Two members of an HNF1 homeoprotein family are expressed in human liver

The role of transcription factors in maturity-onset diabetes of the young

The study of maturity-onset diabetes of the young (MODY), an autosomal dominant form of early-onset diabetes mellitus characterised by defective insulin secretion has been extremely successful in two ways. Firstly it has enabled definitive diagnosis for patients. This allows more accurate prediction of disease and treatment requirements. Secondly it has facilitated an increased understanding of the genes and pathways that are crucial for normal beta-cell function. Five of the six MODY genes, TCF1 (encoding HNF-1alpha), TCF2 (encoding HNF-1beta) HNF4A, insulin promoter factor (IPF)1, and NEUROD1, are transcription factors that operate in a complex network of gene regulation. Several genes have been shown to be regulated by the MODY transcription factors in a beta-cell specific manner. This includes the co-regulation of HNF-1alpha and HNF-4alpha by each other. The exact mechanism of how mutations in these transcription factors result in diabetes in humans remains unknown. However, current opinion favours pleiotropic adverse effects on many genes; extensive in vitro and in vivo studies of these genes has highlighted their importance in both glucose sensing-insulin secretion coupling and maintaining the fully differentiated beta-cell phenotype.

Transcriptional regulation of the ovine intestinal Na+/glucose cotransporter SGLT1 gene. Role of HNF-1 in glucose activation of promoter function

Dietary sugars D-glucose and D-galactose are transported across the intestinal brush-border membrane by the Na+/glucose cotransporter, SGLT1. In various species studied, it has been shown that the activity, and expression, of intestinal SGLT1 is regulated by dietary sugars. We report in this paper that regulation of the intestinal SGLT1 gene by lumenal sugar is due, in part, to an increase in transcription. Using deletion analyses of the -66/+21-bp fragment, we have identified the minimal region of the ovine SGLT1 promoter able to support transcription. Site-directed mutagenesis of the hepatic nuclear factor-1 (HNF-1) consensus motif within this domain eliminates basal promoter function. In addition, we show direct evidence for glucose-induced activation of the -66/+21-bp promoter region. There is a co-ordinated decline in the abundance of ovine intestinal HNF-1 and SGLT1 transcripts during transition from preruminant to adult ruminant. This decline is recovered after glucose infusion of adult sheep intestine. Similarly, as shown using DNA mobility-shift assays, the intensity of the HNF-1-binding complex to the target promoter sequence decreases during maturation of the animal; this is restored after intestinal sugar infusion. These data indicate that HNF-1 plays an important role in the glucose responsiveness of the ovine SGLT1 gene. This is the first report of in vitro glucose-induced activation of the intestinal SGLT1 promoter and identification of a glucose-responsive region of the ovine SGLT1 promoter.

Genetic determinants of type 2 diabetes mellitus

Type 2 diabetes refers to a group of disparate metabolic diseases, which are typically characterized by insulin resistance in peripheral tissues, together with impaired insulin secretion from pancreatic beta-cells. The complexity of type 2 diabetes is related to factors such as genetic heterogeneity, interactions between genes, and the modulating role played by the environment. Recent progress has included defining the molecular basis of monogenic forms of type 2 diabetes, such as familial partial lipodystrophy and the subtypes of maturity-onset diabetes of the young (MODY), and also the identification of chromosomal regions that may harbor type 2 diabetes susceptibility genes. Many common variants in functional and positional candidate genes, including ADRB3, PPARG, ENPP1, and CAPN10, have also been studied for their possible role as determinants of type 2 diabetes, with varying levels of agreement between studies. The availability of a relatively complete sequence of the human genome will increase the amount of genetic information that can be used to evaluate hypotheses for the genetic basis of type 2 diabetes. To make sense of human type 2 diabetes in the post-genomic era, it is essential to have well-defined phenotypes in addition to sufficient numbers of individuals with the appropriate pedigree structure from families and/or communities.

Transcriptional activation by hepatocyte nuclear factor-1 requires synergism between multiple coactivator proteins

Hepatocyte nuclear factor-1 (HNF-1) plays an important role in the regulation of a large number of genes expressed in the liver, kidney, and pancreatic beta-cells. In exploring the molecular mechanism involved in HNF-1-dependent gene activation in the in vivo chromatin context, we found that HNF-1 can physically interact with the histone acetyltransferases (HATs) CREB-binding protein (CBP), p300/CBP-associated factor (P/CAF), Src-1, and RAC3. The transcriptional activation potential of HNF-1 on a genome integrated promoter was strictly dependent on the synergistic action of CBP and P/CAF, which can independently interact with the N-terminal and C-terminal domain of HNF-1, respectively. Moreover, the HAT activity of both coactivators was important, as opposed to the selective requirement for the HAT activity of P/CAF in activation from a transiently transfected reporter. Interaction of CBP with the N-terminal domain of HNF-1 greatly increased the binding affinity for P/CAF with the C-terminal activation domain, which may represent the molecular basis for the observed functional synergism. The results support a model that involves the combined action of multiple coactivators recruited by HNF-1, which activate transcription by coupling nucleosome modification and recruitment of the general transcription machinery.

The mutated human gene encoding hepatocyte nuclear factor 1beta inhibits kidney formation in developing Xenopus embryos

The transcription factor hepatocyte nuclear factor 1beta (HNF1beta) is a tissue-specific regulator that also plays an essential role in early development of vertebrates. In humans, four heterozygous mutations in the HNF1beta gene have been identified that lead to early onset of diabetes and severe primary renal defects. The degree and type of renal defects seem to depend on the specific mutation. We show that the frameshift mutant P328L329fsdelCCTCT associated with nephron agenesis retains its DNA-binding properties and acts as a gain-of-function mutation with increased transactivation potential in transfection experiments. Expression of this mutated factor in the Xenopus embryo leads to defective development and agenesis of the pronephros, the first kidney form of amphibians. Very similar defects are generated by overexpressing in Xenopus the wild-type HNF1beta, which is consistent with the gain-of-function property of the mutant. In contrast, introduction of the human HNF1beta mutant R137-K161del, which is associated with a reduced number of nephrons with hypertrophy of the remaining ones and which has an impaired DNA binding, shows only a minor effect on pronephros development in Xenopus. Thus, the overexpression of both human mutants has a different effect on renal development in Xenopus, reflecting the variation in renal phenotype seen with these mutations. We conclude that mutations in human HNF1beta can be functionally characterized in Xenopus. Our findings imply that HNF1beta not only is an early marker of kidney development but also is functionally involved in morphogenetic events, and these processes can be investigated in lower vertebrates.

Anatomy of a homeoprotein revealed by the analysis of human MODY3 mutations

Hepatocyte nuclear factor 1alpha (HNF1alpha) is an atypical dimeric homeodomain-containing protein that is expressed in liver, intestine, stomach, kidney, and pancreas. Mutations in the HNF1alpha gene are associated with an autosomal dominant form of non-insulin-dependent diabetes mellitus called maturity-onset diabetes of the young (MODY3). More than 80 different mutations have been identified so far, many of which involve highly conserved amino acid residues among vertebrate HNF1alpha. In the present work, we investigated the molecular mechanisms by which MODY3 mutations could affect HNF1alpha function. For this purpose, we analyzed the properties of 10 mutants resulting in amino acid substitutions or protein truncation. Some mutants have a reduced protein stability, whereas others are either defective in the DNA binding or impaired in their intrinsic trans-activation potential. Three mutants, characterized by a complete loss of trans-activation, behave as dominant negatives when transfected with the wild-type protein. These data define a clear causative relationship between MODY3 mutations and functional defects in HNF1alpha trans-activation. In addition, our analysis sheds new light on the structure of a homeoprotein playing a key role in pancreatic beta cell function.

Functional activity of hepatocyte nuclear factor-1 is specifically decreased in amino acid-limited hepatoma cells

Limitation of cultured rat hepatoma cells for an essential amino acid results in a specific decrease in expression of several genes that are preferentially expressed in the liver, including the serum albumin and transthyretin genes. In the work presented here, we examined whether the coordinate repression of these genes is caused by decreased activity of one or more of the liver-enriched transcription factors, hepatocyte nuclear factor-1 (HNF-1), HNF-3, HNF-4 or C/EBP. To address this question, HepG2 human hepatoma cells were transiently transfected with luciferase reporter constructs containing multiple copies of individual transcription factor binding sites. Limitation for an essential amino acid resulted in specific repression of a construct in which luciferase expression was directed by HNF-1. A single HNF-1 binding site located adjacent to the TATA box plays a major role in transcription directed by the serum albumin promoter in transient transfection assays. Amino acid limitation of cells transfected with an albumin promoter/luciferase reporter construct resulted in specific repression of promoter activity. In addition, bacterial methylation or site-directed mutagenesis of the HNF-1 binding site in the albumin proximal promoter region eliminated the regulation of an albumin promoter-luciferase reporter construct under conditions of amino acid limitation. These results demonstrated that the HNF-1 binding site played a major role in regulation of the albumin promoter by amino acid availability. Deletion analysis of the albumin promoter confirmed regulation through the HNF-1 binding site and also identified a second amino acid regulatory element in the upstream region of the albumin promoter, which has been shown previously to contain a functional binding site for HNF-3. The repression of albumin promoter and HNF-1 reporter constructs in amino acid-limited cells occurred without a change in the DNA binding activity of HNF-1. Moreover, HNF-3 DNA binding activity was also not decreased in amino acid-limited cells. These results suggest that the regulation of transcription by amino acids occurs at the level of transcriptional activation by HNF-1 and HNF-3, rather than by alteration of the DNA binding activity of either factor.

Mechanism of suppression of hepatitis B virus precore RNA transcription by a frequent double mutation

A double mutation which converts nucleotide 1765 from A to T and nucleotide 1767 from G to A is frequently found in the hepatitis B virus (HBV) genome isolated from HBV patients with chronic hepatitis symptoms. This double mutation is located in the core promoter that controls the transcription of the precore RNA and the core RNA. In addition, this double mutation also resides in the X protein coding sequence, converting codon 130 from Lys to Met and codon 131 from Val to Ile. Previous studies indicate that this double mutation removes a nuclear receptor binding site in the core promoter, suppresses specifically precore RNA transcription, and enhances viral replication. In this study, we further investigated how this double mutation suppresses precore RNA transcription. We found that this double mutation not only removed the nuclear receptor binding site but also created an HNF1 transcription factor binding site. Further transfection studies using Huh7 hepatoma cells indicate that the removal of the nuclear receptor binding site has no effect on the transcription of HBV RNAs, the two-codon change in the X protein sequence suppresses the transcription of both precore and core RNAs, and the creation of the HNF1 binding site restores the core RNA level. Hence, the specific suppression of precore RNA transcription by this frequent double-nucleotide mutation is the combined result of multiple factors.

Hepatocyte nuclear factor-1 acts as an accessory factor to enhance the inhibitory action of insulin on mouse glucose-6-phosphatase gene transcription

Glucose-6-phosphatase catalyzes the terminal step in the gluconeogenic and glycogenolytic pathways. Transcription of the gene encoding the glucose-6-phosphatase catalytic subunit (G6Pase) is stimulated by cAMP and glucocorticoids whereas insulin strongly inhibits both this induction and basal G6Pase gene transcription. Previously, we have demonstrated that the maximum repression of basal G6Pase gene transcription by insulin requires two distinct promoter regions, designated A (from -271 to -199) and B (from -198 to -159). Region B contains an insulin response sequence because it can confer an inhibitory effect of insulin on the expression of a heterologous fusion gene. By contrast, region A fails to mediate an insulin response in a heterologous context, and the mutation of region B within an otherwise intact promoter almost completely abolishes the effect of insulin on basal G6Pase gene transcription. Therefore, region A is acting as an accessory element to enhance the effect of insulin, mediated through region B, on G6Pase gene transcription. Such an arrangement is a common feature of cAMP and glucocorticoid-regulated genes but has not been previously described for insulin. A combination of fusion gene and protein-binding analyses revealed that the accessory factor binding region A is hepatocyte nuclear factor-1. Thus, despite the usually antagonistic effects of cAMP/glucocorticoids and insulin, all three agents are able to use the same factor to enhance their action on gene transcription. The potential role of G6Pase overexpression in the pathophysiology of MODY3 and 5, rare forms of diabetes caused by hepatocyte nuclear factor-1 mutations, is discussed.

Detailed analysis of a 17q21 microdissection library by sequence bioinformatics and isolation of region-specific clones

A region-specific microdissection library originating from human chromosome 17q21, was constructed using the MboI linker-adaptor microcloning technique. DNA sequencing of 241 microclones resulted in the identification of 74 novel coding sequences, paralogs of known genes, and known, but previously unmapped, genes or expressed sequence tags that were "virtually" mapped to chromosome 17q21. By pooling the microclones as multiplexed hybridization probes, and by virtue of their origin on 17q21, we were able to identify approximately 150 P1 clones from the human Reference Library Data Base P1 Library that potentially map to chromosome 17q21. Verification of the 17q21 location of 16 P1 clones was accomplished by PCR analysis with STS primer pairs to known 17q21 genes or by FISH. Our results demonstrate the substantial advantage of combining the sequence analysis of microclones with multiplex hybridization strategies for gene discovery and mapping specific gene rich regions of the genome.

Expression ratio of hepatocyte nuclear factor-1 to variant hepatocyte nuclear factor-1 in differentiation of hepatocellular carcinoma and hepatoblastoma

BACKGROUND/AIMS

Liver-specific protein genes have multiple cis-/trans-acting elements, but those accountable for hepatocytic differentiation are unclear. An AT-rich core sequence (AT motif) is essential as a cis-acting element for the hepatic transcription. Homologous proteins hepatocyte nuclear factor-1 (HNF-1) and variant HNF-1 (vHNF-1) bind to this motif. The ratio of HNF-1 to vHNF-1 mRNA was examined in various liver tissues with respect to their differentiation.

METHODS

The competitive reverse transcriptional polymerase chain reaction was employed to amplify HNF-1 and vHNF-1 mRNA simultaneously and to examine their expression ratio in total RNA extracted from frozen liver tissues of 37 patients with hepatocellular carcinoma, five patients with hepatoblastoma, and 15 non-neoplastic liver tissues.

RESULTS

The ratio of HNF-1 to vHNF-1 mRNA was higher in well-differentiated cases than in poorly-differentiated and undifferentiated cases, except that one poorly-differentiated hepatoblastoma displayed a high ratio. Non-neoplastic liver tissues had low ratios similar to poorly-differentiated hepatocellular carcinoma, the reason for which remained unknown. However, chronic hepatitis and liver cirrhosis cases also demonstrated low ratios, and hence degenerative changes themselves displayed no obvious influence on such ratios. Thus, the gene expression of HNF-1 and vHNF-1 seemed to be differentially regulated in neoplastic and non-neoplastic hepatocytes.

CONCLUSIONS

These results suggested that the ratio of HNF-1 to vHNF-1 mRNA correlated with histological differentiation of HCC and hepatoblastoma.

Cellular factors controlling the activity of woodchuck hepatitis virus enhancer II

Woodchuck hepatitis virus (WHV) efficiently induces hepatocellular carcinoma in chronically infected hosts. A key step in hepatocarcinogenesis by WHV is insertional activation of the cellular N-myc gene by integrated viral DNA. WHV enhancer II (En II) is the major cis-acting element involved in this activation. Here we characterize this viral enhancer element and define the cellular factors involved in its activity. WHV En II activity is strongly liver specific and maps to an 88-nucleotide DNA segment (nucleotides 1772 to 1859) located 5' to the pregenomic RNA start site. Genetic analyses and electrophoretic mobility shift assays indicate that the enhancer contains three subregions important to its activity. The core elements of the enhancer are recognition sites for the liver-enriched factors HNF1 and HNF4; together, these signals account for the bulk of En II activity as well as its strong liver specificity. Multimerization of either recognition site produced strong activity even in the absence of other En II sequences. 5' to these elements is a binding site for the ubiquitous Oct-1 transcription factor, which further augments enhancer activity ca. twofold.

Role of the liver-enriched transcription factor hepatocyte nuclear factor 1 in transcriptional regulation of the factor V111 gene

Coagulation factor VIII is an essential cofactor required for normal hemostatic function. A deficiency in factor VIII results in the bleeding disorder hemophilia A. Despite the fact that the factor VIII gene was cloned a decade ago, the mechanisms which control its transcription remain unresolved. In our studies, we have characterized 12 protein binding sites within the factor VIII promoter by DNase I protection assays performed with rat liver nuclear extracts. Three of these elements (sites 1 to 3) are situated within the 5' untranslated region of the gene, while three other sites (sites 4 to 6) lie within the first 100 bp upstream of the transcriptional start site. We have identified an additional site (site 7) approximately 300 bp upstream from site 6, as well as a cluster of five sites in a 250-bp region which terminates approximately 1 kb from the transcriptional start site. Seven of these binding sites (sites 2, 3, 4, 6, 7, 9, and 10) bind members of the C/EBP family of transcription factors. DBP also binds to five of these sites (sites 3, 4, 6, 7, and 9). Utilizing transient transfection studies in HepG2 cells, we have shown that deletion of the factor VIII promoter sequences distal to nucleotide -44 results in a significant but small increase in promoter activity. The activity of each of the various 5' deletion constructs is significantly enhanced by cotransfection of C/EBPalpha and D-site-binding protein expression plasmids, while cotransfection of both C/EBPalpha and C/EBPbeta plasmids resulted in a further enhancement of transactivation. These studies also provide evidence of a repressor element located between nucleotides -740 and -1002. Since the minimal promoter sequence (-44 to +148) maintains the transcriptional activity of the full-length promoter sequence, we proceeded to identify additional factors binding to sites 1 to 4. Competition studies revealed that a ubiquitous transcription factor, NF-Y, binds to site 4, while the liver-enriched transcription factor hepatocyte nuclear factor I (HNF-1) binds to site 1. Mutation analysis of the minimal promoter demonstrated that HNF-1 is critical for activating transcription of the factor VIII gene in vitro. Our results also suggest that the multiple upstream elements that we have identified may act as a backup regulatory region in the event of disruption of the HNF-1 element in the 5' untranslated region.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

Role of LFB3 in cell-specific cAMP induction of the urokinase-type plasminogen activator gene

In previous work we suggested that a kidney-specific transcription factor LFB3 cooperates with cAMP-response element (CRE)-binding proteins within a cAMP regulatory unit comprised of three protein-binding domains and located 3.4 kilobase pairs upstream of the urokinase-type plasminogen activator (uPA) gene in LLC-PK1 cells (Menoud, P.-A., Matthies, R., Hofsteenge, J., and Nagamine, Y. (1993) Nucleic Acids Res. 21, 1845-1852). The two domains contain a CRE-like sequence, and the third domain is recognized by LFB3. The absolute requirement of LFB3 as well as the cooperation among the three domains for cAMP regulation were confirmed by transient transfection assays in F9 teratocarcinoma cells, in which the level of LFB3 was negligible. Suspecting a possible feedback regulation of LFB3 mRNA expression during cAMP-dependent uPA gene induction in LLC-PK1 cells, we measured LFB3 mRNA levels after cAMP treatment and found a strong reduction. This reduction was not due to a change in template activity of the LFB3 gene because run-on transcription showed no significant change in LFB3 gene transcription. RNA synthesis inhibitor-chase experiments indicated that the down-regulation was post-transcriptional. Interestingly, when the inhibitor was added at the same time as cAMP, the cAMP-induced decrease in LFB3 mRNA levels was abrogated, suggesting that ongoing RNA synthesis is required for the decrease. Similar effects on LFB3 mRNA metabolism were observed with all agents that induce uPA mRNA in LLC-PK1 cells, including 12-O-tetradecanoylphorbol-13-acetate, okadaic acid, colchicine, and cytochalasin. We discuss the significance of this regulation in uPA gene expression.

LFB1/HNF1 acts as a repressor of its own transcription

LFB1/HNF1 is a hepatocyte-enriched trans-activator involved in the regulation of many liver-specific genes. We report the cloning and characterization of a rat genomic DNA fragment containing about 3.5 kb of the LFB1/HNF1 gene 5'-flanking region. This DNA segment is capable of directing the liver-specific expression of a reporter gene in transfection assays. More interestingly, the basal activity of the LFB1/HNF1 promoter in cultured hepatoma cell lines is down-regulated by exogenously added LFB1/HNF1 protein itself. The ability to repress transcription starting from its own promoter requires the integrity of the N-terminal LFB1/HNF1 DNA-binding domain. Contrary to the expectations, in vitro binding experiments failed to demonstrate any specific and functional interaction of purified LFB1/HNF1 with the -3.5 kb promoter sequence. In addition to the DNA-binding domain, a 60 aa region contained in the C-terminus of the protein and distinct from the previously characterized activation domains, is also required for the repressing function.

Putative nuclear localization signals (NLS) in protein transcription factors

We have recognized about ten distinct forms of strongly basic hexapeptides, containing at least four arginines and lysines, characteristic of nuclear proteins among all eukaryotic species, including yeast, plants, flies and mammals. These basic hexapeptides are considered to be different versions of a core nuclear localization signal, NLS. Core NLSs are present in nearly all nuclear proteins and absent from nearly all "nonassociated" cytoplasmic proteins that have been investigated. We suggest that the few (approximately 10%) protein factors lacking a typical NLS core peptide may enter the nucleus via their strong crosscomplexation with their protein factor partners that possess a core NLS. Those cytoplasmic proteins found to possess a NLS-like peptide are either tightly associated with cell membrane proteins or are integral components of large cytoplasmic protein complexes. On the other hand, some versions of core NLSs are found in many cell membrane proteins and secreted proteins. It is hypothesized that in these cases the N-terminal hydrophobic signal peptide of extracellular proteins and the internal hydrophobic domains of transmembrane proteins are stronger determinants for their subcellular localization. The position of core NLSs among homologous nuclear proteins may or may not be conserved; however, if lost from an homologous site it appears elsewhere in the protein. This search provides a set of rules to our understanding of the nature of core nuclear localization signals: (1) Core NLS are proposed to consist most frequently of an hexapeptide with 4 arginines and lysines; (2) aspartic and glutamic acid residues as well as bulky amino acids (F, Y, W) need not to be present in this hexapeptide; (3) acidic residues and proline or glycine that break the alpha-helix are frequently in the flanking region of this hexapeptide stretch; (4) hydrophobic residues ought not to be present in the core NLS flanking region allowing for the NLS to be exposed on the protein. In this study we attempt to classify putative core NLS from a wealth of nuclear protein transcription factors from diverse species into several categories, and we propose additional core NLS structures yet to be experimentally verified.

HNF1 alpha activates the aminopeptidase N promoter in intestinal (Caco-2) cells

The importance of HNF1 binding proteins for intestinal aminopeptidase N expression was investigated using the Caco-2 cell-line. Aminopeptidase N promoter activity in Caco-2 cells depends on the HNF1 element (positions -85 to -58) and co-transfection with an HNF1 alpha expression vector demonstrates a direct activation of the promoter by HNF1 alpha through this element. Electrophoretic mobility shift assays using nuclear extracts from Caco-2 cells show the presence of high amounts of HNF1 binding proteins irrespective of their state of differentiation.

Developmental regulation of rat serine dehydratase gene expression: evidence for the presence of a repressor in fetal hepatocytes

To determine why the rat serine dehydratase gene becomes transcriptionally activated just after birth, we examined the interactions of DNA binding proteins of fetal and adult rat livers with the serine dehydratase gene promoter by DNase I protection analysis and gel mobility shift assay. Several binding regions of nuclear proteins were found to be common to fetal and adult livers and interaction of factors with the characteristics of Sp1 or NF-Y was suggested. Two additional regions, named regions B and I, were specific to fetal liver. These regions contain GATA-like sequences and competition experiments by gel mobility shift assay suggested that the fetal liver-enriched factor binds to the GATA-like sequences. The function of the regions B and I in transcription regulation was investigated in fetal and adult hepatocytes by transient DNA transfer experiments with serine dehydratase-chloramphenicol acetyltransferase fusions. These experiments showed that these regions functioned as negative cis-acting elements in fetal hepatocytes, but not in adult hepatocytes.

An indirect negative autoregulatory mechanism involved in hepatocyte nuclear factor-1 gene expression

Recent studies have revealed that hepatocyte nuclear factor 4 (HNF-4) is an essential positive regulator of another liver enriched transcription factor HNF-1, defining a transcriptional hierarchy between the two factors operating in hepatocytes. To assess the possible autoregulation of the HNF-1 gene we have examined the effect of HNF-1 on its own transcription. In transient transfection assays, HNF-1 strongly down-regulated transcription driven by its own promoter in HepG2 cells. In addition HNF-1 also repressed the activity of HNF-4 dependent ApoCIII and ApoAI promoters. The same effect was observed using vHNF-1, a distinct but highly related protein to HNF-1. Both HNF-1 and vHNF-1 downregulated HNF-4 activated transcription from intact and chimeric promoter constructs carrying various HNF-4 binding sites implying that they act by impeding HNF-4 binding or activity. DNA binding and cell free transcription experiments however failed to demonstrate any direct or indirect interaction of HNF-1 and vHNF-1 with the above regulatory regions. Both factors repressed HNF-4 induced transcription of the ApoCIII and HNF-1 genes in HeLa cells, arguing against the requirement of a hepatocyte specific function. These findings define an indirect negative autoregulatory mechanism involved in HNF-1 gene expression, which in turn may affect HNF-4 dependent transcription of other liver specific genes.

More potent transcriptional activators or a transdominant inhibitor of the HNF1 homeoprotein family are generated by alternative RNA processing

We report the isolation of cDNAs from human liver encoding several isoforms of the hepatocyte nuclear factor homeoproteins HNF1 and vHNF1 generated by the differential use of polyadenylation sites and by alternative splicing. In the novel isoforms intron sequences that are excised in the previously described forms are translated in the same frame as exon sequences until the first termination codon is encountered. Hence, the newly found isoforms all contain different C-terminal domains. For HNF1 it has been shown that its C-terminal region is responsible for the activation of transcription. In transient transfection assays the two novel HNF1 isoforms, HNF1-B and -C, transactivate 5-fold better than the previously described HNF1 protein (HNF1-A). The newly isolated isoform of vHNF1, designated vHNF1-C, is unable to transactivate and behaves as a transdominant repressor when cotransfected with HNF1-A, -B or -C. All of the different isoforms of HNF1 and vHNF1 can form homo- and heterodimers and their mRNAs are differentially expressed in fetal and adult human liver, kidney and intestine, suggesting distinct roles during development. Our studies show that the transactivation domain of the members of the HNF1 homeoprotein family is organized in modules which can be exchanged to generate either more potent transcriptional activators or a transdominant repressor.

Cartilage homeoprotein 1, a homeoprotein selectively expressed in chondrocytes

We identified a rat cDNA that encodes cartilage homeoprotein 1 (Cart-1). The deduced amino acid sequence of Cart-1 contains a paired-type homeodomain. Northern blot hybridization and RNase protection assay revealed that Cart-1 RNA was present at high levels in a well-differentiated rat chondrosarcoma tumor and in a cell line derived from this tumor. Cart-1 RNA was detected in primary mouse and rat chondrocytes but not in various fibroblasts including mouse 10T1/2 cells, NIH 3T3 cells, BALB 3T3 cells, and rat skin fibroblasts. It was also undetectable in mouse C2 myoblasts, S194 myeloma cells, and embryonic stem cells. Cart-1 RNA was present at a very low level in tested but was not detected in other soft tissues of 8-week-old rats. In situ hybridization of rat embryos between 14.5 and 16.5 days post coitum revealed relatively high levels of Cart-1 RNA in condensed prechondrocytic mesenchymal cells and in early chondrocytes of cartilage primordia. The levels of Cart-1 RNA were lower in mature chondrocytes. No hybridization was observed in brain, spinal cord, heart, spleen, gastrointestinal tract, liver, and muscle. We speculate that Cart-1 has a role in chondrocyte differentiation.

Gene regulation in rodent hepatocytes during development, differentiation and disease

The expression of genes in the liver is mostly controlled at the transcriptional level and depends on the regulatory interactions between cis-acting sequences and trans-acting molecules. Proximal promoters and distant enhancers in combination with a number of hepatocyte-enriched DNA-binding proteins and general transcription factors interact specifically with these elements and control the expression of liver-specific genes. Hepatocyte-enriched regulatory proteins have been isolated from liver nuclear extracts, characterized, and their corresponding genes have been cloned. These include the hepatocyte nuclear factors 1, 3, 4 (HNF-1,3,4), some members of the CAAAT/enhancer binding protein (C/EBP) family, and D site binding protein (DBP). These factors belong to larger families and are able to form heterodimers, perhaps with the exception of the HNF-3 family, with other members of the same family. Interestingly, the majority of the genes encoding such proteins are themselves regulated at the transcriptional level, although both transcriptional and post-transcriptional events modulate their expression during development, hepatocyte differentiation and disease, suggesting that a transcriptional cascade may play a critical role in mammalian liver development and differentiation.

Genomic structure of the POU-related hepatic transcription factor HNF-1 alpha

Hepatocyte Nuclear Factor-1 alpha (HNF-1 alpha) is a vertebrate transcriptional regulatory protein which plays a prominent role in the activation of a large family of hepatocyte-specific genes. It is the prototype of a new subfamily of POU-related proteins important in embryogenesis and cell-type specification. Here we report on the genomic structure and DNA sequence of HNF-1 alpha of the hen. Functional domains are non-uniformly conserved between hen and mammals consistent with the notion of functions in the hen oviduct that were lost in mammals after avian mammalian separation. The unusual homeo-related DNA-binding domain is well preserved between birds and mammals. The exon/intron pattern of the gene changed during evolution. The rat gene consists of 9 exons, the chicken gene of 10. In mammals one intron was eliminated that in hens subdivides the serine-rich transactivation domain. Comparison of the sequences at this junction indicates the involvement of elements of the V(D)J recombination system in the process of intron elimination.

Hepatic nuclear factor 1 alpha (HNF-1 alpha) is expressed in the oviduct of hens and interacts with regulatory elements of the lysozyme gene

HNF-1 alpha is a nuclear transcriptional regulatory protein required for the expression of a variety of liver-specific genes. This factor was previously considered liver-specific but later shown to be expressed also in a few other mammalian tissues. Here we report on the occurrence of HNF-1 alpha in the avian oviduct. This finding is of particular interest because HNF-1 alpha is not expressed in female reproductive organs of mammals. The avian oviduct is the site of assembly for the avian egg and the site of tissue-specific synthesis of the major egg white proteins, such as lysozyme. We also demonstrate that the chicken lysozyme gene contains HNF-1 recognition sites within two of its important upstream regulatory elements. The presence of HNF-1 recognition elements in functionally significant regulatory sites of the lysozyme gene and high levels of HNF-1 alpha in the oviduct is a strong indication for the involvement of HNF-1 alpha in the control of the lysozyme gene and possibly other egg white protein genes in the chicken oviduct. Apparently, HNF-1 alpha performs functions in the avian oviduct that were lost upon development from birds to mammals.

Competition between transcription factors HNF1 and HNF3, and alternative cell-specific activation by DBP and C/EBP contribute to the regulation of the liver-specific aldolase B promoter

The aldolase B proximal promoter is controlled by at least five elements spanning from -190 to -103 bp with respect to the start site of transcription. From 5' to 3', we found: a negative DE element, an activating C/EBP-DBP binding site, a CCAAT box binding NFY that seems to play a negative role, and an activating element consisting of two overlapping binding sites for HNF-1 and HNF-3. Contransfection experiments of aldolase B/CAT constructs and of expression vectors for different transcription factors were carried out in human hepatoma Hep G2 cells. We found that DBP and HNF-1 are strong transactivators of the aldolase B promoter while C/EBP and vHNF-1 are only weak activators and HNF-3 alone does not modify such activity. Deletion of the distal negative element results in a similar transactivation by C/EBP and DBP, enhanced for the former and reduced for the latter. In hepatocytes in primary culture, the strong transactivator is C/EBP while DBP is essentially inactive. This tissue-specificity of C/EBP and DBP action could depend on interaction with tissue-specific proteins bound to a neighbouring site, probably DE. Finally, HNF3 behaves as a very strong anti-activator of the aldolase B promoter. It competitively antagonizes transactivation by HNF-1 and non-competitively transactivation by DBP. This negative effect of HNF-3 and tissue-specificity of the transactivation potential of DBP and C/EBP are unique features of the aldolase B promoter.

Developmental regulation and tissue distribution of the liver transcription factor LFB1 (HNF1) in Xenopus laevis

The transcription factor LFB1 (HNF1) was initially identified as a regulator of liver-specific gene expression in mammals. It interacts with the promoter element HP1, which is functionally conserved between mammals and amphibians, suggesting that a homologous factor, XLFB1, also exists in Xenopus laevis. To study the role of LFB1 in early development, we isolated two groups of cDNAs coding for this factor from a Xenopus liver cDNA library by using a rat LFB1 cDNA probe. A comparison of the primary structures of the Xenopus and mammalian proteins shows that the myosin-like dimerization helix, the POU-A-related domain, the homeo-domain-related region, and the serine/threonine-rich activation domain are conserved between X. laevis and mammals, suggesting that all these features typical for LFB1 are essential for function. Using monoclonal antibodies, we demonstrate that XLFB1 is present not only in the liver but also in the stomach, intestine, colon, and kidney. In an analysis of the expression of XLFB1 in the developing Xenopus embryo, XLFB1 transcripts appear at the gastrula stage. The XLFB1 protein can be identified in regions of the embryo in which the liver diverticulum, stomach, gut, and pronephros are localized. The early appearance of XLFB1 expression during embryogenesis suggests that the tissue-specific transcription factor XLFB1 is involved in the determination and/or differentiation of specific cell types during organogenesis.

Developmental regulation and tissue distribution of the liver transcription factor LFB1 (HNF1) in Xenopus laevis

The transcription factor LFB1 (HNF1) was initially identified as a regulator of liver-specific gene expression in mammals. It interacts with the promoter element HP1, which is functionally conserved between mammals and amphibians, suggesting that a homologous factor, XLFB1, also exists in Xenopus laevis. To study the role of LFB1 in early development, we isolated two groups of cDNAs coding for this factor from a Xenopus liver cDNA library by using a rat LFB1 cDNA probe. A comparison of the primary structures of the Xenopus and mammalian proteins shows that the myosin-like dimerization helix, the POU-A-related domain, the homeo-domain-related region, and the serine/threonine-rich activation domain are conserved between X. laevis and mammals, suggesting that all these features typical for LFB1 are essential for function. Using monoclonal antibodies, we demonstrate that XLFB1 is present not only in the liver but also in the stomach, intestine, colon, and kidney. In an analysis of the expression of XLFB1 in the developing Xenopus embryo, XLFB1 transcripts appear at the gastrula stage. The XLFB1 protein can be identified in regions of the embryo in which the liver diverticulum, stomach, gut, and pronephros are localized. The early appearance of XLFB1 expression during embryogenesis suggests that the tissue-specific transcription factor XLFB1 is involved in the determination and/or differentiation of specific cell types during organogenesis.

Expression patterns of vHNF1 and HNF1 homeoproteins in early postimplantation embryos suggest distinct and sequential developmental roles

The homeoproteins HNF1 (LFB1/HNF1-A) and vHNF1 (LFB3/HNF1 beta) interact with an essential control element of a group of liver-specific genes. During development, these putative target genes are initially expressed in the visceral endoderm of the yolk sac and subsequently in fetal liver. To assess the possible involvement of HNF1 and/or vHNF1 as transcriptional regulators in the early steps of visceral endoderm differentiation, we have analyzed the expression pattern of both factors both in vitro during differentiation of murine F9 embryonal carcinoma cells and in vivo during early postimplantation mouse development. We show here that differentiation of F9 cells into either visceral or parietal endoderm is accompanied by a sharp induction in vHNF1 mRNA and protein. By contrast, only low levels of aberrantly sized HNF1 transcripts, but not DNA-binding protein, are found in F9 cells and its differentiated derivatives. At 6–7.5 days of gestation, high levels of vHNF1 mRNA are present in the visceral extraembryonic endoderm, which co-localize with transcripts of the transthyretin gene. HNF1 transcripts are first detected in the yolk sac roughly two embryonic days later, after the developmental onset of transcription of target genes. As development proceeds, discrepancies are observed between the level of transcripts of both vHNF1 and HNF1 and their respective nuclear binding proteins, notably in the yolk sac and embryonic kidney. In addition, we show that two alternative spliced isoforms of vHNF1 mRNA, vHNF-A and vHNF1-B, are expressed in both embryonic and adult tissues. Taken together, these data suggest that vHNF1 participates as a regulatory factor in the initial transcriptional activation of the target genes in the visceral endoderm of the yolk sac, whereas the later appearance of HNF1 could be required for maintenance of their expression. Our results also provide evidence of a posttranscriptional level of control of vHNF1 and HNF1 gene expression during development, in addition to the spatial restriction in transcription.

Structure of the gene encoding hepatocyte nuclear factor 1 (HNF1)

Genomic clones have been isolated that cover the entire gene for the transcription factor HNF1 (hepatocyte nuclear factor 1). This protein governs the expression of many genes, synthesized in the liver in a tissue-specific manner. We have determined the intron/exon structure of the HNF1 gene, which is strictly conserved between rat and mouse and estimate that it spans not more than 40kb in the rat genome. Whereas most homeoprotein genes do not contain introns within the homeodomain, HNF1 displays an intron between the regions encoding the second and the third helices. We discuss possible evolutionary mechanisms leading to this homeobox intron/exon pattern.

HNF1, a homeoprotein member of the hepatic transcription regulatory network

Numerous liver specific genes are transcriptionally activated by the binding to their promoter or enhancer of Hepatic Nuclear Factor 1 (HNF1). HNF1 contains a variant homeo-domain and binds to DNA as either a homodimer or a heterodimer with the vHNF1 protein. Surprisingly, HNF1 is not restricted to hepatocytes but is expressed in epithelial cells of several endoderm derived organs and in mesoderm derived kidney tubules. Hence, HNF1 alone can not account for the differentiated state of the hepatic cells. In fact, several other liver-enriched transcription factors have been cloned. The hepatic phenotype could result from the combinatorial expression of these regulators. Possible involvement of these trans-acting factors in liver organogenesis and hepatic differentiation is discussed.

vHNF1 is expressed in epithelial cells of distinct embryonic origin during development and precedes HNF1 expression

HNF1 (Hepatic Nuclear Factor 1) and vHNF1 are transcriptional regulators containing a highly divergent homeodomain. The first was initially found in liver nuclear extracts and is crucial for the transcription of albumin and many other hepatocyte specific genes, while the second was found in dedifferentiated hepatoma cells. Both recognize the same DNA binding site and can form homo and heterodimers in vitro and in vivo. In situ hybridization analyses have been performed to delineate the spatial and temporal pattern of expression of vHNF1 relative to HNF1 during mouse embryogenesis. The results show that accumulation of vHNF1 mRNAs expression is detected in several tissues of the embryo of both endodermal and mesodermal origin. Expression occurs in the yolk sac, the primitive gut, the liver primordium, and at different stages of kidney development in polarized epithelial structures and usually precedes that of HNF1. vHNF1 expression seems particularly prevalent with morphogenetic events in the kidney and may be a marker for certain polarized epithelium.