Thyroid hormone response element sequence and the recruitment of retinoid X receptors for thyroid hormone responsiveness

Thyroid-stimulating hormone-thyroid hormone signaling contributes to circadian regulation through repressing clock2/npas2 in zebrafish

Thyroid-stimulating hormone (TSH) is important for the thyroid gland, development, growth, and metabolism. Defects in TSH production or the thyrotrope cells within the pituitary gland cause congenital hypothyroidism (CH), resulting in growth retardation and neurocognitive impairment. While human TSH is known to display rhythmicity, the molecular mechanisms underlying the circadian regulation of TSH and the effects of TSH-thyroid hormone (TH) signaling on the circadian clock remain elusive. Here we show that TSH, thyroxine (T4), triiodothyronine (T3), and tshba display rhythmicity in both larval and adult zebrafish and tshba is regulated directly by the circadian clock via both E'-box and D-box. Zebrafish tshba-/- mutants manifest congenital hypothyroidism, with the characteristics of low levels of T4 and T3 and growth retardation. Loss or overexpression of tshba alters the rhythmicity of locomotor activities and expression of core circadian clock genes and hypothalamic-pituitary-thyroid (HPT) axis-related genes. Furthermore, TSH-TH signaling regulates clock2/npas2 via the thyroid response element (TRE) in its promoter, and transcriptome analysis reveals extensive functions of Tshba in zebrafish. Together, our results demonstrate that zebrafish tshba is a direct target of the circadian clock and in turn plays critical roles in circadian regulation along with other functions.

TREM2 is thyroid hormone regulated making the TREM2 pathway druggable with ligands for thyroid hormone receptor

Triggering receptor expressed on myeloid cells-2 (TREM2) is a cell surface receptor on macrophages and microglia that senses and responds to disease-associated signals to regulate the phenotype of these innate immune cells. The TREM2 signaling pathway has been implicated in a variety of diseases ranging from neurodegeneration in the central nervous system to metabolic disease in the periphery. Here, we report that TREM2 is a thyroid hormone-regulated gene and its expression in macrophages and microglia is stimulated by thyroid hormone and synthetic thyroid hormone agonists (thyromimetics). Our findings report the endocrine regulation of TREM2 by thyroid hormone, and provide a unique opportunity to drug the TREM2 signaling pathway with orally active small-molecule therapeutic agents.

The protective variant rs7173049 at LOXL1 locus impacts on retinoic acid signaling pathway in pseudoexfoliation syndrome

LOXL1 (lysyl oxidase-like 1) has been identified as the major effect locus in pseudoexfoliation (PEX) syndrome, a fibrotic disorder of the extracellular matrix and frequent cause of chronic open-angle glaucoma. However, all known PEX-associated common variants show allele effect reversal in populations of different ancestry, casting doubt on their biological significance. Based on extensive LOXL1 deep sequencing, we report here the identification of a common non-coding sequence variant, rs7173049A>G, located downstream of LOXL1, consistently associated with a decrease in PEX risk (odds ratio, OR = 0.63; P = 6.33 × 10⁻³¹) in nine different ethnic populations. We provide experimental evidence for a functional enhancer-like regulatory activity of the genomic region surrounding rs7173049 influencing expression levels of ISLR2 (immunoglobulin superfamily containing leucine-rich repeat protein 2) and STRA6 [stimulated by retinoic acid (RA) receptor 6], apparently mediated by allele-specific binding of the transcription factor thyroid hormone receptor beta. We further show that the protective rs7173049-G allele correlates with increased tissue expression levels of ISLR2 and STRA6 and that both genes are significantly downregulated in tissues of PEX patients together with other key components of the STRA6 receptor-driven RA signaling pathway. siRNA-mediated downregulation of RA signaling induces upregulation of LOXL1 and PEX-associated matrix genes in PEX-relevant cell types. These data indicate that dysregulation of STRA6 and impaired retinoid metabolism are involved in the pathophysiology of PEX syndrome and that the variant rs7173049-G, which represents the first common variant at the broad LOXL1 locus without allele effect reversal, mediates a protective effect through upregulation of STRA6 in ocular tissues.

Acute microcystin-LR exposure interfere thyroid hormones homeostasis in adult zebrafish (Danio rerio)

Microcystin-LR (MC-LR) in the aquatic environment may disturb thyroid hormone (TH) homeostasis. It is not clear how MC-LR affects downstream biological processes after TH disturbance. After exposure to 50, 100, 200 and 400 μg/L MC-LR for 24, 48, 72, or 96 h, alterations of the TH metabolism of adult zebrafish at thyroxine (T4), triiodothyronine (T3) levels, and iodothyronine deiodinase (Dio) activity, were observed. After exposure to MC-LR at 400 μg/L, T3 and T4 levels decreased significantly in females (p < 0.05) and returned to normal levels at 96 h. In males, T4 levels were not significantly different between groups. The expression of corticotropin releasing hormone, thyroid-stimulating hormone beta subunit, transthyretin, sodium/iodide cotransporter, thrombopoietin, thyroid hormone receptor alpha and beta changed, but not in a dose-dependent manner. Acute MC-LR exposure induced a negative feedback regulation of the hypothalamic-pituitary-thyroid axis in adult zebrafish, and females were more sensitive than males. In conclusion, acute MC-LR exposure disrupted the TH metabolism by altering Dio activity and gene expression of the HPT axis; these changes may affect the complement system through regulation of c9 mRNA synthesis.

Adaptive correction of craniofacial defects in pre-metamorphic Xenopus laevis tadpoles involves thyroid hormone-independent tissue remodeling

While it is well-established that some organisms can regenerate lost structures, the ability to remodel existing malformed structures has been less well studied. Thus, in this study we examined the ability of pre-metamorphic Xenopus laevis tadpoles to self-correct malformed craniofacial tissues and found that tadpoles can adaptively improve and normalize abnormal craniofacial morphology caused by numerous developmental perturbations. We then investigated the tissue-level and molecular mechanisms that mediate the self-correction of craniofacial defects in pre-metamorphic X. laevis tadpoles. Our studies revealed that this adaptive response involves morphological changes and the remodeling of cartilage tissue, prior to metamorphosis. RT-qPCR and RNA-Seq analysis of gene expression suggests a thyroid hormone-independent endocrine signaling pathway as the potential mechanism responsible for triggering the adaptive and corrective remodeling response in these larvae that involves mmp1 and mmp13 upregulation. Thus, investigating how malformed craniofacial tissues are naturally corrected in X. laevis tadpoles has led us to valuable insights regarding the maintenance and manipulation of craniofacial morphology in a vertebrate system. These insights may help in the development of novel therapies for developmental craniofacial anomalies in humans.

Transcriptome Analyses of Female Somatotropes and Lactotropes Reveal Novel Regulators of Cell Identity in the Pituitary

The differentiation of the hormone-producing cell lineages of the anterior pituitary represents an informative model of mammalian cell fate determination. The generation and maintenance of two of these lineages, the GH-producing somatotropes and prolactin (PRL)-producing lactotropes, are dependent on the pituitary-specific transcription factor POU1F1. Whereas POU1F1 is expressed in both cell types, and plays a direct role in the activation of both the Gh and Prl genes, GH expression is restricted to somatotropes and PRL expression is restricted to lactotropes. These observations imply the existence of additional, cell type-enriched factors that contribute to the somatotrope and lactotrope cell identities. In this study, we use transgenic mouse models to facilitate sorting of somatotrope and lactotrope populations based on the expression of fluorescent markers expressed under Gh and Prl gene transcriptional controls. The transcriptomic analyses reveal a concordance of gene expression profiles in the two populations. The limited number of divergent mRNAs between the two populations includes a set of transcription factors that may have roles in pituitary lineage divergence and/or in regulating expression of cell type-specific genes after differentiation. Four of these factors were validated for lineage enrichment at the level of protein expression, two somatotrope enriched and two lactotrope enriched. Three of these four factors were shown to have corresponding activities in appropriate enhancement or repression of landmark genes in a cell culture model system. These studies identify novel regulators of the somatotropes and lactotropes, and they establish a useful database for further study of these lineages in the anterior pituitary.

Wnt Signaling in Thyroid Homeostasis and Carcinogenesis

The Wnt pathway is essential for stem cell maintenance, but little is known about its role in thyroid hormone signaling and thyroid stem cell survival and maintenance. In addition, the role of Wnt signaling in thyroid cancer progenitor cells is also unclear. Here, we present emerging evidence for the role of Wnt signaling in somatic thyroid stem cell and thyroid cancer stem cell function. An improved understanding of the role of Wnt signaling in thyroid physiology and carcinogenesis is essential for improving both thyroid disease diagnostics and therapeutics.

Functional analysis of thyroid hormone receptor beta in Xenopus tropicalis founders using CRISPR-Cas

Amphibians provide an ideal model to study the actions of thyroid hormone (TH) in animal development because TH signaling via two TH receptors, TRα and TRβ, is indispensable for amphibian metamorphosis. However, specific roles for the TRβ isoform in metamorphosis are poorly understood. To address this issue, we generated trβ-disrupted Xenopus tropicalis tadpoles using the CRISPR-Cas system. We first established a highly efficient and rapid workflow for gene disruption in the founder generation (F0) by injecting sgRNA and Cas9 ribonucleoprotein. Most embryos showed severe mutant phenotypes carrying high somatic mutation rates. Utilizing this founder analysis system, we examined the role of trβ in metamorphosis. trβ-disrupted pre-metamorphic tadpoles exhibited mixed responsiveness to exogenous TH. Specifically, gill resorption and activation of several TH-response genes, including trβ itself and two protease genes, were impaired. However, hind limb outgrowth and induction of the TH-response genes, klf9 and fra-2, were not affected by loss of trβ Surprisingly, trβ-disrupted tadpoles were able to undergo spontaneous metamorphosis normally, except for a slight delay in tail resorption. These results indicate TRβ is not required but contributes to the timing of resorptive events of metamorphosis.

Thyroid hormone promotes differentiation of colon cancer stem cells

Tumor formation and maintenance depend on a small fraction of cancer stem cells (CSCs) that can self-renew and generate a wide variety of differentiated cells. CSCs are resistant to chemotherapy and radiation, and can represent a reservoir of cancer cells that often cause relapse after treatment. Evidence suggests that CSCs also give rise to metastases. Thyroid hormone (TH) controls a variety of biological processes including the development and functioning of most adult tissues. Recent years has seen the emergence of an intimate link between TH and multiple steps of tumorigenesis. Thyroid hormone controls the balance between the proliferation and differentiation of CSCs, and may thus be a druggable anti-cancer agent. Here, we review current understanding of the effects of TH on colorectal CSCs, including the cross regulatory loops between TH and regulators of CSC stemness. Targeting TH in the tumor microenvironment may improve treatment strategies.

Refuse leachate exposure causes changes of thyroid hormone level and related gene expression in female goldfish (Carassius auratus)

To elucidate the potential thyroid disrupting effects of refuse leachate on females, female goldfish (Carassius auratus) were exposed to 0.5% diluted leachates from each step of a leachate treatment process (i.e. raw leachate before treatment, after membrane bioreactor treatment, and the final treated leachate) for 21days. Raw leachate exposure caused disturbances in the thyroid cascade of female fish, as evidenced by the elevated plasma 3,3',5-triiodo-l-thyronine (p<0.05) and thyroid-stimulating hormone (p<0.01) levels as well as up-regulated hepatic and gonadal type I deiodinase (p<0.01), type II deiodinase (p<0.01) and thyroid receptor (p<0.05) mRNA levels. Thyroid disrupting potency decreased markedly as raw leachate progressed through the "membrane bioreactor + reverse osmosis" treatment but could still be detected in the treated leachate. As our results indicated, thyroid system in female goldfish was more sensitive to leachate exposure than that of the male fish.

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.

Thyroid disruption in male goldfish (Carassius auratus) exposed to leachate from a municipal waste treatment plant: Assessment combining chemical analysis and in vivo bioassay

Several classes of thyroid-disrupting chemicals (TDCs) have been found in refuse leachate, but the potential impacts of leachate on the thyroid cascade of aquatic organisms are yet not known. In this study, we chemically analyzed frequently reported TDCs, as well as conducted a bioassay, to evaluate the potential thyroid-disrupting effects of leachate. We used radioimmunoassay to determine the effects of leachate exposure on plasma 3,3',5-triiodo-l-thyronine (T3), 3,3',5,5'-l-thyroxine (T4), and thyroid-stimulating hormone (TSH) levels in adult male goldfish (Carassius auratus). We also investigated the impacts of leachate treatment on hepatic and gonadal deiodinases [types I (D1), II (D2), and III (D3)] and gonadal thyroid receptor (TRα-1 and TRβ) mRNA expressions by using real-time polymerase chain reaction. The results indicated the presence of five TDCs (bisphenol A, 4-t-octylphenol, di-n-butyl phthalate, di-n-octyl phthalate, and diethylhexyl phthalate); their mean concentrations in the leachate were 18.11, 2.76, 4.86, 0.21, and 9.16 μg/L, respectively. Leachate exposure induced plasma T3 and TSH levels in male fish, without influencing the plasma T4 levels. The highly elevated D2 mRNA levels in the liver were speculated to be the primary reason for the induction of plasma T3 levels. Disruption of thyroid functions by leachate was also suggested by the up-regulation of D1 and D2 as well as TRα-1 mRNA levels in the gonads. Prominent thyroid disruptions despite the very low TDC concentrations in the exposure media used in the bioassay strongly indicated the existence of unidentified TDCs in the leachate. Our study indicated the necessity of conducting in vivo bioassays to detect thyroid dysfunctions caused by leachate.

High Affinity Heme Binding to a Heme Regulatory Motif on the Nuclear Receptor Rev-erbβ Leads to Its Degradation and Indirectly Regulates Its Interaction with Nuclear Receptor Corepressor

Rev-erbα and Rev-erbβ are heme-binding nuclear receptors (NR) that repress the transcription of genes involved in regulating metabolism, inflammation, and the circadian clock. Previous gene expression and co-immunoprecipitation studies led to a model in which heme binding to Rev-erbα recruits nuclear receptor corepressor 1 (NCoR1) into an active repressor complex. However, in contradiction, biochemical and crystallographic studies have shown that heme decreases the affinity of the ligand-binding domain of Rev-erb NRs for NCoR1 peptides. One explanation for this discrepancy is that the ligand-binding domain and NCoR1 peptides used for in vitro studies cannot replicate the key features of the full-length proteins used in cellular studies. However, the combined in vitro and cellular results described here demonstrate that heme does not directly promote interactions between full-length Rev-erbβ (FLRev-erbβ) and an NCoR1 construct encompassing all three NR interaction domains. NCoR1 tightly binds both apo- and heme-replete FLRev-erbβ·DNA complexes; furthermore, heme, at high concentrations, destabilizes the FLRev-erbβ·NCoR1 complex. The interaction between FLRev-erbβ and NCoR1 as well as Rev-erbβ repression at the Bmal1 promoter appear to be modulated by another cellular factor(s), at least one of which is related to the ubiquitin-proteasome pathway. Our studies suggest that heme is involved in regulating the degradation of Rev-erbβ in a manner consistent with its role in circadian rhythm maintenance. Finally, the very slow rate constant (10(-6) s(-1)) of heme dissociation from Rev-erbβ rules out a prior proposal that Rev-erbβ acts as an intracellular heme sensor.

Thyroid hormone signaling: Contribution to neural function, cognition, and relationship to nicotine

Cigarette smoking is common despite its adverse effects on health, such as cardiovascular disease and stroke. Understanding the mechanisms that contribute to the addictive properties of nicotine makes it possible to target them to prevent the initiation of smoking behavior and/or increase the chance of successful quit attempts. While highly addictive, nicotine is not generally considered to be as reinforcing as other drugs of abuse. There are likely other mechanisms at work that contribute to the addictive liability of nicotine. Nicotine modulates aspects of the endocrine system, including the thyroid, which is critical for normal cognitive functioning. It is possible that nicotine's effects on thyroid function may alter learning and memory, and this may underlie some of its addictive potential. Here, we review the literature on thyroid function and cognition, with a focus on how nicotine alters thyroid hormone signaling and the potential impact on cognition. Changes in cognition are a major symptom of nicotine addiction. Current anti-smoking therapies have modest success at best. If some of the cognitive effects of nicotine are mediated through the thyroid hormone system, then thyroid hormone agonists may be novel treatments for smoking cessation therapies. The content of this review is important because it clarifies the relationship between smoking and thyroid function, which has been ill-defined in the past. This review is timely because the reduction in smoking rates we have seen in recent decades, due to public awareness campaigns and public smoking bans, has leveled off in recent years. Therefore, novel treatment approaches are needed to help reduce smoking rates further.

Thyroxine differentially modulates the peripheral clock: lessons from the human hair follicle

The human hair follicle (HF) exhibits peripheral clock activity, with knock-down of clock genes (BMAL1 and PER1) prolonging active hair growth (anagen) and increasing pigmentation. Similarly, thyroid hormones prolong anagen and stimulate pigmentation in cultured human HFs. In addition they are recognized as key regulators of the central clock that controls circadian rhythmicity. Therefore, we asked whether thyroxine (T4) also influences peripheral clock activity in the human HF. Over 24 hours we found a significant reduction in protein levels of BMAL1 and PER1, with their transcript levels also decreasing significantly. Furthermore, while all clock genes maintained their rhythmicity in both the control and T4 treated HFs, there was a significant reduction in the amplitude of BMAL1 and PER1 in T4 (100 nM) treated HFs. Accompanying this, cell-cycle progression marker Cyclin D1 was also assessed appearing to show an induced circadian rhythmicity by T4 however, this was not significant. Contrary to short term cultures, after 6 days, transcript and/or protein levels of all core clock genes (BMAL1, PER1, clock, CRY1, CRY2) were up-regulated in T4 treated HFs. BMAL1 and PER1 mRNA was also up-regulated in the HF bulge, the location of HF epithelial stem cells. Together this provides the first direct evidence that T4 modulates the expression of the peripheral molecular clock. Thus, patients with thyroid dysfunction may also show a disordered peripheral clock, which raises the possibility that short term, pulsatile treatment with T4 might permit one to modulate circadian activity in peripheral tissues as a target to treat clock-related disease.

Genome-wide binding patterns of thyroid hormone receptor beta

Thyroid hormone (TH) receptors (TRs) play central roles in metabolism and are major targets for pharmaceutical intervention. Presently, however, there is limited information about genome wide localizations of TR binding sites. Thus, complexities of TR genomic distribution and links between TRβ binding events and gene regulation are not fully appreciated. Here, we employ a BioChIP approach to capture TR genome-wide binding events in a liver cell line (HepG2). Like other NRs, TRβ appears widely distributed throughout the genome. Nevertheless, there is striking enrichment of TRβ binding sites immediately 5′ and 3′ of transcribed genes and TRβ can be detected near 50% of T3 induced genes. In contrast, no significant enrichment of TRβ is seen at negatively regulated genes or genes that respond to unliganded TRs in this system. Canonical TRE half-sites are present in more than 90% of TRβ peaks and classical TREs are also greatly enriched, but individual TRE organization appears highly variable with diverse half-site orientation and spacing. There is also significant enrichment of binding sites for TR associated transcription factors, including AP-1 and CTCF, near TR peaks. We conclude that T3-dependent gene induction commonly involves proximal TRβ binding events but that far-distant binding events are needed for T3 induction of some genes and that distinct, indirect, mechanisms are often at play in negative regulation and unliganded TR actions. Better understanding of genomic context of TR binding sites will help us determine why TR regulates genes in different ways and determine possibilities for selective modulation of TR action.

Recognition by the thyroid hormone receptor of canonical DNA response elements

To shed more light on the molecular requirements for recognition of thyroid response elements (TREs) by thyroid receptors (TRs), we compared the specific aspects of DNA TRE recognition by different TR constructs. Using fluorescence anisotropy, we performed a detailed and hierarchical study of TR-TRE binding. This was done by comparing the binding affinities of three different TR constructs for four different TRE DNA elements, including palindromic sequences and direct repeats (F2, PAL, DR-1, and DR-4) as well as their interactions with nonspecific DNA sequences. The effect of MgCl(2) on suppressing of nonselective DNA binding to TR was also investigated. Furthermore, we determined the dissociation constants of the hTRbeta DBD (DNA binding domain) and hTRbeta DBD-LBD (DNA binding and ligand binding domains) for specific TREs. We found that a minimum DNA recognition peptide derived from DBD (H1TR) is sufficient for recognition and interaction with TREs, whereas scrambled DNA sequences were unrecognized. Additionally, we determined that the TR DBD binds to F2, PAL, and DR-4 with high affinity and similar K(d) values. The TR DBD-LBD recognizes all the tested TREs but binds preferentially to F2, with even higher affinity. Finally, our results demonstrate the important role played by LBDs in modulating TR-DNA binding.

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.

Thyroid hormone-dependent gene expression as a biomarker of short-term 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) exposure in European common frog (Rana temporaria) tadpoles

The effects on thyroid hormone-dependent gene biomarker responses of the persistent organochlorine pesticide metabolite 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) were investigated after exposure of 4-week-old European common frog (Rana temporaria) (stage 36) tadpoles to two (0.001 and 0.01 ppm) DDE concentrations. Total body weight, total length, and tail length and width increased after 3-day exposure to DDE. Expression patterns of genes encoding for growth hormone, thyroid-stimulating hormone (TSHbeta) and thyroid hormone receptor (TRalpha and TRbeta) isoforms were evaluated in the head, body and tail regions using a validated real-time polymerase chain reaction (PCR) method. The mRNA expression of growth hormone in the body, and TSHbeta in the head showed significant DDE concentration-dependent decreases. While DDE caused variable effects on TRalpha mRNA steady-state, the expression of TRbeta was significantly decreased in the tail by DDE in a concentration-specific manner. The effect of DDE exposure on TRbeta mRNA expression showed a negative correlation with tail length and width during the exposure period. The unique pattern of a DDE-induced decrease of tail TRbeta expression probably reflects the significant role of this thyroid hormone receptor isoform in tail re-absorption and overall metamorphosis in anuran species. Therefore, the present study shows that the evaluation of thyroid hormone-dependent genes may represent quantitative biomarkers of acute exposure to organochlorine pesticides in anuran species during critical developmental periods such as metamorphosis. Given the widespread environmental levels of DDT and its metabolites, these pollutants will remain a subject of concern and their effects on anuran species should be studied in more detail.

Tumor necrosis factor-alpha attenuates thyroid hormone-induced apoptosis in vascular endothelial cell line XLgoo established from Xenopus tadpole tails

Amphibian metamorphosis induced by T(3) involves programmed cell death and the differentiation of various types of cells in degenerated and reconstructed tissues. However, the signaling pathway that directs the T(3)-dependent cell-fate determinations remains unclear. TNF-alpha is a pleiotropic cytokine that affects diverse cellular responses. Engagement of TNF-alpha with its receptor (TNFR1) causes intracellular apoptotic and/or survival signaling. To investigate TNF signaling functions during anuran metamorphosis, we first identified Xenopus laevis orthologs of TNF (xTNF)-alpha and its receptor. We found that xTNF-alpha activated nuclear factor-kappaB in X. laevis A6 cells through the Fas-associated death domain and receptor-interacting protein 1. Interestingly, xTNF-alpha mRNA in blood cells showed prominent expression at prometamorphosis during metamorphosis. Next, to elucidate the apoptotic and/or survival signaling induced by xTNF-alpha in an in vitro model of metamorphosis, we established a vascular endothelial cell line, XLgoo, from X. laevis tadpole tail. XLgoo cells formed actin stress fibers and elongated in response to xTNF-alpha. T(3) induced apoptosis in these cells, but the addition of xTNF-alpha blocked the T(3)-induced apoptosis. In addition, treatment of the cells with T(3) for 2 d induced the expression of thyroid hormone receptor-beta and caspase-3, and this thyroid hormone receptor-beta induction was drastically repressed by xTNF-alpha. Furthermore, in organ culture of the tail, xTNF-alpha significantly attenuated the tail degeneration induced by T(3). These findings suggested that xTNF-alpha could protect vascular endothelial cells from apoptotic cell death induced by T(3) during metamorphosis and thereby participate in the regulation of cell fate.

Association of genetic variants in GABRA3 gene and thyrotoxic hypokalaemic periodic paralysis in Thai population

BACKGROUND

Genetic predisposition has been suggested to play role in the pathogenesis of thyrotoxic hypokalaemic periodic paralysis (THPP).

OBJECTIVES

In this study, we assessed the differences of single-nucleotide polymorphisms (SNP) allelic frequency between THPP patients and well-characterized controls in order to find the susceptibility genetic variants related to THPP using microarray-based assessments on pooled DNA.

METHODS

Fifty cases of THPP and 50 male hyperthyroid patients without hypokalaemia as controls were recruited. Equal amounts of individual genomic DNA were pooled from each group. Estimated allele frequencies of SNPs were derived by averaging relative allele signal score obtained by Affymetrix GeneChip(R) Mapping 10K Arrays.

RESULTS

Sixty-nine loci that display robust allele frequency differences between THPP and controls were identified. SNP rs750841 (A > T) in intron 3 of the gamma-aminobutyric acid (GABA) receptor alpha3 subunit (GABRA3) gene possessed the most significant difference in allele frequency (27% in THPP case and 5% in controls, P = 0.007). Actual allele frequencies obtained from genotyping in each individual were very similar to the estimated frequency from the pools (28% in THPP and 2% in controls, and P = 0.0002). Nearby DNA sequences of GABRA3 were sequenced and an additional two SNPs were found (A > C at exon 1 and G > T of rs12688128). Allele A of rs750841 and allele G of rs12688128 in intron 3 were predominantly found in THPP with significant genetic relative risk of 19 (P < 0.0002; 95%CI 2.4-151.6).

CONCLUSIONS

Whole-genome scanning on pooled DNA provides an accurate, useful screening tool for elucidating genetic underpinnings of THPP. SNPs at intron 3 of GABRA3 are found to be associated with THPP.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Direct regulation of androgen receptor-associated protein 70 by thyroid hormone and its receptors

Thyroid hormone (T3) regulates multiple physiological processes during development, growth, differentiation, and metabolism. Most T3 actions are mediated via thyroid hormone receptors (TRs) that are members of the nuclear hormone receptor superfamily of ligand-dependent transcription factors. The effects of T3 treatment on target gene regulation was previously examined in TRalpha1-overexpressing hepatoma cell lines (HepG2-TRalpha1). Androgen receptor (AR)-associated protein 70 (ARA70) was one gene found to be up-regulated by T3. The ARA70 is a ligand-dependent coactivator for the AR and was significantly increased by 4- to 5-fold after T3 treatment by Northern blot analyses in the HepG2-TRalpha1 stable cell line. T3 induced a 1- to 2-fold increase in the HepG2-TRbeta1 stable cell line. Both stable cell lines attained the highest fold expression after 24 h treatment with 10 nM T3. The ARA70 protein was increased up to 1.9-fold after T3 treatment in HepG2-TRalpha1 cells. Similar findings were obtained in thyroidectomized rats after T3 application. Cycloheximide treatment did not suppress induction of ARA70 transcription by T3, suggesting that this regulation is direct. A series of deletion mutants of ARA70 promoter fragments in pGL2 plasmid were generated to localize the thyroid hormone response element (TRE). The DNA fragments (-234/-190 or +56/+119) gave 1.55- or 2-fold enhanced promoter activity by T3. Thus, two TRE sites exist in the upstream-regulatory region of ARA70. The TR-TRE interaction was further confirmed with EMSAs. Additionally, ARA70 could interfere with TR/TRE complex formation. Therefore, the data indicated that ARA70 suppresses T3 signaling in a TRE-dependent manner. These experimental results suggest that T3 directly up-regulates ARA70 gene expression. Subsequently, ARA70 negatively regulates T3 signaling.

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.

Thyroid hormone response element organization dictates the composition of active receptor

Thyroid hormone (triiodothyronine, T(3)) is known to activate transcription by binding heterodimers of thyroid hormone receptors (TRs) and retinoid X receptors (RXRs). RXR-TRs bind to T(3) response elements (TREs) composed of direct repeats of the sequence AGGTCA spaced by four nucleotides (DR-4). In other TREs, however, the half-sites can be arranged as inverted palindromes and palindromes (Pal). Here we show that TR homodimers and monomers activate transcription from representative TREs with alternate half-site placements. TR beta activates transcription more efficiently than TR alpha at an inverted palindrome (F2), and this correlates with preferential TR beta homodimer formation at F2 in vitro. Furthermore, reconstruction of TR transcription complexes in yeast indicates that TR beta homodimers are active at F2, whereas RXR-TRs are active at DR-4 and Pal. Finally, analysis of TR beta mutations that block homodimer and/or heterodimer formation reveal TRE-selective requirements for these surfaces in mammalian cells, which suggest that TR beta homodimers are active at F2, RXR-TRs at DR-4, and TR monomers at Pal. TR beta requires higher levels of hormone for activation at F2 than other TREs, and this differential effect is abolished by a dimer surface mutation suggesting that it is related to composition of the TR.TRE complex. We propose that interactions of particular TR oligomers with different elements play unappreciated roles in TRE-selective actions of liganded TRs in vivo.

Indirect regulation of human dehydroepiandrosterone sulfotransferase family 1A member 2 by thyroid hormones

Thyroid hormone, T(3), regulates cell metabolism, differentiation, and development. cDNA microarrays were performed to study the mechanism of target gene regulation after T(3) treatment in a thyroid hormone receptor-alpha (TRalpha)-overexpressing hepatoma cell line (HepG2-TRalpha). The differentially expressed target genes are several metabolic enzymes, including dehydroepiandrosterone-sulfotransferase family 1A member 2 (SULT2A1). Enzyme SULT2A1 was elevated roughly 5-fold at the protein level and 9-fold increase at the mRNA level after 48 h T(3) treatment in HepG2-TRalpha cells. Cycloheximide inhibited T(3)-induced SULT2A1 expression, suggesting that regulation was indirect. SULT2A1 has been reported to be regulated by the two transcription factors, steroidogenic factor 1 (SF1) and GATA, in the human adrenal gland. T(3) induced a 2.5- to 3.5-fold elevation of SF1 at the protein level and a 6.2-fold increase at the RNA level in HepG2-TRalpha cells. About seven SF1 binding sites exist on the SULT2A1 gene. To identify and localize the critical SF1 binding site, series of deletion mutants of SULT2A1 promoter fragments in pGL2 plasmid were constructed. The promoter activity of the SULT2A1 gene was enhanced about 2.8- to 7.1-fold by T(3). The -228 SF1 binding site was identified as the most critical site because deleting this region reduced T(3)-induced expression. Transcription factor SF1 application enhanced the -228 but not -117 reporter plasmid activities. SULT2A1 and SF1 up-regulation at protein and RNA levels in thyroidectomized rats occurred after T(3) application. In summary, this work demonstrated that the SULT2A1 gene was mediated by SF1 and indirectly regulated by T(3). Further study is required to elucidate the physiological importance of SULT2A1 induction mediated by T(3).

Regulation of thyroid hormone receptor alpha2 RNA binding and subcellular localization by phosphorylation

Thyroid hormone receptor alpha2 (TRalpha2) is an alternative splice product of the TRalpha primary transcript whose unique carboxyl terminus does not bind T3 or activate transcription. The physiological function of TRalpha2 is unknown. We have found that TRalpha2 is a single stranded RNA binding protein and that the RNA binding domain localizes to a 41 amino acid region immediately distal to the second zinc finger. TRalpha2 contains a single protein kinase CK2 phosphorylation site in its amino terminus and potentially nine CK2 sites in its unique carboxyl terminus. In vitro CK2 treatment of TRalpha2 eliminated its RNA binding. Mutational analysis indicated that phosphorylations at the N- and C-terminal sites both contribute to this inhibitory effect. Cellular localization studies demonstrated that phosphorylated TRalpha2 is primarily cytoplasmic, whereas unphosphorylated TRalpha2 is primarily nuclear. Since RNA binding is a property of unphosphorylated TRalpha2, the TRalpha2-RNA interaction likely represents a nuclear function of TRalpha2.

Differential organ expression patterns of thyroid hormone receptor isoform genes in p,p'-DDE-treated adult male common frog, Rana temporaria

Using the European common frog, Rana temporaria, as a model, we have studied the organ-specific gene expression patterns of thyroid hormone receptor isoforms after exposure to an organochlorine (OC) compound, p,p'-DDE. Four groups of frogs were subcutaneously injected with p,p'-DDE at 0.01, 0.1, 1 and 10mg/kg body weight, respectively. In addition, one group, serving as the control group, was injected with pure corn oil. TH receptor isoforms (TRα and TRβ) gene expressions were evaluated in the brain, kidney, testis and liver using real-time PCR with gene-specific primers. Our results show that p,p'-DDE doses induced slight elevations of TRα and TRβ mRNA in the brain. In the testis, p,p'-DDE induced an initial significant 3-fold increase of TRα mRNA at 0.01mg/kg and thereafter clear dose-dependent decreases of TRα mRNA levels were observed. For testicular TRβ mRNA levels, p,p'-DDE induced a slight elevation at 0.01mg/kg and thereafter significant decreases in TRβ mRNA levels were observed. p,p'-DDE induced significant 2-4-fold elevations of both TR isoforms in frog kidney. The strongest transcriptional effect of p,p'-DDE on TR isoforms was observed in the kidney. While TRα mRNA was not measurable in the liver, p,p'-DDE induced an initial 1.7-fold increase at 0.01mg/kg of TRβ mRNA and thereafter an apparent dose-dependent decrease was observed. The relative abundance of TRα and TRβ gene expression in different organs are in the order: kidney>testis>brain>liver. While the induction TRα and TRβ might result to hypersensitivity and subsequent gain of biological functions, the inhibition might result to loss of biological function. Given the high persistency in the environment and continued use in developing countries coupled with the tendency for global atmospheric transport, DDT and its metabolites such as p,p'-DDE will remain a focus of concern both for scientific and societal reasons.

Ligand-mediated decrease of thyroid hormone receptor-alpha1 in cardiomyocytes by proteosome-dependent degradation and altered mRNA stability

Tri-iodo-L-thyronine (T3) is essential for maintaining normal cardiac contractile function by regulating transcription of numerous T3-responsive genes. Both hormone availability and relative amounts of nuclear thyroid hormone receptor isoforms (TRalpha1, TRbeta1) determine T3 effectiveness. Cultured neonatal rat ventricular myocytes grown in T3-depleted medium expressed predominantly TRalpha1 protein, but within 4 h of T3 treatment, TRbeta1 protein increased significantly, whereas TRalpha1 was decreased by 46 +/- 5%. Using replication-defective adenoviruses to overexpress TRalpha1 in cardiomyocytes, we studied the mechanisms by which T3 mediated the decrease in TRalpha1 protein. Inhibitors of the proteosome pathway resulted in an accumulation of ubiquitylated TRalpha1 in the nucleus and prevented T3-induced degradation of ubiquitylated TRalpha1, suggesting that T3 induced proteosome-mediated degradation of TRalpha1; however, TR ubiquitylation was T3 independent. TRalpha1 transcriptional activity, measured using transient transfection of a thyroid hormone-responsive element (TRE) reporter plasmid, was T3 dose dependent and inversely proportional to nuclear TRalpha1 content, with 10 nM T3 having maximum effect. Quantitative RT-PCR showed that both endogenous and adenovirus-expressed TRalpha1 mRNAs were significantly decreased to 54 +/- 11 and 25 +/- 5%, respectively, within 4 h of T3 treatment. Measurements of TRalpha1 mRNA half-life in actinomycin D-treated cardiomyocytes showed that T3 treatment significantly decreased TRalpha1 mRNA half-life from 4 h to less than 2 h, whereas it had no effect of TRbeta1 mRNA half-life. These data support a role for both the proteosome degradation pathway and altered mRNA stability in T3-induced decrease of nuclear TRalpha1 in the cardiomyocyte and provide novel cellular targets for therapeutic development.

An RNA-binding Domain in the Thyroid Hormone Receptor Enhances Transcriptional Activation

Thyroid hormone plays important roles in development, differentiation, and metabolic homeostasis by binding to nuclear thyroid hormone receptors, which regulate target gene expression by interacting with DNA response elements and coregulatory proteins. We show that thyroid hormone receptors also are single-stranded RNA binding proteins and that this binding is functionally significant. By using a series of deletion mutants, a novel RNA-binding domain was localized to a 41-amino acid segment of thyroid hormone receptor alpha1 between the second zinc finger and the ligand-binding domain. This RNA-binding domain was necessary and sufficient for thyroid hormone receptor binding to the steroid receptor RNA activator (SRA). Although SRA does not bind directly to steroid receptors, it has been identified as a steroid receptor coactivator, and was thought not to be a coactivator for thyroid hormone receptors. However, transfection studies revealed that SRA enhances thyroid hormone induction of appropriate reporter genes and that the thyroid hormone receptor RNA-binding domain is important for this enhancement. We conclude that thyroid hormone receptors bind RNA through a novel domain and that the interaction of this domain with SRA, and perhaps other RNAs, enhances thyroid hormone receptor function.

Characterization of the human renal Na(+)-sulphate cotransporter gene ( NAS1) promoter

Sulphate (SO(4)(2-)) plays an essential role during growth, development, and cellular metabolism. Recently, we have isolated the human renal Na(+)-SO(4)(2-) cotransporter (hNaSi-1) that is implicated in the regulation of serum SO(4)(2-) levels. To gain an insight into hNaSi-1 regulation, our aims were to clone and characterize functionally the hNaSi-1 gene ( NAS1) promoter. We PCR-amplified 3742 bp of the NAS1 5'-flanking region, which is 64% AT-rich and contains numerous putative cis-acting elements. The NAS1 transcription start site was mapped to 25 bp upstream from the translation start site. NAS1 promoter truncations fused to luciferase gene constructs transfected into renal LLC-PK1, MDCK and OK cells allowed us to establish that the first 169 bp of the NAS1 promoter are sufficient for basal transcription. Furthermore, the NAS1 promoter conferred responsiveness to the polycyclic aromatic hydrocarbon 3-methylcholanthrene (3-MC), but not to thyroid hormone (T(3)) or vitamin D [1,25-(OH)(2)D(3)]. Site-directed mutagenesis of the NAS1 promoter identified a functional xenobiotic response element at -2,052, which conferred 3-MC responsiveness. The human NAS1 gene promoter is not responsive to Vitamin D or T(3), unlike the mouse Nas1 promoter with which it shares approximately 40% sequence similarity, but is transactivated by 3-MC, suggesting that the control of renal SO(4)(2-) reabsorption via the regulation of NAS1 transcription may be important for maintaining the sulphation potential for kidney polycyclic aromatic hydrocarbon metabolism.

[Molecular mechanism of thyroid hormone action]

Thyroid hormones (TH) are involved in normal differentiation, growth, and metabolism in several tissues of all vertebrates. Their actions are mediated by the TH receptors (TRs), members of the nuclear hormone receptor superfamily. These receptors are transcription factors that bind to DNA on specific sequences, the TR response element (TREs), in promoters of target genes. Two genes encode TRs, alpha e beta, located in chromosomes 17 and 3, respectively. These isoforms show different functions and exhibit a tissue specific expression. TRs function as monomers, homodimers or heterodimers with retinoid X receptor (RXR) and modulate transcription activity (repression or activation) by interacting with co-repressor and co-activators, which associate with TR in the absence or presence of T3, respectively. Understanding the molecular mechanism of TR action and the definition of its crystallographic structure will provide new insights into transcription mechanisms and will facilitate the design of new drugs with greater therapeutic value.

Challenges confronting risk analysis of potential thyroid toxicants

Screening and testing for potential thyroid toxicants using endpoints of thyroid function, including circulating levels of thyroid hormones and thyrotropin, will not capture toxicants that directly interfere with thyroid hormone action at the receptor. The goals of the present review are to provide a critique of the literature focused on thyroid hormone and brain development as it relates to testing and evaluating thyroid toxicants, and to propose possible solutions to this perceived dilemma.

The mouse sulfate anion transporter gene Sat1 (Slc26a1): cloning, tissue distribution, gene structure, functional characterization, and transcriptional regulation thyroid hormone

Sulfate (SO(4)(2-)) is required for bone/cartilage formation and cellular metabolism. sat-1 is a SO(4)(2-) anion transporter expressed on basolateral membranes of renal proximal tubules, and is suggested to play an important role in maintaining SO(4)(2-) homeostasis. As a first step towards studying its tissue-specific expression, hormonal regulation, and in preparation for the generation of knockout mice, we have cloned and characterized the mouse sat-1 cDNA (msat-1), gene (sat1; Slc26a1) and promoter region. msat-1 encodes a 704 amino acid protein (75.4 kDa) with 12 putative transmembrane domains that induce SO(4)(2-) (also oxalate and chloride) transport in Xenopus oocytes. msat-1 mRNA was expressed in kidney, liver, cecum, calvaria, brain, heart, and skeletal muscle. Two distinct transcripts were expressed in kidney and liver due to alternative utilization of the first intron, corresponding to an internal portion of the 5'-untranslated region. The Sat1 gene (~6 kb) consists of 4 exons. Its promoter is ~52% G + C rich and contains a number of well-characterized cis-acting elements, including sequences resembling hormone responsive elements T(3)REs and VDREs. We demonstrate that Sat1 promoter driven basal transcription in OK cells was stimulated by tri-iodothyronine. Site-directed mutagenesis identified an imperfect T(3)RE at -454-bp in the Sat1 promoter to be responsible for this activity. This study represents the first characterization of the structure and regulation of the Sat1 gene encoding a SO(4)(2-)/chloride/oxalate anion transporter.