Selenium Increases Thyroid-Stimulating Hormone-Induced Sodium/Iodide Symporter Expression Through Thioredoxin/Apurinic/Apyrimidinic Endonuclease 1-Dependent Regulation of Paired Box 8 Binding Activity.

AIMS: The sodium-iodide symporter (NIS) mediates the uptake of I(-) by the thyroid follicular cell and is essential for thyroid hormone biosynthesis. Nis expression is stimulated by thyroid-stimulating hormone (TSH) and also requires paired box 8 (Pax8) to bind to its promoter. Pax8 binding activity depends on its redox state by a mechanism involving thioredoxin/thioredoxin reductase-1 (Txn/TxnRd1) reduction of apurinic/apyrimidinic endonuclease 1 (Ape1). In this study, we investigate the role of Se in Nis expression. RESULTS: Selenium increases TSH-induced Nis expression and activity in rat thyroid cells. The stimulatory effect of Se occurs at the transcriptional level and is only observed for Nis promoters containing a Pax8 binding site in the Nis upstream enhancer, suggesting that Pax8 is involved in this effect. In fact, Se increases Pax8 expression and its DNA-binding capacity, and in Pax8-silenced rat thyroid cells, Nis is not Se responsive. By inhibiting Ape1 and TxnRd1 functions, we found that both enzymes are crucial for TSH and TSH plus Se stimulation of Pax8 activity and mediate the Nis response to Se treatment. INNOVATION: We describe that Se increases Nis expression and activity. We demonstrate that this effect is dependent on the redox functions of Ape1 and Txn/TxnRd1 through control of the DNA binding activity of Pax8. CONCLUSION: Nis expression is controlled by Txn/Ape1 through a TSH/Se-dependent mechanism. These findings open a new field of study regarding the regulation of Nis activity in thyroid cells. Antioxid. Redox Signal. 24, 855-866.

Regulation of thyroid oxidative state by thioredoxin reductase has a crucial role in thyroid responses to iodide excess.

The phenomenon that supraphysiological doses of iodide (I(-)) temporarily inhibit thyroid hormone synthesis is known as thyroid iodide autoregulation. Recovery of thyroid function has been attributed to sodium-iodide symporter (NIS) inhibition, but the diversity of available data makes it difficult to reach definitive conclusions. Iodide excess induces reactive oxygen species production and cell toxicity. However, the roles of the oxidative state of the cell and antioxidant selenoproteins in I(-) autoregulation have never been explored. Here we analyze the effects of high I(-) doses in rat thyroids and in PCCl3 cells in the period comprising I(-) autoregulation (i.e. 0-72 h after I(-) administration), focusing on NIS expression, redox state, and the expression and activity of selenoproteins. Our results show that NIS mRNA inhibition by I(-) does not occur at the transcriptional level, because neither NIS promoter activity nor Pax8 expression or its binding to DNA was modulated. Because I(-) uptake was inhibited much earlier than NIS protein, and no effect was observed on its subcellular localization, we suggest that I(-) is inhibiting NIS in the plasma membrane. The increased reactive oxygen species production leads to an increase in thioredoxin reductase mRNA levels and enzyme activity, which reduces the oxidative stress. Inhibition of thioredoxin reductase at either gene expression or activity levels prevented NIS recovery, thus illustrating a new role played by this selenoprotein in the regulation of cell homeostasis and consequently in I(-) autoregulation.

Large-scale transcriptome analyses reveal new genetic marker candidates of head, neck, and thyroid cancer.

A detailed genome mapping analysis of 213,636 expressed sequence tags (EST) derived from nontumor and tumor tissues of the oral cavity, larynx, pharynx, and thyroid was done. Transcripts matching known human genes were identified; potential new splice variants were flagged and subjected to manual curation, pointing to 788 putatively new alternative splicing isoforms, the majority (75%) being insertion events. A subset of 34 new splicing isoforms (5% of 788 events) was selected and 23 (68%) were confirmed by reverse transcription-PCR and DNA sequencing. Putative new genes were revealed, including six transcripts mapped to well-studied chromosomes such as 22, as well as transcripts that mapped to 253 intergenic regions. In addition, 2,251 noncoding intronic RNAs, eventually involved in transcriptional regulation, were found. A set of 250 candidate markers for loss of heterozygosis or gene amplification was selected by identifying transcripts that mapped to genomic regions previously known to be frequently amplified or deleted in head, neck, and thyroid tumors. Three of these markers were evaluated by quantitative reverse transcription-PCR in an independent set of individual samples. Along with detailed clinical data about tumor origin, the information reported here is now publicly available on a dedicated Web site as a resource for further biological investigation. This first in silico reconstruction of the head, neck, and thyroid transcriptomes points to a wealth of new candidate markers that can be used for future studies on the molecular basis of these tumors. Similar analysis is warranted for a number of other tumors for which large EST data sets are available.

[Biological markers in thyroid tumors]. / Marcadores biológicos de tumores tiroidianos.

Thyroid tumors originate from two cell types: 1) medullar carcinoma from parafolicullar cells and 2) the tumors derived from follicular epithelial cells, which include multinodular goiter, adenomas, differentiated carcinomas (papillary and follicular carcinoma) and undifferentiated carcinoma (anaplastic carcinoma). Because of the tumors distinct biological behavior, there is a requirement for a specific therapeutic approach. Some thyroid cancer specific mutations have been identified using molecular biology and more recently, genomic methodology. We now understand much of the alterations that occur in the expression of growth factors, receptors and the intracellular signaling pathway. However, none of these have yet proven to be efficient as a marker for diagnosis and prognosis, nor are they helpful in establishing a targeted therapeutic approach. In this review, we will discuss the main aspects of thyroid tumorigenesis and evaluate the potential of these factors as markers for thyroid follicular neoplasia.

A transcript finishing initiative for closing gaps in the human transcriptome.

We report the results of a transcript finishing initiative, undertaken for the purpose of identifying and characterizing novel human transcripts, in which RT-PCR was used to bridge gaps between paired EST clusters, mapped against the genomic sequence. Each pair of EST clusters selected for experimental validation was designated a transcript finishing unit (TFU). A total of 489 TFUs were selected for validation, and an overall efficiency of 43.1% was achieved. We generated a total of 59,975 bp of transcribed sequences organized into 432 exons, contributing to the definition of the structure of 211 human transcripts. The structure of several transcripts reported here was confirmed during the course of this project, through the generation of their corresponding full-length cDNA sequences. Nevertheless, for 21% of the validated TFUs, a full-length cDNA sequence is not yet available in public databases, and the structure of 69.2% of these TFUs was not correctly predicted by computer programs. The TF strategy provides a significant contribution to the definition of the complete catalog of human genes and transcripts, because it appears to be particularly useful for identification of low abundance transcripts expressed in a restricted set of tissues as well as for the delineation of gene boundaries and alternatively spliced isoforms.

Marcadores biológicos de tumores tiroidianos / Biological markers in thyroid tumors

Um marcador biológico ideal deve ser específico e sensível para identificar o tipo tumoral e caracterizar o estágio da progressão neoplásica. Os tumores de tiróide originam-se de dois tipos celulares: 1) carcinoma medular originário de células parafoliculares; e 2) as neoplasias de células epiteliais foliculares, que incluem bócio, adenomas, carcinomas diferenciados (carcinoma papilífero e carcinoma folicular) e carcinoma indiferenciado (carcinoma anaplásico). O comportamento biológico distinto faz com que cada tipo tumoral necessite de uma conduta terapêutica específica. O conhecimento acumulado ao longo destes anos, utilizando métodos de biologia molecular e, mais recentemente, a genômica, identificou mutações específicas de câncer de tiróide e, atualmente, entendemos muito das alterações que ocorrem na expressão de fatores de crescimento, seus receptores e proteínas sinalizadoras intracelular nas neoplasias tiroidianas. Contudo, apesar desses, até o momento não dispomos de um marcador eficiente que auxilie no diagnóstico e prognóstico e, conseqüentemente, para indicação de uma terapêutica mais adequada. Nesta revisão, discutiremos os principais aspectos relacionados à tumorigênese tiroidiana, avaliando o potencial destes fatores como marcador em neoplasia folicular de tiróide.