Thyroid hormone inhibits growth and stimulates terminal differentiation of epiphyseal growth plate chondrocytes

Effects of selenium and iodine on Kashin-Beck disease: an updated review

Kashin-Beck disease (KBD) is an endochondral osteogenesis disorder characterised by epiphysis damage and secondary deformable arthropathy induced by multiple external factors, among which selenium (Se) and iodine deficiency are important influencing factors. Iodine deficiency is usually accompanied by a low Se content in the soil in the KBD areas of China. Se can reverse oxidative damage to chondrocytes. In addition, Se is related to the bone conversion rate and bone mineral density. Low Se will hinder growth and change bone metabolism, resulting in a decrease in the bone conversion rate and bone mineral density. Thyroid hormone imbalance caused by thyroid dysfunction caused by iodine deficiency can damage bone homeostasis. Compared with Se deficiency alone, Se combined with iodine deficiency can reduce the activity of glutathione peroxidase more effectively, which increases the vulnerability of chondrocytes and other target cells to oxidative stress, resulting in chondrocyte death. Clinical studies have shown that supplementation with Se and iodine is helpful for the prevention and treatment of KBD.

Evaluation of Bone Mineral Changes in Panoramic Radiographs of Hypothyroid and Hyperthyroid Patients Using Fractal Dimension Analysis

Objective Hyperthyroidism and hypothyroidism are endocrinopathies that cause a decrease in bone mineral density. The aim of this study is to investigate possible bone changes in the mandible caused by hyperthyroidism and hypothyroidism using fractal analysis (FA) on panoramic radiographs. Material and Methods Panoramic radiographs of a total of 180 patients, including 120 patient groups (60 hyperthyroid, 60 hypothyroid) and 60 healthy control groups, were used. Five regions of interests (ROI) were determined from panoramic radiographs and FA was performed. ROI1: geometric midpoint of mandibular notch and mandibular foramen, ROI2: geometric midpoint of mandibular angle, ROI3: anterior of mental foramen, ROI4: basal cortical area from distal mental foramen to distal root of first molar, ROI5: geometric center of mandibular foramen and mandibular ramus. Results While a significant difference was observed between the patient and control groups regarding ROI1 and ROI2 (p < 0.05); there was no significant difference between the groups in relation to ROI3, ROI4, and ROI5. All FA values were lower in the hyperthyroid group than in the hypothyroid group. Conclusion Fractal analysis proves to be an effective method for early detection of bone mass changes. In the present study, it was concluded that while the mandibular cortical bone was intact, trabecular rich regions were affected by osteoporosis caused by thyroid hormones. Necessary precautions should be taken against the risk of osteoporosis in patients with thyroid hormone disorders.

Maternal Hyperthyroidism in Rats Alters the Composition and Gene Expression of the Matrix Produced In Vitro by Chondrocytes from Offspring with Intrauterine Growth Restriction

Herein, we aimed to evaluate cultures of femoral chondrocytes from offspring of rats with intrauterine growth restriction (IUGR) induced by maternal hyperthyroidism. Fourteen adult female Wistar rats were divided into two groups, a control group and a group treated with daily L-thyroxine administration using an orogastric tube (50 µg/animal/day) during pregnancy. Three days after birth, the offspring were euthanized for chondrocyte extraction. At 7, 14, and 21 days, viability and alkaline-phosphatase (ALP) activity were assessed using the MTT assay and BCIP/NBT method, respectively, in a 2D culture. Pellets (3D cultures) were stained with periodic acid Schiff (PAS) to assess the morphology and percentage of PAS+ areas. The gene transcripts for Col2, Col10, Acan, Sox9, and Runx2 were evaluated by qRT-PCR. The MTT and ALP-assay results showed no significant differences between the groups. Maternal hyperthyroidism did not alter the chondrocyte morphology, but significantly reduced the percentage of PAS+ areas, decreased the expression of the gene transcripts of Col2 and Acan, and increased Sox9 expression. Maternal hyperthyroidism in rats alters the composition and gene expression of the matrix produced by chondrocytes from offspring with IUGR. This may be one of the mechanisms through which excess maternal thyroid hormones reduce offspring bone growth.

Anatomical Assessment of the Adult Skeleton of Zebrafish Reared Under Different Thyroid Hormone Profiles

Thyroid hormone (TH) directs the growth and maintenance of tissues throughout the body during development and into adulthood, and plays a particularly important role in proper ossification and homeostasis of the skeleton. To better understand the roles of TH in the skeletogenesis of a vertebrate model, and to define areas of the skeleton that are particularly sensitive to developmental TH, we examined the effects of hypo- and hyperthyroidism on skeletal development in zebrafish. Performing a bone-by-bone anatomical assessment on the entire skeleton of adult fish, we found that TH is required for proper ossification, growth, morphogenesis, and fusion of numerous bones. We showed that the pectoral girdle, dermatocranium, Weberian apparatus, and dentary are particularly sensitive to TH, and that TH affects development of skeletal element regardless of bone type and developmental origin. Indeed, the hormone does not universally promote ossification: we found that developmental TH prevents ectopic ossification in multiple thin bones and within connective tissue of the jaw. In all, we found that TH regulates proper morphogenesis and ossification in the majority of zebrafish bones, and that the requirement for the hormone extends across bone types and developmental profiles. Anat Rec, 302:1754-1769, 2019. © 2019 American Association for Anatomy.

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.

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.

Excess maternal and postnatal thyroxine alters chondrocyte numbers and the composition of the extracellular matrix of growth cartilage in rats

Purpose/Aim: The aim of this study was to evaluate the effects of excess maternal and postnatal thyroxine on chondrocytes and the extracellular matrix (ECM) of growth cartilage.

MATERIALS AND METHODS

We used 16 adult female Wistar rats divided into two groups: thyroxine treatment and control. From weaning to 40 days of age, offspring of the treated group (n = 8) received L-thyroxine. Plasma free T4 was measured. Histomorphometric analysis was performed on thyroids and femurs of all offspring. Alcian blue histochemical staining and real-time reverse transcription polymerase chain reaction measurements of gene expression levels of Sox9, Runx2, Aggrecan, Col I, Col II, Alkaline phosphatase, Mmp2, Mmp9, and Bmp2 were performed. Data were analyzed for statistical significance by student's t-test.

RESULTS

Excess maternal and postnatal thyroxine reduced the intensity of Alcian blue staining, altered the number of chondrocytes in proliferative and hypertrophic zones in growth cartilage, and reduced the gene expression of Sox9, Mmp2, Mmp9, Col II, and Bmp2 in the growth cartilage of all offspring. Additionally, excess thyroxine altered the gene expression of Runx2, Aggrecan and Col I, and this effect was dependent on age.

CONCLUSIONS

Excess thyroxine in neonates suppresses chondrocyte proliferation, stimulates chondrocyte hypertrophy and changes the ECM composition by reducing the amount of proteoglycans and glycosaminoglycans (GAGs). Prolonged exposure to excess thyroxine suppresses chondrocyte activity in general, with a severe reduction in the proteoglycan content of cartilage and the expression of gene transcripts essential for endochondral growth and characteristics of the chondrocyte phenotype.

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.

The cartilage extracellular matrix as a transient developmental scaffold for growth plate maturation

The cartilage growth plate is a specialized developmental tissue containing characteristic zonal arrangements of chondrocytes. The proliferative and differentiative states of chondrocytes are tightly regulated at all stages including the initial limb bud and rudiment cartilage stages of development, the establishment of the primary and secondary ossification centers, development of the growth plates and laying down of bone. A multitude of spatio-temporal signals, including transcription factors, growth factors, morphogens and hormones, control chondrocyte maturation and terminal chondrocyte differentiation/hypertrophy, cell death/differentiation, calcification and vascular invasion of the growth plate and bone formation during morphogenetic transition of the growth plate. This involves hierarchical, integrated signaling from growth and factors, transcription factors, mechanosensory cues and proteases in the extracellular matrix to regulate these developmental processes to facilitate progressive changes in the growth plate culminating in bone formation and endochondral ossification. This review provides an overview of selected components which have particularly important roles in growth plate biology including collagens, proteoglycans, glycosaminoglycans, growth factors, proteases and enzymes.

Skeletal manifestations of juvenile hypothyroidism and the impact of treatment on skeletal system

Thyroid hormone mediates growth and development of the skeleton through its direct effects and through its permissive effects on growth hormone. The effect of hypothyroidism on bone is well described in congenital hypothyroidism, but the impact of thyroid hormone deficiency on a growing skeleton, as it happens with juvenile hypothyroidism, is less defined. In addition, the extent to which the skeletal defects of juvenile hypothyroidism revert on the replacement of thyroid hormone is not known. A study was undertaken in 29 juvenile autoimmune hypothyroid patients to study the skeletal manifestations of juvenile hypothyroidism and the impact of treatment of hypothyroidism on the skeletal system of juvenile patients. Hypothyroidism has a profound impact on the skeletal system and delayed bone age, dwarfism, and thickened bands at the metaphyseal ends being the most common findings. Post treatment, skeletal findings like delayed bone age and dwarfism improved significantly, but there were no significant changes in enlargement of sella, presence of wormian bones, epihyseal dysgenesis, vertebral changes and thickened band at the metaphyseal ends. With the treatment of hypothyroidism, there is an exuberant advancement of bone age, the catch up of bone age being approximately double of the chronological age advancement.

The effect of dexamethasone and triiodothyronine on terminal differentiation of primary bovine chondrocytes and chondrogenically differentiated mesenchymal stem cells

The newly evolved field of regenerative medicine is offering solutions in the treatment of bone or cartilage loss and deficiency. Mesenchymal stem cells, as well as articular chondrocytes, are potential cells for the generation of bone or cartilage. The natural mechanism of bone formation is that of endochondral ossification, regulated, among other factors, through the hormones dexamethasone and triiodothyronine. We investigated the effects of these hormones on articular chondrocytes and chondrogenically differentiated mesenchymal stem cells, hypothesizing that these hormones would induce terminal differentiation, with chondrocytes and differentiated stem cells being similar in their response. Using a 3D-alginate cell culture model, bovine chondrocytes and chondrogenically differentiated stem cells were cultured in presence of triiodothyronine or dexamethasone, and cell proliferation and extracellular matrix production were investigated. Collagen mRNA expression was measured by real-time PCR. Col X mRNA and alkaline phosphatase were monitored as markers of terminal differentiation, a prerequisite of endochondral ossification. The alginate culture system worked well, both for the culture of chondrocytes and for the chondrogenic differentiation of mesenchymal stem cells. Dexamethasone led to an increase in glycosaminoglycan production. Triiodothyronine increased the total collagen production only in chondrocytes, where it also induced signs of terminal differentiation, increasing both collagen X mRNA and alkaline phosphatase activity. Dexamethasone induced terminal differentiation in the differentiated stem cells. The immature articular chondrocytes used in this study seem to be able to undergo terminal differentiation, pointing to their possible role in the onset of degenerative osteoarthritis, as well as their potential for a cell source in bone tissue engineering. When chondrocyte-like cells, after their differentiation, can indeed be moved on towards terminal differentiation, they can be used to generate a model of endochondral ossification, but this limitation must be kept in mind when using them in cartilage tissue engineering application.

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.

Genetic confirmation for a central role for TNFα in the direct action of thyroid stimulating hormone on the skeleton

Clinical data showing correlations between low thyroid-stimulating hormone (TSH) levels and high bone turnover markers, low bone mineral density, and an increased risk of osteoporosis-related fractures are buttressed by mouse genetic and pharmacological studies identifying a direct action of TSH on the skeleton. Here we show that the skeletal actions of TSH deficiency are mediated, in part, through TNFα. Compound mouse mutants generated by genetically deleting the Tnfα gene on a Tshr(-/-) (homozygote) or Tshr(+/-) (heterozygote) background resulted in full rescue of the osteoporosis, low bone formation, and hyperresorption that accompany TSH deficiency. Studies using ex vivo bone marrow cell cultures showed that TSH inhibits and stimulates TNFα production from macrophages and osteoblasts, respectively. TNFα, in turn, stimulates osteoclastogenesis but also enhances the production in bone marrow of a variant TSHβ. This locally produced TSH suppresses osteoclast formation in a negative feedback loop. We speculate that TNFα elevations due to low TSH signaling in human hyperthyroidism contribute to the bone loss that has traditionally been attributed solely to high thyroid hormone levels.

Thyroid hormone actions in cartilage and bone

Thyroid hormones exert widespread and complex actions in almost all tissues during development, throughout childhood and in adults. The skeleton is an important T3-target tissue that exemplifies these processes, and yet understanding of the specific cellular and molecular mechanisms of T3 action in bone and cartilage remains incomplete. Here, the skeleton is considered as a T3-target tissue. The actions of thyroid hormones during skeletal development and in chondrocytes and growth plate cartilage during post-natal linear growth are outlined. The physiological importance of these actions are discussed in relation to patients with autosomal dominant mutations in genes encoding the thyroid hormone receptors TRα1 and TRβ, and in mice harbouring deletions or mutations of the orthologous genes. The role of thyroid hormones and the control of T3 action in bone turnover and maintenance are also outlined, and T3 action in bone-forming osteoblasts and bone-resorbing osteoclasts discussed. The physiological and functional consequences of T3 action in bone are considered in relation to mutant mouse models and to effects on bone mineral density and fracture susceptibility in humans. Finally, new studies identifying a putative role for thyroid hormone metabolism in articular cartilage maintenance and the pathogenesis of osteoarthritis are considered. The pharmacological context of these new findings is discussed, emphasising the importance of this emerging field of study in thyroid hormone pathophysiology.

The inhibitory effect of salmon calcitonin on tri-iodothyronine induction of early hypertrophy in articular cartilage

OBJECTIVE

Salmon calcitonin has chondroprotective effect both in vitro and in vivo, and is therefore being tested as a candidate drug for cartilage degenerative diseases. Recent studies have indicated that different chondrocyte phenotypes may express the calcitonin receptor (CTR) differentially. We tested for the presence of the CTR in chondrocytes from tri-iodothyronin (T3)-induced bovine articular cartilage explants. Moreover, investigated the effects of human and salmon calcitonin on the explants.

METHODS

Early chondrocyte hypertrophy was induced in bovine articular cartilage explants by stimulation over four days with 20 ng/mL T3. The degree of hypertrophy was investigated by molecular markers of hypertrophy (ALP, IHH, COLX and MMP13), by biochemical markers of cartilage turnover (C2M, P2NP and AGNxII) and histology. The expression of the CTR was detected by qPCR and immunohistochemistry. T3-induced explants were treated with salmon or human calcitonin. Calcitonin down-stream signaling was measured by levels of cAMP, and by the molecular markers.

RESULTS

Compared with untreated control explants, T3 induction increased expression of the hypertrophic markers (p<0.05), of cartilage turnover (p<0.05), and of CTR (p<0.01). Salmon, but not human, calcitonin induced cAMP release (p<0.001). Salmon calcitonin also inhibited expression of markers of hypertrophy and cartilage turnover (p<0.05).

CONCLUSIONS

T3 induced early hypertrophy of chondrocytes, which showed an elevated expression of the CTR and was thus a target for salmon calcitonin. Molecular marker levels indicated salmon, but not human, calcitonin protected the cartilage from hypertrophy. These results confirm that salmon calcitonin is able to modulate the CTR and thus have chondroprotective effects.

The skeletal consequences of thyrotoxicosis

Euthyroid status is essential for normal skeletal development and the maintenance of adult bone structure and strength. Established thyrotoxicosis has long been recognised as a cause of high bone turnover osteoporosis and fracture but more recent studies have suggested that subclinical hyperthyroidism and long-term suppressive doses of thyroxine (T4) may also result in decreased bone mineral density (BMD) and an increased risk of fragility fracture, particularly in postmenopausal women. Furthermore, large population studies of euthyroid individuals have demonstrated that a hypothalamic-pituitary-thyroid axis set point at the upper end of the normal reference range is associated with reduced BMD and increased fracture susceptibility. Despite these findings, the cellular and molecular mechanisms of thyroid hormone action in bone remain controversial and incompletely understood. In this review, we discuss the role of thyroid hormones in bone and the skeletal consequences of hyperthyroidism.

Mechanisms of action of thyroid hormones in the skeleton

BACKGROUND

Thyroid hormones regulate skeletal development, acquisition of peak bone mass and adult bone maintenance. Abnormal thyroid status during childhood disrupts bone maturation and linear growth, while in adulthood it results in altered bone remodeling and an increased risk of fracture

SCOPE OF REVIEW

This review considers the cellular effects and molecular mechanisms of thyroid hormone action in the skeleton. Human clinical and population data are discussed in relation to the skeletal phenotypes of a series of genetically modified mouse models of disrupted thyroid hormone signaling.

MAJOR CONCLUSIONS

Euthyroid status is essential for normal bone development and maintenance. Major thyroid hormone actions in skeletal cells are mediated by thyroid hormone receptor α (TRα) and result in anabolic responses during growth and development but catabolic effects in adulthood. These homeostatic responses to thyroid hormone are locally regulated in individual skeletal cell types by the relative activities of the type 2 and 3 iodothyronine deiodinases, which control the supply of the active thyroid hormone 3,5,3'-L-triiodothyronine (T3) to its receptor.

GENERAL SIGNIFICANCE

Population studies indicate that both thyroid hormone deficiency and excess are associated with an increased risk of fracture. Understanding the cellular and molecular basis of T3 action in skeletal cells will lead to the identification of new targets to regulate bone turnover and mineralization in the prevention and treatment of osteoporosis. This article is part of a Special Issue entitled Thyroid hormone signaling.

The role of FSH and TSH in bone loss and its clinical relevance

Osteoporosis, a global health problem, is now frequently recognized to be secondary to alterations in the pituitary-bone axis. This review examines the current evidence for how dysregulation of the pituitary-bone axis leads to osteoporotic bone loss. Specifically, perimenopausal bone loss in the context of follicle-stimulating hormone action, and hyperthyroid bone loss in the context of thyroid-stimulating hormone action are explored. From the reviewed scientific findings, recommendations for early diagnosis and better clinical management of bone loss are made.

Role of the pituitary-bone axis in skeletal pathophysiology

PURPOSE OF REVIEW

Embedded within textbooks for decades is the hard fact that releasing hormones from the anterior pituitary, namely, follicle-stimulating hormone, thyroid-stimulating hormone and adrenocorticotropic hormone, stimulate master hormone secretion from target endocrine organs. We propose a paradigm shift in endocrine physiology, which is that these hormones act by design on bone directly, also now considered an endocrine organ.

RECENT FINDINGS

Complementary investigations using mouse genetic and cell biological approaches reveal that follicle-stimulating hormone and thyroid-stimulating hormone act on bone cells directly to regulate bone remodeling and bone mass. Thyroid-stimulating hormone inhibits bone remodeling, whereas follicle-stimulating hormone stimulates it. We also find that the posterior pituitary hormone oxytocin is anabolic to the skeleton.

SUMMARY

An ambitious extrapolation is that a plurality of pituitary hormones acts in concert as part of a 'pituitary-bone' axis to regulate skeletal integrity in health and disease. When dysregulated master hormone levels during hypogonadism and hyperthyroidism cause altered pituitary hormone secretion through hypothalamic feedback, the latter hormones contribute to the skeletal loss.

Thyroid-stimulating hormone, thyroid hormones, and bone loss

It has become accepted by virtue of rich anecdotal experience and clinical research that thyrotoxicosis is associated with high-turnover osteoporosis. The bone loss, primarily due to accelerated resorption that is not compensated by a coupled increase in bone formation, has been attributed solely to elevated thyroid hormone levels. Evidence using mice lacking the thyroid hormone receptors alpha and beta establishes a role for thyroid hormones in regulating bone remodeling but does not exclude an independent action of thyroid-stimulating hormone (TSH), levels of which are low in hyperthyroid states, even when thyroid hormones are normal, as after thyroxine supplementation and in subclinical hyperthyroidism. We show that TSH directly suppresses bone remodeling and that TSH receptor null mice have profound bone loss, suggesting that reduced TSH signaling contributes to hyperthyroid osteoporosis. TSH and its receptor could become valuable drug targets in treating bone loss.

Case report: bilateral slipped capital femoral epiphyses and hormone replacement

A 24-year-old woman presented with an 11-year history of bilateral hip pain. Radiographs of the hips revealed severe bilateral slipped upper femoral epiphyses; the left side was more severely slipped than the right. While moving the hips under fluoroscopy we observed motion at the physes and reproduced the patient's pain; the motion confirmed the diagnosis of chronic slipped capital femoral epiphysis. Endocrinology tests showed hypothyroidism. After 1 year of thyroxin therapy, the patient's pain subsided and radiographs of the hips showed fusion of the physes. This case emphasizes the importance of screening for an endocrine disorder in patients with slipped capital femoral epiphysis particularly in adults and shows fusion can occur once the underlying endocrine abnormality is treated.

Model of induction of thyroid dysfunctions in adult female mice

It is described the elaboration of a protocol to induce hyperthyroidism and hypothyroidism in mice by administrating thyroxin and propylthiouracil, respectively, in the drinking water. The drugs were administered to adult female mice of the Swiss strain for 30 days in order to obtain a systemic status of thyroid dysfunction. The induction of hyperthyroidism and hypothyroidism in the animals was confirmed by the histomorphological analysis of the thyroid in the end of the experiment, when the state of gland dysfunction in the animals submitted to the treatment was observed.

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.

Involvement of cyclic guanosine monophosphate-dependent protein kinase II in chondrocyte hypertrophy during endochondral ossification

During vertebrate skeletal development, the appendicular skeleton forms through endochondral ossification, which involves the intricately regulated multistep differentiation of mesenchymal cells. During this process, mesenchymal condensations initially differentiate into chondrocytes. Then chondrocytes in the center further differentiate into hypertrophic chondrocytes. Hypertrophic chondrocytes express a number of osteogenic factors and induce bone formation. Although numerous studies have provided novel insights into the regulation and function of cartilage development, little is known about the intracellular signaling pathways regulating chondrocyte hypertrophy. Recent study revealed that cyclic guanosine monophosphate (cGMP)-dependent protein kinase II (cGKII) coupled the stop of proliferation and the start of hypertrophic differentiation of chondrocytes. Herein, we review the molecular mechanism of regulation of chondrocyte hypertrophy by cGKII and the interaction between cGKII and other signaling pathways.

Effects of 24R,25- and 1α,25-dihydroxyvitamin D3 on mineralizing growth plate chondrocytes

Time- and dosage-dependent effects of 1,25(OH)(2)D(3) and 24,25(OH)(2)D(3) on primary cultures of pre- and post-confluent avian growth plate (GP) chondrocytes were examined. Cultures were grown in either a serum-containing culture medium designed to closely mimic normal GP extracellular fluid (DATP5) or a commercially available serum-free media (HL-1) frequently used for studying skeletal cells. Hoechst DNA, Lowry protein, proteoglycan (PG), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP) activity and calcium and phosphate mineral deposition in the extracellular matrix were measured. In preconfluent cultures grown in DATP5, physiological levels of 24,25(OH)(2)D(3) (0.10-10 nM) increased DNA, protein, and LDH activity significantly more than did 1,25(OH)(2)D(3) (0.01-1.0 nM). However, in HL-1, the reverse was true. Determining ratios of LDH and PG to DNA, protein, and each other, revealed that 1,25(OH)(2)D(3) specifically increased PG, whereas 24,25(OH)(2)D(3) increased LDH. Post-confluent cells were generally less responsive, especially to 24,25(OH)(2)D(3). The positive anabolic effects of 24,25(OH)(2)D(3) required serum-containing GP-fluid-like culture medium. In contrast, effects of 1,25(OH)(2)D(3) were most apparent in serum-free medium, but were still significant in serum-containing media. Administered to preconfluent cells in DATP5, 1,25(OH)(2)D(3) caused rapid, powerful, dosage-dependent inhibition of Ca(2+) and Pi deposition. The lowest level tested (0.01 nM) caused >70% inhibition during the initial stages of mineral deposition; higher levels of 1,25(OH)(2)D(3) caused progressively more profound and persistent reductions. In contrast, 24,25(OH)(2)D(3) increased mineral deposition 20-50%; it required >1 week, but the effects were specific, persistent, and largely dosage-independent. From a physiological perspective, these effects can be explained as follows: 1,25(OH)(2)D(3) levels rise in hypocalcemia; it stimulates gut absorption and releases Ca(2+) from bone to correct this deficiency. We now show that 1,25(OH)(2)D(3) also conserves Ca(2+) by inhibiting mineralization. The slow anabolic effects of 24,25(OH)(2)D(3)are consistent with its production under eucalcemic conditions which enable bone formation. These findings, which implicate serum-binding proteins and accumulation of PG in modulating accessibility of the metabolites to GP chondrocytes, also help explain some discrepancies previously reported in the literature.

A highly conserved enhancer in mammalian type X collagen genes drives high levels of tissue-specific expression in hypertrophic cartilage in vitro and in vivo

Previously we have identified a cis-acting regulatory domain in the human type X collagen gene upstream of the transcription start site which acts as a strong enhancer in hypertrophic, but not in resting chondrocytes. Here we show that this enhancer is highly conserved also in the murine and bovine Col10a1 genes, but not found in the known promoter sequences of chicken Col10a1. It contains a functionally active AP-1 site (TPA Responsive Element, TRE) which is essential for the high transcriptional activity of the COL10A1 enhancer in transiently transfected hypertrophic chondrocytes. Gel-shift experiments with nuclear extracts of hypertrophic chondrocytes revealed FosB and Fra-1 as candidates regulating AP-1 factors binding to the TRE site. In fact, coexpression of FosB and Fra-1 in reporter gene assays greatly stimulated transcriptional activity of enhancer bearing reporter genes. Quantitative analysis of AP-1 factor mRNA levels in distinct fractions of fetal bovine epiphyseal chondrocytes by real-time PCR confirmed significant levels of FosB and Fra-1 mRNA besides other AP-1 factors in hypertrophic chondrocytes. A key role of the enhancer element in regulating tissue-specific expression of the Col10a1 gene was shown by establishing transgenic mouse lines with a reporter gene containing a 4.6 kb murine Col10a1 promoter fragment which included the enhancer, exon 1, part of exon 2 and the first intron. Reporter gene expression was seen exclusively in hypertrophic cartilages in the growth plates of long bones, ribs, vertebrae, sternum and mandibles of 17.5-18.5 dpc embryos, confirming that the 4.6 kb promoter is able to drive specific expression of Col10a1 in hypertrophic cartilage. These established transgenic lines should facilitate the genetic analysis of regulatory pathways of chondrocyte maturation and Col10a1 gene expression in the future.

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.

Chondrocytes isolated from tibial dyschondroplasia lesions and articular cartilage revert to a growth plate-like phenotype when cultured in vitro

We report here a comparative study of the development and behavior of chondrocytes isolated from normal growth plate tissue, tibial dyschondroplasic lesions, and from articular cartilage. The objective of these studies was to determine whether the properties exhibited by chondrocytes in dysplasic lesions or in articular cartilage were due to their cellular phenotype, their environment, or both. We had previously analyzed the electrolytes and amino acid levels in the extracellular fluid of avian growth plate chondrocytes. Using these data, we constructed a culture medium (DATP5) in which growth plate cells essentially recapitulate their normal behavior in vivo. Here, we used DATP5 to examine the behavior of chondrocytes isolated from lesions of tibial dyschondroplasia (TD). We found that once isolated from lesion and grown in this supportive medium, dysplasic chondrocytes behaved essentially like normal growth plate cells. These findings suggest that the cause of TD is local factors operating in vivo to prevent these cells from developing normally. With respect to articular chondrocytes, our data indicate that they more closely retain normal protein and proteoglycan synthesis when grown in serum-free media. These cells readily induced mineral formation in vitro, both in the presence and absence of serum. However, in serum-containing media, mineralization was significantly enhanced when the cells were exposed to retinoic acid (RA) or osteogenic protein-1 (OP-1). Our studies support previous work indicating the presence of autocrine factors produced by articular chondrocytes in vivo that prevent mineralization and preserve matrix integrity. The lack of inhibitory factors and the presence of supporting factors are likely reasons for the induction of mineralization by articular chondrocytes in vitro.

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.

Efeito da associação hipotireoidismo-castração no osso e nas paratireóides de ratas adultas

O efeito do hipotireoidismo sobre o metabolismo ósseo e as paratireóides na deficiência ou suficiência dos esteróides ovarianos foi estudado em 32 ratas Wistar, com 2 meses de idade, distribuídas em 4 grupos de 8: eutireóideo não castrado (ENC), eutireóideo castrado (EC), hipotireóideo não castrado (HNC) e hipotireóideo castrado (HC). Após 120 dias de tratamento, as ratas foram sacrificadas e o plasma colhido para dosagem de T4 livre. Foi evidenciada hipertrofia das paratireóides somente no grupo HNC. As ratas do grupo HNC apresentaram osteopenia de maior extensão e intensidade, decorrente do menor crescimento, da inibição da aposição e do aumento da reabsorção ósseas. Nas ratas EC, a osteopenia foi causada por menor aposição e aumento da reabsorção ósseas. Embora a osteopenia na associação hipotireoidismo-castração tenha sido quase sempre mais intensa em relação à das ratas EC, sua intensidade, quando comparada à osteopenia dos animais HNC, foi variável e dependente do sítio ósseo estudado. Apesar de causar necrose dos ossos de maior metabolismo, a associação hipotireoidismo-castração não potencializou a osteopenia decorrente da ação isolada do hipotireoidismo até os 120 dias de tratamento.

Growth factor regulation of human growth plate chondrocyte proliferation in vitro

Linear growth occurs as the result of growth plate chondrocytes undergoing proliferative and hypertrophic phases. Paracrine feedback loops that regulate the entry of chondrocytes into the hypertrophic phase have been shown and similar pathways likely exist for the proliferative phase. Human long-bone growth plate chondrocytes were cultured in vitro. The proliferative effects of a variety of factors were determined by [3H]thymidine uptake and the gene expression profile of these cells was determined by DNA microarray analysis. Serum, insulin-like growth factor (IGF)-I and -II, transforming growth factor-beta (TGF-beta, fibroblast growth factor (FGF)-1, -2, and -18, and platelet-derived growth factor (PDGF)-BB were potent stimulators of proliferation. FGF-10, testosterone, and bone morphogenetic proteins (BMP)-2, -4, and -6 inhibited proliferation. Microarray analysis showed that the genes for multiple members of the IGF-I, TGF-beta, FGF, and BMP pathways were expressed, suggesting the presence of autocrine/paracrine pathways that regulate the proliferative phase of growth plate-mediated growth.

The molecular actions of thyroid hormone in bone

Thyroid hormone (T(3)) is essential for the normal development of endochondral and intramembranous bone and plays an important role in the linear growth and maintenance of bone mass. Childhood T(3) deficiency results in retardation of skeletal development and growth arrest, whereas T(3) excess leads to accelerated growth and bone formation. In adult thyrotoxicosis, there is increased bone remodelling, characterized by an imbalance between bone resorption and formation, which results in net bone loss and an increased risk for posteoporotic fracture. These clinical observations demonstrate the importance of T(3) in skeletal development and metabolism. Nevertheless, the molecular mechanisms of T(3) action in bone are poorly understood. Here, we provide an overview of T(3) regulation of chondrocytes, osteoblasts and osteoclasts, and the actions of thyroid hormone receptor (TR) isoforms in skeletal development. The possible roles of T(3) and TRs in nuclear receptor crosstalk, prereceptor ligand metabolism, heparan sulfate proteoglycan synthesis and angiogenesis are also considered.

A osteoporose e os distúrbios endócrinos da tireóide e das gônadas

Apesar da dedicação incessante dos pesquisadores no estudo da osteoporose, muito ainda necessita ser elucidado. A deficiência dos esteróides sexuais, principalmente a de estrógeno, é considerada a principal causa de osteoporose, embora existam inúmeros outros fatores envolvidos. O hipertireoidismo, por exemplo, é considerado um dos fatores de risco para indução ou agravamento da osteoporose e tem despertado o interesse para o estudo dos efeitos de T3 e T4 sobre o metabolismo ósseo. Embora o hipotireoidismo e a afuncionalidade das gônadas seja uma associação freqüente na mulher, a hipofunção da tireóide não é considerada fator de risco para a osteoporose da menopausa. Assim, o estudo da inter-relação entre os distúrbios endócrinos, tão comuns na idade avançada, e a osteoporose é fundamental, pois deste conhecimento poderão advir meios de controle e tratamento adequados, bem como a definição da real natureza do distúrbio ósseo. O objetivo desta revisão é apresentar e discutir alguns aspectos da osteoporose e sua inter-relação com os distúrbios endócrinos da tireóide e das gônadas.

Thyroid hormone enhances aggrecanase-2/ADAM-TS5 expression and proteoglycan degradation in growth plate cartilage

Effects of thyroid hormone on proteoglycan degradation in various regions of cartilage were investigated. In propylthiouracil-treated rats with hypothyroidism, proteoglycan degradation in epiphyseal cartilage during endochondral ossification was markedly suppressed. However, injections of T(4) reversed this effect of propylthiouracil on proteoglycan degradation. In pig growth plate explants, T(3) also induced breakdown of proteoglycan. T(3) increased the release of aggrecan monomer and core protein from the explants into the medium. Accordingly, the level of aggrecan monomer remaining in the tissue decreased after T(3) treatment, and the monomer lost hyaluronic acid-binding capacity, suggesting that the cleavage site is in the interglobular domain. The aggrecan fragment released from the T(3)-exposed explants underwent cleavage at Glu(373)-Ala(374), the major aggrecanase-cleavage site. The stimulation of proteoglycan degradation by T(3) was less prominent in resting cartilage explants than in growth plate explants and was barely detectable in articular cartilage explants. Using rabbit growth plate chondrocyte cultures, we explored proteases that may be involved in T(3)-induced aggrecan degradation and found that T(3) enhanced the expression of aggrecanase-2/ADAM-TS5 (a disintegrin and a metalloproteinase domain with thrombospondin type I domains) mRNA, whereas we could not detect any enhancement of stromelysin, gelatinase, or collagenase activities or any aggrecanase-1/ADAM-TS4 mRNA expression. We also found that the aggrecanse-2 mRNA level, but not aggrecanase-1, increased at the hypertrophic stage during endochondral ossification. These findings suggest that aggrecanse-2/ADAM-TS5 is involved in aggrecan breakdown during endochondral ossification, and that thyroid hormone stimulates the aggrecan breakdown partly via the enhancement of aggrecanase-2/ADAM-TS5.

Transport of inorganic phosphate in primary cultures of chondrocytes isolated from the tibial growth plate of normal adolescent chickens

This report describes Pi transport activity in chondrocytes isolated from the growth plate (GP) of normal adolescent chickens grown in primary cell culture. Our recent work showed that Pi transport in matrix vesicles (MV) isolated from normal GP cartilage was not strictly Na+-dependent, whereas previously characterized Pi transport from rachitic GP cartilage MV was. This Na+-dependent Pi transporter (NaPiT), a member of the Type III Glvr-1 gene family, is expressed only transiently during early differentiation of GP cartilage, is enhanced by Pi-deficiency, and is most active at pH 6.8. Since GP mineralization requires abundant Pi and occurs under slightly alkaline conditions, it seemed unlikely that this type of Pi transporter was solely responsible for Pi uptake during normal GP development. Therefore we asked whether the lack of strict Na+-dependency in Pi transport seen in normal MV was also evident in normal GP chondrocytes. In fact, cellular Pi transport was found not to be strictly Na+-dependent, except for a brief period early in the culture. Choline could equally serve as a Na+ substitute. Activity of choline-supported Pi transport was optimum at pH 7.6-8.0. In addition, prior exposure of the cells to elevated extracellular Pi (2-3 mM) strongly enhanced subsequent Pi uptake, which appeared to depend on prior loading of the cells with mineral ions. Prevention of Pi loading by pretreatment with Pi transport inhibitors not only inhibited subsequent cellular Pi uptake, it also blocked mineral formation. Treatment with elevated extracellular Pi did not induce apoptosis in these GP chondrocytes.

Dexamethasone inhibits and thyroid hormone promotes differentiation of mouse chondrogenic ATDC5 cells

The effects of glucocorticoid (GC) excess, thyrotoxicosis, and hypothyroidism on linear growth indicate that growth plate chondrocytes are exquisitely sensitive to GC and thyroid hormone (T(3)). Murine ATDC5 cells undergo chondrogenesis in vitro and were used to evaluate the effects of dexamethasone (Dex) and T(3) on cell proliferation and differentiation. Immature and differentiated ATDC5 cells expressed glucocorticoid and T(3)-receptor mRNAs. Cells proliferated and organized into cartilage-like nodules after 7 days. Chondrocyte maturation progressed over 9-40 days, with increasing alkaline phosphatase (ALP) activity, secretion of an Alcian blue-positive matrix, and mineralization of cartilage-like nodules. Dex reduced cell number over the 40 day period, causing inhibition of ALP activity and matrix production with failure of mineralization. Following withdrawal of Dex, chondrocytes proliferated and re-entered the differentiation and mineralization program, indicating that GC inhibition of chondrogenesis is reversible. In contrast, T(3) reduced cell proliferation, but induced ALP activity and increased matrix secretion earlier than in control cultures. Thus, GCs and T(3) regulate growth plate chondrocyte differentiation by distinct mechanisms. GCs arrest cell proliferation, differentiation, and cartilage mineralization and maintain chondrocyte precursors in a state of quiescence with the capacity to re-enter chondrogenesis. T(3) inhibits cell proliferation but accelerates differentiation to stimulate chondrogenesis.

Interactions between GH, IGF-I, glucocorticoids, and thyroid hormones during skeletal growth

Linear growth occurs during development and the childhood years until epiphyseal fusion occurs. This process results from endochondral ossification in the growth plates of long bones and is regulated by systemic hormones and paracrine or autocrine factors. The major regulators of developmental and childhood growth are GH, IGF-I, glucocorticoids, and thyroid hormone. Sex steroids are responsible for the pubertal growth spurt and epiphyseal fusion. This review will consider interactions between GH, IGF-I, glucocorticoids, and thyroid hormone during linear growth. It is well known from physiologic and clinical studies that these hormones interact at the level of the hypothalamus and pituitary. Interacting effects on peripheral tissues such as liver are also well understood, but we concentrate here on the epiphyseal growth plate as an important and newly appreciated target organ for convergent hormone action.

Thyroid hormones promote chondrocyte differentiation in mouse ATDC5 cells and stimulate endochondral ossification in fetal mouse tibias through iodothyronine deiodinases in the growth plate

Thyroid hormones (THs), 3,3',5-triiodo-L-thyronine (T3) and L-thyroxine (T4), are important for the normal development of the growth plate (GP); congenital TH deficiency leads to severe dwarfism. In mouse chondrogenic cell line, ATDC5, T3 enhanced differentiation and increased Alizarin red staining, but did not affect Alcian blue staining. In organ-cultured mouse tibias, THs stimulated the cartilage growth, especially in the hypertrophic zone. Interestingly, T4 was as equally potent as T3 in organ-cultured tibias, which suggests that T4 is metabolized locally to T3, because T4 is a prohormone and must be converted to T3 for its activity. Two enzymes catalyze the conversion; type I deiodinase (D1) and type II deiodinase (D2). D1 has a ubiquitous distribution and D2, with a high affinity for T4, is present where the maintenance of intracellular T3 concentration is critical. Messenger RNAs (mRNAs) for D1 and D2 were detected in neonatal mouse tibias and ATDC5 cells. The enzyme activity was unaffected by the D1 inhibitor 6-propyl-2-thiouracil, suggesting that D2 mainly catalyzes the reaction. D2 mRNA was detected in differentiated ATDC5 cells. In organ-cultured mouse tibias, D2 activity was greater at later stages. In contrast, thyroid hormone receptors (TRs) were expressed in neonatal mouse tibias and ATDC5 cells, but their expression levels in ATDC5 cells were stable throughout the culture periods. Therefore, increased T3 production at later stages by D2 is likely to contribute to the preferential effects of THs in the terminal differentiation of GP. This article is the first to show that T4 is activated locally in GP and enhances the understanding of TH effects in GP.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Expression of cyclin-dependent kinase inhibitors in epiphyseal chondrocytes induced to terminally differentiate with thyroid hormone

A growing body of evidence suggests that systemic hormones and peptide growth factors may exert their effects on cell growth and differentiation in part through regulation of the cell division cycle. We hypothesized that thyroid hormone regulates terminal differentiation of growth plate chondrocytes in part through controlling cell cycle progression at the G1/S restriction point. Our results support this hypothesis by demonstrating that treatment of epiphyseal chondrocytes with thyroid hormone under chemically defined conditions results in the arrest of DNA synthesis and the onset of terminal differentiation, indicating that thyroid hormone is one factor capable of regulating the transition between cell growth and differentiation in these cells. This terminal differentiation process is associated with induction of the cyclin/cyclin-dependent kinase inhibitors p21(cip-1 waf-1) and p27kip1, suggesting that thyroid hormone may regulate terminal differentiation in part by arresting cell cycle progression through induction of cyclin-dependent kinase inhibitors.