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Interactions among receptors, thyroid hormone response elements, and ligands in the regulation of the rat uncoupling protein gene expression by thyroid hormone.

Uncoupling protein (UCP) is essential to the thermogenic function of brown adipose tissue (BAT). Thyroid hormone stimulates the rat UCP gene through two thyroid hormone response elements (TRE) located upstream of -2,300 and separated by 27 bp. They are an everted repeat (upstream TRE or upTRE) and a direct repeat (down-stream TRE or dnTRE). The goal of the present studies was to investigate whether these TREs interact and how such an interaction could contribute to explain the UCP responsiveness to T3 in vivo. We therefore aimed to define: the heterodimeric partner of the T3 receptor (T3R); the role of T3 in the receptor-receptor and receptors-DNA interactions; how such in vitro interactions relate to the enhancer function of TREs; and how the two TREs interact. Studies included electrophoretic mobility shift assays, utilizing T3R and retinoid X receptors (RXR); DNA footprinting; and transient transfections of HIB-1B cells, a BAT-derived cell line. As in many previously described TREs, the partner of the T3R is RXR. The unliganded T3Rs bind to the TREs as homodimers, which act as repressors of transcription. T3 reduces the binding of T3R homodimers, hence relieving the repression, and stimulates the binding of heterodimers and transcription in proportion to the heterodimer binding to the elements. Although qualitatively similar in these regards, there were important quantitative differences between both TREs. The upTRE binds more T3R homodimers and less T3R-RXR heterodimers than the dnTRE, and T3 more readily facilitates heterodimer binding to the dn- than to the upTRE. These in vitro characteristics are reflected in a lower efficiency of T3 to relieve T3R homodimer-mediated repression and to stimulate transcription through up-than through dnTRE. There were also significant interactions between the two TREs both in the binding of the receptors, T3R and RXR, and in the responsiveness to T3. By itself, each TRE responded modestly to T3, upTRE with lower sensitivity and dnTRE with higher sensitivity than traditional TREs, whereas together, in the context of the gene sequence, they mediated a response greater than the sum of those mediated by each TRE separately, with an intermediate sensitivity to T3. Thus, two TREs that are inadequate to explain the responsiveness of the UCP gene to T3, together form a complex unit appropriate for the regulation of the gene by thyroid hormone. These interactions represent yet another way TREs can shape up the responsiveness of genes to thyroid hormone.

Adipocytes↗

A mutant thyroid hormone receptor beta 1 identified in a patient with resistance to thyroid hormone inhibits the activities of not only the wild-type TRs, but also other nuclear receptors.

Although mutations of human thyroid hormone receptor beta (hTR beta) have been associated with resistance to thyroid hormone (RTH), the molecular basis by which the mutant TRs cause the various clinical symptoms is unknown. We show here that a mutant TR beta [corrected] identified in a patient with RTH inhibited the transcriptional activities of, not only the wild-type TR beta, but also other nuclear receptors including retinoid X receptor alpha (RXR alpha), vitamin D3 receptor (VDR) and retinoic acid receptor (RAR alpha). We provide evidence that these inhibitions by the mutant TR beta [corrected] occur by different mechanisms. Namely, the mutant TR beta interferes with VDR and RAR alpha by competition for binding to the corresponding response elements, but the pathway through RXR alpha is mainly inhibited by squelching of RXR alpha in solution. These findings suggest that in patients with RTH, not only the T3 responsive genes but also other responsive genes are inhibited by the mutant TRs, which might explain the variety of clinical symptoms in RTH.

Child↗

Amiodarone, thyroid hormone indexes, and altered thyroid function: long-term serial effects in patients with cardiac arrhythmias.

Amiodarone, a drug that has electrophysiologic actions resembling those of hypothyroidism, increases serum levels of T4 and reverse T3 (rT3) and decreases T3. The drug's long-term effects on thyroid function are poorly defined. Serial thyroid hormone indexes in 76 patients given amiodarone for 6 to 32 months (mean +/- standard deviation 16 +/- 7) for arrhythmias were determined serially. Over this period, 68 patients (89%) remained euthyroid; hypothyroidism developed in 6 (8%) and hyperthyroidism developed in 2 (3%). In patients who remained euthyroid, thyroid hormone alterations attained steady-state values at 3 months: T4 increased 42% (p less than 0.01), rT3 increased 172% (p less than 0.01) and T3 decreased 16% (p less than 0.05), without significant effect on thyroid stimulating hormone. For the euthyroid patients, the 90% tolerance limits (95% confidence) over the follow-up period for T4 was 5 to 19 micrograms/dl (normal 4 to 12), for T3 36 to 163 ng/dl (normal 60 to 160), for rT3 22 to 131 ng/dl (normal 15 to 50) and for thyroid stimulating hormone 0 to 14 microU/ml (normal 1 to 6). The changes in hormone indexes in hyperthyroid or hypothyroid patients were unrelated to the cumulative dose or duration of drug therapy. The most reliable diagnostic indexes for amiodarone-induced altered thyroid state were: thyroid stimulating hormone level over 20 microU for hypothyroidism and T4 over 20 ng/dl or high T3 over 200 ng/dl for hyperthyroidism. All levels were within the 90% tolerance limits derived for these hormones from patients remaining euthyroid on amiodarone long-term.(ABSTRACT TRUNCATED AT 250 WORDS)

Amiodarone↗

[Syndrome of resistance to thyroid hormones].

Although rare, thyroid hormone resistance syndrome should be suspected on a biological profile combining high thyroid hormone and non-suppressed TSH plasma levels. Resistance to thyroid hormone can be classified into 3 forms: generalized, pituitary and peripheral, all three showing tissue resistance heterogeneity from one person to another, and from one tissue to another in the same subject. In the generalized and pituitary forms, thyroid hormone levels are high with paradoxically normal or increased TSH levels. The TRH test still stimulates TSH secretion, and only the highest doses of T3 successfully suppress TSH secretion. In the generalized form, euthyroidism is usual, whereas in the pituitary form hyperthyroidism requires treatment in order to lower TSH secretion. In the peripheral form, various symptoms of thyroid hormone deficiency may be observed, contrasting with normal T3, T4 and TSH serum levels, and requiring supraphysiological doses of T3 for correction. In most cases, familial occurrence can be evidenced, with an autosomal dominant or sometimes recessive mode of inheritance. Genetic analysis has identified, in the generalized form, more than 10 mutations of the thyroid hormone receptor beta gene, all resulting in an alteration in the T3 binding domain of the receptor. In the autosomal dominant form, tissue resistance may result from a "dominant inhibitory effect" of the normal receptor function by the mutant receptor. All these thyroid hormone resistance syndromes constitute exceptional models for studying the mechanisms of action of thyroid hormones. Simultaneous observations of the mutated receptors with various clinical and biological phenotypes should further our understanding of thyroid hormone receptor function.

Drug Resistance↗

[Clinical studeis on the regulatory system of thyroid hormone secretion and serum triiodothyronine. Part III. Studies on thyroid hormone change and its relationship when TSH in cases with subclinical hypothyroidism (Hashimoto's thyroiditis and 131I treated Graves' disease)].

Changes in thyroid hormone levels and their relationship with TSH in cases with Hashimoto's thyroiditis and 131I treated Graves' disease in subclinical hypothyroid state were investigated: 1) In 34 cases with Hashimoto's thyroiditis, TSH levels was 14.7 +/- 26.8 muU/ml (mean +/- s.d.), T3 144.5 +/- 33.6 Rg/100 ml; T4 6.7 +/- 2.4 MUG/100 ML; T3RU 25.4 +/- 3.2%; F.T.I. 2.0g +/- O.89; T3/T4 2.46+/- 1.04%. It was noted that TSH levels revealed significant positive correlation with T3/T4 (r=0.77); significant inverse correlation with F.T.I. (r= - 0.51); and non-significant inverse correlation with T3 levels (r= - 0.06). These results indicate that T3 might play a more important role than T4 in maintaining normal thyroid function (so-called T3 euthyroidism). This is probably based on metabolic disturbance of iodine which is common in Hashimoto's thyroid. In several cases with rather high normal basal T3 levels, no significant elevation in T3 was observed in spite of elevated TSH level following TRH administration. This might indicate that the thyroid is fully functioning and that no more reserve is left for the thyroid to respond further to elevated TSH. 2) In 30 cases with 131I treated Graves' disease, TSH concentration was 22.3 +/- 29.8 muU/ml (mean +/- s.d.); T3 100.7 +/- 35.4 ng/100 ml: T4 6.2 +/- 2.9 MUG/100 ML; T3RU 25.6 +/- 2.3%; F.T.I. 1.87 +/- 0.85; T3/T4 1.80 +/- 0.64. Significant inverse correlation was noted between TSH levels and F.T.I. as well as T3 levels. Although TSH levels showed significant positive correlation with T3/T4 (r=0.44), its degree of correlation was not as good as that with F.T.I. (r= - 0.71) and T3 (r= - 0.53). These results indicate that there is no preference in T3 production. From the responsiveness of thyroid and pituitary following TRH administration, assumption can be drawn that 131I treated Graves' patients, who became euthyroid, may in fact be on the way to hypothyroidism.

Follow-Up Studies↗

An identical neo-mutation in the thyroid hormone receptor beta gene (A317T) in 2 unrelated Thai families with resistance to thyroid hormone.

We reported two unrelated Thai girls with resistance to thyroid hormone. The affected patients presented with goiter and no other stigmata of hyperthyroidism. Their serum T4, T3, free T4 and free T3 concentrations were high and they had normal levels of TSH. The affected girl in family 1 was treated with an antithyroid drug for 1-9/12 years. The affected girl in family 2 was only observed her thyroid function tests. TRH test showed normal TSH response in both girls. Analysis of the thyroid hormone receptor beta gene of both affected girls revealed the same missense mutation, changing the guanine in nucleotide 1234 to an adenine which results in the replacement of the normal alanine (GCT) with a threonine (ACT) at codon 317. Two proposita were heterozygous, and this mutation was not present in their parents compatible with a neo-mutation.

Adolescent↗

Identification of a thyroid hormone response element in the phosphoenolpyruvate carboxykinase (GTP) gene. Evidence for synergistic interaction between thyroid hormone and cAMP cis-regulatory elements.

Transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK) in the liver is regulated by many hormones including thyroid hormone (T3). In order to identify the elements in the promoter which are required for transcriptional induction by T3, we cotransfected a T3 receptor expression vector with a PEPCK-CAT reporter gene into HepG2 cells. Using vectors with deletions in the PEPCK promoter, we identified a single T3 response element (TRE) between positions -332 and -308. This element binds [125I]T3-labeled T3 receptor contained in nuclear extracts prepared from rat liver. Furthermore, the P3(I) element (-250 to -234), a previously described cis-sequence involved in mediating the induction of PEPCK gene transcription by cAMP, is also required for the T3 responsiveness of the promoter. In the absence of either the TRE or the P3(I) binding sites, no stimulation of transcription from the PEPCK promoter by T3 was observed, indicating that both elements are required for the T3 transcriptional regulation. Finally, a synergistic induction of PEPCK gene transcription by T3 and cAMP is described. This interaction requires both T3- and cAMP-responsive cis-acting elements.

Base Sequence↗

[Two cases of Graves' disease with anti-thyroid hormone autoantibodies: the effect of subtotal thyroidectomy on the titers of anti-thyroid hormone autoantibodies and anti-Tg antibodies].

We experienced two cases of Graves' disease associated with anti-thyroid hormone autoantibodies (Case 1: anti-T4, Case 2: anti-T3). Both cases underwent subtotal thyroidectomy, and titers of anti-thyroid hormone antibodies and anti-Tg antibodies were compared before and after operation. In case 1, titers of both anti-T4 and anti-Tg antibodies decreased after operation, whereas in Case 2, titers of both anti-T3 and anti-Tg were unchanged before and after operation. There was a significant positive correlation between titers of anti-T4 and anti-Tg antibodies in Case 1 (r = 0.90, P less than 0.01), and anti-T3 and anti-Tg antibodies in Case 2 (r = 0.64, P less than 0.01). These results strengthen the possibility that the antigen of anti-thyroid hormone autoantibodies in both cases is the Tg molecule.

Autoantibodies↗

[TSH state after thyroid hormone deprivation in athyrotic thyroid cancer patients (author's transl)].

The duration of thyroid hormone deprivation necessary on the one hand to achieve a sufficiently high endogenous TSH level for the identification of 131I-storing metastases or local relapses, and on the other hand to keep the consequences of thyroid hormone deprivation as low as possible, was determined in 111 athyrotic patients with thyroid carcinoma by means of the basal TSH level or a TRH stimulation test. From the first follow-up examination, a T3 deprivation of 8 days is to be recommended. Low basal TSH levels and marked stimulation in the TRH test should be sufficient cause to prolong the T3 deprivation to 10 days with further examinations. After a longterm suppression of athyrotic patients with thyroid hormone, a decrease in the TSH rise was observed in the course of 2 years, after appropriate interruption of the substitution.

Female↗

Thyroid hormone and norepinephrine signaling in brown adipose tissue. II: Differential effects of thyroid hormone on beta 3-adrenergic receptors in brown and white adipose tissue.

UNLABELLED: beta 3-adrenergic receptors (AR) are predominantly expressed in brown (BAT) and white adipose tissue (WAT), being more abundant in the former. There is growing interest in these receptors because of their potential to be pharmacologically targeted to control energy expenditure and lipid accretion. We have examined, in BAT and WAT of rats, the effect of thyroid hormone on the expression of beta 3-AR and their contribution to cAMP generation. Receptor density was assessed with the nonselective beta 3-AR ligand [125I]-cyanopindolol ([125I]-CYP) and the highly selective beta 3-AR agonist CL3216,243. beta 3-AR messenger RNA (mRNA) was analyzed by Northern blotting of total BAT and WAT RNA. Generation of cAMP by brown adipocytes in response to norepinephrine (NE) and CL316,243 was measured by RIA. In BAT, beta 3-AR can account for as much as 50% of the maximal cAMP response to NE, whereas in WAT these receptors probably account for all the effect of NE. In hypothyroidism. BAT beta 3-AR number is increased, as are beta 3-AR mRNA (4- to 6-fold) and the relative contribution of these receptors to the maximal cAMP production. In contrast, both beta 3-AR number and mRNA levels are reduced in WAT of hypothyroid rats. The injection of T3 to hypothyroid rats reverts the changes in beta 3-AR within 24 h, and T3 excess causes a greater than 90% reduction in beta 3-AR mRNA in BAT but a 5-fold increase in WAT. In both tissues, hypothyroidism is associated with a marked reduction in the capacity to generate cAMP, but this is not completely restored even after 2 days of a receptor-saturating dose of T3. CONCLUSIONS: 1) thyroid hormone differentially affects the expression of beta 3-AR in BAT and WAT; 2) these effects of T3 are both rapid and marked and seem to take place at a pretranslational level; 3) in both tissues there is a postreceptor defect in the generation of cAMP that is corrected by T3 much later after the changes in beta 3-AR are reversed; and 4) thyroid hormone is among the known factors that most vigorously affect the expression of beta 3-AR.

Adipose Tissue↗

Stimulation of facilitated [3H]uridine transport by thyroid hormone in GH1 cells. Evidence for regulation by the thyroid hormone nuclear receptor.

We have previously shown that 3,5,3'-triiodo-L-thyronine (L-T3) stimulates cell growth and a 4- to 8-fold increase in growth hormone mRNA in GH1 cells. These effects appear to be mediated by a thyroid hormone nuclear receptor with an equilibrium dissociation constant for L-T3 of 0.2 nM and an abundance of about 10,000 receptors per cell nucleus. In this report, we show that L-T3 exerts a pleiotypic effect on GH1 cells to rapidly (within 2 h) stimulate [3H]uridine uptake to a maximal value of 2.5- to 3-fold after 24 h. This results from an increase in the number of functional uridine "transport sites" as shown by studies documenting an increase in the apparent Vmax with no change in the Km, 17 microM. Although the labeling of the cellular uridine pool and pools of all phosphorylated uridine derivatives was increased by L-T3, there was no change in the relative amounts of the individual pools in cells incubated with or without hormone. The intracellular concentration of [3H]uridine was estimated to be similar to that of the medium, suggesting that facilitated transport mediates [3H]uridine uptake. That this increase in [3H]uridine transport was nuclear receptor-mediated is supported by the excellent correspondence of the L-T3 dose-response curve for [3H]uridine uptake and that for L-T3 binding to receptor. Finally, inhibition of protein synthesis by cycloheximide and RNA synthesis by actinomycin D demonstrated that the L-T3 effect required continuing protein and RNA synthesis. These results are consistent with an effect of the L-T3-nuclear receptor complex to increase uridine uptake in GH1 cells by altering the expression of gene(s) essential for the transport process.

Animals↗

3,5,3'-L-triiodothyronine (thyroid hormone)-induced protein-DNA interactions in the thyroid hormone response elements and cell type-specific elements of the rat growth hormone gene revealed by in vivo dimethyl sulfate footprinting.

The cell type-specific element and 3,5,3'-L-triiodothyronine (thyroid hormone) (T3) response element of the rat growth hormone gene act synergistically to produce cell type-specific, T3-regulated expression. Pit-1 is a pituitary cell type-specific transcription factor that binds the cell type-specific element and is essential to its activity. T3 receptors bind as homodimers and heterodimers to the T3 response element and are essential for its activity. Here, we report the use of ligation-mediated polymerase chain reaction in vivo dimethyl sulfate footprinting to study the effects of T3 on protein-DNA interactions in the rat growth hormone gene promoter in vivo. T3-responsive guanine methylation was detected only in and near the T3 response element and the Pit-1 binding sites. The results indicate that within 2 h, T3 induces occupancy of the T3 response element and Pit-1 sites by their respective trans-acting factors in vivo.

Animals↗

Cell cycle-dependent expression of thyroid hormone receptor-beta is a mechanism for variable hormone sensitivity.

Thyroid hormone receptors (TRs) are ligand-regulatable transcription factors. Currently, little is known about the expression of TRs or other nuclear hormone receptors during the cell cycle. We thus developed a stable expression system to express green fluorescent protein-TRbeta in HeLa cells under tetracycline regulation, and studied TR expression during the cell cycle by laser scanning cytometry. Only approximately 9-15% of the nonsynchronized cell population expressed TR because the majority of cells were in G(1) phase and did not express detectable amounts of TR. However, when cells were synchronized in early S phase with hydroxyurea and then released, TR expression levels increased in a cell cycle-dependent manner and peaked to 30-40% cells expressing TR at late G(2)/M phase before declining to nonsynchronized levels. Moreover, we observed a direct correlation between transcriptional activity and TR expression during the cell cycle. Similar cell cycle-dependent findings also were observed for endogenous TR in rat pituitary GH(3) cells. Last, cycloheximide studies demonstrated that the increase in TR expression was primarily due to increased translation. These novel observations of cell cycle-dependent expression of TR suggest that differential hormone sensitivity can occur during the cell cycle and may contribute to cell cycle progression during normal development and oncogenesis.

Animals↗

[The nature of thyroid hormone receptors. Intracellular transport and compartmentalization of chromatin receptors and thyroid hormones].

It is well established that in vivo administered labelled TBPA penetrates into liver, brain and lung cells, is translocated from cytosol into the nucleus and is accepted by chromatin without being affected by modifications touching upon the antigenic determinants of this protein. Electron microscopic autoradiography demonstrated that 125I-TBPA translocated from cytosol into the nucleus is localized on the border between hetero- and euchromatin. The data obtained may serve as an additional proof of the universal structure of intracellular thyroid hormone receptors and suggest that TBPA participate in manifestation of genetic effects of thyroid hormones.

Animals↗

Thyroid hormone responsive protein (THRP) mediates thyroid hormone-induced cytotoxicity in primary neuronal cultures.

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone (TH) in the cerebral cortex of adult rats. To study the effects of THRP on neuronal cell survival, primary neurons cultured from rats at embryonic day 19 were treated with either 10(-7) mol L(-1) 3,5,3'-triiodothyronine (T(3)), or 10(-7) mol L(-1) L: -thyroxine (T(4)). This resulted in decreasing neuronal cell number starting 48 h after treatment. T(3) -related cytotoxicity was also documented by measurement of lactate dehydrogenase release into the medium and by propidium iodide staining. Treatment of cells with 10(-7) mol L(-1) T(3) resulted in a significant increase in THRP mRNA levels as early as 24 h of treatment in a concentration-dependent manner. T(3) treatment did not alter glyceraldehyde 3-phosphate dehydrogenase (G3PDH) mRNA levels. Exogenous expression of THRP by transfecting cells with a THRP expression construct (pSVL-THRP) was associated with a significant increase in cell death as measured by the increased number of propidium iodide staining cells (18.0+/-2.1 cells per field) compared with mock-transfected cells (3.3+/-0.2), P<0.002. To further document THRP-induced cytotoxicity, the cells were either transfected with pSVL (empty vector)+pSV2neo (neomycin resistance vector for cell labeling), pSVL-THRP+pSV2neo, or pSVL-THRP+pc-Abl (cAbl tyrosine kinase expressing vector)+pSV2neo. After 24 h the cells were treated with 500 microg mL(-1) G418 (a congener of neomycin) to eliminate the non-transfected cells. Transfection with pSVL-THRP reduced neuronal survival relative to cells transfected with pSVL (356+/-15.6 compared with 145+/-16.9, P<0.05). Co-transfection of THRP with wild-type c-Abl did not alter the effect of THRP on cell survival. It is concluded that THRP is an important factor in TH-induced neuronal cell death.

Animals↗

Thyroid-hormone therapy and thyroid cancer: a reassessment.

Experimental studies and clinical data have demonstrated that thyroid-cell proliferation is dependent on thyroid-stimulating hormone (TSH), thereby providing the rationale for TSH suppression as a treatment for differentiated thyroid cancer. Several reports have shown that hormone-suppressive treatment with the L-enantiomer of tetraiodothyronine (L-T(4)) benefits high-risk thyroid cancer patients by decreasing progression and recurrence rates, and cancer-related mortality. Evidence suggests, however, that complex regulatory mechanisms (including both TSH-dependent and TSH-independent pathways) are involved in thyroid-cell regulation. Indeed, no significant improvement has been obtained by suppressing TSH in patients with low-risk thyroid cancer. Moreover, TSH suppression implies a state of subclinical thyrotoxicosis. In low-risk patients, the goal of L-T(4) treatment is therefore to obtain a TSH level in the normal range (0.5-2.5 mU/l). Only selected patients with high-risk papillary and follicular thyroid cancer require long-term TSH-suppressive doses of L-T(4). In these patients, careful monitoring is necessary to avoid undesirable effects on bone and heart.

Humans↗

Thyroid hormone receptors form distinct nuclear protein-dependent and independent complexes with a thyroid hormone response element.

We have examined the binding of nuclear proteins and recombinant thyroid hormone receptors (TRs) to the palindromic thyroid hormone responsive element AGGTCATGACCT (TREp) using a gel electrophoretic mobility shift assay. Four specific protein-DNA complexes were detected after incubation of nuclear extracts (NE) from T3-responsive pituitary (GH3) cells with a TREp-containing DNA fragment. This was compared with the TREp binding of reticulocyte lysate-synthesized TRs. TR alpha 1 and TR beta 2 each formed a single major TR:TREp complex which comigrated with the least retarded complex formed by GH3 NE, while TR beta 1 formed multiple complexes suggesting that it can bind to TREp as an oligomer. Interestingly, coincubation of 35S-TR alpha 1, GH3 NE, and unlabeled TREp resulted in not only the 35S-TR:TREp complex, but in two additional more greatly retarded complexes containing 35S-TR alpha 1 and comigrating with those formed by GH3 extract alone. Incubation of each of the TRs with NE from COS-7 cells, which do not possess sufficient endogenous TRs to mediate T3-responses, resulted in formation of a new, more greatly shifted complex. A similar, heat labile activity which altered mobility of the TR:TRE complex was also present in NE from T3-unresponsive JEG-3 cells. At high concentration of NE, all of the TR bound to TREp was more greatly retarded than in the absence of NE. Truncation of TR alpha 1 at amino acid 210 prevented additional complex formation in the presence of NE without affecting DNA binding, suggesting that the carboxyl-terminus of the TRs is essential for interaction with nuclear proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗