Upregulation of miR-21 by Ras in vivo and its role in tumor growth

miR-21 is a microRNA (miRNA) frequently overexpressed in human cancers. Here we show that miR-21 is upregulated both in vitro and in vivo by oncogenic Ras, thus linking this miRNA to one of the most frequently activated oncogenes in human cancers. Ras regulation of miR-21 occurs with a delayed kinetic and requires at least two Ras downstream pathways. A screen of human thyroid cancers and non-small-cell lung cancers for the expression of miR-21 reveals that it is overexpressed mainly in anaplastic thyroid carcinomas, the most aggressive form of thyroid cancer, whereas in lung its overexpression appears to be inversely correlated with tumor progression. We also show that a LNA directed against miR-21 slows down tumor growth in mice. Consistently, a search for mRNAs downregulated by miR-21 shows an enrichment for mRNAs encoding cell cycle checkpoints regulators, suggesting an important role for miR-21 in oncogenic Ras-induced cell proliferation.

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

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

Role of the thyroid-stimulating hormone receptor signaling in development and differentiation of the thyroid gland

The thyroid-stimulating hormone/thyrotropin (TSH) is the most relevant hormone in the control of thyroid gland physiology in adulthood. TSH effects on the thyroid gland are mediated by the interaction with a specific TSH receptor (TSHR). We studied the role of TSHTSHR signaling on gland morphogenesis and differentiation in the mouse embryo using mouse lines deprived either of TSH (pit(dw)pit(dw)) or of a functional TSHR (tshr(hyt)tshr(hyt) and TSHR-knockout lines). The results reported here show that in the absence of either TSH or a functional TSHR, the thyroid gland develops to a normal size, whereas the expression of thyroperoxidase and the sodium/iodide symporter are reduced greatly. Conversely, no relevant changes are detected in the amounts of thyroglobulin and the thyroid-enriched transcription factors TTF-1, TTF-2, and Pax8. These data suggest that the major role of the TSH/TSHR pathway is in controlling genes involved in iodide metabolism such as sodium/iodide symporter and thyroperoxidase. Furthermore, our data indicate that in embryonic life TSH does not play an equivalent role in controlling gland growth as in the adult thyroid.

Cititf1 and endoderm differentiation in Ciona intestinalis

Studies on the initial formation of the endoderm have lead to the identification, mostly in Xenopus, of numerous genes relevant for the formation of this tissue during early embryogenesis (reviewed by Date, Curr. Biol. 9 (1999) R812-R815 and by Yasuo and Lemaire, Curr. Biol. 9 (1999) 869-879). In ascidians, the most primitive chordates, endoderm differentiation is an autonomous process which is almost complete at 64 cells stage. Cititf1, a gene homologous to mammalian Titf1, is the first specific endodermal marker isolated from the ascidian Ciona intestinalis (Ristatore et al., Development 126 (1999) 5149-5159). Here we study the in vivo role of Cititf1 by using two different approaches: interference and ectopic expression. The results presented here show that interference with Cititf1 function seems to affect gastrulation movements, while ectopic expression of Cititf1 mRNA into the notochord alters differentiation of this tissue probably by recruiting notochord blastomeres to an endodermal fate. These data together with studies, still in progress in our laboratory, on the transcriptional regulation of Cititf1, strongly indicate that Cititf1 plays an important role in the process of endoderm formation in Ciona embryogenesis.

Characterization of the upstream enhancer of the rat sodium/iodide symporter gene

We previously demonstrated the presence of an enhancer that is located between nucleotides - 2264 and - 2495 in the 5' flanking region of the rat sodium/iodide symporter (NIS) gene (Ohno et al., 1999). When attached to NIS or heterologous promoters, this 232 bp fragment, which we call NUE, is able to stimulate transcription in a thyroid-specific and cAMP-dependent manner. A paired-domain transcription factor Pax8 binds to this enhancer and can stimulate the transcription in non-thyroid cells that do not normally support the NUE activities. Cotransfection of PKA, a downstream effector of cAMP, further potentiates the Pax8-mediated transactivation. However, this transcriptional machinery containing pax8 seems to require contributions from the neighboring cis-acting element that is similar to CRE/AP-1 consensus sequences. Modification of this putative CRE/AP-1 site not only represses the NUE transcriptional activities by 90% in FRTL-5 cells, but also nullifies the synergistic effect of PKA on pax8-mediated transactivation in HeLa cells. In this report, we have further characterized the putative CRE/AP-1 site within the NIS upstream enhancer using gel mobility shift assay. An oligonucleotide probe with NIS CRE/AP-1 sequence produced complex binding patterns in both FRTL-5 and HeLa cell, reflecting the presence of diverse classes of binding factors. When compared with CRE or AP-1 elements in other genes, the mobility shift pattern of NIS CRE/AP-1 was similar to those of collagenase TRE, c-Jun TRE, and somatostatin CRE, but the relative intensities of the binding complexes were quite different. This observation raises a possibility that the NIS CRE/AP-site is regulated by a novel mechanism.

Hormonal control of the transcription factor Pax8 and its role in the regulation of thyroglobulin gene expression in thyroid cells

The transcription factor Pax8 plays an important role in the expression of the differentiated phenotype of thyroid follicular cells. It has recently been shown that Pax8 is necessary for thyroglobulin (Tg) gene expression in the fully differentiated rat thyroid cell line PC. We have used the PC model system to investigate the role of Pax8 as a mediator of TSH regulation of Tg gene expression. We have demonstrated that Pax8 expression, as well as Tg expression, is severely reduced in cells grown in the absence of hormones and serum. The re-addition of TSH or forskolin to the culture medium is able to restore to wild-type levels the expression of both Pax8 and Tg. We have determined that the action of TSH/forskolin on Pax8 is at the transcriptional level. However, the re-expression of Pax8 can be observed several hours before that of Tg, suggesting that either another factor is needed or that Pax8 itself must be post-translationally modified by a newly synthesized protein to become active. To distinguish between these two possibilities we have stably transfected into PC cells an exogenous Pax8 that is expressed independently of TSH. Our results indicate that in these cells the Tg promoter is still dependent on TSH despite the constitutive presence of Pax8. Furthermore, we also show that in this condition Tg gene transcription requires de novo protein synthesis. In conclusion, TSH regulates the expression of Pax8 at a transcriptional level and also regulates the activity of Pax8 by controlling the expression of one or more as yet unknown factors.

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.

Immediate early genes induced by H-Ras in thyroid cells

Expression of oncogenic v-H-Ras in the thyroid cell line FRTL-5 (FRTL-5(Ras)) results in uncontrolled proliferation, loss of thyroid-specific gene expression and tumorigenicity. Concomitant expression of constitutively activated MEK and Rac, two major H-Ras downstream effectors, in FRTL-5 (FRTL-5(MEK/Rac)) recapitulates H-Ras effects on proliferation and morphology. In contrast to FRTL-5(Ras), however, FRTL-5(MEK/Rac) cells remain differentiated and are not tumorigenic. To find H-Ras induced genes potentially responsible for tumorigenicity and loss of differentiation, we have used subtractive suppression hybridization (SSH), a PCR-based cDNA subtraction technique, between de-differentiated and tumorigenic FRTL-5(Ras) cells and differentiated and non-tumorigenic FRTL-5(MEK/Rac) cells. We examined 800 of the cDNA clones obtained after subtraction and verified their levels of expression in the two cell lines by reverse northern, identifying 337 H-Ras induced genes. By sequence analysis, we clustered 57 different genes. Among these, 39 were known genes (involved in diverse signal transduction processes regulating mitogenic activity, cell survival, cytoskeletal reorganization, stress response and invasion) while the remaining 18 clones were novel genes. Among the 57 H-Ras specific clones, we identified those genes whose expression is induced early by H-Ras. We suggest that these immediate-early genes may play a crucial role in H-Ras-mediated transformation in thyroid epithelial cells.

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.

Identification and developmental expression of three Distal-less homeobox containing genes in the ascidian Ciona intestinalis

Several homeobox-containing genes related to Drosophila Distal-less (Dll) have been isolated from a wide variety of organisms and have been shown to function as developmental regulators. While in Drosophila only one Dll gene has been described so far, in Vertebrates many components of the Dlx multigenic family have been characterized. This suggests that, during the evolution of the Chordate phylum, the Dlx genes arose from an ancestral Dll/Dlx gene via gene duplication. We have previously reported the isolation of two Dll-related homeoboxes from the protochordate Ciona intestinalis, and described their clustered arrangement (Gene 156 (1995) 253). Here we present the detailed genomic organization and spatial-temporal expression of these two genes, Ci-Dll-A and Ci-Dll-B, and describe the isolation and characterization of another member of the ascidian family of Dll-related genes, which we tentatively named Ci-Dll-C.

Pax8 has a key role in thyroid cell differentiation

Transformation of rat thyroid cells with polyoma virus middle T antigen results in loss of the thyroid-differentiated phenotype, measured as the expression of the thyroglobulin (Tg), thyroperoxidase (TPO), and sodium/iodide symporter (NIS) genes. Among the transcription factors involved in the regulation of these genes, TTF-1 and TTF-2 were still detected at nearly wild-type levels, while a specific loss of the paired domain transcription factor Pax8 was observed. In this study, we used the PCPy cell line as a model system to study the role of Pax8 in thyroid differentiation. We demonstrate that the reintroduction of Pax8 in PCPy cells is sufficient to activate expression of the endogenous genes encoding thyroglobulin, thyroperoxidase, and sodium/iodide symporter. Thus, this cell system provides direct evidence for the ability of Pax8 to activate transcription of thyroid-specific genes at their chromosomal locus and strongly suggests a fundamental role of this transcription factor in the maintenance of functional differentiation in thyroid cells. Moreover, we show that Pax8 and TTF-1 cooperate in the activation of the thyroglobulin promoter and that additional thyroid-specific mechanism(s) are involved in such a cooperation. To identify the Pax8 domain able to mediate the specific activation of the thyroglobulin promoter, we transfected in PCPy cells three different Pax8 isoforms. The results of such experiments indicate that for the transcriptional activation of thyroid-specific genes, Pax8 uses an as yet unidentified functional domain.

The thyroid transcription factor 2 (TTF-2) is a promoter-specific DNA-binding independent transcriptional repressor

The thyroid transcription factor TTF-2 is a forkhead-containing protein involved in thyroid-specific gene expression and necessary for thyroid morphogenesis. In this paper, we demonstrate that TTF-2 is able to inhibit the activity of the thyroid-specific transcription factors TTF-1 and Pax-8 only on certain promoters. We identified the minimal protein domain responsible for repressor activity, which behaves as an independent functional domain, and we show that repression by TTF-2 is DNA-binding independent. We suggest that TTF-2 is able to interfere with a specific cofactor required for TTF-1 and Pax-8 activity.

Multiple ras downstream pathways mediate functional repression of the homeobox gene product TTF-1

Expression of oncogenic Ras in thyroid cells results in loss of expression of several thyroid-specific genes and inactivation of TTF-1, a homeodomain-containing transcription factor required for normal development of the thyroid gland. In an effort to understand how signal transduction pathways downstream of Ras may be involved in suppression of the differentiated phenotype, we have tested mutants of the Ras effector region for their ability to affect TTF-1 transcriptional activity in a transient-transfection assay. We find that V12S35 Ras, a mutant known to interact specifically with Raf but not with RalGDS or phosphatidylinositol 3-kinase (PI3 kinase) inhibits TTF-1 activity. Expression of an activated form of Raf (Raf-BXB) also inhibits TTF-1 function to a similar extent, while the MEK inhibitors U0126 and PD98059 partially relieve Ras-mediated inactivation of TTF-1, suggesting that the extracellular signal-regulated kinase (ERK) pathway is involved in this process. Indeed, ERK directly phosphorylates TTF-1 at three serine residues, and concomitant mutation of these serines to alanines completely abolishes ERK-mediated phosphorylation both in vitro and in vivo. Since activation of the Raf/MEK/ERK pathway accounts for only part of the activity elicited by oncogenic Ras on TTF-1, other downstream pathways are likely to be involved in this process. We find that activation of PI3 kinase, Rho, Rac, and RalGDS has no effect on TTF-1 transcriptional activity. However, a poorly characterized Ras mutant, V12N38 Ras, can partially repress TTF-1 transcriptional activity through an ERK-independent pathway. Importantly, concomitant expression of constitutive activated Raf and V12N38 Ras results in almost complete loss of TTF-1 activity. Our data indicate that the Raf/MEK/ERK cascade may act in concert with an as-yet-uncharacterized signaling pathway activated by V12N38 Ras to repress TTF-1 function and ultimately to inhibit thyroid cell differentiation.

Expression and functional analysis of Cititf1, an ascidian NK-2 class gene, suggest its role in endoderm development

In solitary ascidians the fate of endoderm is determined at a very early stage of development and depends on cytoplasmic factors whose nature has not been determined. We have isolated a member of the NK-2 gene family, Cititf1, from the ascidian Ciona intestinalis, showing high sequence homology to mammalian TITF1. The Cititf1 gene was expressed in all endodermal precursors at the pregastrula and gastrula stages, and is thus the first specific regulatory endodermal marker to be isolated from an ascidian. Cititf1 expression was downregulated at the end of gastrulation to reappear at middle tailbud and larval stages in the most anterior and ventral parts of head endoderm, regions which give rise, after metamorphosis, to the adult endostyle, where Cititf1 mRNA was still present. Microinjection of Cititf1 mRNA into fertilized eggs resulted in tadpole larvae with abnormalities in head-trunk development consequent to the formation of excess endoderm, perhaps due to recruitment of notochord precursors to an endodermal fate. These data suggest that Cititf1 plays an important role in normal endoderm differentiation during ascidian embryogenesis.

Ascidian homologs of mammalian thyroid peroxidase genes are expressed in the thyroid-equivalent region of the endostyle

The endostyle is a pharyngeal organ for the internal filter feeding of urochordates, cephalochordates, and larval lamprey. This organ is also considered to be homologous to the follicular thyroid gland of higher vertebrates. Thyroglobulin (Tg) and thyroid peroxidase (TPO) are specifically expressed in the thyroid gland of higher vertebrates, and they play an important role in iodine metabolism for the synthesis of thyroid hormones. Previous histochemical observations showed that iodine-concentrating and peroxidase activities were detected in zones 7, 8, and 9 of the ascidian endostyle, suggesting that these zones contains cells that are equivalent to those in the vertebrate follicular thyroid. In order to investigate the molecular developmental mechanisms involved in the formation and function of the endostyle, with special reference to the evolution of the thyroid gland, in the present study, we isolated and characterized cDNA clones for TPO genes, CiTPO from Ciona intestinalis and HrTPO from Halocynthia roretzi. Northern blot and in situ hybridization analyses revealed that the expression of the ascidian TPO genes was restricted to zone 7, one of the elements equivalent to the thyroid. These results provide the first evidence at the gene expression level for shared function between a part of the ascidian endostyle and the vertebrate follicular thyroid gland. J. Exp. Zool. ( Mol. Dev. Evol. ) 285:158-169, 1999.

Structural defects of a Pax8 mutant that give rise to congenital hypothyroidism

Pax proteins are transcriptional regulators that play important roles during embryogenesis. These proteins recognize specific DNA sequences via a conserved element: the paired domain (Prd domain). The low level of organized secondary structure, in the free state, is a general feature of Prd domains; however, these proteins undergo a dramatic gain in alpha-helical content upon interaction with DNA ('induced fit'). Pax8 is expressed in the developing thyroid, kidney and several areas of the central nervous system. In humans, mutations of the Pax8 gene, which are mapped to the coding region of the Prd domain, give rise to congenital hypothyroidism. Here, we have investigated the molecular defects caused by a mutation in which leucine at position 62 is substituted for an arginine. Leu62 is conserved among Prd domains, and contributes towards the packing together of helices 1 and 3. The binding affinity of the Leu62Arg mutant for a specific DNA sequence (the C sequence of thyroglobulin promoter) is decreased 60-fold with respect to the wild-type Pax8 Prd domain. However, the affinities with which the wild-type and the mutant proteins bind to a non-specific DNA sequence are very similar. CD spectra demonstrate that, in the absence of DNA, both wild-type Pax8 and the Leu62Arg mutant possess a low alpha-helical content; however, in the Leu62Arg mutant, the gain in alpha-helical content upon interaction with DNA is greatly reduced with respect to the wild-type protein. Thus the molecular defect of the Leu62Arg mutant causes a reduced capability for induced fit upon DNA interaction.

Molecular genetics of congenital hypothyroidism

Congenital thyroid gland defects - resulting in reduced production of the hormones triiodothyronine (T3) and thyroxine (T4) - can be a consequence of either reduced or absent thyroid tissue (thyroid dysgenesis) or, less frequently, of impairment in the biochemical mechanisms responsible for hormone biosynthesis (thyroid dyshormonogenesis). Recent studies have revealed how mutations in the genes encoding either transcription factors or the thyroid stimulating hormone receptor cause, in humans or in mouse models, thyroid dysgenesis. This demonstrates, for the first time, the heritability of this condition. New genes responsible for thyroid dyshormonogenesis have also been discovered.

Cloning, chromosomal localization and identification of polymorphisms in the human thyroid transcription factor 2 gene (TITF2)

The human gene encoding the thyroid transcription factor 2 (TTF-2) was cloned and mapped to human chromosome 9q22. Three polymorphisms were identified in the gene by SSCP and direct sequencing: two consist of a third base substitution in the triplet encoding Leu129 and Ser273, and the third is an alanine stretch that varies from 12 to 17 residues. TTF-2 plays a critical role during thyroid morphogenesis in mice, and in man the TITF2 gene is associated with congenital hypothyroidism and cleft palate with thyroid dysgenesis. The polymorphisms identified in this study can be used as markers to study the role of the TITF2 gene in other cases of thyroid dysgenesis, especially in familial cases.

Calreticulin enhances the transcriptional activity of thyroid transcription factor-1 by binding to its homeodomain

Transcription factors are often regulated by associated protein cofactors that are able to modify their activity by several different mechanisms. In this study we show that calreticulin, a Ca2+-binding protein with chaperone activity, binds to thyroid transcription factor-1 (TTF-1), a homeodomain-containing protein implicated in the differentiation of lung and thyroid. The interaction between calreticulin and TTF-1 appears to have functional significance because it results in increased transcriptional stimulation of TTF-1-dependent promoters. Calreticulin binds to the TTF-1 homeodomain and promotes its folding, suggesting that the mechanism involved in stimulation of transcriptional activity is an increase of the steady-state concentration of active TTF-1 protein in the cell. We also demonstrate that calreticulin mRNA levels in thyroid cells are under strict control by the thyroid-stimulating hormone, thus implicating calreticulin in the modulation of thyroid gene expression by thyroid-stimulating hormone.

Concomitant activation of MEK-1 and Rac-1 increases the proliferative potential of thyroid epithelial cells, without affecting their differentiation

Activating point mutations in the Ras oncogene occur in a large number of human tumors, especially of epithelial origin. In thyroid follicular cells, ectopic expression of oncogenic H-Ras results in growth factor-independent proliferation, loss of differentiation and tumor formation in nude mice. In fibroblasts concomitant activation of the MAP kinase cascade and the small GTPase Rac-1 leads to full malignant transformation. We have tested the effects of these key downstream mediators of Ras in thyroid epithelial cells, by stably expressing either a constitutively active form of MEK-1 (MEK(deltaN3/S218E/S222D)), a constitutively active form of Rac-1 (Val12-Rac), or both. While the activation of one molecule or the other results in a weak phenotype, concomitant activation of both MEK-1 and Rac-1 in thyroid cells leads to growth factor-independent proliferation, morphological transformation and anchorage-independent growth. However, in contrast to Ras-transformed thyroid cells, the ones expressing the constitutively active forms of MEK-1 and Rac-1 maintain their differentiate phenotype and fail to form tumors when injected into nude mice. Thus, in thyroid epithelial cells, concomitant activation of MEK-1 and Rac-1 can reproduce only a subset of the Ras-induced effects and is not sufficient to cause full malignant transformation. Significantly, Ras-mediated increased proliferation and loss of differentiation can be dissociated in these cells.

Ciona intestinalis nuclear receptor 1: a member of steroid/thyroid hormone receptor family

Nuclear hormone receptors comprise a large family of zinc finger transcription factors, some with hydrophobic ligands, such as thyroid hormone, vitamin D, steroids, etc., and others for which no ligand has been found. Thyroid hormone receptors (TRs) generally are considered to be confined to the vertebrata that possess a thyroid gland. Tunicates represent the most primitive of the chordates, and there are data supporting a role for thyroid hormone in their metamorphosis, but no data are available on TRs in this genus; hence, we have studied Ciona intestinalis. Screening of a Ciona library with the DNA binding domain of Xenopus laevis TR (xTR) resulted in the isolation of a nuclear hormone receptor, C. intestinalis nuclear receptor 1 (CiNR1). CiNR1 is similar to TRs of more evolved species with a conserved DNA binding domain whereas the ligand binding domain shows poor homology to vertebrate sequences. The C-terminal part of CiNR1 spans approximately 200 amino acids more than other TRs, lacks the AF2 transactivation domain, and is not able to bind triiodothyronine. Phylogenetically, CiNR1 appears to be close to the common ancestral gene of TRs. Expression of CiNR1 was limited to the developing embryo and the larval stage, which suggests a role during development and metamorphosis. In transfection experiments, CiNR1 down-regulated basal transcription of a reporter gene driven by the TR palindrome responsive element. When CiNR1 was cotransfected with chicken TRalpha, it attenuated the normal thyroid hormone response in a dominant negative fashion. This attenuation required the C-terminal portion of the molecule.

A mouse model for hereditary thyroid dysgenesis and cleft palate

Alteration of thyroid gland morphogenesis (thyroid dysgenesis) is a frequent human malformation. Among the one in three to four thousand newborns in which congenital hypothyroidism is detected, 80% have either an ectopic, small and sublingual thyroid, or have no thyroid tissue. Most of these cases appear sporadically, although a few cases of recurring familial thyroid dysgenesis have been described. The lack of evidence for hereditary thyroid dysgenesis may be due to the severity of the hypothyroid phenotype. Neonatal screening and early thyroid hormone therapy have eliminated most of the clinical consequences of hypothyroidism such that the heritability of this condition may become apparent in the near future. We have recently cloned cDNA encoding a forkhead domain-containing transcription factor, TTF-2, and have located the position of the gene, designated Titf2, to mouse chromosome 4 (ref. 3). Titf2 is expressed in the developing thyroid, in most of the foregut endoderm and in craniopharyngeal ectoderm, including Rathke's pouch. Expression of Titf2 in thyroid cell precursors is down-regulated as they cease migration, suggesting that this factor is involved in the process of thyroid gland morphogenesis. Here we show that Titf2-null mutant mice exhibit cleft palate and either a sublingual or completely absent thyroid gland. Thus, mutation of Titf2-/- results in neonatal hypothyroidism that shows similarity to thyroid dysgenesis in humans.

p53 genes mutated in the DNA binding site or at a specific COOH-terminal site exert divergent effects on thyroid cell growth and differentiation

Expression of mutated versions of the p53 gene deranged the differentiation program of thyroid cells and resulted in deregulated growth. Specifically, p53 mutants in several residues of the DNA-binding region induced thyrotropin (TSH) -independent growth and inhibition of the expression of thyroid-specific genes. The loss of the differentiated phenotype invariably correlated with the blockage of the expression of the genes coding for the thyroid transcriptional factors PAX-8 and TTF2. Conversely, thyroid cells transfected with a p53 gene mutated at codon 392, located outside the DNA-binding region, stimulated the expression of differentiation genes in the absence of the TSH, and induced TSH-independent growth. cAMP intracellular levels were higher in thyroid cells transfected with the p53 gene mutated at the 392 site than in the untransfected thyroid cells, but lower in the cells transfected with the other mutated p53 genes. Fra-1 and c-jun were induced by p53, resulting in increased AP-1 levels. The results of this study suggest that p53 exerts effects on cAMP transduction pathway in thyroid cells, which are exquisitely sensitive to cAMP.

Ha-ras interference with thyroid cell differentiation is associated with a down-regulation of thyroid transcription factor-1 phosphorylation

Mechanisms responsible for the lack of thyroid-specific differentiation markers in Ha-ras transformed FRTL-5 cells have been investigated. In vivo cell labeling and immunoprecipitation demonstrate that phosphorylation of the thyroid transcription factor-1 (TTF-1) is clearly reduced in thyroid cells transformed with the Ha-ras oncogene. Fingerprinting analysis of phosphotryptic peptides from FRTL-5 and Ha-ras-FRTL-5 cells also reveals a heterogeneous pattern of TTF-1 phosphorylation in the transformed cell line. This heterogeneity is localized in the amino terminal cluster of phosphoserines, as determined by transfection of HeLa cells with TTF-1 mutants in which serine residues have been replaced by alanines. Amplification and nucleotide sequence of the 5'-coding region of the TTF-1 gene in Ha-ras-FRTL-5 cells rule out the possibility that differences in phosphorylation were the consequence of any mutational event affecting residues within the N-terminal protein sequence. Hypophosphorylated TTF-1 is still able to bind its DNA consensus sequence within the thyroglobulin promoter, although a reporter construct whose expression is exclusively dependent on TTF-1 is not transactivated. Transfection of Ha-ras-FRTL-5 cells with an expression vector encoding the cAMP dependent protein kinase A (PKA) catalytic subunit partially reestablishes TTF-1 transcriptional activity. Taken together, these results indicate that the lack of specific thyroid gene expression in Ha-ras-FRTL-5 cells could be a direct consequence of the inability of TTF-1 to promote transcription.

Molecular events involved in differentiation of thyroid follicular cells

Organogenesis is a complex event, often dependent on inductive tissue interactions, that ultimately promote expression and activation of a combinatorial sequence of transcription factors which are involved in controlling migration, growth and differentiation.

PAX8 mutations associated with congenital hypothyroidism caused by thyroid dysgenesis

Permanent congenital hypothyroidism (CH) is a common disease that occurs in 1 of 3,000-4,000 newborns. Except in rare cases due to hypothalamic or pituitary defects, CH is characterized by elevated levels of thyroid-stimulating hormone (TSH) resulting from reduced thyroid function. When thyroid hormone therapy is not initiated within the first two months of life, CH can cause severe neurological, mental and motor damage. In 80-85% of cases, CH is associated with and presumably is a consequence of thyroid dysgenesis (TD). In these cases, the thyroid gland can be absent (agenesis, 35-40%), ectopically located (30-45%) and/or severely reduced in size (hypoplasia, 5%). Familial cases of TD are rare, even though ectopic or absent thyroid has been occasionally observed in siblings. The pathogenesis of TD is still largely unknown. Although a genetic component has been suggested, mutations in the gene encoding the receptor for the thyroid-stimulating hormone (TSHR) have been identified in only two cases of TD with hypoplasia. We report mutations in the coding region of PAX8 in two sporadic patients and one familial case of TD. All three point mutations are located in the paired domain of PAX8 and result in severe reduction of the DNA-binding activity of this transcription factor. These genetic alterations implicate PAX8 in the pathogenesis of TD and in normal thyroid development.

Cihox5, a new Ciona intestinalis Hox-related gene, is involved in regionalization of the spinal cord

In this paper we report the cloning, sequence and expression analysis of a new Ciona intestinalis hox gene. On the basis of sequence comparison with mammalian and Amphioxus homologues, we called this gene Cihox5. Northern blot analysis reveals a single transcript of about 1.3 kb in length, that is present from neurula until larva stage. Whole-mount in situ hybridization shows restricted expression of this gene in putative blood cells precursors and in a regional domain of the spinal chord. Expression in the spinal cord is attributed to ependymal cells. This result implies a role for this gene in primitive regionalization of spinal cord along the anteroposterior axis in the absence of neuronal bodies.

Identification of the thyroid transcription factor-1 as a target for rat MST2 kinase

Thyroid transcription factor-1 (TTF-1) is a homeodomain-containing transcription factor that is required for thyroid-specific expression of the thyroglobulin and thyroperoxidase genes as well as for lung-specific expression of the surfactant protein A, B, and C and the CC10 and the HNF-3 alpha genes. TTF-1 is a phosphoprotein, and the phosphorylation of TTF-1 has been studied already. However, the kinase(s) that could be responsible for this phosphorylation have not been known. In this paper we report the identification by in-gel kinase assay of a 56-kDa serine/threonine kinase that is able to phosphorylate TTF-1 in thyroid cells. The cloning of this kinase revealed that we had identified the rat homolog of the human MST2 kinase. The pathway in which human MST2 functions is not known; however, it does not appear to involve either mitogen-activated protein kinases such as Erk1 and Erk2 nor the stress-activated protein kinases such as JNK and p38. We show that the activity responsible for TTF-1 phosphorylation is constitutive in thyroid cells. Furthermore, we demonstrate that TTF-1 is phosphorylated in vivo by rMST2 at the same residues that had been identified previously as the major phosphorylation sites. Thus, TTF-1 represents the first identified target of this class of protein kinases.

Expression of the RET/PTC1 oncogene impairs the activity of TTF-1 and Pax-8 thyroid transcription factors

The most frequent genetic alterations described thus far in human papillary thyroid carcinomas are somatic rearrangements of the RET proto-oncogene, which generate the chimeric RET/PTC oncogenes. We recently found that the expression of the RET/PTC1 oncogene blocked the expression of the thyroid-differentiated phenotype in rat thyroid epithelial cell line PC CI 3 (PC). Here, we show that this block occurs at a transcriptional level; indeed, the thyroid-specific thyroglobulin and thyroperoxidase gene promoters were inactive in PC-PTC cells. Specific transcription factors, namely, TTF-1 and Pax-8, regulate the expression of differentiated functions in thyroid cells. Here, we show that Pax-8 is expressed at reduced levels in PC-PTC cells and that its adoptive overexpression is unable to restore the activity of target promoters. In contrast, TTF-1 expression is unaltered in PC-PTC cells; however, by using a synthetic promoter that contains its specific target sequence, we demonstrate that TTF-1 is inactive in PC-PTC cells. We conclude that the RET/PTC1 oncogene alters the expression of the thyroid-differentiated phenotype by at least two different mechanisms, ie., down-regulation of Pax-8 protein and mRNA expression and impaired function of TTF-1 and Pax-8, which occurs at a posttranslational level.

Transfection of TTF-1 gene induces thyroglobulin gene expression in undifferentiated FRT cells

The thyroglobulin gene, the substrate for thyroid hormone biosynthesis, is not expressed in the FRT cell line, which, even though it manifests the polarised epithelial phenotype, does not express any of the thyroid functional properties. Two transcription factors, TTF-1 and Pax-8, have been implicated in thyroid specific expression of the thyroglobulin gene. FRT cells contain Pax-8 but they lack TTF-1. In this paper, we show that transfection of TTF-1 expression vectors in FRT cells results in activation of thyroglobulin gene expression. If the expression vector encoded for TTF-1-ER, a fusion gene coding for the entire TTF-1 protein fused to the hormone-binding domain of the steroid receptor, under the control of the RSV promoter, thyroglobulin gene expression was controlled by estrogen. These data provide a direct demonstration that TTF-1 activates the chromosomal thyroglobulin promoter. Since transfection of TTF-1 expression vectors in non-thyroid cell types did not result in thyroglobulin gene expression, it is suggested that Pax-8, in addition, perhaps, to a specific cellular environment, might be required for thyroid specific expression of the thyroglobulin gene.

Transcriptional control of the forkhead thyroid transcription factor TTF-2 by thyrotropin, insulin, and insulin-like growth factor I

The hormonal regulation of both thyroglobulin and thyroperoxidase promoter activity in FRTL-5 thyroid cells takes place, at least in part, through a hormone-responsive element to which the thyroid transcription factor TTF-2 binds. The TTF-2 cDNA, encoded by the titf2 locus, has recently been cloned and classified as a member of the forkhead transcription factor family. Here, we demonstrate that TTF-2 mRNA levels become undetectable in FRTL-5 thyroid cells cultured for 4 days in 0.2% serum and in the absent of thyrotropin (TSH) and insulin. Addition of TSH, insulin or insulin-like growth factor I (IGF-I) to the culture medium increases the levels of this transcription factor in a dose- and time- dependent manner and requires ongoing protein synthesis. The TSH effect is greater than that produced by insulin or IGF-I and is similar to the effect produced by the cAMP analog forskolin. The TSH and insulin effects are additive. In all cases, the mRNA levels increase is accompanied by an increase in transcription rate, as demonstrated by run-off assays. These data demonstrate that the TTF-2 mRNA is under tight hormonal control. This is consistent with an important role for TTF-2 as a mediator of the transcriptional activation of thyroid-specific genes (thyroglobulin and thyroperoxidase) by TSH via cAMP and by insulin through the IGF-I receptor.

Specific cellular localization of tyrosinase mRNA during Ciona intestinalis larval development

A Ciona intestinalis cDNA clone that encodes a protein highly homologous to other tyrosinases was isolated. Northern blot analysis showed that expression of Ciona tyrosinase starts at the early neurula stage and continues throughout the tail-bud and tadpole larval stages. The earliest tyrosinase expression was detected, by in situ hybridization, at the neural plate stage, in pigment precursor cells located along the two neural folds, in the animal region of the embryo. In the course of embryonic development the strong hybridization signal was always localized, within the rostral part of the developing brain, in the pigment precursor cells and was later detected in the otolith and ocellus. These results are discussed in relation to tyrosinase as an early marker of neural induction.

TTF-2, a new forkhead protein, shows a temporal expression in the developing thyroid which is consistent with a role in controlling the onset of differentiation

Expression of thyroglobulin (Tg) and thyroperoxidase (TPO) genes in thyroid follicular cells occurs in the mouse at embryonic day (E)14.5. Two transcription factors, TTF-1 and Pax-8, have been implicated in transcriptional activation of Tg and TPO, even though the onset of their expression is at E9.5, suggesting that additional events are necessary for transcriptional activation of Tg and TPO genes. We report in this paper the cloning of TTF-2, a DNA binding protein that recognizes sites on both Tg and TPO promoters. TTF-2 is a new forkhead domain-containing protein whose expression is restricted to the endodermal lining of the foregut and to the ectoderm that will give rise to the anterior pituitary. TTF-2 shows transient expression in the developing thyroid and anterior pituitary. In the thyroid, TTF-2 expression is down-regulated just before the onset of Tg and TPO gene expression, suggesting that this transcription factor plays the role in development of a negative controller of thyroid-specific gene expression.

Hydrogen-deuterium exchange studies of the rat thyroid transcription factor 1 homeodomain

The 1H NMR solution structure of the rat thyroid transcription factor 1 homeodomain (TTF-1 HD) showed that the molecule folds like classical homeodomains. The C-terminal extension of helix III (fragment 51-59) appeared to adopt a helical geometry, albeit not as rigid as the preceding portion, but the hydrogen-deuterium exchange of backbone amides and the NOE data provided evidence of a discontinuity between the two moieties of helix III at the highly conserved fragment Asn51-His52-Arg53. Analysis of quantitative measurements of isotope exchange rates allows one to recognize the general occurrence, in that region of HD motifs, of opposite effects to helix III stability. Asparagine, histidine and arginine residues occur most frequently at the beginning and end of protein helices. In TTF-1 HD a local fluctuation is observed in the fragment 51-53 which either kinks or tightens the alpha-helix. A search through the protein structure database reveals that the three most common variants of HD fragments 51-53 are often involved in helices and, frequently, in helix initiation or termination. For homeodomains in general, the nature of the fragment 51-53 may be related to the conformational dynamics of their DNA-recognition helix (helix III). Besides the specific results on fragment 51-53, the complete isotope exchange analysis of TTF-1 HD data shows that the partially solvent-exposed recognition helix is stabilized by hydrophobic interactions, like most of the structured regions of the molecule. Hydrophobic stabilization of the contacting regions meets the requirements of a DNA-interaction mechanism which, as shown with other DNA-protein complexes, should entail negative heat capacity variations due to changes in solvent exposure of the nonpolar protein surface.

Mutations in the gene encoding thyroid transcription factor-1 (TTF-1) are not a frequent cause of congenital hypothyroidism (CH) with thyroid dysgenesis

Permanent congenital hypothyroidism (CH) has an incidence of 1/3000-4000 newborns and is among the most frequent cause of mental retardation and neurological alterations in children. In 80% to 85% of cases CH is associated with thyroid dysgenesis. A group of 61 patients with CH (22 with agenesis, 18 with ectopy, 1 with hypoplasia, and 20 cases with CH without thyroid enlargement but not further characterized) and 30 normal subjects were examined for the presence of mutations in the gene encoding the thyroid transcription factor 1 (TTF-1). The coding-region of the TTF-1 gene was analyzed in all cases by the single stranded conformational polymorphism (SSCP) and no mutations were detected. Direct sequencing also carried out in patients with thyroid agenesis confirmed the absence of mutations or polymorphisms in the TTF-1 gene. The absence of mutations in the TTF-1 gene in our samples indicates that the mutations in the TTF-1 gene are not a frequent cause of CH.

Transglutaminase activity is related to CAG repeat length in patients with Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder associated with CAG repeat expansion. We measured transglutaminase (TGase) activity in lymphocytes from 35 HD patients and from healthy individuals to ascertain whether it was altered in this condition. TGase activity was above maximum control levels in 25% of HD patients; it was correlated with the age of the patient and inversely correlated with the CAG repeat length. These results suggest that: (1) HD could be biochemically heterogeneous, and (2) the length of the CAG repeat expansion/TGase ratio could be important in the manifestation of HD.

Analysis of the solution structure of the homeodomain of rat thyroid transcription factor 1 by 1H-NMR spectroscopy and restrained molecular mechanics

The solution structure of the rat thyroid transcription factor 1 (TTF-1) homeodomain has been elucidated by 1H-NMR and restrained modeling. The TTF-1 homeodomain folds in the same manner as classical homeodomains, with three helices, a loose loop between the first two helices, and a tight turn between helix II and helix III. The typical assembly of the hydrophobic core is maintained and N-capping motifs are identified in helix I and helix III. The N-terminal stretch of helix II exhibits some mobility, similar to the preceding loop region, which may be related to its anomalous capping. The N-terminal decapeptide and the C-terminal octapeptide of the molecule (68 residues long) are disordered. All the previous characteristics are shared by all known isolated homeodomain structures. An important difference among these structures occurs at the C-terminal extension of helix III, which is either disordered or helically folded. In the TTF-1 homeodomain, the C-terminal extension of helix III (residues 51-59) appears structured, albeit not as rigidly as the preceding portion. Analysis of the NOEs and hydrogendeuterium exchange of backbone amides provides evidence for discontinuity between the two moieties of helix III, which is introduced by a tightening or a kink of residues 51-53.

A molecular code dictates sequence-specific DNA recognition by homeodomains

Most homeodomains bind to DNA sequences containing the motif 5'-TAAT-3'. The homeodomain of thyroid transcription factor 1 (TTF-1HD) binds to sequences containing a 5'-CAAG-3' core motif, delineating a new mechanism for differential DNA recognition by homeodomains. We investigated the molecular basis of the DNA binding specificity of TTF-1HD by both structural and functional approaches. As already suggested by the three-dimensional structure of TTF-1HD, the DNA binding specificities of the TTF-1, Antennapedia and Engrailed homeodomains, either wild-type or mutants, indicated that the amino acid residue in position 54 is involved in the recognition of the nucleotide at the 3' end of the core motif 5'-NAAN-3'. The nucleotide at the 5' position of this core sequence is recognized by the amino acids located in position 6, 7 and 8 of the TTF-1 and Antennapedia homeodomains. These data, together with previous suggestions on the role of amino acids in position 50, indicate that the DNA binding specificity of homeodomains can be determined by a combinatorial molecular code. We also show that some specific combinations of the key amino acid residues involved in DNA recognition do not follow a simple, additive rule.

A network of specific minor-groove contacts is a common characteristic of paired-domain-DNA interactions

Pax proteins are a family of transcription factors conserved during evolution and able to bind specific DNA sequences through a domain called a "paired domain'. The DNA-binding specificity of the Pax-8 paired domain was investigated. Site-selection experiments indicate that Pax-8 binds to a consensus sequence similar to those bound by Pax-2 and Pax-5. When consensus sequences of various paired domains are observed in light of recent structural studies describing paired-domain-DNA interaction [Xu, Rould, Jun, Desplan and Pabo (1995) Cell 80, 639-650], it appears that base-pairs contacted in the minor groove are conserved, while most of the base-pairs contacted in the major groove are not. Therefore a network of specific minor groove contacts is a common characteristic of paired-domain-DNA interactions. The functional importance of such a network was successfully tested by analysing the effect of consensus-based mutations on the Pax-8 binding site of the thyroglobulin promoter.

Surfactant proteins and thyroid transcription factor-1 in pulmonary and breast carcinomas

Antibodies to the pulmonary epithelial cell-specific proteins surfactant proteins A and B (SP-A and SP-B) and to thyroid transcription factor-1 (TTF-1), a homeodomain nuclear transcription protein, were used as immunohistochemical markers to asses their ability to distinguish primary pulmonary non-small cell carcinomas (n = 57) from carcinomas of the breast (n = 51). SP-A, SP-B, and TTF-1 were detected in 49%, 53%, and 63% of non-small cell carcinomas, respectively. These three antibodies stained pulmonary adenocarcinomas in 54%, 63% and 76% of specimens, respectively. Squamous cell carcinomas rarely stained using these markers. Antibodies to SP-B and TTF-1 never stained any of the 51 breast carcinomas, whereas four of these tumors stained for SP-A. To better define the potential diagnostic value of these antibodies, 13 breast carcinomas metastatic to the lung were studied. Of the three antibodies tested, only TTF-1 seemed useful, because none of the 13 metastatic tumors showed immunoreactivity to this antibody, whereas six specimens (46%) showed reactivity for both SP-A and SP-B. To emphasize further the potential usefulness of antibodies to TTF-1, sections of adenocarcinomas of the colon (n = 18) and prostate (n = 9), renal cell carcinomas (n = 8), and epithelioid mesotheliomas (n = 4) were evaluated; none was positive. Only one of 66 gastric and one of eight endometrial adenocarcinomas showed focal positivity. These results demonstrate the usefulness of immunodetection of a pulmonary cell selective transcription protein (TTF-1) in the diagnosis of pulmonary adenocarcinoma, readily distinguishing breast carcinomas from primary pulmonary adenocarcinomas. In contrast, staining for SP-A and SP-B is of limited value, because there is an unacceptably high rate of cross-reactivity between breast carcinomas metastatic to the lung and primary pulmonary carcinomas. The latter finding illustrates and supports the fact that tumor marker phenotypes might differ in primary and secondary tissue sites.

Hepatocyte nuclear factor 3beta participates in the transcriptional regulation of the thyroperoxidase promoter

The promoters of the thyroglobulin (Tg) and thyroperoxidase (TPO) genes both contain sequences that match the consensus recognized by the fork-head family of transcription factors. The fork-head recognition element in the TPO promoter is recognized by two proteins, one of which is the transcription factor HNF-3beta, a member of the hepatocyte nuclear factor 3 (HNF-3) family, and the other is a thyroid-specific binding activity previously described as thyroid transcription factor 2 (TTF-2). Differently, only TTF-2 is capable of recognizing the fork-head recognition element in the thyroglobulin promoter, HNF-3beta protein is present in cultured thyroid cells and in the adult thyroid gland, suggesting that it does indeed have a role in modulating thyroid-specific gene expression. The transcriptional activities of TPO promoter mutants impaired in either TTF-2 or HNF-3beta binding suggest that both factors participate in regulating transcription from the TPO promoter.

Reexpression of thyroid peroxidase in a derivative of an undifferentiated thyroid carcinoma cell line by introduction of wild-type p53

Loss of function of p53 is believed to result in transformation through impairment of its properties as a transcription factor, which interferes with the regulation of the cell cycle and under certain conditions, with programmed cell death. We report that stable transfection of clonal undifferentiated thyroid carcinoma cell lines harboring endogenous p53 mutations with a wild-type p53 expression vector only rarely yields transfectants expressing authentic wild-type p53. Among these, most exhibited an increase in doubling time and an impairment of colony formation in soft agar. Only one clonal wild-type p53-overexpressing derivative of the NPA papillary carcinoma cell line was obtained, and these cells were found to reexpress thyroid peroxidase (TPO). This clone also demonstrated reexpression of the paired box domain transcription factor Pax-8, which specifically activates transcription of TPO. Wild-type p53 did not directly stimulate transcriptional activity of a TPO promoter construct. Although the low frequency of authentic wild-type p53 stable transfectants limits the power of this analysis, these data suggest that in addition to its role in malignant transformation, p53 may be significant in the determination or maintenance of cell differentiation in thyroid neoplasms.

Expression of thyroid transcription factor 1 gene can be regulated at the transcriptional and posttranscriptional levels

The complete structure of the gene for thyroid transcription factor 1 (TTF-1), both in rats and humans, has been determined. The rat TTF-1 gene shows three transcriptional start sites and contains two introns, one of which is alternatively spliced. Nuclear run-on and transient transfection experiments indicate that TTF-1 gene expression can be controlled at different levels. Using thyroid and nonthyroid cell lines, it can be shown that transcriptional mechanisms are involved in controlling thyroid-specific expression of the TTF-1 gene. In contrast, in thyroid cells expressing an activated Ki-ras oncogene, the steady-state level of TTF-1 mRNA is greatly reduced, while transcription of the TTF-1 gene is only moderately affected, suggesting that the accumulation of TTF-1 mRNA can be regulated by a posttranscriptional, Ras-sensitive mechanism.

Mapping and functional role of phosphorylation sites in the thyroid transcription factor-1 (TTF-1)

The phosphorylation of thyroid transcription factor-1 (TTF-1), is homeodomain-containing transcription factor that is required for thyroid-specific expression of the thyroglobulin and thyroperoxidase gene promoters, has been studied. Phosphorylation occurs on a maximum of seven serine residues that are distributed in three tryptic peptides. Mutant derivatives of TTF-1, with alanine sites, have been constructed and used to assess the functional relevance of TTF-1 phosphorylation. The DNA binding activity of TTF-1 appears to be phosphorylation-independent, as indicated also by the performance of TTF-1 purified from an overexpressing Escherichia coli strain. Transcriptional activation by TTF-1 could require phosphorylation only in specific cell types since in a co-transfection assay in heterologous cells both wild-type and mutant proteins show a similar transcriptional activity.