Distinct tissue-specific roles for thyroid hormone receptors beta and alpha1 in regulation of type 1 deiodinase expression

Main Factors Involved in Thyroid Hormone Action

The thyroid hormone receptors are the mediators of a multitude of actions by the thyroid hormones in cells. Most thyroid hormone activities require interaction with nuclear receptors to bind DNA and regulate the expression of target genes. In addition to genomic regulation, thyroid hormones function via activation of specific cytosolic pathways, bypassing interaction with nuclear DNA. In the present work, we reviewed the most recent literature on the characteristics and roles of different factors involved in thyroid hormone function in particular, we discuss the genomic activity of thyroid hormone receptors in the nucleus and the functions of different thyroid hormone receptor isoforms in the cytosol. Furthermore, we describe the integrin αvβ3-mediated thyroid hormone signaling pathway and its rapid nongenomic action in the cell. We furthermore reviewed the thyroid hormone transporters enabling the uptake of thyroid hormones in the cell, and we also include a paragraph on the proteins that mediate thyroid receptors’ shuttling from the nucleus to the cytosol.

An update on non-thyroidal illness syndrome

The non-thyroidal illness syndrome (NTIS) was first reported in the 1970s as a remarkable ensemble of changes in serum TH (TH) concentrations occurring in probably any severe illness. Ever since, NTIS has remained an intriguing phenomenon not only because of the robustness of the decrease in serum triiodothyronine (T3), but also by its clear correlation with morbidity and mortality. In recent years, it has become clear that (parenteral) feeding in patients with critical illness should be taken into account as a major determinant not only of NTIS but also of clinical outcome. Moreover, both experimental animal and clinical studies have shown that tissue TH concentrations during NTIS do not necessarily reflect serum low TH concentrations and may decrease, remain unaltered, or even increase according to the organ and type of illness studied. These differential changes now have a solid basis in molecular studies on organ-specific TH transporters, receptors and deiodinases. Finally, the role of inflammatory pathways in these non-systemic changes has begun to be clarified. A fascinating role for TH metabolism in innate immune cells, including neutrophils and monocytes/macrophages, was reported in recent years, but there is no evidence at this early stage that this may be a determinant of susceptibility to infections. Although endocrinologists have been tempted to correct NTIS by TH supplementation, there is at present insufficient evidence that this is beneficial. Thus, there is a clear need for adequately powered randomized clinical trials (RCT) with clinically relevant endpoints to fill this knowledge gap.

Murine intestinal stem cells are highly sensitive to modulation of the T3/TRα1-dependent pathway

The thyroid hormone T3 and its nuclear receptor TRα1 control gut development and homeostasis through the modulation of intestinal crypt cell proliferation. Despite increasing data, in-depth analysis on their specific action on intestinal stem cells is lacking. By using ex vivo 3D organoid cultures and molecular approaches, we observed early responses to T3 involving the T3-metabolizing enzyme Dio1 and the transporter Mct10, accompanied by a complex response of stem cell- and progenitor-enriched genes. Interestingly, specific TRα1 loss-of-function (inducible or constitutive) was responsible for low ex vivo organoid development and impaired stem cell activity. T3 treatment of animals in vivo not only confirmed the positive action of this hormone on crypt cell proliferation but also demonstrated its key action in modulating the number of stem cells, the expression of their specific markers and the commitment of progenitors into lineage-specific differentiation. In conclusion, T3 treatment or TRα1 modulation has a rapid and strong effect on intestinal stem cells, broadening our perspectives in the study of T3/TRα1-dependent signaling in these cells.

A coregulator shift, rather than the canonical switch, underlies thyroid hormone action in the liver

In this study, Shabtai et al. investigated the mechanism of thyroid hormone (TH)-dependent gene repression, generated a mouse line in which endogenous thyroid hormone receptor TRβ1 was epitope-tagged to allow precise chromatin immunoprecipitation at the low physiological levels of thyroid hormone receptors (TR), and defined high-confidence binding sites where TR functioned at enhancers regulated in the same direction as the nearest gene in a TRβ-dependent manner. Their results demonstrate that, in contrast to the canonical “all or none” coregulator switch model, TH regulates gene expression by orchestrating a shift in the relative binding of corepressors and coactivators.

Thyroid hormones (THs) are powerful regulators of metabolism with major effects on body weight, cholesterol, and liver fat that have been exploited pharmacologically for many years. Activation of gene expression by TH action is canonically ascribed to a hormone-dependent “switch” from corepressor to activator binding to thyroid hormone receptors (TRs), while the mechanism of TH-dependent repression is controversial. To address this, we generated a mouse line in which endogenous TRβ1 was epitope-tagged to allow precise chromatin immunoprecipitation at the low physiological levels of TR and defined high-confidence binding sites where TRs functioned at enhancers regulated in the same direction as the nearest gene in a TRβ-dependent manner. Remarkably, although positive and negative regulation by THs have been ascribed to different mechanisms, TR binding was highly enriched at canonical DR4 motifs irrespective of the transcriptional direction of the enhancer. The canonical NCoR1/HDAC3 corepressor complex was reduced but not completely dismissed by TH and, surprisingly, similar effects were seen at enhancers associated with negatively as well as positively regulated genes. Conversely, coactivator CBP was found at all TH-regulated enhancers, with transcriptional activity correlating with the ratio of CBP to NCoR rather than their presence or absence. These results demonstrate that, in contrast to the canonical “all or none” coregulator switch model, THs regulate gene expression by orchestrating a shift in the relative binding of corepressors and coactivators.

Jinlida Granules Improve Dysfunction of Hypothalamic-Pituitary-Thyroid Axis in Diabetic Rats Induced by STZ

Objective

We aim to explore the effects and mechanisms of Jinlida granules on the dysfunction of hypothalamic-pituitary-thyroid (HPT) axis in diabetic rats induced by streptozotocin.

Methods

A total of 48 SD rats were randomized into normal control group (NC, n = 6) and diabetic group (n = 42). Rats in diabetic group were randomly divided into diabetes mellitus (DM) control group, low, medium, and high doses of Jinlida group (JL, JM, and JH), medium dose of Jinlida plus Tongxinluo group (JM + T), metformin group (Met), and Saxagliptin group (Sax) (n = 6 in each group). Diabetic rats were obtained by intraperitoneal injection of streptozotocin and sacrificed at 8 weeks to examine the function of HPT axis.

Results

Levels of fasting blood glucose (P < 0.05), pIκB, TNFα (P < 0.05), pNF-κB, and IL-6 (P < 0.01) in liver tissue and TSHR mRNA expression (P < 0.01) in diabetic group were significantly increased, while levels of serum T3 and T4, thyroid hormone receptor (TR) mRNA and Dio1 mRNA in liver tissue, and sodium iodide symporter (NIS) mRNA in thyroid tissue in diabetic group were significantly decreased compared with those in NC group (P < 0.01). Among diabetic groups, level of fasting blood glucose in JH, JM + T and Met group was lower (P < 0.05) compared with DM group. However, levels of serum T3 and T4, TR mRNA in liver tissue, TSHR, and NIS mRNA in thyroid tissue in JH, JM + T, Met, and Sax group were significantly increased (P < 0.01) compared to DM group. In contrast, levels of Dio1 mRNA, pI-κB in Met and JM + T groups, pNF-κB in JH, JM + T, and Met group, and TNFα and IL-6 in JM, JH, JM + T, and Met group were significantly decreased (P < 0.05). HE staining showed reduced thyroid follicular epithelium and follicular area, as well as increased colloid area in DM group, indicating impaired synthesis, reabsorption, and secretory of TH in diabetes, which was significantly improved in JH, JM + T, and Met groups.

Conclusion

HPT axis dysfunction in DM could be significantly improved by Jinlida granules. The mechanism might be associated with the anti-inflammatory effects involving NF-κB pathway. Our findings suggested the potential benefit of Jinlida granules for patients with HPT axis dysfunction and DM, which was to be verified by more experimental and clinical studies.

Thyroid hormone receptor α mutation causes a severe and thyroxine-resistant skeletal dysplasia in female mice

A new genetic disorder has been identified that results from mutation of THRA, encoding thyroid hormone receptor α1 (TRα1). Affected children have a high serum T3:T4 ratio and variable degrees of intellectual deficit and constipation but exhibit a consistently severe skeletal dysplasia. In an attempt to improve developmental delay and alleviate symptoms of hypothyroidism, patients are receiving varying doses and durations of T4 treatment, but responses have been inconsistent so far. Thra1(PV/+) mice express a similar potent dominant-negative mutant TRα1 to affected individuals, and thus represent an excellent disease model. We hypothesized that Thra1(PV/+) mice could be used to predict the skeletal outcome of human THRA mutations and determine whether prolonged treatment with a supraphysiological dose of T4 ameliorates the skeletal abnormalities. Adult female Thra1(PV/+) mice had short stature, grossly abnormal bone morphology but normal bone strength despite high bone mass. Although T4 treatment suppressed TSH secretion, it had no effect on skeletal maturation, linear growth, or bone mineralization, thus demonstrating profound tissue resistance to thyroid hormone. Despite this, prolonged T4 treatment abnormally increased bone stiffness and strength, suggesting the potential for detrimental consequences in the long term. Our studies establish that TRα1 has an essential role in the developing and adult skeleton and predict that patients with different THRA mutations will display variable responses to T4 treatment, which depend on the severity of the causative mutation.

In vivo activity of the thyroid hormone receptor beta- and α-selective agonists GC-24 and CO23 on rat liver, heart, and brain

Thyroid hormone analogs with selective actions through specific thyroid hormone receptor (TR) subtypes are of great interest. They might offer the possibility of mimicking physiological actions of thyroid hormone with receptor subtype or tissue specificity with therapeutic aims. They are also pharmacological tools to dissect biochemical pathways mediated by specific receptor subtypes, in a complementary way to mouse genetic modifications. In this work, we studied the in vivo activity in developing rats of two thyroid hormone agonists, the TRβ-selective GC-24 and the TRα-selective CO23. Our principal goal was to check whether these compounds were active in the rat brain. Analog activity was assessed by measuring the expression of thyroid hormone target genes in liver, heart, and brain, after administration to hypothyroid rats. GC-24 was very selective for TRβ and lacked activity on the brain. On the other hand, CO23 was active in liver, heart, and brain on genes regulated by either TRα or TRβ. This compound, previously shown to be TRα-selective in tadpoles, displayed no selectivity in the rat in vivo.

Changes in gene expression of DOR and other thyroid hormone receptors in rat liver during acute-phase response

Non-thyroidal illness is characterized by low tri-iodothyronine (T3) serum level under acute-phase conditions. We studied hepatic gene expression of the newly identified thyroid hormone receptor (TR) cofactor DOR/TP53INP2 together with TRs in a rat model of aseptic abscesses induced by injecting intramuscular turpentine-oil into each hind limb. A fast (4-6 h) decrease in the serum level of free thyroxine and free T3 was observed. By immunohistology, abundant DOR protein expression was detected in the nuclei of hepatocytes and ED-1(+) (mononuclear phagocytes), CK-19(+) (biliary cells), and SMA(+) (mesenchymal cells of the portal tract) cells. DOR signal was reduced with a minimum at 6-12 h after the acute-phase reaction (APR). Immunohistology also showed a similar pattern of protein expression in TRα1 but without a significant change during APR. Transcripts specific for DOR, nuclear receptor co-repressor 1 (NCoR-1), and TRβ1 were down-regulated with a minimum at 6-12 h, whereas expression for TRα1 and TRα2 was slightly and significantly up-regulated, respectively, with a maximum at 24 h after APR was initiated. In cultured hepatocytes, acute-phase cytokines interleukin-1β (IL-1β) and IL-6 down-regulated DOR and TRβ1 at the mRNA level. Moreover, gene expression of DOR and TRs (TRα1, TRα2, and TRβ1) was up-regulated in hepatocytes by adding T3 to the culture medium; this up-regulation was almost completely blocked by treating the cells with IL-6. Thus, TRβ1, NCoR-1, and the recently identified DOR/TP53INP2 are abundantly expressed and down-regulated in liver cells during APR. Their down-regulation is attributable to the decreased serum level of thyroid hormones and most probably also to the direct action of the main acute-phase cytokines.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

The nuclear corepressor, NCoR, regulates thyroid hormone action in vivo

The thyroid hormone receptor (TR) has been proposed to regulate expression of target genes in the absence of triiodothyronine (T(3)) through the recruitment of the corepressors, NCoR and SMRT. Thus, NCoR and SMRT may play an essential role in thyroid hormone action, although this has never been tested in vivo. To accomplish this, we developed mice that express in the liver a mutant NCoR protein (L-NCoRDeltaID) that cannot interact with the TR. L-NCoRDeltaID mice appear grossly normal, however, when made hypothyroid the repression of many positively regulated T(3)-target genes is abrogated, demonstrating that NCoR plays a specific and sufficient role in repression by TR in the absence of T(3). Remarkably, in the euthyroid state, expression of many T(3)-targets is also up-regulated in L-NCoRDeltaID mice, demonstrating that NCoR also determines the magnitude of the response to T(3) in euthyroid animals. Although positive T(3) targets were up-regulated in L-NCoRDeltaID mice in the hypo- and euthyroid state, there was little effect seen on negatively regulated T(3) target genes. Thus, NCoR is a specific regulator of T(3)-action in vivo and mediates repression by the unliganded TR in hypothyroidism. Furthermore, NCoR appears to play a key role in determining the tissue-specific responses to similar levels of circulating T(3). Interestingly, NCoR recruitment to LXR is also impaired in this model, leading to activation of LXR-target genes, further demonstrating that NCoR recruitment regulates multiple nuclear receptor signaling pathways.

Effects of substitution and high-dose thyroid hormone therapy on deiodination, sulfoconjugation, and tissue thyroid hormone levels in prolonged critically ill rabbits

To delineate the metabolic fate of thyroid hormone in prolonged critically ill rabbits, we investigated the impact of two dose regimes of thyroid hormone on plasma 3,3'-diiodothyronine (T(2)) and T(4)S, deiodinase type 1 (D1) and D3 activity, and tissue iodothyronine levels in liver and kidney, as compared with saline and TRH. D2-expressing tissues were ignored. The regimens comprised either substitution dose or a 3- to 5- fold higher dose of T(4) and T(3), either alone or combined, targeted to achieve plasma thyroid hormone levels obtained by TRH. Compared with healthy animals, saline-treated ill rabbits revealed lower plasma T(3) (P=0.006), hepatic T(3) (P=0.02), and hepatic D1 activity (P=0.01). Substitution-dosed thyroid hormone therapy did not affect these changes except a further decline in plasma (P=0.0006) and tissue T(4) (P=0.04). High-dosed thyroid hormone therapy elevated plasma and tissue iodothyronine levels and hepatic D1 activity, as did TRH. Changes in iodothyronine tissue levels mimicked changes in plasma. Tissue T(3) and tissue T(3)/reverse T(3) ratio correlated with deiodinase activities. Neither substitution- nor high-dose treatment altered plasma T(2). Plasma T(4)S was increased only by T(4) in high dose. We conclude that in prolonged critically ill rabbits, low plasma T(3) levels were associated with low liver and kidney T(3) levels. Restoration of plasma and liver and kidney tissue iodothyronine levels was not achieved by thyroid hormone in substitution dose but instead required severalfold this dose. This indicates thyroid hormone hypermetabolism, which in this model of critical illness is not entirely explained by deiodination or by sulfoconjugation.

Age-related changes in renal and hepatic cellular mechanisms associated with variations in rat serum thyroid hormone levels

Aging is associated with changes in thyroid gland physiology. Age-related changes in the contribution of peripheral tissues to thyroid hormone serum levels have yet to be systematically assessed. Here, we investigated age-related alterations in the contributions of the liver and kidney to thyroid hormone homeostasis using 6-, 12-, and 24-mo-old male Wistar rats. A significant and progressive decline in plasma thyroxine occurred with age, but triiodothyronine (T(3)) was decreased only at 24 mo. This was associated with an unchanged protein level of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in the kidney and with a decreased MCT8 level in the liver at 24 mo. Hepatic type I deiodinase (D1) protein level and activity declined progressively with age. Renal D1 levels were decreased at both 12 and 24 mo but D1 activity was decreased only at 24 mo. In the liver, no changes occurred in thyroid hormone receptor (TR) TRalpha(1), whereas a progressive increase in TRbeta(1) occurred at both mRNA and total protein levels. In the kidney, both TRalpha(1) and TRbeta(1) mRNA and total protein levels were unchanged between 6 and 12 mo but increased at 24 mo. Interestingly, nuclear TRbeta1 levels were decreased in both liver and kidney at 12 and 24 mo, whereas nuclear TRalpha(1) levels were unchanged. Collectively, our data show differential age-related changes among hepatic and renal MCT8 and D1 and TR expressions, and they suggest that renal D1 activity is maintained with age to compensate for the decrease in hepatic T(3) production.

Hepatocyte nuclear factor 4alpha contributes to thyroid hormone homeostasis by cooperatively regulating the type 1 iodothyronine deiodinase gene with GATA4 and Kruppel-like transcription factor 9

Type 1 iodothyronine deiodinase (Dio1), a selenoenzyme catalyzing the bioactivation of thyroid hormone, is highly expressed in the liver. Dio1 mRNA and enzyme activity levels are markedly reduced in the livers of hepatocyte nuclear factor 4alpha (HNF4alpha)-null mice, thus accounting for its liver-specific expression. Consistent with this deficiency, serum T4 and rT3 concentrations are elevated in these mice compared with those in HNF4alpha-floxed control littermates; however, serum T3 levels are unchanged. Promoter analysis of the mouse Dio1 gene demonstrated that HNF4alpha plays a key role in the transactivation of the mouse Dio1 gene. Deletion and substitution mutation analyses demonstrated that a proximal HNF4alpha site (direct repeat 1 [TGGACAAAGGTGC]; HNF4alpha-RE) is crucial for transactivation of the mouse Dio1 gene by HNF4alpha. Mouse Dio1 is also stimulated by thyroid hormone signaling, but a direct role for thyroid hormone receptor action has not been reported. We also showed that thyroid hormone-inducible Krüppel-like factor 9 (KLF9) stimulates the mouse Dio1 promoter very efficiently through two CACCC sequences that are located on either side of HNF4alpha-RE. Furthermore, KLF9 functions together with HNF4alpha and GATA4 to synergistically activate the mouse Dio1 promoter, suggesting that Dio1 is regulated by thyroid hormone in the mouse through an indirect mechanism requiring prior KLF9 induction. In addition, we showed that physical interactions between the C-terminal zinc finger domain (Cf) of GATA4 and activation function 2 of HNF4alpha and between the basic domain adjacent to Cf of GATA4 and a C-terminal domain of KLF9 are both required for this synergistic response. Taken together, these results suggest that HNF4alpha regulates thyroid hormone homeostasis through transcriptional regulation of the mouse Dio1 gene with GATA4 and KLF9.

Effects of the synthetic liver X receptor agonist T0901317 on the growth hormone and thyroid hormone axes in male rats

Liver X receptors (LXRs), activated by oxysterols, play an important role in the regulation of lipid and glucose metabolism, which is also markedly dependent on thyroid hormone and growth hormone (GH) status. Here, we investigated how a 1-week exposure to the synthetic LXR agonist T0901317 affected GH secretion and thyroid hormone status in male rats. While the pulse frequency of GH secretion was marginally affected there was a highly significant decrease in the triiodo-L-thyronine/thyroxine (T3/T4) ratio in plasma. This effect was associated with decreased expression of deiodinase 1 (DIO1) and 2 (DIO2) mRNA in the liver and thyroid gland, respectively. Expression of sterol regulatory element binding protein-1c (SREBP-1c), the hallmark of stimulated lipogenesis, was markedly increased in both thyroid and pituitary implying that protracted pharmacological LXR activation may promote lipid accumulation in these endocrine tissues. These findings suggest that attention must be given to pituitary hormone dependent axes when developing therapeutic strategies based on agonism of the LXRs, e.g. for treatment of atherosclerosis.

Thermogenic effect of triiodothyroacetic acid at low doses in rat adipose tissue without adverse side effects in the thyroid axis

Triiodothyroacetic acid (TRIAC) is a physiological product of triiodothyronine (T(3)) metabolism, with high affinity for T(3) nuclear receptors. Its interest stems from its potential thermogenic effects. Thus this work aimed 1) to clarify these thermogenic effects mediated by TRIAC vs. T(3) in vivo and 2) to determine whether they occurred predominantly in adipose tissues. To examine this, control rats were infused with equimolar T(3) or TRIAC doses (0.8 or 4 nmolx100 g body wt(-1) x day(-1)) or exposed for 48 h to cold. Both T(3) doses and only the highest TRIAC dose inhibited plasma and pituitary thyroid-stimulating hormone (TSH) and thyroxine (T(4)) in plasma and tissues. Interestingly, the lower TRIAC dose marginally inhibited plasma T(4). T(3) infusion increased plasma and tissue T(3) in a tissue-specific manner. The highest TRIAC dose increased TRIAC concentrations in plasma and tissues, decreasing plasma T(3). TRIAC concentrations in tissues were <10% those of T(3). Under cold exposure or high T(3) doses, TRIAC increased only in white adipose tissue (WAT). Remarkably, only the lower TRIAC dose activated thermogenesis, inducing ectopic uncoupling protein (UCP)-1 expression in WAT and maximal increases in UCP-1, UCP-2, and lipoprotein lipase (LPL) expression in brown adipose tissue (BAT), inhibiting UCP-2 in muscle and LPL in WAT. TRIAC, T(3), and cold exposure inhibited leptin secretion and mRNA in WAT. In summary, TRIAC, at low doses, induces thermogenic effects in adipose tissues without concomitant inhibition of TSH or hypothyroxinemia, suggesting a specific role regulating energy balance. This selective effect of TRIAC in adipose tissues might be considered a potential tool to increase energy metabolism.

Thyroid hormone induces cerebellar Purkinje cell dendritic development via the thyroid hormone receptor alpha1

The thyroid hormone l-3,3',5-triiodothyronine (T3) plays an important role during cerebellar development. Perinatal T3 deficiency leads to severe cellular perturbations, among them a striking reduction in the growth and branching of Purkinje cell dendritic arborization. The molecular mechanisms underlying these effects are poorly understood. Despite the well documented broad expression of thyroid hormone receptors (TRs), analysis of different TR-deficient mice has failed to provide detailed information about the function of distinct TRs during neuronal development. The cerebellar cell culture systems offer an excellent model by which to study the effects of T3, because differentiation of cerebellar neurons in mixed and purified cultures proceeds in the absence of serum that contains T3. Addition of T3 to cerebellar cultures causes a dramatic increase in Purkinje cell dendrite branching and caliber in a dose- and time-dependent manner. Furthermore, we demonstrate for the first time that T3 acts on Purkinje cells directly through TRalpha1 expressed on the Purkinje cell and not on the granule cell, the presynaptic partner of Purkinje cells. In contrast, TRbeta isoforms are not involved, because Purkinje cells derived from TRbeta-/- mice show the same T3 responsiveness as wild-type cells. T3-promoted Purkinje cell differentiation was not mediated via neurotrophins, as suggested previously, because dendritogenesis of Purkinje cells from BDNF-/- mice could be effectively stimulated in vitro by T3 treatment. Furthermore, the effects of T3 observed were not abolished by tyrosine kinase receptor B (TrkB)-IgG, TrkC-IgG, or K252a, agents known to block the actions of neurotrophin. These results indicate that T3 directly affects Purkinje cell differentiation through activation of the TRalpha1.

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.