Dissection of functional domains of the pituitary-specific transcription factor GHF-1

MRE11 as a molecular signature and therapeutic target for cancer treatment with radiotherapy

MRE11, the core of the MRE11/RAD50/NBS1 complex, is one of key DNA damage response proteins. Increasing evidence suggests that its expression in cancer cells is critical to developing radioresistance; as such, MRE11 is an emerging marker for targeted radiosensitization strategies. Elevated MRE11 in tumor tissues has been associated with poor survival in patients undergoing radiotherapy, although in some cancer types, the opposite has been noted. The recent discovery of ionizing radiation-induced truncation of MRE11, which decreases its efficacy, may explain some of these paradoxical findings. The progress of research on the biological modulation of MRE11 expression is also discussed, with the potential application of small molecule or large molecule inhibitors of MRE11 for enhancing radiosensitivity. Current research has further highlighted both nuclease and non-nuclease activities of MRE11 in cancer cells treated with ionizing radiation, and differentiation between these is essential to verify the targeting effects of radiosensitizing agents. These updates clarify our understanding of how MRE11 expression may be utilized in future stratification of cancer patients for radiotherapy, and how it may be leveraged in shaping novel radiosensitization strategies.

Precocious or early puberty in patients with combined pituitary hormone deficiency due to POU1F1 gene mutation: case report and review of possible mechanisms

Central precocious puberty (CPP) or early puberty (EP) is a rare entity in combined pituitary hormone deficiency (CPHD), the latter caused by mutations in pituitary transcription factor genes. The early onset of puberty in two patients with CPHD with POU1F1 gene mutation was evaluated. A 3-month-old boy was diagnosed with central hypothyroidism, and L-thyroxine was commenced. He was referred for the evaluation of short stature at 20 months of age. Anthropometric evaluation revealed severe short stature (- 6.1 SDS), and growth hormone (GH) and prolactin deficiencies were diagnosed. Homozygous POU1F1 gene mutation (c.731T>G, p. I244S) was also detected. Testicular enlargement and high luteinizing hormone (LH) levels were observed at 7 years and 9 months of age while he was on GH and L-thyroxine treatment. Due to rapid progression of puberty, gonadotropin-releasing hormone analogue (GnRHa) was initiated at 11.3 years of age. This patient recently turned 19.2 years old, and his final height was - 2.3 SDS. The second patient, a 6-month-old boy, was also referred for growth retardation. His height was - 2.7 SDS, and GH and thyroid-stimulating hormone (TSH) deficiencies were diagnosed. He also had homozygous (c.10C>T, p.Q4*) POU1F1 gene mutation. Onset of puberty was relatively early, at 10 years, with advanced bone age. He was on GnRHa treatment between 11.5 and 12.5 years of age. Recent evaluation of the patient was at 13.6 years of age, and he is still on levothyroxine and GH treatment. The relationship between the POU1F1 genotype and CPP or EP has not as yet been firmly established in humans. Animal studies have revealed that the Pou1f1 gene has a major effect on regulation of GnRH receptor function and the Gata2 gene. It has also been demonstrated that this gene controls gonadotrope evolution and prevents excess gonadotropin levels. Further studies are, however, needed to elucidate the relation between POU1F1 function and CPP.

Evolution of the POU1F1 transcription factor in mammals: Rapid change of the alternatively-spliced β-domain

The POU1F1 (Pit-1) transcription factor is important in regulating expression of growth hormone, prolactin and TSH β-subunit, and controlling development of the anterior pituitary cells in which these hormones are produced. POU1F1 is a conserved protein comprising three main domains, an N-terminal transcription activation domain (TAD), a POU-specific domain and a C-terminal homeodomain. Within the TAD, a β-domain can be inserted by alternative splicing, giving an extended 'β-variant' with altered properties. Here sequence data from over 100 species were used to assess the variability of POU1F1 in mammals. This showed that the POU-specific domain and homeodomain are very strongly conserved, and that the TAD is somewhat less conserved, as are linker and hinge regions between these main domains. On the other hand, the β-domain is very variable, apparently evolving at a rate not significantly different from that expected for unconstrained, neutral evolution. In several species stop and/or frameshift mutations within the β-domain would prevent expression of the β-variant as a functional protein. In most species expression of the β-variant is low (<5% of total POU1F1 expression). The rate of evolution of POU1F1 in mammals shows little variation, though the lineage leading to dog does show an episode of accelerated change. This comparative genomics study suggests that in most mammalian species POU1F1 variants produced by alternative splicing may have little physiological significance.

Combined pituitary hormone deficiency: current and future status

Over the last two decades, the understanding of the mechanisms involved in pituitary ontogenesis has largely increased. Since the first description of POU1F1 human mutations responsible for a well-defined phenotype without extra-pituitary malformation, several other genetic defects of transcription factors have been reported with variable degrees of phenotype-genotype correlations. However, to date, despite the identification of an increased number of genetic causes of isolated or multiple pituitary deficiencies, the etiology of most (80-90 %) congenital cases of hypopituitarism remains unsolved. Identifying new etiologies is of importance as a post-natal diagnosis to better diagnose and treat the patients (delayed pituitary deficiencies, differential diagnosis of a pituitary mass on MRI, etc.), and as a prenatal diagnosis to decrease the risk of early death (undiagnosed corticotroph deficiency for instance). The aim of this review is to summarize the main etiologies and phenotypes of combined pituitary hormone deficiencies, associated or not with extra-pituitary anomalies, and to suggest how the identification of such etiologies could be improved in the near future.

Identification of novel SNPs in the Sarda breed goats POU1F1 gene and their association with milk productive performance

The aim of the study was to detect polymorphism in the POU1F1 gene in Sarda breed goat, as well as to establish if SNPs could be associated with milk productive traits. The research was conducted on 129 Sarda breed goats from 4 to 5 years old, multiparous, lactating and in their third to fifth lactation. We report nine exonic and seven non-coding regions SNPs within the Sarda goat POU1F1 gene, namely, Ex 1 61 G>C; Ex 1 108 G>A; Ex 3 C>T; Ex 3 92 C>T; Ex 4 110 A>G; Ex 5 34 G>A resulting in Arg213Lys change; IVS4 641 G>A, IVS4 643 A>C, IVS4 659 G>A, IVS4 677 A>C, IVS4 G699Del, IVS4 709 C>G, Ex 6 17 G>A resulting in Arg228Ser change, Ex 6 58 G>T, Ex 6 172 T>C, 3'UTR 110 T>C. A statistically significant association was found between genotype TT, in position 17 of the exon 6 (3.1 % of frequency), and increased milk yield (P < 0.01) while genotype GT (25.6 % of frequency) was associated with a higher fat content. Genotype TT in position 58 of the exon 6 (3.9 % of frequency) was found to be associated with a higher fat (P < 0.01) and protein content (P < 0.05). Twenty-eight haplotypes were detected, but no significant association between the haplotypes and the milk production traits have been found. Our data, as well as providing new SNPs extending the POU1F1 gene characterization, evidence a relationship between polymorphism and milk production traits in Sarda goat breed.

Insulin-like growth factor 1 mediates negative feedback to somatotroph GH expression via POU1F1/CREB binding protein interactions

Circulating insulin-like growth factor 1 (IGF-1) has been shown to act as a negative feedback regulator of growth hormone (GH) gene expression; however, the mechanism of this negative feedback is poorly understood. Activation and regulation of GH gene expression require the binding of the transcription factor POU1F1 to the GH promoter along with cyclic AMP (cAMP) response element binding protein (CREB) binding protein (CBP). We investigate the role of CBP as a target of IGF-1 somatotroph regulation using the MtT/S somatotroph cell line. IGF-1 significantly inhibits basal GH mRNA levels but not POU1F1 levels. Chromatin immunoprecipitation assays demonstrate inhibition of CBP binding to the GH promoter after IGF-1 treatment. We hypothesized that IGF-1 receptor (IGF-1R) signaling disrupts the POU1F1/CBP complex to inhibit gene expression. In support, the use of a mutant CBP (S436A) construct, which lacks a critical phosphorylation site, leads to the loss of IGF-1 inhibition. The studies of CBP (S436A) knock-in mice show elevated serum GH levels, a greater response to GH releasing hormone (GHRH) stimulation along with lower weight gain, and decreased body fat. Our data confirm the inhibitory effects of IGF-1 on GH expression at the level of the promoter and provide evidence of CBP's role as a target of IGF-1R signaling.

Differential abilities of chicken Pit1 isoforms to regulate the GH promoter: evidence for synergistic activation

Pit1, pituitary-specific transcription factor 1, regulates differentiation of cells of the Pit1 lineage in the anterior pituitary and the synthesis of peptide hormones by these cell types, including GH. Pit1 is characterized by an N-terminal transactivation domain and a C-terminal POU domain. Alternative forms of Pit1, differing from each other in the N-terminal domain, have been reported in several species, but the functional implication of having multiple isoforms is not known. Several PIT1 mRNA transcripts exist in chickens that have not been characterized. This study was conducted to determine which, if any, of the chicken Pit1 isoforms regulate the chicken GH (cGH) promoter. During the course of this work, Pit1β2, a novel isoform of chicken Pit1, was discovered. Effects of known and novel isoforms (Pit1α, Pit1β1, Pit1β2, and Pit1γ) on cGH promoter activity were characterized in chicken Leghorn male hepatoma cells. Three of the isoforms, Pit1α, Pit1β1, and Pit1β2, activated the cGH promoter, whereas Pit1γ did not. Results from gel-shift assays indicated that Pit1γ does not bind to the proximal Pit1-bindng site of the cGH promoter, suggesting a possible mechanism underlying its inactivity. We found a functional advantage for having multiple isoforms expressed. When Pit1β1 was coexpressed with Pit1α or Pit1β2, significantly greater activation of the cGH promoter occurred than with any one isoform alone, with synergistic activation occurring when Pit1α and Pit1β1 were coexpressed. Whether this increased activation required, or was facilitated by, heterodimerization of two isoforms is not known. Identification of isoforms with specific functions will facilitate identification of their respective interacting partners that are essential for GH gene expression.

Identification of a novel mutation in the exon 2 splice donor site of the POU1F1/PIT-1 gene in Japanese identical twins with mild combined pituitary hormone deficiency

CONTEXT

To date, approximately 35 different POU1F1 mutations have been described in patients with familial and sporadic combined pituitary hormone deficiency (CPHD) from different ethnic backgrounds. The majority are missense mutations clustered within the conserved POU-specific and POU-homeo domains, encoded by exons 4 and 6, respectively.

OBJECTIVES

This study aimed to identify the molecular basis and clinical characteristics of a Japanese CPHD family with a novel POU1F1 mutation.

DESIGN

The POU1F1 gene was sequenced in identical twin brothers with mild CPHD. The mutation identified was also evaluated in family members as well as 188 Japanese controls and then examined in functional studies.

RESULTS

A novel heterozygous splice site mutation (Ex2 + 1G>T; c.214 + 1G>T) was detected. This mutation was also present in their undiagnosed mother, but not in any of the controls. In vitro splicing studies suggested this mutation to result in an in-frame skipping of exon 2, thus producing an internally deleted protein lacking most of the R2 transactivation subdomain (TAD-R2). Heterologous expression studies of the mutated POU1F1 protein showed only modest reductions in its transactivation activities in HEK293T cells, while acting as a dominant-negative inhibitor of the endogenous activities of POU1F1 in pituitary GH3 cells.

CONCLUSIONS

This is the first report of a mutation at the exon 2 donor splice site of POU1F1, affecting TAD-R2. The addition of this mutation to the growing list of pathological POU1F1 mutations may provide deeper insights into clinical heterogeneity in the expressions of individual mutations and a better understanding of the structure-function relationships of POU1F1.

Identification of mammalian Pit-1w, possibly involved in spermatogenesis in mice

Pit-1 is a pituitary-specific transcription factor responsible for pituitary development and hormone expression in mammals. Alternative splicing of Pit-1 gene transcripts has been shown to give rise to several variants with discrete transactivation properties; however, those arising from alternative promoters such as avian Pit-1 w have not yet been identified in mammals. Here, comparative genomics analysis followed by reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of 5' cDNA ends (5'RACE) were used in identifying Pit-1 w mRNA in the mouse pituitary. The mouse Pit-1 w mRNA is generated by using an alternative promoter located in the first intron, as with chicken Pit-1 w, and is expressed in a wide variety of tissues besides the pituitary. In the testis, Pit-1 w is expressed as the predominant variant and a protein of 33 kDa. During the first wave of spermatogenesis, expression of Pit-1 w mRNA at substantial levels was observed from 3 weeks, but not at 1 or 2 weeks after birth. A combination of immunohistochemistry and in situ hybridization detected Pit-1 mRNA and Pit-1 immunoreactivity in the spermatogonia, spermatocytes, and spermatids in the testis of adult mice. Because secondary spermatocytes and haploid spermatids increase in number between 18 and 20 days after birth in mice, it is possible that mouse Pit-1 w plays a role in spermatogenesis. This is the first report demonstrating the expression of Pit-1 variants arising from alternative promoters in mammals.

Role of chicken Pit-1 isoforms in activating growth hormone gene

In the present study, we expressed chicken (ch) Pit-1α (chPit-1α) and chPit-1γin vitro to compare the roles of chPit-1s in the transcription of the chicken growth hormone (chGH) gene. Both green fluorescence protein (GFP)-fused chPit-1γ and GFP-fused chPit-1α were localized in the nuclei of COS-7 cells. In a luciferase reporter gene assay, both chPit-1α and chPit-1γ transactivated the chGH promoter, and chPit-1α showed a more potent effect than chPit-1γ. On the other hand, an increase of cellular cAMP induced by forskolin promoted transactivation of the chGH gene with chPit-1α and chPit-1γ to similar extents. These results suggest that chPit-1γ may modulate the basal promoter activity of the chGH gene to the same degree as chPit-1α; however, a structural difference observed at the N-terminus transactivation domains in chPit-1α and chPit-1γ could be associated with the efficiency of basal activation of the chGH promoter.

Sipl1 and Rbck1 are novel Eya1-binding proteins with a role in craniofacial development

The eyes absent 1 protein (Eya1) plays an essential role in the development of various organs in both invertebrates and vertebrates. Mutations in the human EYA1 gene are linked to BOR (branchio-oto-renal) syndrome, characterized by kidney defects, hearing loss, and branchial arch anomalies. For a better understanding of Eya1's function, we have set out to identify new Eya1-interacting proteins. Here we report the identification of the related proteins Sipl1 (Shank-interacting protein-like 1) and Rbck1 (RBCC protein interacting with PKC1) as novel interaction partners of Eya1. We confirmed the interactions by glutathione S-transferase (GST) pulldown analysis and coimmunoprecipitation. A first mechanistic insight is provided by the demonstration that Sipl1 and Rbck1 enhance the function of Eya proteins to act as coactivators for the Six transcription factors. Using reverse transcriptase PCR (RT-PCR) and in situ hybridization, we show that Sipl1 and Rbck1 are coexpressed with Eya1 in several organs during embryogenesis of both the mouse and zebrafish. By morpholino-mediated knockdown, we demonstrate that the Sipl1 and Rbck1 orthologs are involved in different aspects of zebrafish development. In particular, knockdown of one Sipl1 ortholog as well as one Rbck1 ortholog led to a BOR syndrome-like phenotype, with characteristic defects in ear and branchial arch formation.

The 26-amino acid beta-motif of the Pit-1beta transcription factor is a dominant and independent repressor domain

The POU-homeodomain transcription factor Pit-1 governs the pituitary cell-specific expression of Pit-1, GH, prolactin (PRL), and TSHbeta genes. Alternative splicing generates Pit-1beta, which contains a 26-amino acid beta-domain inserted at amino acid 48, in the middle of the Pit-1 transcription activation domain (TAD). Pit-1beta represses GH, PRL, and TSHbeta promoters in a pituitary-specific manner, because Pit-1beta activates these same promoters in HeLa nonpituitary cells. Here we comprehensively analyze the role of beta-domain sequence, position, and context, to elucidate the mechanism of beta-dependent repression. Repositioning the beta-motif to the Pit-1 amino terminus, hinge, linker, and carboxyl terminus did not affect its ability to repress basal rat (r) PRL promoter activity in GH4 pituitary cells, but all lost the ability to repress Ras-induced rPRL promoter activity. To determine whether beta-domain repression is independent of Pit-1 protein and DNA binding sites, we generated Gal4-Pit-1TAD, Gal4-Pit-1betaTAD, and Gal4-beta-domain fusions and demonstrated that the beta-motif is sufficient to actively repress VP16-mediated transcription of a heterologous promoter. Moreover, beta-domain point mutants had the same effect whether fused to Gal4 or within the context of intact Pit-1beta. Surprisingly, Gal4-beta repression lost histone deacetylase sensitivity and pituitary specificity. Taken together, these results reveal that the beta-motif is a context-independent, modular, transferable, and dominant repressor domain, yet the beta-domain repressor activity within Pit-1beta contains cell type, promoter, and Pit-1 protein context dependence.

Identification of functional domains in TdIF1 and its inhibitory mechanism for TdT activity

TdT interacting factor 1 (TdIF1) was identified as a protein that binds to terminal deoxynucleotidyltransferase (TdT) to negatively regulate TdT activity. TdT is a template-independent DNA polymerase that catalyzes the incorporation of deoxynucleotides to the 3'-hydroxyl end of DNA templates to increase the junctional diversity of immunoglobulin or T-cell receptor (TcR) genes. Here, using bioinformatics analysis, we identified the TdT binding, DNA binding and dimerization regions, and nuclear localization signal (NLS) in TdIF1. TdIF1 bound to double-stranded DNA (dsDNA) through three DNA binding regions: residues 1-75, the AT-hook-like motif (ALM) and the predicted helix-turn-helix (HTH) motif. ALM in TdIF1 preferentially bound to AT-rich DNA regions. NLS was of the bipartite type and overlapped ALM. TdIF1 bound to the Pol beta-like region in TdT and blocked TdT access to DNA ends. In the presence of dsDNA, however, TdIF1 bound to dsDNA to release TdT from the TdIF1/TdT complex and to exhibit TdT activity, implying that active TdT released microenvironmentally concentrates around AT-rich DNA to synthesize DNA.

Differential Utilization of Transcription Activation Subdomains by Distinct Coactivators Regulates Pit-1 Basal and Ras Responsiveness

The POU-homeodomain transcription factor Pit-1 governs ontogeny and cell-specific gene expression of pituitary lactotropes, somatotropes, and thyrotropes. The splice isoform, Pit-1β, inserts a 26-amino acid (AA) repressor at AA48 in the Pit-1 transcription activation domain (TAD). The Pit-1 TAD contains a basal regulatory subregion, R1 (AA1–45), and a basal and Ras-responsive region, R2 (AA46–80). To precisely map these activities, we generated GAL4-Pit-1/Pit-1βTAD fusions and, in full-length HA-Pit-1, a series of substitution mutants of R2. Analysis in GH4 cells identified an activation domain at AA50–70, followed by an overlapping, dual-function, Ras-responsive-inhibitory domain, located from AA60–80. In contrast, GAL4-Pit-1βTAD repressed both basal and Ras-mediated TAD activity. To determine the functional interplay between TAD subregions and the β-domain, we inserted the β-domain every 10 AA across the 80-AA Pit-1 TAD. Like wild-type Pit-1β, each construct retained transcriptional activity in HeLa cells and repressed the Ras response in GH4 cells. However, β-domain insertion at AA61 and 71 resulted in greater repression of Ras responsiveness, defining a critical R2 TAD spanning AA61–71 of Pit-1. Furthermore, Ras activation is augmented by steroid receptor coactivator 1, whereas cAMP response element binding protein-binding protein is not a Ras mediator in this system. In summary, the Pit-1/Pit-1β TADs are composed of multiple, modular, and transferable subdomains, including a regulatory R1 domain, a basal activation region, a selective inhibitory-Ras-responsive segment, and a β-specific repressor domain. These data provide novel insights into the mechanisms by which the Pit-1 TAD integrates DNA binding, protein partner interactions, and distinct signaling pathways to fine-tune Pit-1 activity.

Identification and characterization of four splicing variants of ovine POU1F1 gene

Expression of POU1F1 gene, a member of the POU homeodomain family of transcription factors, is necessary for normal differentiation, development and survival of three anterior pituitary cell types (thyrotrophs, somatotrophs and lactotrophs) and for the proper expression of growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH) genes and POU1F1 gene itself. Alternative splicing forms of this gene have been reported in different species, with few functional studies. Apart from the POU1F1-Wild-type with the expected length, in this work we isolated three additional splicing variants: POU1F1-beta, with a 78 bp insert in the trans-activation domain; POU1F1-gamma that lacks exon 3 and POU1F1-delta that lacks exons 3, 4 and 5. Four different protein isoforms were also detected by Western blot in the sheep pituitary tissue. Functional assays were performed to study the trans-activation of GH and PRL promoters by the splicing variants. Regarding the PRL promoter, the beta variant presented only 12% of the Wild-type trans-activation capacity. Variants gamma and delta showed no capacity to trans-activate PRL promoter. Both gamma and delta variants acted as repressors of Wt, reducing significantly the trans-activation made by Wt alone (p<0.05). Concerning the GH promoter, the beta variant presented a trans-activation capacity 10% higher than Wt. Wt and beta variants strongly interact in the activation of GH promoter doubling the trans-activation potential of Wt. Variants gamma and delta showed no capacity to trans-activate the GH promoter and both acted as repressors, reducing significantly (p<0.001) the trans-activation performed by Wt. This work presents, for the first time, the characterization of four splicing forms of Ovis aries POU1F1 gene.

The Transcription Factor PLA-1/SKN-1A is Expressed in Human Placenta and Regulates the Placental Lactogen-3 Gene Expression

Here we report the selective expression of two POU transcription factor genes, PLA-1 and OCT-1, in human placenta and choriocarcinoma cell lines JAR, JEG-3 and BeWo. Pla-1 protein binds to a POU-consensus DNA sequence in the human placental lactogen-3 (PL-3) promoter and it is capable of trans-activating its transcription up to 18-fold. Other tissue-specific or ubiquitous POU transcription factors such as Pit-1/GHF-1 or Oct-1 showed none or low levels of trans-activation of the PL-3 promoter. In addition, we identified an unique and highly charged region in the N-terminal portion of Pla-1 protein required for full trans-activation of the PL-3 promoter.

Ovis aries POU1F1 Gene: Cloning, Characterization and Polymorphism Analysis

POU1F1 (PIT-1/GHF-1) is a transcription factor with critical role in the transcriptional regulation of multiple genes in the pituitary and also important for the survival, differentiation and proliferation of three pituitary cell types. To understand the regulation of POU1F1 gene in Ovis aries we report its cloning, sequencing and characterization. The sequenced 5787 bp included six exons and two complete introns. Ovine POU1F1 gene has a high level of conservation with its bovine, human and rat counterparts showing 98.2%, 91.2% and 86.2% of similarity at the coding level, respectively. All six exons were analyzed for polymorphism detection in 100 animals of the Portuguese indigenous ovine breed 'Churra da Terra Quente'. One polymorphism was found at codon 58 in exon 2, in one allele of 4 animals leading to a change from cysteine to tyrosine (2% allelic frequency). In exon 3 two polymorphisms were detected: a G to A transition altering a glycine to an asparagine at codon 89 in one allele of one animal (0.5% allelic frequency) and another G to A transition at codon 105 converting an alanine into a threonine in one allele of 3 animals (1.5% allelic frequency). These polymorphisms might change the structure of the POU1F1 protein and modify gene-expression. In intron 4, an A to G transition was detected in one allele of six animals (3% allelic frequency). Exons 1, 4 and 6 showed no polymorphisms.

Novel mutations within the POU1F1 gene associated with variable combined pituitary hormone deficiency

CONTEXT

Mutations within the gene encoding the pituitary-specific transcription factor POU1F1 are associated with combined pituitary hormone deficiency (CPHD). Most of the affected individuals manifest GH, prolactin, and TSH deficiency.

OBJECTIVE

We have now screened 129 individuals with CPHD and isolated GH deficiency for mutations within POU1F1.

RESULTS

Causative mutations were identified in 10 of 129 individuals (7.8%). Of these, five patients harbored the dominant negative R271W mutation, which is a well-recognized mutational hot spot. We have also identified a second frequently occurring mutation, E230K, which appears to be common in Maltese patients. Additionally, we describe two novel mutations within POU1F1, an insertion of a single base pair (ins778A) and a missense mutation (R172Q). Functional studies have revealed that POU1F1 (E230K) is associated with a reduction in transactivation, although DNA-binding affinity is similar to the wild-type protein. On the other hand, POU1F1 (R172Q) is associated with a reduction in DNA binding and transactivation, whereas POU1F1 (ins778A) is associated with loss of DNA binding and a reduction in transactivation.

CONCLUSIONS

Our data suggest that the phenotype associated with POU1F1 mutations may be more variable, with the occasional preservation of TSH secretion. Additionally, our data revealed POU1F1 mutations in three patients who were diagnosed as having ACTH deficiency but who, on further evaluation, were found to have normal cortisol secretion. Hence, elucidation of the genotype led to further evaluation of the phenotype, with the cessation of cortisol replacement that had been commenced unnecessarily. These data reflect the importance of mutational analysis in patients with CPHD.

In SilicoIdentification of Regulatory Elements of GRIN1 Genes

The ionotropic receptor of glutamate activated by N-methyl-D-aspartate (iGluR-NMDA) is a multiheteromeric complex constituted by at least three different types of subunits, encoded by seven different genes. The subunits of iGluR-NMDA have a complex system of regulation of their gene expression. Their expression is specific for each type of neural cell, as well as for the age of the organism. Moreover, there are reports that iGluR-NMDA expression is species-specific. Even though this macromolecular complex is very important in physiology and pathology of the central nervous system, knowledge to date about the regulatory elements controlling expression is scarce. We present the results of an in silico prediction of potential regulatory elements, some of which coincide with the few known experimentally. We also present the important differences regarding the presence and the localization of the regulatory elements among human, rat, and mouse species.

Structural and Functional Analysis of the Differential Effects of c-Jun and v-Jun on Prolactin Gene Expression

The protooncogene c-Jun and its oncogenic isoform v-Jun are members of the activator protein 1 family of transcription factors that have been shown to have differential transcriptional effects that are both promoter specific and cell type specific. Previously, we have demonstrated that whereas c-Jun inhibits pituitary-specific rat prolactin (rPRL) promoter activity, expression of v-Jun stimulates the rPRL promoter in GH4 pituitary cells. In this report, we have conducted an extensive structure-function analysis of c-Jun vs. v-Jun to determine which regions of these proteins are responsible for their differential transcriptional effects in this pituitary model system. We show that isoform-specific responses are mediated by complex interactions between the delta-domain, serine 243, and the amino-terminal transcriptional activation domains. Thus, in contrast to previous reports, no single domain is responsible for the differential transcriptional activities of c-Jun and v-Jun. Mutation of c-Jun serine 243 to phenylalanine and replacement of the c-Jun amino terminus with the corresponding region from v-Jun, thereby removing the delta-domain, are necessary and sufficient to confer a functional switch from the c-Jun-inhibitory to the v-Jun-activating phenotype. Thus, we propose that isoform-specific subdomains in c-Jun and v-Jun dictate discrete interactions with distinct protein partners, which underlie the differential Jun-dependent transcriptional responses of the rPRL promoter.

Characterization of the 5'-flanking transcriptional regulatory region of chicken growth hormone gene

A 1727-bp fragment of 5'-flanking region of chicken growth hormone (cGH) gene has been cloned and sequenced. Various lengths of the 5'-flanking region (122 to 1775 bp) was linked to a luciferase reporter gene, and its transcriptional regulation was examined by an in vitro transient transfection coupled with luciferase assay. Our results demonstrated that pituitary-specific transcription factor, Pit-1, is necessary and sufficient to confer a strong tissue-specific expression. Co-transfection with goldfish or chicken Pit-1 expression vectors significantly restored the luciferase expression in HeLa cells. Site-directed mutagenesis and mobility gel-shift assays further confirmed the position of the Pit-1 binding site at -113/-104. Moreover, a repressive thyroid hormone response element (TRE) was identified at -137/-74, and we propose that interactions between the TRE and Pit-1 sites may be required for its repressive effect.

Ras signaling and transcriptional synergy at a flexible Ets-1/Pit-1 composite DNA element is defined by the assembly of selective activation domains

Pit-1 and Ets-1 binding to a composite element synergistically activates and targets Ras-mitogen-activated protein kinase signaling to the rat prolactin promoter. These transcriptional responses appear to depend on three molecular features: organization of the Ets-1/Pit-1 composite element, physical interaction of these two factors via the Pit-1 homeodomain (amino acids 199-291) and the Ets-1 regulatory III domain (amino acids 190-257), and assembly of their transcriptional activation domains (TADs). Here we show that the organization of the Ets-1/Pit-1 composite element tolerates significant flexibility with regard to Ras stimulation and synergy. Specifically, the putative monomeric Pit-1 binding site can be substituted with bona fide binding sites for either a Pit-1 monomer or dimer, and these sites tolerated a separation of 28 bp. Additionally, we show that the physical interaction of Ets-1 and Pit-1 is not required for Ras responsiveness or synergy because block mutations of the Pit-1 interaction surface in Ets-1, which reduced Ets-1/Pit-1 binding in vitro, did not significantly affect Ets-1 stimulation of Ras responsiveness or synergy. We also show differential use of distinct TAD subtypes and Pit-1 TAD subregions to mediate either synergy or Ras responsiveness. Specifically, TADs from Gal4, VP16, or Ets-2 regulatory III domain linked to Ets-1 DNA binding domain constructs restored synergy to these TAD/Ets-1 DNA binding domain fusions. Conversely, deletion of the defined Pit-1 TAD (amino acids 2-80) retained synergy, but not Ras responsiveness. Consequently, we further defined the Pit-1 amino-terminal TAD into region 1 (R1, amino acids 2-45) and region 2 (R2, amino acids 46-80). R1 appears to regulate basal and synergistic responses, whereas the Ras response was mapped to R2. In summary, Ras responsiveness and Pit-1/Ets-1 synergy are mediated through the assembly of distinct TADs at a flexible composite element, indicating that different mechanisms underlie these two transcriptional responses and that the Pit-1 R2 subregion represents a novel, tissue-specific Ras-responsive TAD.

The alpha-helical D1 domain of the tobacco bZIP transcription factor BZI-1 interacts with the ankyrin-repeat protein ANK1 and is important for BZI-1 function, both in auxin signaling and pathogen response

The tobacco (Nicotiana tabacum) bZIP transcription factor BZI-1 is involved in auxin-mediated growth responses and in establishing pathogen defenses. Transgenic plants expressing a dominant-negative BZI-1-DeltaN derivative, which lacks the N-terminal activation domain, showed altered vegetative growth. In particular auxin-induced rooting and formation of tobacco mosaic virus-induced hypersensitive response lesions are affected. BZI-1-related proteins described in various plant species share the conserved domains D1, D2, BD, and D4. To define those BZI-1 domains involved in transcription factor function, BZI-1 deletion derivatives were expressed in transgenic plants. The domains D1 or BD are crucial for BZI-1-DeltaN function in planta. The basic BD domain is mediating DNA binding of BZI-1. Yeast two-hybrid and in vitro binding studies reveal the ankyrin-repeat protein ANK1, which specifically interacts with a part of the BZI-1 protein (amino acids 73-222) encoding the D1 domain. ANK1 does not bind DNA or act as a co-activator of BZI-1-mediated transcription. Moreover, green fluorescence protein localization studies propose that ANK1 is acting mainly inside the cytosol. Transcription analysis reveals that ANK1 is ubiquitously expressed, but after pathogen attack transcription is transiently down-regulated. Along these lines, ANK1 homologous proteins in Arabidopsis thaliana have been reported to function in pathogen defense. We therefore propose that the D1 domain serves as an interaction surface for ANK1, which appears to regulate BZI-1 function in auxin signaling as well as pathogen response.

A novel nonsense mutation in the Pit-1 gene: evidence for a gene dosage effect

The POU transcription factor Pit-1 functions in the development of somatotrophs, lactotrophs, and thyrotrophs of the anterior pituitary gland. It also plays a role in cell-specific gene expression and regulation of the gene products from these cell types, GH, prolactin, and TSH, respectively. In the present report we studied a patient with severe growth failure. Provocative studies revealed undetectable GH, prolactin, and TSH levels, and her pituitary gland was hypoplastic on magnetic resonance imaging. She had a novel homozygous nonsense mutation in the 3' end of the first alpha-helix of the POU-specific domain of the Pit-1 gene. This mutation results in a truncated protein with loss of most of the Pit-1 DNA-binding domains. Interestingly, her parents, who each have one mutant allele, have evidence of mild endocrine dysfunction. Thus, two normal copies of the Pit-1 gene appear necessary for full Pit-1 gene function.

A PIT-1 homeodomain mutant blocks the intranuclear recruitment of the CCAAT/enhancer binding protein alpha required for prolactin gene transcription

The pituitary-specific homeodomain protein Pit-1 cooperates with other transcription factors, including CCAAT/enhancer binding protein alpha (C/EBPalpha), in the regulation of pituitary lactotrope gene transcription. Here, we correlate cooperative activation of prolactin (PRL) gene transcription by Pit-1 and C/EBPalpha with changes in the subnuclear localization of these factors in living pituitary cells. Transiently expressed C/EBPalpha induced PRL gene transcription in pituitary GHFT1-5 cells, whereas the coexpression of Pit-1 and C/EBPalpha in HeLa cells demonstrated their cooperativity at the PRL promoter. Individually expressed Pit-1 or C/EBPalpha, fused to color variants of fluorescent proteins, occupied different subnuclear compartments in living pituitary cells. When coexpressed, Pit-1 recruited C/EBPalpha from regions of transcriptionally quiescent centromeric heterochromatin to the nuclear regions occupied by Pit-1. The homeodomain region of Pit-1 was necessary for the recruitment of C/EBPalpha. A point mutation in the Pit-1 homeodomain associated with the syndrome of combined pituitary hormone deficiency in humans also failed to recruit C/EBPalpha. This Pit-1 mutant functioned as a dominant inhibitor of PRL gene transcription and, instead of recruiting C/EBPalpha, was itself recruited by C/EBPalpha to centromeric heterochromatin. Together our results suggest that the intranuclear positioning of these factors determines whether they activate or silence PRL promoter activity.

Domains of Pit-1 required for transcriptional synergy with GATA-2 on the TSH beta gene

Previous studies showed that Pit-1 functionally cooperates with GATA-2 to stimulate transcription of the TSH beta gene. Pit-1 and GATA-2 are uniquely coexpressed in pituitary thyrotropes and activate transcription by binding to a composite promoter element. To define the domains of Pit-1 important for functional cooperativity with GATA-2, we cotransfected a set of Pit-1 deletions with an mTSH beta-luciferase reporter. Plasmids were titrated to express equivalent amounts of protein. A mutant containing a deletion of the hinge region between the POU and homeodomains retained the ability to fully synergize with GATA-2. In contrast, mutants containing deletions of amino acids 2-80 or 72-125 demonstrated 56 or 34% of the synergy found with the full-length protein, suggesting that these regions contributed to cooperativity. Mutants with deletions of the POU-specific or homeodomain further reduced the effect signifying the requirement for DNA binding. GST interaction studies demonstrated that only the homeodomain of Pit-1 interacted with GATA-2. Finally, several mutations between the Pit-1 and GATA-2 sites on the TSH beta promoter reduced binding for each factor and greatly reduced ternary complex formation. Thus multiple domains of Pit-1 are required for full synergy with GATA-2 and sequences between the two binding sites contribute to co-occupancy with both factors on the proximal TSH beta promoter.

Molecular basis of combined pituitary hormone deficiencies

Pituitary gland commitment from oral ectoderm occurs in response to inductive signals from the neuroepithelium of the ventral diencephalon. Invagination of the oral ectoderm leads to the creation of Rathke's pouch. Intensified cell proliferation within Rathke's pouch results in formation of the anterior pituitary lobe. Subsequently, highly differentiated cell types arise sequentially due to overlapping, but distinct, spatial and temporal patterns of signaling molecules and transcription factors. Mutations in some of the pituitary-specific transcription factors have been identified in patients with hypopituitarism, confirming the role of these factors in pituitary development.

Characterization of transactivation domain and developmental expression of pituitary specific transcription factor, Pit-1 of ayu (Plecoglossus altivelis)

Pit-1 is a pituitary-specific transcription factor, which regulates the expression of growth hormone, prolactin, and thyroid stimulating hormone-beta genes. We previously reported the expression of a Pit-1 gene from ayu (Plecoglossus altivelis), which is an important cultivated food fish in Taiwan and Japan. Comparison of ayu Pit-1 with that of salmon, turkey, and rodent, revealed that the Pit-1 structure is highly conserved through vertebrates, especially in POU-specific and POU-homeo domains. The variation among fish, bird, and mammal are mainly found in transactivation domain by alternative splicing and initiation. Three insertions were found. The gamma-insert in fish Pit-1 is homologous to the exon 2a of avian Pit-1, which is not found in mammals. The beta-insert of fish Pit-1 is homologous to the 28 amino acids (a.a.) and 26 a.a. insert of avian Pit-1 beta(*) and mammalian Pit-1 beta, respectively. An additional similarity was noticed between fish and bird, as both of them contain 7 a.a. insert that is not present in mammalian Pit-1. By site directed mutagenesis, we demonstrated that the beta, gamma, and the 7 a.a. inserts of ayu Pit-1 are critical for activation of zebrafish growth hormone promoter. The ayu Pit-1 protein was found to be expressed specifically in pituitary gland, and its mRNA was first detected at embryonic day 4, significantly increased at embryonic day 5, then sustained to time of hatching at day 8.

Target integration by a chimeric Sp1 zinc finger domain-Moloney murine leukemia virus integrase in vivo

A specificity protein 1 (Sp1) zinc finger domain containing two tandem zinc fingers was fused to the C terminus of the integrase (IN) protein of the Moloney murine leukemia virus (MuLV). The integrity of the MuLV IN was completely preserved, since the fusion was conducted at the last amino acid residue of the protein. The vector pMIN-Sp1, which carried the fused MuLV IN-Sp1 zinc finger domain gene, was cotransfected with a wild-type MuLV vector pMLV-K to NIH/3T3 cells. A nonradioactive reverse transcriptase assay was performed on culture supernatants collected from the cotransfected cells to confirm the production of recombinant viruses. The expression of the fusion protein and the integration of the MuLV genome by the fusion protein were confirmed by a Northern and then a Southern hybridization analysis on the total RNA or genomic DNA extracted from cells infected by viruses collected from the supernatants of the cotransfected cells. Regions of the host chromosome that were selected by the fusion protein as the integration targets were sequenced using the TOPO(TM) cloning method on a series of PCR products generated with a nested set of primers. The percentage of positive clones screened that contained the DNA-binding sequence of the fused Sp1 zinc finger domain was around 13% (5 out of 39 clones). It was found that the Sp1 DNA-binding sequence was only present in regions that were proximal to one of the long terminal repeats of the integrated viral genome, suggesting that the fusion protein could select a target sequence for integration. The host flanking sequences determined for all the positive clones were also used as queries to perform a BLAST search on the GenBank mouse EST entries. Although matching scores for sequences of some of the clones computed were more significant than others, it was difficult to judge whether or not the integration in these clones had been targeted to some gene sequences. Most of the integration sites might exist in the introns, since we found that the probability of the gene sequences containing an Sp1 DNA-binding site was low.

Expression of ayu (Plecoglossus altivelis) Pit-1 in Escherichia coli: its purification and immunohistochemical detection using monoclonal antibody

The pituitary-specific transcription factor Pit-1 belongs to the family of POU-domain proteins and is known to play an important role in the differentiation of pituitary cells. Here we report the complete nucleotide sequence of cDNA encoding Pit-1 from the brackish water fish, ayu (Plecoglossus altivelis). Nucleotide sequence analysis of 1910 bp of ayu Pit-1 cDNA revealed an open reading frame of 1074 bp that encodes a protein of 358 amino acids containing a POU-specific domain, POU homeodomain, and an STA (Ser/Thr-rich activation) transactivation domain. We inserted the coding region of Pit-1 cDNA, obtained by PCR, into a pET-20b(+) plasmid to produce recombinant Pit-1 in Escherichia coli BL21 (DE3) pLysS cells. Upon induction with isopropyl beta-D-thiogalactopyranoside, Pit-1 was expressed and accumulated as inclusion bodies in E. coli. The protein was then purified in one step by affinity chromatography on a nickel-nitrilotriacetic acid agarose column under denaturing conditions. This method yielded 0.7 mg of highly pure and stable protein per 200 ml of bacterial culture. A band of 40 kDa, resolved as recombinant ayu Pit-1 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, agrees well with the molecular mass calculated from the translated cDNA sequence. The purified recombinant Pit-1 was confirmed in vitro through Western blot analysis, using its monoclonal antibody. This monoclonal antibody detected Pit-1 in the nuclei of ayu developing pituitary by immunohistochemical reaction. It serves as a good reagent for the detection of ayu Pit-1 in situ.

A defined locus control region determinant links chromatin domain acetylation with long-range gene activation

Gene activation in higher eukaryotes is often under the control of regulatory elements quite distant from their target promoters. It is unclear how such long-range control is mediated. Here we show that a single determinant of the human growth hormone locus control region (hGH LCR) located 14.5 kb 5prime prime or minute to the hGH-N promoter has a critical, specific, and nonredundant role in facilitating promoter trans factor binding and activating hGH-N transcription. Significantly, this same determinant plays an essential role in establishing a 32 kb acetylated domain that encompasses the entire hGH LCR and the contiguous hGH-N promoter. These data support a model for long-range gene activation via LCR-mediated targeting and extensive spreading of core histone acetylation.

BZI-1 specifically heterodimerises with the tobacco bZIP transcription factors BZI-2, BZI-3/TBZF and BZI-4, and is functionally involved in flower development

The tobacco (Nicotiana tabacum) protein BZI-1 is closely related to the plant bZIP transcription factors CPRF2, G/HBF-1 and OHP1. Using the C-terminal part of BZI-1, which includes the bZIP domain, as a bait in a yeast two-hybrid screen, three BZI-1 interacting bZIP transcription factors, referred to as BZI-2, BZI-3/TBZF and BZI-4, were isolated. The observed interactions are due to the leucine zipper dimerisation domain and have been found to be specific, in so far as other bZIP transcription factors do not interact with BZI-1. The formation of heterodimers is favoured to homodimerisation. Furthermore, physical protein-protein interaction was confirmed by in vitro binding studies. Expression analysis reveals that BZI-2 mRNA is predominantly located in the stems and parts of the flower. BZI-3/TBZF expression has been observed predominantly in flowers and, to a lesser extent, in the vegetative parts of the plant. In particular, BZI-4 is transcribed specifically in the stamen, the petals and the pistils of the tobacco flower. Since BZI-1 is expressed ubiquitously in tobacco plants, co-localisation with BZI-2, BZI-3/TBZF or BZI-4 might influence BZI-1 heterodimerisation and consequently target gene selection. Analysis of transgenic plants displaying changes in BZI-1 protein level revealed that BZI-1 regulates BZI-4 expression. Moreover, a reduction in functional BZI-1 protein resulted in flowers having a reduced size; in particular, the stamen and the petals are affected. Consequently, BZI-1 homo- or heterodimers are involved in the control of the size of the organs of flowers. Based on these data, we discuss a model postulating BZI transcription factor heterodimerisation as a mechanism determining target gene selection and regulating processes involved in plant development.

The turkey transcription factor Pit-1/GHF-1 can activate the turkey prolactin and growth hormone gene promoters in vitro but is not detectable in lactotrophs in vivo

The transcription factor Pit-1/GHF-1 plays an important role in regulating the prolactin (Prl) and growth hormone (GH) genes in mammals. In this study, the role that Pit-1 plays in regulating the prolactin and growth hormone genes in avian species was examined by cotransfection assays and immunofluorescence staining of pituitary sections. In cotransfection assays, turkey Pit-1 activated the turkey Prl, turkey GH, and rat Prl promoters 3.8-, 3.7-, and 12.5-fold, respectively. This activation was comparable to rat Pit-1 activation of these same promoters. A point mutation in the turkey Pit-1 cDNA, which changed leu-219 to ser-219, resulted in a 2-, 2-, and 10-fold reduction in the activation of the turkey Prl, turkey GH, and rat Prl promoters, respectively. Unexpectedly, coexpression of tPit-1 (leu-219) and tPit-1(ser-219) activated turkey Prl and rat Prl promoters 9.4- and 35.9-fold, respectively, but had no effect on the turkey GH promoter. Dual-label immunofluorescence analysis of turkey pituitary sections revealed that Pit-1 was not detectable in prolactin-staining cells but was detectable in GH-staining cells. Taken together, these data indicate that in the domestic turkey, Pit-1 can activate the turkey Prl promoter in vitro, but does not appear to play a role in regulating Prl gene expression in vivo. Pit-1, however, still likely plays a role in regulating GH gene expression.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

Compartmentalization of regulatory proteins in the cell nucleus

The cell nucleus is increasingly recognized as a spatially organized structure. In this review, the nature and controversies associated with nuclear compartmentalization are discussed. The relationship between nuclear structure and organization of proteins involved in the regulation of RNA polymerase II-transcribed genes is then discussed. Finally, very recent data on the mobility of these proteins within the cell nucleus is considered and their implications for regulation through compartmentalization of proteins and genomic DNA are discussed.

Complementary DNA cloning and ontogenic expression of pituitary-specific transcription factor of chickens (Gallus domesticus) from the pituitary gland

Pit-1 is a pituitary-specific POU-domain DNA binding factor, which binds to and transactivates promoters of growth hormone (GH)-, prolactin (PRL)-, and thyroid-stimulating hormone-beta (TSHbeta)-encoding genes. Pit-1 has been identified in several mammalian species. In birds, it has been identified only in the turkey and chicken pituitary. Several (gg) Pit-1 cDNA sequences have been identified in the anterior pituitary of the chicken (Gallus domesticus). As in turkey, three different transcripts (ggPit-1*, ggPit-1beta*, and ggPit-1W*) of the Pit-1 gene were identified. This suggests that the chicken pituitary expresses more variants than reported earlier. Previous studies have concentrated on expressions during posthatch life of the turkey and chicken. The present study has determined the ontogeny of Pit-1 during the embryonic life of the chicken (day 1-day 21 of incubation). The mRNA for Pit-1 was first detected in the pituitary on day 5 of embryonic life. The expression of the mRNA was maintained until hatch. The presence of Pit-1 at this stage of embryonic development suggests that Pit-1 may be physiologically important during embryonic development of birds, as it precedes the gene expression and secretion of GH, PRL, and TSHbeta. A similar sequence for the expression of Pit-1 relative to GH and PRL during embryonic/fetal development has been reported for rodents.

Genetic regulation of the embryology of the pituitary gland and somatotrophs

Extrinsic and intrinsic signaling gradients determine expression patterns of pituitary-specific factors in the developing anterior pituitary gland. The temporal and spatial relations of these developmental factors are required for the determination of each of the pituitary cell lineages. Rpx is required for early differentiation of the anterior pituitary. The determination of the somatotroph cell line is dependent on the transcription factors Lhx3, Prop-1, and Pit-1. Pit-1 also plays a role in the activation and regulation of the somatotroph gene product, GH. Additional factors such as CREB and the GHRH receptor, may be involved in somatotroph determination, while Zn-15 and Pitx2 may be involved in GH gene activation.

Identification of SP3 as a negative regulatory transcription factor in the monocyte expression of growth hormone

A number of studies from different laboratories clearly show that cells of the immune system produce a GH molecule indistinguishable from that produced in the pituitary. A more recent finding from our studies suggests that monocytes use the same first exon and promoter sequence for the expression of lymphocyte GH as that reported for the expression of pituitary GH. In this report we have extended these results by determining that two members of the SP family of transcription factors, SP1 and SP3, bind to the region at -138/-133 bp containing a GGGAGG motif. Confirmation that this region of the monocyte GH promoter-bound SP1 and SP3 was accomplished using electrophoretic mobility shift assays with SP1 consensus and mutant probes as well as specific antibodies to SP1 and SP3. Selective mutation of the SP1/SP3 site increased basal transcription by 73%, indicating that this site is important in transcriptional inhibition. Overexpression of SP1 had no demonstrable effect on the GH promoter, whereas overexpression of SP3 caused inhibition of expression in P-388 monocyte cells. Cotransfection of P-388 cells with overexpression vectors for both SP1 and SP3 transcription factors also resulted in inhibition of basal expression. Transfection experiments in Drosophila SL-2 cells overexpressing SP1 and/or SP3 suggest that both factors repress the basal expression of GH promoter luciferase constructs and that the effect together was additive. Taken together, the results demonstrate that basal expression of monocyte GH may be negatively regulated by SP3.

The Pit-1 homeodomain and beta-domain interact with Ets-1 and modulate synergistic activation of the rat prolactin promoter

Pit-1/GHF-1 is a pituitary-specific, POU homeodomain transcription factor required for development of somatotroph, lactotroph, and thyrotroph cell lineages and regulation of the temporal and spatial expression of the growth hormone, prolactin (PRL), and thyrotropin-beta genes. Synergistic interaction of Pit-1 with a member of the Ets family of transcription factors, Ets-1, has been shown to be an important mechanism regulating basal and Ras-induced lactotroph-specific rat (r) PRL promoter activity. Pit-1beta/GHF-2, an alternatively spliced isoform containing a 26-amino acid insert (beta-domain) within its transcription-activation domain, physically interacts with Ets-1 but fails to synergize. By using a series of Pit-1 internal-deletion constructs in a transient transfection protocol to reconstitute rPRL promoter activity in HeLa cells, we have determined that the functional and physical interaction of Pit-1 and Ets-1 is mediated via the POU homeodomain, which is common to both Pit-1 and Pit-1beta. Although the Pit-1 homeodomain is both necessary and sufficient for direct binding to Ets-1 in a DNA-independent manner, an additional interaction surface was mapped to the beta-domain, specific to the Pit-1beta isoform. Thus, the unique transcriptional properties of Pit-1 and Pit-1beta on the rPRL promoter may be due to the formation of functionally distinct complexes of these two Pit-1 isoforms with Ets-1.

Pit-1 binding sites at the somatotrope-specific DNase I hypersensitive sites I, II of the human growth hormone locus control region are essential for in vivo hGH-N gene activation

The human growth hormone gene cluster is composed of five closely related genes. The 5'-most gene in the cluster, hGH-N, is expressed exclusively in somatotropes and lactosomatotropes of the anterior pituitary. Although the hGH-N promoter contains functional binding sites for multiple transcription factors, including Sp1, Zn-15, and Pit-1, predictable and developmentally appropriate expression of hGH-N transgenes in the mouse pituitary requires the presence of a previously characterized locus control region (LCR) composed of multiple chromatin DNase I hypersensitive sites (HS). LCR determinant(s) necessary for hGH-N transgene activation are largely conferred by two closely spaced HS (HS I,II) located 14.5 kilobase pairs upstream of the hGH-N gene. The region sufficient to mediate this activity has recently been sublocalized to a 404-base pair segment of HS I,II (F14 segment). In the present study, we identified multiple binding sites for the pituitary POU domain transcription factor Pit-1 within this segment. Using a transgenic founder assay, these sites were shown to be required for high level, position-independent, and somatotrope-specific expression of a linked hGH-N transgene. Because the Pit-1 sites in the hGH-N gene promoter are insufficient for such gene activation in vivo, these data suggested a unique chromatin-mediated developmental role for Pit-1 in the hGH LCR.

CREB-independent regulation by CBP is a novel mechanism of human growth hormone gene expression

Hypothalamic growth hormone-releasing hormone (GHRH) stimulates growth hormone (GH) gene expression in anterior pituitary somatotrophs by binding to the GHRH receptor, a G-protein-coupled transmembrane receptor, and by mediating a cAMP-mediated protein kinase A (PKA) signal-transduction pathway. Two nonclassical cAMP-response element motifs (CGTCA) are located at nucleotides -187/-183 (distal cAMP-response element; dCRE) and -99/-95 (proximal cAMP-response element; pCRE) of the human GH promoter and are required for cAMP responsiveness, along with the pituitary-specific transcription factor Pit-1 (official nomenclature, POU1F1). Although a role for cAMP-response element binding protein (CREB) in GH stimulation by PKA has been suggested, it is unclear how the effect may be mediated. CREB binding protein (CBP) is a nuclear cofactor named for its ability to bind CREB. However, CBP also binds other nuclear proteins. We determined that CBP interacts with Pit-1 and is a cofactor for Pit-1-dependent activation of the human GH promoter. This pathway appears to be independent of CREB, with CPB being the likely target of phosphorylation by PKA.

Critical residues of the homeodomain involved in contacting DNA bases also specify the nuclear accumulation of thyroid transcription factor-1

The N-terminal end of thyroid transcription factor-1 (TTF-1) homeodomain is composed of a stretch of five basic amino-acids that is conserved in both POU- and NK2-class homeodomains and constitutes a functional nuclear localization signal. By analyzing the cellular distribution of fusion proteins, composed of a jellyfish green fluorescent variant and different parts of TTF-1, we show here that the presence of this basic sequence is not sufficient by itself to confer complete nuclear accumulation. By mutagenesis, we identified a second region located in the center of the DNA recognition helix of the homeodomain that is also able to specify a predominantly nuclear localization of the chimeric proteins, independently of the presence of the basic NLS. The destruction, by mutagenesis, of both the basic stretch and the motif in the DNA recognition helix led to the total loss of nuclear accumulation, indicating that complete nuclear accumulation of TTF-1 results from the concerted action of these two proteic signals. Both of the regions of the homeodomain that are involved in nuclear targeting also encompass critical amino-acids responsible for DNA binding site recognition, as evidenced by the loss of DNA binding activity in vitro upon mutagenesis. Specifically, residues in the central part of the DNA recognition helix are involved in contacting bases in the major groove of DNA and are the most conserved in homeodomain proteins, suggesting that this part of the homeodomain could play a general role in the nuclear localization of members of this family of proteins.

Transcription Factors and Hypopituitarism

Several homeodomain factors are found in the developing anterior pituitary lobe. The production of these developmental transcription factors has distinct temporal and spatial patterns. By interacting with each other, as well as with other extrinsic and intrinsic signals, they control cell determination and specification. Here, we discuss transcription factors that have been shown to have an in vivo role in pituitary cell-type specification.

Synergistic activation of the prolactin promoter by vitamin D receptor and GHF-1: role of the coactivators, CREB-binding protein and steroid hormone receptor coactivator-1 (SRC-1)

PRL gene expression is dependent on the presence of the pituitary-specific transcription factor GHF-1/Pit-1, which is transcribed in a highly restricted manner in cells of the anterior pituitary. In pituitary GH3 cells, vitamin D increases the levels of PRL transcripts and stimulates the PRL promoter. We have analyzed the role of GHF-1 and of the vitamin D receptor (VDR) to confer vitamin D responsiveness to the PRL promoter. For this purpose we have used nonpituitary HeLa cells, which do not express GHF-1. We found that VDR activates the PRL promoter both in a ligand-dependent and -independent manner through a sequence located between positions -45/-27 in the proximal 5'-flanking region. This sequence also confers VDR and vitamin D responsiveness to a heterologous promoter. In the context of the PRL gene, VDR requires the presence of GHF-1 to activate the promoter. Truncation of the last 12 C-terminal amino acids of VDR, which contain the ligand-dependent activation function (AF2), abolishes regulation by vitamin D, suggesting that binding of coactivators to this region mediates ligand-dependent stimulation of the PRL promoter by the receptor. Indeed, expression of the coactivators, steroid hormone receptor coactivator-1 (SRC-1) and CREB-binding protein (CBP), significantly enhances the stimulatory effect of vitamin D mediated by the wild-type VDR but not by the AF2 mutant receptor. Furthermore, CBP also increases the activation of the PRL promoter by GHF-1 and the ligand-independent activation by both wild-type and mutant VDR.

The tissue-specific transcription factor Pit-1/GHF-1 binds to the c-fos serum response element and activates c-fos transcription

Pit-1, a POU domain-containing transcription factor, is involved in two functions in the pituitary: PRL and GH tissue-specific expression and somatolactotroph cells expansion. To analyze the molecular basis of the latter function, we tested whether Pit-1 can directly transactivate expression of an early marker of cell cycle initiation, the c-fos gene. We show that Pit-1 overexpression in PC12 cells, which do not express Pit-1, increases c-fos expression. Moreover, cAMP-induced c-fos promoter activity is decreased in the somatolactotroph cell line GH3 when Pit-1 expression is reduced by hybrid arrest with an antisense sequence complementary to Pit-1 cDNA. In contrast to hormonal genes regulation, where it has been shown that any Pit-1 phosphorylation site is involved, we show that the Pit-1 phosphorylation sites are required to allow increase of c-fos promoter activity by Pit-1. We further show, by gel shift analyses, that Pit-1 is able to specifically bind the serum response element sequence present within the c-fos promoter but with a lesser affinity than the Pit-1 response element. Taken together, these results demonstrate that the tissue-specific transcription factor Pit-1 is able to enhance expression of genes involved in cell cycle initiation, suggesting that this mechanism allows Pit-1 to increase somato-lactotroph cell proliferation.

Defective retinoic acid regulation of the Pit-1 gene enhancer: a novel mechanism of combined pituitary hormone deficiency

Pit-1 is a pituitary-specific transcription factor responsible for pituitary development and hormone expression in mammals. Pit-1 contains two protein domains, termed POU-specific and POU-homeo, which are both necessary for DNA binding and activation of the GH and PRL genes and regulation of the PRL, TSH-beta subunit (TSH-beta), and Pit-1 genes. Pit-1 is also necessary for retinoic acid induction of its own gene during development through a Pit-1-dependent enhancer. Combined pituitary hormone deficiency is caused by defective transactivation of target genes in the anterior pituitary. In the present report, we provide in vivo evidence that retinoic acid induction of the Pit-1 gene can be impaired by a Pit-1 gene mutation, suggesting a new molecular mechanism for combined pituitary hormone deficiency in man.