A thyroid-specific far-upstream enhancer in the human sodium/iodide symporter gene requires Pax-8 binding and cyclic adenosine 3',5'-monophosphate response element-like sequence binding proteins for full activity and is differentially regulated in normal and thyroid cancer cells

Retinoic acid stimulation of the sodium/iodide symporter in MCF-7 breast cancer cells is mediated by the insulin growth factor-I/phosphatidylinositol 3-kinase and p38 mitogen-activated protein kinase signaling pathways

CONTEXT

All-trans retinoic acid (tRA) induces differentiation in MCF-7 breast cancer cells, stimulates sodium/iodide symporter (NIS) gene expression, and inhibits cell proliferation. Radioiodine administration after systemic tRA treatment has been proposed as an approach to image and treat some differentiated breast cancer.

OBJECTIVE

The objective of this work was to study the relative role of genomic and nongenomic pathways in tRA stimulation of NIS expression in MCF-7 cells.

DESIGN

We inspected the human NIS gene locus for retinoic acid-responsive elements and tested them for function. The effects of signal transduction pathway inhibitors were also tested in tRA-treated MCF-7 cells and TSH-stimulated FRTL-5 rat thyroid cells, followed by iodide uptake assay, quantitative RT-PCR of NIS, and cell cycle phase analysis.

RESULTS

Multiple retinoic acid response elements around the NIS locus were identified by sequence inspection, but none of them was a functional tRA-induced element in MCF-7 cells. Inhibitors of the IGF-I receptor, Janus kinase, and phosphatidylinositol 3-kinase (PI3K), significantly reduced NIS mRNA expression and iodide uptake in tRA-stimulated MCF-7 cells but not FRTL-5 cells. An inhibitor of p38 MAPK significantly reduced iodide uptake in both tRA-stimulated MCF-7 cells and TSH-stimulated FRTL-5 cells. IGF-I and PI3K inhibitors did not significantly reduce the basal NIS mRNA expression in MCF-7 cells. Despite the chronic inhibitory effects on cell proliferation, tRA did not reduce the S-phase distribution of MCF-7 cells during the period of NIS induction.

CONCLUSION

The IGF-I receptor/PI3K pathway mediates tRA-stimulated NIS expression in MCF-7 but not FRTL-5 thyroid cells.

Molecular mechanisms of epithelial cell-specific expression and regulation of the human anion exchanger (pendrin) gene

Pendrin, a Cl(-)/anion exchanger encoded by the gene PDS, is highly expressed in the kidney, thyroid, and inner ear epithelia and is essential for bicarbonate secretion, iodide accumulation, and endolymph ion balance, respectively. This study aimed to define promoter regulatory elements essential for renal, thyroid, and inner ear epithelial cell-specific expression of human PDS (hPDS) and to explore the effect of ambient pH and aldosterone on hPDS promoter activity. Endogenous pendrin mRNA and protein were detected in renal HEK293, thyroid LA2, and inner ear VOT36 epithelial cell lines, but not in the fibroblast cell line, NIH3T3. A 4.2-kb hPDS 5'-flanking DNA sequence and consecutive 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the above cell lines. Distinct differences in expression/activity of deduced positive/negative regulatory elements within the hPDS promoter between HEK293, LA2, and VOT36 cells were demonstrated, with only basal activity in NIH3T3 cells. Acidic pH (7.0-7.1) decreased and alkaline pH (7.6-7.7) increased hPDS promoter activity in transfected HEK293 and VOT36, but not in LA2 cells. Aldosterone (10(-8) M) reduced hPDS promoter activity in HEK293 but had no effect in LA2 and VOT36 cells. These pH and aldosterone-induced effects on the hPDS promoter occurred within 96-bp and 89-bp regions, respectively, which likely contain distinct response elements to these modulators. Acidic pH and aldosterone decreased, and alkaline pH increased, endogenous pendrin mRNA level in HEK293 cells. In conclusion, pendrin-mediated HCO3(-) secretion in the renal tubule and anion transport in the endolymph may be regulated transcriptionally by systemic pH and aldosterone.

Effect of Iodide on Human Choriogonadotropin, Sodium-Iodide Symporter Expression, and Iodide Uptake in BeWo Choriocarcinoma Cells

CONTEXT

Active placental transport of maternal iodide by the thyroidal sodium iodide symporter (NIS) provides an essential substrate for fetal thyroid hormone synthesis. NIS is expressed in trophoblast and is regulated by human choriogonadotropin (hCG). In thyroid, iodide down-regulates expression of several genes including NIS. Placentas of iodine-deficient rats demonstrate up-regulation of NIS mRNA, suggesting a role for iodide in regulating placental NIS.

OBJECTIVES AND METHODS

The objectives were to examine effects of iodide on expression of NIS and hCG in BeWo choriocarcinoma cells. Gene expression was studied by quantitative real-time PCR. Effects on NIS protein expression were assessed by Western blotting. Functional activity of NIS was measured by (125)I uptake. Expression of hCG protein was assessed by immunoassay of secreted hormone.

RESULTS

Iodide inhibited NIS mRNA and membrane protein expression as well as (125)I uptake, which were paralleled by decreased betahCG mRNA expression and protein secretion. Iodide had no effects on pendrin expression. Addition of hCG increased NIS mRNA expression. This effect was partially inhibited by addition of iodide. The inhibitory effects of iodide on NIS mRNA expression were abolished by propylthiouracil and dithiothreitol.

CONCLUSIONS

We conclude that expression of placental NIS is modulated by maternal iodide. This may occur through modulation of hCG effects on NIS and hCG gene expression.

Down-regulation of the Sodium/Iodide Symporter Explains 131I-Induced Thyroid Stunning

(131)I radiation therapy of differentiated thyroid cancer may be compromised by thyroid stunning (i.e., a paradoxical inhibition of radioiodine uptake caused by radiation from a pretherapeutic diagnostic examination). The stunning mechanism is yet uncharacterized at the molecular level. We therefore investigated whether the expression of the sodium/iodide symporter (NIS) gene is changed by irradiation using (131)I. Confluent porcine thyroid cells on filter were stimulated with thyroid-stimulating hormone (TSH; 1 milliunit/mL) or insulin-like growth factor-I (IGF-I; 10 ng/mL) and simultaneously exposed to (131)I in the culture medium for 48 h, porcine NIS mRNA was quantified by real-time reverse transcription-PCR using 18S as reference, and transepithelial iodide transport was monitored using (125)I(-) as tracer. TSH increased the NIS expression >100-fold after 48 h and 5- to 20-fold after prolonged stimulation. IGF-I enhanced the NIS transcription at most 15-fold but not until 5 to 7 days. (131)I irradiation (7.5 Gy) decreased both TSH-stimulated and IGF-I-stimulated NIS transcription by 60% to 90% at all investigated time points. TSH and IGF-I stimulated NIS synergistically 15- to 60-fold after 5 days. NIS expression was reduced by (131)I also in costimulated cells, but the transcription level remained higher than in nonirradiated cells stimulated with TSH alone. Changes in NIS mRNA always correlated with altered (125)I(-) transport in cultures with corresponding treatments. It is concluded that down-regulation of NIS is the likely explanation of (131)I-induced thyroid stunning. Enhanced NIS expression by synergistically acting agents (TSH and IGF-I) partly prevents the loss of iodide transport expected from a given absorbed dose, suggesting that thyroid stunning might be pharmacologically treatable.

PTTG and PBF repress the human sodium iodide symporter

The ability of the thyroid to accumulate iodide provides the basis for radioiodine ablation of differentiated thyroid cancers and their metastases. Most thyroid tumours exhibit reduced iodide uptake, although the mechanisms accounting for this remain poorly understood. Pituitary tumour transforming gene (PTTG) is a proto-oncogene implicated in the pathogenesis of thyroid tumours. We now show that PTTG and its binding factor PBF repress expression of sodium iodide symporter (NIS) messenger RNA (mRNA), and inhibit iodide uptake. This process is mediated at least in part through fibroblast growth factor-2. In detailed studies of the NIS promoter in rat FRTL-5 cells, PTTG and PBF demonstrated specific inhibition of promoter activity via the human upstream enhancer element (hNUE). Within this approximately 1 kb element, a complex PAX8-upstream stimulating factor 1 (USF1) response element proved critical both to basal promoter activity and to PTTG and PBF repression of NIS. In particular, repression by PTTG was contingent upon the USF1, but not the PAX8, site. Finally, in human primary thyroid cells, PTTG and PBF similarly repressed the NIS promoter via hNUE. Taken together, our data suggest that the reported overexpression of PTTG and PBF in differentiated thyroid cancer has profound implications for activity of the NIS gene, and hence significantly impacts upon the efficacy of radioiodine treatment.

HEX, PAX-8 and TTF-1 gene expression in human thyroid tissues: a comparative analysis with other genes involved in iodide metabolism

OBJECTIVE

Benign and malignant thyroid tumours are characterized by alterations of the expression level of thyroid-specific genes involved in the iodide metabolism. Imbalance in the levels of transcription factors has been recognized as a critical molecular event in the development of neoplasm. The delineation of eventual correlations existing between the expression of transcription factors and of putative target genes in physiological and pathological conditions could be relevant to better understand tumorigenesis.

PATIENTS AND METHODS

We examined the expression levels of transcription factors involved in thyroid development [thyroid transcription factor 1 (TTF-1), paired box gene 8 (PAX-8) and haematopoietically expressed homeobox (HEX)] in 101 thyroid tissues, including 14 normal thyroid tissues, 13 hyperfunctioning tissues, 27 benign adenomas and 47 follicular or papillary carcinomas. Then, we compared their expression levels with those of thyroid-specific genes involved in iodide metabolism.

RESULTS

In benign tumours, PAX-8 and TTF-1 gene expression levels were not significantly different from the expression levels in normal tissues. However, a significant decrease was found in carcinomas. Interestingly, HEX gene expression was significantly decreased in both hyper- and hypofunctioning benign tissues and also in carcinomas. Expression levels of Pendred syndrome (PDS), natrium iodine symporter (NIS), thyroglobulin (Tg), thyroid peroxidase (TPO) and dual oxidase 1 or 2 (DUOX2) genes were significantly correlated with the expression of PAX-8 and with that of HEX. Expression level of TTF-1 was weakly correlated only with the expression levels of PDS and DUOX2.

CONCLUSION

Our findings suggest that alterations in the transcription factors PAX-8, TTF-1 and HEX gene expression, by acting individually or together, have a role in both thyroidal tumorigenesis and in the dedifferentiation process.

Genetics, clinical management and natural history of congenital hypothyroidism

Congenital hypothyroidism has an incidence of 1:3000-4000 newborns. In the past, it was a common cause of mental retardation, although newborn screening has improved the prognosis. Several transcription factors are crucial to the early organogenesis of the thyroid, including TITF1, FOXE1, PAX8 and HHEX. Nevertheless, a small minority of humans with congenital hypothyroidism carry mutations in these genes. Long-term follow-up studies show that with appropriate therapy, the mental and physical development of congenital hypothyroidism patients is in the normal range, although somewhat delayed when compared with appropriate controls. Critical issues of treatment include early diagnosis and rapid correction of hypothyroidism by ensuring rapid initiation of treatment and rapid normalization of thyroid-stimulating hormone levels, especially in cases of severe congenital hypothyroidism. There has been recent question of the need to treat premature infants with hypothyroxinemia but no elevation of the thyroid-stimulating hormone concentration. Numerous controlled studies of the short- and long-term effects of thyroid hormone supplementation have not shown a consistent benefit in this group of patients. Future research will investigate further the genetic underpinnings of the condition. There is a worldwide need to prevent congenital hypothyroidism by ensuring adequate dietary iodine and to develop infrastructure to diagnose and treat congenital hypothyroidism in developing countries.

Expression, regulation, and function of paired-box gene 8 in the human placenta and placental cancer cell lines

Pax proteins are transcriptional regulators that control a variety of developmental decisions in vertebrates. During development, the paired-box gene 8 (PAX8) is expressed in the thyroid, kidney, and several areas of the central nervous system. It is also expressed in the adult thyroid gland, in which it mediates TSH-induced modulation of the expression of important genes, such as those encoding thyroglobulin, thyroperoxidase, and the sodium/iodide symporter (NIS). Thus far, placental expression of PAX8 has been described only in mice. In the present study, we show that PAX8 is also expressed in the human placenta at term. In an in vitro model of placental cancer, the JAR choriocarcinoma cell line, human chorionic gonadotropin (hCG) increased levels of PAX8 mRNA and protein, and gel retardation assays indicated that the up-regulation of PAX8 protein expression is associated with an increase in its DNA-binding activity. The effects of hCG were mimicked by forskolin, indicating that they are cAMP dependent. Levels of mRNA for the Wilms' tumor 1 (WT1) and NIS genes were increased in JAR cells by hCG treatment, whereas overexpression of PAX8 increased only levels of WT1 mRNA. In cells transfected with PAX8-specific small interfering RNA, the stimulatory effects of hCG on WT1 mRNA levels were abolished, but hormonal enhancement of NIS mRNA levels was unchanged. These findings indicate that, in JAR cells, hCG activates a cAMP-dependent pathway that can up-regulate WT1 expression through PAX8.

Recovery of NIS expression in thyroid cancer cells by overexpression of Pax8 gene

BACKGROUND

Recovery of iodide uptake in thyroid cancer cells by means of obtaining the functional expression of the sodium/iodide symporter (NIS) represents an innovative strategy for the treatment of poorly differentiated thyroid cancer. However, the NIS gene expression alone is not always sufficient to restore radioiodine concentration ability in these tumour cells.

METHODS

In this study, the anaplastic thyroid carcinoma ARO cells were stably transfected with a Pax8 gene expression vector. A quantitative RT-PCR was performed to assess the thyroid specific gene expression in selected clones. The presence of NIS protein was detected by Western blot and localized by immunofluorescence. A iodide uptake assay was also performed to verify the functional effect of NIS induction and differentiation switch.

RESULTS

The clones overexpressing Pax8 showed the re-activation of several thyroid specific genes including NIS, Pendrin, Thyroglobulin, TPO and TTF1. In ARO-Pax8 clones NIS protein was also localized both in cell cytoplasm and membrane. Thus, the ability to uptake the radioiodine was partially restored, associated to a high rate of efflux. In addition, ARO cells expressing Pax8 presented a lower rate of cell growth.

CONCLUSION

These finding demonstrate that induction of Pax8 expression may determine a re-differentiation of thyroid cancer cells, including a partial recovery of iodide uptake, fundamental requisite for a radioiodine-based therapeutic approach for thyroid tumours.

Differential regulation of sodium/iodide symporter gene expression by nuclear receptor ligands in MCF-7 breast cancer cells

The sodium/iodide symporter (NIS) mediates iodide uptake in lactating breast tissue and is expressed in some breast cancers. We have previously demonstrated that all-trans retinoic acid (tRA) stimulates NIS gene expression and the selective cytotoxic effect of beta-emitting radioiodide-131 ((131)I) in both in vitro and in vivo MCF-7 breast cancer cell systems. We studied the ability of natural and synthetic retinoids, in combination with other nuclear receptor ligands, to achieve greater and more sustained induction of NIS in MCF-7 cells and enhance (131)I-mediated cytotoxicity. Selective stimulation of retinoic acid receptor (RAR) beta/gamma produced marked NIS induction; and selective stimulation of RARalpha, RARgamma, or retinoid X receptor produced more modest induction. Maximal NIS induction was seen with 9-cis retinoic acid and AGN190168, a RAR beta/gamma-agonist. Dexamethasone (Dex), but not the other nuclear receptor ligands, in combination with tRA synergistically induced iodide uptake and NIS mRNA expression, predominantly by prolonging NIS mRNA half-life. The addition of Dex reduced the EC(50) of tRA for NIS stimulation to approximately 7%, such that 10(-7) m tRA with addition of Dex enhanced iodide uptake and selective cytotoxicity of (131)I greater than 10(-6) m tRA alone. AGN190168 combined with Dex synergistically increased iodide uptake and significantly prolonged induction (5 d) of iodide uptake compared with that induced by the combination of tRA/Dex or 9-cis retinoic acid/Dex. The addition of Dex reduced the effective dose of retinoid and prolonged the induction of NIS, especially with AGN190168, suggesting higher efficacy of (131)I after combination treatment.

Activation of nicotinamide N-methyltransferase gene promoter by hepatocyte nuclear factor-1beta in human papillary thyroid cancer cells

We previously demonstrated that the human nicotinamide N-methytransferase (NNMT) gene was highly expressed in many papillary thyroid cancers and cell lines. The expression in other papillary and follicular cancers or cell lines and normal thyroid cells was low or undetectable. To gain an understanding of the molecular mechanism of this cell-specific expression, the NNMT promoter was cloned and studied by luciferase reporter gene assay. The promoter construct was expressed highly in papillary cancer cell lines, including those with higher (e.g. BHP 2-7) and lower (e.g. BHP 14-9) NNMT gene expression, and expressed weakly in follicular thyroid cancer cell lines. Further study with 5'-deletion promoter construct suggested that the NNMT promoter was regulated differently in BHP 2-7 and BHP 14-9 cells. In BHP 2-7 cells, promoter activity was dependent on an upstream sequence. In BHP 14-9 cells, sequence in the basal promoter region contributed notably to the overall promoter activity. RT-PCR or Western blot analysis indicated that hepatocyte nuclear factor-1beta (HNF-1beta) was expressed in only papillary cancer cell lines with high NNMT gene expression. HNF-1beta was not expressed or expressed very weakly in other papillary, follicular, and Hurthle cancer cell lines and primary cultures of normal thyroid cells and benign thyroid conditions. A HNF-1 binding site was identified in the NNMT basal promoter region. Mutations in this site decreased NNMT promoter activity in the HNF-1beta-positive BHP 2-7 cells, but not in the HNF-1beta-negative BHP 14-9 cells. HNF-1beta bound to the HNF-1 site specifically as a homodimer as determined by gel retardation assays with HNF-1beta-specific antibody. Cotransfection of a HNF-1beta expression plasmid increased NNMT promoter activity significantly in both HNF-1beta-positive and -negative thyroid cancer cell lines and Hep G2 liver cancer cells. Furthermore, transient expression of HNF-1beta in BHP 14-9 cells increased endogenous NNMT protein levels. In summary, HNF-1beta functions as a transcription activator for NNMT gene expression in some papillary thyroid cancer cells.

Transcription factor Nkx-2.5 induces sodium/iodide symporter gene expression and participates in retinoic acid- and lactation-induced transcription in mammary cells

The sodium/iodide symporter (NIS) is a plasma membrane protein that mediates active iodide transport in thyroid and mammary cells. It is a prerequisite for radioiodide treatment of thyroid cancer and a promising diagnostic and therapeutic tool for breast cancer. We investigated the molecular mechanisms governing NIS expression in mammary cells. Here we report that Nkx-2.5, a cardiac homeobox transcription factor that is also expressed in the thyroid primordium, is a potent inducer of the NIS promoter. By binding to two specific promoter sites (N2 and W), Nkx-2.5 induced the rNIS promoter (about 50-fold over the basal level). Interestingly, coincident with NIS expression, Nkx-2.5 mRNA and protein were present in lactating, but not virgin, mammary glands in two human breast cancer samples and in all-trans retinoic acid (tRA)-stimulated MCF-7 breast cancer cells. A cotransfected dominant-negative Nkx-2.5 mutant abolished tRA-induced endogenous NIS induction, which shows that Nkx-2.5 activity is critical for this process. Remarkably, in MCF-7 cells, Nkx-2.5 overexpression alone was sufficient to induce NIS and iodide uptake. In conclusion, Nkx-2.5 is a novel relevant transcriptional regulator of mammary NIS and could thus be exploited to manipulate NIS expression in breast cancer treatment strategies.

The CRE-like element inside the 5'-upstream region of the rat sodium/iodide symporter gene interacts with diverse classes of b-Zip molecules that regulate transcriptional activities through strong synergy with Pax-8

We previously demonstrated that transcription of the rat sodium/iodide symporter (NIS) gene is regulated by NUE, an upstream enhancer located between nucleotides -2264 and -2495 of the 5'-flanking region. To elucidate the mechanism of TSH/cAMP-mediated regulation of NIS gene expression, we have characterized the putative cAMP response element (CRE)/activator protein (AP)-1 site (termed NUC) that is closely located between the two Pax-8 (paired box domain transcription factor-8) binding sites within NUE. In two different approaches using either gel supershift analyses or dominant-negative inhibitors of b-Zip molecules, we have shown that NUC can be recognized by several members of the AP-1 and CREB family transcription factors that modulate the transcriptional activity of NUE. Using tethered dimers of b-Zip molecules, we have also demonstrated that specific homo- or heterodimers of AP-1 can synergistically stimulate NUE activity in concert with Pax-8. To demonstrate further that NUC is a bona fide CRE, we made an artificial promoter with the five-time tandem repeat of this sequence (5xNUC). In comparison to the canonical CRE (5xCRE), 5xNUC manifested greater transcriptional activity and broader response to cAMP signaling. Hence, we postulate that the significance of this evolutionally conserved CRE-like site may lie in its broader cell type specificity.

Histone deacetylase inhibitors restore radioiodide uptake and retention in poorly differentiated and anaplastic thyroid cancer cells by expression of the sodium/iodide symporter thyroperoxidase and thyroglobulin

Iodide uptake by the thyroid is mediated by the sodium/iodide symporter. Upon iodide uptake, thyroperoxidase catalyzes iodination of tyrosine residues in thyroglobulin, retaining iodide within thyroid follicles. Dedifferentiation-induced loss of these functions in cancers, rendering them unresponsive to radioiodide, occurs with most poorly differentiated and anaplastic tumors. We focused on the histone deacetylase (HDAC) inhibitors (HDACI) as a way to induce differentiation of thyroid cancer cells. We assessed re-expression of thyroid-specific genes mRNA induced by HDACI using quantitative RT-PCR and immunostaining in poorly differentiated papillary and anaplastic thyroid cancer cells. HDACI induced expression of thyroid-specific gene mRNAs and proteins, and accumulation of radioiodide through iodination of generic cellular proteins were detected. HDACI-treated tumors could specifically accumulate (125)I as revealed by imaging experiments and radioiodide concentration in vivo. In an attempt to determine the mechanism by which these gene expressions occurred, we detected the inhibition of protein synthesis by cycloheximide, which up-regulated the expression of thyroperoxidase and thyroglobulin mRNA in HDACI-treated cells and down-regulated that of sodium/iodide symporter mRNA. Together, our results suggest that HDACI-induced expression of thyroid-specific genes, some of which is mediated by some protein synthesis, may contribute to development of novel strategy against thyroid cancer.

Transcriptional regulation of human sodium/iodide symporter gene: a role for redox factor-1

The transcriptional regulation of the human sodium/iodide symporter (NIS) gene in normal and transformed thyroid cells is a crucial issue in attempting to restore iodide uptake and use radioiodine as a therapeutic treatment of thyroid cancer. Previous investigations have shown that the multifunctional protein apurinic apyrimidinic endonuclease/redox factor 1 (APE/Ref-1) plays an important role in regulation of thyroid-specific gene transcription. In this study, we investigated the effects of APE/Ref-1 on human NIS promoter activity. Cotransfection experiments performed in nonthyroid HeLa cells demonstrated that APE/Ref-1 exerts both PAX8-dependent and PAX8-independent effects. In fact, in the absence of PAX8, overexpression of APE/Ref-1 enhanced NIS promoter activity 2-fold. When the expression plasmid of APE/Ref-1 was transfected together with an expression plasmid for PAX8, a strong cooperative effect was detected with an increase of NIS promoter activity 9-fold over control. The PAX8-independent effect of APE/Ref-1 was specific for the NIS promoter, resulting not present for the promoter of the thyroperoxidase (TPO) gene. It was, at least in part, due to the up-regulation of the transcriptional activity of the ubiquitous factor early growth response-1 (Egr-1). In the thyroid tumor cell lines TPC-1 and B-CPAP, APE/Ref-1 was not effective by itself, and it also failed to increase PAX8 stimulation on NIS promoter activity. These data demonstrate a role for APE/Ref-1 protein in the transcriptional regulation of NIS gene expression by itself and in cooperation with PAX8. However, restoring the PAX8-APE/Ref-1 expression in tumor cells may not be sufficient to obtain adequate levels of NIS gene expression.

Systemic Retinoic Acid Treatment Induces Sodium/Iodide Symporter Expression and Radioiodide Uptake in Mouse Breast Cancer Models

Lactating breast tissue and some breast cancers express the sodium/iodide symporter (NIS) and concentrate iodide. We recently demonstrated that all-trans retinoic acid (tRA) induces both NIS gene expression and iodide accumulation in vitro in well-differentiated human breast cancer cells (MCF-7). In the present study, we investigated the in vivo efficacy and specificity of tRA-stimulated iodide accumulation in mouse breast cancer models. Immunodeficient mice with MCF-7 xenograft tumors were treated with systemic tRA for 5 days. Iodide accumulation in the xenograft tumors was markedly increased, approximately 15-fold greater than levels without treatment, and the effects were tRA dose dependent. Iodide accumulation in other organs was not significantly influenced by tRA treatment. Significant induction of NIS mRNA and protein in the xenograft tumors was observed after tRA treatment. Iodide accumulation and NIS mRNA expression were also selectively induced in breast cancer tissues in transgenic mice expressing the oncogene, polyoma virus middle T antigen. These data demonstrate selective induction of functional NIS in breast cancer by tRA. Treatment with short-term systemic retinoic acid, followed by radioiodide administration, is a potential tool in the diagnosis and treatment of some differentiated breast cancer.

Cloning of the 5'-flanking region of mouse sodium/iodide symporter and identification of a thyroid-specific and TSH-responsive enhancer

The sodium/iodide symporter (NIS) mediates active iodide uptake into thyroid follicular cells and is important for the diagnosis and radioiodide treatment of thyroid cancers. In order to better investigate the transcriptional regulation of the NIS gene, we cloned the 3.2 kb 5'-flanking region of the mouse NIS (mNIS) gene in this study. The cloned 5'-flanking region of mNIS shares 68% identity with that of rat NIS (rNIS), yet has little similarity to that of human NIS (hNIS). Based on sequence homology to rNIS, the putative mNIS transcriptional start site is mapped to -97 nt relative to the ATG site. The minimal promoter of mNIS is located within 650 bp of the 5'-flanking region as determined by the transient expression analysis of promoter-reporter constructs. The mNIS upstream enhancer (mNUE) was identified based on sequence homology to rNUE. The mNUE is localized to the region between -3042 and -2809 nt relative to the ATG site and shares 94.4% identity with rat NUE (rNUE), while only 67.8% identity with human NUE (hNUE). It contains two Pax-8 binding sites and a Tax/CREB binding site. The mNUE is also demonstrated to confer thyroid-specific and TSH-responsive transcriptional activity. The high degree of homology in the 5'-flanking region between mNIS and rNIS suggests that mNIS and rNIS share similar mechanisms for transcriptional regulation.

Advances in Na(+)/I(-) symporter (NIS) research in the thyroid and beyond

The Na(+)/I(-) symporter (NIS) is a plasma membrane glycoprotein that mediates active iodide uptake in the thyroid-the essential first step in thyroid hormone biosynthesis-and in other tissues, such as salivary and lactating mammary glands. Thyroidal radioiodide uptake has been used for over 60 years in the diagnosis and effective treatment of thyroid cancer and other diseases. However, the NIS cDNA was only isolated in 1996 by expression cloning in Xenopus laevis oocytes, marking the beginning of the molecular characterization of NIS and the study of its regulation, both in the thyroid and other tissues. One of the most exciting current areas of NIS research-radioiodide treatment of extrathyroidal cancers-was launched by the discovery of functional expression of endogenous NIS in breast cancer and by the ectopic transfer of the NIS gene into otherwise non NIS-expressing cancers. This review summarizes the main findings in NIS research, emphasizing the most recent developments.

Graves' IgG activate upstream enhancer of the sodium/iodide symporter

Graves' thyroid tissue has been shown to express elevated levels of human sodium/iodide symporter (hNIS) mRNA and protein. In the present work, we demonstrate for the first time that hNIS gene expression in Graves' disease (GD) is up-regulated by Graves' IgG. Here, in transient transfection experiments using FRTL-5 cells, hNIS promoter and enhancer/luciferase construct showed an up to six-fold increase in transcriptional activity after incubation with purified Graves' IgG. Mutation of a CRE site in hNIS enhancer inhibited Graves' IgG response. In addition, mutation of a novel putative regulatory region in hNIS promoter reduced the stimulation three-fold. This discovered putative regulatory sequence might play a role in hNIS up-regulation by Graves' IgG and TSH. The data presented here complement our current knowledge of the pathogenesis of GD and will contribute to a better understanding of mechanisms regulating the thyroid iodide concentrating activity.

The functional interaction between the paired domain transcription factor Pax8 and Smad3 is involved in transforming growth factor-beta repression of the sodium/iodide symporter gene

Transforming growth factor-beta (TGF-beta) is a secreted protein that regulates proliferation, differentiation, and death in various cell types, including thyroid cells, although few details are known about its mechanisms of action in this cell type. Here, we studied the role of TGF-beta on the regulation of sodium/iodide symporter (NIS) gene expression in PC Cl3 thyroid cells. TGF-beta inhibits thyroid-stimulated hormone (TSH)-induced NIS mRNA and protein levels in a dose-dependent manner. This effect takes place at the transcriptional level, as TGF-beta inhibits TSH-induced transcription of a luciferase reporter construct containing a 2.8-kb DNA fragment of the rat NIS promoter. The inhibitory effect of TGF-beta was partially overcome by inhibitory Smad7 and mimicked by overexpression of either Smad3 or a constitutively activated mutant of TGF-beta receptor I (acALK-5). Using internal deletions of the promoter, we defined a region between -2,841 to -1,941, which includes the NIS upstream enhancer (NUE), as responsible for the TGF-beta/Smad inhibitory effect. NUE contains two binding sites for the paired domain transcription factor Pax8, the main factor controlling NIS transcription. The physical interaction observed between Pax8 and Smad3 appears to be responsible for the decrease in Pax8 binding to DNA. Expression of Pax8 mRNA and protein was also decreased by TGF-beta treatment. The results suggest that, through activation of Smad3, TGF-beta decreases Pax8 DNA binding activity as well as Pax8 mRNA and protein levels, which are at least partially involved in TGF-beta-induced down-regulation of NIS gene expression in thyroid follicular cells. Our results thus demonstrate a novel mechanism of Smad3 function in regulating thyroid cell differentiation by functionally antagonizing the action of the paired domain transcription factor Pax8.

Biology and clinical application of the NIS gene

The sodium iodide symporter (NIS) is a plasma basolateral membrane protein that actively transports iodide to the thyroid follicular cells as the first step of thyroid hormone biosynthesis. NIS also mediates active iodide transport in other human tissues including the salivary glands, lactating mammary gland and gastric mucosa. NIS expression has been recently reported also in several other human tissues but its physiological role is still unclear. Cloning of the NIS gene and the development of specific NIS antibodies have allowed the characterization of the pathogenic role of NIS in thyroid cancer, thyroid autoimmune diseases, congenital hypothyroidism and other, non-thyroidal human diseases. The possibility to increase its levels of expression or to reinduce its expression in thyroid carcinomas that have lost the ability to take up radioiodine is one of the most promising clinically related fields of research. The recent discovery that more than 80% of human breast carcinomas endogenously express NIS protein has opened a very interesting new area of research into the possibility of using radioiodide in the diagnosis and treatment of breast cancer. In an attempt to make tumor cells susceptible to radioiodide destruction, several types of cancer cells have been transfected with the NIS gene. This has demonstrated the feasibility of the in vitro technique but also raised the problem of the absence of the iodide organification machinery in non-thyroidal cells, which, at the moment, represents the major limit of this strategy.

The modified firefly luciferase reporter gene (luc+) but not Renilla luciferase is induced by all-trans retinoic acid in MCF-7 breast cancer cells

Luciferase genes are widely used as reporters to analyze promoter and regulatory elements. We found that a luciferase reporter gene vector with a modified firefly luciferase gene (luc+), but not Renilla luciferase (Rluc), was induced by all-trans retinoic acid (tRA) in the MCF-7 breast cancer cell line. tRA (5 x 10(-6) M) increased luciferase activity of the pGL3 promoter vector (containing luc+) up to approximately 3.8-fold in MCF-7 cells, but not in LNCaP prostate cancer cells or JEG-3 choriocarcinoma cells. Chimeric plasmids were constructed and showed that tRA-induction required the luc+ gene, but not any specific promoter or vector sequence. Time course and dose-response studies of tRA-induction indicated that longer treatment (> 24h) and higher tRA dose (> 10(-6) M) were required for luc+ induction compared with those for a positive retinoic acid response element (maximum induction at 6 h and 10(-8) M tRA). Studies with the translation inhibitor, cycloheximide, indicated the half-life of the luc+ protein was increased from 9.7 +/- 1.5 to 22.1 +/- 3.1 h with tRA treatment. Other retinoids, TTNPB, a retinoic acid receptor beta/gamma-specific ligand, and a retinoid X receptor ligand, did not significantly increase luc+ expression. Caution is needed in analysis of retinoid responsive gene regulation with the luciferase reporter system in MCF-7 cells, especially at high retinoid concentrations.