Dual functions of the steroid hormone receptor coactivator 3 in modulating resistance to thyroid hormone

Mutations of the thyroid hormone receptor beta (TRbeta) gene cause resistance to thyroid hormone (RTH). RTH is characterized by increased serum thyroid hormone associated with nonsuppressible thyroid-stimulating hormone (TSH) and impaired growth. It is unclear how the actions of TRbeta mutants are modulated in vivo to affect the manifestation of RTH. Using a mouse model of RTH that harbors a knockin mutation of the TRbeta gene (TRbetaPV mouse), we investigated the effect of the steroid hormone receptor coactivator 3 (SRC-3) on RTH. In TRbetaPV mice deficient in SRC-3, dysfunction of the pituitary-thyroid axis and hypercholesterolemia was lessened, but growth impairment of RTH was worsened. The lessened dysfunction of the pituitary-thyroid axis was attributed to a significant decrease in growth of the thyroid and pituitary. Serum insulin-like growth factor 1 (IGF-1) was further reduced in TRbetaPV mice deficient in SRC-3. This effect led to reduced signaling of the IGF-1/phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway that is known to mediate cell growth and proliferation. Thus, SRC-3 modulates RTH by at least two mechanisms, one via its role as a receptor coregulator and the other via its growth regulatory role through the IGF-1/PI3K/AKT/mTOR signaling.     

Marked potentiation of the dominant negative action of a mutant thyroid hormone receptor beta in mice by the ablation of one wild-type beta allele

Mutations in the thyroid hormone receptor (TR) beta gene result in resistance to thyroid hormone (RTH), characterized by reduced sensitivity of tissues to thyroid hormone. To understand which physiological TR pathways are affected by mutant receptors, we crossed mice with a dominantly negative TRbeta mutation (TRbetaPV) with mice carrying a TRbeta null mutation (TRbeta(-/-)) to determine the consequences of the TRbetaPV mutation in the absence of wild-type TRbeta. TRbeta(PV/-) mice are distinct from TRbeta(+/-) mice that did not show abnormalities in thyroid function tests. TRbeta(PV/-) mice are also distinct from TRbeta(PV/+) and TRbeta(-/-) mice in that the latter shows mild dysfunction in the pituitary-thyroid axis, whereas the former exhibit very severe abnormalities, including extensive papillary hyperplasia of the thyroid epithelium, indistinguishable from that observed in TRbeta(PV/PV) mice. Similar to TRbeta(PV/PV) mice, TRbeta(PV/-) mice exhibited impairment in weight gain. Moreover, the abnormal regulation patterns of T3-target genes in the tissues of TRbeta(PV/-) and TRbeta(PV/PV) mice were strikingly similar. Using TR isoforms and PV-specific antibodies in gel shift assays, we found that in vivo, PV competed with TRalpha1 for binding to thyroid hormone response elements in TRbeta(PV/-) mice as effectively as in TRbeta(PV/PV) mice. Thus, the actions of mutant TRbeta are markedly potentiated by the ablation of the second TRbeta allele, suggesting that interference with wild-type TRalpha1-mediated gene regulation by mutant TRbeta leads to severe RTH.     

Differential expression of thyroid hormone receptor isoforms dictates the dominant negative activity of mutant Beta receptor

Mutations in the thyroid hormone receptor beta gene (TRbeta) cause resistance to thyroid hormone (RTH). Genetic analyses indicate that phenotypic manifestation of RTH is due to the dominant negative action of mutant TRbeta. However, the molecular mechanisms underlying the dominant negative action of mutants and how the same mutation results in marked variability of resistance in different tissues in vivo are not clear. Here we used a knock-in mouse (TRbetaPV mouse) that faithfully reproduces human RTH to address these questions. We demonstrated directly that TRbeta1 protein was approximately 3-fold higher than TRalpha1 in the liver of TRbeta(+/+) mice but was not detectable in the heart of wild-type and TRbetaPV mice. The abundance of PV in the liver of TRbeta(PV/PV) was more than TRbeta(PV/+) mice but not detectable in the heart. TRalpha1 in the liver was approximately 6-fold higher than that in the heart of wild-type and TRbetaPV mice. Using TR isoforms and PV-specific antibodies in gel shift assays, we found that in vivo, PV competed not only with TR isoforms for binding to thyroid hormone response elements (TRE) but also competed with TR for the retinoid X receptors in binding to TRE. These competitions led to the inhibition of the thyroid hormone (T(3))-positive regulated genes in the liver. In the heart, however, PV was significantly lower and thus could not effectively compete with TRalpha1 for binding to TRE, resulting in activation of the T(3)-target genes by higher levels of circulating thyroid hormones. These results indicate that in vivo, differential expression of TR isoforms in tissues dictates the dominant negative activity of mutant beta receptor, thereby resulting in variable phenotypic expression in RTH.     

Cell type-dependent modulation of the dominant negative action of human mutant thyroid hormone beta 1 receptors.

BACKGROUND: Mutations in the ligand-binding domain of the thyroid hormone receptor beta (TR beta) gene cause the syndrome of resistance to thyroid hormone (RTH). The clinical phenotype results from the antagonism of the normal TR alpha and the non-mutated TR beta alleles by the TR beta 1 mutants, via a dominant negative effect. There is, however, marked heterogeneity of organ resistance within and among kindreds with RTH. This study examines the potential role of cell type in modulating the dominant negative potency of human TR beta 1 (h-TR beta 1) mutants. MATERIALS AND METHODS: Transient transfections were performed in HeLa and NIH3T3 cells, using a wild type (WT) and three naturally occurring mutant h-TR beta 1 constructs, and three natural thyroid hormone response elements (TREs). Immunocytochemistry was performed to detect levels of TR beta 1 expression in these two cell types. In order to determine how TR beta 1 interacts with other cellular partners, gel-shift analyses using HeLa and NIH3T3 nuclear extracts were performed. RESULTS: Transfection studies using WT h-TR beta 1 in HeLa and NIH3T3 cells, showed that the 3,3',5-triiodothyronine (T3)-induced transactivation of the different TREs varied between cell types. Unlike the non-T3-binding h-TR beta 1 mutant, PV, mutants ED and OK displayed the expected T3-induced dose responsiveness in these two cell types. For each TRE examined, the magnitude of the dominant negative effect varied between the cell types. The levels of receptor expression in HeLa and NIH3T3 cells were identical, as determined by immunocytochemistry. Gel-shift analyses showed differences in the formation of hetero- and homodimers depending on both the cell type and TRE motif. CONCLUSIONS: The cell type in which a mutant receptor operates affects the relative amounts of hetero- and homodimers. Together with the nature of the mutation and the TRE-motif, this could modulate the dominant negative action of mutant receptors in different tissues, which, in turn, could contribute to the variable phenotypic characteristics of RTH.     

Resistance to thyroid hormone mediated by defective thyroid hormone receptor alpha

Background

Thyroid hormone acts via receptor subtypes (TRα1, TRβ1, TRβ2) with differing tissue distributions, encoded by distinct genes (THRA, THRB). THRB mutations cause a disorder with central (hypothalamic–pituitary) resistance to thyroid hormone action with markedly elevated thyroid hormone and normal TSH levels.

Scope of review

This review describes the clinical features, genetic and molecular pathogenesis of a homologous human disorder mediated by defective THRA. Clinical features include growth retardation, skeletal dysplasia and constipation associated with low-normal T4 and high-normal T3 levels and a low T4/T3 ratio, together with subnormal reverse T3 levels. Heterozygous TRa1 mutations in affected individuals generate defective mutant receptors which inhibit wild-type receptor action in a dominant negative manner.

Major conclusions

Mutations in human TRα1 mediate RTH with features of hypothyroidism in particular tissues (e.g. skeleton, gastrointestinal tract), but are not associated with a markedly dysregulated pituitary–thyroid axis.

General significance

Human THRA mutations could be more common but may have eluded discovery due to the absence of overt thyroid dysfunction. Nevertheless, in the appropriate clinical context, a thyroid biochemical signature (low T4/T3 ratio, subnormal reverse T3 levels), may enable future identification of cases.This article is part of a Special Issue entitled Thyroid hormone signalling.     

Serum copper as a novel biomarker for resistance to thyroid hormone

Thyroid hormone action is mediated by the thyroid hormone receptors TRα1 and TRβ. Defects in TRβ lead to RTH (resistance to thyroid hormone) β, a syndrome characterized by high levels of thyroid hormone and non-suppressed TSH (thyroid-stimulating hormone). However, a correct diagnosis of RTHβ patients is difficult as the clinical picture varies. A biochemical serum marker indicative of defects in TRβ signalling is needed and could simplify the diagnosis of RTHβ, in particular the differentiation to TSH-secreting pituitary adenomas, which present with clinically similar symptoms. In the present paper we show that serum copper levels are regulated by thyroid hormone, which stimulates the synthesis and the export of the hepatic copper-transport protein ceruloplasmin into the serum. This is accompanied by a concerted reduction in the mRNA levels of other copper-containing proteins such as metallothioneins 1 and 2 or superoxide dismutase 1. The induction of serum copper is abolished in genetically hyperthyroid mice lacking TRβ and human RTHβ patients, demonstrating an important role of TRβ for this process. Together with a previously reported TRα1 specific regulation of serum selenium, we show that the ratio of serum copper and selenium, which is largely independent of thyroid hormone levels, volume changes or sample degradation, can constitute a valuable novel biomarker for RTHβ. Moreover, it could also provide a suitable large-scale screening parameter to identify RTHα patients, which have not been identified to date.     

The T3R alpha gene encoding a thyroid hormone receptor is essential for post-natal development and thyroid hormone production.

The diverse functions of thyroid hormones are thought to be mediated by two nuclear receptors, T3R alpha1 and T3R beta, encoded by the genes T3R alpha and T3R beta respectively. The T3R alpha gene also produces a non-ligand-binding protein T3R alpha2. The in vivo functions of these receptors are still unclear. We describe here the homozygous inactivation of the T3R alpha gene which abrogates the production of both T3R alpha1 and T3R alpha2 isoforms and that leads to death in mice within 5 weeks after birth. After 2 weeks of life, the homozygous mice become progressively hypothyroidic and exhibit a growth arrest. Small intestine and bones showed a strongly delayed maturation. In contrast to the negative regulatory function of the T3R beta gene on thyroid hormone production, our data show that the T3R alpha gene products are involved in up-regulation of thyroid hormone production at weaning time. Thus, thyroid hormone production might be balanced through a positive T3R alpha and a negative T3R beta pathway. The abnormal phenotypes observed on the homozygous mutant mice strongly suggest that the T3R alpha gene is essential for the transformation of a mother-dependent pup to an 'adult' mouse. These data define crucial in vivo functions for thyroid hormones through a T3R alpha pathway during post-natal development.     

Effect of fasting on thyroid hormone levels, and TR(alpha) and TR(beta) mRNA accumulation in late-stage embryo and juvenile rainbow trout, Oncorhynchus mykiss

The accumulation of mRNA encoding for hepatic and intestinal T3-receptor (TR) and body and liver masses were measured in fed and 3-week fasted juvenile and swim up stage rainbow trout embryos. Plasma and total body thyroid hormone (TH) levels were measured for juvenile and swim up stages, respectively. Fasted juveniles exhibited a lower hepatosomatic index (HSI), liver mass and plasma T4 and T3 concentrations than fed animals, but there were no changes in body mass or the accumulation of mRNA encoding for either of the TR(alpha) or TR(beta) isoforms in liver or intestine. TR(beta) mRNA accumulation was greater than TR(alpha) mRNA accumulation in both tissues. Fasted embryos had lower whole body TH levels and body, liver and intestinal tract masses, in addition to a lower intestinosomatic index. However, there was no change in HSI. Fasting did not affect whole body or hepatic TR(alpha) and TR(beta) mRNA accumulation, although intestinal tract TR(alpha) and TR(beta) mRNA accumulation was lower in the fasted embryos. The HSI and body mass changes in fasted juvenile and embryo stages, respectively, indicated that both developmental stages were impacted by fasting. Both stages also showed evidence of decreased TH production. The lower TR gene expression in the intestinal tract of fasted embryos may suggest a role for THs in the transitional stage of intestinal development during this period of development.     

Functional regulation of thyroid hormone receptor variant TR alpha 2 by phosphorylation.

The thyroid hormone (T3) receptor (TR) variant TR alpha 2 is abundant in brain but does not bind T3 because of its unique C terminus. The only known function of TR alpha 2, inhibition of TR-dependent transactivation, involves competition for T3 response elements. Paradoxically, in vitro-translated TR alpha 2 bound poorly to these sites. We report here that dephosphorylation of TR alpha 2 restored its DNA binding. Mutation of C-terminal serine residues to alanine (TR alpha 2-SA) was equally effective. The C terminus of TR alpha 2 was phosphorylated in a human cell line, whereas that of TR alpha 2-SA was not. Conversely, TR alpha 2-SA was a much better inhibitor of T3 action than was wild-type TR alpha 2. The dominant negative activity of TR alpha 2-SA was less than stoichiometric with TR concentration, possibly because it was unable to heterodimerize with retinoid X receptor, which enhances the binding of other TRs. Purified casein kinase II as well as a reticulocyte casein kinase II-like activity phosphorylated TR alpha 2 on serines 474 and 475. Mutation of these two residues to alanine was sufficient to restore DNA binding. Thus, DNA binding by TR alpha 2 is regulated by phosphorylation at a site distant from the DNA-binding domain. The increased dominant negative activity of a nonphosphorylatable form of TR alpha 2 suggests that phosphorylation may provide a rapid, T3-independent mechanism for cell-specific modulation of the expression of T3-responsive genes.     

The thyroid hormone receptors: molecular basis of thyroid hormone resistance.

Major progress has been achieved in the mechanism of action of thyroid hormones thanks to the identification of the T3 receptor as the product of the proto-oncogene c-erbA. Recognition of subsets of receptors with and without T3-binding properties and of the interaction of different receptors with each other leads to new insights in cell regulation and development. In thyroid hormone resistance, distinct mutations in the T3-binding domain of thyroid hormone receptor (TR)beta have been identified in unrelated families. No correlation between the type of mutation and tissue resistance has been established. Mutant TRs bind to thyroid hormone response elements (TREs) on both negative or positive T3-controlled genes. Subjects with heterozygous TR beta gene deletion are not affected, supporting the hypothesis that mutant TRs act through a dominant negative effect. In generalized thyroid hormone resistance, mutated TR beta may interfere through competition for TREs and/or formation of inactive dimers. Finally, deficiency in T3 receptor auxiliary protein or other accessory proteins or competition between mutant and normal TRs for these factors is not excluded.     

Abnormal heart rate and body temperature in mice lacking thyroid hormone receptor alpha 1.

Thyroid hormone, acting through several nuclear hormone receptors, plays important roles in thermogenesis, lipogenesis and maturation of the neonatal brain. The receptor specificity for mediating these effects is largely unknown, and to determine this we developed mice lacking the thyroid hormone receptor TR alpha 1. The mice have an average heart rate 20% lower than that of control animals, both under normal conditions and after thyroid hormone stimulation. Electrocardiograms show that the mice also have prolonged QRS- and QTend-durations. The mice have a body temperature 0.5 degrees C lower than normal and exhibit a mild hypothyroidism, whereas their overall behavior and reproduction are normal. The results identify specific and important roles for TR alpha 1 in regulation of tightly controlled physiological functions, such as cardiac pacemaking, ventricular repolarisation and control of body temperature.