Mutant Thyroid Hormone Receptor Research Articles

The Impact of Cancer‐Associated Mutations in Thyroid Hormone Receptor (TR) on TR Localization

The thyroid hormone receptor (TR)—a nuclear receptor that is essential for the proper regulation of metabolism and development, as it regulates gene expression in response to thyroid hormone—contains nuclear localization signals (NLSs) and nuclear export signals (NESs) that allow for TR transport into and out of the nucleus, respectively. Previous research has shown that nuclear import, nuclear retention, and nuclear export of TR are associated with modulation of gene expression, the alteration of which can contribute to various diseases, including Resistance to Thyroid Hormone (RTH) Syndrome, type II diabetes mellitus, and several types of cancer. There are three main subtypes of TR: TRα1, TRβ1, and TRβ2. Here, through mammalian cell transfection of expression plasmids for green fluorescent protein (GFP)‐tagged TR and fluorescence microscopy, we examined particular groups of TRα1 mutations that were observed in patients with hepatocellular carcinoma and renal carcinoma and are associated with NLSs and NESs of TRα1. More specifically, we calculated the nuclear‐to‐cytoplasmic ratios of TRα1 variants to determine the impact of TRα1 mutations on receptor localization within HeLa cells. We found that, while wild‐type TR has a primarily nuclear localization with a smooth distribution in human cells grown in culture, the TRα1 mutations S40T, K136R, L251P, V390A and A225G, D246N, G350K lead to significantly decreased nuclear‐to‐cytoplasmic TRα1 ratios (greater cytoplasmic distribution) and formation of TRα1 aggregates. We observed that the distribution of a TRα1 mutant with just one of the four mutations (L251P) is very similar to the distribution of the TRα1 mutant with all four mutations (S40T, K136R, L251P, V390A), suggesting that L251P plays a major role in the mislocalization of TRα1. In addition, we found that TRα1 with a G24E mutation had a significantly greater cytoplasmic distribution, compared to wild‐type TRα1. In conclusion, our findings suggest that certain TRα1 mutations that are associated with carcinoma lead to TRα1 mislocalization, which may alter the expression of thyroid hormone‐dependent genes. These results contribute to an improved understanding of TR structure and function, which could lead to novel strategies for discovering therapies for TR‐related illnesses.Support or Funding InformationNIH #2R15DK058028‐04This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Nuclear Receptor Corepressor 1 Regulates Thyroid Hormone Receptor Nuclear Retention

The thyroid hormone receptor (TR) mediates gene transcription in response to thyroid hormone (T3) and significantly affects cell growth, development and metabolism. Although TR shuttles rapidly between the nucleus and cytoplasm, it is primarily nuclear localized at steady state. A shift in the balance between export and import leading to TR mislocalization may correlate with certain cancers or endocrine disorders. Previously, we have identified multiple nuclear localization signals and nuclear export signal within TR that interact with importins and exportins, respectively, to mediate shuttling. Given that TR contains multiple, competing import and export signals, the question arises: what factors are involved in nuclear retention of TR? In addition to DNA‐binding sites, the lab's prior research indicated that nuclear coactivator MED1 facilitates TR nuclear localization. Thus, we hypothesize that certain transcriptional coregulators, including Nuclear Receptor Corepressor 1 (NCoR1), contribute to TR nuclear retention. Using transient transfection assays and fluorescence microscopy, we measured the fluorescent intensity of GFP or mCherry‐tagged TR in the nucleus (N) and cytoplasm (C) of HeLa cells, and then calculated the N/C ratio as an indicator of intracellular localization patterns. We discovered that when NCoR1 is knocked down by shRNA, the amount of TR in the nucleus decreased significantly (lower N/C ratio). Inversely, when NCoR1 is over‐expressed, the amount of TR in the nucleus significantly increased (higher N/C ratio). An even more dramatic cytosolic shift was observed for a TR mutant that cannot bind to NCoR1 (AHT‐TR). Taken together, our results suggest that NCoR1 is a key regulator of TR nuclear retention. This finding will contribute to the development of a dynamic mathematical model that aims to predict TR's N/C ratio during the interaction with different complexes and TR localization's effect on transcriptional output of T3‐dependent genes.Support or Funding InformationNIH 2R15DK058028This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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

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. 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. 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. 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.

Novel THRB mutation analysis in congenital hypothyroidism with thyroid dysgenesis.

Thyroid dysgenesis (TD) accounts for most cases of congenital hypothyroidism. Although mutations in thyroid hormone receptor β (THRB) have been identified in TD, the mutational spectrum of THRB and phenotype-genotype correlations have not been fully elucidated. In this study, we aimed to find mutations of THRB, examine the functions of these mutations, and attempt to elucidate the relationship between THRB and TD. Thus, we screened the exons of THRB in 280 patients with TD and 200 normal subjects in samples collected from China. We performed cell morphology assays, MTT assays, flow cytometric analyses, and a quantitative reverse-transcription polymerase chain reaction in human thyroid follicular epithelial cells (Nthy-ori cell line) to examine the impact of THRB mutations. In two unrelated patients, two novel missense mutations, c.76G>A (p.D26N) and c.107G>A (p.C36Y), were identified in THRB. Functional studies suggested that the C36Y mutant caused changes in morphology, inhibiting cell proliferation and promoting apoptosis in a human thyroid cell line. In addition, we found that messenger RNA expressions of thyroglobulin (TG) and the Na+ /I- symporter (NIS) were decreased in a time-dependent manner in mutant THRB compared with the wild type. To our knowledge, this is the first study to document the prevalence of THRB mutations and the genotype-phenotype spectrum of TD in a Chinese population. We characterized the function of a C36Y mutation, which reduced cell proliferation and increased cell death in thyroid epithelial cells. This study provides further evidence for genetic THRB defects and disease mechanisms in TD.

A Possible Novel Mechanism of Action of Genistein and Daidzein for Activating Thyroid Hormone Receptor-Mediated Transcription

Thyroid hormone receptors (TRs) are members of the nuclear receptor superfamily that regulate their target genes for controlling organ development and functional maintenance. Soybean isoflavones, especially genistein and daidzein, modulate various hormone-mediated pathways. However, their effects on TRs have not yet been extensively studied. In this study, the effects of these isoflavones on TR action were evaluated using transient transfection-based reporter gene assays and molecular docking studies. Genistein and daidzein augmented T3-liganded TR-mediated transcription in a concentration-dependent manner. In the mammalian 2-hybrid study, these isoflavones augmented the recruitment of steroid receptor coactivator-1 and nuclear corepressor to liganded or unliganded TRs. Using a series of mutant TRs, we also showed that the activation function-2 domain of TRs was responsible for the augmentation by these isoflavones. CV-1 cells had expressed TRα, TRβ1, and ERα mRNAs. However, neither the overexpression nor the knocking down of ERα altered the augmentation of TR action by isoflavones, indicating that the effects of isoflavones are exerted through their direct action on TRs. In silico molecular docking studies showed that genistein and daidzein can directly bind to the TR-ligand-binding domain. These findings indicate that the augmentation of the TR-mediated transcription by genistein and daidzein is due to their direct binding to TR-ligand-binding domain to induce the recruitment of steroid receptor coactivator-1. Our study reports a novel mode of action of soybean isoflavones on TR function. The biological effects and the relevance of these isoflavones to human health may be partially attributable to the activation of thyroid hormone signaling.

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.

NCOR1 modulates erythroid disorders caused by mutations of thyroid hormone receptor \u03b11

Thyroid hormone receptor α (THRA) gene mutations, via dominant negative mode, cause erythroid abnormalities in patients. Using mice expressing a dominant negative TRα1 mutant (TRα1PV; Thra1PV/+ mice), we showed that TRα1PV acted directly to suppress the expression of key erythroid genes, causing erythroid defects. The nuclear receptor corepressor 1 (NCOR1) was reported to mediate the dominant negative effects of mutated TRα1. However, how NCOR1 could regulate TRα1 mutants in erythroid defects in vivo is not known. In the present study, we crossed Thra1PV/+ mice with mice expressing a mutant Ncor1 allele (NCOR1ΔID; Ncor1ΔID mice). TRα1PV mutant cannot bind to NCOR1ΔID. The expression of NCOR1ΔID ameliorated abnormalities in the peripheral blood indices, and corrected the defective differentiation potential of progenitors in the erythroid lineage. The defective terminal erythropoiesis of lineage-negative bone marrow cells of Thra1PV/+ mice was rescued by the expression of NCOR1ΔID. De-repression of key erythroid genes in Thra1PV/+Ncor1ΔID/ΔID mice led to partial rescue of terminal erythroid differentiation. These results indicate that the inability of TRα1PV to recruit NCOR1ΔID to form a repressor complex relieved the deleterious actions of TRα1 mutants in vivo. NCOR1 is a critical novel regulator underpining the pathogenesis of erythroid abnormalities caused by TRα1 mutants.

A resistance to thyroid hormone syndrome mutant operates through the target gene repertoire of the wild-type thyroid hormone receptor

Thyroid hormone receptors (TRs) play crucial roles in vertebrates. Wild-type (WT) TRs function primarily as hormone-regulated transcription factors. A human endocrine disease, Resistance to Thyroid Hormone (RTH)-Syndrome, is caused by inheritance of mutant TRs impaired in the proper regulation of target gene expression. To better understand the molecular basis of RTH we compared the target genes regulated by an RTH-TRβ1 mutant (R429Q) to those regulated by WT-TRβ1. With only a few potential exceptions, the vast majority of genes we were able to identify as regulated by the WT-TRβ1, positively or negatively, were also regulated by the RTH-TRβ1 mutant. We conclude that the actions of R429Q-TRβ1 in RTH-Syndrome most likely reflect the reduced hormone affinity observed for this mutant rather than an alteration in target gene repertoire. Our results highlight the importance of target gene specificity in defining the disease phenotype and improve our understanding of how clinical treatments impact RTH-Syndrome.

Impaired sensitivity to thyroid hormone

In conditions with impaired sensitivity to thyroid hormone, reduced effect of thyroid hormone is attributable to various defects. The purpose of this article is to give an overview of these conditions, as well as to provide updated knowledge on impaired sensitivity to thyroid hormone, also known as thyroid hormone resistance, with mutations in thyroid hormone receptor β (TRβ). This article is based on a selection of English-language articles, and Norwegian original and review articles found in PubMed, and the authors' own experiences with this patient group. Thyroid hormone resistance has long been a recognised cause of the reduced effect of thyroid hormone. Several other conditions that involve impaired sensitivity to thyroid hormone have been described in recent decades, and mutations have been identified in genes that code for thyroid hormone receptor α (TRα), a cell membrane transporter, as well as in the deiodinases that metabolise thyroxine (T4) to the bioactive form triiodothyronine (T3). The conditions vary in terms of their clinical picture and biochemical profile. Based on clinical and biochemical findings, thyroid hormone resistance may be erroneously interpreted as hyperthyroidism. In patients with thyroid hormone resistance, the condition may be exacerbated if it is treated as hyperthyroidism. It is therefore essential to recognise the conditions and their differential diagnoses.

Cardiovascular impact of thyroid hormone receptor mutations

Cardiovascular impact of thyroid hormone receptor mutations

G478(P) A case report of trbeta mutation leading to raised t4 levels

Introduction We present a five year old girl with thyroid hormone resistance, subsequently discovered to be heterozygous for TRbeta mutation. This case highlights the necessity to investigate, in detail, all children with persistently high thyroxin with normal TSH levels in order to aid future management and the necessity to follow them up. Case report The patient was born at term by normal delivery, weighing 3.34kg (50th centile). She was referred at six months of age for poor weight gain (2nd centile). Blood tests showed an elevated free T4 (43.4) with normal TSH (3.10). Systemic examination was normal and remained so over the following months. Repeat thyroid function tests showed persistently elevated T4 with normal TSH. At 2 ½ years old, genetic analysis revealed she is heterozygous for TRbeta mutation (thyroid hormone receptive gene). Mum has no mutation detected and her father cannot be tested for unavoidable reason. Since then, she has had slow growth, idiopathic thrombocytopenic purpura, vitamin D deficiency, and coeliac disease. Broader antibody testing has not revealed an underlying autoimmune aetiology to date. Recently, the patient has been investigated for recurrent falls and abnormal gait. She has right sided hemi hypotrophy with drooped shoulder and pelvis, along with winged scapulae, flared ribs and prominent abdomen. Her gross motor skills are generally delayed. Conclusion Mutation of the beta thyroid hormone receptor is usually either autosomal dominantly inherited or is a de novo mutation, resulting in defective patterns of gene expression. This is a rare disorder, usually presenting with goitre. TRbeta mutation should be considered in children with persistently elevated T4 levels in conjunction with a normal TSH. The other immune conditions like ITP and changes in body habitus are new associations, cause of which is yet not identified. ** Photos are available **

Familial dysalbuminaemic hyperthyroxinaemia: a novel mass spectrometry approach to diagnosis

Background and aims Familial dysalbuminaemic hyperthyroxinaemia (FDH) is an important cause of discordant thyroid function tests (TFT). Free thyroxine (T4) levels are falsely elevated on some assays and current diagnostic tests for FDH are laborious. We aimed to investigate a novel, mass spectrometry approach to diagnosis of FDH in a 51-year-old man with persistently elevated free T4 levels, discordant normal TSH and normal free triiodothyronine. He was clinically euthyroid and had a daughter with similar TFT profile. Methods Serum albumin was assessed using electrospray time-of-flight (TOF) liquid chromatography/mass spectrometry (LC/MS). Targeted sequencing of exons 3 and 7 of the albumin gene was used to confirm mutations and to screen 15 archived DNA samples that were negative for thyroid hormone receptor mutations. Results LC/MS demonstrated heterozygosity for an albumin variant with a 19 Da decrease in mass, indicative of an Arg→His substitution. This was confirmed by DNA sequencing which showed a guanine to adenine transition at codon 218 of exon 7, a mutation known to increase T4 binding. Discussion TOF LC/MS is a novel procedure for diagnosing FDH. The test is rapid (taking

Activation of the RhoB signaling pathway by thyroid hormone receptor β in thyroid cancer cells.

Thyroid hormone receptor (TR) mediates the crucial effects of the thyroid hormone (T3) on cellular growth, development, and differentiation. Decreased expression or inactivating somatic mutations of TRs have been found in human cancers of the liver, breast, lung, and thyroid. The mechanisms of TR-associated carcinogenesis are still not clear. To establish the function of TRβ in thyroid cancer cell proliferation, we constructed a recombinant adenovirus vector, AdTRβ, which expresses human TRβ1 cDNA. Thyroid cancer cell lines in which TRβ protein levels were significantly decreased as compared to intact thyroid tissues were infected with AdTRβ and the function of TRβ on cell proliferation and migration was analyzed. Ligand-bound TRβ induced HDAC1 and HDAC3 dissociation from, and histone acetylation associated with the RhoB promoter and enhanced the expression of RhoB mRNA and protein. In AdTRβ-infected cells, T3 and farnesyl transferase inhibitor (FTI)-treatment induced the distribution of RhoB on the cell membrane and enhanced the abundance of active GTP-bound RhoB. This RhoB protein led to p21-associated cell-cycle arrest in the G0/G1 phase, following inhibition of cell proliferation and invasion. Conversely, lowering cellular RhoB by small interfering RNA knockdown in AdTRβ-infected cells led to downregulation of p21 and inhibited cell-cycle arrest. The growth of BHP18-21v tumor xenografts in vivo was significantly inhibited by AdTRβ injection with FTIs-treatment, as compared to control virus-injected tumors. This novel signaling pathway triggered by ligand-bound TRβ provides insight into possible mechanisms of proliferation and invasion of thyroid cancer and may provide new therapeutic targets for thyroid cancers.

RXR agonist modulates TR: corepressor dissociation upon 9-cis retinoic acid treatment.

Transcriptional regulation controlled by thyroid hormone receptor (TR) drives events such as development, differentiation, and metabolism. TRs may act either as homodimers or as heterodimers with retinoid X receptor (RXR). Thyroid hormone T3 preferentially binds TR-RXR heterodimers, which activate transcription through coactivator recruitment. However, it is unclear whether TR-RXR heterodimers may also be responsive to the canonical RXR agonist 9-cis retinoic acid (9C) in the context of physiological gene regulation. New structural studies suggest that 9C promotes the displacement of bound coactivators from the heterodimer, modifying TR-RXR activity. To shed light on the molecular mechanisms that control TR-RXR function, we used biophysical approaches to characterize coregulator recruitment to TR-TR or to TR-RXR in the presence of T3 and/or 9C as well as cell-based assays to establish the functional significance of biophysical findings. Using cell-based and fluorescence assays with mutant and wild-type TR, we show that 9C does indeed have a function in the TR-RXR heterodimer context, in which it induces the release of corepressors. Furthermore, we show that 9C does not promote detectable conformational changes in the structure of the TR-RXR heterodimer and does not affect coactivator recruitment. Finally, our data support the view that DNA binding domain and Hinge regions are important to set up NR-coactivator binding interfaces. In summary, we showed that the RXR agonist 9C can regulate TR function through its modulation of corepressor dissociation.

A novel mutation of thyroid hormone receptor β in exon 10 in a case of thyroid hormone-resistant non-Hodgkin's lymphoma of the thyroid.

Only a few previous studies have demonstrated an association between resistance to thyroid hormone (RTH) and thyroid cancer. The current study presents the case of a 67-year-old female who was referred to the Third Xiangya Hospital of Central South University with an enlargement of the neck that had grown gradually over two years and subsequently, rapidly enlarged over the two months prior to admission, alongside a slight sensation of shortness of breath. Laboratory data revealed a significantly increased level of thyroid-stimulating hormone (TSH), total triiodothyronine, total thyroxine, free triiodothyronine, free thyroxine, thyroprotein and thyroglobulin antibody; however, the levels of thyroperoxidase and TSH receptor antibody were within the normal ranges. A thyroid hormone suppression test revealed a TSH reduction of 32%, Magnetic resonance imaging of the pituitary gland was negative for abnormalities. The patient’s thyroid pathology revealed a non-Hodgkin’s lymphoma of the thyroid. CHOP + nimustine chemotherapy significantly reduced the clinical symptoms. The genetic analysis revealed a novel point mutation of the thyroid hormone receptor β (THRB) gene in exon 10 (g1680 G to A) in the 3′-untranslated region of the patient. To the best of our knowledge, this is the first case report of RTH with thyroid non-Hodgkin’s lymphoma, which involved a mutation (g1680 G to A) in exon 10 of THRB.

Synergistic Signaling of KRAS and Thyroid Hormone Receptor β Mutants Promotes Undifferentiated Thyroid Cancer through MYC Up-Regulation

Undifferentiated thyroid carcinoma is one of the most aggressive human cancers with frequent RAS mutations. How mutations of the RAS gene contribute to undifferentiated thyroid cancer remains largely unknown. Mice harboring a potent dominant negative mutant thyroid hormone receptor β, TRβPV (ThrbPV/PV), spontaneously develop well-differentiated follicular thyroid cancer similar to human cancer. We genetically targeted the KrasG12D mutation to thyroid epithelial cells of ThrbPV/PV mice to understand how KrasG12D mutation could induce undifferentiated thyroid cancer in ThrbPV/PVKrasG12D mice. ThrbPV/PVKrasG12D mice exhibited poorer survival due to more aggressive thyroid tumors with capsular invasion, vascular invasion, and distant metastases to the lung occurring at an earlier age and at a higher frequency than ThrbPV/PV mice did. Importantly, ThrbPV/PVKrasG12D mice developed frequent anaplastic foci with complete loss of normal thyroid follicular morphology. Within the anaplastic foci, the thyroid-specific transcription factor paired box gene 8 (PAX8) expression was virtually lost and the loss of PAX8 expression was inversely correlated with elevated MYC expression. Consistently, co-expression of KRASG12D with TRβPV upregulated MYC levels in rat thyroid pccl3 cells, and MYC acted to enhance the TRβPV-mediated repression of the Pax8 promoter activity of a distant upstream enhancer, critical for thyroid-specific Pax8 expression. Our findings indicated that synergistic signaling of KRASG12D and TRβPV led to increased MYC expression. Upregulated MYC contributes to the initiation of undifferentiated thyroid cancer, in part, through enhancing TRβPV-mediated repression of the Pax8 expression. Thus, MYC might serve as a potential target for therapeutic intervention.

Analysis of the roles of mutations in thyroid hormone receptor-β by a bacterial biosensor system

Mutations in thyroid hormone receptors (TRs) often lead to metabolic and developmental disorders, but patients with these mutations are difficult to treat with existing thyromimetic drugs. In this study, we analyzed six clinically observed mutations in the ligand-binding domain of the human TRβ using an engineered bacterial hormone biosensor. Six agonist compounds, including triiodothyronine (T3), thyroxine (T4), 3,5,3'-triiodothyroacetic acid (Triac), GC-1, KB-141, and CO-23, and the antagonist NH-3 were examined for their ability to bind to each of the TRβ mutants. The results indicate that some mutations lead to the loss of ability to bind to native ligands, ranging from several fold to several hundred fold, while other mutations completely abolish the ability to bind to any ligand. Notably, the effect of each ligand on each TRβ mutant in this bacterial system is highly dependent on both the mutation and the ligand; some ligands were bound well by a wide variety of mutants, while other ligands lost their affinity for all but the WT receptor. This study demonstrates the ability of our bacterial system to differentiate agonist compounds from antagonist compounds and shows that one of the TRβ mutations leads to an unexpected increase in antagonist ability relative to other mutations. These results indicate that this bacterial sensor can be used to rapidly determine ligand-binding ability and character for clinically relevant TRβ mutants.

Inappropriate heat dissipation ignites brown fat thermogenesis in mice with a mutant thyroid hormone receptor α1

Thyroid hormone is a major regulator of thermogenesis, acting both in peripheral organs and on central autonomic pathways. Mice heterozygous for a point mutation in thyroid hormone receptor α1 display increased thermogenesis as a consequence of high sympathetic brown fat stimulation. Surprisingly, despite the hypermetabolism, their body temperature is not elevated. Here we show, using isolated tail arteries, that defective thyroid hormone receptor α1 signaling impairs acetylcholine-mediated vascular relaxation as well as phenylephrine-induced vasoconstriction. Using infrared thermography on conscious animals, we demonstrate that these defects severely interfere with appropriate peripheral heat conservation and dissipation, which in turn leads to compensatory alterations in brown fat activity. Consequently, when the vasoconstrictive defect in mice heterozygous for a point mutation in thyroid hormone receptor α1 was reversed with the selective α1-adrenergic agonist midodrine, the inappropriate heat loss over their tail surface was reduced, normalizing brown fat activity and energy expenditure. Our analyses demonstrate that thyroid hormone plays a key role in vascular heat conservation and dissipation processes, adding a unique aspect to its well-documented functions in thermoregulation. The data thus facilitate understanding of temperature hypersensitivity in patients with thyroid disorders. Moreover, the previously unrecognized connection between cardiovascular regulation and metabolic activity revealed in this study challenges the interpretation of several experimental paradigms and questions some of the currently derived hypotheses on the role of thyroid hormone in thermogenesis.

Inhibition of Tumorigenesis by the Thyroid Hormone Receptor β in Xenograft Models

Previous studies showed a close association between several types of human cancers and somatic mutations of thyroid hormone receptor β (TRβ) and reduced expression of TRβ due to epigenetic inactivation and/or deletion of the THRB gene. These observations suggest that TRβ could act as a tumor suppressor in carcinogenesis. However, the mechanisms by which TRβ could function to inhibit tumorigenesis are less well understood. We used the human follicular thyroid cancer cell lines (FTC-133 and FTC-236 cells) to elucidate how functional expression of the THRB gene could affect tumorigenesis. We stably expressed the THRB gene in FTC cells and evaluated the effects of the expressed TRβ on cancer cell proliferation, migration, and tumor growth in cell-based studies and xenograft models. Expression of TRβ in FTC-133 cells, as compared with control FTC cells without TRβ, reduced cancer cell proliferation and impeded migration of tumor cells through inhibition of the AKT-mTOR-p70 S6K pathway. TRβ expression in FTC-133 and FTC-236 led to less tumor growth in xenograft models. Importantly, new vessel formation was significantly suppressed in tumors induced by FTC cells expressing TRβ compared with control FTC cells without TRβ. The decrease in vessel formation was mediated by the downregulation of vascular endothelial growth factor in FTC cells expressing TRβ. These findings indicate that TRβ acts as a tumor suppressor through downregulation of the AKT-mTOR-p70 S6K pathway and decreased vascular endothelial growth factor expression in FTC cells. The present results raise the possibility that TRβ could be considered as a potential therapeutic target for thyroid cancer.

Nuclear receptor corepressors confer the actions of mutant thyroid hormone receptor a

Nuclear receptor corepressors confer the actions of mutant thyroid hormone receptor a