Hormone selectivity in thyroid hormone receptors

Electrochemical degradation of diclofenac generates unexpected thyroidogenic transformation products: Implications for environmental risk assessment

Diclofenac (DCF) is an environmentally persistent, nonsteroidal anti-inflammatory drug (NSAID) with thyroid disrupting properties. Electrochemical advanced oxidation processes (eAOPs) can efficiently remove NSAIDs from wastewater. However, eAOPs can generate transformation products (TPs) with unknown chemical and biological characteristics. In this study, DCF was electrochemically degraded using a boron-doped diamond anode. Ultra-high performance liquid chromatography coupled with high-resolution mass spectrometry was used to analyze the TPs of DCF and elucidate its potential degradation pathways. The biological impact of DCF and its TPs was evaluated using the Xenopus Eleutheroembryo Thyroid Assay, employing a transgenic amphibian model to assess thyroid axis activity. As DCF degradation progressed, in vivo thyroid activity transitioned from anti-thyroid in non-treated samples to pro-thyroid in intermediately treated samples, implying the emergence of thyroid-active TPs with distinct modes of action compared to DCF. Molecular docking analysis revealed that certain TPs bind to the thyroid receptor, potentially triggering thyroid hormone-like responses. Moreover, acute toxicity occurred in intermediately degraded samples, indicating the generation of TPs exhibiting higher toxicity than DCF. Both acute toxicity and thyroid effects were mitigated with a prolonged degradation time. This study highlights the importance of integrating in vivo bioassays in the environmental risk assessment of novel degradation processes.

Per- and polyfluoroalkyl substances and the associated thyroid cancer risk: A case-control study in China

The role of perfluoroalkyl and polyfluoroalkyl substances (PFAS) as thyroid carcinogens is unclear. Therefore, we intended to identify associations between each PFAS congener and their mixture with thyroid cancer risk. This case-control study of thyroid cancer was conducted in Shijiazhuang, Hebei Province, China. Three hundred participants were recruited from January to May 2022 and were matched according to sex and age. Twelve PFAS were assessed using ultra-high-performance liquid chromatography-tandem mass spectrometry. Associations between PFAS congeners and thyroid cancer risk were considered under conditional logistic regression analysis and a restricted cubic spline model. Mixture effects were also assessed with quantile g-computation and a Bayesian kernel machine regression model. Compared to the first tertile, third tertile PFOA, PFNA, PFHxS, PFDA, and PFUnDA concentrations were associated with lower thyroid cancer risk (ORPFOA: 0.32, 95% confidence interval (CI): 0.15-0.69; ORPFNA: 0.18, 95% CI: 0.07-0.46; ORPFHxS: 0.37, 95% CI: 0.15-0.92; ORPFDA: 0.07, 95% CI: 0.02-0.23; ORPFUnDA: 0.12, 95% CI: 0.05-0.30) after adjusting for confounding factors. PFNA, PFDA, and PFUnDA had a negative dose-response relationship with thyroid cancer risk. Mixture analysis also showed that thyroid cancer risk is negatively associated with the overall mixture and carboxylates. In the overall mixture, PFOS and PFDA contributed most to positive and negative changes in thyroid cancer risk, respectively. However, PFOS, PFNA, PFDA, and PFUnDA were of equally high importance. This study is the first to confirm the effects of the PFAS mixture on thyroid cancer, and further large-scale prospective studies are still warranted to test these inverse associations.

TRβ Agonism Induces Tumor Suppression and Enhances Drug Efficacy in Anaplastic Thyroid Cancer in Female Mice

Thyroid hormone receptor beta (TRβ) is a recognized tumor suppressor in numerous solid cancers. The molecular signaling of TRβ has been elucidated in several cancer types through re-expression models. Remarkably, the potential impact of selective activation of endogenous TRβ on tumor progression remains largely unexplored. We used cell-based and in vivo assays to evaluate the effects of the TRβ agonist sobetirome (GC-1) on a particularly aggressive and dedifferentiated cancer, anaplastic thyroid cancer (ATC). Here we report that GC-1 reduced the tumorigenic phenotype, decreased cancer stem-like cell populations, and induced redifferentiation of the ATC cell lines with different mutational backgrounds. Of note, this selective activation of TRβ amplified the effects of therapeutic agents in blunting the aggressive cell phenotype and stem cell growth. In xenograft assays, GC-1 alone inhibited tumor growth and was as effective as the kinase inhibitor, sorafenib. These results indicate that selective activation of TRβ not only induces a tumor suppression program de novo but enhances the effectiveness of anticancer agents, revealing potential novel combination therapies for ATC and other aggressive solid tumors.

Thyroid hormone action and liver disease, a complex interplay

Thyroid hormone action is involved in virtually all physiological processes. It is well known that the liver and thyroid are intimately linked, with thyroid hormone playing important roles in de novo lipogenesis, beta-oxidation (fatty acid oxidation), cholesterol metabolism, and carbohydrate metabolism. Clinical and mechanistic research studies have shown that thyroid hormone can be involved in chronic liver diseases, including alcohol-associated or NAFLD and HCC. Thyroid hormone action and synthetic thyroid hormone analogs can exert beneficial actions in terms of lowering lipids, preventing chronic liver disease and as liver anticancer agents. More recently, preclinical and clinical studies have indicated that some analogs of thyroid hormone could also play a role in the treatment of liver disease. These synthetic molecules, thyromimetics, can modulate lipid metabolism, particularly in NAFLD/NASH. In this review, we first summarize the thyroid hormone signaling axis in the context of liver biology, then we describe the changes in thyroid hormone signaling in liver disease and how liver diseases affect the thyroid hormone homeostasis, and finally we discuss the use of thyroid hormone-analog for the treatment of liver disease.

Thyroid Hormone Receptor-β Agonists in NAFLD Therapy: Possibilities and Challenges

Nonalcoholic fatty liver disease (NAFLD) is a progressive metabolic liver disease with an unknown pathogenesis and no FDA-approved drug treatment to date. Hypothyroidism has been identified as a risk factor for NAFLD as thyroxine is required for regulating metabolism in adults. Thyroxine has been shown to reduce fat in the livers of murine models with experimentally induced NAFLD. The use of synthetic thyroxine has been shown to increase lipid metabolism leading to weight loss; however, thyroxine has also been shown to cause many side effects, especially in the heart. Overcoming these cardiac side effects involves designing agonists specific to one of the 2 gene subtypes for the thyroid hormone (TH) receptor (TR), TRβ. While the other TH receptor subtype, TRα, is mainly expressed in the heart and is responsible for thyroxine's cardiac function, TRβ is mainly expressed in the liver and is involved in liver function. Using TRβ-specific agonists to treat NAFLD can prevent cardiac and other adverse side effects. Several TRβ-specific agonists have shown positive therapeutic effects in NAFLD animal models and have entered clinical trials. We seek to provide a comprehensive updated reference of TRβ-specific agonists in this review and explore the future therapeutic potential of TRβ-specific activation in the treatment of NAFLD.

Regulation of gene transcription by thyroid hormone receptor β agonists in clinical development for the treatment of non-alcoholic steatohepatitis (NASH)

Thyroid hormones are important modulators of metabolic activity in mammals and alter cholesterol and fatty acid levels through activation of the nuclear thyroid hormone receptor (THR). Currently, there are several THRβ agonists in clinical trials for the treatment of non-alcoholic steatohepatitis (NASH) that have demonstrated the potential to reduce liver fat and restore liver function. In this study, we tested three THRβ-agonism-based NASH treatment candidates, GC-1 (sobetirome), MGL-3196 (resmetirom), and VK2809, and compared their selectivity for THRβ and their ability to modulate the expression of genes specific to cholesterol and fatty acid biosynthesis and metabolism in vitro using human hepatic cells and in vivo using a rat model. Treatment with GC-1 upregulated the transcription of CPT1A in the human hepatocyte-derived Huh-7 cell line with a dose-response comparable to that of the native THR ligand, triiodothyronine (T3). VK2809A (active parent of VK2809), MGL-3196, and VK2809 were approximately 30-fold, 1,000-fold, and 2,000-fold less potent than T3, respectively. Additionally, these relative potencies were confirmed by quantification of other direct gene targets of THR, namely, ANGPTL4 and DIO1. In primary human hepatocytes, potencies were conserved for every compound except for VK2809, which showed significantly increased potency that was comparable to that of its active counterpart, VK2809A. In high-fat diet fed rats, a single dose of T3 significantly reduced total cholesterol levels and concurrently increased liver Dio1 and Me1 RNA expression. MGL-3196 treatment resulted in concentration-dependent decreases in total and low-density lipoprotein cholesterol with corresponding increases in liver gene expression, but the compound was significantly less potent than T3. In conclusion, we have implemented a strategy to rank the efficacy of THRβ agonists by quantifying changes in the transcription of genes that lead to metabolic alterations, an effect that is directly downstream of THR binding and activation.

Selective Thyroid Hormone Receptor-Beta (TRβ) Agonists: New Perspectives for the Treatment of Metabolic and Neurodegenerative Disorders

Thyroid hormones (THs) elicit significant effects on numerous physiological processes, such as growth, development, and metabolism. A lack of thyroid hormones is not compatible with normal health. Most THs effects are mediated by two different thyroid hormone receptor (TR) isoforms, namely TRα and TRβ, with the TRβ isoform known to be responsible for the main beneficial effects of TH on liver. In brain, despite the crucial role of TRα isoform in neuronal development, TRβ has been proposed to play a role in the remyelination processes. Consequently, over the past two decades, much effort has been applied in developing thyroid hormone analogs capable of uncoupling beneficial actions on liver (triglyceride and cholesterol lowering) and central nervous system (CNS) (oligodendrocyte proliferation) from deleterious effects on the heart, muscle and bone. Sobetirome (GC-1) and subsequently Eprotirome (KB2115) were the first examples of TRβ selective thyromimetics, with Sobetirome differing from the structure of thyronines because of the absence of halogens, biaryl ether oxygen, and amino-acidic side chain. Even though both thyromimetics showed encouraging actions against hypercholesterolemia, non-alcoholic steatohepatitis (NASH) and in the stimulation of hepatocytes proliferation, they were stopped after Phase 1 and Phase 2–3 clinical trials, respectively. In recent years, advances in molecular and structural biology have facilitated the design of new selective thyroid hormone mimetics that exhibit TR isoform-selective binding, and/or liver- and tissue-selective uptake, with Resmetirom (MGL-3196) and Hep-Direct prodrug VK2809 (MB07811) probably representing two of the most promising lipid lowering agents, currently under phase 2–3 clinical trials. More recently the application of a comprehensive panel of ADME-Toxicity assays enabled the selection of novel thyromimetic IS25 and its prodrug TG68, as very powerful lipid lowering agents both in vitro and in vivo. In addition to dyslipidemia and other liver pathologies, THs analogs could also be of value for the treatment of neurodegenerative diseases, such as multiple sclerosis (MS). Sob-AM2, a CNS- selective prodrug of Sobetirome has been shown to promote significant myelin repair in the brain and spinal cord of mouse demyelinating models and it is rapidly moving into clinical trials in humans. Taken together all these findings support the great potential of selective thyromimetics in targeting a large variety of human pathologies characterized by altered metabolism and/or cellular differentiation.

Structural insights revealed by two novel THRB mutations

PURPOSE

Among the inheritable forms of impaired sensitivity to thyroid hormone, resistance to thyroid hormone (RTH) due to mutations in the thyroid hormone receptor beta gene (THRB) is the first and best known described defect, revealing a wide phenotypic variability with an incompletely understood physiopathology. The objective of this study was to evaluate two novel mutations in THRB, N331H and L346R, in an attempt to provide a rational understanding of the harmful effects caused by them.

METHODS

The mutations of two patients with RTHβ were reproduced for analysis of gene transactivation by dual-luciferase reporter assay, and for molecular modeling for crystallography-based structural assessment. Serum measurements of TSH and FT4 were performed to compare the thyrotrophic resistance to thyroid hormone between RTHβ patients and controls.

RESULTS

Both mutants showed impaired gene transactivation, with greater damage in L346R. Molecular modeling suggested that the damage occurring in N331H is primarily due to reduced strength of the hydrogen bonds that stabilize T3 in its ligand-binding cavity (LBC), whereas in L346R, the damage is more marked and is mainly due to changes in hydrophobicity and molecular volume inside the LBC. Hormonal dosages indicated that the L346R mutant exhibited greater thyrotrophic resistance than N331H.

CONCLUSIONS

This study provides a rational understanding of the effects of mutations, indicating deleterious structural changes in the LBC in both THR, and discloses that not only the position of the mutation but, notably, the nature of the amino acid exchange, has a cardinal role in the functional damage of the receptor.

Identification of highly active natural thyroid hormone receptor agonists by pharmacophore-based virtual screening

Thyroid hormone receptor (TR) is an important target for the treatment of metabolic diseases. The X-ray crystallographic data for the TR complexed with different ligands were employed to generate feature-based pharmacophore models of the active site of TR receptor. The derived hypothesis was then used to find novel hit compounds through an in silico virtual screening on the Universal Natural Products Database (UNPD). The binding mode and action mechanism of the hit compounds were further investigated by molecular docking and molecular dynamics studies, then compounds possessing similar binding site with the crystal ligand were subjected to binding activity assay. Finally, UNPD19665 and UNPD184785 were proved to be more active than crystal TR ligands, with a binding activity value of 9.82 nM and 12.62 nM, respectively. Communicated by Ramaswamy H. Sarma.

Revealing a Mutant-Induced Receptor Allosteric Mechanism for the Thyroid Hormone Resistance

Resistance to thyroid hormone (RTH) is a clinical disorder without specific and effective therapeutic strategy, partly due to the lack of structural mechanisms for the defective ligand binding by mutated thyroid hormone receptors (THRs). We herein uncovered the prescription drug roxadustat as a novel THRβ-selective ligand with therapeutic potentials in treating RTH, thereby providing a small molecule tool enabling the first probe into the structural mechanisms of RTH. Despite a wide distribution of the receptor mutation sites, different THRβ mutants induce allosteric conformational modulation on the same His435 residue, which disrupts a critical hydrogen bond required for the binding of thyroid hormones. Interestingly, roxadustat retains hydrophobic interactions with THRβ via its unique phenyl extension, enabling the rescue of the activity of the THRβ mutants. Our study thus reveals a critical receptor allosterism mechanism for RTH by mutant THRβ, providing a new and viable therapeutic strategy for the treatment of RTH.

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We identified a novel THR ligand that effectively binds to THRβ mutants

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Structures revealed mechanisms for the RTH controlled by a key residue switch

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Roxadustat retains unique hydrophobic interactions with THRβ mutants

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We provide a promising approach to design THR ligands in treating RTH

Alternative ligands for thyroid hormone receptors

Thyroid hormone receptors (TRs) are ligand-dependent transcription factors that activate or repress gene transcription, resulting in the regulation of numerous physiological programs. While 3,3',5-L-triiodothyronine is the TR cognate ligand, these receptors can also be activated by various alternative ligands, including endogenous and synthetic molecules capable of inducing diverse active receptor conformations that influence thyroid hormone-dependent signaling pathways. This review mainly discusses current knowledge on 3,5-diiodo-L-thyronine and 3,5,3'-triiodothyroacetic acid, two endogenous molecules that bind to TRs and regulate gene expression; and the molecular interactions between TRs and ligands, like synthetic thyromimetics developed to target specific TR isoforms for tissue-specific regulation of thyroid-related disorders, or endocrine disruptors that have allowed the design of new analogues and revealed essential amino acids for thyroid hormone binding.

Potential Role of Thyroid Receptor β Agonists in the Treatment of Hyperlipidemia

Thyroid hormones have important effects on cellular development, growth, and metabolism and are necessary for the healthy function of almost all tissues. Hyperthyroid patients with excess thyroid hormone levels experience tachycardia, fatigue, muscle wasting, and osteoporosis. However, although high thyroid hormone levels have adverse effects, efforts have been made to harness the beneficial effects, such as reduced serum low-density lipoprotein (LDL) cholesterol levels, elevated basal metabolic rate, and weight loss. Thyroid hormones interact with nuclear thyroid hormone receptors (TRs), and cholesterol levels are reduced through TRβ, whereas extrahepatic adverse actions are primarily connected to TRα. Thus, to develop a useful compound for clinical use, efforts have been focusing on developing compounds with isomer-specific functions based on the structure of thyroid hormones, i.e., thyromimetics that are liver and/or TRβ specific. In this short review, we discuss the development of the early thyromimetics that enabled, through modern molecular techniques, the progress towards improved design of TRβ-selective thyromimetics. We also address the early promise shown in human clinical trials and the current status of these drugs and other emerging compounds.

Pathological Interactions Between Mutant Thyroid Hormone Receptors and Corepressors and Their Modulation by a Thyroid Hormone Analogue with Therapeutic Potential

BACKGROUND

Thyroid hormone receptors (TRs) are tightly regulated by the corepressors nuclear receptor corepressor (NCoR) and silencing mediator of retinoic acid and thyroid hormone receptors. Three conserved corepressor/NR signature box motifs (CoRNR1-3) forming the nuclear receptor interaction domain have been identified in these corepressors. Whereas TRs regulate multiple normal physiological and developmental pathways, mutations in TRs can result in endocrine diseases and be associated with cancers due to impairment of corepressor release. Three mutants that are located in helix H11 of TRs are of special interest: TRα-M388I, a mutant associated with the development of renal clear cell carcinomas (RCCCs), and TRβ-Δ430 and TRβ-Δ432, two deletion mutants causing resistance to thyroid hormone syndrome.

METHODS

Several cell-based and biophysical methods were used to measure the affinity between wild-type and mutant TRα and TRβ and all the CoRNR motifs from corepressors to quantify the effects of different thyroid hormone analogues on these interactions. This study was coupled with the measurement of interactions between wild-type and mutant TRs in the context of a heterodimer with RXR to a NCoR fragment in the presence of the same ligands. Structural insights into the binding mode of corepressors to TRs were assessed in parallel by nuclear magnetic resonance spectroscopy.

RESULTS

The study shows that TRs interact more avidly with the silencing mediator of retinoic acid and thyroid hormone receptors than with NCoR peptides, and that TRα binds most avidly to S-CoRNR3, whereas TRβ binds preferentially to S-CoRNR2. In the studied TR mutants, a transfer of the CoRNR-specificity toward CoRNR1 was observed, coupled with a significant increase in the binding strength. In contrast to 3,5,3'-triiodothyronine (T3), the agonist TRIAC and the antagonist NH-3 were very efficient at dissociating the abnormally strong interactions between mutant TRβs and corepressors. A strong impairment of T3-binding for TRβ mutants was shown compared to TRIAC and NH-3 and could explain the different efficiencies of the different ligands in releasing corepressors from the studied TRβ mutants. Consequently, TRIAC was found to be more effective than T3 in facilitating coactivator recruitment and decreasing the dominant activity of TRβ-Δ430.

CONCLUSION

This study helps to clarify the specific interaction surfaces involved in the pathologic phenotype of TR mutants and demonstrates that TRIAC is a potential therapeutic agent for patients suffering from resistance to thyroid hormone syndromes.

Quantification of Thyromimetic Sobetirome Concentration in Biological Tissue Samples

Thyroid hormone is a principal regulator of essential processes in vertebrate physiology and homeostasis. Synthetic derivatives of thyroid hormone, known as thyromimetics, display desirable therapeutic properties. Thoroughly understanding how thyromimetics distribute throughout the body is crucial for their development and this requires appropriate bioanalytical techniques to quantify drug levels in different tissues. Here, we describe a detailed protocol for the quantification of the thyromimetic sobetirome using liquid chromatography tandem-mass spectrometry (LC-MS/MS).

Therapeutic applications of thyroid hormone analogues in resistance to thyroid hormone (RTH) syndromes

Thyroid hormone (TH) is crucial for normal development and metabolism of virtually all tissues. TH signaling is predominantly mediated through binding of the bioactive hormone 3,3',5-triiodothyronine (T3) to the nuclear T3-receptors (TRs). The intracellular TH levels are importantly regulated by transporter proteins that facilitate the transport of TH across the cell membrane and by the three deiodinating enzymes. Defects at the level of the TRs, deiodinases and transporter proteins result in resistance to thyroid hormone (RTH) syndromes. Compounds with thyromimetic potency but with different (bio)chemical properties compared to T3 may hold therapeutic potential in these syndromes by bypassing defective transporters or binding to mutant TRs. Such TH analogues have the potential to rescue TH signaling. This review describes the role of TH analogues in the treatment of RTH syndromes. In particular, the application of 3,3',5-triiodothyroacetic acid (Triac) in RTH due to defective TRβ and the role of 3,5-diiodothyropropionic acid (DITPA), 3,3',5,5'-tetraiodothyroacetic acid (Tetrac) and Triac in MCT8 deficiency will be highlighted.

Structural aspects of thyroid hormone binding to proteins and competitive interactions with natural and synthetic compounds

Thyroid hormones and their metabolites constitute a vast class of related iodothyronine compounds that contribute to the regulation of metabolic activity and cell differentiation. They are in turn transported, transformed and recognized as signaling molecules through binding to a variety of proteins from a wide range of evolutionary unrelated protein families, which renders these proteins and their iodothyronine binding sites an example for extensive convergent evolution. In this review, we will briefly summarize what is known about iodothyronine binding sites in proteins, the modes of protein/iodothyronine interaction, and the ligand conformations. We will then discuss physiological and synthetic compounds, including popular drugs and food components, that can interfere with iodothyronine binding and recognition by these proteins. The discussion also includes compounds persisting in the environment and acting as endocrine disrupting chemicals.

GC-1: A Thyromimetic With Multiple Therapeutic Applications in Liver Disease

Thyroid hormones (THs), namely, 3,5,3'-triiodo-l-thyronine (T3) and 3,5,3',5'-tetraiodo-l-thyronine (thyroxine or T4), influence a variety of physiological processes that have important implications in fetal development, metabolism, cell growth, and proliferation. While THs elicit several beneficial effects on lipid metabolism and improve myocardial contractility, these therapeutically desirable effects are associated to a thyrotoxic state that severely limits the possible use of THs as therapeutic agents. Therefore, several efforts have been made to develop T3 analogs that could retain the beneficial actions (triglyceride, cholesterol, obesity, and body mass lowering) without the adverse TH-dependent side effects. This goal was achieved by the synthesis of TRβ-selective agonists. In this review, we summarize the current knowledge on the effects of one of the best characterized TH analogs, the TRβ1-selective thyromimetic, GC-1. In particular, we review some of the effects of GC-1 on different liver disorders, with reference to its possible clinical application. A brief comment on the possible therapeutic use of GC-1 in extrahepatic disorders is also included.

Naturally Occurring Amino Acids in Helix 10 of the Thyroid Hormone Receptor Mediate Isoform-Specific TH Gene Regulation

Thyroid hormone (TH) action is mediated by the products of two genes, TH receptor (THR)α (THRA) and THRβ (THRB) that encode several closely related receptor isoforms with differing tissue distributions. The vast majority of THR isoform-specific effects are thought to be due to tissue-specific differences in THR isoform expression levels. We investigated the alternative hypothesis that intrinsic functional differences among THR isoforms mediate these tissue-specific effects. To achieve the same level of expression of each isoform, we created tagged THR isoforms and tested their DNA and functional properties in vitro. We found significant homodimerization and functional differences among the THR isoforms. THRA1 was unable to form homodimers on direct repeat separated by 4 bp DNA elements and was also defective in TH-dependent repression of Tshb and Rxrg in a thyrotroph cell line, TαT1.1. In contrast, THRB2 was both homodimer sufficient and fully functional on these negatively regulated genes. Using domain exchanges and individual amino acid switches between THRA1 and THRB2, we identified three amino acids in helix 10 of the THRB2 ligand-binding domain that are required for negative regulation and are absent in THRA1.

Triiodothyroacetic acid in health and disease

Thyroid hormone (TH) is crucial for development and metabolism of many tissues. The physiological relevance and therapeutic potential of TH analogs have gained attention in the field for many years. In particular, the relevance and use of 3,3',5-triiodothyroacetic acid (Triac, TA₃) has been explored over the last decades. Although TA₃ closely resembles the bioactive hormone T₃, differences in transmembrane transport and receptor isoform-specific transcriptional activation potency exist. For these reasons, the application of TA₃ as a treatment for resistance to TH (RTH) syndromes, especially MCT8 deficiency, is topic of ongoing research. This review is a summary of all currently available literature about the formation, metabolism, action and therapeutic applications of TA₃.

3,5-diiodothyronine (3,5-T2) reduces blood glucose independently of insulin sensitization in obese mice

AIM

Thyroid hormones regulate metabolic response. While triiodothyronine (T3) is usually considered to be the active form of thyroid hormone, one form of diiodothyronine (3,5-T2) exerts T3-like effects on energy consumption and lipid metabolism. 3,5-T2 also improves glucose tolerance in rats and 3,5-T2 levels correlate with fasting glucose in humans. Presently, however, little is known about mechanisms of 3,5-T2 effects on glucose metabolism. Here, we set out to compare effects of T3, 3,5-T2 and another form of T2 (3,3-T2) in a mouse model of diet-induced obesity and determined effects of T3 and 3,5-T2 on markers of classical insulin sensitization to understand how diiodothyronines influence blood glucose.

METHODS

Cell- and protein-based assays of thyroid hormone action. Assays of metabolic parameters in mice. Analysis of transcript and protein levels in different tissues by qRT-PCR and Western blot.

RESULTS

T3 and 3,5-T2 both reduce body weight, adiposity and body temperature despite increased food intake. 3,3'-T2 lacks these effects. T3 and 3,5-T2 reduce blood glucose levels, whereas 3,3'-T2 worsens glucose tolerance. Neither T3 nor 3,5-T2 affects markers of insulin sensitization in skeletal muscle or white adipose tissue (WAT), but both reduce hepatic GLUT2 glucose transporter levels and glucose output. T3 and 3,5-T2 also induce expression of mitochondrial uncoupling proteins (UCPs) 3 and 1 in skeletal muscle and WAT respectively.

CONCLUSIONS

3,5-T2 influences glucose metabolism in a manner that is distinct from insulin sensitization and involves reductions in hepatic glucose output and changes in energy utilization.

Extending Halogen-based Medicinal Chemistry to Proteins: IODO-INSULIN AS A CASE STUDY

Insulin, a protein critical for metabolic homeostasis, provides a classical model for protein design with application to human health. Recent efforts to improve its pharmaceutical formulation demonstrated that iodination of a conserved tyrosine (TyrB26) enhances key properties of a rapid-acting clinical analog. Moreover, the broad utility of halogens in medicinal chemistry has motivated the use of hybrid quantum- and molecular-mechanical methods to study proteins. Here, we (i) undertook quantitative atomistic simulations of 3-[iodo-TyrB26]insulin to predict its structural features, and (ii) tested these predictions by X-ray crystallography. Using an electrostatic model of the modified aromatic ring based on quantum chemistry, the calculations suggested that the analog, as a dimer and hexamer, exhibits subtle differences in aromatic-aromatic interactions at the dimer interface. Aromatic rings (TyrB16, PheB24, PheB25, 3-I-TyrB26, and their symmetry-related mates) at this interface adjust to enable packing of the hydrophobic iodine atoms within the core of each monomer. Strikingly, these features were observed in the crystal structure of a 3-[iodo-TyrB26]insulin analog (determined as an R6 zinc hexamer). Given that residues B24-B30 detach from the core on receptor binding, the environment of 3-I-TyrB26 in a receptor complex must differ from that in the free hormone. Based on the recent structure of a "micro-receptor" complex, we predict that 3-I-TyrB26 engages the receptor via directional halogen bonding and halogen-directed hydrogen bonding as follows: favorable electrostatic interactions exploiting, respectively, the halogen's electron-deficient σ-hole and electronegative equatorial band. Inspired by quantum chemistry and molecular dynamics, such "halogen engineering" promises to extend principles of medicinal chemistry to proteins.

Chemical Hybridization of Glucagon and Thyroid Hormone Optimizes Therapeutic Impact for Metabolic Disease

Glucagon and thyroid hormone (T₃) exhibit therapeutic potential for metabolic disease but also exhibit undesired effects. We achieved synergistic effects of these two hormones and mitigation of their adverse effects by engineering chemical conjugates enabling delivery of both activities within one precisely targeted molecule. Coordinated glucagon and T₃ actions synergize to correct hyperlipidemia, steatohepatitis, atherosclerosis, glucose intolerance, and obesity in metabolically compromised mice. We demonstrate that each hormonal constituent mutually enriches cellular processes in hepatocytes and adipocytes via enhanced hepatic cholesterol metabolism and white fat browning. Synchronized signaling driven by glucagon and T₃ reciprocally minimizes the inherent harmful effects of each hormone. Liver-directed T₃ action offsets the diabetogenic liability of glucagon, and glucagon-mediated delivery spares the cardiovascular system from adverse T₃ action. Our findings support the therapeutic utility of integrating these hormones into a single molecular entity that offers unique potential for treatment of obesity, type 2 diabetes, and cardiovascular disease.

Nongenomic actions of thyroid hormone

The nongenomic actions of thyroid hormone begin at receptors in the plasma membrane, mitochondria or cytoplasm. These receptors can share structural homologies with nuclear thyroid hormone receptors (TRs) that mediate transcriptional actions of T3, or have no homologies with TR, such as the plasma membrane receptor on integrin αvβ3. Nongenomic actions initiated at the plasma membrane by T4 via integrin αvβ3 can induce gene expression that affects angiogenesis and cell proliferation, therefore, both nongenomic and genomic effects can overlap in the nucleus. In the cytoplasm, a truncated TRα isoform mediates T4-dependent regulation of intracellular microfilament organization, contributing to cell and tissue structure. p30 TRα1 is another shortened TR isoform found at the plasma membrane that binds T3 and mediates nongenomic hormonal effects in bone cells. T3 and 3,5-diiodo-L-thyronine are important to the complex nongenomic regulation of cellular respiration in mitochondria. Thus, nongenomic actions expand the repertoire of cellular events controlled by thyroid hormone and can modulate TR-dependent nuclear events. Here, we review the experimental approaches required to define nongenomic actions of the hormone, enumerate the known nongenomic effects of the hormone and their molecular basis, and discuss the possible physiological or pathophysiological consequences of these actions.

Futures Challenges in Thyroid Hormone Signaling Research

The canonical pathway of thyroid hormone signaling involves its binding to nuclear receptors (TRs) acting directly on the transcription of a number of genes. Recent genome-wide studies revealed that chromatin occupancy by TR is not sufficient for transactivation of gene expression. Reciprocally, in some cases, DNA binding by TR may not be required for cellular response. This leaves many new questions to be addressed in future research.

Identification of Thyroid Hormones and Functional Characterization of Thyroid Hormone Receptor in the Pacific Oyster Crassostrea gigas Provide Insight into Evolution of the Thyroid Hormone System

Thyroid hormones (THs) play important roles in development, metamorphosis, and metabolism in vertebrates. During the past century, TH functions were regarded as a synapomorphy of vertebrates. More recently, accumulating evidence has gradually convinced us that TH functions also occur in invertebrate chordates. To date, however, TH-related studies in non-chordate invertebrates have been limited. In this study, THs were qualitatively detected by two reliable methods (HPLC and LC/MS) in a well-studied molluscan species, the Pacific oyster Crassostrea gigas. Quantitative measurement of THs during the development of C. gigas showed high TH contents during embryogenesis and that oyster embryos may synthesize THs endogenously. As a first step in elucidating the TH signaling cascade, an ortholog of vertebrate TH receptor (TR), the most critical gene mediating TH effects, was cloned in C. gigas. The sequence of CgTR has conserved DNA-binding and ligand-binding domains that normally characterize these receptors. Experimental results demonstrated that CgTR can repress gene expression through binding to promoters of target genes and can interact with oyster retinoid X receptor. Moreover, CgTR mRNA expression was activated by T4 and the transcriptional activity of CgTR promoter was repressed by unliganded CgTR protein. An atypical thyroid hormone response element (CgDR5) was found in the promoter of CgTR, which was verified by electrophoretic mobility shift assay (EMSA). These results indicated that some of the CgTR function is conserved. However, the EMSA assay showed that DNA binding specificity of CgTR was different from that of the vertebrate TR and experiments with two dual-luciferase reporter systems indicated that l-thyroxine, 3,3′,5-triiodothyronine, and triiodothyroacetic acid failed to activate the transcriptional activity of CgTR. This is the first study to functionally characterize TR in mollusks. The presence of THs and the functions of CgTR in mollusks contribute to better understanding of the evolution of the TH system.

Src-dependent phosphorylation at Y406 on the thyroid hormone receptor β confers the tumor suppressor activity

Association studies suggest that the thyroid hormone receptor β1 (TRβ1) could function as a tumor suppressor in cancer cells. However, the underlying molecular mechanisms remain to be elucidated. We explored how TRβ1 acted as a tumor suppressor in breast cancer MDA cells. Proliferation and invasiveness were markedly inhibited in cells stably expressing TRβ1 (MDA-TRβ1 cells). cSrc-phosphorylated TRβ1 at Y406 signaled T3-induced degradation. Mutation of Y406 to Phe (TRβ1Y406F) did not affect T3 binding affinity, but blocked T3-induced degradation in cells. Importantly, cell-based studies showed that TRβ1Y406F lost the inhibitory effects by TRβ1 on cell proliferation and invasion. Consistently, in xenograft models, MDA-TRβ1 cells exhibited significantly slower tumor growth rates than those of Neo control cells. In contrast, the tumor growth rates of MDA-TRβ1Y406F cells were indistinguishable from those of Neo control cells. We further showed that markedly more TRβ1Y406F than TRβ1 was physically associated with cSrc in cells, leading to constitutive activation of cSrc-FAK-ERK signaling. In contrast, degradation of T3-bound TRβ1 complexed with cSrc attenuated signaling to decrease cell proliferation and invasiveness, thus confirming TRβ1 as a tumor suppressor. Thus, the present studies suggested that TRβ1 could be tested as a novel potential therapeutic target.

Identification of a new hormone-binding site on the surface of thyroid hormone receptor

Thyroid hormone receptors (TRs) are members of the nuclear receptor superfamily of ligand-activated transcription factors involved in cell differentiation, growth, and homeostasis. Although X-ray structures of many nuclear receptor ligand-binding domains (LBDs) reveal that the ligand binds within the hydrophobic core of the ligand-binding pocket, a few studies suggest the possibility of ligands binding to other sites. Here, we report a new x-ray crystallographic structure of TR-LBD that shows a second binding site for T3 and T4 located between H9, H10, and H11 of the TRα LBD surface. Statistical multiple sequence analysis, site-directed mutagenesis, and cell transactivation assays indicate that residues of the second binding site could be important for the TR function. We also conducted molecular dynamics simulations to investigate ligand mobility and ligand-protein interaction for T3 and T4 bound to this new TR surface-binding site. Extensive molecular dynamics simulations designed to compute ligand-protein dissociation constant indicate that the binding affinities to this surface site are of the order of the plasma and intracellular concentrations of the thyroid hormones, suggesting that ligands may bind to this new binding site under physiological conditions. Therefore, the second binding site could be useful as a new target site for drug design and could modulate selectively TR functions.

Understanding nuclear receptor form and function using structural biology

Nuclear receptors (NRs) are a major transcription factor family whose members selectively bind small-molecule lipophilic ligands and transduce those signals into specific changes in gene programs. For over two decades, structural biology efforts were focused exclusively on the individual ligand-binding domains (LBDs) or DNA-binding domains of NRs. These analyses revealed the basis for both ligand and DNA binding and also revealed receptor conformations representing both the activated and repressed states. Additionally, crystallographic studies explained how NR LBD surfaces recognize discrete portions of transcriptional coregulators. The many structural snapshots of LBDs have also guided the development of synthetic ligands with therapeutic potential. Yet, the exclusive structural focus on isolated NR domains has made it difficult to conceptualize how all the NR polypeptide segments are coordinated physically and functionally in the context of receptor quaternary architectures. Newly emerged crystal structures of the peroxisome proliferator-activated receptor-γ-retinoid X receptor α (PPARγ-RXRα) heterodimer and hepatocyte nuclear factor (HNF)-4α homodimer have recently revealed the higher order organizations of these receptor complexes on DNA, as well as the complexity and uniqueness of their domain-domain interfaces. These emerging structural advances promise to better explain how signals in one domain can be allosterically transmitted to distal receptor domains, also providing much better frameworks for guiding future drug discovery efforts.

Thyroid hormone receptor-β agonists prevent hepatic steatosis in fat-fed rats but impair insulin sensitivity via discrete pathways

Liver-specific thyroid hormone receptor-β (TRβ)-specific agonists are potent lipid-lowering drugs that also hold promise for treating nonalcoholic fatty liver disease and hepatic insulin resistance. We investigated the effect of two TRβ agonists (GC-1 and KB-2115) in high-fat-fed male Sprague-Dawley rats treated for 10 days. GC-1 treatment reduced hepatic triglyceride content by 75%, but the rats developed fasting hyperglycemia and hyperinsulinemia, attributable to increased endogenous glucose production (EGP) and diminished hepatic insulin sensitivity. GC-1 also increased white adipose tissue lipolysis; the resulting increase in glycerol flux may have contributed to the increase in EGP. KB-2115, a more TRβ- and liver-specific thyromimetic, also prevented hepatic steatosis but did not induce fasting hyperglycemia, increase basal EGP rate, or diminish hepatic insulin sensitivity. Surprisingly, insulin-stimulated peripheral glucose disposal was diminished because of a decrease in insulin-stimulated skeletal muscle glucose uptake. Skeletal muscle insulin signaling was unaffected. Instead, KB-2115 treatment was associated with a decrease in GLUT4 protein content. Thus, although both GC-1 and KB-2115 potently treat hepatic steatosis in fat-fed rats, they each worsen insulin action via specific and discrete mechanisms. The development of future TRβ agonists must consider the potential adverse effects on insulin sensitivity.

Thyroid hormone receptors and cancer

BACKGROUND

Thyroid hormone receptors (TRs) are ligand-dependent transcription factors that mediate the actions of the thyroid hormone (T3) in development, growth, and differentiation. The THRA and THRB genes encode several TR isoforms that express in a tissue- and development-dependent manner. In the past decades, a significant advance has been made in the understanding of TR actions in maintaining normal cellular functions. However, the roles of TRs in human cancer are less well understood. The reduced expression of TRs because of hypermethylation, or deletion of TR genes found in human cancers suggests that TRs could function as tumor suppressors. A close association of somatic mutations of TRs with human cancers further supports the notion that the loss of normal functions of TR could lead to uncontrolled growth and loss of cell differentiation.

SCOPE OF REVIEW

In line with the findings from association studies in human cancers, mice deficient in total functional TRs (Thra1(-/-)Thrb(-/-) mice) or with a targeted homozygous mutation of the Thrb gene (denoted PV; Thrb(PV/PV) mice) spontaneously develop metastatic thyroid carcinoma. This review will examine the evidence learned from these genetically engineered mice that provided strong evidence to support the critical role of TRs in human cancer.

MAJOR CONCLUSIONS

Loss of normal functions of TR by deletion or by mutations could contribute to cancer development, progression and metastasis.

GENERAL SIGNIFICANCE

Novel mechanistic insights are revealed in how aberrant TR activities lead to carcinogenesis. Mouse models of thyroid cancer provide opportunities to identify molecular targets as potential treatment modalities. This article is part of a Special Issue entitled Thyroid hormone signalling.

Identical gene regulation patterns of T3 and selective thyroid hormone receptor modulator GC-1

Synthetic selective thyroid hormone (TH) receptor (TR) modulators (STRM) exhibit beneficial effects on dyslipidemias in animals and humans and reduce obesity, fatty liver, and insulin resistance in preclinical animal models. STRM differ from native TH in preferential binding to the TRβ subtype vs. TRα, increased uptake into liver, and reduced uptake into other tissues. However, selective modulators of other nuclear receptors exhibit important gene-selective actions, which are attributed to differential effects on receptor conformation and dynamics and can have profound influences in animals and humans. Although there are suggestions that STRM may exhibit such gene-specific actions, the extent to which they are actually observed in vivo has not been explored. Here, we show that saturating concentrations of the main active form of TH, T(3), and the prototype STRM GC-1 induce identical gene sets in livers of euthyroid and hypothyroid mice and a human cultured hepatoma cell line that only expresses TRβ, HepG2. We find one case in which GC-1 exhibits a modest gene-specific reduction in potency vs. T(3), at angiopoietin-like factor 4 in HepG2. Investigation of the latter effect confirms that GC-1 acts through TRβ to directly induce this gene but this gene-selective activity is not related to unusual T(3)-response element sequence, unlike previously documented promoter-selective STRM actions. Our data suggest that T(3) and GC-1 exhibit almost identical gene regulation properties and that gene-selective actions of GC-1 and similar STRM will be subtle and rare.

Thyromimetics: a journey from bench to bed-side

Selective thyromimetics are synthetic analogs of thyroid hormones with tissue-specific thyroid hormone actions. Tissue selectivity is partly mediated by selectivity for the thyroid hormone receptor-β isoform, but is also enhanced by tissue-selective uptake. Several preclinical animal models and recent human clinical trials have provided sound evidence that thyromimetics can serve as pharmacological tools to improve serum lipids without affecting heart rate. Thyromimetics consistently and efficiently lowered low-density lipoprotein cholesterol and lipoprotein (a) plasma levels without positive chronotropic effects. Most importantly, thyromimetics had a synergistic action when used in addition to 3-hydroxy-3-methylglutaryl CoA reductase inhibitors. Animal data have further suggested that thyromimetics might be useful in the treatment of obesity, hepatic steatosis and atherosclerosis. However, only long-term phase III clinical trials will tell if the observed lipid lowering effects of thyromimetics will improve cardiovascular outcome in humans, too. At the moment, the treatment of dyslipidemia seems to be the major indication for the therapeutic use of thyromimetics, which are now rapidly moving from bench to bed-side.

Analysis of agonist and antagonist effects on thyroid hormone receptor conformation by hydrogen/deuterium exchange

Thyroid hormone receptors (TRs) are ligand-gated transcription factors with critical roles in development and metabolism. Although x-ray structures of TR ligand-binding domains (LBDs) with agonists are available, comparable structures without ligand (apo-TR) or with antagonists are not. It remains important to understand apo-LBD conformation and the way that it rearranges with ligands to develop better TR pharmaceuticals. In this study, we conducted hydrogen/deuterium exchange on TR LBDs with or without agonist (T(3)) or antagonist (NH3). Both ligands reduce deuterium incorporation into LBD amide hydrogens, implying tighter overall folding of the domain. As predicted, mass spectroscopic analysis of individual proteolytic peptides after hydrogen/deuterium exchange reveals that ligand increases the degree of solvent protection of regions close to the buried ligand-binding pocket. However, there is also extensive ligand protection of other regions, including the dimer surface at H10-H11, providing evidence for allosteric communication between the ligand-binding pocket and distant interaction surfaces. Surprisingly, C-terminal activation helix H12, which is known to alter position with ligand, remains relatively protected from solvent in all conditions suggesting that it is packed against the LBD irrespective of the presence or type of ligand. T(3), but not NH3, increases accessibility of the upper part of H3-H5 to solvent, and we propose that TR H12 interacts with this region in apo-TR and that this interaction is blocked by T(3) but not NH3. We present data from site-directed mutagenesis experiments and molecular dynamics simulations that lend support to this structural model of apo-TR and its ligand-dependent conformational changes.

The binding of synthetic triiodo l-thyronine analogs to human transthyretin: molecular basis of cooperative and non-cooperative ligand recognition

Transthyretin (TTR) is a tetrameric β-sheet-rich transporter protein directly involved in human amyloid diseases. Several classes of small molecules can bind to TTR delaying its amyloid fibril formation, thus being promising drug candidates to treat TTR amyloidoses. In the present study, we characterized the interactions of the synthetic triiodo L-thyronine analogs and thyroid hormone nuclear receptor TRβ-selective agonists GC-1 and GC-24 with the wild type and V30M variant of human transthyretin (TTR). To achieve this aim, we conducted in vitro TTR acid-mediated aggregation and isothermal titration calorimetry experiments and determined the TTR:GC-1 and TTR:GC-24 crystal structures. Our data indicate that both GC-1 and GC-24 bind to TTR in a non-cooperative manner and are good inhibitors of TTR aggregation, with dissociation constants for both hormone binding sites (HBS) in the low micromolar range. Analysis of the crystal structures of TTRwt:GC-1(24) complexes and their comparison with the TTRwt X-ray structure bound to its natural ligand thyroxine (T4) suggests, at the molecular level, the basis for the cooperative process displayed by T4 and the non-cooperative process provoked by both GC-1 and GC-24 during binding to TTR.

Essential molecular determinants for thyroid hormone transport and first structural implications for monocarboxylate transporter 8

Monocarboxylate transporter 8 (MCT8, SLC16A2) is a thyroid hormone (TH) transmembrane transport protein mutated in Allan-Herndon-Dudley syndrome, a severe X-linked psychomotor retardation. The neurological and endocrine phenotypes of patients deficient in MCT8 function underscore the physiological significance of carrier-mediated TH transmembrane transport. MCT8 belongs to the major facilitator superfamily of 12 transmembrane-spanning proteins and mediates energy-independent bidirectional transport of iodothyronines across the plasma membrane. Structural information is lacking for all TH transmembrane transporters. To gain insight into structure-function relations in TH transport, we chose human MCT8 as a paradigm. We systematically performed conventional and liquid chromatography-tandem mass spectrometry-based uptake measurements into MCT8-transfected cells using a large number of compounds structurally related to iodothyronines. We found that human MCT8 is specific for L-iodothyronines and requires at least one iodine atom per aromatic ring. Neither thyronamines, decarboxylated metabolites of iodothyronines, nor triiodothyroacetic acid and tetraiodothyroacetic acid, TH derivatives lacking both chiral center and amino group, are substrates for MCT8. The polyphenolic flavonoids naringenin and F21388, potent competitors for TH binding at transthyretin, did not inhibit T(3) transport, suggesting that MCT8 can discriminate its ligand better than transthyretin. Bioinformatic studies and a first molecular homology model of MCT8 suggested amino acids potentially involved in substrate interaction. Indeed, alanine mutation of either Arg(445) (helix 8) or Asp(498) (helix 10) abrogated T(3) transport activity of MCT8, supporting their predicted role in substrate recognition. The MCT8 model allows us to rationalize potential interactions of amino acids including those mutated in patients with Allan-Herndon-Dudley syndrome.

Structural modeling of high-affinity thyroid receptor-ligand complexes

Understanding the molecular basis of the binding modes of natural and synthetic ligands to nuclear receptors is fundamental to our comprehension of the activation mechanism of this important class of hormone regulated transcription factors and to the development of new ligands. Thyroid hormone receptors (TR) are particularly important targets for pharmaceuticals development because TRs are associated with the regulation of metabolic rates, body weight, and circulating levels of cholesterol and triglycerides in humans. While several high-affinity ligands are known, structural information is only partially available. In this work we obtain structural models of several TR-ligand complexes with unknown structure by docking high affinity ligands to the receptors' ligand binding domain with subsequent relaxation by molecular dynamics simulations. The binding modes of these ligands are discussed providing novel insights into the development of TR ligands. The experimental binding free energies are reasonably well-reproduced from the proposed models using a simple linear interaction energy free-energy calculation scheme.

Structural overview of the nuclear receptor superfamily: insights into physiology and therapeutics

As ligand-regulated transcription factors, the nuclear hormone receptors are nearly ideal drug targets, with internal pockets that bind to hydrophobic, drug-like molecules and well-characterized ligand-induced conformational changes that recruit transcriptional coregulators to promoter elements. Yet, due to the multitude of genes under the control of a single receptor, the major challenge has been the identification of ligands with gene-selective actions, impacting disease outcomes through a narrow subset of target genes and not across their entire gene-regulatory repertoire. Here, we summarize the concepts and work to date underlying the development of steroidal and nonsteroidal receptor ligands, including the use of crystal structures, high-throughput screens, and rational design approaches for finding useful therapeutic molecules. Difficulties in finding selective receptor modulators require a more complete understanding of receptor interdomain communications, posttranslational modifications, and receptor-protein interactions that could be exploited for target gene selectivity.

Gaining ligand selectivity in thyroid hormone receptors via entropy

Nuclear receptors are important targets for pharmaceuticals, but similarities between family members cause difficulties in obtaining highly selective compounds. Synthetic ligands that are selective for thyroid hormone (TH) receptor beta (TRbeta) vs. TRalpha reduce cholesterol and fat without effects on heart rate; thus, it is important to understand TRbeta-selective binding. Binding of 3 selective ligands (GC-1, KB141, and GC-24) is characterized at the atomic level; preferential binding depends on a nonconserved residue (Asn-331beta) in the TRbeta ligand-binding cavity (LBC), and GC-24 gains extra selectivity from insertion of a bulky side group into an extension of the LBC that only opens up with this ligand. Here we report that the natural TH 3,5,3'-triodothyroacetic acid (Triac) exhibits a previously unrecognized mechanism of TRbeta selectivity. TR x-ray structures reveal better fit of ligand with the TRalpha LBC. The TRbeta LBC, however, expands relative to TRalpha in the presence of Triac (549 A(3) vs. 461 A(3)), and molecular dynamics simulations reveal that water occupies the extra space. Increased solvation compensates for weaker interactions of ligand with TRbeta and permits greater flexibility of the Triac carboxylate group in TRbeta than in TRalpha. We propose that this effect results in lower entropic restraint and decreases free energy of interactions between Triac and TRbeta, explaining subtype-selective binding. Similar effects could potentially be exploited in nuclear receptor drug design.

Differential effects of TR ligands on hormone dissociation rates: evidence for multiple ligand entry/exit pathways

Some nuclear receptor (NR) ligands promote dissociation of radiolabeled bound hormone from the buried ligand binding cavity (LBC) more rapidly than excess unlabeled hormone itself. This result was interpreted to mean that challenger ligands bind allosteric sites on the LBD to induce hormone dissociation, and recent findings indicate that ligands bind weakly to multiple sites on the LBD surface. Here, we show that a large fraction of thyroid hormone receptor (TR) ligands promote rapid dissociation (T(1/2)<2h) of radiolabeled T(3) vs. T(3) (T(1/2) approximately 5-7h). We cannot discern relationships between this effect and ligand size, activity or affinity for TRbeta. One ligand, GC-24, binds the TR LBC and (weakly) to the TRbeta-LBD surface that mediates dimer/heterodimer interaction, but we cannot link this interaction to rapid T(3) dissociation. Instead, several lines of evidence suggest that the challenger ligand must interact with the buried LBC to promote rapid T(3) release. Since previous molecular dynamics simulations suggest that TR ligands leave the LBC by several routes, we propose that a subset of challenger ligands binds and stabilizes a partially unfolded intermediate state of TR that arises during T(3) release and that this effect enhances hormone dissociation.