Thyroid hormone receptor mutations and disease: beyond thyroid hormone resistance

Truncated variants of thyroid hormone receptor beta display disease-inflicting malfunctioning at cellular level

Thyroid hormone receptor β (THRβ) is a member of the nuclear receptor superfamily of ligand-modulated transcription factors. Upon ligand binding, THRβ sequentially recruits the components of transcriptional machinery to modulate target gene expression. In addition to regulating diverse physiological processes, THRβ plays a crucial role in hypothalamus-pituitary-thyroid axis feedback regulation. Anomalies in THRβ gene/protein structure are associated with onset of diverse disease states. In this study, we investigated disease-inflicting truncated variants of THRβ using in-silico analysis and cell-based assays. We examined the THRβ truncated variants on multiple test parameters, including subcellular localization, ligand-receptor interactions, transcriptional functions, interaction with heterodimeric partner RXR, and receptor-chromatin interactions. Moreover, molecular dynamic simulation approaches predicted that shortened THRβ-LBD due to point mutations contributes proportionally to the loss of structural integrity and receptor stability. Deviant subcellular localization and compromised transcriptional function were apparent with these truncated variants. Present study shows that 'mitotic bookmarking' property of some THRβ variants is also affected. The study highlights that structural and conformational attributes of THRβ are necessary for normal receptor functioning, and any deviations may contribute to the underlying cause of the inflicted diseases. We anticipate that insights derived herein may contribute to improved mechanistic understanding to assess disease predisposition.

Targeting Thyroid Hormone/Thyroid Hormone Receptor Axis: An Attractive Therapy Strategy in Liver Diseases

Thyroid hormone/thyroid hormone receptor (TH/TR) axis is characterized by TH with the assistance of plasma membrane transporters to combine with TR and mediate biological activities. Growing evidence suggests that TH/TR participates in plenty of hepatic metabolism. Thus, this review focuses on the role of the TH/TR axis in the liver diseases. To be specific, the TH/TR axis may improve metabolic-associated fatty liver disease, hepatitis, liver fibrosis, and liver injury while exacerbating the progression of acute liver failure and alcoholic liver disease. Also, the TH/TR axis has paradoxical roles in hepatocellular carcinoma. The TH/TR axis may be a prospecting target to cure hepatic diseases.

Thyroid hormone dependent transcriptional programming by TRβ requires SWI/SNF chromatin remodelers

Transcriptional regulation in response to thyroid hormone (3,5,3'-triiodo-l-thyronine, T3) is a dynamic and cell-type specific process that maintains cellular homeostasis and identity in all tissues. However, our understanding of the mechanisms of thyroid hormone receptor (TR) actions at the molecular level are actively being refined. We used an integrated genomics approach to profile and characterize the cistrome of TRβ, map changes in chromatin accessibility, and capture the transcriptomic changes in response to T3 in normal human thyroid cells. There are significant shifts in TRβ genomic occupancy in response to T3, which are associated with differential chromatin accessibility, and differential recruitment of SWI/SNF chromatin remodelers. We further demonstrate selective recruitment of BAF and PBAF SWI/SNF complexes to TRβ binding sites, revealing novel differential functions in regulating chromatin accessibility and gene expression. Our findings highlight three distinct modes of TRβ interaction with chromatin and coordination of coregulator activity.

Neutrophil-to-lymphocyte Ratio is Associated with LV Diastolic Dysfunction in the Overt Hyperthyroid Patients

BACKGROUND

Recent studies have shown that the neutrophil-to-lymphocyte ratio (NLR) has gradually been identified as a more reliable marker of inflammation, with predictive value for the development of many diseases. However, its association with left ventricular (LV) diastolic dysfunction in overt hyperthyroid patients is unclear. Here, we aimed to explore the relationship between NLR and LV diastolic dysfunction in overt hyperthyroid patients.

METHODS

For this study, we retrospected the consecutive medical files of 350 overt hyperthyroid patients. Their medical data and laboratory findings were recorded. According to the presence or absence of LV diastolic dysfunction, the patients with overt hyperthyroidism were divided into two groups. One group with LV diastolic dysfunction included 104 patients and another group with non-LV diastolic dysfunction included 246 patients. The NLR values between the two groups were compared, and the relationship between NLR levels and the prevalence of LV diastolic dysfunction was also explored.

RESULTS

The NLR value in LV diastolic dysfunction group in the overt hyperthyroid subjects was significantly higher than that in non-LV diastolic dysfunction group [1.100 (0.907-1.580) vs 1.000 (0.761-1.405), P=0.016]. The prevalence of LV diastolic dysfunction in Low- (NLR<0.879), Medium- (0.879< NLR<1.287), and High- (NLR >1.287) NLR level groups were 20.9%, 32.5% and 35.7% respectively. Moreover, increased NLR is associated with increased prevalence of LV diastolic dysfunction, and after adjustment for potential associated factors, NLR remained significantly associated with LV diastolic dysfunction. (OR = 11.753, 95%CI = 1.938-71.267, P = 0.007).

CONCLUSIONS

Our findings demonstrated that the NLR was associated with LV diastolic dysfunction in the overt hyperthyroid patients, and the prevalence of LV diastolic dysfunction may be positively correlated with NLR levels.

Hypercholesterolemia in Two Siblings with Resistance to Thyroid Hormones Due to Disease-Causing Variant in Thyroid Hormone Receptor (THRB) Gene

Resistance to thyroid hormone beta (RTHβ) is a syndrome characterized by a reduced response of target tissues to thyroid hormones. In 85% of cases, a pathogenic mutation in the thyroid hormone receptor beta (THRB) gene is found. The clinical picture of RTHβ is very diverse; the most common findings are goiter and tachycardia, but the patients might be clinically euthyroid. The laboratory findings are almost pathognomonic with elevated free thyroxin (fT4) levels and high or normal thyrotropin (TSH) levels; free triiodothyronin (fT3) levels may also be elevated. We present three siblings with THRB mutation (heterozygous disease-variant c.727C>T, p.Arg243Trp); two of them also had hypercholesterolemia, while all three had several other clinical characteristics of RTHβ. This is the first description of the known Slovenian cases with RTHβ due to the pathogenic mutation in the THRB gene. Hypercholesterolemia might be etiologically related with RTHβ, since the severity of hormonal resistance varies among different tissues and hypercholesterolemia in patients with THRB variants might indicate the relatively hypothyroid state of the liver. We suggest that cholesterol levels are measured in all RTHβ patients.

Roundup causes embryonic development failure and alters metabolic pathways and gut microbiota functionality in non-target species

BACKGROUND

Research around the weedkiller Roundup is among the most contentious of the twenty-first century. Scientists have provided inconclusive evidence that the weedkiller causes cancer and other life-threatening diseases, while industry-paid research reports that the weedkiller has no adverse effect on humans or animals. Much of the controversial evidence on Roundup is rooted in the approach used to determine safe use of chemicals, defined by outdated toxicity tests. We apply a system biology approach to the biomedical and ecological model species Daphnia to quantify the impact of glyphosate and of its commercial formula, Roundup, on fitness, genome-wide transcription and gut microbiota, taking full advantage of clonal reproduction in Daphnia. We then apply machine learning-based statistical analysis to identify and prioritize correlations between genome-wide transcriptional and microbiota changes.

RESULTS

We demonstrate that chronic exposure to ecologically relevant concentrations of glyphosate and Roundup at the approved regulatory threshold for drinking water in the US induce embryonic developmental failure, induce significant DNA damage (genotoxicity), and interfere with signaling. Furthermore, chronic exposure to the weedkiller alters the gut microbiota functionality and composition interfering with carbon and fat metabolism, as well as homeostasis. Using the "Reactome," we identify conserved pathways across the Tree of Life, which are potential targets for Roundup in other species, including liver metabolism, inflammation pathways, and collagen degradation, responsible for the repair of wounds and tissue remodeling.

CONCLUSIONS

Our results show that chronic exposure to concentrations of Roundup and glyphosate at the approved regulatory threshold for drinking water causes embryonic development failure and alteration of key metabolic functions via direct effect on the host molecular processes and indirect effect on the gut microbiota. The ecological model species Daphnia occupies a central position in the food web of aquatic ecosystems, being the preferred food of small vertebrates and invertebrates as well as a grazer of algae and bacteria. The impact of the weedkiller on this keystone species has cascading effects on aquatic food webs, affecting their ability to deliver critical ecosystem services. Video Abstract.

The Role of Thyroid Hormone in the Innate and Adaptive Immune Response during Infection

In the past decades, there has been growing evidence for a functional interaction between the thyroid hormone and the immune system. This article provides an overview of the mechanisms by which thyroid hormones affect the innate and adaptive immune response during infection. The influence of thyroid hormone on the most important players of the innate [neutrophils, macrophages, natural killer (NK) cells, and dendritic cells (DCs)] and adaptive immune system (B- and T-lymphocytes) is reviewed here based on both clinical and preclinical studies. The effects of modulation of the immune system by drugs, such as monoclonal antibodies, tyrosine kinase inhibitors, and interferons on thyroid function, are beyond the scope of this article. Thyroid hormones regulate the activity of neutrophils which is reflected by higher numbers of neutrophils outside the bloodstream and enhanced activity of the respiratory burst following stimulation with thyroid hormone. Hyperthyroidism affects neutrophil function to a larger extent than hypothyroidism. In addition to neutrophil function, macrophage function is strongly affected by thyroid hormones, with triiodothyronine having a pro-inflammatory effect in these cells. NK cell proliferation and cytotoxic activity are also dependent on thyroid hormone levels. Finally, thyroid hormones enhance DC proliferation and maturation. In the adaptive immune system, a hyperthyroid state leads to increased activation of lymphocytes. This effect of thyroid hormone is mediated by various factors including NF-κB and protein kinase C signaling pathways and the β-adrenergic receptor. In general, a hyperthyroid state leads to a more activated immune system whereas hypothyroidism leads to a less activated immune system. © 2020 American Physiological Society. Compr Physiol 10:1277-1287, 2020.

Triiodothyronine (T3) upregulates the expression of proto-oncogene TGFA independent of MAPK/ERK pathway activation in the human breast adenocarcinoma cell line, MCF7

OBJECTIVE

To verify the physiological action of triiodothyronine T3 on the expression of transforming growth factor α (TGFA) mRNA in MCF7 cells by inhibition of RNA Polymerase II and the MAPK/ERK pathway.

MATERIALS AND METHODS

The cell line was treated with T3 at a physiological dose (10-9M) for 10 minutes, 1 and 4 hour (h) in the presence or absence of the inhibitors, α-amanitin (RNA polymerase II inhibitor) and PD98059 (MAPK/ERK pathway inhibitor). TGFA mRNA expression was analyzed by RT-PCR. For data analysis, we used ANOVA, complemented with the Tukey test and Student t-test, with a minimum significance of 5%.

RESULTS

T3 increases the expression of TGFA mRNA in MCF7 cells in 4 h of treatment. Inhibition of RNA polymerase II modulates the effect of T3 treatment on the expression of TGFA in MCF7 cells. Activation of the MAPK/ERK pathway is not required for T3 to affect the expression of TGFA mRNA.

CONCLUSION

Treatment with a physiological concentration of T3 after RNA polymerase II inhibition altered the expression of TGFA. Inhibition of the MAPK/ERK pathway after T3 treatment does not interfere with the TGFA gene expression in a breast adenocarcinoma cell line.

Revisiting available knowledge on teleostean thyroid hormone receptors

Teleosts are the most numerous class of living vertebrates. They exhibit great diversity in terms of morphology, developmental strategies, ecology and adaptation. In spite of this diversity, teleosts conserve similarities at molecular, cellular and endocrine levels. In the context of thyroidal systems, and as in the rest of vertebrates, thyroid hormones in fish regulate development, growth and metabolism by actively entering the nucleus and interacting with thyroid hormone receptors, the final sensors of this endocrine signal, to regulate gene expression. In general terms, vertebrates express the functional thyroid hormone receptors alpha and beta, encoded by two distinct genes (thra and thrb, respectively). However, different species of teleosts express thyroid hormone receptor isoforms with particular structural characteristics that confer singular functional traits to these receptors. For example, teleosts contain two thra genes and in some species also two thrb; some of the expressed isoforms can bind alternative ligands. Also, some identified isoforms contain deletions or large insertions that have not been described in other vertebrates and that have not yet been functionally characterized. As in amphibians, the regulation of some of these teleost isoforms coincides with the climax of metamorphosis and/or life transitions during development and growth. In this review, we aimed to gain further insights into thyroid signaling from a comparative perspective by proposing a systematic nomenclature for teleost thyroid hormone receptor isoforms and summarize their particular functional features when the information was available.

Thyroid hormone receptor β1 stimulates ABCB4 to increase biliary phosphatidylcholine excretion in mice

The ATP-binding cassette transporter ABCB4/MDR3 is critical for biliary phosphatidylcholine (PC) excretion at the canalicular membrane of hepatocytes. Defective ABCB4 gene expression and protein function result in various cholestatic liver and bile duct injuries. Thyroid hormone receptor (THR) is a major regulator of hepatic lipid metabolism; we explored its potential role in ABCB4 regulation. Thyroid hormone T3 stimulation to human hepatocyte models showed direct transcriptional activation of ABCB4 in a dose- and time-dependent manner. To determine whether THRβ1 (the main THR isoform of the liver) is involved in regulation, we tested THRβ1-specific agonists (e.g., GC-1, KB-141); these agonists resulted in greater stimulation than the native hormone. KB-141 activated hepatic ABCB44 expression in mice, which enhanced biliary PC secretion in vivo. We also identified THR response elements 6 kb upstream of the ABCB4 locus that were conserved in humans and mice. Thus, T3-via THRβ1 as a novel transcriptional activator regulates ABCB4 to increase ABCB4 protein levels at the canalicular membrane and promote PC secretion into bile. These findings may have important implications for understanding thyroid hormone function as a potential modifier of bile duct homeostasis and provide pharmacologic opportunities to improve liver function in hepatobiliary diseases caused by low ABCB4 expression.

FUNNEL-GSEA: FUNctioNal ELastic-net regression in time-course gene set enrichment analysis

Motivation

Gene set enrichment analyses (GSEAs) are widely used in genomic research to identify underlying biological mechanisms (defined by the gene sets), such as Gene Ontology terms and molecular pathways. There are two caveats in the currently available methods: (i) they are typically designed for group comparisons or regression analyses, which do not utilize temporal information efficiently in time-series of transcriptomics measurements; and (ii) genes overlapping in multiple molecular pathways are considered multiple times in hypothesis testing.

Results

We propose an inferential framework for GSEA based on functional data analysis, which utilizes the temporal information based on functional principal component analysis, and disentangles the effects of overlapping genes by a functional extension of the elastic-net regression. Furthermore, the hypothesis testing for the gene sets is performed by an extension of Mann-Whitney U test which is based on weighted rank sums computed from correlated observations. By using both simulated datasets and a large-scale time-course gene expression data on human influenza infection, we demonstrate that our method has uniformly better receiver operating characteristic curves, and identifies more pathways relevant to immune-response to human influenza infection than the competing approaches.

Availability and Implementation

The methods are implemented in R package FUNNEL, freely and publicly available at: https://github.com/yunzhang813/FUNNEL-GSEA-R-Package.

Supplementary information

Supplementary data are available at Bioinformatics online.

Nuclear receptors in neural stem/progenitor cell homeostasis

In the central nervous system, embryonic and adult neural stem/progenitor cells (NSCs) generate the enormous variety and huge numbers of neuronal and glial cells that provide structural and functional support in the brain and spinal cord. Over the last decades, nuclear receptors and their natural ligands have emerged as critical regulators of NSC homeostasis during embryonic development and adult life. Furthermore, substantial progress has been achieved towards elucidating the molecular mechanisms of nuclear receptors action in proliferative and differentiation capacities of NSCs. Aberrant expression or function of nuclear receptors in NSCs also contributes to the pathogenesis of various nervous system diseases. Here, we review recent advances in our understanding of the regulatory roles of steroid, non-steroid, and orphan nuclear receptors in NSC fate decisions. These studies establish nuclear receptors as key therapeutic targets in brain diseases.

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.

Resistance to Thyroid Hormone due to Heterozygous Mutations in Thyroid Hormone Receptor Alpha

BACKGROUND

Thyroid hormone (TH) acts via nuclear thyroid hormone receptors (TRs). TR isoforms (TRα1, TRα2, TRβ1, TRβ2) are encoded by distinct genes (THRA and THRB) and show differing tissue distributions. Patients with mutations in THRB, exhibiting resistance within the hypothalamic-pituitary-thyroid axis with elevated TH and nonsuppressed thyroid-stimulating hormone (TSH) levels, were first described decades ago. In 2012, the first patients with mutations in THRA were identified. Scope of this review: This review describes the clinical and biochemical characteristics of patients with resistance to thyroid hormone alpha (RTHα) due to heterozygous mutations in THRA. The genetic basis and molecular pathogenesis of the disorder together with effects of levothyroxine treatment are discussed.

CONCLUSIONS

The severity of the clinical phenotype of RTHα patients seems to be associated with the location and type of mutation in THRA. The most frequent abnormalities observed include anemia, constipation, and growth and developmental delay. In addition, serum (F)T3 levels can be high-normal to high, (F)T4 and rT3 levels normal to low, while TSH is normal or mildly raised. Despite heterogeneous consequences of mutations in THRA, RTHα should be suspected in subjects with even mild clinical features of hypothyroidism together with high/high-normal (F)T3, low/low-normal (F)T4, and normal TSH.

Mitochondrial Actions of Thyroid Hormone

The hypermetabolic effects of thyroid hormones (THs), the major endocrine regulators of metabolic rate, are widely recognized. Although, the cellular mechanisms underlying these effects have been extensively investigated, much has yet to be learned about how TH regulates diverse cellular functions. THs have a profound impact on mitochondria, the organelles responsible for the majority of cellular energy production, and several studies have been devoted to understand the respective importance of the nuclear and mitochondrial pathways for organelle activity. During the last decades, several new aspects of both THs (i.e., metabolism, transport, mechanisms of action, and the existence of metabolically active TH derivatives) and mitochondria (i.e., dynamics, respiratory chain organization in supercomplexes, and the discovery of uncoupling proteins other than uncoupling protein 1) have emerged, thus opening new perspectives to the investigation of the complex relationship between thyroid and the mitochondrial compartment. In this review, in the light of an historical background, we attempt to point out the present findings regarding thyroid physiology and the emerging recognition that mitochondrial dynamics as well as the arrangement of the electron transport chain in mitochondrial cristae contribute to the mitochondrial activity. We unravel the genomic and nongenomic mechanisms so far studied as well as the effects of THs on mitochondrial energetics and, principally, uncoupling of oxidative phosphorylation via various mechanisms involving uncoupling proteins. The emergence of new approaches to the question as to what extent and how the action of TH can affect mitochondria is highlighted. © 2016 American Physiological Society. Compr Physiol 6:1591-1607, 2016.

Resistance to Thyroid Hormone Alpha in an 18-Month-Old Girl: Clinical, Therapeutic, and Molecular Characteristics

BACKGROUND

Recently, the first patients with resistance to thyroid hormone alpha (RTHα) due to inactivating mutations in the thyroid hormone receptor alpha (TRα) were identified. These patients are characterized by growth retardation, variable motor and cognitive defects, macrocephaly, and abnormal thyroid function tests. The objective was to characterize a young girl (18 months old) with a mutation in both TRα1 and TRα2, and to study the effects of early levothyroxine (LT4) treatment.

METHODS

The patient was assessed clinically and biochemically before and during 12 months of LT4 treatment. In addition, the consequences of the mutation for TRα1/2 receptor function were studied in vitro.

RESULTS

At 18 months of age, the patient presented with axial hypotonia, delayed motor development, severe growth retardation, and abnormally elevated triiodothyronine (T3)/thyroxine (T4) ratios. RTHα was suspected, and concomitantly a c.632A>G/p.D211G missense mutation was identified, affecting both the TRα1 and TRα2 proteins. This mutation was also found in the girl's father. LT4 treatment was started, resulting in a marked improvement of her hypotonia, motor skills, and growth. Functionally, the missense mutation led to decreased transcriptional activity of TRα1, which could be overcome by higher T3 levels in vitro. The mutant TRα1 showed a moderate dominant negative activity on wild type (WT) TRα1. In contrast, WT TRα2 and mutant TRα2 had negligible transcriptional activity and showed no dominant-negative effect over TRα1.

CONCLUSIONS

This report describes the phenotype of a young RTHα patient with a mild TRα mutation before and during early LT4 treatment. Treatment had beneficial effects on her muscle tone, motor development, and growth.

The expression of thyroid hormone receptors (THR) is regulated by the progesterone receptor system in first trimester placental tissue and in BeWo cells in vitro

BACKGROUND

Thyroid hormones are essential for the maintenance of pregnancy and a deficiency in maternal thyroid hormones has been associated with early pregnancy losses. The aim of this study was a systematic investigation of the influence of mifepristone (RU 486) on the expression of the thyroid hormone receptor (THR) isoforms THRα1, THRα2, THRβ1 and THRβ2 on protein and mRNA-level.

METHODS

Samples of placental tissue were obtained from patients with mifepristone induced termination of pregnancy (n=13) or mechanical induced termination of normal pregnancy (n=20), each from the 4th to 13th week of pregnancy. Expression of THRα1, THRα2, THRβ1 and THRβ2 was analysed on protein level by immunohistochemistry and on mRNA level by real time RT-PCR (TaqMan). The influence of progesterone on THR gene expression was analysed in the trophoblast tumour cell line BeWo by real time RT-PCR (TaqMan).

RESULTS

Nuclear expression of THRα1, THRα2 and THRβ1 is downregulated on protein level in mifepristone (RU 486) treated villous trophoblast tissue. In decidual tissue, we found a significant downregulation only for THRα1 in mifepristone treated tissue. On mRNA level, we also found a significantly reduced expression of THRA but no significant downregulation for THRB in placental tissue. The gene THRA encodes the isoform THRα and the gene THRB encodes the isoform THRβ. The majority of cells expressing the thyroid hormone receptors in the decidua are decidual stromal cells. In addition, in vitro experiments with trophoblast tumour cells showed that progesterone significantly induced THRA but not THRB expression.

CONCLUSIONS

Termination of pregnancy with mifepristone (RU 486) leads to a downregulation of THRα1, THRα2 and THRβ1 in villous trophoblasts and in addition to a decreased expression of THRA in placental tissue. Decreased expression of THRα1 induced by RU486 could also be found in the decidua. Therefore inhibition of the progesterone receptor may be responsible for this downregulation. This assumption is supported by the finding, that stimulation of the progesterone receptor by progesterone itself up-regulated THRA in trophoblast cells in vitro.

Clinical Consequences of Mutations in Thyroid Hormone Receptor-α1

Thyroid hormone (TH) exerts its biological activity via the TH receptors TRα1 and TRβ1/2, which are encoded by the THRA and THRB genes. The first patients with mutations in THRB were identified decades ago. These patients had a clinical syndrome of resistance to TH associated with high serum TH and nonsuppressed thyroid-stimulating hormone levels. Until recently, no patients with mutations in THRA had been identified. In an attempt to predict the clinical phenotype of such patients, different TRα1 mutant mouse models have been generated. These mice have a variable phenotype depending on the location and severity of the mutation. Recently, the first humans with mutations in THRA were identified. Their phenotype consists of relatively low serum T4 and high serum T3 levels (and thus an elevated T3/T4 ratio), growth retardation, delayed mental and bone development, and constipation. While, in retrospect, certain features present in humans can also be found in mouse models, the first humans carrying a defect in TRα1 were not suspected of having a THRA gene mutation initially. The current review focuses on the clinical consequences of TRα1 mutations.

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.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Clinical phenotype of a new type of thyroid hormone resistance caused by a mutation of the TRα1 receptor: consequences of LT4 treatment

CONTEXT

Recently the first patients with inactivating mutations in T₃ receptor (TR)-α1 have been identified. These patients have low free T₄, low T₄, high T₃, low rT₃, and normal TSH serum levels, in combination with growth retardation, delayed bone development, and constipation.

OBJECTIVE

The aim of the current study was to report the effects of levothyroxine (LT4) treatment on the clinical phenotype of 2 patients (father and daughter) with a heterozygous inactivating mutation in TRα1.

SETTING AND PARTICIPANTS

Both patients were treated with LT4 for the last 5 years. To evaluate the effect of LT4 treatment, LT4 was withdrawn for 35 days and subsequently reinitiated. Data were collected from medical records, by reanalysis of serum collected over the last 6 years, and by a detailed clinical evaluation.

RESULTS

Treatment with LT4 resulted in a suppression of serum TSH and normalization of serum free T₄ and rT₃, whereas T₃ levels remained elevated in both patients. In addition, there was a normalization of the dyslipidemia as well as a response in serum IGF-I, SHBG, and creatine kinase in the index patient. All these parameters returned to pretreatment values when LT4 was briefly stopped. LT4 also resulted in an improvement of certain clinical features, such as constipation and nerve conductance. However, cognitive and fine motor skill defects remained.

CONCLUSION

This study reports the consequences of LT4 treatment over a prolonged period of time in 2 of the first patients with a heterozygous mutation in TRα1. LT4 therapy leads to an improvement of certain but not all features of the clinical phenotype.

Thyroid hormone actions in liver cancer

The thyroid hormone 3,3',5-triiodo-L-thyronine (T3) mediates several physiological processes, including embryonic development, cellular differentiation, metabolism, and the regulation of cell proliferation. Thyroid hormone receptors (TRs) generally act as heterodimers with the retinoid X receptor (RXR) to regulate target genes. In addition to their developmental and metabolic functions, TRs have been shown to play a tumor suppressor role, suggesting that their aberrant expression can lead to tumor transformation. Conversely, recent reports have shown an association between overexpression of wild-type TRs and tumor metastasis. Signaling crosstalk between T3/TR and other pathways or specific TR coregulators appear to affect tumor development. Since TR actions are complex as well as cell context-, tissue- and time-specific, aberrant expression of the various TR isoforms has different effects during diverse tumorigenesis. Therefore, elucidation of the T3/TR signaling mechanisms in cancers should facilitate the identification of novel therapeutic targets. This review provides a summary of recent studies focusing on the role of TRs in hepatocellular carcinomas (HCCs).

Thyroid hormone receptor actions on transcription in amphibia: The roles of histone modification and chromatin disruption

Thyroid hormone (T3) plays diverse roles in adult organ function and during vertebrate development. The most important stage of mammalian development affected by T3 is the perinatal period when plasma T3 level peaks. Amphibian metamorphosis resembles this mammalian postembryonic period and is absolutely dependent on T3. The ability to easily manipulate this process makes it an ideal model to study the molecular mechanisms governing T3 action during vertebrate development. T3 functions mostly by regulating gene expression through T3 receptors (TRs). Studies in vitro, in cell cultures and reconstituted frog oocyte transcription system have revealed that TRs can both activate and repress gene transcription in a T3-dependent manner and involve chromatin disruption and histone modifications. These changes are accompanied by the recruitment of diverse cofactor complexes. More recently, genetic studies in mouse and frog have provided strong evidence for a role of cofactor complexes in T3 signaling in vivo. Molecular studies on amphibian metamorphosis have also revealed that developmental gene regulation by T3 involves histone modifications and the disruption of chromatin structure at the target genes as evidenced by the loss of core histones, arguing that chromatin remodeling is an important mechanism for gene activation by liganded TR during vertebrate development.

Common genetic variants in pituitary-thyroid axis genes and the risk of differentiated thyroid cancer

Thyroid hormone receptors, THRA and THRB, together with the TSH receptor, TSHR, are key regulators of thyroid function. Alterations in the genes of these receptors (THRA, THRB and TSHR) have been related to thyroid diseases, including thyroid cancer. Moreover, there is evidence suggesting that predisposition to differentiated thyroid cancer (DTC) is related to common genetic variants with low penetrance that interact with each other and with environmental factors. In this study, we investigated the association of single nucleotide polymorphisms (SNPs) in the THRA (one SNP), THRB (three SNPs) and TSHR (two SNPs) genes with DTC risk. A case-control association study was conducted with 398 patients with sporadic DTC and 479 healthy controls from a Spanish population. Among the polymorphisms studied, only THRA-rs939348 was found to be associated with an increased risk of DTC (recessive model, odds ratio=1.80, 95% confidence interval=1.03-3.14, P=0.037). Gene-gene interaction analysis using the genotype data of this study together with our previous genotype data on TG and TRHR indicated a combined effect of the pairwises: THRB-TG (P interaction=0.014, THRB-rs3752874 with TG-rs2076740; P interaction=0.099, THRB-rs844107 with TG-rs2076740) and THRB-TRHR (P interaction=0.0024, THRB-rs3752874 with TRHR-rs4129682) for DTC risk in a Spanish population. Our results confirm that THRA is a risk factor for DTC, and we show for the first time the combined effect of THRB and TG or TRHR on DTC susceptibility, supporting the importance of gene-gene interaction in thyroid cancer risk.

Thyroid hormone receptors: the challenge of elucidating isotype-specific functions and cell-specific response

BACKGROUND

Thyroid hormone receptors TRα1, TRβ1 and TRβ2 are broadly expressed and exert a pleiotropic influence on many developmental and homeostatic processes. Extensive genetic studies in mice precisely defined their respective function.

SCOPE OF REVIEW

The purpose of the review is to discuss two puzzling issues:

MAJOR CONCLUSIONS

Mouse genetics support a balanced contribution of expression pattern and receptor intrinsic properties in defining the receptor respective functions. The molecular mechanisms sustaining cell specific response remain hypothetical and based on studies performed with other nuclear receptors.

GENERAL SIGNIFICANCE

The isoform-specificity and cell-specificity questions have many implications for clinical research, drug development, and endocrine disruptor studies. This article is part of a Special Issue entitled Thyroid hormone signalling.

Ligand-dependent corepressor acts as a novel corepressor of thyroid hormone receptor and represses hepatic lipogenesis in mice

BACKGROUND & AIMS

Transcriptional co-regulators assist nuclear receptors to control the transcription and maintain the metabolic homeostasis. Ligand-dependent corepressor (LCOR) was reported to function as a transcriptional corepressor in vitro. We found LCOR expression decreased in fatty livers of leptin-deficient (ob/ob) mice, diet-induced obese mice, as well as patients, suggesting LCOR may play a role in lipid homeostasis. We sought to investigate the physiological role of LCOR in vivo and elucidate the underlining molecular mechanisms.

METHODS

The effect of LCOR on hepatic lipid accumulation and thyroid hormone receptor (TR) mediated expression of lipogenic genes was studied in vitro and in vivo.

RESULTS

Ectopic expression of LCOR via intravenous infection with LCOR adenovirus decreased the hepatic triglyceride level in wild type, ob/ob, and diet-induced obese mice. Interestingly, overexpression of LCOR repressed the thyroid hormone induced expression of lipogenic genes and non-lipogenic genes, and ameliorated hepatic steatosis in obese mice, suggesting that LCOR might regulate lipogenesis as a novel TR corepressor. Furthermore, our study revealed that LCOR could interact with TRβ1 in the presence of the ligand, which resulted in competitive binding and reduced recruitment of steroid receptor coactivator-1/3 (SRC-1/3) to the promoter region of TR target genes.

CONCLUSIONS

Our data suggest that LCOR is likely to suppress TRβ1-mediated hepatic lipogenesis by decreasing binding and recruitment of SRCs to TRβ1. Our study reveals the physiological function of hepatic LCOR in lipid metabolism and the mechanism by which LCOR regulates lipogenesis. Hepatic LCOR may be a potential target for treating hepatic steatosis.

Thyroid hormone potentiates insulin signaling and attenuates hyperglycemia and insulin resistance in a mouse model of type 2 diabetes

BACKGROUND AND PURPOSE

The thyroid hormone, triiodothyronine (T3) has many metabolic functions. Unexpectedly, exogenous T3 lowered blood glucose in db/db mice, a model of type 2 diabetes. Here, we have explored this finding and its possible mechanisms further.

EXPERIMENTAL APPROACH

db/db and lean mice were treated with T3, the phosphoinositide 3- kinase (PI3-kinase) inhibitor, LY294002, plus T3, or vehicles. Blood glucose, insulin sensitivity, levels and synthesis were measured. Effects of T3 on intracellular insulin signaling were analyzed in 3T3-L1 pre-adipocytes with Western blotting. Knock-down of the thyroid hormone receptor α1 (TRα1) in 3T3-L1 cells was achieved with an appropriate silencing RNA (siRNA).

KEY RESULTS

Single injections of T3 (7 ng·g⁻¹ i.p.) rapidly and markedly attenuated hyperglycemia. Treatment with T3 (14 ng·g⁻¹·day⁻¹, 18 days) dose-dependently attenuated blood glucose and increased insulin sensitivity in db/db mice. Higher doses of T3 (28 ng·g⁻¹·day⁻¹) reversed insulin resistance in db/db mice. T3 also increased insulin levels in plasma and the neurogenic differentiation factor (an insulin synthesis transcription factor) and insulin storage in pancreatic islets in db/db mice. These anti-diabetic effects of T3 were abolished by the PI3-kinase inhibitor (LY294002). In 3T3-L1 preadipocytes, T3 enhanced insulin-induced tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and activation of PI3-kinase, effects blocked by siRNA for TRα1.

CONCLUSIONS AND IMPLICATIONS

T3 potentiated insulin signaling, improved insulin sensitivity, and increased insulin synthesis, which may contribute to its anti-diabetic effects. These findings may provide new approaches to the treatment of type 2 diabetes.

Thyroid hormone receptor mutations in cancer and resistance to thyroid hormone: perspective and prognosis

Thyroid hormone, operating through its receptors, plays crucial roles in the control of normal human physiology and development; deviations from the norm can give rise to disease. Clinical endocrinologists often must confront and correct the consequences of inappropriately high or low thyroid hormone synthesis. Although more rare, disruptions in thyroid hormone endocrinology due to aberrations in the receptor also have severe medical consequences. This review will focus on the afflictions that are caused by, or are closely associated with, mutated thyroid hormone receptors. These include Resistance to Thyroid Hormone Syndrome, erythroleukemia, hepatocellular carcinoma, renal clear cell carcinoma, and thyroid cancer. We will describe current views on the molecular bases of these diseases, and what distinguishes the neoplastic from the non-neoplastic. We will also touch on studies that implicate alterations in receptor expression, and thyroid hormone levels, in certain oncogenic processes.

A mechanism for pituitary-resistance to thyroid hormone (PRTH) syndrome: a loss in cooperative coactivator contacts by thyroid hormone receptor (TR)beta2

Thyroid hormone receptors (TR) are hormone-modulated transcription factors that regulate overall metabolic rate, lipid utilization, heart rate, and development. TR are expressed as a mix of interrelated receptor isoforms. The TRβ2 isoform is expressed in the hypothalamus and pituitary, where it plays an important role in the feedback regulation of thyroid hormone levels. TRβ2 exhibits unique transcriptional properties that parallel the ability of this isoform to bind to certain coactivators cooperatively through multiple contact surfaces. The more peripherally expressed TRβ1 isoform, in contrast, appears to recruit these coactivators through a single contact mechanism. We report here that clusters of charged amino acids in the TR hormone-binding domain are required for this enhanced mode of coactivator recruitment and that mutations in these charge clusters, by disrupting TRβ2 coactivator binding, are a molecular basis for pituitary resistance to thyroid hormone, a disease characterized by inappropriate thyroid hormone feedback regulation. We propose that the charge clusters allow wild-type TRβ2 to assume a conformation compatible with its mode of multiple contact coactivator recruitment, whereas disruption of these charge clusters disrupts normal T(3) homeostasis by reducing TRβ2 to a TRβ1-like, single contact mode of coactivator binding.

Differential recruitment of nuclear coregulators directs the isoform-dependent action of mutant thyroid hormone receptors

Studies using mice deficient in thyroid hormone receptors (TR) indicate that the two TR isoforms, TRα1 and TRβ1, in addition to mediating overlapping biological activities of the thyroid hormone, T3, also mediate distinct functions. Mice harboring an identical dominant negative mutation (denoted PV) at the C terminus of TRα1 (Thra1(PV) mice) or β1 (Thrb(PV) mice) also exhibit distinct phenotypes. These knockin mutant mice provide an opportunity to understand the molecular basis of isoform-dependent functions in vivo. Here we tested the hypothesis that the distinct functions of TR mutant isoforms are directed by a subset of nuclear regulatory proteins. Tandem-affinity chromatography of HeLa nuclear extracts showed that distinct 33 nuclear proteins including nuclear receptor corepressor (NCoR1) and six other proteins preferentially associated with TRα1PV or TRβ1PV, respectively. These results indicate that recruitment of nuclear regulatory proteins by TR mutants is subtype dependent. The involvement of NCoR1 in mediating the distinct liver phenotype of Thra1(PV) and Thrb(PV) mice was further explored. NCoR1 preferentially interacted with TRα1PV rather than with TRβ1PV. NCoR1 was recruited more avidly to the thyroid hormone response element-bound TRα1PV than to TRβ1PV in the promoter of the CCAAT/enhancer-binding protein α gene to repress its expression in the liver of Thra1(PV) mice, but not in Thrb(PV) mice. This preferential recruitment of NCoR1 by mutant isoforms could contribute, at least in part, to the distinct liver lipid phenotype of these mutant mice. The present study highlights a novel mechanism by which TR isoforms direct their selective functions via preferential recruitment of a subset of nuclear coregulatory proteins.

Amphibian metamorphosis as a model for studying endocrine disruption on vertebrate development: effect of bisphenol A on thyroid hormone action

Thyroid hormone (TH) is essential for proper development in vertebrates. TH deficiency during gestation and early postnatal development produces severe neurological, skeletal, metabolism and growth abnormalities. It is therefore important to consider environmental chemicals that may interfere with TH signaling. Exposure to environmental contaminants that disrupt TH action may underlie the increasing incidence of human developmental disorders worldwide. One contaminant of concern is the xenoestrogen bisphenol A (BPA), a chemical widely used to manufacture polycarbonate plastics and epoxy resins. The difficulty in studying uterus-enclosed mammalian embryos has hampered the analysis on the direct effects of BPA during vertebrate development. As TH action at the cellular level is highly conserved across vertebrate species, amphibian metamorphosis serves as an important TH-dependent in vivo vertebrate model for studying potential contributions of BPA toward human developmental disorders. Using Xenopus laevis as a model, we and others have demonstrated the inhibitory effects of BPA exposure on metamorphosis. Genome-wide gene expression analysis revealed that surprisingly, BPA primarily targets the TH-signaling pathway essential for metamorphosis in Xenopus laevis. Given the importance of the genomic effects of TH during metamorphosis and the conservation in its regulation in higher vertebrates, these observations suggest that the effect of BPA in human embryogenesis is through the inhibition of the TH pathway and warrants further investigation. Our findings further argue for the critical need to use in vivo animal models coupled with systematic molecular analysis to determine the developmental effects of endocrine disrupting compounds.

Growth activation alone is not sufficient to cause metastatic thyroid cancer in a mouse model of follicular thyroid carcinoma

TSH is the major stimulator of thyrocyte proliferation, but its role in thyroid carcinogenesis remains unclear. To address this question, we used a mouse model of follicular thyroid carcinoma (FTC) (TRbeta(PV/PV) mice). These mice, harboring a dominantly negative mutation (PV) of the thyroid hormone-beta receptor (TRbeta), exhibit increased serum thyroid hormone and elevated TSH. To eliminate TSH growth-stimulating effect, TRbeta(PV/PV) mice were crossed with TSH receptor gene knockout (TSHR(-/-)) mice. Wild-type siblings of TRbeta(PV/PV) mice were treated with an antithyroid agent, propylthiouracil, to elevate serum TSH for evaluating long-term TSH effect (WT-PTU mice). Thyroids from TRbeta(PV/PV)TSHR(-/-) showed impaired growth with no occurrence of FTC. Both WT-PTU and TRbeta(PV/PV) mice displayed enlarged thyroids, but only TRbeta(PV/PV) mice developed metastatic FTC. Molecular analyses indicate that PV acted, via multiple mechanisms, to activate the integrins-Src-focal adhesion kinase-p38 MAPK pathway and affect cytoskeletal restructuring to increase tumor cell migration and invasion. Thus, growth stimulated by TSH is a prerequisite but not sufficient for metastatic cancer to occur. Additional genetic alterations (such as PV), destined to alter focal adhesion and migration capacities, are required to empower hyperplastic follicular cells to invade and metastasize. These in vivo findings provide new insights in understanding carcinogenesis of the human thyroid.

Molecular aspects of thyroid hormone actions

Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T(3) via transcriptional regulation. Two TR genes, alpha and beta, encode four T(3)-binding receptor isoforms (alpha1, beta1, beta2, and beta3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T(3), transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T(3) target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T(3)-dependent manner. In the absence of T(3), corepressors act to repress the basal transcriptional activity, whereas in the presence of T(3), coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRbeta gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T(3). Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism. Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin alphavbeta3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na(+)/H(+) exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T(4) is an agonist at the plasma membrane without conversion to T(3). Tetraiodothyroacetic acid is a T(4) analog that inhibits the actions of T(4) and T(3) at the integrin, including angiogenesis and tumor cell proliferation. T(3) can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alphavbeta3. Downstream consequences of phosphatidylinositol 3-kinase activation by T(3) include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase. Thyroid hormone, chiefly T(3) and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T(4) and rT(3), but not T(3), is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth. A truncated TRalpha1 isoform that resides in the extranuclear compartment mediates the action of thyroid hormone on the cytoskeleton.

A conserved lysine in the thyroid hormone receptor-alpha1 DNA-binding domain, mutated in hepatocellular carcinoma, serves as a sensor for transcriptional regulation

Nuclear receptors are hormone-regulated transcription factors that play key roles in normal physiology and development; conversely, mutant nuclear receptors are associated with a wide variety of neoplastic and endocrine disorders. Typically, these receptor mutants function as dominant negatives and can interfere with wild-type receptor activity. Dominant-negative thyroid hormone receptor (TR) mutations have been identified in over 60% of the human hepatocellular carcinomas analyzed. Most of these mutant TRs are defective for corepressor release or coactivator binding in vitro, accounting for their transcriptional defects in vivo. However, two HCC-TR mutants that function as dominant-negative receptors in cells display near-normal properties in vitro, raising questions about the molecular basis behind their transcriptional defects. We report here that a single amino acid substitution, located at the same position in the DNA-binding domain of both mutants, is responsible for their impaired transcriptional activation and dominant-negative properties. Significantly, this amino acid, K74 in TRalpha, is highly conserved in all known nuclear receptors and seems to function as an allosteric sensor that regulates the transcriptional activity of these receptors in response to binding to their DNA recognition sequences. We provide evidence that these two human hepatocellular carcinoma mutants have acquired dominant-negative function as a result of disruption of this allosteric sensing. Our results suggest a novel mechanism by which nuclear receptors can acquire transcriptional defects and contribute to neoplastic disease.

A novel mutation of thyroid hormone receptor beta (I431V) impairs corepressor release, and induces thyroid hormone resistance syndrome

Resistance to thyroid hormone (RTH) is a rare disorder characterized by variable tissue hyporesponsiveness to thyroid hormone, usually caused by mutations in the thyroid hormone receptor beta (TRbeta). We describe a large Brazilian family harboring a novel mutation affecting TRbeta gene and inducing RTH. A 14-year-old girl was found to have elevated free T4 and free T3 plasma concentrations in coexistence with unsuppressed TSH and a questionable goiter. The diagnosis of RTH was verified by identification of a novel mutation (I431V) in the TRbeta gene. Sixteen asymptomatic relatives of the proposita are also affected by the mutation. Functional studies showed that I431V mutation exerts dominant-negative effect on wild type TRbeta, mainly by impairment of ligand-dependent release of corepressor SMRT. The presence of this mutation reduces potency, but does not affect efficacy of thyroid hormone action, in accordance with the clinical picture of eumetabolism of the affected individuals.

Novel non-genomic signaling of thyroid hormone receptors in thyroid carcinogenesis

The thyroid hormone receptors (TRs) are transcription factors that mediate the pleiotropic activities of the thyroid hormone, T3. Four T3-binding isoforms, TRalpha1, TRbeta1, TRbeta2, and TRbeta3, are encoded by two genes, THRA and THRB. Mutations and altered expression of TRs have been reported in human cancers. A targeted germ-line mutation of the Thrbeta gene in the mouse leads to spontaneous development of follicular thyroid carcinoma (TRbeta(PV/PV) mouse). The TRbetaPV mutant has lost T3-binding activity and displays potent dominant negative activity. The striking phenotype of thyroid cancer exhibited by TRbeta(PV/PV) mice has recently led to the discovery of novel non-genomic actions of TRbetaPV that contribute to thyroid carcinogenesis. These actions involve direct physical interaction of TRbetaPV with cellular proteins, namely the regulatory subunit of the phosphatidylinositol 3-kinase (p85alpha), the pituitary tumor transforming gene (PTTG) and beta-catenin, that are critically involved in cell proliferation, motility, migration, and metastasis. Thus, a TRbeta mutant (TRbetaPV), via a novel mode of non-genomic action, acts as an oncogene in thyroid carcinogenesis.

Thyroid hormone receptor mutants implicated in human hepatocellular carcinoma display an altered target gene repertoire

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that control multiple aspects of normal physiology and development. Mutations in TRs have been identified at high frequency in certain cancers, including human hepatocellular carcinomas (HCCs). The majority of HCC-TR mutants bear lesions within their DNA recognition domains, and we have hypothesized that these lesions change the mutant receptors' target gene repertoire in a way crucial to their function as oncoproteins. Using stable cell transformants and expression array analysis, we determined that mutant TRs isolated from two different HCCs do, as hypothesized, display a target gene repertoire distinct from that of their normal TR progenitors. Only a subset of genes regulated by wild-type TRs was regulated by the corresponding HCC-TR mutants. More surprisingly, the HCC-TR mutants also gained the ability to regulate additional target genes not recognized by the wild-type receptors, and were not simply restricted to repression, but could also activate a subset of their target genes. We conclude that the TR mutants isolated from HCC have sustained multiple alterations from their normal progenitors that include not only changes in their transcriptional outputs, but also changes in the genes they target; both are likely to contribute to neoplasia.

Endocrine-disrupting equivalents in industrial effluents discharged into Yangtze River

The endocrine-disrupting equivalents in effluents from three chemical industry wastewater treatment systems in the vicinity of Yangtze River were determined by several transactivation reporter gene assays. Transient transfections of African green monkey kidney cell line (CV-1) were used to determine the estrogenic, anti-androgenic and anti-thyroid equivalents in the effluents. Organic extracts of the effluents contained compounds that were potent anti-androgens and the activities measured as an equivalent concentration of flutamide were 45.53, 34.65 and 91.61 nM, respectively. The extracts also contained detectable concentrations of thyroid antagonists. Estrogenic activities, measured with the reporter gene assay, were near or below the method detection limit (0.58 pM as E2). Concentrations of some of the major constituents such as di(2-ethylhexyl)phthalate, dibutyl phthalate, 2,6-dinitrotoluene and nitrobenzene were quantified. The data suggest that the reporter gene assay is useful to predication of endocrine disrupting effects in polluted aquatic body.

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.

Endocrine-disrupting chemicals: an Endocrine Society scientific statement

There is growing interest in the possible health threat posed by endocrine-disrupting chemicals (EDCs), which are substances in our environment, food, and consumer products that interfere with hormone biosynthesis, metabolism, or action resulting in a deviation from normal homeostatic control or reproduction. In this first Scientific Statement of The Endocrine Society, we present the evidence that endocrine disruptors have effects on male and female reproduction, breast development and cancer, prostate cancer, neuroendocrinology, thyroid, metabolism and obesity, and cardiovascular endocrinology. Results from animal models, human clinical observations, and epidemiological studies converge to implicate EDCs as a significant concern to public health. The mechanisms of EDCs involve divergent pathways including (but not limited to) estrogenic, antiandrogenic, thyroid, peroxisome proliferator-activated receptor gamma, retinoid, and actions through other nuclear receptors; steroidogenic enzymes; neurotransmitter receptors and systems; and many other pathways that are highly conserved in wildlife and humans, and which can be modeled in laboratory in vitro and in vivo models. Furthermore, EDCs represent a broad class of molecules such as organochlorinated pesticides and industrial chemicals, plastics and plasticizers, fuels, and many other chemicals that are present in the environment or are in widespread use. We make a number of recommendations to increase understanding of effects of EDCs, including enhancing increased basic and clinical research, invoking the precautionary principle, and advocating involvement of individual and scientific society stakeholders in communicating and implementing changes in public policy and awareness.

Thyroid hormone receptor mutations found in renal clear cell carcinomas alter corepressor release and reveal helix 12 as key determinant of corepressor specificity

Thyroid hormone receptors (TRs) regulate multiple normal physiological and developmental pathways, whereas mutations in TRs can result in endocrine and neoplastic disease. A particularly high rate of TR mutations has been found in human renal clear cell carcinomas (RCCCs). We report here that the majority of these RCCC TR mutants tested are defective for transcriptional activation and behave as dominant-negative inhibitors of wild-type receptor function. Although several of the dominant-negative RCCC TR mutants are impaired for hormone binding, all fail to release from corepressors appropriately in response to T(3), a trait that closely correlates with their defective transcriptional properties. Notably, many of these mutants exhibit additional changes in their specificity for different corepressor splice forms that may further contribute to the disease phenotype. Mapping of the relevant mutations reveals that the C-terminal receptor helix 12 is not simply a hormone-operated switch that either permits or prevents all corepressor binding, but is instead a selective gatekeeper that actively discriminates between different forms of corepressor even in the absence of T(3).

The thyroid hormone receptor (TR) beta-selective agonist GC-1 inhibits proliferation but induces differentiation and TR beta mRNA expression in mouse and rat osteoblast-like cells

Previous studies showed anabolic effects of GC-1, a triiodothyronine (T3) analogue that is selective for both binding and activation functions of thyroid hormone receptor (TR) beta1 over TRalpha1, on bone tissue in vivo. The aim of this study was to investigate the responsiveness of rat (ROS17/2.8) and mouse (MC3T3-E1) osteoblast-like cells to GC-1. As expected, T3 inhibited cellular proliferation and stimulated mRNA expression of osteocalcin or alkaline phosphatase in both cell lineages. Whereas equimolar doses of T3 and GC-1 equally affected these parameters in ROS17/2.8 cells, the effects of GC-1 were more modest compared to those of T3 in MC3T3-E1 cells. Interestingly, we showed that there is higher expression of TRalpha1 than TRbeta1 mRNA in rat (approximately 20-90%) and mouse (approximately 90-98%) cell lineages and that this difference is even higher in mouse cells, which highlights the importance of TRalpha1 to bone physiology and may partially explain the modest effects of GC-1 in comparison with T3 in MC3T3-E1 cells. Nevertheless, we showed that TRbeta1 mRNA expression increases (approximately 2.8- to 4.3-fold) as osteoblastic cells undergo maturation, suggesting a key role of TRbeta1 in mediating T3 effects in the bone forming cells, especially in mature osteoblasts. It is noteworthy that T3 and GC-1 induced TRbeta1 mRNA expression to a similar extent in both cell lineages (approximately 2- to 4-fold), indicating that both ligands may modulate the responsiveness of osteoblasts to T3. Taken together, these data show that TRbeta selective T3 analogues have the potential to directly induce the differentiation and activity of osteoblasts.

Genotyping of resistance to thyroid hormone in South American population. Identification of seven novel missense mutations in the human thyroid hormone receptor beta gene

Thyroid Hormone Receptor beta (THRB) defects, typically transmitted as autosomal dominant traits, cause Resistance to Thyroid Hormone (RTH). We analyzed the THRB gene in thirteen South American patients with clinical evidence RTH from eleven unrelated families. Sequence analysis revealed seven novel missense mutations. Four novel mutations were identified in exon 9. The first, a c.991A>G transition which originates a substitution of asparagine by aspartic acid (p.N331D). The second nucleotide alteration consists of a guanine to cytosine transversion at position 1003 (c.1003G>C) and results in substitution of the alanine at codon 335 by proline (p.A335P). The third mutation, a c.1022T>C transition produces a change of leucine by proline (p.L341P). The fourth mutation detected in exon 9 was a c.1036C>T transition which replaces the leucine at codon 346 by phenylalanine (p.L346F). The sequencing of the exon 10 detected three novel missense mutations. The first, a c.1293A>G transition changing isoleucine 431 for methionine (p.I431M). The second, the cytosine at position 1339 was replaced by adenine (c.1339C>A) resulting in the replacement of proline by threonine (p.P447T). The third mutation detected in exon 10 was a c.1358C>T transition resulting in the substitution of proline at codon 453 by leucine (p.P453L). Finally, sequencing analysis of the THRB gene revealed three substitutions previously described (p.A268G, p.P453T and p.F459C). The p.P453T was found in two patients. In conclusion, we report thirteen patients with RTH caused by heterozygous mutations of the THRB gene. Seven of the identified mutations correspond to novel substitutions.

Current and Potential Rodent Screens and Tests for Thyroid Toxicants

This article reviews current rodent screens and tests to detect thyroid toxicants. Many points of disruption for thyroid toxicants are outlined and include: (a) changes in serum hormone level; (b) thyroperoxidase inhibitors; (c) the perchlorate discharge test; (d) inhibitors of iodide uptake; (e) effects on iodothyronine deiodinases; (f) effects on thyroid hormone action; and (g) role of binding proteins (e.g., rodent transthyretin). The major thyroid endpoints currently utilized in existing in vivo assay protocols of the Organization for Economic Cooperation and Development (OECD), Japanese researchers, and U.S. Environmental Protection Agency (EPA) include thyroid gland weight, histopathology, circulating thyroid hormone measurements, and circulating thyroid-stimulating hormone (TSH). These endpoints can be added into the existing in vivo assays for reproduction, development, and neurodevelopment that are outlined in this chapter. Strategic endpoints for possible addition to existing protocols to detect effects on developmental and adult thyroid endpoints are discussed. Many of these endpoints for detecting thyroid system disruption require development and additional research before they can be considered in existing assays. Examples of these endpoints under development include computer-assisted morphometry of the brain and evaluation of treatment-related changes in gene expression, thyrotropin-releasing hormone (TRH) and TSH challenge tests, and tests to evaluate thyroid hormone (TH)-dependent developmental events, especially in the rodent brain (e.g., measures of cerebellar and cortical proliferation, differentiation, migration, apoptosis, planimetric measures and gene expression, and oligodendrocyte differentiation). Finally, TH-responsive genes and proteins as well as enzyme activities are being explored. Existing in vitro tests are also reviewed, for example, thyroid hormone (TH) metabolism, receptor binding, and receptor activation assays, and their restrictions are described. The in vivo assays are currently the most appropriate for understanding the potential effects of a thyroid toxicant on the thyroid system. The benefits and potential limitations of the current in vivo assays are listed, and a discussion of the rodent thyroid system in the context of human health is touched upon. Finally, the importance of understanding the relationship between timing of exposure, duration of dose, and time of acquisition of the endpoints in interpreting the results of the in vivo assays is emphasized.

Identification and consequences of polymorphisms in the thyroid hormone receptor alpha and beta genes

OBJECTIVE

Genetic factors exert considerable influence on thyroid function variables. Single nucleotide polymorphisms (SNPs) in thyroid hormone pathway genes have been associated with serum thyroid parameters implying small alterations in the hypothalamus-pituitary-thyroid axis. However, little is known about SNPs in the THRA (17q11.2) and THRB (3p24.2) genes. The aim of this study was to map THRA and THRB for the occurrence and frequencies of SNPs and relate these to thyroid parameters.

DESIGN AND METHODS

SNPs were identified by sequencing all THRA and THRB exons and flanking regions in 52 randomly selected subjects. SNPs were genotyped in 1116 healthy Danish twins by TaqMan assays and related to thyroid parameters. One SNP in THRB was additionally genotyped in the elderly population of the Rotterdam Scan Study (n = 940).

MAIN OUTCOME

15 SNPs (7 novel) in THRA and THRB were identified. Two SNPs in the 3' untranslated region of THRA were genotyped: a novel SNP (2390A/G) and 1895C/A (rs12939700). In THRB, a synonymous (735C/T; rs3752874) and an intronic SNP (in9-G/A; rs13063628) were genotyped. No associations between SNPs and thyroid hormone levels (total and free 3,3',5-triiodo-L-thyronine [T3] and thyroxine, reverse T3) were found. THRB-in9-G/A was significantly associated with higher serum thyroid stimulating hormone (TSH) (p(lnTSH) = 0.01) in the Danish twins, but not in subjects of the Rotterdam Scan Study, although it showed a similar trend.

CONCLUSIONS

Analysis of the T3 receptor genes revealed 15 SNPs, including 7 novel. Only THRB-in9-G/A was associated with higher serum TSH in a large population of Danish twins.

Carbaryl, 1-naphthol and 2-naphthol inhibit the beta-1 thyroid hormone receptor-mediated transcription in vitro

Effects of pesticides on the function of thyroid have attracted lots of attention because thyroid hormones (THs) play a major role in mammalian brain development. In order to screen for compounds that acted on the thyroid hormone receptor (TR) signaling pathway, we transiently transfected the vector pGal4-L-TRbeta1 (Gal4 DBD fused to hTRbeta1 LBD) and Gal4-responsive luciferase reporter pUAS-tk-Luc into HepG2 cell, developing a reporter gene assay which showed good response to triiodothyronine (T3) and thyroxine (T4) with the median effective concentration (EC(50)) of 0.46 and 25.53 nM, respectively. Bisphenol A exhibited weak anti-thyroid hormone activity with median inhibitory concentration (IC(50)) value of 6.45 x 10(-5)M. The assay showed acceptable repeatability to T3 with intra coefficient of variability (CV) of 5.9% and inter CV of 11.7%. Carbaryl, 1-naphthol (1-NAP) and 2-naphthol (2-NAP) were tested for their agonist and antagonist activities. As a result, we found that all the three related chemicals possessed TR antagonist activity and none of them showed the agonist activity. These results further indicated that TR might be the targets of industrial chemicals. And this assay provided a useful tool for investigating the effects of environment chemicals on thyoid function.

Resistance to thyroid hormone with missense mutation (V349M) in the thyroid hormone receptor beta gene

Resistance to thyroid hormone (RTH) is a syndrome characterized by reduced sensitivity to the thyroid hormone. It is generally caused by mutations in the thyroid hormone receptor beta (TR beta) gene. On the basis of its clinical features, two different forms of this syndrome have been described: generalized resistance and pituitary resistance. A total of 122 TR beta gene mutations have been identified thus far. A 38-year-old woman presented with intermittent palpitation. Thyroid function tests revealed elevated levels of free T4 and TSH. TSH a-subunit levels were 0.41 mlU/mL, and magnetic resonance images of the sellar region evidenced no abnormal findings. The TSH response to TRH stimulation was found to be normal. The sequence analysis of the TR beta gene verified a missense mutation in exon 11, and the observed amino acid alteration was a substitution of a valine for a methionine at codon 349. We report the first case of a woman with RTH, which was found to be caused by a missense mutation (V349M) in the TR beta gene.