Differential responses of femoral and vertebral bones to long-term excessive L-thyroxine administration in adult rats

The Thyroid Hormone Transporter MCT10 Is a Novel Regulator of Trabecular Bone Mass and Bone Turnover in Male Mice

Thyroid hormones (TH) are essential for skeletal development and adult bone homeostasis. Their bioavailability is determined by specific transporter proteins at the cell surface. The TH-specific transporter monocarboxylate transporter 8 (MCT8) was recently reported as a regulator of bone mass in mice. Given that high systemic triiodothyronine (T3) levels in Mct8 knockout (KO) mice are still able to cause trabecular bone loss, alternative TH transporters must substitute for MCT8 function in bone. In this study, we analyzed the skeletal phenotypes of male Oatp1c1 KO and Mct10 KO mice, which are euthyroid, and male Mct8/Oatp1c1 and Mct8/Mct10 double KO mice, which have elevated circulating T3 levels, to unravel the role of TH transport in bone. MicroCT analysis showed no significant trabecular bone changes in Oatp1c1 KO mice at 4 weeks and 16 weeks of age compared with wild-type littermate controls, whereas 16-week-old Mct8/Oatp1c1 double KO animals displayed trabecular bone loss. At 12 weeks, Mct10 KO mice, but not Mct8/Mct10 double KO mice, had decreased trabecular femoral bone volume with reduced osteoblast numbers. By contrast, lack of Mct10 in 24-week-old mice led to trabecular bone gain at the femur with increased osteoblast numbers and decreased osteoclast numbers whereas Mct8/Mct10 double KO did not alter bone mass. Neither Mct10 nor Mct8/Mct10 deletion affected vertebral bone structures at both ages. In vitro, osteoblast differentiation and activity were impaired by Mct10 and Mct8/Mct10-deficiency. These data demonstrate that MCT10, but not OATP1C1, is a site- and age-dependent regulator of bone mass and turnover in male mice.

Low TSH Levels Within Euthyroid Range Could Play a Negative Role on Bone Mineral Density in Postmenopausal Women with Type 2 Diabetes

OBJECTIVE

We aimed to evaluate the relationship between thyroid-stimulating hormone (TSH) and bone mineral density (BMD) in euthyroid type 2 diabetes (T2D).

METHODS

This retrospective analysis enrolled 439 T2D patients with normal thyroid function, including 226 males and 213 females. All the female patients were postmenopausal. Serum glycosylated hemoglobin A1c (HbA1c), TSH, free triiodothyronine (FT₃), and free thyroxine (FT₄) concentrations were analyzed. BMD of the lumbar spine (L₁-L₄), femoral neck, and hip joint was determined using dual-energy X-ray absorptiometry.

RESULTS

The patients were grouped based on tri-sectional quantiles of the TSH levels: 0.55~1.70mIU/L (Group 1), 1.71~2.58mIU/L (Group 2), and 2.59~4.74mIU/L (Group 3). Our data showed that, in male patients, no difference in BMD was identified among groups. In postmenopausal women, unlike at the lumbar spine (P = 0.459), the mean BMD at the femoral neck (P = 0.014) and hip joint (P = 0.014) had a statistical difference among groups and increased with TSH level. In addition, our analysis demonstrated that TSH levels shown no correlation with BMD at all sites in males. However, in females, BMD at the femoral neck (r = 0.156, P = 0.023) and hip joint (r = 0.172, P = 0.012) had a positive correlation with TSH levels. After adjusting for age and BMI, multiple regression analysis showed that TSH levels influenced BMD at the femoral neck (β = 0.188, P = 0.001) and hip joint (β = 0.204, P = 0.001) in female patients.

CONCLUSION

In summary, our data demonstrates that low TSH levels are associated with decreased BMD at the femoral neck and hip joint in postmenopausal T2D women with euthyroidism.

Thyrotoxicosis Involves β2-Adrenoceptor Signaling to Negatively Affect Microarchitecture and Biomechanical Properties of the Femur

Background: Thyrotoxicosis increases bone turnover, resulting in net bone loss. Sympathetic nervous system (SNS) activation, via β2-adrenoceptor (β2-AR) signaling, also has osteopenic effects. Because thyroid hormones (TH) interact with the SNS to regulate several physiological processes, we hypothesized that this interaction also occurs to regulate bone mass. Previous studies support this hypothesis, as α2-AR knockout (KO) mice are less susceptible to thyrotoxicosis-induced osteopenia. Here, we evaluated whether TH-SNS interactions in bone involve β2-AR signaling. Methods: Thyrotoxicosis was induced in 120-day-old female and male mice with β2-AR gene inactivation (β2-AR^-/-) by daily treatment with supraphysiological doses of triiodothyronine (T3) for 12 weeks. The impact of thyrotoxicosis on femoral bone microarchitecture, remodeling, fracture risk, and gene expression of the receptor activator of nuclear factor-kappa-B (RANK)-RANK ligand (RANKL)-osteoprotegerin (OPG) pathway was evaluated. In addition, the effect of the β2-AR-specific agonist clenbuterol (CL) on cAMP accumulation was determined in osteoblastic (MC3T3-E1) cells treated with T3 and/or 17β-estradiol (E2). Results: Thyrotoxicosis negatively affected trabecular bone microarchitecture in wild-type (WT) females, but this effect was milder or nonexistent in β2-AR^-/- animals, whereas the opposite was seen in males. T3 treatment increased the femoral RANKL/OPG mRNA ratio and the endosteal perimeter and medullary area of the diaphysis in WT females and males, but not in β2-AR^-/- mice, suggesting that T3 promotes endosteal resorption in cortical bone, in a mechanism that involves β2-AR signaling. T3 treatment increased endocortical mineral apposition rate only in WT females but not in β2-AR^-/- mice, suggesting that TH also induce bone formation in a β2-AR signaling-dependent mechanism. T3 treatment decreased femoral resistance to fracture only in WT females, but not in KO mice. E2 and CL similarly increased cAMP accumulation in MC3T3-E1 cells; whereas T3 alone had no effect, but it completely blocked E2-stimulated cAMP accumulation, suggesting that some T3 effects on bone may involve E2/cAMP signaling in osteoblasts. Conclusions: These findings sustain the hypothesis that T3 interacts with the SNS to regulate bone morphophysiology in a β2-AR signaling-dependent mechanism. The data also reveal sex as an important modifier of skeletal manifestations of thyrotoxicosis, as well as a modifier of the TH-SNS interactions to control bone microarchitecture, remodeling, and resistance to fracture.

Excesso de tiroxina materna associado ao hipertireoidismo pós-natal reduz o crescimento ósseo e o perfil proliferativo e angiogênico das cartilagens de crescimento de ratos

RESUMO Foram estudados os efeitos do excesso da tiroxina materna associado ao hipertireoidismo pós-natal sobre o crescimento ósseo e o perfil proliferativo e angiogênico das cartilagens. Dezesseis ratas Wistar adultas foram distribuídas nos grupos tratados com L-tiroxina e controle. A prole do grupo tratado recebeu L-tiroxina do desmame até 40 dias de idade. Ao desmame, foi realizada dosagem plasmática de T4 livre nas mães. Na prole, foram realizados: dosagem plasmática de T3 total e T4 livre, morfometria das tireoides, mensuração do comprimento e largura do fêmur. Nas cartilagens, foi avaliada a expressão imuno-histoquímica e gênica de CDC-47, VEGF, Flk-1, Ang1, Ang2 e Tie2. As médias entre grupos foram comparadas pelo teste T de Student. As concentrações de T4 livre das mães tratadas e de T3 total e T4 livre da prole foram significativamente mais elevadas. A largura do fêmur foi menor nos animais tratados. Houve também redução da imunoexpressão de CDC-47 e de VEGF e dos transcritos gênicos para VEGF e Ang1 nas cartilagens. Conclui-se que o excesso de tiroxina materna associado ao hipertireoidismo pós-natal reduz a largura da diáfise femoral, a proliferação celular e a expressão de VEGF e de Ang1 nas cartilagens de crescimento de ratos.

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.

Constitutively Elevated Blood Serotonin Is Associated with Bone Loss and Type 2 Diabetes in Rats

Reduced peripheral serotonin (5HT) in mice lacking tryptophan hydroxylase (TPH1), the rate limiting enzyme for 5HT synthesis, was reported to be anabolic to the skeleton. However, in other studies TPH1 deletion either had no bone effect or an age dependent inhibition of osteoclastic bone resorption. The role of 5HT in bone therefore remains poorly understood. To address this issue, we used selective breeding to create rat sublines with constitutively high (high-5HT) and low (low-5HT) platelet 5HT level (PSL) and platelet 5HT uptake (PSU). High-5HT rats had decreased bone volume due to increased bone turnover characterized by increased bone formation and mineral apposition rate, increased osteoclast number and serum C-telopeptide level. Daily oral administration of the TPH1 inhibitor (LX1032) for 6 weeks reduced PSL and increased the trabecular bone volume and trabecular number of the spine and femur in high-5HT rats. High-5HT animals also developed a type 2 diabetes (T2D) phenotype with increased: plasma insulin, glucose, hemoglobin A1c, body weight, visceral fat, β-cell pancreatic islets size, serum cholesterol, and decreased muscle strength. Serum calcium accretion mediated by parathyroid hormone slightly increased, whereas treatment with 1,25(OH)2D3 decreased PSL. Insulin reduction was paralleled by a drop in PSL in high-5HT rats. In vitro, insulin and 5HT synergistically up-regulated osteoblast differentiation isolated from high-5HT rats, whereas TPH1 inhibition decreased the number of bone marrow-derived osteoclasts. These results suggest that constitutively elevated PSL is associated with bone loss and T2D via a homeostatic interplay between the peripheral 5HT, bone and insulin.

Lack of α2C-Adrenoceptor Results in Contrasting Phenotypes of Long Bones and Vertebra and Prevents the Thyrotoxicosis-Induced Osteopenia

A series of studies have demonstrated that activation of the sympathetic nervous system (SNS) causes osteopenia via β₂-adrenoceptor (β₂-AR) signaling. However, in a recent study, we found an unexpected and generalized phenotype of high bone mass in female mice with chronic sympathetic hyperactivity, due to double gene inactivation of adrenoceptors that negatively regulate norepinephrine release, α_2A-and α_2C-AR (α_2A/2C-AR^-/-). These findings suggest that β₂-AR is not the single adrenoceptor involved in bone turnover regulation and show that α₂-AR signaling may also mediate the SNS actions in the skeleton. In addition, we found that α_2A/2C-AR^-/- animals are resistant to the thyrotoxicosis-induced osteopenia, suggesting that thyroid hormone (TH), when in supraphysiological levels, interacts with the SNS to control bone mass and structure, and that this interaction may also involve α₂-AR signaling. In the present study, to further investigate these hypotheses and to discriminate the roles of α₂-AR subtypes, we have evaluated the bone phenotype of mice with the single gene inactivation of α_2C-AR subtype, which mRNA expression was previously shown to be down regulated by triiodothyronine (T3). A cohort of 30 day-old female α_2CAR^-/- mice and their wild-type (WT) controls were treated with a supraphysiological dose of T3 for 30 or 90 days, which induced a thyrotoxic state in both mouse lineages. The micro-computed tomographic (μCT) analysis showed that α_2C-AR^-/- mice present lower trabecular bone volume (BV/TV) and number (Tb.N), and increased trabecular separation (Tb.Sp) in the femur compared with WT mice; which was accompanied by decreased bone strength (determined by the three-point bending test) in the femur and tibia. The opposite was observed in the vertebra, where α_2C-AR^-/- mice show increased BV/TV, Tb.N and trabecular thickness (Tb.Th), and decreased Tb.Sp, compared with WT animals. In spite of the contrasting bone phenotypes of the femur and L5, thyrotoxicosis negatively regulated most of the micro architectural features of the trabecular bone in both skeletal sites of WT, but not of α_2C-AR^-/- mice. T3 treatment also decreased biomechanical properties (maximum load and ultimate load) in the femur and tibia of WT, but not of knockout mice. The mRNA expression of osteocalcin, a marker of mature osteoblasts, and tartrate-resistant acid phosphatase, which is expressed by osteoclasts and is involved in collagen degradation, was increased by T3 treatment only in WT, and not in α_2C-AR^-/- mice. Altogether, these findings suggest that α_2C-AR subtype mediates the effects of the SNS in the bone in a skeletal site-dependent manner, and that thyrotoxicosis depends on α_2C-AR signaling to promote bone loss, which sustains the hypothesis of a TH-SNS interaction to modulate bone remodeling and structure.

Bone Indices in Thyroidectomized Patients on Long-Term Substitution Therapy with Levothyroxine Assessed by DXA and HR-pQCT

Background. Studies on bone effects of long-term substitution therapy with levothyroxine (LT4) have shown discrepant results. Previous studies have, however, not evaluated volumetric bone mineral densities (vBMD), bone structure, and strength using high resolution peripheral quantitative computed tomography (HR-pQCT) and finite element analysis (FEA). Using a cross-sectional design, we aimed to determine whether BMD, structure, and strength are affected in hypothyroid patients on LT4 substitution therapy. Methods. We compared 49 patients with well-substituted hypothyroidism with 49 age- and gender-matched population based controls. Areal BMD was assessed by DXA, vBMD and bone geometry by HR-pQCT, and bone strength by FEA. Results. Patients had been thyroidectomized due to thyroid cancer (10%) and nontoxic (33%) or toxic goiter (57%). 82% were women. TSH levels did not differ between groups, but patients had significantly higher levels of T4 (p < 0.001) and lower levels of T3 (p < 0.01). Compared to controls, patients had higher levels of magnesium (p < 0.05), whereas ionized calcium and PTH were lower (p < 0.05). Bone scans did not reveal any differences in BMD, bone geometry, or strength. Conclusion. If patients with hypothyroidism are well-substituted with LT4, the disease does not affect bone indices to any major degree.

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.

Frax score calculations in postmenopausal women with subclinical hypothyroidism

OBJECTIVE

The aim of our study was to evaluate the relationship between the elevated TSH and fracture risk in postmenopausal women with subclinical hypothyroidism for evaluation of individuals with a high risk for osteoporotic fractures.

DESIGN

FRAX score calculation (10-year estimated risk for bone fracture) and measurement of bone markers (osteocalcin and beta cross-laps) were performed in 82 postmenopausal women with newly discovered subclinical hypothyroidism (mean age 59.17±7.07, mean BMI 27.89±3.46kg/m2, menopause onset in 48.05±4.09 years of age) and 51 matched controls (mean age 59.69±5.72, mean BMI 27.68±4.66kg/m2, menopause onset in 48.53±4.58 years of age) with normal thyroid function.

RESULTS

The main FRAX score was significantly higher in the group with subclinical hypothyroidism than in the controls (6.50±4.58 vs. 4.35±1.56; p=0.001). Hip FRAX score was significantly higher in the group with subclinical hypothyroidism (1.11±1.94 vs. 0.50±0.46; p=0.030). There was no significant difference in bone markers: osteocalcin (23.99±12.63 vs. 21.79±5.34 ng/mL; p=0.484) and beta cross-laps (365.76±184.84 vs. 306.88±110.73 pg/mL; p=0.21) between the two groups.

CONCLUSIONS

Postmenopausal patients with subclinical hypothyroidism, in particular of autoimmune origin, have higher FRAX scores and a thus greater risk for low-trauma hip fracture than euthyroid postmenopausal women. Our results point to the need to monitor postmenopausal patients with subclinical hypothyroidism for avoidance of fractures.

Thyroid function within the upper normal range is associated with reduced bone mineral density and an increased risk of nonvertebral fractures in healthy euthyroid postmenopausal women

CONTEXT

The relationship between thyroid function and bone mineral density (BMD) is controversial. Existing studies are conflicting and confounded by differences in study design, small patient numbers, and sparse prospective data.

OBJECTIVE

We hypothesized that variation across the normal range of thyroid status in healthy postmenopausal women is associated with differences in BMD and fracture susceptibility.

DESIGN

The Osteoporosis and Ultrasound Study (OPUS) is a 6-yr prospective study of fracture-related factors.

SETTING

We studied a population-based cohort from five European cities.

PARTICIPANTS

A total of 2374 postmenopausal women participated. Subjects with thyroid disease and nonthyroidal illness and those receiving drugs affecting thyroid status or bone metabolism were excluded, leaving a study population of 1278 healthy euthyroid postmenopausal women.

INTERVENTIONS

There were no interventions.

MAIN OUTCOME MEASURES

We measured free T(4) (fT4) (picomoles/liter), free T(3) (fT3) (picomoles/liter), TSH (milliunits/liter), bone turnover markers, BMD, and vertebral, hip, and nonvertebral fractures.

RESULTS

Higher fT4 (beta = -0.091; P = 0.004) and fT3 (beta = -0.087; P = 0.005) were associated with lower BMD at the hip, and higher fT4 was associated with increasing bone loss at the hip (beta = -0.09; P = 0.015). After adjustment for age, body mass index, and BMD, the risk of nonvertebral fracture was increased by 20% (P = 0.002) and 33% (P = 0.006) in women with higher fT4 or fT3, respectively, whereas higher TSH was protective and the risk was reduced by 35% (P = 0.028). There were independent associations between fT3 and pulse rate (beta = 0.080; P = 0.006), increased grip strength (beta = 0.171; P<0.001), and better balance (beta = 0.099; P < 0.001), indicating that the relationship between thyroid status and fracture risk is complex.

CONCLUSIONS

Physiological variation in normal thyroid status is related to BMD and nonvertebral fracture.

Efeitos sítio-ósseo dependentes no fêmur e vértebra de ratas com disfunções tireoidianas

OBJETIVO: Avaliar as diferenças sítio-ósseo dependentes no efeito das disfunções tireoidianas no fêmur e vértebras lombares de ratas. MÉTODOS: 33 ratas Wistar com dois meses de idade foram distribuídas em três grupos: eutireoideas (controle), hipotireoideas e hipertireoideas. Após 90 dias de tratamento para indução do hipo e hipertireoidismo, as ratas foram eutanasiadas, o sangue foi colhido para dosagem de T4 livre e os fêmures e as vértebras lombares (L1-L3) foram descalcificados e processados para análise da porcentagem de tecido ósseo trabecular. RESULTADOS: O grupo hipertireoideo apresentou porcentagem de tecido ósseo trabecular significativamente mais elevada na metáfise femoral, em comparação ao controle. Mas o hipertireoidismo não alterou a porcentagem de tecido ósseo trabecular na vértebra. O hipotireoidismo reduziu significativamente a porcentagem de tecido ósseo trabecular em comparação aos demais grupos nos segmentos 1-3 das vértebras lombares, mas não alterou a porcentagem de tecido ósseo trabecular no fêmur. CONCLUSÃO: O efeito do hipotireoidismo e do hipertireoidismo sobre a histomorfometria óssea é diferente e dependente do sítio ósseo.

Low cortical bone density measured by computed tomography in children and adolescents with untreated hyperthyroidism

OBJECTIVE

To determine whether increased thyroid hormones levels have an effect on various bone components (cortical vs cancellous bone).

STUDY DESIGN

The anthropometric and 3-dimensional quantitative computed tomography (CT) bone measurements, including bone density (BD), cross-sectional area (CSA) of the lumbar spine and femur, and cortical bone area (CBA) of the femur, of 18 children and adolescents with untreated hyperthyroidism were reviewed and compared with those of age-, sex-, and ethnicity-matched historical controls.

RESULTS

No significant differences in height, weight, body mass index (BMI), or pubertal staging between patients and controls were found. Cortical BD was significantly lower (P < .001) in children and adolescents with hyperthyroidism compared with historical controls. After adjusting for weight and height, no difference in femur CSA between hyperthyroid children and historical controls was evident. No significant correlations among thyroid hormone levels, antithyroid antibody levels, and cortical BD values were found.

CONCLUSIONS

As determined by CT, cortical bone is the preferential site of bone loss in children and adolescents with untreated hyperthyroidism.

Relationship between Subclinical Thyroid Dysfunction and Femoral Neck Bone Mineral Density in Women

BACKGROUND

Osteoporosis associated with thyroid dysfunction has been traditionally viewed as a secondary consequence of altered thyroid function, but there was recently a report about the direct effects of thyroid-stimulating hormone (TSH) on bone remodeling, which was mediated via the TSH receptor found on osteoblast and osteoclast precursor cells. Endogenous subclinical thyroid dysfunction seems to be an appropriate model to examine the direct effect of TSH on bone metabolism while ruling out the direct effect of thyroid hormone on bone metabolism. Thus, we aimed to investigate the relationship between subclinical thyroid dysfunction and bone mineral metabolism in women.

METHODS

We enrolled 413 women (mean age: 52.2 +/- 6.6 years) in our study. Serum levels of TSH, free T4 and the biochemical markers of bone turnover were measured by the standard methods. BMD at the lumbar spine and femoral neck were measured by dual energy X-ray absorptiometry.

RESULTS

Femoral neck BMD was significantly reduced both in the subclinical hyperthyroid group and in the subclinical hypothyroid group as compared with the euthyroid group (one-way ANOVA, p <0.001; post-hoc analysis, p = 0.041, p = 0.033). In contrast to the femoral neck BMD, the lumbar spine BMD showed no difference between the two groups. Additionally, serum calcium and alkaline phosphatase levels, urine deoxypyridinoline levels, and urine calcium to creatinine ratio showed no differences between the two groups.

CONCLUSIONS

Women having subclinical hyperthyroidism and women with subclinical hypothyroidsm have reduced femoral neck BMD. Additional studies are required to elucidate the mechanism for this finding.

A frequent regulatory variant of the estrogen-related receptor alpha gene associated with BMD in French-Canadian premenopausal women

Genes are important BMD determinants. We studied the association of an ESRRA gene functional variant with BMD in 1335 premenopausal women. The ESRRA genotype was an independent predictor of L2-L4 BMD, with an effect similar to smoking and equivalent to a 10-kg difference in weight.

INTRODUCTION

Several genetic polymorphisms have been associated with osteoporosis or osteoporosis fractures, but no functional effect has been shown for most of these gene variants. Because functional studies have implicated estrogen-related receptor alpha (ESRRA) in bone metabolism, we evaluated whether a recently described regulatory variant of the ESRRA gene is associated with lumbar and hip BMD as measured by DXA and with heel bone parameters as measured by quantitative ultrasound (QUS).

MATERIALS AND METHODS

Heel bone parameters were measured by right calcaneal QUS in 1335 healthy French-Canadian premenopausal women, and one-half of these women also had their BMD evaluated at two sites: femoral neck and lumbar spine (L2-L4) by DXA. All bone measures were tested separately for association with the ESRRA genotype by analysis of covariance. The significance of the ESRRA contribution to the model was also assessed by two different permutation tests.

RESULTS

A statistically significant association between ESRRA genotype and lumbar spine BMD was observed: women carrying the long ESRRA genotype had a 3.9% (0.045 g/cm2) higher lumbar spine BMD than those carrying the short ESRRA genotype (p = 0.004), independently of other risk factors measured. This effect of ESRRA genotype is similar to the effect of smoking and equivalent to a 10-kg difference in weight. This association was confirmed by permutation tests (p = 0.004). The same trend was observed for femoral neck BMD (2.6%, p = 0.07). However, no association was observed between ESRRA and QUS heel bone measures.

CONCLUSION

These results support the genetic influence of this ESRRA regulatory variant on BMD.

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.

O efeito molecular e estrutural do hormônio tiroideano no esqueleto

O hormônio tiroideano é essencial para o desenvolvimento, maturação e metabolismo ósseos normais. Durante o desenvolvimento, a deficiência do hormônio tiroideano resulta em atraso na maturação do esqueleto e disgênese das epífises, resultando em redução do crescimento e anormalidades esqueléticas. O hormônio tiroideano também tem efeito no osso do adulto. A tirotoxicose é freqüentemente associada ao aumento do metabolismo ósseo e diminuição da massa óssea. Embora a importância do hormônio tiroideano no desenvolvimento e metabolismo ósseos seja clara, os mecanismos que medeiam os efeitos desse hormônio no tecido ósseo apenas começam a ser desvendados. O hormônio tiroideano pode atuar indiretamente no esqueleto, aumetando a secreção de hormônio do crescimento (GH) e insulin-like growth factor-1 (IGF-1); ou diretamente, modulando genes alvo via receptores nucleares específicos. Não se sabe, entretanto, se os principais efeitos do hormônio tiroideano no osso são resultado de ações diretas ou indiretas. Achados in vitro, tais como a presença de receptores de hormônio tiroideano (TR) e a indução de genes e proteínas em células esqueléticas pelo hormônio tiroideano, evidenciam a importância de ações diretas. Esta revisão tem como meta sumarizar os achados in vivo e in vitro relacionados aos efeitos do hormônio tiroideano no esqueleto.

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

Thyrotoxicosis is frequently associated with increased bone turnover and decreased bone mass. To investigate the role of thyroid hormone receptor-beta (TR beta) in mediating the osteopenic effects of triiodothyronine (T3), female adult rats were treated daily (64 days) with GC-1 (1.5 microg/100 g body wt), a TR beta-selective thyromimetic compound. Bone mass was studied by dual-energy X-ray absorptiometry of several skeletal sites and histomorphometry of distal femur, and the results were compared with T3-treated (3 microg/100 g body wt) or control animals. As expected, treatment with T3 significantly reduced bone mineral density (BMD) in the lumbar vertebrae (L2-L5), femur, and tibia by 10-15%. In contrast, GC-1 treatment did not affect the BMD in any of the skeletal sites studied. The efficacy of GC-1 treatment was verified by a reduction in serum TSH (-52% vs. control, P < 0.05) and cholesterol (-21% vs. control, P < 0.05). The histomorphometric analysis of the distal femur indicated that T3 but not GC-1 treatment reduced the trabecular volume, thickness, and number. We conclude that chronic, selective activation of the TR beta isoform does not result in bone loss typical of T3-induced thyrotoxicosis, suggesting that the TR beta isoform is not critical in this process. In addition, our findings suggest that the development of TR-selective T3 analogs that spare bone mass represents a significant improvement toward long-term TSH-suppressive therapy.

Insulin-like growth factor binding proteins in femoral and vertebral bone marrow stromal cells: expression and regulation by thyroid hormone and dexamethasone

Insulin-like growth factor (IGF)-I is an important regulator of bone metabolism. Clinical observations suggest that different anatomic sites within the adult skeleton respond differently to hormonal and therapeutic treatment, and recent studies on bone marrow stromal cells in culture show that there are skeletal site-dependent differences in the gene expression of IGF-I. The actions of IGF-I and -II on bone cells are known to be modulated by the IGF binding proteins (IGFBP)-1 through -6 and the Type I and Type II IGF receptors. Therefore, we compared the expression of IGFBP-1 through -6 in adult female rat bone marrow stromal cell cultures derived from two separate skeletal sites: vertebrae and femurs. The cultures were maintained simultaneously under conditions that support osteoblast differentiation from osteoprogenitors present in the femoral and vertebral marrow cell populations. We also addressed whether IGFBP messenger RNA levels are regulated by thyroid hormone (T(3)) and dexamethasone (dex) treatment in femoral vs. vertebral marrow stromal cells in vitro, since steroid hormones play an important role in skeletal function. Northern blot analyses revealed that there are distinct skeletal site differences in the gene expression of IGFBPs. The vertebral marrow cultures express IGFBP-2 through -6 mRNAs, with IGFBP-2, IGFBP-4, and IGFBP-6 mRNAs predominating. The femoral marrow stromal cell cultures express only IGFBP-4 and IGFBP-6. Importantly, vertebral marrow cultures have much higher IGFBP mRNA steady-state levels than femoral cultures for all the detected IGFBP transcripts. IGFBP-1 is not detected in either femoral or vertebral cultures. In addition to a skeletal site difference, we show that T(3) and dex regulate the expression of specific IGFBP mRNAs. T(3) treatment also upregulates IGF-I protein secretion by vertebral marrow stromal cell cultures. Interestingly, the type I receptor for IGF-I was expressed equivalently in cultures from the two skeletal sites. These findings have important implications for the anatomical site specificities of hormonal responses that are noted in the skeleton.

Insulin-like growth factor I production is essential for anabolic effects of thyroid hormone in osteoblasts

Thyroid hormone (T3) and insulin-like growth factor I (IGF-I) are critical regulators of skeletal function. T3 increases IGF-I production in bone. To assess the potential role of IGF-I as a mediator of T3 actions, we characterized phenotypic markers of osteoblast activity in two osteoblast models, normal mouse osteoblasts and MC3T3-E1 cells, exposed to T3 alone or under conditions that interfere with IGF-I actions. T3 significantly increased osteoblast 3H-proline incorporation, alkaline phosphatase (ALP), and osteocalcin. Both alphaIR3, a neutralizing monoclonal antibody to the IGF-I receptor, and JB1, an IGF-I analogue antagonist, attenuated the stimulatory effects of T3. T3 effects also were decreased in cells transfected with antisense oligonucleotide (AS-ODN) to the IGF-I receptor gene. Both IGF-I and T3 had mitogenic effects that were inhibited by the antagonists. IGF-I by itself did not stimulate 3H-proline incorporation, ALP, and osteocalcin in the models used, revealing that although IGF-I is essential for the anabolic effects of T3, it acts in concert with other factors to elicit these phenotypic responses.

Thyroid hormone excess increases insulin-like growth factor I transcripts in bone marrow cell cultures: divergent effects on vertebral and femoral cell cultures

Thyroid hormones (T3 and T4) regulate bone development, growth, and turnover. Studies have suggested that different skeletal sites respond differently to thyroid hormones. Therefore, we examined the in vitro T3 responsiveness of cells committed to the osteoblast lineage as a function of skeletal location. Bone marrow cells derived from female rat femurs and vertebrae were cultured using conditions that induce osteogenic differentiation. Cells from both sites formed mineralized bone nodules in primary and secondary culture. In femoral cultures, collagen type I (coll I) and osteocalcin (OC) messenger RNA (mRNA) levels increased from the earliest time point examined (day 3) to a maximum on day 12 and thereafter declined to undetectable levels. T3 increased both OC and coll I mRNA, resulting in a continuous expression throughout the culture period. Insulin-like growth factor I (IGF-I) gene expression was detected at very low levels by Northern analysis of femoral total RNA, and T3 only marginally enhanced IGF-I mRNA levels. In vertebral cultures, OC and coll I mRNA levels also increased with time in culture, but remained expressed throughout the culture period. OC and coll I mRNA levels were not markedly altered in response to T3. In contrast to femoral cells, IGF-I gene expression was easily visualized in Northern blots from untreated vertebral cultures and was markedly increased by the addition of T3. The continuous presence of T3 (10(-7) M) in the medium for 18 days caused a marked decrease in the number of alkaline phosphatase-positive colonies formed in femoral secondary cultures, but only a slight decrease in the number in vertebral cultures. In addition, short term (6 days) exposure to T3 (10(-7) M) at the beginning of the culture period decreased alkaline phosphatase activity in femoral cultures, but not in vertebral cultures. These findings indicate that there are skeletal site-dependent differences in the in vitro responses of cells of the osteoblastic lineage to thyroid hormone.

Effects of thyroid hormone administration and estrogen deficiency on bone mass of female rats

To investigate the effects of thyroxine (T4) administration on bone mass, five 81-day-old female rats were treated with T4 (25 microg of T4/100 g of body weight [bw]/day), and bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry (DXA) 28 days later. The BMD values for the total skeleton, femoral, and tibial subsegments were lower than in controls (p < or = 0.05). The lumbar spine (L2-L5) was not significantly affected by T4 treatment. Next, thirty-seven 211 +/- 1.5 (mean +/- SEM)-day-old female rats were divided into six groups as follows: (1) control; (2) ovariectomized (OVX); (3) 1xT4 (approximately 1.0 microg of T4/100 g of bw/day; approximately physiological replacement dose); (4) OVX + 1xT4; (5) 2xT4 (approximately 2.0 microg of T4/100 g of bw/day); (6) OVX + 2xT4. DXA scans were performed at days 0 and 85. Control rats showed a generalized BMD increase, as opposed to a decrease in OVX rats. The trabecular bone volume of the fifth lumbar vertebra was also lower in OVX rats than in controls (p < 0.05). The 1xT4 treatment had no effect on BMD of intact rats, while treatment with 2xT4 impaired the expected BMD increase. Unexpectedly, the OVX + 1xT4 group presented a generalized BMD increase that was significant for the total skeleton, L2-L5, and femoral subsegments (p < 0.05), comparable to controls. Treating OVX animals with 2xT4 did not potentiate the osteopenic effects of estrogen deficiency, nor did it reverse the osteopenic effects of OVX. In conclusion, treatment with high doses of T4 caused BMD to decrease substantially, particularly at the femur, whereas near physiological doses of T4 prevented bone loss associated with OVX, and regardless of bone type (trabecular or cortical), the skeleton site seems to be a more important determinant of the effects of thyroid hormone on bone mass.

Site selectivity of osteoblast gene expression response to thyroid hormone localized by in situ hybridization

We have previously reported that thyroid-stimulating hormone (TSH)-suppressive doses of L-thyroxine (L-T4) decrease femoral, but not vertebral, bone mineral density (BMD) in rats. L-T4-induced decreases in BMD were associated with increased expression of genes, reflecting osteoblast activity in mRNA extracted from whole femurs but not from vertebrae. To document that this skeletal selectivity reflected altered osteoblast activity, we studied gene expression by in situ hybridization in 8-wk-old rats treated with L-T4 (20 microg x 100 g body wt(-1) x day(-1)) for 4 wk. TSH-suppressive doses of L-T4 were associated with decreased femoral (0.299 +/- 0.005 vs. 0.273 +/- 0.005 g/cm2, P < 0.01), but not vertebral (0.222 +/- 0.004 vs. 0.218 +/- 0.003 g/cm2), BMD. In situ hybridization documented that L-T4 administration for 4 wk increased expression of osteocalcin and alkaline phosphatase mRNA in femoral, but not vertebral, osteoblasts. This study demonstrates a differential gene expression response of vertebral and femoral osteoblasts to L-T4. This altered degree of gene expression markers of osteoblast activity documented by in situ hybridization may in part explain the apparent clinical differences in the effect of L-T4 on femoral and vertebral BMD.