Giorgio Napolitano

The Thyrotropin Receptor

This chapter has outlined the complex process required for thyroid growth and function. Both events are regulated by TSHR via a multiplicity of signals, with the aid of and requirement for a multiplicity of hormones that regulate the TSHR via receptor cross-talk: insulin, IGF-I, adrenergic receptors, and purinergic receptors. Cross-talk appears to regulate G-protein interactions or activities induced by TSH as well as TSHR gene expression. The TSHR structure and its mechanism of signal transduction is being rapidly unraveled in several laboratories, since the recent cloning of the receptor. In addition, the epitopes for autoantibodies against the receptor that can subvert the normal regulated synthesis and secretion of thyroid hormones, causing hyper- or hypofunction, have been defined. Studies of regulation of the TSHR minimal promotor have uncovered a better understanding of the mechanisms by which TSH regulates both growth and function of the thyroid cell. A key novel component of this phenomenon involves TSH AMP positive and negative regulation of the TSHR. Negative transcriptional regulation is a common feature of MHC class I genes in the thyroid. Subversion of negative regulation or too little negative regulation is suggested to result in autoimmune disease. Methimazole and iodide at autoregulatory levels may be important in reversing this process and returning thyroid function to normal. Their action appears to involve factors that react with the IREs on both the TSHR and the TG promoter. Too much negative regulation, as in the case of ras transformation, results in abnormal growth without function. TTF-1 is implicated as a critical autoregulatory component in both positive and negative regulation of the TSHR and appears to be the link between TSH, the TSHR, TSHR-mediated signals, TG and TPO biosynthesis, and thyroid hormone formation. Differentially regulated expression of the TSHR and TG by cAMP and insulin depend on differences in the specificity of the TTF-1 site, that is, the lack of Pax-8 interactions with the TSHR, and the IRE sites. Single-strand binding proteins will become important in determining how TSHR transcription is controlled mechanistically.

Regulation of Major Histocompatibility Complex Class I Gene Expression in Thyroid Cells ROLE OF THE cAMP RESPONSE ELEMENT-LIKE SEQUENCE

The major histocompatibility complex (MHC) class I gene cAMP response element (CRE)-like site, −107 to −100 base pairs, is a critical component of a previously unrecognized silencer, −127 to −90 bp, important for thyrotropin (TSH)/cAMP-mediated repression in thyrocytes. TSH/cAMP induced-silencer activity is associated with the formation of novel complexes with the 38-base pair silencer, whose appearance requires the CRE and involves ubiquitous and thyroid-specific proteins as follows: the CRE-binding protein, a Y-box protein termed thyrotropin receptor (TSHR) suppressor element protein-1 (TSEP-1); thyroid transcription factor-1 (TTF-1); and Pax-8. TTF-1 is an enhancer of class I promoter activity; Pax-8 and TSEP-1 are suppressors. TSH/cAMP decreases TTF-1 complex formation with the silencer, thereby decreasing maximal class I expression; TSH/cAMP enhance TSEP-1 and Pax-8 complex formation in association with their repressive actions. Oligonucleotides that bind TSEP-1, not Pax-8, prevent formation of the TSH/cAMP-induced complexes associated with TSH-induced class I suppression, i.e. TSEP-1 appears to be the dominant repressor factor associated with TSH/cAMP-decreased class I activity and formation of the novel complexes. TSEP-1, TTF-1, and/or Pax-8 are involved in TSH/cAMP-induced negative regulation of the TSH receptor gene in thyrocytes, suppression of MHC class II, and up-regulation of thyroglobulin. TSH/cAMP coordinate regulation of common transcription factors may, therefore, be the basis for self-tolerance and the absence of autoimmunity in the face of TSHR-mediated increases in gene products that are important for thyroid growth and function but are able to act as autoantigens.

Zinc Sulfate Supplementation Improves Thyroid Function in Hypozincemic Down Children

In subjects affected by trisomy 21 (Down syndrome), hypothyroidism is the most common endocrinological deficit. Plasma zinc levels, which are commonly detected below the normal range in Down patients, are related to some endocrinological and immunological functions; in fact, zinc deficiency has been shown to impair immune response and growth rate. Aims of this study were to evaluate (1) the role of zinc deficiency in subclinical hypothyroidism and (2) thyroid function changes in Down children cyclically supplemented with zinc sulfate. Inverse correlations have been observed between age and triiodotironine (T3) and between zinc and thyroid-stimulating hormone (TSH); higher TSH levels have been found in hypozincemic patients at the beginning of the study. After 6 mo of supplementation, an improvement of thyroid function (TSH levels: 3.96 ± 1.84 vs 2.64 ± 1.33 mUI/mL basally and after 6 mo, respectively) was observed in hypozincemic patients. In the second cycle of supplementation, a similar trend of TSH was observed. At the end of the study, TSH significantly decreased in treated hypozincemic subjects (4.48 ± 1.93 vs 2.96 ± 1.20 mUI/mL) and it was no longer different in comparison to normozincemic patients. We suggest zinc supplementation to the diet in hypozincemic Down children as a simple and useful therapeutic tool.

Walking training in postmenopause: effects on both spontaneous physical activity and training-induced body adaptations.

ObjectiveBecause physical exercise has been widely used for primary and secondary preventions of cardiometabolic diseases arising with menopause, the aim of our study was to determine whether participation in aerobic physical exercise is linked to the modification of spontaneous physical activity and whether this compensation affects aerobic training–related body adaptations. MethodsBoth before and after a 13-week walking training program, 34 postmenopausal women (mean ± SD age, 55.89 ± 3.57 y) were analyzed for lipids, adipokines, glucose, and insulin plasma levels, as well as for body measures, heart rate and blood pressure at rest, maximal aerobic capacity, total daily energy expenditure, mean intensity of daily physical activities, and time and energy spent on physical activities with an intensity of more than three metabolic equivalents. ResultsAerobic training induced significant reductions in body mass, body mass index, heart rate, systolic blood pressure, basal cardiac double product, plasma glucose, leptin, and resistin. Aerobic fitness, the reserve of the cardiac double product, and the quantitative insulin sensitivity index were significantly improved. Cluster analysis of the variations in the total daily energy expenditure, the mean intensity of daily physical activities, and the time and energy spent on physical activities with an intensity of more than three metabolic equivalents identified two subgroups: one showed reduced spontaneous physical activity (GROUP−), whereas the other did not (GROUP+). The subgroups differed significantly only for plasma lipid variation. GROUP+ showed significantly reduced low-density lipoprotein cholesterol and total cholesterol, whereas GROUP− did not show significantly modified plasma lipids. ConclusionsIn postmenopause, participation in a program of aerobic physical exercise can result in a reduction of spontaneous physical activity, which inhibits the positive effects of the aerobic exercise on plasma lipids and lipoproteins.

Graves’ Disease: A Host Defense Mechanism Gone Awry

In this report we summarize evidence to support a model for the development of Graves’ disease. The model suggests that Graves’ disease is initiated by an insult to the thyrocyte in an individual with a normal immune system. The insult, infectious or otherwise, causes double strand DNA or RNA to enter the cytoplasm of the cell. This causes abnormal expression of major histocompatibility (MHC) class I as a dominant feature, but also aberrant expression of MHC class II, as well as changes in genes or gene products needed for the thyrocyte to become an antigen presenting cell (APC). These include increased expression of proteasome processing proteins (LMP2), transporters of antigen peptides (TAP), invariant chain (Ii), HLA-DM, and the co-stimulatory molecule, B7, as well as STAT and NF-kB activation. A critical factor in these changes is the loss of normal negative regulation of MHC class I, class II, and thyrotropin receptor (TSHR) gene expression, which is necessary to maintain self-tolerance during the normal changes in gene expression involved in hormonally-increased growth and function of the cell. Self-tolerance to the TSHR is maintained in normals because there is a population of CD8+ cells which normally suppresses a population of CD4+ cells that can interact with the TSHR if thyrocytes become APCs. This is a host self-defense mechanism that we hypothesize leads to autoimmune disease in persons, for example, with a specific viral infection, a genetic predisposition, or even, possibly, a TSHR polymorphism. The model is suggested to be important to explain the development of other autoimmune diseases including systemic lupus or diabetes.

Major Histocompatibility Class II HLA-DRα Gene Expression in Thyrocytes: Counter Regulation by the Class II Transactivator and the Thyroid Y Box Protein

Aberrant expression of major histocompatibility complex (MHC) class II proteins on thyrocytes, which is associated with autoimmune thyroid disease, is mimicked by γ-interferon (γ-IFN). To define elements and factors that regulate class II gene expression in thyrocytes and that might be involved in aberrant expression, we have studied γ-IFN-induced HLA-DRα gene expression in rat FRTL-5 thyroid cells. The present report shows that class II expression in FRTL-5 thyrocytes is positively regulated by the class II transactivator (CIITA), and that CIITA mimics the action of γ-IFN. Thus, as is the case for γ-IFN, several distinct and highly conserved elements on the 5′-flanking region of the HLA-DRα gene, the S, X1, X2, and Y boxes between −137 to −65 bp, are required for class II gene expression induced by pCIITA transfection in FRTL-5 thyroid cells. CIITA and γ-IFN do not cause additive increases in HLA-DRα gene expression in FRTL-5 cells, consistent with the possibility that CIITA is an intermediate factor in ...

THYROID FUNCTION AND PLASMA SELENIUM IN CHRONIC UREMIC PATIENTS ON HEMODIALYSIS TREATMENT

It has been shown recently that Selenium (Se), an essential trace element for humans, is involved in the regulation of thyroid function, since the enzyme that catalyzes the liver conversion of the thyroid hormone T4 to the more active form T3 is a selenoenzyme. In chronic uremic patients, low blood Se levels as well as thyroid function abnormalities are often found. The present study was carried out to verify whether any correlation exists between Se levels and thyroid function, and to evaluate possible changes in hormonal pattern during Se supplementation in 10 chronic uremic patients on hemodialysis (HD) treatment. Se was supplemented orally as sodium selenite over six consecutive months. Basic plasma Se levels were significantly lower in patients than in normal controls. Right from the start of Se supplementation, plasma Se concentration promptly normalized and leveled off in the normal range throughout the study. Significant increase of FT3 and reduction of TSH levels were detected during Se supplementation. In Se-supplemented patients, a significant direct correlation was also found between reverse T3 (rT3) and TSH, and a significant inverse correlation was found between Se and TSH. Our results suggest that Se deficiency in chronic uremic patients represents a factor influencing the thyroid function and that the Se status should be determined in the evaluation of thyroid metabolism in these patients.

The flavonoid quercetin inhibits thyroid-restricted genes expression and thyroid function.

Quercetin is the most abundant flavonoid present in a broad range of fruit and vegetables. Furthermore, quercetin is available as dietary supplements that are based on its antioxidant, antiproliferative and anti-inflammatory properties. However, concerns have been raised about the potential toxic effects of excessive intake of quercetin, and several studies have demonstrated that flavonoids, included quercetin, can interfere with thyroid function. In a previous report, we showed that quercetin inhibits thyroid-cell growth and iodide uptake. The latter effect was associated with down-regulation of sodium/iodide symporter gene expression. In the present study, we have evaluated the effects of quercetin on the expression of other thyroid-restricted genes, and we show that quercetin decreases the expression of the thyrotropin receptor, thyroid peroxidase and thyroglobulin genes. We further investigated the inhibitory effects of quercetin on thyroid function in vivo through evaluation of radioiodine uptake in the Sprague-Dawley rat, which was significantly decreased after 14 days of quercetin treatment. These data confirm that quercetin can act as a thyroid disruptor, and they suggest that caution is needed in its supplemental and therapeutic use.

Regulation of Major Histocompatibility (MHC) Class II Human Leukocyte Antigen-DRα Gene Expression in Thyrocytes by Single Strand Binding Protein-1, a Transcription Factor That Also Regulates Thyrotropin Receptor and MHC Class I Gene Expression

The single strand binding protein (SSBP-1) is a positive regulator of TSH receptor gene expression and binds to an element with a GXXXXG motif. The S box of the mouse major histocompatibility class II gene has multiple GXXXXG motifs and can also bind SSBP-1. The S box is one of four highly conserved elements on the 5′-flanking region of class II genes that are necessary for interferon-γ (IFNγ) to overcome the normally suppressed state of the gene and induce aberrant class II expression. In this report we show that SSBP-1, when overexpressed in FRTL-5 thyroid cells, is a positive regulator of human leukocyte antigen (HLA)-DRα class II gene expression, as is IFNγ or the class II trans-activator (CIITA). This is evidenced by increased exogenous promoter activity, increased endogenous RNA levels, and increased endogenous antigen expression after transfecting full-length SSBP-1 complementary DNA together with a HLA-DRα promoter-reporter gene chimera into TSH-treated FRTL-5 thyroid cells whose endogenous SSBP-1...

Walking training affects dehydroepiandrosterone sulfate and inflammation independent of changes in spontaneous physical activity.

ObjectiveWe hypothesized that physical exercise in postmenopausal women could interfere with the molecular interrelationship of the immune-endocrine system and be effective even in women in whom training determined a reduction of spontaneous physical activity (SPA). For this reason, we investigated the effects of an aerobic program on plasma dehydroepiandrosterone sulfate (DHEA-S) and cytokine levels in relationship to SPA modification. MethodsThirty-two postmenopausal women (mean [SD] age, 56.38 [4.33] y) were enrolled in the study. Inclusion criteria were as follows: age younger than 65 years, body mass index higher than 18.5 and lower than 35 kg/m2, no pharmacological treatments, and no history of chronic, cardiovascular, or orthopedic diseases. Before and after 3 months of walking training at moderate intensity (40-50 min, 4 d/wk), they were evaluated for SPA, body composition, energy intake, and levels of plasma cytokines (tumor necrosis factor &agr; [TNF-&agr;], interleukin [IL]-1&agr;, IL-1&bgr;, IL-2, IL-8, and IL-10), C-reactive protein, DHEA-S, cortisol, and estrogen. ResultsAt baseline, SPA did not correlate with either DHEA-S level or cytokine levels. There was negative correlation between DHEA-S and both TNF-&agr; and IL-2. After the intervention program, 16 women showed increased SPA, and 16 women showed decreased SPA. Independent of these changes in SPA, both TNF-&agr; levels and cortisol-to-DHEA-S ratio decreased, whereas DHEA-S levels increased. ConclusionsIn postmenopausal women, walking training, rather than SPA, influences DHEA-S and cytokine concentrations and their correlations, thus interfering with adrenal steroids and the inflammatory markers network. Physical exercise acts in parallel on menopausal neuroendocrine alterations and on the systemic inflammatory profile independent of SPA changes.