Thyrotropin receptors in brown adipose tissue: thyrotropin stimulates type II iodothyronine deiodinase and uncoupling protein-1 in brown adipocytes

Targeting Thyroid-Stimulating Hormone Receptor: A Perspective on Small-Molecule Modulators and Their Therapeutic Potential

TSHR is a member of the glycoprotein hormone receptors, a subfamily of class A G-protein-coupled receptors and plays pivotal roles in various physiological and pathological processes, particularly in thyroid growth and hormone production. The aberrant TSHR function has been implicated in several human diseases including Graves' disease and orbitopathy, nonautoimmune hyperthyroidism, hypothyroidism, cancer, neurological disorders, and osteoporosis. Consequently, TSHR is recognized as an attractive therapeutic target, and targeting TSHR with small-molecule modulators including agonists, antagonists, and inverse agonists offers great potential for drug discovery. In this perspective, we summarize the structures and biological functions of TSHR as well as the recent advances in the development of small-molecule TSHR modulators, highlighting their chemotypes, mode of actions, structure-activity relationships, characterizations, in vitro/in vivo activities, and therapeutic potential. The challenges, new opportunities, and future directions in this area are also discussed.

Understanding the Roles of Selenium on Thyroid Hormone-Induced Thermogenesis in Adipose Tissue

Brown adipose tissue (BAT) and white adipose tissue (WAT) are known to regulate lipid metabolism. A lower amount of BAT compared to WAT, along with adipose tissue dysfunction, can result in obesity. Studies have shown that selenium supplementation protects against adipocyte dysfunction, decreases WAT triglycerides, and increases BAT triiodothyronine (T3). In this review, we discuss the relationship between selenium and lipid metabolism regulation through selenoprotein deiodinases and the role of deiodinases and thyroid hormones in the induction of adipose tissue thermogenesis. Upon 22 studies included in our review, we found that studies investigating the relationship between selenium and deiodinases demonstrated that selenium supplementation affects the iodothyronine deiodinase 2 (DIO2) protein and the expression of its associated gene, DIO2, proportionally. However, its effect on DIO1 is inconsistent while its effect on DIO3 activity is not detected. Studies have shown that the activity of deiodinases especially DIO2 protein and DIO2 gene expression is increased along with other browning markers upon white adipose tissue browning induction. Studies showed that thermogenesis is stimulated by the thyroid hormone T3 as its activity is correlated to the expression of other thermogenesis markers. A proposed mechanism of thermogenesis induction in selenium supplementation is by autophagy control. However, more studies are needed to establish the role of T3 and autophagy in adipose tissue thermogenesis, especially, since some studies have shown that thermogenesis can function even when T3 activity is lacking and studies related to autophagy in adipose tissue thermogenesis have contradictory results.

Dopaminergic and adrenergic receptors synergistically stimulate browning in 3T3-L1 white adipocytes

The browning of white adipose tissue (WAT) has attracted considerable attention in the scientific community as a popular strategy for enhancing energy expenditure to combat obesity. As a part of this strategy, β3-adrenergic receptor (β3-AR) is the most widely studied receptor that mediates thermogenesis. Parenthetically, further studies in search for additional receptors expressed in adipocytes that can mediate thermogenesis has been appearing, and this paper reports that dopaminergic receptor 1 (DRD1) and β3-AR synergistically stimulate browning in 3T3-L1 white adipocytes. qRT-PCR and immunoblot analysis methods were applied to evaluate the effects of DRD1 on the target proteins downstream of β3-AR and other markers involved in lipid metabolism, mitochondrial biogenesis, and browning events. These results show that DRD1 is expressed in epididymal WAT (eWAT), brown adipose tissue (BAT), and inguinal WAT (iWAT) of normal and high-fat-fed mice, and a deficiency of DRD1 downregulates the expression of brown adipocyte-specific proteins. Silencing of DRD1 affected lipid metabolic activity in 3T3-L1 adipocytes by reducing mitochondrial biogenesis as well as levels of lipolytic and fat oxidative marker proteins in a similar pattern to β3-AR. Moreover, mechanistic studies showed that the depletion of DRD1 downregulates β3-AR and its downstream molecules, suggesting both receptors might synergistically stimulate browning. Parallel to the UCP1-dependent thermogenesis, the depletion of DRD1 also downregulates the expression of core proteins responsible for UCP1-independent thermogenesis. Overall, DRD1 mediates β3-AR-dependent 3T3-L1 browning and UCP1-independent thermogenesis.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

Inherited Disorders of Thyroid Hormone Metabolism Defect Caused by the Dysregulation of Selenoprotein Expression

Consistent activation and functioning of thyroid hormones are essential to the human body as a whole, especially in controlling the metabolic rate of all organs and systems. Impaired sensitivity to thyroid hormones describes any process that interferes with the effectiveness of thyroid hormones. The genetic origin of inherited thyroid hormone defects and the investigation of genetic defects upon the processing of thyroid hormones are of utmost importance. Impaired sensitivity to thyroid hormone can be categorized into three conditions: thyroid hormone cell membrane transport defect (THCMTD), thyroid hormone metabolism defect (THMD), and thyroid hormone action defect (THAD). THMD is caused by defects in the synthesis and processing of deiodinases that convert the prohormone thyroxine (T4) to the active hormone triiodothyronine (T3). Deiodinase, a selenoprotein, requires unique translation machinery that is collectively composed of the selenocysteine (Sec) insertion sequence (SECIS) elements, Sec-insertion sequence-binding protein 2 (SECISBP2), Sec-specific eukaryotic elongation factor (EEFSEC), and Sec-specific tRNA (TRU-TCA1-1), which leads to the recognition of the UGA codon as a Sec codon for translation into the growing polypeptide. In addition, THMD could be expanded to the defects of enzymes that are involved in thyroid hormone conjugation, such as glucuronidation and sulphation. Paucity of inherited disorders in this category leaves them beyond the scope of this review. This review attempts to specifically explore the genomic causes and effects that result in a significant deficiency of T3 hormones due to inadequate function of deiodinases. Moreover, along with SECISBP2, TRU-TCA1-1, and deiodinase type-1 (DIO1) mutations, this review describes the variants in DIO2 single nucleotide polymorphism (SNP) and thyroid stimulating hormone receptor (TSHR) that result in the reduced activity of DIO2 and subsequent abnormal conversion of T3 from T4. Finally, this review provides additional insight into the general functionality of selenium supplementation and T3/T4 combination treatment in patients with hypothyroidism, suggesting the steps that need to be taken in the future.

The Mysterious Universe of the TSH Receptor

The thyroid-stimulating hormone receptor (TSH-R) is predominantly expressed in the basolateral membrane of thyrocytes, where it stimulates almost every aspect of their metabolism. Several extrathyroidal locations of the receptor have been found including: the pituitary, the hypothalamus, and other areas of the central nervous system; the periorbital tissue; the skin; the kidney; the adrenal; the liver; the immune system cells; blood cells and vascular tissues; the adipose tissue; the cardiac and skeletal muscles, and the bone. Although the functionality of the receptor has been demonstrated in most of these tissues, its physiological importance is still a matter of debate. A contribution to several pathological processes is evident in some cases, as is the case of Grave's disease in its multiple presentations. Conversely, in the context of other thyroid abnormalities, the contribution of the TSH-R and its ligand is still a matter of debate. This article reviews the several different sites of expression of the TSH-R and its potential role in both physiological and pathological processes.

Obesity and Thyroid Axis

Development of obesity is primarily the result of imbalance between energy intake and energy expenditure. Thyroid hormones influence energy expenditure by regulating cellular respiration and thermogenesis and by determining resting metabolic rate. Triiodothyronine influences lipid turnover in adipocytes and impacts appetite regulation through the central nervous system, mainly the hypothalamus. Thyroid-stimulating hormone may also influence thermogenesis, suppress appetite and regulate lipid storage through lipolysis and lipogenesis control. Subclinical hypothyroidism may induce changes in basal metabolic rate with subsequent increase in BMI, but obesity can also affect thyroid function via several mechanisms such as lipotoxicity and changes in adipokines and inflammatory cytokine secretion. The present study investigated the complex and mutual relationships between the thyroid axis and adiposity.

Hungry Bone Syndrome Secondary to Subtotal Thyroidectomy in A Patient With Thyrotoxicosis

Patients with thyrotoxicosis are prone to transient hypocalcemia after thyroidectomy, which may be due in part to surgical damage to the parathyroid glands. Hungry bone syndrome (HBS) can also cause hypocalcemia after thyroidectomy. HBS is due to increased osteoblast-mediated bone formation activity and normal or decreased bone resorption activity. As HBS is uncommon in patients after thyroidectomy, we herein present a case of hypocalcemia secondary to HBS after subtotal thyroidectomy for thyrotoxicosis in a 25-year-old woman with a two-month history of tingling extremities and carpopedal spasms after subtotal thyroidectomy for thyrotoxicosis. Diagnostic tests showed hypocalcemia and hyperphosphatemia with elevated parathyroid hormone levels and moderately decreased serum 25-hydroxyvitamin D levels. In addition to thyroid hormone replacement therapy, she was given calcitriol and Caltrate D (600 mg calcium plus 125 IU cholecalciferol). After two months of treatment, she no longer had spasms and her paresthesia improved. Meanwhile, serum electrolytes and parathyroid hormone levels had almost returned to the normal ranges. This is a rare case of HBS presented as a complication of subtotal thyroidectomy in a patient with thyrotoxicosis.

TSH promotes adiposity by inhibiting the browning of white fat

Adiposity is caused by an imbalance between energy intake and consumption. Promotion of the browning of white fat increases energy expenditure and could combat adiposity. Thyroid-stimulating hormone (TSH) has been confirmed to positively correlate with adiposity. However, the putative connection between TSH and white adipose browning has never been explored. In this study, we sought to assess the effect of TSH on white adipose tissue browning and energy metabolism. Subclinical hypothyroidism mice, thyroid-specific Tshr-knockout mice injected with TSH, adipocyte-specific and global Tshr-knockout micewere subjected to morphological, physiological, genetic or protein expression analyses and metabolic cages to determine the role of TSH on the browning of white adipose tissue and metabolism. 3T3-L1 and primary SVF cells were used to verify the effects and mechanism of TSH on the browning of white adipocytes. We show that increased circulation TSH level decreases energy expenditure, promotes adiposity, impairs glucose and lipid metabolism. Knockout of Tshr decreases adiposity, increases energy expenditureand markedly promotes the development of beige adipocytesin both epididymal and inguinal subcutaneous white fat via a mechanism that likely involves AMPK/PRDM16/PGC1α. Our results reveal an important role of TSH in regulating energy balance and adiposity by inhibiting the browning of white fat.

The Molecular Function and Clinical Role of Thyroid Stimulating Hormone Receptor in Cancer Cells

The thyroid stimulating hormone (TSH) and its cognate receptor (TSHR) are of crucial importance for thyrocytes to proliferate and exert their functions. Although TSHR is predominantly expressed in thyrocytes, several studies have revealed that functional TSHR can also be detected in many extra-thyroid tissues, such as primary ovarian and hepatic tissues as well as their corresponding malignancies. Recent advances in cancer biology further raise the possibility of utilizing TSH and/or TSHR as a therapeutic target or as an informative index to predict treatment responses in cancer patients. The TSH/TSHR cascade has been considered a pivotal modulator for carcinogenesis and/or tumor progression in these cancers. TSHR belongs to a sub-group of family A G-protein-coupled receptors (GPCRs), which activate a bundle of well-defined signaling transduction pathways to enhance cell renewal in response to external stimuli. In this review, recent findings regarding the molecular basis of TSH/TSHR functions in either thyroid or extra-thyroid tissues and the potential of directly targeting TSHR as an anticancer strategy are summarized and discussed.

A compendium of G-protein-coupled receptors and cyclic nucleotide regulation of adipose tissue metabolism and energy expenditure

With the ever-increasing burden of obesity and Type 2 diabetes, it is generally acknowledged that there remains a need for developing new therapeutics. One potential mechanism to combat obesity is to raise energy expenditure via increasing the amount of uncoupled respiration from the mitochondria-rich brown and beige adipocytes. With the recent appreciation of thermogenic adipocytes in humans, much effort is being made to elucidate the signaling pathways that regulate the browning of adipose tissue. In this review, we focus on the ligand-receptor signaling pathways that influence the cyclic nucleotides, cAMP and cGMP, in adipocytes. We chose to focus on G-protein-coupled receptor (GPCR), guanylyl cyclase and phosphodiesterase regulation of adipocytes because they are the targets of a large proportion of all currently available therapeutics. Furthermore, there is a large overlap in their signaling pathways, as signaling events that raise cAMP or cGMP generally increase adipocyte lipolysis and cause changes that are commonly referred to as browning: increasing mitochondrial biogenesis, uncoupling protein 1 (UCP1) expression and respiration.

The Association of Obesity with Autoimmune Thyroiditis and Thyroid Function-Possible Mechanisms of Bilateral Interaction

A growing number of patients suffer from autoimmune diseases, including autoimmune thyroid disease. There has simultaneously been a significant increase in the prevalence of obesity worldwide. It is still an open question whether adiposity can directly influence activation of inflammatory processes affecting the thyroid in genetically predisposed individuals. Adipokines, biologically active substances derived from the adipocytes, belong to a heterogenic group of compounds involved in numerous physiological functions, including the maintenance of metabolism, hormonal balance, and immune response. Notably, the presence of obesity worsens the course of selected autoimmune diseases and impairs response to treatment. Moreover, the excess of body fat may result in the progression of autoimmune diseases. Nutritional status, body weight, and energy expenditure may influence thyroid hormone secretion. Interestingly, thyroid hormones might influence the activity of adipose tissue as metabolic alterations related to fat tissue are observed under pathological conditions in which there are deficits or overproduction of thyroid hormones. Functioning TSH receptors are expressed on adipocytes. Thermogenesis may presumably be stimulated by TSH binding to its receptor on brown adipocytes. There could be a bilateral interaction between the thyroid and adipose. Obesity may influence the onset and course of autoimmune disease.

Brown Adipose Tissue Response to Cold Stimulation Is Reduced in Girls With Autoimmune Hypothyroidism

Objective

The interaction between thyroid status and brown adipose tissue (BAT) activation is complex. We assessed the effect of autoimmune hypothyroidism (ATD) in female children on BAT activation, measured using infrared thermography.

Design

Twenty-six female participants (14 with ATD and 12 healthy controls) between 5 and 17 years of age attended a single study session. Thermal images were taken of the supraclavicular region before, and after, the introduction of a cool stimulus.

Results

Participants with ATD had lower resting (hypothyroid, 34.9 ± 0.7°C; control, 35.4 ± 0.5°C; P = 0.03) and stimulated (hypothyroid, 35.0 ± 0.6°C; control, 35.5 ± 0.5°C; P = 0.04) supraclavicular temperatures compared with controls, but there was no difference between groups in the temperature increase with stimulation. BAT activation, calculated as the relative temperature change comparing the supraclavicular temperature to a sternal reference region, was reduced in participants with ATD (hypothyroid, 0.1 ± 0.1°C; control, 0.2 ± 0.2°C; P = 0.04). Children with ATD were frequently biochemically euthyroid due to replacement therapy, but, despite this, increased relative supraclavicular temperature was closely associated with increased TSH (r = 0.7, P = 0.01) concentrations.

Conclusions

Girls with ATD had an attenuated thermogenic response to cold stimulation compared with healthy controls, but, contrary to expectation, those with suboptimal biochemical control (with higher TSH) showed increased BAT activation. This suggests that the underlying disease process may have a negative effect on BAT response, but high levels of TSH can mitigate, and even stimulate, BAT activity. In summary, thyroid status is a complex determinant of BAT activity in girls with ATD.

Resolution of Hypothyroidism Restores Cold-Induced Thermogenesis in Humans

BACKGROUND

Hypothyroidism is a frequent endocrine disorder with common symptoms of increased cold sensitivity and unintended weight gain, indicating changes in energy expenditure (EE) and response to cold exposure. Thyroid hormones (TH) play an important role for proper function of brown adipose tissue (BAT) and cold-induced thermogenesis (CIT) in rodents, but the role of hypothyroidism on CIT in humans is uncertain.

METHODS

This was a prospective observational study. Forty-two patients presenting with subclinical or overt hypothyroidism in whom TH replacement was planned were recruited. Thirty-three patients completed the study. Thermogenesis was measured by indirect calorimetry during warm conditions and after a mild cold stimulus of 90 minutes, both during the hypothyroid state and after at least three months of sufficient TH replacement. CIT was determined as the difference between EE during mildly cold and warm conditions. The primary endpoint was the change of CIT between the hypothyroid and euthyroid state.

RESULTS

EE during warm conditions increased from a median of 1330 (interquartile range [IQR] 1251-1433) kcal/24 hours in the hypothyroid state to a median of 1442 (IQR 1294-1579) kcal/24 hours in the euthyroid state (+8.5%; p = 0.0002). EE during mild cold exposure increased from 1399 (IQR 1346-1571) kcal/24 hours to 1610 (IQR 1455-1674) kcal/24 hours (+15%; p < 0.0001). The median CIT was 55 (IQR 1-128) kcal/24 hours at the baseline visit, after restoration of euthyroidism CIT increased by 102% to a median of 111 (IQR 15.5-200) kcal/24 hours (p = 0.011). Serum levels of free thyroxine at the respective visit and mean outdoor temperature during the preceeding 30 days were significantly associated with CIT (p = 0.021 and p = 0.001, respectively).

CONCLUSION

Restoring euthyroidism significantly increases CIT in hypothyroid humans.

Concurrent TSHR mutations and DIO2 T92A polymorphism result in abnormal thyroid hormone metabolism

Deiodinase 2 (DIO2) plays an important role in thyroid hormone metabolism and its regulation. However, molecular mechanism that regulates DIO2 activity remains unclear; only mutaions in selenocysteine insertion sequence binding protein 2 and selenocysteine tranfer RNA (tRNA[Ser]Sec) are reported to result in decreased DIO2 activity. Two patients with clinical evidence of abnormal thyroid hormone metabolism were identified and found to have TSHR mutations as well as DIO2 T92A single nucleotide polymorphism (SNP). Primary-cultured fibroblasts from one patient present a high level of basal DIO2 enzymatic activity, possibly due to compensation by augmented DIO2 expression. However, this high enzymatic active state yet fails to respond to accelerating TSH. Consequently, TSHR mutations along with DIO2 T92A SNP ("double hit") may lead to a significant reduction in DIO2 activity stimulated by TSH, and thereby may have clinical relevance in a select population of hypothyroidism patients who might benefit from a T3/T4 combination therapy.

Effects of intravenous thyrotropin-releasing hormone on 18F-fluorodeoxyglucose uptake in human brown adipose tissue: a randomized controlled trial

OBJECTIVE

Brown adipose tissue (BAT) activity in humans is stimulated by cold and by a limited number of pharmacological agents, including β3-adrenergic agonists and bile acids. Although thyrotropin-releasing hormone (TRH) is known to activate BAT in several mammals, this has not been reported in humans.

DESIGN

A randomized, placebo-controlled, double-blind, cross-over trial.

METHODS

We investigated the effects of intravenous bolus administration of 400 µg TRH or 2 mL saline on BAT activity in healthy, lean men. BAT activity was measured as standardized ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) uptake and glucose metabolic rate (MRglu) using dynamic PET/CT imaging. The first six individuals were studied at room temperature, while subsequently nine were exposed to mild cold (17°C ± 1°C) for 60 min before imaging. During the dynamic scan, blood was withdrawn for measurement of thyroid hormone and catecholamine concentrations. This trial is registered with The Netherlands National Trial Register (number NTR5512).

RESULTS

Sixteen participants were recruited. Six men studied at room temperature showed no visible BAT activity during either session. After exposure to mild cold, four of nine men (44.4%) showed clear increase of ¹⁸F-FDG uptake after TRH administration compared to placebo. Maximal standardized ¹⁸F-FDG uptake showed a trend toward increase after TRH compared to placebo (P = 0.066). MRglu showed a significant increase after TRH administration (P = 0.014). The increase in ¹⁸F-FDG uptake was not paralleled by changes in plasma thyroid hormone or catecholamine concentrations.

CONCLUSION

Systemic TRH administration can increase the activity of cold-stimulated BAT in adult men. These findings may assist developing pharmacological strategies for modulating BAT activity in the management of obesity.

Administration of Thyrotropin-Releasing Hormone in the Hypothalamic Paraventricular Nucleus of Male Rats Mimics the Metabolic Cold Defense Response

BACKGROUND

Cold exposure increases thyrotropin-releasing hormone (TRH) expression primarily in the hypothalamic paraventricular nucleus (PVN). The PVN is a well-known hypothalamic hub in the control of energy metabolism. TRH terminals and receptors are found on PVN neurons. We hypothesized that TRH release in the PVN plays an important role in the control of thermogenesis and energy mobilization during cold exposure.

METHODS

Male Wistar rats were exposed to a cold environment (4°C) or TRH retrodialysis in the PVN for 2 h. We compared the effects of cold exposure and TRH administration in the PVN on plasma glucose, corticosterone, and thyroid hormone concentrations, body temperature, locomotor activity, as well as metabolic gene expression in the liver and brown adipose tissue.

RESULTS

Cold exposure increased body temperature, locomotor activity, and plasma corticosterone concentrations, but blood glucose concentrations were similar to that of room temperature control animals. TRH administration in the PVN also promptly increased body temperature, locomotor activity and plasma corticosterone concentrations. However, TRH administration in the PVN markedly increased blood glucose concentrations and endogenous glucose production (EGP) compared to saline controls. Selective hepatic sympathetic or parasympathetic denervation reduced the TRH-induced increase in glucose concentrations and EGP. Gene expression data indicated increased gluconeogenesis in liver and lipolysis in brown adipose tissue, both after cold exposure and TRH administration.

CONCLUSIONS

We conclude that TRH administration in the rat PVN largely mimics the metabolic and behavioral changes induced by cold exposure indicating a potential link between TRH release in the PVN and cold defense.

Targeted Foxe1 Overexpression in Mouse Thyroid Causes the Development of Multinodular Goiter But Does Not Promote Carcinogenesis

Recent genome-wide association studies have identified several single nucleotide polymorphisms in the forkhead box E1 gene (FOXE1) locus, which are strongly associated with the risk for thyroid cancer. In addition, our recent work has demonstrated FOXE1 overexpression in papillary thyroid carcinomas. To assess possible contribution of Foxe1 to thyroid carcinogenesis, transgenic mice overexpressing Foxe1 in their thyroids under thyroglobulin promoter (Tg-Foxe1) were generated. Additionally, Tg-Foxe1 mice were exposed to x-rays at the age of 5 weeks or crossed with Pten(+/-) mice to examine the combined effect of Foxe1 overexpression with radiation or activated phosphatidylinositol-3-kinase/Akt pathway, respectively. In 5- to 8-week-old Tg-Foxe1 mice, severe hypothyroidism was observed, and mouse thyroids exhibited hypoplasia of the parenchyma. Adult 48-week-old mice were almost recovered from hypothyroidism, their thyroids were enlarged, and featured colloid microcysts and multiple benign nodules of macrofollicular-papilloid growth pattern, but no malignancy was found. Exposure of transgenic mice to 1 or 8 Gy of x-rays and Pten haploinsufficiency promoted hyperplastic nodule formation also without carcinogenic effect. These results indicate that Foxe1 overexpression is not directly involved in the development of thyroid cancer and that proper Foxe1 dosage is essential for achieving normal structure and function of the thyroid.

TSH effects on thermogenesis in rat brown adipocytes

TSH receptor (TSHR) is present in the thyroid and other tissues, as adipose tissue. In brown adipose tissue (BAT) TSH increases UCP1 expression and lipolysis. We have studied the regulation of Tshr mRNA expression and the effect of TSH on Ucp1 and Dio2 mRNA, on D2 activity and O2 consumption in rat brown adipocytes and the TSH signaling pathways. Tshr increased during brown adipocyte differentiation, was up-regulated by insulin and low TSH concentrations and down-regulated by high TSH concentrations, T3 and/or NE. TSH increased basal Ucp1 mRNA in a dose-dependent way acting synergistically with T3, while had no effect when NE was present. High TSH concentrations increased basal Dio2 mRNA (12-fold) and were synergistic with T3 (100-fold), but decreased Dio2 mRNA in T3+NE-treated cells. TSH increased D2 activities in T3-treated cells and inhibition of ERK pathway decreased the TSH effect by 55%. In T3+NE treated-cells TSH decreased D2 activity by 50%, in a dose-dependent manner. TSH activated Akt and Erk phosphorylation, while inhibition of PKA promoted Akt phosphorylation. TSH inhibited leptin mRNA. TSH increased O2 consumption by 20% and T3 enhanced its effect. Tshr is expressed in brown adipocytes and is regulated by insulin, TSH, T3 and NE. TSH increases basal and T3-stimulated Ucp1 and Dio2 expression and D2 activity only when T3 is present, but decreases Dio2 mRNA and D2 activity stimulated by NE+T3. TSH increases O2 consumption, confirming the role of TSH in the maintenance of thermogenesis.

Transcriptome profiling of brown adipose tissue during cold exposure reveals extensive regulation of glucose metabolism

We applied digital gene expression profiling to determine the transcriptome of brown and white adipose tissues (BAT and WAT, respectively) during cold exposure. Male C57BL/6J mice were exposed to cold for 2 or 4 days. A notable induction of genes related to glucose uptake, glycolysis, glycogen metabolism, and the pentose phosphate pathway was observed in BAT from cold-exposed animals. In addition, glycerol-3-phosphate dehydrogenase 1 expression was induced in BAT from cold-challenged mice, suggesting increased synthesis of glycerol from glucose. Similarly, expression of lactate dehydrogenases was induced by cold in BAT. Pyruvate dehydrogenase kinase 2 (Pdk2) and Pdk4 were expressed at significantly higher levels in BAT than in WAT, and Pdk2 was induced in BAT by cold. Of notice, only a subset of the changes detected in BAT was observed in WAT. Based on changes in gene expression during cold exposure, we propose a model for the intermediary glucose metabolism in activated BAT: 1) fluxes through glycolysis and the pentose phosphate pathway are induced, the latter providing reducing equivalents for de novo fatty acid synthesis; 2) glycerol synthesis from glucose is increased, facilitating triacylglycerol synthesis/fatty acid re-esterification; 3) glycogen turnover and lactate production are increased; and 4) entry of glucose carbon into the tricarboxylic acid cycle is restricted by PDK2 and PDK4. In summary, our results demonstrate extensive and diverse gene expression changes related to glucose handling in activated BAT.

Homeostatic Control of the Thyroid-Pituitary Axis: Perspectives for Diagnosis and Treatment

The long-held concept of a proportional negative feedback control between the thyroid and pituitary glands requires reconsideration in the light of more recent studies. Homeostatic equilibria depend on dynamic inter-relationships between thyroid hormones and pituitary thyrotropin (TSH). They display a high degree of individuality, thyroid-state-related hierarchy, and adaptive conditionality. Molecular mechanisms involve multiple feedback loops on several levels of organization, different time scales, and varying conditions of their optimum operation, including a proposed feedforward motif. This supports the concept of a dampened response and multistep regulation, making the interactions between TSH, FT4, and FT3 situational and mathematically more complex. As a homeostatically integrated parameter, TSH becomes neither normatively fixed nor a precise marker of euthyroidism. This is exemplified by the therapeutic situation with l-thyroxine (l-T4) where TSH levels defined for optimum health may not apply equivalently during treatment. In particular, an FT3-FT4 dissociation, discernible FT3-TSH disjoint, and conversion inefficiency have been recognized in l-T4-treated athyreotic patients. In addition to regulating T4 production, TSH appears to play an essential role in maintaining T3 homeostasis by directly controlling deiodinase activity. While still allowing for tissue-specific variation, this questions the currently assumed independence of the local T3 supply. Rather it integrates peripheral and central elements into an overarching control system. On l-T4 treatment, altered equilibria have been shown to give rise to lower circulating FT3 concentrations in the presence of normal serum TSH. While data on T3 in tissues are largely lacking in humans, rodent models suggest that the disequilibria may reflect widespread T3 deficiencies at the tissue level in various organs. As a consequence, the use of TSH, valuable though it is in many situations, should be scaled back to a supporting role that is more representative of its conditional interplay with peripheral thyroid hormones. This reopens the debate on the measurement of free thyroid hormones and encourages the identification of suitable biomarkers. Homeostatic principles conjoin all thyroid parameters into an adaptive context, demanding a more flexible interpretation in the accurate diagnosis and treatment of thyroid dysfunction.

Insights into Brown Adipose Tissue Physiology as Revealed by Imaging Studies

There has been resurgence in interest in brown adipose tissue (BAT) following radiological and histological identification of metabolically active BAT in adult humans. Imaging enables BAT to be studied non-invasively and therefore imaging studies have contributed a significant amount to what is known about BAT function in humans. In this review the current knowledge (derived from imaging studies) about the prevalence, function, activity and regulation of BAT in humans (as well as relevant rodent studies), will be summarized.

An acute injection of corticosterone increases thyrotrophin-releasing hormone expression in the paraventricular nucleus of the hypothalamus but interferes with the rapid hypothalamus pituitary thyroid axis response to cold in male rats

The activity of the hypothalamic-pituitary-thyroid (HPT) axis is rapidly adjusted by energy balance alterations. Glucocorticoids can interfere with this activity, although the timing of this interaction is unknown. In vitro studies indicate that, albeit incubation with either glucocorticoid receptor (GR) agonists or protein kinase A (PKA) activators enhances pro-thyrotrophin-releasing hormone (pro-TRH) transcription, co-incubation with both stimuli reduces this enhancement. In the present study, we used primary cultures of hypothalamic cells to test whether the order of these stimuli alters the cross-talk. We observed that a simultaneous or 1-h prior (but not later) activation of GR is necessary to inhibit the stimulatory effect of PKA activation on pro-TRH expression. We tested these in vitro results in the context of a physiological stimulus on the HPT axis in adult male rats. Cold exposure for 1 h enhanced pro-TRH mRNA expression in neurones of the hypophysiotrophic and rostral subdivisions of the paraventricular nucleus (PVN) of the hypothalamus, thyrotrophin (TSH) serum levels and deiodinase 2 (D2) activity in brown adipose tissue (BAT). An i.p. injection of corticosterone stimulated pro-TRH expression in the PVN of rats kept at ambient temperature, more pronouncedly in hypophysiotrophic neurones that no longer responded to cold exposure. In corticosterone-pretreated rats, the cold-induced increase in pro-TRH expression was detected only in the rostral PVN. Corticosterone blunted the increase in serum TSH levels and D2 activity in BAT produced by cold in vehicle-injected animals. Thus, increased serum corticosterone levels rapidly restrain cold stress-induced activation of TRH hypophysiotrophic neurones, which may contribute to changing energy expenditure. Interestingly, TRH neurones of the rostral PVN responded to both corticosterone and cold exposure with an amplified expression of pro-TRH mRNA, suggesting that these neurones integrate stress and temperature distinctly from the hypophysiotrophic neurones.

Thyrostimulin deficiency does not alter peripheral responses to acute inflammation-induced nonthyroidal illness

Thyrostimulin, a putative glycoprotein hormone, comprises the subunits GPA2 and GPB5 and activates the TSH receptor (TSHR). The observation that proinflammatory cytokines stimulate GPB5 transcription suggested a role for thyrostimulin in the pathogenesis of nonthyroidal illness syndrome (NTIS). In the present study, we induced acute inflammation by LPS administration to GPB5(-/-) and WT mice to evaluate the role of thyrostimulin in peripheral thyroid hormone metabolism during NTIS. In addition to serum thyroid hormone concentrations, we studied mRNA expression and activity of deiodinase types I, II, and III (D1, D2, and D3) in peripheral T3 target tissues, including liver, muscle, and white and brown adipose tissue (WAT and BAT), of which the latter three express the TSHR. LPS decreased serum free (f)T4 and fT3 indexes to a similar extent in GPB5(-/-) and WT mice. Serum reverse (r)T3 did not change following LPS administration. LPS also induced significant alterations in tissue D1, D2, and D3 mRNA and activity levels, but only the LPS-induced increase in WAT D2 mRNA expression differed between GPB5(-/-) and WT mice. In conclusion, lacking GPB5 during acute illness does not affect the LPS-induced decrease of serum thyroid hormones while resulting in subtle changes in tissue D2 expression that are unlikely to be mediated via the TSHR.

Presence of brown adipose tissue in an adolescent with severe primary hypothyroidism

CONTEXT

Brown adipose tissue (BAT) generates heat during adaptive thermogenesis in response to cold temperature. Thyroid hormone (TH) receptors, type 2 deiodinase, and TSH receptors are present on brown adipocytes, indicating that the thyroid axis regulates BAT. It is unknown whether absent TH in humans would down-regulate development of BAT and its thermogenic function.

OBJECTIVE

The objective of the study was to examine BAT by magnetic resonance imaging (MRI) and infrared thermal imaging (IRT) in a pediatric patient with severe primary hypothyroidism before and after TH treatment.

DESIGN/SETTING

This study was a case report with longitudinal follow-up in a tertiary center.

MAIN OUTCOME MEASURES

BAT fat fraction (FF) by MRI and skin temperature by IRT were measured.

RESULTS

An 11.5-year-old female was severely hypothyroid (TSH, 989 μIU/mL; free T4, 0.10 ng/dL; low thyroglobulin, 3.0 ng/mL). Low MRI measures of FF (56.1% ± 3.7%) indicated that BAT was abundantly present in the supraclavicular fossa. IRT showed higher supraclavicular temperature (36.0°C ±0.16°C) than the suprasternal area (34.3°C ± 0.19°C). After 2 months of TH replacement, she was euthyroid (TSH, 4.3 μIU/mL; free T4, 1.49 ng/dL; T3, 102 ng/dL) at which time supraclavicular BAT decreased (increased FF 60.7% ± 3.8%). IRT showed a higher, more homogeneous skin temperature throughout the upper thorax (supraclavicular, 37.1°C ± 0.23°C; suprasternal, 36.4°C ± 0.13°C). The overall size of the supraclavicular fat depot decreased from 84.79 cm(3) to 41.21 cm(3).

CONCLUSIONS

These findings document the presence of BAT and thermogenesis in profound hypothyroidism and suggest a role for TSH and/or TRH as a potential regulator of BAT.

Effects of recombinant human thyrotropin administration on 24-hour arterial pressure in female undergoing evaluation for differentiated thyroid cancer

Objective. Thyroid-stimulating-hormone (TSH) receptors are expressed in endothelial cells. We investigated whether elevated TSH levels after acute recombinant TSH (rhTSH) administration may result in alterations in blood pressure (BP) in premenopausal women with well-differentiated thyroid carcinoma (DTC). Designs. Thirty euthyroid DTC female patients were evaluated by rhTSH stimulation test (mean age 40.4 ± 8.6 years). A 24 h ambulatory systolic and diastolic blood pressure (SBP, DBP) monitoring (24 hr ABPM) was performed on days 2-3(D2-3). TSH was measured on day 1(D1), day 3(D3), and day 5(D5). Central blood pressure was evaluated on D3. Twenty-three patients were studied 1-4 weeks earlier (basal measurements). Results. TSH levels were D1: median 0.2 mU/L, D3: median 115.0 mU/L, and D5: median 14.6 mU/L. There were no significant associations between TSH on D1 and D3 and any BP measurements. Median D5 office-SBP and 24 h SBP, DBP, and central SBP were correlated with D5-TSH (P < 0.04). In those where a basal 24 h ABPM had been performed median pulse pressure was higher after rhTSH-test (P = 0.02). Conclusions. TSH, when acutely elevated, may slightly increase SBP, DBP, and central SBP. This agrees with previous reports showing positive associations of BP with TSH.

TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis

This paper provides the reader with an overview of our current knowledge of hypothalamic-pituitary-thyroid feedback from a cybernetic standpoint. Over the past decades we have gained a plethora of information from biochemical, clinical, and epidemiological investigation, especially on the role of TSH and other thyrotropic agonists as critical components of this complex relationship. Integrating these data into a systems perspective delivers new insights into static and dynamic behaviour of thyroid homeostasis. Explicit usage of this information with mathematical methods promises to deliver a better understanding of thyrotropic feedback control and new options for personalised diagnosis of thyroid dysfunction and targeted therapy, also by permitting a new perspective on the conundrum of the TSH reference range.

Supraclavicular swelling in hypothyroidism

Hypothyroidism can have a variety of presentations. We report here a case of acquired hypothyroidism in a pediatric patient who first presented with bilateral supraclavicular swelling. Hypothyroidism and its presenting signs and symptoms are discussed with a focus on the less common findings that can be associated with hypothyroidism in children.

Extrathyroidal expression of TSH receptor

The TSH receptor expressed on the cell surface of thyroid follicular cells plays a pivotal role in the regulation of thyroid status and growth of the thyroid gland. In recent years it has become evident that the TSH receptor is also expressed widely in a variety of extrathyroidal tissues including: anterior pituitary; hypothalamus; ovary; testis; skin; kidney; immune system; bone marrow and peripheral blood cells; white and brown adipose tissue; orbital preadipocyte fibroblasts and bone. A large body of evidence is emerging to describe the functional roles of the TSH receptor at these various sites but their physiological importance in many cases remains a subject of controversy and much interest. Current understanding of the actions of the TSH receptor in extrathyroidal tissues and their possible physiological implications is discussed.

Neuroendocrine mechanism of seasonal reproduction in birds and mammals

In temperate zones, animals use changes in day length as a calendar to time their breeding season. However, the photoreceptive and neuroendocrine mechanisms of seasonal reproduction are considered to differ markedly between birds and mammals. This can be understood from the fact that the eye is the only photoreceptive organ, and melatonin mediates the photoperiodic information in mammals, whereas in birds, photoperiodic information is directly received by the deep brain photoreceptors and melatonin is not involved in seasonal reproduction. Recent molecular and functional genomics analysis uncovered the gene cascade regulating seasonal reproduction in birds and mammals. Long day-induced thyroid stimulating hormone in the pars tuberalis of the pituitary gland regulates thyroid hormone catabolism within the mediobasal hypothalamus. Further, this local thyroid hormone catabolism appears to regulate seasonal gonadotropin-releasing hormone secretion. These findings suggest that although the light input pathway is different between birds and mammals (i.e. light or melatonin), the core mechanisms are conserved in these vertebrates.

Type 2 iodothyronine deiodinase is highly expressed in medullary thyroid carcinoma

Type II deiodinase (D2) plays a critical role in controlling intracellular T3 concentration and early studies indicated a follicular but not a parafollicular C-cell origin of D2 activity in the thyroid gland. Here, we show that D2 is highly expressed in human medullary thyroid carcinoma (MTC), a tumor that arises from the C-cells. D2 transcripts were detected in all MTC samples obtained from 12 unselected MTC patients and the levels of D2 activity were comparable to those found in surrounding normal follicular tissue (0.41+/-0.10 fmol min mg protein vs. 0.43+/-0.41 fmol min mg protein, P=0.91). Additional analysis in the TT cells, a human MTC cell line, demonstrated that the D2 expression is downregulated by thyroid hormones and enhanced by cAMP analogs and dexamethasone. The thyroid hormone receptor alpha1 and beta isoforms were also detected in all MTC samples and in TT cells, thus suggesting a potential role of T3 locally produced by D2 in this neoplastic tissue.

High serum bile acids cause hyperthyroidism and goiter

BACKGROUND/AIMS

In the duodenal content reflux model of rats, we noted an elevation of serum bile acid and swelling of the thyroid gland. This study was designed to elucidate whether bile acids (BAs) also enhance thyroid function.

METHODS

In varying lengths of period after esophago-jejunostomy without gastrectomy, which causes duodenal content reflux, rats were sacrificed and blood samples were taken from the heart for analyses of BAs and triiodothyronine (T3), thyroxine (T4), free T3 (fT3), free T4 (fT4), and thyroid-stimulating hormone (TSH) in the serum.

RESULTS

Macroscopically, at 10 and 30 weeks after operation, thyroid glands in the reflux model showed a symmetric enlargement because of the presence of diffuse hypertrophy of the thyroid follicular epithelium. At both time points, no significant differences were detected in T3, T4, fT3, and fT4 levels between the reflux model and the control group, whereas, at 10 weeks after operation, the animals with the reflux showed significantly lower serum TSH levels and greater thyroid weight than those in the control group. An inverse correlation between serum BAs and TSH levels was noted in the reflux model but not in the control group. Microscopically, thyroid follicles were greater in size and number, with paler colloids in the reflux model than the control group.

CONCLUSIONS

The present results suggest that high serum BAs cause hyperplasia of the thyroid follicles and the reduction of TSH. The effects of BAs on thyroid hormones, thus, include the induction of overall hyperthyroidism. Therefore, the strict monitoring of serum TSH levels is of vital importance if BAs are used for the treatment of obesity.

Thyrotrophin in the pars tuberalis triggers photoperiodic response

Molecular mechanisms regulating animal seasonal breeding in response to changing photoperiod are not well understood. Rapid induction of gene expression of thyroid-hormone-activating enzyme (type 2 deiodinase, DIO2) in the mediobasal hypothalamus (MBH) of the Japanese quail (Coturnix japonica) is the earliest event yet recorded in the photoperiodic signal transduction pathway. Here we show cascades of gene expression in the quail MBH associated with the initiation of photoinduced secretion of luteinizing hormone. We identified two waves of gene expression. The first was initiated about 14 h after dawn of the first long day and included increased thyrotrophin (TSH) beta-subunit expression in the pars tuberalis; the second occurred approximately 4 h later and included increased expression of DIO2. Intracerebroventricular (ICV) administration of TSH to short-day quail stimulated gonadal growth and expression of DIO2 which was shown to be mediated through a TSH receptor-cyclic AMP (cAMP) signalling pathway. Increased TSH in the pars tuberalis therefore seems to trigger long-day photoinduced seasonal breeding.

Synchronized changes in transcript levels of genes activating cold exposure-induced thermogenesis in brown adipose tissue of experimental animals

To identify genes whose expression in brown adipose tissue (BAT) is up- or down-regulated in cold-exposed rats, we performed microarray analysis of RNA samples prepared from the BAT of cold-exposed rats and of rats kept at room temperature. Previously reported elevations of transcript levels of uncoupling protein (UCP1), type II iodothyronine deiodinase (DIO2), and type III adenylate cyclase (AC3) in the BAT of cold-exposed rats over those in that of rats maintained at room temperature were confirmed. In addition to these changes, remarkable elevations of the transcript levels of several genes that seemed to be associated with the processes of cell-cycle regulation and DNA replication were detected in the BAT of cold-exposed rats, possibly reflecting the significant proliferation of brown adipocytes in response to cold exposure. Up-regulation of the gene encoding sarcomeric mitochondrial type creatine kinase in the BAT of cold-exposed rats was also detected by microarray analysis, but subsequent Northern analysis revealed that the expression of not only the sarcomeric mitochondrial type enzyme, but also that of 2 other subtypes, i.e., cytoplasmic brain type and cytoplasmic muscle type, was elevated in the BAT of cold-exposed rats. Microarray analysis also revealed a significant expression of myoglobin in BAT and its elevation in the BAT of cold-exposed rats, and this result was supported by calibrated Northern analysis. On the contrary, several genes such as regulator of G-protein signaling 2 and IMP dehydrogenase 1 were down-regulated in the BAT of cold-exposed rats. The physiological meaning of these changes accompanying cold exposure was discussed.

The Asp727Glu polymorphism in the TSH receptor is associated with insulin resistance in healthy elderly men

BACKGROUND

Variations in thyroid function within the normal range are associated with differences in metabolism and body composition. For instance, TSH is positively associated with body mass index (BMI). This could be due to alterations in thyroid hormone activity, or to direct effects of TSH, as the TSH receptor (TSHR) is also expressed in adipose tissue. The TSHR-Asp727Glu polymorphism is associated with lower serum TSH levels in vivo. In this study, we analysed whether serum thyroid parameters and the TSHR-Asp727Glu polymorphism were associated with glucose metabolism and insulin resistance. In addition, we analysed the Thr92Ala polymorphism in the type 2 deiodinase (D2), which was recently associated with insulin resistance.

METHODS

Genotypes were determined in a population of 349 elderly men (age 77.7 +/- 3.5 years), for whom serum thyroid parameters and data on insulin resistance, such as fasting blood glucose, serum insulin and homeostasis model assessment (HOMA) values, were available.

RESULTS

In nondiabetic, euthyroid subjects, TSH was positively associated with leptin levels, whereas FT4 and rT3 were significantly negatively correlated with insulin and HOMA. Carriers of the TSHR-Glu727 allele had a significantly higher glucose (P = 0.01), insulin (P = 0.001), glycated haemoglobin (HbA1c) (P = 0.002), HOMA (P = 0.001) and leptin (P = 0.008). The D2-Ala(92) allele showed a trend towards higher levels of insulin (P = 0.07) and a higher HOMA (P = 0.09).

CONCLUSION

In this population of nondiabetic elderly men, serum thyroid parameters and the TSHR-Asp727Glu polymorphism were associated with relative insulin resistance. Our study suggests that genetic variation in TSHR plays a role in insulin resistance and thereby influences glucose metabolism.

Recombinant thyrotropin in the diagnosis of congenital hypothyroidism

CONTEXT

A modern approach to congenital hypothyroidism requires a definitive diagnosis of the underlying mechanisms; this can be achieved within the first weeks of life. When uncertainty persists, treatment is commenced, and the definitive diagnosis of congenital hypothyroidism is deferred to the age of 3 yr.

OBJECTIVES

The interruption of thyroid replacement treatment is perceived as risky by parents and physicians. The aim of this pilot study was to test the possibility of a definitive diagnosis during thyroid replacement treatment, using stimulation of thyroid tissue by recombinant human (rh)TSH.

SUBJECTS

Eight patients, three boys and five girls, age 5-15 yr (mean, 9.5+/-3.7 yr), with congenital hypothyroidism that had been diagnosed by the neonatal screening program, and having their diagnosis verified between the ages of 3-4 yr, were reevaluated while on thyroid replacement therapy.

INTERVENTIONS

Patients received im 0.6 mg/m2 rhTSH on two consecutive days.

RESULTS

rhTSH pharmacokinetics, maximal concentration, t1/2, and area under the curve in children were different as compared with adults. In the patients with intact TSH receptors, free T4 levels decreased after the first and the second injection of rhTSH (P=0.0137 and P=0.0149, respectively). All eight children showed identical scintigraphy after rhTSH administration as compared with thyroid replacement withdrawal.

CONCLUSIONS

The use of rhTSH is effective for definitive diagnosis of congenital hypothyroidism during thyroid replacement treatment, and no safety issues were encountered.

Change in Ucp1 mRNA Expression Following Long-Term Cold Exposure under Normal or High-Fat Diet Regimes in the Cold-Intolerant Mammal, Suncus murinus

The house musk shrew (Suncus murinus), or suncus, is a unique experimental mammal that is cold intolerant. However, even basic knowledge of brown adipose tissue (BAT), which is important for non-shivering thermogenesis (NST), is minimal. Therefore, we exposed suncus for 18 days to mild cold temperatures (8-14 degrees C) and/or a high-fat diet, which are factors that increase NST, and measured two mRNAs that are critical for NST in BAT, uncoupling protein 1 (Ucp1) and type II 5'-deiodinase (D2). Neither mild cold exposure nor a high-fat diet alone induced up-regulation of the mRNAs. However, combinations of cold exposure and high-fat diet significantly increased both mRNAs. Therefore, cold intolerance in suncus may be partly caused by dietary components.

The clinicopathologic basis of Graves' ophthalmopathy: a review

PURPOSE

Graves' ophthalmopathy (GO) is a controversial disease, with disagreement within the medical community regarding its pathogenesis, diagnosis, and treatment.

METHODS

We reviewed recent literature on clinical and pathological aspects of GO from both the endocrinologist's and ophthalmologist's perspective.

RESULTS

Investigations into the pathogenesis of GO have included possible antigenic targets, orbital cell types, and development of animal models. Diagnosis has been improved recently with new tools and grading systems, but can be complicated by conditions that may simulate one or more of the findings of GO. The new findings of clinical studies also compel practitioners to reassess commonly used GO treatments such as orbital irradiation.

CONCLUSIONS

Improved understanding of the pathogenic mechanisms of GO should hopefully lead to new diagnostic and therapeutic approaches to this problematic condition.

Expression of type 2 iodothyronine deiodinase in human osteoblast is stimulated by thyrotropin

Thyroid hormones play important roles in bone growth, development, and turnover. To exert its biological activity, T(4) needs to be converted to T(3) by iodothyronine deiodinase. In human thyroid gland as well as rat brown adipose tissue, type 2 iodothyronine deiodinase (D2) expression is regulated by a TSH receptor-cAMP-mediated mechanism. TSH receptor knockout mice demonstrated the direct effects of TSH on bone via TSH receptors found on osteoblast and osteoclast precursors. In the present study we investigated the possible expression and function of iodothyronine deiodinase and TSH receptors in human osteoblast-like osteosarcoma (SaOS-2) cells and normal human osteoblast (NHOst) cells. Iodothyronine deiodinase activity was detected in SaOS-2 cells and NHOst cells, and all of the characteristics of deiodinating activity were compatible with those of D2. Northern analysis demonstrated D2 mRNA expression in SaOS-2 cells and NHOst cells. D2 mRNA levels as well as D2 activities were rapidly increased by dibutyryl cAMP or forskolin in SaOS-2 cells and NHOst cells. TSH receptor mRNA was demonstrated in SaOS-2 cells and NHOst cells, and D2 mRNA and D2 activity were stimulated by TSH in both cells. In addition, all T(3) receptor isoforms were detected by RT-PCR in SaOS-2 cells and NHOst cells. The present results indicate the expression of functional TSH receptors and D2 in human osteoblasts and suggest previously unrecognized roles of TSH receptors and local T(3) production by D2 in the pathophysiology of human osteoblasts.