Spared bone mass in rats treated with thyroid hormone receptor TR beta-selective compound GC-1

Thyroid hormone receptor Thra and Thrb knockout differentially affects osteoblast biology and thyroid hormone responsiveness in vitro

Thyroid hormones (TH) are important modulators of bone remodeling and thus, thyroid diseases, in particular hyperthyroidism, are able to compromise bone quality and fracture resistance. TH actions on bone are mediated by the thyroid hormone receptors (TR) TRα1 and TRβ1, encoded by Thra and Thrb, respectively. Skeletal phenotypes of mice lacking Thra (Thra0/0 ) and Thrb (Thrb-/- ) are well-described and suggest that TRα1 is the predominant mediator of TH actions in bone. Considering that bone cells might be affected by systemic TH changes seen in these mutant mice, here we investigated the effects of TR knockout on osteoblasts exclusively at the cellular level. Primary osteoblasts obtained from Thra0/0 , Thrb-/- , and respective wildtype (WT) mice were analyzed regarding their differentiation potential, activity and TH responsiveness in vitro. Thra, but not Thrb knockout promoted differentiation and activity of early, mature and late osteoblasts as compared to respective WT cells. Interestingly, while mineralization capacity and expression of osteoblast marker genes and TH target gene Klf9 was increased by TH in WT and Thra-deficient osteoblasts, Thrb knockout mitigated the responsiveness of osteoblasts to short (48 h) and long term (10 d) TH treatment. Further, we found a low ratio of Rankl, a potent osteoclast stimulator, over osteoprotegerin, an osteoclast inhibitor, in Thrb-deficient osteoblasts and in line, supernatants obtained from Thrb-/- osteoblasts reduced numbers of primary osteoclasts in vitro. In accordance to the increased Rankl/Opg ratio in TH-treated WT osteoblasts only, supernatants from these cells, but not from TH-treated Thrb-/- osteoblasts increased the expression of Trap and Ctsk in osteoclasts, suggesting that osteoclasts are indirectly stimulated by TH via TRβ1 in osteoblasts. In conclusion, our study shows that both Thra and Thrb differentially affect activity, differentiation and TH response of osteoblasts in vitro and emphasizes the importance of TRβ1 to mediate TH actions in bone.

Thyroid Hormone Receptor-β Agonists in NAFLD Therapy: Possibilities and Challenges

Nonalcoholic fatty liver disease (NAFLD) is a progressive metabolic liver disease with an unknown pathogenesis and no FDA-approved drug treatment to date. Hypothyroidism has been identified as a risk factor for NAFLD as thyroxine is required for regulating metabolism in adults. Thyroxine has been shown to reduce fat in the livers of murine models with experimentally induced NAFLD. The use of synthetic thyroxine has been shown to increase lipid metabolism leading to weight loss; however, thyroxine has also been shown to cause many side effects, especially in the heart. Overcoming these cardiac side effects involves designing agonists specific to one of the 2 gene subtypes for the thyroid hormone (TH) receptor (TR), TRβ. While the other TH receptor subtype, TRα, is mainly expressed in the heart and is responsible for thyroxine's cardiac function, TRβ is mainly expressed in the liver and is involved in liver function. Using TRβ-specific agonists to treat NAFLD can prevent cardiac and other adverse side effects. Several TRβ-specific agonists have shown positive therapeutic effects in NAFLD animal models and have entered clinical trials. We seek to provide a comprehensive updated reference of TRβ-specific agonists in this review and explore the future therapeutic potential of TRβ-specific activation in the treatment of NAFLD.

Nonalcoholic fatty liver disease and osteoporosis: A potential association with therapeutic implications

Nonalcoholic fatty liver disease (NAFLD) and osteoporosis are two highly prevalent metabolic diseases. Increasing experimental evidence supports a pathophysiological link between NAFLD and osteoporosis. A key feature could be chronic, low-grade inflammation, which characterizes NAFLD and possibly affects bone metabolism. In this context, several factors, including but not limited to receptor activator of nuclear factor kappa-B ligand, osteoprotegerin, osteopontin and osteocalcin, may serve as mediators. In the clinical setting, most but not all epidemiological evidence indicates that NAFLD is associated with lower bone mineral density or osteoporosis in adults. Although an association between NAFLD and osteoporosis has not yet been established, and thus remains speculative, pharmacological considerations already exist. Some of the current and emerging pharmacological options for NAFLD have shown possible anti-osteoporotic properties (eg, vitamin E, obeticholic acid, semaglutide), while others (eg, pioglitazone, canagliflozin) have been associated with increased risk of fractures and may be avoided in patients with NAFLD and concomitant osteoporosis, especially those at high fracture risk. Conversely, some anti-osteoporotic medications (denosumab) might benefit NAFLD, while others (raloxifene) might adversely affect it and, consequently, may be avoided in patients with osteoporosis and NAFLD. If an association between NAFLD and osteoporosis is established, a medication that could target both diseases would be a great advancement. This review summarizes the main experimental and clinical evidence on the potential association between NAFLD and osteoporosis and focuses on treatment considerations derived from this potential association.

Nonalcoholic Fatty Liver Disease and Hypercholesterolemia: Roles of Thyroid Hormones, Metabolites, and Agonists

Background: Thyroid hormones (THs) exert a strong influence on mammalian lipid metabolism at the systemic and hepatic levels by virtue of their roles in regulating circulating lipoprotein, triglyceride (TAG), and cholesterol levels, as well as hepatic TAG storage and metabolism. These effects are mediated by intricate sensing and feedback systems that function at the physiological, metabolic, molecular, and transcriptional levels in the liver. Dysfunction in the pathways involved in lipid metabolism disrupts hepatic lipid homeostasis and contributes to the pathogenesis of metabolic diseases, such as nonalcoholic fatty liver disease (NAFLD) and hypercholesterolemia. There has been strong interest in understanding and employing THs, TH metabolites, and TH mimetics as lipid-modifying drugs. Summary: THs regulate many processes involved in hepatic TAG and cholesterol metabolism to decrease serum cholesterol and intrahepatic lipid content. TH receptor β analogs designed to have less side effects than the natural hormone are currently being tested in phase II clinical studies for NAFLD and hypercholesterolemia. The TH metabolites, 3,5-diiodo-l-thyronine (T2) and T1AM (3-iodothyronamine), have different beneficial effects on lipid metabolism compared with triiodothyronine (T3), although their clinical application is still under investigation. Also, prodrugs and glucagon/T3 conjugates have been developed that direct TH to the liver. Conclusions: TH-based therapies show clinical promise for the treatment of NAFLD and hypercholesterolemia. Strategies for limiting side effects of TH are being developed and may enable TH metabolites and analogs to have specific effects in the liver for treatments of these conditions. These liver-specific effects and potential suppression of the hypothalamic/pituitary/thyroid axis raise the issue of monitoring liver-specific markers of TH action to assess clinical efficacy and dosing of these compounds.

Alternative ligands for thyroid hormone receptors

Thyroid hormone receptors (TRs) are ligand-dependent transcription factors that activate or repress gene transcription, resulting in the regulation of numerous physiological programs. While 3,3',5-L-triiodothyronine is the TR cognate ligand, these receptors can also be activated by various alternative ligands, including endogenous and synthetic molecules capable of inducing diverse active receptor conformations that influence thyroid hormone-dependent signaling pathways. This review mainly discusses current knowledge on 3,5-diiodo-L-thyronine and 3,5,3'-triiodothyroacetic acid, two endogenous molecules that bind to TRs and regulate gene expression; and the molecular interactions between TRs and ligands, like synthetic thyromimetics developed to target specific TR isoforms for tissue-specific regulation of thyroid-related disorders, or endocrine disruptors that have allowed the design of new analogues and revealed essential amino acids for thyroid hormone binding.

Hypothalamic-Pituitary-Thyroid Axis Perturbations in Male Mice by CNS-Penetrating Thyromimetics

Thyromimetics represent a class of experimental drugs that can stimulate tissue-selective thyroid hormone action. As such, thyromimetics should have effects on the hypothalamic-pituitary-thyroid (HPT) axis, but details of this action and the subsequent effects on systemic thyroid hormone levels have not been reported to date. Here, we compare the HPT-axis effects of sobetirome, a well-studied thyromimetic, with Sob-AM2, a newly developed prodrug of sobetirome that targets sobetirome distribution to the central nervous system (CNS). Similar to endogenous thyroid hormone, administration of sobetirome and Sob-AM2 suppress HPT-axis gene transcript levels in a manner that correlates to their specific tissue distribution properties (periphery vs CNS, respectively). Dosing male C57BL/6 mice with sobetirome and Sob-AM2 at concentrations ≥10 μg/kg/d for 29 days induces a state similar to central hypothyroidism characterized by depleted circulating T4 and T3 and normal TSH levels. However, despite the systemic T4 and T3 depletion, the sobetirome- and Sob-AM2-treated mice do not show signs of hypothyroidism, which may result from the presence of the thyromimetic in the thyroid hormone-depleted background.

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.

Excess Maternal Thyroxine Alters the Proliferative Activity and Angiogenic Profile of Growth Cartilage of Rats at Birth and Weaning

Objective The aim of this study was to unravel the mechanisms by which thyroxine affects skeletal growth by evaluating proliferative activity and angiogenic profile of growth cartilage of neonatal and weanling rats. Methods Sixteen adult Wistar rats were equally divided into 2 groups: control and treated with thyroxine during pregnancy and lactation. The weight, measurement of plasma free T4 and thyroids, femurs' histomorphometric analysis, and proliferative activity and angiogenic profile by immunohistochemical or real-time reverse transcriptase-polymerase chain reaction in growth cartilage was performed. Data were analyzed using Student's t test. Results The free T4 was significantly higher in the treated rats. However, the height of the follicular epithelium of the thyroid in newborns was significantly lower in the treated group. The excess maternal thyroxine significantly reduced the body weight and length of the femur in the offspring but significantly increased the thickness of trabecular bone and changed the height of the zones of the growth plate. Furthermore, excess maternal thyroxine reduced cell proliferation and vascular endothelial growth factor (VEGF) expression in the growth cartilage of newborn and 20-day-old rats ( P < 0.05). There was also a reduction in the immunohistochemical expression of Tie2 in the cartilaginous epiphysis of the newborns and FLK-1 in the articular cartilage of 20-day-old rats. No significant difference was observed in Ang2 expression. Conclusions The excess maternal thyroxine during pregnancy and lactation reduced endochondral bone growth in the progeny and reduced the proliferation rate and VEGF, Flk-1, and Tie2 expression in the cartilage of growing rats without altering the mRNA expression of Ang1 and Ang2.

GC-1: A Thyromimetic With Multiple Therapeutic Applications in Liver Disease

Thyroid hormones (THs), namely, 3,5,3'-triiodo-l-thyronine (T3) and 3,5,3',5'-tetraiodo-l-thyronine (thyroxine or T4), influence a variety of physiological processes that have important implications in fetal development, metabolism, cell growth, and proliferation. While THs elicit several beneficial effects on lipid metabolism and improve myocardial contractility, these therapeutically desirable effects are associated to a thyrotoxic state that severely limits the possible use of THs as therapeutic agents. Therefore, several efforts have been made to develop T3 analogs that could retain the beneficial actions (triglyceride, cholesterol, obesity, and body mass lowering) without the adverse TH-dependent side effects. This goal was achieved by the synthesis of TRβ-selective agonists. In this review, we summarize the current knowledge on the effects of one of the best characterized TH analogs, the TRβ1-selective thyromimetic, GC-1. In particular, we review some of the effects of GC-1 on different liver disorders, with reference to its possible clinical application. A brief comment on the possible therapeutic use of GC-1 in extrahepatic disorders is also included.

Physiological Bone Remodeling: Systemic Regulation and Growth Factor Involvement

Bone remodeling is essential for adult bone homeostasis. It comprises two phases: bone formation and resorption. The balance between the two phases is crucial for sustaining bone mass and systemic mineral homeostasis. This review highlights recent work on physiological bone remodeling and discusses our knowledge of how systemic and growth factors regulate this process.

Sobetirome prodrug esters with enhanced blood-brain barrier permeability

There is currently great interest in developing drugs that stimulate myelin repair for use in demyelinating diseases such as multiple sclerosis. Thyroid hormone plays a key role in stimulating myelination during development and also controls the expression of important genes involved in myelin repair in adults. Because endogenous thyroid hormone in excess lacks a generally useful therapeutic index, it is not used clinically for indications other than hormone replacement; however, selective thyromimetics such as sobetirome offer a therapeutic alternative. Sobetirome is the only clinical-stage thyromimetic that is known to cross the blood-brain-barrier (BBB) and we endeavored to increase the BBB permeability of sobetirome using a prodrug strategy. Ester prodrugs of sobetirome were prepared based on literature reports of improved BBB permeability with other carboxylic acid containing drugs and BBB permeability was assessed in vivo. One sobetirome prodrug, ethanolamine ester 11, was found to distribute more sobetirome to the brain compared to an equimolar peripheral dose of unmodified sobetirome. In addition to enhanced brain levels, prodrug 11 displayed lower sobetirome blood levels and a brain/serum ratio that was larger than that of unmodified sobetirome. Thus, these data indicate that an ester prodrug strategy applied to sobetirome can deliver increased concentrations of the active drug to the central nervous system (CNS), which may prove useful in the treatment of CNS disorders.

Sustained zero-order delivery of GC-1 from a nanochannel membrane device alleviates metabolic syndrome

BACKGROUND/OBJECTIVES

Our objective was to assess the sustained, low-dose and constant administration of the thyroid receptor-β (TRβ)-selective agonist GC-1 (sobetirome) from a novel nanochannel membrane device (NMD) for drug delivery. As it known to speed up metabolism, accomplish weight loss, improve cholesterol levels and possess anti-diabetic effects, GC-1 was steadily administered by our NMD, consisting of an implantable nanochannel membrane, as an alternative to conventional daily administration, which is subject to compliance issues in clinical settings.

SUBJECTS/METHODS

Diet-induced obese C57BL/J6 male mice were fed a very high-fat diet (VHFD) and received NMD implants subcutaneously. Ten mice per group received capsules containing GC-1 or phosphate-buffered saline (control). Weight, lean and fat mass, as well as cholesterol, triglycerides, insulin and glucose, were monitored for 24 days. After treatment, plasma levels of thyroid-stimulating hormone (TSH) and thyroxine were compared. mRNA levels of a panel of thermogenic markers were examined using real-time PCR in white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue, liver and local inflammatory response to the implant were examined histologically. Pancreatic islet number and β-cell area were assessed.

RESULTS

GC-1 released from the NMD reversed VHFD-induced obesity and normalized serum cholesterol and glycemia. Significant reductions in body weight and fat mass were observed within 10 days, whereas reductions in serum cholesterol and glucose levels were seen within 7 days. The significant decrease in TSH was consistent with TRβ selectivity for GC-1. Levels of transcript for Ucp1 and thermogenic genes PGC1a, Cidea, Dio2 and Cox5a showed significant upregulation in WAT in NMD-GC-1-treated mice, but decreased in BAT. Although mice treated by NMD-GC-1 showed a similar number of pancreatic islets, they exhibited significant increase in β-cell area.

CONCLUSIONS

Our data demonstrate that the NMD implant achieves steady administration of GC-1, offering an effective and tightly controlled molecular delivery system for treatment of obesity and metabolic disease, thereby addressing compliance.

Somatostatin Analogue Treatment of a TSH-Secreting Adenoma Presenting With Accelerated Bone Metabolism and a Pericardial Effusion: A Case Report

Increased bone turnover and other less frequent comorbidities of hyperthyroidism, such as heart failure, have only rarely been reported in association with central hyperthyroidism due to a thyrotropin (TSH)-secreting pituitary adenoma (TSHoma). Treatment is highly empirical and relies on eliminating the tumor and the hyperthyroid state.We report here an unusual case of a 39-year-old man who was initially admitted for management of pleuritic chest pain and fever of unknown origin. Diagnostic work up confirmed pericarditis and pleural effusion both refractory to treatment. The patient had a previous history of persistently elevated levels of alkaline phosphatase (ALP), indicative of increased bone turnover. He had also initially been treated with thyroxine supplementation due to elevated TSH levels. During the diagnostic process a TSHoma was revealed. Thyroxine was discontinued, and resection of the pituitary tumor followed by treatment with a somatostatin analog led to complete recession of the effusions, normalization of ALP, and shrinkage of pituitary tumor.Accelerated bone metabolism and pericardial and pleural effusions attributed to a TSHoma may resolve after successful treatment of the tumor. The unexpected clinical course of this case highlights the need for careful long-term surveillance in patients with these rare pituitary adenomas.

Thyroid Hormone Potentially Benefits Multiple Sclerosis via Facilitating Remyelination

Myelin destruction due to inflammatory damage of oligodendrocytes (OLs) in conjunction with axonal degeneration is one of the major histopathological hallmarks of multiple sclerosis (MS), a common autoimmune disorder affecting the central nervous system (CNS). Therapies over the last 20 years mainly focus on the immune system and, more specifically, on the modulation of immune cell behavior. It seems to be effective in MS with relapse, while it is of little benefit to progressive MS in which neurodegeneration following demyelination outweighs inflammation. Otherwise, remyelination, as a result of oligodendrocyte production from oligodendrocyte precursor cells (OPCs), is considered to be a potential target for the treatment of progressive MS. In this review, positive effects of remyelination on MS will be discussed in view of the critical role played by thyroid hormone (TH), focusing on the following points: (1) promising treatment of TH on MS that potentially targets to remyelination; (2) the active role of TH that is able to promote remyelination; (3) the regulative role of TH that works on endogenous stem and precursor cells; (4) the effect of TH on gene transcription; and (5) a working hypothesis which is developed that TH can alleviate MS by promoting remyelination, and the mechanism of which is its regulative role in gene transcription of OPCs.

Guidelines for the treatment of hypothyroidism: prepared by the american thyroid association task force on thyroid hormone replacement

BACKGROUND

A number of recent advances in our understanding of thyroid physiology may shed light on why some patients feel unwell while taking levothyroxine monotherapy. The purpose of this task force was to review the goals of levothyroxine therapy, the optimal prescription of conventional levothyroxine therapy, the sources of dissatisfaction with levothyroxine therapy, the evidence on treatment alternatives, and the relevant knowledge gaps. We wished to determine whether there are sufficient new data generated by well-designed studies to provide reason to pursue such therapies and change the current standard of care. This document is intended to inform clinical decision-making on thyroid hormone replacement therapy; it is not a replacement for individualized clinical judgment.

METHODS

Task force members identified 24 questions relevant to the treatment of hypothyroidism. The clinical literature relating to each question was then reviewed. Clinical reviews were supplemented, when relevant, with related mechanistic and bench research literature reviews, performed by our team of translational scientists. Ethics reviews were provided, when relevant, by a bioethicist. The responses to questions were formatted, when possible, in the form of a formal clinical recommendation statement. When responses were not suitable for a formal clinical recommendation, a summary response statement without a formal clinical recommendation was developed. For clinical recommendations, the supporting evidence was appraised, and the strength of each clinical recommendation was assessed, using the American College of Physicians system. The final document was organized so that each topic is introduced with a question, followed by a formal clinical recommendation. Stakeholder input was received at a national meeting, with some subsequent refinement of the clinical questions addressed in the document. Consensus was achieved for all recommendations by the task force.

RESULTS

We reviewed the following therapeutic categories: (i) levothyroxine therapy, (ii) non-levothyroxine-based thyroid hormone therapies, and (iii) use of thyroid hormone analogs. The second category included thyroid extracts, synthetic combination therapy, triiodothyronine therapy, and compounded thyroid hormones.

CONCLUSIONS

We concluded that levothyroxine should remain the standard of care for treating hypothyroidism. We found no consistently strong evidence for the superiority of alternative preparations (e.g., levothyroxine-liothyronine combination therapy, or thyroid extract therapy, or others) over monotherapy with levothyroxine, in improving health outcomes. Some examples of future research needs include the development of superior biomarkers of euthyroidism to supplement thyrotropin measurements, mechanistic research on serum triiodothyronine levels (including effects of age and disease status, relationship with tissue concentrations, as well as potential therapeutic targeting), and long-term outcome clinical trials testing combination therapy or thyroid extracts (including subgroup effects). Additional research is also needed to develop thyroid hormone analogs with a favorable benefit to risk profile.

Thyroid hormone interacts with the sympathetic nervous system to modulate bone mass and structure in young adult mice

To investigate whether thyroid hormone (TH) interacts with the sympathetic nervous system (SNS) to modulate bone mass and structure, we studied the effects of daily T3 treatment in a supraphysiological dose for 12 wk on the bone of young adult mice with chronic sympathetic hyperactivity owing to double-gene disruption of adrenoceptors that negatively regulate norepinephrine release, α(2A)-AR, and α(2C)-AR (α(2A/2C)-AR(-/-) mice). As expected, T3 treatment caused a generalized decrease in the areal bone mineral density (aBMD) of WT mice (determined by DEXA), followed by deleterious effects on the trabecular and cortical bone microstructural parameters (determined by μCT) of the femur and vertebra and on the biomechanical properties (maximum load, ultimate load, and stiffness) of the femur. Surprisingly, α(2A/2C)-AR(-/-) mice were resistant to most of these T3-induced negative effects. Interestingly, the mRNA expression of osteoprotegerin, a protein that limits osteoclast activity, was upregulated and downregulated by T3 in the bone of α(2A/2C)-AR(-/-) and WT mice, respectively. β1-AR mRNA expression and IGF-I serum levels, which exert bone anabolic effects, were increased by T3 treatment only in α(2A/2C)-AR(-/-) mice. As expected, T3 inhibited the cell growth of calvaria-derived osteoblasts isolated from WT mice, but this effect was abolished or reverted in cells isolated from KO mice. Collectively, these findings support the hypothesis of a TH-SNS interaction to control bone mass and structure of young adult mice and suggests that this interaction may involve α2-AR signaling. Finally, the present findings offer new insights into the mechanisms through which TH regulates bone mass, structure, and physiology.

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.

Thyroid hormone regulation of metabolism

Thyroid hormone (TH) is required for normal development as well as regulating metabolism in the adult. The thyroid hormone receptor (TR) isoforms, α and β, are differentially expressed in tissues and have distinct roles in TH signaling. Local activation of thyroxine (T4), to the active form, triiodothyronine (T3), by 5'-deiodinase type 2 (D2) is a key mechanism of TH regulation of metabolism. D2 is expressed in the hypothalamus, white fat, brown adipose tissue (BAT), and skeletal muscle and is required for adaptive thermogenesis. The thyroid gland is regulated by thyrotropin releasing hormone (TRH) and thyroid stimulating hormone (TSH). In addition to TRH/TSH regulation by TH feedback, there is central modulation by nutritional signals, such as leptin, as well as peptides regulating appetite. The nutrient status of the cell provides feedback on TH signaling pathways through epigentic modification of histones. Integration of TH signaling with the adrenergic nervous system occurs peripherally, in liver, white fat, and BAT, but also centrally, in the hypothalamus. TR regulates cholesterol and carbohydrate metabolism through direct actions on gene expression as well as cross-talk with other nuclear receptors, including peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR), and bile acid signaling pathways. TH modulates hepatic insulin sensitivity, especially important for the suppression of hepatic gluconeogenesis. The role of TH in regulating metabolic pathways has led to several new therapeutic targets for metabolic disorders. Understanding the mechanisms and interactions of the various TH signaling pathways in metabolism will improve our likelihood of identifying effective and selective targets.

A selective thyroid hormone β receptor agonist enhances human and rodent oligodendrocyte differentiation

Nerve conduction within the mammalian central nervous system is made efficient by oligodendrocyte-derived myelin. Historically, thyroid hormones have a well described role in regulating oligodendrocyte differentiation and myelination during development; however, it remains unclear which thyroid hormone receptors are required to drive these effects. This is a question with clinical relevance since nonspecific thyroid receptor stimulation can produce deleterious side-effects. Here we report that GC-1, a thyromimetic with selective thyroid receptor β action and a potentially limited side-effect profile, promotes in vitro oligodendrogenesis from both rodent and human oligodendrocyte progenitor cells. In addition, we used in vivo genetic fate tracing of oligodendrocyte progenitor cells via PDGFαR-CreER;Rosa26-eYFP double-transgenic mice to examine the effect of GC-1 on cellular fate and find that treatment with GC-1 during developmental myelination promotes oligodendrogenesis within the corpus callosum, occipital cortex and optic nerve. GC-1 was also observed to enhance the expression of the myelin proteins MBP, CNP and MAG within the same regions. These results indicate that a β receptor selective thyromimetic can enhance oligodendrocyte differentiation in vitro and during developmental myelination in vivo and warrants further study as a therapeutic agent for demyelinating models.

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.

Cross-talk between the thyroid and liver: A new target for nonalcoholic fatty liver disease treatment

Nonalcoholic fatty liver disease (NAFLD) has been recognized as the most common liver metabolic disease, and it is also a burgeoning health problem that affects one-third of adults and is associated with obesity and insulin resistance now. Thyroid hormone (TH) and its receptors play a fundamental role in lipid metabolism and lipid accumulation in the liver. It is found that thyroid receptor and its isoforms exhibit tissue-specific expression with a variety of functions. TRβ1 is predominantly expressed in the brain and adipose tissue and TRβ2 is the major isoform in the liver, kidney and fat. They have different functions and play important roles in lipid metabolism. Recently, there are many studies on the treatment of NAFLD with TH and its analogues. We review here that thyroid hormone and TR are a potential target for pharmacologic treatments. Lipid metabolism and lipid accumulation can be regulated and reversed by TH and its analogues.

Thyroid hormone analogues for the treatment of metabolic disorders: new potential for unmet clinical needs?

OBJECTIVE

To provide a comprehensive review of the discovery and development of selective thyroid hormone receptor agonists and provide a discussion of their use in hyperlipidemia, obesity, and type 2 diabetes mellitus.

METHODS

Preclinical and clinical English language literature from 1930 to present was reviewed and thematically summarized.

RESULTS

Human trials have shown that thyroid hormone receptor β (TRβ) agonists effectively lower low-density lipoprotein, triglycerides, apolipoprotein B, and lipoprotein(a) levels. In preclinical studies, TRβ agonists enhance reverse cholesterol transport and decrease atherosclerosis in selected models. While animal data suggest these drugs may have additional utility to modulate weight and improve glucose homeostasis, human studies have not shown similar results.

CONCLUSION

TRβ agonists are a novel therapeutic class for lipid management. Their mechanism of action for lipid lowering is distinct from statin drugs, suggesting a strong possibility for synergistic effects with combined therapy. The long-term effects of these drugs on cardiovascular outcomes, however, are unknown. Recently, the development of the most promising agent in this class, eprotirome, was halted over toxicology concerns following long-term canine studies. Consequently, the future of contemporary TRβ agonists is unclear. The creation of a next generation of TRβ agonists that provide additional tissue specific effects or bind TRβ with even higher selectivity may lead to improved safety and efficacy and allow for their application to other metabolic disorders like obesity and type 2 diabetes mellitus.

Mechanisms of thyroid hormone action

Our understanding of thyroid hormone action has been substantially altered by recent clinical observations of thyroid signaling defects in syndromes of hormone resistance and in a broad range of conditions, including profound mental retardation, obesity, metabolic disorders, and a number of cancers. The mechanism of thyroid hormone action has been informed by these clinical observations as well as by animal models and has influenced the way we view the role of local ligand availability; tissue and cell-specific thyroid hormone transporters, corepressors, and coactivators; thyroid hormone receptor (TR) isoform-specific action; and cross-talk in metabolic regulation and neural development. In some cases, our new understanding has already been translated into therapeutic strategies, especially for treating hyperlipidemia and obesity, and other drugs are in development to treat cardiac disease and cancer and to improve cognitive function.

Thyroid hormone receptor β mediates thyroid hormone effects on bone remodeling and bone mass

Excess thyroid hormone (TH) in adults causes osteoporosis and increases fracture risk. However, the mechanisms by which TH affects bone turnover are not elucidated. In particular, the roles of thyroid hormone receptor (TR) isotypes in the mediation of TH effects on osteoblast-mediated bone formation and osteoclast-mediated bone resorption are not established. In this study we have induced experimental hypothyroidism or hyperthyroidism in adult wild-type, TRα- or TRβ-deficient mice and analyzed the effects of TH status on the structure and remodeling parameters of trabecular bone. In wild-type mice, excess TH decreased bone volume and mineralization. High TH concentrations were associated with a high bone-resorption activity, assessed by increased osteoclast surfaces and elevated concentrations of serum bone-resorption markers. Serum markers of bone formation also were higher in TH-treated mice. TRα deficiency did not prevent TH action on bone volume, bone mineralization, bone formation, or bone resorption. In contrast, TRβ deficiency blocked all the early effects of excess TH observed in wild-type mice. However, prolonged exposure to low or high TH concentrations of TRβ-deficient mice induced mild modifications of bone structure and remodeling parameters. Together our data suggest that TRβ receptors mediate the acute effects produced by transient changes of TH concentrations on bone remodeling, whereas TRα receptors mediate long-term effects of chronic alterations of TH metabolism. These data shed new light on the respective roles of TRs in the control of bone metabolism by TH.

Impaired metabolic effects of a thyroid hormone receptor beta-selective agonist in a mouse model of diet-induced obesity

BACKGROUND

The use of selective agonists of the thyroid hormone receptor isoform beta (TRbeta) has been linked to metabolic improvement in animal models of diet-induced obesity, nonalcoholic liver disease, and genetic hypercholesterolemia.

METHODS

To identify potential target tissues of such compounds, we exposed primary murine brown adipocytes and skeletal myocytes for 24 hours to 50 nM GC-24, a highly selective TRbeta agonist. GC-24 (17 ng/[g BW.day] for 36 days) was also tested in a mouse model of diet-induced obesity.

RESULTS

While the brown adipocytes responded to GC-24, with 17%-400% increases in the expression of 12 metabolically relevant genes, the myocytes remained largely unresponsive to GC-24 treatment. In control mice kept on chow diet, GC-24 treatment accelerated energy expenditure by about 15% and limited body weight gain by about 50%. However, in the obese animals the GC-24-mediated reduction in body weight gain dropped to only 20%, while energy expenditure remained unaffected. In addition, an analysis of gene expression in the skeletal muscle, brown adipose tissue, and liver of these obese animals failed to identify a conclusive GC-24 transcriptome footprint.

CONCLUSION

Feeding a high-fat diet impairs most of the beneficial metabolic effects associated with treatment with TRbeta-selective agonists.

A TRbeta-selective agonist confers resistance to diet-induced obesity

Thyroid hormone receptor beta (TRbeta also listed as THRB on the MGI Database)-selective agonists activate brown adipose tissue (BAT) thermogenesis, while only minimally affecting cardiac activity or lean body mass. Here, we tested the hypothesis that daily administration of the TRbeta agonist GC-24 prevents the metabolic alterations associated with a hypercaloric diet. Rats were placed on a high-fat diet and after a month exhibited increased body weight (BW) and adiposity, fasting hyperglycemia and glucose intolerance, increased plasma levels of triglycerides, cholesterol, nonesterified fatty acids and interleukin-6. While GC-24 administration to these animals did not affect food ingestion or modified the progression of BW gain, it did increase energy expenditure, eliminating the increase in adiposity without causing cardiac hypertrophy. Fasting hyperglycemia remained unchanged, but treatment with GC-24 improved glucose tolerance by increasing insulin sensitivity, and also normalized plasma triglyceride levels. Plasma cholesterol levels were only partially normalized and liver cholesterol content remained high in the GC-24-treated animals. Gene expression in liver, skeletal muscle, and white adipose tissue was only minimally affected by treatment with GC-24, with the main target being BAT. In conclusion, during high-fat feeding treatment with the TRbeta-selective agonist, GC-24 only partially improves metabolic control probably as a result of accelerating the resting metabolic rate.

Thyroid hormone mimetics: potential applications in atherosclerosis, obesity and type 2 diabetes

Thyroid hormones influence heart rate, serum lipids, metabolic rate, body weight and multiple aspects of lipid, carbohydrate, protein and mineral metabolism. Although increased thyroid hormone levels can improve serum lipid profiles and reduce fat, these positive effects are counterbalanced by harmful effects on the heart, muscle and bone. Thus, attempts to use thyroid hormones for cholesterol-lowering and weight loss purposes have so far been limited. However, over the past decade, thyroid hormone analogues that are capable of uncoupling beneficial effects from deleterious effects have been developed. Such drugs could serve as powerful new tools to address two of the largest medical problems in developed countries--atherosclerosis and obesity.

The thyroid hormone receptor (TR) beta-selective agonist GC-1 inhibits proliferation but induces differentiation and TR beta mRNA expression in mouse and rat osteoblast-like cells

Previous studies showed anabolic effects of GC-1, a triiodothyronine (T3) analogue that is selective for both binding and activation functions of thyroid hormone receptor (TR) beta1 over TRalpha1, on bone tissue in vivo. The aim of this study was to investigate the responsiveness of rat (ROS17/2.8) and mouse (MC3T3-E1) osteoblast-like cells to GC-1. As expected, T3 inhibited cellular proliferation and stimulated mRNA expression of osteocalcin or alkaline phosphatase in both cell lineages. Whereas equimolar doses of T3 and GC-1 equally affected these parameters in ROS17/2.8 cells, the effects of GC-1 were more modest compared to those of T3 in MC3T3-E1 cells. Interestingly, we showed that there is higher expression of TRalpha1 than TRbeta1 mRNA in rat (approximately 20-90%) and mouse (approximately 90-98%) cell lineages and that this difference is even higher in mouse cells, which highlights the importance of TRalpha1 to bone physiology and may partially explain the modest effects of GC-1 in comparison with T3 in MC3T3-E1 cells. Nevertheless, we showed that TRbeta1 mRNA expression increases (approximately 2.8- to 4.3-fold) as osteoblastic cells undergo maturation, suggesting a key role of TRbeta1 in mediating T3 effects in the bone forming cells, especially in mature osteoblasts. It is noteworthy that T3 and GC-1 induced TRbeta1 mRNA expression to a similar extent in both cell lineages (approximately 2- to 4-fold), indicating that both ligands may modulate the responsiveness of osteoblasts to T3. Taken together, these data show that TRbeta selective T3 analogues have the potential to directly induce the differentiation and activity of osteoblasts.

[Thyroid hormone analogs: an important biological supply and new therapeutic possibilities]

Thyroid hormones [predominantly 3, 5, 3 -I- iodothyronine (T3)] regulate cholesterol and lipoprotein metabolism but cardiac effects restrict their use as hypolipidemic drugs. New molecules have been developped which target specifically the thyroid hormone receptor ss, predominant isoform in liver. The first thyroid hormone agonist, called GC1, has selective actions compared to T3. In animals, GC1 reduced serum cholesterol and serum triglycerides, probably by stimulation important steps in reverse cholesterol transport. Other selective thyromimetic, KB- 2115 and KB - 141 have similar effects. Another class of thyroid hormone analogs, the thyronamines have emerged recently but the basic biology of this new class of endogenous thyroid hormone remains to better understood. Therefore, these molecules may be a potentially treatment for obesity and reduction cholesterol, triglycerides and lipoprotein (a). To date the studies in human are preliminary. Tolerance and efficacy of these drugs are still under investigation.

Metabolic effects of thyroid hormone derivatives

The processes and pathways mediating the intermediary metabolism of carbohydrates, lipids, and proteins are all affected by thyroid hormones (THs) in almost all tissues. Particular attention has been devoted by scientists to the effects of THs on lipid metabolism. Among others, effects related to cholesterol, lipid handling, and cardiac performance have been the subject of study. Many reports are present in the literature concerning the calorigenic effect of THs, with most of them aimed at identifying the molecular basis of this effect. However, at the moment the mechanism(s) underlying the metabolic effects of THs remain to be elucidated. THs exert most of their effects though TH receptors (TRs). However, some effects of THs cannot be explained by a nuclear-mediated pathway, and recently an increasing number of nonnuclear actions have been described, which can provide a regulatory system of which the effects differ from those mediated on the transcriptional level by TRs. Some of the TH derivatives (naturally occurring metabolites and analogs) possess biological activities. TH-related biological effects have been described for physiological products such as tetraiodothyroacetic acid (Tetrac) and triiodothyroacetic acid (Triac) (via oxidative deamination and decarboxylation of thyroxine [T4] and triiodothyronine [T3] alanine chain), 3,3',5'-triiodothyronine (rT3) (via T4 and T3 deiodination), 3,3'-diiodothyronine (3,3'-T2) and 3,5-diiodothyronine (T2) (via T4, T3, and rT3 deiodination), and 3-iodothyronamine (T1AM) and thyronamine (T0AM) (via T4 and T3 deiodination and amino acid decarboxylation), as well as for TH structural analogs, such as 3,5,3'-triiodothyropropionic acid (Triprop), 3,5-dibromo-3-pyridazinone-l-thyronine (L-940901), N-[3,5-dimethyl-4-(4'-hydroxy-3'-isopropylphenoxy)-phenyl]-oxamic acid (CGS 23425), 3,5-dimethyl-4[(4'-hydroxy-3'-isopropylbenzyl)-phenoxy] acetic acid (GC-1), 3,5-dichloro-4[(4-hydroxy-3-isopropylphenoxy)phenyl] acetic acid (KB-141), and 3,5-diiodothyropropionic acid (DITPA). Most of these compounds have interesting properties: counteracting lipid accumulation, reducing cholesterol level, and increasing lipid metabolism without cardiotoxic effects. Hopefully, further studies on basic mechanisms of such compounds will be harbingers of more knowledge on the metabolic effects of TH derivatives and on their possible clinical application.

Effects of thyroid hormone analogs on lipid metabolism and thermogenesis

Thyroid hormone affects in a myriad of biological processes such as development, growth, and metabolic control. Triiodothyronine (T3) is the biologically active form of thyroid hormone that acts through nuclear receptors, TRalpha and TRbeta, regulating gene expression. Given that the distribution of these receptors is heterogeneous amongst the different tissues, it is not surprising that some physiological effects of T3 are isoform specific. For example, while TRalpha is the dominant receptor in the brain and skeletal system and mediates most of the synergism between T3 and the sympathetic signaling pathway in the heart, TRbeta is abundant in liver and is probably the isoform that mediates most of the T3 effects on lipid metabolism. Thus, it makes sense to develop compounds that selectively act on either one of the TRs, allowing for the activation of specific T3-dependent pathways. This article reviews the recent progress made in this area, focusing on the physiological effects of compounds that lower serum cholesterol and decrease fat mass, as they spare skeletal muscle and bone masses, as well as the heart. The available studies indicate that achieving selective activation of different TR-mediated pathways is a promising strategy for treating lipid disorders and obesity.

A child with resistance to thyroid hormone without thyroid hormone receptor gene mutation: a 20-year follow-up

We report here the 20-year follow-up study of a male subject diagnosed at 15 months of age as a sporadic case of pituitary resistance to thyroid hormone on the combination of clinical hyperthyroidism, elevated serum thyroid hormone (TH) levels and inappropriate thyrotropin (TSH). On D-thyroxine (D-T(4)) therapy from 30 months of age to 12.5 years, hyperactivity and hyperthyroid signs and symptoms as well as growth abnormalities improved, serum L-thyroxine (L-T(4)) enantiomer normalized, and basal and stimulated TSH decreased significantly without complete suppression. After 8 years off D-T(4), at 20 years of age, clinical status was normal despite persisting high TH levels and inappropriate TSH. Evolution of serum markers of TH action and echocardiography measurements followed up from 15 months to 20 years of age either in basal condition or on triiodothyronine (T(3)), as well as the sequential determination of bone mineral density suggest differences in the tissue responses to T(3): normal in bone with a high remodelling rate, heterogeneity for various hepatic markers, and decreased at heart level. No mutations were found in the coding sequence of TRbeta1, TRbeta2, TRalpha1, RXRgamma, SMRT, NCoR1, and NCoA1. In this patient the putative long-term effects of the persisting high bone resorption are unknown.

Mitochondrial uncoupling as a target in the treatment of obesity

PURPOSE OF REVIEW

Obesity is associated with many health problems and its prevalence is rapidly increasing worldwide. Very few pharmaceutical compounds are available for obesity treatment. Strategies for the development of compounds can be targeted to the outcomes of reduced dietary energy intake and/or increased energy expenditure/thermogenesis. In this review, we focus on recent discoveries that advance our understanding of mitochondrial uncoupling as a target for the treatment of obesity. There are various mechanisms whereby uncoupling can occur and for the purpose of this review, we elaborate upon the uncoupling that can occur (1) through the original uncoupling protein, UCP1, in brown adipocytes, or in 'converted' white adipose tissue, and (2) in skeletal muscle.

RECENT FINDINGS

Studies have identified a number of novel receptors and regulatory proteins involved in the emergence of brown adipocytes in white adipose tissue. Molecular and pharmacologic approaches in knockout and transgenic mice have demonstrated their relevance to obesity treatment. Recent research into uncoupling mechanisms in skeletal muscle indicates that uncoupling can occur through basal and inducible processes.

SUMMARY

Uncoupling is a naturally occurring phenomenon whose underlying mechanisms require substantial further study for the development of antiobesity therapies.

Thyroid and bone

This article provides a summary of the numerous interactions between the thyroid gland and the skeleton, in the normal state, in disorders of thyroid function and as a result of thyroid malignancy. It recaps the current understanding of bone growth and development in the endochondral growth plate and the normal mechanisms of mature bone remodeling. The actions of thyroid hormones on these processes are described, and the clinical impact of thyroid disorders and their treatments on the bone are summarized. Finally, our current understanding of the physiology of bone metastases from thyroid cancer is covered.

Isoflavone-supplemented soy yoghurt associated with resistive physical exercise increase bone mineral density of ovariectomized rats

OBJECTIVE

To determine the effects produced by the ingestion of a fermented soy product (soy yoghurt), supplemented with isoflavones and associated with a resistive exercise program, on the bone metabolism of mature ovariectomized (Ovx) and sham-ovariectomized (sham-Ovx) rats.

METHODS

A total of 56 rats were used. They were divided into 2 sedentary control groups, the Ovx control group (C-Ovx) and the sham-Ovx control group (C-Sovx), each with 7 sedentary animals, and 2 treated groups, Ovx and sham-Ovx, with 21 animals each. These two treated groups were subdivided into three subgroups of seven animals each, which received the following treatments: consuming the soy yoghurt+sedentary, only subjected to resistive exercise, and consuming the soy yoghurt+resistive exercise. Both the program of resistive exercise and the consumption of soy yoghurt (at 3 mL/(kg body weight day)) continued for 12 weeks. The soy yoghurt was supplemented with isoflavones at 50mg/100g of product. The animals were sacrificed and their right-side femurs and tibias removed and assessed for bone mineral density (BMD). The alkaline phosphatase activity (AP) was determined in the blood serum.

RESULTS

There was a significant increase in both femur and tibia BMD values and in serum alkaline phosphatase activity in all the treated subgroups, compared with the control groups (p<0.05).

CONCLUSION

The ingestion of the soy yoghurt supplemented with isoflavones was capable of preventing a loss of bone mass in Ovx rats and of increasing bone mass in sham rats, whilst the resistive exercise program was effective in augmenting the bone mass in sham and Ovx rats.

Thyroid hormone-induced demineralisation of the vertebral skeleton of the eel, Anguilla anguilla

The role of thyroid hormones (TH) in bone remodelling is controversial. Indeed, in humans, while they are necessary for normal growth and development, their overproduction can induce important mineral bone loss and osteoporosis. Intense bone resorption is a natural phenomenon also observed in some teleosts, during reproductive migration and fasting. Our work aimed at investigating the effects of chronic treatments with TH (thyroxin, T4 or triiodothyronine, T3) on bone resorption in a migratory fish, the European eel (Anguilla anguilla), a representative species of an ancient group of teleosts (Elopomorphs). The incineration method showed that TH induced a significant mineral loss in eel vertebral skeleton. Histology and histophysical (qualitative and quantitative microradiographs) methods were then applied to vertebral sections to determine which types of resorption were induced by TH. Quantitative image analysis of microradiographs showed that TH significantly increased the porosity of the vertebrae, demonstrating the induction of a severe bone loss. Histology revealed the appearance of large osteoclastic lacunae, indicating a stimulation of osteoclastic resorption. Quantitative image analysis of ultrathin microradiographs showed a significant increase of the size of osteocytic lacunae, indicating a stimulation of periosteocytic osteolysis. Finally, quantitative microradiographs indicated a significant fall of mineralisation degree. TH treatments did not stimulate the production of the calcium-bonded lipo-phospho-protein vitellogenin, indicating that TH-induced bone demineralisation was not mediated by any indirect effect on vitellogenesis. Our study demonstrates that TH may participate in the mobilisation of bone mineral stores in the eel, by inducing different types of vertebral bone resorption, such as osteoclastic resorption and periosteocytic osteolysis. These data suggest that the stimulatory action of TH on bone resorption may be an ancient regulatory mechanism in vertebrates.

Selective agonists and antagonists to thyroid hormone action

Selective thyromimetics have been designed and shown to exhibit some of the beneficial effects of thyroid hormones, such as lowering of cholesterol and weight reduction, without the adverse thyroid hormone action on muscle, bone, and heart rate. Progress has also been made in attempting to treat hyperthyroidism by synthesizing antagonists that block thyroid hormone action, at the level of the thyroid hormone receptor or of the thyrotropin receptor. Clinical trials are still awaited, however, to verify whether these potentially promising agents will indeed prove to be of clinical therapeutic value.

The thyroid hormone receptor beta-specific agonist GC-1 selectively affects the bone development of hypothyroid rats

We investigated the effects of GC-1, a TRbeta-selective thyromimetic, on bone development of hypothyroid rats. Whereas T3 reverted the IGF-I deficiency and the skeletal defects caused by hypothyroidism, GC-1 had no effect on serum IGF-I or on IGF-I protein expression in the epiphyseal growth plate of the femur, but induced selective effects on bone development. Our findings indicate that T3 exerts some essential effects on bone development that are mediated by TRbeta1.

INTRODUCTION

We investigated the role of the thyroid hormone receptor beta1 (TRbeta1) on skeletal development of rats using the TRbeta-selective agonist GC-1.

MATERIALS AND METHODS

Twenty-one-day-old female rats (n = 6/group) were rendered hypothyroid (Hypo) and treated for 5 weeks with 0.3 ug/100 g BW/day of T3 (1xT3), 5xT3, or equimolar doses of GC-1 (1xGC-1 and 5xGC-1). Serum triiodothyronine (T3), thyroxine (T4), thyroid-stimulating hormone (TSH), and insulin-like growth factor (IGF)-I concentrations were determined by radioimmunoassay (RIA). BMD and longitudinal bone growth were determined by DXA. Trabecular bone histomorphometry and epiphyseal growth plate (EGP) morphometry were performed in the distal femur. Expressions of IGF-I protein and of collagen II and X mRNA were evaluated by immunohistochemistry and in situ hybridization, respectively. To determine hormonal effects on ossification, skeletal preparations of hypothyroid-, 5xGC-1-, and 5xT3-treated neonatal rats were compared.

RESULTS

Hypothyroidism impaired longitudinal body growth and BMD gain, delayed ossification, reduced the number of hypertrophic chondrocytes (HCs; 72% versus Euthyroid [Eut] rats; p < 0.001), and resulted in disorganized columns of EGP chondrocytes. Serum IGF-I was 67% reduced versus Eut rats (p < 0.001), and the expression of IGF-I protein and collagen II and X mRNA were undetectable in the EGP of Hypo rats. T3 completely or partially normalized all these parameters. In contrast, GC-1 did not influence serum concentrations or EGP expression of IGF-I, failed to reverse the disorganization of proliferating chondrocyte columns, and barely affected longitudinal growth. Nevertheless, GC-1 induced ossification, HC differentiation, and collagen II and X mRNA expression and increased EGP thickness to Eut values. GC-1-treated rats had higher BMD gain in the total tibia, total femur, and in the femoral diaphysis than Hypo animals (p < 0.05). These changes were associated with increased trabecular volume (48%, p < 0.01), mineralization apposition rate (2.3-fold, p < 0.05), mineralizing surface (4.3-fold, p < 0.01), and bone formation rate (10-fold, p < 0.01).

CONCLUSIONS

Treatment of hypothyroid rats with the TRbeta-specific agonist GC-1 partially reverts the skeletal development and maturation defects resultant of hypothyroidism. This finding suggests that TRbeta1 has an important role in bone development.

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

Thyroid hormone affects multiple aspects of bone metabolism, but little is known about thyroid hormone deiodination in bone cells except that cultures of skeletal cells and bone organ express types 1 and 2 iodothyronine deiodinases (D1 and D2) mRNAs. In the present study, outer ring deiodination (ORD) activity was detected in bone extracts of multiple sites of the mouse skeleton, bone marrow, and the MC3T3-E1 osteoblastic cell line. In all tissues, ORD was detected using 125I-rT3 or 125I-T4 as substrates and was found to be 6-n-propylthiouracil insensitive, display a Michaelis constant (T4) of approximately 1 nM, increase about 3-fold in hypo- and virtually disappear in thyrotoxicosis. Extracts of calvaria had the lowest ORD activity, whereas tibial and femoral extracts had roughly three times as much. The absence of ORD activity in bone extracts from mice with targeted disruption of the Dio2 gene confirms the principal role of D2 in this tissue. In the MC3T3-E1 osteoblasts, D2 activity increased in a time-dependent manner after plating, and with the content of selenium in the media, reaching a maximum 5-7 d later as cells attained more than 90% confluence. In these cells D2 half-life is about 30-40 min, which is further accelerated by exposure to substrate and stabilized by the proteasome inhibitor, MG132. Treatment with vitamin D [1,25(OH)2VD]-induced D2 activity by 2- to 3-fold as early as 24 h, regardless of the level of cell confluence, but estradiol, PTH, forskolin, leptin, TNFalpha, TGFbeta, and dexamethasone did not affect D2. Given the role of D2 in other cell types and processes, it is likely that bone ORD not only plays a role in bone development and adult bone T3 homeostasis but also contributes to extrathyroidal T3 production and maintenance of serum T3.

Efeito da associação hipertireoidismo-castração no osso de ratas adultas

Foi estudada a relação tireóide-gônadas e sua influência sobre a morfologia óssea de ratas Wistar, com cinco meses de idade, castradas e induzidas ao hipertireoidismo ou mantidas em eutireoidismo por período de 30, 60 e 90 dias. Ratas não castradas foram mantidas nas mesmas condições e serviram como controle. Ao final de cada período, foram determinadas as concentrações plasmáticas de T4 livre, progesterona e estradiol. Os ossos de cada grupo foram submetidos às análises radiológica e histológica. O hipertireoidismo nas ratas não castradas levou à alteração da morfologia do osso, variável ao longo do período experimental, conduzindo, aos 60 dias, à perda de osso trabecular por aumento da reabsorção óssea. Aos 90 dias não houve perda óssea porque o aumento da reabsorção foi acompanhado por maior aposição óssea. Nas ratas eutireóideas castradas, a diminuição dos níveis de progesterona inibiu a aposição óssea, causando, aos 30 dias, pequena perda do osso trabecular das vértebras lombares e do osso alveolar, que se intensificou aos 60 e 90 dias, atingindo também o osso cortical. A administração de tiroxina nas ratas castradas reduziu a osteopenia decorrente da castração aos 60 dias, mas não aos 90 dias, quando a perda óssea foi mais extensa. Conclui-se que o hipoprogesteronismo e o hipoestrogenismo alteram o metabolismo ósseo e que a resposta do osso ao hipertireoidismo depende do perfil plasmático dos esteróides sexuais, do tempo de exposição e da configuração do tecido ósseo.

[Molecular mechanism of thyroid hormone action]

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