Molecular and structural biology of thyroid hormone receptors

Thyroid hormone resistance: Mechanisms and therapeutic development

As an essential primary hormone, thyroid hormone (TH) is indispensable for human growth, development and metabolism. Impairment of TH function in several aspects, including TH synthesis, activation, transportation and receptor-dependent transactivation, can eventually lead to thyroid hormone resistance syndrome (RTH). RTH is a rare syndrome that manifests as a reduced target cell response to TH signaling. The majority of RTH cases are related to thyroid hormone receptor β (TRβ) mutations, and only a few RTH cases are associated with thyroid hormone receptor α (TRα) mutations or other causes. Patients with RTH suffer from goiter, mental retardation, short stature and bradycardia or tachycardia. To date, approximately 170 mutated TRβ variants and more than 20 mutated TRα variants at the amino acid level have been reported in RTH patients. In addition to these mutated proteins, some TR isoforms can also reduce TH function by competing with primary TRs for TRE and RXR binding. Fortunately, different treatments for RTH have been explored with structure-activity relationship (SAR) studies and drug design, and among these treatments. With thyromimetic potency but biochemical properties that differ from those of primary TH (T3 and T4), these TH analogs can bypass specific defective transporters or reactive mutant TRs. However, these compounds must be carefully applied to avoid over activating TRα, which is associated with more severe heart impairment. The structural mechanisms of mutation-induced RTH in the TR ligand-binding domain are summarized in this review. Furthermore, strategies to overcome this resistance for therapeutic development are also discussed.

µ-Crystallin: A thyroid hormone binding protein

µ-Crystallin is a NADPH-regulated thyroid hormone binding protein encoded by the CRYM gene in humans. It is primarily expressed in the brain, muscle, prostate, and kidney, where it binds thyroid hormones, which regulate metabolism and thermogenesis. It also acts as a ketimine reductase in the lysine degradation pathway when it is not bound to thyroid hormone. Mutations in CRYM can result in non-syndromic deafness, while its aberrant expression, predominantly in the brain but also in other tissues, has been associated with psychiatric, neuromuscular, and inflammatory diseases. CRYM expression is highly variable in human skeletal muscle, with 15% of individuals expressing ≥13 fold more CRYM mRNA than the median level. Ablation of the Crym gene in murine models results in the hypertrophy of fast twitch muscle fibers and an increase in fat mass of mice fed a high fat diet. Overexpression of Crym in mice causes a shift in energy utilization away from glycolysis towards an increase in the catabolism of fat via β-oxidation, with commensurate changes of metabolically involved transcripts and proteins. The history, attributes, functions, and diseases associated with CRYM, an important modulator of metabolism, are reviewed.

Organo-Halogens and their Possible Involvement in Prebiotic Chemistry

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In this review, we examined the possibility that some halogenated organic derivatives were used in the primitive ocean at the beginning of life on Earth. Firstly, we described the existence of extraterrestrial halogenated molecules, then we studied their nonbiological syntheses on the present Earth, especially in volcanic environments. In order to demonstrate the diversity of today’s halogenated biomolecules, representative examples are given and the biosynthesis of some of them is summarized. Finally, we proposed two aspects of the chemistry of halogenated compounds that may have been useful en route to biomolecules, firstly the use of methyl chloride as the first methylation reagent, secondly the synthesis and use of α-chloro-carbonyl derivatives.

Chlorinated Polyfluoroalkylether Sulfonates Exhibit Similar Binding Potency and Activity to Thyroid Hormone Transport Proteins and Nuclear Receptors as Perfluorooctanesulfonate

Chlorinated polyfluoroalkylether sulfonates (Cl-PFAESs) have been used as perfluorooctanesulfonate (PFOS) alternatives in the chrome plating industry for years. Although Cl-PFAESs have become ubiquitous environmental contaminants, knowledge on their toxicological mechanism remains very limited. We compared potential thyroid hormone (TH) disruption effects of Cl-PFAESs and PFOS via the mechanisms of competitive binding to TH transport proteins and activation of TH receptors (TRs). Fluorescence binding assays revealed that 6:2 Cl-PFAES, 8:2 Cl-PFAES and F-53B (a mixture of 6:2 and 8:2 Cl-PFAES) all interacted with a TH transport protein transthyretin (TTR), with 6:2 Cl-PFAES showing the highest affinity. It was also found that the chemicals interacted with TRs, with the affinity following the order of 6:2 Cl-PFAES > PFOS > 8:2 Cl-PFAES. In reporter gene assays the chemicals exhibited agonistic activity toward TRs, with the potency of 6:2 Cl-PFAES comparable to that of PFOS. The chemicals also promoted GH3 cell proliferation, with 6:2 Cl-PFAES displaying the highest potency. Molecular docking and molecular dynamic simulation revealed that both Cl-PFAESs fit into the ligand binding pockets of TTR and TRs with the binding modes similar to PFOS. Collectively, our results demonstrate that Cl-PFAESs might cause TH disruption effects through competitive binding to transport proteins and activation of TRs.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

Thyroid hormone analogs for the treatment of dyslipidemia: past, present, and future

OBJECTIVE

Treatment of dyslipidemia is a major burden for public health. Thyroid hormone regulates lipid metabolism by binding the thyroid hormone receptor (TR), but the use of thyroid hormone to treat dyslipidemia is not indicated due to its deleterious effects on heart, bone, and muscle. Thyroid hormone analogs have been conceived to selectively activate TR in the liver, thus reducing potential side-effects.

METHODS

The authors searched the PubMed database to review TR and the action of thyromimetics in vitro and in animal models. Then, all double-blind, placebo controlled trials that analyzed the use of thyroid hormone analog for the treatment of dyslipidemia in humans were included. Finally, the ongoing research on the use of TR agonists was searched, searching the US National Institutes of Health Registry and the WHO International Clinical Trial Registry Platform (ICTRP).

RESULTS

Thyromimetics were tested in humans for the treatment of dyslipidemia, as a single therapeutic agent or as an add-on therapy to the traditional lipid-lowering drugs. In most trials, thyromimetics lowered total cholesterol, low-density lipoprotein cholesterol, and triglycerides, but their use has been associated with adverse side-effects, both in pre-clinical studies and in humans.

CONCLUSIONS

The use of thyromimetics for the treatment of dyslipidemia is not presently recommended. Future possible clinical applications might include their use to promote weight reduction. Thyromimetics might also represent an interesting alternative, both for the treatment of non-alcoholic steatohepatitis, and type 2 diabetes due to their positive effects on insulin sensitivity. Finally, additional experimental and clinical studies are needed for a better comprehension of the effect(s) of a long-term therapy.

Subclinical hypothyroidism, lipid metabolism and cardiovascular disease

Subclinical hypothyroidism is defined by elevated serum thyrotropin in presence of normal free thyroid hormones. Lipid metabolism is influenced by thyroid hormone and many reports showed that lipids status worsen along with TSH level. Subclinical hypothyroidism has been also linked to other cardiovascular risk factors such as alteration in blood pressure and increased atherosclerosis. Further evidences suggested that mild dysfunction of thyroid gland is associated with metabolic syndrome and heart failure. Thyrotropin level seems the best predictor of cardiovascular disease, in particular when its levels are above 10mU/L. However, despite these observations, there is no clear evidence that levothyroxine therapy in subjects with milder form of subclinical hypothyroidism could improve lipid status and the other cardiovascular risk factors. In this review, we address the effect of thyroid hormone and cardiovascular risk, with a focus on lipid metabolism.

The identification of nuclear receptors associated with hepatic steatosis to develop and extend adverse outcome pathways

The development of adverse outcome pathways (AOPs) is becoming a key component of twenty-first century toxicology. AOPs provide a conceptual framework that links the molecular initiating event to an adverse outcome through organized toxicological knowledge, bridging the gap from chemistry to toxicological effect. As nuclear receptors (NRs) play essential roles for many physiological processes within the body, they are used regularly as drug targets for therapies to treat many diseases including diabetes, cancer and neurodegenerative diseases. Due to the heightened development of NR ligands, there is increased need for the identification of related AOPs to facilitate their risk assessment. Many NR ligands have been linked specifically to steatosis. This article reviews and summarizes the role of NR and their importance with links between NR examined to identify plausible putative AOPs. The following NRs are shown to induce hepatic steatosis upon ligand binding: aryl hydrocarbon receptor, constitutive androstane receptor, oestrogen receptor, glucocorticoid receptor, farnesoid X receptor, liver X receptor, peroxisome proliferator-activated receptor, pregnane X receptor and the retinoic acid receptor. A preliminary, putative AOP was formed for NR binding linked to hepatic steatosis as the adverse outcome.

The ability of thyroid hormone receptors to sense t4 as an agonist depends on receptor isoform and on cellular cofactors

T4 (3,5,3',5'-tetraiodo-l-thyronine) is classically viewed as a prohormone that must be converted to the T3 (3,5,3'-triiodo-l-thyronine) form for biological activity. We first determined that the ability of reporter genes to respond to T4 and to T3 differed for the different thyroid hormone receptor (TR) isoforms, with TRα1 generally more responsive to T4 than was TRβ1. The response to T4 vs T3 also differed dramatically in different cell types in a manner that could not be attributed to differences in deiodinase activity or in hormone affinity, leading us to examine the role of TR coregulators in this phenomenon. Unexpectedly, several coactivators, such as steroid receptor coactivator-1 (SRC1) and thyroid hormone receptor-associated protein 220 (TRAP220), were recruited to TRα1 nearly equally by T4 as by T3 in vitro, indicating that TRα1 possesses an innate potential to respond efficiently to T4 as an agonist. In contrast, release of corepressors, such as the nuclear receptor coreceptor NCoRω, from TRα1 by T4 was relatively inefficient, requiring considerably higher concentrations of this ligand than did coactivator recruitment. Our results suggest that cells, by altering the repertoire and abundance of corepressors and coactivators expressed, may regulate their ability to respond to T4, raising the possibility that T4 may function directly as a hormone in specific cellular or physiological contexts.

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.

A review of literatures on the adverse effects of thyroid abnormalities and liver disorders: an overview on liver dysfunction and hypothyroidism

The healthy thyroid is vital for the liver metabolism. The liver also plays an important role in the metabolism of thyroid hormones. Thyroid and liver diseases can apparently have an adverse effects on each other organs. The main concept behind this present review is to analyze the coordination existed among thyroid and liver and the pathophysiology surrounding these two vital organs in human metabolism.

Thyroid hormone analogues: where do we stand in 2013?

Thyroid hormones (THs) are important in the development and maintenance of lipid and energy homeostasis. THs act through two closely related TH receptors (TRs α and β), which are conditional transcription factors. Recently, TH analogues or thyromimetics with varying degrees of TR subtype and liver uptake selectivity have been developed. These compounds exert beneficial effects of TH excess states without many undesirable TR-dependent side effects. Several selective TR modulators (STRMs) showed exceptionally promising results in lowering serum cholesterol in preclinical animal models and human clinical studies. Moreover, some first generation STRMs elicit other potentially beneficial effects on obesity, glucose metabolism, and nonalcoholic fatty liver disease (NAFLD). While it was initially thought that STRMs would be an effective long-term therapy to combat elevated cholesterol, possibly in conjunction with another cholesterol-lowering therapy, the statins, three major first generation STRMs failed to progress beyond early phase III human trials. The aim of this review is to discuss how STRMs work, their actions in preclinical animal models and human clinical trials, why they did not progress beyond clinical trials as cholesterol-lowering therapeutics, whether selective TR modulation continues to hold promise for dyslipidemias, and whether members of this drug class could be applied to the treatment of other aspects of metabolic syndrome and human genetic disease.

Thyroid hormones in male reproductive development: evidence for direct crosstalk between the androgen and thyroid hormone axes

Thyroid hormones (THs) exert a broad range of effects on development in vertebrate species, demonstrating connections in nearly every biological endocrine system. In particular, studies have shown that THs play a role in sexual differentiation and gonadal development in mammalian and non-mammalian species. There is considerable evidence that the effects of THs on reproductive development are mediated through the female hormonal axis; however, recent findings suggest a more direct crosstalk between THs and the androgen axis. These findings demonstrate that THs have considerable influence in the sexual ontogeny of male vertebrates, through direct interactions with select sex-determining-genes and regulation of gonadotropin production in the hypothalamus-pituitary-gonad axis. THs also regulate androgen biosynthesis and signaling through direct and indirect regulation of steroidogenic enzyme expression and activity. Novel promoter analysis presented in this work demonstrates the potential for direct and vertebrate wide crosstalk at the transcriptional level in mice (Mus musculus), Western clawed frogs (Silurana tropicalis) and medaka (Oryzias latipes). Cumulative evidence from previous studies; coupled with novel promoter analysis suggests mechanisms for a more direct crosstalk between the TH and male reproductive axes across vertebrate species.

Genome-wide analysis of thyroid hormone receptors shared and specific functions in neural cells

TRα1 and TRβ1, the two main thyroid hormone receptors in mammals, are transcription factors that share similar properties. However, their respective functions are very different. This functional divergence might be explained in two ways: it can reflect different expression patterns or result from different intrinsic properties of the receptors. We tested this second hypothesis by comparing the repertoires of 3,3',5-triiodo-L-thyronine (T3)-responsive genes of two neural cell lines, expressing either TRα1 or TRβ1. Using transcriptome analysis, we found that a substantial fraction of the T3 target genes display a marked preference for one of the two receptors. So when placed alone in identical situations, the two receptors have different repertoires of target genes. Chromatin occupancy analysis, performed at a genome-wide scale, revealed that TRα1 and TRβ1 cistromes were also different. However, receptor-selective regulation of T3 target genes did not result from receptor-selective chromatin occupancy of their promoter regions. We conclude that modification of TRα1 and TRβ1 intrinsic properties contributes in a large part to the divergent evolution of the receptors' function, at least during neurodevelopment.

Bacterial biosensors for screening isoform-selective ligands for human thyroid receptors α-1 and β-1

Subtype-selective thyromimetics have potential as new pharmaceuticals for the prevention or treatment of heart disease, high LDL cholesterol and obesity, but there are only a few methods that can detect agonistic behavior of TR-active compounds. Among these are the rat pituitary GH₃ cell assay and transcriptional activation assays in engineered yeast and mammalian cells. We report the construction and validation of a newly designed TRα-1 bacterial biosensor, which indicates the presence of thyroid active compounds through their impacts on the growth of an engineered Escherichia coli strain in a simple defined medium. This biosensor couples the configuration of a hormone receptor ligand-binding domain to the activity of a thymidylate synthase reporter enzyme through an engineered allosteric fusion protein. The result is a hormone-dependent growth phenotype in the expressing E. coli cells. This sensor can be combined with our previously published TRβ-1 biosensor to detect potentially therapeutic subtype-selective compounds such as GC-1 and KB-141. To demonstrate this capability, we determined the half-maximal effective concentration (EC₅₀) for the compounds T₃, Triac, GC-1 and KB-141 using our biosensors, and determined their relative potency in each biosensor strain. Our results are similar to those reported by mammalian cell reporter gene assays, confirming the utility of our assay in identifying TR subtype-selective therapeutics. This biosensor thus provides a high-throughput, receptor-specific, and economical method (less than US$ 0.10 per well at laboratory scale) for identifying important therapeutics against these targets.

A literature review on the adverse effects of hypothyroidism on kidney function

Thyroid produce two important hormone of thyroxine or tetraidothyronine (T4) and triidothyronine (T3), which are involved in whole aspect of metabolism. T4 and T3 play vital role in all biochemical function, growth and development in human body. The basic metabolic pathways in kidney and every organ in human controlled by these hormones. T4 and T3 are involved in kidney function in health and diseases condition therefore the pathophysiology of kidney can be directly influenced and regulated by thyroid hormones. Kidney growth, haemodynamic, blood circulation, tubular, electrolyte balance and glomerular filtration rate (GFR) are among such crucial process. Hypothyroidism which accompanied with reduced thyroid hormone production adversely affect the renal functions, development and eventually leading to reduced weight, kidney vascular disorders, electrolyte, tubular transport imbalances, lower filtration rate and other adverse consequences of hypothyroidism. On other hand kidney diseases can also disrupt the thyroid function metabolism resulting in the subsequent hypothyroidism. It is an interesting subject in how thyroid and kidney in health and diseases closely interacted. For the ideal clinical follow up of either of thyroid and renal diseases the two organs should be simultaneously examined for a proper patient management. Close correlation of thyroid and kidney clinical teams are essential to check the cross reactions and adverse interactions which might be produced between these two vital organs to avoid misdiagnosis either of thyroid or kidney abnormalities.

Gene regulation of CYP4F11 in human keratinocyte HaCaT cells

Mechanisms regulating CYP4F genes remain under investigation, although characterization of CYP4F regulatory modalities would facilitate the discovery of new drug targets. This present study shows that all-trans- and 9-cis-retinoic acids can inhibit CYP4F11 expression in human keratinocyte-derived HaCaT cells. Transrepression of many genes by retinoic acids is mediated by interactions between retinoid receptors and the activator protein 1 (AP-1) complex. Proinflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin 1beta, which can activate the AP-1 complex, induce CYP4F11 transcription in HaCaT cells. The c-Jun N-terminal kinase (JNK)-specific inhibitor 1,9-pyrazoloanthrone (SP600125) blocked the induction of CYP4F11 by both cytokines, indicating involvement of the JNK pathway. Furthermore, TNF-alpha failed to induce CYP4F11 transcription when HaCaT cells were preincubated with retinoic acids. Retinoic acids are ligands for the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). The RXR agonist 6-(1(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl)cyclopropyl) nicotinic acid (LG268) greatly induced CYP4F11 transcription, whereas the RAR agonist 4-(2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-propenyl)benzoic acid (TTNPB) markedly inhibited CYP4F11 transcription, indicating that down-regulation of CYP4F11 transcription by retinoic acid is mediated by RARs and may also be related to ligand competition for RXRs. Thus, the CYP4F11 gene is positively regulated by multiple signaling pathways in HaCaT keratinocytes, including RXR and JNK signaling pathways.

Isoform-specific transcriptional activity of overlapping target genes that respond to thyroid hormone receptors alpha1 and beta1

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that control multiple aspects of physiology and development. TRs are expressed in vertebrates as a series of distinct isoforms that exert distinct biological roles. We wished to determine whether the two most widely expressed isoforms, TR alpha 1 and TR beta 1, exert their different biological effects by regulating different sets of target genes. Using stably transformed HepG2 cells and a microarray analysis, we were able to demonstrate that TR alpha 1 and TR beta 1 regulate a largely overlapping repertoire of target genes in response to T(3) hormone. However, these two isoforms display very different transcriptional properties on each individual target gene, ranging from a much greater T(3)-mediated regulation by TR alpha 1 than by TR beta 1, to near equal regulation by both isoforms. We also identified TR alpha 1 and TR beta 1 target genes that were regulated by these receptors in a hormone-independent fashion. We suggest that it is this gene-specific, isoform-specific amplitude of transcriptional regulation that is the likely basis for the appearance and maintenance of TR alpha 1 and TR beta 1 over evolutionary time. In essence, TR alpha 1 and TR beta 1 adjust the magnitude of the transcriptional response at different target genes to different levels; by altering the ratio of these isoforms in different tissues or at different developmental times, the intensity of T(3) response can be individually tailored to different physiological and developmental requirements.

The p160 coactivator PAS-B motif stabilizes nuclear receptor binding and contributes to isoform-specific regulation by thyroid hormone receptors

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that play multiple roles in vertebrate endocrinology and development. TRs are expressed as a series of distinct receptor isoforms that mediate different biological functions. The TRbeta2 isoform is expressed primarily in the hypothalamus, pituitary, cochlea, and retina, and displays an enhanced response to hormone agonist relative to the other TR isoforms. We report here that the unusual transcriptional properties of TRbeta2 parallel the ability of this isoform to bind p160 coactivators cooperatively through multiple contact surfaces; the more broadly expressed TRbeta1 isoform, in contrast, utilizes a single contact mechanism. Intriguingly, the PAS-B domain in the p160 N terminus plays a previously unanticipated role in permitting TRbeta2 to recruit coactivator at limiting triiodothyronine concentrations. The PAS-B sequences also play an important role in coactivator binding by estrogen receptor-alpha. We propose that the PAS-B domain of the p160 coactivators is an important modulator of coactivator recruitment for a specific subset of nuclear receptors, permitting stronger transcriptional activation at lower hormone concentrations than would otherwise occur, and allowing isoform-specific mRNA splicing to customize the hormone response in different tissues.

Molecular basis for dimer formation of TRbeta variant D355R

Protein quality and stability are critical during protein purification for X-ray crystallography. A target protein that is easy to manipulate and crystallize becomes a valuable product useful for high-throughput crystallography for drug design and discovery. In this work, a single surface mutation, D355R, was shown to be crucial for converting the modestly stable monomeric ligand binding domain of the human thyroid hormone receptor (TR LBD) into a stable dimer. The structure of D335R TR LBD mutant was solved using X-ray crystallography and refined to 2.2 A resolution with R(free)/R values of 24.5/21.7. The crystal asymmetric unit reveals the TR dimer with two molecules of the hormone-bound LBD related by twofold symmetry. The ionic interface between the two LBDs comprises residues within loop H10-H11 and loop H6-H7 as well as the C-terminal halves of helices 8 of both protomers. Direct intermolecular contacts formed between the introduced residue Arg 355 of one TR molecule and Glu 324 of the second molecule become a part of the extended dimerization interface of 1330 A(2) characteristic for a strong complex assembly that is additionally strengthened by buffer solutes.

Only subtle protein conformational adaptations are required for ligand binding to thyroid hormone receptors: simulations using a novel multipoint steered molecular dynamics approach

Thyroid hormone receptors (TR) are hormone-dependent transcription regulators that play a major role in human health, development, and metabolic functions. The thyroid hormone resistance syndrome, diabetes, obesity, and some types of cancer are just a few examples of important diseases that are related to TR malfunctioning, particularly impaired hormone binding. Ligand binding to and dissociation from the receptor ultimately control gene transcription and, thus, detailed knowledge of binding and release mechanisms are fundamental for the comprehension of the receptor's biological function and development of pharmaceuticals. In this work, we present the first computational study of ligand entry into the ligand binding domain (LBD) of a nuclear receptor. We report molecular dynamics simulations of ligand binding to TRs using a generalization of the steered molecular dynamics technique designed to perform single-molecule pulling simulations along arbitrarily nonlinear driving pathways. We show that only gentle protein movements and conformational adaptations are required for ligand entry into the LBDs and that the magnitude of the forces applied to assist ligand binding are of the order of the forces involved in ligand dissociation. Our simulations suggest an alternative view for the mechanisms ligand binding and dissociation of ligands from nuclear receptors in which ligands can simply diffuse through the protein surface to reach proper positioning within the binding pocket. The proposed picture indicates that the large-amplitude protein motions suggested by the apo- and holo-RXRalpha crystallographic structures are not required, reconciling conformational changes of LBDs required for ligand entry with other nuclear receptors apo-structures that resemble the ligand-bound LBDs.

Biometrology of physical properties of skin in thyroid dysfunction

OBJECTIVE

There is ample clinical evidence that skin is responsive to physiopathological levels of circulating thyroid hormones. The aim of the study was to assess some physical changes of the skin in the presence of moderate thyroid dysfunction.

SETTING

University Hospital.

PATIENTS AND METHODS

A total of 119 adults suffering from hypothyroidism or hyperthyroidism and 60 healthy controls were enrolled in this study. Hormonal dosages (TSH, fT3, fT4) were assessed in the serum. A series of biometrological assessments were also performed on the volar and dorsal aspects of the forearms. These included electrometric assessments (Nova Dermal Phase Meter, Corneometer), evaporimetry (Tewameter)), colorimetry (Mexameter), ultrasound shear wave propagation (Reviscometer) and squamometry X. Correlations were searched between each of the serum hormonal dosages and each of the biometrological parameters.

RESULTS

The hormonal changes in the untreated patients with thyroid dysfunction were modest in intensity. A few outlier values with regard to the normal range were found for each biometrological parameter. No correlations were found between fT3 or fT4 and each of the physical parameters. By contrast, significant negative linear correlations were found between thyroid-stimulating hormone (TSH) and skin hydration measured by the Corneometer and the Nova DPM.

CONCLUSION

This multipronged exploratory study shows that direct or indirect effects of TSH may influence the stratum corneum hydration. This correlation seemed very sensitive, as no other specific biophysical parameter was significantly correlated with the thyroid hormonal concentrations in the serum. However, our findings do not exclude the possibility of some other skin changes supervening in case of more severe thyroid dysfunction. The mechanism by which TSH alters the stratum corneum hydration is yet unknown.

Coactivator recruitment is enhanced by thyroid hormone receptor trimers

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors. TRs are generally thought to bind to their DNA target sites as homodimers or as TR/retinoid X receptor (RXR) heterodimers. However, we have shown that certain TR isoforms, such as TRbeta0, can bind as trimers to a subset of naturally occurring DNA elements. We report here that this trimeric mode of DNA recognition by TRbeta0 also results in an enhanced recruitment of coactivators in vitro and increased transcriptional activation in cells compared to TRbeta0 dimers. At least part of this enhanced coactivator recruitment reflects a selectively enhanced avidity of the TRbeta0 trimer for a specific LXXLL interaction motif within the p160 coactivators. TRbeta0 trimers also recruit certain coactivators at lower concentrations of T3 hormone and exhibit distinct coactivator stoichiometries than do TRbeta0 dimers. We conclude that trimer formation confers isoform-specific DNA recognition and transcriptional regulatory properties that are not observed for TR dimers.

An improved high throughput protein-protein interaction assay for nuclear hormone receptors

The Glutathione-S-Transferase (GST) “pulldown” assay has been used extensively to assay protein interactions in vitro. This methodology has been especially useful for investigating the interactions of nuclear hormone receptors with a wide variety of their interacting partners and coregulatory proteins. Unfortunately, the original GST-pulldown technique relies on multiple binding, washing and elution steps performed in individual microfuge tubes, and requires repeated centrifugation, aspiration, and suspension steps. This type of batch processing creates a significant liquid handling bottleneck, limiting the number of sample points that can be incorporated into one experiment and producing inherently less efficient washing and elution than would a flow-through methodology. In this manuscript, we describe the adaptation of this GST-pulldown assay to a 96-well filter plate format. The use of a multi-well filter plate makes it possible to assay more samples in significantly less time using less reagents and more efficient sample processing than does the traditional single tube assay.

The dominant negative thyroid hormone receptor beta-mutant {Delta}337T alters PPAR{alpha} signaling in heart

PPARalpha and TR independently regulate cardiac metabolism. Although ligands for both these receptors are currently under evaluation for treatment of congestive heart failure, their interactions or signaling cooperation have not been investigated in heart. We tested the hypothesis that cardiac TRs interact with PPARalpha regulation of target genes and used mice exhibiting a cardioselective Delta337T TRbeta1 mutation (MUT) to reveal cross-talk between these nuclear receptors. This dominant negative transgene potently inhibits DNA binding for both wild-type (WT) TRalpha and TRbeta. We used UCP3 and MTE-1 as principal reporters and analyzed gene expression from hearts of transgenic (MUT) and nontransgenic (WT) littermates 6 h after receiving either specific PPARalpha ligand (WY-14643) or vehicle. Interactions were determined through qRT-PCR analyses, and the extent of these interactions across multiple genes was determined using expression arrays. In the basal state, we detected no differences between groups for protein content for UCP3, PPARalpha, TRalpha2, RXRbeta, or PGC-1alpha. However, protein content for TRalpha1 and the PPARalpha heterodimeric partner RXRalpha was diminished in MUT, whereas PPARbeta increased. We demonstrated cross-talk between PPAR and TR for multiple genes, including the reporters UCP3 and MTE1. WY-14643 induced a twofold increase in UCP3 gene expression that was totally abrogated in MUT. We demonstrated variable cross-talk patterns, indicating that multiple mechanisms operate according to individual target genes. The non-ligand-binding TRbeta1 mutation alters expression for multiple nuclear receptors, providing a novel mechanism for interaction that has not been previously demonstrated. These results indicate that therapeutic response to PPARalpha ligands may be determined by thyroid hormone state and TR function.

New and improved glucocorticoid receptor ligands

The therapeutic and prophylactic use of glucocorticoids is widespread due to their powerful anti-inflammatory, antiproliferative and immunomodulatory activity. However, long-term use of these drugs can result in severe dose-limiting side effects. One of the most critical and debilitating side effects is osteoporosis, which leads to increased risk of fractures. Glucocorticoids damage bone through several different mechanisms. The search for novel glucocorticoids that have reduced side effects in bone and other tissues is being driven by the identification of new mechanisms of action of the glucocorticoid receptor. This may facilitate the detection of new, safer therapies with efficacies equivalent to currently prescribed steroids.

Thyroid hormone receptors mutated in liver cancer function as distorted antimorphs

Aberrant thyroid hormone receptors (TRs) are found in over 70% of the human hepatocellular carcinomas (HCCs) analysed. To better understand the role(s) of these TR mutants in this neoplasia, we analysed a panel of HCC mutant receptors for their molecular properties. Virtually all HCC-associated TR mutants tested retained the ability to repress target genes in the absence of T3, yet were impaired in T3-driven gene activation and functioned as dominant-negative inhibitors of wild-type TR activity. Intriguingly, the HCC TRalpha1 mutants exerted dominant-negative interference at all T3 concentrations tested, whereas the HCC TRbeta1 mutants were dominant-negatives only at low and intermediate T3 concentrations, reverting to transcriptional activators at higher hormone levels. The relative affinity for the SMRT versus N-CoR corepressors was detectably altered for several of the HCC mutant TRs, suggesting changes in corepressor preference and recruitment compared to wild type. Several of the TRalpha HCC mutations also altered the DNA recognition properties of the encoded receptors, indicating that these HCC TR mutants may regulate a distinct set of target genes from those regulated by wild-type TRs. Finally, whereas wild-type TRs interfere with c-Jun/AP-1 function in a T3-dependent fashion and suppress anchorage-independent growth when ectopically expressed in HepG2 cells, at least certain of the HCC mutants did not exert these inhibitory properties. These alterations in transcriptional regulation and DNA recognition appear likely to contribute to oncogenesis by reprogramming the differentiation and proliferative properties of the hepatocytes in which the mutant TRs are expressed.

Molecular dynamics simulations reveal multiple pathways of ligand dissociation from thyroid hormone receptors

Nuclear receptor (NR) ligands occupy a pocket that lies within the core of the NR ligand-binding domain (LBD), and most NR LBDs lack obvious entry/exit routes upon the protein surface. Thus, significant NR conformational rearrangements must accompany ligand binding and release. The precise nature of these processes, however, remains poorly understood. Here, we utilize locally enhanced sampling (LES) molecular dynamics computer simulations to predict molecular motions of x-ray structures of thyroid hormone receptor (TR) LBDs and determine events that permit ligand escape. We find that the natural ligand 3,5,3'-triiodo-L-thyronine (T(3)) dissociates from the TRalpha1 LBD along three competing pathways generated through i), opening of helix (H) 12; ii), separation of H8 and H11 and the Omega-loop between H2 and H3; and iii), opening of H2 and H3, and the intervening beta-strand. Similar pathways are involved in dissociation of T(3) and the TRbeta-selective ligand GC24 from TRbeta; the TR agonist IH5 from the alpha- and beta-TR forms; and Triac from two natural human TRbeta mutants, A317T and A234T, but are detected with different frequencies in simulations performed with the different structures. Path I was previously suggested to represent a major pathway for NR ligand dissociation. We propose here that Paths II and III are also likely ligand escape routes for TRs and other NRs. We also propose that different escape paths are preferred in different situations, implying that it will be possible to design NR ligands that only associate stably with their cognate receptors in specific cellular contexts.

Rearrangements in Thyroid Hormone Receptor Charge Clusters That StabilizeBound 3,5′,5-Triiodo-L-thyronine and Inhibit HomodimerFormation

In this study, we investigated how thyroid hormone (3,5',5-triiodo-l-thyronine, T3) inhibits binding of thyroid hormone receptor (TR) homodimers, but not TR-retinoid X receptor heterodimers, to thyroid hormone response elements. Specifically we asked why a small subset of TRbeta mutations that arise in resistance to thyroid hormone syndrome inhibit both T3 binding and formation of TRbeta homodimers on thyroid hormone response elements. We reasoned that these mutations may affect structural elements involved in the coupling of T3 binding to inhibition of TR DNA binding activity. Analysis of TR x-ray structures revealed that each of these resistance to thyroid hormone syndrome mutations affects a cluster of charged amino acids with potential for ionic bond formation between oppositely charged partners. Two clusters (1 and 2) are adjacent to the dimer surface at the junction of helices 10 and 11. Targeted mutagenesis of residues in Cluster 1 (Arg338, Lys342, Asp351, and Asp355) and Cluster 2 (Arg429, Arg383, and Glu311) confirmed that the clusters are required for stable T3 binding and for optimal TR homodimer formation on DNA but also revealed that different arrangements of charged residues are needed for these effects. We propose that the charge clusters are homodimer-specific extensions of the dimer surface and further that T3 binding promotes specific rearrangements of these surfaces that simultaneously block homodimer formation on DNA and stabilize the bound hormone. Our data yield insight into the way that T3 regulates TR DNA binding activity and also highlight hitherto unsuspected T3-dependent conformational changes in the receptor ligand binding domain.

A Novel Mechanism of Thyroid Hormone-dependent Negative Regulation by Thyroid Hormone Receptor, Nuclear Receptor Corepressor (NCoR), and GAGA-binding Factor on the Rat CD44 Promoter

CD44 is an adhesion molecule in the extracellular matrix that shows various functions, including tumor genesis and metastasis. A recent study showed that CD44 expression level was strongly correlated with the generation of papillary thyroid carcinomas, the most prevalent malignancy of the thyroid gland. We report here that CD44 is negatively regulated by thyroid hormone (T(3)) through a novel mechanism. We demonstrate that nuclear receptor corepressor (NCoR) enhances thyroid hormone receptor (TR)-mediated basal transactivation by a weak TR.DNA interaction in the absence of T(3), which is repressed by T(3) through a transient TR .DNA interaction. Initially, we identified that CD44 was negatively directly transcriptionally T(3) -responsive. Deletion and mutation analysis indicated that both a weak TR and a GAGA-binding factor (GAF) binding sites on the CD44 promoter were required for negative regulation by T(3). The weak TR.DNA interaction was further confirmed by electrophoretic gel mobility shift assay, chromatin immunoprecipitation, and transfection assays using a non-DNA-binding TRalpha1 mutant. More interestingly, NCoR acted as a co-activator to enhance TR-mediated basal transactivation in the absence of T(3). This effect was eliminated by removal of TR or NCoR binding. Most strikingly, T(3) induced a remarkable increase in TR.DNA binding at 40-60 min after T(3) exposure that rapidly returned to basal levels, suggesting a T(3)-induced remodeling of chromatin structure at the early stage of T(3) stimulation resulting in repression. Therefore, we propose a mechanism by which NCoR, GAF, and TR interact with the CD44 negative T(3)-responsive element to enhance basal transactivation, whereas T(3) induces the remodeling of chromatin structure for repression.

Endocardial and epicardial derived FGF signals regulate myocardial proliferation and differentiation in vivo

The epicardium regulates growth and survival of the underlying myocardium. This activity depends on intrinsic retinoic acid (RA) and erythropoietin signals. However, these signals do not act directly on the myocardium and instead are proposed to regulate the production of an unidentified soluble epicardial derived mitogen. Here, we show that Fgf9, Fgf16, and Fgf20 are expressed in the endocardium and epicardium and that RA can induce epicardial expression of Fgf9. Using knockout mice and an embryonic heart organ culture system, we show that endocardial and epicardial derived FGF signals regulate myocardial proliferation during midgestation heart development. We further show that this FGF signal is received by both FGF receptors 1 and 2 acting redundantly in the cardiomyoblast. In the absence of this signal, premature differentiation results in cellular hypertrophy and newborn mice develop a dilated cardiomyopathy. FGFs thus constitute all or part of the epicardial signal regulating myocardial growth and differentiation.

Mutant-Selective Thyromimetics for the Chemical Rescue of Thyroid Hormone Receptor Mutants Associated with Resistance to Thyroid Hormone†

The thyroid hormone receptors (TRs) are ligand-dependent transcription factors that control the expression of multiple genes involved in development and homeostasis in response to thyroid hormone (triiodothyronine, T3). Mutations to TRbeta that reduce or abolish ligand-dependent transactivation function are associated with resistance to thyroid hormone (RTH), an autosomal dominant human genetic disease. A series of neutral alcohol-based compounds, based on the halogen-free thyromimetic GC-1, have been designed, synthesized, and evaluated in cell-based assays for their ability to selectively rescue three of the most common RTH-associated mutations (i.e., Arg320 --> Cys, Arg320 --> His, and Arg316 --> His) that affect the basic carboxylate-binding arginine cluster of TRbeta. Several analogues show improved potency and activity in the mutant receptors relative to the parent compound GC-1. Most significantly, two of these mutant-complementing thyromimics show high potency and activity with a strong preference for the mutant receptors over wild-type TRalpha(wt), that is associated with the cardiotoxic actions of T3. The compounds were evaluated in reporter gene assays using the four common thyroid hormone response elements, DR4, PAL, F2 (LAP), and TSH, and show activities and selectivites consistent with their unique potential as agents to selectively rescue thyroid function to these RTH-associated mutants.

Nuclear hormone receptor co-repressors

Gene silencing is an essential transcriptional regulatory process. Co-repressors mediate gene repression through their recruitment by DNA bound transcriptional silencer proteins. Co-repressors repress gene expression through several mechanisms, mostly investigated on the level of chromatin. Lack or aberrant gene silencing is associated with many defects both on cellular and organismic level. Several human diseases are based on dysregulated co-repressor binding to transcriptional silencers indicating that co-repressor recruitment and the strength of gene silencing must be under strict control. In line with that gene silencing is important for animal development, cellular proliferation and transformation. Co-repressors play also a major role in the treatment of hormone-dependent growing cancers, such as for breast and prostate cancer therapy. The molecular basis of anti-hormone therapy lies in the recruitment of co-repressors to the estrogen or androgen receptors, respectively, which leads to their inactivation and to inhibition of cancer growth. The molecular mechanisms of selected topics are summarized here.

Heterodimers of retinoic acid receptors and thyroid hormone receptors display unique combinatorial regulatory properties

Nuclear receptors are ligand-regulated transcription factors that regulate key aspects of metazoan development, differentiation, and homeostasis. Nuclear receptors recognize target genes by binding to specific DNA recognition sequences, denoted hormone response elements (HREs). Many nuclear receptors can recognize HREs as either homodimers or heterodimers. Retinoid X receptors (RXRs), in particular, serve as important heterodimer partners for many other nuclear receptors, including thyroid hormone receptors (TRs), and RXR/TR heterodimers have been proposed to be the primary mediators of target gene regulation by T3 hormone. Here, we report that the retinoic acid receptors (RARs), a distinct class of nuclear receptors, are also efficient heterodimer partners for TRs. These RAR/TR heterodimers form with similar affinities as RXR/TR heterodimers on an assortment of consensus and natural HREs, and preferentially assemble with the RAR partner 5' of the TR moiety. The corepressor and coactivator recruitment properties of these RAR/TR heterodimers and their transcriptional activities in vivo are distinct from those observed with the corresponding RXR heterodimers. Our studies indicate that RXRs are not unique in their ability to partner with TRs, and that RARs can also serve as robust heterodimer partners and combinatorial regulators of T3-modulated gene expression.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

Gene silencing by the thyroid hormone receptor

The thyroid hormone receptors (TR) are able to bind DNA and to repress transcription in the absence of thyroid hormone. This repression function is an important feature of TRs as aberrant silencing can lead to severe diseases and developmental abnormalities. TR utilizes different mechanisms to achieve repression of target genes including the recruitment of cofactors called corepressors and interference with the basal transcriptional machinery. Recent studies have revealed an important role of chromatin in TR silencing involving different histone modifications and the responsible enzymes. Furthermore, the transcriptional properties of TR depend on the type of the TR DNA-binding elements. This review will focus on the molecular basis of gene silencing by TR and diseases caused by aberrant functioning.

Thyroid-stimulating hormone receptor and its role in Graves' disease

The thyroid-stimulating hormone (TSH, or thyrotropin) receptor (TSHR) mediates the activating action of TSH to the thyroid gland, resulting in the growth and proliferation of thyrocytes and thyroid hormone production. In Graves' disease, thyroid-stimulating autoantibodies can mimic TSH action and stimulate thyroid cells. This leads to hyperthyroidism and abnormal overproduction of thyroid hormone. TSHR-antibodies-binding epitopes on the receptor molecule are well studied. Mechanism of TSHR-autoantibodies production is more or less clear but a susceptibility gene, which is linked to their production, is still unknown. Genetic studies show no linkage between the TSHR gene and Graves' disease. Among three common polymorphisms in the TSHR gene, only the D727E germline polymorphism in the cytoplasmic tail of the receptor showed an association with the disease, and this association is weak. The absence of a strong genetic effect of the TSHR polymorphisms in such a common and complex disorder as Graves' disease may be explained by a high degree of evolutionary conservation in TSHR. This can be shown by naturally existing germline and somatic mutations in the TSHR gene that cause various types of nonautoimmune and hereditary thyroid disease.

Thyroid hormone induction of the adrenoleukodystrophy-related gene (ABCD2)

X-linked adrenoleukodystrophy (X-ALD) is a demyelinating disorder associated with impaired very-long-chain fatty-acid (VLCFA) beta-oxidation caused by mutations in the ABCD1 (ALD) gene that encodes a peroxisomal membrane ABC transporter. ABCD2 (ALDR) displays partial functional redundancy because when overexpressed, it is able to correct the X-ALD biochemical phenotype. The ABCD2 promoter contains a putative thyroid hormone-response element conserved in rodents and humans. In this report, we demonstrate that the element is capable of binding retinoid X receptor and 3,5,3'-tri-iodothyronine (T3) receptor (TRbeta) as a heterodimer and mediating T3 responsiveness of ABCD2 in its promoter context. After a T3 treatment, an induction of the ABCD2 gene was observed in the liver of normal rats but not that of TRbeta-/- mice. ABCD2 was not induced in the brain of the T3-treated rats. However, we report for the first time that induction of the ABCD2 redundant gene is feasible in myelin-producing cells (differentiated CG4 oligodendrocytes). The induction was specific for this cell type because it did not occur in astrocytes. Furthermore, we observed T3 induction of ABCD2 in human and mouse ABCD1-deficient fibroblasts, which was correlated with normalization of the VLCFA beta-oxidation. Finally, ABCD3 (PMP70), a close homolog of ABCD2, was also induced by T3 in the liver of control rats, but not that of TRbeta-/- mice, and in CG4 oligodendrocytes.

Triiodothyronine decreases the activity of the proximal promoter (PII) of the aromatase gene in the mouse Sertoli cell line, TM4

Estrogens and thyroid hormones play a significant role in regulating functions and development of the testis. The synthesis of estrogens from androgens is catalyzed by the enzyme complex termed aromatase, which in the testis displays an age-related cellular compartmentalization, primarily in Sertoli cells in immature animals, whereas in adults it is expressed in Leydig and germ cells. T3 induces a precocious terminal differentiation of prepubertal Sertoli cells together with a dramatic decrease of their aromatase activity. In the present work, we have examined the mechanism by which T3 exerts this inhibitory action on aromatase expression. As an experimental model, we used the mouse Sertoli cell line TM4, which conserves a large spectrum of functional features present in immature Sertoli cells. For instance, after revealing the presence of aromatase by immunocytochemistry and measuring its enzymatic activity, we confirmed in this cell line the functional events previously characterized in primary cultures of immature rat Sertoli cells: 1) a strong stimulation of aromatase activity by dibutyryl-cAMP [(Bu)2cAMP] (simulating FSH action); and 2) the inhibition of aromatase activity by incubation with T3 under basal condition and after (Bu)2cAMP stimulation. After identifying promoter II as the regulatory region located immediately upstream of the transcriptional initiation site in the TM4 cell line by rapid amplification of cDNA ends analysis, we conducted experiments to examine the molecular mechanism by which thyroid hormones modulate aromatase gene expression in this cell line. TM4 cells were transfected with plasmids containing different segments of the rat promoter II sequence ligated to a luciferase reporter gene. Analysis of the activities of these promoter fusions demonstrated that T3 inhibits basal and (Bu)2cAMP-stimulated activity of the aromatase promoter. This effect was not revealed in T3-treated cells transfected with construct in which the steroidogenic factor-1 (SF-1) response element was mutated. These results indicate that the inhibitory effect of T3 requires the integrity of the SF-1 response element and are further supported in the EMSA. The EMSA experiments demonstrated that thyroid hormone/thyroid receptor alpha1 complex (TH/TRalpha1) is able to compete with SF-1 in binding to oligonucleotides containing an SF-1 motif, an element essential for the activity of the PII aromatase promoter. The findings suggest that the binding of the thyroid hormone/thyroid receptor alpha1 complex to the SF-1 motif is the molecular mechanism by which T3 exerts an inhibitory effect on aromatase gene expression in the TM4 cell line.

Retinoic acid receptors beta and gamma do not repress, but instead activate target gene transcription in both the absence and presence of hormone ligand

Retinoic acid receptors (RARs) are important mediators of retinoid signaling in morphogenesis, development, and cell differentiation. Three major isotypes of RARs, denoted alpha, beta, and gamma, have been identified, each encoded by a distinct genetic locus. Although RARalpha, RARbeta, and RARgamma share many structural and functional features, these three isotypes are known to play unique, as well as overlapping, roles in physiology and development. We report here that the three RAR isotypes display different transcriptional properties in the absence of hormone ligand; under these conditions, RARalpha is a strong repressor of target gene expression, whereas both RARbeta and RARgamma fail to repress and instead are able to mediate substantial levels of hormone-independent transcriptional activation. These differing transcriptional properties appear to reflect the differing abilities of the three RAR isotypes to interact with the SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) corepressor protein: RARalpha binds to SMRT strongly both in vitro and in vivo, whereas RARbeta and RARgamma interact only weakly with SMRT. The ability to repress or to activate transcription in the absence of hormone maps predominantly to isotype-specific differences in the sequence of helix 3 within the hormone binding domain of the RARs, and the transcriptional properties of one isotype can be exchanged with that of another by exchanging portions of helix 3. The different transcriptional properties of RARalpha, RARbeta, and RARgamma in the absence of hormone contribute to the distinctive biological functions of these proteins and provide a rationale for the strong conservation of the three distinct isotypes during the vertebrate evolutionary radiation.

Design of thyroid hormone receptor antagonists from first principles

It is desirable to obtain TR antagonists for treatment of hyperthyroidism and other conditions. We have designed TR antagonists from first principles based on TR crystal structures. Since agonist ligands are buried in the fold of the TR ligand binding domain (LBD), we reasoned that ligands that resemble agonists with large extensions should bind the LBD, but would prevent its folding into an active conformation. In particular, we predicted that extensions at the 5' aryl position of ligand should reposition helix (H) 12, which forms part of the co-activator binding surface, and thereby inhibit TR activity. We have found that some synthetic ligands with 5' aryl ring extensions behave as antagonists (DIBRT, NH-3), or partial antagonists (GC-14, NH-4). Moreover, one compound (NH-3) represents the first potent TR antagonist with nanomolar affinity that also inhibits TR action in an animal model. However, the properties of the ligands also reveal unexpected aspects of TR behavior. While nuclear receptor antagonists generally promote binding of co-repressors, NH-3 blocks co-activator binding and also prevents co-repressor binding. More surprisingly, many compounds with extensions behave as full or partial agonists. We present hypotheses to explain both behaviors in terms of dynamic equilibrium of H12 position.

Competitive cofactor recruitment by orphan receptor hepatocyte nuclear factor 4alpha1: modulation by the F domain

For most ligand-dependent nuclear receptors, the status of endogenous ligand modulates the relative affinities for corepressor and coactivator complexes. It is less clear what parameters modulate the switch between corepressor and coactivator for the orphan receptors. Our previous work demonstrated that hepatocyte nuclear factor 4alpha1 (HNF4alpha1, NR2A1) interacts with the p160 coactivator GRIP1 and the cointegrators CBP and p300 in the absence of exogenously added ligand and that removal of the F domain enhances these interactions. Here, we utilized transient-transfection analysis to demonstrate repression of HNF4alpha1 activity by the corepressor silencing mediator of retinoid and thyroid receptors (SMRT) in several cell lines and on several HNF4alpha-responsive promoter elements. Glutathione S-transferase pulldown assays confirmed a direct interaction between HNF4alpha1 and receptor interaction domain 2 of SMRT. Loss of the F domain resulted in marked reduction of the ability of SMRT to interact with HNF4alpha1 in vitro and repress HNF4alpha1 activity in vivo, although the isolated F domain itself failed to interact with SMRT. Surprisingly, loss of both the A/B and F domains restored full repression by SMRT, suggesting involvement of both domains in the SMRT interaction. Finally, we show that when coexpressed along with HNF4alpha1 and GRIP1, CBP, or p300, SMRT can titer out HNF4alpha1-mediated transactivation in a dose-dependent manner and that this competition derives from mutually exclusive binding. Collectively, these results suggest that HNF4alpha can functionally interact with both a coactivator and a corepressor without altering the status of any putative ligand and that the presence of the F domain may play a role in discriminating between the different coregulators.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.