The paired-domain transcription factor Pax8 binds to the upstream enhancer of the rat sodium/iodide symporter gene and participates in both thyroid-specific and cyclic-AMP-dependent transcription

Thyroid transcription factors in development, differentiation and disease

Identification of the thyroid transcription factors (TTFs), NKX2-1, FOXE1, PAX8 and HHEX, has considerably advanced our understanding of thyroid development, congenital thyroid disorders and thyroid cancer. The TTFs are fundamental to proper formation of the thyroid gland and for maintaining the functional differentiated state of the adult thyroid; however, they are not individually required for precursor cell commitment to a thyroid fate. Although knowledge of the mechanisms involved in thyroid development has increased, the full complement of genes involved in thyroid gland specification and the signals that trigger expression of the genes that encode the TTFs remain unknown. The mechanisms involved in thyroid organogenesis and differentiation have provided clues to identifying the genes that are involved in human congenital thyroid disorders and thyroid cancer. Mutations in the genes that encode the TTFs, as well as polymorphisms and epigenetic modifications, have been associated with thyroid pathologies. Here, we summarize the roles of the TTFs in thyroid development and the mechanisms by which they regulate expression of the genes involved in thyroid differentiation. We also address the implications of mutations in TTFs in thyroid diseases and in diseases not related to the thyroid gland.

A de novo 10.79 Mb interstitial deletion at 2q13q14.2 involving PAX8 causing hypothyroidism and mullerian agenesis: a novel case report and literature review

Reports of interstitial deletions involving proximal long arm of chromosome 2 are limited. Based on early chromosomal analysis studies, the phenotypic consequence of deletions at the ancestral chromosome fusion site at chromosome 2q13q14.1 remains unclear. A recurrent 1.71 Mb deletion at 2q13 has recently been proposed as a new genomic disorder, associated with an increased risk of intellectual disability and craniofacial dysmorphism. Herein, we report the case of a 12 year-old girl with unique clinical features including global developmental delay, mullerian agenesis, and hypothyroidism associated with a normal size and position of the thyroid gland, as well as negative thyroid antibodies. Microarray-based comparative genomic hybridization study revealed a de novo 10.79 Mb deletion at 2q13q14.2 (111,548,932–122,336,492), which involves more than 88 UCSC genes, 38 of which are OMIM genes, 7 of which are disease-causing and 3 of which (including GLI2, IL1B and PAX8) show a dominant inheritance pattern.. Interestingly, PAX8 (chr2:113,973,574–114,036,498), a member of the paired-box gene family, is essential for the formation of thyroxine-producing follicular cells. Autosomal dominant transmission of congenital thyroid hypoplasia due to loss-of-function mutation of PAX8 suggests a possible haploinsufficiency effect. Additionally, PAX8 is also expressed in the tissue primordia that form both the mullerian duct derivatives and the upper urinary tracts. A recent study has associated a novel PAX8 mutation with a severe form of hypothyroidism and abnormalities in the urogenital tract. Taken together, the unique clinical manifestation seen in this patient could be attributed to the heterozygous deletion of PAX8 gene. A prospective investigation is merited to fully evaluate the pathogenic effect of the interstitial deletion of 2q13q14.2.

The paired box transcription factor Pax8 is essential for function and survival of adult thyroid cells

The transcription factor Pax8 is already known to be essential at very early stages of mouse thyroid gland development, before the onset of thyroid hormone production. In this paper we show, using a conditional inactivation strategy, that the removal of the Pax8 protein late in gland development results in severe hypothyroidism, consequent to a reduced gland size and a deranged differentiation. These results demonstrate that Pax8 is also an essential player in controlling survival and differentiation of adult thyroid follicular cells.

Modulation of sodium iodide symporter in thyroid cancer

Radioactive iodine (RAI) is a key therapeutic modality for thyroid cancer. Loss of RAI uptake in thyroid cancer inversely correlates with patient's survival. In this review, we focus on the challenges encountered in delivering sufficient doses of I-131 to eradicate metastatic lesions without increasing the risk of unwanted side effects. Sodium iodide symporter (NIS) mediates iodide influx, and NIS expression and function can be selectively enhanced in thyroid cells by thyroid-stimulating hormone. We summarize our current knowledge of NIS modulation in normal and cancer thyroid cells, and we propose that several reagents evaluated in clinical trials for other diseases can be used to restore or further increase RAI accumulation in thyroid cancer. Once validated in preclinical mouse models and clinical trials, these reagents, mostly small-molecule inhibitors, can be readily translated into clinical practice. We review available genetically engineered mouse models of thyroid cancer in terms of their tumor development and progression as well as their thyroid function. These mice will not only provide important insights into the mechanisms underlying the loss of RAI uptake in thyroid tumors but will also serve as preclinical animal models to evaluate the efficacy of candidate reagents to selectively increase RAI uptake in thyroid cancers. Taken together, we anticipate that the optimal use of RAI in the clinical management of thyroid cancer is yet to come in the near future.

NIS mediates iodide uptake in the female reproductive tract and is a poor prognostic factor in ovarian cancer

CONTEXT

The sodium iodide symporter (NIS) mediates active transport of iodide into the thyroid and the lactating mammary glands and is highly expressed in thyroid and breast carcinomas. NIS is clinically very relevant because it allows the treatment with radioiodine of thyroid cancer patients.

OBJECTIVE

In this study we wanted to explore whether NIS is expressed in the ovary and in ovarian cancer.

METHODS/PATIENTS

Methods included NIS and paired box 8 expression and function in ovarian cancer patients and rats by immunochemistry, immunoblot, RT-PCR, and iodide uptake.

RESULTS

Here we demonstrate for the first time that NIS is expressed in the ovary and fallopian tube and actively accumulates significant levels of radioiodide in vivo. In a large survey of menstruating women receiving radioiodide for medical purposes, 15% showed significant uptake in the normal reproductive tract. Ovarian NIS activity is influenced by the estrous cycle stage in rats, being up-regulated during peak levels of estrogens occurring immediately before the ovulation. We unveil that the regulatory mechanism underlying this phenomenon is based on the functional cooperation of estrogen receptor-α and paired box 8. We also show that NIS is highly expressed in ovarian cancer, predicting a poor prognosis in these patients.

CONCLUSIONS

These results provide the basis that will help minimize the impact of therapeutic doses of radioiodide on gonadal function. We also suggest that NIS is a new ovarian cancer marker, opening a door for the use of radioiodide in the diagnosis and treatment of ovarian cancer patients.

Essential role of the ERK/MAPK pathway in blood-placental barrier formation

The mammalian genome contains two ERK/MAP kinase kinase genes, Map2k1 and Map2k2, which encode dual-specificity kinases responsible for ERK activation. Loss of Map2k1 function in mouse causes embryonic lethality due to placental defects, whereas Map2k2 mutants have a normal lifespan. The majority of Map2k1(+/-) Map2k2(+/-) embryos die during gestation from the underdevelopment of the placenta labyrinth, demonstrating that both kinases are involved in placenta formation. Map2k1(+/-) Map2k2(+/-) mutants show reduced vascularization of the labyrinth and defective formation of syncytiotrophoblast layer II (SynT-II) leading to the accumulation of multinucleated trophoblast giant cells (MTGs). To define the cell type-specific contribution of the ERK/MAPK pathway to placenta development, we performed deletions of Map2k1 function in different Map2k1 Map2k2 allelic backgrounds. Loss of MAP kinase kinase activity in pericytes or in allantois-derived tissues worsens the MTG phenotype. These results define the contribution of the ERK/MAPK pathway in specific embryonic and extraembryonic cell populations for normal placentation. Our data also indicate that MTGs could result from the aberrant fusion of SynT-I and -II. Using mouse genetics, we demonstrate that the normal development of SynT-I into a thin layer of multinucleated cells depends on the presence of SynT-II. Lastly, the combined mutations of Map2k1 and Map2k2 alter the expression of several genes involved in cell fate specification, cell fusion and cell polarity. Thus, appropriate ERK/MAPK signaling in defined cell types is required for the proper growth, differentiation and morphogenesis of the placenta.

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.

The Na+/I- symporter (NIS): mechanism and medical impact

The Na(+)/I(-) symporter (NIS) is the plasma membrane glycoprotein that mediates active I(-) transport in the thyroid and other tissues, such as salivary glands, stomach, lactating breast, and small intestine. In the thyroid, NIS-mediated I(-) uptake plays a key role as the first step in the biosynthesis of the thyroid hormones, of which iodine is an essential constituent. These hormones are crucial for the development of the central nervous system and the lungs in the fetus and the newborn and for intermediary metabolism at all ages. Since the cloning of NIS in 1996, NIS research has become a major field of inquiry, with considerable impact on many basic and translational areas. In this article, we review the most recent findings on NIS, I(-) homeostasis, and related topics and place them in historical context. Among many other issues, we discuss the current outlook on iodide deficiency disorders, the present stage of understanding of the structure/function properties of NIS, information gleaned from the characterization of I(-) transport deficiency-causing NIS mutations, insights derived from the newly reported crystal structures of prokaryotic transporters and 3-dimensional homology modeling, and the novel discovery that NIS transports different substrates with different stoichiometries. A review of NIS regulatory mechanisms is provided, including a newly discovered one involving a K(+) channel that is required for NIS function in the thyroid. We also cover current and potential clinical applications of NIS, such as its central role in the treatment of thyroid cancer, its promising use as a reporter gene in imaging and diagnostic procedures, and the latest studies on NIS gene transfer aimed at extending radioiodide treatment to extrathyroidal cancers, including those involving specially engineered NIS molecules.

Impact of Metformin and compound C on NIS expression and iodine uptake in vitro and in vivo: a role for CRE in AMPK modulation of thyroid function

BACKGROUND

Although adenosine monophosphate activated protein kinase (AMPK) plays a crucial role in energy metabolism, a direct effect of AMPK modulation on thyroid function has only recently been reported, and much of its function in the thyroid is currently unknown. The aim of this study was to investigate the mechanism of AMPK modulation in iodide uptake. Furthermore, we wanted to investigate the potential of the AMPK inhibitor compound C as an enhancer of iodide uptake by thyrocytes.

METHODS

The in vitro and in vivo effects of AMPK modulation on sodium-iodide symporter (NIS) protein levels and iodide uptake were examined in follicular rat thyroid cell-line cells and C57Bl6/J mice. Activation of AMPK by metformin resulted in a strong reduction of iodide uptake (up to sixfold with 5 mM metformin after 96 h) and NIS protein levels in vitro, whereas AMPK inhibition by compound C not only stimulated iodide uptake but also enhanced NIS protein levels both in vitro (up to sevenfold with 1 μM compound C after 96 h) and in vivo (1.5-fold after daily injections with 20 mg/kg for 4 days). We investigated the regulation of NIS expression by AMPK using a range of promoter constructs consisting of either the NIS promoter or isolated CRE (cAMP response element) and NF-κB elements, which are present within the NIS promoter.

RESULTS

Metformin reduced NIS promoter activity (0.6-fold of control), whereas compound C stimulated its activity (3.4-fold) after 4 days. This largely coincides with CRE activation (0.6- and 3.0-fold). These experiments show that AMPK exerts its effects on iodide uptake, at least partly, through the CRE element in the NIS promoter. Furthermore, we have used AMPK-alpha1 knockout mice to determine the long-term effects of AMPK inhibition without chemical compounds. These mice have a less active thyroid, as shown by reduced colloid volume and reduced responsiveness to thyrotropin.

CONCLUSION

NIS expression and iodine uptake in thyrocytes can be modulated by metformin and compound C. These compounds exert their effect by modulation of AMPK, which, in turn, regulates the activation of the CRE element in the NIS promoter. Overall, this suggests that the use of AMPK modulating compounds may be useful for the enhancement of iodide uptake by thyrocytes, which could be useful for the treatment of thyroid cancer patients with radioactive iodine.

The thyroid cancer PAX8-PPARG fusion protein activates Wnt/TCF-responsive cells that have a transformed phenotype

A chromosomal translocation results in the production of a paired box 8-peroxisome proliferator-activated receptor gamma (PAX8-PPARG) fusion protein (PPFP) in ∼35% of follicular thyroid carcinomas. To examine the role of PPFP in thyroid oncogenesis, the fusion protein was stably expressed in the non-transformed rat thyroid cell line PCCL3. PPFP conferred on PCCL3 cells the ability to invade through Matrigel and to form colonies in anchorage-independent conditions. PPFP also increased the fraction of cells with Wnt/TCF-responsive green fluorescent protein reporter gene expression. This Wnt/TCF-activated population was enriched for colony-forming and invading cells. These actions of PPFP required a functional PPARG DNA binding domain (DBD) within PPFP and were further stimulated by PPARG agonists. These data indicate that PPFP, through its PPARG DBD, induces Wnt/TCF pathway activation in a subpopulation of cells, and these cells have properties of cellular transformation including increased invasiveness and anchorage-independent growth.

An epistatic interaction between the PAX8 and STK17B genes in papillary thyroid cancer susceptibility

Papillary Thyroid Cancer (PTC) is a heterogeneous and complex disease; susceptibility to PTC is influenced by the joint effects of multiple common, low-penetrance genes, although relatively few have been identified to date. Here we applied a rigorous combined approach to assess both the individual and epistatic contributions of genetic factors to PTC susceptibility, based on one of the largest series of thyroid cancer cases described to date. In addition to identifying the involvement of TSHR variation in classic PTC, our pioneer study of epistasis revealed a significant interaction between variants in STK17B and PAX8. The interaction was detected by MD-MBR (p = 0.00010) and confirmed by other methods, and then replicated in a second independent series of patients (MD-MBR p = 0.017). Furthermore, we demonstrated an inverse correlation between expression of PAX8 and STK17B in a set of cell lines derived from human thyroid carcinomas. Overall, our work sheds additional light on the genetic basis of thyroid cancer susceptibility, and suggests a new direction for the exploration of the inherited genetic contribution to disease using association studies.

The sodium/iodide symporter NIS is a transcriptional target of the p53-family members in liver cancer cells

Thyroid iodide accumulation via the sodium/iodide symporter (NIS; SLC5A5) has been the basis for the longtime use of radio-iodide in the diagnosis and treatment of thyroid cancers. NIS is also expressed, but poorly functional, in some non-thyroid human cancers. In particular, it is much more strongly expressed in cholangiocarcinoma (CCA) and hepatocellular carcinoma (HCC) cell lines than in primary human hepatocytes (PHH). The transcription factors and signaling pathways that control NIS overexpression in these cancers is largely unknown. We identified two putative regulatory clusters of p53-responsive elements (p53REs) in the NIS core promoter, and investigated the regulation of NIS transcription by p53-family members in liver cancer cells. NIS promoter activity and endogenous NIS mRNA expression are stimulated by exogenously expressed p53-family members and significantly reduced by member-specific siRNAs. Chromatin immunoprecipitation analysis shows that the p53–REs clusters in the NIS promoter are differentially occupied by the p53-family members to regulate basal and DNA damage-induced NIS transcription. Doxorubicin strongly induces p53 and p73 binding to the NIS promoter, leading to an increased expression of endogenous NIS mRNA and protein in HCC and CCA cells, but not in PHH. Silencing NIS expression reduced doxorubicin-induced apoptosis in HCC cells, pointing to a possible role of a p53-family-dependent expression of NIS in apoptotic cell death. Altogether, these results indicate that the NIS gene is a direct target of the p53 family and suggests that the modulation of NIS by DNA-damaging agents is potentially exploitable to boost NIS upregulation in vivo.

Hypothyroxinemia induced by mild iodine deficiency deregulats thyroid proteins during gestation and lactation in dams

The main object of the present study was to explore the effect on thyroidal proteins following mild iodine deficiency (ID)-induced maternal hypothyroxinemia during pregnancy and lactation. In the present study, we established a maternal hypothyroxinemia model in female Wistar rats by using a mild ID diet. Maternal thyroid iodine content and thyroid weight were measured. Expressions of thyroid-associated proteins were analyzed. The results showed that the mild ID diet increased thyroid weight, decreased thyroid iodine content and increased expressions of thyroid transcription factor 1, paired box gene 8 and Na+/I- symporter on gestational day (GD) 19 and postpartum days (PN) 21 in the maternal thyroid. Moreover, the up-regulated expressions of type 1 iodothyronine deiodinase (DIO1) and type 2 iodothyronine deiodinase (DIO2) were detected in the mild ID group on GD19 and PN21. Taken together, our data indicates that during pregnancy and lactation, a maternal mild ID could induce hypothyroxinemia and increase the thyroidal DIO1 and DIO2 levels.

Pax8 has a critical role in epithelial cell survival and proliferation

The transcription factor Pax8, a member of the Paired-box gene family, is a critical regulator required for proper development and differentiation of thyroid follicular cells. Despite being Pax8 well characterized with respect to its role in regulating genes responsible for thyroid differentiation, its involvement in cell survival and proliferation has been hypothesized but remains unclear. Here, we show that Pax8 overexpression significantly increases proliferation and colony-forming efficiency of Fischer rat thyroid line 5 epithelial cells, although it is not sufficient to overcome their hormone dependence. More interestingly, we show that Pax8-specific silencing induces apoptosis through a p53-dependent pathway that involves caspase-3 activation and cleavage of poly(ADP)ribose polymerase. Our data indicate that tumor protein 53 induced nuclear protein 1 (tp53inp1), a positive regulator of p53-dependent cell cycle arrest and apoptosis, is a transcriptional target of Pax8 and is upregulated by Pax8 knockdown. Remarkably, tp53inp1 silencing significantly abolishes Pax8-induced apoptosis thus suggesting that tp53inp1 may be the mediator of the observed effects. In conclusion, our data highlight that Pax8 is required for the survival of differentiated epithelial cells and its expression levels are able to modulate the proliferation rate of such cells.

New insights into FoxE1 functions: identification of direct FoxE1 targets in thyroid cells

BACKGROUND

FoxE1 is a thyroid-specific forkhead transcription factor essential for thyroid gland development, as well as for the maintenance of the thyroid differentiated state in adults. FoxE1 recognizes and binds to a short DNA sequence present in thyroglobulin (Tg) and thyroperoxidase (Tpo) promoters, but FoxE1 binding to regulatory regions other than Tg and Tpo promoters remains almost unexplored. Improving knowledge of the regulatory functions of FoxE1 is necessary to clarify its role in endocrine syndromes and cancer susceptibility.

METHODOLOGY/PRINCIPAL FINDING

In order to further investigate downstream FoxE1 targets, we performed a genome-wide expression screening after knocking-down FoxE1 and obtained new insights into FoxE1 transcriptional networks in thyroid follicular cells. After validation, we confirmed Adamts9, Cdh1, Duox2 and S100a4 as upregulated genes and Casp4, Creld2, Dusp5, Etv5, Hsp5a, Nr4a2 and Tm4sf1 as downregulated genes when FoxE1 was silenced. In promoter regions of putative FoxE1-regulated genes and also in the promoters of the classical thyroid genes Nis, Pax8 and Titf1, we performed an in silico search of the FoxE1 binding motif that was in close proximity to the NF1/CTF binding sequence, as previously described for other forkhead factors. Using chromatin immunoprecipitation we detected specific in vivo FoxE1 binding to novel regulatory regions in two relevant thyroid genes, Nis and Duox2. Moreover, we demonstrated simultaneous binding of FoxE1 and NF1/CTF to the Nis upstream enhancer region, as well as a clear functional activation of the Nis promoter by both transcription factors.

CONCLUSIONS/SIGNIFICANCE

In search for potential downstream mediators of FoxE1 function in thyroid cells, we identified two novel direct FoxE1 target genes. To our knowledge, this is the first evidence regarding the implication of Nis and Duox2 in executing the transcriptional program triggered by FoxE1. Furthermore, this study points out the important role of FoxE1 in the regulation of a large number of genes in thyroid cells.

Functional analysis of the murine Pax8 promoter reveals autoregulation and the presence of a novel thyroid-specific DNA-binding activity

BACKGROUND

Organogenesis of the thyroid gland requires the Pax8 protein. Absence or reduction of Pax8 results in congenital hypothyroidism in animal models and humans, respectively. This study aims at elucidating the regulatory mechanism leading to the expression of Pax8 in thyroid cells.

METHODS

The murine Pax8 gene promoter was functionally dissected by mutagenesis and transfection in the thyroid cell line FRTL-5. Nuclear factors important for thyroid-specific gene expression were identified by DNA-binding assays.

RESULTS

We show that Pax8 binds to and controls the expression of its own promoter. Furthermore, we identify a novel, thyroid-specific, DNA-binding activity (denominated nTTF [for novel Thyroid Transcription Factor]) that recognizes a specific region of the Pax8 promoter.

CONCLUSIONS

The Pax8 promoter appears to be autoregulated, a feature that might be responsible for the haploinsufficiency displayed by this gene.

Sterol regulatory element-binding proteins are regulators of the NIS gene in thyroid cells

The uptake of iodide into the thyroid, an essential step in thyroid hormone synthesis, is an active process mediated by the sodium-iodide symporter (NIS). Despite its strong dependence on TSH, the master regulator of the thyroid, the NIS gene was also reported to be regulated by non-TSH signaling pathways. In the present study we provide evidence that the rat NIS gene is subject to regulation by sterol regulatory element-binding proteins (SREBPs), which were initially identified as master transcriptional regulators of lipid biosynthesis and uptake. Studies in FRTL-5 thyrocytes revealed that TSH stimulates expression and maturation of SREBPs and expression of classical SREBP target genes involved in lipid biosynthesis and uptake. Almost identical effects were observed when the cAMP agonist forskolin was used instead of TSH. In TSH receptor-deficient mice, in which TSH/cAMP-dependent gene regulation is blocked, the expression of SREBP isoforms in the thyroid was markedly reduced when compared with wild-type mice. Sterol-mediated inhibition of SREBP maturation and/or RNA interference-mediated knockdown of SREBPs reduced expression of NIS and NIS-specific iodide uptake in FRTL-5 cells. Conversely, overexpression of active SREBPs caused a strong activation of the 5'-flanking region of the rat NIS gene mediated by binding to a functional SREBP binding site located in the 5'-untranslated region of the rat NIS gene. These findings show that TSH acts as a regulator of SREBP expression and maturation in thyroid epithelial cells and that SREBPs are novel transcriptional regulators of NIS.

Screening of PAX8 mutations in Chinese patients with congenital hypothyroidism

BACKGROUND

Congenital hypothyroidism (CH) is a neonatal endocrine disease with an incidence of 1:2000 to 1:4000 worldwide. In about 85% of patients CH is secondary to thyroid dysgenesis, but its pathogenesis remains unclear. Thyroid transcription factors, such as paired box transcription factor 8 (PAX8), play an important role in thyroid organogenesis and development.

AIM

To screen PAX8 mutations in Chinese CH patients and characterize the features of PAX8 mutations in China.

MATERIALS AND METHODS

Blood samples were collected from 300 CH patients in Shandong Province, China, and genomic DNA was extracted from peripheral blood leukocytes. Using PCR and direct sequencing, exon 3 and exon 4 of PAX8 were analyzed.

RESULTS

Analysis of PAX8 in 300 CH patients revealed heterozygous missense mutations or variations in two unrelated patients; one was a known missense mutation G92A, resulting in an arginine to histidine substitution at codon 31, the other was a missense variation G122T, resulting in the substitution of a glycine at position 41 by a valine residue. The patient with the R31H mutation had CH with thyroid hypoplasia, while the patient with the G41V variation had CH with a eutopic and normal-sized thyroid gland.

CONCLUSION

We report a heterozygous missense mutation and a variation in PAX8 in two out of 300 unrelated Chinese CH patients, showing that the PAX8 mutation rate is very low in CH patients in China.

The sodium iodide symporter (NIS): regulation and approaches to targeting for cancer therapeutics

Expression of the sodium iodide symporter (NIS) is required for efficient iodide uptake in thyroid and lactating breast. Since most differentiated thyroid cancer expresses NIS, β-emitting radioactive iodide is routinely utilized to target remnant thyroid cancer and metastasis after total thyroidectomy. Stimulation of NIS expression by high levels of thyroid-stimulating hormone is necessary to achieve radioiodide uptake into thyroid cancer that is sufficient for therapy. The majority of breast cancer also expresses NIS, but at a low level insufficient for radioiodine therapy. Retinoic acid is a potent NIS inducer in some breast cancer cells. NIS is also modestly expressed in some non-thyroidal tissues, including salivary glands, lacrimal glands and stomach. Selective induction of iodide uptake is required to target tumors with radioiodide. Iodide uptake in mammalian cells is dependent on the level of NIS gene expression, but also successful translocation of NIS to the cell membrane and correct insertion. The regulatory mechanisms of NIS expression and membrane insertion are regulated by signal transduction pathways that differ by tissue. Differential regulation of NIS confers selective induction of functional NIS in thyroid cancer cells, as well as some breast cancer cells, leading to more efficient radioiodide therapy for thyroid cancer and a new strategy for breast cancer therapy. The potential for systemic radioiodide treatment of a range of other cancers, that do not express endogenous NIS, has been demonstrated in models with tumor-selective introduction of exogenous NIS.

Cyclic AMP-response element modulator inhibits the promoter activity of the sodium iodide symporter gene in thyroid cancer cells

BACKGROUND

Comprehension of the regulatory mechanism involved in the sodium iodide symporter (NIS) expression is of great relevance for thyroid cancer. In fact, restoration of NIS expression would be a strategy to treat undifferentiated thyroid cancer. Previous in vitro findings suggest that the cyclic AMP-response element (CRE) modulator (CREM) is involved in control of NIS expression. In this work, we examined the expression of CREM in a series of thyroid cancer tissues and its action on NIS promoter in human thyroid cancer cells.

METHODS

Expression of mRNA levels for CREM, PAX8 and NIS was measured by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in 6 normal thyroid tissues, 22 papillary, 12 follicular and 4 anaplastic thyroid cancers. The effect of CREM on transcriptional activity of the NIS promoter was investigated by transient transfection of human thyroid cell lines.

RESULTS

Compared to normal tissues, NIS and PAX8 mRNA levels were significantly reduced in all types of thyroid cancer. As expected, the maximal decrease was detected in anaplastic thyroid cancer. Conversely, CREM mRNA levels were increased in all types of thyroid cancer, reaching statistical significance for follicular and anaplastic thyroid carcinoma (p=0.0157 and 0.0045, respectively). Transfection experiments showed an inhibitory effect of CREM on NIS promoter activity in various thyroid cancer cell lines.

CONCLUSIONS

These data demonstrate that CREM expression is increased in thyroid cancer tissue and may play a role in the downregulation of NIS expression in thyroid cancer acting at the transcriptional level.

Genome-wide analysis of Pax8 binding provides new insights into thyroid functions

Background

The transcription factor Pax8 is essential for the differentiation of thyroid cells. However, there are few data on genes transcriptionally regulated by Pax8 other than thyroid-related genes. To better understand the role of Pax8 in the biology of thyroid cells, we obtained transcriptional profiles of Pax8-silenced PCCl3 thyroid cells using whole genome expression arrays and integrated these signals with global cis-regulatory sequencing studies performed by ChIP-Seq analysis

Results

Exhaustive analysis of Pax8 immunoprecipitated peaks demonstrated preferential binding to intragenic regions and CpG-enriched islands, which suggests a role of Pax8 in transcriptional regulation of orphan CpG regions. In addition, ChIP-Seq allowed us to identify Pax8 partners, including proteins involved in tertiary DNA structure (CTCF) and chromatin remodeling (Sp1), and these direct transcriptional interactions were confirmed in vivo. Moreover, both factors modulate Pax8-dependent transcriptional activation of the sodium iodide symporter (Nis) gene promoter. We ultimately combined putative and novel Pax8 binding sites with actual target gene expression regulation to define Pax8-dependent genes. Functional classification suggests that Pax8-regulated genes may be directly involved in important processes of thyroid cell function such as cell proliferation and differentiation, apoptosis, cell polarity, motion and adhesion, and a plethora of DNA/protein-related processes.

Conclusion

Our study provides novel insights into the role of Pax8 in thyroid biology, exerted through transcriptional regulation of important genes involved in critical thyrocyte processes. In addition, we found new transcriptional partners of Pax8, which functionally cooperate with Pax8 in the regulation of thyroid gene transcription. Besides, our data demonstrate preferential location of Pax8 in non-promoter CpG regions. These data point to an orphan CpG island-mediated mechanism that represents a novel role of Pax8 in the transcriptional output of the thyrocyte.

Combining transfer of TTF-1 and Pax-8 gene: a potential strategy to promote radioiodine therapy of thyroid carcinoma

Cotransfer of thyroid-specific transcription factor (TTF)-1 and Pax-8 gene to tumor cells, resulting in the re-expression of iodide metabolism-associated proteins, such as sodium iodide symporter (NIS), thyroglobulin (Tg), thyroperoxidase (TPO), offers the possibility of radioiodine therapy to non-iodide-concentrating tumor because the expression of iodide metabolism-associated proteins in thyroid are mediated by the thyroid transcription factor TTF-1 and Pax-8. The human TTF-1 and Pax-8 gene were transducted into the human thyroid carcinoma (K1 and F133) cells by the recombinant adenovirus, AdTTF-1 and AdPax-8. Re-expression of NIS mRNA and protein, but not TPO and Tg mRNA and protein, was detected in AdTTF-1-infected F133 cells, following with increasing radioiodine uptake (6.1-7.4 times), scarcely iodide organification and rapid iodide efflux (t(1/2) ≈ 8-min in vitro, t(1/2) ≈ 4.7-h in vivo). On contrast, all of the re-expression of NIS, TPO and Tg mRNA and proteins were detected in F133 cells coinfected with AdTTF-1 and AdPax-8. AdTTF-1- and AdPax-8-coinfected K1 and F133 cells could effectively accumulate radioiodine (6.6-7.5 times) and obviously retarded radioiodine retention (t(1/2) ≈ 25-30-min in vitro, t(1/2) ≈ 12-h in vivo) (P<0.05). Accordingly, the effect of radioiodine therapy of TTF-1 and Pax-8 cotransducted K1 and F133 cells (21-25% survival rate in vitro) was better than that of TTF-1-transducted cells (40% survival rate in vitro) (P<0.05). These results indicate that single TTF-1 gene transfer may have limited efficacy of radioiodine therapy because of rapid radioiodine efflux. The cotransduction of TTF-1 and Pax-8 gene, with resulting NIS-mediated radioiodine accumulation and TPO and Tg-mediated radioiodine organification and intracellular retention, may lead to effective radioiodine therapy of thyroid carcinoma.

An essential role for Pax8 in the transcriptional regulation of cadherin-16 in thyroid cells

Cadherin-16 was originally identified as a tissue-specific cadherin present exclusively in kidney. Only recently, Cadherin-16 has been detected also on the plasma membrane of mouse thyrocytes. This last finding prompted us to note that the expression profile of Cadherin-16 resembles that of the transcription factor Pax8, a member of the Pax (paired-box) gene family, predominantly expressed in the developing and adult kidney and thyroid. Pax8 has been extensively characterized in the thyroid and shown to be a master gene for thyroid development and differentiation. In this study, we determined the role of the transcription factor Pax8 in the regulation of Cadherin-16 expression. We demonstrate that the Cadherin-16 minimal promoter is transcriptionally active in thyroid cells as well as in kidney cells, that Pax8 is able to activate transcription from a Cadherin-16 promoter reporter construct, and more importantly, that indeed Pax8 is able to bind in vivo the Cadherin-16 promoter region. In addition, by means of Pax8 RNA interference in thyroid cells and by analyzing Pax8 null mice, we demonstrate that Pax8 regulates also in vivo the expression of Cadherin-16. Finally, we reveal that the expression of Cadherin-16 is TSH dependent in FRTL-5 thyroid cells and significantly reduced in mouse thyroid carcinomas. Therefore, we conclude that Cadherin-16 is a novel downstream target of the transcription factor Pax8, likely since the early steps of thyroid development, and that its expression is associated with the fully differentiated state of the thyroid cell.

Left ventricular remodeling in swine after myocardial infarction: a transcriptional genomics approach

Despite the apparent appropriateness of left ventricular (LV) remodeling following myocardial infarction (MI), it poses an independent risk factor for development of heart failure. There is a paucity of studies into the molecular mechanisms of LV remodeling in large animal species. We took an unbiased molecular approach to identify candidate transcription factors (TFs) mediating the genetic reprogramming involved in post-MI LV remodeling in swine. Left ventricular tissue was collected from remote, non-infarcted myocardium, 3 weeks after MI-induction or sham-surgery. Microarray analysis identified 285 upregulated and 278 downregulated genes (FDR < 0.05). Of these differentially expressed genes, the promoter regions of the human homologs were searched for common TF binding sites (TFBS). Eighteen TFBS were overrepresented >two-fold (p < 0.01) in upregulated and 13 in downregulated genes. Left ventricular nuclear protein extracts were assayed for DNA-binding activity by protein/DNA array. Out of 345 DNA probes, 30 showed signal intensity changes >two-fold. Five TFs were identified in both TFBS and protein/DNA array analyses, which showed matching changes for COUP-TFII and glucocorticoid receptor (GR) only. Treatment of swine with the GR antagonist mifepristone after MI reduced the post-MI increase in LV mass, but LV dilation remained unaffected. Thus, using an unbiased approach to study post-MI LV remodeling in a physiologically relevant large animal model, we identified COUP-TFII and GR as potential key mediators of post-MI remodeling.

Pioglitazone induces a proadipogenic antitumor response in mice with PAX8-PPARgamma fusion protein thyroid carcinoma

Approximately 35% of follicular thyroid carcinomas harbor a chromosomal translocation that results in expression of a paired box gene 8-peroxisome proliferator-activated receptor γ gene (PPARγ) fusion protein (PPFP). To better understand the oncogenic role of PPFP and its relationship to endogenous PPARγ, we generated a transgenic mouse model that combines Cre-dependent PPFP expression (PPFP;Cre) with homozygous deletion of floxed Pten (PtenFF;Cre), both thyroid specific. Although neither PPFP;Cre nor PtenFF;Cre mice develop thyroid tumors, the combined PPFP;PtenFF;Cre mice develop metastatic thyroid cancer, consistent with patient data that PPFP is occasionally found in benign thyroid adenomas and that PPFP carcinomas have increased phosphorylated AKT/protein kinase B. We then tested the effects of the PPARγ agonist pioglitazone in our mouse model. Pioglitazone had no effect on PtenFF;Cre mouse thyroids. However, the thyroids in pioglitazone-fed PPFP;PtenFF;Cre mice decreased 7-fold in size, and metastatic disease was prevented. Remarkably, pioglitazone caused an adipogenic response in the PPFP;PtenFF;Cre thyroids characterized by lipid accumulation and the induction of a broad array of adipocyte PPARγ target genes. These data indicate that, in the presence of pioglitazone, PPFP has PPARγ-like activity that results in trans-differentiation of thyroid carcinoma cells into adipocyte-like cells. Furthermore, the data predict that pioglitazone will be therapeutic in patients with PPFP-positive carcinomas.

Identification of novel Pax8 targets in FRTL-5 thyroid cells by gene silencing and expression microarray analysis

Background

The differentiation program of thyroid follicular cells (TFCs), by far the most abundant cell population of the thyroid gland, relies on the interplay between sequence-specific transcription factors and transcriptional coregulators with the basal transcriptional machinery of the cell. However, the molecular mechanisms leading to the fully differentiated thyrocyte are still the object of intense study. The transcription factor Pax8, a member of the Paired-box gene family, has been demonstrated to be a critical regulator required for proper development and differentiation of thyroid follicular cells. Despite being Pax8 well-characterized with respect to its role in regulating genes involved in thyroid differentiation, genomics approaches aiming at the identification of additional Pax8 targets are lacking and the biological pathways controlled by this transcription factor are largely unknown.

Methodology/Principal Findings

To identify unique downstream targets of Pax8, we investigated the genome-wide effect of Pax8 silencing comparing the transcriptome of silenced versus normal differentiated FRTL-5 thyroid cells. In total, 2815 genes were found modulated 72 h after Pax8 RNAi, induced or repressed. Genes previously reported to be regulated by Pax8 in FRTL-5 cells were confirmed. In addition, novel targets genes involved in functional processes such as DNA replication, anion transport, kinase activity, apoptosis and cellular processes were newly identified. Transcriptome analysis highlighted that Pax8 is a key molecule for thyroid morphogenesis and differentiation.

Conclusions/Significance

This is the first large-scale study aimed at the identification of new genes regulated by Pax8, a master regulator of thyroid development and differentiation. The biological pathways and target genes controlled by Pax8 will have considerable importance to understand thyroid disease progression as well as to set up novel therapeutic strategies.

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

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

Radioiodine therapy of thyroid carcinoma following Pax-8 gene transfer

The thyroid transcription factor Pax-8 could bind with the promoter/enhancer of thyroid-specific genes such as thyroglobulin (Tg), thyroperoxidase (TPO) and sodium iodide symporter (NIS), and regulate the expression of these proteins in thyrocyte. Promoting iodide accumulation in tumor cells by re-expression of Pax-8 provides a possible strategy for radioiodine therapy of tumor. Therefore, we investigated the effect of Pax-8 gene transfer on radioiodine therapy of thyroid carcinoma. The human Pax-8 gene was transfected into the human thyroid carcinoma (K1 and F133) cells by the recombinant adenovirus vector. Although the NIS mRNA was not detected, the expression of mRNA and proteins of Tg and TPO in AdPax-8-infected F133 cells were activated by Pax-8. Iodide uptake in thyroid carcinoma cells was reactivated by Pax-8 (increasing 3.3-fold in K1 cells and 5.7-fold in F133 cells). Moreover, Pax-8 promoted iodide organification and the retention time of iodine in Pax-8-expressing cells apparently prolonged in vitro and in vivo (P<0.05). Pax-8-expressing thyroid carcinoma cells were selectively killed by radioiodine. The AdPax-8-infected tumors in vivo clearly visualized in scanning images at 12 h after administration of radioiodine. These results indicate that Pax-8 can promote iodide uptake, and specifically prolong the retention time of iodide in thyroid cancer in vitro and in vivo by promoting the expression of TPO and Tg proteins. Pax-8 gene transfection may lead to effective radioiodine therapy of tumor.

PAX8 promotes tumor cell growth by transcriptionally regulating E2F1 and stabilizing RB protein

The retinoblastoma protein (RB)–E2F1 pathway has a central role in regulating the cell cycle. Several PAX proteins (tissue-specific developmental regulators), including PAX8, interact with the RB protein, and thus regulate the cell cycle directly or indirectly. Here, we report that PAX8 expression is frequent in renal cell carcinoma, bladder, ovarian and thyroid cancer cell lines, and that silencing of PAX8 in cancer cell lines leads to a striking reduction in the expression of E2F1 and its target genes, as well as a proteasome-dependent destabilization of RB protein, with the RB1 mRNA level remaining unaffected. Cancer cells expressing PAX8 undergo a G₁/S arrest and eventually senesce following PAX8 silencing. We demonstrate that PAX8 transcriptionally regulates the E2F1 promoter directly, and E2F1 transcription is enhanced after RB depletion. RB is recruited to the PAX8-binding site, and is involved in PAX8-mediated E2F1 transcription in cancer cells. Therefore, our results suggest that, in cancer, frequent and persistent expression of PAX8 is required for cell growth control through transcriptional activation of E2F1 expression and upregulation of the RB–E2F1 pathway.

Thyroid-specific transcription factors and their roles in thyroid cancer

Homeodomain, forkhead domain, and paired domain-containing transcription factors play a major role in development, tissue-specific gene expression, and tissue homeostasis in organs where they are expressed. Recently, their roles in stem cell and cancer biology are emerging. In the thyroid, NKX2-1, FOXE1, and PAX8 transcription factors are responsible for thyroid organogenesis and expression of thyroid-specific genes critical for thyroid hormone synthesis. In contrast to their known roles in gene regulation, thyroid development and homeostasis, their involvement in stem cell, and/or cancer biology are still elusive. In order to further understand the nature of thyroid cancer, it is critical to determine their roles in thyroid cancer.

Characterization of a novel loss-of-function mutation of PAX8 associated with congenital hypothyroidism

BACKGROUND

Congenital hypothyroidism (CH) is a common endocrine disease that occurs in about 1:3000 newborns. In 80-85% of the cases, CH is presumably secondary to thyroid dysgenesis (TD), a defect in the organogenesis of the gland leading to an ectopic (30-45%), absent (agenesis, 35-40%) or hypoplastic (5%) thyroid gland. The pathogenesis of TD is still largely unknown. Most cases of TD are sporadic, although familial occurrences have occasionally been described. Recently, mutations in the PAX8 transcription factor have been identified in patients with TD.

OBJECTIVE

Our aim was to identify and functionally characterize novel PAX8 mutations with autosomal dominant transmission responsible for TD.

DESIGN

The PAX8 gene was sequenced in a mother and child both suffering from congenital hypothyroidism (CH) because of thyroid hypoplasia. Subsequently, expression vectors encoding the mutated PAX8 were generated, and the effects of the mutation on both the DNA-binding capability and the transcriptional activity were evaluated.

RESULTS

PAX8 gene sequencing revealed a heterozygous mutation that consists of the substitution of a histidine residue with a glutamine at position 55 of the PAX8 protein (H55Q). When tested in cotransfection experiments with a thyroglobulin promoter reporter construct, the mutant protein turned out to be still able to bind DNA in Electrophoretic Mobility Shift Assay assays but transcriptionally inactive.

CONCLUSIONS

Our findings confirm the important role of PAX8 in normal thyroid development and support the evidence that in humans haploinsufficiency of PAX8 is associated with TD.

The effects of amitrole on thyroglobulin and iodide uptake in FRTL-5 cells

Thyroid is a frequent target for endocrine effects of pesticides. Thyroglobulin (TG) and iodide uptake are crucial to thyroid hormone synthesis and may be targets of thyroid-disrupting chemicals. In our study, thyroid follicular FRTL-5 cells were treated with amitrole, an inhibitor of the thyroid peroxidase (TPO), and the effects on TG and total iodide uptake were observed. The results showed that 1-100 mg/L amitrole had no marked effects on FRTL-5 cell proliferation and DNA synthesis. However, it significantly increased the transcription of tg gene and inhibited the total iodide uptake. And 10-100 mg/L amitrole significantly decreased TG in the culture medium. The data suggests amitrole may disrupt the expression and secretion of TG and iodide uptake.

Mutations in the gene encoding paired box domain (PAX8) are not a frequent cause of congenital hypothyroidism (CH) in Iranian patients with thyroid dysgenesis

OBJECTIVE: Congenital hypothyroidism (CH) may be caused by defects in the thyroid or in one of the stages in the synthesis of thyroid hormones. Thyroid dysgenesis may be associated with mutation in the paired box transcription factor 8 (PAX8) gene. We attempted to screen PAX8 gene mutation in 50 CH patients with thyroid dysgenesis. SUBJECTS AND METHODS: The patients were classified in two groups as agenesis and ectopic based on biochemical and para clinical tests. By employing PCR, Single Strand Conformation Polymorphism (SSCP) and sequencing, exons 3 to 12 of PAX8 gene with their exon-intron boundaries were studied. RESULTS: No mutation was found in these patients in any of the exons. CONCLUSION: Our results, once again, indicate that the PAX8 mutation rate is very low and can only explain a minority of the cases. Therefore, it is highly needed to further investigate the genes controlling development and function of thyroid.

NF-kappaB p65 subunit mediates lipopolysaccharide-induced Na(+)/I(-) symporter gene expression by involving functional interaction with the paired domain transcription factor Pax8

The Gram-negative bacterial endotoxin lipopolysaccharide (LPS) elicits a variety of biological responses. Na(+)/I(-) symporter (NIS)-mediated iodide uptake is the main rate-limiting step in thyroid hormonogenesis. We have recently reported that LPS stimulates TSH-induced iodide uptake. Here, we further analyzed the molecular mechanism involved in the LPS-induced NIS expression in Fisher rat thyroid cell line 5 (FRTL-5) thyroid cells. We observed an increase in TSH-induced NIS mRNA expression in a dose-dependent manner upon LPS treatment. LPS enhanced the TSH-stimulated NIS promoter activity denoting the NIS-upstream enhancer region (NUE) as responsible for the stimulatory effects. We characterized a novel putative conserved kappaB site for the transcription factor nuclear factor-kappaB (NF-kappaB) within the NUE region. NUE contains two binding sites for the transcription factor paired box 8 (Pax8), main regulator of NIS transcription. A physical interaction was observed between the NF-kappaB p65 subunit and paired box 8 (Pax8), which appears to be responsible for the synergic effect displayed by these transcription factors on NIS gene transcription. Moreover, functional blockage of NF-kappaB signaling and site-directed mutagenesis of the kappaB cis-acting element abrogated LPS stimulation. Silencing expression of p65 confirmed its participation as an effector of LPS-induced NIS stimulation. Furthermore, chromatin immunoprecipitation corroborated that NIS is a novel target gene for p65 transactivation in response to LPS. Moreover, we were able to corroborate the LPS-stimulatory effect on thyroid cells in vivo in LPS-treated rats, supporting that thyrocytes are capable of responding to systemic infections. In conclusion, our results reveal a new mechanism involving p65 in the LPS-induced NIS expression, denoting a novel aspect in thyroid cell differentiation.

The biology of the sodium iodide symporter and its potential for targeted gene delivery

The sodium iodide symporter (NIS) is responsible for thyroidal, salivary, gastric, intestinal and mammary iodide uptake. It was first cloned from the rat in 1996 and shortly thereafter from human and mouse tissue. In the intervening years, we have learned a great deal about the biology of NIS. Detailed knowledge of its genomic structure, transcriptional and post-transcriptional regulation and pharmacological modulation has underpinned the selection of NIS as an exciting approach for targeted gene delivery. A number of in vitro and in vivo studies have demonstrated the potential of using NIS gene therapy as a means of delivering highly conformal radiation doses selectively to tumours. This strategy is particularly attractive because it can be used with both diagnostic (99mTc, 125I, 124I)) and therapeutic (131I, 186Re, 188Re, 211At) radioisotopes and it lends itself to incorporation with standard treatment modalities, such as radiotherapy or chemoradiotherapy. In this article, we review the biology of NIS and discuss its development for gene therapy.

Effect of thyroid statuses on sodium/iodide symporter (NIS) gene expression in the extrathyroidal tissues in mice

BACKGROUND

Iodide that is essential for thyroid hormone synthesis is actively transported into the thyroid follicular cells via sodium/iodide symporter (NIS) protein in vertebrates. It is well known that NIS expression in thyroid is regulated by the thyroid statuses mainly through thyroid stimulating hormone (TSH). Although NIS mRNA expressions in extrathyroidal tissues have been qualitatively reported, their regulation by thyroid statuses has not been well clarified.

METHODS

Male ICR mice aged four weeks were assigned into three groups (control, hypothyroid, and hyperthyroid). Hypothyroid group of mice were treated with 0.02% methimazole in drinking water and hyperthyroid group of mice received intraperitoneal injection (4 mug L-T4 twice a week) for four weeks. NIS mRNA expression levels in the tissues were evaluated using Northern blot hybridization and quantitative real-time RTPCR (qPCR). Additionally, end-point RTPCR for the thyroid follicular cell-characteristic genes (TSH receptor, TSHR; thyroid transcription factor-1, TTF1; and paired box gene 8, Pax8) was carried out.

RESULTS

By Northern blot analysis, NIS mRNA was detected in thyroid and stomach. In addition to these organs, qPCR revealed the expression also in the submandibular gland, colon, testis, and lung. Expression of NIS mRNA in thyroid was significantly increased in hypothyroid and decreased in hyperthyroid group. Trends of NIS mRNA expression in extrathyroidal tissues were not in line with that in the thyroid gland in different thyroid statuses. Only in lung, NIS mRNA was regulated by thyroid statuses but in opposite way compared to the manner in the thyroid gland. There were no extrathyroidal tissues that expressed all three characteristic genes of thyroid follicular cells.

CONCLUSIONS

NIS mRNA expression in the thyroid gland was up-regulated in hypothyroid mice and was down-regulated in hyperthyroid mice, suggesting that NIS mRNA in the thyroid gland is regulated by thyroid statuses. In contrast, NIS mRNA expression in extrathyroidal tissues was not altered by thyroid statuses although it was widely expressed. Lack of responsiveness of NIS mRNA expressions in extrathyroidal tissues reemphasizes additional functions of NIS protein in extrathyroidal tissues other than iodide trapping.

Comparative genomics reveals a functional thyroid-specific element in the far upstream region of the PAX8 gene

Background

The molecular mechanisms leading to a fully differentiated thyrocite are still object of intense study even if it is well known that thyroglobulin, thyroperoxidase, NIS and TSHr are the marker genes of thyroid differentiation. It is also well known that Pax8, TTF-1, Foxe1 and Hhex are the thyroid-enriched transcription factors responsible for the expression of the above genes, thus are responsible for the differentiated thyroid phenotype. In particular, the role of Pax8 in the fully developed thyroid gland was studied in depth and it was established that it plays a key role in thyroid development and differentiation. However, to date the bases for the thyroid-enriched expression of this transcription factor have not been unraveled yet. Here, we report the identification and characterization of a functional thyroid-specific enhancer element located far upstream of the Pax8 gene.

Results

We hypothesized that regulatory cis-acting elements are conserved among mammalian genes. Comparison of a genomic region extending for about 100 kb at the 5'-flanking region of the mouse and human Pax8 gene revealed several conserved regions that were tested for enhancer activity in thyroid and non-thyroid cells. Using this approach we identified one putative thyroid-specific regulatory element located 84.6 kb upstream of the Pax8 transcription start site. The in silico data were verified by promoter-reporter assays in thyroid and non-thyroid cells. Interestingly, the identified far upstream element manifested a very high transcriptional activity in the thyroid cell line PC Cl3, but showed no activity in HeLa cells. In addition, the data here reported indicate that the thyroid-enriched transcription factor TTF-1 is able to bind in vitro and in vivo the Pax8 far upstream element, and is capable to activate transcription from it.

Conclusions

Results of this study reveal the presence of a thyroid-specific regulatory element in the 5' upstream region of the Pax8 gene. The identification of this regulatory element represents the first step in the investigation of upstream regulatory mechanisms that control Pax8 transcription during thyroid differentiation and are relevant to further studies on Pax8 as a candidate gene for thyroid dysgenesis.

Intronic elements in the Na+/I- symporter gene (NIS) interact with retinoic acid receptors and mediate initiation of transcription

Activity of the sodium/iodide symporter (NIS) in lactating breast is essential for iodide (I(-)) accumulation in milk. Significant NIS upregulation was also reported in breast cancer, indicating a potential use of radioiodide treatment. All-trans-retinoic acid (tRA) is a potent ligand that enhances NIS expression in a subset of breast cancer cell lines and in experimental breast cancer models. Indirect tRA stimulation of NIS in breast cancer cells is very well documented; however, direct upregulation by tRA-activated nuclear receptors has not been identified yet. Aiming to uncover cis-acting elements directly regulating NIS expression, we screened evolutionary-conserved non-coding genomic sequences for responsiveness to tRA in MCF-7. Here, we report that a potent enhancer in the first intron of NIS mediates direct regulation by tRA-stimulated nuclear receptors. In vitro as well as in vivo DNA-protein interaction assays revealed direct association between retinoic acid receptor-alpha (RARalpha) and retinoid-X-receptor (RXR) with this enhancer. Moreover, using chromatin immunoprecipitation (ChIP) we uncovered early events of NIS transcription in response to tRA, which require the interaction of several novel intronic tRA responsive elements. These findings indicate a complex interplay between nuclear receptors, RNA Pol-II and multiple intronic RAREs in NIS gene, and they establish a novel mechanistic model for tRA-induced gene transcription.

Posttranscriptional regulation of sodium-iodide symporter mRNA expression in the rat thyroid gland by acute iodide administration

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

Congenital Hypothyroidism Caused by a PAX8 Gene Mutation Manifested as Sodium/Iodide Symporter Gene Defect

Loss-of-function mutations of the PAX8 gene are considered to mainly cause congenital hypothyroidism (CH) due to thyroid hypoplasia. However, some patients with PAX8 mutation have demonstrated a normal-sized thyroid gland. Here we report a CH patient caused by a PAX8 mutation, which manifested as iodide transport defect (ITD). Hypothyroidism was detected by neonatal screening and L-thyroxine replacement was started immediately. Although ¹²³I scintigraphy at 5 years of age showed that the thyroid gland was in the normal position and of small size, his iodide trapping was low. The ratio of the saliva/plasma radioactive iodide was low. He did not have goiter; however laboratory findings suggested that he had partial ITD. Gene analyses showed that the sodium/iodide symporter (NIS) gene was normal; instead, a mutation in the PAX8 gene causing R31H substitution was identified. The present report demonstrates that individuals with defective PAX8 can have partial ITD, and thus genetic analysis is useful for differential diagnosis.

Endogenous expression of the sodium iodide symporter mediates uptake of iodide in murine models of colorectal carcinoma

The sodium iodide symporter (NIS) mediates iodide uptake into the thyroid. Because of this mechanism, differentiated thyroid cancer is susceptible for radioiodine therapy. Functional NIS expression in extrathyroidal tumors has been reported mainly in breast cancer. We screened colorectal tumors for NIS expression and investigated the mechanisms regulating NIS activity. Cell lines were screened for iodide uptake in vitro and NIS expression was evaluated by real-time RT-PCR, immunocytochemistry and immunoblotting. Iodide and pertechnetate uptake were evaluated in allograft tumors by biodistribution studies and scintigraphy. Tumors of transgenic mouse models for colorectal cancer harboring mutations in the oncogenes KRAS, beta-catenin or the tumor-suppressor gene adenomatous-polyposis coli (APC) were screened for NIS expression by RT-PCR. In vitro, functional NIS activity was detected in murine CMT93 rectal carcinoma cells and NIS expression was verified on mRNA and protein level. Inhibition of tyrosine kinases increased iodide uptake. Inhibition of tyrosine phosphatases decreased iodide uptake. In vivo, functional NIS expression was preserved in CMT93 tumors and tumor uptake could be enhanced by treatment of mice with tyrosine kinase inhibitors. In transgenic murine models of colorectal cancer, 14% of endogenous tumors expressed elevated levels of NIS mRNA. We conclude that NIS is functionally expressed in a subset of murine colorectal tumors and its activity is regulated by tyrosine phosphorylation. Therefore, with specific tyrosine kinase inhibition, these tumors might be susceptible for radioiodine treatment. Further studies are justified to identify the specific pathways regulating NIS activity and to transfer these findings to human cell lines and tissues.

Waterborne exposure to PFOS causes disruption of the hypothalamus-pituitary-thyroid axis in zebrafish larvae

Thyroid hormones (THs) play an important role in the normal development and physiological functions in fish. Environmental chemicals may adversely affect thyroid function by disturbing gene transcription. Perfluorooctane sulfonate (PFOS), a persistent compound, is widely distributed in the aquatic environment and wildlife. In the present study, we investigated whether PFOS could disrupt the hypothalamic-pituitary-thyroid (HPT) axis. Zebrafish embryos were exposed to various concentrations of PFOS (0, 100, 200 and 400 microgL(-1)) and gene expression patterns were examined 15d post-fertilization. The expression of several genes in the HPT system, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid peroxidase (TPO), transthyretin (TTR), iodothyronine deiodinases (Dio1 and Dio2) and thyroid receptor (TRalpha and TRbeta), was quantitatively measured using real-time PCR. The gene expression levels of CRF and TSH were significantly up-regulated and down-regulated, respectively, upon exposure to 200 and 400 microg L(-1) PFOS. A significant increase in NIS and Dio1 gene expression was observed at 200 microg L(-1) PFOS exposure, while TG gene expression was down-regulated at 200 and 400 microg L(-1) PFOS exposure. TTR gene expression was down-regulated in a concentration-dependent manner. Up-regulation and down-regulation of TRalpha and TRbeta gene expression, respectively, was observed upon exposure to PFOS. The whole body thyroxine (T(4)) content remained unchanged, whereas triiodothyronine (T(3)) levels were significantly increased, which could directly reflect disrupted thyroid hormone status after PFOS exposure. The overall results indicated that PFOS exposure could alter gene expression in the HPT axis and that mechanisms of disruption of thyroid status by PFOS could occur at several steps in the synthesis, regulation, and action of thyroid hormones.

Paired box gene 8-peroxisome proliferator-activated receptor-gamma fusion protein and loss of phosphatase and tensin homolog synergistically cause thyroid hyperplasia in transgenic mice

Approximately 35% of follicular thyroid carcinomas and a small fraction of follicular adenomas are associated with a t(2;3)(q13;p25) chromosomal translocation that fuses paired box gene 8 (PAX8) with the peroxisome proliferator-activated receptor-gamma gene (PPARG), resulting in expression of a PAX8-PPARgamma fusion protein, PPFP. The mechanism by which PPFP contributes to follicular thyroid neoplasia is poorly understood. Therefore, we have created mice with thyroid-specific expression of PPFP. At 1 yr of age, 25% of PPFP mice demonstrate mild thyroid hyperplasia. We bred these mice to mice with thyroid-specific single-allele deletion of the tumor suppressor Pten, denoted ThyPten(+/-). In humans, PTEN deletion is associated with follicular adenomas and carcinomas, and in mice, deletion of one Pten allele causes mild thyroid hyperplasia. We found that PPFP synergizes with ThyPten(+/-) to cause marked thyroid hyperplasia, but carcinomas were not observed. AKT phosphorylation was increased as expected in the ThyPten(+/-) thyroids, and also was increased in the PPFP thyroids and in human PPFP follicular cancers. Staining for the cell cycle marker Ki-67 was increased in the PPFP, ThyPten(+/-), and PPFP;ThyPten(+/-) thyroids compared with wild-type thyroids. Several genes with increased expression in PPFP cancers also were found to be increased in the thyroids of PPFP mice. This transgenic mouse model of thyroidal PPFP expression exhibits properties similar to those of PPFP thyroid cancers. However, the mice develop thyroid hyperplasia, not carcinoma, suggesting that additional events are required to cause follicular thyroid cancer.

Single nucleotide variants of the TGACTCA motif modulate energetics and orientation of binding of the Jun-Fos heterodimeric transcription factor

The Jun-Fos heterodimeric transcription factor is the terminal link between the transfer of extracellular information in the form of growth factors and cytokines to the site of DNA transcription within the nucleus in a wide variety of cellular processes central to health and disease. Here, using isothermal titration calorimetry, we report detailed thermodynamics of the binding of bZIP domains of Jun-Fos heterodimer to synthetic dsDNA oligos containing the TGACTCA cis element and all possible single nucleotide variants thereof encountered widely within the promoters of a diverse array of genes. Our data show that Jun-Fos heterodimer tolerates single nucleotide substitutions and binds to TGACTCA variants with affinities in the physiologically relevant micromolar to submicromolar range. The energetics of binding are richly favored by enthalpic forces and opposed by entropic changes across the entire spectrum of TGACTCA variants in agreement with the notion that protein-DNA interactions are largely driven by electrostatic interactions and intermolecular hydrogen bonding. Of particular interest is the observation that the Jun-Fos heterodimer binds to specific TGACTCA variants in a preferred orientation. Our 3D atomic models reveal that such orientational preference results from asymmetric binding and may in part be attributable to chemically distinct but structurally equivalent residues R263 and K148 located within the basic regions of Jun and Fos, respectively. Taken together, our data suggest that the single nucleotide variants of the TGACTCA motif modulate energetics and orientation of binding of the Jun-Fos heterodimer and that such behavior may be a critical determinant of differential regulation of specific genes under the control of this transcription factor. Our study also bears important consequences for the occurrence of single nucleotide polymorphisms within the TGACTCA cis element at specific gene promoters between different individuals.

Oncogenic ras blocks the cAMP pathway and dedifferentiates thyroid cells via an impairment of pax8 transcriptional activity

A deranged differentiation is often a landmark of transformed cells. We used a thyroid cell line expressing an inducible Ras oncoprotein in order to study the hierarchy of molecular events leading to suppression of thyroid-specific gene expression. We find that, upon Ras activation, there is an immediate global down-regulation of thyroid differentiation, which is associated with an inhibition of the cAMP signaling pathway. We demonstrate that an unusual negative cross talk between Ras oncogene and the cAMP pathway induces inactivation of the transcription factor Pax8 that we propose as a crucial event in Ras-induced dedifferentiation.

The solution structure of DNA-free Pax-8 paired box domain accounts for redox regulation of transcriptional activity in the pax protein family

Pax-8 is a transcription factor belonging to the PAX genes superfamily and its crucial role has been proven both in embryo and in the adult organism. Pax-8 activity is regulated via a redox-based mechanism centered on the glutathionylation of specific cysteines in the N-terminal region (Cys45 and Cys57). These residues belong to a highly evolutionary conserved DNA binding site: the Paired Box (Prd) domain. Crystallographic protein-DNA complexes of the homologues Pax-6 and Pax-5 showed a bipartite Prd domain consisting of two helix-turn-helix (HTH) motifs separated by an extended linker region. Here, by means of nuclear magnetic resonance, we show for the first time that the HTH motifs are largely defined in the unbound Pax-8 Prd domain. Our findings contrast with previous induced fit models, in which Pax-8 is supposed to largely fold upon DNA binding. Importantly, our data provide the structural basis for the enhanced chemical reactivity of residues Cys45 and Cys57 and explain clinical missense mutations that are not obviously related to the DNA binding interface of the paired box domain. Finally, sequence conservation suggests that our findings could be a general feature of the Pax family transcription factors.