Dietary iodide controls its own absorption through post-transcriptional regulation of the intestinal Na+/I- symporter

Combined Vorinostat and Chloroquine Inhibit Sodium-Iodide Symporter Endocytosis and Enhance Radionuclide Uptake In Vivo

PURPOSE

Patients with aggressive thyroid cancer are frequently failed by the central therapy of ablative radioiodide (RAI) uptake, due to reduced plasma membrane (PM) localization of the sodium/iodide symporter (NIS). We aimed to understand how NIS is endocytosed away from the PM of human thyroid cancer cells, and whether this was druggable in vivo.

EXPERIMENTAL DESIGN

Informed by analysis of endocytic gene expression in patients with aggressive thyroid cancer, we used mutagenesis, NanoBiT interaction assays, cell surface biotinylation assays, RAI uptake, and NanoBRET to understand the mechanisms of NIS endocytosis in transformed cell lines and patient-derived human primary thyroid cells. Systemic drug responses were monitored via 99mTc pertechnetate gamma counting and gene expression in BALB/c mice.

RESULTS

We identified an acidic dipeptide within the NIS C-terminus that mediates binding to the σ2 subunit of the Adaptor Protein 2 (AP2) heterotetramer. We discovered that the FDA-approved drug chloroquine (CQ) modulates NIS accumulation at the PM in a functional manner that is AP2 dependent. In vivo, CQ treatment of BALB/c mice significantly enhanced thyroidal uptake of 99mTc pertechnetate in combination with the histone deacetylase (HDAC) inhibitor vorinostat/SAHA, accompanied by increased thyroidal NIS mRNA. Bioinformatic analyses validated the clinical relevance of AP2 genes with disease-free survival in RAI-treated DTC, enabling construction of an AP2 gene-related risk score classifier for predicting recurrence.

CONCLUSIONS

NIS internalization is specifically druggable in vivo. Our data, therefore, provide new translatable potential for improving RAI therapy using FDA-approved drugs in patients with aggressive thyroid cancer. See related commentary by Lechner and Brent, p. 1220.

Mechanism of multi-organ compensation under different iodine intake in pregnant rats: results from a repeated-measures study of iodine metabolism

PURPOSE

This study aimed to explore the differences in iodine metabolism and expression of NIS and Pendrin in pregnant rats under different iodine nutritional status.

METHODS

Female Wistar rats were divided into four groups: low iodine (LI), normal iodine (NI), ten fold high iodine (10HI), and fifty fold high iodine (50HI). The intervention began after one week of adaptive feeding. Iodine metabolism experiments were performed beginning on the 15th day of pregnancy. 24-h iodine intake and excretion were calculated. The concentrations of iodine in urine, fecal, thyroid, and placenta were measured by ICP-MS. PCR and Western Blot were used to detect the mRNA levels and cell membrane protein of sodium/iodide symporter (NIS) and Pendrin in the small intestine, thyroid, kidney, and placenta.

RESULTS

Fecal iodine excretion (FIE) and urinary iodine excretion (UIE) in the 50HI group were significantly higher than those in the NI group (P < 0.05). The NIS protein and mRNA in the kidney and small intestine have an upward trend in iodine deficiency and a downward trend in iodine excess. Thyroid and placental iodine storage in the 50HI group were significantly higher than those in the NI group (P < 0.05). NIS, Pendrin protein, and mRNA in the thyroid and placenta tend to increase when iodine is deficient and decrease when there is excess.

CONCLUSION

Iodine excretion and iodine stores in the placenta and thyroid gland are positively correlated with iodine intake. NIS and Pendrin are also regulated by iodine intake.

The master role of polarized NIS expression in regulating iodine metabolism in the human body

Objective

The aim of this study was to investigate how polarized sodium iodide symporter (NIS) expression may regulate iodide metabolism in vivo.

Materials and methods

Polarized NIS expression was analyzed in tissues that accumulate iodide by the use of immunohistochemistry and polyclonal antibody against the C-terminal end of human NIS (hNIS).

Results

Iodide absorption in the human intestine occurs via NIS expressed in the apical membrane. Iodide is secreted into the lumen of the stomach and salivary glands via NIS expressed in the basolateral membrane and then circulates back from the small intestine to the bloodstream via NIS expressed in the apical membrane.

Conclusion

Polarized NIS expression in the human body regulates intestinal-bloodstream recirculation of iodide, perhaps prolonging the availability of iodide in the bloodstream. This leads to more efficient iodide trapping by the thyroid gland. Understanding the regulation and manipulating gastrointestinal iodide recirculation could increase radioiodine availability during theranostic NIS applications.

Iodine Deficiency and Iodine Prophylaxis: An Overview and Update

The thyroid gland requires iodine to synthesize thyroid hormones, and iodine deficiency results in the inadequate production of thyroxine and related thyroid, metabolic, developmental, and reproductive disorders. Iodine requirements are higher in infants, children, and during pregnancy and lactation than in adult men and non-pregnant women. Iodine is available in a wide range of foods and water and is susceptible to almost complete gastric and duodenal absorption as an iodide ion. A healthy diet usually provides a daily iodine consumption not exceeding 50% of the recommended intake. Iodine supplementation is usually necessary to prevent iodine deficiency disorders (IDDs), especially in endemic areas. The community-based strategy of iodine fortification in salt has eradicated IDDs, such as endemic goiter and cretinism, in countries providing adequate measures of iodine prophylaxis over several decades in the 20th century. Iodized salt is the cornerstone of iodine prophylaxis in endemic areas, and the continuous monitoring of community iodine intake and its related clinical outcomes is essential. Despite the relevant improvement in clinical outcomes, subclinical iodine deficiency persists even in Western Europe, especially among girls and women, being an issue in certain physiological conditions, such as pregnancy and lactation, and in people consuming unbalanced vegetable-based or salt-restricted diets. Detailed strategies to implement iodine intake (supplementation) could be considered for specific population groups when iodized salt alone is insufficient to provide adequate requirements.

PROBLEMATIC ASPECTS, COMPLICATIONS, MISCONCEPTIONS AND DEBATABLE ISSUES OF IODINE PROPHYLAXIS IN RADIATION EVENTS (REVIEW)

Prerequisite. Since the advent of nuclear energy, industry and weapons, a possibility of radiation events i.e. incidents and accidents had emerged. Given the presence of radioactive iodine as part of environmental contamination, the response of authorities and medical services consists, in particular, in carrying out the emergency iodine prophylaxis among specialists and general population. And along with the fact that emergency iodine prophylaxis is a generally accepted measure in radiation events accompanied by the release of radioactive iodine, some methods of its implementation were and remain in certain sources and instructions/recommendations contradictory and even false. Such inconsistency increases the potential risks of health effects of radioactive iodine and exacerbates the sense of fear and uncertainty among the population involved in the incident.

OBJECTIVE

to consider and review the essence of emergency iodine prophylaxis during radiation events, physiological aspects of iodine metabolism in the body, properties of individual iodine prophylaxis agents that are recommended, and to justify the rationality of using some of them along with absurdity/inadmissibility of others; substantiate the creation of a unified preventive information strategy regarding the event in order to reduce anxiety and other negative psychological consequences among the affected population.

MATERIALS AND METHODS

The review was performed by searching the abstract and scientometric databases and printed publications.

RESULTS

In the event of serious radiation events at nuclear power plants and industry facilities, radioactive iodine is highly likely to enter the environment. With the threat of radioactive iodine incorporation or with its incorporation that has already begun, it is absolutely necessary to carry out the emergency iodine prophylaxis. Such prevention should be carried out with stable iodine preparations such as potassium iodide or potassium iodate in special pharmaceutical formulas. Dosing of drugs in age and population groups should be carried out by specialists in radiation medicine and radiation safety in accordance with internationally recognized guidelines. The use of iodinecontaining food additives, iodine solution for external use and Lugol's solution is categorically unacceptable due to complete ineffectiveness, impracticality of implementation, and sometimes due to the threat of serious harm to health.

CONCLUSIONS

Clear preparedness plans for possible radiation accidents and incidents, as well as successfullyimplemented appropriate preventive measures, including emergency iodine prophylaxis, are crucial for the effective and successful response to such events. Emergency iodine prophylaxis during radiation events should be carried out exclusively under the guidance of specialists in radiation medicine and radiation safety using special pharmaceutical formulas of potassium iodide or potassium iodate in doses recognized by the international scientific community. Other means of emergency iodine prophylaxis, including «handicraft»/home preparations, are absolutely unacceptable. Implementation of this protective measure should be accompanied by a coordinated information campaign in order to minimize purely radiation risks and to preserve the psychological well-being of the population.

Nutritional iodine status and obesity

Iodine is an essential component of the thyroid hormones, thyroxine and triiodothyronine. Its availability strictly depends on iodine content of foods, which may vary from very low to very high. Inadequate iodine intake (deficiency or excess) may affect thyroid function resulting in hypothyroidism or hyperthyroidism. Based on median urinary iodine concentrations, epidemiological criteria have been established for the categorization and monitoring of nutritional iodine status of a population (or subgroups of populations). Additional methods for iodine intake assessment include measurement of thyroid size (by thyroid palpation or ultrasonography) and of biochemical parameters, such as neonatal thyroid stimulating hormone, thyroglobulin and thyroid hormones.

Recent studies carried out in overweight/obese children and adults provide evidence that body mass index (BMI) may significantly influence the above indicators, thus theoretically affecting the epidemiological evaluation of nutritional iodine status in populations.

In this short review, we analyze current knowledge on the effects of overweight and obesity on indicators of adequacy and monitoring of iodine status, namely urinary iodine excretion and thyroid volume and echogenicity.

Data on urinary iodine excretion in overweight/obese children are divergent, as both increased and reduced levels have been reported in overweight/obese children compared to normal-weight controls.

Whether gastrointestinal surgery may affect iodine absorption and lead to iodine deficiency in patients undergoing bariatric surgery has been evaluated in a limited number of studies, which excluded iodine deficiency, thus suggesting that supplements usually recommended after bariatric surgery do not need to include iodine.

Albeit limited, evidence on thyroid volume and obesity is consistent with a direct relationship between thyroid volume and BMI, irrespective of nutritional iodine status. Finally, a higher frequency of thyroid hypoechoic pattern has been described in overweight/obese children. This finding has been recently related to an increased adipocyte infiltration and thyroid parenchyma imbibition mediated by inflammatory cytokines and should be considered when the frequency of thyroid hypoechoic pattern is used as non-invasive marker to indirectly assess thyroid autoimmunity in monitoring Universal Salt Iodization programs. Further studies, specifically addressing the role of schoolchildren body mass index as a factor potentially influencing iodine intake indicators are needed.

Na+/I- symporter expression, function, and regulation in non-thyroidal tissues and impact on thyroid cancer therapy

For the past 80 years, radioiodine (131I) has been used to ablate thyroid tissue not removed by surgery or to treat differentiated thyroid cancer that has metastasized to other parts of the body. However, the Na+/I- symporter (NIS), which mediates active iodide uptake into thyroid follicular cells, is also expressed in several non-thyroidal tissues. This NIS expression permits 131I accumulation and radiation damage in these non-target tissues, which accounts for the adverse effects of radioiodine therapy. We will review the data regarding the expression, function, and regulation of NIS in non-thyroidal tissues and explain the seemingly paradoxical adverse effects induced by 131I, the self-limited gastrointestinal adverse effects in contrast to the permanent salivary dysfunction that is seen after 131I therapy. We propose that prospective studies are needed to uncover the time-course of pathological processes underlying development and progression or ultimate resolution of 131I-induced salivary ductal obstruction and nasolacrimal duct obstruction. Finally, preventive measures and early therapeutic interventions that can be applied potentially to eliminate or alleviate long-term radioiodine adverse effects will be discussed.

Serum Iodine Is Correlated with Iodine Intake and Thyroid Function in School-Age Children from a Sufficient-to-Excessive Iodine Intake Area

BACKGROUND

An alternative feasible and convenient method of assessing iodine intake is needed.

OBJECTIVE

The aim of this study was to examine the utility of serum iodine for assessing iodine intake in children.

METHODS

One blood sample and 2 repeated 24-h urine samples (1-mo interval) were collected from school-age children in Shandong, China. Serum free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone (TSH), thyroglobulin (Tg), total iodine (StI), and non-protein-bound iodine (SnbI) concentrations and urine iodine (UIC) and creatinine (UCr) concentrations were measured. Iodine intake was estimated based on two 24-h urine iodine excretions (24-h UIE). Associations between serum iodine and other factors were analyzed using the Spearman rank correlation test. Receiver operating characteristic (ROC) curves were used to illustrate diagnostic ability of StI and SnbI.

RESULTS

In total, 1686 children aged 7-14 y were enrolled. The median 24-h UIC for the 2 collections was 385 and 399 μg/L, respectively. The median iodine intake was estimated to be 299 μg/d and was significantly higher in boys than in girls (316 μg/d compared with 283 μg/d; P < 0.001). StI and SnbI were both positively correlated with FT4 (ρ = 0.30, P < 0.001; and ρ = 0.21, P < 0.001), Tg (ρ = 0.21, P < 0.001; and ρ = 0.19, P < 0.001), 24-h UIC (ρ = 0.56, P < 0.001; and ρ = 0.47, P < 0.001), 24-h UIE (ρ = 0.46, P < 0.001; and ρ = 0.49, P < 0.001), urine iodine-to-creatinine ratio (ρ = 0.58, P < 0.001; and ρ = 0.62, P < 0.001), and iodine intake (ρ = 0.49, P < 0.001; and ρ = 0.53, P < 0.001). The areas under the ROC curves for StI and SnbI for the diagnosis of excessive iodine intake in children were 0.76 and 0.77, respectively. The optimal StI and SnbI threshold values for defining iodine excess in children were 101 and 56.2 μg/L, respectively.

CONCLUSIONS

Serum iodine was positively correlated with iodine intake and the serum FT4 concentration in children. It is a potential biomarker for diagnosing excessive iodine intake in children. This trial was registered at clinicaltrials.gov as NCT02915536.

Risks and benefits of consuming edible seaweeds

Recent interest in seaweeds as a source of macronutrients, micronutrients, and bioactive components has highlighted prospective applications within the functional food and nutraceutical industries, with impetus toward the alleviation of risk factors associated with noncommunicable diseases such as obesity, type 2 diabetes, and cardiovascular disease. This narrative review summarizes the nutritional composition of edible seaweeds; evaluates the evidence regarding the health benefits of whole seaweeds, extracted bioactive components, and seaweed-based food products in humans; and assesses the potential adverse effects of edible seaweeds, including those related to ingestion of excess iodine and arsenic. If the potential functional food and nutraceutical applications of seaweeds are to be realized, more evidence from human intervention studies is needed to evaluate the nutritional benefits of seaweeds and the efficacy of their purported bioactive components. Mechanistic evidence, in particular, is imperative to substantiate health claims.

Effects of perinatal di (2-ethylhexyl) phthalate exposure on thyroid function in rat offspring

Di (2-ethylhexyl) phthalate (DEHP) is a commonly used plasticizer in industry and displays the characteristics of an endocrine disruptor. Disorders of the maternal thyroid hormone (TH) during pregnancy can cause adverse effects on the fetus. We investigated the effects and possible mechanism of perinatal DEHP exposure on the thyroid function of pups. Pregnant female Wistar rats were randomly divided into four groups and received doses of DEHP of 0, 30, 300, 750 mg/kg/day by gavage at from gestational day (GD) 0 to postnatal day (PN) 21. The concentration of serum THs and the ultrastructure of thyroid follicular cells in the offspring were examined. Related protein level and gene expression of thyroid proteins in pups were analyzed by western blotting and real-time PCR. We found that DEHP significantly reduced total thyroxine (TT₄) and increased thyroid stimulating hormone (TSH) in pups, while total triiodothyronine (TT₃) showed no change. Thyroid follicular cells ultrastructure was damaged in DEHP exposed pups as viewed by electron microscopy. Furthermore, exposure to DEHP significantly increased protein and mRNA levels of thyroid transcription factor 1 (TTF-1), paired box 8 (PAX8), sodium iodide symporter (NIS) and thyroid peroxidase (TPO) in pups. In addition, levels of deiodinases of pups were also affected. These findings indicated that DEHP can disrupt thyroid function by damaging thyroid follicles and affecting TTF-1, PAX8, NIS, TPO and the deiodinase protein family.

Implications of Na+/I- Symporter Transport to the Plasma Membrane for Thyroid Hormonogenesis and Radioiodide Therapy

Iodine is a crucial component of thyroid hormones; therefore, a key requirement for thyroid hormone biosynthesis is that iodide (I⁻) be actively accumulated in the thyroid follicular cell. The ability of the thyroid epithelia to concentrate I⁻ is ultimately dependent on functional Na⁺/ I⁻ symporter (NIS) expression at the plasma membrane. Underscoring the significance of NIS for thyroid physiology, loss-of-function mutations in the NIS-coding SLC5A5 gene cause an I⁻ transport defect, resulting in dyshormonogenic congenital hypothyroidism. Moreover, I⁻ accumulation in the thyroid cell constitutes the cornerstone for radioiodide ablation therapy for differentiated thyroid carcinoma. However, differentiated thyroid tumors often exhibit reduced (or even undetectable) I⁻ transport compared with normal thyroid tissue, and they are diagnosed as cold nodules on thyroid scintigraphy. Paradoxically, immunohistochemistry analysis revealed that cold thyroid nodules do not express NIS or express normal, or even higher NIS levels compared with adjacent normal tissue, but NIS is frequently intracellularly retained, suggesting the presence of posttranslational abnormalities in the transport of the protein to the plasma membrane. Ultimately, a thorough comprehension of the mechanisms that regulate NIS transport to the plasma membrane would have multiple implications for radioiodide therapy, opening the possibility to identify new molecular targets to treat radioiodide-refractory thyroid tumors. Therefore, in this review, we discuss the current knowledge regarding posttranslational mechanisms that regulate NIS transport to the plasma membrane under physiological and pathological conditions affecting the thyroid follicular cell, a topic of great interest in the thyroid cancer field.

Na+ coordination at the Na2 site of the Na+/I- symporter

The sodium/iodide symporter (NIS) mediates active I(-) transport in the thyroid-the first step in thyroid hormone biosynthesis-with a 2 Na(+): 1 I(-) stoichiometry. The two Na(+) binding sites (Na1 and Na2) and the I(-) binding site interact allosterically: when Na(+) binds to a Na(+) site, the affinity of NIS for the other Na(+) and for I(-) increases significantly. In all Na(+)-dependent transporters with the same fold as NIS, the side chains of two residues, S353 and T354 (NIS numbering), were identified as the Na(+) ligands at Na2. To understand the cooperativity between the substrates, we investigated the coordination at the Na2 site. We determined that four other residues-S66, D191, Q194, and Q263-are also involved in Na(+) coordination at this site. Experiments in whole cells demonstrated that these four residues participate in transport by NIS: mutations at these positions result in proteins that, although expressed at the plasma membrane, transport little or no I(-) These residues are conserved throughout the entire SLC5 family, to which NIS belongs, suggesting that they serve a similar function in the other transporters. Our findings also suggest that the increase in affinity that each site displays when an ion binds to another site may result from changes in the dynamics of the transporter. These mechanistic insights deepen our understanding not only of NIS but also of other transporters, including many that, like NIS, are of great medical relevance.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Nitric oxide-repressed Forkhead factor FoxE1 expression is involved in the inhibition of TSH-induced thyroid peroxidase levels

Thyroid peroxidase (TPO) is essential for thyroid hormone synthesis mediating the covalent incorporation of iodine into tyrosine residues of thyroglobulin process known as organification. Thyroid-stimulating hormone (TSH) via cAMP signaling is the main hormonal regulator of TPO gene expression. In thyroid cells, TSH-stimulated nitric oxide (NO) production inhibits TSH-induced thyroid-specific gene expression, suggesting a potential autocrine role of NO in modulating thyroid function. Indeed, NO donors downregulate TSH-induced iodide accumulation and organification in thyroid cells. Here, using FRTL-5 thyroid cells as model, we obtained insights into the molecular mechanism underlying the inhibitory effects of NO on iodide organification. We demonstrated that NO donors inhibited TSH-stimulated TPO expression by inducing a cyclic guanosine monophosphate-dependent protein kinase-mediated transcriptional repression of the TPO gene. Moreover, we characterized the FoxE1 binding site Z as mediator of the NO-inhibited TPO expression. Mechanistically, we demonstrated that NO decreases TSH-induced FoxE1 expression, thus repressing the transcripcional activation of TPO gene. Taken together, we provide novel evidence reinforcing the inhibitory role of NO on thyroid cell function, an observation of potential pathophysiological relevance associated with human thyroid pathologies that come along with changes in the NO production.

S-Nitrosylation of NF-κB p65 Inhibits TSH-Induced Na(+)/I(-) Symporter Expression

Nitric oxide (NO) is a ubiquitous signaling molecule involved in a wide variety of cellular physiological processes. In thyroid cells, NO-synthase III-endogenously produced NO reduces TSH-stimulated thyroid-specific gene expression, suggesting a potential autocrine role of NO in modulating thyroid function. Further studies indicate that NO induces thyroid dedifferentiation, because NO donors repress TSH-stimulated iodide (I(-)) uptake. Here, we investigated the molecular mechanism underlying the NO-inhibited Na(+)/I(-) symporter (NIS)-mediated I(-) uptake in thyroid cells. We showed that NO donors reduce I(-) uptake in a concentration-dependent manner, which correlates with decreased NIS protein expression. NO-reduced I(-) uptake results from transcriptional repression of NIS gene rather than posttranslational modifications reducing functional NIS expression at the plasma membrane. We observed that NO donors repress TSH-induced NIS gene expression by reducing the transcriptional activity of the nuclear factor-κB subunit p65. NO-promoted p65 S-nitrosylation reduces p65-mediated transactivation of the NIS promoter in response to TSH stimulation. Overall, our data are consistent with the notion that NO plays a role as an inhibitory signal to counterbalance TSH-stimulated nuclear factor-κB activation, thus modulating thyroid hormone biosynthesis.

Dietary iodine absorption is not influenced by malabsorptive bariatric surgery

BACKGROUND

Bariatric surgery is accompanied by malabsorption of protein, carbohydrates, fats, vitamins, and trace elements. Iodine is essential to the synthesis of thyroid hormones. The aim of this study was to estimate the daily iodine intake in severely obese patients before and after bariatric surgery.

METHODS

Thirty-five severely obese patients (obese group) with a BMI of 51.3 ± 8.3 kg/m(2) were studied before, 3 months, and 6 months after bariatric surgery. Eleven out of 35 patients were subjected to gastric bypass operation Roux-en-Y and 24 were subjected to a variant of biliopancreatic diversion with long limb procedure. The patients did not use any iodine supplements and no iodine antiseptics were administered during the operation. The messmates of the patients, following a similar diet (control group) with a BMI of 31.2 ± 10.7 kg/m(2), were also studied. Serum T3, T4, TSH, thyroid peroxidase antibodies, urinary iodine excretion (UIE) in a spot urine, and thyroid volume were measured in all subjects, at baseline and at 3- and 6-month follow-up in the obese group.

RESULTS

UIE at baseline was similar in obese and control group (median (min-max), 129.5 (24.9-462) vs. 138.9 (30.8-381) μg/L, ns). In the obese group, a transient increase of UIE was observed 3 months after the operation and returned to baseline levels 6-months postsurgery.

CONCLUSIONS

The UIE is not reduced after malabsorptive bariatric surgery, although all stomach, duodenum, and a substantial part of jejunum were bypassed. It appears that iodine is absorbed sufficiently along the remaining gastrointestinal tract.

Sodium/Iodide Symporter Mutant V270E Causes Stunted Growth but No Cognitive Deficiency

CONTEXT

Iodide (I(-)), an essential constituent of the thyroid hormones, is actively accumulated in the thyroid by the Na(+)/I(-) symporter (NIS), a key plasma membrane protein encoded by the slc5a5 gene. Mutations in slc5a5 cause I(-) transport defects (ITDs), autosomal-recessive disorders in which I(-) accumulation is totally or partially impaired, leading to congenital hypothyroidism. The characterization of NIS mutants has yielded significant insights into the molecular mechanism of NIS.

OBJECTIVE

This study aimed to determine the basis of a patient's ITD clinical phenotype, by sequencing her slc5a5 gene.

DESIGN

Genomic DNA was purified and the slc5a5 gene sequence determined. Functional in vitro studies were performed to characterize the V270E NIS mutant.

PATIENT

The index patient was diagnosed with hypothyroidism with minimal radioiodide uptake in a normally located, although enlarged, thyroid gland.

RESULTS

We identified a new NIS mutation: V270E. The patient had the compound heterozygous NIS mutation R124H/V270E. R124H NIS has been characterized previously. We show that V270E markedly reduces I(-) uptake via a pronounced (but not total) impairment of the protein's plasma membrane targeting. Remarkably, V270E is intrinsically active. Therefore, a negative charge at position 270 interferes with NIS cell surface trafficking. The patient's minimal I(-) uptake enabled sufficient thyroid hormone biosynthesis to prevent cognitive impairment.

CONCLUSIONS

A nonpolar residue at position 270, which all members of the SLC5A family have, is required for NIS plasma membrane targeting.

Role of precursor mRNA splicing in nutrient-induced alterations in gene expression and metabolism

Precursor mRNA (pre-mRNA) splicing is a critical step in gene expression that results in the removal of intronic sequences from immature mRNA, leading to the production of mature mRNA that can be translated into protein. Alternative pre-mRNA splicing is the process whereby alternative exons and/or introns are selectively included or excluded, generating mature mRNAs that encode proteins that may differ in function. The resulting alterations in the pattern of protein isoform expression can result in changes in protein-protein interaction, subcellular localization, and flux through metabolic pathways. Although basic mechanisms of pre-mRNA splicing of introns and exons are reasonably well characterized, how these mechanisms are regulated remains poorly understood. The goal of this review is to highlight selected recent advances in our understanding of the regulation of pre-mRNA splicing by nutrients and modulation of nutrient metabolism that result from changes in pre-mRNA splicing.

Mutation screening of the sodium iodide symporter gene in a cohort of 105 China patients with congenital hypothyroidism

OBJECTIVE

Dyshormonogenetic congenital hypothyroidism (CH) was reported to be associated with a mutation in the sodium iodide symporter (NIS) gene. The present study was undertaken in the Guangxi Zhuang Autonomous Region of China, to determine the nature and frequency of NIS gene mutations among patients with CH due to dyshormonogenesis.

SUBJECTS AND METHODS

Blood samples were collected from 105 dyshormonogenetic CH patients in Guangxi Zhuang Autonomous Region, China, and genomic DNA was extracted from peripheral blood leukocytes. All exons of the NIS gene together with their exon-intron boundaries were screened by next-generation sequencing.

RESULTS

Two silent variations (T221T and T557T) and one missense variation (M435L), as well as two polymorphisms (rs200587561 and rs117626343) were found.

CONCLUSIONS

Our results indicate that the NIS mutation rate is very low in the Guangxi Zhuang Autonomous Region, China, and it is necessary to study mutations of other genes that have major effects on thyroid dyshormonogenesis and have not as yet been studied in this population.

Physiological sodium concentrations enhance the iodide affinity of the Na+/I- symporter

The Na(+)/I(-) symporter (NIS) mediates active I(-) transport--the first step in thyroid hormonogenesis--with a 2Na(+):1I(-) stoichiometry. NIS-mediated (131)I(-) treatment of thyroid cancer post-thyroidectomy is the most effective targeted internal radiation cancer treatment available. Here to uncover mechanistic information on NIS, we use statistical thermodynamics to obtain Kds and estimate the relative populations of the different NIS species during Na(+)/anion binding and transport. We show that, although the affinity of NIS for I(-) is low (Kd=224 μM), it increases when Na(+) is bound (Kd=22.4 μM). However, this Kd is still much higher than the submicromolar physiological I(-) concentration. To overcome this, NIS takes advantage of the extracellular Na(+) concentration and the pronounced increase in its own affinity for I(-) and for the second Na(+) elicited by binding of the first. Thus, at physiological Na(+) concentrations, ~79% of NIS molecules are occupied by two Na(+) ions and ready to bind and transport I(-).

Hypermethylation of a New Distal Sodium/Iodide Symporter (NIS) enhancer (NDE) is associated with reduced NIS expression in thyroid tumors

CONTEXT

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.

OBJECTIVES

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.

DESIGN

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.

MAIN OUTCOME MEASURES

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.

RESULTS

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.

CONCLUSIONS

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: implications for health and disease

Disorders of the thyroid gland are among the most common conditions diagnosed and managed by pediatric endocrinologists. Thyroid hormone synthesis depends on normal iodide transport and knowledge of its regulation is fundamental to understand the etiology and management of congenital and acquired thyroid conditions such as hypothyroidism and hyperthyroidism. The ability of the thyroid to concentrate iodine is also widely used as a tool for the diagnosis of thyroid diseases and in the management and follow up of the most common type of endocrine cancers: papillary and follicular thyroid cancer. More recently, the regulation of iodide transport has also been the center of attention to improve the management of poorly differentiated thyroid cancer. Iodine deficiency disorders (goiter, impaired mental development) due to insufficient nutritional intake remain a universal public health problem. Thyroid function can also be influenced by medications that contain iodide or interfere with iodide metabolism such as iodinated contrast agents, povidone, lithium and amiodarone. In addition, some environmental pollutants such as perchlorate, thiocyanate and nitrates may affect iodide transport. Furthermore, nuclear accidents increase the risk of developing thyroid cancer and the therapy used to prevent exposure to these isotopes relies on the ability of the thyroid to concentrate iodine. The array of disorders involving iodide transport affect individuals during the whole life span and, if undiagnosed or improperly managed, they can have a profound impact on growth, metabolism, cognitive development and quality of life.

The acute inhibitory effect of iodide excess on sodium/iodide symporter expression and activity involves the PI3K/Akt signaling pathway

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.

The iodide-transport-defect-causing mutation R124H: a δ-amino group at position 124 is critical for maturation and trafficking of the Na+/I- symporter

Na(+)/I(-) symporter (NIS)-mediated active accumulation of I(-) in thyrocytes is a key step in the biosynthesis of the iodine-containing thyroid hormones T3 and T4. Several NIS mutants have been identified as a cause of congenital I(-) transport defect (ITD), and their investigation has yielded valuable mechanistic information on NIS. Here we report novel findings derived from the thorough characterization of the ITD-causing mutation R124H, located in the second intracellular loop (IL-2). R124H NIS is incompletely glycosylated and colocalizes with endoplasmic reticulum (ER)-resident protein markers. As a result, R124H NIS is not targeted to the plasma membrane and therefore does not mediate any I(-) transport in transfected COS-7 cells. Strikingly, however, the mutant is intrinsically active, as revealed by its ability to mediate I(-) transport in membrane vesicles. Of all the amino acid substitutions we carried out at position 124 (K, D, E, A, W, N and Q), only Gln restored targeting of NIS to the plasma membrane and NIS activity, suggesting a key structural role for the δ-amino group of R124 in the transporter's maturation and cell surface targeting. Using our NIS homology model based on the structure of the Vibrio parahaemolyticus Na(+)/galactose symporter, we propose an interaction between the δ-amino group of either R or Q124 and the thiol group of C440, located in IL-6. We conclude that the interaction between IL-2 and IL-6 is critical for the local folding required for NIS maturation and plasma membrane trafficking.

Asn441 plays a key role in folding and function of the Na+/I- symporter (NIS)

The Na(+)/I(-) symporter (NIS) is a plasma membrane glycoprotein that mediates active I(-) transport in the thyroid, the first step in the biosynthesis of the iodine-containing thyroid hormones T3 and T4. Several NIS mutants have been identified as a cause of congenital I(-) transport defect (ITD), and their investigation has yielded valuable mechanistic information on NIS. Here we report a thorough characterization of the ITD-causing NIS mutation in which the sixth intracellular loop residues 439-443 are missing. This mutant protein was intracellularly retained, incompletely glycosylated, and intrinsically inactive. Engineering 5 Ala at positions 439-443 partially recovered cell surface targeting and activity (∼15%). Strikingly, NIS with the sequence 439-AANAA-443, in which Asn was restored at position 441, was targeted to the plasma membrane and exhibited ∼95% the transport activity of WT NIS. Based on our NIS homology model, we propose that the side chain of N441, a residue conserved throughout most of the SLC5 family, interacts with the main chain amino group of G444, capping the α-helix of transmembrane segment XII and thus stabilizing the structure of the molecule. Our data provide insight into a critical interhelical interaction required for NIS folding and activity.