Biomedical subjects

S Y Cheng

Publications and source records attributed to S Y Cheng.

At least 55 records · Page 3Linked to original sources

PubMed1996

Modulation of the transcriptional activity of thyroid hormone receptors by the tumor suppressor p53.

Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcriptional factors that regulate growth, differentiation, and development. The molecular mechanisms by which TRs mediate these effects are unclear. One prevailing hypothesis suggests that TRs may cooperate with other transcriptional factors to mediate their biological effects. In this study, we tested this hypothesis by examining whether the activity of TRs is modulated by the tumor suppressor p53. p53 is a nuclear protein that regulates gene expression via sequence-specific DNA binding and/or direct protein-protein interaction. We found that the human TR subtype beta 1 (h-TR beta 1) physically interacted with p53 via its DNA binding domain. As a result of this physical interaction, binding of h-TR beta 1 to its hormone response elements either as homodimer or as a heterodimer with the retinoic X receptor was inhibited by p53 in a concentration-dependent manner. In transfected cells, wild-type p53 repressed the hormone-dependent transcriptional activation of h-TR beta 1. In contrast, mutant p53 either had no effect or activated the transcriptional activity of h-TR beta 1 depending on the type of hormone response elements. These results indicate the gene regulating activity of TRs was modulated by p53, suggesting that the cross talk between these two transcriptional factors may play an important role in the biology of normal and cancer cells.

Animals↗

PubMed1996

Respiratory symptoms and pulmonary function in mill workers exposed to wood dust.

This study surveyed wood dust exposure levels and pulmonary hazards among wood mill workers. Dust concentrations as measured by six-stage cascade impactors were high in work areas of grinding and screening. Total dust concentrations for these dusty activities ranged from 4.4 to 22.4 mg/m3, and the respirable proportions were between 2.4% and 50.2%. The dust level in the sawing work was 2.9 mg/m3. Although symptoms of cough and phlegm were higher in smoking workers than in nonsmoking workers, the prevalence of respiratory symptoms in the exposed workers was not significantly higher than in the controls. However, the incidence of symptoms such as chronic phlegm and chronic bronchitis in the nonsmoking high-exposure workers was significantly higher than in nonsmoking controls. The mean values of MMF, PEFR, and FEF25% were significantly lower in the exposed workers than in controls for both smokers and nonsmokers. The pulmonary function deficits, with the exception of FEV1.0/FVC, also showed a significant trend with increasing levels of wood dusts exposure classified by job titles for both smokers and nonsmokers. After adjustment for age, sex, height, and smoking status, all parameters of pulmonary function were significantly lower in exposed workers than in controls and showed a declining trend with increasing exposure levels classified by job titles. These results indicate that high level of wood dust exposure in the wood mill industries may lead to pulmonary hazards. Engineering control and industrial hygiene are mandatory for dusty activities.

Adult↗

PubMed1996

Respiratory symptoms and pulmonary function among wood dust-exposed joss stick workers.

Health hazards associated with wood dust exposure have been investigated in various industries. This study surveyed wood dust exposure levels and pulmonary effects among joss stick workers. Greater dust concentrations, as measured by six-stage cascade impactors, were observed in work areas where joss sticks were produced and incense was mixed than in other work areas. Total dust concentrations for these two high dust activities ranged from 9.9 to 42.7 mg/m3, and respirable proportions were between 2.0% and 54.6%. Higher dust levels were observed for dry joss stick production methods than for wet production methods. Dust levels for all other performance areas were lower than the permissible exposure level of 10 mg/m3. Although symptoms of cough and phlegm were higher in smoking workers than in nonsmoking workers, the prevalence of respiratory symptoms for exposed workers was not significantly higher than for the controls. The prevalence of pulmonary function deficits and the values of FEF25% and FEF75% in the exposed workers were significantly worse than those in the controls. But no difference was found between the male controls and the male exposed workers, the high-exposure group. Respiratory symptoms and pulmonary function also did not show a dose-response trend with the exposure levels estimated by correlation with worker job titles and duration of employment. No suspected case of pneumoconiosis was found from the chest radiographs. These results suggest that wood dust exposure in the joss stick industries might not lead to significant pulmonary damage.

Adult↗

PubMed1996

Determination of zinc fractions in human blood and seminal plasma by ultrafiltration and atomic absorption spectrophotometry.

A simple and reliable method is described which combines ultrafiltration technique with atomic absorption spectrophotometry to determine the Zn fractions in human blood plasma and seminal plasma. Ultrafiltrable, loosely bound, and firmly bound Zn can be measured using this method in the presence or absence of ethylenediaminetetraacetic acid (EDTA). The YMT membranes for the ultrafiltration must be rinsed thoroughly before use. In contrast to Zn in blood plasma, a large part of Zn in the seminal plasma was found to be ultrafiltrable. This method can be applied to study the physiologically active part of Zn in body fluids related to various disease states.

Adult↗

PubMed1996

Identification of naturally occurring dominant negative mutants of thyroid hormone alpha 1 and beta 1 receptors in a human hepatocellular carcinoma cell line.

To understand the function of thyroid hormone nuclear receptors (TRs) in human hepatocellular carcinoma cells (HCC), we characterized the hormone binding and transactivational activity of TRs in a HCC cell line, J7. TR alpha 1 (J7-TR alpha 1) and TR beta 1 (J7-TR beta 1) complementary DNAs were cloned from this cell line, and the binding activity to the hormone response elements (TREs) and to the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3) of the expressed TR proteins were evaluated. J7-TR alpha 1 and J7-TR beta 1 bound to TREs similarly as the TRs isolated from other tissues. However, J7-TR alpha 1 did not bind to T3, and J7-TR beta 1 bound to T3 with only about 10% the affinity of the wild-type TR beta 1. Sequencing of the complementary DNAs shows a single Met259Ile mutation in J7-TR alpha 1 and Met334Val in J7-TR beta 1. Using reporters containing TREs, we found that J7-TR alpha 1 and J7-TR beta 1 had virtually lost their transactivational activity. Moreover, these two mutants inhibited the transactivational activity of the wild-type TRs by a dominant negative effect not only on the transfected TRs, but also on endogenous TRs in other two HCC cell lines, SK-Hep-1 and HepG2. The potency of the dominant negative effect of these two mutants inversely correlated with the expression level of endogenous TRs. The present studies identified two novel naturally occurring TR mutants that have potent dominant negative action. The identification of both the alpha and beta dominant negative mutants in J7 made this cell line a useful model system to further understand the molecular mechanism of the dominant negative action of TR mutants.

Carcinoma, Hepatocellular↗

PubMed1996

Understanding the molecular mechanism of dominant negative action of mutant thyroid hormone beta 1-receptors: the important role of the wild-type/mutant receptor heterodimer.

The clinical manifestations of patients with resistance to thyroid hormone result from inhibition of the functions of wild-type thyroid hormone receptors (wTRs) by the dominant negative effect of mutant TR beta 1 receptors (mTR beta 1). One of the proposed mechanisms by which mTR beta 1 exerts its dominant negative action is via formation of the putative inactive wTR beta 1/mTR beta 1 heterodimer. However, the nature of the wTR beta 1/mTR beta 1 heterodimer is poorly understood. The present study characterizes the wTR beta 1/mTR beta 1 heterodimer by electrophoretic mobility shift assay. The mutant TR beta 1 used was PV, which contains a frame shift mutation in the C-terminal part of TR beta 1 and has less than 1% of the T3 binding affinity of the wTR beta 1. Because of the difficulty in resolving wTR beta 1 and mutant PV dimers, we used a truncated wTR beta 1 in which the A/B domain was deleted (delta TR beta 1) to demonstrate the formation of the heterodimer on thyroid hormone response elements (TREs) in which the half-site binding motifs are oriented in an inverted repeat (F2), a direct repeat separated by four nucleotides (DR4), or an inverted repeat (Pal). Deletion of the A/B domain had no effect on the binding of T3 and TREs to wTR beta 1. In the presence of equal amounts of delta TR beta 1 and PV, three types of molecular complexes. delta TR beta 1 homodimer, delta TR beta 1/PV heterodimer, and PV homodimer bound to each TRE in a ratio of approximately 1:2:1. The identities of these complexes were confirmed by their ability to be supershifted by anti-TR beta 1 and/or anti-PV antibodies. delta TR beta 1/PV heterodimer formation varied with different TREs. The ratio of apparent affinity constant (Ka) in the binding of delta TR beta 1/PV to TREs was F2:DR4:Pal = approximately 6:2:1. The effect of T3 on delta TR beta 1/PV heterodimer formation was TRE dependent. No T3-induced dissociation was observed for the delta TR beta 1/PV heterodimer when bound to F2 and Pal. In contrast, the delta TR beta 1/PV heterodimer bound to DR4 was dissociated by T3 with an ED50 of 3.9 +/- 0.9 nM. The T3-induced dissociation of delta TR beta 1 homodimer bound to F2, DR4, and Pal had ED50 values of 4.1 +/- 1.2, 1.3 +/- 0.3, and more than 100 nM, respectively. By transfection assays, the dominant negative action of PV was found to be TRE dependent with the rank order of F2 >> Pal > ME (a DR4-like TRE in the rat malic enzyme gene). Taken together, these results indicate a strong correlation between wTR beta 1/mTR beta 1 heterodimer formation and the dominant negative potency of PV. These results suggest that the wTR beta 1/mTR beta 1 heterodimer could play an important role in the dominant negative action of mTR beta 1.

Animals↗

PubMed1995

Increased invasive activity of human hepatocellular carcinoma cells is associated with an overexpression of thyroid hormone beta 1 nuclear receptor and low expression of the anti-metastatic nm23 gene.

To understand the role of thyroid hormone in metastasis, we studied the expression of the anti-metastatic nm23 gene in eight human hepatocellular carcinoma (HCC) cell lines. These cells differentially expressed the anti-metastatic nm23 gene. A low level of nm23 proteins was found to have a high in vitro invasive activity which correlated closely with an overexpression of the thyroid hormone beta 1 nuclear receptor (h-TR beta 1). Concurrent with the down-regulation of h-TR beta 1, the invasive activity of HCC cells was suppressed by the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3). These results indicate that the invasive activity of HCC cells was regulated by T3, suggesting that T3 could be involved in modulating the functions of nm23.

Carcinoma, Hepatocellular↗

PubMed1995

Structure of the carboxy-terminal region of thyroid hormone nuclear receptors and its possible role in hormone-dependent intermolecular interactions.

The thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors. To understand the molecular basis of ligand-dependent transactivation, we studied the structure of their carboxy-terminal activation domain. We analyzed the structures of the peptides derived from the C-terminal sequences of human TR subtypes beta 1 (h-TR beta 1) and alpha 1 (h-TR alpha 1) and a human TR mutant, PV, by circular dichroism (CD). Mutant PV has a C-terminal frameshift mutation and does not bind to the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3). Analyses of the secondary structures of the peptides by CD indicate that five amino acids, EVFED, are part of an amphipathic alpha-helix which is required to maintain the structural integrity of the hormone binding domain. A monoclonal antibody, C4 (mAb C4), which recognizes both h-TR beta 1 and h-TR alpha 1 was developed. Using a series of truncated mutants and synthetic peptides, we mapped the epitope of mAb C4 to the conserved C-terminal amino acids, EVFED. Analysis of the binding data indicates that binding of T3 to either h-TR beta 1 or h-TR alpha 1 was competitively inhibited by mAb C4. Deletion of C-terminal amino acids including EVFED led to a total loss of T3 binding activity. Thus, part of the T3 binding site is located in this five amino acid segment. T3 may transduce its hormonal signal to the transcriptional machinery via interaction with EVFED at the C-terminus of TRs.

Amino Acid Sequence↗

PubMed1995

Interaction of thyroid hormone nuclear receptor with antibody: characterization of the thyroid hormone binding site.

To understand the structural basis in the hormone-dependent transcriptional regulation of human beta 1 thyroid hormone receptor (h-TR beta 1), we characterized the region which interacted with the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3). Using the hormone binding domain of h-TR beta 1 (K206-D461) as an immunogen, we screened for monoclonal antibodies which inhibited the binding of T3 to h-TR beta 1. mAb C3, which recognized native h-TR beta 1, was obtained. Analyses of the binding data indicate that binding of T3 to h-TR beta 1 was competitively inhibited by mAb C3. Using a series of truncated mutants of h-TR beta 1 and synthetic peptides, we mapped the binding site of mAb C3 to the region of E248-V256. Thus, part of T3 binding site in h-TR beta 1 is in this nine-amino acid segment, which was shown by circular dichroism spectroscopy to be a random coil. Based on the proposed model of the hormone binding domain as an alpha/beta barrel, E248-V256 contains part of Loop 1 which is on the same side of the DNA binding domain. These results raise the possibility that Loop 1 could be in direct contact with the nearby DNA binding domain to affect the interaction of DNA binding domain with the T3 target genes.

Amino Acid Sequence↗

PubMed1995

Dose effects on the excretion of urinary metabolites of 2-[1,2,methoxy-13C]methoxyethanol in rats and mice.

The administration of 2-methoxyethanol (2-ME) to pregnant rats, mice, or primates results in developmental toxicity. To assess the role of metabolism in the adverse response of 2-ME, carbon-13 NMR spectroscopy was used to examine, directly in the urine, metabolites produced after administering high (250 mg/kg) and low (25 mg/kg) doses of 2-[1,2,methoxy-13C]ME to pregnant CD-1 mice and male Fischer 344 rats. The high dose elicits teratogenic effects in mice and testicular toxicity in rats. The urinary disposition was also examined after dosing pregnant CD-1 mice with a developmentally toxic level of 2-ME together with serine or acetate (known attenuators of 2-ME embryotoxicity). Seven novel metabolites were found in rat urine, consistent with those assigned in our previous studies with mice. Metabolite composition was compared for the different dosing regimens. A lower percentage of metabolites derived after conversion of 2-ME to 2-methoxyacetic acid (2-MAA) was found following concurrent administration of 2-ME with acetate, D-serine, or L-serine. These differences are mainly attributed to higher levels of ethylene glycol and/or glycolic acid that arise for the 2-ME administrations with any of the attenuators. Acetate together with 2-ME also reduced the percentage of metabolites incorporated into intermediary metabolism. These data indicate that attenuators of 2-ME teratogenic effects may alter metabolism and distribution by decreasing the conversion of 2-ME to 2-MAA, decreasing the conversion of 2-MAA to a coenzyme A thioester (2-methoxyacetyl approximately CoA), altering the utilization of the coenzyme A thioester, and/or increasing the conversion of 2-ME to ethylene glycol and its further metabolism. These changes in metabolism may contribute to the attenuating effects of these agents on 2-ME.

Animals↗

PubMed1995

Modulation of thyroid hormone action by mutant thyroid hormone receptors, c-erbA alpha 2 and peroxisome proliferator-activated receptor: evidence for different mechanisms of inhibition.

Thyroid hormone action is not only determined by hormone availability, but also by target organ sensitivity. A dominant negative interaction is known to occur between thyroid hormone receptors (TRs) and the non-ligand binding splicing variant c-erbA alpha 2 as well as mutant TR beta 1 from kindreds with resistance to thyroid hormone. We compared the inhibitory effect of naturally occurring mutant hTR beta 1, artificially created hTR alpha 1 mutants, c-erbA alpha 2 and the human peroxisome proliferator-activated receptor (hPPAR) on three prototypic T3-response elements (TREs), TRE-PAL, DR + 4 and TRE-LAP. The inhibitory effect of mutant hTR alpha 1 and beta 1 occurred only on TRE-LAP and to a minor degree on DR + 4 when equimolar ratios of mutant/wildtype receptor were present. In contrast, the c-erbA alpha 2 splicing variant and the hPPAR inhibited TR action on all three TREs. Gel mobility shift experiments in the presence of T3 showed increased binding of mutant hTR alpha 1 and beta 1 only to TRE-LAP compared to the binding of wildtype hTRs, thereby explaining their TRE-selective dominant negative potency. Contrarily, equal amounts of c-erbA alpha 2 or hPPAR protein did not bind to either of the three response elements even in the presence of RXR. Since the TR:RXR heterodimers were only partially displaced from DNA in the presence of excess amounts of c-erbA alpha 2, it is likely that the TRE-unspecific dominant negative action of c-erbA alpha 2 is due in part to competition for DNA-binding and for TR-auxiliary proteins. In contrast, equimolar amounts of hPPAR completely inhibited the DNA-binding of hTR beta 1:RXR heterodimers, but not of TR:TR homodimers, suggesting that hPPAR has a higher RXR-binding affinity and is therefore a potent competitor for intranuclear RXR. Since thyroid hormones and peroxisome proliferators regulate in part a similar subset of target genes involved in fatty acid metabolism, these results suggest the possibility of cross-talk among the thyroid hormone and peroxisome proliferator signalling pathways. In summary, the results suggest that thyroid hormone action can be modulated by at least three different mechanisms: (i) increased binding of mutant hTRs to specific TREs; (ii) efficient competition for limiting amounts of RXR through the preferential formation of hPPAR:RXR, rather than TR:RXR heterodimers; and (iii) competition for binding to DNA and to auxiliary proteins other than RXR in the case of c-erbA alpha 2.

DNA↗

PubMed1995

POEMS syndrome in Chinese.

POEMS (polyneuropathy, organomegaly, endocrine disturbance, M-protein and skin hyperpigmentation) syndrome occurring in Chinese has been inadequately described. Here we report our experience of 12 Chinese patients with the syndrome. This series is similar to those reported from Japan and from the Western world by the predominance of middle-aged males. Diabetic mellitus, a common feature in POEMS syndrome, was not present in our patients. M-protein of the lambda type was demonstrated in 7 patients by immunoelectrophoresis or by immunofixation electrophoresis. Osteosclerosis in Chinese patients is not as common as in patients reported from Japan or from the Western world; a common radiographic finding in our patients was a compression fracture of the thoraco-lumbar spine with or without sclerotic and lytic lesions. The condition has a grave prognosis; 9 patients die within 4 years after onset of the disease despite various kinds of treatment.

Adult↗

PubMed1995

Cell type-dependent modulation of the dominant negative action of human mutant thyroid hormone beta 1 receptors.

BACKGROUND: Mutations in the ligand-binding domain of the thyroid hormone receptor beta (TR beta) gene cause the syndrome of resistance to thyroid hormone (RTH). The clinical phenotype results from the antagonism of the normal TR alpha and the non-mutated TR beta alleles by the TR beta 1 mutants, via a dominant negative effect. There is, however, marked heterogeneity of organ resistance within and among kindreds with RTH. This study examines the potential role of cell type in modulating the dominant negative potency of human TR beta 1 (h-TR beta 1) mutants. MATERIALS AND METHODS: Transient transfections were performed in HeLa and NIH3T3 cells, using a wild type (WT) and three naturally occurring mutant h-TR beta 1 constructs, and three natural thyroid hormone response elements (TREs). Immunocytochemistry was performed to detect levels of TR beta 1 expression in these two cell types. In order to determine how TR beta 1 interacts with other cellular partners, gel-shift analyses using HeLa and NIH3T3 nuclear extracts were performed. RESULTS: Transfection studies using WT h-TR beta 1 in HeLa and NIH3T3 cells, showed that the 3,3',5-triiodothyronine (T3)-induced transactivation of the different TREs varied between cell types. Unlike the non-T3-binding h-TR beta 1 mutant, PV, mutants ED and OK displayed the expected T3-induced dose responsiveness in these two cell types. For each TRE examined, the magnitude of the dominant negative effect varied between the cell types. The levels of receptor expression in HeLa and NIH3T3 cells were identical, as determined by immunocytochemistry. Gel-shift analyses showed differences in the formation of hetero- and homodimers depending on both the cell type and TRE motif. CONCLUSIONS: The cell type in which a mutant receptor operates affects the relative amounts of hetero- and homodimers. Together with the nature of the mutation and the TRE-motif, this could modulate the dominant negative action of mutant receptors in different tissues, which, in turn, could contribute to the variable phenotypic characteristics of RTH.

3T3 Cells↗

PubMed1994

Tissue-dependent developmental expression of a cytosolic thyroid hormone protein gene in Xenopus: its role in the regulation of amphibian metamorphosis.

We have cloned the cDNA encoding the Xenopus laevis homolog of mammalian cytosolic thyroid hormone binding protein (CTHBP). We found that while its mRNA level varies little in whole animals during development, the expression of CTHBP is inversely correlated with tissue-specific transformations during metamorphosis. A high level of its mRNA was observed in the tail of premetamorphic tadpoles. However, the expression is dramatically repressed with the onset of rapid tail resorption. In the hindlimb, the expression of CTHBP is very low during morphogenesis. Subsequently, its expression continuously increases during the period of limb growth. In contrast, a low level of CTHBP expression was detected in the intestine throughout metamorphosis. These results suggest that CTHBP could function to modulate the metamorphic process by regulating the level of intracellular thyroid hormones.

Amino Acid Sequence↗

PubMed1994

Stimulation of proliferation by 3,3',5-triiodo-L-thyronine in poorly differentiated human hepatocarcinoma cells overexpressing beta 1 thyroid hormone receptor.

To understand the role of thyroid hormone nuclear receptors (TRs) in hepatocarcinogenesis, we characterized the TRs in nine human hepatocarcinoma cell lines. The expression of TR proteins is receptor subtype- and cell type-dependent. TR alpha 1 protein expresses similarly at a low level in each of the nine cell lines. In contrast, TR beta 1 is overexpressed in hepatocarcinoma cells which are poorly differentiated. Furthermore, thyroid hormone was found to stimulate the proliferation of cells in which TR beta 1 is overexpressed. These results suggest that TR beta 1 is most likely involved in the differentiation and proliferation of hepatocarcinoma cells. Our studies have shed new light in the understanding of the role of TRs in liver carcinogenesis.

Biomarkers↗

PubMed1994

Phosphorylation enhances the target gene sequence-dependent dimerization of thyroid hormone receptor with retinoid X receptor.

To understand the molecular basis of the phosphorylation-enhanced transcriptional activity of human thyroid hormone nuclear receptor subtype beta 1 (hTR beta 1), we studied the effect of phosphorylation on the interaction of hTR beta 1 with the retinoid X receptor beta (RXR beta), we studied the effect of phosphorylation on the interaction of hTR beta 1 with the retinoid X receptor beta (RXR beta). In vitro, the extent of hTR beta 1.RXR beta heterodimer bound to various thyroid hormone response elements (TREs) was compared before and after phosphorylation of hTR beta 1. Without phosphorylation, hTR beta 1.RXR beta heterodimer was barely detectable under the experimental conditions. After phosphorylation of hTR beta 1, heterodimer bound to (i) the chicken lysozyme gene TRE, (ii) a TRE consisting of direct repeats of half-site binding motifs separated by four gaps, and (iii) a palindromic TRE was enhanced by approximately 10-, 7-, and 6-fold, respectively. The effect of phosphorylation on hTR beta 1.RXR beta heterodimerization was reversible. Dephosphorylation of the phosphorylated hTR beta 1 by alkaline phosphatase led to loss of the ability of hTR beta 1 to form a heterodimer with RXR beta in either the absence or the presence of DNA. These results indicate that the heterodimerization is enhanced by phosphorylation. To evaluate the effect of phosphorylation on the interaction of hTR beta 1 with RXR beta in vivo, we cotransfected hTR beta 1, RXR beta and TRE-chloramphenicol acetyltransferase (CAT) expression plasmids into CV-1 cells. CAT activity was assessed in the presence or absence of okadaic acid. Okadaic acid is a potent inhibitor of phosphatases 1 and 2A and increases the in vivo phosphorylation of hTR beta 1 by approximately 10-fold. Using the CAT reporter gene under control of the TRE from the malic enzyme gene, we found that RXR beta increased the okadaic acid-enhanced hTR beta 1-mediated CAT activity by 2- to 3-fold in the presence of 3,3',5-triiodo-L-thyronine. However, 9-cis-retinoic acid did not enhance the effect of okadaic acid. Our results indicate that phosphorylation is essential for the interaction of hTR beta 1 with RXR beta. Thus, phosphorylation plays a pivotal role in the gene-regulating activity of hTR beta 1.

Alkaline Phosphatase↗

PubMed1994

Analysis of the binding of 3,3',5-triiodo-L-thyronine and its analogues to mutant human beta 1 thyroid hormone receptors: a model of the hormone binding site.

To understand the nature of the thyroid hormone binding site, we characterized the binding of 3,3',5-triiodo-L-thyronine (T3) and its analogues to eight naturally occurring mutated human beta 1 thyroid hormone receptors (h-TR beta 1). The mutant receptors were derived from patients with the syndrome of generalized thyroid hormone resistance, and each has a point mutation in the hormone binding domain (KT, R338W; TP, L450H; IR, D322H; NN, G347E; AH, P453H; OK, M442V; RL, F459C; and ED, A317T). Compared to the wild-type h-TR beta 1, binding of T3 was reduced by as much as 97% for the mutants. The order of binding affinity of wild-type h-TR beta 1 to the analogues is T3 > D-T3 > L-thyroxine > 3,5-diiodo-L-thyronine > 3,3',5'-triiodo-L-thyronine. The mutant receptors showed essentially the same order of reduced affinities for the analogues, but the amounts of the reductions varied in each case. These results suggest specific local interactions (interplay) of analogues with the mutated residues in the receptors. On the basis of these data and a putative structure of the hormone binding domains as an eight-stranded alpha/beta barrel, we propose the location of the hormone in the binding site of h-TR beta 1. Ionic bonds anchor the hormone's alanine side chain to loop 4 of the 8-fold alpha/beta barrel. The phenyl ring lies across the amino-terminal face of the domain with the phenoxy ring pointing downward into the barrel interacting with beta-strand 8 on the opposite side.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites↗

PubMed1994

Thyroid hormone influences beta myosin heavy chain (beta MHC) expression.

3,5,3'-Triiodo-L-thyronine (T3) regulation of beta-MHC expression was studied in rat fetal heart cells using deletion mutants of both the rat and human promoter regions fused to a CAT expression vector. T3 inhibited the expression of human and rat beta-MHC constructs with an IC50 of about 1nM, which was similar to the EC50 for beta MHC-mRNA observed in cardiomyocytes. Deletion analysis of beta MHC promoter constructs suggested that a T3 response element (TRE) is located near the start of transcription. Possibly, T3-receptor binding at this site interferes with formation of the transcriptional initiation complex.

Animals↗