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Groeneweg S, Peeters RP, Visser TJ, Visser WE. Triiodothyroacetic acid in health and disease. J Endocrinol 2017; 234:R99-R121. [PMID: 28576869 DOI: 10.1530/joe-17-0113] [Citation(s) in RCA: 45] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/26/2017] [Accepted: 05/30/2017] [Indexed: 12/16/2022]
Abstract
Thyroid hormone (TH) is crucial for development and metabolism of many tissues. The physiological relevance and therapeutic potential of TH analogs have gained attention in the field for many years. In particular, the relevance and use of 3,3',5-triiodothyroacetic acid (Triac, TA3) has been explored over the last decades. Although TA3 closely resembles the bioactive hormone T3, differences in transmembrane transport and receptor isoform-specific transcriptional activation potency exist. For these reasons, the application of TA3 as a treatment for resistance to TH (RTH) syndromes, especially MCT8 deficiency, is topic of ongoing research. This review is a summary of all currently available literature about the formation, metabolism, action and therapeutic applications of TA3.
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Affiliation(s)
- Stefan Groeneweg
- Department of Internal Medicine and Academic Center for Thyroid DiseasesErasmus University Medical Center, Rotterdam, The Netherlands
| | - Robin P Peeters
- Department of Internal Medicine and Academic Center for Thyroid DiseasesErasmus University Medical Center, Rotterdam, The Netherlands
| | - Theo J Visser
- Department of Internal Medicine and Academic Center for Thyroid DiseasesErasmus University Medical Center, Rotterdam, The Netherlands
| | - W Edward Visser
- Department of Internal Medicine and Academic Center for Thyroid DiseasesErasmus University Medical Center, Rotterdam, The Netherlands
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Visser WE, Friesema ECH, Visser TJ. Minireview: thyroid hormone transporters: the knowns and the unknowns. Mol Endocrinol 2010; 25:1-14. [PMID: 20660303 DOI: 10.1210/me.2010-0095] [Citation(s) in RCA: 266] [Impact Index Per Article: 19.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
The effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field.
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Affiliation(s)
- W Edward Visser
- Erasmus University Medical Center, Molewaterplein 50, Rotterdam, The Netherlands
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Wagner MS, Wajner SM, Maia AL. Is there a role for thyroid hormone on spermatogenesis? Microsc Res Tech 2009; 72:796-808. [DOI: 10.1002/jemt.20759] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
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Wajner SM, Wagner MS, Maia AL. Clinical implications of altered thyroid status in male testicular function. ACTA ACUST UNITED AC 2009; 53:976-82. [PMID: 20126850 DOI: 10.1590/s0004-27302009000800011] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2009] [Accepted: 10/16/2009] [Indexed: 11/22/2022]
Abstract
Thyroid hormones are involved in the development and maintenance of virtually all tissues. Although for many years the testis was thought to be a thyroid-hormone unresponsive organ, studies of the last decades have demonstrated that thyroid dysfunction is associated not only with abnormalities in morphology and function of testes, but also with decreased fertility and alterations of sexual activity in men. Nowadays, the participation of triiodothyronine (T3) in the control of Sertoli and Leydig cell proliferation, testicular maturation, and steroidogenesis is widely accepted, as well as the presence of thyroid hormone transporters and receptors in testicular cells throughout the development process and in adulthood. But even with data suggesting that T3 may act directly on these cells to bring about its effects, there is still controversy regarding the impact of thyroid diseases on human spermatogenesis and fertility, which can be in part due to the lack of well-controlled clinical studies. The current review aims at presenting an updated picture of recent clinical data about the role of thyroid hormones in male gonadal function.
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Rodrigues MCS, Santos GM, da Silva CA, Baxter JD, Webb P, Lomri N, Neves FAR, Ribeiro RCJ, Simeoni LA. Thyroid Hormone Transport is Disturbed in Erythrocytes from Patients with Chronic Renal Failure on Hemodialysis. Ren Fail 2009; 26:461-6. [PMID: 15462116 DOI: 10.1081/jdi-200026760] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022] Open
Abstract
AIMS To now, there are no studies reporting whether thyroid hormones (THs) transport play a role in thyroid hormone dysfunction observed in chronic renal failure (CRF). Therefore, the aim of this study was to investigate the transport of THs in erythrocytes from patients with CRF on hemodialysis (HD). METHODS [125I]-L-triiodothyronine ([125I]T3) and [125I]-L-thyroxine ([125I]T4) erythrocytes uptake was measured at 1 min and 5 min. To study L-triiodothyronine (LT3) and L-thyroxine (LT4) efflux from erythrocytes, we preloaded the cells during 180 min with [125I]T3 or [125I]T4 and measured their [125I]T3 or [125I]T4 efflux during 60 min. RESULTS [125I]T3 uptake in erythrocytes from uremic patients pre-HD was higher than control subjects by 50% at 1 min and by 55% at 5 min. However, [125I]T4 uptake in erythrocytes from uremic patients was significantly lower at 1min (88%) and at 5 min (63%). LT3 efflux rate was lower and LT4 efflux was significantly higher than in control subjects. After 60-min of efflux, LT3 remained in erythrocytes was 80% higher and LT4 was 57% lower than in normal individuals. Neither [125I]T3 and [125I]T4 uptake, nor efflux rates were changed by hemodialysis. CONCLUSION Despite the fact that uremic patients on hemodialysis show low serum levels of LT3, changes in LT3 influx and efflux could act as a compensatory mechanism that neutralize thyroid hormone dysfunction in order to maintain the euthyroid state.
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Affiliation(s)
- Maria C S Rodrigues
- Department of Pharmaceutical Sciences, University of Brasilia, Brasilia, Brazil
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Visser WE, Friesema ECH, Visser TJ. Transport of thyroxine and 3,3',5-triiodothyronine in human umbilical vein endothelial cells. Endocrinology 2009; 150:1552-7. [PMID: 18948392 DOI: 10.1210/en.2008-0627] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
The prerequisite for the uptake of thyroid hormone (TH) in peripheral tissues is the exit of TH from the bloodstream. The first step in this process is transport across the endothelium. Little is known about this important step in TH physiology. Therefore, we aimed to characterize the TH transport processes across the endothelium using human umbilical vein endothelial cells as a model. Transport studies showed rapid uptake of 1 nm [(125)I]T(3) and [(125)I]T(4) in these cells. The apparent Michaelis constant value for [(125)I]T(3) uptake was about 1 microm, and the IC(50) for T(4) inhibition of T(3) uptake was about 3 microm. The aromatic amino acids phenylalanine, tyrosine, and tryptophan and the L-type amino acid transporter-specific ligand 2-aminobicyclo-(2, 2, 1)-heptane-2-carboxylic acid did not inhibit [(125)I]T(3) or [(125)I]T(4) uptake. Verapamil was capable of reversibly reducing transport of [(125)I]T(3) and [(125)I]T(4). Human umbilical vein endothelial cells incubated with the affinity label BrAcT(3) resulted in a labeling of multiple proteins, which are probably protein disulfide isomerase related. Extrapolating our findings to the endothelial lining of blood vessels suggests that T(3) and T(4) uptake is mediated by the same transport system. Because TH transport characteristics do not correspond to known TH transporters, further studies are required to identify the TH transporter protein(s) at the molecular level. Possible candidates may be widely expressed Na(+)-independent transporter proteins.
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Affiliation(s)
- W Edward Visser
- Department of Internal Medicine, Erasmus University Medical Center, Dr Molewaterplein 50, 3015 GE Rotterdam, The Netherlands.
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Abstract
Thyroid hormone is a critical regulator of growth, development, and metabolism in virtually all tissues, and altered thyroid status affects many organs and systems. Although for many years testis has been regarded as a thyroid hormone unresponsive organ, it is now evident that thyroid hormone plays an important role in testicular development and function. A considerable amount of data show that thyroid hormone influences steroidogenesis as well as spermatogenesis. The involvement of tri-iodothyronine (T(3)) in the control of Sertoli cell proliferation and functional maturation is widely accepted, as well as its role in postnatal Leydig cell differentiation and steroidogenesis. The presence of thyroid hormone receptors in testicular cells throughout development and in adulthood implies that T(3) may act directly on these cells to bring about its effects. Several recent studies have employed different methodologies and techniques in an attempt to understand the mechanisms underlying thyroid hormone effects on testicular cells. The current review aims at presenting an updated picture of the recent advances made regarding the role of thyroid hormones in male gonadal function.
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Affiliation(s)
- Márcia Santos Wagner
- Endocrine Division, Thyroid Section, Hospital de Clínicas de Porto Alegre, Universidade Federal do Rio Grande do Sul, 90035-033, Porto Alegre, RS, Brasil
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Zoeller RT, Tyl RW, Tan SW. Current and Potential Rodent Screens and Tests for Thyroid Toxicants. Crit Rev Toxicol 2008; 37:55-95. [PMID: 17364705 DOI: 10.1080/10408440601123461] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
This article reviews current rodent screens and tests to detect thyroid toxicants. Many points of disruption for thyroid toxicants are outlined and include: (a) changes in serum hormone level; (b) thyroperoxidase inhibitors; (c) the perchlorate discharge test; (d) inhibitors of iodide uptake; (e) effects on iodothyronine deiodinases; (f) effects on thyroid hormone action; and (g) role of binding proteins (e.g., rodent transthyretin). The major thyroid endpoints currently utilized in existing in vivo assay protocols of the Organization for Economic Cooperation and Development (OECD), Japanese researchers, and U.S. Environmental Protection Agency (EPA) include thyroid gland weight, histopathology, circulating thyroid hormone measurements, and circulating thyroid-stimulating hormone (TSH). These endpoints can be added into the existing in vivo assays for reproduction, development, and neurodevelopment that are outlined in this chapter. Strategic endpoints for possible addition to existing protocols to detect effects on developmental and adult thyroid endpoints are discussed. Many of these endpoints for detecting thyroid system disruption require development and additional research before they can be considered in existing assays. Examples of these endpoints under development include computer-assisted morphometry of the brain and evaluation of treatment-related changes in gene expression, thyrotropin-releasing hormone (TRH) and TSH challenge tests, and tests to evaluate thyroid hormone (TH)-dependent developmental events, especially in the rodent brain (e.g., measures of cerebellar and cortical proliferation, differentiation, migration, apoptosis, planimetric measures and gene expression, and oligodendrocyte differentiation). Finally, TH-responsive genes and proteins as well as enzyme activities are being explored. Existing in vitro tests are also reviewed, for example, thyroid hormone (TH) metabolism, receptor binding, and receptor activation assays, and their restrictions are described. The in vivo assays are currently the most appropriate for understanding the potential effects of a thyroid toxicant on the thyroid system. The benefits and potential limitations of the current in vivo assays are listed, and a discussion of the rodent thyroid system in the context of human health is touched upon. Finally, the importance of understanding the relationship between timing of exposure, duration of dose, and time of acquisition of the endpoints in interpreting the results of the in vivo assays is emphasized.
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Affiliation(s)
- R Thomas Zoeller
- Biology Department, Morrill Science Center, University of Massachusetts-Amherst, Amherst, Massachusetts 01003, USA.
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Abstract
PURPOSE OF REVIEW Chronic kidney disease is characterized by multiple abnormalities in the thyroid hormone physiology. In the present review, we will briefly discuss the effects of uremia on thyroid hormone synthesis, metabolism, transport, and action. RECENT FINDINGS Uremic toxins have been shown to interfere at various levels of the thyroid hormone action, including thyroid hormone transport across plasma membrane and thyroid hormone receptor activity. These abnormalities could explain the resistance to thyroid hormone action in uremia, at least in some tissues. SUMMARY The pathogenesis of thyroid axis abnormalities in uremia is incompletely understood, and its clinical significance remains unclear. The increasing prevalence of chronic kidney disease underscores the need for further efforts to understand the metabolic consequences of uremia and address questions such as the impact of thyroid hormone therapy.
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Affiliation(s)
- Angélica Amorim Amato
- Molecular Pharmacology Laboratory, Faculty of Health Sciences, University of Brasília, Brasília, Brazil
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Kurose K, Saeki M, Tohkin M, Hasegawa R. Thyroid hormone receptor mediates human MDR1 gene expression-Identification of the response region essential for gene expression. Arch Biochem Biophys 2008; 474:82-90. [PMID: 18395509 DOI: 10.1016/j.abb.2008.03.020] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2007] [Revised: 03/18/2008] [Accepted: 03/20/2008] [Indexed: 11/15/2022]
Abstract
P-glycoprotein, encoded by the MDR1 gene, is a drug efflux transporter that is expressed in various tissues and plays an important role in the absorption and elimination of many drugs and xenobiotics. Induction of the MDR1 gene affects drug disposition and the efficacy of drug treatment. In this study, we demonstrated that the thyroid hormone receptor (TR) induces MDR1 gene expression in a thyroid hormone (TH)-dependent manner. The 5'-upstream region of the human MDR1 gene was examined for the presence of TH-responsive elements. Luciferase-reporter gene assays revealed that the TH response region is located between -7.9 and -7.8kb upstream from the transcription start site of MDR1. The region contains two TH response clusters, one of which includes a direct repeat with a three-nucleotide spacer (DR3) and a four-nucleotide spacer DR4(I), and the other of which includes two DR4s (II and III). Mutation analyses indicated that every direct repeat has a unique contribution to the TH response. In particular, DR4(I) was shown to be the most important element. Chromatin immunoprecipitation assays revealed that TR and retinoid X receptor (RXR) bind to the TH response region, and gel mobility shift assays confirmed that one molecule of TR/RXR heterodimer binds to each of the clusters in this region, with preferential binding to the upstream one. We furthermore demonstrated that two molecules of TR/RXR could bind simultaneously to the TH response region. The order of binding affinity to the direct repeats was DR4(I)>DR4(II)>DR4(III) approximately DR3. Our results indicate that these two closely spaced TR/RXR-binding clusters are both required for the maximal induction of MDR1 gene expression mediated by TR.
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Affiliation(s)
- Kouichi Kurose
- Division of Medicinal Safety Science, National Institute of Health Sciences, 1-18-1 Kamiyoga, Setagaya-ku, Tokyo 158-8501, Japan.
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Turgut G, Baştemir M, Turgut S, Akin F, Kursunluoglu R, Kaptanoğlu B. P-glycoprotein polymorphism in hypo- and hyper-thyroidism patients. Mol Biol Rep 2007; 35:693-8. [PMID: 17891478 DOI: 10.1007/s11033-007-9142-y] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2007] [Accepted: 08/29/2007] [Indexed: 10/22/2022]
Abstract
P-glycoprotein (Pgp) is encoded by the multidrug resistance gene (MDR1) in humans and is the product of MDR1. It is expressed in various tissues and is related to drug distribution in intestinal erythrocytes, capillary endotel of brain, proximal tubules cells of kidneys and liver canalicular cells. Expression of Pgp is affected by Pgp polymorphism, and exon 26 C3435T polymorphism is the most common one. It has been thought that expression of Pgp is high in C-allele subjects and this situation is responsible for the resistance against some drugs and substances. Pgp may have a role in the distribution of thyroid hormones, drugs used for hypo- and hyperthyroidism and the resistance occurred. For this purpose possible relationship between T and C alleles and frequency of Pgp polymorphism as well as thyroid hormone distribution in patients with hypo- and hyperthyroidism was investigated. Thirty five hyperthyroidism patients diagnosed as Graves' disease, 78 hypothyroidism patients diagnosed as Hashimoto's thyroiditis and 100 healthy volunteers were included in the study. According to the results obtained no statistically significant difference was found in Pgp C3435T polymorphism between hypo- and hyperthyroidism patients. In addition, the serum free T3 levels of hyperthyroidism patients with C alleles was higher than those of subjects with T alleles. No statistically significant difference was seen in the CC, CT and TT genotype frequencies between the patients and control groups. In conclusion, it seems that Pgp polymorphism is not a predictor factor for the occurrence of hypo- and hyperthyroidism. There is a significant relationship between Pgp and the elevated serum free T3 levels of hyperthyroidism patients, and further research will help understand this situation.
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Affiliation(s)
- Günfer Turgut
- Faculty of Medicine, Department of Physiology, University of Pamukkale, Denizli, 20070, Turkey.
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Kassem NA, Deane R, Segal MB, Chen R, Preston JE. Thyroxine (T4) transfer from CSF to choroid plexus and ventricular brain regions in rabbit: contributory role of P-glycoprotein and organic anion transporting polypeptides. Brain Res 2007; 1181:44-50. [PMID: 17915195 DOI: 10.1016/j.brainres.2007.08.052] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2007] [Revised: 07/18/2007] [Accepted: 08/26/2007] [Indexed: 10/22/2022]
Abstract
This study investigated the transfer of T4 from cerebrospinal fluid (CSF) into the choroid plexuses (CP) and ventricular brain regions, and the role of P-glycoprotein (P-gp), multidrug resistance protein 1 (mrp1) and organic anion transporting polypeptides (oatps). During in vivo ventriculo-cisternal (V-C) perfusion in the anesthetized rabbit (meditomidine hydrochloride 0.5 mg kg(-1), pentobarbitone 10 mg kg(-1) i.v.), 125I-T4 was perfused continuously into ventricular CSF with reference molecules 14C-mannitol and blue dextran. Over 2 h, 36.9+/-4.6% 125I-T4 was recovered in cisternal CSF. Addition of P-gp substrate verapamil increased CSF 125I-T4 recovery to 51.4+/-2.8%, although mrp1 and oatp substrates had no significant effect. In brain, 125I-T4 showed greatest accumulation in the CP (1.52+/-0.31 ml g(-1)), followed by ventricular regions (caudate putamen, ependyma, hippocampus, 0.05-0.14 ml g(-1)). At the CP, verapamil and probenecid (but not indomethacin) significantly increased 125I-T4 accumulation, implicating a role for P-gp and oatps. Of other brain regions, all three drugs increased accumulation in caudate putamen 3-5 times, and indomethacin and probenecid increased accumulation in ependyma 4-5 times. The role of P-gp was investigated further in isolated incubated CPs using 5 microg/ml C219 anti-P-gp antibody. Both 125I-T4 and 3H-cyclosporin accumulation increased by 80%, suggesting that P-gp is functional in the CP and has a role in removal of T4. Combined with the in vivo results, these studies suggest that P-gp provides a local homeostatic mechanism, maintaining CSF T4 levels. We conclude that P-gp and oatps contribute to the transfer of 125I-T4 between the CSF, CP and brain, hence regulating 125I-T4 availability in CSF.
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Affiliation(s)
- Nouhad A Kassem
- King's College London, Pharmaceutical Sciences Research Division, Hodgkin Building, Guy's Campus London SE1 1UL, UK.
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Ohara N, Tsujino T, Maruo T. The Role of Thyroid Hormone in Trophoblast Function, Early Pregnancy Maintenance, and Fetal Neurodevelopment. JOURNAL OF OBSTETRICS AND GYNAECOLOGY CANADA 2004; 26:982-90. [PMID: 15560861 DOI: 10.1016/s1701-2163(16)30420-0] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
OBJECTIVE To review the literature on the roles of thyroid hormone in trophoblast function, early pregnancy maintenance, and fetal neurodevelopment. METHODS MEDLINE was searched for English-language papers published from 1971 to 2003, using the key words "brain," "hypothyroidism," "placenta," "pregnancy," "threatened abortion," "thyroid hormone," "thyroid hormone receptor," "thyroid hormone replacement therapy," "thyroid hormone-responsive gene," and "trophoblast." RESULTS Transplacental transfer of thyroid hormone occurs before the onset of fetal thyroid hormone secretion. Thyroid hormone receptors and iodothyronine deiodinases are present in the placenta and the fetal central nervous system early in pregnancy, and thyroid hormone plays a crucial role both in trophoblast function and fetal neurodevelopment. Maternal hypothyroxinemia is associated with a high rate of spontaneous abortion and long-term neuropsychological deficits in children born of hypothyroid mothers. Maternal iodine deficiency also causes a wide spectrum of neuropsychological disorders in children, ranging from subclinical deficits in cognitive motor and auditory functions to hypothyroid-induced cognitive impairment in infants. However, these conditions are preventable when iodine supplementation is initiated before the second trimester. Although thyroid hormone replacement therapy is effective for reducing the adverse effects complicated by maternal hypothyroidism, the appropriate dose of thyroid hormone is mandatory in protecting the early stage of pregnancy. CONCLUSIONS Close monitoring of maternal thyroid hormone status and ensuring adequate maternal thyroid hormone levels in early pregnancy are of great importance to prevent miscarriage and neuropsychological deficits in infants.
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Affiliation(s)
- Noriyuki Ohara
- Department of Obstetrics and Gynecology, Kobe University Graduate School of Medicine, Kobe, Japan
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Barra GB, Velasco LFR, Pessanha RP, Campos AM, Moura FN, Dias SMG, Polikarpov I, Ribeiro RCJ, Simeoni LA, Neves FAR. [Molecular mechanism of thyroid hormone action]. ACTA ACUST UNITED AC 2004; 48:25-39. [PMID: 15611816 DOI: 10.1590/s0004-27302004000100005] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Thyroid hormones (TH) are involved in normal differentiation, growth, and metabolism in several tissues of all vertebrates. Their actions are mediated by the TH receptors (TRs), members of the nuclear hormone receptor superfamily. These receptors are transcription factors that bind to DNA on specific sequences, the TR response element (TREs), in promoters of target genes. Two genes encode TRs, alpha e beta, located in chromosomes 17 and 3, respectively. These isoforms show different functions and exhibit a tissue specific expression. TRs function as monomers, homodimers or heterodimers with retinoid X receptor (RXR) and modulate transcription activity (repression or activation) by interacting with co-repressor and co-activators, which associate with TR in the absence or presence of T3, respectively. Understanding the molecular mechanism of TR action and the definition of its crystallographic structure will provide new insights into transcription mechanisms and will facilitate the design of new drugs with greater therapeutic value.
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Affiliation(s)
- Gustavo B Barra
- Laboratório de Farmacologia Molecular, Departamento de Ciências Farmacêuticas, Faculdade de Ciências da Saúde, Universidade de Brasília, Brasília, DF
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Ritchie JWA, Shi YB, Hayashi Y, Baird FE, Muchekehu RW, Christie GR, Taylor PM. A role for thyroid hormone transporters in transcriptional regulation by thyroid hormone receptors. Mol Endocrinol 2003; 17:653-61. [PMID: 12554788 DOI: 10.1210/me.2002-0179] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
Thyroid hormones (THs) must be taken up by target cells to act at the genomic level through binding to nuclear thyroid hormone receptors (TRs). Extensive study has been made of mechanisms by which TH-bound TRs regulate transcription, yet little is known about the critical upstream step, i.e. how THs enter the cell. Growing evidence suggests that saturable transport mechanisms mediate the greater part of TH movement across the plasma membrane and have important roles in the regulation of TH bioavailability. For example, System L is a multifunctional transport system serving as a plasma membrane transporter of THs and amino acids in mammalian cells. We have used two complementary systems, the Xenopus oocyte (which has negligible basal System L activity) and the mammalian BeWo cell line (which has System L activity for TH transport), to investigate the role of this representative TH transporter in nuclear action of THs. We demonstrate that overexpression of System L in Xenopus oocytes increases both cytoplasmic and nuclear delivery of THs from external medium and also enhances transcriptional activation by TRs. Conversely, blocking endogenous System L activity in BeWo cells with specific inhibitors reduces both TH uptake and TR function. These results indicate that plasma membrane TH transporters such as System L may have important roles in gene regulation by TRs.
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Affiliation(s)
- J W A Ritchie
- School of Life Sciences, University of Dundee, MSI/WTB Complex, Dundee DD1 5EH, Scotland, United Kingdom
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