Overproduction of a truncated hepatocyte nuclear factor 3 protein inhibits expression of liver-specific genes in hepatoma cells

Identification of a transthyretin enhancer site that selectively binds the hepatocyte nuclear factor-3 beta isoform

The upstream proximal region of the transthyretin (TTR) promoter and a distal enhancer are sufficient to drive liver-specific expression of the TTR gene, as demonstrated by experiments in transgenic mice. Previous analyses have characterized the binding of a number of liver-enriched transcription factors of the TTR promoter including hepatocyte nuclear factors one (HNF-1), HNF-4, and three distinct HNF-3 proteins (alpha, beta, and gamma), which are members of the winged helix (fork head) family. The TTR enhancer was shown to bind members of the CCAAT/enhancer binding protein (C/EBP) family at two distinct sites (TTR-2 and TTR-3), and an oligonucleotide containing the activation protein one (AP-1) binding sequence competed for recognition to a third enhancer site (TTR-1). In this study, we have carried out a detailed analysis of the transcription factors that recognize the TTR enhancer elements (TTR-1, TTR-2, and TTR-3 oligonucleotide sequences). Analysis of the TTR-1 site demonstrates that the putative AP-1 site in the TTR enhancer binds a ubiquitously expressed factor that is distinct from the AP-1 family of proteins. Next we demonstrate, via gel shift analysis, that the TTR-3 site is recognized by the C/EBP family in liver nuclear extracts. We also show that whereas the TTR-2 enhancer site is capable of binding recombinant C/EBP proteins, it does not bind C/EBP proteins from liver nuclear extracts. The TTR-2 site does, however, contain a variant HNF-3 recognition sequence that exclusively binds the HNF-3 beta isoform. Mutation of this HNF-3 beta-specific recognition sequence caused reductions in TTR enhancer activity. We had previously observed a 95% decrease in HNF-3 alpha expression and a 20% reduction in HNF-3 beta expression in acute phase livers, which correlated with a 60% decrease in TTR gene transcription. We propose that the HNF-3 beta-specific binding site in the TTR enhancer may play a role in maintaining TTR gene expression during the acute phase response in spite of the dramatic reduction in HNF-3 alpha protein levels.

SNAIL1-mediated downregulation of FOXA proteins facilitates the inactivation of transcriptional enhancer elements at key epithelial genes in colorectal cancer cells

Phenotypic conversion of tumor cells through epithelial-mesenchymal transition (EMT) requires massive gene expression changes. How these are brought about is not clear. Here we examined the impact of the EMT master regulator SNAIL1 on the FOXA family of transcription factors which are distinguished by their particular competence to induce chromatin reorganization for the activation of transcriptional enhancer elements. We show that the expression of SNAIL1 and FOXA genes is anticorrelated in transcriptomes of colorectal tumors and cell lines. In cellular EMT models, ectopically expressed Snail1 directly represses FOXA1 and triggers downregulation of all FOXA family members, suggesting that loss of FOXA expression promotes EMT. Indeed, cells with CRISPR/Cas9-induced FOXA-deficiency acquire mesenchymal characteristics. Furthermore, ChIP-seq data analysis of FOXA chromosomal distribution in relation to chromatin structural features which characterize distinct states of transcriptional activity, revealed preferential localization of FOXA factors to transcriptional enhancers at signature genes that distinguish epithelial from mesenchymal colon tumors. To validate the significance of this association, we investigated the impact of FOXA factors on structure and function of enhancers at the CDH1, CDX2 and EPHB3 genes. FOXA-deficiency and expression of dominant negative FOXA2 led to chromatin condensation at these enhancer elements. Site-directed mutagenesis of FOXA binding sites in reporter gene constructs and by genome-editing in situ impaired enhancer activity and completely abolished the active chromatin state of the EPHB3 enhancer. Conversely, expression of FOXA factors in cells with inactive CDX2 and EPHB3 enhancers led to chromatin opening and de novo deposition of the H3K4me1 and H3K27ac marks. These findings establish the pioneer function of FOXA factors at enhancer regions of epithelial genes and demonstrate their essential role in maintaining enhancer structure and function. Thus, by repressing FOXA family members, SNAIL1 targets transcription factors at strategically important positions in gene-regulatory hierarchies, which may facilitate transcriptional reprogramming during EMT.

Cancer patient mortality is overwhelmingly due to distant organ metastases. Epithelial-mesenchymal transition is a process thought to facilitate local invasion and dissemination of cancer cells, thereby promoting metastasis. The conversion of epithelial cells into mesenchymal, fibroblast-like cells requires profound gene expression changes. A few transcription factors like SNAIL1 can initiate these changes, but are unlikely to be solely responsible for all of them. In our study we asked, whether destabilization of epithelial gene expression programs could involve FOXA transcription factors. FOXA factors represent a special subgroup of regulatory proteins, so-called pioneer factors, with unique roles in the activation of transcriptional enhancers which are key regulatory DNA elements that orchestrate spatio-temporal gene expression. In a model of colorectal cancer we found that SNAIL1 represses FOXA factors, and demonstrate that FOXA factors are associated with enhancer elements at epithelial signature genes. Indeed, FOXA factors are sufficient to initiate enhancer activation and necessary to maintain their activity. Our findings indicate that SNAIL1 induces pervasive repression of epithelial genes through a hierarchical scheme of alterations in transcription factor expression which may be applicable to other instances of cell fate changes and transcriptional reprogramming.

Inhibition of Wnt/β-catenin signaling by a soluble collagen-derived frizzled domain interacting with Wnt3a and the receptors frizzled 1 and 8

The Wnt/β-catenin pathway controls cell proliferation, death and differentiation. Several families of extracellular proteins can antagonize Wnt/β-catenin signaling, including the decoy receptors known as secreted frizzled related proteins (SFRPs), which have a cysteine-rich domain (CRD) structurally similar to the extracellular Wnt-binding domain of the frizzled receptors. SFRPs inhibit Wnt signaling by sequestering Wnts through the CRD or by forming inactive complexes with the frizzled receptors. Other endogenous molecules carrying frizzled CRDs inhibit Wnt signaling, such as V3Nter, which is proteolytically derived from the cell surface component collagen XVIII and contains a biologically active frizzled domain (FZC18) inhibiting in vivo cell proliferation and tumor growth in mice. We recently showed that FZC18 expressing cells deliver short-range signals to neighboring cells, decreasing their proliferation in vitro and in vivo through the Wnt/β-catenin signaling pathway. Here, using low concentrations of soluble FZC18 and Wnt3a, we show that they physically interact in a cell-free system. In addition, soluble FZC18 binds the frizzled 1 and 8 receptors' CRDs, reducing cell sensitivity to Wnt3a. Conversely, inhibition of Wnt/β-catenin signaling was partially rescued by the expression of full-length frizzled 1 and 8 receptors, but enhanced by the expression of a chimeric cell-membrane-tethered frizzled 8 CRD. Moreover, soluble, partially purified recombinant FZC18_CRD inhibited Wnt3a-induced β-catenin activation. Taken together, the data indicate that collagen XVIII-derived frizzled CRD shifts Wnt sensitivity of normal cells to a lower pitch and controls their growth.

Foxa2 (HNF-3beta) regulates expression of hepatotrophic factor ALR in liver cells

Liver regeneration is a multistep and well-orchestrated process which is initiated by injuries such as tissue loss, infectious or toxic insults. Augmenter of liver regeneration (ALR) is a hepatotrophic growth factor which has been shown to stimulate hepatic regeneration after partial hepatectomy and therefore seems to be regulated during the regenerative process in the liver. Our aim was to analyze how ALR is regulated in hepatic tissues and which transcription factors might regulate its tissue-specific expression. Promoter studies of ALR (-733/+527 bp) revealed potential regulatory elements for various transcription factors like Foxa2, IL-6 RE-BP and C/EBPbeta. Analysis of the promoter activity by performing luciferase assays revealed that co-transfection with Foxa2 significantly induced the activity of ALR promoter in HepG2 cells. EMSA and Supershift analysis using anti-Foxa2 antibody confirmed the specific binding of Foxa2 to ALR promoter and this binding was inducible when the cells were simultaneously stimulated with IL-6. The increased binding after activation with IL-6 and/or Foxa2 was confirmed by elevated ALR protein levels using Western blot technique. In addition, we could not detect any binding of C/EBPbeta and IL-6 RE-BP to the promoter of ALR. In conclusion, these results indicate that ALR is regulated by Foxa2, and this regulation may be amplified by IL-6.

The Fox genes in the liver: from organogenesis to functional integration

Formation and function of the liver are highly controlled, essential processes. Multiple signaling pathways and transcriptional regulatory networks cooperate in this complex system. The evolutionarily conserved FOX, for Forkhead bOX, class of transcriptional regulators is critical to many aspects of liver development and function. The FOX proteins are small, mostly monomeric DNA binding factors containing the so-called winged helix DNA binding motif that distinguishes them from other classes of transcription factors. We discuss the biochemical and genetic roles of Foxa, Foxl1, Foxm1, and Foxo, as these have been shown to regulate many processes throughout the life of the organ, controlling both formation and function of the liver.

Tumour necrosis factor-alpha attenuates insulin action on phosphoenolpyruvate carboxykinase gene expression and gluconeogenesis by altering the cellular localization of Foxa2 in HepG2 cells

Circulating tumour necrosis factor-alpha (TNFalpha) levels, which are elevated in obesity-associated insulin resistance and diabetes, inhibit insulin signalling at several points in the signalling cascade. The liver is critical in maintaining circulating glucose levels and, in a preliminary investigation using the human hepatoma (HepG2) cell line in this study, we demonstrated the role of TNFalpha in the regulation of this phenomenon and determined the underlying molecular mechanisms. As the transcription factor Foxa2 has been implicated, in part, in the regulation of gluconeogenic genes, we studied the effects of TNFalpha and/or insulin on its cellular status in hepatocytes, followed by an assessment of its occupancy on the phosphoenolpyruvate carboxykinase (PEPCK) promoter. Preincubation of cells with TNFalpha, followed by insulin, significantly prevented insulin-mediated nuclear exclusion of Foxa2 and substantially increased its nuclear concentration. Foxa2 was subsequently found to occupy its binding element on the PEPCK promoter. TNFalpha alone, however, did not alter the status of cellular Foxa2 or its occupancy on the PEPCK promoter. TNFalpha preincubation also significantly attenuated insulin-induced inhibition of the expression of gluconeogenic enzymes and hepatic glucose production. Insulin inhibition of PEPCK expression and the preventive effect of TNFalpha could be partially but significantly restored in the presence of Foxa2 siRNA. Several other well-known mediators of insulin action in the liver in general and of gluconeogenic genes in particular include Foxo1, PGC-1 and SREBP-1c. Our results indicate that another transcription factor, Foxa2, is at least partly responsible for the attenuating effect of TNFalpha on insulin action on PEPCK expression and glucose production in HepG2 cells.

Global analysis of in vivo Foxa2-binding sites in mouse adult liver using massively parallel sequencing

Foxa2 (HNF3β) is a one of three, closely related transcription factors that are critical to the development and function of the mouse liver. We have used chromatin immunoprecipitation and massively parallel Illumina 1G sequencing (ChIP–Seq) to create a genome-wide profile of in vivo Foxa2-binding sites in the adult liver. More than 65% of the ∼11.5 k genomic sites associated with Foxa2 binding, mapped to extended gene regions of annotated genes, while more than 30% of intragenic sites were located within first introns. 20.5% of all sites were further than 50 kb from any annotated gene, suggesting an association with novel gene regions. QPCR analysis demonstrated a strong positive correlation between peak height and fold enrichment for Foxa2-binding sites. We measured the relationship between Foxa2 and liver gene expression by overlapping Foxa2-binding sites with a SAGE transcriptome profile, and found that 43.5% of genes expressed in the liver were also associated with Foxa2 binding. We also identified potential Foxa2-interacting transcription factors whose motifs were enriched near Foxa2-binding sites. Our comprehensive results for in vivo Foxa2-binding sites in the mouse liver will contribute to resolving transcriptional regulatory networks that are important for adult liver function.

Prostate epithelial cell fate

Androgen receptor (AR) within prostatic mesenchymal cells, with the absence of AR in the epithelium, is still sufficient to induce prostate development. AR in the luminal epithelium is required to express the secretory markers associated with differentiation. Nkx3.1 is expressed in the epithelium in early prostatic embryonic development and expression is maintained in the adult. Induction of the mouse prostate gland by the embryonic mesenchymal cells results in the organization of a sparse basal layer below the luminal epithelium with rare neuroendocrine cells that are interdispersed within this basal layer. The human prostate shows similar glandular organization; however, the basal layer is continuous. The strong inductive nature of embryonic prostatic and bladder mesenchymal cells is demonstrated in grafts where embryonic stem (ES) cells are induced to differentiate and organize as a prostate and bladder, respectively. Further, the ES cells can be driven by the correct embryonic mesenchymal cells to form epithelium that differentiates into secretory prostate glands and differentiated bladders that produce uroplakin. This requires the ES cells to mature into endoderm that gives rise to differentiated epithelium. This process is control by transcription factors in both the inductive mesenchymal cells (AR) and the responding epithelium (FoxA1 and Nkx3.1) that allows for organ development and differentiation. In this review, we explore a molecular mechanism where the pattern of transcription factor expression controls cell determination, where the cell is assigned a developmental fate and subsequently cell differentiation, and where the assigned cell now emerges with it's own unique character.

A cryptic frizzled module in cell surface collagen 18 inhibits Wnt/beta-catenin signaling

Collagens contain cryptic polypeptide modules that regulate major cell functions, such as cell proliferation or death. Collagen XVIII (C18) exists as three amino terminal end variants with specific amino terminal polypeptide modules. We investigated the function of the variant 3 of C18 (V3C18) containing a frizzled module (FZC18), which carries structural identity with the extracellular cysteine-rich domain of the frizzled receptors. We show that V3C18 is a cell surface heparan sulfate proteoglycan, its topology being mediated by the FZC18 module. V3C18 mRNA was expressed at low levels in 21 normal adult human tissues. Its expression was up-regulated in fibrogenesis and in small well-differentiated liver tumors, but decreased in advanced human liver cancers. Low FZC18 immunostaining in liver cancer nodules correlated with markers of high Wnt/β−catenin activity. V3C18 (M_r = 170 kD) was proteolytically processed into a cell surface FZC18-containing 50 kD glycoprotein precursor that bound Wnt3a in vitro through FZC18 and suppressed Wnt3a-induced stabilization of β−catenin. Ectopic expression of either FZC18 (35 kD) or its 50 kD precursor inhibited Wnt/β−catenin signaling in colorectal and liver cancer cell lines, thus downregulating major cell cycle checkpoint gatekeepers cyclin D1 and c-myc and reducing tumor cell growth. By contrast, full-length V3C18 was unable to inhibit Wnt signaling. In summary, we identified a cell-surface signaling pathway whereby FZC18 inhibits Wnt/β−catenin signaling. The signal, encrypted within cell-surface C18, is released by enzymatic processing as an active frizzledcysteine-rich domain (CRD) that reduces cancer cell growth. Thus, extracellular matrix controls Wnt signaling through a collagen-embedded CRD behaving as a cell-surface sensor of proteolysis, conveying feedback cues to control cancer cell fate.

Contribution of the amino and carboxyl termini for PHA-4/FoxA function in Caenorhabditis elegans

FoxA transcription factors are central regulators of gut development in all animals that have been studied. Here we examine the sole Caenorhabditis elegans FoxA protein, which is called pha-4. We describe the molecular characterization of five pha-4 mutations and characterize their associated phenotypes. Two nonsense mutations are predicted to truncate PHA-4 after the DNA binding domain and remove the conserved carboxyl terminus. Surprisingly, animals harboring these mutations are viable, provided the mutant mRNAs are stabilized by inactivating the nonsense-mediated decay pathway. Two additional nonsense mutations reveal that the DNA binding domain is critical for activity. A missense mutation predicted to alter the PHA-4 amino terminus leads to a dramatic reduction in pha-4 activity even though the protein is expressed appropriately. We suggest that the PHA-4 amino terminus is essential for PHA-4 function in vivo, possibly as a transactivation domain, and can compensate for loss of the carboxyl terminus. We also provide evidence for autoregulation by PHA-4.

Role of hepatocyte nuclear factor 3γ in the expression of human CYP2C genes

Hepatocyte nuclear factor 3 gamma (HNF-3 gamma) is an important transcription factor for the maintenance of specific liver functions. However, its relevance in the expression of human cytochrome P450 (CYP) genes has not yet been explored. Several HNF3 putative binding sites can be identified in human CYP2C 5'-flanking regions. Gene reporter experiments with proximal promoters revealed that HNF-3 gamma transactivated CYP2C8, CYP2C9, and CYP2C19 (25-, 4-, and 4-fold, respectively), but it did not transactivate CYP2C18. However, overexpression of HNF-3 gamma in hepatoma cells by means of a recombinant adenovirus induced CYP2C9, CYP2C18, and CYP2C19 mRNA (4.5-, 20-, and 50-fold, respectively) but did not activate endogenous CYP2C8. The lack of effect of HNF-3 gamma on endogenous CYP2C8 could be reversed by treating cells with the deacetylase inhibitor, trichostatin A, suggesting the existence of chromatin condensation around functional HNF3 elements in this gene. We conclude that HNF3 gamma is an important transcription factor for the hepatic-specific expression of human CYP2C genes. Our results also evidence that efficient transfection tools, such as adenoviral vectors, may be decisive for assessing the role of transcription factor on chromatin organized genes.

About GATA3, HNF3A, and XBP1, three genes co-expressed with the oestrogen receptor-alpha gene (ESR1) in breast cancer

In breast tumours and breast cancer cell (BCC) lines, microarray analyses have revealed that a series of genes are expressed in close association with the oestrogen receptor-alpha (ER-alpha) gene, ESR1. Three of them, GATA3, HNF3A (also known as FOXA1), and XBP1 encode transcription factors. Here, we present these factors and we discuss their potential involvement in the ER-alpha-mediated actions in BCC. We notably show the relations that exist, or that might exist, between these factors and the oestrogen-inducible trefoil factor TFF1.

The human organic anion transporting polypeptide 8 (SLCO1B3) gene is transcriptionally repressed by hepatocyte nuclear factor 3beta in hepatocellular carcinoma

BACKGROUND/AIMS

The organic anion transporting polypeptides (OATPs) mediate the uptake of numerous amphipathic compounds into hepatocytes. Our aim was to study the expression and regulation of OATP8 (OATP1B3, SLC21A8/SLCO1B3) and OATP-C (OATP1B1, SLC21A6/SLCO1B1) in hepatocellular carcinomas (HCC).

METHODS

RNA and protein levels in 13 paired HCC and adjacent non-tumor liver samples were quantified by real-time polymerase chain reaction or Western blot, respectively. The OATP8 and OATP-C gene promoters were characterized by luciferase reporter assays and electrophoretic mobility shift assays (EMSA).

RESULTS

The expression of OATP8 was decreased in 60% of HCC compared to surrounding non-tumor liver tissue, on both the mRNA and protein levels. Expression of the liver-enriched transcription factor hepatocyte nuclear factor 3beta (HNF3beta) was increased in 70% of HCC and correlated inversely with OATP8 mRNA (r=-0.75, P<0.05) and protein. In contrast to OATP8, expression of OATP-C was not significantly decreased in HCC. In transfected Huh7 cells, OATP8 promoter activity was inhibited by 70% when HNF3beta was cotransfected. An HNF3beta binding site was located at nt -39/-23 by EMSA. The OATP-C promoter was not inhibited by HNF3beta.

CONCLUSIONS

HNF3beta represses transcription of the OATP8 but not the OATP-C gene, providing a mechanism for reduced expression of OATP8 in HCC.

Inhibition of Foxo1 function is associated with improved fasting glycemia in diabetic mice

Excessive hepatic glucose production is a contributing factor to fasting hyperglycemia in diabetes. Insulin suppresses hepatic glucose production by inhibiting the expression of two gluconeogenic enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G-6-Pase). The forkhead transcription factor Foxo1 has been implicated as a mediator of insulin action in regulating hepatic gluconeogenesis, and a Foxo1 mutant (Foxo1-Delta256), devoid of its carboxyl domain, has been shown to interfere with Foxo1 function and inhibit gluconeogenic gene expression in cultured cells. To study the effect of Foxo1-Delta256 on glucose metabolism in animals, the Foxo1-Delta256 cDNA was delivered to the livers of mice by adenovirus-mediated gene transfer. Hepatic Foxo1-Delta256 production resulted in inhibition of gluconeogenic activity, as evidenced by reduced PEPCK and G-6-Pase expression in the liver. Mice treated with the Foxo1-Delta256 vector exhibited significantly reduced blood glucose levels. In contrast, blood glucose levels in control vector-treated animals remained unchanged, which coincided with the lack of alterations in the expression levels of PEPCK and G-6-Pase. When tested in diabetic db/db mice, hepatic production of Foxo1-Delta256 was shown to reduce fasting hyperglycemia. Furthermore, we showed that hepatic Foxo1 expression was deregulated as a result of insulin resistance in diabetic mice and that Foxo1-Delta256 interfered with Foxo1 function via competitive binding to target promoters. These results demonstrated that functional inhibition of Foxo1, caused by hepatic expression of its mutant, is associated with reduced hepatic gluconeogenic activity and improved fasting glycemia in diabetic mice.

The role of hepatocyte nuclear factor-3 alpha (Forkhead Box A1) and androgen receptor in transcriptional regulation of prostatic genes

Androgens and mesenchymal factors are essential extracellular signals for the development as well as the functional activity of the prostate epithelium. Little is known of the intraepithelial determinants that are involved in prostatic differentiation. Here we found that hepatocyte nuclear factor-3 alpha (HNF-3 alpha), an endoderm developmental factor, is essential for androgen receptor (AR)-mediated prostatic gene activation. Two HNF-3 cis-regulatory elements were identified in the rat probasin (PB) gene promoter, each immediately adjacent to an androgen response element. Remarkably, similar organization of HNF-3 and AR binding sites was observed in the prostate-specific antigen (PSA) gene core enhancer, suggesting a common functional mechanism. Mutations that disrupt these HNF-3 motifs significantly abolished the maximal androgen induction of PB and PSA activities. Overexpressing a mutant HNF-3 alpha deleted in the C-terminal region inhibited the androgen-induced promoter activity in LNCaP cells where endogenous HNF-3 alpha is expressed. Chromatin immunoprecipitation revealed in vivo that the occupancy of HNF-3 alpha on PSA enhancer can occur in an androgen-depleted condition, and before the recruitment of ligand-bound AR. A physical interaction of HNF-3 alpha and AR was detected through immunoprecipitation and confirmed by glutathione-S-transferase pull-down. This interaction is directly mediated through the DNA-binding domain/hinge region of AR and the forkhead domain of HNF-3 alpha. In addition, strong HNF-3 alpha expression, but not HNF-3 beta or HNF-3 gamma, is detected in both human and mouse prostatic epithelial cells where markers (PSA and PB) of differentiation are expressed. Taken together, these data support a model in which regulatory cues from the cell lineage and the extracellular environment coordinately establish the prostatic differentiated response.

The mouse alpha-fetoprotein promoter is repressed in HepG2 hepatoma cells by hepatocyte nuclear factor-3 (FOXA)

The mouse alpha-fetoprotein gene is expressed at high levels in the fetal liver and is transcriptionally silenced at birth. The repression is governed, at least in part, by the 250 base pair (bp) AFP promoter. We show here that the AFP promoter is dramatically repressed by HNF3 in HepG2 hepatoma cells. This repression is governed by the region between -205 and -150. Furthermore, this fragment can confer HNF3-mediated repression on a heterologous promoter. The repression is abolished by a mutation that is centered at -165. EMSA analyses using in vivo and in vitro synthesized proteins indicate that HNF3 proteins do not bind DNA from the -205 to -150 region. We propose that HNF3 represses AFP promoter activity through indirect mechanisms that modulate the binding or activity of a liver-enriched factor that interacts with the -165 region of the AFP promoter.

Cytochrome P450 expression in human hepatocytes and hepatoma cell lines: molecular mechanisms that determine lower expression in cultured cells

1. Cultured hepatic cells have reduced cytochrome P450 (CYP) activities in comparison with human liver, but the mechanism(s) that underlies this circumstance is not clear. We investigated the causes of this low CYP activity by analysing the activity, protein, mRNA and heterologous nuclear RNA contents of the most important CYPs involved in drug metabolism (1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5) in cultured human hepatocytes, and in HepG2 and Mz-Hep-1 hepatoma cell lines. 2. After 24 h of culture, hepatocytes retained most of their CYP activities and protein contents, but the mRNA decreased 20-fold. However, the mRNA content of most CYPs in 24-h hepatocytes was still 400-fold higher than in hepatoma cells. When we examined the transcriptional activity of the CYP genes, this decreased during culture time in hepatocytes and it was poor in hepatoma cell lines. 3. We investigated the abundance of key hepatic transcription factors that govern CYP transcription (C/EBP-beta: LAP and LIP, HNF-3alpha, HNF-4alpha, RXR-alpha) and observed that the expression of some factors was altered in the hepatoma cells. 4. In conclusion, the loss of biotransformation activity in cultured hepatic cells is caused by a decrease in CYP transcription, which correlates with an alteration in the expression of key transcription factors.

Foxa2 (HNF3beta ) controls multiple genes implicated in metabolism-secretion coupling of glucose-induced insulin release

The transcription factor Foxa2 is implicated in blood glucose homeostasis. Conditional expression of Foxa2 or its dominant-negative mutant DN-Foxa2 in INS-1 cells reveals that Foxa2 regulates the expression of genes important for glucose sensing in pancreatic beta-cells. Overexpression of Foxa2 results in blunted glucose-stimulated insulin secretion, whereas induction of DN-Foxa2 causes a left shift of glucose-induced insulin release. The mRNA levels of GLUT2 and glucokinase are drastically decreased after induction of Foxa2. In contrast, loss of Foxa2 function leads to up-regulation of hexokinase (HK) I and II and glucokinase (HK-IV) mRNA expression. The glucokinase and the low K(m) hexokinase activities as well as glycolysis are increased proportionally. In addition, induction of DN-Foxa2 also reduces the expression of beta-cell K(ATP) channel subunits Sur1 and Kir6.2 by 70%. Furthermore, in contrast to previous reports, induction of Foxa2 causes pronounced decreases in the HNF4alpha and HNF1alpha mRNA levels. Foxa2 fails to regulate the expression of Pdx1 transcripts. The expression of insulin and islet amyloid polypeptide is markedly suppressed after induction of Foxa2, while the glucagon mRNA levels are significantly increased. Conversely, Foxa2 is required for glucagon expression in these INS-1-derived cells. These results suggest that Foxa2 is a vital transcription factor evolved to control the expression of genes essential for maintaining beta-cell glucose sensing and glucose homeostasis.

Cell-specific involvement of HNF-1beta in alpha(1)-antitrypsin gene expression in human respiratory epithelial cells

The synergistic action of hepatocyte nuclear factor (HNF)-1alpha and HNF-4 plays an important role in expression of the alpha(1)-antitrypsin (alpha(1)-AT) gene in human hepatic and intestinal epithelial cells. Recent studies have indicated that the alpha(1)-AT gene is also expressed in human pulmonary alveolar epithelial cells, a potentially important local site of the lung antiprotease defense. In this study, we examined the possibility that alpha(1)-AT gene expression in a human pulmonary epithelial cell line H441 was also directed by the synergistic action of HNF-1alpha and HNF-4 and/or by the action of HNF-3, which has been shown to play a dominant role in gene expression in H441 cells. The results show that alpha(1)-AT gene expression in H441 cells is predominantly driven by HNF-1beta, even though HNF-1beta has no effect on alpha(1)-AT gene expression in human hepatic Hep G2 and human intestinal epithelial Caco-2 cell lines. Expression of alpha(1)-AT and HNF-1beta was also demonstrated in primary cultures of human respiratory epithelial cells. HNF-4 has no effect on alpha(1)-AT gene expression in H441 cells, even when it is cotransfected with HNF-1beta or HNF-1alpha. HNF-3 by itself has little effect on alpha(1)-AT gene expression in H441, Hep G2, or Caco-2 cells but tends to have an upregulating effect when cotransfected with HNF-1 in Hep G2 and Caco-2 cells. These results indicate the unique involvement of HNF-1beta in alpha(1)-AT gene expression in a cell line and primary cultures derived from human respiratory epithelium.

Hepatic nuclear factor-3 (HNF-3 or Foxa2) regulates glucagon gene transcription by binding to the G1 and G2 promoter elements

Glucagon gene expression in the endocrine pancreas is controlled by three islet-specific elements (G3, G2, and G4) and the alpha-cell-specific element G1. Two proteins interacting with G1 have previously been identified as Pax6 and Cdx2/3. We identify here the third yet uncharacterized complex on G1 as hepatocyte nuclear factor 3 (HNF-3)beta, a member of the HNF-3/forkhead transcription family, which plays an important role in the development of endoderm-related organs. HNF-3 has been previously demonstrated to interact with the G2 element and to be crucial for glucagon gene expression; we thus define a second binding site for this transcription on the glucagon gene promoter. We demonstrate that both HNF-3alpha and -beta produced in heterologous cells can interact with similar affinities to either the G1 or G2 element. Pax6, which binds to an overlapping site on G1, exhibited a greater affinity as compared with HNF-3alpha or -beta. We show that both HNF-3beta and -alpha can transactivate glucagon gene transcription through the G2 and G1 elements. However, HNF-3 via its transactivating domains specifically impaired Pax6-mediated transactivation of the glucagon promoter but had no effect on transactivation by Cdx2/3. We suggest that HNF-3 may play a dual role on glucagon gene transcription by 1) inhibiting the transactivation potential of Pax6 on the G1 and G3 elements and 2) direct activation through G1 and G2.

Mutation analysis of left-right axis determining genes in NOD and ICR, strains susceptible to maternal diabetes

BACKGROUND

Genetic background of the fetus contributes to the pathogenesis of congenital malformation after teratogen exposure. Such contribution is illustrated in left-right axis malformations observed in the F1 offspring of nonobese diabetic (NOD) mouse dams and sires from different strains. When sires of the NOD, ICR, or C57BL/6J were mated with NOD dams, incidence varied depending on the fetal genotype, with 65% in NOD x NOD, 24% in NOD x ICR, and 7% in NOD x C57BL/6J.

METHODS

As a first step in elucidating the molecular basis of the interstrain differences in susceptibility to situs defects, we compared genomic sequences of six genes HNF3beta, Acvr2b, Nodal, ZIC3, Lefty1, and Smad2, which are involved in the normal development of left-right axis among NOD, ICR, and C57BL/6J strains.

RESULTS

The outbred strain ICR had 1) a 0.2-kb insertion in the putative promoter region of the isoform E of HNF3beta together with a G to A change that could create a potential splice acceptor in the exon 3 of HNF3beta (gene frequency P = 0.36), 2) five single base substitutions within the 5' controlling element and a proline to serine substitution (P2S) of Lefty1 (P = 0.77), and 3) a tyrosine to histidine substitution within the prodomain of Nodal (P = 0.48). The inbred strain NOD had the same G to A change as ICR and a three-base deletion in the putative promoter of isoform E of HNF3beta.

CONCLUSIONS

We suggest that sequence variations in HNF3beta, Lefty1, and Nodal might account, in part, for the interstrain differences in susceptibility to situs abnormalities among the offspring of diabetic dams.

The promoter of the long variant of collagen XVIII, the precursor of endostatin, contains liver-specific regulatory elements

The endostatin precursor collagen XVIII is expressed at high levels in human livers, the main source being hepatocytes. We have studied the regulatory elements in the promoter 2 of the Col18a1 gene that directs the transcription of the NC1-517 variant of collagen alpha1(XVIII), which is the main form expressed in the liver. The 5'-flanking region of Col18a1 gene was cloned, and a series of 5'-deletions from -3286 bp to +285 bp were linked to the luciferase reporter gene. Transfection experiments in HepG2 cells allowed to identify a silencer-like element containing putative HNF1 and HNF3 sites and activator elements containing stretches of GC-rich sequences. Another putative HNF3 site in close apposition to a NF1/CTF site was localized upstream of the silencer-like element. Cotransfection experiments showed that the Col18a1 promoter 2 was transactivated by Sp1 and HNF3alpha. Gel-shift analyses showed that HNF3, NF1/CTF, and Sp1-like sites specifically recognized nuclear factors. Super-shift experiments indicated that HNF3beta was the major form of HNF3 interacting with the HNF3/NF1 site. The well-differentiated hepatoma cell line mhATFS315 transfected with a truncated form of HNF3beta, which competitively blocks HNF3 transactivating activity, expressed the Col18a1gene at a very low level. Taken together, these data strongly suggest that Col18a1 is a liver-specific gene. Furthermore, gel-shift analyses performed with nuclear factors prepared from well-differentiated hepatocellular carcinomas showed increased HNF3/NF1 binding activity compared with normal livers. Consequently, the precursor of endostatin might be differently expressed according to the differentiated and/or transformed state of hepatocytes.

Elevated levels of hepatocyte nuclear factor 3beta in mouse hepatocytes influence expression of genes involved in bile acid and glucose homeostasis

The winged helix transcription factor, hepatocyte nuclear factor-3beta (HNF-3beta), mediates the hepatocyte-specific transcription of numerous genes important for liver function. However, the in vivo role of HNF-3beta in regulating these genes remains unknown because homozygous null HNF3beta mouse embryos die in utero prior to liver formation. In order to examine the regulatory function of HNF-3beta, we created transgenic mice in which the -3-kb transthyretin promoter functions to increase hepatocyte expression of the rat HNF-3beta protein. Postnatal transgenic mice exhibit growth retardation, depletion of hepatocyte glycogen storage, and elevated levels of bile acids in serum. The retarded growth phenotype is likely due to a 20-fold increase in hepatic expression of insulin-like growth factor binding protein 1 (IGFBP-1), which results in elevated levels in serum of IGFBP-1 and limits the biological availability of IGFs required for postnatal growth. The defects in glycogen storage and serum bile acids coincide with diminished postnatal expression of hepatocyte genes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase and glycogen synthase) and sinusoidal bile acid uptake (Ntcp), respectively. These changes in gene transcription may result from the disruptive effect of HNF-3beta on the hepatic expression of the endogenous mouse HNF-3alpha,-3beta, -3gamma, and -6 transcription factors. Furthermore, adult transgenic livers lack expression of the canalicular phospholipid transporter, mdr2, which is consistent with ultrastructure evidence of damage to transgenic hepatocytes and bile canaliculi. These transgenic studies represent the first in vivo demonstration that the HNF-3beta transcriptional network regulates expression of hepatocyte-specific genes required for bile acid and glucose homeostasis, as well as postnatal growth.

Transducin-like enhancer of split proteins, the human homologs of Drosophila groucho, interact with hepatic nuclear factor 3beta

Members of the hepatic nuclear factor 3 (HNF3) family, including HNF3alpha, HNF3beta, and HNF3gamma, play important roles in embryonic development, the establishment of tissue-specific gene expression, and the regulation of gene expression in differentiated tissues. We found, using the glutathione S-transferase pull-down method, that the transducin-like Enhancer of split (TLE) proteins, which are the human homologs of Drosophila Groucho, directly associate with HNF3beta. Conserved region II of HNF3beta (amino acids 361-388) is responsible for the interaction with TLE1. A mammalian two-hybrid assay was used to confirm that this interaction occurs in vivo. Overexpression of TLE1 in HepG2 and HeLa cells decreases transactivation mediated through the C-terminal domain of HNF3beta, and Grg5, a naturally occurring dominant negative form of Groucho/TLE, also increases the transcriptional activity of this region of HNF3. These results lead us to suggest that TLE proteins could influence the expression of mammalian genes regulated by HNF3.

The molecular physiology of hepatic nuclear factor 3 in the regulation of gluconeogenesis

Glucocorticoids stimulate gluconeogenesis by increasing the rate of transcription of genes that encode gluconeogenic enzymes such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase. Previous studies have shown that hepatic nuclear factor 3 (HNF3) is required as an accessory factor for several glucocorticoid-stimulated genes, including PEPCK. Here, we show that adenovirus-mediated expression of an HNF3beta protein with a deleted C-terminal transactivation domain (HNF3betaDeltaC) reduces the glucocorticoid-induced expression of the PEPCK and glucose-6-phosphatase genes in H4IIE hepatoma cells. Furthermore, expression of this truncated HNF3 protein results in a proportionate reduction of glucocorticoid-stimulated glucose production from lactate and pyruvate in these cells. The expression of HNF3betaDeltaN, in which the N-terminal transactivation domain is deleted, does not exhibit any of these effects. These results provide direct evidence that members of the HNF3 family are required for proper regulation of hepatic gluconeogenesis. Modulation of the function of the HNF3 family of proteins might be used to reduce the excessive hepatic production of glucose that is an important pathophysiologic feature of diabetes mellitus.

Hepatocyte nuclear factor 3 relieves chromatin-mediated repression of the alpha-fetoprotein gene

The alpha-fetoprotein gene (AFP) is tightly regulated at the tissue-specific level, with expression confined to endoderm-derived cells. We have reconstituted AFP transcription on chromatin-assembled DNA templates in vitro. Our studies show that chromatin assembly is essential for hepatic-specific expression of the AFP gene. While nucleosome-free AFP DNA is robustly transcribed in vitro by both cervical (HeLa) and hepatocellular (HepG2) carcinoma extracts, the general transcription factors and transactivators present in HeLa extract cannot relieve chromatin-mediated repression of AFP. In contrast, preincubation with either HepG2 extract or HeLa extract supplemented with recombinant hepatocyte nuclear factor 3 alpha (HNF3alpha), a hepatic-enriched factor expressed very early during liver development, is sufficient to confer transcriptional activation on a chromatin-repressed AFP template. Transient transfection studies illustrate that HNF3alpha can activate AFP expression in a non-liver cellular environment, confirming a pivotal role for HNF3alpha in establishing hepatic-specific gene expression. Restriction enzyme accessibility assays reveal that HNF3alpha promotes the assembly of an open chromatin structure at the AFP promoter. Combined, these functional and structural data suggest that chromatin assembly establishes a barrier to block inappropriate expression of AFP in non-hepatic tissues and that tissue-specific factors, such as HNF3alpha, are required to alleviate the chromatin-mediated repression.

Biochemical and functional properties of a palindromic sequence motif within the hepatitis B virus enhancer 1

The hepatitis B virus (HBV) enhancer 1 is a transcriptional element that contributes to the liver-specific regulation of HBV gene expression. We previously identified a novel protein binding site within the enhancer that contains an 8-bp palindromic sequence motif. This motif partially overlaps the binding sites for nuclear factor 1 and hepatocyte nuclear factor 3beta (HNF3beta). Moreover, we demonstrated that this novel site is recognized by a protein or proteins, tentatively designated as palindrome-binding factor (PBF), that cooperatively interact with HNF3beta. In the present work, we have further examined the biochemical and functional attributes of PBF. Protein-DNA interaction studies indicate that three thymidine residues located at the 3'-end of the palindromic sequence motif are important for maximal PBF-binding activity. When protein-DNA complexes were photocrosslinked by exposure to ultraviolet (UV) light, a prominent polypeptide with an apparent molecular mass of 50 kDa was found to associate with the PBF-binding site. Furthermore, transient transfection studies support the hypothesis that PBF contributes to enhancer 1 activity by a combinatorial mechanism that involves at least one other cis-acting sequence motif, the HNF3beta-binding site.

The chum salmon IGF-II gene promoter is activated by hepatocyte nuclear factor 3beta

IGF-II plays an important role in growth and development of vertebrates and is highly expressed in adult salmon liver. In the present study, we demonstrate that a liver-enriched transcription factor, hepatocyte nuclear factor 3beta (HNF-3beta), is an activator of the chum salmon IGF-II gene. Multiple binding sites for HNF-3beta were identified within the 5'-UTR using electrophoretic mobility shift assays and mutation of these sites prevents binding of HNF-3beta. In transient transfection assays it was shown that mutation of the HNF-3beta binding sites results in a substantial decrease of HNF-3beta-activated salmon IGF-II gene expression. This is the first identified transcription factor that is functionally involved in the regulation of fish IGF-II expression.

Growth and differentiation of cultured fetal hepatocytes isolated various developmental stages

We examined the relationship between cell proliferation and differentiation of cultured rat fetal and newborn hepatocytes isolated from various developmental stages. The albumin production rate increased along with cell growth under in vitro culture and became maximal two days after the growth cessation. AFP was secreted by both fetal and newborn hepatocytes with growth ability. Furthermore, the responses to HGF addition in fetal hepatocyte cultures were observed in terms of growth stimulation and down-regulated of the Met receptor. We also studied the changes in RB and liver enriched transcription factors (C/EBPs) for investigating the mechanism underlying proliferation and differentiation of fetal hepatocytes. Western blot analysis of hepatocytes taken from various gestation stages of rat liver showed that the expression of RB and C/EBP beta increased as gestation stage proceeded. When RB antisense S-oligonucleotide was added to the culture medium, proliferation and AFP expression increased, while C/EBP alpha and albumin expressions decreased. These results indicated that the tumor suppressor gene product RB had a profound role not only in cell proliferation but also hepatocyte differentiation.

In situ hybridization with 33P-labeled RNA probes for determination of cellular expression patterns of liver transcription factors in mouse embryos

Murine hepatocyte nuclear factor-3beta (HNF-3beta) protein is a member of a large family of developmentally regulated transcription factors that share homology in the winged helix/fork head DNA binding domain and that participate in embryonic pattern formation. HNF-3beta also mediates cell-specific transcription of genes important for the function of hepatocytes, intestinal and bronchiolar epithelium, and pancreatic acinar cells. We have previously identified a hepatocyte and pancreatic cut-homeodomain transcription factor, HNF-6, which is required for HNF-3beta promoter activity. In this study, we used in situ hybridization studies of stage-specific embryos to demonstrate that HNF-6 and its target gene, HNF-3beta, are coexpressed in the foregut endoderm and in the pancreatic and hepatic diverticulum. More detailed analysis of HNF-6 and HNF-3beta's developmental expression patterns provides evidence of colocalization in hepatocytes, intestinal epithelium, and pancreatic ductal epithelium and exocrine acinar cells. In support of the role of HNF-6 in regulating HNF-3beta expression in developing hepatocytes, their liver expression levels are both transiently reduced between 14 and 15 days of gestation. At day 18 of gestation and in adult pancreas, HNF-6 and HNF-3beta transcripts remain colocalized in the exocrine acinar cells, but their expression patterns diverge in endocrine cells. HNF-3beta expression is restricted to the endocrine cells of the islets of Langerhans, whereas the ductal epithelium expresses HNF-6. We discuss these expression patterns with respect to specification of hepatocytes and differentiation of the endocrine and exocrine pancreas.

Expression of hepatocyte nuclear factor-3alpha in rat prostate, seminal vesicle, and bladder

Hepatocyte nuclear factor-3alpha (HNF-3alpha), a member of the hepatocyte-forkhead-homolog family of transcription factors, regulates gene expression in the endoderm-derived liver and lung. To determine if HNF-3alpha might also play a role in endodermal derivatives of the urogenital sinus, the expression of HNF-3alpha in male accessory sex organs was assessed by Northern blotting, in situ hybridization, and electrophoretic mobility shift analysis. RNA from the dorsolateral prostate (DP), ventral prostate (VP), anterior prostate (AP), seminal vesicle (SV), and bladder was compared with RNA from the liver and spleen as positive and negative controls, respectively. HNF-3alpha mRNA levels in the DP, VP, AP, and bladder were 20, 14, 5, and 6 times higher than the SV equivalent in the liver. HNF-3alpha mRNA was detected in 8 of 10 prostate epithelial cell lines (rat NRP 152 and 154, mouse Pr14, and human DU-145, PC3, LNCaP, ND-1, and BPH-1) but not in rat Dunning epithelial or mouse Pr12 cells. Addition of testosterone to castrated rats was found to prevent a drastic loss of HNF-3alpha mRNA in the VP. This result suggests that HNF-3alpha mRNA levels are at least indirectly regulated by testosterone. The HNF-3alpha mRNA is expressed in epithelial cells of the urogenital sinus derivatives VP, AP, DP, and bladder and Wolffian duct derivative, the SV. To confirm that functional HNF-3alpha protein is produced in the VP, electrophoretic mobility shift assays were performed with whole-cell extracts and high-affinity oligonucleotide (TTR-S) from the transthyretin promoter. Binding to TTR-S was disrupted when the extract was incubated with HNF-3alpha, but not with HNF-3beta, antibody. Taken together, the results using VP, AP, DP, SV, and bladder suggest that HNF-3alpha may play an important role in development and maintenance of urogenital tract epithelial cells.

Characterization of rat liver-specific methionine adenosyltransferase gene promoter. Role of distal upstream cis-acting elements in the regulation of the transcriptional activity

Methionine adenosyltransferase is a ubiquitous enzyme that catalyzes the only known route of biosynthesis of S-adenosylmethionine, the major methyl group donor in cell metabolism. In mammals, two different methionine adenosyltransferases exist: one is confined to the liver, and the other one is distributed in extrahepatic tissues. In the present study, we report the cloning of the 5'-flanking region of liver-specific methionine adenosyltransferase gene from rat. Two closely spaced sites for transcriptional initiation were identified by primer extension analysis. The major transcription start site was determined to be 29 nucleotides downstream from the putative TATA box. Transient transfection assays of constructs containing sequentially deleted 5'-flanking sequences fused to the luciferase gene showed that rat hepatic methionine adenosyltransferase promoter was able to efficiently drive reporter expression not only in liver-type cells (rat hepatoma H35 cells and human hepatoblastoma HepG2 cells) but also in Chinese hamster ovary cells. Two regions spanning nucleotides -1251 to -958 and -197 to +65 were found to be crucial for the promoter efficiency. The distal upstream region contains elements that positively regulate promoter activity in H35 and HepG2 cells but are ineffective in Chinese hamster ovary cells. Eight protein binding sites were characterized in both regions by DNase I footprinting analysis. Two of these elements, sites A and B, located in the distal region, were found to be essential for the regulation of promoter activity. Electrophoretic mobility shift assays and competition experiments showed that site A is recognized by an NF1 protein. Site B was able to interact with a member of HNF-3 family when nuclear extracts from rat liver and H35 cells were used in the in vitro assay, but an additional binding activity to an NHF1-like protein was obtained with the hepatoma cell extracts. It is suggested that this differential binding can contribute to the cell specificity of promoter function.

A new gene encoding a putative transcription factor regulated by the Drosophila circadian clock

Circadian rhythms of locomotor activity and eclosion in Drosophila depend upon the reciprocal autoregulation of the period (per) and timeless (tim) genes. As part of this regulatory loop, per and tim mRNA levels oscillate in a circadian fashion. Other cycling transcripts may participate in this central pacemaker mechanism or represent outputs of the clock. In this paper, we report the isolation of Crg-1, a new circadianly regulated gene. Like per and tim transcript levels, Crg-1 transcript levels oscillate with a 24 h period in light:dark (LD) conditions, with a maximal abundance at the beginning of the night. These oscillations persist in complete darkness and depend upon per and tim proteins. The putative CRG-1 proteins show some sequence similarity with the DNA-binding domain of the HNF3/fork head family of transcription factors. In the adult head, in situ hybridization analysis reveals that per and Crg-1 have similar expression patterns in the eyes and optic lobes.

The hepatocyte nuclear factor 3beta stimulates the transcription of the human insulin-like growth factor I gene in a direct and indirect manner

Promoter 1 (P1) of the human insulin-like growth factor I (IGF-I) gene is most active in adult liver. In this study we show that HNF-3beta, a member of the winged helix protein family of liver-enriched transcription factors, has a strong stimulatory effect on the activity of P1. Transient transfection experiments in combination with bandshift and DNase I footprinting analysis revealed the presence of two HNF-3 binding sites in the proximal promoter region of P1. Both binding sites, which are well conserved in evolution, are required for maximal transactivation. Studies employing HNF-3 mutant constructs indicated that IGF-I expression is also regulated indirectly by HNF-3beta as a consequence of enhanced expression of HNF-1alpha. This liver-enriched transcription factor has previously been shown to transactivate P1. Thus, HNF-3beta regulates the expression of the human IGF-I gene via two distinct mechanisms.

The transcriptional activator hepatocyte nuclear factor 6 regulates liver gene expression

The hepatocyte nuclear factor 3(alpha) (HNF-3(alpha)), -3(beta), and -3(gamma) proteins share homology in the winged-helix/fork head DNA binding domain and mediate hepatocyte-enriched transcription of numerous genes whose expression is necessary for organ function. In this work, we identify a liver-enriched transcription factor, HNF-6, which recognizes the -138 to -126 region of the HNF-3(beta) promoter and binds the original HNF-3 site of the transthyretin promoter (-94 to -106). We show that HNF-6 and HNF-3 possess different DNA binding specificities by competition and methylation interference studies and are immunologically distinct. Site-directed mutagenesis of the HNF-6 sites in the HNF-3(beta) and transthyretin promoters diminishes reporter gene expression, suggesting that HNF-6 activates transcription of these promoters. Using the HNF-6 binding sequence DHWATTGAYTWWD (where W = A or T, Y = T or C, H is not G, and D is not C) determined by sequence comparison and methylation interference, we predicted that HNF-6 will bind to 22 additional hepatocyte-enriched genes. Of these potential target genes, we selected seven of the HNF-6 binding sequences and demonstrated that they bind the HNF-6 protein. These include promoter sequences from alpha-2 urinary globulin, alpha-1 antitrypsin, cytochrome P-450 2C13, L-type 6-phosphofructo-2-kinase, mouse major urinary protein, tryptophan oxygenase, and alpha-fetoprotein genes. HNF-6 binding activity was also found in the intestinal epithelial cell line HT29, and potential HNF-6 binding sites were present in intestinal sucrase isomaltase, cdx-2 homeodomain protein, and intestinal fatty acid binding protein promoter regions. These studies suggest that HNF-6 may regulate hepatocyte-specific genes and may play a role in epithelial cell differentiation of gut endoderm via regulation of HNF-3(beta).

Purification and properties of rat liver nuclear proteins that interact with the hepatitis B virus enhancer 1

The hepatitis B virus enhancer 1 element plays a fundamental role in the liver-specific regulation of hepatitis B virus gene expression. A central region of enhancer 1, the enhancer core domain, contains at least four cis-acting sequence motifs that are essential for enhancer 1 activity. In this study, we have investigated an essential motif within the core domain previously defined as footprint V (FPV). Transient transfection analyses demonstrate that FPV is capable of independently functioning in a liver-specific manner to activate transcription. Therefore, to further examine the liver-specific properties of FPV-mediated enhancer 1 activity, we have carried out the biochemical purification and characterization of FPV binding activity from rat liver nuclei. This study has conclusively identified hepatocyte nuclear factor 3beta (HNF-3beta), a liver-enriched member of the HNF-3/forkhead gene family, as the predominant purified protein that interacts with the FPV motif. Moreover, a cellular protein(s) that copurified with HNF-3beta specifically interacts with a novel sequence motif that partially overlaps FPV. Since this novel motif contains a palindromic sequence, we have tentatively referred to the protein(s) that binds to this site as palindrome-binding factor (PBF). Additional evidence indicates that HNF-3beta and PBF cooperatively interact with enhancer 1. Therefore, this study supports the hypothesis that FPV-mediated enhancer activity involves a cooperative interplay between HNF-3beta and at least one other enhancer 1-binding protein, PBF.