beta-OH-Tib were not effective antagonists of E(2). Both metabolites acted through the ER because the addition of an E(2) antagonist blocked their activity. These results show that 3alpha-OH-Tib and 3beta-OH-Tib exert agonist activity, yet lack antagonist or additive activity, through the ERalpha and ERbeta on an ERE-driven but not on an AP-1-driven promoter in a glial cell model and in normal human astrocytes. This contrasts with the effects of 4OHT, which exerted little or no agonist activity, but reduced E(2)-stimulated activity through ERalpha on the ERE, in the same cells.

Determining the transrepression activity of xenoestrogen on nuclear factor-kappa B in Cos-1 cells by estrogen receptor-alpha

Functional assays have been used to define the estrogenicity of xenoestrogens in cotransfection studies employing estrogen receptors in various cell lines. It is known that estrogen is able to affect transcription from other nuclear transcription factors, especially the nuclear factor-kappa B (NF-kappa B). The ability of selected xenoestrogens (methoxychlor [MXC], dieldrin, and o',p'-DDT) to transrepress the NF-kappa B-mediated transcription in Cos-1 cells was evaluated by cotransfection of human estrogen receptor-alpha (hERalpha). These xenoestrogens have been described as comparably potent xenoestrogens, whereas their relative binding activity (RBA) has been relegated to a lower order as compare to estrogen. The two NF-kappa B response element-containing SV40 promoter and -242/+54 cytomegalovirus (CMV)-expressing firefly luciferase (2 x NRE-PV-Luc and 2 x NRE-CMV-Luc, respectively) were transfected into Cos-1 cells with pRL-tk, expressing the renilla luciferase as internal control. The estrogen receptor was expressed from cytomegalovirus major immediate early promoter (CMV-MIEP) (CMV5-hERalpha). Treatment with 1 nM estrogen (E(2)) (26.2%), 5 nM E(2) (41.4%; p < .05), and xenoestrogens (methoxychlor [1 nM: 29.6%, p < .05; 10 nM: 22.6%), dieldrin [1 nM: 10.3%; 10 nM: 36.06%, p < .05], and o',p'-DDT [1 nM: 17.0%; 10 nM: 7.15%]) repressed transcription from 2 x NREX-PV-Luc. The antiestrogen, ICI 182,780, failed to antagonize the effects of xenoestrogens. The effects of xenoestrogens in transrepression of NF-kappa B by ERalpha were similar when 2 x NRE-CMV-Luc was employed as reporter. Statistically significant (p < .01) repression by 1 nM E(2) (69.2%), 5 nM E(2) (69.1%), 1 nM o',p'-DDT (51.4%), 1 nM dieldrin (47.3%), and 1 nM MXC (73.3%) were observed. The effect of these xenoestrogens without ERalpha cotransfection on 2 x NRE-PV-Luc- and 2 x NRE-CMV-Luc-mediated NF-kappa B transcription was not affected by the treatment alone. It is concluded that xenoestrogens, like estrogens, are capable of producing transrepression of NF-kappa B by hERalpha.

MSH2 118T>C and MSH6 159C>T promoter polymorphisms and the risk of colorectal cancer

The most important indicator of colorectal cancer (CRC) risk is the presence of family history of the disease. Inherited genetic changes, such as single nucleotide polymorphisms, in key candidate genes may contribute to CRC risk. We investigated whether promoter polymorphisms in DNA mismatch repair (MMR) genes MSH2 and MSH6 are associated with the risk of CRC. We genotyped 929 CRC patients and 1098 control subjects from Ontario, and 467 patients and 344 controls from Newfoundland and Labrador, for two promoter polymorphisms in the MMR genes MSH2 and MSH6 using the fluorogenic 5' nuclease assay. We used unconditional logistic regression to evaluate the association between each polymorphism and CRC after adjusting for age and sex. The associations between polymorphisms and tumor clinicopathological features were evaluated with a Pearson's chi-squared test or Fisher's exact test. All statistical tests were two sided. We observed strong associations between the MSH2 -118T>C polymorphism and family history of CRC based on the Amsterdam criteria I (P = 0.005) and Amsterdam criteria I and II (P = 0.036) among cases from Ontario. This association was especially evident among female CRC patients in Ontario (for Amsterdam criteria I, and I and II combined, P = 0.003 and P = 0.0001, respectively). The MSH2 -118T>C polymorphism was associated with strong family history of CRC in Ontario patients.

The discovery and mechanism of action of letrozole

Because estrogen contributes to the promotion and progression of breast cancer, a greater understanding of the role of estrogen in breast cancer has led to therapeutic strategies targeting estrogen synthesis, the estrogen receptor, and intracellular signaling pathways. The enzyme aromatase catalyses the final step in estrogen biosynthesis and was identified as an attractive target for selective inhibition. Modern third-generation aromatase inhibitors (AIs) effectively block the production of estrogen without exerting effects on other steroidogenic pathways. The discovery of letrozole (Femara^®) achieved the goal of discovering a highly potent and totally selective AI. Letrozole has greater potency than other AIs, including anastrozole, exemestane, formestane, and aminoglutethimide. Moreover, letrozole produces near complete inhibition of aromatase in peripheral tissues and is associated with greater suppression of estrogen than is achieved with other AIs. The potent anti-tumor effects of letrozole were demonstrated in several animal models. Studies with MCF-7Ca xenografts successfully predicted that letrozole would be clinically superior to the previous gold standard tamoxifen and also indicated that it may be more effective than other AIs. An extensive program of randomized clinical trials has demonstrated the clinical benefits of letrozole across the spectrum of hormone-responsive breast cancer in postmenopausal women.

Estrogen receptors: how do they signal and what are their targets

During the past decade there has been a substantial advance in our understanding of estrogen signaling both from a clinical as well as a preclinical perspective. Estrogen signaling is a balance between two opposing forces in the form of two distinct receptors (ER alpha and ER beta) and their splice variants. The prospect that these two pathways can be selectively stimulated or inhibited with subtype-selective drugs constitutes new and promising therapeutic opportunities in clinical areas as diverse as hormone replacement, autoimmune diseases, prostate and breast cancer, and depression. Molecular biological, biochemical, and structural studies have generated information which is invaluable for the development of more selective and effective ER ligands. We have also become aware that ERs do not function by themselves but require a number of coregulatory proteins whose cell-specific expression explains some of the distinct cellular actions of estrogen. Estrogen is an important morphogen, and many of its proliferative effects on the epithelial compartment of glands are mediated by growth factors secreted from the stromal compartment. Thus understanding the cross-talk between growth factor and estrogen signaling is essential for understanding both normal and malignant growth. In this review we focus on several of the interesting recent discoveries concerning estrogen receptors, on estrogen as a morphogen, and on the molecular mechanisms of anti-estrogen signaling.

Estrogen receptor beta increases the efficacy of antiestrogens by effects on apoptosis and cell cycling in breast cancer cells

Clinical evidence indicates that higher levels of estrogen receptor beta (ERbeta) predicts improved disease-free and overall survival in patients treated with adjuvant tamoxifen therapy. To better understand the mechanisms in which ERbeta can modulate breast cancer therapies, we introduced ERbeta under an inducible promoter into MCF-7 breast cancer cells. In these cells, induction of ERbeta expression led to a shift in the potency and an increase in the efficacy of tamoxifen to inhibit proliferation. A similar effect on breast cancer cells was observed for two other antiestrogens, raloxifene, and fulvestrant. Induced expression of ERbeta did not enhance the antiproliferative effects of small molecule inhibitors that target the epidermal growth factor receptor, insulin growth factor receptor-1 and histone deacetylase, indicating ERbeta specifically cooperates with antiestrogens. The combination of ERbeta expression, which arrests cells in G2, and tamoxifen, which arrests cells in G1, led to a potent blockade of the cell cycle. ERbeta also increased tamoxifen-induced cell death and cooperated with tamoxifen to induce expression of the pro-apoptotic gene bik. In summary, our data indicates that ERbeta increases the efficacy of antiestrogens by effects on apoptosis and on cell cycling and, together with clinical observations, suggests ERbeta could be a valuable prognostic marker and potential therapeutic target.

Coactivation of estrogen receptor alpha (ER alpha)/Sp1 by vitamin D receptor interacting protein 150 (DRIP150)

Vitamin D receptor interacting protein (DRIP150) coactivates estrogen receptor alpha (ERalpha)-mediated transactivation in breast cancer cell lines transfected with a construct (pERE(3)) containing three estrogen responsive elements (EREs). In this study, we show that DRIP150 also coactivates ERalpha/Sp1-mediated transactivation in ZR-75, MCF-7, and MDA-MB-231 breast cancer cells transfected with a construct (pSp1(3)) containing three consensus GC-rich motifs. Studies on coactivation of wild-type and variant ERalpha/Sp1 by DRIP150 indicates that the DNA-binding domain and helix 12 in the ligand binding domain of ERalpha are required and the coactivation response is squelched by overexpressing an NR-box peptide that contains two LXXLL motifs from GRIP2. In contrast, coactivation of ERalpha/Sp1 by wild-type and mutant DRIP150 expression plasmids show that coactivation of ERalpha/Sp1 by DRIP150 is independent of the NR-boxes. Deletion analysis of DRIP150 demonstrates that coactivation requires an alpha-helical NIFSEVRVYN (amino acids 795-804) motif within 23 amino acid sequence (789-811) in the central region of DRIP150 and similar results were obtained for coactivation of ERalpha by DRIP150. Thus, although different domains of ERalpha are required for hormone-dependent activation of ERalpha and ERalpha/Sp1, coactivation of these transcription factors by DRIP150 requires the alpha-helical amino acids 795-804. This is the first report of a coactivator that enhances ERalpha/Sp1-mediated transactivation in breast cancer cells.

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.

Effect of estrogens on skin aging and the potential role of SERMs

In humans, structural and functional changes attributable to aging are more visibly evident in the skin than in any other organ. Estrogens have significant effects on skin physiology and modulate epidermal keratinocytes, dermal fibroblasts and melanocytes, in addition to skin appendages including the hair follicle and the sebaceous gland. Importantly, skin aging can be significantly delayed by the administration of estrogen. This paper reviews the effects of estrogens on skin and the mechanisms by which estrogens can alleviate the changes due to aging that occur in human skin. The relevance of estrogen replacement therapy (HRT) in postmenopausal women and the potential value of selective estrogen receptor modulators (SERMs) as a therapy for diminishing skin aging are also highlighted.

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.

Inhibition of histone deacetylase enhances the anti-proliferative action of antiestrogens on breast cancer cells and blocks tamoxifen-induced proliferation of uterine cells

Here we report a novel potential therapeutic strategy using histone deacetylase (HDAC) inhibitors to enhance the action of hormonal therapy agents in estrogen receptor alpha (ER alpha)-positive breast cancer. HDAC inhibitors [trichostatin A (TSA), suberoylanilide hydroxamic acid (SAHA) and valproic acid (VPA)], inhibited proliferation of MCF-7 breast cancer cells and, in combination with tamoxifen inhibited proliferation better than with either agent alone. VPA, an anti-convulsant drug with HDAC inhibitory activity, enhanced tamoxifen action at doses within the concentration range used for anti-convulsive therapy. VPA cooperated with tamoxifen in a variety of ER alpha-positive cell lines and was also effective when combined with other antiestrogens, and with aromatase inhibition. VPA enhanced antiestrogen action by promoting cell death via apoptosis without affecting cell cycling. Some of this action may be due to VPA's ability to induce the pro-apoptotic gene Bik, which is also induced by antiestrogens. Remarkably, VPA blocked the undesirable pro-proliferative action of tamoxifen on uterine endometrial cells. Our in vitro results suggest that VPA and other HDAC inhibitors have the potential to enhance hormonal therapy for ER alpha-positive breast cancer and simultaneously reverse the adverse effects of antiestrogens in the uterus.

The protein kinase C-eta isoform induces proliferation in glioblastoma cell lines through an ERK/Elk-1 pathway

Glioblastoma multiforme (GBM) is the highest grade of astrocytoma. GBM pathogenesis has been linked to receptor tyrosine kinases and kinases further down signal-transduction pathways - in particular, members of the protein kinase C (PKC) family. The expression and activity of various PKC isoforms are increased in malignant astrocytomas, but not in non-neoplastic astrocytes. This suggests that PKC activity contributes to tumor progression. The level of PKC-eta expressed correlates with the degree of phorbol-12-myristate-13-acetate (PMA)-induced proliferation of two glioblastoma cell lines, U-1242 MG and U-251 MG. Normally, U-1242 cells do not express PKC-eta, and PMA inhibits their proliferation. Conversely, PMA increases proliferation of U-1242 cells that are stably transfected with PKC-eta (U-1242-PKC-eta). PMA treatment also stimulates proliferation of U-251 cells, which express PKC-eta. Here, we determined that extracellular signal-regulated kinase (ERK) and Elk-1 are downstream targets of PKC-eta. Elk-1-mediated transcriptional activity correlates with the PKC-eta-mediated mitogenic response. Pretreatment of U-1242-PKC-eta cells with inhibitors of PKC or MAPK/ERK kinase (MEK) (bisindolyl maleimide (BIM) or U0126, respectively) blocked both PMA-induced Elk-1 transcriptional activity and PMA-stimulated proliferation. An overexpressed dominant-negative PKC-eta reduced the mitogenic response in U-251 cells, as did reduction of Elk-1 by small interfering RNA. Taken together, these results strongly suggest that PKC-eta-mediated glioblastoma proliferation involves MEK/mitogen-activated protein (MAP) kinase phosphorylation, activation of ERK and subsequently of Elk-1. Elk-1 target genes involved in GBM proliferative responses have yet to be identified.

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.

Genome-wide analysis of estrogen receptor binding sites

The estrogen receptor is the master transcriptional regulator of breast cancer phenotype and the archetype of a molecular therapeutic target. We mapped all estrogen receptor and RNA polymerase II binding sites on a genome-wide scale, identifying the authentic cis binding sites and target genes, in breast cancer cells. Combining this unique resource with gene expression data demonstrates distinct temporal mechanisms of estrogen-mediated gene regulation, particularly in the case of estrogen-suppressed genes. Furthermore, this resource has allowed the identification of cis-regulatory sites in previously unexplored regions of the genome and the cooperating transcription factors underlying estrogen signaling in breast cancer.

Estrogen receptor alpha regulates matrix metalloproteinase-13 promoter activity primarily through the AP-1 transcriptional regulatory site

Many females develop bone diseases such as osteoporosis, and joint diseases such as osteoarthritis after menopause when estrogen levels decline. As estrogen receptors (ER) are present in such tissues, it is possible that the loss of estrogen at menopause influences the expression of enzymes such as members of the MMP family of proteinases to affect bone and connective tissue metabolism. The present study was undertaken to assess a possible relationship between ER-alpha and MMP-13 expression at the promoter level, and to determine how such a relationship could be modulated by ligands such as estrogen. Using a rabbit synovial cell line lacking endogenous ER, a transient transfection system with an ER-alpha construct, and a series of MMP-13 promoter-luciferase constructs of varying lengths and with specific mutations in transcription factor binding sites, it was found that ER-alpha can significantly enhance MMP-13 promoter activity via the AP-1 site, with modulatory influences by the Runx and PEA-3 sites on this ER-alpha dependent enhancement of the promoter activity. This enhancement by ER-alpha was significantly depressed in the presence of 17-ss-estradiol in a dose dependent manner. The influence of tamoxifen and raloxifen on the activity of the ER-alpha was consistent with their known agonist/antagonist activity. These findings indicate that loss of estrogen in vivo could potentially lead to enhanced expression of MMP-13, a proteinase that has been implicated in both osteoporosis and osteoarthritis, and thus contribute to the development and progression of these conditions.

Chromatin immunoprecipitation assay detects ERalpha recruitment to gene specific promoters in uterus

Chromatin immunoprecipitation (ChIP) technique allows detection of proteins that bind to chromatin. While this technique has been applied extensively in cell-based studies, its tissue-based application remains poorly explored. We are specifically interested in examining estrogen-dependent transcriptional mechanism in respect of recruitment of estrogen receptor-alpha (ERα), a ligand-activated transcription factor, to uterine gene promoters in mice. Recent gene-array studies, utilizing ERα knock-out vs. wild-type mice, have revealed that estrogen regulates numerous uterine genes temporally and most importantly via ERα during the phase-II response, including three well characterized genes viz., lactoferrin (Ltf), progesterone receptor (Pgr) and cyclinD1 (Ccnd1). Here, utilizing systematic ChIP studies, we demonstrate endogenous recruitment of ERα to above uterine gene promoters following estradiol-17β (E₂) injection in mice.

The pure estrogen receptor antagonist ICI 182,780 promotes a novel interaction of estrogen receptor-alpha with the 3',5'-cyclic adenosine monophosphate response element-binding protein-binding protein/p300 coactivators

Estrogen receptor-alpha (ERalpha) is a member of the nuclear receptor superfamily of ligand-activated transcription factors. Abundant evidence demonstrates that ERalpha agonists promote, whereas antagonists inhibit, receptor binding to coactivators. In this report we demonstrate that binding of the ICI 182,780 (ICI) pure antiestrogen to ERalpha promotes its interaction with the cAMP response element-binding protein-binding protein (CBP)/p300 but not the p160 family of coactivators, demonstrating the specificity of this interaction. Amino acid mutations within the coactivator binding surface of the ERalpha ligand-binding domain revealed that CBP binds to this region of the ICI-liganded receptor. The carboxy-terminal cysteine-histidine rich domain 3 of CBP, rather than its amino-terminal nuclear interacting domain, shown previously to mediate agonist-dependent interactions of CBP with nuclear receptors, is required for binding to ICI-liganded ERalpha. Chromatin immunoprecipitation assays revealed that ICI but not the partial agonist/antagonist 4-hydroxytamoxifen is able to recruit CBP to the pS2 promoter, and this distinguishes ICI from this class of antiestrogens. Chromatin immunoprecipitation assays for pS2 and cytochrome P450 1B1 promoter regions revealed that ICI-dependent recruitment of CBP, but not receptor, to ERalpha targets is gene specific. ICI treatment did not recruit the steroid receptor coactivator 1 to the pS2 promoter, and it failed to induce the expression of this gene. Taken together, these data indicate that recruitment of the CBP coactivator/cointegrator without steroid receptor coactivator 1 to ERalpha is insufficient to promote transcription of ERalpha target genes.

A novel estradiol/estrogen receptor alpha-dependent transcriptional mechanism controls expression of the human prolactin receptor

Prolactin exerts diverse functions in target tissues through its membrane receptors, and is a potent mitogen in normal and neoplastic breast cells. Estradiol (E(2)) induces human prolactin receptor (hPRLR) gene expression through stimulation of its generic promoter (PIII). This study identifies a novel E(2)-regulated non-estrogen responsive element-dependent transcriptional mechanism that mediates E(2)-induced hPRLR expression. E(2) stimulated transcriptional activity in MCF7A(2) cells transfected with PIII lacking an estrogen responsive element, and increased hPRLR mRNA and protein. The abolition of the E(2) effect by mutation of Sp1 or C/EBP elements that bind Sp1/Sp3 and C/EBPbeta within PIII indicated the cooperation of these transfactors in E(2)-induced transcription of the hPRLR. DNA affinity protein assay showed that E(2) induced estrogen receptor alpha (ERalpha) binding to Sp1/Sp3 and C/EBPbeta DNA-protein complexes. The ligand-binding domain of ERalpha was essential for its physical interaction with C/EBPbeta, and E(2) promoted this association, and its DNA binding domain was required for transactivation of PIII. Co-immunoprecipitation studies revealed tethering of C/EBPbeta to Sp1 by E(2)-activated ERalpha. Chromatin immunoprecipitation analysis showed that E(2) induced recruitment of C/EBPbeta, ERalpha, SRC1, p300, pCAF, TFIIB, and Pol II, with no change in Sp1/Sp3. E(2) also induced promoter-associated acetylation of H3 and H4. These findings demonstrate that an E(2)/ERalpha, Sp1, and C/EBPbeta complex with recruitment of coactivators and TFIIB and Pol II are required for E(2)-activated transcriptional expression of the hPRLR through PIII. Estradiol produced in breast stroma and adipose tissue, which are major sources of estrogen in post-menopausal women, could up-regulate hPRLR gene expression and stimulate breast tumor growth.

Functional characterization of a novel variant of estrogen receptor beta identified in screening of DNA derived from African Americans

OBJECTIVE

Information on single nucleotide polymorphisms in the estrogen receptor beta (ERbeta) gene is lacking for the African American population.

METHODS

In this study, we systematically screened the coding and flanking intron regions of the ERbeta gene in 49 healthy African American individuals.

RESULTS

We detected four novel variants, of which one variant (963T-->C) resulted in amino acid change from phenylalanine to leucine at position 289, referred to as ERbetaF289L. This receptor variant was characterized in vitro for transcriptional activity and ligand-binding. These studies revealed that ERbetaF289L had reduced estrogen binding affinity and impaired response to 17beta-estradiol induced transactivation compared to the wild-type ERbeta.

CONCLUSION

This novel variant might confer genetic susceptibility to certain endocrine related diseases in African Americans.

Reduction of QM protein expression correlates with tumor grade in prostatic adenocarcinoma

The QM protein is a transcription cofactor inhibiting the activity of AP-1 transcription factors and is also a ribosomal protein participating in protein synthesis. While protein synthesis is known to be increased in many cancers, inhibition of AP-1 activity presumably suppresses development and growth of sex-hormone-regulated tumor cells. The present study is the first report on immunohistochemical data of QM in human prostatic tissues. Paraffin sections of human prostate cancer samples were immunohistochemically stained for QM. The staining scores were analyzed with the clinicopathologic data of the patients. QM protein expression was found in all normal prostate glands adjacent to prostate cancer and in various intraepithelial neoplasia (PIN). In prostate cancer, the staining intensity and stained areas were decreased, compared to the normal glands and PIN lesions; in high-grade tumors only some patches of tumor cells showed positivity. Intense (3+) staining was mostly observed in the Gleason grade three areas (48%) compared to grade 4 and 5 areas (22%), although both low and high-grade tumors showed similar percentages of weakly stained areas. Moreover, staining in prostatic adenocarcinoma was often topographically patchy and varied from negative or weak (1+) to intense (3+). There was an inverse correlation from normal to low-grade tumors and then to high-grade tumors. However, in high-grade tumors, the positive areas were mostly confined to peripheral aspects of tumors and were particularly strong in foci of perineural invasion. This preliminary study suggests that decreased QM expression may be associated with early development of prostate cancer, but later a high level of QM may facilitate progression of the tumors to a more aggressive phenotype.

Oral contraceptives and skin cancer: is there a link?

The skin expresses estrogen, progesterone, and androgen receptors. In the presence of steroid hormones, such as those contained in oral contraceptives, the skin likely responds to hormonal signals that control the cell cycle, apoptosis, DNA replication, and other cellular functions. Some estrogen-responsive pathways have the potential to promote tumor development, including the augmentation of epidermal growth factor signaling, the expression of proto-oncogenes, and inhibition of apoptosis. The question of whether oral contraceptives increase the risk for the development of skin cancer, particularly melanoma, is still an area of concern. This paper reviews the available evidence, the bulk of which suggests that while the skin responds to estrogens, progestins, and androgens, these responses do not significantly increase the risk of developing skin cancer when estrogen exposure is not excessive.

Multiple pathways transmit neuroprotective effects of gonadal steroids

Numerous preclinical studies suggest that gonadal steroids, particularly estrogen, may be neuroprotective against insult or disease progression. This paper reviews the mechanisms contributing to estrogen-mediated neuroprotection. Rapid signaling pathways, such as MAPK, PI3K, Akt, and PKC, are required for estrogen's ability to provide neuroprotection. These rapid signaling pathways converge on genomic pathways to modulate transcription of E2-responsive genes via ERE-dependent and ERE-independent mechanisms. It is clear that both rapid signaling and transcription are important for estrogen's neuroprotective effects. A mechanistic understanding of estrogen-mediated neuroprotection is crucial for the development of therapeutic interventions that enhance quality of life without deleterious side effects.

Metal complex SERMs (selective oestrogen receptor modulators). The influence of different metal units on breast cancer cell antiproliferative effects

The selective oestrogen receptor modulator tamoxifen is a leading agent in the adjuvant treatment of breast cancer. Several organometallic moieties have been vectorised with tamoxifen, in order to improve on the latter's antiproliferative properties by the addition of a potentially cytotoxic moiety, and have been evaluated versus both oestrogen receptor positive (MCF7) and oestrogen receptor negative (MDA-MB231) breast cancer cells. For tamoxifen analogues with ((R,R)-trans-1,2-diaminocyclohexane)platinum(II), cyclopentadienyl rhenium tricarbonyl, and ruthenocene tethers, there was no enhancement of the antiproliferative effect on oestrogen receptor positive cells, nor any cytotoxic effect on oestrogen receptor negative cells, while those containing cyclopentadienyl titanium dichloride showed an oestrogenic effect. However, compounds where ferrocene replaces tamoxifen's phenyl ring were strongly cytotoxic against both cell lines. The synthesis and biological results of these compounds is reviewed and placed in the historic context of inorganic compounds in therapy.

Transcriptional regulation of tyrosine hydroxylase by estrogen: opposite effects with estrogen receptors alpha and beta and interactions with cyclic AMP

Reported effects of estrogen administration on tyrosine hydroxylase (TH) gene expression are confusing. Therefore, we studied the mechanism of regulation of TH transcription by estrogen with different estradiol receptor (ER) subtypes. PC12 cells, transiently co-transfected with expression vector for ERalpha or ERbeta, and luciferase gene under control of the TH promoter, were treated with 17 beta-estradiol (E2). E2 doubled luciferase activity with ERalpha; however, it was decreased with ERbeta. Mapping the TH promoter showed that the putative half estrogen response element (ERE) motif at - 675, as well as the activation protein 1 motif at - 205, were not required for response to E2 with either ER. The specificity protein 1/early growth response gene 1 (Egr 1) motif was required for the E2-elicited response with ERbeta, but not with ERalpha. Deletion of the cyclic AMP/Ca2+ response element (CRE/CaRE) nearly abolished E2-triggered responses with either ER. Further analysis revealed an imperfect canonical putative ERE overlapping with CRE/CaRE and Nurr1 response element. Oligonucleotides spanning this ERE displayed binding to ER, Cyclic AMP Response Element Binding Protein (CREB) and other proteins. Moreover, E2 attenuated the increase in TH transcription seen with cyclic AMP analogs. Thus, TH is transcriptionally regulated by estradiol in opposite directions depending on ER subtype. The overlapping ERE and CRE/CaRE may integrate interactions elicited by various regulators of TH transcription including cAMP and estrogens.

Aryl hydrocarbon receptor-mediated transcription: ligand-dependent recruitment of estrogen receptor alpha to 2,3,7,8-tetrachlorodibenzo-p-dioxin-responsive promoters

Using chromatin immunoprecipitation assays, we studied the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated recruitment of the aryl hydrocarbon receptor (AhR) and several co-regulators to the CYP1A1 promoter. AhR displayed a time-dependent recruitment, reaching a peak at 75 min and maintaining promoter occupancy for the remainder of the time course. Recruitment of AhR was followed by TIF2/SRC2, which preceded CBP, histone H3 acetylation, and RNA polymerase II (RNAPII). Simultaneous recruitment to the enhancer and the TATA box region suggests the formation of a large multiprotein complex bridging the two promoter regions. Interestingly, estrogen receptor alpha (ERalpha) displayed a TCDD- and time-dependent recruitment to the CYP1A1 promoter, which was increased by co-treatment with estradiol. Transfection in HuH7 human liver cells confirmed previously reported ERalpha enhancement of AhR activity. In contrast, TCDD did not induce the recruitment of ERalpha to the estrogen-responsive pS2 promoter, and after 120 min of co-treatment with estradiol, ERalpha is still present on the CYP1A1 promoter but no longer at pS2. RNA interference studies with T47D cells support a role for ERalpha in TCDD-dependent CYP1A1 expression. Our data suggest that ERalpha acts as a coregulator of AhR-mediated transcriptional activation and that the recruitment of ERalpha by AhR represents a novel mechanism AhR-ERalpha cross talk.

cFos is critical for MCF-7 breast cancer cell growth

The activating protein-1 (AP-1) transcription factor is a converging point of multiple signal transduction pathways in many cells. We have previously demonstrated that overexpressing Tam67, a dominant-negative (DN) form of cJun, blocks AP-1 activity and inhibits breast cancer cell growth. We hypothesized that Tam67 forms dimers with other AP-1 proteins to suppress the growth of breast cancer cells. In the present study, we used immunoprecipitation-Western blotting to demonstrate that Tam67 binds all Jun and Fos proteins in breast cancer cells. In addition, we used two variants of the Tam67 mutant to investigate whether Jun or Fos protein was required for breast cancer cell growth. We created a Tam/Fos mutant in which the cJun dimerization domain was replaced by the cFos dimerization domain, and a Tam/Squelcher mutant in which the cJun dimerization domain was deleted. We then isolated MCF-7 cell lines that stably expressed these cJun-DN mutants under the control of an inducible promoter. Using AP-1-dependent reporter assays, we observed that Tam67 and Tam/Fos mutants inhibited AP-1 transcriptional activity, while the Tam/Squelcher mutant did not. We then determined whether Tam/Fos or Tam/Squelcher inhibited breast cell growth as well as Tam67. We found that while Tam67 repressed cell growth, neither Tam/Fos nor Tam/Squelcher mutant affected cell growth. These results indicate that Tam67 likely inactivates Fos family member proteins to suppress breast cancer cell growth. Finally, we performed antisense experiments to knock down the expression of individual family members (cJun or cFos). Our results demonstrated that antisense cFos inhibited breast cancer cell proliferation and colony formation, while antisense cJun did not. These results suggest that Tam67 suppresses breast cancer cell growth by interacting with Fos family members, specifically with cFos, to produce an inactive AP-1 complex.

Estradiol activates group I and II metabotropic glutamate receptor signaling, leading to opposing influences on cAMP response element-binding protein

In addition to mediating sexual maturation and reproduction through stimulation of classical intracellular receptors that bind DNA and regulate gene expression, estradiol is also thought to influence various brain functions by acting on receptors localized to the neuronal membrane surface. Many intracellular signaling pathways and modulatory proteins are affected by estradiol via this unconventional route, including regulation of the transcription factor cAMP response element-binding protein (CREB). However, the mechanisms by which estradiol acts at the membrane surface are poorly understood. Because both estradiol and CREB have been implicated in regulating learning and memory, we characterized the effects of estradiol on this transcription factor in cultured rat hippocampal neurons. Within minutes of administration, estradiol triggered mitogen-activated protein kinase (MAPK)-dependent CREB phosphorylation in unstimulated neurons. Furthermore, after brief depolarization, estradiol attenuated L-type calcium channel-mediated CREB phosphorylation. Thus, estradiol exhibited both positive and negative influences on CREB activity. These effects of estradiol were sex specific and traced to membrane-localized estrogen receptors that stimulated group I and II metabotropic glutamate receptor (mGluR) signaling. Activation of estrogen receptor alpha (ERalpha) led to mGluR1a signaling, triggering CREB phosphorylation through phospholipase C regulation of MAPK. In addition, estradiol stimulation of ERalpha or ERbeta triggered mGluR2/3 signaling, decreasing L-type calcium channel-mediated CREB phosphorylation. These results not only characterize estradiol regulation of CREB but also provide two putative signaling mechanisms that may account for many of the unexplained observations regarding the influence of estradiol on nervous system function.

Analysis of estrogen receptor alpha-Sp1 interactions in breast cancer cells by fluorescence resonance energy transfer

Estrogen-dependent regulation of several genes associated with cell cycle progression, proliferation, and nucleotide metabolism in breast cancer cells is associated with interactions of estrogen receptor (ER)alpha/Sp1 with GC-rich promoter elements. This study investigates ligand-dependent interactions of ERalpha and Sp1 in MCF-7 breast cancer cells using fluorescence resonance energy transfer (FRET). Chimeric ERalpha and Sp1 proteins fused to cyan fluorescent protein or yellow fluorescent protein were transfected into MCF-7 cells, and a FRET signal was induced after treatment with 17beta-estradiol, 4'-hydroxytamoxifen, or ICI 182,780. Induction of FRET by these ERalpha agonists/antagonists was paralleled by their activation of gene expression in cells transfected with a construct (pSp1(3)) containing three tandem Sp1 binding sites linked to a luciferase reporter gene. In contrast, interactions between ERalpha and Sp1DeltaDBD [a DNA binding domain (DBD) deletion mutant of Sp1] are not observed, and this is consistent with the critical role of the C-terminal DBD of Sp1 for interaction with ERalpha. Results of the FRET assay are consistent with in vitro studies on ERalpha/Sp1 interactions and transactivation, and confirm that ERalpha and Sp1 interact in living breast cancer cells.

Prolactin and estrogen enhance the activity of activating protein 1 in breast cancer cells: role of extracellularly regulated kinase 1/2-mediated signals to c-fos

Despite the important roles of both prolactin (PRL) and 17beta-estradiol (E2) in normal mammary development as well as in breast cancer, and coexpression of the estrogen receptor (ER) and PRL receptor in many mammary tumors, the interactions between PRL and E2 in breast cancer have not been well studied. The activating protein 1 (AP-1) transcription factor, a known regulator of processes essential for normal growth and development as well as carcinogenesis, is a potential site for cross-talk between these hormones in breast cancer cells. Here we demonstrate that PRL and E2 cooperatively enhance the activity of AP-1 in MCF-7-derived cells. In addition to the acute PRL-induced ERK1/2 activation, PRL and E2 also individually elicited delayed, sustained rises in levels of phosphorylated p38 and especially ERK1/2. Together, these hormones increased the dynamic phosphorylation of ERK1/2 and c-Fos, and induced c-fos promoter activity. Synergistic activation of the transcription factor, Elk-1, reflected the PRL-E2 interaction at ERK1/2 and is a likely mechanism for activation of the c-fos promoter via the serum response element. The enhanced AP-1 activity resulting from the interaction of these hormones may increase expression of many target genes that are critical for oncogenesis and may contribute to neoplastic progression.

Estrogen Receptor ?? in Breast Cancer: Associations Between ER??, Hormonal Receptors, and Other Prognostic Biomarkers

The estrogen receptor (ER)-beta isoform has been recently identified to be distinct from ERalpha isoform and regulates separate sets of genes, and can exert opposite signaling functions depending on the ligand and response elements. Previous studies of ERbeta have been at the mRNA level and few by immunohistochemistry, and the results are inconsistent. In this study the authors compared expression of ERbeta with those of other prognostic biomarkers by immunohistochemistry on tissue microarray slides, and with morphologic parameters on 147 cases of primary breast cancer. Immunoreactivity of more than 10% of cancer cells was considered to be positive. Associations between categoric variables were analyzed using the chi test, and a P value less than 0.05 was considered to be significant. ERbeta was expressed in benign epithelium and stromal cells, and breast cancer cells in 59% of different histologic types of breast cancer. ERbeta was coexpressed with ERalpha in 45% of cases. There was a statistically significant association between expression of ERbeta and Her-2/neu (P<0.000), cathepsin D (P<0.02), p53 (P<0.03), and PS2 (P<0.002). Ki-67 was almost exclusively expressed in ERbeta-positive cells. No statistically significant association was seen between ERbeta expression and histologic grade, DNA ploidy, or S-phase.

Mechanisms of estrogen receptor signaling: convergence of genomic and nongenomic actions on target genes

Estrogen receptors (ERs) act by regulating transcriptional processes. The classical mechanism of ER action involves estrogen binding to receptors in the nucleus, after which the receptors dimerize and bind to specific response elements known as estrogen response elements (EREs) located in the promoters of target genes. However, ERs can also regulate gene expression without directly binding to DNA. This occurs through protein-protein interactions with other DNA-binding transcription factors in the nucleus. In addition, membrane-associated ERs mediate nongenomic actions of estrogens, which can lead both to altered functions of proteins in the cytoplasm and to regulation of gene expression. The latter two mechanisms of ER action enable a broader range of genes to be regulated than the range that can be regulated by the classical mechanism of ER action alone. This review surveys our knowledge about the molecular mechanism by which ERs regulate the expression of genes that do not contain EREs, and it gives examples of the ways in which the genomic and nongenomic actions of ERs on target genes converge. Genomic and nongenomic actions of ERs that do not depend on EREs influence the physiology of many target tissues, and thus, increasing our understanding of the molecular mechanisms behind these actions is highly relevant for the development of novel drugs that target specific receptor actions.

Global Gene Expression Analysis of Estrogen Receptor Transcription Factor Cross Talk in Breast Cancer: Identification of Estrogen-Induced/Activator Protein-1-Dependent Genes

There is a growing body of literature supporting estrogen’s ability to affect gene expression through a nonclassical pathway, in which estrogen receptor (ER) modulates the activity of other transcription factors such as activator protein (AP)-1, specificity protein (Sp-1), or nuclear factor-κB (NFκB). We hypothesized that many estrogen-induced genes are dependent on AP-1 for their expression and that these genes can be identified using genomic strategies. Using cells expressing an inducible cJun dominant negative, we studied the estrogen induction of genes under conditions in which AP-1 was normal or blocked. We show that the expression of AP-1-dependent genes was inhibited by the cJun dominant negative and that AP-1 blockade does not affect mRNA ERα expression or estrogen induction of estrogen-responsive element activity. Using a microarray approach, we then identified 20 new estrogen-induced/AP-1-dependent genes. These estrogen-induced/AP-1-dependent genes contain a higher frequency of consensus AP-1 sites in their promoters and have increased sensitivity to the AP-1 stimulant tetradecanoyl phorbol acetate when compared with estrogen-induced genes whose expression was not affected by AP-1 blockade. We also show estrogen and AP-1-dependent recruitment of ER, steroid receptor coactivator-1, and p300 to the promoter of these genes by chromatin immunoprecipitation. These studies demonstrate that microarrays can be used in a reverse genetics approach to predict the functional promoter structure of large numbers of genes that are regulated by multiple transcription factors.

The androgen metabolite, 5alpha-androstane-3beta, 17beta-diol, is a potent modulator of estrogen receptor-beta1-mediated gene transcription in neuronal cells

5alpha-Androstane-3beta, 17beta-diol (3betaAdiol) is a metabolite of the potent androgen, 5alpha-dihydrotestosterone. Recent studies showed that 3betaAdiol binds to estrogen receptor (ER)-beta and regulates growth of the prostate gland through an estrogen, and not androgen, receptor-mediated pathway. These data raise the possibility that 3betaAdiol could regulate important physiological processes in other tissues that produce 3betaAdiol, such as the brain. Although it is widely accepted that the brain is a target for 5alpha-dihydrotestosterone action, there is no evidence that 3betaAdiol has a direct action in neurons. To explore the molecular mechanisms by which 3betaAdiol might act to modulate gene transcription in neuronal cells, we examined whether 3betaAdiol activates ER-mediated promoter activity and whether ER transactivation is facilitated by a classical estrogen response element (ERE) or an AP-1 complex. The HT-22 neuronal cell line was cotransfected with an expression vector containing ERalpha, ER-beta1, or the ERbeta splice variant, ER-beta2 and one of two luciferase-reporter constructs containing either a consensus ERE or an AP-1 enhancer site. Cells were treated with 100 nM 17beta-estradiol, 100 nM 3betaAdiol, or vehicle for 15 h. We show that 3betaAdiol activated ER-beta1-induced transcription mediated by an ERE equivalent to that of 17beta-estradiol. By contrast, 3betaAdiol had no effect on ERalpha- or ER-beta2-mediated promoter activity. Moreover, ER-beta1 stimulated transcription mediated by an ERE and inhibited transcription by an AP-1 site in the absence of ligand binding. These data provide evidence for activation of ER signaling pathways by an androgen metabolite in neuronal cells.

AP-1 blockade in breast cancer cells causes cell cycle arrest by suppressing G1 cyclin expression and reducing cyclin-dependent kinase activity

The AP-1 transcription factor is a central component of signal transduction pathways in many cells, although the exact role of AP-1 in controlling cell growth and malignant transformation is unknown. We have previously shown that AP-1 complexes are activated by peptide and steroid growth factors in both normal and malignant breast cells, and that blocking AP-1 by overexpressing a dominant-negative form of cJun (cJun-DN, TAM67) inhibits breast cancer cell growth both in vivo and in vitro. We hypothesized that TAM67 inhibits cell growth by altering the expression of cell cycle regulatory proteins, thus causing a cell cycle block. In the present study, we used clones of MCF7 breast cancer cells that express TAM67 under the control of an inducible promoter. First, we determined the effect of AP-1 blockade on cell growth, then we performed 3H-thymidine incorporation and flow cytometry assays to investigate whether TAM67 inhibits the cell cycle. We observed that in the presence of serum TAM67 inhibited cell growth and caused a block in the G1 phase of the cell cycle. Next, we performed Western-blotting and CDK kinase assays to determine the effects of TAM67 on retinoblastoma (Rb) phosphorylation, the expression of cell cycle regulatory proteins, and CDK activity. We discovered that TAM67 inhibited Rb phosphorylation and reduced E2F activity. We also found that TAM67 decreased the expression of D and E cyclins, reduced CDK2 and CDK4 activity, and increased the CDK inhibitor p27. The studies of gene expression at the RNA level showed that TAM67 decreased cyclin Ds mRNA expression. Our study suggests that in the presence of serum, TAM67 inhibits breast cancer growth predominantly by inducing inhibitors of cyclin-dependent kinases (such as p27) and by reducing the expression of the cyclins involved in transitioning from G1 into S phase of the cell cycle. These studies lay the foundation for future attempt to develop new agents for the treatment and prevention of breast cancer.

Estrogen receptor mutations in human disease

As early as the 1800s, the actions of estrogen have been implicated in the development and progression of breast cancer. The estrogen receptor (ER) was identified in the late 1950s and purified a few years later. However, it was not until the 1980s that the first ER was molecularly cloned, and in the mid 1990s, a second ER was cloned. These two related receptors are now called ERalpha and ERbeta, respectively. Since their discovery, much research has focused on identifying alterations within the coding sequence of these receptors in clinical samples. As a result, a large number of naturally occurring splice variants of both ERalpha and ERbeta have been identified in normal epithelium and diseased or cancerous tissues. In contrast, only a few point mutations have been identified in human patient samples from a variety of disease states, including breast cancer, endometrial cancer, and psychiatric diseases. To elucidate the mechanism of action for these variant isoforms or mutant receptors, experimental mutagenesis has been used to analyze the function of distinct amino acid residues in the ERs. This review will focus on ERalpha and ERbeta alterations in breast cancer.

Prenylation inhibitors stimulate both estrogen receptor alpha transcriptional activity through AF-1 and AF-2 and estrogen receptor beta transcriptional activity

Introduction

We showed in a previous study that prenylated proteins play a role in estradiol stimulation of proliferation. However, these proteins antagonize the ability of estrogen receptor (ER) α to stimulate estrogen response element (ERE)-dependent transcriptional activity, potentially through the formation of a co-regulator complex. The present study investigates, in further detail, how prenylated proteins modulate the transcriptional activities mediated by ERα and by ERβ.

Methods

The ERE-β-globin-Luc-SV-Neo plasmid was either stably transfected into MCF-7 cells or HeLa cells (MELN cells and HELN cells, respectively) or transiently transfected into MCF-7 cells using polyethylenimine. Cells deprived of estradiol were analyzed for ERE-dependent luciferase activity 16 hours after estradiol stimulation and treatment with FTI-277 (a farnesyltransferase inhibitor) or with GGTI-298 (a geranylgeranyltransferase I inhibitor). In HELN cells, the effect of prenyltransferase inhibitors on luciferase activity was compared after transient transfection of plasmids coding either the full-length ERα, the full-length ERβ, the AF-1-deleted ERα or the AF-2-deleted ERα. The presence of ERα was then detected by immunocytochemistry in either the nuclei or the cytoplasms of MCF-7 cells. Finally, Clostridium botulinum C3 exoenzyme treatment was used to determine the involvement of Rho proteins in ERE-dependent luciferase activity.

Results

FTI-277 and GGTI-298 only stimulate ERE-dependent luciferase activity in stably transfected MCF-7 cells. They stimulate both ERα-mediated and ERβ-mediated ERE-dependent luciferase activity in HELN cells, in the presence of and in the absence of estradiol. The roles of both AF-1 and AF-2 are significant in this effect. Nuclear ERα is decreased in the presence of prenyltransferase inhibitors in MCF-7 cells, again in the presence of and in the absence of estradiol. By contrast, cytoplasmic ERα is mainly decreased after treatment with FTI-277, in the presence of and in the absence of estradiol. The involvement of Rho proteins in ERE-dependent luciferase activity in MELN cells is clearly established.

Conclusions

Together, these results demonstrate that prenylated proteins (at least RhoA, RhoB and/or RhoC) antagonize the ability of ERα and ERβ to stimulate ERE-dependent transcriptional activity, potentially acting through both AF-1 and AF-2 transcriptional activities.

Estrogen modulates microglial inflammatory mediator production via interactions with estrogen receptor beta

Estrogens are well known to exert antiinflammatory effects outside the central nervous system (CNS). They have also been shown to exert neuroprotective effects in the CNS after several types of injury, including neurodegeneration. However, the molecular mechanisms by which these effects occur remain unclear. Because microglial hyperactivation and their production of neurotoxins is associated with many types of brain injury for which estrogens are beneficial, we sought to investigate the ability of estrogen to modulate microglial function. Furthermore, because little is known regarding the role of each of the two known estrogen receptors (ERs) in microglia, our studies were designed to test the hypothesis that 17beta-estradiol (E(2)) exerts antiinflammatory effects in microglia, specifically via interactions with ERbeta. We tested this hypothesis using the murine microglial cell line BV-2, which naturally expresses only ERbeta. Our results indicate that not only does E(2) decrease lipopolysaccharide (LPS)-stimulated nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression, it also reduces the expression of cyclooxygenase-2, a target for estrogen that has not previously been reported for ERbeta. We also observed that LPS-stimulated TNFalpha mRNA was increased by estrogen. E(2) exerts these effects within 30 min compared with typical estrogen transcriptional responses. Tamoxifen and ICI 182,780 differentially blocked the inhibitory effects of E(2) on LPS-stimulated iNOS and cyclooxygenase-2. In addition, we show that E(2) alters LPS-stimulated MAPK pathway activation, supporting the idea that alterations in the MAPKs may be a potential mechanism by which ERbeta mediates decreased microglial activation.

Gene expression profiles in breast tumors regarding the presence or absence of estrogen and progesterone receptors

Estrogen acts via its receptor (ER) to stimulate cell growth and differentiation in the mammary gland. ER and progesterone receptor (PR), which is regulated by estrogen via ER, have been used as prognostic markers in clinical management of breast cancer patients. Patients with ER- breast tumors have a poorer prognosis than patients with ER+ tumors. The aim of the present study was the identification of tumor-associated genes differentially expressed in breast tumors regarding the presence or absence of ER and PR hybridized with cDNA microarrays containing 4,500 tumor-derived expressed sequence tags generated using the ORESTES technique. Samples of human primary breast carcinomas from 38 patients were analyzed. The experiments were performed in triplicates and data from each element were acquired by phosphoimage scanning. Data acquisition was performed using the ArrayVision software. After normalization statistical analysis was applied. In a preliminary analysis, 98 differentially expressed transcripts were identified, 46 were found to be more expressed in ER+/PR+ and 52 were found to be more expressed in ER-/PR- breast tumors. The biochemical functions of the genes in the reported expression profile are diverse and include metabolic enzymes, protein kinases, helicases, transcription factors, cell cycle regulators and apoptotic factors. ER-/PR- breast tumors displayed increased levels of transcripts of genes associated with neurodegeneration and genes associated with proliferation were found in ER+/PR+ tumors.

An Alternative Splicing Variant of the Selenoprotein Thioredoxin Reductase Is a Modulator of Estrogen Signaling

The selenoprotein thioredoxin reductase (TrxR1) is an integral part of the thioredoxin system. It serves to transfer electrons from NADPH to thioredoxin leading to its reduction. Interestingly, recent work has indicated that thioredoxin reductase can regulate the activity of transcription factors such as p53, hypoxia-inducible factor, and AP-1. Here, we describe that an alternative splicing variant of thioredoxin reductase (TrxR1b) containing an LXXLL peptide motif, is implicated in direct binding to nuclear receptors. In vitro interaction studies revealed direct interaction of the TrxR1b with the estrogen receptors alpha and beta. Confocal microscopy analysis showed nuclear colocalization of the TrxR1b with both estrogen receptor alpha and beta in estradiol-17beta-treated cells. Transcriptional studies demonstrated that TrxR1b can affect estrogen-dependent gene activation differentially at classical estrogen response elements as compared with AP-1 response elements. Based on these results, we propose a model where thioredoxin reductase directly influences the estrogen receptor-coactivator complex assembly on non-classical estrogen response elements such as AP-1. In summary, our results suggest that TrxR1b is an important modulator of estrogen signaling.