The thyroid hormone receptor variant alpha2 is a weak antagonist because it is deficient in interactions with nuclear receptor corepressors

TRα2—An Untuned Second Fiddle or Fine-Tuning Thyroid Hormone Action?

Thyroid hormones (THs) control a wide range of physiological functions essential for metabolism, growth, and differentiation. On a molecular level, TH action is exerted by nuclear receptors (TRs), which function as ligand-dependent transcription factors. Among several TR isoforms, the function of TRα2 remains poorly understood as it is a splice variant of TRα with an altered C-terminus that is unable to bind T3. This review highlights the molecular characteristics of TRα2, proposed mechanisms that regulate alternative splicing and indications pointing towards an antagonistic function of this TR isoform in vitro and in vivo. Moreover, remaining knowledge gaps and major challenges that complicate TRα2 characterization, as well as future strategies to fully uncover its physiological relevance, are discussed.

What is the Role of Thyroid Hormone Receptor Alpha 2 (TRα2) in Human Physiology?

Thyroid hormone receptors are nuclear receptors that function as transcription factors and are regulated by thyroid hormones. To date, a number of variants and isoforms are known. This review focuses on the thyroid hormone receptor α (TRα), in particular TRα2, an isoform that arises from alternative splicing of the THRA mRNA transcript. Unlike the TRα1 isoform, which can bind T3, the TRα2 isoform lacks a ligand-binding domain but still binds to DNA thereby antagonizing the transcriptional activity of TRα1. Although a regulatory role has been proposed, the physiological function of this TRα2 antagonism is still unclear due to limited in vitro and mouse model data. Recently, the first patients with resistance to thyroid hormone due to mutations in THRA, the TRα encoding gene, affecting the antagonistic function of TRα2 were described, suggesting a significant role of this particular isoform in human physiology.

A Direct Comparison of Thyroid Hormone Receptor Protein Levels in Mice Provides Unexpected Insights into Thyroid Hormone Action

Background: Thyroid hormone (TH) action is mediated by three major thyroid hormone receptor (THR) isoforms α1, β1, and β2 (THRA1, THRB1, and THRB2). These THRs and a fourth major but non-TH binding isoform, THRA2, are encoded by two genes Thra and Thrb. Reliable antibodies against all THR isoforms are not available, and THR isoform protein levels in mammalian tissues are often inferred from messenger RNA (mRNA) levels. Methods: We generated knock-in mouse models expressing endogenously and identically 2X hemagglutenin epitope (HA)-tagged THRs (THRA1/2, THRB1, and THRB2), which could then be detected by commercially available anti-HA antibodies. Using nuclear enrichment, immunoprecipitation, and Western blotting, we determined relative THR protein expression in 16 mouse organs. Results: In all peripheral organs tested except the liver, the predominant THR isoform was THRA1. Surprisingly, in metabolically active organs such as fat and muscle, THRB1 protein levels were up to 10 times lower than that of THRA1, while their mRNA levels appeared similar. In contrast to peripheral organs, the central nervous system (CNS) had a unique pattern with relatively low levels of both THRB1 and THRA1, and high levels of THRA2 expression. As expected, THRB2 was highly expressed in the pituitary, but a previously unknown sex-specific difference in THRB2 expression was found (female mice having higher pituitary expression than male mice). Higher THRB2 expression appears to make the central axis more sensitive to TH as both serum thyrotropin and Tshb mRNA levels were lower in female mice. Conclusions: Direct comparison of THR protein abundance in different organs using endogenously tagged HA-THR mouse lines shows that expression of THR isoforms is regulated at transcriptional and posttranscriptional levels, and in organ-specific manner. The prevalence of THRA1 and low abundance of THRB1 in majority of peripheral tissues suggest that peripheral actions of these isoforms should be revisited. A unique pattern of high THRA2 in CNS warrants further exploration of this non-TH binding isoform in brain development. Finally, THRB2, in addition to cell-specific control, is also regulated in a sex-specific manner, which may change the hypothalamus-pituitary-thyroid axis set point and perhaps metabolism in males and females.

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.

Diverse Genotypes and Phenotypes of Three Novel Thyroid Hormone Receptor-α Mutations

CONTEXT

Recently several patients with resistance to thyroid hormone (RTH)-α due to T3 receptor-α (TRα) mutations were identified. The phenotype of these patients consists of varying degrees of growth impairment, delayed bone, mental and motor development, constipation, macrocephaly, and near-normal thyroid function tests.

OBJECTIVE

The objective of the study was to describe the clinical phenotype of three new families with RTHα and thereby gain more detailed knowledge on this novel syndrome.

DESIGN, SETTING, AND PARTICIPANTS

RTHα was suspected in three index patients from different families. Detailed clinical and biochemical assessment and imaging and genetic analyses were performed in the patients and their relatives. In addition, functional consequences of TRα mutations were investigated in vitro.

RESULTS

We studied 22 individuals from three families and identified 10 patients with heterozygous TRα mutations: C380fs387X, R384H, and A263S, respectively. The frame-shift mutation completely inactivated TRα, whereas the missense mutations produced milder defects. These mutations were associated with decreasing severity of the clinical phenotype: the patient in family 1 showed severe defects in growth, mental, and motor development, whereas the seven patients in family 3 had only mild clinical features. The most frequent abnormalities were anemia, constipation, and a delay in at least one of the developmental milestones. Serum free T3 ranged from high-normal to high and serum free T4 and rT3 from normal to low. TSH levels were normal in all patients.

CONCLUSIONS

This large case series underlines the variation in the clinical phenotype of RTHα patients. RTHα should be suspected in subjects when even mild clinical and laboratory features of hypothyroidism are present along with high/high-normal free T3, low/normal free T4, and normal TSH.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Molecular characterization of human thyroid hormone receptor β isoform 4

Thyroid hormone exerts a pleiotropic effect on development, differentiation, and metabolism through thyroid hormone receptor (TR). A novel thyroid hormone receptor β isoform (TRβ4) was cloned using PCR from a human pituitary cDNA library as a template. We report here the characterization of TRβ4 from a molecular basis. Temporal expression of TRβ4 during the fetal period is abundant in the brain and kidney, comparable with the adult pattern. Western blot analysis revealed that TRs are ubiquitination labile proteins, while TRβ1 is potentially stable. TRβ1, peroxisome proliferator-activated receptors (PPAR), and vitamin D receptor (VDR), which belong to class II transcription factors that function via the formation of heterodimeric complexes with retinoid X receptor (RXR), were suppressed by TRβ4 in a dose-dependent manner. Thus, TRβ4 exhibits ligand-independent transcriptional silencing, possibly as a substitute for dimerized RXR. In this study, TRβ1 and TRβ4 transcripts were detected in several cell lines. Quantitative RT-PCR assay showed that the expression of TRβ4 in human embryonic carcinoma cells of the testis was suppressed by sex hormone in a reciprocal manner to TRβ1. In contrast, TRβ4 was expressed under a high dose of triiodothyronine (T3) in a reciprocal manner to TRβ1. Finally, in transiently transfected NIH-3T3 cells, green fluorescence protein (GFP)-tagged TRβ4 was mostly nuclear in both the absence and the presence of T3. By mutating defined regions of both TRβs, we found that both TRβ1 and TRβ4 had altered nuclear/cytoplasmic distribution as compared with wild-type, and different to T3 and the nuclear receptor corepressor (NCoR). Thus, site-specific DNA binding is not essential for maintaining TRβs within the nucleus.

Resistance to thyroid hormone due to defective thyroid receptor alpha

Thyroid hormones act via nuclear receptors (TRα1, TRβ1, TRβ2) with differing tissue distribution; the role of α2 protein, derived from the same gene locus as TRα1, is unclear. Resistance to thyroid hormone alpha (RTHα) is characterised by tissue-specific hypothyroidism associated with near-normal thyroid function tests. Clinical features include dysmorphic facies, skeletal dysplasia (macrocephaly, epiphyseal dysgenesis), growth retardation, constipation, dyspraxia and intellectual deficit. Biochemical abnormalities include low/low-normal T4 and high/high-normal T3 concentrations, a subnormal T4/T3 ratio, variably reduced reverse T3, raised muscle creatine kinase and mild anaemia. The disorder is mediated by heterozygous, loss-of-function, mutations involving either TRα1 alone or both TRα1 and α2, with no discernible phenotype attributable to defective α2. Whole exome sequencing and diagnostic biomarkers may enable greater ascertainment of RTHα, which is important as thyroxine therapy reverses some metabolic abnormalities and improves growth, constipation, dyspraxia and wellbeing. The genetic and phenotypic heterogeneity of RTHα and its optimal management remain to be elucidated.

Cinnamaldehyde Contributes to Insulin Sensitivity by Activating PPARδ, PPARγ, and RXR

Cinnamon is a traditional folk herb used in Asia and has been reported to have antidiabetic effects. Our previous study showed that cinnamaldehyde (CA), a major effective compound in cinnamon, exhibited hypoglycemic and hypolipidemic effects together in db/db mice. The aim of the present study was to elucidate the molecular mechanisms of the effects of CA on the transcriptional activities of three peroxisome proliferator-activated receptors, (PPAR) α, δ, and γ. We studied the effects of CA through a transient expression assay with TSA201 cells, derivatives of human embryonic kidney cell line (HEK293). Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis was also performed to evaluate mRNA expression levels. We show here that CA induced PPARδ, PPARγ and retinoid X receptor (RXR) activation. CA may activate PPARγ in a different manner than pioglitazone, as CA selectively stimulated PPARγ S342A mutant while pioglitazone did not. In addition, CA and L-165041 had a synergistic effect on PPARδ activation. To gather the biological evidence that CA increases PPARs transcription, we further measured the expressions of PPARδ and PPARγ target genes in 3T3-L1 adipocytes. The data showed CA induced the expression of PPARδ and PPARγ target genes, namely aP2 and CD36, in differentiated adipocytes. As a result, PPARδ, PPARγ and their heterodimeric partner RXR appear to play a part in the CA action in the target tissues, thereby enhancing insulin sensitivity and fatty acid β-oxidation and energy uncoupling in skeletal muscle and adipose tissue.

Resistance to thyroid hormone caused by a mutation in thyroid hormone receptor (TR)α1 and TRα2: clinical, biochemical, and genetic analyses of three related patients

BACKGROUND

The thyroid hormone receptor α gene (THRA) transcript is alternatively spliced to generate either thyroid hormone receptor (TR)α1 or a non-hormone-binding variant protein, TRα2, the function of which is unknown. Here, we describe the first patients identified with a mutation in THRA that affects both TRα1 and TRα2, and compare them with patients who have resistance to thyroid hormone owing to a mutation affecting only TRα1, to delineate the relative roles of TRα1 and TRα2.

METHODS

We did clinical, biochemical, and genetic analyses of an index case and her two sons. We assessed physical and radiological features, thyroid function, physiological and biochemical markers of thyroid hormone action, and THRA sequence.

FINDINGS

The patients presented in childhood with growth failure, developmental delay, and constipation, which improved after treatment with thyroxine, despite normal concentrations of circulating thyroid hormones. They had similar clinical (macrocephaly, broad faces, skin tags, motor dyspraxia, slow speech), biochemical (subnormal ratio of free thyroxine:free tri-iodothyronine [T3], low concentration of total reverse T3, high concentration of creatine kinase, mild anaemia), and radiological (thickened calvarium) features to patients with TRα1-mediated resistance to thyroid hormone, although our patients had a heterozygous mis-sense mutation (Ala263Val) in both TRα1 and TRα2 proteins. The Ala263Val mutant TRα1 inhibited the transcriptional function of normal receptor in a dominant-negative fashion. By contrast, function of Ala263Val mutant TRα2 matched its normal counterpart. In vitro, high concentrations of T3 restored transcriptional activity of Ala263Val mutant TRα1, and reversed the dominant-negative inhibition of its normal counterpart. High concentrations of T3 restored expression of thyroid hormone-responsive target genes in patient-derived blood cells.

INTERPRETATION

TRα1 seems to be the principal functional product of the THRA gene. Thyroxine treatment alleviates hormone resistance in patients with mutations affecting this gene, possibly ameliorating the phenotype. These findings will help the diagnosis and treatment of other patients with resistance to thyroid hormone resulting from mutations in THRA.

FUNDING

Wellcome Trust, NIHR Cambridge Biomedical Research Centre, Marie Curie Actions, Foundation for Development of Internal Medicine in Europe.

TR alpha 2 exerts dominant negative effects on hypothalamic Trh transcription in vivo

Mammalian thyroid hormone receptors (TRs) have multiple isoforms, including the bona fide receptors that bind T₃ (TRα1, TRβ1 and TRβ2) and a non-hormone-binding variant, TRα2. Intriguingly, TRα2 is strongly expressed in the brain, where its mRNA levels exceed those of functional TRs. Ablation of TRα2 in mice results in over-expression of TRα1, and a complex phenotype with low levels of free T₃ and T₄, without elevated TSH levels, suggesting an alteration in the negative feedback at the hypothalamic-pituitary level. As the hypothesis of a potential TRH response defect has never been tested, we explored the functional role of TRα2 in negative feedback on transcription of hypothalamic thyrotropin, Trh. The in vivo transcriptional effects of TRα2 on hypothalamic Trh were analysed using an in vivo reporter gene approach. Effects on Trh-luc expression were examined to that of two, T₃ positively regulated genes used as controls. Applying in vivo gene transfer showed that TRα2 over-expression in the mouse hypothαlamus abrogates T₃-dependent repression of Trh and T₃ activation of positively regulated promoters, blocking their physiological regulation. Surprisingly, loss of function studies carried out by introducing a shTRα2 construct in the hypothalamus also blocked physiological T₃ dependent regulation. Thus, modulating hypothalamic TRα2 expression by either gain or loss of function abrogated T₃ dependent regulation of Trh transcription, producing constant transcriptional levels insensitive to feedback. This loss of physiological regulation was reflected at the level of the endogenous Trh gene, were gain or loss of function held mRNA levels constant. These results reveal the as yet undescribed dominant negative role of TRα2 over TRα1 effect on hypothalamic Trh transcription.

Thyroid hormone, thyromimetics, and metabolic efficiency

Thyroid hormone (TH) has long been recognized as a major modulator of metabolic efficiency, energy expenditure, and thermogenesis. TH effects in regulating metabolic efficiency are transduced by controlling the coupling of mitochondrial oxidative phosphorylation and the cycling of extramitochondrial substrate/futile cycles. However, despite our present understanding of the genomic and nongenomic modes of action of TH, its control of mitochondrial coupling still remains elusive. This review summarizes historical and up-to-date findings concerned with TH regulation of metabolic energetics, while integrating its genomic and mitochondrial activities. It underscores the role played by TH-induced gating of the mitochondrial permeability transition pore (PTP) in controlling metabolic efficiency. PTP gating may offer a unified target for some TH pleiotropic activities and may serve as a novel target for synthetic functional thyromimetics designed to modulate metabolic efficiency. PTP gating by long-chain fatty acid analogs may serve as a model for such strategy.

Thyroid hormone function in the rat testis

Thyroid hormones are emerging regulators of testicular function since Sertoli, germ, and Leydig cells are found to express thyroid hormone receptors (TRs). These testicular cells also express deiodinases, which are capable of converting the pro-hormone T4 to the active thyroid hormone T3, or inactivating T3 or T4 to a non-biologically active form. Furthermore, thyroid hormone transporters are also found in the testis. Thus, the testis is equipped with the transporters and the enzymes necessary to maintain the optimal level of thyroid hormone in the seminiferous epithelium, as well as the specific TRs to execute thyroid hormone action in response to different stages of the epithelial cycle of spermatogenesis. Studies using genetic models and/or goitrogens (e.g., propylthiouracil) have illustrated a tight physiological relationship between thyroid hormone and testicular function, in particular, Sertoli cell differentiation status, mitotic activity, gap junction function, and blood-testis barrier assembly. These findings are briefly summarized and discussed herein.

Fundamentally distinct roles of thyroid hormone receptor isoforms in a thyrotroph cell line are due to differential DNA binding

Thyroid hormones have a profound influence on human development and disease. The hypothalamic-pituitary-thyroid axis involves finely tuned feedback mechanisms to maintain thyroid hormone (TH) levels. Despite the important role of TH-negative feedback in regulating this axis, the mechanism by which this occurs is not clearly defined. Previous in vivo studies suggest separate roles for the two thyroid hormone receptor isoforms, THRA and THRB, in this axis. We performed studies using a unique pituitary thyrotroph cell line (TαT1.1) to determine the relative roles of THRA and THRB in the regulation of Tshb. Using chromatin immunoprecipitation assays, we found that THRB, not THRA, bound to the Tshb promoter. By selectively depleting THRB, THRA, or both THRA and THRB in TαT1.1 cells, we found that simultaneous knockdown of both THRB and THRA abolished T(3)-mediated down-regulation of Tshb at concentrations as high as 100 nm T(3). In contrast, THRA knockdown alone had no effect on T(3)-negative regulation, whereas THRB knockdown alone abolished T(3)-mediated down-regulation of Tshb mRNA levels at 10 nm but not 100 nm T(3) concentrations. Interestingly, chromatin immunoprecipitation assays showed that THRA becomes enriched on the Tshb promoter after knockdown of THRB. Thus, a likely mechanism for the differential effects of THR isoforms on Tshb may be based on their differential DNA-binding affinity to the promoter.

Thyroid hormone receptor α and regulation of type 3 deiodinase

Mice deficient in thyroid hormone receptor α (TRα) display hypersensitivity to thyroid hormone (TH), with normal serum TSH but diminished serum T(4). Our aim was to determine whether altered TH metabolism played a role in this hypersensitivity. TRα knockout (KO) mice have lower levels of rT(3), and lower rT(3)/T(4) ratios compared with wild-type (WT) mice. These alterations could be due to increased type 1 deiodinase (D1) or decreased type 3 deiodinase (D3). No differences in D1 mRNA expression and enzymatic activity were found between WT and TRαKO mice. We observed that T(3) treatment increased D3 mRNA in mouse embryonic fibroblasts obtained from WT or TRβKO mice, but not in those from TRαKO mice. T(3) stimulated the promoter activity of 1.5 kb 5'-flanking region of the human (h) DIO3 promoter in GH3 cells after cotransfection with hTRα but not with hTRβ. Moreover, treatment of GH3 cells with T(3) increased D3 mRNA after overexpression of TRα. The region necessary for the T(3)-TRα stimulation of the hD3 promoter (region -1200 to -1369) was identified by transfection studies in Neuro2A cells that stably overexpress either TRα or TRβ. These results indicate that TRα mediates the up-regulation of D3 by TH in vitro. TRαKO mice display impairment in the regulation of D3 by TH in both brain and pituitary and have reduced clearance rate of TH as a consequence of D3 deregulation. We conclude that the absence of TRα results in decreased clearance of TH by D3 and contributes to the TH hypersensitivity.

Structure and expression of two nuclear receptor genes in marsupials: insights into the evolution of the antisense overlap between the α-thyroid hormone receptor and Rev-erbα

Background

Alternative processing of α-thyroid hormone receptor (TRα, NR1A1) mRNAs gives rise to two functionally antagonistic nuclear receptors: TRα1, the α-type receptor, and TRα2, a non-hormone binding variant that is found only in mammals. TRα2 shares an unusual antisense coding overlap with mRNA for Rev-erbα (NR1D1), another nuclear receptor protein. In this study we examine the structure and expression of these genes in the gray short-tailed opossum, Monodelphis domestica, in comparison with that of eutherian mammals and three other marsupial species, Didelphis virginiana, Potorous tridactylus and Macropus eugenii, in order to understand the evolution and regulatory role of this antisense overlap.

Results

The sequence, expression and genomic organization of mRNAs encoding TRα1 and Rev-erbα are very similar in the opossum and eutherian mammals. However, the sequence corresponding to the TRα2 coding region appears truncated by almost 100 amino acids. While expression of TRα1 and Rev-erbα was readily detected in all tissues of M. domestica ages 0 days to 18 weeks, TRα2 mRNA was not detected in any tissue or stage examined. These results contrast with the widespread and abundant expression of TRα2 in rodents and other eutherian mammals. To examine requirements for alternative splicing of TRα mRNAs, a series of chimeric minigenes was constructed. Results show that the opossum TRα2-specific 5' splice site sequence is fully competent for splicing but the sequence homologous to the TRα2 3' splice site is not, even though the marsupial sequences are remarkably similar to core splice site elements in rat.

Conclusions

Our results strongly suggest that the variant nuclear receptor isoform, TRα2, is not expressed in marsupials and that the antisense overlap between TRα and Rev-erbα thus is unique to eutherian mammals. Further investigation of the TRα and Rev-erbα genes in marsupial and eutherian species promises to yield additional insight into the physiological function of TRα2 and the role of the associated antisense overlap with Rev-erbα in regulating expression of these genes.

An exonic splicing enhancer within a bidirectional coding sequence regulates alternative splicing of an antisense mRNA

The discovery of increasing numbers of genes with overlapping sequences highlights the problem of expression in the context of constraining regulatory elements from more than one gene. This study identifies regulatory sequences encompassed within two genes that overlap in an antisense orientation at their 3' ends. The genes encode the alpha-thyroid hormone receptor gene (TRalpha or NR1A1) and Rev-erbalpha (NR1D1). In mammals TRalpha pre-mRNAs are alternatively spliced to yield mRNAs encoding functionally antagonistic proteins: TRalpha1, an authentic thyroid hormone receptor; and TRalpha2, a non-hormone-binding variant that acts as a repressor. TRalpha2-specific splicing requires two regulatory elements that overlap with Rev-erbalpha sequences. Functional mapping of these elements reveals minimal splicing enhancer elements that have evolved within the constraints of the overlapping Rev-erbalpha sequence. These results provide insight into the evolution of regulatory elements within the context of bidirectional coding sequences. They also demonstrate the ability of the genetic code to accommodate multiple layers of information within a given sequence, an important property of the code recently suggested on theoretical grounds.

The transforming acidic coiled coil (TACC1) protein modulates the transcriptional activity of the nuclear receptors TR and RAR

Background

The transcriptional activity of Nuclear hormone Receptors (NRs) is regulated by interaction with coactivator or corepressor proteins. Many of these cofactors have been shown to have a misregulated expression or to show a subcellular mislocalization in cancer cell lines or primary tumors. Therefore they can be factors involved in the process of oncogenesis.

Results

We describe a novel NR coregulator, TACC1, which belongs to the Transforming Acidic Coiled Coil (TACC) family. The interaction of TACC1 with Thyroid Hormone Receptors (TR) and several other NRs has been shown in a yeast two-hybrid screen and confirmed by GST pulldown, colocalization and co-immunoprecipitation experiments. TACC1 interacts preferentially with unliganded NRs. In F9 cells, endogenous TACC1 localized in the chromatin-enriched fraction of the nucleus and interacted with Retinoid Acid Receptors (RARα) in the nucleus. TACC1 depletion in the cell led to decreased RARα and TRα ligand-dependent transcriptional activity and to delocalization of TR from the nucleus to the cytoplasm.

Conclusions

From these experimental studies we propose that TACC1 might be a scaffold protein building up a transcriptional complex around the NRs we studied. This function of TACC1 might account for its involvement in several forms of tumour development.

A selective peroxisome proliferator-activated receptor-gamma modulator, telmisartan, binds to the receptor in a different fashion from thiazolidinediones

Angiotensin type 1 receptor blockers are widely used for the treatment of hypertension, and one angiotensin type 1 receptor blocker, telmisartan, specifically activates the peroxisome proliferator-activated receptor (PPAR)-gamma. We studied the impact of PPARgamma mutants on transcriptional control and interaction with cofactors to elucidate differences in the molecular mechanism between telmisartan and other PPARgamma agonists, thiazolidinediones (TZDs). We created several amino acid substitutions in the ligand binding domain of PPARgamma that, based on molecular modeling, may affect the binding of these agents. In transient expression experiments, wild-type PPARgamma-mediated transcription stimulated by telmisartan was more than one third of that stimulated by TZDs. The activation stimulated by TZDs was impaired, whereas activation stimulated by telmisartan was retained, in the H323Y, S342A, and H449A mutants. In the Y473A mutant, the TZD-induced activation was further impaired and lower than that of telmisartan-induced activation. Coexpression of coactivators enhanced the activation by both telmisartan and TZDs, but activation by telmisartan always exceeded that of TZDs in the Y473A mutant. Based on a mammalian two-hybrid assay, the interaction with corepressors was retained in Y473A. Telmisartan and TZDs, but not 9cis retinoic acid, dissociated corepressors from the wild-type PPARgamma. Telmisartan most effectively dissociated corepressors from Y473A. The interaction with coactivators was enhanced by TZD activation of wild-type PPARgamma and both telmisartan and TZD activation of Y473A. Thus, the Y473A mutant is selectively stimulated by telmisartan but not TZDs, suggesting that telmisartan and TZDs have differential effects on the transcriptional control. In conclusion, these PPARgamma mutants could be powerful tools for developing novel therapeutic agents that retain the metabolic efficacy of PPARgamma activation with fewer adverse effects, such as the increase in body weight associated with TZDs.

Preferential physical and functional interaction of pregnane X receptor with the SMRTalpha isoform

The silencing mediator for retinoid and thyroid hormone receptors (SMRT) serves as a platform for transcriptional repression elicited by several steroid/nuclear receptors and transcription factors. SMRT exists in two major splicing isoforms, alpha and tau, with SMRTalpha containing only an extra 46-amino acid sequence inserted immediately downstream from the C-terminal corepressor motif. Little is known about potential functional differences between these two isoforms. Here we show that the pregnane X receptor (PXR) interacts more strongly with SMRTalpha than with SMRTtau both in vitro and in vivo. It is interesting that the PXR-SMRTalpha interaction is also resistant to PXR ligand-induced dissociation, in contrast to the PXR-SMRTtau interaction. SMRTalpha consistently inhibits PXR activity more efficiently than does SMRTtau in transfection assays, although they possess comparable intrinsic repression activity and association with histone deacetylase. We further show that the mechanism for the enhanced PXR-SMRTalpha interaction involves both the 46-amino acid insert and the C-terminal corepressor motif. In particular, the first five amino acids of the SMRTalpha insert are essential and sufficient for the enhanced binding of SMRTalpha to PXR. Furthermore, we demonstrate that Tyr2354 and Asp2355 residues of the SMRTalpha insert are most critical for the enhanced interaction. In addition, expression data show that SMRTalpha is more abundantly expressed in most human tissues and cancer cell lines, and together these data suggest that SMRTalpha may play a more important role than SMRTtau in the negative regulation of PXR.

The rat thyroid hormone receptor (TR) Deltabeta3 displays cell-, TR isoform-, and thyroid hormone response element-specific actions

The THRB gene encodes the well-described thyroid hormone (T3) receptor (TR) isoforms TRbeta1 and TRbeta2 and two additional variants, TRbeta3 and TRDeltabeta3, of unknown physiological significance. TRbeta1, TRbeta2, and TRbeta3 are bona fide T3 receptors that bind DNA and T3 and regulate expression of T3-responsive target genes. TRDeltabeta3 retains T3 binding activity but lacks a DNA binding domain and does not activate target gene transcription. TRDeltabeta3 can be translated from a specific TRDeltabeta3 mRNA or is coexpressed with TRbeta3 from a single transcript that contains an internal TRDeltabeta3 translation start site. In these studies, we provide evidence that the TRbeta3/Deltabeta3 locus is present in rat but not in other vertebrates, including humans. We compared the activity of TRbeta3 with other TR isoforms and investigated mechanisms of action of TRDeltabeta3 at specific thyroid hormone response elements (TREs) in two cell types. TRbeta3 was the most potent isoform, but TR potency was TRE dependent. TRDeltabeta3 acted as a cell-specific and TRE-dependent modulator of TRbeta3 when coexpressed at low concentrations. At higher concentrations, TRDeltabeta3 was a TRE-selective and cell-specific antagonist of TRalpha1, -beta1, and -beta3. Both TRbeta3 and TRDeltabeta3 were expressed in the nucleus in the absence and presence of hormone, and their actions were determined by cell type and TRE structure, whereas TRDeltabeta3 actions were also dependent on the TR isoform with which it interacted. Analysis of these complex responses implicates a range of nuclear corepressors and coactivators as cell-, TR isoform-, and TRE-specific modulators of T3 action.

Behavioral inhibition and impaired spatial learning and memory in hypothyroid mice lacking thyroid hormone receptor alpha

Thyroid hormone insufficiency leads to impaired neurogenesis, behavioral alterations and cognitive deficits. Thyroid hormone receptors, expressed in brain regions involved in these behaviors, mediate the effects of thyroid hormone deficiency or excess. To determine the contribution of thyroid hormone receptor alpha (TRalpha) in these behaviors, we examined the behavior of euthyroid as well as hypo- and hyperthyroid mice lacking all isoforms of the TRalpha (TRalpha(o/o)). The hypothyroxinemic TRalpha(o/o) mice demonstrated behavioral inhibition, manifested in decreased activity and increased anxiety/fear in the open field test (OFT) and increased immobility in the forced swim test (FST) compared to C57BL/6J mice. TRalpha(o/o) mice also showed learning and recall impairments in the Morris water maze (MWM), which were exaggerated by hypothyroidism in TRalpha(o/o) mice. These impairments were concurrent with increased thigmotaxis, suggesting an increased anxiety-like state of the TRalpha(o/o) mice in the MWM. Expression of genes, known to be involved in processes modulating learning and memory, such as glucocorticoid receptor (GR), growth-associated protein 43 (GAP-43) and neurogranin (RC3), were significantly decreased in the hippocampus of TRalpha(o/o) mice. GR expression was also decreased in the frontal cortex and amygdala of TRalpha(o/o) mice, indicating that expression of GR is regulated, probably developmentally, by one or more isoforms of TRalpha in the mouse brain. Taken together these data demonstrate behavioral alterations in the TRalpha(o/o) mice, indicating the functional role of TRalpha, and a delicate interaction between TRalpha and TRbeta-regulated genes in these behaviors. Thyroid hormone-regulated genes potentially responsible for the learning deficit found in TRalpha(o/o) mice include GR, RC3 and GAP-43.

Region-specific effects of hypothyroidism on the relative expression of thyroid hormone receptors in adult rat brain

The aim of this study was to determine whether changes in the circulating thyroid hormone (TH) and brain synaptosomal TH content affected the relative levels of mRNA encoding different thyroid hormone receptor (TR) isoforms in adult rat brain. Northern analysis of polyA+RNA from cerebral cortex, hippocampus and cerebellum of control and hypothyroid adult rats was performed in order to determine the relative expression of all TR isoforms. Circulating and synaptosomal TH concentrations were determined by radioimmunoassay. Region-specific quantitative differences in the expression pattern of all TR isoforms in euthyroid animals and hypothyroid animals were recorded. In hypothyroidism, the levels of TRalpha2 mRNA (non-T3-binding isoform) were decreased in all brain regions examined. In contrast the relative expression of TRalpha1 was increased in cerebral cortex and hippocampus, whereas in cerebellum remained unaffected. The TRbeta1 relative expression in cerebral cortex and hippocampus of hypothyroid animals was not affected, whereas this TR isoform was not detectable in cerebellum. The TR isoform mRNA levels returned to control values following T4 intraperitoneal administration to the hypothyroid rats. The obtained results show that in vivo depletion of TH regulates TR gene expression in adult rat brain in a region-specific manner.

Environmental xenobiotics and nuclear receptors--interactions, effects and in vitro assessment

A group of intracellular nuclear receptors is a protein superfamily including arylhydrocarbon AhR, estrogen ER, androgen AR, thyroid TR and retinoid receptors RAR/RXR as well as molecules with unknown function known as orphan receptors. These proteins play an important role in a wide range of physiological as well as toxicological processes acting as transcription factors (ligand-dependent signalling macromolecules modulating expression of various genes in a positive or negative manner). A large number of environmental pollutants and other xenobiotics negatively affect signaling pathways, in which nuclear receptors are involved, and these modulations were related to important in vivo toxic effects such as immunosuppression, carcinogenesis, reproduction or developmental toxicity, and embryotoxicity. Presented review summarizes current knowledge on major nuclear receptors (AhR, ER, AR, RAR/RXR, TR) and their relationship to known in vivo toxic effects. Special attention is focused on priority organic environmental contaminants and experimental approaches for determination and studies of specific toxicity mechanisms.

Athyroid Pax8-/- mice cannot be rescued by the inactivation of thyroid hormone receptor alpha1

The Pax8(-/-) mouse provides an ideal animal model to study the consequences of congenital hypothyroidism, because its only known defect is the absence of thyroid follicular cells. Pax8(-/-) mice are, therefore, completely athyroid in postnatal life and die around weaning unless they are substituted with thyroid hormones. As reported recently, Pax8(-/-) mice can also be rescued and survive to adulthood by the additional elimination of the entire thyroid hormone receptor alpha (TRalpha) gene, yielding Pax8(-/-)TRalpha(o/o) double-knockout animals. This observation has led to the hypothesis that unliganded TRalpha1 might be responsible for the lethal phenotype observed in Pax8(-/-) animals. In this study we report the generation of Pax8(-/-)TRalpha1(-/-) double-knockout mice that still express the non-T(3)-binding TR isoforms alpha2 and Deltaalpha2. These animals closely resemble the phenotype of Pax8(-/-) mice, including growth retardation and a completely distorted appearance of the pituitary with thyrotroph hyperplasia and hypertrophy, extremely high TSH mRNA levels, reduced GH mRNA expression, and the almost complete absence of lactotrophs. Like Pax8(-/-) mice, Pax8(-/-)TRalpha1(-/-) compound mutants die around weaning unless they are substituted with thyroid hormones. These findings do not support the previous interpretation that the short life span of Pax8(-/-) mice is due to the negative effects of the TRalpha1 aporeceptor, but, rather, suggest a more complex mechanism involving TRalpha2 and an unliganded TR isoform.

Diazepam affects the nuclear thyroid hormone receptor density and their expression levels in adult rat brain

Thyroid hormones (THs) are involved in the occurrence of anxiety and affective disorders; however, the effects following an anxiolytic benzodiazepine treatment, such as diazepam administration, on the mechanism of action of thyroid hormones has not yet been investigated. The effect of diazepam on the in vitro nuclear T3 binding, on the relative expression of the TH receptors (TRs) and on the synaptosomal TH availability were examined in adult rat cerebral hemispheres 24 h after a single intraperitoneal dose (5 mg/kg BW) of this tranquillizer. Although, diazepam did not affect the availability of TH either in blood circulation or in the synaptosomal fraction, it decreased (33%) the nuclear T3 maximal binding density (B(max)). No differences were observed in the equilibrium dissociation constant (K(d)). The TRalpha2 variant (non-T3-binding) mRNA levels were increased by 33%, whereas no changes in the relative expression of the T3-binding isoforms of TRs (TRalpha1, TRbeta1) were observed. This study shows that a single intraperitoneal injection of diazepam affects within 24 h, the density of the nuclear TRs and their expression pattern. The latest effect occurs in an isoform-specific manner involving specifically the TRalpha2 mRNA levels in adult rat brain.

Tissue-specific regulation of cytochrome c oxidase subunit expression by thyroid hormone

The influence of thyroid hormone (T(3)) on respiration is partly mediated via its effect on the cytochrome c oxidase (COX) enzyme, a multi-subunit complex within the mitochondrial respiratory chain. We compared the expression of COX subunits I, III, Vb, and VIc and thyroid receptors (TR)alpha1 and TRbeta1 with functional changes in COX activity in tissues that possess high oxidative capacities. In response to 5 days of T(3) treatment, TRbeta1 increased 1.6-fold in liver, whereas TRalpha1 remained unchanged. T(3) also induced concomitant increases in the protein and mRNA expression of nuclear-encoded subunit COX Vb in liver, matched by a 1.3-fold increase in binding to a putative thyroid response element (TRE) within the COX Vb promoter in liver, suggesting transcriptional regulation. In contrast, T(3) had no effect on COX Vb expression in heart. T(3) produced a significant increase in COX III mRNA in liver but decreased COX III mRNA in heart. These changes were matched by parallel alterations in mitochondrial transcription factor A expression in both tissues. In contrast, COX I protein increased in both liver and heart 1.7- and 1.5-fold (P < 0.05), respectively. These changes in COX I closely paralleled the T(3)-induced increases in COX activity observed in both of these tissues. In liver, T(3) induced a coordinated increase in the expression of the nuclear (COX Vb) and mitochondrial (COX I) genomes at the protein level. However, in heart, the main effect of T(3) was restricted to the expression of mitochondrial DNA subunits. Thus our data suggest that T(3) regulates the expression of COX subunits by both transcriptional and posttranscriptional mechanisms. The nature of this regulation differs between tissues possessing a high mitochondrial content, like liver and heart.

Diurnal variation in rat liver thyroid hormone receptor (TR)-alpha messenger ribonucleic acid (mRNA) is dependent on the biological clock in the suprachiasmatic nucleus, whereas diurnal variation of TR beta 1 mRNA is modified by food intake

Previous studies have shown a diurnal variation of certain isoforms of thyroid hormone receptors (TR) in rat liver. The genesis of these diurnal changes is still unknown. To clarify whether the biological clock, located in the hypothalamic suprachiasmatic nucleus (SCN), is involved, we made selective SCN lesions. Rats with an SCN lesion lost their circadian rhythm of plasma corticosterone and TSH when compared with intact animals. TR alpha 1 and TR alpha 2 mRNA expression of control rats was higher in the light period than in the dark period; changes that were abolished in the rats with SCN lesions. In contrast, liver TR beta 1 mRNA of intact rats showed a diurnal variation that failed to reach statistical significance. To evaluate whether these effects could be explained indirectly by the disappearance of rhythmic feeding behavior in rats with SCN lesions, we performed a second experiment in which otherwise intact animals were subjected to a regular feeding (RF) schedule, with one meal every 4 h. When compared with rats with free access to food, RF only affected TR beta 1 mRNA expression and had no effect on the diurnal changes in TR alpha 1 and TR alpha 2. We conclude that liver TR beta 1 expression is most clearly affected by food intake. Diurnal changes in liver TR alpha 1 and TR alpha 2 are controlled by the biological clock in the SCN but not via changes in the daily rhythm of food intake. The findings may have physiological relevance for diurnal variation of T(3)-dependent gene expression, which is supported by a diurnal variation in the expression of the 5'-deiodinase gene.

Estrogen and thyroid hormone receptor interactions: physiological flexibility by molecular specificity

The influence of thyroid hormone on estrogen actions has been demonstrated both in vivo and in vitro. In transient transfection assays, the effects of liganded thyroid hormone receptors (TR) on transcriptional facilitation by estrogens bound to estrogen receptors (ER) display specificity according to the following: 1) ER isoform, 2) TR isoform, 3) the promoter through which transcriptional facilitation occurs, and 4) cell type. Some of these molecular phenomena may be related to thyroid hormone signaling of seasonal limitations upon reproduction. The various combinations of these molecular interactions provide multiple and flexible opportunities for relations between two major hormonal systems important for neuroendocrine feedbacks and reproductive behaviors.

Nutrition-hormone receptor-gene interactions: implications for development and disease

Nutrition profoundly alters the phenotypic expression of a given genotype, particularly during fetal and postnatal development. Many hormones act as nutritional signals and their receptors play a key role in mediating the effects of nutrition on numerous genes involved in differentiation, growth and metabolism. Polypeptide hormones act on membrane-bound receptors to trigger gene transcription via complex intracellular signalling pathways. By contrast, nuclear receptors for lipid-soluble molecules such as glucocorticoids (GC) and thyroid hormones (TH) directly regulate transcription via DNA binding and chromatin remodelling. Nuclear hormone receptors are members of a large superfamily of transcriptional regulators with the ability to activate or repress many genes involved in development and disease. Nutrition influences not only hormone synthesis and metabolism but also hormone receptors, and regulation is mediated either by specific nutrients or by energy status. Recent studies on the role of early environment on development have implicated GC and their receptors in the programming of adult disease. Intrauterine growth restriction and postnatal undernutrition also induce striking differences in TH-receptor isoforms in functionally-distinct muscles, with critical implications for gene transcription of myosin isoforms. glucose transporters, uncoupling proteins and cation pumps. Such findings highlight a mechanism by which nutritional status can influence normal development, and modify nutrient utilization. thermogenesis. peripheral sensitivity to insulin and optimal cardiac function. Diet and stage of development will also influence the transcriptional activity of drugs acting as ligands for nuclear receptors. Potential interactions between nuclear receptors, including those for retinoic acid and vitamin D, should not be overlooked in intervention programmes using I or vitamin A supplementation of young and adult human populations

Impaired interaction of mutant thyroid hormone receptors associated with human hepatocellular carcinoma with transcriptional coregulators

Thyroid hormone (T(3)) exerts its many biological activities through interaction with specific nuclear receptors (TRs) that function as ligand-dependent transcription factors at genes that contain a thyroid hormone response element (TRE). Mutant TRs have been detected in human hepatocellular carcinoma cell lines and tissue, but their contribution to carcinogenesis has remained unclear. The interaction of four such mutant TRs (J7-TRalpha1, J7-TRbeta1, H-TRalpha1, and L-TRalpha1) with transcriptional coregulators has now been investigated. With the exception of J7-TRalpha1, which in the absence of T(3) exhibited transcriptional silencing activity with a TRE-reporter gene construct in transfected cells, the mutant TRs had little effect (compared with that of wild-type receptors) on transcriptional activity of the reporter gene in the absence or presence of T(3), of the transcriptional corepressors SMRT, NCoR or of the transcriptional coactivator SRC. Electrophoretic mobility-shift assays revealed that, in the presence of T(3), the J7-TRss1 mutant did not interact with SRC, whereas J7-TRalpha1 and H-TRalpha1 exhibited reduced abilities to associate with this coactivator and L-TRalpha1 showed an ability to interact with SRC similar to that of wild-type TRalpha1. The dominant negative activity of the mutant TRs in transfected cells appeared inversely related to the ability of the receptors to interact with SRC. Whereas J7-TRss1, H-TRalpha1, and L-TRalpha1 did not interact with SMRT, and NCoR. J7-TRalpha1 bind to corepressors but failed to dissociate from them in the presence of T(3). These aberrant interactions between the mutant TRs and transcriptional coregulators may contribute to the highly variable clinical characteristics of human hepatocellular carcinoma.

Increased sensitivity to thyroid hormone in mice with complete deficiency of thyroid hormone receptor

Only three of the four thyroid hormone receptor (TR) isoforms, alpha1, beta1, and beta2, bind thyroid hormone (TH) and are considered to be true TRs. TRalpha2 binds to TH response elements on DNA, but its role in vivo is still unknown. We produced mice completely deficient in TRalpha (TRalpha(o/o)) that maintain normal serum thyroid-stimulating hormone (TSH) concentration despite low serum thyroxine (T(4)), suggesting increased sensitivity to TH. We therefore examined the effects of TH (L-3,3',5-triiodothyronine, L-T3) given to TH-deprived and to intact TRalpha(o/o) mice. Controls were wild-type (WT) mice of the same strain and mice resistant to TH due to deficiency in TRbeta (TRbeta(-/-)). In liver, T3 produced significantly greater responses in TRalpha(o/o) and smaller responses in TRbeta(-/-) as compared with WT mice. In contrast, cardiac responses to L-T3 were absent or reduced in TRalpha(o/o), whereas they were similar in WT and TRbeta(-/-) mice, supporting the notion that TRalpha1 is the dominant TH-dependent TR isoform in heart. 5-Triiodothyronine (L-T3) given to intact mice produced a greater suppression of serum T(4) in TRalpha(o/o) than it did in WT mice and reduced by a greater amount the TSH response to TSH-releasing hormone. This is an in vivo demonstration that a TR deficiency can enhance sensitivity to TH. This effect is likely due to the abrogation of the constitutive "silencing" effect of TRalpha2 in tissues expressing the TRbeta isoforms.

Increased sensitivity to thyroid hormone in mice with complete deficiency of thyroid hormone receptor alpha

Only three of the four thyroid hormone receptor (TR) isoforms, alpha1, beta1, and beta2, bind thyroid hormone (TH) and are considered to be true TRs. TRalpha2 binds to TH response elements on DNA, but its role in vivo is still unknown. We produced mice completely deficient in TRalpha (TRalpha(o/o)) that maintain normal serum thyroid-stimulating hormone (TSH) concentration despite low serum thyroxine (T(4)), suggesting increased sensitivity to TH. We therefore examined the effects of TH (L-3,3',5-triiodothyronine, L-T3) given to TH-deprived and to intact TRalpha(o/o) mice. Controls were wild-type (WT) mice of the same strain and mice resistant to TH due to deficiency in TRbeta (TRbeta(-/-)). In liver, T3 produced significantly greater responses in TRalpha(o/o) and smaller responses in TRbeta(-/-) as compared with WT mice. In contrast, cardiac responses to L-T3 were absent or reduced in TRalpha(o/o), whereas they were similar in WT and TRbeta(-/-) mice, supporting the notion that TRalpha1 is the dominant TH-dependent TR isoform in heart. 5-Triiodothyronine (L-T3) given to intact mice produced a greater suppression of serum T(4) in TRalpha(o/o) than it did in WT mice and reduced by a greater amount the TSH response to TSH-releasing hormone. This is an in vivo demonstration that a TR deficiency can enhance sensitivity to TH. This effect is likely due to the abrogation of the constitutive "silencing" effect of TRalpha2 in tissues expressing the TRbeta isoforms.

Cloning and characterization of two novel thyroid hormone receptor beta isoforms

Thyroid hormone (T(3)) activates nuclear receptor transcription factors, encoded by the TRalpha (NR1A1) and TRbeta (NR1A2) genes, to regulate target gene expression. Several TR isoforms exist, and studies of null mice have identified some unique functions for individual TR variants, although considerable redundancy occurs, raising questions about the specificity of T(3) action. Thus, it is not known how diverse T(3) actions are regulated in target tissues that express multiple receptor variants. I have identified two novel TRbeta isoforms that are expressed widely and result from alternative mRNA splicing. TRbeta3 is a 44.6-kDa protein that contains an unique 23-amino-acid N terminus and acts as a functional receptor. TRDeltabeta3 is a 32.8-kDa protein that lacks a DNA binding domain but retains ligand binding activity and is a potent dominant-negative antagonist. The relative concentrations of beta3 and Deltabeta3 mRNAs vary between tissues and with changes in thyroid status, indicating that alternative splicing is tissue specific and T(3) regulated. These data provide novel insights into the mechanisms of T(3) action and define a new level of specificity that may regulate thyroid status in tissue.

Post-transcriptional regulation of thyroid hormone receptor expression by cis-acting sequences and a naturally occurring antisense RNA

Thyroid hormone (T(3)) coordinates growth, differentiation, and metabolism by binding to nuclear thyroid hormone receptors (TRs). The TRalpha gene encodes T(3)-activated TRalpha1 (NR1A1a) as well as an antagonistic, non-T(3)-binding alternatively spliced product, TRalpha2 (NR1A1b). Thus, the TRalpha1/TRalpha2 ratio is a critical determinant of T(3) action. However, the mechanisms underlying this post-transcriptional regulation are unknown. We have identified a non-consensus, TRalpha2-specific 5' splice site and conserved intronic sequences as key determinants of TRalpha mRNA processing. In addition to these cis-acting elements, a novel regulatory feature is the orphan receptor RevErbAalpha (NR1D1) gene, which is transcribed from the opposite direction at the same locus and overlaps the TRalpha2 coding region. RevErbAalpha gene expression correlates with a high TRalpha1/TRalpha2 ratio in a number of tissues. Here we demonstrate that coexpression of RevErbAalpha and TRalpha regulates the TRalpha1/TRalpha2 ratio in intact cells. Thus, both cis- and trans-regulatory mechanisms contribute to cell-specific post-transcriptional regulation of TR gene expression and T(3) action.

Functions of thyroid hormone receptors in mice

Thyroid hormone receptors (TRs) play a central role in mediating the actions of thyroid hormone in development and homeostasis in vertebrate species. The TRs are nuclear receptors that act as ligand-regulated transcription factors. There are two TR genes (TRalpha and TRbeta), each capable of generating different variant products, suggesting a potentially complex array of TR pathways. Targeted mutagenesis in the mouse has indicated that there are specific individual functions for the TR genes in vivo. The deletion of combinations of TRalpha and TRbeta variants has revealed that additional functions are convergently regulated by both TR genes and indicates that control of an extended range of functions is facilitated by a network of specific and common TR pathways. The TR-deficient mouse models have allowed investigation of the TR pathways underlying many functions of thyroid hormone and provide a unique perspective on receptor-mediated mechanisms of biological control.

A fusion protein of the estrogen receptor (ER) and nuclear receptor corepressor (NCoR) strongly inhibits estrogen-dependent responses in breast cancer cells

Nuclear receptor corepressor (NCoR) mediates repression (silencing) of basal gene transcription by nuclear receptors for thyroid hormone and retinoic acid. The goal of this study was to create novel estrogen receptor (ER) mutants by fusing transferable repressor domains from the N-terminal region of NCoR to a functional ER fragment. Three chimeric NCoR-ER proteins were created and shown to lack transcriptional activity. These fusion proteins silenced basal transcription of the ERE2-tk-Luc reporter gene and inhibited the activity of co-transfected wild-type ER (wtER), indicating that they possess dominant negative activity. One of the fusion proteins (CDE-RD1), containing the ER DNA-binding and ligand-binding domains linked to the NCoR repressor domain (RD1), was selected for detailed examination. Its hormone affinity, intracellular localization, and level of expression in transfected cells were similar to wtER, and it bound to the estrogen response element (ERE) DNA in gel shift assays. Glutathione-S-transferase pull-down assays showed that CDE-RD1 retains the ability to bind to steroid receptor coactivator-1. Introduction of a DNA-binding domain mutation into the CDE-RD1 fusion protein eliminated silencing and dominant negative activity. Thus, the RD1 repressor domain prevents transcriptional activation despite the apparent ability of CDE-RD1 to bind DNA, ligand, and coactivators. Transcriptional silencing was incompletely reversed by trichostatin A, suggesting a histone deacetylase-independent mechanism for repression. CDE-RD1 inhibited ER-mediated transcription in T47D and MDA-MB-231 breast cancer cells and repressed the growth of T47D cells when delivered to the cells by a retroviral vector. These ER-NCoR fusion proteins provide a novel means for inhibiting ER-mediated cellular responses, and analogous strategies could be used to create dominant negative mutants of other transcription factors.

The human RC3 gene homolog, NRGN contains a thyroid hormone-responsive element located in the first intron

NRGN is the human homolog of the neuron-specific rat RC3/neurogranin gene. This gene encodes a postsynaptic 78-amino acid protein kinase substrate that binds calmodulin in the absence of calcium, and that has been implicated in dendritic spine formation and synaptic plasticity. In the rat brain RC3 is under thyroid hormone control in specific neuronal subsets in both developing and adult animals. To evaluate whether the human gene is also a target of thyroid hormone we have searched for T3-responsive elements in NRGN cloned genomic fragments spanning the whole gene. Labeled DNA fragments were incubated with T3 receptors (T3R) and 9-cis-retinoic acid receptors and immunoprecipitated using an anti T3R antibody. A receptor-binding site was localized in the first intron, 3000 bp downstream from the origin of transcription. Footprinting analysis revealed the sequence GGATTAAATGAGGTAA, closely related to the consensus T3-responsive element of the direct repeat (DR4) type. This sequence binds the T3R-9-cis-retinoic acid receptors heterodimers, but not T3R monomers or homodimers, and is able to confer regulation by T3R and T3 when fused upstream of the NRGN or thymidine kinase promoters. The data reported in this work suggest that NRGN is a direct target of thyroid hormone in human brain, and that control of expression of this gene could underlay many of the consequences ofhypothyroidism on mental states during development as well as in adult subjects.