Thyroid-specific enhancer-binding protein (T/EBP): cDNA cloning, functional characterization, and structural identity with thyroid transcription factor TTF-1

Advances in Thyroid Organoids Research and Applications

INTRODUCTION

Organoids are three-dimensional cellular aggregates derived from stem cells or primary tissues that can self-organize into organotypic structures and showcase the physiological functions of that organ. Organoids typically comprise multiple organ-specific cell types that are responsible for organ function in vivo. They may also incorporate various cellular and molecular stromal components to recapitulate the in vivo microenvironment of the target organ.

METHODS

All articles related to thyroid-like organs were synthesized. Articles published between 1959 and 2023 were assessed, categorized, and analyzed using relevant keywords.

RESULTS

As such, organoids provide a model of greater physiological relevance than 2D cell culture for basic and translational research. Murine and human organoids of the thyroid have been established from embryonic stem cells (ESCs), pluripotent stem cells (PSCs) and from various healthy or diseased thyroid tissues. These thyroid organoids have been used in basic and translation research on thyroid-related diseases including hyperthyroidism, Graves' disease, and Hashimoto's thyroiditis. In addition, organoids derived from patients with thyroid cancer retain histopathological features and mutational profile of the original tumor. These patient-derived organoids have been successfully used in in vitro evaluation of drug response of individual patients, demonstrating their potential application in personalized treatment of thyroid cancer.

CONCLUSION

In this review article, we have discussed various techniques for establishing thyroid organoids and their applications in thyroid-related diseases as disease models, regenerative medicines, or a tool for drug testing.

Transcription factors in microcephaly

Higher cognition in humans, compared to other primates, is often attributed to an increased brain size, especially forebrain cortical surface area. Brain size is determined through highly orchestrated developmental processes, including neural stem cell proliferation, differentiation, migration, lamination, arborization, and apoptosis. Disruption in these processes often results in either a small (microcephaly) or large (megalencephaly) brain. One of the key mechanisms controlling these developmental processes is the spatial and temporal transcriptional regulation of critical genes. In humans, microcephaly is defined as a condition with a significantly smaller head circumference compared to the average head size of a given age and sex group. A growing number of genes are identified as associated with microcephaly, and among them are those involved in transcriptional regulation. In this review, a subset of genes encoding transcription factors (e.g., homeobox-, basic helix-loop-helix-, forkhead box-, high mobility group box-, and zinc finger domain-containing transcription factors), whose functions are important for cortical development and implicated in microcephaly, are discussed.

Unraveling the Complex Interplay Between Transcription Factors and Signaling Molecules in Thyroid Differentiation and Function, From Embryos to Adults

Thyroid differentiation of progenitor cells occurs during embryonic development and in the adult thyroid gland, and the molecular bases of these complex and finely regulated processes are becoming ever more clear. In this Review, we describe the most recent advances in the study of transcription factors, signaling molecules and regulatory pathways controlling thyroid differentiation and development in the mammalian embryo. We also discuss the maintenance of the adult differentiated phenotype to ensure the biosynthesis of thyroid hormones. We will focus on endoderm-derived thyroid epithelial cells, which are responsible for the formation of the thyroid follicle, the functional unit of the thyroid gland. The use of animal models and pluripotent stem cells has greatly aided in providing clues to the complicated puzzle of thyroid development and function in adults. The so-called thyroid transcription factors - Nkx2-1, Foxe1, Pax8 and Hhex - were the first pieces of the puzzle identified in mice. Other transcription factors, either acting upstream of or directly with the thyroid transcription factors, were subsequently identified to, almost, complete the puzzle. Among them, the transcription factors Glis3, Sox9 and the cofactor of the Hippo pathway Taz, have emerged as important players in thyroid differentiation and development. The involvement of signaling molecules increases the complexity of the puzzle. In this context, the importance of Bmps, Fgfs and Shh signaling at the onset of development, and of TSH, IGF1 and TGFβ both at the end of terminal differentiation in embryos and in the adult thyroid, are well recognized. All of these aspects are covered herein. Thus, readers will be able to visualize the puzzle of thyroid differentiation with most - if not all - of the pieces in place.

A transgene targeted to the zebrafish nkx2.4b locus drives specific green fluorescent protein expression and disrupts thyroid development

BACKGROUND

With the goal of labeling and manipulating the zebrafish hypothalamus, we sought to target a green fluorescent protein (gfp) transgene to the expression domains of nkx2.4b, a gene expressed during hypothalamic and thyroid development. We combined transcription activator-like effector nucleases (TALENs)-mediated mutagenesis with a targeting construct to enable insertion of a gfp transgene into the endogenous nkx2.4b genomic locus.

RESULTS

Injection of TALENs targeted to the first exon of nkx2.4b created a predicted null allele, and homozygous mutant embryos displayed loss of thyroid markers. From embryos injected with both TALENs and a targeting construct carrying a gfp transgene, we recovered a line in which GFP was expressed specifically in the hypothalamus and thyroid. Fish homozygous for this allele lacked exon 1 of nkx2.4b and exhibited hypothyroid phenotypes.

CONCLUSIONS

By combining TALENs injections with a targeting construct that contained a gfp transgene, we were able to recover an allele in which GFP is expressed in the nkx2.4b expression domains, with homozygous phenotypes suggesting the creation of a loss-of-function transgenic line. These results demonstrate the creation of a useful tool for studying hypothalamus and thyroid development.

Haploinsufficiency of NKX2-1 in Brain-Lung-Thyroid Syndrome with Additional Multiple Pituitary Dysfunction

INTRODUCTION

Heterozygous mutations or haploinsufficiency of NKX2-1 are associated with the brain-lung-thyroid syndrome incorporating primary hypothyroidism, respiratory distress, and neurological disturbances.

CASE PRESENTATION

We report a patient presenting in the neonatal period with multiple pituitary hormone deficiency including central hypothyroidism and hypoadrenalism, growth hormone deficiency, undetectable gonadotrophins, and a small anterior pituitary on MRI. CGH microarray revealed haploinsufficiency for NKX2.1 and during subsequent follow-up, she has exhibited the classic triad of brain-lung-thyroid syndrome with undetectable tissue on thyroid ultrasonography. Whilst the role of NKX2-1 is well described in murine pituitary development, this report constitutes the first description of multiple pituitary dysfunction in humans associated with the syndrome and haploinsufficiency NKX2-1.

CONCLUSION

The report highlights a potential need for pituitary screening in patients with established brain-lung-thyroid syndrome and implicates NKX2.1 in human pituitary disease.

Delineating the early transcriptional specification of the mammalian trachea and esophagus

The genome-scale transcriptional programs that specify the mammalian trachea and esophagus are unknown. Though NKX2-1 and SOX2 are hypothesized to be co-repressive master regulators of tracheoesophageal fates, this is untested at a whole transcriptomic scale and their downstream networks remain unidentified. By combining single-cell RNA-sequencing with bulk RNA-sequencing of Nkx2-1 mutants and NKX2-1 ChIP-sequencing in mouse embryos, we delineate the NKX2-1 transcriptional program in tracheoesophageal specification, and discover that the majority of the tracheal and esophageal transcriptome is NKX2-1 independent. To decouple the NKX2-1 transcriptional program from regulation by SOX2, we interrogate the expression of newly-identified tracheal and esophageal markers in Sox2/Nkx2-1 compound mutants. Finally, we discover that NKX2-1 binds directly to Shh and Wnt7b and regulates their expression to control mesenchymal specification to cartilage and smooth muscle, coupling epithelial identity with mesenchymal specification. These findings create a new framework for understanding early tracheoesophageal fate specification at the genome-wide level.

Thyrotropin, but Not Thyroid-Stimulating Antibodies, Induces Biphasic Regulation of Gene Expression in Human Thyrocytes

Background: Thyrotropin (TSH) and thyroid-stimulating antibodies (TSAbs) activate TSH receptor (TSHR) signaling by binding to its extracellular domain. TSHR signaling has been studied extensively in animal thyrocytes and in engineered cell lines, and differences in signaling have been observed in different cell systems. We, therefore, decided to characterize and compare TSHR signaling mediated by TSH and monoclonal TSAbs in human thyrocytes in primary culture. Methods: We used quantitative reverse transcription-polymerase chain reaction to measure mRNA levels of thyroid-specific genes thyroglobulin (TG), thyroperoxidase (TPO), iodothyronine deiodinase type 2 (DIO2), sodium-iodide symporter (NIS), and TSHR after stimulation by TSH or two monoclonal TSAbs, KSAb1 and M22. We also compared secreted TG protein after TSHR activation by TSH and TSAbs using an enzyme-linked immunosorbent assay. TSHR cell surface expression was determined using fluorescence activated cell sorting (FACS). Results: We found that TSH at low doses increases and at high doses (>1 mU/mL) decreases levels of gene expression for TSHR, TG, TPO, NIS, and DIO2. The biphasic effect of TSH on signaling was not caused by downregulation of cell surface TSHRs. This bell-shaped biphasic dose-response curve has been termed an inverted U-shaped dose-response curve (IUDRC). An IUDRC was also found for TSH-induced regulation of TG secretion. In contrast, KSAb1- and M22-induced regulation of TSHR, TG, TPO, NIS, and DIO2 gene expression, and secreted TG followed a monotonic dose-response curve that plateaus at high doses of activating antibody. Conclusions: Our data demonstrate that the physiological activation of TSHRs by TSH in primary cultures of human thyrocytes is characterized by a regulatory mechanism that may inhibit thyrocyte overstimulation. In contrast, TSAbs do not exhibit biphasic regulation. Although KSAb1 and M22 may not be representative of all TSAbs found in patients with Graves' disease, we suggest that persistent robust stimulation of TSHRs by TSAbs, unrelieved by a decrease at high TSAb levels, fosters chronic stimulation of thyrocytes in Graves' hyperthyroidism.

Murine hypothalamic destruction with vascular cell apoptosis subsequent to combined administration of human papilloma virus vaccine and pertussis toxin

Vaccination is the most powerful way to prevent human beings from contracting infectious diseases including viruses. In the case of the human papillomavirus (HPV) vaccine, an unexpectedly novel disease entity, HPV vaccination associated neuro-immunopathetic syndrome (HANS), has been reported and remains to be carefully verified. To elucidate the mechanism of HANS, we applied a strategy similar to the active experimental autoimmune encephalitis (EAE) model - one of the most popular animal models used to induce maximum immunological change in the central nervous system. Surprisingly, mice vaccinated with pertussis toxin showed neurological phenotypes that include low responsiveness of the tail reflex and locomotive mobility. Pathological analyses revealed the damage to the hypothalamus and circumventricular regions around the third ventricle, and these regions contained apoptotic vascular endothelial cells. These data suggested that HPV-vaccinated donners that are susceptible to the HPV vaccine might develop HANS under certain environmental factors. These results will give us the new insight into the murine pathological model of HANS and help us to find a way to treat of patients suffering from HANS.

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.

nkx2.1 and nkx2.4 genes function partially redundant during development of the zebrafish hypothalamus, preoptic region, and pallidum

During ventral forebrain development, orthologs of the homeodomain transcription factor Nkx2.1 control patterning of hypothalamus, preoptic region, and ventral telencephalon. However, the relative contributions of Nkx2.1 and Nkx2.4 to prosencephalon development are poorly understood. Therefore, we analyzed functions of the previously uncharacterized nkx2.4-like zgc:171531 as well as of the presumed nkx2.1 orthologs nkx2.1a and nkx2.1b in zebrafish forebrain development. Our results show that zgc:171531 and nkx2.1a display overlapping expression patterns and a high sequence similarity. Together with a high degree of synteny conservation, these findings indicate that both these genes indeed are paralogs of nkx2.4. As a result, we name zgc:171531 now nkx2.4a, and changed the name of nkx2.1a to nkx2.4b, and of nkx2.1b to nkx2.1. In nkx2.1, nkx2.4a, and nkx2.4b triple morpholino knockdown (nkx2TKD) embryos we observed a loss of the rostral part of prosomere 3 and its derivative posterior tubercular and hypothalamic structures. Furthermore, there was a loss of rostral and intermediate hypothalamus, while a residual preoptic region still develops. The reduction of the ventral diencephalon was accompanied by a ventral expansion of the dorsally expressed pax6, revealing a dorsalization of the basal hypothalamus. Within the telencephalon we observed a loss of pallidal markers, while striatum and pallium are forming. At the neuronal level, nkx2TKD morphants lacked several neurosecretory neuron types, including avp, crh, and pomc expressing cells in the hypothalamus, but still form oxt neurons in the preoptic region. Our data reveals that, while nkx2.1, nkx2.4a, and nkx2.4b genes act partially redundant in hypothalamic development, nkx2.1 is specifically involved in the development of rostral ventral forebrain including the pallidum and preoptic regions, whereas nkx2.4a and nkx2.4b control the intermediate and caudal hypothalamus.

Comprehensive genotyping and clinical characterisation reveal 27 novel NKX2-1 mutations and expand the phenotypic spectrum

BACKGROUND

NKX2-1 encodes a transcription factor with large impact on the development of brain, lung and thyroid. Germline mutations of NKX2-1 can lead to dysfunction and malformations of these organs. Starting from the largest coherent collection of patients with a suspected phenotype to date, we systematically evaluated frequency, quality and spectrum of phenotypic consequences of NKX2-1 mutations.

METHODS

After identifying mutations by Sanger sequencing and array CGH, we comprehensively reanalysed the phenotype of affected patients and their relatives. We employed electrophoretic mobility shift assay (EMSA) to detect alterations of NKX2-1 DNA binding. Gene expression was monitored by means of in situ hybridisation and compared with the expression level of MBIP, a candidate gene presumably involved in the disorders and closely located in close genomic proximity to NKX2-1.

RESULTS

Within 101 index patients, we detected 17 point mutations and 10 deletions. Neurological symptoms were the most consistent finding (100%), followed by lung affection (78%) and thyroidal dysfunction (75%). Novel symptoms associated with NKX2-1 mutations comprise abnormal height, bouts of fever and cardiac septum defects. In contrast to previous reports, our data suggest that missense mutations in the homeodomain of NKX2-1 not necessarily modify its DNA binding capacity and that this specific type of mutations may be associated with mild pulmonary phenotypes such as asthma. Two deletions did not include NKX2-1, but MBIP, whose expression spatially and temporarily coincides with NKX2-1 in early murine development.

CONCLUSIONS

The high incidence of NKX2-1 mutations strongly recommends the routine screen for mutations in patients with corresponding symptoms. However, this analysis should not be confined to the exonic sequence alone, but should take advantage of affordable NGS technology to expand the target to adjacent regulatory sequences and the NKX2-1 interactome in order to maximise the yield of this diagnostic effort.

Respiratory insufficiency in a newborn with congenital hypothyroidism due to a new mutation of TTF-1/NKX2.1 gene

NK2 homeobox-1 (NKX2.1) gene encoding the thyroid transcription factor-1 (TTF-1) plays a critical role in lung, thyroid, and central nervous system morphogenesis and function; mutations cause a rare form of progressive respiratory failure associated with alterations of surfactant synthesis, composition, and homeostasis. Molecular mechanisms are heterogeneous and poorly explored. A 28 days old male, soon after birth, presented respiratory failure requiring oxygen treatment at FiO2 27%, prolonged for 2 weeks. Routine neonatal screenings detected a high thyroid stimulating hormone concentration. On day 27 congenital hypothyroidism was confirmed and substitutive treatment was begun. Since the persistence of respiratory symptoms sweat test, CFTR mutation, lymphocyte subpopulations, and sputum cultures were tested, resulting negative. Brain and cardiac defects were also ruled out. Bronchoscopy and BAL analysis were normal. Computed tomography showed bilateral multiple ground glass attenuation, consolidative areas and diffuse bronchial wall thickening. Based on the severity of symptoms, the exclusion of other causes of respiratory disease and the CT findings of interstitial lung disease, we investigated genes affecting the surfactant homeostasis. Sequencing analysis of the three exons of the TTF1 revealed a heterozygous mutation c.334G > T that results in the replacement of glycine in position 112 with a stop codon, generating a nonsense protein that lacks the correct transactivation domain in the C-terminal region. Genetic analysis of the family showed that the father, who was asymptomatic, carried the mutation. Screening for TTF-1 deletions or mutations should always be considered in children with congenital hypothyroidism and an unexplained neonatal respiratory distress or neurodevelopmental deficits.

Regulation of LH/FSH expression by secretoglobin 3A2 in the mouse pituitary gland

Secretoglobin (SCGB) 3A2 was originally identified as a downstream target for the homeodomain transcription factor NKX2-1 in the lung. NKX2-1 plays a role in the genesis and expression of genes in the thyroid, lung and ventral forebrain; Nkx2-1-null mice have no thyroid and pituitary and severely hypoplastic lungs and hypothalamus. To demonstrate whether SCGB3A2 plays any role in pituitary hormone production, NKX2-1 and SCGB3A2 expression in the mouse pituitary gland was examined by immunohistochemical analysis and RT-PCR. NKX2-1 was localized in the posterior pituitary lobe, whereas SCGB3A2 was observed in both anterior and posterior lobes as shown by immunohistochemistry and RT-PCR. Expression of CCAAT-enhancer binding proteins (C/EBPs), which regulate mouse Scgb3a2 transcription, was also examined by RT-PCR. C/EBPβ, γ, δ and ζ were expressed in the adult mouse pituitary gland. SCGB3A2 was expressed in the anterior and posterior lobes from postnatal days 1 and 5, respectively and the areas where SCGB3A2 expression was found coincided with the area where FSH-secreting cells were found. Double-staining for SCGB3A2 and pituitary hormones revealed that SCGB3A2 was mainly localized in gonadotrophs in 49 % of FSH-secreting cells and 47 % of LH-secreting cells. In addition, SCGB3A2 dramatically inhibited LH and FSH mRNA expression in rat pituitary primary cell cultures. These results suggest that SCGB3A2 regulates FSH/LH production in the anterior pituitary lobe and that transcription factors other than NKX2-1 may regulate SCGB3A2 expression.

Thyroid regeneration: how stem cells play a role?

Many tissues if not all are thought to contain stem cells that are responsible for regeneration and repair of the tissue after injury. Dysregulation of tissue regeneration may result in various pathological conditions, among which cancer is the most extensively studied. Notably, the so-called cancer stem cells or tumor-initiating cells, have been studied in order to understand the mechanisms of carcinogenesis and/or metastasis. However, the nature of cancer stem cells, let alone normal stem/progenitor cells, particularly those of the thyroid remains elusive. There remains a gap in knowledge between adult thyroid stem/progenitor cells and cancer stem cells of the thyroid, and if and/or how they are related to each other. Understanding of the mechanism for thyroid regeneration and mode of participation of normal adult thyroid stem/progenitor cells in this process will hopefully yield a more complete understanding of the nature of thyroid cancer stem cells, and/or help understand the pathogenesis of other thyroid diseases. This review summarizes the current understanding of adult thyroid stem/progenitor cells, with particular emphasis on how they contribute to thyroid regeneration.

Cytoplasmic staining of TTF-1 in the differential diagnosis of hepatocellular carcinoma

Thyroid transcription factor 1 (TTF-1) is a widely used biomarker in surgical pathology. Its nuclear staining is sensitive and specific for the diagnosis of primary pulmonary and thyroid adenocarcinoma as well as small cell carcinomas arising in many organs. The cytoplasmic staining of TTF-1 is also observed, particularly in the benign and malignant hepatic cells. It has been controversial whether TTF-1 cytoplasmic staining is reliable enough to have diagnostic value. This review focuses on this issue and explores the potential application of TTF-1 cytoplasmic staining in the differential diagnosis of hepatocellular carcinoma from other primary and metastatic malignancies in the liver. The mechanism of TTF-1 cytoplasmic staining is also discussed.

Negative NKX2-1 (TTF-1) as temporary surrogate marker for treatment selection during EGFR-mutation analysis in patients with non-small-cell lung cancer

INTRODUCTION

In the past decade, major progress has been made toward personalized medical treatment of non-small-cell lung cancer (NSCLC) through the discovery of epithelial growth factor receptor (EGFR) mutations. However, mutation analysis takes extra time and additional costs in the diagnostic evaluation of lung cancer patients. It has been hypothesized that EGFR mutations are restricted to terminal respiratory unit -type adenocarcinoma expressing thyroid transcription factor-1 (official symbol NKX2-1) as determined by immunohistochemistry. The aim of the current study is to evaluate the potential of NKX2-1 immunohistochemistry as a prescreening test for EGFR mutation analysis.

METHODS

From 2004 to December 2010, 810 consecutive NSCLC tumor specimens were tested for EGFR mutations in a routine diagnostic procedure. Immunohistochemistry for NKX2-1 was performed (clone 8G7G3/1 [Dako]) and the results were compared with tumor EGFR-mutation status and clinicopathological characteristics.

RESULTS

EGFR mutations were detected in 114 specimens (14%). NKX2-1 expression was present in 68%. In the cases with EGFR mutation, NKX2-1 staining was positive in 92%. NKX2-1 immunohistochemical (IHC) staining was significantly associated with the presence of EGFR mutations (p = 5.3×10). NKX2-1 increased the negative predictive value in NSCLC to more than 95%.

CONCLUSIONS

In case of a negative NKX2-1 IHC staining, and only if clinically urgent, the high negative predictive value of more than 95% for EGFR mutations is a suitable temporary surrogate marker for the choice of starting with chemotherapy. In case of positive NKX2-1 IHC, the best strategy is to wait for the outcome of EGFR-mutation analysis and then choose the appropriate treatment.

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.

Development of thyroid gland and ultimobranchial body cyst is independent of p63

The ultimobranchial body (UBB) and thyroid primordium are the origins of the thyroid gland that fuse around embryonic day 14.5 of mouse gestation, ultimately giving rise to calcitonin-producing C cells and thyroglobulin-producing follicular cells, respectively. A homeodomain transcription factor NKX2-1 is expressed both in the UBB and the thyroid primordium, and is critical for development of the thyroid gland. In this study, the role of p63 in development of UBB and the thyroid gland was analyzed by histological, immunohistochemical, and electron microscopic analyses using mice with various combinations of Nkx2-1 and p63 wild-type, heterozygous, and null alleles. In the absence of p63, a normal thyroid gland develops, as revealed by expression of thyroglobulin and calcitonin, thus showing that p63 is not required for thyroid development. However, in mice carrying the Nkx2-1-null allele, the UBB remains as a cystic vesicular structure and/or in nested patterns consisting of p63-positive cells surrounding the vesicle and undifferentiated immature cells with occasional cilia lying inside. The cystic UBB was present even in the Nkx2-1;p63 double-null mice. The structure and p63 expression pattern of the UBB cyst strikingly resemble the solid cell nest. These results show that in the absence of NKX2-1, UBB becomes cystic independent of p63, which is likely the origin of SCN.

TTF-1 regulates α5 nicotinic acetylcholine receptor (nAChR) subunits in proximal and distal lung epithelium

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels comprised of five similar subunits that influence signal transduction and cell turnover. α5 is a structural subunit detected in many non-neuronal tissues; however, its function during pulmonary development is unknown.

RESULTS

α5 was assessed by immunohistochemistry and RT-PCR in mouse lungs from embryonic day (E)13.5 to post-natal day (PN)20. From E13.5 to E18.5, α5 expression was primarily observed in primitive airway epithelial cells while mesenchymal expression was faint and sporadic. α5 expression was detected throughout the proximal lung at PN1 and extensively expressed in the peripheral lung at PN4, an early stage of murine alveologenesis. An interesting shift occurred wherein α5 expression was almost undetectable in the proximal lung from PN4-PN10, but significant localization was again observed at PN20. Transcriptional control of α5 was determined by assessing the activity of reporters containing 2.0-kb and 850-bp of the mouse α5 promoter. Because perinatal expression of α5 was abundant in bronchiolar and alveolar epithelium, we assessed transcriptional control of α5 in Beas2B cells, a human bronchiolar epithelial cell line, and A-549 cells, an alveolar type II cell-like human epithelial cell line. Thyroid Transcription Factor-1 (TTF-1), a key transcription regulator of pulmonary morphogenesis, significantly increased α5 transcription by acting on both the 2.0-kb and 850-bp α5 promoters. Site-directed mutagenesis revealed that TTF-1 activated α5 transcription by binding specific TTF-1 response elements. Exogenous TTF-1 also significantly induced α5 transcription.

CONCLUSIONS

These data demonstrate that α5 is specifically controlled in a temporal and spatial manner during pulmonary morphogenesis. Ongoing research may demonstrate that precise regulation of α5 is important during normal organogenesis and misexpression correlates with tobacco related lung disease.

Disruption of hypothalamic leptin signaling in mice leads to early-onset obesity, but physiological adaptations in mature animals stabilize adiposity levels

Distinct populations of leptin-sensing neurons in the hypothalamus, midbrain, and brainstem contribute to the regulation of energy homeostasis. To assess the requirement for leptin signaling in the hypothalamus, we crossed mice with a floxed leptin receptor allele (Leprfl) to mice transgenic for Nkx2.1-Cre, which drives Cre expression in the hypothalamus and not in more caudal brain regions, generating LeprNkx2.1KO mice. From weaning, LeprNkx2.1KO mice exhibited phenotypes similar to those observed in mice with global loss of leptin signaling (Leprdb/db mice), including increased weight gain and adiposity, hyperphagia, cold intolerance, and insulin resistance. However, after 8 weeks of age, LeprNkx2.1KO mice maintained stable adiposity levels, whereas the body fat percentage of Leprdb/db animals continued to escalate. The divergence in the adiposity phenotypes of Leprdb/db and LeprNkx2.1KO mice with age was concomitant with increased rates of linear growth and energy expenditure in LeprNkx2.1KO mice. These data suggest that remaining leptin signals in LeprNkx2.1KO mice mediate physiological adaptations that prevent the escalation of the adiposity phenotype in adult mice. The persistence of severe adiposity in LeprNkx2.1KO mice, however, suggests that compensatory actions of circuits regulating growth and energy expenditure are not sufficient to reverse obesity established at an early age.

Ongoing expression of Nkx2.1 in the postnatal mouse forebrain: potential for understanding NKX2.1 haploinsufficiency in humans?

Coordinated movements require the caudate-putamen and the globus pallidus, two nuclei belonging to the basal ganglia, to be intact and functioning properly. Many neurons populating these regions derive from the medial ganglionic eminence, a transient structure that expresses the transcription factor Nkx2.1 during prenatal development. Accordingly, the basal ganglia of Nkx2.1(-/-) mice are heavily affected and a substantial loss of several types of GABAergic interneurons has been observed. Interestingly, heterozygous mutation of the NKX2.1 gene in humans has been described as causing an unusual disorder from the second year of life onwards, which is mainly characterized by disturbances of motor abilities and delayed speech development. In the present study, we therefore investigated whether Nkx2.1 is still expressed in the young adult and aged mouse forebrain. After birth, the most intense immunolabeling for Nkx2.1 was detected in several components of the hypothalamic region, in the subventricular zone of the ventral tips lining the lateral ventricles, and in neighboring structures including the striatum, the globus pallidus and the various nuclei of the septal complex. Surprisingly, this staining pattern was substantially maintained into adulthood. Double immunocytochemistry for Nkx2.1 and various neuronal markers revealed that mainly parvalbumin-containing GABAergic neurons, but also cholinergic neurons, of the ventral forebrain express this protein. Moreover, in situ hybridization confirmed that these neurons maintain synthesis of Nkx2.1 throughout life. The robust expression of Nkx2.1 by these neurons points to a broad functional spectrum within the adult forebrain.

Thyroid transcription factor-1 (TTF-1/Nkx2.1/TITF1) gene regulation in the lung

TTF-1 [thyroid transcription factor-1; also known as Nkx2.1, T/EBP (thyroid-specific-enhancer-binding protein) or TITF1] is a homeodomain-containing transcription factor essential for the morphogenesis and differentiation of the thyroid, lung and ventral forebrain. TTF-1 controls the expression of select genes in the thyroid, lung and the central nervous system. In the lung, TTF-1 controls the expression of surfactant proteins that are essential for lung stability and lung host defence. Human TTF-1 is encoded by a single gene located on chromosome 14 and is organized into two/three exons and one/two introns. Multiple transcription start sites and alternative splicing produce mRNAs with heterogeneity at the 5' end. The 3' end of the TTF-1 mRNA is characterized by a rather long untranslated region. The amino acid sequences of TTF-1 from human, rat, mouse and other species are very similar, indicating a high degree of sequence conservation. TTF-1 promoter activity is maintained by the combinatorial or co-operative actions of HNF-3 [hepatocyte nuclear factor-3; also known as FOXA (forkhead box A)], Sp (specificity protein) 1, Sp3, GATA-6 and HOXB3 (homeobox B3) transcription factors. There is limited information on the regulation of TTF-1 gene expression by hormones, cytokines and other biological agents. Glucocorticoids, cAMP and TGF-beta (transforming growth factor-beta) have stimulatory effects on TTF-1 expression, whereas TNF-alpha (tumour necrosis factor-alpha) and ceramide have inhibitory effects on TTF-1 DNA-binding activity in lung cells. Haplo-insufficiency of TTF-1 in humans causes hypothyroidism, respiratory dysfunction and recurring pulmonary infections, underlining the importance of optimal TTF-1 levels for the maintenance of thyroid and lung function. Recent studies have implicated TTF-1 as a lineage-specific proto-oncogene for lung cancer.

A germline mutation (A339V) in thyroid transcription factor-1 (TITF-1/NKX2.1) in patients with multinodular goiter and papillary thyroid carcinoma

BACKGROUND

The genetic factors that determine the risk of papillary thyroid carcinoma (PTC) among patients with multinodular goiter (MNG) remain undefined. Because thyroid transcription factor-1 (TTF-1) is important to thyroid development, we evaluated whether the gene that encodes it, TITF-1/NKX2.1, is a genetic determinant of MNG/PTC predisposition.

METHODS

Twenty unrelated PTC patients with a history of MNG (MNG/PTC), 284 PTC patients without a history of MNG (PTC), and 349 healthy control subjects were screened for germline mutation(s) in TITF-1/NKX2.1 by sequencing of amplified DNA from blood. The effects of the mutation on the growth and differentiation of thyroid cells were demonstrated by ectopic expression of wild-type (WT) and mutant proteins in PCCL3 normal rat thyroid cells, followed by tests of cell proliferation, activation of cell growth pathways, and transcription of TTF-1 target genes. All statistical tests were two-sided.

RESULTS

A missense mutation (1016C>T) was identified in TITF-1/NKX2.1 that led to a mutant TTF-1 protein (A339V) in four of the 20 MNG/PTC patients (20%). These patients developed substantially more advanced tumors than MNG/PTC or PTC patients without the mutation (P = .022, Fisher exact test). Notably, this germline mutation was dominantly inherited in two families, with some members bearing the mutation affected with MNG, associated with either PTC or colon cancer. The mutation encoding the A339V substitution was not found among the 349 healthy control subjects nor among the 284 PTC patients who had no history of MNG. Overexpression of A339V TTF-1 in PCCL3 cells, as compared with overexpression of WT TTF-1, was associated with increased cell proliferation including thyrotropin-independent growth (average A339V proliferation rate = 134.27%, WT rate = 104.43%, difference = 34.3%, 95% confidence interval = 12.0% to 47.7%, P = .010), enhanced STAT3 activation, and impaired transcription of the thyroid-specific genes Tg, TSH-R, and Pax-8.

CONCLUSION

This is the first germline mutation identified in MNG/PTC patients. It could contribute to predisposition for MNG and/or PTC and to the pathogenesis of PTC.

Review: the role of neural crest cells in the endocrine system

The neural crest is a pluripotent population of cells that arises at the junction of the neural tube and the dorsal ectoderm. These highly migratory cells form diverse derivatives including neurons and glia of the sensory, sympathetic, and enteric nervous systems, melanocytes, and the bones, cartilage, and connective tissues of the face. The neural crest has long been associated with the endocrine system, although not always correctly. According to current understanding, neural crest cells give rise to the chromaffin cells of the adrenal medulla, chief cells of the extra-adrenal paraganglia, and thyroid C cells. The endocrine tumors that correspond to these cell types are pheochromocytomas, extra-adrenal paragangliomas, and medullary thyroid carcinomas. Although controversies concerning embryological origin appear to have mostly been resolved, questions persist concerning the pathobiology of each tumor type and its basis in neural crest embryology. Here we present a brief history of the work on neural crest development, both in general and in application to the endocrine system. In particular, we present findings related to the plasticity and pluripotency of neural crest cells as well as a discussion of several different neural crest tumors in the endocrine system.

NKX2.1 specifies cortical interneuron fate by activating Lhx6

In the ventral telencephalon, the medial ganglionic eminence (MGE) is a major source of cortical interneurons. Expression of the transcription factor NKX2.1 in the MGE is required for the specification of two major subgroups of cortical interneurons - those that express parvalbumin (PV) or somatostatin (SST) - but direct targets of NKX2.1 remain to be established. We find that electroporation of Nkx2.1 cDNA into the ventral telencephalon of slice cultures from Nkx2.1-/- mouse embryos, followed by transplantation into neonatal cortex to permit postnatal analysis of their fate, rescues the loss of PV- and SST-expressing cells. The LIM-homeobox gene Lhx6 is induced by this rescue experiment, and gain- and loss-of-function studies suggest that Lhx6 is necessary and sufficient to rescue these and other interneuron phenotypes in cells transplanted from Nkx2.1-/- slices. Finally, NKX2.1 protein binds a highly conserved sequence in the Lhx6 promoter, and this sequence appears to mediate the direct activation of Lhx6 by NKX2.1. The slice transfection and transplantation methods employed here are beginning to uncover embryonic mechanisms for specifying neuronal fates that only become definable postnatally.

Origin of the ultimobranchial body cyst: T/ebp/Nkx2.1 expression is required for development and fusion of the ultimobranchial body to the thyroid

The ultimobranchial body (UBB) is an outpocketing of the fourth pharyngeal pouch that fuses with the thyroid diverticulum, giving rise to calcitonin-producing C-cells. In this study, we demonstrate that the UBB is composed of two types of cells: one expressing T/ebp/Nkx2.1 and the other expressing p63. The former cell type, accounting for a majority of the UBB, requires T/ebp/Nkx2.1 for their survival. In contrast, the p63-positive cells, even in the absence of T/ebp/Nkx2.1 expression, can proliferate and give rise to a vesicular structure that is lined by a monolayer of p63-negative cells, surrounded by a cluster and/or single layer of p63-positive cells, displaying the basal/stem cell phenotype. T/ebp/Nkx2.1 haploinsufficiency causes abnormal fusion of the UBB with the thyroid diverticulum, which stays as a cluster of C-cells around the vesicular structure, similar to the one observed in mice null for T/ebp/Nkx2.1 expression. These results demonstrate that T/ebp/Nkx2.1 plays a role in the survival of UBB cells, their dissemination into the thyroid diverticulum, and the formation of UBB-derived vesicular structure.

Identification and expression of alternative splice variants of the mouse Ppp1r3b gene in lung epithelial cells

Hepatic glycogen-targeting subunit GL of protein phosphatase-1 (PP1), which is encoded by the gene Ppp1r3b, is one of a family of proteins that target PP1 to glycogen and regulate its activity on enzymes of glycogen metabolism. GL is primarily expressed in the liver where it promotes hepatic glycogen synthesis in response to insulin. Here, we show that GL is highly expressed in embryonic mouse lungs. GL utilizes two alternative promoters and 5' non-coding exons, which produce at least three alternatively spliced transcripts that encode identical proteins. Expression of GL in the lung is detectable as early as embryonic day (E) 12.5 and increases by E16.5, however, it declines before birth. In situ hybridization of mouse embryonic lung revealed that GL is expressed in bronchial epithelial cells. Thus, the temporal and spatial expression of GL in the lung suggests that it has a potential role in glycogen metabolism during lung development.

Thyroid-specific enhancer-binding protein/NKX2.1 is required for the maintenance of ordered architecture and function of the differentiated thyroid

Thyroid-specific enhancer-binding protein (T/ebp)/Nkx2.1-null mouse thyroids degenerate by embryonic day (E) 12-13 through apoptosis whereas T/ebp/Nkx2.1-heterogyzgous mice exhibit hypothyroidism with elevated TSH levels. To understand the role of T/ebp/Nkx2.1 in the adult thyroid, a thyroid follicular cell-specific conditional knockout (KO) mouse line, T/ebp(fl/fl);TPO-Cre, was established that expresses Cre recombinase under the human thyroid peroxidase (TPO) gene promoter. These mice appeared to be healthy and exhibited loss of T/ebp/Nkx2.1 expression in many, but not all, thyroid follicular cells as determined by immunohistochemistry and real-time PCR, thus presenting a T/ebp-thyroid-conditional hypomorphic mice. Detailed analysis of the thyroids from T/ebp(fl/fl), T/ebp(fl/fl);TPO-Cre, and T/ebp(fl/ko) mice, where the latter mouse line is derived from crosses with the original T/ebp/Nkx2.1-heterozygous mice, revealed that T/ebp(fl/fl);TPO-Cre mice can be classified into two groups with different phenotypes: one having atrophic/degenerative thyroid follicles with frequent presence of adenomas and extremely high serum TSH levels, and the other having an altered thyroid structure with reduced numbers of extraordinary dilated follicles consisting of excessive numbers of follicular cells as compared with those usually found in the normal thyroid. The latter phenotype was also observed in aged T/ebp(fl/ko) mouse thyroids. In vitro three-dimensional thyroid primary cultures using thyroids from T/ebp(fl/fl);TPO-Cre, T/ebp(fl/ko), and T/ebp(fl/fl) mice, and the latter treated with recombinant adenovirus with and without Cre expression, demonstrated that only cells from T/ebp(fl/fl) mice without adeno-Cre treatment formed follicular structures. Taken together, these results suggest that T/ebp/Nkx2.1 is required for maintenance of the normal architecture and function of differentiated thyroids.

Temporal-spatial expression and transcriptional regulation of alpha7 nicotinic acetylcholine receptor by thyroid transcription factor-1 and early growth response factor-1 during murine lung development

Nicotinic acetylcholine receptors are ligand-gated ion channels formed by five homologous subunits that are involved in processes including signal transduction, proliferation, and apoptosis. The developmental role of these receptors, however, is unclear. In the present investigation, alpha(7) nicotinic acetylcholine receptor expression was assessed by immunohistochemistry in mouse lungs from embryonic day (E)13.5 to postnatal day (PN)20. Transcriptional mechanisms that regulate alpha(7) were assessed by the transfection of murine bronchiolar cells with a reporter containing 1.1 kb of the mouse alpha(7) promoter, TTF-1, and Egr-1. alpha(7) was initially detected at E13.5 in pulmonary mesenchymal cells and in the epithelium of the primitive tubules at E15.5. From E18.5 to PN1, alpha(7) was expressed in conducting airway and saccule epithelial cells. By PN10, expression was observed in the peripheral epithelium and on luminal membranes of bronchiolar epithelial cells in the proximal lung, a pattern that continued through PN20. From E15.5 to PN20, type II alveolar cells expressed both prosurfactant protein C and alpha(7). From E18.5 to PN20, Clara cells in the bronchiolar epithelium co-expressed Clara cell secretory protein and alpha(7). TTF-1 dose-dependently activated alpha(7) transcription in vitro by binding specific TTF-1 regulatory elements in the mouse alpha(7) promoter. Furthermore, alpha(7) was not detected in TTF-1-null mice and markedly increased in TTF-1-overexpressing mice. Conversely, Egr-1 inhibited alpha(7) expression. Temporal-spatial alpha(7) expression supports the concept that these receptors function during normal pulmonary morphogenesis. A model is also supported whereby alpha(7) is induced by the essential pulmonary transcription factor TTF-1 and suppressed by Egr-1 during pulmonary development.

Interleukin-10 Induces Uteroglobin-related Protein (UGRP) 1 Gene Expression in Lung Epithelial Cells through Homeodomain Transcription Factor T/EBP/NKX2.1

UGRP1 is a downstream target gene for homeodomain transcription factor T/EBP/NKX2.1, which is predominantly expressed in lung epithelial cells, and may play an anti-inflammatory role in lung inflammation. To understand the role of UGRP1 in inflammation, its expression was investigated in relation to cytokine signaling. In vivo experiments using mouse embryonic lung organ culture and intranasal administration of interleukin (IL) 10 revealed that constitutive expression of Ugrp1 mRNA is enhanced by IL-10. Increase of protein levels was also demonstrated by immunohistochemistry using embryonic lungs. This IL-10 induction of Ugrp1 gene expression occurs at the transcriptional level when examined using mouse embryonic lung primary cultures. In human lung NCI-H441 cells that in contrast to mouse lung cells, do not exhibit constitutive expression of the gene, expression of the UGRP1 gene was induced in a rapid and stable fashion. Two T/EBP, but not STAT3, binding sites located in the human UGRP1 gene promoter are responsible for IL-10 induction of the UGRP1 gene as judged by transfection, gel shift, and chromatin immunoprecipitation analyses. The IL-10 receptor chains, IL-10R1 and IL-10R2, are expressed in H441 cells, however, STAT3 was only weakly activated upon IL-10 treatment. In contrast, STAT3 was strongly activated when the cells were treated with other cytokines such as IL-22 and interferon-beta but UGRP1 expression was not increased. Together these results demonstrate that IL-10 induces UGRP1 gene expression in lung epithelial cells through a T/EBP/NKX2.1-dependent pathway. The results further suggest that UGRP1 might be a target for IL-10 anti-inflammatory activities in the lung.

Thyroid development and its disorders: genetics and molecular mechanisms

Thyroid gland organogenesis results in an organ the shape, size, and position of which are largely conserved among adult individuals of the same species, thus suggesting that genetic factors must be involved in controlling these parameters. In humans, the organogenesis of the thyroid gland is often disturbed, leading to a variety of conditions, such as agenesis, ectopy, and hypoplasia, which are collectively called thyroid dysgenesis (TD). The molecular mechanisms leading to TD are largely unknown. Studies in murine models and in a few patients with dysgenesis revealed that mutations in regulatory genes expressed in the developing thyroid are responsible for this condition, thus showing that TD can be a genetic and inheritable disease. These studies open the way to a novel working hypothesis on the molecular and genetic basis of this frequent human condition and render the thyroid an important model in the understanding of molecular mechanisms regulating the size, shape, and position of organs.

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.

NKX2.1 regulates transcription of the gene for human bone morphogenetic protein-4 in lung epithelial cells

Bone morphogenetic protein 4, BMP4, plays an important role in the development of various organs including the lungs. Little is known regarding the regulation of Bmp4 gene expression in any organ. In the lung, indirect evidence indicates that NKX2.1, a homeodomain transcriptional factor with a demonstrated role in lung morphogenesis, may be a potential upstream regulator of Bmp4 gene expression. In particular, Bmp4 mRNA is reduced or absent in Nkx2.1(-/-) lungs. The human Bmp4 gene has been reported to include two regions of promoter activity in an embryonal carcinoma cell line, Tera2EC. The hBmp4.1 promoter is located upstream of exon I, whereas the second promoter, hBmp4.2, is localized within intron 1 and upstream of exon II. In the current study, we used a co-transfection assay in lung epithelial cells to examine the response of the two hBmp4 promoters to transcriptional stimulation by NKX2.1. Two DNA sequences were identified on the hBmp4.1 promoter that bind NKX2.1 and serve as functional cis-active NKX2.1-responsive elements. Similarly, NKX2.1 stimulated transcription from the hBmp4.2 promoter through two consensus binding sites localized within 412 nucleotides from the site of transcriptional initiation. Thus, both hBmp4 promoters include specific cis-active elements that bind to and mediate transcriptional regulation by NKX2.1. These findings bear functional implications regarding the regulation of a key signaling molecule by a homeodomain transcriptional regulator of lung epithelial morphogenesis.

Thyroid transcription factor (TTF) -1 regulates the expression of midkine (MK) during lung morphogenesis

Midkine (MK) is a 13-kDa heparin-binding growth factor that is thought to mediate developmental processes, including vasculogenesis, cell migration, and proliferation in various organs. To determine whether MK plays a role during lung morphogenesis, immunostaining for MK was assessed in mouse lung from embryonic day (E) 13 to postnatal day (PN) 24. MK was detected in mesenchymal and respiratory epithelial cells of the peripheral mouse lung from E13.0 to E15.5. From E18.5 to PN1, MK was observed primarily in epithelial cells lining conducting airways and peripheral lung saccules. By PN10, expression was no longer observed in respiratory epithelial cells but was readily detected in small blood vessels in the alveolar region of the lung. Although most respiratory epithelial cells uniformly expressed MK before E13.0, MK was restricted to subsets of cells by E18.5, colocalizing with the Clara cell secretory protein (CCSP) marker in conducting airways and with pro-SPC, a marker specific for alveolar type II pneumocytes. By PN10, MK was not detected in respiratory epithelial cells of the conducting airways and was closely associated with capillary networks. The sites of intense MK staining in the respiratory epithelial cells correlated with sites of expression of thyroid transcription factor (TTF) -1, a transcription factor regulating formation and gene expression in the lung parenchyma. TTF-1 enhanced transcription of the mouse MK gene promoter, acting on TTF-1 regulatory elements located in the 5'-region of the gene. Furthermore, MK expression was not detected in lungs of TTF-1 null mice. TTF-1 regulates expression of MK in the lung. The temporal/spatial distribution of midkine is consistent with a potential role in paracrine signaling during lung morphogenesis.

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

The sodium/iodide symporter (NIS) gene is highly expressed in the thyroid gland and is important for the diagnosis and radioiodide therapy of differentiated thyroid cancers. We investigated a human NIS (hNIS) gene 5'-far-upstream enhancer (hNUE) (-9847 to -8968). The hNUE is TSH responsive in both FRTL-5 cells and primary normal thyroid cells, but not in human papillary thyroid cancer cells (BHP cells). The hNUE enhanced expression of the basal hNIS promoter 15-fold and required both a Pax-8 binding site and a cAMP response element (CRE)-like sequence for full activity. The hNUE activated transcription in a thyroid-selective and cAMP-dependent manner, mediated by both protein kinase A (PKA)-dependent and PKA-independent pathways. Pax-8 and two CRE-like sequence binding proteins bind to the hNUE. Supershift binding assay indicated that one of the CRE-like sequence binding protein(s) was CRE-binding protein-1, activation transcription factor-1, and/or CRE modulator, and the other was an unknown factor(s) that is absent in BHP 2-7 cells. A far-upstream enhancer is important for hNIS regulation in the thyroid. Deficient CRE-like sequence binding protein(s) that bind to the hNUE in normal thyroid cells may be responsible for reduced NIS gene expression in some thyroid carcinomas.

Thyroid transcription factor-1: a review

Thyroid transcription factor-1 (TTF-1) is a 38-kd homeodomain containing DNA-binding protein originally identified in follicular cells of the thyroid and subsequently in pneumocytes. This review focuses on the utility of antisera in TTF-1 immunohistochemical staining in the diagnosis of neoplastic conditions. Based on published studies to date, anti-TTF-1 is a very useful reagent in distinguishing pulmonary adenocarcinoma from other primary carcinomas, identifying differentiated thyroid neoplasms, distinguishing mesothelioma from pulmonary adenocarcinoma, and distinguishing small cell carcinoma of the lung from Merkel cell carcinoma. It may also be useful in distinguishing neuroendocrine (NE) tumors of the lung from well-differentiated NE tumors from other sites, such as the intestine.

The synergistic activity of thyroid transcription factor 1 and Pax 8 relies on the promoter/enhancer interplay

The transcription factors, thyroid transcription factor 1 (TTF-1) and Pax 8, play a pivotal role in the transcriptional regulation of the thyroid differentiation marker genes and in the differentiation of the thyroid follicular cells. They have a very restricted tissue distribution, and the thyrocyte is the only cell type with the simultaneous expression of these factors. Here we show that TTF-1 and Pax 8 cooperatively activate their target genes and that their synergistic activity requires the cross-talk between enhancer and gene promoter. We have characterized the cis and trans requirements of the TTF1/Pax 8 synergistic activity on the thyroperoxidase gene. We show that their synergy is also important for thyroglobulin gene transcription.

Immunohistochemical analyses of thyroid-specific enhancer-binding protein in the fetal and adult rat hypothalami and pituitary glands

Thyroid-specific enhancer-binding protein (T/EBP), also known as NKX2.1 or TTF-1, regulates the expression of thyroid- and lung-specific genes. The t/ebp/Nkx2.1-null mutant mouse was stillborn but lacked the thyroid gland, pituitary gland, ventral region of the forebrain and normal lungs. These data demonstrated that T/EBP/NKX2.1 plays an important role not only in tissue-specific gene expressions in adults but also in genesis of these organs during development. Although the expression of t/ebp/Nkx2.1 in the brain has been reported, its function in the brain remains unknown. The present study was designed to determine the localization of T/EBP/NKX2.1 in the hypothalamus and pituitary gland of fetal and adult rats by immunohistochemistry as the first step toward understanding the function of T/EBP/NKX2.1 in the rat brain. In the fetal rat hypothalamus, T/EBP/NKX2.1 was localized widely in the ventral hypothalamic areas. In the adult rat brain, T/EBP/NKX2.1 was localized in the ventromedial hypothalamic nucleus, medial tuberal nucleus, arcuate nucleus and mammillary body. No T/EBP/NKX2.1 immunoreactivity was observed in the anterior or intermediate lobe of the pituitary gland in either fetal or adult rats. On the other hand, immunoreactive T/EBP/NKX2.1 was found in the posterior lobe of the pituitary gland. This paper presents results of detailed analyses of the distributions of T/EBP/NKX2.1 protein in the fetal and adult rat hypothalami and pituitary glands, and these results should provide important information for understanding the function of T/EBP/NKX2.1 in the brain.

The DNA glycosylase T:G mismatch-specific thymine DNA glycosylase represses thyroid transcription factor-1-activated transcription

The transcription factor thyroid transcription factor-1 (TTF-1) is a homeodomain-containing protein that belongs to the NK2 family of genes involved in organogenesis. TTF-1 is required for normal development of the forebrain, lung, and thyroid. In a search for factors that regulate TTF-1 transcriptional activity, we isolated three genes (T:G mismatch-specific thymine DNA glycosylase (TDG), homeodomain-interacting protein kinase 2 (HIPK2), and Ajuba), whose products can interact with TTF-1 in yeast and in mammalian cells. TDG is an enzyme involved in base excision repair. In the present paper, we show that TDG acts as a strong repressor of TTF-1 transcriptional activity in a dose-dependent manner, while HIPK2 and Ajuba display no effect on TTF-1 activity, at least under the tested conditions. TDG-mediated inhibition occurs specifically on TTF-1-responsive promoters in thyroid and non thyroid cells. TDG associates with TTF-1 in mammalian cells through the TTF-1 carboxyl-terminal activation domain and is independent of the homeodomain. These findings reveal a previously unsuspected role for the repair enzyme TDG as a transcriptional repressor and open new routes toward the understanding of the regulation of TTF-1 transcriptional activity.

Genetic regulation of thyroid development

Normal thyroid function is essential for development, growth, and metabolic homeostasis. The prerequisites for an euthyroid metabolic state include a normally developed thyroid gland, a properly functioning system for thyroid hormone synthesis, and sufficient iodine intake. Defects in any of the essential steps in thyroid development or thyroid hormone synthesis may result in morphologic abnormalities and impaired hormonogenesis. These defects can be partial or complete, leading to varying degrees of hypothyroidism. Morphologic alterations associated with congenital hypothyroidism include the absence of detectable thyroid tissue, ectopic tissue, thyroid hypoplasia, or a goitrous thyroid. However, in some patients with hypothyroidism, the thyroid is of normal size. This article focuses on defects in thyroid development. Recent insights into the developmental regulation of the calcitonin-producing C cells will not be discussed, and defects in hormone synthesis are discussed in an accompanying article.

Differential regulation of the human sodium/iodide symporter gene promoter in papillary thyroid carcinoma cell lines and normal thyroid cells

The absence of TSH-stimulated radioiodide uptake in differentiated thyroid cancer is associated with a high recurrence rate and reduced survival. We studied regulation of the sodium/iodide symporter gene in human papillary thyroid cancer cell lines (BHP) and primary human thyroid cells. BHP cells expressed very low levels of sodium/iodide symporter mRNA and did not concentrate iodide, but iodide uptake was restored to levels seen in FRTL-5 rat thyroid cells by stable transfection of a sodium/iodide symporter cDNA. Sodium/iodide symporter gene expression, therefore, was necessary and sufficient for iodide uptake in BHP cells. We cloned the human sodium/iodide symporter gene 5'-flanking region and analyzed progressive 5'-deletions in transient transfections. We identified a region, -596 to -268, essential to confer full promoter activity in primary normal human thyroid cells. Sodium/iodide symporter promoter activity in four BHP cell lines, however, was markedly reduced, consistent with down-regulation of the endogenous sodium/iodide symporter gene. Nuclear extracts from BHP 2-7 cells had reduced or absent binding to regions of the sodium/iodide symporter promoter shown to be critical for expression, compared with nuclear extracts from FRTL-5 cells. Competition studies indicated that these nuclear proteins were not known thyroid transcription factors. Modifications of the sodium/iodide symporter promoter with demethylation or histone acetylation did not increase sodium/iodide symporter expression, and no deletions of the critical regulatory region were identified in the endogenous gene in BHP cells. Regulation of the sodium/iodide symporter 5'-flanking region in transient transfection paralleled endogenous sodium/iodide symporter expression. Reduced expression of potential novel nuclear factor(s) in these cell lines may contribute to reduced sodium/iodide symporter expression resulting in absence of iodide uptake in some papillary thyroid cancers.

A bipotential precursor population for pancreas and liver within the embryonic endoderm

The pancreas emerges independently from dorsal and ventral domains of embryonic gut endoderm. Gene inactivation experiments in mice have identified factors required for dorsal pancreas development, but factors that initiate the ventral pancreas have remained elusive. In this study, we investigated the hypothesis that the emergence of the ventral pancreas is related to the emergence of the liver. We find that the liver and ventral pancreas are specified at the same time and in the same general domain of cells. Using embryo tissue explantation experiments, we find that the default fate of the ventral foregut endoderm is to activate the pancreas gene program. FGF signalling from the cardiac mesoderm diverts this endoderm to express genes for liver instead of those for pancreas. No evidence was found to indicate that the cell type choice for pancreas or liver involves a selection for growth or viability. Cardiac mesoderm or FGF induces the local expression of sonic hedgehog, which in turn is inhibitory to pancreas but not to liver. The bipotential precursor cell population for pancreas and liver in embryonic development and its fate selection by FGF has features that appear to be recapitulated in the adult pancreas and are reflected in the evolution of these organs.

Immunoreactivity for thyroid transcription factor-1 in stage I non-small cell carcinomas of the lung

Thyroid transcription factor-1 (TTF-1) is a nuclear protein regulating the transcriptional activity of lung-specific genes in the normal and neoplastic bronchioloalveolar cells. It has been implicated in the normal growth and development of the lung, and the disruption of the TTF-1 locus leads to neonatal death with pulmonary hypoplasia. We evaluated retrospectively the prevalence and clinical significance of TTF-1 immunoreactivity in 222 patients with stage I non-small cell lung carcinoma (NSCLC) with a follow-up time of at least 5 years, and we investigated its relationship with other markers of tumor growth, namely cell proliferation and angiogenesis. TTF-1 immunoreactivity was documented by using the commercially available monoclonal antibody 8G7G3/1 in 72% of 97 adenocarcinomas, 5% of 119 squamous cell carcinomas, and in the glandular component of two adenosquamous carcinomas. Four large cell carcinomas were completely unreactive. In adenocarcinomas, but not squamous cell carcinomas, TTF-1 immunoreactivity correlated significantly with microvessel density (p = 0.04) and inversely with the tumor proliferation fraction assessed by Ki-67 immunostaining (p = 0.03). Also, TTF-1-immunoreactive adenocarcinomas showed a trend for a size less than 3 cm (p = 0.08). TTF-1 expression was not related to specific growth patterns, tumor grade, or tumor cell typing. TTF-1 immunoreactivity did not significantly affect patient survival, although patients with more than 75% immunoreactive neoplastic cells showed a trend for longer overall and disease-free survival. Our findings suggest that TTF-1 could be involved in the development of small pulmonary adenocarcinomas, but it has not prognostic implications in patients with stage I NSCLC.

A unique combination of transcription factors controls differentiation of thyroid cells

The thyroid follicular cell type is devoted to the synthesis of thyroid hormones. Several genes, whose protein products are essential for efficient hormone biosynthesis, are uniquely expressed in this cell type. A set of transcriptional regulators, unique to the thyroid follicular cell type, has been identified as responsible for thyroid specific gene expression; it comprises three transcription factors, named TTF-1, TTF-2, and Pax8, each of which is expressed also in cell types different from the thyroid follicular cells. However, the combination of these factors is unique to the thyroid hormone producing cells, strongly suggesting that they play an important role in differentiation of these cells. An overview of the molecular and biological features of these transcription factors is presented here. Data demonstrating that all three play also an important role in early thyroid development, at stages preceding expression of the differentiated phenotype, are also reviewed. The wide temporal expression, from the beginning of thyroid organogenesis to the adult state, is suggestive of a recycling of the thyroid-specific transcription factors, that is, the control of different sets of target genes at diverse developmental stages. The identification of molecular mechanisms leading to specific gene expression in thyroid cells renders this cell type an interesting model in which to address several aspects of cell differentiation and organogenesis.

Retinoic acid stimulation of the human surfactant protein B promoter is thyroid transcription factor 1 site-dependent

Surfactant B (SP-B) is a 79-amino acid peptide critical to postnatal respiratory adaptation. Expression of SP-B by respiratory epithelial cells is regulated by developmental and hormonal influences at the level of gene transcription. Previous studies supported the role of retinoic acids (RA) and their receptors (RARs) in SP-B gene transcription. In the present study, RARalpha was detected in mouse alveolar type II epithelial cells where SP-B is synthesized and processed. Deletion and site-specific mutagenesis analysis identified clustered retinoic acid-responsive element sites in the 5'-flanking enhancer region of the hSP-B gene that bound RARalpha proteins. RAR coactivators ACTR, SRC-1, and transcriptional intermediary factor 2 (TIF2) stimulated human (h) SP-B promoter activity in a dose-dependent fashion in pulmonary adenocarcinoma H441 cells. In addition, an RAR-associated protein, CREB-binding protein (CBP), potentiated the effects of RAR on hSP-B promoter activity in H441 cells. Importantly, RA stimulation of the hSP-B promoter depends on tissue-specific thyroid transcription factor (TTF-1) DNA-binding sites. TTF-1 protein synergistically stimulated the hSP-B promoter with RARalpha, CBP, and nuclear receptor coactivators in H441 cells. In addition, TTF-1 interacted directly with RARalpha and TIF2 in the mammalian two-hybrid system. These findings support a model in which RAR/retinoid X receptor, TTF-1, coactivators, and CBP form a transcription activation complex in the upstream enhancer region of the hSP-B gene.

Tumor necrosis factor-alpha regulation of thyroid transcription factor-1 and Pax-8 in rat thyroid FRTL-5 cells

Tumor necrosis factor-alpha (TNF-alpha) is known to modulate the expression of thyroid-specific genes, such as thyroglobulin (TG), contributing to the pathogenesis of autoimmune thyroid disease. In the present study, we show that TNF-alpha suppresses DNA-binding activity of thyroid transcription factors, Pax-8 and thyroid transcription factor-1 (TTF-1), which is, in part, involved in TNF-alpha-induced decrease in TG gene expression. Transfected into rat thyroid FRTL-5 cells, the activity of reporter plasmid containing the rat TG promoter ligated to a luciferase gene was significantly suppressed in the presence of TNF-alpha. In gel mobility shift analyses, protein-DNA complexes formed by TTF-1 and Pax-8 were reduced when the nuclear extracts prepared from TNF-alpha-treated FRTL-5 cells were used. The suppressive effect of TNF-alpha on TTF-1-DNA complex formation is, in part, caused by suppression of TTF-1 gene transcription by TNF-alpha. Expressions of TTF-1 messenger RNA and protein, which were assessed by Northern blot and Western blot analyses, respectively, were decreased by TNF-alpha treatment of FRTL-5 cells. In contrast, TNF-alpha did not affect the expression of Pax-8 messenger RNA. Treatment of FRTL-5 cells with TNF-alpha caused a decrease in Pax-8 protein in nuclear extracts and accumulation of the protein in the cytoplasm, as assessed by Western blot analyses. Mutation of the TTF-1/Pax-8-binding site lost the TNF-alpha-induced decrease in TG promoter activity in a transfection experiment. These results indicate that TNF-alpha suppresses the activity of TTF-1 and Pax-8 by different mechanisms, which, in part, seem to be involved in TNF-alpha-induced decrease in TG gene expression.