Thyroid hormone receptor alpha sumoylation modulates white adipose tissue stores.

Thyroid hormone (TH) and thyroid hormone receptor (THR) regulate stem cell proliferation and differentiation during development, as well as during tissue renewal and repair in the adult. THR undergoes posttranslational modification by small ubiquitin-like modifier (SUMO). We generated the THRA (K283Q/K288R)-/- mouse model for in vivo studies and used human primary preadipocytes expressing the THRA sumoylation mutant (K283R/K288R) and isolated preadipocytes from mutant mice for in vitro studies. THRA mutant mice had reduced white adipose stores and reduced adipocyte cell diameter on a chow diet, compared to wild-type, and these differences were further enhanced after a high fat diet. Reduced preadipocyte proliferation in mutant mice, compared to wt, was shown after in vivo labeling of preadipocytes with EdU and in preadipocytes isolated from mice fat stores and studied in vitro. Mice with the desumoylated THRA had disruptions in cell cycle G1/S transition and this was associated with a reduction in the availability of cyclin D2 and cyclin-dependent kinase 2. The genes coding for cyclin D1, cyclin D2, cyclin-dependent kinase 2 and Culin3 are stimulated by cAMP Response Element Binding Protein (CREB) and contain CREB Response Elements (CREs) in their regulatory regions. We demonstrate, by Chromatin Immunoprecipitation (ChIP) assay, that in mice with the THRA K283Q/K288R mutant there was reduced CREB binding to the CRE. Mice with a THRA sumoylation mutant had reduced fat stores on chow and high fat diets and reduced adipocyte diameter.

Noncanonical Thyroid Hormone Receptor α Action Mediates Arterial Vasodilation.

CONTEXT: Hypothyroidism impairs cardiovascular health and contributes to endothelial dysfunction with reduced vasodilation. How 3,5,3'-triiodothyronine (T3) and its receptors are involved in the regulation of vasomotion is not yet fully understood. In general, thyroid hormone receptors (TRs) either influence gene expression (canonical action) or rapidly activate intracellular signaling pathways (noncanonical action). OBJECTIVE: Here we aimed to characterize the T3 action underlying the mechanism of arterial vasodilation and blood pressure (BP) regulation. METHODS: Mesenteric arteries were isolated from male rats, wild-type (WT) mice, TRα knockout (TRα 0) mice, and from knockin mice with a mutation in the DNA-binding domain (TRα GS). In this mutant, DNA binding and thus canonical action is abrogated while noncanonical signaling is preserved. In a wire myograph system, the isolated vessels were preconstricted with norepinephrine. The response to T3 was measured, and the resulting vasodilation (Δ force [mN]) was normalized to maximum contraction with norepinephrine and expressed as percentage vasodilation after maximal preconstriction with norepinephrine (%NE). Isolated vessels were treated with T3 (1 × 10-15 to 1 × 10-5 mol/L) alone and in combination with the endothelial nitric oxide-synthase (eNOS) inhibitor L-NG-nitroarginine methyl ester (L-NAME) or the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin. The endothelium was removed to determine the contribution of T3 to endothelium-dependent vasodilation. The physiological relevance of T3-induced vasodilation was determined by in vivo arterial BP measurements in male and female mice. RESULTS: T3 treatment induced vasodilation of mesenteric arteries from WT mice within 2 minutes (by 21.5 ±â€…1.7%NE). This effect was absent in arteries from TRα 0 mice (by 5.3 ±â€…0.6%NE, P < .001 vs WT) but preserved in TRα GS arteries (by 17.2 ±â€…1.1%NE, not significant vs WT). Inhibition of either eNOS or PI3K reduced T3-mediated vasodilation from 52.7 ±â€…4.5%NE to 28.5 ±â€…4.1%NE and 22.7 ±â€…2.9%NE, respectively. Removal of the endothelium abolished the T3-mediated vasodilation in rat mesenteric arteries (by 36.7 ±â€…5.4%NE vs 3.5 ±â€…6.2%NE). In vivo, T3 injection led to a rapid decrease of arterial BP in WT (by 13.9 ±â€…1.9 mm Hg) and TRα GS mice (by 12.4 ±â€…1.9 mm Hg), but not in TRα 0 mice (by 4.1 ±â€…1.9 mm Hg). CONCLUSION: These results demonstrate that T3 acting through noncanonical TRα action affects cardiovascular physiology by inducing endothelium-dependent vasodilation within minutes via PI3K and eNOS activation.

p43, a Truncated Form of Thyroid Hormone Receptor α, Regulates Maturation of Pancreatic ß Cells.

P43 is a truncated form of thyroid hormone receptor α localized in mitochondria, which stimulates mitochondrial respiratory chain activity. Previously, we showed that deletion of p43 led to reduction of pancreatic islet density and a loss of glucose-stimulated insulin secretion in adult mice. The present study was designed to determine whether p43 was involved in the processes of ß cell development and maturation. We used neonatal, juvenile, and adult p43-/- mice, and we analyzed the development of ß cells in the pancreas. Here, we show that p43 deletion affected only slightly ß cell proliferation during the postnatal period. However, we found a dramatic fall in p43-/- mice of MafA expression (V-Maf Avian Musculoaponeurotic Fibrosarcoma Oncogene Homolog A), a key transcription factor of beta-cell maturation. Analysis of the expression of antioxidant enzymes in pancreatic islet and 4-hydroxynonenal (4-HNE) (a specific marker of lipid peroxidation) staining revealed that oxidative stress occurred in mice lacking p43. Lastly, administration of antioxidants cocktail to p43-/- pregnant mice restored a normal islet density but failed to ensure an insulin secretion in response to glucose. Our findings demonstrated that p43 drives the maturation of ß cells via its induction of transcription factor MafA during the critical postnatal window.

The Prognostic Impact of Retinoid X Receptor and Thyroid Hormone Receptor alpha in Unifocal vs. Multifocal/Multicentric Breast Cancer.

The aim of this study was to assess the prognostic value of the steroid hormone receptor expression, counting the retinoid X receptor (RXR) and thyroid hormone receptors (THRs), on the two different breast cancer (BC) entities: multifocal/multicentric versus unifocal. The overall and disease-free survival were considered as the prognosis determining aspects and analyzed by uni- and multi-variate analysis. Furthermore, histopathological grading and TNM staging (T = tumor size, N = lymph node involvement, M = distant metastasis) were examined in relation to RXR and THRs expression. A retrospective statistical analysis was carried out on survival-related events in a series of 319 sporadic BC patients treated at the Department of Gynecology and Obstetrics at the Ludwig-Maximillian's University in Munich between 2000 and 2002. The expression of RXR and THRs, including its two major isoforms THRα1 and THRα2, was analyzed by immunohistochemistry and showed to have a significant correlation for both BC entities in regard to survival analysis. Patients with multifocal/multicentric BC were exposed to a significantly worse disease-free survival (DFS) when expressing RXR. Patients with unifocal BC showed a significantly worse DFS when expressing THRα1. In contrast, a statistically significant positive association between THRα2 expression and enhanced DFS in multifocal/multicentric BC was shown. Especially the RXR expression in multifocal/multicentric BC was found to play a remarkably contradictory role for BC prognosis. The findings imply the need for a critical review of possible molecular therapies targeting steroid hormone receptors in BC treatment. Our results strengthen the need to further investigate the behavior of the nuclear receptor family, especially in relation to BC focality.

Thyroid hormone receptor α in skeletal muscle is essential for T3-mediated increase in energy expenditure.

Thyroid hormones are important for homeostatic control of energy metabolism and body temperature. Although skeletal muscle is considered a key site for thyroid action, the contribution of thyroid hormone receptor signaling in muscle to whole-body energy metabolism and body temperature has not been resolved. Here, we show that T3-induced increase in energy expenditure requires thyroid hormone receptor alpha 1 (TRα1 ) in skeletal muscle, but that T3-mediated elevation in body temperature is achieved in the absence of muscle-TRα1 . In slow-twitch soleus muscle, loss-of-function of TRα1 (TRαHSACre ) alters the fiber-type composition toward a more oxidative phenotype. The change in fiber-type composition, however, does not influence the running capacity or motivation to run. RNA-sequencing of soleus muscle from WT mice and TRαHSACre mice revealed differentiated transcriptional regulation of genes associated with muscle thermogenesis, such as sarcolipin and UCP3, providing molecular clues pertaining to the mechanistic underpinnings of TRα1 -linked control of whole-body metabolic rate. Together, this work establishes a fundamental role for skeletal muscle in T3-stimulated increase in whole-body energy expenditure.

Downregulation of Type 3 Deiodinase in the Hypothalamus During Inflammation.

Background: Inflammation is associated with marked changes in cellular thyroid hormone (TH) metabolism in triiodothyronine (T3) target organs. In the hypothalamus, type 2 deiodinase (D2), the main T3 producing enzyme, increases upon inflammation, leading to an increase in local T3 availability, which in turn decreases thyrotropin releasing hormone expression in the paraventricular nucleus. Type 3 deiodinase (D3), the T3 inactivating enzyme, decreases during inflammation, which might also contribute to the increased T3 availability in the hypothalamus. While it is known that D2 is regulated by nuclear factor κB (NF-κB) during inflammation, the underlying mechanisms of D3 regulation are unknown. Therefore, the aim of the present study was to investigate inflammation-induced D3 regulation using in vivo and in vitro models. Methods: Mice were injected with a sublethal dose of bacterial endotoxin (lipopolysaccharide [LPS]) to induce a systemic acute-phase response. A human neuroblastoma (SK-N-AS) cell line was used to test the involvement of the thyroid hormone receptor alpha 1 (TRα1) as well as the activator protein-1 (AP-1) and NF-κB inflammatory pathways in the inflammation-induced decrease of D3. Results: D3 expression in the hypothalamus was decreased 24 hours after LPS injection in mice. This decrease was similar in mice lacking the TRα. Incubation of SK-N-AS cells with LPS robustly decreased both D3 mRNA expression and activity. This led to increased intracellular T3 concentrations. The D3 decrease was prevented when NF-κB or AP-1 was inhibited. TRα1 mRNA expression decreased in SK-N-AS cells incubated with LPS, but knockdown of the TRα in SK-N-AS cells did not prevent the LPS-induced D3 decrease. Conclusions: We conclude that the inflammation-induced D3 decrease in the hypothalamus is mediated by the inflammatory pathways NF-κB and AP-1, but not TRα1. Furthermore, the observed decrease modulates intracellular T3 concentrations. Our results suggest a concerted action of inflammatory modulators to regulate both hypothalamic D2 and D3 activities to increase the local TH concentrations.

Genetic and Pharmacological Targeting of Transcriptional Repression in Resistance to Thyroid Hormone Alpha.

Background: Thyroid hormones act in bone and cartilage via thyroid hormone receptor alpha (TRα). In the absence of triiodothyronine (T3), TRα interacts with co-repressors, including nuclear receptor co-repressor-1 (NCoR1), which recruit histone deacetylases (HDACs) and mediate transcriptional repression. Dominant-negative mutations of TRα cause resistance to thyroid hormone alpha (RTHα; OMIM 614450), characterized by excessive repression of T3 target genes leading to delayed skeletal development, growth retardation, and bone dysplasia. Treatment with thyroxine has been of limited benefit, even in mildly affected individuals, and there is a need for new therapeutic strategies. It was hypothesized that (i) the skeletal manifestations of RTHα are mediated by the persistent TRα/NCoR1/HDAC repressor complex containing mutant TRα, and (ii) treatment with the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) would ameliorate these manifestations. Methods: The skeletal phenotypes of (i) Thra1PV/+ mice, a well characterized model of RTHα; (ii) Ncor1ΔID/ΔID mice, which express an NCoR1 mutant that fails to interact with TRα; and (iii) Thra1PV/+Ncor1ΔID/ΔID double-mutant adult mice were determined. Wild-type, Thra1PV/+, Ncor1ΔID/ΔID, and Thra1PV/+Ncor1ΔID/ΔID double-mutant mice were also treated with SAHA to determine whether HDAC inhibition results in amelioration of skeletal abnormalities. Results:Thra1PV/+ mice had a severe skeletal dysplasia, characterized by short stature, abnormal bone morphology, and increased bone mineral content. Despite normal bone length, Ncor1ΔID/ΔID mice displayed increased cortical bone mass, mineralization, and strength. Thra1PV/+Ncor1ΔID/ΔID double-mutant mice displayed only a small improvement of skeletal abnormalities compared to Thra1PV/+ mice. Treatment with SAHA to inhibit histone deacetylation had no beneficial or detrimental effects on bone structure, mineralization, or strength in wild-type or mutant mice. Conclusions: These studies indicate treatment with SAHA is unlikely to improve the skeletal manifestations of RTHα. Nevertheless, the findings (i) confirm that TRα1 has a critical role in the regulation of skeletal development and adult bone mass, (ii) suggest a physiological role for alternative co-repressors that interact with TR in skeletal cells, and (iii) demonstrate a novel role for NCoR1 in the regulation of adult bone mass and strength.

TRα inhibits arterial renin-angiotensin system expression and prevents cholesterol accumulation in vascular smooth muscle cells.

OBJECTIVES: The tissue renin-angiotensin system (tRAS) plays a key role in the maintenance of cellular homeostasis but is also implicated in atherosclerosis. Thyroid hormone (TH) contributes, via genomic effects, to control of tRAS gene expression in the arterial wall and vascular smooth muscle cells (VSMCs). We investigated the specific functions of TH receptors-α and -ß (TRα and TRß) on tRAS gene expression in the aorta and VSMCs, and the potential protective effect of TRα against atherosclerosis. MATERIAL AND METHODS: Using aorta and cultured aortic VSMCs from TRα and TRß deficient mice, tRAS gene expression was analyzed by determining mRNA levels on real-time PCR. Gene regulation under cholesterol loading mimicking atherosclerosis conditions was also examined in VSMCs in vitro. RESULTS: TRα deletion significantly increased expression of angiotensinogen (AGT) and angiotensin II receptor type 1 subtype a (AT1Ra) at transcriptional level in aorta, a tissue with high TRα expression level. TRα activity thus seems to be required for maintenance of physiological levels of AGTand AT1Raexpression in the arterial wall. In addition, during cholesterol loading, TRα deletion significantly increased cholesterol content in VSMCs, with a weaker decrease in AGTexpression. CONCLUSION: TRα seems to have an inhibitory impact on AGTand AT1Raexpression, and loss of TRα function in TRα0/0 mice increases tRAS expression in the aortic wall. More importantly, TRα deletion significantly increases VSMC cholesterol content. Our results are consistent with a protective role of TRα against atherosclerosis.

Thyroid Hormone Receptor Alpha Deletion in ApoE-/- Mice Alters the Arterial Renin-Angiotensin System and Vascular Smooth Muscular Cell Cholesterol Metabolism.

Thyroid hormone (TH) regulates gene transcription by binding to TH receptors (TRs). TRs regulate the genes of lipid metabolism and the renin-angiotensin system (RAS). We examined the effect of TRα deletion in ApoE-/- mice (DKO mice) on the following: (i) the expression of genes controlling cholesterol metabolism and tissue (t)RAS in the liver and aorta and (ii) the expression of these genes and the regulation of cholesterol content in cultured vascular smooth muscle cells (VSMCs). TRα deletion in ApoE-/- mice led to the repression of genes involved in the synthesis and influx of cholesterol in the liver. However, TRα deletion in the arterial wall suppressed the expression of genes involved in the esterification and excretion of cholesterol and enhanced the expression of angiotensinogen (AGT). The VSMCs of the ApoE-/- and DKO mice increased their cholesterol content during cholesterol loading, but failed to increase the expression of ATP-binding cassette transporter A1 (ABCA1). T3 addition partially corrected these abnormalities in the cells of the ApoE-/- mice but not those of the DKO mice. In conclusion, TRα deletion in ApoE-/- mice slightly increases the expression of tRAS in the aorta and aggravates the dysregulation of cholesterol content in the VSMCs.

Effects of hyper- and hypothyroidism on the development and proliferation of testicular cells in prepubertal rats.

Thyroid hormones are important in the development and regulation of testes. This study was conducted to determine the effects of hyper- and hypothyroidism on testicular development in prepubertal rats aged 20-70 days. Weaning male rats (20 days old) until day 70 age were randomly divided into four groups: control, hyperthyroid (hyper-T), hypothyroid (hypo-T) and hypothyroid treated with thyroxine (T4) (hypo-T+T4). The results indicated that thyroid hormones caused a significant effect in body and testis weights, and food and water consumption. In addition there were changes in serum concentrations of tri-iodothyronine, T4, thyroid stimulating hormone (TSH) and testosterone. Histomorphology showed a significant decrease in seminiferous tubule diameter in hyper-T compared to the other groups. Leydig cell numbers showed a significant elevation in hyper-T but not in hypo-T groups. Immunostaining indicated that TSH receptor (TSHR), thyroid hormone receptors α/ß (TRαß) and proliferating cell nuclear antigen (PCNA) have the roles in testicular development. Our findings suggest that hyper- and hypo-thyroidism regulate testicular cell proliferation and spermatogenesis in prepubertal rats, indicating that expression of TSHR, TRαß and PCNA may be regulated by thyroid hormones that are involved in testicular development; and that the administration of T4 to the hypo-T+T4 group leads to an improvement in the testicular condition.

Thyroid Hormone Receptor α Controls Developmental Timing and Regulates the Rate and Coordination of Tissue-Specific Metamorphosis in Xenopus tropicalis.

Thyroid hormone (T3) receptors (TRs) mediate the effects of T3 on organ metabolism and animal development. There are two TR genes, TRα and TRß, in all vertebrates. During animal development, TRα expression is activated earlier than zygotic T3 synthesis and secretion into the plasma, implicating a developmental role of TRα both in the presence and absence of T3. Using T3-dependent amphibian metamorphosis as a model, we previously proposed a dual-function model for TRs, in particular TRα, during development. That is, unliganded TR represses the expression of T3-inducible genes during premetamorphosis to ensure proper animal growth and prevent premature metamorphosis, whereas during metamorphosis, liganded TR activates target gene transcription to promote the transformation of the tadpole into a frog. To determine if TRα has such a dual function, we generated homozygous TRα-knockout animal lines. We show that, indeed, TRα knockout affects both premetamorphic animal development and metamorphosis. Surprisingly, we observed that TRα is not essential for amphibian metamorphosis, given that homozygous knockout animals complete metamorphosis within a similar time period after fertilization as their wild-type siblings. On the other hand, the timing of metamorphosis for different organs is altered by the knockout; limb metamorphosis occurs earlier, whereas intestinal metamorphosis is completed later than in wild-type siblings. Thus, our studies have demonstrated a critical role of endogenous TRα, not only in regulating both the timing and rate of metamorphosis, but also in coordinating temporal metamorphosis of different organs.

Translating thyroid hormone effects into clinical practice: the relevance of thyroid hormone receptor α1 in cardiac repair.

Thyroid hormone (TH) appears to have a critical role in cardiac repair after injury beyond its role in development and metabolism homeostasis. This unique action is due to the fact that TH effect on the heart is shown to be differentiated depending on its administration on injured or healthy myocardium. Thus, TH can limit ischemia-reperfusion injury via a fine balance between pro-apoptotic and pro-survival signaling pathways. This response is thyroid hormone receptor (TRα1) dependent. Furthermore, an interaction between stress-induced growth kinase signaling and TRα1 is shown to occur and determine postischemic remodeling and cardiac recovery depending on the availability of TH. This new evidence is consistent with clinical observations showing the cardioprotective effect of TH treatment in cardiac surgery, transplantation and heart failure. TH and/or thyroid analogs may be novel agents in treating heart diseases.

Chondrocytes play a major role in the stimulation of bone growth by thyroid hormone.

Thyroid hormone (T3) is required for postnatal skeletal growth. It exerts its effect by binding to nuclear receptors, TRs including TRα1 and TRß1, which are present in most cell types. These cell types include chondrocytes and osteoblasts, the interactions of which are known to regulate endochondral bone formation. In order to analyze the respective functions of T3 stimulation in chondrocytes and osteoblasts during postnatal growth, we use Cre/loxP recombination to express a dominant-negative TRα1(L400R) mutant receptor in a cell-specific manner. Phenotype analysis revealed that inhibiting T3 response in chondrocytes is sufficient to reproduce the defects observed in hypothyroid mice, not only for cartilage maturation, but also for ossification and mineralization. TRα1(L400R) in chondrocytes also results in skull deformation. In the meantime, TRα1(L400R) expression in mature osteoblasts has no visible effect. Transcriptome analysis identifies a number of changes in gene expression induced by TRα1(L400R) in cartilage. These changes suggest that T3 normally cross talks with several other signaling pathways to promote chondrocytes proliferation, differentiation, and skeletal growth.

[Thyroid hormone and skeletal metabolism].

The role of the hypothalamic-pituitary-thyroid axis is important in normal skeletal development, gain of bone mass, and regulation of adult bone metabolism. Hypothyroidism in childhood causes delayed bone maturation and growth disturbance and thyroid dysfunction in adult induces altered bone remodeling and an increased risk of bone fracture. Thyroid hormone actions in skeletal cells are mainly mediated by thyroid hormone receptor α (TRα) . The responses to thyroid hormone are regulated by type 2 and 3 iodothyronine deiodinase (DIO2 and DIO3) , which convert prohormone (T4) to active hormone (T3) . Euthyroid status is necessary for the homeostasis of human bone metabolism.

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, TRß1 and TRß2) 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.

Vascular smooth muscle cell apoptosis is an early trigger for hypothyroid atherosclerosis.

AIMS: Endothelial dysfunction is an initial and vascular smooth muscle cell (VSMC) apoptosis, a later step of atherosclerosis. Hypothyroidism accelerates atherosclerosis. However, the early events responsible for this pro-atherosclerotic effect are unclear. METHODS AND RESULTS: Rats were resistant to induction of atherosclerosis by high cholesterol diet alone, but became susceptible in hypothyroid state achieved by administration of propylthiouracil (PTU) for 6 weeks. VSMC dysfunction and apoptosis were obvious within 1 week after PTU treatment, without signs of endothelial dysfunction. This early VSMC damage was caused by hypothyroidism but not the high cholesterol diet. In ApoE knockout mice, PTU-induced hypothyroidism triggered early VSMC apoptosis, increased oxidative stress, and accelerated atherosclerosis development. Thyroid hormone supplementation (T4, 10, or 50 µg/kg) prevented atherogenic phenotypes in hypothyroid rats and mice. In rats, thyroidectomy caused severe hypothyroidism 5 days after operation, which also led to rapid VSMC dysfunction and apoptosis. In vitro studies did not show a direct toxic effect of PTU on VSMCs. In contrast, thyroid hormone (T3, 0.75 µg/L plus T4, 50 nmol/L) exerted a direct protection against VSMC apoptosis, which was reduced by knockdown of TRα1, rather than TRß1 and TRß2 receptors. TRα1-mediated inhibition of apoptotic signalling of JNKs and caspase-3 contributed to the anti-apoptotic action of thyroid hormone. CONCLUSION: These findings provide an in vivo example for VSMC apoptosis as an early trigger of hypothyroidism-associated atherosclerosis, and reveal activation of TRα1 receptors to prevent VSMC apoptosis as a therapeutic strategy in this disease.

Resistance to thyroid hormone mediated by defective thyroid hormone receptor alpha.

BACKGROUND: Thyroid hormone acts via receptor subtypes (TRα1, TRß1, TRß2) 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.

Role of thyroid hormone receptor subtypes α and ß on gene expression in the cerebral cortex and striatum of postnatal mice.

The effects of thyroid hormones (THs) on brain development and function are largely mediated by the control of gene expression. This is achieved by the binding of the genomically active T3 to transcriptionally active nuclear TH receptors (TRs). T3 and the TRs can either induce or repress transcription. In hypothyroidism, the reduction of T3 lowers the expression of a set of genes, the positively regulated genes, and increases the expression of negatively regulated genes. Two mechanisms may account for the effect of hypothyroidism on genes regulated directly by T3: first, the loss of T3 signaling and TR transactivation, and second, an intrinsic activity of the unliganded TRs directly responsible for repression of positive genes and enhancement of negative genes. To analyze the contribution of the TR subtypes α and ß, we have measured by RT-PCR the expression of a set of positive and negative genes in the cerebral cortex and the striatum of TR-knockout male and female mice. The results indicate that TRα1 exerts a predominant but not exclusive role in the regulation of positive and negative genes. However, a fraction of the genes analyzed are not or only mildly affected by the total absence of TRs. Furthermore, hypothyroidism has a mild effect on these genes in the absence of TRα1, in agreement with a role of unliganded TRα1 in the effects of hypothyroidism.

Mechanisms of action of thyroid hormones in the skeleton.

BACKGROUND: Thyroid hormones regulate skeletal development, acquisition of peak bone mass and adult bone maintenance. Abnormal thyroid status during childhood disrupts bone maturation and linear growth, while in adulthood it results in altered bone remodeling and an increased risk of fracture SCOPE OF REVIEW: This review considers the cellular effects and molecular mechanisms of thyroid hormone action in the skeleton. Human clinical and population data are discussed in relation to the skeletal phenotypes of a series of genetically modified mouse models of disrupted thyroid hormone signaling. MAJOR CONCLUSIONS: Euthyroid status is essential for normal bone development and maintenance. Major thyroid hormone actions in skeletal cells are mediated by thyroid hormone receptor α (TRα) and result in anabolic responses during growth and development but catabolic effects in adulthood. These homeostatic responses to thyroid hormone are locally regulated in individual skeletal cell types by the relative activities of the type 2 and 3 iodothyronine deiodinases, which control the supply of the active thyroid hormone 3,5,3'-L-triiodothyronine (T3) to its receptor. GENERAL SIGNIFICANCE: Population studies indicate that both thyroid hormone deficiency and excess are associated with an increased risk of fracture. Understanding the cellular and molecular basis of T3 action in skeletal cells will lead to the identification of new targets to regulate bone turnover and mineralization in the prevention and treatment of osteoporosis. This article is part of a Special Issue entitled Thyroid hormone signaling.