Thyroid hormones and thyroid hormone receptors: Effects of thyromimetics on reverse cholesterol transport

Reverse cholesterol transport (RCT) is a complex process which transfers cholesterol from peripheral cells to the liver for subsequent elimination from the body via feces. Thyroid hormones (THs) affect growth, develop- ment, and metabolism in almost all tissues. THs exert their actions by binding to thyroid hormone receptors (TRs). There are two major subtypes of TRs, TRα and TRβ, and several isoforms (e.g. TRα1, TRα2, TRβ1, and TRβ2). Activation of TRα1 affects heart rate, whereas activation of TRβ1 has positive effects on lipid and lipoprotein metabolism. Consequently, particular interest has been focused on the development of thyromimetic compounds targeting TRβ1, not only because of their ability to lower plasma cholesterol but also due their ability to stimulate RCT, at least in pre-clinical models. In this review we focus on THs, TRs, and on the effects of TRβ1-modulating thyromimetics on RCT in various animal models and in humans.

Molecular Characterization of Thyroid Hormone Receptors (TRs) and their Responsiveness to T3 in Microhylafissipes

To explore and enrich the molecular mechanisms of thyroid hormone receptors （TRs） in the metamorphosis of amphibians, the cDNA sequences of TRa and TRβ in Microhyla fissipes were cloned and characterized. TRa was 1 706 bp in length with an open reading frame （ORF） of 1 257 bp encoding a predicted protein of 418 amino acids and TRβ was 1 422 bp with an ORF of 1 122 bp encoding a predicted protein of 373 amino acids. Their protein sequences contained 4 conserved domains of the nuclear receptor superfamily with two highly conserved cysteine-rich zinc fingers in the DNA-binding domain, whereas TRβ was 42 amino acids shorter in its A/B domain than TRot. Highly-conserved sequences and structures indicated their conserved functions during metamorphosis. TRa expression reached peak at 12 h and then decreased from 12 h to 48 h. While dramatically up-regulated TRβ was observed after exposure of T3 within 24 h, and it was down-regulated from 24 h to 48 h. The expression pattern of TRβ is similar to that in the natural metamorphosis. Furthermore, tadpoles treated 24 h also resembled the climax of metamorphosis tadpoles and TRβ expression had higher responsiveness than TRa to T3 in M. fissipes. These results suggest M. fissipes may serve as the model to assay environmental compounds on TH signaling disruption.

A new mutation in the thyroid hormone receptor gene of a Chinese family with resistance to thyroid hormone

Background Resistance to thyroid hormone （RTH） is a dominant inherited syndrome of reduced tissue responsiveness to thyroid hormone. It is usually due to mutations located at the ligand-binding domain and adjacent hinge region of the thyroid hormone receptor β（TRβ）. We report the clinical and laboratory characteristics and the genetic analysis of a patient with this rare disorder and his family members. Methods The clinical presentations and changes of thyroid function tests （TFTs） including magnetic resonance imaging （MRI） of pituitary and other laboratory tests were analysed. TFTs of his family＇s members were detected as well. Direct DNA sequencing of the TRβ gene was done for those with abnormal TFTs. Results The RTH child had goiter, irritability, aggressiveness, and sudoresis. His TFTs showed high levels of circulating free thyroid hormones （FT4 and FT3） and normal thyroid-stimulating hormone （TSH） concentrations. He felt worse when treated as hyperthyroidism （Grave disease） with thiamazole and his clinical presentations got improved obviously when treated as RTH with bromocriptine without obvious advert effect. We identified a novel missense mutation, A317D, located in exon 9 of the gene of this boy and his mother. His mother had not any clinical presentation, but having abnormal TFTs results. Conclusions This patient reported here was concordant with the criteria of RTH. The feature is dysfunction of hypothalamus-pituitary-thyroid axis. A novel mutation was found in the TRβ, A317D, of this family. This research verified the phenomena that there is a clinical heterogeneity within the same mutation of different RTH patients.

Lost expression of thyroid hormone receptor-β1 mRNA in esophageal cancer

Thyroid hormone receptors （TR）, ligand-mediated transcription factors, regulate cell growth, differentiation, and apoptosis. In humans, two different genes encode TR-α and TR-β and they are often co-expressed in various tissues at different levels. To explore the role of TR in esophageal cancer, we analyzed expression of TR-β1 mRNA （most abundantly expressed in the majority of normal cells） in normal and malignant esophageal tissue specimens using in situ hybridization. The TR-β1 mRNA was detected in 92.3% （96 of 104） of normal esophageal mucosa, whereas TR-β1 mRNA was only detected in 55.8% （58 of 104 cases） of esophageal squamous cell carcinoma specimens （P〈 0.00001）. Expression of TR-β1 mRNA was associated with well-differentiated cancers （P〈 0.001）. Furthermore, we determined whether the loss of heterozygosity （LOH） in TR-β1 gene locus would be responsible for the lost TR-β1 expression. Analysis of 73 esophageal tissue specimens generated 39 informative cases, 17 of which showed LOH （43.6%） but only 9 of these 17 cases were correlated with lost TR-β1 expression. This study demonstrated that expression of TR-β1 mRNA was lost in esophageal cancer tissues, which may be due to multiple mechanisms.

Molecular characterization and developmental expression patterns of thyroid hormone receptors(TRs) and their responsiveness to TR agonist and antagonist in Rana nigromaculata

Considering some advantages of Rana nigromaculata as an experimental species, we propose that this species, like Xenopus laevis, could be used to assay thyroid hormone（TH） signaling disrupting actions. To validate the utilizability of R. nigromaculata, we investigated the responsiveness of R. nigromaculata to a TH receptor（TR） agonist（T3） and antagonist（amiodarone） by analyzing expression, based on characterizing TR cDNA and developmental expression patterns. With high levels of identity with the corresponding genes in X. laevis, both TRα and TRβ in R. nigromaculata exhibited roughly similar developmental expression patterns to those of X. laevis, in spite of some species-specific differences. Both TRα and TRβ expression had greater changes in the liver and intestine than in the tail and brain during metamorphosis. T3 exposure for 2 days induced more dramatic increases of TRβ expression in stage 27 than in stage34 tadpoles but not in stage 42 tadpoles, showing that the responsiveness of R. nigromaculata to TH decreased with development and disappeared at the onset of metamorphic climax.Corresponding to greater changes of TRβ expression in the liver and intestine than in the tail and brain during metamorphosis, the liver and intestine had higher responsiveness to exogenous T3 than the tail and brain. Amiodarone inhibited T3-induced TRβ expression. Our results show that R. nigromaculata can be used as a model species for assaying TH signaling disrupting actions by analyzing TRβ expression, and intestine tissues at stage 27 are ideal test materials due to high responsiveness and easy accessibility.

Dechloranes exhibit binding potency and activity to thyroid hormone receptors

Dechloranes are a group of halogenated flame retardants with a basic bicyclo[2.2.1]heptene,including Dechlorane Plus(DP),Dechlorane 602(Dec 602),Dechlorane 603(Dec 603)and Dechlorane 604(Dec 604).A few epidemiological investigations and animal experiments have shown that DP exhibited thyroid-interfering effects.In the present study,we investigated whether DP and three other dechloranes could interfere the thyroid function through thyroid hormone receptors(TRs,TRαand TRβ)signaling pathways.The binding affinities of the four dechloranes to the two TRs were determined by fluorescence competitive binding assay.It was found that all the four dechloranes could bind with the two TRs.The relative potency(RP)values ranged from nd(not detectable)to 0.0667.Between the two TRs,dechloranes were more inclined to bind with TRβ,which implies that the thyroid interference effect of dechloranes may have selectivity in different tissues and organs.TRs-mediated luciferase reporter gene assay and T-screen assay showed that all the four dechloranes exhibited antagonistic activity to TRs in the cells.Taken together,our results demonstrated that dechloranes might interfere with thyroid function by binding with TRs and acting as TR antagonists.The health risk of highly exposed human populations should be of serious concern because of the high hazard quotient calculated from our cell assay results.

Stimulating effect of thyroid hormones in peripheral nerve regeneration:research history and future direction toward clinical therapy

Injury to peripheral nerves is often observed in the clinic and severe injuries may cause loss of motor and sensory functions.Despite extensive investigation,testing various surgical repair techniques and neurotrophic molecules,at present,a satisfactory method to ensuring successful recovery does not exist.For successful molecular therapy in nerve regeneration,it is essential to improve the intrinsic ability of neurons to survive and to increase the speed of axonal outgrowth.Also to induce Schwann cell phenotypical changes to prepare the local environment favorable for axonal regeneration and myelination.Therefore,any molecule that regulates gene expression of both neurons and Schwann cells could play a crucial role in peripheral nerve regeneration.Clinical and experimental studies have reported that thyroid hormones are essential for the normal development and function of the nervous system,so they could be candidates for nervous system regeneration.This review provides an overview of studies devoted to testing the effect of thyroid hormones on peripheral nerve regeneration.Also it emphasizes the importance of combining biodegradable tubes with local administration of triiodothyronine for future clinical therapy of human severe injured nerves.We highlight that the local and single administration of triiodothyronine within biodegradable nerve guide improves significantly the regeneration of severed peripheral nerves,and accelerates functional recovering.This technique provides a serious step towards future clinical application of triiodothyronine in human severe injured nerves.The possible regulatory mechanism by which triiodothyronine stimulates peripheral nerve regeneration is a rapid action on both axotomized neurons and Schwann cells.

Thyroid disruption by technical decabromodiphenyl ether (DE-83R) at low concentrations in Xenopus laevis

Decabromodiphenyl ether （decaBDE）,as a flame retardant,is widely produced and used.To study the thyroid disruption by technical decaBDE at low concentrations,Xenopus laevis tadpoles were exposed to technical decaBDE mixture DE-83R （1-1000 ng/L） in water from stage 46/47 （free swimming larvae,system of Nieuwkoop and Faber） to stage 62.DE-83R at concentration of 1000 ng/L significantly delayed the time to metamorphosis （presented by forelimb emergence,FLE）.Histological examination showed that DE83R at all tested concentrations caused histological alterations-multilayer follicular epithelial cell and markedly increased follicle size accompanied by partial colloid depletion and increase in the peripheral colloid vacuolation,in thyroid glands.All tested concentrations of DE-83R also induced a down-regulation of thyroid receptor mRNA expression.These results demonstrated that technical decaBDE disrupted the thyroid system in X.laevis tadpoles.Analysis of polybrominated diphenyl ethers （PBDEs） （sum of 39 congeners） in X.laevis indicated that mean concentrations of total PBDEs in X.laevis exposed to 1,10,100,1000 ng/L were 11.0,128.1,412.1,1400.2 ng/g wet weight,respectively.Considering that PBDEs burden of X.laevis tadpoles was close to PBDEs levels in amphibians as reported in previous studies,our study has raised new concerns for thyroid disruption in amphibians of technical decaBDE at environmentally relevant concentrations.

Daily exposure to low concentrations Tetrabromobisphenol A interferes with the thyroid hormone pathway in HepG2 cells

Tetrabromobisphenol A(TBBPA)is a flame retardant that adversely affects the environment and human health.The present study exposed HepG2 cells to low concentrations of TBBPA daily to investigate the changes in gene regulation,mainly related to pathways associated with the endocrine system.The quantitative polymerase chain reaction(qPCR)confirmed that prolonged exposure gradually activated the thyroid hormone and parathyroid hormone signaling pathways.The expression levels of genes related to the thyroid hormone signaling pathway were upregulated(1.15-8.54 times)after five generations of exposure to 1 and 81 nM TBBPA.Furthermore,co-exposure to 81 nM TBBPA and 0.5 nM thyroid hormone receptor antagonist for five generations significantly reduced the expression of thyroid hormone and parathyroid hormone receptors.Meanwhile,81 nM TBBPA inhibited the activation of the Ras pathway and downregulated Ras gene expression level(3.7 times),indicating the association between the toxic effect and thyroid hormone receptors.Additionally,our experiments revealed that the thyroid hormone pathway regulated the induction of the Ras signaling pathway by TBBPA.The study thus proves that daily exposure to TBBPA interferes with the thyroid hormone signaling pathway and subsequently the endocrine system.

Thyroid hormone action in metabolic regulation

Thyroid hormone plays pivotal roles in growth,differentiation,development and metabolic homeostasis via thyroid hormone receptors(TRs)by controlling the expression of TR target genes.The transcriptional activity of TRs is modulated by multiple factors including various TR isoforms,diverse thyroid hormone response elements,different heterodimeric partners,coregulators,and the cellular location of TRs.In the present review,we summarize recent advance in understanding the molecular mechanisms of thyroid hormone action obtained from human subject research,thyroid hormone mimetics application,TR isoform-specific knock-in mouse models,and mitochondrion study with highlights in metabolic regulations.Finally,as future perspectives,we share our thoughts about current challenges and possible approaches to promote our knowledge of thyroid hormone action in metabolism.

The effects and mechanisms of thyroid hormones in the cardiovascular system

Background Thyroid hormones(THs) including thyroxine(T4) and triiodothyronine(T3) with high biological activities have important effects on cardiovascular system by acting on renin-angiotensin-aldosterone system(RAAS), oxidative stress, mitochondria, endothelial cells, vascular smooth muscle cells(VSMC), cardiomyocytes, thyroid hormone receptor(TRs), cholesterol metabolism, insulin sensitivity, blood coagulation, etc. Excess or lack of THs is detrimental to cardiovascular function, so this article reviews the mechanism of THs on cardiovascular system.[S Chin J Cardiol 2019;20(4):269-279]

Association between TSHR gene polymorphism and the risk of Graves' disease:a meta-analysis

Thyroid stimulating hormone receptor（TSHR） is thought to be a significant candidate for genetic susceptibility to Graves＇ disease（GD）.However,the association between TSHR gene polymorphism and the risk of GD remains controversial.In this study,we investigated the relationship between the two conditions by meta-analysis.We searched all relevant case-control studies in PubMed,Web of Science,CNKI and Wanfang for literature available until May2015,and chose studies on two single nucleotide polymorphisms（SNPs）：rs 179247 and rsl2101255,within TSHR intron-1.Bias of heterogeneity test among studies was determined by the fixed or random effect pooled measure,and publication bias was examined by modified Begg＇s and Egger＇s test.Eight eligible studies with 15 outcomes were involved in this meta-analysis,including 6,976 GD cases and 7,089 controls from China,Japan,Poland,UK and Brazil.Pooled odds ratios（ORs） for allelic comparisons showed that both TSHR rsl79247A/G and rsl2101255T/C polymorphism had significant association with GD（OR=1.422,95%CI=1.353—1.495,P〈0.001,P_（heterogeneity）=0.448;OR= 1.502,95%CI：1.410-1.600,P〈0.001,P_（heterogeneity）=0.642）,and the associations were the same under dominant,recessive and co-dominant models.In subgroup analyses,the conclusions are also consistent with all those in Asian,European and South America subgroups（P〈0.001）.Our meta-analysis revealed a significant association between TSHR rsl79247A/G and rsl2101255T/C polymorphism with GD in five different populations from Asia,Europe and South America.Further studies are needed in other ethnic backgrounds to independently confirm our findings.

Targeting transcriptional regulators for treatment of anaplastic thyroid cancer

Dysregulation of genes perpetuates cancer progression.During carcinogenesis,cancer cells acquire dependency of aberrant transcriptional programs(known as“transcription addiction”)to meet the high demands for uncontrolled proliferation.The needs for particular transcription programs for cancer growth could be cancer-type-selective.The dependencies of certain transcription regulators could be exploited for therapeutic benefits.Anaplastic thyroid cancer(ATC)is an extremely aggressive human cancer for which new treatment modalities are urgently needed.Its resistance to conventional treatments and the lack of therapeutic options for improving survival might have been attributed to extensive genetic heterogeneity due to subsequent evolving genetic alterations and clonal selections during carcinogenesis.Despite this genetic complexity,mounting evidence has revealed a characteristic transcriptional addiction of ATC cells resulting in evolving diverse oncogenic signaling for cancer cell survival.The transcriptional addiction has presented a huge challenge for effective targeting as shown by the failure of previous targeted therapies.However,an emerging notion is that many different oncogenic signaling pathways activated by multiple upstream driver mutations might ultimately converge on the transcriptional responses,which would provide an opportunity to target transcriptional regulators for treatment of ATC.Here,we review the current understanding of how genetic alterations in cancer distorted the transcription program,leading to acquisition of transcriptional addiction.We also highlight recent findings from studies aiming to exploit the opportunity for targeting transcription regulators as potential therapeutics for ATC.

Hsa-miR-637 inhibits human hepatocyte proliferation by targeting Med1-interacting proteins

Background:Recent studies have shown that mediator complex subunit 1(Med1)can significantly affect hepatocyte proliferation and differentiation.Acting as a tumor suppressor,microRNA-637(hsa-miR-637)can inhibit the growth of hepatocarcinoma cells and further induce cell apoptosis.However,the function of hsa-miR-637 and its target genes during liver regeneration remains to be elucidated.Methods:This study used co-immunoprecipitation(Co-IP)assay,transfection,luciferase reporter assay,functional assay by cell counting kit-8(CCK-8),Annexin V-FITC/propidium iodide apoptosis assay,and quantitative polymerase chain reaction analysis of chromatin immunoprecipitation(ChIP)for analysis.Results:Hsa-miR-637 has been suggested to suppress the expression of two Med1-interacting nuclear receptors,identified as the peroxisome proliferator-activated receptor alpha(PPARA)and thyroid hormone receptor alpha(THRA)at the transcriptional and translational levels in the human liver HL-7702 cell line.The interaction between Med1 and PPARA/THRA in HL-7702 cells was then confirmed.The transcriptional repression of hsa-miR-637 on PPARA and THRA was also demonstrated.Moreover,hsamiR-637 has been determined to suppress the proliferation of HL-7702 cells.Furthermore,cell cycle arrest of HL-7702 cells was induced by transfection of hsa-miR-637 at the S phase,but its apoptosis failed.Finally,PPARA was indicated to directly bind to the promoter of some transcription factors,like bcatenin,mouse double minute 2(MDM2),and p53.Conclusions:This study has confirmed that hsa-miR-637 plays an antiproliferative role during liver regeneration,which may contribute in understanding the regenerative process of the liver.

A Study on Neonatal Tolerance Against Graves＇ Disease in BALB/c Mice

INTRODUCTION Autoimmunity is defined as, a condition characterized by a specific humoral or cell-mediated immune response against the constituents of the body＇s own tissues （autoantigens）. In numerous autoimmune diseases, such an immune response is well recognized that causes damage to the self-constituents of body tissues by the products of the immune system. Graves＇ hyperthyroidism occurs after the loss of tolerance to the thyroid stimulating hormone receptor （TSHR） and the generation of thyroid stimulatory antibodies that mimic the action of thyroid-stimulating hormone （TSH）.