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.
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 cotransfected 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 PLOS ONE | https://doi.org/10.
Introduction: Hyperfunctioning papillary thyroid carcinoma (PTC) is rare and consequently, little information on its molecular etiology is available. Although BRAF V600E (BRAF c.1799T>A, p.V600E) is a prominent oncogene in PTC, its mutation has not yet been reported in hyperfunctioning PTC. Case Presentation: Ultrasonography detected a 26-mm nodule in the right lobe of the thyroid gland of a 48-year-old man. Thyroid function tests indicated that he was hyperthyroid with a TSH level of 0.01 mIU/L (reference range: 0.05–5.00) and a free thyroxine level of 23.2 pmol/L (reference range: 11.6–21.9). TSHR autoantibodies were <0.8 IU/L (reference value: <2.0 IU/L). The 99mTc thyroid scintigram revealed a round, right-sided focus of tracer uptake by the nodule with a decreased uptake in the remainder of the gland. The patient underwent total thyroidectomy because fine-needle aspiration cytology revealed a malignancy. The histopathological diagnosis was conventional PTC. Subsequent mutational analysis of BRAF (exon 15), TSHR (exons 1–10), GNAS (exons 7–10), EZH1 (exon 16), KRAS, NRAS, HRAS (codons 12, 13, and 61), and TERT promoter (C250T and C228T) identified a heterozygous point mutation in BRAF V600E in a tumor tissue sample. In addition, we identified a TSHR D727E polymorphism (TSHR c.2181C>G, p.D727E) in both the tumor and the surrounding normal thyroid tissue. Discussion and Conclusions: We report a case of hyperfunctioning PTC with a BRAF V600E mutation for the first time. Our literature search yielded 16 cases of hyperfunctioning thyroid carcinoma in which a mutational analysis was conducted. We identified TSHR mutations in 13 of these cases. One case revealed a combination of TSHR and KRAS mutations; the other case revealed a TSHR mutation with a PAX8/PPARG rearrangement. These findings suggest that the concomitant activation of oncogenes (in addition to constitutive activation of the TSHR-cyclic AMP cascade) are associated with the malignant phenotype in hyperfunctioning thyroid nodules.
T3 negatively regulates thyrotropin-releasing hormone (TRH) secreted from hypothalamus paraventricular nucleus (PVN). As T3 receptor (TR) _2 is known to mediate the negative regulation of prepro-TRH gene, we previously investigated the T3-dependent negative regulation of the prepro-TRH gene. We reported that T3-bound TR_2 inhibits the transcription of this gene induced by transcription factor GATA2 via tethering mechanism but not negative T3-responsive element (site4), as in the case of thyrotropin (TSH) _ gene (Vitam Horm. 106:97-127, 2018). We also confirmed that GATA2 is expressed in the TRH neuron in PVN. Interestingly, it was reported that T3-dependent repression of TSH_ gene is blunted in the mice whose steroid receptor coactivator (SRC)-1 and/or 2 were genetically ablated (Endocrinology 143(4) 1554-1557, 2002), suggesting the involvement of SRCs in the T3-dependent negative regulation. Indeed, T3-dependent inhibition of TSH_ secretion is impaired in the knock-in mice, of which TR_1 was substituted with mutant TR_1 E457A, a mutant lacking for the interaction with SRC-1 without defect with T3-binding (J. Clin. Invest. 115(9)2517-2523, 2005). Because similar TR_1 mutants have been reported (Science. 12;280(5370):1747-9, 1998), we attempted here to study their function in the context of the negative regulation of the prepro-TRH promoter in monkey kidney-derived CV-1 cells. Compared with wild-type TR_2, T3-dependent inhibition of the CAT-reporter gene, which was fused with prepro-TRH promoter (nt. -547 to nt. +84bp ), was relieved in all the mutant TR_2s including V337R, K341A, I355R, L507R, L509R, E510K and E510A (corresponding to E457A in TR_1). Unexpectedly, however, over-expression of SRC-1 and SRC-2 did not affect the T3-dependent inhibition mediated by wild-type TR_2. These results indicate that negative regulation of preproTRH gene by T3 may be mediated by unknown factor that is able to interact with the surface of T3-bound TR_2 as in the case of SRC-1 or 2. Alternatively, effect by over-expressed SRC-1 and 2 may be quickly metabolized by ubiquitin-proteasome system or exported to cytosol as previously reported. We are currently investigating the effect of SI-2, bufalin and gossypol, which were reported to degradate the SRC proteins potently (PNAS 113(18) 4970-4975, 2016 and references therein).
Background: Hyperfunctioning papillary thyroid carcinoma (PTC) is a rare tumor and accounts for less than 0.1% of all thyroid tumors. Information about its driver mutations is limited. Our literature search yielded 16 cases wherein a mutational analysis was conducted. Thyrotropin receptor (TSHR) mutations were identified in 11 of these cases. One case revealed a combination of TSHR and KRAS mutations. No mutations were identified in the other four cases. BRAFV600E is a prominent oncogene in PTC; however, hyperfunctioning PTC with this mutation has not yet been reported. Clinical Case: In a 48-year-old man, ultrasonography (US) during an annual medical checkup revealed a nodule at the right lobe of the thyroid gland. He visited the outpatient clinic for further evaluation. Thyroid function tests indicated that he was hyperthyroid with TSH level of 0.01 mIU/L (reference range: 0.05-5.00), free thyroxine level of 1.8 ng/dL (reference range: 0.9-1.7), and free triiodothyronine level of 4.3 pg/mL (reference range: 2.3-4.0). Serum thyroglobulin was 62.1 ng/mL (reference range: <33.7) and TSHR autoantibodies (TRAb) was <0.8 IU/L (reference range: <2.0 IU/L). B-mode US revealed a hypoechoic, heterogeneous nodule with largest diameter of 25 mm, and it had a jagged border and microcalcification. Color Doppler US revealed increased intranodular vascularity. The 99mTc thyroid scintigram revealed a round, right-sided focus of tracer uptake by the nodule with suppression in the remainder of the gland. These findings were consistent with an autonomously-functioning thyroid nodule. The patient underwent total thyroidectomy because fine-needle aspiration cytology revealed a malignant cytological diagnosis. The histopathological diagnosis of the patient was PTC, tall cell variant, pT2, pEx0, pN1b, and M0. Subsequent mutational analysis of BRAF (exon 15), TSHR (exons 9 and 10), GNAS (exons 7-10), KRAS, NRAS, HRAS (codons 12, 13, and 61), and TERT promoter (C250T and C228T) only identified a heterozygous point mutation in BRAFV600E in tissue samples. Conclusion: We report for the first time a case of hyperfunctioning papillary thyroid carcinoma with a BRAF mutation.
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