Type 2 iodothyronine selenodeiodinase is expressed throughout the mouse skeleton and in the MC3T3-E1 mouse osteoblastic cell line during differentiation

Sustained Pituitary T3 Production Explains the T4-mediated TSH Feedback Mechanism

The regulation of thyroid activity and thyroid hormone (TH) secretion is based on feedback mechanisms that involve the anterior pituitary TSH and medial basal hypothalamus TSH-releasing hormone. Plasma T3 levels can be "sensed" directly by the anterior pituitary and medial basal hypothalamus; plasma T4 levels require local conversion of T4 to T3, which is mediated by the type 2 deiodinase (D2). To study D2-mediated T4 to T3 conversion and T3 production in the anterior pituitary gland, we used mouse pituitary explants incubated with 125I-T4 for 48 hours to measure T3 production at different concentrations of free T4. The results were compared with cultures of D1- or D2-expressing cells, as well as freshly isolated mouse tissue. These studies revealed a unique regulation of the D2 pathway in the anterior pituitary gland, distinct from that observed in nonpituitary tissues. In the anterior pituitary, increasing T4 levels reduced D2 activity slightly but caused a direct increase in T3 production. However, the same changes in T4 levels decreased T3 production in human HSkM cells and murine C2C12 cells (both skeletal muscle) and mouse bone marrow tissue, which reached zero at 50 pM free T4. In contrast, the increase in T4 levels caused the pig kidney LLC-PK1 cells and kidney fragments to proportionally increase T3 production. These findings have important implications for both physiology and clinical practice because they clarify the mechanism by which fluctuations in plasma T4 levels are transduced in the anterior pituitary gland to mediate the TSH feedback mechanism.

How does Hashimoto's thyroiditis affect bone metabolism?

Bone marrow contains resident cellular components that are not only involved in bone maintenance but also regulate hematopoiesis and immune responses. The immune system and bone interact with each other, coined osteoimmunology. Hashimoto's thyroiditis (HT) is one of the most common chronic autoimmune diseases which is accompanied by lymphocytic infiltration. It shows elevating thyroid autoantibody levels at an early stage and progresses to thyroid dysfunction ultimately. Different effects exert on bone metabolism during different phases of HT. In this review, we summarized the mechanisms of the long-term effects of HT on bone and the relationship between thyroid autoimmunity and osteoimmunology. For patients with HT, the bone is affected not only by thyroid function and the value of TSH, but also by the setting of the autoimmune background. The autoimmune background implies a breakdown of the mechanisms that control self-reactive system, featuring abnormal immune activation and presence of autoantibodies. The etiology of thyroid autoimmunity and osteoimmunology is complex and involves a number of immune cells, cytokines and chemokines, which regulate the pathogenesis of HT and osteoporosis at the same time, and have potential to affect each other. In addition, vitamin D works as a potent immunomodulator to influence both thyroid immunity and osteoimmunology. We conclude that HT affects bone metabolism at least through endocrine and immune pathways.

Vitamin D, Thyroid Hormones and Cardiovascular Risk: Exploring the Components of This Novel Disease Triangle

The role of thyroid hormones (THs) in the cardiovascular (CV) system, through several direct and indirect effects is recognized. Even very small modification in TH levels (as those observed in subclinical hypothyroidism or hyperthyroidism, and low triiodothyronine syndrome) may adversely affect the CV system, whereas thyroid hormones benefit the CV system and improve the prognosis. There is also evidence of vitamin D effects on cardiometabolic disease (e.g., through modulation of endothelial and smooth muscle cell activity, renin-angiotensin-aldosterone system, nitric oxide, oxidative stress, and inflammatory response), as well as an association between vitamin D [25(OH)D] deficiency and autoimmune thyroid diseases or cancer, and a relationship between vitamin D concentration and titers of antibodies and thyroid autoimmunity replacement. Interestingly, experimental data indicate a direct effect of vitamin D on Type 2 deiodinase expression causing subsequential peripheral conversion of T4 into T3. However, the functional links among THs, vitamin D and the cardiovascular system, and clinical effects of coexisting abnormalities in this new troublesome triad, have not yet been reviewed. The main aim of this review is to discuss pathophysiology of this relationship, proposing new mechanistic insights involving vitamin D in the modulation of cardiometabolic disease and thyroid profile.

Selenium in Endocrinology-Selenoprotein-Related Diseases, Population Studies, and Epidemiological Evidence

Selenium (Se), apart from iodine, iron, and calcium, is one of the nutrient-derived key elements strongly affecting the endocrine system. However, no specific hormonal "feedback" regulation for Se status has yet been identified, in contrast to the fine-tuned hormone network regulating Ca2+ and phosphate balance or hepcidin-related iron status. Since its discovery as an essential trace element, the effects of Se excess or deficiency on the endocrine system or components of the hypothalamic-pituitary-periphery feedback circuits, the thyroid hormone axis, glucoregulatory and adrenal hormones, male and female gonads, the musculoskeletal apparatus, and skin have been identified. Analysis of the Se status in the blood or via validated biomarkers such as the hepatically derived selenoprotein P provides valuable diagnostic insight and a rational basis for decision making on required therapeutic or preventive supplementation of risk groups or patients. Endocrine-related epidemiological and interventional evidence linking Se status to beneficial or potentially adverse actions of selected selenoproteins mediating most of the (patho-) physiological effects are discussed in this mini-review. Autoimmune thyroid disease, diabetes and obesity, male fertility, as well as osteoporosis are examples for which observational or interventional studies have indicated Se effects. The currently prevailing concept relating Se and selenoproteins to "oxidative stress," reactive oxygen species, radical hypotheses, and related strategies of pharmacological approaches based on various selenium compounds will not be the focus. The crucial biological function of several selenoproteins in cellular redox-regulation and specific enzyme reactions in endocrine pathways will be addressed and put in clinical perspective.

Paradigms of Dynamic Control of Thyroid Hormone Signaling

Thyroid hormone (TH) molecules enter cells via membrane transporters and, depending on the cell type, can be activated (i.e., T4 to T3 conversion) or inactivated (i.e., T3 to 3,3'-diiodo-l-thyronine or T4 to reverse T3 conversion). These reactions are catalyzed by the deiodinases. The biologically active hormone, T3, eventually binds to intracellular TH receptors (TRs), TRα and TRβ, and initiate TH signaling, that is, regulation of target genes and other metabolic pathways. At least three families of transmembrane transporters, MCT, OATP, and LAT, facilitate the entry of TH into cells, which follow the gradient of free hormone between the extracellular fluid and the cytoplasm. Inactivation or marked downregulation of TH transporters can dampen TH signaling. At the same time, dynamic modifications in the expression or activity of TRs and transcriptional coregulators can affect positively or negatively the intensity of TH signaling. However, the deiodinases are the element that provides greatest amplitude in dynamic control of TH signaling. Cells that express the activating deiodinase DIO2 can rapidly enhance TH signaling due to intracellular buildup of T3. In contrast, TH signaling is dampened in cells that express the inactivating deiodinase DIO3. This explains how THs can regulate pathways in development, metabolism, and growth, despite rather stable levels in the circulation. As a consequence, TH signaling is unique for each cell (tissue or organ), depending on circulating TH levels and on the exclusive blend of transporters, deiodinases, and TRs present in each cell. In this review we explore the key mechanisms underlying customization of TH signaling during development, in health and in disease states.

Analysis of Physiological Responses to Thyroid Hormones and Their Receptors in Bone

Thyroid hormone has profound effects on skeletal development and adult bone maintenance. Here, we review the current literature concerning thyroid hormone action in bone and cartilage in relation to human disease and animal models. We describe state-of-the-art imaging and biomechanical methods used to determine structural and functional parameters in the skeletal phenotyping of mouse models.

Abnormal Thyroid Hormone Status Differentially Affects Bone Mass Accrual and Bone Strength in C3H/HeJ Mice: A Mouse Model of Type I Deiodinase Deficiency

C3H/HeJ (C3H) mice are deficient of type I deiodinase (D1), an enzyme that activates thyroid hormone (TH), converting thyroxine (T4) to triiodothyronine (T3). Nevertheless, C3H mice present normal serum T3 and a gross euthyroid phenotype. To investigate if a global D1 deficiency interferes in the TH effects on bone, we compared bone growth, bone mass accrual and bone strength of C3H and C57BL/6J (B6) mice under abnormal TH status. Four-week-old female mice of both strains were grouped as Euthyroid, Hypothyroid (pharmacologically-induced), 1xT4 and 10xT4 (hypothyroid animals receiving 1- or 10-fold the physiological dose of T4 /day/16 weeks). Hypothyroidism and TH excess similarly impaired body weight (BW) gain and body growth in both mice strains. In contrast, whereas hypothyroidism only slightly impaired bone mineral density (BMD) accrual in B6 mice, it severely impaired BMD accrual in C3H mice. No differences were observed in serum and bone concentrations of T3 between hypothyroid animals of both strains. Interestingly, treatment with 10xT4 was less deleterious to BMD accrual in C3H than in B6 mice and resulted in less elevated T3 serum levels in B6 than in C3H mice, which is probably explained by the lower D1 activity in C3H mice. In addition, hypothyroidism decreased bone strength only in C3H but not in B6 mice, while TH excess decreased this parameter in both strains. These findings indicate that D1 deficiency contributes to the TH excess-induced differences in bone mass accrual in C3H vs. B6 mice and suggest that deiodinase-unrelated genetic factors might account for the different skeleton responses to hypothyroidism between strains.

Current concepts and challenges to unravel the role of iodothyronine deiodinases in human neoplasias

Thyroid hormones (THs) are essential for the regulation of several metabolic processes and the energy consumption of the organism. Their action is exerted primarily through interaction with nuclear receptors controlling the transcription of thyroid hormone-responsive genes. Proper regulation of TH levels in different tissues is extremely important for the equilibrium between normal cellular proliferation and differentiation. The iodothyronine deiodinases types 1, 2 and 3 are key enzymes that perform activation and inactivation of THs, thus controlling TH homeostasis in a cell-specific manner. As THs seem to exert their effects in all hallmarks of the neoplastic process, dysregulation of deiodinases in the tumoral context can be critical to the neoplastic development. Here, we aim at reviewing the deiodinases expression in different neoplasias and exploit the mechanisms by which they play an essential role in human carcinogenesis. TH modulation by deiodinases and other classical pathways may represent important targets with the potential to oppose the neoplastic process.

Imbalance of dietary nutrients and the associated differentially expressed genes and pathways may play important roles in juvenile Kashin-Beck disease

BACKGROUND

Kashin-Beck disease (KBD) is a childhood-onset endemic osteoarthropathy in China. Nutrients including trace elements may play active roles in the development of KBD.

OBJECTIVE

This study aimed to estimate the nutrient intakes of children in endemic areas and to identify the imbalanced nutrients associated differentially expressed genes in the juvenile patients with KBD.

METHODS

In this cross-sectional study, a consecutive 3 day 24 h semi-quantitative dietary retrospect questionnaire was conducted to estimate the daily nutrient intakes of children using CDGSS 3.0 software. Gene profile analysis was employed to identify differentially expressed genes in peripheral blood mononuclear cells of children with KBD. GOC, CTD, KEGG, and REACTOME databases were used to establish the relationship between nutrients and nutrients-associated differentially expressed genes and pathways. Statistical analyses were accomplished by SPSS 18.0 software.

RESULTS

Daily Se intakes without supplementation of children were significantly lower in Se-supplemented (Se + ) KBD areas (29.3 ∼ 29.6 mg/d) and non-endemic area (27.8 ± 7.9 mg/d) compared to non-Se-supplemented (Se-) KBD area (32.9 ± 7.9 mg/d, c² = 20.24, P < .01). Children in Se+ KBD areas were suffering more serious insufficient intake of multiple nutrients, including vitamins-B₂/-C/-E, Ca, Fe, Zn and I. Gene profile analysis combined with bioinformatics technique identified 34 nutrients associated differentially expressed genes and 10 significant pathways which are related to the pathological changes in juvenile KBD.

CONCLUSIONS

Imbalance of dietary nutrients and nutrients-associated differentially expressed genes and pathways may play important roles in the development of juvenile KBD.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

The Deiodinase Trio and Thyroid Hormone Signaling

Thyroid hormone signaling is customized in a time and cell-specific manner by the deiodinases, homodimeric thioredoxin fold containing selenoproteins. This ensures adequate T3 action in developing tissues, healthy adults and many disease states. D2 activates thyroid hormone by converting the pro-hormone T4 to T3, the biologically active thyroid hormone. D2 expression is tightly regulated by transcriptional mechanisms triggered by endogenous as well as environmental cues. There is also an on/off switch mechanism that controls D2 activity that is triggered by catalysis and functions via D2 ubiquitination/deubiquitination. D3 terminates thyroid hormone action by inactivation of both T4 and T3 molecules. Deiodinases play a role in thyroid hormone homeostasis, development, growth and metabolic control by affecting the intracellular levels of T3 and thus gene expression on a cell-specific basis. In many cases, tight control of these pathways by T3 is achieved with coordinated reciprocal changes in D2-mediated thyroid hormone activation D3-mediated thyroid hormone inactivation.

Thyroid Hormone Signaling in the Development of the Endochondral Skeleton

Thyroid hormone (TH) is an established regulator of skeletal growth and maintenance both in clinical studies and in laboratory models. The clinical consequences of altered thyroid status on the skeleton during development and in adulthood are well known, and genetic mouse models in which elements of the TH signaling axis have been manipulated illuminate the mechanisms which underlie TH regulation of the skeleton. TH is involved in the regulation of the balance between proliferation and differentiation in several skeletal cell types including chondrocytes, osteoblasts, and osteoclasts. The effects of TH are mediated primarily via the thyroid hormone receptors (TRs) α and β, ligand-inducible nuclear receptors which act as transcription factors to regulate target gene expression. Both TRα and TRβ signaling are important for different stages of skeletal development. The molecular mechanisms of TH action in bone are complex and include interaction with a number of growth factor signaling pathways. This review provides an overview of the regulation and mechanisms of TH action in bone, focusing particularly on the role of TH in endochondral bone formation during postnatal growth.

Thyroid hormone promotes differentiation of colon cancer stem cells

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

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Effect of vitamin D3 on thyroid function and de-iodinase 2 expression in diabetic rats

OBJECTIVE

This study aimed to assess the effect of vitamin D3 administration to diabetic rats on thyroid profile and deiodinase 2 (D2).

METHODS

Thirty male Wistar rats were included into three groups; control, streptozotocin-induced diabetic and diabetic supplemented with vitamin D3 groups. Ten weeks later, serum levels of free T4, free T3 and TSH were measured. Tissue homogenates from liver, kidney, muscle, femur bone, heart and brain were obtained and assessed for D2 mRNA.

RESULTS

Diabetic rats demonstrated significant increase in free T4 and significant decrease in free T3. These changes were ameliorated by vitamin D3 administration. D2 mRNA was significantly reduced in all tissue homogenates obtained from diabetic rats, while vitamin D3 treatment significantly enhanced D2 in liver and brain homogenates.

CONCLUSION

Diabetes mellitus inhibited peripheral conversion of T4 into T3 secondary to reduction in D2 expression. Vitamin D3 greatly corrected the alterations in thyroid profile and D2 expression.

Isolated vitamin D deficiency is not associated with nonthyroidal illness syndrome, but with thyroid autoimmunity

AIM

This study aimed to compare thyroid functions, thyroid autoantibodies, and the existence of nonthyroidal illness syndrome (NTIS) according to vitamin D level.

MATERIALS AND METHODS

The study included age- and BMI-matched healthy volunteers with and without vitamin D deficiency. In addition, the nonthyroidal illness syndrome status was evaluated.

RESULTS

Anti-TPO positivity was significantly more common in those with severe and moderate vitamin D deficiency, as compared to those with a normal 25(OH)D level. Furthermore, TSH levels were significantly lower in those with severe and moderate vitamin D deficiency than in those with a normal 25(OH)D level. In addition, there was a significant weak inverse correlation between anti-TPO positivity and the 25(OH)D level and a positive correlation between the TSH level and 25(OH)D level. Only 1 thyroid function test result was compatible with NTIS among the participants with moderate vitamin D deficiency; therefore the difference was not significant.

CONCLUSIONS

The prevalence of thyroid autoantibody positivity was higher in those with severe and moderate vitamin D deficiency than in those with a normal 25(OH)D level. Additional large-scale studies must be conducted to determine if vitamin D deficiency plays a causal role in the pathogenesis of Hashimoto's thyroiditis and NTIS.

Defending plasma T3 is a biological priority

Triiodothyronine (T3), the active form of thyroid hormone is produced predominantly outside the thyroid parenchyma secondary to peripheral tissue deiodination of thyroxine (T4), with <20% being secreted directly from the thyroid. In healthy individuals, plasma T3 is regulated by the negative feedback loop of the hypothalamus-pituitary-thyroid axis and by homoeostatic changes in deiodinase expression. Therefore, with the exception of a minimal circadian rhythmicity, serum T3 levels are stable over long periods of time. Studies in rodents indicate that different levels of genetic disruption of the feedback mechanism and deiodinase system are met with increase in serum T4 and thyroid-stimulating hormone (TSH) levels, while serum T3 levels remain stable. These findings have focused attention on serum T3 levels in patients with thyroid disease, with important clinical implications affecting therapeutic goals and choice of therapy for patients with hypothyroidism. Although monotherapy with levothyroxine is the standard of care for hypothyroidism, not all patients normalize serum T3 levels with many advocating for combination therapy with levothyroxine and liothyronine. The latter could be relevant for a significant number of patients that remain symptomatic on monotherapy with levothyroxine, despite normalization of serum TSH levels.

Thyroid hormone deiodinases D1, D2, and D3 are expressed in human endothelial dermal microvascular line: effects of thyroid hormones

Endothelial system acts as a large endocrine organ in the human body; however, little is still known about the regulative role of THs on endothelial cells. Aim of the present study was to investigate the expression of the TH deiodinases (D1, D2, and D3) and TH receptors (TRα1, TRα2, and TRβ1) in an endothelial microvascular cultured cell model (HMEC-1), after stimulation with triiodothyronine (T3, 10-100 nM), thyroxine (T4, 10-100 nM), and reverse T3 (rT3, 1-10 nM). DIO1 was significantly inhibited by T4 at 10 and 100 nM (p < 0.001). rT3 significantly inhibited DIO1 at 1 nM concentration (p < 0.01) and stimulated DIO1 at 10 nM dosage (p < 0.001). T4 and rT3 significantly inhibited DIO2 at all concentrations. DIO3 was induced at 100 nM T3 (p < 0.05) and 100 nM rT3 (p < 0.01), and TRα1 and TRα2 mRNAs were significantly increased after 100 nM T3 treatment (p < 0.05) and decreased after 1 and 10 nM rT3 (p < 0.05). TRβ1 was significantly increased by all THs at different concentrations: 10 nM T3 and 100 nM T3 (p < 0.05), 1 nM rT3 (p < 0.001), and 100 nM T4 (p < 0.01). D1 and D2 protein levels were evaluated, but no significant difference was observed for any hormonal treatment. For the first time, we found that the TH deiodinases and receptors are expressed in endothelial HMEC-1 cells. These findings might be of significant clinical relevance, given the important regulatory role of the endothelium as first barrier to the bloodstream.

The multiple effects of thyroid disorders on bone and mineral metabolism

Differently from most hormones, which commonly are specialized molecules able to influence other cells, tissues and systems, thyroid hormones (TH) are pleiotropic peptides, whose primordial function is difficult to identify. The complex action of TH on human economy can be easily witnessed by examining the diverse consequences of TH excess and deficiency during development and after maturity. In particular, different manifestations in bone modeling and remodeling reflect the circumstantial consequences of thyroid disturbances, which are age dependent. While hyperthyroidism during childhood enhances bone mineralization and accelerates epiphyseal maturation, in adults it induces bone loss by predominant activation of osteoclast activity. Furthermore, the syndrome of TH resistance is a multifaceted condition in which different sites exhibit signs of hormone excess or deficiency depending on the configuration of the TH receptor isoform. The investigation of the impact of TH resistance on the skeleton still remains to be elucidated. We present here a thorough review of the action of TH on bone and of the impact of thyroid disorders, including hyper- and hypothyroidism and the syndrome of TH resistance, on the skeleton.

Tissue-specific inactivation of type 2 deiodinase reveals multilevel control of fatty acid oxidation by thyroid hormone in the mouse

Type 2 deiodinase (D2) converts the prohormone thyroxine (T4) to the metabolically active molecule 3,5,3'-triiodothyronine (T3), but its global inactivation unexpectedly lowers the respiratory exchange rate (respiratory quotient [RQ]) and decreases food intake. Here we used FloxD2 mice to generate systemically euthyroid fat-specific (FAT), astrocyte-specific (ASTRO), or skeletal-muscle-specific (SKM) D2 knockout (D2KO) mice that were monitored continuously. The ASTRO-D2KO mice also exhibited lower diurnal RQ and greater contribution of fatty acid oxidation to energy expenditure, but no differences in food intake were observed. In contrast, the FAT-D2KO mouse exhibited sustained (24 h) increase in RQ values, increased food intake, tolerance to glucose, and sensitivity to insulin, all supporting greater contribution of carbohydrate oxidation to energy expenditure. Furthermore, FAT-D2KO animals that were kept on a high-fat diet for 8 weeks gained more body weight and fat, indicating impaired brown adipose tissue (BAT) thermogenesis and/or inability to oxidize the fat excess. Acclimatization of FAT-D2KO mice at thermoneutrality dissipated both features of this phenotype. Muscle D2 does not seem to play a significant metabolic role given that SKM-D2KO animals exhibited no phenotype. The present findings are unique in that they were obtained in systemically euthyroid animals, revealing that brain D2 plays a dominant albeit indirect role in fatty acid oxidation via its sympathetic control of BAT activity. D2-generated T3 in BAT accelerates fatty acid oxidation and protects against diet-induced obesity.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Role and Mechanisms of Actions of Thyroid Hormone on the Skeletal Development

The importance of the thyroid hormone axis in the regulation of skeletal growth and maintenance has been well established from clinical studies involving patients with mutations in proteins that regulate synthesis and/or actions of thyroid hormone. Data from genetic mouse models involving disruption and overexpression of components of the thyroid hormone axis also provide direct support for a key role for thyroid hormone in the regulation of bone metabolism. Thyroid hormone regulates proliferation and/or differentiated actions of multiple cell types in bone including chondrocytes, osteoblasts and osteoclasts. Thyroid hormone effects on the target cells are mediated via ligand-inducible nuclear receptors/transcription factors, thyroid hormone receptor (TR) α and β, of which TRα seems to be critically important in regulating bone cell functions. In terms of mechanisms for thyroid hormone action, studies suggest that thyroid hormone regulates a number of key growth factor signaling pathways including insulin-like growth factor-I, parathyroid hormone related protein, fibroblast growth factor, Indian hedgehog and Wnt to influence skeletal growth. In this review we describe findings from various genetic mouse models and clinical mutations of thyroid hormone signaling related mutations in humans that pertain to the role and mechanism of action of thyroid hormone in the regulation of skeletal growth and maintenance.

Desiccated thyroid extract compared with levothyroxine in the treatment of hypothyroidism: a randomized, double-blind, crossover study

CONTEXT

Patients previously treated with desiccated thyroid extract (DTE), when being switched to levothyroxine (L-T₄), occasionally did not feel as well despite adequate dosing based on serum TSH levels.

OBJECTIVE

Our objective was to investigate the effectiveness of DTE compared with L-T₄ in hypothyroid patients.

DESIGN AND SETTING

We conducted a randomized, double-blind, crossover study at a tertiary care center.

PATIENTS

Patients (n = 70, age 18-65 years) diagnosed with primary hypothyroidism on a stable dose of L-T₄ for 6 months were included in the study.

INTERVENTION

Patients were randomized to either DTE or L-T₄ for 16 weeks and then crossed over for the same duration.

OUTCOME MEASURES

Biochemical and neurocognitive tests at baseline and at the end of each treatment period were evaluated.

RESULTS

There were no differences in symptoms and neurocognitive measurements between the 2 therapies. Patients lost 3 lb on DTE treatment (172.9 ± 36.4 lb vs 175.7 ± 37.7 lb, P < .001). At the end of the study, 34 patients (48.6%) preferred DTE, 13 (18.6%) preferred L-T₄, and 23 (32.9%) had no preference. In the subgroup analyses, those patients who preferred DTE lost 4 lb during the DTE treatment, and their subjective symptoms were significantly better while taking DTE as measured by the general health questionnaire-12 and thyroid symptom questionnaire (P < .001 for both). Five variables were predictors of preference for DTE.

CONCLUSION

DTE therapy did not result in a significant improvement in quality of life; however, DTE caused modest weight loss and nearly half (48.6%) of the study patients expressed preference for DTE over L-T₄. DTE therapy may be relevant for some hypothyroid patients.

Coordination of hypothalamic and pituitary T3 production regulates TSH expression

Type II deiodinase (D2) activates thyroid hormone by converting thyroxine (T4) to 3,5,3'-triiodothyronine (T3). This allows plasma T4 to signal a negative feedback loop that inhibits production of thyrotropin-releasing hormone (TRH) in the mediobasal hypothalamus (MBH) and thyroid-stimulating hormone (TSH) in the pituitary. To determine the relative contributions of these D2 pathways in the feedback loop, we developed 2 mouse strains with pituitary- and astrocyte-specific D2 knockdown (pit-D2 KO and astro-D2 KO mice, respectively). The pit-D2 KO mice had normal serum T3 and were systemically euthyroid, but exhibited an approximately 3-fold elevation in serum TSH levels and a 40% reduction in biological activity. This was the result of elevated serum T4 that increased D2-mediated T3 production in the MBH, thus decreasing Trh mRNA. That tanycytes, not astrocytes, are the cells within the MBH that mediate T4-to-T3 conversion was defined by studies using the astro-D2 KO mice. Despite near-complete loss of brain D2, tanycyte D2 was preserved in astro-D2 KO mice at levels that were sufficient to maintain both the T4-dependent negative feedback loop and thyroid economy. Taken together, these data demonstrated that the hypothalamic-thyroid axis is wired to maintain normal plasma T3 levels, which is achieved through coordination of T4-to-T3 conversion between thyrotrophs and tanycytes.

Selenium protects bone marrow stromal cells against hydrogen peroxide-induced inhibition of osteoblastic differentiation by suppressing oxidative stress and ERK signaling pathway

Osteoporosis is a bone disease that leads to an increased risk of fracture. Oxidative stress may play a major role in the development of osteoporosis in part by inhibiting osteoblastic differentiation of bone marrow stromal cells (MSCs). Some evidence suggested that antioxidant selenium could prevent osteoporosis, but the underlying mechanism remains unclear. In this work, the effect of sodium selenite on H₂O₂-induced inhibition of osteoblastic differentiation of primary rat bone MSCs and the related mechanisms were examined. Pretreatment with selenite inhibited the adverse effect of H₂O₂ on osteoblastic differentiation of MSCs, based on alkaline phosphatase activity, gene expression of type I collagen and osteocalcin, and matrix mineralization. In addition, selenite pretreatment also suppressed the activation of extracellular signal-regulated kinase (ERK) induced by H₂O₂. The above effects were mediated by the antioxidant effect of selenite. Selenite enhanced the gene expression and activity of glutathione peroxidase, reversed the decreased total antioxidant capacity and reduced glutathione, and suppressed reactive oxygen species production and lipid peroxidation level in H₂O₂-treated MSCs. These results showed that selenite protected MSCs against H₂O₂-induced inhibition of osteoblastic differentiation through inhibiting oxidative stress and ERK activation, which provided, for the first time, the mechanistic explanation for the negative association of selenium status and risk of osteoporosis in terms of bone formation.

Resistance to obesity by repression of VEGF gene expression through induction of brown-like adipocyte differentiation

Adipose tissues are classified into white adipose tissue (WAT) and brown adipose tissue (BAT). WAT is responsible for energy storage, and malfunction is associated with obesity. BAT, on the contrary, consumes fat to generate heat through uncoupling mitochondrial respiration and is important in body weight control. Vascular endothelial growth factor (VEGF)-A is the founding member of the VEGF family and has been found highly expressed in adipose tissue. A genetic mouse model of an inducible VEGF (VEGF-A) repression system was used to study VEGF-regulated energy metabolism in WAT. VEGF-repressed mice demonstrated lower food efficiency, lower body weight, and resistance to high-fat diet-induced obesity. Repression of VEGF expression caused morphological and molecular changes in adipose tissues. VEGF repression induced brown-like adipocyte development in WAT, up-regulation of BAT-specific genes including PRDM16, GATA-1, BMP-7, CIDEA, and UCP-1 and down-regulation of leptin, a WAT-specific gene. VEGF repression up-regulated expression of VEGF-B and its downstream fatty acid transport proteins. Relative levels of VEGF/VEGF-B may be important switches in energy metabolism and of pharmaceutical significances.

Triiodothyronine stimulates glucose transport in bone cells

Thyroid hormones increase energy expenditure and bone turnover in vivo. To study whether 3,3',5-triiodo-l-thyronine (T₃) stimulates the uptake of glucose in osteoblastic cells, PyMS (a cell line derived from rat bone) cells were kept in serum-free culture medium and treated with T₃. We measured [1-¹⁴C]-2-deoxy-D: -glucose (2DG) uptake and looked for expression of the high-affinity glucose transporters GLUT1 and GLUT3 by northern and western analysis. T₃ did not influence the cell number but slightly (1.3-fold) increased the protein content of the cell cultures. 2DG uptake was low in serum-deprived cell cultures and was increased by T₃ (up to 2.5-fold at 1 nmol l⁻¹ after 4 days) in a dose- and time-dependent manner. Triiodothyronine at 1 nmol l⁻¹ increased GLUT1 and GLUT3 abundance in membranes. Therefore, increased glucose uptake induced by T₃ in osteoblasts may be mediated by the known high-affinity glucose transporters GLUT1 and GLUT3.

Thyroid hormone metabolism in skeletal development and adult bone maintenance

Metabolism of thyroid hormones by the type 2 and type 3 iodothyronine deiodinases (D2, D3) in T3-responsive target cells is a sophisticated mechanism that helps to maintain local T3 concentrations and facilitates T3 action in a cell-specific manner that is independent of circulating thyroid hormone concentrations. Recent findings have demonstrated an essential physiological role for the thyroid hormone-activating enzyme D2 in the optimization of bone mineralization and strength. Emerging population studies have also identified the genes encoding D2 and the thyroid hormone-inactivating enzyme D3 as susceptibility loci for osteoarthritis. These new data reveal an essential role for the local control of T3 availability in osteoblasts and chondrocytes during maintenance and repair of bone and cartilage.

Significant association between thyroid hormones and erythrocyte indices in euthyroid subjects

OBJECTIVE

Hypothyroidism and hyperthyroidism are each associated with anaemia, but relationships between thyroid function and erythrocyte indices in euthyroid subjects have not been examined. The aim of this study was to examine these relationships in a community-based cohort.

DESIGN, SUBJECTS AND MEASUREMENTS

Linear regression models with free T4, free T3 and TSH as predictors of erythrocyte indices and serum iron parameters were fitted to data from a cohort of 1179 participants in the 1994 Busselton health study and a subset of 1011 euthyroid participants. All models were adjusted for age, age(2), sex and an age-sex interaction.

RESULTS

In the full cohort and euthyroid subset, there were significant, positive linear relationships between free T4 and each of haemoglobin, haematocrit and erythrocyte count (P < 0·01 for each), such that in euthyroid participants, each 1·0 pM increase in free T4 was associated with an increase in haemoglobin of 0·39 g/l. There were significant relationships between free T3 and each of haemoglobin, haematocrit and erythrocyte count (P < 0·001 for each), with the best model fits obtained using free T3(2), indicating curved relationships. TSH had a significant (P < 0·05) inverse relationship with serum iron and transferrin saturation in the full cohort and the euthyroid subset. Serum iron concentrations were lower in participants with subclinical hypothyroidism (n = 87) than euthyroid subjects [mean (SD) 15·9 (4·7) vs 18·4 (6·0) μM, P = 0·001].

CONCLUSION

In euthyroid subjects, small differences in thyroid function are associated with significant differences in erythrocyte indices.

Thyroid hormone deiodinases and cancer

Deiodinases constitute a group of thioredoxin fold-containing selenoenzymes that play an important function in thyroid hormone homeostasis and control of thyroid hormone action. There are three known deiodinases: D1 and D2 activate the pro-hormone thyroxine (T4) to T3, the most active form of thyroid hormone, while D3 inactivates thyroid hormone and terminates T3 action. A number of studies indicate that deiodinase expression is altered in several types of cancers, suggesting that (i) they may represent a useful cancer marker and/or (ii) could play a role in modulating cell proliferation - in different settings thyroid hormone modulates cell proliferation. For example, although D2 is minimally expressed in human and rodent skeletal muscle, its expression level in rhabdomyosarcoma (RMS)-13 cells is threefold to fourfold higher. In basal cell carcinoma (BCC) cells, sonic hedgehog (Shh)-induced cell proliferation is accompanied by induction of D3 and inactivation of D2. Interestingly a fivefold reduction in the growth of BCC in nude mice was observed if D3 expression was knocked down. A decrease in D1 activity has been described in renal clear cell carcinoma, primary liver cancer, lung cancer, and some pituitary tumors, while in breast cancer cells and tissue there is an increase in D1 activity. Furthermore D1 mRNA and activity were found to be decreased in papillary thyroid cancer while D1 and D2 activities were significantly higher in follicular thyroid cancer tissue, in follicular adenoma, and in anaplastic thyroid cancer. It is conceivable that understanding how deiodinase dysregulation in tumor cells affect thyroid hormone signaling and possibly interfere with tumor progression could lead to new antineoplastic approaches.

Metabolic effects of liothyronine therapy in hypothyroidism: a randomized, double-blind, crossover trial of liothyronine versus levothyroxine

CONTEXT

Levothyroxine (L-T(4)) therapy is based on the assumption that the conversion of T(4) into T(3) provides adequate amounts of active hormone at target tissues. However, in rodents, L-T(4) alone does not restore a euthyroid state in all tissues. Previous combination L-T(4)/liothyronine (L-T(3)) therapy trials focused on quality-of-life endpoints, and limited information is available on the effects on other measures of thyroid hormone action.

OBJECTIVE

Our objective was to evaluate the efficacy of thyroid hormone replacement with L-T(4) or L-T(3) at doses producing equivalent normalization of TSH.

PARTICIPANTS, DESIGN, AND SETTING

Fourteen hypothyroid patients participated in this randomized, double-blind, crossover intervention at the National Institutes of Health Clinical Center.

INTERVENTIONS

L-T(3) or L-T(4) were administered thrice daily to achieve a target TSH from 0.5-1.5 mU/liter. Volunteers were studied as inpatients after 6 wk on a stable dose and at the target TSH.

MAIN OUTCOME MEASURES

Serum thyroid hormones, lipid parameters, and indices of glucose metabolism were evaluated.

RESULTS

No difference was observed in TSH between L-T(3) and L-T(4) treatments. L-T(3) resulted in significant weight loss [L-T(4), 70.6 ± 12.5, vs. L-T(3), 68.5 ± 11.9 kg (P = 0.009)] and in a 10.9 ± 10.0% decrease in total cholesterol (P = 0.002), 13.3 ± 12.1% decrease in low-density lipoprotein-cholesterol (P = 0.002), and an 18.3 ± 28.6% decrease in apolipoprotein B (P = 0.018). No significant differences were observed in high-density lipoprotein-cholesterol, heart rate, blood pressure, exercise tolerance, or insulin sensitivity.

CONCLUSIONS

The substitution of L-T(3) for L-T(4) at equivalent doses (relative to the pituitary) reduced body weight and resulted in greater thyroid hormone action on the lipid metabolism, without detected differences in cardiovascular function or insulin sensitivity.

Type 2 deiodinase at the crossroads of thyroid hormone action

Thyroid hormone action can be customized on a cell-specific fashion through the controlled action of the deiodinase group of enzymes, which are homodimeric thioredoxin fold containing selenoproteins. Whereas the type II deiodinase (D2) initiates thyroid hormone signaling by activating the pro-hormone thyroxine (T4) to the biologically active T3 molecule, the type III deiodinase (D3) terminates thyroid hormone action by catalyzing the inactivation of both T4 and T3 molecules. Deiodinases play a role in thyroid hormone homeostasis, development, growth and metabolic control by affecting the intracellular levels of T3 and thus gene expression on a cell-specific basis. Whereas both Dio2 and Dio3 are transcriptionally regulated, ubiquitination of D2 is a switch mechanism that controls D2 activity and intracellular T3 production. The hedgehog-inducible WSB-1 and the yeast Doa10 mammalian ortholog TEB4 are two E3 ligases that inactivate D2 via ubiquitination. Inactivation involves disruption of the D2:D2 dimer and can be reversed via two ubiquitin-specific proteases, USP20 and USP33, rescuing catalytic activity and T3 production. The ubiquitin-based switch mechanism that controls D2 activity illustrates how different cell types fine-tune thyroid hormone signaling, making D2 a suitable target for pharmacological intervention. This article reviews the cellular and molecular aspects of D2 regulation and the current models of D2-mediated thyroid hormone signaling.

Minireview: cracking the metabolic code for thyroid hormone signaling

Cells are not passive bystanders in the process of hormonal signaling and instead can actively customize hormonal action. Thyroid hormone gains access to the intracellular environment via membrane transporters, and while diffusing from the plasma membrane to the nucleus, thyroid hormone signaling is modified via the action of the deiodinases. Although the type 2 deiodinase (D2) converts the prohormone T(4) to the biologically active T(3), the type 3 deiodinase (D3) converts it to reverse T(3), an inactive metabolite. D3 also inactivates T(3) to T(2), terminating thyroid hormone action. Therefore, D2 confers cells with the capacity to produce extra amounts of T(3) and thus enhances thyroid hormone signaling. In contrast expression of D3 results in the opposite action. The Dio2 and Dio3 genes undergo transcriptional regulation throughout embryonic development, childhood, and adult life. In addition, the D2 protein is unique in that it can be switched off and on via an ubiquitin regulated mechanism, triggered by catalysis of T(4). Induction of D2 enhances local thyroid hormone signaling and energy expenditure during activation of brown adipose tissue by cold exposure or high-fat diet. On the other hand, induction of D3 in myocardium and brain during ischemia and hypoxia decreases energy expenditure as part of a homeostatic mechanism to slow down cell metabolism in the face of limited O(2) supply.

Novel molecular targets for prevention of obesity and osteoporosis

Evidence from both epidemiological studies and basic research suggests that obesity and osteoporosis are interrelated. Though there is an increase in the prevalence of these disorders, a limited number of treatments are available, one of the reasons being the complexity of the pathways involved and difficulty in identifying a single molecular target. Due to adverse effects of pharmaceuticals, intake of herbal drugs by patients without a physician's recommendation is increasing globally. Lack of success with targeted monotherapy has encouraged scientists to determine whether combinations of phytochemicals that interfere with numerous cell-signaling pathways can be a more effective approach to treat complex diseases. For example, evidence is emerging that specific combinations of phytochemicals are far more effective than single compounds in decreasing adipogenesis and promoting bone formation. Since multiple pathways are dysfunctional in obesity and osteoporosis, an ideal approach for preventing and treating these diseases may be to use a combination of phytochemicals to address several targets simultaneously.

The type 2 deiodinase Thr92Ala polymorphism is associated with increased bone turnover and decreased femoral neck bone mineral density

The role of type 2 deiodinase (D2) in the human skeleton remains unclear. The D2 polymorphism Thr92Ala has been associated with lower enzymatic activity, which could result in lower local triiodothyronine (T(3)) availability in bone. We therefore hypothesized that the D2 Thr92Ala polymorphism may influence bone mineral density (BMD) and bone turnover. We studied 154 patients (29 men, 125 women: 79 estrogen-replete, 46 estrogen-deficient) with cured differentiated thyroid carcinoma. BMD and bone turnover markers [bone-specific alkaline phosphatase (BAP), cross-linking terminal C-telopeptide of type I collagen (CTX), procollagen type 1 amino-terminal propeptide (P1NP), and cross-linked N-telopeptide of type I collagen (NTX)] were measured. Effects of the D2 Thr92Ala polymorphism on BMD and bone turnover markers were assessed by a linear regression model, with age, gender, estrogen state, body mass index (BMI), serum calcium, 25-hydroxyvitamin D, parathyroid hormone (PTH), thyroid-stimulating hormone (TSH), and free triiodothyroxine (T(4)) as covariables. Sixty patients were wild type (Thr/Thr), 66 were heterozygous (Thr/Ala), and 28 were homozygous (Ala/Ala) for the D2 polymorphism. There were no significant differences in any covariables between the three genotypes. Subjects carrying the D2 Thr92Ala polymorphism had consistently lower femoral neck and total hip densities than wild-type subjects (p = .028), and this was accompanied by significantly higher serum P1NP and CTX and urinary NTX/creatinine levels. We conclude that in patients with cured differentiated thyroid carcinoma, the D2 Thr92Ala polymorphism is associated with a decreased femoral neck BMD and higher bone turnover independent of serum thyroid hormone levels, which points to a potential functional role for D2 in bone.

Germain DL, Galton VA, Williams GR. Optimal bone strength and mineralization requires the type 2 iodothyronine deiodinase in osteoblasts

Hypothyroidism and thyrotoxicosis are each associated with an increased risk of fracture. Although thyroxine (T4) is the predominant circulating thyroid hormone, target cell responses are determined by local intracellular availability of the active hormone 3,5,3'-L-triiodothyronine (T3), which is generated from T4 by the type 2 deiodinase enzyme (D2). To investigate the role of locally produced T3 in bone, we characterized mice deficient in D2 (D2KO) in which the serum T3 level is normal. Bones from adult D2KO mice have reduced toughness and are brittle, displaying an increased susceptibility to fracture. This phenotype is characterized by a 50% reduction in bone formation and a generalized increase in skeletal mineralization resulting from a local deficiency of T3 in osteoblasts. These data reveal an essential role for D2 in osteoblasts in the optimization of bone strength and mineralization.

New insights into BMP-7 mediated osteoblastic differentiation of primary human mesenchymal stem cells

Bone Morphogenetic Proteins (BMPs) are members of the TGF-beta superfamily of growth factors. Several BMPs exhibit osteoinductive bioactivities, and are critical for bone formation in both developing and mature skeletal systems. BMP-7 (OP-1) is currently used clinically in revision of posterolateral spine fusions and long bone non-unions. The current study characterizes BMP-7 induced gene expression during early osteoblastic differentiation of human mesenchymal stem cells (hMSC). Primary hMSC were treated with BMP-7 for 24 or 120 h and gene expression across the entire human genome was evaluated using Affymetrix HG-U133 Plus 2.0 Arrays. 955 probe sets representing 655 genes and 95 ESTs were identified as differentially expressed and were organized into three major expression profiles (Profiles A, B and C) by hierarchical clustering. Genes from each profile were classified according to biochemical pathway analyses. Profile A, representing genes upregulated by BMP-7, revealed strong enrichment for established osteogenic marker genes, as well as several genes with undefined roles in osteoblast function, including MFI2, HAS3, ADAMTS9, HEY1, DIO2 and FGFR3. A functional screen using siRNA suggested roles for MFI2, HEY1 and DIO2 in osteoblastic differentiation of hMSC. Profile B contained genes transiently downregulated by BMP-7, including numerous genes associated with cell cycle regulation. Follow-up studies confirmed that BMP-7 attenuates cell cycle progression and cell proliferation during early osteoblastic differentiation. Profile C, comprised of genes continuously downregulated by BMP-7, exhibited strong enrichment for genes associated with chemokine/cytokine activity. Inhibitory effects of BMP-7 on cytokine secretion were verified by analysis of enriched culture media. Potent downregulation of CHI3L1, a potential biomarker for numerous joint diseases, was also observed in Profile C. A focused evaluation of BMP, GDF and BMP inhibitor expression elucidated feedback loops modulating BMP-7 bioactivity. BMP-7 was found to induce BMP-2 and downregulate GDF5 expression. Transient knockdown of BMP-2 using siRNA demonstrated that osteoinductive properties associated with BMP-7 are independent of endogenous BMP-2 expression. Noggin was identified as the predominant inhibitor induced by BMP-7 treatment. Overall, this study provides new insight into key bioactivities characterizing early BMP-7 mediated osteoblastic differentiation.

Type 2 deiodinase expression is induced by peroxisomal proliferator-activated receptor-gamma agonists in skeletal myocytes

The thyroid hormone activating type 2 deiodinase (D2) is known to play a role in brown adipose tissue-mediated adaptive thermogenesis in rodents, but the finding of D2 in skeletal muscle raises the possibility of a broader metabolic role. In the current study, we examined the regulation of the D2 pathway in primary skeletal muscle myoblasts taken from both humans and mice. We found that pioglitazone treatment led to a 1.6- to 1.9-fold increase in primary human skeletal myocyte D2 activity; this effect was seen with other peroxisomal proliferator-activated receptor-gamma agonists. D2 activity in primary murine skeletal myotubes increased 2.8-fold in response to 5 microM pioglitazone and 1.6-fold in response to 5 nM insulin and increased in a dose-dependent manner in response to lithocholic acid (maximum response at 25 microM was approximately 3.8-fold). We compared Akt phosphorylation in primary myotubes derived from wild-type and D2 knockout (D2KO) mice: phospho-Akt was reduced by 50% in the D2KO muscle after 1 nM insulin exposure. Expression of T(3)-responsive muscle genes via quantitative RT-PCR suggests that D2KO cells have decreased thyroid hormone signaling, which could contribute to the abnormalities in insulin signaling. D2 activity in skeletal muscle fragments from both murine and human sources was low, on the order of about 0.01 fmol/min . mg of muscle protein. The phenotypic changes seen with D2KO cells support a metabolic role for D2 in muscle, hinting at a D2-mediated linkage between thyroid hormone and insulin signaling, but the low activity calls into question whether skeletal muscle D2 is a major source of plasma T(3).

The monocarboxylate transporter 8 and L-type amino acid transporters 1 and 2 are expressed in mouse skeletons and in osteoblastic MC3T3-E1 cells

BACKGROUND

Several plasma membrane transporters have been shown to mediate the cellular influx and/or efflux of iodothyronines, including the sodium-independent organic anion co-transporting polypeptide 1 (OATP1), the sodium taurocholate co-transporting polypeptide (NTCP), the L-type amino acid transporter 1 (LAT1) and 2 (LAT2), and the monocarboxylate transporter 8 (MCT8). The aim of this study was to investigate if the mRNAs of these transporters were expressed and regulated by thyroid hormone (TH) in mouse calvaria-derived osteoblastic MC3T3-E1 cells and in the fetal and postnatal bones of mice.

METHODS

The mRNA expression of the iodothyronine transporters was investigated with real-time polymerase chain reaction analysis in euthyroid and hypothyroid fetuses and litters of mice and in MC3T3-E1 cells treated with increasing doses of triiodothyronine (T(3); 10(-10) to 10(-6) M) or with 10(-8) M T(3) for 1-9 days.

RESULTS

MCT8, LAT1, and LAT2 mRNAs were detected in fetal and postnatal femurs and in MC3T3-E1 cells, while OATP1 and NTCP mRNAs were not. LAT1 and LAT2 mRNAs were not affected by TH status in vivo or in vitro or by the stage of bone development or osteoblast maturation (analyzed by the expression of osteocalcin and alkaline phosphatase, which are key markers of osteoblastic differentiation). In contrast, the femoral mRNA expression of MCT8 decreased significantly during post-natal development, whereas MCT8 mRNA expression increased as MC3T3-E1 cells differentiated. We also showed that MCT8 mRNA was up-regulated in the femur of hypothyroid animals, and that it was down-regulated by treatment with T(3) in MC3T3-E1 cells.

CONCLUSIONS

This is the first study to demonstrate the mRNA expression of LAT1, LAT2, and MCT8 in the bone tissue of mice and in osteoblast-like cells. In addition, the pattern of MCT8 expression observed in vivo and in vitro suggests that MCT8 may be important to modulate TH effects on osteoblast differentiation and on bone development and metabolism.

Deiodinase-mediated thyroid hormone inactivation minimizes thyroid hormone signaling in the early development of fetal skeleton

Thyroid hormone (TH) plays a key role on post-natal bone development and metabolism, while its relevance during fetal bone development is uncertain. To study this, pregnant mice were made hypothyroid and fetuses harvested at embryonic days (E) 12.5, 14.5, 16.5 and 18.5. Despite a marked reduction in fetal tissue concentration of both T4 and T3, bone development, as assessed at the distal epiphyseal growth plate of the femur and vertebra, was largely preserved up to E16.5. Only at E18.5, the hypothyroid fetuses exhibited a reduction in femoral type I and type X collagen and osteocalcin mRNA levels, in the length and area of the proliferative and hypertrophic zones, in the number of chondrocytes per proliferative column, and in the number of hypertrophic chondrocytes, in addition to a slight delay in endochondral and intramembranous ossification. This suggests that up to E16.5, thyroid hormone signaling in bone is kept to a minimum. In fact, measuring the expression level of the activating and inactivating iodothyronine deiodinases (D2 and D3) helped understand how this is achieved. D3 mRNA was readily detected as early as E14.5 and its expression decreased markedly ( approximately 10-fold) at E18.5, and even more at 14 days after birth (P14). In contrast, D2 mRNA expression increased significantly by E18.5 and markedly ( approximately 2.5-fold) by P14. The reciprocal expression levels of D2 and D3 genes during early bone development along with the absence of a hypothyroidism-induced bone phenotype at this time suggest that coordinated reciprocal deiodinase expression keeps thyroid hormone signaling in bone to very low levels at this early stage of bone development.

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.

Iodothyronine deiodinase enzyme activities in bone

Euthyroid status is essential for normal skeletal development and maintenance of the adult skeleton, but the mechanisms which control supply of thyroid hormone to bone cells are poorly understood. Thyroid hormones enter target cells via monocarboxylate transporter-8 (MCT8), which provides a functional link between thyroid hormone uptake and metabolism in the regulation of T3-action but has not been investigated in bone. Most circulating active thyroid hormone (T3) is derived from outer ring deiodination of thyroxine (T4) mediated by the type 1 deiodinase enzyme (D1). The D2 isozyme regulates intra-cellular T3 supply and determines saturation of the nuclear T3-receptor (TR), whereas a third enzyme (D3) inactivates T4 and T3 to prevent hormone availability and reduce TR-saturation. The aim of this study was to determine whether MCT8 is expressed in the skeleton and whether chondrocytes, osteoblasts and osteoclasts express functional deiodinases. Gene expression was analyzed by RT-PCR and D1, D2 and D3 function by sensitive and highly specific determination of enzyme activities. MCT8 mRNA was expressed in chondrocytes, osteoblasts and osteoclasts at all stages of cell differentiation. D1 activity was undetectable in all cell types, D2 activity was only present in mature osteoblasts whereas D3 activity was evident throughout chondrocyte, osteoblast and osteoclast differentiation in primary cell cultures. These data suggest that T3 availability especially during skeletal development may be limited by D3-mediated catabolism rather than by MCT8 mediated cellular uptake or D2-dependent T3 production.

Iodine deficiency mitigates growth retardation and osteopenia in selenium-deficient rats

Selenium deficiency is associated with impaired bone metabolism and osteopenia in rats. However, it is not known how combined selenium and iodine deficiency affects bone metabolism. Therefore, we investigated the effect of selenium and iodine deficiency on bone metabolism in 2nd-generation selenium- and iodine-deficient rats. Selenium-deficient (Se-), iodine-deficient (I-), selenium- and iodine-deficient (Se-/I-), and control rats (Se+/I+), were pair-fed their respective diets until they were 74 d old. Each pair-fed rat was fed a selenium-adequate diet in the same amount as that consumed the day before by its selenium-deficient counterpart, taking food spillage into account. The skeletal phenotype was analyzed by dual energy X-ray absorptiometry, histomorphometry, and bone metabolism markers. Erythrocyte glutathione peroxidase activity (Gpx) and plasma thyroid hormones were measured to assess selenium and iodine status, respectively. In both Se-/I+ and Se-/I- rats, Gpx was reduced by 99% compared with pair-fed Se+/I+ and Se+/I- rats (P < 0.001). Iodine deficiency reduced plasma thyroxine by 64% in the 2 iodine-deficient groups (P < 0.001). Body weight, tail length, plasma insulin-like growth factor, pituitary growth hormone concentration, and femur and tibia bone mineral density were significantly greater in the Se-/I- rats than in the Se-/I+ rats. This study shows that iodine deficiency mitigated growth retardation and osteopenia in 2nd-generation selenium-deficient rats and suggests that adequate selenium status should be ensured before measures are taken to correct iodine deficiency.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.