Iodide transport in the thyroid gland

Human pendrin expressed in Xenopus laevis oocytes mediates chloride/formate exchange

Pendred syndrome, characterized by congenital sensorineural hearing loss and goiter, is one of the most common forms of syndromic deafness. The gene causing Pendred syndrome (PDS) encodes a protein designated pendrin, which is expressed in the thyroid, kidney, and fetal cochlea. Pendrin functions as an iodide and chloride transporter, but its role in the development of hearing loss and goiter is unknown. In this study, we examined the mechanism of pendrin-mediated anion transport in Xenopus laevis oocytes. Unlabeled formate added to the uptake medium inhibited pendrin-mediated (36)Cl uptake in X. laevis oocytes. In addition, the uptake of [(14)C]formate was stimulated in oocytes injected with PDS cRNA compared with water-injected controls. These results indicate that formate is a substrate for pendrin. Furthermore, chloride stimulated the efflux of [(14)C]formate and formate stimulated the efflux of (36)Cl in oocytes expressing pendrin, results consistent with pendrin-mediated chloride/formate exchange. These data demonstrate that pendrin is functionally similar to the renal chloride/formate exchanger, which serves as an important mechanism of chloride transport in the proximal tubule. A similar process could participate in the development of ion gradients within the inner ear.

Estradiol increases proliferation and down-regulates the sodium/iodide symporter gene in FRTL-5 cells

Goiter (increased thyroid gland size) is more prevalent in women than men, even in areas where iodine levels in the diet are sufficient. We investigated a possible role of estrogen on thyroid follicular cell growth using rat FRTL-5 thyroid follicular cells as a model. Estrogen receptor-alpha (ERalpha) messenger RNA was present in FRTL-5 cells using a RT-PCR assay and was confirmed by Western blot analysis. An estrogen-responsive reporter gene was transfected into FRTL-5 cells to test the functionality of the endogenous ERs. Estradiol increased the activity of the reporter gene, and the antagonist, ICI182780, inhibited ER-dependent transcription. To extend this analysis, we examined the effect of estradiol on FRTL-5 cell growth. Estradiol increased FRTL-5 cell growth in a time- and concentration-dependent manner in either the absence or presence of TSH. Because iodine is known to inhibit thyroid cell growth, the effect of estradiol on the expression of the sodium/iodide symporter (NIS) was assessed as a potential target of estrogen action. Estradiol blocked TSH-induced NIS expression, and treatment of cells with estradiol and ICI182780 restored TSH-induced NIS expression to normal levels. These data demonstrate that FRTL-5 cells contain functional ERs that enhance cell growth and inhibit expression of the NIS. The demonstration of a direct effect of estradiol on thyroid follicular cells raises the possibility that it may play a role in the sexually dimorphic prevalence of goiter.

Regulation of sodium iodide symporter gene expression in FRTL-5 rat thyroid cells

The sodium iodide symporter (NIS), first identified in FRTL-5 cells, plays a critical role in iodide transport in the thyroid gland and in the production of the iodine-containing thyroid hormones. The aim of our study was to examine the regulation of NIS RNA steady-state levels and protein expression as well as functional activity in FRTL-5 cells. FRTL-5 cells cycling in media containing thyrotropin (TSH) were incubated for 48 hours with dexamethasone (10(-8)-10(-5) M), triiodothyronine (T3; 10(-9)-10(-6) M), methimazole (100 microM), propylthiouracil (PTU; 100 microM), perchlorate (10 microM) and potassium iodide (40 microM). In other experiments, cells were treated for 48 hours with various cytokines including interleukin-6 (IL-6) (100 U/mL), interferon-gamma (IFN-gamma) (100 U/mL), tumor necrosis factor-alpha (TNF-alpha) (10 ng/ml), IL-1alpha (100 U/mL), and IL-1beta (100 U/mL). Northern blot analysis using a 32P-labeled rat NIS-specific cDNA probe (nucleotides 1397-1937) revealed NIS mRNA as a single species of approximately 3 kb. When normalized for beta-actin mRNA signal intensities, NIS RNA steady-state levels in viable FRTL-5 cells were suppressed by approximately 80% after incubation with dexamethasone and T3 in a concentration-dependent manner. Iodide accumulation was decreased by up to 40% after incubation with dexamethasone and T3, respectively, in a concentration-dependent manner. Using a rabbit polyclonal rNIS-specific antibody, Western blot analysis of FRTL-5 cell membranes revealed a 60% and 70% suppression of NIS protein expression after treatment with T3 (0.1 microM) and dexamethasone (1 microM), respectively. In additon, NIS RNA steady-state levels were decreased by approximately 50% after treatment of monolayers with methimazole, PTU, and potassium iodide, respectively. Incubation with methimazole and PTU resulted in a 20% and 25% decrease of iodide accumulation, respectively, whereas potassium iodide suppressed iodide accumulation by approximately 50%. Treatment of FRTL-5 cells with IL-6 and IL-1beta resulted in a 30% decrease of NIS RNA steady-state levels. IL-6 did not alter NIS functional activity, but IL-1beta suppressed iodide accumulation by approximately 25%. IFN-gamma and perchlorate failed to alter NIS RNA steady-state levels. In contrast to IFN-gamma that had no effect on iodide accumulation, perchlorate almost completely suppressed iodide accumulation. TNF-alpha and IL-1alpha failed to alter NIS RNA steady-state levels in higher passage numbers of FRTL-5 cells, whereas treatment with TNF-alpha and IL-1alpha of early passages of FRTL-5 cells (<20 cell passages) resulted in a 70% and 40% decrease of NIS RNA steady-state levels, respectively, and in a 20% suppression of NIS functional activity. In conclusion, our data suggest that various agents known to affect iodide transport are capable of differentially altering NIS gene expression and function in cultured thyroid cells. Suppression of NIS gene expression and function by certain cytokines may be responsible, at least in part, for the impaired radioiodine uptake by thyroid tissue in certain forms of thyroiditis.

Restoration of iodide uptake in dedifferentiated thyroid carcinoma: relationship to human Na+/I-symporter gene methylation status

Disseminated dedifferentiated thyroid epithelial carcinoma, which cannot sufficiently concentrate therapeutic radioiodide, is a terminal disease without any effective systemic treatment or chemotherapy. This is a likely consequence of loss of human sodium-iodide symporter (hNIS) function. We hypothesized that hNIS transcriptional failure in thyroid carcinoma could be consequent to methylation of DNA in critical regulatory regions and could be reversed with chemical demethylation treatment. Analysis of hNIS messenger ribonucleic acid (mRNA) expression in 23 tumor samples revealed that although loss of this expression corresponded to loss of clinical radioiodide uptake, some thyroid carcinomas with hNIS mRNA expression did not concentrate iodide, suggesting additional posttranscriptional mechanisms for loss of hNIS function. In addition, analysis of DNA methylation in CpG-rich regions of the hNIS promoter extending to the first intron failed to define specific methylation patterns associated with transcriptional failure in human thyroid tumor samples. In seven human thyroid carcinoma cell lines lacking hNIS mRNA, treatment with 5-azacytidine or sodium butyrate was able to restore hNIS mRNA expression in four cell lines and iodide transport in two cell lines. Investigation of methylation patterns in these cell lines revealed that successful restoration of hNIS transcription was associated with demethylation of hNIS DNA in the untranslated region within the first exon. This was also associated with restoration of expression of thyroid transcription factor-1. These results suggest a role for DNA methylation in loss of hNIS expression in thyroid carcinomas as well as a potential application for chemical demethylation therapy in restoring responsiveness to therapeutic radioiodide.

The Pendred syndrome gene encodes a chloride-iodide transport protein

Pendred syndrome is the most common form of syndromic deafness and characterized by congenital sensorineural hearing loss and goitre. This disorder was mapped to chromosome 7 and the gene causing Pendred syndrome (PDS) was subsequently identified by positional cloning. PDS encodes a putative transmembrane protein designated pendrin. Pendrin is closely related to a family of sulfate transport proteins that includes the rat sulfate-anion transporter (encoded by Sat-1; 29% amino acid sequence identity), the human diastrophic dysplasia sulfate transporter (encoded by DTD; 32%) and the human sulfate transporter 'downregulated in adenoma' (encoded by DRA; 45%). On the basis of this homology and the presence of a slightly modified sulfate-transporter signature sequence comprising its putative second transmembrane domain, pendrin has been proposed to function as a sulfate transporter. We were unable to detect evidence of sulfate transport following the expression of pendrin in Xenopus laevis oocytes by microinjection of PDS cRNA or in Sf9 cells following infection with PDS-recombinant baculovirus. The rates of transport for iodide and chloride were significantly increased following the expression of pendrin in both cell systems. Our results demonstrate that pendrin functions as a transporter of chloride and iodide, but not sulfate, and may provide insight into thyroid physiology and the pathophysiology of Pendred syndrome.

Expression of thyroid-related genes in human thymus

There are several thyroid antigens including human sodium iodide symporter (hNIS), thyrotropin receptor (TSH-R), thyroid peroxidase (TPO), and thyroglobulin (Tg) that have been considered to be thyroid-specific proteins involved in the pathogenesis of autoimmune thyroid diseases. We examined the expression of these thyroid-tolerance related genes in normal human thymus, the lymphoid organ responsible for the induction of central T-cell self. Reverse transcription-polymerase chain reaction (RT-PCR) amplifications were performed with 4 pairs of oligonucleotide primers specific for the hNIS, TSH-R, TPO, and Tg genes, respectively. Gene-specific transcripts were confirmed by Southern hybridization using digoxigenin-labeled internal oligonucleotide probes. To monitor cDNA integrity and quantity, all samples were coamplified with a pair of intron-spanning human beta-actin-specific oligonucleotide primers. Furthermore, using a highly sensitive immunostaining technique and antibodies specific for these 4 antigens, we examined whether NIS-, TSH-R-, TPO-, and Tg-specific immunoreactivity can be detected and localized in normal human thymus. RT-PCR and Southern hybridization revealed expression of each of these 4 thyroid-related genes in normal human thymus. In addition, immunohistochemical analysis of frozen tissue sections derived from normal human thymus showed marked immunoreactivity for NIS, TSH-R, and Tg as well as weaker staining for TPO. Control reactions using isotype matched nonimmune immunoglobulins were consistently negative. Taken together, our results suggest that NIS-, TSH-R-, TPO-, and Tg-RNA are present and actively processed to immunoreactive NIS-, TSH-R-, TPO-, and Tg-like protein in human thymus. These data support the concept that pre-T lymphocytes may be educated to recognize thyroid-related epitopes expressed in thymus, and, thus, to generate self-tolerance against these thyroid-related antigens.

Molecular and cellular mechanisms of transepithelial iodide transport in the thyroid

Thyroid hormone is an essential regulator of developmental growth and metabolism in vertebrates. Iodine is a necessary constituent of thyroid hormone. Due to the scarcity and uneven distribution of iodine on the Earth's crust, the structure of the thyroid gland is adjusted to collect and store this element in order to secure a continuous supply of thyroid hormone throughout life. Still, disease resulting from hypothyroidism due to iodine deficiency is a global health problem, illustrating the great biological significance that iodine saving mechanisms have evolved. Iodide is accumulated together with prohormone (thyroglobulin) in the lumen of the thyroid follicles. The rate-limiting step of this transport is the sodium/iodide symporter located in the basolateral plasma membrane of the thyroid follicular cells. Iodide is also transferred across the apical plasma membrane into the lumen where hormonogenesis takes place. In this review, recent progress in the understanding of transepithelial iodide transport in the thyroid is summarized.

Characterization of gastric Na+/I- symporter of the rat

Characterization of gastric Na+/I- symporter (NIS) of the rat was carried out. Sequencing of the open reading frame of gastric NIS mRNA showed only three nucleotide changes when compared with FRTL-5 NIS cDNA, and two of these changes led to amino acid changes. The results of Northern blot analysis showed that abundant NIS mRNA was expressed in the stomach when compared with other organs. Western blot analysis using gastric mucosa and FRTL-5 lysates detected the difference in molecular weight between FRTL-5 and gastric mucosa lysates, suggesting abnormal posttranslational modification of gastric NIS protein. Immunohistochemically, gastric NIS protein was located in the cornification layer of the stratified squamous epithelium of the pars proventricularis and in parietal cells and on the apical border of surface epithelial cells of the pars glandularis. Gastric NIS protein was present in tubulovesicular structures and lysosomes in parietal cells by immunoelectron microscopy. Gastric NIS protein exists to trap I- from the gastric lumen, except in parietal cells. Results indicated that a very large amount of gastric NIS mRNA is expressed to be translated, whereas only a small amount of immature gastric NIS protein is detected. This may indicate that immature gastric NIS protein rapidly degrades to peptides.

Expression of alpha- and beta-subunits and activity of Na+K+ ATPase in pig thyroid cells in primary culture: modulation by thyrotropin and thyroid hormones

Na+ K+ ATPase located at the basolateral pole of thyroid epithelial cells, contributes to thyroid hormone synthesis by generating the driving force for the uptake of the substrate, iodide. We have investigated whether the expression of the alpha- and beta-subunits and activity of Na+ K+ ATPase were subjected to variations in response, (a) to TSH, that controls the expression of differentiation in thyroid cells and (b) to thyroid hormones as potential autocrine factors. Studies were carried out on pig thyroid cells cultured (a) without TSH to obtain thyroid cell monolayers (TCM) in basal state or (b) with TSH in the form of cell monolayers (TCM-T) or as reconstituted thyroid follicles (RTF). Iodide uptake activity, thyroperoxidase protein and thyroglobulin mRNA taken as parameters of thyroid cell differentiation were 6 to 25-fold higher in RTF and TCM-T than in TCM. Western blot analyses of Na+ K+ ATPase subunits revealed that the alpha-subunit (105 kDa) content of TCM-T and RTF was similar but 8-fold higher than that of TCM. In contrast, the beta-subunit (50 kDa) content of TCM-T and RTF was only about twice that of TCM. Similar relative variations were observed at the mRNA level for both alpha- and beta-subunits. Na+ K+ ATPase activity was only 40% higher in RTF and TCM-T than in TCM. A 48 h treatment of RTF by either T4 or T3 (1-100 nM) induced a 3-fold increase of the alpha-subunit but did neither alter the beta-subunit nor the Na+ K+ ATPase activity. In conclusion, Na+ K+ ATPase activity and the level of expression of its beta-subunit, known to control the assembly and targetting of alpha-beta oligomers and thus the amount of functional sodium pump at the plasma membrane, are only moderately altered when thyroid cells undergo major changes in their differentiation status. Our data show that the expression of the alpha-subunit of Na+ K+ ATPase by thyroid cells is up-regulated by TSH and thyroid hormones.

Differences in the electrophysiological response to I- and the inhibitory anions SCN- and ClO-4, studied in FRTL-5 cells

The electrophysiological properties of the Na+/I- symporter (NIS) were examined in a cloned rat thyroid cell line (FRTL-5) using the whole-cell patch-clamp technique. When the holding potential was between -40 mV and -80 mV, 1 mM NaI and NaSCN induced an immediate inward current which was greater with SCN- than with I-. The reversal potential for I- and SCN- induced membrane currents was +50 mV. This is close to the value of +55 mV calculated by the Nernst equation for Na+. These results are consistent with I- and SCN- translocation via the NIS that is energized by the electrochemical gradient of Na+ and coupled to the transport of two or more Na+. There was no change in the membrane current recording with ClO-4 indicating that ClO-4 was either not transported into the cell, or the translocation was electroneutral. ClO-4 addition, however, did reverse the inward currents induced by I- or SCN-. These effects of I-, SCN- and ClO-4 on membrane currents reflect endogenous NIS activity since the responses duplicated those seen in CHO cells transfected with NIS. There were additional currents elicited by SCN- in FRTL-5 cells under certain conditions. For example at holding potentials of 0 and +30 mV, 1 mM SCN- produced an increasingly greater outward current. This outward current was transient. In addition, when SCN- was washed off the cells a transient inward current was detected. Unlike SCN-, 1-10 mM I- had no observable effect on the membrane current at holding potentials of 0 and +30 mV. The results indicate FRTL-5 cells may have a specific SCN- translocation system in addition to the SCN- translocation by the I- porter. Differences demonstrated in current response may explain some of the complicated influx and efflux properties of I-, SCN- and ClO-4 in thyroid cells.

Na+/I- symporter distribution in human thyroid tissues: an immunohistochemical study

Antipeptide antibodies raised against the carboxyl-terminal region of the human sodium/iodide (Na+/I-) symporter (hNIS) were used to investigate by immunohistochemistry the presence and distribution of the hNIS protein in normal thyroid tissues, in some pathological nonneoplastic thyroid tissues, and in different histotypes of thyroid neoplasms. In normal thyroid tissue, staining of hNIS protein was heterogeneous and limited to a minority of follicular cells that were in close contact with capillary vessels. In positive cells, immunostaining was limited to the basolateral membrane. In contrast, in Graves' disease the majority of follicular cells expressed the hNIS protein. In autoimmune thyroiditis, the number of hNIS-positive cells, was similar to that found in normal tissue. These positive cells were found essentially close to lymphocytic infiltrates. This observation supports the concept of hNIS as an autoantigen. In diffuse nodular hyperplasia, hNIS staining was heterogeneous, but the number of hNIS-positive cells exceeded that found in normal tissue. In well differentiated follicular or papillary carcinoma, the number of hNIS-positive cells was significantly lower than in normal tissue. In poorly differentiated follicular carcinoma, the number ofhNIS-positive cells was less than that found in well differentiated carcinoma, or there were no positive cells. Interestingly, in all of these thyroid tissues, the number of follicular cells exhibiting TSH receptor (TSHR) immunoreactivity was greater than the number ofhNIS-positive cells. As hNIS expression appears to be related to TSHR stimulation, the decreased number of TSHR-positive cells in cancers may contribute to the reduced capacity of neoplastic cells to concentrate iodide. In one patient with a follicular cancer with an absence of hNIS immunostaining, the total body 131I scan showed no uptake in metastatic tissue. In three cancers with positive hNIS cells, the 131I scan showed uptake in lymph node metastases. This suggests that immunodetection of hNIS could predict radioiodine uptake in thyroid cancers.

Regulation and tissue distribution of the human sodium iodide symporter gene

OBJECTIVE

Iodide uptake by the thyroid gland is mediated by the sodium iodide symporter (NIS). In the present report, we have analysed the factors that modulate human NIS mRNA expression and iodide uptake in primary thyroid follicular cell (TFC) cultures. In addition, NIS mRNA tissue distribution was investigated.

METHODS

Primary thyroid follicular cell cultures were treated with human recombinant TSH with or without cytokines for 72 h. Subsequently, NIS gene expression and iodide uptake were analysed using reverse transcription-polymerase chain reaction (RT-PCR) and 125I uptake, respectively. Human tissue samples were investigated for NIS gene expression using both RT-PCR and Northern blotting.

RESULTS

Human TSH increased both NIS gene expression and iodide uptake in TFC cultures in a dose-dependent manner. Using concentrations of 0.1 U/l of hTSH, a minor increase in NIS gene expression was detected without a detectable increase in iodide uptake. IL-1 alpha, TNF alpha and IFN gamma at concentrations of 10(5) U/l all inhibited TSH-induced NIS gene expression and iodide uptake. In these experiments, there was a good correlation between NIS mRNA expression and iodide uptake. Using RT-PCR higher levels of NIS mRNA were detected in Graves' disease (GD) compared to multi-nodular goitre tissue samples. Stomach and salivary gland tissue also expressed NIS mRNA, whereas low levels were found in the mammary gland and extraocular muscle tissue. No expression was detected in the ovary, oesophagus, colon, extraocular fat or skin. In contrast, Northern blot analysis failed to detect NIS in stomach, salivary gland, intestinal fat or non-toxic multi-nodular goitre tissue samples, although this was present in GD thyroid tissue.

CONCLUSION

TSH upregulates sodium iodide symporter gene expression and iodide uptake in primary thyroid follicular cell cultures, and this induction is modulated by cytokines. Variable levels of sodium iodide symporter mRNA are present in different tissue samples, with high expression evident in Graves' disease thyroid tissue.

N-linked glycosylation of the thyroid Na+/I- symporter (NIS). Implications for its secondary structure model

The Na+/I- symporter (NIS), a 618-amino acid membrane glycoprotein that catalyzes the active accumulation of I- into thyroid cells, was identified and characterized at the molecular level in our laboratory (Dai, G., Levy, O., and Carrasco, N. (1996) Nature 379, 458-460). Because mature NIS is highly glycosylated, it migrates in SDS-polyacrylamide gel electrophoresis as a broad polypeptide of higher molecular mass (approximately 90-110 kDa) than nonglycosylated NIS (approximately 50 kDa). Using site-directed mutagenesis, we substituted both separately and simultaneously the asparagine residues in all three putative N-linked glycosylation consensus sequences of NIS with glutamine and assessed the effects of the mutations on function and stability of NIS in COS cells. All mutants were active and displayed 50-90% of wild-type NIS activity, including the completely nonglycosylated triple mutant. This demonstrates that to a considerable extent, function and stability of NIS are preserved in the partial or even total absence of N-linked glycosylation. We also found that Asn225 is glycosylated, thus proving that the hydrophilic loop that contains this amino acid residue faces the extracellular milieu rather than the cytosol as previously suggested. We demonstrated that the NH2 terminus faces extracellularly as well. A new secondary structure model consistent with these findings is proposed.

Differential effects of protein kinase A on Ras effector pathways

Ras mutants with the ability to interact with different effectors have played a critical role in the identification of Ras-dependent signaling pathways. We used two mutants, RasS35 and RasG37, which differ in their ability to bind Raf-1, to examine Ras-dependent signaling in thyroid epithelial cells. Wistar rat thyroid cells are dependent upon thyrotropin (TSH) for growth. Although TSH-stimulated mitogenesis requires Ras, TSH activates protein kinase A (PKA) and downregulates signaling through Raf and the mitogen-activated protein kinase (MAPK) cascade. Cells expressing RasS35, a mutant which binds Raf, or RasG37, a mutant which binds RalGDS, exhibited TSH-independent proliferation. RasS35 stimulated morphological transformation and anchorage-independent growth. RasG37 stimulated proliferation but not transformation as measured by these indices. TSH exerted markedly different effects on the Ras mutants and transiently repressed MAPK phosphorylation in RasS35-expressing cells. In contrast, TSH stimulated MAPK phosphorylation and growth in cells expressing RasG37. The Ras mutants, in turn, exerted differential effects on TSH signaling. RasS35 abolished TSH-stimulated changes in cell morphology and thyroglobulin expression, while RasG37 had no effect on these activities. Together, the data indicate that cross talk between Ras and PKA discriminates between distinct Ras effector pathways.

A novel thyroid transcription factor is essential for thyrotropin-induced up-regulation of Na+/I- symporter gene expression

The stimulation of iodide (I-) transport by TSH in FRTL-5 thyroid cells is partly due to an increase in Na+/I- symporter (NIS) gene expression. The identification of a TSH-responsive element (TRE) in the NIS promoter and its relationship to the action of thyroid transcription factor-1 (TTF-1) on the promoter are the subjects of this report. By transfecting NIS promoter-luciferase chimeric plasmids into FRTL-5 cells in the presence or absence of TSH, we identify a TRE between -420 and -370 bp of the NIS 5'-flanking region. Nuclear extracts from FRTL-5 cells cultured in the absence of TSH form two groups of protein-DNA complexes, A and B, in gel mobility shift assays using an oligonucleotide having the sequence from -420 to -385 bp. Only the A complex is increased by exposure of FRTL-5 cells to TSH or forskolin. The addition of TSH to FRTL-5 cells can increase the A complex at 3-6 h, reaching a maximum at 12 h. FRTL-5, but not nonfunctioning FRT thyroid or Buffalo rat liver (BRL) cell nuclear extracts, form the A complex. The TSH-increased nuclear factor in FRTL-5 cells interacting with the NIS TRE is distinct from TTF-1, thyroid transcription factor-2, or Pax-8, as evidenced by the absence of competition using oligonucleotides specific for these factors in gel shift assays. Neither is it the nuclear protein interacting with cAMP response element. The TRE is in the upstream of a TTF-1-binding site, -245 to -230 bp. Mutation of the TRE causing a loss of TSH responsiveness also decreases TTF-1-induced promoter activity in a transfection experiment. The formation of the A complex between FRTL-5 nuclear extracts and the NIS TRE is redox-regulated. In sum, TSH/cAMP-induced up-regulation of the NIS requires a novel thyroid transcription factor, which also appears to be involved in TTF-1-mediated thyroid-specific NIS gene expression.

The Na+/I- symporter (NIS): recent advances

The Na+/I- symporter (NIS) catalyzes the accumulation of iodide into thyroid cells, an essential step in the biosynthesis of thyroid hormones. As a result of the isolation of the rat NIS cDNA, steadfast advances in the study of NIS at the molecular level have resulted in the following accomplishments: generation of high-affinity anti-NIS antibodies, elucidation of NIS stoichiometry and specificity by electrophysiological analysis, biochemical and immunological experimental testing of the proposed NIS secondary structure model, monitoring the regulation of NIS protein expression by thyroid stimulating hormone and iodide, characterization of the rat NIS gene promoter, isolation of the cDNA clone encoding human NIS and subsequent determination of human NIS genomic organization, description of NIS mutations in patients with congenital lack of iodide transport, and the molecular identification of NIS in extrathyroidal tissues.

Biochemical and functional changes during the bovine fetal thyroid development

To establish biochemical and functional relations during thyroid development, the activity of thyroid peroxidase (TPO), nicotinamide adenine dinucleotide phosphate (NADPH)-cytochrome c reductase and monoamine oxidase (MAO) in a particulate fraction and the iodide transport and organification in slices of bovine fetal thyroid were examined throughout gestation. The cytochemical localization of TPO, H2O2 generating sites and MAO was also studied. Fetal glands were grouped in stages I to V according to increasing developmental features; adult tissues were also analyzed. TPO activity in each of the fetal stages was higher than in the adult; a marked increase was observed in stages IV and V. Iodide transport (T/M) was similar in stages I to V and the adult. Iodide organification in fetal thyroids showed a similar pattern to that of TPO activity. When compared with the adult, at midgestation (stages II to III), a lower iodination coexisted with a higher TPO activity. The activity of NADPH-cytochrome c reductase and MAO, two enzymes previously proposed to participate in thyroid H2O2 generation, did not parallel the level of iodide organification. Cells from stages II to V exhibited a positive cytochemical reaction for TPO in the rough endoplasmic reticulum (RER) and the perinuclear cisternae (PC). In stages IV, V, and adult, TPO was occasionally found in apical vesicles and microvilli, whereas H2O2 was detected within the RER and the PC. MAO reaction was positive in adult, but not in fetal thyroid. These results indicate that a high TPO activity accompanied the onset of the organification process during fetal thyroid development. The level of iodination was associated with the presence of TPO at a proper site rather than to the level of TPO activity. Evidence against a role of NADPH-cytochrome c reductase and MAO in the iodide organification was obtained.

A homozygous missense mutation of the sodium/iodide symporter gene causing iodide transport defect

Iodide transport defect is a disorder characterized by an inability of the thyroid to maintain an iodide concentration difference between the plasma and the thyroid. The recent cloning of the sodium/iodide symporter (NIS) gene enabled us to characterize the NIS gene in this disorder. We identified a homozygous missense mutation of A-->C at nucleotide +1060 in NIS complementary DNA in a male patient who was born from consanguineous marriage, had a huge goiter, and lacked the ability to accumulate iodide but was essentially euthyroid. The mutation results in an amino acid replacement of Thr354-->Pro in the middle of the ninth transmembrane domain. COS-7 cells transfected with the mutant NIS complementary DNA showed markedly decreased iodide uptake, confirming that this mutation was the direct cause of the disorder in the patient. Northern analysis of thyroid ribonucleic acid revealed that NIS messenger ribonucleic acid level was markedly increased (> 100-fold) compared with that in the normal thyroid, suggesting possible compensation by overexpression.

Retinoic acid increases sodium/iodide symporter mRNA levels in human thyroid cancer cell lines and suppresses expression of functional symporter in nontransformed FRTL-5 rat thyroid cells

Decreased iodide uptake in de-differentiated thyroid carcinomas impedes radioiodide therapy. RTPCR analysis revealed reduced expression of Na+/I- symporter (NIS) mRNA in human thyroid carcinomas as compared to normal thyroid. However, in follicular thyroid carcinoma cell lines FTC-133 and FTC-238, treatment with 1 microM all-trans retinoic acid (RA) markedly increased NIS mRNA levels. Anaplastic thyroid carcinoma cell lines HTh74 and C643 showed basal expression of NIS mRNA, but no RA-stimulation. All four cell lines contained the approximately 80 kD NIS protein as judged by Western blot, although they did not accumulate iodide. In contrast, in nontransformed rat FRTL-5 cells, 1 microM RA downregulated NIS mRNA levels, inhibited the TSH- or forskolin-triggered induction of NIS message after TSH-depletion, and reduced iodide uptake to 38% after 5 d. This divergent RA-responsivity of NIS may provide the means to target radioiodide to thyroid carcinomas by upregulating iodide transport into tumor tissue while simultaneously inhibiting iodide accumulation in normal thyrocytes and may thus re-establish the potential for radioiodide therapy.

Thyroid Na+/I- symporter. Mechanism, stoichiometry, and specificity

The rat thyroid Na+/I- symporter (NIS) was expressed in Xenopus laevis oocytes and characterized using electrophysiological, tracer uptake, and electron microscopic methods. NIS activity was found to be electrogenic and Na+-dependent (Na+ >> Li+ >> H+). The apparent affinity constants for Na+ and I- were 28 +/- 3 mM and 33 +/- 9 microM, respectively. Stoichiometry of Na+/anion cotransport was 2:1. NIS was capable of transporting a wide variety of anions (I-, ClO3-, SCN-, SeCN-, NO3-, Br-, BF4-, IO4-, BrO3-, but perchlorate (ClO4-) was not transported. In the absence of anion substrate, NIS exhibited a Na+-dependent leak current (approximately 35% of maximum substrate-induced current) with an apparent Na+ affinity of 74 +/- 14 mM and a Hill coefficient (n) of 1. In response to step voltage changes, NIS exhibited current transients that relaxed with a time constant of 8-14 ms. Presteady-state charge movements (integral of the current transients) versus voltage relations obey a Boltzmann relation. The voltage for half-maximal charge translocation (V0.5) was -15 +/- 3 mV, and the apparent valence of the movable charge was 1. Total charge was insensitive to [Na+]o, but V0.5 shifted to more negative potentials as [Na+]o was reduced. NIS charge movements are attributed to the conformational changes of the empty transporter within the membrane electric field. The turnover rate of NIS was >/=22 s-1 in the Na+ uniport mode and >/=36 s-1 in the Na+/I- cotransport mode. Transporter density in the plasma membrane was determined using freeze-fracture electron microscopy. Expression of NIS in oocytes led to a approximately 2. 5-fold increase in the density of plasma membrane protoplasmic face intramembrane particles. On the basis of the kinetic results, we propose an ordered simultaneous transport mechanism in which the binding of Na+ to NIS occurs first.

Transforming growth factor-beta1 suppresses thyrotropin-induced Na+/I- symporter messenger RNA and protein levels in FRTL-5 rat thyroid cells

Iodide transport into the thyroid catalyzed by the Na+/I- symporter (NIS), is the first and main rate-limiting step in thyroid hormone synthesis. Recently, we have demonstrated that thyrotropin (TSH) increases NIS messenger RNA (mRNA) and protein levels, as well as iodide uptake activity. Although transforming growth factor-beta1 (TGFbeta1) is known to affect thyroid cell function, it is still unclear how TGFbeta1 regulates TSH-stimulated iodide accumulation. Therefore, the effects of TGFbeta1 on TSH-stimulated NIS mRNA and protein levels were examined in FRTL-5 rat thyroid cells by Northern and Western blot analyses, and iodide uptake was assessed. Northern blot analysis revealed that TGFbeta1 suppressed TSH-stimulated NIS mRNA levels in a dose- and time-dependent manner. Western blot analysis demonstrated that TGFbeta1 suppressed TSH-stimulated NIS protein levels. TGFbeta1 also suppressed (Bu)2 cyclic adenosine monophosphate (cAMP)- and forskolin-stimulated NIS mRNA and protein levels, indicating a role for TGFbeta1 downstream of cAMP production. As predicted, TGFbeta1 inhibited TSH-stimulated iodide uptake activity. These results suggest that the inhibitory effect of TGFbeta1 on TSH-stimulated iodide uptake is at least in part due to a suppression of NIS specific transcription. Therefore, TGFbeta1 may act as an autocrine or paracrine local modulator of thyroid hormone synthesis by influencing NIS mRNA levels in the thyroid.

Moderate doses of iodide in vivo inhibit cell proliferation and the expression of thyroperoxidase and Na+/I- symporter mRNAs in dog thyroid

The function and the growth of adult thyroid gland is controlled by the opposite actions of thyrotropin (TSH) and iodide, the main substrate of the gland. Iodide deprivation leads to stimulation of the thyroid, improving the efficiency of iodide transport for hormone biosynthesis. We have investigated cell proliferation and thyroid specific gene expression 24 and 48 h after administering KI to dogs previously treated with goitrogens and perchlorate. In the hypothyroid dogs T3 and T4 serum levels decreased from 53 +/- 4 to < 30 ng/dl and from 1.6 +/- 0.6 to < 1 microg/dl respectively; TSH concentration increased from 0.16 +/- 0.02 to 2.7 +/- 0.4 ng/ml. After a 24 h moderate KI treatment (300 microg KI/dog of +/- 10 kg) serum T3 concentrations rose higher than the initial normal values, while T4 concentrations increased to reach values equivalent to the normal level. The high TSH concentration did not change significantly. The hyperplasia of the chronically stimulated thyroid resulting from goitrogens/NaClO4 treatment was not modified by this short term treatment with KI. In contrast, KI decreased the weight of the total gland and the level of cell proliferation, as determined by the fraction of cells incorporating BrdU. The effect of acute administration of KI on the expression of four major thyroid genes, the TSH receptor (TSHr), thyroglobulin (Tg), thyroperoxidase (TPO), and Na+/I- symporter (NIS) was analyzed by Northern blot. Tg, TPO and NIS mRNA expressions were up-regulated by chronic stimulation. The expression of the mRNAs of TSHr and Tg did not significantly differ between hyperstimulated and KI-treated dogs while TPO and NIS mRNA expression decreased after a 48 h KI treatment. TPO and NIS are therefore the only of these four genes whose expression is acutely modulated by iodide in vivo. Under TSH stimulation low doses of iodide resulted in: (1) decreased cell proliferation, (2) reestablished synthesis and secretion of thyroid hormones, (3) diminished TPO and NIS mRNA expression. Notably low doses of iodide under the same conditions had no effect on Tg and TSHr mRNA expression.

Regulation by thyroid-stimulating hormone of sodium/iodide symporter gene expression and protein levels in FRTL-5 cells

To investigate the mechanism of I- transport stimulation by TSH, we studied the effects of TSH on Na+/I- symporter (NIS) messenger RNA (mRNA) and protein levels in FRTL-5 cells and correlated these with I- transport activity. When 1 mU/ml TSH was added to quiescent FRTL-5 cells, a 12-h latency was observed before the onset of increased I- transport activity, which reached a maximum [approximately 27 times basal (5H medium) levels] at 72 h. In contrast, Northern blot analysis, using rat NIS complementary DNA as a probe, revealed that addition of TSH to these cells significantly increased NIS mRNA at 3-6 h, reaching a maximum after 24 h (approximately 5.9 times basal levels). Forskolin and (Bu)2cAMP mimicked this stimulatory effect on both the I- transport activity and mRNA levels. D-ribofranosylbenzimidazole, a transcription inhibitor, almost completely blocked TSH-induced stimulation of I- transport and NIS mRNA levels. Western blot analysis demonstrated that TSH increased NIS protein levels at 36 h, reaching a maximum at 72 h, in parallel with the kinetics of TSH-induced I- transport activity. However, it also showed that the amount of NIS protein already present in FRTL-5 cell membranes before the addition of TSH was about one third of the maximum level induced by TSH. These results indicate that stimulation of I- transport activity by TSH in thyrocytes is partly due to a rapid increase in NIS gene expression, followed by a relatively slow NIS protein synthesis. However, the existence of an abundant amount of protein in quiescent FRTL-5 cells with very low I- transport activity also suggests that this activity is controlled by another TSH-regulated factor(s).

Characterization of the thyroid Na+/I- symporter with an anti-COOH terminus antibody

The Na+/I- symporter (NIS) is the plasma membrane protein that catalyzes active I- transport in the thyroid, the first step in thyroid hormone biogenesis. The cDNA encoding NIS was recently cloned in our laboratory and a secondary structure model proposed, suggesting that NIS is an intrinsic membrane protein (618 amino acids; approximately 65.2 kDa predicted molecular mass) with 12 putative transmembrane domains. Here we report the generation of a site-directed polyclonal anti-COOH terminus NIS antibody (Ab) that immunoreacts with a approximately 87 kDa-polypeptide present in membrane fractions from a rat thyroid cell line (FRTL-5). The model-predicted cytosolic-side location of the COOH terminus was confirmed by indirect immunofluorescence experiments using anti-COOH terminus NIS Ab in permeabilized FRTL-5 cells. Immunoreactivity was competitively blocked by the presence of excess synthetic peptide. Treatment of membrane fractions from FRTL-5 cells, Xenopus laevis oocytes, and COS cells expressing NIS with peptidyl N-glycanase F converted the approximately 87 kDa-polypeptide into a approximately 50 kDa-species, the same relative molecular weight exhibited by NIS expressed in E. coli. Anti-NIS Ab immunoprecipitated both the NIS precursor molecule (approximately 56 kDa) and the mature approximately 87 kDa form. Furthermore, a direct correlation between circulating levels of thyroid-stimulating hormone and NIS expression in vivo was demonstrated.

Iodine metabolism and thyroid physiology: current concepts

Iodine plays a central role in thyroid physiology, being both a major constituent of thyroid hormones (THS) and a regulator of thyroid gland function. This review concerns those aspects of thyroid physiology in which significant advances have been made in recent years. We have known for decades that the thyroid gland concentrates iodine (I-) against an electrochemical gradient by a carrier-mediated mechanism driven by ATP. A similar I- uptake mechanism is found in other organs, including salivary glands, stomach, choroid plexus, and mammary glands, but only in the thyroid does TSH regulate the process. This past year saw a major advance with the cloning of the thyroid I- transporter. This development opens the way to an elucidation of the regulation of I- transport in the normal gland and in thyroid neoplasms that lack this property ("cold" nodules). All of the subsequent steps in TH biosynthesis, from oxidation and organification of iodide to the secretion of T4 and T3 into the circulation, are stimulated by TSH and inhibited by excess iodine. Recently, some of the regulatory mechanisms have been clarified. The function of the major TH-binding proteins in plasma is to maintain an equilibrium between extracellular and cellular hormone pools. Transthyretin, the principal T4-binding protein in cerebrospinal fluid, may play a similar role in the central nervous system. Although it generally is agreed that cellular uptake of TH is a function of the unbound (free) form of the hormone, there is evidence that certain TH-binding plasma proteins (i.e., apolipoproteins) may serve specific transport functions. The intracellular concentration of T3, the active TH, is determined by the rates of cellular uptake of T4 and T3, the rates of metabolic transformation, including conversion of T4 to T3, and the rate of T3 efflux. The latter has been assumed to be a passive process. However, recent studies by our group in San Francisco have shown that T3 is transported out of cells by a specific, saturable, verapamil-inhibitable mechanism. This T3 efflux system is widespread among cells from many tissues, and, at least in liver, modulates intracellular and nuclear concentration of the hormone and thereby influences TH action.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Cloning and characterization of the thyroid iodide transporter

Iodide (I-) is an essential constituent of the thyroid hormones T3 and T4, and is accumulated by the thyroid. The transport of iodide, the first step in thyroid hormogenesis, is catalysed by the Na+/I- symporter, an intrinsic membrane protein that is crucial for the evaluation, diagnosis and treatment of thyroid disorders. Although several other important thyroid proteins involved in hormogenesis have been characterized, the Na+/I- symporter has not. Here we report the isolation of a complementary DNA clone that encodes this symporter, as a result of functional screening of a cDNA library from a rat thyroid-derived cell line (FRTL-5) in Xenopus laevis oocytes. Oocyte microinjection of an RNA transcript made in vitro from this cDNA clone elicited a more than 700-fold increase in perchlorate-sensitive Na+/I- symport activity over background. To our knowledge, this is the first iodide-transporting molecule to have its cDNA cloned, providing a missing link in the thyroid hormone biosynthetic pathway.

Inhibition of iodide transport in rat thyroid cells using N-substituted anthranilic acid derivatives

The purpose of this study was to test the effects of chloride channel blockers on iodide uptake in thyroid cells, in the hope of eventually using these blockers to identify and isolate a putative iodide transporter. The chloride channel blockers used in this report are derivatives of N-substituted anthranilic acid and were synthesized using published procedures. For these studies FRTL-5 cells, a line of continuous-growing rat thyroid cells, were used as a model system to study effects on iodide transport. In these cells, there are at least two ways for transmembrane iodide movements, a sodium-dependent influx step and a proposed channel that normally mediates iodide efflux. Two derivatives studied decreased iodide accumulation in FRTL-5 cells, but were found also to lower intracellular pH and ATP levels. To simplify interpretation of the effect of the drugs on iodide transport, we extended the studies using plasma membrane vesicles made from pig thyroid. Iodide entry in these vesicles depended on a sodium gradient and was independent of ATP levels. Iodide transport in plasma membrane vesicles and FRTL-5 cells was measured at 30 sec when the uptake was nearly linear and therefore likely to reflect iodide entry. The uptake was measured using three concentrations of iodide and three of drug. Kinetic analysis of the data described a competitive inhibition by the drugs with a Ki of approximately 250 microM. In summary, N-substituted anthranilic acid derivatives reversibly inhibit iodide entry in FRTL-5 cells and pig plasma membrane vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of Na(+)-K(+)-ATPase by thyrotropin in cultured thyroid follicular cells

Thyroid-stimulating hormone (TSH; thyrotropin) produces a pleiotropic response in the thyroid gland, accelerating nearly every aspect of metabolic turnover within the follicular epithelia. We examined the effects of TSH on expression of Na(+)-K(+)-ATPase in FRTL-5 cells, a cell line derived from rat thyroid. TSH (10 mU/ml) produced a nearly twofold increase in abundance of the mRNA encoding the catalytic alpha 1-subunit within 6 h of treatment. With the four mRNAs encoding the beta 1-subunit, TSH produced a striking increase in abundance, but this regulation was discoordinate, and some species increased more than others. Similar increases in mRNA abundance were elicited by activators of the adenosine 3',5'-cyclic monophosphate second messenger system. In contrast to the alpha 1- and beta 1-mRNAs, the abundance of the mRNA encoding the beta 2-subunit was unchanged with TSH after 6 h, indicating that the effects of thyrotropin were not universal or indiscriminate. Thyrotropin also caused a 76% increase in Na(+)-K(+)-ATPase activity and a 46% increase in pump-mediated transport after 48 h. These studies suggest that the changes in metabolic turnover initiated by TSH during hormone synthesis include upregulation of the N(+)-K+ pump.

Na(+)-I- symport activity is present in membrane vesicles from thyrotropin-deprived non-I(-)-transporting cultured thyroid cells

The active accumulation of I- in the thyroid gland is mediated by the Na(+)-I- symporter and driven by the Na+ gradient generated by the Na+/K(+)-ATPase. Thyrotropin (TSH) stimulates thyroidal I- accumulation. Rat thyroid-derived FRTL-5 cells require TSH to accumulate I-. TSH withdrawal for over 7 days results in complete loss of Na(+)-I-symport activity in these cells [Weiss, S. J., Philp, N. J. and Grollman, E. F. (1984) Endocrinology 114, 1090-1098]. Surprisingly, membrane vesicles prepared from FRTL-5 cells maintained in TSH-free medium [TSH(-)cells]accumulate I-, suggesting that the absence of Na(+)-I- symport activity in TSH(-) cells cannot be due solely to a decrease in the biosynthesis of either the symporter or a putative activating factor. This finding indicates that the Na(+)-I- symporter is present, probably in an inactive state, in TSH(-) cells despite their lack of Na(+)-I- symport activity. Na(+)-I- symport activity in thyroid membrane vesicles is enhanced when conditions for vesicle preparation favor proteolysis. Subcellular fractionation studies in both TSH(+) and TSH(-) cells show that Na(+)-I- symport activity is mostly associated with fractions enriched in plasma membrane rather than in intracellular membranes, suggesting that the Na(+)-I- symporter may constitutively reside in the plasma membrane and may be activated by TSH.