The Mechanism of Negative Transcriptional Regulation by Thyroid Hormone: Lessons From the Thyrotropin β Subunit Gene

Thyroid hormone-regulated chromatin landscape and transcriptional sensitivity of the pituitary gland

Thyroid hormone (3,5,3'-triiodothyronine, T3) is a key regulator of pituitary gland function. The response to T3 is thought to hinge crucially on interactions of nuclear T3 receptors with enhancers but these sites in pituitary chromatin remain surprisingly obscure. Here, we investigate genome-wide receptor binding in mice using tagged endogenous thyroid hormone receptor β (TRβ) and analyze T3-regulated open chromatin using an anterior pituitary-specific Cre driver (Thrbb2Cre). Strikingly, T3 regulates histone modifications and chromatin opening primarily at sites that maintain TRβ binding regardless of T3 levels rather than at sites where T3 abolishes or induces de novo binding. These sites associate more frequently with T3-activated than T3-suppressed genes. TRβ-deficiency blunts T3-regulated gene expression, indicating that TRβ confers transcriptional sensitivity. We propose a model of gene activation in which poised receptor-enhancer complexes facilitate adjustable responses to T3 fluctuations, suggesting a genomic basis for T3-dependent pituitary function or pituitary dysfunction in thyroid disorders.

Thyroid endocrine disruption induced by [C8mim]Br: An integrated in vivo, in vitro, and in silico study

Conventional thyroid-disrupting chemicals (TDCs) such as polybrominated diphenyl ethers, polychlorinated biphenyls, and bisphenols perturb animal's thyroid endocrine system by mimicking the action of endogenous thyroid hormones (THs), since they share a similar backbone structure of coupled benzene rings with THs. 1-methyl-3-octylimidazolium bromide ([C₈mim]Br), a commonly used ionic liquid (IL), has no structural similarity to THs. Whether it interferes with thyroid function and how its mode of action differs from conventional TDCs is largely unknown. Herein, zebrafish embryo-larvae experiments (in vivo), GH3 cell line studies (in vitro), and molecular simulation analyses (in silico) were carried out to explore the effect of [C₈mim]Br on thyroid homeostasis and its underlying mechanism. Molecular docking results suggested that [C₈mim]⁺ likely bound to retinoid X receptors (RXRs), which may compromise the formation of TH receptor/RXR heterodimers. This then perturbed the negative regulation of thyroid-stimulating hormone β (tshβ) transcription by T3 in GH3 cell line. The resulting enhancement of tshβ expression further caused hyperthyroidism and developmental toxicity in larval zebrafish. These findings provided a crucial aspect of the ecological risks of ILs, and presented a new insight into the thyroid-disrupting mechanisms for emerging pollutants that do not have structural similarity to THs.

The Role of Genetic Polymorphisms in Differentiated Thyroid Cancer: A 2023 Update

Thyroid cancer is the most common endocrine malignancy, with an increasing trend in the past decades. It has a variety of different histological subtypes, the most frequent one being differentiated thyroid cancer, which refers to papillary carcinoma, the most common histological type, followed by follicular carcinoma. Associations between genetic polymorphisms and thyroid cancer have been investigated over the years and are an intriguing topic for the scientific world. To date, the results of associations of single nucleotide polymorphisms, the most common genetic variations in the genome, with thyroid cancer have been inconsistent, but many promising results could potentially influence future research toward developing new targeted therapies and new prognostic biomarkers, thus consolidating a more personalized management for these patients. This review focuses on emphasizing the existing literature data regarding genetic polymorphisms investigated for their potential association with differentiated thyroid cancer and highlights the opportunity of using genetic variations as biomarkers of diagnosis and prognosis for thyroid cancer patients.

Peripubertal soy isoflavone consumption leads to subclinical hypothyroidism in male Wistar rats

Exposure to endocrine-disrupting chemicals during critical windows of development may lead to functional abnormalities in adulthood. Isoflavones are a flavonoid group of phytoestrogens that are recognized by their estrogenic activity and are highly abundant in soybean. Since the thyroid gland presents estrogen receptors and infants, toddlers and teenagers may consume isoflavones from soy-based infant formula and beverages as alternatives to animal milk, we propose to investigate the potential effects of relevant concentrations of soy isoflavones in the regulation of the hypothalamic-pituitary (HP) thyroid axis using peripubertal male rats as an experimental model. Thirty-two 23-day-old male rats were exposed to 0.5, 5, or 50 mg of soy isoflavones/kg from weaning to 60 days of age, when they were euthanized, and the tissues were collected to evaluate the mRNA expression of genes involved in the regulation of the HP thyroid axis and dosages of thyroid hormones (THs). Serum TSH concentrations were increased, while alterations were not observed in serum concentrations of triiodothyronine and thyroxine. Regarding mRNA gene expression, Mct-8 was increased in the hypothalamus, Mct-8, Thra1, and Thrb2 were decreased in the pituitary, and Nis and Pds were reduced in the thyroid. In the heart, Mct8 and Thrb2 were increased, and Thra1 was decreased. In the liver, Mct8, Thra1, and Thrb2 were decreased. These results suggest that the consumption of relevant doses of soy isoflavones during the peripubertal period in males may induce subclinical hypothyroidism, with alterations in the regulation of the HP thyroid axis, modulation of TH synthesis, and peripheral alterations in TH target organs.

Impairment of the Hypothalamus-Pituitary-Thyroid Axis Caused by Naturally Occurring GATA2 Mutations In Vitro

The transcription factor GATA2 regulates gene expression in several cells and tissues, including hematopoietic tissues and the central nervous system. Recent studies revealed that loss-of-function mutations in GATA2 are associated with hematological disorders. Our earlier in vitro studies showed that GATA2 plays an essential role in the hypothalamus–pituitary–thyroid axis (HPT axis) by regulating the genes encoding prepro-thyrotropin-releasing hormone (preproTRH) and thyroid-stimulating hormone β (TSHβ). However, the effect of GATA2 mutants on the transcriptional activity of their promoters remains unelucidated. In this study, we created five human GATA2 mutations (R308P, T354M, R396Q, R398W, and S447R) that were reported to be associated with hematological disorders and analyzed their functional properties, including transactivation potential and DNA-binding capacity toward the preproTRH and the TSHβ promoters. Three mutations (T354M, R396Q, and R398W) within the C-terminal zinc-finger domain reduced the basal GATA2 transcriptional activity on both the preproTRH and the TSHβ promoters with a significant loss of DNA binding affinity. Interestingly, only the R398W mutation reduced the GATA2 protein expression. Subsequent analysis demonstrated that the R398W mutation possibly facilitated the GATA2 degradation process. R308P and S447R mutants exhibited decreased transcriptional activity under protein kinase C compared to the wild-type protein. In conclusion, we demonstrated that naturally occurring GATA2 mutations impair the HPT axis through differential functional mechanisms in vitro.

Thyroid hormone receptor β sumoylation is required for thyrotropin regulation and thyroid hormone production

Thyroid hormone receptor β (THRB) is posttranslationally modified by small ubiquitin-like modifier (SUMO). We generated a mouse model with a mutation that disrupted sumoylation at lysine 146 (K146Q) and resulted in desumoylated THRB as the predominant form in tissues. The THRB K146Q mutant mice had normal serum thyroxine (T4), markedly elevated serum thyrotropin-stimulating hormone (TSH; 81-fold above control), and enlargement of both the pituitary and the thyroid gland. The marked elevation in TSH, despite a normal serum T4, indicated blunted feedback regulation of TSH. The THRB K146Q mutation altered the recruitment of transcription factors to the TSHβ gene promoter, compared with WT, in hyperthyroidism and hypothyroidism. Thyroid hormone content (T4, T3, and rT3) in the thyroid gland of the THRB K146Q mice was 10-fold lower (per gram tissue) than control, despite normal TSH bioactivity. The expression of thyroglobulin and dual oxidase 2 genes in the thyroid was reduced and associated with modifications of cAMP response element-binding protein DNA binding and cofactor interactions in the presence of the desumoylated THRB. Therefore, thyroid hormone production had both TSH-dependent and TSH-independent components. We conclude that THRB sumoylation at K146 was required for normal TSH feedback regulation and TH synthesis in the thyroid gland, by a TSH-independent pathway.

An Update on the Pathophysiology and Diagnosis of Inappropriate Secretion of Thyroid-Stimulating Hormone

Inappropriate secretion of thyroid-stimulating hormone (IST), also known as central hyperthyroidism, is a clinical condition characterized by elevated free thyroxine and triiodothyronine concentrations concurrent with detectable thyroid-stimulating hormone (TSH) concentrations. Similarly, the term syndrome of IST (SITSH) is widely used in Japan to refer to a closely related condition; however, unlike that for IST, an elevated serum free triiodothyronine concentration is not a requisite criterion for SITSH diagnosis. IST or SITSH is an important indicator of resistance to thyroid hormone β (RTHβ) caused by germline mutations in genes encoding thyroid hormone receptor β (TRβ) and TSH-secreting pituitary adenoma. Recent evidence has accumulated for several conditions associated with IST, including RTH without mutations in the TRβ gene (non-TR-RTH), the phenomenon of hysteresis involving the hypothalamus-pituitary-thyroid axis (HPT-axis), methodological interference, and Cushing’s syndrome after surgical resection. However, little information is available on the systematic pathophysiological aspects of IST in previous review articles. This report presents an overview of the recent advances in our understanding of the etiological aspects of IST that are relevant for diagnosis and treatment. Moreover, the report focuses on the potential mechanism of IST caused by hysteresis in the HPT-axis (lagging TSH recovery) in terms of epigenetic regulation.

G ATA2 mediates the negative regulation of the prepro-thyrotropin-releasing hormone gene by liganded T3 receptor β2 in the rat hypothalamic paraventricular nucleus

Thyroid hormone (T3) inhibits thyrotropin-releasing hormone (TRH) synthesis in the hypothalamic paraventricular nucleus (PVN). Although the T3 receptor (TR) β2 is known to mediate the negative regulation of the prepro-TRH gene, its molecular mechanism remains unknown. Our previous studies on the T3-dependent negative regulation of the thyrotropin β subunit (TSHβ) gene suggest that there is a tethering mechanism, whereby liganded TRβ2 interferes with the function of the transcription factor, GATA2, a critical activator of the TSHβ gene. Interestingly, the transcription factors Sim1 and Arnt2, the determinants of PVN differentiation in the hypothalamus, are reported to induce expression of TRβ2 and GATA2 in cultured neuronal cells. Here, we confirmed the expression of the GATA2 protein in the TRH neuron of the rat PVN using immunohistochemistry with an anti-GATA2 antibody. According to an experimental study from transgenic mice, a region of the rat prepro-TRH promoter from nt. -547 to nt. +84 was able to mediate its expression in the PVN. We constructed a chloramphenicol acetyltransferase (CAT) reporter gene containing this promoter sequence (rTRH(547)-CAT) and showed that GATA2 activated the promoter in monkey kidney-derived CV1 cells. Deletion and mutation analyses identified a functional GATA-responsive element (GATA-RE) between nt. -357 and nt. -352. When TRβ2 was co-expressed, T3 reduced GATA2-dependent promoter activity to approximately 30%. Unexpectedly, T3-dependent negative regulation was maintained after mutation of the reported negative T3-responsive element, site 4. T3 also inhibited the GATA2-dependent transcription enhanced by cAMP agonist, 8-bromo-cAMP. A rat thyroid medullary carcinoma cell line, CA77, is known to express the preproTRH mRNA. Using a chromatin immunoprecipitation assay with this cell line where GATA2 expression plasmid was transfected, we observed the recognition of the GATA-RE by GATA2. We also confirmed GATA2 binding using gel shift assay with the probe for the GATA-RE. In CA77 cells, the activity of rTRH(547)-CAT was potentiated by overexpression of GATA2, and it was inhibited in a T3-dependent manner. These results suggest that GATA2 transactivates the rat prepro-TRH gene and that liganded TRβ2 interferes with this activation via a tethering mechanism as in the case of the TSHβ gene.

Liganded T3 receptor β2 inhibits the positive feedback autoregulation of the gene for GATA2, a transcription factor critical for thyrotropin production

The serum concentration of thyrotropin (thyroid stimulating hormone, TSH) is drastically reduced by small increase in the levels of thyroid hormones (T3 and its prohormone, T4); however, the mechanism underlying this relationship is unknown. TSH consists of the chorionic gonadotropin α (CGA) and the β chain (TSHβ). The expression of both peptides is induced by the transcription factor GATA2, a determinant of the thyrotroph and gonadotroph differentiation in the pituitary. We previously reported that the liganded T3 receptor (TR) inhibits transactivation activity of GATA2 via a tethering mechanism and proposed that this mechanism, but not binding of TR with a negative T3-responsive element, is the basis for the T3-dependent inhibition of the TSHβ and CGA genes. Multiple GATA-responsive elements (GATA-REs) also exist within the GATA2 gene itself and mediate the positive feedback autoregulation of this gene. To elucidate the effect of T3 on this non-linear regulation, we fused the GATA-REs at -3.9 kb or +9.5 kb of the GATA2 gene with the chloramphenicol acetyltransferase reporter gene harbored in its 1S-promoter. These constructs were co-transfected with the expression plasmids for GATA2 and the pituitary specific TR, TRβ2, into kidney-derived CV1 cells. We found that liganded TRβ2 represses the GATA2-induced transactivation of these reporter genes. Multi-dimensional input function theory revealed that liganded TRβ2 functions as a classical transcriptional repressor. Then, we investigated the effect of T3 on the endogenous expression of GATA2 protein and mRNA in the gonadotroph-derived LβT2 cells. In this cell line, T3 reduced GATA2 protein independently of the ubiquitin proteasome system. GATA2 mRNA was drastically suppressed by T3, the concentration of which corresponds to moderate hypothyroidism and euthyroidism. These results suggest that liganded TRβ2 inhibits the positive feedback autoregulation of the GATA2 gene; moreover this mechanism plays an important role in the potent reduction of TSH production by T3.