Molecular mechanisms of action of steroid/thyroid receptor superfamily members

FRAP reveals that mobility of oestrogen receptor-alpha is ligand- and proteasome-dependent

Here we report the use of fluorescence recovery after photobleaching (FRAP) to examine the intranuclear dynamics of fluorescent oestrogen receptor-alpha (ER). After bleaching, unliganded ER exhibits high mobility (recovery t1/2 < 1 s). Agonist (oestradiol; E2) or partial antagonist (4-hydroxytamoxifen) slows ER recovery (t1/2 approximately 5-6 s), whereas the pure antagonist (ICI 182,780) and, surprisingly, proteasome inhibitors each immobilize ER to the nuclear matrix. Dual FRAP experiments show that fluorescent ER and SRC-1 exhibit similar dynamics only in the presence of E2. In contrast to reports that several nuclear proteins show uniform dynamics, ER exhibits differential mobility depending upon several factors that are linked to its transcription function.

Synergistic, p160 coactivator-dependent enhancement of estrogen receptor function by CARM1 and p300

Members of the p160 coactivator family (steroid receptor coactivator-1 (SRC-1), glucocorticoid receptor interacting protein 1 (GRIP1), and activator of thyroid and retinoic acid receptors (ACTR)) mediate transcriptional activation by nuclear receptors. After being recruited to the promoter by nuclear receptors, the p160 coactivator transmits the activating signal via two C-terminal activation domains, AD1 and AD2. AD1 is a binding site for the related coactivators cAMP-response element binding protein binding protein (CBP) and p300, whereas AD2 binds to another coactivator, coactivator-associated arginine methyltransferase 1 (CARM1), a protein-arginine methyltransferase. The current study explored the cooperative functional and mechanistic relationships among GRIP1, CARM1, and p300 in transient transfection assays, where they enhanced the ability of the estrogen receptor (ER) to activate transcription of a reporter gene. The coactivator functions of p300 and CARM1 depended on the co-expression of GRIP1. Simultaneous co-expression of all three coactivators caused a synergistic enhancement of ER function. Deletion of the AD1 domain of GRIP1 abolished the ability of p300 to potentiate ER activity but had no effect on CARM1-mediated stimulation. In contrast, when the AD2 domain of GRIP1 was deleted, p300 still stimulated ER function through the mutant GRIP1, but CARM1 failed to do so. Thus, both binding of p300 to AD1 and binding of CARM1 to AD2 are required for their respective coactivator functions and for their synergy. Furthermore, CARM1 and p300 function independently through different activating domains of GRIP1, and their synergy suggests that they enhance transcription by different, complementary mechanisms.

Ductal growth is impeded in mammary glands of C-neu transgenic mice

The steroid hormone, estradiol, is essential for both the growth of normal breast and induction of mammary carcinomas. The growth promoting effects of estrogen are presumed to be mediated by growth factors, in particular, epidermal growth factor, which mediates its effects through erbB receptors, erbB1 and erbB2/C-neu. C-neu is amplified and over-expressed in a large number of human cancers and transgenic mice over-expressing C-neu also develop mammary tumors. However, as yet, the impact of C-neu over-expression on estrogen action during normal mammary development and hence, its precise role in carcinogenesis, remains unclear. In the present studies, we demonstrate that estradiol-dependent mammary ductal growth accompanying puberty is impaired in transgenic mice expressing wild type Cneu, and is intrinsic to the tissue. The impairment is not due to an overall impairment in estrogen action, since progesterone receptor expression is unaffected in C-neu mice. It is also not due to an intrinsic inability of the epithelial cells to proliferate, since impeded ductal growth co-exists with alveolar growth during pregnancy. Therefore, we propose that, depending on the physiological state, C-neu may either promote or inhibit the growth of mammary epithelial cells, and discuss its potential significance to carcinogenesis.

Evolution of structure, ontogeny of gene expression, and function of Xenopus laevis transthyretin

Xenopus laevis transthyretin (xTTR) cDNA was cloned and sequenced. The derived amino acid sequence was very similar to those of other vertebrate transthyretins (TTR). TTR gene expression was observed during metamorphosis in X. laevis tadpole liver but not in tadpole brain nor adult liver. Recombinant xTTR was synthesized in Pichia pastoris and identified by amino acid sequence, subunit molecular mass, tetramer formation, and binding to retinol-binding protein. Contrary to mammalian xTTRs, the affinity of xTTR was higher for L-triiodothyronine than for L-thyroxine. The regions of the TTR genes coding for the NH(2)-terminal sections of the polypeptide chains of TTR seem to have evolved by stepwise shifts of mRNA splicing sites between exons 1 and 2, resulting in shorter and more hydrophilic NH(2) termini. This may be one molecular mechanism of positive Darwinian evolution. Open reading frames with xTTR-like sequences in the genomes of C. elegans and several microorganisms suggested evolution of the TTR gene from ancestor TTR gene-like "DNA modules." Increasing preference for binding of L-thyroxine over L-triiodothyronine may be associated with evolving tissue-specific regulation of thyroid hormone action by deiodination.

Molecular mechanisms of dissociative glucocorticoid activity

BACKGROUND

Glucocorticoids mediate their effects on target cells via transactivation and transrepression of certain target genes. While conventional glucocorticoids do not distinguish between transactivation and transrepression, new glucocoticoids should be able to dissociate these effects, thus lowering the potential of unwanted side-effects of glucocorticoids in clinical use. In this study, we developed a new experimental system to test potentially selective glucocorticoids in normal lymphocytes.

MATERIALS AND METHODS

Following pretreatment with phytohaemagglutinin, normal lymphocytes were transfected, using electroporation, with pGL3 luciferase reporter vectors under the control: (1) of the human IL-2 promoter; and (2) of a glucocorticoid response element (GRE). Luciferase activity was measured in response to various steroid compounds, including the potentially dissociative glucocorticoid medroxyprogesterone acetate (MPA).

RESULTS

The IL-2 promoter was induced 267.2 +/- 27.5-fold (mean +/- SD) by phorbol ester and ionomycin. In these cells, hydrocortisone and dexamethasone caused a 22.9 +/- 3.6% and a 38.4 +/- 10% reduction in luciferase activity, respectively. Under GRE control, hydrocortisone stimulated luciferase activity 6.4 +/- 0.50-fold and dexamethasone 8.2 +/- 0.4-fold. MPA-induced transrepression was 73.3 +/- 7.2% for the IL-2 promoter, and transactivation was 2.4 +/- 0.4-fold with the GRE-driven construct. The natural progestin progesterone did not have significant effects on either construct.

CONCLUSIONS

This is the first system that allows efficient analysis of glucocorticoid-dependent transactivation and transrepression in normal human lymphocytes. Compared to conventional glucocorticoids, MPA can be referred to as a dissociative glucocorticoid, its transrepression/transactivation ratio being 6.6 (transrepression 1.91/transactivation 0.29), with dexamethasone being the standard (transrepression 1/transactivation 1). We conclude that MPA is a highly promising substance for the treatment of autoimmune/inflammatory diseases.

Temporally distinct and ligand-specific recruitment of nuclear receptor-interacting peptides and cofactors to subnuclear domains containing the estrogen receptor

Ligand binding to estrogen receptor (ER) is presumed to regulate the type and timing of ER interactions with different cofactors. Using fluorescence microscopy in living cells, we characterized the recruitment of five different green fluorescent protein (GFP)-labeled ER-interacting peptides to the distinct subnuclear compartment occupied by blue fluorescent protein (BFP)-labeled ER alpha. Different ligands promoted the recruitment of different peptides. One peptide was recruited in response to estradiol (E2), tamoxifen, raloxifene, or ICI 182,780 incubation whereas other peptides were recruited specifically by E2 or tamoxifen. Peptides containing different sequences surrounding the ER-interacting motif LXXLL were recruited with different time courses after E2 addition. Complex temporal kinetics also were observed for recruitment of the full-length, ER cofactor glucocorticoid receptor-interacting protein 1 (GRIP1); rapid, E2-dependent recruitment of GRIP1 was blocked by mutation of the GRIP1 LXXLL motifs to LXXAA whereas slower E2 recruitment persisted for the GRIP1 LXXAA mutant. This suggested the presence of multiple, temporally distinct GRIP 1 recruitment mechanisms. E2 recruitment of GRIP1 and LXXLL peptides was blocked by coincubation with excess ICI 182,780. In contrast, preformed E2/ER/GRIP1 and E2/ER/LXXLL complexes were resistant to subsequent ICI 182,780 addition whereas ICI 182,780 dispersed preformed complexes containing the GRIP1 LXXAA mutant. This suggested that E2-induced LXXLL binding altered subsequent ligand/ER interactions. Thus, alternative, ligand-selective recruitment and dissociation mechanisms with distinct temporal sequences are available for ER alpha action in vivo.

Identification and characterization of estrogen receptor variants in prostate cancer cell lines

A sensitive semi-nested reverse transcriptase-polymerase chain reaction (RT-PCR) amplification was performed to evaluate estrogen receptor-alpha (ER-alpha) mRNA expression in prostate cancer cell lines. We demonstrated the presence of wild-type ER-alpha (wt ER-alpha) and five ER-alpha variants, designated ER-alphaA, B, C, D, and E. Unlike ER-alphaA and D, ER-alphaB, C, and E were not previously reported in normal or cancerous mammalian cells. DNA sequencing analysis of these ER-alpha variants revealed the genetic changes to be either in-frame or out-of-frame deletions. The expression of each ER-alpha variant differs significantly depending on the androgen responsiveness, tumorigenic and metastatic potentials of each prostate cancer cell line. The potential functional significance of ER-alpha variants was assessed in yeast two-hybrid and ERE promoter-reporter mammalian transcription assay systems. The results of these studies indicated that none of the ER-alpha variants can form homo- or heterodimers either with wt ER-alpha or among themselves in vivo, and that these ER-alpha variants have no demonstrable transcriptional or dominant-negative activity, as assessed in vitro.

Combined corticosteroid/granulocyte colony-stimulating factor (G-CSF) therapy in the treatment of severe congenital neutropenia unresponsive to G-CSF: Activated glucocorticoid receptors synergize with G-CSF signals

More than 90% of patients with severe congenital neutropenia (SCN) respond to granulocyte colony-stimulating factor (G-CSF) therapy. The basis for the refractory state in the remaining patients is unknown. To address this issue, we studied a child with SCN who was totally unresponsive to G-CSF and had a novel point mutation in the extracellular domain of the G-CSF receptor (GCSF-R). Marrow stromal support of granulopoiesis was evaluated by plating CD34(+) cells on preformed stromal layers. Nonadherent cells were harvested and assayed in clonogenic assays for granulocytic colony production. The in vitro effect of G-CSF and corticosteroids on granulopoiesis was evaluated in clonogenic assays of marrow mononuclear cells, by proliferation studies of the murine myeloid cell line 32D expressing the patient's mutated G-CSFR, and by measuring STAT5 activation in nuclear extracts from stimulated cells.Patient's stroma supported granulopoiesis derived from control marrow CD34(+) cells in a normal manner. Normal stroma, however, failed to induce granulopoiesis from patient's CD34(+) cells. Clonogenic assays of the patient's marrow mononuclear cells incorporating either G-CSF or hydrocortisone produced little neutrophil growth. In contrast, inclusion of both G-CSF and hydrocortisone in the cytokine "cocktail" markedly increased the neutrophil numbers. Proliferation of 32D cells expressing the mutated receptor and STAT5 activation were improved by a combination of G-CSF and dexamethasone. When small daily doses of oral prednisone were then administered to the patient with conventional doses of subcutaneous G-CSF, the patient responded with increased neutrophil numbers and with a complete reversal of the infectious problems. These data provide insight into SCN unresponsive to standard G-CSF treatment and to the potential corrective action of combined treatment with G-CSF and corticosteroids through synergistic activation of STAT5.

Development of peptide antagonists that target estrogen receptor beta-coactivator interactions

The biological actions of estrogen are manifest through two genetically distinct estrogen receptors (ER alpha and ER beta) that display nonidentical expression patterns in target tissues. The phenotypic alterations in response to estrogens in mice disrupted for either or both of these receptors are not identical, suggesting that each subtype plays a unique role in ER-action. However, the lack of subtype-specific agonists and antagonists has made it difficult to define the processes that are regulated by ER alpha and/or ER beta. Previously, we have reported the identification and characterization of a series of LXXLL-containing peptide antagonists that block estrogen signaling by preventing the association of ER alpha with required coactivators. As expected, given the similarity of the coactivator binding pockets among nuclear receptors, most of the peptide antagonists identified inhibited the activity of multiple receptors. However, by altering sequences flanking the core LXXLL motif, some receptor selectivity was afforded. Building on this observation, we have screened combinatorial phage libraries, expressing peptides in the format X7LXXLLX7, for peptides that interact in a specific manner with ER beta. Using this approach, a series of highly specific, potent peptide antagonists have been identified that efficiently inhibit ER beta-mediated estrogen signaling when introduced into target cells. Interestingly, in cells where both ER subtypes were expressed, these ER beta antagonists were capable of attenuating ER action, suggesting that ER alpha and ER beta do indeed form functional heterodimeric complexes. We believe that suitably formulated versions of these peptides can be used to study ER beta action in vitro and in vivo. In addition, the unanticipated specificity of the peptides identified should serve as an impetus to investigate the use of this approach to develop peptide antagonists of other nuclear receptors and unrelated transcription factors.

Thoughts on tamoxifen resistant breast cancer. Are coregulators the answer or just a red herring?

The antiestrogen tamoxifen is an effective treatment for estrogen receptor positive breast cancers, slowing tumor growth and preventing disease recurrence, with relatively few side effects. However, many patients who initially respond to treatment, later become resistant to treatment. Tamoxifen has both agonist and antagonist activities, which are manifested in a tissue-specific pattern. Development of tamoxifen resistance can be characterized by an increase in the partial agonist properties of the antiestrogen in the breast, resulting in loss of growth inhibition and even inappropriate tumor stimulation. Nuclear receptor function is modulated by transcriptional coregulators, which either enhance or repress receptor activity. Using a mixed antagonist-biased two-hybrid screening strategy, we identified two such proteins: the human homolog of the nuclear receptor corepressor, N-CoR, and a novel coactivator, L7/SPA (Switch Protein for Antagonists). In transcriptional studies N-CoR suppressed the agonist properties of tamoxifen and RU486, while L7/SPA increased agonist effects. We speculated that the relative level of these coactivators and corepressors might determine the balance of agonist and antagonist properties of mixed antagonists such as tamoxifen. Using quantitative RT-PCR we therefore measured the levels of transcripts encoding these coregulators, as well as the corepressor SMRT, and the coactivator SRC-1, in a small cohort of tamoxifen resistant and sensitive breast tumors. The results suggest that tumor sensitivity to mixed antagonists may be governed by a complex set of transcription factors, which we are only now beginning to understand.

Identification of estrogen target genes in human neural cells

In mammals, estrogens have a multiplicity of effects ranging from control of differentiation of selected brain nuclei, reproductive functions, sexual behavior. In addition, these hormones influence the manifestation of disorders like depression and Alzheimer's. Study of the cells target for the hormone has shown that estrogen receptors (ERs) are expressed in all known neural cells, including microglia. In view of the potential interest in the use of estrogens in the therapy of several pathologies of the nervous system, it would be of interest to fully understand the mechanism of estrogen activity in the various neural target cells and get an insight on the molecular means allowing the hormone to display such a variety of effects. We have proposed the use of a reductionist approach for the systematic understanding of the estrogen activities in each specific type of target cell. Thus, we have generated a model system in which to study the activation of one of the known (ERs), estrogen receptor alpha. This system allowed us to identify a number of novel genes which expression may be influenced following the activation of this receptor subtype by estradiol (E(2)). We here report on data recently obtained by the study of one of these target genes, nip2, which encodes a proapoptotic protein product. We hypothesize that nip2 might be an important molecular determinant for estrogen anti-apoptotic activity in cells of neural origin and represents a potential target for drugs aimed at mimicking the E(2) beneficial effects in neural cells.

Estrogen receptor beta in the breast: role in estrogen responsiveness and development of breast cancer

Breast cancer is one of the most common forms of cancer observed in women. Endogenous estrogen is thought to play a major role in its development and estrogen receptor blockers are the most important drugs in its treatment. It has long been thought that any conditions or exposures, which enhance estrogenic responses, would result in an increased risk for breast cancer. The discovery of the second estrogen receptor, ERbeta, which can have effects opposite to those of the well-known 'original' estrogen receptor (now called ERalpha) challenges this simplistic view. In order to understand breast cancer one must first understand how the normal breast is maintained. The functions of ERbeta in the breast remain to be defined but from what we have learnt about its activities in in vitro systems, this estrogen receptor may have a protective role in the breast. Studies in human and rodent breasts as well as in human breast cancer biopsies reveal that ERbeta is by far the more abundant of the two ERs. Despite the role of estrogen in proliferation of the breast, neither of the two ERs appears to located in epithelial cells which divide in response to estrogen. In order to define the functions of ERbeta in the normal and malignant breast, we have created mice in which the ERbeta gene has been inactivated. Studies of the breasts of ERbeta knock out mice (BERKO) revealed abnormal epithelial growth, overexpression of Ki67 and severe cystic breast disease as mice age.

A structural biologist's view of the oestrogen receptor

Here we review the results that have emerged from our structural studies on the oestrogen receptor ligand-binding domain (ER-LBD). The effects of agonists and antagonists on the structure of ERalpha- and ERbeta-LBDs are examined. In addition, the findings from structural studies of ER-LBD in complex with peptide fragments corresponding to the NR-box II and III modules of the p160 coactivator TIF2 are discussed in the context of the assembly of ER:coactivator complexes. Together these studies have broadened our understanding of ER function by providing a unique insight into ER's ligand specificity, it's ability to interact with coactivators and the structural changes that underlie receptor agonism and antagonism.

From ligand to response: generating diversity in nuclear receptor coregulator function

Nuclear receptor ligands regulate diverse developmental and physiological processes by activating intracellular members of the nuclear receptor superfamily. Activated nuclear receptors mediate the expression of distinct gene networks in vivo by an as yet unspecified mechanism. Central to the process is the recruitment by these receptors of coactivators, a functionally diversified set of factors shown to be required for efficient transcriptional regulation by activated receptors. This article will highlight recent advances in selected mechanistic aspects of receptor function, as well as discussing the potential of coactivators to act as mediators of the intricate pharmacology of nuclear receptor ligands.

Analysis of estrogen receptor interaction with a repressor of estrogen receptor activity (REA) and the regulation of estrogen receptor transcriptional activity by REA

The transcriptional activity of nuclear hormone receptors is known to be modulated by coregulator proteins. We found that the repressor of estrogen receptor activity (REA), a protein recruited to the hormone-occupied estrogen receptor (ER), decreased the transcriptional activity of ER, both when ER was acting directly through DNA response elements as well as when it was tethered to other transcription factors. Administration of antisense REA resulted in a 2-4-fold increase in ER transactivation, implying that endogenous REA normally dampens the stimulatory response to estradiol. To define the interaction regions between ER and REA, we used glutathione S-transferase pull-down assays. We found that REA bound to the ligand-binding domain (E domain) of ER, but not to other regions of ER, and that REA interaction with ER involved a region in the C-terminal half of REA. REA and the coactivator SRC-1 were involved in a functional competition for regulation of ER transcriptional activity, which we show results from competition between these two coregulators for interaction with ER. REA contains an LXXLL motif near its N terminus, but this motif was not involved in its binding to ER. Rather, this sequence was required for the competitive binding of REA and SRC-1 to ER and thus for optimal repression of ER activity. Our findings show that the regions of REA required for its interaction with ER and for its repression of ER activity are different.

The vitamin D response element-binding protein. A novel dominant-negative regulator of vitamin D-directed transactivation

Vitamin D resistance in certain primate genera is associated with the constitutive overexpression of a non-vitamin D receptor (VDR)-related, vitamin D response element-binding protein (VDRE-BP) and squelching of vitamin d-directed transactivation. We used DNA affinity chromatography to purify proteins associated with non-VDR-VDRE binding activity from vitamin d-resistant New World primate cells. In electrophoretic mobility shift assays, these proteins bound specifically to either single-strand or double-strand oligonucleotides harboring the VDRE. Amino acid sequencing of tryptic peptides from a 34-kDa (VDRE-BP1) and 38-kDa species (VDRE-BP-2) possessed sequence homology with human heterogeneous nuclear ribonucleoprotein (hnRNP) A1 and hnRNPA2, respectively. cDNAs bearing the open reading frame for both VDRE-BPs were cloned and used to transfect wild-type, hormone-responsive primate cells. Transient and stable overexpression of the VDRE-BP2 cDNA, but not the VDRE-BP1 cDNA, in wild-type cells with a VDRE-luciferase reporter resulted in significant reduction in reporter activity. These data suggest that the hnRNPA2-related VDRE-BP2 is a dominant-negative regulator of vitamin D action.

A membrane-associated progesterone-binding protein, 25-Dx, is regulated by progesterone in brain regions involved in female reproductive behaviors

The ventromedial hypothalamus (VMH) plays a central role in the regulation of the female reproductive behavior lordosis, a behavior dependent upon the sequential activation of receptors for the ovarian steroid hormones estradiol (E) and progesterone (P). These receptors function as transcription factors to alter the expression of target genes. To discover behaviorally relevant genes targeted by E and P in the VMH, we used the differential display PCR to identify messenger RNAs that are differentially expressed in the hypothalamus of ovariectomized (ovx) rats treated with E alone compared with ovariectomized rats treated with E and P. We show here that one interesting mRNA within the hypothalamus that is repressed by P after E priming encodes the protein 25-Dx, the rat homolog of the human membrane-associated P-binding protein Hpr6.6. Neurons in the brain containing the highest levels of 25-Dx are located in several nuclei of the basal forebrain, including the VMH. 25-Dx expression is also higher in the hypothalamus of female P receptor "knockout" mice than in their wild-type littermates. These findings suggest a mechanism in which the activation of nuclear P receptor represses expression of a membrane P receptor, 25-Dx, during lordosis facilitation.

Antiestrogenicity of beta-naphthoflavone and PAHs in cultured rainbow trout hepatocytes: evidence for a role of the arylhydrocarbon receptor

The aims of the present study were to assess, (1) if polyaromatic hydrocarbons (PAHs) are able to inhibit estradiol-regulated vitellogenin synthesis in fish; and (2) if this antiestrogenic activity is mediated through the binding of PAHs to the arylhydrocarbon receptor (AhR). Cultured liver cells of rainbow trout, Oncorhynchus mykiss, were co-exposed to PAHs and 17beta-estradiol (E2), and the resulting effects on induction of AhR-regulated 7-ethoxyresorufin-O-deethylase (EROD) activity and on E2-regulated vitellogenesis were investigated. The following test compounds were compared: the PAH 3-methylcholanthrene (3MC), which is a strong EROD inducer, the PAH anthracene (ANT), which is not an inducer of EROD activity, and the model EROD inducer, beta-naphthoflavone (betaNF). 3MC and betaNF led to significant decreases of E2-triggered hepatocellular VTG synthesis, whereas ANT exerted no antiestrogenic activity. The rank order of the antiestrogenic activity of the test substances agreed with their EROD-inducing potency suggesting that their antiestrogenicity might be mediated through the AhR. Further evidence for this assumption comes from the observation that inhibitors such as alpha-naphthoflavone which interferes with ligand-AhR binding, and 8-methoxypsoralen (8MP), which prevents binding of the occupied AhR to responsive DNA elements, clearly reduced the antiestrogenic effects of the xenobiotics. Furthermore, from the comparison of estradiol concentrations in media of liver cells exposed to the CYP 1A-inducing agents and in media of control cells it is unlikely that the observed antiestrogenic effects were caused by an enhanced E2 catabolism. In conclusion, the results from this study indicate that, (1) AhR-binding PAHs possess an antiestrogenic activity; and (2) that the antiestrogenic activity is mediated through the AhR.

Inhibition of mineralocorticoid-mediated transcription by NF-kappaB

Nuclear factor-kappaB (NF-kappaB) is a ubiquitous transcription factor that regulates the expression of multiple inflammatory and immune response genes and plays a critical role in host defense and in chronic inflammatory diseases. The mineralocorticoid receptor (MR) belongs to the steroid/thyroid hormone receptor super-family of ligand-induced transcription factors. We demonstrate a dose-dependent, mutual transcriptional antagonism between NF-kappaB and MR in transient transfection experiments. We also show that the antagonism is limited to the p65 subunit of NF-kappaB heterodimer but not p50. Transient cotransfection experiments with MR deletion constructs reveal the necessity of various N-terminal MR domains for this phenomenon. Inhibition of NF-kappaB by IkappaB relieves the repression of NF-kappaB function by MR. These data suggest that the p65 subunit of NF-kappaB interacts with MR indirectly and transrepresses MR activation.

Estrogen stimulation of ovarian surface epithelial cell proliferation

Ovarian cancer is the leading cause of gynecological cancer mortality, and 85-90% of this malignancy originates from the ovarian surface epithelium (OSE). The etiology of ovarian epithelial cancer is unknown, but a role for estrogens has been suspected. However, the effect of estrogens on OSE cell proliferation remains to be determined. Using the rabbit model, our studies have demonstrated that 17beta-estradiol stimulates OSE cell proliferation and the formation of a papillary ovarian surface morphology similar to that seen in human ovarian serous neoplasms of low malignant potential. Immunohistochemical staining of ovarian tissue sections with an antibody to the estrogen receptor alpha demonstrates its expression in both OSE cells and stromal interstitial cells. In primary ovarian cell cultures, the proliferative response of the epithelial cells to 17beta-estradiol depends on the expression of the estrogen receptor alpha in the epithelial cells. However, when the epithelial cells are grown together with ovarian stromal cells, their proliferative response to this hormone is greatly enhanced, suggesting the involvement of stromal-epithelial interactions. These studies suggest a role for estrogens and the estrogen receptor alpha in OSE growth.

Transcriptional regulation of steroid receptor coactivator-1 (SRC-1) in glucocorticoid action

Diverse mechanisms of steroid receptor action have been clarified in recent years, as a consequence of the discovery of multiple coactivators. Among them, steroid receptor coactivator-1 (SRC-1) is a member of the p160 coactivator families, which are 160 kDa proteins that interact with steroid receptors in a hormone-sensitive manner. Since coactivators function as transcriptional power boosters, subtle changes in coactivator expression levels in certain cells markedly change of receptor-mediated transcriptional activity. Expression of the glucocorticoid receptor (GR) has been shown to be autoregulated in glucocorticoid action, i.e., GR is downregulated by its cognate ligand, indicating that this autoregulation of GR may protect target cells against excessive hormone action. In the present study, we examined whether coactivator expression levels are also regulated by glucocorticoids. Among several coactivators, the SRC-1 mRNA level was downregulated by dexamethasone treatment in rat tissues, such as liver, heart, kidney, stomach, and cerebrum, in vivo. We also demonstrated dexamethasone-mediated downregulation of SRC-1 mRNA and its protein levels in rat renal mesangial cells in vitro. These results suggest that ligand-mediated downregulation of SRC-1 is crucial in the physiology of glucocorticoid action.

Repression of chick multidrug resistance-associated protein 1 (chMRP1) gene expression by estrogen

Although a number of genes have been identified whose transcriptional activities are stimulated by estrogen, relatively few have been discovered that are repressed. In an effort to determine whether estrogen can directly repress gene expression, attempts were made to identify genes that are direct targets of the estrogen receptor and whose activities are repressed by it. Because the development and differentiation of the chick oviduct are exquisitely dependent upon estrogen, this seemed an appropriate model system for testing this hypothesis. RNA was isolated from estrogen-treated and estrogen-withdrawn chick oviducts and was subjected to differential display analysis. Surprisingly, one of the products repressed by estrogen encoded the chick homolog of the multidrug resistance-associated protein 1 (MRP1) gene. Further cloning resulted in a chick MRP1 (chMRP1) cDNA clone that is 72% identical with human MRP1. Translation of the chMRP1 sequence indicates a 77% amino acid identity with both the human and mouse MRP1 proteins. Treatment of estrogen-withdrawn chicks with 17beta-estradiol decreased chMRP1 mRNA levels to 50% within 30 min and to 70% by 1h, which is comparable to the level observed with chronic repression by estrogen. ChMRP1 mRNA is present in many other tissues, including the heart, lung, brain, kidney, skeletal muscle, and intestine, but is undetectable in the liver. This study indicates that in estrogen-responsive tissues such as chick oviduct, the regulation of chMRP1 gene expression is controlled by estrogen.

Apparent coactivation due to interference of expression constructs with nuclear receptor expression

Transient cotransfection in COS-7 cells, a standard approach to demonstrate coactivation, was used to study the coactivation properties of NuRIP183, a new nuclear receptor interacting protein of 183 kDa, isolated by a yeast two-hybrid screening. Transfection with a NuRIP183 expression construct strongly increased the ligand-dependent response of reporter constructs for several nuclear receptors when compared to transfection with the empty expression vector. A more detailed study, however, revealed major changes in the expression level of the nuclear receptors in cotransfection experiments, indicating that the observed changes in receptor activity were not due to coactivation but to differences in receptor concentration caused by interference from the cotransfected expression constructs with the expression of the receptor. Such interference, which is inversely related to the length of the insert, was observed, not only in COS-7 cells but also in CV-1 and MCF-7 cells, using different transfection techniques (FuGENE-6 and calcium phosphate) and different expression vectors (pSG5, pcDNA1.1 and pIRESneo). These data cast some doubt on coactivation of nuclear receptors based on similar cotransfection experiments without measurement of receptor concentration. Moreover, it is recommended to limit the amounts of (co)transfected expression plasmid and to avoid the use of empty expression plasmid as a control. Finally, one should be aware of similar misleading results in other experimental set-ups based on cotransfection.

Thyroid hormone regulation of apoptotic tissue remodeling: implications from molecular analysis of amphibian metamorphosis

Organogenesis and tissue remodeling are critical processes during postembryonic animal development. Anuran metamorphosis has for nearly a century served as an excellent model to study these processes in vertebrates. Frogs not only have essentially the same organs with the same functions as higher vertebrates such as humans, but also employ similar organogenic processes involving highly conserved genes. Development of frog organs takes place during metamorphosis, which is free of any maternal influences but absolutely dependent on the presence of thyroid hormone. Furthermore, this process can be easily manipulated both in intact tadpoles and in organ cultures by controlling the availability of thyroid hormone. These interesting properties have led to extensive morphological, cellular, and biochemical studies on amphibian metamorphosis. More recently, the cloning of thyroid hormone receptors and the demonstration that they are transcription factors have encouraged enormous interest in the molecular pathways controlling tissue remodeling induced by thyroid hormone during metamorphosis. This article summarizes some of the recent studies on the mechanisms of gene regulation by thyroid hormone receptors and isolation and functional characterization of genes induced by thyroid hormone during Xenopus metamorphosis. Particular focus is placed on the remodeling of the animal intestine, which involves both apoptosis (programmed cell death) of larval cells and de novo development of adult tissues, and the roles of thyroid hormone-induced genes that encode matrix metalloproteinases during this process.

Streptozotocin-induced diabetes and glucocorticoid receptor regulation: tissue- and age-specific variation

Streptozotocin (STZ) -induced diabetic effects were analyzed for glucocorticoid receptor (GR) level and for in vitro activation of GR by specific binding analysis, using [3H]dexamethasone, a synthetic glucocorticoid, and by DNA cellulose and nuclear binding assay, in the liver and kidney of 15- (immature) and 120-day-old (mature) male mice. Comparison of GR level (fmol/mg protein) among the control mice reveals decreased (22-33%) specific binding in the liver and kidney of mature mice compared with immature ones. Scatchard analyses, however, reveal no change in the affinity (K(d)) of receptor at these two ages of mice. STZ-induced diabetes did not alter the level of GR in either of the tissues at both the ages studied. The GR from both the tissues underwent thermal activation, albeit the extent of activation was more pronounced in mature liver compared to immature, with no such difference of activation in the kidney. In diabetic mice, the activation of hepatic GR exhibits reduced DNA cellulose ( approximately 20-23%) and nuclear (24-30%) binding compared to control mice. In contrast, thermal activation of kidney GR does not show marked differences in diabetic mice at either of the ages studied. Cross-mixing experiments (i.e. binding of activated GR from diabetic mice to nuclei of control and vice-versa) performed on the mature liver, indicate receptor specificity. These findings reveal tissue- and age- specific variations in the level of GR that is not influenced under diabetic conditions. However, the activation of hepatic GR is reduced during STZ-induced diabetes that might play a role in controlling glucose homeostasis in diabetic animals.

Regulation of nuclear factor-kappa B, activator protein-1, and glutathione levels by tumor necrosis factor-alpha and dexamethasone in alveolar epithelial cells

The development of an oxidant/antioxidant imbalance in lung inflammation may activate redox-sensitive transcription factors such as nuclear factor-kappa B (NF-kappa B) and activator protein-1 (AP-1), which regulate the genes for proinflammatory mediators and protective antioxidant genes. GSH, a ubiquitous tripeptide thiol, is a vital intra- and extracellular protective antioxidant against oxidative stress, which plays a key role in the control of proinflammatory processes in the lungs. The rate-limiting enzyme in GSH synthesis is gamma-glutamylcysteine synthetase (gamma-GCS), which consists of a catalytic heavy and a regulatory light subunit. The promoter regions of the human gamma-GCS subunits contain AP-1, NF-kappa B, and antioxidant response elements and are regulated by oxidants, growth factors, inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha), and anti-inflammatory agent (dexamethasone) in lung cells. TNF-alpha depletes intracellular GSH, concomitant with an increase in oxidised glutathione levels in alveolar epithelial cells. TNF-alpha also induces the activation of NF-kappa B and AP-1 and the subsequent increase in gamma-GCS heavy subunit transcription in these cells. Dexamethasone depleted both basal and TNF-alpha-stimulated GSH levels by down-regulating the gamma-GCS-heavy subunit transcription via a mechanism involving AP-1 (c-Jun). The existence of this fine tuning between the redox GSH levels and the activation of transcription factors may determine the balance of transcription for proinflammatory and antioxidant gamma-GCS genes in inflammation. More studies are required to understand the signalling mechanism of the redox regulation of NF-kappa B and AP-1 and gene transcription in inflammation. This could lead to the development of therapeutic strategies based on the pharmacological manipulation of the production of this important antioxidant in inflammation.

Regulation of the promoters for the human bone morphogenetic protein 2 and 4 genes

The bone morphogenetic proteins 2 and 4 are known to be important in bone formation and are expressed in both the developing and adult mammalian bone. Understanding the regulation of these genes in osteoblasts may yield methods by which we can control expression to induce bone formation. We have isolated and characterized the human BMP-2 and BMP-4 promoters and report substantially more upstream sequence information than that which has been published. Human osteoblasts were found to have a single transcript initiation site that is conserved across species, rather than multiple start sites, as has previously been reported (Feng, J.Q., Harris, M.A., Ghosh-Choudhury, N., Feng, M., Mundy, G.R., Harris, S.E., 1994. Structure and sequence of mouse morphogenetic protein-2 gene (BMP-2): comparison of the structures and promoter regions of BMP-2 and BMP-4 genes. Biochim. Biophys. Acta 1218, 221-224; Heller, L.C., Li, Y., Abrams, K.L., Rogers, M.B., 1999. Transcriptional regulation of the Bmp2 gene. J. Biol. Chem. 274, 1394-1400; Sugiura, T., 1999. Cloning and functional characterization of the 5'-flanking region of the human bone morphogenetic protein-2 gene. Biochem. J. 338, 433-440). A series of promoter deletions for both human BMP-2 and BMP-4 fused to the luciferase reporter gene were analyzed thoroughly in human and murine osteoblastic cell lines. Several compounds and growth factors that stimulate general or osteogenic pathways were used to treat cells transfected with the promoter constructs. Retinoic acid compounds and the phorbol ester, PMA were found to stimulate BMP-2 and, to a lesser degree, BMP-4. The combination of all trans-RA and PMA caused a synergistic increase in BMP-2 promoter activity and endogenous mRNA. The RA stimulation appears to be an indirect effect on the BMP-2 promoter, as the most highly conserved RRE in the BMP-2 promoter was unable to functionally bind or compete for protein binding. Potential binding sites in both promoters for the bone-specific transcription factor, Cbfa-1, were found to specifically bind Cbfa-1 protein in osteoblast nuclear extracts; however, deletion of these sites did not significantly affect transcriptional activity of the promoters in osteoblasts. These data thus present new sequence and regulatory information for the human BMP-2 and BMP-4 promoters and clarify the human BMP-2 gene transcriptional start site in osteoblasts.

Identification of a novel FAS/ER-alpha fusion transcript expressed in human cancer cells

A semi-nested reverse transcriptase-polymerase chain reaction (RT-PCR) was applied to evaluate the presence of estrogen receptor-alpha (ER-alpha) in human prostate cancer cells. Unexpectedly, a novel fatty acid synthase (FAS)/ER-alpha fusion transcript was identified, in which the N-terminus of FAS was fused in-frame with the C-terminus of ER-alpha. The existence of the FAS/ER-alpha transcript was further confirmed by RT-PCR analysis using various sets of amplification primers and different reverse-transcribed primers in the presence of dimethyl sulfoxide to eliminate the secondary structure of RNA. The predicted FAS/ER-alpha protein would contain largely domain I of FAS and the entire ligand binding domain of ER-alpha. The FAS/ER-alpha was expressed in a variety of human cancer cell lines including prostate, breast, cervical and bladder cancer cell lines. Our data suggest that the presence of FAS/ER-alpha may complicate the FAS and the ER-alpha signalling pathway.

Differential effects of diethylstilbestrol and 2,3,7,8-tetrachlorodibenzo-p-dioxin on thymocyte differentiation, proliferation, and apoptosis in bcl-2 transgenic mouse fetal thymus organ culture

Both the estrogenic drug diethylstilbestrol (DES) and the pervasive environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibit thymocyte development. The mechanisms by which either agent induces thymic atrophy are still undetermined. We previously found that TCDD and DES inhibited C57BL/6 murine fetal thymocyte organ cultures (FTOC) at different stages of development. Now, using bcl-2 transgenic (TG) mice, we have further investigated their effects on FTOC proliferation, differentiation, maturation, and apoptosis. As with C57BL/6 mice, thymocyte development in C3H/bcl-2 FTOCs was inhibited by either TCDD (10 nM) or DES (20 microM) in both bcl-2 TG- and TG+ littermates. However, the percentage reduction of cell number induced by DES in bcl-2 TG+ FTOCs was significantly less than the level of inhibition in TG- FTOCs. There was no difference in the level of reduction from TCDD-exposed TG+ or TG- FTOC. Whereas TCDD increased production of mature CD8 cells in either strain, DES mainly yielded cells in the CD4(-)CD8(-)(DN) stage in TG- mice. The anti-apoptotic bcl-2 transgene overcame some DES blocking of DN thymocyte development, allowing more cells to differentiate into CD4 single-positive cells. Analysis of cell cycle showed that TCDD inhibited entry into S phase, whereas DES blocked cell cycling in the G2/M phase. TCDD did not induce detectable apoptosis in FTOC. However, unlike the effects of 17 beta-estradiol (E2) in vivo, DES induced apoptosis in the TG- FTOC, and these apoptotic cells were mainly in the DN subpopulation. This apoptosis could be prevented by the overexpression of bcl-2 in the TG+ mice. Our results demonstrate that, in addition to inhibition of fetal thymocytes at different stages of development by TCDD and DES, DES also induces thymic atrophy by both bcl-2-inhibitable apoptosis and by inducing cell cycle arrest in G2/M in the latest stage in the stem cell compartment. TCDD, on the other hand, does not induce apoptosis, but inhibits entry into cell cycle in the earliest stage in the stem cell compartment.

Analysis of estrogen receptor alpha and beta in endometrial carcinomas: correlation with ER beta and clinicopathologic findings in 45 cases

Estrogens play important roles in the pathogenesis of the great majority of endometrial endometrioid adenocarcinoma. Recently, a novel estrogen receptor (ER), ER beta, has been characterized, but little is known about the status of ER beta in endometrial carcinoma. We therefore examined expression of both ER alpha and ER beta in 45 cases of endometrioid endometrial adenocarcinoma using mRNA in situ hybridization, reverse transcription and polymerase chain reaction (RT-PCR), and immunohistochemistry. We also correlated the findings with various clinicopathologic parameters in these cases to examine their possible biologic significance. Accumulation of mRNA hybridization signals for both ER alpha and ER beta was detected predominantly in the cytoplasm of carcinoma cells, and to a lesser extent in some stromal cells. ER beta mRNA was detected in 16/45 cases (35.6%), and ER alpha mRNA hybridization signals were detected in 36/45 cases (80.0%). Among the 16 ER beta positive cases, 15 cases also had ER alpha mRNA hybridization signals. In the cases that expressed both ER alpha and ER beta, ER alpha mRNA hybridization signals were more widely distributed than ER beta mRNA. In 21 cases, carcinoma cells had ER alpha mRNA hybridization signals but not ER beta mRNA. There was a statistically significant positive correlation between the results of mRNA in situ hybridization and semiquantitative RT-PCR or immunohistochemistry for both ER alpha and ER beta. There were no significant correlations between ER beta mRNA expression and PR labeling index, Ki67 LI, age, or histologic grade. The results from our study indicate that ER beta is coexpressed with ER alpha, and that the estrogenic effects occur predominantly through ER alpha in endometrial carcinomas.

Modulation of the far-upstream enhancer of the rat alpha-fetoprotein gene by members of the ROR alpha, Rev-erb alpha, and Rev-erb beta groups of monomeric orphan nuclear receptors

Expression of the oncodevelopmental alpha-fetoprotein (AFP) gene is tightly regulated and occurs in the yolk sac, fetal liver and intestine, and cancerous liver cells. Transcription of the AFP gene is under the control of three enhancers that are very tissue specific. We have shown that the most upstream of these enhancers, located at -6 kb, works through the combined action of liver-enriched factors and nuclear receptors that bind to three regions of this DNA regulatory element. This study showed that orphan nuclear receptors of the ROR alpha, Re-verb alpha, and Rev-erb beta groups can bind as monomers with high affinity and specificity to an evolutionarily conserved AGGTCA motif in the functionally important region 1 of this AFP enhancer. Transient transfection experiments performed with human HepG2 hepatoma cells showed that overproduction of ROR alpha 4 stimulated the activity of the AFP enhancer in a dose-dependent manner, while that of Rev-erb alpha and Rev-erb beta had the opposite effect. These effects were highly specific and required the integrity of the AGGTCA motif. The action of these nuclear receptors also occurred in the context of the entire 7-kb regulatory region of the rat AFP gene. These results suggest that altering the amounts or activities of these orphan receptors in cells of hepatic or endodermal origin could modulate AFP gene expression in response to a variety of developmental or carcinogenic stimuli.

Nuclear receptor conformation, coregulators, and tamoxifen-resistant breast cancer

The development of tamoxifen resistance and consequent disease progression are common occurrences in breast cancers, often despite the continuing expression of estrogen receptors (ER). Tamoxifen is a mixed antagonist, having both agonist and antagonist properties. We have suggested that the development of tamoxifen resistance is associated with an increase in its agonist-like properties, resulting in loss of antagonist effects or even inappropriate tumor stimulation. Nuclear receptor function is influenced by a family of transcriptional coregulators, that either enhance or suppress transcriptional activity. Using a mixed antagonist-biased two-hybrid screening strategy, we identified two such proteins: the human homolog of the nuclear receptor corepressor, N-CoR, and a novel coactivator, L7/SPA (Switch Protein for Antagonists). In transcriptional studies, N-CoR suppressed the agonist properties of tamoxifen and RU486, and L7/SPA increased agonist effects. We speculated that the relative levels of these coactivators and corepressors may determine the balance of agonist and antagonist properties of mixed antagonists, such as tamoxifen. Using quantitative RT-PCR, we, therefore, measured the levels of transcripts encoding these coregulators, as well as the corepressor SMRT, and the coactivator SRC-1, in a small cohort of tamoxifen-resistant and sensitive breast tumors. The results suggest that tumor sensitivity to mixed antagonists may be governed by a complex set of transcription factors, which we are only now beginning to understand.

The mechanism of action of thyroid hormones

Thyroid hormone is essential for normal development, differentiation, and metabolic balance. Thyroid hormone action is mediated by multiple thyroid hormone receptor isoforms derived from two distinct genes. The thyroid hormone receptors belong to a nuclear receptor superfamily that also includes receptors for other small lipophilic hormones. Thyroid hormone receptors function by binding to specific thyroid hormone-responsive sequences in promoters of target genes and by regulating transcription. Thyroid hormone receptors often form heterodimers with retinoid X receptors. Heterodimerization is regulated through distinct mechanisms that together determine the specificity and flexibility of the sequence recognition. Amino-terminal regions appear to modulate thyroid hormone receptor function in an isoform-dependent manner. Unliganded thyroid hormone receptor represses transcription through recruitment of a corepressor complex, which also includes Sin3A and histone deacetylase. Ligand binding alters the conformation of the thyroid hormone receptor in such a way as to release the corepressor complex and recruit a coactivator complex that includes multiple histone acetyltransferases, including a steroid receptor family coactivator, p300/CREB-binding protein-associated factor (PCAF), and CREB binding protein (CBP). The existence of histone-modifying activities in the transcriptional regulatory complexes indicates an important role of chromatin structure. Stoichiometric, structural, and sequence-specific rules for coregulator interaction are beginning to be understood, as are aspects of the tissue specificity of hormone action. Moreover, knockout studies suggest that the products of two thyroid hormone receptor genes mediate distinct functions in vivo. The increased understanding of the structure and function of thyroid hormone receptors and their interacting proteins has markedly clarified the molecular mechanisms of thyroid hormone action.

A novel, conditionally replicative adenovirus for the treatment of breast cancer that allows controlled replication of E1a-deleted adenoviral vectors

The efficiency of gene therapy strategies against cancer is limited by the poor distribution of the vectors in the malignant tissues. To solve this problem, a new generation of tumor-specific, conditionally replicative adenoviruses is being developed. To direct the replication of the virus to breast cancer, we have considered one characteristic present in a great proportion of these cancers, which is the expression of estrogen receptors (ERs). On the basis of the wild-type adenovirus type 5, we have constructed a conditionally replicative adenovirus (Ad5ERE2) in which the E1a and E4 promoters have been replaced by a portion of the pS2 promoter containing two estrogen-responsive elements (EREs). This promoter induces transcriptional activation of the E1a and E4 units in response to estrogens in cells that express the ERs. Ad5ERE2 is able to kill ER(+) human breast cancer cell lines as efficiently as the wild-type virus, but has decreased capacity to affect ER(-) cells. By complementation of the E1a protein in trans, Ad5ERE2 allows restricted replication of a conventional E1a-deleted adenoviral vector. When a virus expressing the proapoptotic gene Bc1-xs (Clarke et al., Proc. Natl. Acad. Sci. U.S.A. 1995;92:11024-11028) is used in combination with Ad5ERE2, the ability of both viruses to induce cell death is dramatically increased, and the effect can be modulated by addition of the antiestrogen tamoxifen.

The glucocorticoid receptor inhibits NFkappaB by interfering with serine-2 phosphorylation of the RNA polymerase II carboxy-terminal domain

Glucocorticoids repress NFkappaB-mediated activation of proinflammatory genes such as interleukin-8 (IL-8) and ICAM-1. Our experiments suggest that the glucocorticoid receptor (GR) confers this effect by associating through protein-protein interactions with NFkappaB bound at each of these genes. That is, we show that the GR zinc binding region (ZBR), which includes the DNA binding and dimerization functions of the receptor, binds directly to the dimerization domain of the RelA subunit of NFkappaB in vitro and that the ZBR is sufficient to associate with RelA bound at NFkappaB response elements in vivo. Moreover, we demonstrate in vivo and in vitro that GR does not disrupt DNA binding by NFkappaB. In transient transfections, we found that the GR ligand binding domain is essential for repression of NFkappaB but not for association with it and that GR can repress an NFkappaB derivative bearing a heterologous activation domain. We used chromatin immunoprecipitation assays in untransfected A549 cells to infer the mechanism by which the tethered GR represses NFkappaB-activated transcription. As expected, we found that the inflammatory signal TNFalpha stimulated preinitiation complex (PIC) assembly at the IL-8 and ICAM-1 promoters and that the largest subunit of RNA polymerase II (pol II) in those complexes became phosphorylated at serines 2 and 5 in its carboxy-terminal domain (CTD) heptapeptide repeats (YSPTSPS); these modifications are required for transcription initiation. Remarkably, GR did not inhibit PIC assembly under repressing conditions, but rather interfered with phosphorylation of serine 2 of the pol II CTD.

Estrogen targets genes involved in protein processing, calcium homeostasis, and Wnt signaling in the mouse uterus independent of estrogen receptor-alpha and -beta

Estrogen actions in target organs are normally mediated via activation of nuclear estrogen receptors (ERs). By using mRNA differential display technique, we show, herein, that estradiol-17beta (E(2)) and its catechol metabolite 4-hydroxy-E(2) (4OHE(2)) can modulate uterine gene expression in ERalpha(-/-) mice. Whereas administration of E(2) or 4OHE(2) rapidly up-regulated (4-8-fold) the expression of immunoglobulin heavy chain binding protein (Bip), calpactin I (CalP), calmodulin (CalM), and Sik similar protein (Sik-SP) genes in ovariectomized wild-type or ERalpha(-/-) mice, the expression of secreted frizzled related protein-2 (SFRP-2) gene was down-regulated (4-fold). Bip, CalP, and CalM are calcium-binding proteins and implicated in calcium homeostasis, whereas SFRP-2 is a negative regulator of Wnt signaling. Bip and Sik-SP also possess chaperone-like functions. Administration of ICI-182,780 or cycloheximide failed to influence these estrogenic responses, demonstrating that these effects occur independent of ERalpha, ERbeta, or protein synthesis. In situ hybridization showed differential cell-specific expression of these genes in wild-type and ERalpha(-/-) uteri. Although progesterone can antagonize or synergize estrogen actions, it had minimal effects on these estrogenic responses. Collectively, the results demonstrate that estrogens have a unique ability to influence specific genes in the uterus not involving classical nuclear ERs.

Differential expression of estrogen receptor beta splice variants in rat brain: identification and characterization of a novel variant missing exon 4

Estrogen receptor beta (ER-beta) mRNA is found in abundance in rat brain. The distribution of ER-beta mRNA in brain differs from that of ER-alpha suggesting they subserve different functions. ER-beta mRNA has been reported to be variably spliced, in contrast to ER-alpha, resulting in numerous isoforms that possess different functional properties. The present study was undertaken to determine whether the isoforms of ER-beta mRNA are differentially distributed in different brain regions. In order to assess the range of transcript forms expressed in various brain regions in the same assay, a micropunch dissection technique was combined with semiquantitative RT-PCR. The relative abundance of each ER-beta isoform (beta1>beta2>beta1delta3>beta2delta3) was similar in all ER-beta positive brain regions with the exception of the hippocampus, which contained low levels of most isoforms and a fifth ER-beta isoform, which we are calling ER-beta1delta4. Based on its sequence, ER-beta1delta4 encodes an ER-beta that is missing exon 4. Initial characterization studies of this showed that it did not bind estrogen, and that, unlike ER-beta1, it localized to the cytoplasm when expressed in cultured cells. The distribution of ER-beta1delta4 was different from that of the other isoforms in that it was expressed at high levels in the hippocampus, where the other isoforms were low, and that it was nearly undetectable in the brain regions that expressed the highest levels of the other ER-beta splice variants. These data suggest that a highly complex pattern of estrogen signaling can occur in a region specific manner in the rat brain.

Subgroup of reproductive functions of progesterone mediated by progesterone receptor-B isoform

Progesterone regulates reproductive function through two intracellular receptors, progesterone receptor-A (PR-A) and progesterone receptor-B (PR-B), that arise from a single gene and function as transcriptional regulators of progesterone-responsive genes. Although in vitro studies show that PR isoforms can display different transcriptional regulatory activities, their physiological significance is unknown. By selective ablation of PR-A in mice, we show that the PR-B isoform modulates a subset of reproductive functions of progesterone by regulation of a subset of progesterone-responsive target genes. Thus, PR-A and PR-B are functionally distinct mediators of progesterone action in vivo and should provide suitable targets for generation of tissue-selective progestins.

The SMRT corepressor is regulated by a MEK-1 kinase pathway: inhibition of corepressor function is associated with SMRT phosphorylation and nuclear export

The SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) corepressor participates in the repression of target gene expression by a variety of transcription factors, including the nuclear hormone receptors, promyelocytic leukemia zinc finger protein, and B-cell leukemia protein 6. The ability of SMRT to associate with these transcription factors and thereby to mediate repression is strongly inhibited by activation of tyrosine kinase signaling pathways, such as that represented by the epidermal growth factor receptor. We report here that SMRT function is potently inhibited by a mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) cascade that operates downstream of this growth factor receptor. Intriguingly, the SMRT protein is a substrate for phosphorylation by protein kinases operating at multiple levels in this MAPKKK pathway, including the MAPKs, MAPK-extracellular signal-regulated kinase 1 (MEK-1), and MEK-1 kinase (MEKK-1). Phosphorylation of SMRT by MEKK-1 and, to a lesser extent, MEK-1 inhibits the ability of SMRT to physically tether to its transcription factor partners. Notably, activation of MEKK-1 or MEK-1 signaling in transfected cells also leads to a redistribution of the SMRT protein from a nuclear compartment to a more perinuclear or cytoplasmic compartment. We suggest that SMRT-mediated repression is regulated by the MAPKKK cascade and that changes both in the affinity of SMRT for its transcription factors and in the subcellular distribution of SMRT contribute to the loss of SMRT function that is observed in response to kinase signal transduction.

Binding of xenobiotics to hepatic estrogen receptor and plasma sex steroid binding protein in the teleost fish, the common carp (Cyprinus carpio)

Competitive receptor binding assays have been suggested as an in vitro screening tool for assessing the activity of alleged estrogenic substances. In this study, we determined the ability of steroidal and nonsteroidal substances to inhibit the binding of [(3)H]17 beta-estradiol (E2) to the hepatic estrogen receptor (ER) and the plasma sex steroid binding protein (SBP) of the teleost fish, the common carp (Cyprinus carpio). The objectives of the study were (1) to characterize ER binding in the liver cytosol of male and female carp, (2) to establish complete [(3)H]E2 displacement curves from carp ER for a range of natural and xenobiotic substances and to compare the ligand data of carp ER with published data from other vertebrate species to reveal possible species differences, and (3) to determine the interaction of natural and xenobiotic substances with the steroid binding site of SBP in carp plasma. The results indicate the presence of a single class of estrogen binding sites with high affinity and limited capacity in liver cytosol of carp. The various test agents showed partly quantitative differences in their binding affinities, with the xenobiotics generally showing limited ability to displace [(3)H]E2 from the hepatic ER or from plasma SBP of carp. However, we found no evidence that a compound is an ER ligand exclusively in one species. The findings of this study indicate that interspecies extrapolation of steroid receptor binding data is possible on a yes/no basis but not on a quantitative basis.

Two medfly promoters that have originated by recent gene duplication drive distinct sex, tissue and temporal expression patterns

Genes encoding predominantly male-specific serum polypeptides (MSSPs) in the medfly Ceratitis capitata are members of a multigene family that are structurally similar to the genes encoding odorant binding proteins of insects. To study the transcriptional regulation of the genes MSSP-alpha2 and MSSP-beta2, overlapping fragments of their promoters, containing the 5' UTRs and 5' flanking regions, were fused to the lacZ reporter gene and introduced into the medfly genome via Minos-mediated germline transformation. Transgenic flies were functionally assayed for beta-galactosidase activity. Despite their extensive sequence similarity, the two gene promoters show distinct expression patterns of the reporter gene, consistent with previously reported evidence for analogous transcriptional activity of the corresponding endogenous genes. The MSSP-alpha2 promoter drives gene expression specifically in the fat body of the adult males, whereas the MSSP-beta2 promoter directs gene expression in the midgut of both sexes. In contrast, similar transformation experiments in Drosophila melanogaster showed that both promoters drive the expression of the reporter gene in the midgut of adult flies of both sexes. Thus, the very same MSSP-alpha2 promoter fragment directs expression in the adult male fat body in Ceratitis, but in the midgut of both sexes in Drosophila. Our data suggest that through the evolution of the MSSP gene family a limited number of mutations that occurred within certain cis-acting elements, in combination with new medfly-specific trans-acting factors, endowed these recently duplicated genes with distinct sex-, tissue-, and temporal-specific expression patterns.

Prothymosin alpha selectively enhances estrogen receptor transcriptional activity by interacting with a repressor of estrogen receptor activity

We find that prothymosin alpha (PTalpha) selectively enhances transcriptional activation by the estrogen receptor (ER) but not transcriptional activity of other nuclear hormone receptors. This selectivity for ER is explained by PTalpha interaction not with ER, but with a 37-kDa protein denoted REA, for repressor of estrogen receptor activity, a protein that we have previously shown binds to ER, blocking coactivator binding to ER. We isolated PTalpha, known to be a chromatin-remodeling protein associated with cell proliferation, using REA as bait in a yeast two-hybrid screen with a cDNA library from MCF-7 human breast cancer cells. PTalpha increases the magnitude of ERalpha transcriptional activity three- to fourfold. It shows lesser enhancement of ERbeta transcriptional activity and has no influence on the transcriptional activity of other nuclear hormone receptors (progesterone receptor, glucocorticoid receptor, thyroid hormone receptor, or retinoic acid receptor) or on the basal activity of ERs. In contrast, the steroid receptor coactivator SRC-1 increases transcriptional activity of all of these receptors. Cotransfection of PTalpha or SRC-1 with increasing amounts of REA, as well as competitive glutathione S-transferase pulldown and mammalian two-hybrid studies, show that REA competes with PTalpha (or SRC-1) for regulation of ER transcriptional activity and suppresses the ER stimulation by PTalpha or SRC-1, indicating that REA can function as an anticoactivator in cells. Our data support a model in which PTalpha, which does not interact with ER, selectively enhances the transcriptional activity of the ER but not that of other nuclear receptors by recruiting the repressive REA protein away from ER, thereby allowing effective coactivation of ER with SRC-1 or other coregulators. The ability of PTalpha to directly interact in vitro and in vivo with REA, a selective coregulator of the ER, thereby enabling the interaction of ER with coactivators, appears to explain its ability to selectively enhance ER transcriptional activity. These findings highlight a new role for PTalpha as a coregulator activity-modulating protein that confers receptor specificity. Proteins such as PTalpha represent an additional regulatory component that defines a novel paradigm enabling receptor-selective enhancement of transcriptional activity by coactivators.

Tamoxifen enhances myoepithelial cell suppression of human breast carcinoma progression in vitro by two different effector mechanisms

Our previous studies have indicated that myoepithelial cells surrounding ductal and acinar epithelium of glandular organs, such as the breast, exert multiple paracrine suppressive effects on incipient and developing cancers that arise from this epithelium. Myoepithelial cells and derived cell lines (HMS 1-6) exert these effects through the secretion of a number of different effector molecules that exert anti-invasive, anti-proliferative, and anti-angiogenic activities. Since previous basic and clinical studies have examined the role of estrogen agonists and antagonists on human breast cancer cells and because issues of hormone replacement therapy (HRT) and tamoxifen chemoprevention are such timely issues in breast cancer, we wondered whether or not hormonal manipulations might affect myoepithelial cells in vitro as far as their paracrine suppressive activities on breast cancer were concerned. The present in vitro study demonstrates that treatment of myoepithelial cells with tamoxifen but not 17beta-estradiol increases both maspin secretion and invasion-blocking ability. Furthermore tamoxifen but not 17beta-estradiol increases inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) production by myoepithelial cells when they are co-cultured with conditioned media from or breast carcinoma cells directly. This increased myoepithelial NO exerts both autocrine and paracrine antiproliferative effects which can be blocked by inhibition of iNOS. 17beta-Estradiol, however, competes with all of these suppressive effects of tamoxifen suggesting that the mechanism of tamoxifen action is estrogen receptor mediated. Myoepithelial cells lack ER-alpha but express ER-beta. Tamoxifen, but not 17beta-estradiol, increases AP-1 CAT but not ERE-CAT activity. Again, 17beta-estradiol competes with the transcription-activating effects of tamoxifen. These experiments collectively suggest that the actions of tamoxifen on the increased secretion of maspin and increased production of NO by myoepithelial cells are mediated through ER-beta and the transcription-activation of an ER-dependent AP-1 response element.

Antiestrogenic effect of opioid peptides in rat uterus

The effects of a single injection or continuous infusion of opioid peptide, [D-Met(2),pro(5)]enkephalinamide (ENK) on the hormone binding and transcriptional properties of estrogen receptors were investigated in estradiol (E(2)) treated rat uterus. The level of estrogen- (ER) and progesterone receptor (PR) proteins, the hormone binding of E(2) receptors and the effects of single injection of ENK with or without naltrexone (NAL) on the E(2)-induced changes in the level of Fos and Jun proteins and the binding of AP-1 proteins to DNA were studied. The receptor proteins levels were determined by Western blots and the binding of AP-1 to DNA by electrophoretic mobility shift assay. Both the ER and PR protein concentrations and the [3H]Estradiol binding to the high affinity nuclear receptors decreased after ENK treatment during the first two days. At 72 h the PR concentration decreased further, while no significant changes were found in the level of ER, however, at this time the former competitive E(2) binding turned into positive cooperativity. The E(2)-induced increase in the level of Fos proteins and the binding of AP-1 proteins to DNA was inhibited by a single injection of ENK. We conclude that the endogenous opioid peptides may interact with E(2) in the gene regulation of rat uterus.

Heat and chemical shock potentiation of glucocorticoid receptor transactivation requires heat shock factor (HSF) activity. Modulation of HSF by vanadate and wortmannin

Heat shock and other forms of stress increase glucocorticoid receptor (GR) activity in cells, suggesting cross-talk between the heat shock and GR signal pathways. An unresolved question concerning this cross-talk is whether heat shock factor (HSF1) activity is required for this response. We addressed this issue by modulating HSF1 activity with compounds acting by distinct mechanisms: sodium vanadate (SV), an inhibitor of protein phosphatases; and wortmannin, an inhibitor of DNA-dependent protein kinase. Using HSF1- and GR-responsive CAT reporters, we demonstrate that SV inhibits both HSF1 activity and the stress potentiation of GR, while having no effect on the hormone-free GR or HSF1. Paradoxically, SV increased hormone-induced GR activity in the absence of stress. In contrast, wortmannin increased HSF1 activity in stressed cells and had no effect on HSF1 in the absence of stress. Using the pMMTV-CAT reporter containing the negative regulatory element 1 site for DNA-dependent protein kinase, wortmannin was found to increase the GR response. However, in cells expressing a minimal promoter lacking negative regulatory element 1 sites, wortmannin had no effect on the GR in the absence of stress but increased the stress potentiation of GR. Our results show that the mechanism by which GR activity is increased in stressed cells requires intrinsic HSF1 activity.

WISP-2 as a novel estrogen-responsive gene in human breast cancer cells

In order to search for novel estrogen-responsive genes, we performed serial analysis of gene expression (SAGE) for estrogen-treated MCF-7 human breast cancer cells. SAGE analysis of 31,000 and 30,856 tags from non-treated and 17 beta-estradiol (E2)-treated cells for 24 h, respectively, facilitated the identification of 15,037 different transcripts. Comparison of these two SAGE libraries indicated a remarkable similarity in expression profiles. Among the identified transcripts, four genes were found to be markedly increased for E2-treated cells compared with control cells. Three of the transcripts were cathepsin D, pS2 and high mobility group 1 protein, which have been described as estrogen-inducible genes. The fourth gene was WISP-2 (Wnt-1 inducible signaling pathway protein 2) which has recently been reported as an up-regulated gene in the mammary epithelial cell line C57 MG transformed by the Wnt-1 oncogene. The increase in WISP-2 mRNA was completely prevented by co-incubation with a pure anti-estrogen ICI 182,780, but not by coincubation with cycloheximide, indicating that WISP-2 is directly regulated by the estrogen receptor. The WISP-2 gene was also induced by treating with environmental estrogens, such as bisphenol-A or nonylphenol. This study represents the first comprehensive gene expression analysis of estrogen-treated human breast cancer cells.

Cytochrome c release and apoptosis induced by mitochondrial targeting of nuclear orphan receptor TR3

TR3, an immediate-early response gene and an orphan member of the steroid-thyroid hormone-retinoid receptor superfamily of transcription factors, regulates apoptosis through an unknown mechanism. In response to apoptotic stimuli, TR3 translocates from the nucleus to mitochondria to induce cytochrome c release and apoptosis. Mitochondrial targeting of TR3, but not its DNA binding and transactivation, is essential for its proapoptotic effect. Our results reveal a mechanism by which a nuclear transcription factor translocates to mitochondria to initiate apoptosis.

Role of MAP kinase in the enhanced cell proliferation of long term estrogen deprived human breast cancer cells

Women with estrogen receptor (ER) positive breast cancers frequently respond initially to inhibition of estrogen action but later relapse with re-growth of tumor. Previously, we have utilized MCF-7 human breast cancer cells deprived of estradiol long term (LTED cells) as the model system to study the regrowth phenomenon and have demonstrated that these cells exhibited increased cell proliferation rate and increased ER functionality during the adaptive processes. In this report, we examined the hypothesis that the mitogen-activated protein kinase (MAP kinase) signal was involved. We found that activated MAP kinase was elevated in LTED cells and that the MAP kinase specific inhibitor PD98059 was able to inhibit the elevated MAP kinase and [3H]thymidine uptake in LTED cells, suggesting mediation of DNA synthesis and proliferation by the MAP kinase pathway. Other MAP kinase upstream inhibitors, including genestein, RG13022, and mevastatin were also able to inhibit the [3H]thymidine uptake in LTED cells. Interestingly, the antiestrogen, ICI 182,780 was able to block the activated MAP kinase in LTED cells. Treatment with PD98059 did not block elevated basal ERE-CAT activity while at the same time inhibiting [3H]thymidine uptake in LTED cells. Furthermore, treatment with PD98059 partially blocked the E2-stimulated ERE-CAT activity and [3H]thymidine uptake in both LTED and in wild type cells, indicating that both MAP kinase-dependent and MAP kinase-independent pathways are involved in the transactivation function of ER. Taken together, our data suggest that the MAP kinase pathway is, in part, involved in the adaptive process which results in enhanced DNA synthesis and cell proliferation in the absence of exogenous estrogen in LTED cells.

Crucial role of the H11-H12 loop in stabilizing the active conformation of the human mineralocorticoid receptor

The crystal structures of ligand-free and agonist-associated ligand-binding domain (LBD) of nuclear receptors (NRs) reveal that the amphipathic helix H12 is folded back toward the LBD core in the agonist-associated conformation, allowing the binding of coactivators. We used alanine scanning mutagenesis to explore the role of the residues of the loop connecting H11 and H12 in the activation of the human mineralocorticoid receptor (hMR), a member of the NRs family. H950A retained the ligand binding and transcriptional activities of the wild-type receptor and interacted with coactivators. In contrast F956A had no receptor functions. Aldosterone bound to the mutant hMRs (L952A, K953A, V954A, E955A, P957A) with nearly the same affinity as to the wild-type receptor and caused a receptor conformational change in these mutant hMRs as it does for the wild-type receptor. But the aldosterone-induced transcriptional activity of the mutant hMRs was lower (L952A, E955A, P957A) than that of the wild-type receptor or completely abolished (K953A, V954A) and their interaction with coactivators was impaired (E955A) or suppressed (L952A, K953A, V954A, P957A). In the light of a hMR-LBD model based on the structure of the progesterone-associated receptor-LBD, we propose that the integrity of the H11-H12 loop is crucial for folding the receptor into a ligand-binding competent state and for establishing the network of contacts that stabilize the active receptor conformation.