The steroid/nuclear receptors: from three-dimensional structure to complex function

Development and characterization of a human cell line-based transactivation assay to assess thyroid EDCs

Thyroid hormones (THs) are one of the most important hormones, playing key roles in the regulation of various physiological functions. Although THs have important function in human, in vitro test methods based on human cells are currently insufficient to effectively screen and test TH-related endocrine disrupting chemicals (EDCs). We established a TH agonist TA assay using the adenocarcinomic human alveolar basal epithelial cell line A549 to test and screen potential TH agonists. To establish the TH agonist TA assay, a TRE-secNluc-IRES-EGFP reporter cassette was constructed and transfected into the A549 cell line using a retrovirus. We evaluated the TH agonistic properties of several chemicals which were tested by existing thyroid agonists testing method (OECD GD 207). Comparing the results of the TH agonist TA assay with the OECD GD 207, T3, T4, tiratricol, and tetrac (natural TH and 3,3',5,5'-tetraiodothyroacetic acid derivatives), which are TH agonists according to the OECD GD 207, also tested positive in the TH agonist TA assay using the A549 cell line. These results suggested that the TH agonist TA assay developed in this study using a human cell line can provide the information, such as accuracy and specificity to TH agonistic properties of chemicals.

The molecular basis of conformational instability of the ecdysone receptor DNA binding domain studied by in silico and in vitro experiments

The heterodimer of the ecdysone receptor (EcR) and ultraspiracle (Usp), members of the nuclear receptors superfamily, regulates gene expression associated with molting and metamorphosis in insects. The DNA binding domains (DBDs) of the Usp and EcR play an important role in their DNA-dependent heterodimerization. Analysis of the crystal structure of the UspDBD/EcRDBD heterocomplex from Drosophila melanogaster on the hsp27 gene response element, suggested an appreciable similarity between both DBDs. However, the chemical denaturation experiments showed a categorically lower stability for the EcRDBD in contrast to the UspDBD. The aim of our study was an elucidation of the molecular basis of this intriguing instability. Toward this end, we mapped the EcRDBD amino acid sequence positions which have an impact on the stability of the EcRDBD. The computational protein design and in vitro analyses of the EcRDBD mutants indicate that non-conserved residues within the α-helix 2, forming the EcRDBD hydrophobic core, represent a specific structural element that contributes to instability. In particular, the L58 appears to be a key residue which differentiates the hydrophobic cores of UspDBD and EcRDBD and is the main reason for the low stability of the EcRDBD. Our results might serve as a benchmark for further studies of the intricate nature of the EcR molecule.

Nuclear hormone receptors in podocytes

Nuclear receptors are a family of ligand-activated, DNA sequence-specific transcription factors that regulate various aspects of animal development, cell proliferation, differentiation, and homeostasis. The physiological roles of nuclear receptors and their ligands have been intensively studied in cancer and metabolic syndrome. However, their role in kidney diseases is still evolving, despite their ligands being used clinically to treat renal diseases for decades. This review will discuss the progress of our understanding of the role of nuclear receptors and their ligands in kidney physiology with emphasis on their roles in treating glomerular disorders and podocyte injury repair responses.

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

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

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.

Maternal exposure to estradiol and endocrine disrupting compounds alters the sensitivity of sea urchin embryos and the expression of an orphan steroid receptor

Endocrine disrupting compounds (EDCs) are known to affect reproduction and development in marine invertebrates. In previous work, we have shown that developing sea urchin embryos were sensitive to estradiol and estrogenic EDCs at environmentally relevant concentrations in a tamoxifen-sensitive manner (Roepke et al. 2005. Aquat Toxicol 71:155-173). In this study, we report the effects of maternal exposure to EDCs on embryo sensitivity and regulation of an orphan steroid receptor in sea urchin eggs. Maternal exposures were conducted by injecting female Strongylocentrotus purpuratus sea urchins initiating oogenesis with two concentrations of estradiol, octylphenol, tributyltin and o, p-DDD for 8 weeks with an induced spawning before and after the injection cycle. Developing embryos were less sensitive to estradiol following maternal exposure to estradiol, octylphenol and DDD. The steroidogenesis inhibitor, spironolactone, and the aromatase inhibitor, formestane, affected normal sea urchin development with EC50 values of 18 and 2 microM, respectively. Binding of estradiol was demonstrated in homogenates supernatants of sea urchin embryos by filtration centrifugation and column chromatography, but saturation was not reached until 4-6 hr and was highly variable. Analysis of eggs from pre- and post-injection spawns using real-time Q-PCR for the mRNA of an orphan steroid receptor, SpSHR2, shows that receptor mRNA increased in eggs with estradiol, octylphenol and tributyltin but decreased with DDD. RIA showed that estradiol may be present during gastrulation. In summary, maternal exposure to estradiol and EDCs alters embryo sensitivity and regulates the expression of an orphan steroid receptor in the egg.

Targeting estrogen responsive elements (EREs): design of potent transactivators for ERE-containing genes

The estrogen hormone (E2) plays an important role in the physiology and pathophysiology of target tissues. The effects of E2 are conveyed by the estrogen receptors (ER) alpha and beta. The E2-ER complex mediates an array of genomic and non-genomic events that orchestrate the expression of a number of genes involved in the regulation of cell proliferation and differentiation. The interaction of with the regulatory DNA sequence, estrogen responsive element (ERE), of each responsive gene constitutes a critical genomic signaling pathway. However, the relative importance of ERE-dependent E2-ER signaling in cell proliferation remains to be elucidated. To address this issue, we engineered ERE-binding activators (EBAs) that specifically and potently regulate ERE-containing genes. The modular nature of ER allowed us to initially design a monomeric ERE-binding module by genetically joining two DNA-binding domains with the hinge domain. Integration of strong activation domains from other transcription factors into this module generated constitutively active EBAs. These transactivators robustly induced the expression of only ERE-containing promoter constructs in transfected cells independent of ligand, dimerization, ER-subtype and -status. Moreover, EBAs altered cell cycle progression in breast cancer cell lines in a manner similar to E2-ER. These results demonstrate the importance of ERE-containing genes in the regulation of cell proliferation. These novel ERE-binding transregulators could also be a basis for the targeted regulation of ERE-containing genes, the identification of estrogen responsive gene networks, and the development of alternative/complementary therapeutic approaches for estrogen target tissue cancers.

Structural basis of androgen receptor binding to selective androgen response elements

Steroid receptors bind as dimers to a degenerate set of response elements containing inverted repeats of a hexameric half-site separated by 3 bp of spacer (IR3). Naturally occurring selective androgen response elements have recently been identified that resemble direct repeats of the hexameric half-site (ADR3). The 3D crystal structure of the androgen receptor (AR) DNA-binding domain bound to a selective ADR3 reveals an unexpected head-to-head arrangement of the two protomers rather than the expected head-to-tail arrangement seen in nuclear receptors bound to response elements of similar geometry. Compared with the glucocorticoid receptor, the DNA-binding domain dimer interface of the AR has additional interactions that stabilize the AR dimer and increase the affinity for nonconsensus response elements. This increased interfacial stability compared with the other steroid receptors may account for the selective binding of AR to ADR3 response elements.

Vitamin D Receptor–DNA Interactions

The vitamin D receptor (VDR) is a member of the steroid and nuclear hormone receptor superfamily of eukaryotic transcription factors and binds target DNA, or response elements, as a homodimer or heterodimer with the 9-cis retinoid X receptor (RXR). In this chapter, we survey the current understanding of VDR-DNA interactions, emphasizing recent structural insights. We highlight the stereochemical interactions that dictate DNA binding and hexameric half-site sequence affinity as well as the protein-protein interactions that account for preferential binding to a direct repeat of half-sites with three base pairs of spacer DNA (DR3). In addition, we review alternative response element arrangements other than those with DR3. Finally, the chapter discusses the VDR DNA binding domain (DBD) and suggests that it violates classical canons because it does not heterodimerize with the RXR DBD. This unique behavior of VDR is considered in light of recent results demonstrating the formation of VDR DBD-DNA and DR3 DBD-DNA complexes with RXR using a mutant VDR protomer.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Novel mechanism of steroid action in skin through glucocorticoid receptor monomers

Glucocorticoids (GCs), important regulators of epidermal growth, differentiation, and homeostasis, are used extensively in the treatment of skin diseases. Using keratin gene expression as a paradigm of epidermal physiology and pathology, we have developed a model system to study the molecular mechanism of GCs action in skin. Here we describe a novel mechanism of suppression of transcription by the glucocorticoid receptor (GR) that represents an example of customizing a device for transcriptional regulation to target a specific group of genes within the target tissue, in our case, epidermis. We have shown that GCs repress the expression of the basal-cell-specific keratins K5 and K14 and disease-associated keratins K6, K16, and K17 but not the differentiation-specific keratins K3 and K10 or the simple epithelium-specific keratins K8, K18, and K19. We have identified the negative recognition elements (nGREs) in all five regulated keratin gene promoters. Detailed footprinting revealed that the function of nGREs is to instruct the GR to bind as four monomers. Furthermore, using cotransfection and antisense technology we have found that, unlike SRC-1 and GRIP-1, which are not involved in the GR complex that suppresses keratin genes, histone acetyltransferase and CBP are. In addition, we have found that GR, independently from GREs, blocks the induction of keratin gene expression by AP1. We conclude that GR suppresses keratin gene expression through two independent mechanisms: directly, through interactions of keratin nGREs with four GR monomers, as well as indirectly, by blocking the AP1 induction of keratin gene expression.

Activation of the human estrogen receptor by the antiestrogens ICI 182,780 and tamoxifen in yeast genetic systems: implications for their mechanism of action

The antiestrogens tamoxifen and ICI 182,780 have been portrayed as competitive antagonists of the estrogen binding site of the alpha-form of the human estrogen receptor (ER). However, in functional studies, neither compound has consistently been able to block estradiol-induced transcription. In this report, three yeast genetic systems were used to investigate the effects of tamoxifen and ICI 182,780 on ER dimerization, transcriptional activation, and the interaction of the receptor with a coactivator, RIP140. Tamoxifen and ICI 182,780 were able to induce ER dimerization and ER-dependent transcription, albeit at up to 15,000-fold higher concentrations than that of estradiol. In the presence of RIP140, the transcription response maximum was increased up to 30-fold for estradiol and both antiestrogens. Whole yeast cell [(3)H]estradiol binding studies demonstrated that tamoxifen could displace the estradiol from the ER, whereas ICI 182,780 treatment resulted in a 4-fold increase in [(3)H]estradiol binding to the receptor. No antagonism of estradiol was observed with tamoxifen or ICI 182,780 in any of the yeast models employed. We have concluded that the antiestrogen activity of compounds like tamoxifen and ICI 182,780 is not caused by their ability to competitively antagonize estradiol binding to the hormone binding site, but possibly by their ability to induce ER-dependent transcription, which in mammalian systems would result in receptor down-regulation. Compounds such as tamoxifen act through the hormone binding site, whereas ICI 182,780 may cause receptor activation through an allosteric binding site.

Activation of the human estrogen receptor by the antiestrogens ICI 182,780 and tamoxifen in yeast genetic systems: Implications for their mechanism of action

The antiestrogens tamoxifen and ICI 182,780 have been portrayed as competitive antagonists of the estrogen binding site of the alpha-form of the human estrogen receptor (ER). However, in functional studies, neither compound has consistently been able to block estradiol-induced transcription. In this report, three yeast genetic systems were used to investigate the effects of tamoxifen and ICI 182,780 on ER dimerization, transcriptional activation, and the interaction of the receptor with a coactivator, RIP140. Tamoxifen and ICI 182,780 were able to induce ER dimerization and ER-dependent transcription, albeit at up to 15,000-fold higher concentrations than that of estradiol. In the presence of RIP140, the transcription response maximum was increased up to 30-fold for estradiol and both antiestrogens. Whole yeast cell [(3)H]estradiol binding studies demonstrated that tamoxifen could displace the estradiol from the ER, whereas ICI 182,780 treatment resulted in a 4-fold increase in [(3)H]estradiol binding to the receptor. No antagonism of estradiol was observed with tamoxifen or ICI 182,780 in any of the yeast models employed. We have concluded that the antiestrogen activity of compounds like tamoxifen and ICI 182,780 is not caused by their ability to competitively antagonize estradiol binding to the hormone binding site, but possibly by their ability to induce ER-dependent transcription, which in mammalian systems would result in receptor down-regulation. Compounds such as tamoxifen act through the hormone binding site, whereas ICI 182,780 may cause receptor activation through an allosteric binding site.

Expression of nuclear hormone receptors in Escherichia coli

Nuclear hormone receptors are ligand-activated transcription factors. The study of nuclear hormone receptor--ligand interactions requires expression of soluble receptor, but the production of functional soluble receptors in E. coli has generally proved difficult. Recent studies on coexpression of nuclear hormone receptors have yielded active soluble proteins, which were not readily generated when expressed individually.

DHR3, an ecdysone-inducible early-late gene encoding a Drosophila nuclear receptor, is required for embryogenesis

Response to the steroid hormone ecdysone in Drosophila is controlled by genetic regulatory hierarchies that include eight members of the nuclear receptor protein family. The DHR3 gene, located within the 46F early-late ecdysone-inducible chromosome puff, encodes an orphan nuclear receptor that recently has been shown to exert both positive and negative regulatory effects in the ecdysone-induced genetic hierarchies at metamorphosis. We used a reverse genetics approach to identify 11 DHR3 mutants from a pool of lethal mutations in the 46F region on the second chromosome. Two DHR3 mutations result in amino acid substitutions within the conserved DNA binding domain. Analysis of DHR3 mutants reveals that DHR3 function is required to complete embryogenesis. All DHR3 alleles examined result in nervous system defects in the embryo.

YY1 regulates vitamin D receptor/retinoid X receptor mediated transactivation of the vitamin D responsive osteocalcin gene

The responsiveness of genes to steroid hormones is principally mediated by functional interactions between DNA-bound hormone receptors and components of the transcriptional initiation machinery, including TATA-binding protein, TFIIB, or other RNA polymerase II associated factors. This interaction can be physiologically modulated by promoter context-specific transcription factors to facilitate optimal responsiveness of gene expression to hormone stimulation. One postulated regulatory mechanism involves the functional antagonism between hormone receptors and nonreceptor transcription factors interacting at the same hormone response element. Here we demonstrate that the multifunctional regulator YY1 represses 1,25-dihydroxyvitamin D3 (vitamin D)-induced transactivation of the bone tissue-specific osteocalcin gene. We identify YY1 recognition sequences within the vitamin D response element (VDRE) of the osteocalcin gene that are critical for YY1-dependent repression of vitamin D-enhanced promoter activity. We show that YY1 and vitamin D receptor (VDR)/retinoid X receptor heterodimers compete for binding at the osteocalcin VDRE. In addition, we find that YY1 interacts directly with TFIIB, and that one of the two tandemly repeated polypeptide regions of TFIIB spanning the basic domain is responsible for this interaction. TFIIB and VDR can also interact directly, and these factors synergize to mediate transactivation. Our results suggest that YY1 regulates vitamin D enhancement of osteocalcin gene transcription in vivo by interfering with the interactions of the VDR with both the VDRE and TFIIB.