Thyroid disruptors are found in food, atmosphere, soil, and water. These contaminants interfere with the thyroid function through the impairment of thyroid hormone synthesis, plasma transport, peripheral metabolism, transport into the target cells, and thyroid hormone action. It is well known that iodide uptake mediated by the sodium-iodide symporter (NIS) is the first limiting step involved in thyroid hormones production. Therefore, it has been described that several thyroid disruptors interfere with the thyroid function through the regulation of NIS expression and/or activity. Perchlorate, nitrate, and thiocyanate competitively inhibit the NIS-mediated iodide uptake. These contaminants are mainly found in food, water and in the smoke of cigarettes. Although the impact of the human exposure to these anions is highly controversial, some studies indicated their deleterious effects in the thyroid function, especially in individuals living in iodine deficient areas. Considering the critical role of thyroid function and the production of thyroid hormones for growth, metabolism, and development, this review summarizes the impact of the exposure to these NIS-inhibitors on thyroid function and their consequences for human health.

Iodide (I(-)) is an irreplaceable constituent of thyroid hormones and an important regulator of thyroid function, because high concentrations of I(-) down-regulate sodium/iodide symporter (NIS) expression and function. In thyrocytes, activation of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) cascade also inhibits NIS expression and function. Because I(-) excess and PI3K/Akt signaling pathway induce similar inhibitory effects on NIS expression, we aimed to study whether the PI3K/Akt cascade mediates the acute and rapid inhibitory effect of I(-) excess on NIS expression/activity. Here, we reported that the treatment of PCCl3 cells with I(-) excess increased Akt phosphorylation under normal or TSH/insulin-starving conditions. I(-) stimulated Akt phosphorylation in a PI3K-dependent manner, because the use of PI3K inhibitors (wortmannin or 2-(4-Morpholinyl)-8-phenyl-4H-1-benzopyran-4-one) abrogated the induction of I(-) effect. Moreover, I(-) inhibitory effect on NIS expression and function were abolished when the cells were previously treated with specific inhibitors of PI3K or Akt (Akt1/2 kinase inhibitor). Importantly, we also found that the effect of I(-) on NIS expression involved the generation of reactive oxygen species (ROS). Using the fluorogenic probes dihydroethidium and mitochondrial superoxide indicator (MitoSOX Red), we observed that I(-) excess increased ROS production in thyrocytes and determined that mitochondria were the source of anion superoxide. Furthermore, the ROS scavengers N-acetyl cysteine and 2-phenyl-1,2-benzisoselenazol-3-(2H)-one blocked the effect of I(-) on Akt phosphorylation. Overall, our data demonstrated the involvement of the PI3K/Akt signaling pathway as a novel mediator of the I(-)-induced thyroid autoregulation, linking the role of thyroid oxidative state to the Wolff-Chaikoff effect.

In thyroid tumors, reduced radioiodine uptake is associated with worse patient outcome concomitantly with low sodium/iodide symporter (NIS) mRNA expression. Previous studies showed that CpG-island methylation in the NIS proximal promoter does not correlate with mRNA expression. The aim of the study was to identify new CpG-islands within the NIS 5'region and investigate the involvement of their methylation in NIS expression. DNA was obtained from 30 pairs of thyroid samples: tumor (T) and surrounding nontumor (NT) samples. All T samples had reduced NIS mRNA expression compared to NT samples. Methylation degree was quantified by bisulfite sequencing, and NIS expression by real-time PCR and Western blot. Reporter gene assays were performed to determine CpG-island functionality. Tumor cell cultures were treated with 5-Aza demethylating agent to determine NIS expression, methylation status, and (125)I uptake. We identified a new CpG-island2 with 14 CpG sites, located -2152/-1887 relative to ATG site. CpG-island2 was hypermethylated in T compared to NT samples, in both benign and malignant tumor groups. There was a significant inverse correlation between NIS mRNA expression and degree of CpG-island2 methylation in NT and T samples. This sequence increased the expression of a reporter gene; thus, it was considered a new enhancer. Cell culture treatments with 5-Aza reduced CpG-island2 methylation levels concomitantly with restoration of NIS mRNA and protein expression and (125)I uptake. We identified a new distal enhancer, NIS distal enhancer, that regulates gene expression through DNA methylation. This enhancer is hypermethylated in T compared to NT samples and is associated with decreased NIS expression in tumors. This epigenetic deregulation may be an early event in tumorigenesis.

Iodide transport defect (ITD) is an autosomal recessive disorder caused by impaired Na(+)/I(-) symporter (NIS)-mediated active iodide accumulation into thyroid follicular cells. Clinical manifestations comprise a variable degree of congenital hypothyroidism and goiter, and low to absent radioiodide uptake, as determined by thyroid scintigraphy. Hereditary molecular defects in NIS have been shown to cause ITD. Our objective was to perform molecular studies on NIS in a patient with congenital hypothyroidism presenting a clinical ITD phenotype. The genomic DNA encoding NIS was sequenced, and an in vitro functional study of a newly identified NIS mutation was performed. The analysis revealed the presence of an undescribed homozygous C to T transition at nucleotide -54 (-54C>T) located in the 5'-untranslated region in the NIS sequence. Functional studies in vitro demonstrated that the mutation was associated with a substantial decrease in iodide uptake when transfected into Cos-7 cells. The mutation severely impaired NIS protein expression, although NIS mRNA levels remained similar to those in cells transfected with wild-type NIS, suggesting a translational deficiency elicited by the mutation. Polysome profile analysis demonstrated reduced levels of polyribosomes-associated mutant NIS mRNA, consistent with reduced translation efficiency. We described a novel mutation in the 5'-untranslated region of the NIS gene in a newborn with congenital hypothyroidism bearing a clinical ITD phenotype. Functional evaluation of the molecular mechanism responsible for impaired NIS-mediated iodide concentration in thyroid cells indicated that the identified mutation reduces NIS translation efficiency with a subsequent decrease in protein expression and function.

Iodide is an important regulator of thyroid activity. Its excess elicits the Wolff-Chaikoff effect, characterized by an acute suppression of thyroid hormone synthesis, which has been ascribed to serum TSH reduction or TGF-β increase and production of iodolipids in the thyroid. These alterations take hours/days to occur, contrasting with the promptness of Wolff-Chaikoff effect. We investigated whether acute iodide administration could trigger events that precede those changes, such as reduction of sodium-iodide symporter (NIS) mRNA abundance and adenylation, and if perchlorate treatment could counteract them. Rats subjected or not to methylmercaptoimidazole treatment (0.03%) received NaI (2,000 μg/0.5 ml saline) or saline intraperitoneally and were killed 30 min up to 24 h later. Another set of animals was treated with iodide and perchlorate, in equimolar doses. NIS mRNA content was evaluated by Northern blotting and real-time PCR, and NIS mRNA poly(A) tail length by rapid amplification of cDNA ends–poly(A) test (RACE-PAT). We observed that NIS mRNA abundance and poly(A) tail length were significantly reduced in all periods of iodide treatment. Perchlorate reversed these effects, indicating that iodide was the agent that triggered the modifications observed. Since the poly(A) tail length of mRNAs is directly associated with their stability and translation efficiency, we can assume that the rapid decay of NIS mRNA abundance observed was due to a reduction of its stability, a condition in which its translation could be impaired. Our data show for the first time that iodide regulates NIS mRNA expression at posttranscriptional level, providing a new mechanism by which iodide exerts its autoregulatory effect on thyroid.