Thyroid-stimulating hormone-induced down-regulation of thyroid transcription factor 1 in rat thyroid FRTL-5 cells

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

Unraveling the Complex Interplay Between Transcription Factors and Signaling Molecules in Thyroid Differentiation and Function, From Embryos to Adults

Thyroid differentiation of progenitor cells occurs during embryonic development and in the adult thyroid gland, and the molecular bases of these complex and finely regulated processes are becoming ever more clear. In this Review, we describe the most recent advances in the study of transcription factors, signaling molecules and regulatory pathways controlling thyroid differentiation and development in the mammalian embryo. We also discuss the maintenance of the adult differentiated phenotype to ensure the biosynthesis of thyroid hormones. We will focus on endoderm-derived thyroid epithelial cells, which are responsible for the formation of the thyroid follicle, the functional unit of the thyroid gland. The use of animal models and pluripotent stem cells has greatly aided in providing clues to the complicated puzzle of thyroid development and function in adults. The so-called thyroid transcription factors - Nkx2-1, Foxe1, Pax8 and Hhex - were the first pieces of the puzzle identified in mice. Other transcription factors, either acting upstream of or directly with the thyroid transcription factors, were subsequently identified to, almost, complete the puzzle. Among them, the transcription factors Glis3, Sox9 and the cofactor of the Hippo pathway Taz, have emerged as important players in thyroid differentiation and development. The involvement of signaling molecules increases the complexity of the puzzle. In this context, the importance of Bmps, Fgfs and Shh signaling at the onset of development, and of TSH, IGF1 and TGFβ both at the end of terminal differentiation in embryos and in the adult thyroid, are well recognized. All of these aspects are covered herein. Thus, readers will be able to visualize the puzzle of thyroid differentiation with most - if not all - of the pieces in place.

Thyrotropin, but Not Thyroid-Stimulating Antibodies, Induces Biphasic Regulation of Gene Expression in Human Thyrocytes

Background: Thyrotropin (TSH) and thyroid-stimulating antibodies (TSAbs) activate TSH receptor (TSHR) signaling by binding to its extracellular domain. TSHR signaling has been studied extensively in animal thyrocytes and in engineered cell lines, and differences in signaling have been observed in different cell systems. We, therefore, decided to characterize and compare TSHR signaling mediated by TSH and monoclonal TSAbs in human thyrocytes in primary culture. Methods: We used quantitative reverse transcription-polymerase chain reaction to measure mRNA levels of thyroid-specific genes thyroglobulin (TG), thyroperoxidase (TPO), iodothyronine deiodinase type 2 (DIO2), sodium-iodide symporter (NIS), and TSHR after stimulation by TSH or two monoclonal TSAbs, KSAb1 and M22. We also compared secreted TG protein after TSHR activation by TSH and TSAbs using an enzyme-linked immunosorbent assay. TSHR cell surface expression was determined using fluorescence activated cell sorting (FACS). Results: We found that TSH at low doses increases and at high doses (>1 mU/mL) decreases levels of gene expression for TSHR, TG, TPO, NIS, and DIO2. The biphasic effect of TSH on signaling was not caused by downregulation of cell surface TSHRs. This bell-shaped biphasic dose-response curve has been termed an inverted U-shaped dose-response curve (IUDRC). An IUDRC was also found for TSH-induced regulation of TG secretion. In contrast, KSAb1- and M22-induced regulation of TSHR, TG, TPO, NIS, and DIO2 gene expression, and secreted TG followed a monotonic dose-response curve that plateaus at high doses of activating antibody. Conclusions: Our data demonstrate that the physiological activation of TSHRs by TSH in primary cultures of human thyrocytes is characterized by a regulatory mechanism that may inhibit thyrocyte overstimulation. In contrast, TSAbs do not exhibit biphasic regulation. Although KSAb1 and M22 may not be representative of all TSAbs found in patients with Graves' disease, we suggest that persistent robust stimulation of TSHRs by TSAbs, unrelieved by a decrease at high TSAb levels, fosters chronic stimulation of thyrocytes in Graves' hyperthyroidism.

Evaluation on the thyroid disrupting mechanism of malathion in Fischer rat thyroid follicular cell line FRTL-5

Thyroid hormones are involved in many important physiological activities including regulation of energy metabolism, development of nervous system, maintenance of cerebral functions, and so on. Endocrine-disrupting chemicals (EDCs) that interfere with thyroid functions raise serious concerns due to their frequent misuse in areas where regulations are poorly implemented. In addition, chemicals that are originally regarded safe may now be considered as toxic with the development of life sciences. Malathion is an organophosphate insecticide that is widely applied and distributed in agricultural and residential settings. Due to the low acute toxicity and rapid degradation, malathion is not listed as a primary thyroid disrupting chemical. However, emerging evidences reported that malathion affected thyroperoxidase catalyzed iodide oxidation which in turn influenced thyroid hormone transportation, and enhanced parathyroid hyperplasia prevalence. Nevertheless, direct effect of malathion on thyroid hormone biosynthesis remains to be elucidated. This study investigated the effects of thyroid disruption of malathion in Fischer rat thyroid follicular cell line, FRTL-5. Transcriptional and translational analyses on thyroglobulin demonstrated that both mRNA and protein expression levels were significantly inhibited by malathion. Cellular cAMP level and TSH receptor expression were distinctly reduced by malathion (6.0 µg/ml). These results suggested that malathion directly disrupted the biosynthesis of thyroid hormone and the mechanism involved down-regulation of TSH receptor and cellular cAMP. This subsequently led to the suppression of TSH dependent signal transduction, TG transcription inhibition, and obstruction of thyroid hormone biosynthesis.

Thyroid disruption by triphenyl phosphate, an organophosphate flame retardant, in zebrafish (Danio rerio) embryos/larvae, and in GH3 and FRTL-5 cell lines

Triphenyl phosphate (TPP), one of the most widely used organophosphate flame retardants (OPFRs), has frequently been detected in the environment and biota. However, knowledge of its toxicological effects is limited. The present study was conducted to determine the adverse effects of TPP on the thyroid endocrine system of embryonic/larval zebrafish, and the underlying mechanisms for these effects were studied using rat pituitary (GH3) and thyroid follicular (FRTL-5) cell lines. In the GH3 cells, TPP up-regulated the expression of the tshβ, trα, and trβ genes, while T3, a positive control, down-regulated the expression of these genes. In the FRTL-5 cells, the expression of the nis and tpo genes was significantly up-regulated, suggesting that TPP stimulates thyroid hormone synthesis in the thyroid gland. In zebrafish larvae at 7 days post-fertilization (dpf), TPP exposure led to significant increases in both T3 and T4 concentrations and expression of the genes involved in thyroid hormone synthesis. Exposure to TPP also significantly up-regulated the expression of the genes related to the metabolism (dio1), transport (ttr), and elimination (ugt1ab) of thyroid hormones. The down-regulation of the crh and tshβ genes in the zebrafish larvae suggests the activation of a central regulatory feedback mechanism induced by the increased T3 levels in vivo. Taken together, our observations show that TPP could increase the thyroid hormone concentrations in the early life stages of zebrafish by disrupting the central regulation and hormone synthesis pathways.

Modulation of thyroid-specific gene expression in normal and nodular human thyroid tissues from adults: an in vivo effect of thyrotropin

CONTEXT

Evidence from in vitro studies or animal models has shown that TSH affects thyrocytes by thyroid-specific expression modulation.

OBJECTIVE

The objective of our study was to analyze the role of TSH in human thyroid gene expression in vivo.

DESIGN/SETTING

Thirty-nine normal thyroid tissues were collected at the same center.

STUDY SUBJECTS

Patients were divided into two groups based on serum TSH levels: 17 with normal TSH levels (1-4 mU/liter; group 1) and 22 with TSH levels below 0.5 mU/liter (group 2).

INTERVENTION

Group 2 underwent thyroidectomy after suppressive L-T4 therapy.

MAIN OUTCOME MEASURES

mRNA levels of thyroid genes such as sodium/iodide symporter (NIS), apical iodide transporter, pendrin, thyroglobulin, thyroperoxidase, TSH receptor, paired box transcription factor 8, and thyroid transcription factor-1 were evaluated by quantitative PCR.

RESULTS

The reduction of TSH stimulation causes decreases in NIS and apical iodide transporter gene expression in normal tissues and more limited reductions in thyroglobulin, thyroperoxidase, and paired box transcription factor 8, but it has no significant effect on TSH receptor, pendrin, or thyroid transcription factor-1. Comparison of NIS levels in normal and nodular tissues from the same patient confirmed that it is differentially expressed in nodules only in the presence of normal TSH (P < 0.01). In patients with suppressed TSH, nodular NIS levels were similar to those in normal tissues.

CONCLUSIONS

Our data represent the first demonstration in human thyroid tissues that TSH contributes to the regulation of thyrocyte differentiation by modulating thyroid gene levels. It exerts a particularly important effect on the transcription of NIS, which becomes very low after prolonged TSH suppression.

A thyroid-specific far-upstream enhancer in the human sodium/iodide symporter gene requires Pax-8 binding and cyclic adenosine 3',5'-monophosphate response element-like sequence binding proteins for full activity and is differentially regulated in normal and thyroid cancer cells

The sodium/iodide symporter (NIS) gene is highly expressed in the thyroid gland and is important for the diagnosis and radioiodide therapy of differentiated thyroid cancers. We investigated a human NIS (hNIS) gene 5'-far-upstream enhancer (hNUE) (-9847 to -8968). The hNUE is TSH responsive in both FRTL-5 cells and primary normal thyroid cells, but not in human papillary thyroid cancer cells (BHP cells). The hNUE enhanced expression of the basal hNIS promoter 15-fold and required both a Pax-8 binding site and a cAMP response element (CRE)-like sequence for full activity. The hNUE activated transcription in a thyroid-selective and cAMP-dependent manner, mediated by both protein kinase A (PKA)-dependent and PKA-independent pathways. Pax-8 and two CRE-like sequence binding proteins bind to the hNUE. Supershift binding assay indicated that one of the CRE-like sequence binding protein(s) was CRE-binding protein-1, activation transcription factor-1, and/or CRE modulator, and the other was an unknown factor(s) that is absent in BHP 2-7 cells. A far-upstream enhancer is important for hNIS regulation in the thyroid. Deficient CRE-like sequence binding protein(s) that bind to the hNUE in normal thyroid cells may be responsible for reduced NIS gene expression in some thyroid carcinomas.

Differential regulation of the human sodium/iodide symporter gene promoter in papillary thyroid carcinoma cell lines and normal thyroid cells

The absence of TSH-stimulated radioiodide uptake in differentiated thyroid cancer is associated with a high recurrence rate and reduced survival. We studied regulation of the sodium/iodide symporter gene in human papillary thyroid cancer cell lines (BHP) and primary human thyroid cells. BHP cells expressed very low levels of sodium/iodide symporter mRNA and did not concentrate iodide, but iodide uptake was restored to levels seen in FRTL-5 rat thyroid cells by stable transfection of a sodium/iodide symporter cDNA. Sodium/iodide symporter gene expression, therefore, was necessary and sufficient for iodide uptake in BHP cells. We cloned the human sodium/iodide symporter gene 5'-flanking region and analyzed progressive 5'-deletions in transient transfections. We identified a region, -596 to -268, essential to confer full promoter activity in primary normal human thyroid cells. Sodium/iodide symporter promoter activity in four BHP cell lines, however, was markedly reduced, consistent with down-regulation of the endogenous sodium/iodide symporter gene. Nuclear extracts from BHP 2-7 cells had reduced or absent binding to regions of the sodium/iodide symporter promoter shown to be critical for expression, compared with nuclear extracts from FRTL-5 cells. Competition studies indicated that these nuclear proteins were not known thyroid transcription factors. Modifications of the sodium/iodide symporter promoter with demethylation or histone acetylation did not increase sodium/iodide symporter expression, and no deletions of the critical regulatory region were identified in the endogenous gene in BHP cells. Regulation of the sodium/iodide symporter 5'-flanking region in transient transfection paralleled endogenous sodium/iodide symporter expression. Reduced expression of potential novel nuclear factor(s) in these cell lines may contribute to reduced sodium/iodide symporter expression resulting in absence of iodide uptake in some papillary thyroid cancers.

TTF-1, a homeodomain gene required for diencephalic morphogenesis, is postnatally expressed in the neuroendocrine brain in a developmentally regulated and cell-specific fashion

TTF-1 is a member of the Nkx family of homeodomain genes required for morphogenesis of the hypothalamus. Whether TTF-1, or other Nkx genes, contributes to regulating differentiated hypothalamic functions is not known. We now report that postnatal hypothalamic TTF-1 expression is developmentally regulated and associated with the neuroendocrine process of female sexual development. Lesions of the hypothalamus that cause sexual precocity transiently activate neuronal TTF-1 expression near the lesion site. In intact animals, hypothalamic TTF-1 mRNA content also increases transiently, preceding the initiation of puberty. Postnatal expression of the TTF-1 gene was limited to subsets of hypothalamic neurons, including LHRH neurons, which control sexual maturation, and preproenkephalinergic neurons of the lateroventromedial nucleus of the basal hypothalamus, which restrain sexual maturation and facilitate reproductive behavior. TTF-1 mRNA was also detected in astrocytes of the median eminence and ependymal/subependymal cells of the third ventricle, where it colocalized with erbB-2, a receptor involved in facilitating sexual development. TTF-1 binds to and transactivates the erbB-2 and LHRH promoters, but represses transcription of the preproenkephalin gene. The singular increase in hypothalamic TTF-1 gene expression that precedes the initiation of puberty, its highly specific pattern of cellular expression, and its transcriptional actions on genes directly involved in neuroendocrine reproductive regulation suggest that TTF-1 may represent one of the controlling factors that set in motion early events underlying the central activation of mammalian puberty.

Down-regulation of thyroid transcription factor-1 gene expression in fetal lung hypoplasia is restored by glucocorticoids

The thyroid transcription factor (TTF)-1 has an essential role in lung morphogenesis and development. It is involved in the transcription of surfactant proteins (SP), which are critical in respiratory function. Neonates with congenital diaphragmatic hernia die of respiratory failure caused by pulmonary hypoplasia with associated biochemical immaturity. To gain new insights into the causes of this disorder and the effect of prenatal hormonal treatment on reducing mortality in these infants, we evaluated the expression of TTF-1 as marker of lung morphogenesis and SP-B as marker of lung maturity. Using a rat model of lung immaturity, we show that TTF-1 and SP-B messenger RNA (mRNA) levels are drastically reduced in congenital lung hypoplasia. Interestingly, prenatal dexamethasone (Dex) treatment increased both TTF-1 and SP-B mRNAs over control levels when administered to rats with lung hypoplasia, but it had no effect on TTF-1 or a moderate effect on SP-B mRNA when administered to control rats. TRH alone also increases TTF-1 and SP-B mRNA levels but to a lesser extent than Dex. When administered together with Dex, TRH counteracts the induction observed with the glucocorticoid. The decrease in TTF-1 mRNA levels in lung hypoplasia is paralleled by a down-regulation of TTF-1 protein levels, as well as by a decrease in the TTF-1/DNA complex when the TTF-1-binding site of the SP-B promoter was used as a probe. Both parameters were reestablished after glucocorticoid treatment. Moreover, the regulation of TTF-1 gene expression described in this report is accompanied by the same regulation in its promoter activity, as demonstrated in transfection experiments performed in H-441 human lung-derived adenocarcinoma cells. In conclusion, our data demonstrate, for the first time, that lung hypoplasia and the associated respiratory dysfunction caused by SP-B deficiency are caused, in part, by down-regulation of TTF-1 gene expression. The observations that prenatal glucocorticoid treatment induces the expression of TTF-1 supports routine in utero glucocorticoid treatment of patients expected to have lung hypoplasia.

Tumor necrosis factor-alpha regulation of thyroid transcription factor-1 and Pax-8 in rat thyroid FRTL-5 cells

Tumor necrosis factor-alpha (TNF-alpha) is known to modulate the expression of thyroid-specific genes, such as thyroglobulin (TG), contributing to the pathogenesis of autoimmune thyroid disease. In the present study, we show that TNF-alpha suppresses DNA-binding activity of thyroid transcription factors, Pax-8 and thyroid transcription factor-1 (TTF-1), which is, in part, involved in TNF-alpha-induced decrease in TG gene expression. Transfected into rat thyroid FRTL-5 cells, the activity of reporter plasmid containing the rat TG promoter ligated to a luciferase gene was significantly suppressed in the presence of TNF-alpha. In gel mobility shift analyses, protein-DNA complexes formed by TTF-1 and Pax-8 were reduced when the nuclear extracts prepared from TNF-alpha-treated FRTL-5 cells were used. The suppressive effect of TNF-alpha on TTF-1-DNA complex formation is, in part, caused by suppression of TTF-1 gene transcription by TNF-alpha. Expressions of TTF-1 messenger RNA and protein, which were assessed by Northern blot and Western blot analyses, respectively, were decreased by TNF-alpha treatment of FRTL-5 cells. In contrast, TNF-alpha did not affect the expression of Pax-8 messenger RNA. Treatment of FRTL-5 cells with TNF-alpha caused a decrease in Pax-8 protein in nuclear extracts and accumulation of the protein in the cytoplasm, as assessed by Western blot analyses. Mutation of the TTF-1/Pax-8-binding site lost the TNF-alpha-induced decrease in TG promoter activity in a transfection experiment. These results indicate that TNF-alpha suppresses the activity of TTF-1 and Pax-8 by different mechanisms, which, in part, seem to be involved in TNF-alpha-induced decrease in TG gene expression.

Calreticulin enhances the transcriptional activity of thyroid transcription factor-1 by binding to its homeodomain

Transcription factors are often regulated by associated protein cofactors that are able to modify their activity by several different mechanisms. In this study we show that calreticulin, a Ca2+-binding protein with chaperone activity, binds to thyroid transcription factor-1 (TTF-1), a homeodomain-containing protein implicated in the differentiation of lung and thyroid. The interaction between calreticulin and TTF-1 appears to have functional significance because it results in increased transcriptional stimulation of TTF-1-dependent promoters. Calreticulin binds to the TTF-1 homeodomain and promotes its folding, suggesting that the mechanism involved in stimulation of transcriptional activity is an increase of the steady-state concentration of active TTF-1 protein in the cell. We also demonstrate that calreticulin mRNA levels in thyroid cells are under strict control by the thyroid-stimulating hormone, thus implicating calreticulin in the modulation of thyroid gene expression by thyroid-stimulating hormone.

Autoregulation of thyroid-specific gene transcription by thyroglobulin

Thyroglobulin (TG), the primary synthetic product of the thyroid, is the macromolecular precursor of thyroid hormones. TG synthesis, iodination, storage in follicles, and degradation control thyroid hormone formation and secretion into the circulation. Thyrotropin (TSH), via its receptor (TSHR), increases thyroid hormone levels by up-regulating expression of the sodium iodide symporter (NIS), thyroid peroxidase (TPO), and TG genes. TSH does this by modulating the expression and activity of several thyroid-specific transcription factors, thyroid transcription factor (TTF)-1, TTF-2, and Pax-8, which coordinately regulate NIS, TPO, TG, and the TSHR. Major histocompatibility complex class I gene expression, which also is regulated by TTF-1 and Pax-8 in the thyroid, is decreased simultaneously. This helps maintain self-tolerance in the face of TSH-increased gene products necessary for thyroid hormone formation. In this report we show that follicular TG counter-regulates TSH-increased, thyroid-specific gene transcription by suppressing expression of the TTF-1, TTF-2, and Pax-8 genes. This decreases expression of the TG, TPO, NIS, and TSHR genes, but increases class I expression. TG acts transcriptionally, targeting, for example, a sequence within 1.15 kb of the 5' flanking region of TTF-1. TG does not affect ubiquitous transcription factors regulating TG, TPO, NIS, and/or TSHR gene expression. The inhibitory effect of TG on gene expression is not duplicated by thyroid hormones or iodide and may be mediated by a TG-binding protein on the apical membrane. We hypothesize that TG-initiated, transcriptional regulation of thyroid-restricted genes is a normal, feedback, compensatory mechanism that limits follicular function and contributes to follicular heterogeneity.

PAX 8 activates the enhancer of the human thyroperoxidase gene

In this study we report on a novel natural target of the paired domain transcription factor PAX 8 in the enhancer element of the human thyroperoxidase gene, one of the most important thyroid differentiation markers. It is the primary enzyme involved in thyroid hormone synthesis and PAX 8 has been previously identified as an activating factor of the rat thyroperoxidase gene promoter. In vitro, PAX 8 binds a cis element of the human enhancer and its exogenous expression induces the enhancer activity in co-transfection experiments in Cos-7 cells. When mutated at this binding site, the enhancer is no longer activated by PAX 8. Our finding strengthens the PAX 8 role in the maintenance of thyroid differentiation and in particular in the tissue-specific thyroperoxidase gene expression.

Phorbol ester down-regulation of lung surfactant protein B gene expression by cytoplasmic trapping of thyroid transcription factor-1 and hepatocyte nuclear factor 3

The lung-specific surfactant protein B (SP-B) is essential for surfactant function and normal respiration. We investigated the role of thyroid transcription factor-1 (TTF-1) and hepatocyte nuclear factor 3 (HNF3) in the down-regulation of SP-B gene expression by phorbol ester in pulmonary adenocarcinoma H441 cells. Responsiveness to 12-O-tetradecanoylphorbol-13-acetate (TPA) localized to the SP-B proximal promoter (-140/-65 bp) and specifically to binding sites for TTF-1 and HNF3, which act as cell-specific enhancers of SP-B expression. Treatment of cells with TPA (10 nM) caused a time-dependent decrease in both TTF-1 and HNF3 in nuclear extracts and accumulation of both factors in the cytoplasm as assessed by electromobility shift, Western, Southwestern, and immunofluorescence assays. Treatment did not alter the mRNA content or DNA binding activity for either transcription factor. We conclude that down-regulation of SP-B gene expression by phorbol ester involves cytoplasmic trapping and loss of TTF-1 and HNF3 from the nucleus. This mechanism of action is independent of AP-1 and other transcription factors known to be influenced by phorbol ester.