Mechanisms for synergistic activation of thyroid hormone receptor and retinoid X receptor on different response elements

6PPD, Not 6PPD-Quinone, Induced Serious Zebrafish Eye Damage by Disrupting the Thyroid Signaling Pathway

N-(1,3-Dimethylbutyl)-N'-phenyl-1,4-phenylenediamine (6PPD) and its oxidation product 6PPD-quinone (6PPDQ) showed different acute toxicities and bioaccumulation potencies in fish. In this study, we compared the thyroid disrupting effects of 6PPD and 6PPDQ through in vitro, in silico, and in vivo assays. Interestingly, although 6PPD and 6PPDQ showed similar docking affinities with thyroid hormone receptor (TR) isoforms and GH3 cell inhibition effects, the thyroid signaling pathway, eye development, phototactic behaviors, and cell density in the retinal layer in the larval zebrafish were significantly affected only following 6PPD exposure. Further investigation demonstrates that 6PPD can act as a TR antagonist to reduce the opsin protein abundance and inhibit the cone photoreceptor cell proliferation, which finally alters the retinal layer structure and causes microphthalmus in zebrafish. Especially, under environmental relevant concentration exposure, 6PPD induced alterations of trβ, opn1lw1, opn1mw1, rpe65a, nr2e3 gene expressions although no significant eye histopathological change was observed. This study illustrates for the first time the more serious visual system impairment of 6PPD compared to 6PPDQ, with thyroid signaling disruption being a contributing factor, while other important toxic targets still require further research.

Integrated thyroid endocrine disrupting effect on zebrafish (Danio rario) larvae via simultaneously repressing type II iodothyronine deiodinase and activating thyroid receptor-mediated signaling following waterborne exposure to trace azocyclotin

As a widely used organotin acaricide nowadays, azocyclotin (ACT) could induce thyroidal endocrine disruption in fishes and amphibians, but its dominant disrupting mode remains unknown. In this study, zebrafish were firstly exposed to ACT (0.18-0.36 ng/mL) from 2 hpf (hours post fertilization) to 30 dpf (days post fertilization), and a series of developmental toxicological endpoints and thyroid hormones were measured. Result showed that no developmental toxicity to zebrafish was found in 0.18 and 0.24 ng/mL groups except decreased body weight (30 dpf, 0.24 ng/mL). However, exposed to 0.36 ng/mL ACT led to reductions in heartbeat (48 hpf), hatching rate (72 hpf) and bodyweight (30 dpf). General tendencies of decreases in free T3 but increases in free T4 and reductions in ratio of free T3/T4 were also found, inferring that type II deiodinase (Dio2) was repressed. This inference was confirmed by Western analysis that Dio2 expression reduced by 42.7% after 0.36 ng/mL ACT treatment. Moreover, RNA-Seq analysis implied that exposed to 0.36 ng/mL ACT altered the genome-wide gene expression profiles of zebrafish. Totally 5660 genes (involving 3154 down-regulated and 2596 up-regulated genes) were differentially expressed, and 13 deferentially expressed genes including down-regulated dio2 were significantly enriched in thyroid hormone signaling pathway. Subsequently, an in vitro thyroid receptor-reporter gene assay using GH3 cells was performed to further explore the potential disrupting mechanism. Result showed that luciferase activity slightly increased after exposure to ACT alone or ACT combined with low level T3, but was suppressed when combined with high level T3. It indicted there probably existed a competitive relationship in some extent between ACT and T3 in vivo. Overall, the present study provided preliminary evidences that long-term exposure to trace ACT repressed Dio2 expression, declined T3 and then activated thyroid receptor-mediated signaling, thereby leading to integrated thyroid endocrine disruption in zebrafish larvae.

Retinoic X receptor subtypes exert differential effects on the regulation of Trh transcription

How Retinoid X receptors (RXR) and thyroid hormone receptors (TR) interact on negative TREs and whether RXR subtype specificity is determinant in such regulations is unknown. In a set of functional studies, we analyzed RXR subtype effects in T3-dependent repression of hypothalamic thyrotropin-releasing hormone (Trh). Two-hybrid screening of a hypothalamic paraventricular nucleus cDNA bank revealed specific, T3-dependent interaction of TRs with RXRβ. In vivo chromatin immuno-precipitation showed recruitment of RXRs to the TRE-site 4 region of the Trh promoter in the absence of T3. In vivo overexpression of RXRα in the mouse hypothalamus heightened T3-independent Trh transcription, whereas RXRβ overexpression abrogated this activity. Loss of function of RXRα and β by shRNAs induced inverse regulations. Thus, RXRα and RXRβ display specific roles in modulating T3-dependent regulation of Trh. These results provide insight into the actions of these different TR heterodimerization partners within the context of a negatively regulated gene.

Oxidative stress and heart failure in altered thyroid States

Increased or reduced action of thyroid hormone on certain molecular pathways in the heart and vasculature causes relevant cardiovascular derangements. It is well established that hyperthyroidism induces a hyperdynamic cardiovascular state, which is associated with a faster heart rate, enhanced left ventricular systolic and diastolic function whereas hypothyroidism is characterized by the opposite changes. Hyperthyroidism and hypothyroidism represent opposite clinical conditions, albeit not mirror images. Recent experimental and clinical studies have suggested the involvement of ROS tissue damage under altered thyroid status. Altered-thyroid state-linked changes in heart modify their susceptibility to oxidants and the extent of the oxidative damage they suffer following oxidative challenge. Chronic increase in the cellular levels of ROS can lead to a catastrophic cycle of DNA damage, mitochondrial dysfunction, further ROS generation and cellular injury. Thus, these cellular events might play an important role in the development and progression of myocardial remodeling and heart failure in altered thyroid states (hypo- and hyper-thyroidism). The present review aims at elucidating the various signaling pathways mediated via ROS and their modulation under altered thyroid state and the possibility of antioxidant therapy.

Transcription factor effector domains

The last decade has seen an incredible breakthrough in technologies that allow histones, transcription factors (TFs), and RNA polymerases to be precisely mapped throughout the genome. From this research, it is clear that there is a complex interaction between the chromatin landscape and the general transcriptional machinery and that the dynamic control of this interface is central to gene regulation. However, the chromatin remodeling enzymes and general TFs cannot, on their own, recognize and stably bind to promoter or enhancer regions. Rather, they are recruited to cis regulatory regions through interaction with site-specific DNA binding TFs and/or proteins that recognize epigenetic marks such as methylated cytosines or specifically modified amino acids in histones. These "recruitment" factors are modular in structure, reflecting their ability to interact with the genome via one region of the protein and to simultaneously bind to other regulatory proteins via "effector" domains. In this chapter, we provide examples of common effector domains that can function in transcriptional regulation via their ability to (a) interact with the basal transcriptional machinery and general co-activators, (b) interact with other TFs to allow cooperative binding, and (c) directly or indirectly recruit histone and chromatin modifying enzymes.

Hyperthyroid heart disease

The heart is an organ sensitive to the action of thyroid hormone, and measurable changes in cardiac performance are detected with small variations in thyroid hormone serum concentrations. Most patients with hyperthyroidism experience cardiovascular manifestations, and the most serious complications of hyperthyroidism occur as a result of cardiac involvement. Recent studies provide important insights into the molecular pathways that mediate the action of thyroid hormone on the heart and allow a better understanding of the mechanisms that underlie the hemodynamic and clinical manifestations of hyperthyroidism. Several cardiovascular conditions and drugs can interfere with thyroid hormone levels and may pose a difficulty in interpretation of laboratory data in patients with suspected thyroid heart disease. The focus of this report is a review of the current knowledge of thyroid hormone action on the heart and the clinical and hemodynamic laboratory findings as well as therapeutic management of patients with hyperthyroid heart disease.

Cloning and expression pattern of peroxisome proliferator-activated receptors, estrogen receptor alpha and retinoid X receptor alpha in the thicklip grey mullet Chelon labrosus

Aquatic organisms are exposed to diverse xenobiotics that cause peroxisome proliferation and/or endocrine disruption, both modulated in vertebrates by transcription factors of the nuclear receptor (NR) superfamily. Peroxisome proliferators are agonists of peroxisome proliferator-activated receptors (PPARs) that heterodimerize with the retinoid X receptor (RXR). Many xenoestrogens activate the estrogen receptor (ER). Here, 1090 bp of PPARalpha, 1255 bp of PPARgamma, 278 bp of RXRalpha, and 578 bp of ERalpha of thicklip grey mullet Chelon labrosus were cloned. Sequences were highly conserved, although relevant changes with respect to mammalian homologs were identified in PPARgamma and ERalpha. Semi-quantitative RT-PCR was used to determine if these NRs were expressed in different tissues of male, female and undifferentiated mullets captured in January and June. Expression of PPARs was highest in liver and lowest in muscle. RXRalpha expression was homogeneous excepting a low expression in male and female gill in January and brain and heart of undifferentiated fish in January and June. ERalpha expression predominated in liver and female gonad in June. The expression level of PPARs and ERalpha was significantly higher in liver in January than in gills in January or June. The present results show tissue-dependent modulation of expression of NRs in mullets.

Thyroid hormone response element organization dictates the composition of active receptor

Thyroid hormone (triiodothyronine, T(3)) is known to activate transcription by binding heterodimers of thyroid hormone receptors (TRs) and retinoid X receptors (RXRs). RXR-TRs bind to T(3) response elements (TREs) composed of direct repeats of the sequence AGGTCA spaced by four nucleotides (DR-4). In other TREs, however, the half-sites can be arranged as inverted palindromes and palindromes (Pal). Here we show that TR homodimers and monomers activate transcription from representative TREs with alternate half-site placements. TR beta activates transcription more efficiently than TR alpha at an inverted palindrome (F2), and this correlates with preferential TR beta homodimer formation at F2 in vitro. Furthermore, reconstruction of TR transcription complexes in yeast indicates that TR beta homodimers are active at F2, whereas RXR-TRs are active at DR-4 and Pal. Finally, analysis of TR beta mutations that block homodimer and/or heterodimer formation reveal TRE-selective requirements for these surfaces in mammalian cells, which suggest that TR beta homodimers are active at F2, RXR-TRs at DR-4, and TR monomers at Pal. TR beta requires higher levels of hormone for activation at F2 than other TREs, and this differential effect is abolished by a dimer surface mutation suggesting that it is related to composition of the TR.TRE complex. We propose that interactions of particular TR oligomers with different elements play unappreciated roles in TRE-selective actions of liganded TRs in vivo.

Schistosoma mansonimale–female interactions

Schistosome parasites are muticellular eucaryotic organisms with a complex life cycle that involves mammalian and snail hosts. Unlike other trematode parasites, schistosomes (along with the Didymozoidae) have evolved separate sexes or dioecy. Sex is determined by a chromosomal mechanism. The dioecious state created an opportunity for the sexes to play a role in schistosome evolution that has resulted in an interesting interplay between the sexes. The classical observation, made more than 50 years ago, is that female schistosomes do not develop unless a male worm is present. Studies up through the 1990s focused on dissecting the role of the sexes in mate attraction, mate choice, mating behavior, female growth, female reproductive development, egg production, and other sex-evolved functions. In the mid-1980s, studies began to address the molecular events of male–female interactions. The classic morphological observation that female schistosomes do not complete reproductive development unless a male worm is present has been redefined in molecular terms. The male by an unknown mechanism transduces a signal that regulates female gene expression in a stage-, tissue-, and temporal-specific manner. A number of female-specific genes have been identified, along with signaling pathways and nuclear receptors, that play a role in female reproductive development. In addition, a number of host factors such as cytokines have also been demonstrated to affect adult male and female development and egg production. This review focuses on the biological interactions of the male and female schistosome and the role of parasite and host factors in these interactions as they contribute to the life cycle of Schistosoma mansoni.

Perspectives on mammalian cardiovascular aging: humans to molecules

Age-related changes in cardiovascular function and structure in healthy adult volunteer community dwelling subjects (from 20 to 85 years) is remarkable for changes in pump function [impaired left ventricular (LV) ejection reserve capacity manifest by a reduced ejection fraction and accompanied by diminished cardioacceleration, LV dilation at end diastole and an altered diastolic filling pattern] and increased vascular afterloading. There is also evidence for a reduction in the number of cardiac myocytes with advancing age. Subcellular changes with aging (best understood in rodents) include certain regulatory factors of excitation-contraction-relaxation coupling (i.e. calcium handling), modulation by adrenergic receptor (AR) stimulation, and changes in the generation and sensitivity to the damaging effects of ROS. Coordinated changes in gene expression and/or protein function with aging result in a prolonged action potential (AP), Ca(i) transient, and contraction. L-type Ca(2+) current (I(Ca)) inactivates more slowly, and outwardly-directed K(+) currents are reduced, and likely contribute to AP-prolongation. The rate of Ca(2+) sequestration by the sarcoplasmic reticulum (SR) decreases in the senescent myocardium, in part underlying the prolonged Ca(i) transient. An age-associated reduction in transcription of the SERCA2 gene, coding for the SR Ca(2+) pump, accounts in part for a decrease in the SR pump site density. The contractile response to both beta(1)-AR and beta(2)-AR stimulation diminishes with aging due to decreased adrenergic augmentation of I(Ca), and thus the Ca(i) transient, in senescent vs. young hearts. The age-associated reduction in the postsynaptic response of myocardial cells to beta(1)-AR stimulation appears to be due to multiple changes in molecular and biochemical receptor coupling and post-receptor mechanisms. An increased basal production of ROS is paralleled by increased ROS-sensitivity, markers of chronic ROS damage and mitochondrial functional decline. Overall, these changes lead to a diminished (but not necessarily exhausted) capacity of the heart to adapt to physiological or pathological stress with advancing age.

Cardiovascular ageing in health sets the stage for cardiovascular disease

The incidence and prevalence of coronary disease, hypertension, heart failure and stroke increase exponentially with advancing age. While epidemiologic studies have discovered that aspects of lifestyle and genetics are risk factors for these diseases, age, per se, confers the major risk. Thus, it is reasonable to hypothesise that specific pathophysiological mechanisms that underlie these diseases become superimposed on cardiac and vascular substrates that have been modified by an 'ageing process', and that the latter modulates disease occurrence and severity. In order to unravel this age-disease interaction, the nature of the ageing process in the heart and vasculature requires elucidation. Some aspects of the current understanding of ageing of the heart and blood vessels in the absence of apparent disease are the focus of this review.

BRCA1 mediates ligand-independent transcriptional repression of the estrogen receptor

Mutational inactivation of BRCA1 confers a cumulative lifetime risk of breast and ovarian cancers. However, the underlying basis for the tissue-restricted tumor-suppressive properties of BRCA1 remains poorly defined. Here we show that BRCA1 mediates ligand-independent transcriptional repression of the estrogen receptor alpha (ERalpha), a principal determinant of the growth, differentiation, and normal functional status of breasts and ovaries. In Brca1-null mouse embryo fibroblasts and BRCA1-deficient human ovarian cancer cells, ERalpha exhibited ligand-independent transcriptional activity that was not observed in Brca1-proficient cells. Ectopic expression in Brca1-deficient cells of wild-type BRCA1, but not clinically validated BRCA1 missense mutants, restored ligand-independent repression of ERalpha in a manner dependent upon apparent histone deacetylase activity. In estrogen-dependent human breast cancer cells, chromatin immunoprecipitation analysis revealed the association of BRCA1 with ERalpha at endogenous estrogen-response elements before, but not after estrogen stimulation. Collectively, these results reveal BRCA1 to be a ligand-reversible barrier to transcriptional activation by unliganded promoter-bound ERalpha and suggest a possible mechanism by which functional inactivation of BRCA1 could promote tumorigenesis through inappropriate hormonal regulation of mammary and ovarian epithelial cell proliferation.

Thyroid hormone stimulates acetyl-coA carboxylase-alpha transcription in hepatocytes by modulating the composition of nuclear receptor complexes bound to a thyroid hormone response element

Triiodothyronine (T3) stimulates a 7-fold increase in transcription of the acetyl-CoA carboxylase-alpha (ACCalpha) gene in chick embryo hepatocytes. Here, we characterized an ACCalpha T3 response element (ACCalpha-T3RE) with unique functional and protein binding properties. ACCalpha-T3RE activated transcription both in the absence and presence of T3, with a greater activation observed in the presence of T3. In nuclear extracts from hepatocytes incubated in the absence of T3, ACCalpha-T3RE bound protein complexes (complexes 1 and 2) containing the liver X receptor (LXR) and the retinoid X receptor (RXR). In nuclear extracts from hepatocytes incubated in the presence of T3 for 24 h, ACCalpha-T3RE bound a different set of complexes. One complex contained LXR and RXR (complex 3) and another contained the nuclear T3 receptor (TR) and RXR (complex 4). Mutations of ACCalpha-T3RE that inhibited the binding of complexes 1 and 2 decreased transcriptional activation in the absence of T3, and mutations of ACCalpha-T3RE that inhibited the binding of complexes 3 and 4 decreased transcriptional activation in the presence of T3. The stimulation of ACCalpha transcription caused by T3 was closely associated with changes in the binding of complexes 1-4 to ACCalpha-T3RE. These data suggest that T3 regulates ACCalpha transcription by a novel mechanism involving changes in the composition of nuclear receptor complexes bound to ACCalpha-T3RE. We propose that complexes containing LXR/RXR ensure a basal level of ACCalpha expression for the synthesis of structural lipids in cell membranes and that complexes containing LXR/RXR and TR/RXR mediate the stimulation of ACCalpha expression caused by T3.

Thyroid hormone regulation of apoptosis induced by retinoic acid in promyeloleukemic HL-60 cells: studies with retinoic acid receptor-specific and retinoid x receptor-specific ligands

3,5,3'-Triiodo-L-thyronine (T3) potentiates apoptosis during the all-trans-retinoic acid-induced differentiation of promyeloleukemic HL-60 cells. We examined whether the retinoid receptor-specific thyroid hormone action is present during differentiation of HL-60 cells in this study. We used two distinct retinoid receptor agonists. T3 potentiates G1 arrest induced by Am80, a retinoic acid receptor (RAR)-specific agonist, but had no effect on G1 arrest induced by HX600, a retinoid x receptor (RXR)-specific agonist. Am80 alone induces the apoptosis, and T3 enhances it. Although HX600 alone fails to increase the apoptotic fraction, T3 enables the compounds to induce apoptosis. Am80-induced expression of CD11b, a marker for the differentiation, is enhanced by T3. However, T3 or HX600 or both do not affect the expression of CD11b. T3 does not alter the amount of mRNAs of various members of the bcl-2 family. T3, however, enhances the Am80-induced expression of bfl-1 and suppression of bcl-2. In contrast, T3 does not alter either bfl-1 and bcl-2 expression in the presence of HX600. Our observations suggest that cooperative action of T3 with an RXR-specific ligand is different from that with an RAR ligand in cellular apoptotic regulation and that thyroid hormone may be available as a chemotherapeutic agent in acute leukemia.

Responsiveness to thyroid hormone is enhanced in rat hepatocytes cultured as spheroids compared with that in monolayers: altered responsiveness to thyroid hormone possibly involves complex formed on thyroid hormone response elements

We previously reported that the expression of type I iodothyronine 5'-deiodinase (5'DI) gene was increased by 3,3,',5-triiodothyronine (T3) in isolated rat hepatocytes when cultured as spherical aggregates (spheroids), whereas this effect was greatly attenuated in conventional monolayer cultures. In the current study, we examined whether the enhanced T3 responsiveness in spheroid cultures extends to other T3-responsive genes. As observed for 5'DI, T3 increased spot 14, malic enzyme and fibronectin messenger RNAs (mRNAs) by fourfold to fivefold in spheroid cultures, while the effect in monolayer cultures was blunted. This difference in T3 responsiveness was also observed when T3-responsive reporters consisting of the luciferase gene under the control of triiodothyronine response element (TRE) were introduced into hepatocytes using a replication-defective adenovirus vector. These results suggest that the factors required for T3-dependent transcriptional activation are preserved in spheroid cultures and that they must exert their effect by interacting with TRE. Maximal binding capacity of nuclear T3 receptor was not different between monolayer and spheroid cultures while the expression of retinoid X receptor-alpha (RXR alpha) mRNA was higher in spheroid cultures compared with that in monolayers. The difference in RXR alpha mRNA expression, together with enhanced proteolytic cleavage in monolayers that we demonstrated recently, may account for the difference in T3 responsiveness between the two hepatocyte culture systems.

RXR-2, a member of the retinoid x receptor family in Schistosoma mansoni

A cDNA encoding a second full-length member of the Schistosoma mansoni RXR family (SmRXR-2) was identified. The nucleotide sequence of SmRXR-2 translates into a protein of 784 amino acids with a pI of 7.63 and an approximate mass of 78kDa making it the largest reported RXR to date. Phylogenetic tree analysis provides evidence that SmRXR-2 is the most ancient full-length RXR identified. SmRXR-2 exhibits unique sequence features compared with other RXRs. RT-PCR results demonstrate that the SmRXR-2 gene is constitutively expressed and thus must play multiple roles throughout schistosome development in the vertebrate host.

A steroid receptor coactivator, SRA, functions as an RNA and is present in an SRC-1 complex

Nuclear receptors play critical roles in the regulation of eukaryotic gene expression. We report the isolation and functional characterization of a novel transcriptional coactivator, termed steroid receptor RNA activator (SRA). SRA is selective for steroid hormone receptors and mediates transactivation via their amino-terminal activation function. We provide functional and mechanistic evidence that SRA acts as an RNA transcript; transfected SRA, unlike other steroid receptor coregulators, functions in the presence of cycloheximide, and SRA mutants containing multiple translational stop signals retain their ability to activate steroid receptor-dependent gene expression. Biochemical fractionation shows that SRA exists in distinct ribonucleoprotein complexes, one of which contains the nuclear receptor coactivator steroid receptor coactivator 1. We suggest that SRA may act to confer functional specificity upon multiprotein complexes recruited by liganded receptors during transcriptional activation.

Identification of a cDNA encoding a retinoid X receptor homologue from Schistosoma mansoni. Evidence for a role in female-specific gene expression

Schistosoma mansoni, a multicelluar eukaryotic blood fluke, is a major cause of morbidity worldwide in humans. The study of female parasite growth, development, and gene regulation is important because the eggs produced are responsible for the pathogenesis observed in schistosomiasis. p14, an eggshell precursor gene expressed only in sexually mature females in response to a male stimulus, is a model for female-specific gene regulation. The upstream region of the p14 gene shares sequences present in insect genes known to be regulated in a sex-, temporal-, and tissue-specific manner by members of the steroid receptor superfamily. Herein, we report the identification and characterization of a cDNA that encodes the S. mansoni (Sm) RXR homologue. Sequence analysis predicts and Western blot analysis confirms the synthesis of a 74-kDa protein, the largest member of the RXR family reported to date. We show by electrophoretic mobility shift assay analysis that SmRXR binds to cis-elements of the p14 gene including a direct repeat that follows the "3-4-5" rule of binding elements recognized by members of the steroid receptor superfamily. Furthermore, we demonstrate that SmRXR can act as a transcription activator in the yeast one-hybrid system. Through quantitative reverse transcriptase-polymerase chain reaction, we show that the SmRXR gene is constitutively expressed and thus must play multiple roles throughout the schistosome life cycle.

Identification of thyroid hormone response elements in the human fatty acid synthase promoter

To investigate the regulation of the human fatty acid synthase gene by the thyroid hormone triiodothyronine, various constructs of the human fatty acid synthase promoter and the luciferase reporter gene were transfected in combination with plasmids expressing the thyroid hormone and the retinoid X receptors in HepG2 cells. The reporter gene was activated 25-fold by the thyroid hormone in the presence of the thyroid hormone receptor. When both the thyroid hormone and the retinoid X receptors were expressed in HepG2 cells, there was about a 100-fold increase in reporter gene expression. 5'-Deletion analysis disclosed two thyroid hormone response elements, TRE1 (nucleotides -870 to -650) and TRE2 (nucleotides -272 to -40), in the human fatty acid synthase promoter. The presence of thyroid hormone response elements in these two regions of the promoter was confirmed by cloning various fragments of these two regions in the minimal thymidine kinase promoter-luciferase reporter gene plasmid construct and determining reporter gene expression. The results of this cloning procedure and those of electrophoretic mobility shift assays indicated that the sequence GGGTTAcgtcCGGTCA (nucleotides -716 to -731) represents TRE1 and that the sequence GGGTCC (nucleotides -117 to -112) represents TRE2. The sequence of TRE1 is very similar to the consensus sequence of the thyroid hormone response element, whereas the sequence of TRE2 contains only a half-site of the thyroid hormone response element consensus motif because it lacks the direct repeat. The sequences on either side of TRE2 seem to influence its response to the thyroid hormone and retinoid X receptors.

Properties of overlapping EREs: synergistic activation of transcription and cooperative binding of ER

We have designed a novel estrogen-responsive unit, overERE, which consists of two overlapping ERE separated by 5 bp (center-to-center). In gel retardation assays, this sequence forms a low-mobility complex that migrates like an estrogen receptor tetramer. The receptor-overERE complex was specific and was supershifted by anti-ER H222 antibodies. Dose response studies showed that the formation of the receptor tetramer-overERE complex was cooperative. Truncated receptors were used to assess the contribution of the receptor domains. Deletion of the E domain of the ER prevented the formation of an ER-tetramer complex, which reflects a novel function of this receptor domain. In transfection experiments, 17-beta-estradiol activated transcription from an overERE-containing promoter 4-6 times better than from an ERE-containing promoter. This synergistic effect was observed using either the natural hormone (17-beta-estradiol) or xenoestrogens (phenol red, chlordane). We conclude that two overlapping estrogen-responsive elements can elicit synergistic induction of transcription.

Characterization of thyroid hormone response elements in the gene for chicken malic enzyme. Factors that influence triiodothyronine responsiveness

Transcription of the gene for malic enzyme in chick embryo hepatocytes is stimulated about 30-fold by triiodothyronine (T3). T3 responsiveness is mediated by seven direct repeat hexamers that resemble T3 response elements (T3REs); these elements are located far upstream in the 5'-flanking DNA (Hodnett, D. W., Fantozzzi, D. A., Thurmond, D. C., Klautky, S. A., MacPhee, K. G., Estrem, S. T., Xu, G., and Goodridge, A. G. (1996) Arch. Biochem. Biophys. 334, 309-324). In transiently transfected hepatocytes, single copies of six of these elements conferred varying degrees of T3 responsiveness to linked reporter genes. In gel electrophoretic mobility shift analyses, the T3REs bound retinoid X receptor (RXR)-T3 receptor (TR) heterodimers and non-RXR/TR factors present in nuclear extracts prepared from hepatocytes. Binding of the non-RXR/TR factors was specific to individual T3REs and was unaffected by antibodies to TR or RXR. Mutagenesis of binding sites for proteins specific for T3REs 2-5 altered binding of the proteins and T3 responsiveness. These factors appear to bind to and alter function of T3REs without binding directly to TR, differentiating their actions from other TR cofactors; they were tentatively characterized as co-repressors, inhibitors, and activators of T3RE function. Together with RXR and TR, they modulate T3 responsiveness of the gene for chicken malic enzyme.

A thyroid hormone receptor coactivator negatively regulated by the retinoblastoma protein

The retinoblastoma protein (Rb) plays a critical role in cell proliferation, differentiation, and development. To decipher the mechanism of Rb function at the molecular level, we have systematically characterized a number of Rb-interacting proteins, among which is the clone C5 described here, which encodes a protein of 1,978 amino acids with an estimated molecular mass of 230 kDa. The corresponding gene was assigned to chromosome 14q31, the same region where genetic alterations have been associated with several abnormalities of thyroid hormone response. The protein uses two distinct regions to bind Rb and thyroid hormone receptor (TR), respectively, and thus was named Trip230. Trip230 binds to Rb independently of thyroid hormone while it forms a complex with TR in a thyroid hormone-dependent manner. Ectopic expression of the protein Trip230 in cells, but not a mutant form that does not bind to TR, enhances specifically TR-dependent transcriptional activity. Coexpression of wild-type Rb, but not mutant Rb that fails to bind to Trip230, inhibits such activity. These results not only identify a coactivator molecule that modulates TR activity, but also uncover a role for Rb in a pathway that responds to thyroid hormone.

Ultraspiracle, a Drosophila retinoic X receptor alpha homologue, can mobilize the human thyroid hormone receptor to transactivate a human promoter

We have analyzed the functional domains of the Drosophila orphan receptor Ultraspiracle (usp), a homologue of the vertebrate retinoic X receptor alpha, as well as the ability of heterodimers between usp and the thyroid hormone receptor beta (T3Rbeta) to transactivate the human apolipoprotein A-II (apoA-II) promoter. DNA binding assays demonstrated that heterodimers of usp and the human T3Rbeta can bind to the hormone response element (HRE) of the regulatory element AIIJ (-734 to -716) of the human apoA-II promoter. Cotransfection experiments have shown that the combination of usp and T3Rbeta can transactivate the human apoA-II promoter in COS-1 cells 7-8-fold in the presence of thyroid hormone (T3). The observed transactivation was not affected by the deletion of the amino-terminal residues 1-85 of usp, which represent a putative transactivation domain, suggesting that the function of usp is to recruit T3Rbeta. Furthermore, a mutant usp, with impaired DNA binding properties, can form heterodimers with T3Rbeta in vitro but has reduced ability to transactivate the human apoA-II promoter. A minimal thymidine kinase (tk) promoter driven by four AIIJ regulatory elements is repressed to 20% of its original activity by T3Rbeta and the repression is relieved by usp/T3Rbeta heterodimers. Deletion analysis demonstrated that factors bound to the regulatory elements AIIJ, AIIAB, and AIIH participate in the usp/T3Rbeta-mediated transactivation of the human apoA-II promoter. Similarly to element AIIJ, element AIIAB binds usp/T3Rbeta heterodimers, whereas element AIIH binds a COS-1 nuclear activity that is supershifted with anti-hepatic nuclear factor 1 antibodies. The findings suggest that optimal transactivation of the apoA-II promoter by usp/T3Rbeta heterodimers requires complex interactions between these heterodimers and factors bound to other regulatory elements. The observed transcriptional activation through heterodimer formation between nuclear receptors from species as divergent in the evolutionary scale as insects and mammals indicates that the functional domains of these proteins have been highly conserved.

Activation of the orphan receptor RIP14 by retinoids

Retinoids are crucial regulators of a wide variety of processes in both developing and adult animals. These effects are thought to be mediated by the retinoic acid (RA) receptors and the retinoid X receptors (RXRs). We have identified an additional retinoid-activated receptor that is neither a retinoic acid receptors nor an RXR. RXR-interacting protein 14 (RIP14), a recently described orphan member of the nuclear receptor superfamily, can be activated by either all-trans-RA (tRA) or the synthetic retinoid TTNPB [[E]-4-[2-(5, 6, 7, 8-tetrahydro-5, 5, 8, 8-tetramethyl-2-naphthalenyl)propen-1-yl]benzoic acid].RIP14 binds to DNA as a heterodimer with RXR. In the presence of either tRA or TTNPB, the addition of 9-cis-RA or the RXR-specific agonist LG1069 [4-[1-(3, 5, 5, 8, 8-pentamethyl-5, 6, 7, 8-tertrahydro-2-naphthyl)ethenyl]benzoic acid] results in additional activation. Mutations of the ligand-dependent transcriptional activation functions indicate that TTNPB activates the RIP14 component of the RIP14-RXR heterodimer, that 9-cis-RA and LG1069 activate RXR, and that tRA activates via both RIP14 and RXR. Despite the very effective activation of RIP14 by tRA or TTNPB, relatively high concentrations of these compounds are required, and no evidence for direct binding of either compound was obtained using several approaches. These results suggest that RIP14 is the receptor for an as-yet-unidentified retinoid metabolite.

Infant survival is improved by oral iodine supplementation

Although reports suggest that infant mortality is increased during iodine deficiency, the effect of iodine supplementation on infant mortality is unknown. A double-masked, randomized, placebo-controlled, clinical trial of oral iodized oil was conducted in Subang, West Java, Indonesia to evaluate the effect of iodine supplementation on infant mortality. Infants were allocated to receive placebo or oral iodized oil (100 mg) at about 6 wk of age and were followed to 6 mo of age. Six hundred seventeen infants were enrolled in the study. Infant survival was apparently improved, as indicated by a 72% reduction in the risk of death during the first 2 mo of follow-up (P < 0.05) and a delay in the mean time to death among infants who died in the iodized oil group compared with infants who died in the placebo group (48 days vs. 17.5 d, P = 0.06). Other infant characteristics associated with reduced risk of death included weight-for-age at base line, consumption of solid foods, female gender and recent history of maternal iodine supplementation. Oral iodized oil supplementation had a stronger effect on the mortality of males compared with females. This study suggests that oral iodized oil supplementation of infants may reduce infant mortality in populations at risk for iodine deficiency.

Retinoid X receptor-specific ligands synergistically upregulate 1, 25-dihydroxyvitamin D3-dependent transcription in epidermal keratinocytes in vitro and in vivo

We have examined the mechanism by which endogenous retinoid X receptor (RXR), vitamin D3 receptor (VDR), and cognate ligands regulate nuclear 1,25-dihydroxyvitamin D3 (D3) signaling in epidermal keratinocytes from skin, a physiologic D3 target. In vitro, RXR and VDR-specific antibodies identified endogenous RXR and VDR bound to a vitamin D3-responsive element (DR3) as heterodimers (VDR-RXR). In cultured keratinocytes, 9-cis retinoic acid (9cRA), a panagonist for RXR and retinoic acid receptor (RAR), and an RXR-selective agonist, SR11237, synergized with D3 to activate DR3 via endogenous as well as overexpressed VDR-RXR, whereas both of these RXR agonists alone were ineffective. In contrast, SR11237 did not synergize with but antagonized an RAR-selective ligand activation of a retinoic acid-responsive element (DR5) via endogenous RAR-RXR. Furthermore, expression of RXR mutated in transactivation domain AF-2 inhibited endogenous VDR-RXR activity over DR3. This mutant efficiently bound to DR3 as VDR-RXR but showed reduced capacity to transactivate DR3 in response to D3 and SR11237. In vivo, D3 and SR11237 synergistically induced the naturally occurring D3-responsive 24-hydroxylase gene in epidermis of mouse skin, whereas SR11237 alone was ineffective. Our data suggest that allosteric changes caused by VDR in DR3-bound VDR-RXR do not block access of ligands to RXR. RXR ligand-induced conformational changes permit VDR-RXR, via both VDR and RXR activation function domains, to mediate maximal D3 signaling in keratinocytes.

Distal apolipoprotein C-III regulatory elements F to J act as a general modular enhancer for proximal promoters that contain hormone response elements. Synergism between hepatic nuclear factor-4 molecules bound to the proximal promoter and distal enhancer sites

Transient transfection assays have shown that the distal apoC-III promoter segments that contain the regulatory elements F to J enhance the strength of the tandemly linked proximal apoA-I promoter 5- to 13-fold in hepatic (HepG2) cells. Activation in intestinal (CaCo-2) cells to levels comparable to those obtained in HepG2 cells requires a larger apoA-I promoter sequence that extends to nucleotide -1500 as well as the presence of hepatic nuclear factor-4 (HNF-4). The distal apoC-III regulatory elements can also enhance 4- to 8-fold the strength of the heterologous apoB promoter in HepG2 and CaCo-2 cells. Finally, these elements in the presence of HNF-4 enhance 14.5- to 18.5-fold the strength of the minimal adenovirus major late promoter linked to two copies of the hormone response element (HRE) AID of apoA-I in both HepG2 and CaCo-2 cells. In vitro mutagenesis of the promoter/enhancer cluster established that the enhancer activity is lost by a mutation in the HRE present in the 3' end of the regulatory element I (-736 to -714) and is reduced significantly by point mutations or deletions in one or more of the regulatory elements F to J of the apoC-III enhancer. The enhancer activity also requires the HREs of the proximal apoA-I promoter. The apoC-III enhancer can also restore the activity of the proximal apoA-I and apoB promoters that have been inactivated by mutations in CCAAT/enhancers binding protein binding sites, indicating that C/EBP may not participate in the synergistic activation of the promoter/enhancer cluster. The findings suggest that the regulatory elements F to J of the apoC-III promoter act as a general modular enhancer that can potentiate the strength of proximal promoters that contain HREs. Such potentiation in the HepG2 cells can be accounted for by synergistic interactions between HNF-4 or other nuclear hormone receptors bound to the proximal and distal HREs and SP1 or other factors bound to the apoC-III enhancer. Additional factors may be required for optimal activity in CaCo-2 cells as well as for the function of this region as an intestinal enhancer.

Different DNA elements can modulate the conformation of thyroid hormone receptor heterodimer and its transcriptional activity

Thyroid-hormone receptors (TRs) form heterodimers with retinoid-X receptors (RXRs) on thyroid-hormone-response elements (TREs). However, it is not known whether the formation of liganded TR/RXR heterodimer on a TRE alone is sufficient to dictate transcriptional activity. We designed several mutated DR4s (half-sites arranged as direct repeats with a nucleotide gap of 4) that bound TR/RXR heterodimers preferentially, and employed them to characterize functional and biochemical properties of the heterodimers on DNA. Although TR/RXR heterodimer binding was similar on some of the mutated DR4s, transient transfection assays showed that TRalpha failed to support triiodothyronine (T3)-stimulated transcription on "inactive" DR4s but mediated basal repression on both "active" and inactive mutated DR4. T3 binding assays showed that the mutated DR4s did not affect T3 binding to the heterodimer. Finally, partial proteolysis studies revealed that binding of active DR4 elements and T3 to the heterodimer synergistically enhanced heterodimerization-induced protease resistance of TR, but not RXR, in the heterodimer. These results suggest that: 1) liganded TR/RXR heterodimer binding to a DR4 is not sufficient for transcriptional activation of the target gene, and 2) DNA sequences in specific TREs may modify T3-mediated transcription by affecting the conformation of the liganded heterodimer.

Binding specificity and modulation of the ApoA-I promoter activity by homo- and heterodimers of nuclear receptors

Three proximal regulatory elements, AIB, AIC, and AID, of the apoA-I gene are necessary and sufficient for its hepatic expression in vivo and in vitro. DNA binding and competition assays showed that elements AIB and AID contain hormone response elements composed of imperfect direct repeats that support the binding of the hepatic nuclear factor-4, other nuclear orphan receptors, and the ligand-dependent nuclear receptors retinoic X receptor (RXRalpha), RXRalpha/RARalpha, and RXRalpha/T3Rbeta. Substitution mutations on repeats 1 and 2 in the hormone response sites of elements AIB and AID, respectively, abolished the binding of all nuclear receptors and reduced promoter activity to background levels, indicating the importance of both hormone response elements for the hepatic expression of the apoA-I gene. Cotransfection experiments in HepG2 cells with normal and mutated promoter constructs and plasmids expressing nuclear hormone receptors showed that RXRalpha homodimers transactivated the wild type promoter 150% of control, in the presence of 9-cis-retinoic acid (RA), whereas RXR alpha/T3R beta heterodimers repressed transcription to 60% of control, in the presence of T3. RXR alpha/RAR alpha and hepatic nuclear factor-4 did not affect the transcription, driven by the proximal apoA-I promoter. Potassium permanganate and dimethyl sulfate interference experiments showed that RXRalpha homodimers, RXRalpha/RARalpha, and RXRalpha/T3Rbeta heterodimers participate in protein-DNA interactions with 12, 13, and 11 out of the 14 nucleotides, respectively, that span repeats 1 and 2 and the spacer region separating them on the hormone response element of element AID. The binding of RXRalpha homodimers and RXRalpha/T3Rbeta heterodimers is associated with ligand-dependent activation by 9-cis-RA or repression by T3. Upon deletion or mutation of repeat 1, homodimeric binding of RXRalpha is lost whereas heterodimeric binding is retained. This heterodimeric binding to the mutated element AID is mediated solely by interactions with repeat 2 and one adjacent nucleotide and is confined to a heptameric core recognition motif. The interactions of the RXRalpha heterodimers with repeat 2 are associated with low levels of ligand-independent transcriptional activity. The findings suggest that the specific types of homo- and heterodimers of nuclear hormone receptors occupying the hormone response elements of apoA-I and the availability of the ligand may play an important role in the transcriptional regulation of the human apoA-I gene.

Regulation of corticosteroid-binding globulin synthesis by 1alpha,25-dihyroxy-vitamin D3 (calcitriol), 9-cis-retinoic acid and triiodothyronine in cultured rat fetal hepatocytes

Evidence regarding the nature of the regulatory factors which directly act upon liver cells and extra-hepatic tissues to alter CBG synthesis is scarce. The present study used cultured rat fetal hepatocytes to investigate the involvement and possible interplay in this process of several members of the nuclear receptors superfamily: vitamin D (VDR), retinoic acids (RAR/RXR) and thyroid hormones (TR). Treatment of cells with 1alpha,25-(OH)2D3 (1,25-D) elicited a dose-dependent inhibition of basal CBG concentration in culture medium. Maximum inhibition to about 15% of control level was achieved with 0.1-1.0 nM, with an IC50 of 3.8 x 10(-12) M and with no significant change in binding affinity. Differential activation of RAR and RXR with either 9-cis-retinoic acid (9-cis-RA) or the RAR-selective synthetic retinoid TTNPB revealed that high doses of both drugs diminished CBG expression, though the former proved about 10-times more potent than the latter in this regard. Amplification by triiodothyronine (T3) of CBG synthesis failed to block the inhibitory effects of either 1,25-D or retinoids, as revealed by both binding capacity and mRNA measurements. Relative to CBG, 1,25-D similarly depressed the synthesis of alpha-fetoprotein (AFP), while on the contrary, retinoids and T3 were shown to cause opposite effects, as 9-cis-RA and TTNPB elevated and T3 decreased AFP expression. The present findings identify for the first time ligands of VDR and RAR/RXR as powerful negative regulators of both basal and T3-stimulated CBG biosynthesis in fetal hepatocytes and suggest lack of a functional interplay between TR and VR or RAR/RXR in these processes.

Ligands of four receptors in the nuclear steroid/thyroid hormone superfamily inhibit induction of rat cytosolic aldehyde dehydrogenase-3 (ALDH3c) by 3-methylcholanthrene

Using six ligands that bind to four different receptors in the nuclear steroid/thyroid hormone superfamily, we have examined the effects of these chemicals on induction of the cytosolic aldehyde dehydrogenase (ALDH3c) activity by 3-methylcholanthrene (3MC) in rat liver and uterus. In contrast to negligible activities in the untreated rat, ALDH3c enzyme activities are induced after a single dose of 3MC. Hepatic ALDH3c induction is decreased 60% to 90% when 3MC is administered together with any of the following ligands: estradiol, testosterone, progesterone, hydrocortisol, diethylstilbestrol, or tamoxifen. None of these same doses of chemicals, administered alone, affects ALDH3c enzyme activity. In addition, when these ligands are injected 2 days after 3MC, no changes are observed in liver or uterus ALDH3c induction. These results suggest that ligands that bind to different receptors in the nuclear steroid/thyroid hormone superfamily might inhibit the ALD3H3c induction process by polycyclic aromatic hydrocarbons; the molecular mechanism(s) of this inhibitory effect is not yet understood.

Sequence and characterization of a coactivator for the steroid hormone receptor superfamily

A yeast two-hybrid system was used to identify a protein that interacts with and enhances the human progesterone receptor (hPR) transcriptional activity without altering the basal activity of the promoter. Because the protein stimulated transactivation of all the steroid receptors tested, it has been termed steroid receptor coactivator-1 (SRC-1). Coexpression of SRC-1 reversed the ability of the estrogen receptor to squelch activation by hPR. Also, the amino terminal truncated form of SRC-1 acted as a dominant-negative repressor. Together, these results indicate that SRC-1 encodes a coactivator that is required for full transcriptional activity of the steroid receptor superfamily.

Retinoid receptors and keratinocytes

In 1987, a tremendous boost in our understanding of the action of dietary vitamin A occurred with the discovery and characterization of nuclear receptors for retinoic acid, the active form of the vitamin, in the laboratories of P. Chambon and R. Evans. They have shown that the nuclear receptors are ligand-activated transcription factors capable of specific gene regulation. Since that discovery, it has been determined that there are at least six retinoic acid receptors belonging to two families, RARs and RXRs, that they are differentially expressed in various mammalian tissues, and that they act as homo- and heterodimers interacting with other ligand-activated nuclear receptors. The domain structure of the receptors has been described, and their DNA-binding, ligand-binding, dimerization, and transcriptional activation regions characterized. Among the most important retinoid-regulated genes are the homeobox proteins, regulatory transcription factors which are responsible for body axis formation, patterning, limb formation, and other crucial processes during development. Retinoic acid and its receptors also regulate many differentiation markers which are particularly important in stratified epithelia, such as skin and oral epithelia. Our increased understanding led to improved therapy of a large number of skin disorders, ranging from acne to wrinkles and including epidermal and oral carcinomas.

Functional properties of a mutant T3 receptor beta (R338W) identified in a subject with pituitary resistance to thyroid hormone

Previously, we identified a point mutation of the T3 receptor (TR) beta gene (R338W) in a patient with pituitary resistance to thyroid hormone (PRTH). The mutation existed in one of two hot spot areas in TRbeta gene where clusters of mutations have been found in subjects with generalized resistance to thyroid hormone (GRTH). Interestingly, R338W induces the phenotypical features responsible for PRTH. In the present study, we examined the functional properties of R338W in comparison with those of a GRTH-mutant, K443E. The levels of thyroid hormones and inappropriately elevated TSH (SITSH) were similar between subjects with K443E and R338W. Transcriptional activities and dominant negatives potencies were measured by CAT assay in CV1 cells transfected with each mutant TRbeta1 or along with wild-type TR. When a reporter gene containing T3-responsive elements (TRE), TRE-pal2, DR4 or myosin heavy chain alpha subunit, was used, transcriptional activation induced by R338W was higher than that by K443E. At 50 nM T3, K443E decreased the transcriptional activity of wild-type TRbeta1 on TRE-pal2 by 31.5%, while R338W reduced by 13.6% (n = 15, P < 0.05). Co-expression of retinoid X receptor (RXR) alpha increased transcriptional activity of R338W and K443E, but not of wild-type TRbeta1. Dominant negative activity on TRE-TSHalpha subunit of R338W was milder than that of K443E. When T3-binding activities of mutant TRbeta1s expressed in the cells were assayed under the same cell conditions for CAT assay, both mutant TRbeta1 showed remarkably reduced activity with no difference between the two. Gel mobility shift assay using TRE-DR4 showed poor homodimer formation of R338W. Heterodimerization with RXRalpha was similar between R338W, K443E and wild-type TRbeta1. The result of the present study suggested that R338W had relatively mild transcriptional and dominant negative activities on several TREs including TRE-TSHalpha subunit. We also showed poor homodimerization of R338W, which might be related to its weak dominant negative potency.