Cloning and characterization of a novel human hepatocyte transcription factor, hB1F, which binds and activates enhancer II of hepatitis B virus

Characterization of nonconventional hepatitis B viruses lacking the core promoter

The core gene (C-gene) promoter and regulatory sequences play a central role in the hepatitis B virus (HBV) life cycle. They are essential for the synthesis of the pregenomic and precore mRNA. The pregenomic RNA is the template required for replication and also the template for the synthesis of the core protein and polymerase. Here, we report the in vivo existence and functional characterization of HBV variants that lack the C-gene promoter region and the regulatory sequences located therein. HBV promoter fragments were isolated by PCR from sera of chronic carriers and characterized. Truncated promoter elements were identified, and then tested in the context of wild-type genomes in the HuH-7 cell line. The expression of the recombinant HBV genome resulted in the synthesis of surface proteins, and low level of core protein as well as a transcript pattern similar to, but smaller in size to wild-type virus. The recombinant HBV genome with the truncated promoter region produced pregenomic RNA-like transcripts. These transcripts were encapsidated and reverse transcribed when complemented by sufficient core and polymerase protein. These date provide an explanation as to why such deletion mutants of HBV can be produced at all, they highlight the functional potentials of viral sequences activated by mutations and may be of relevance for viral evolution and persistence.

Pathogenesis of hepatitis B virus-related hepatocellular carcinoma: old and new paradigms

Chronic infection with the hepatitis B virus (HBV) is a major risk factor for development of hepatocellular carcinoma (HCC). The pathogenesis of cancer in HBV infection has been extensively analyzed, and multiple factors appear to play a role. A major factor is chronic inflammation and the effects of cytokines in the development of fibrosis and liver cell proliferation. Also important is the role of integration of HBV DNA into host cellular DNA, which, in some situations, acts to disrupt or promote expression of cellular genes that are important in cell growth and differentiation. In addition, expression of HBV proteins may have a direct effect on cellular functions, and some of these gene products can favor malignant transformation. Several HBV genes have been found in infected tissues more frequently than others, including truncated pre-S2/S, hepatitis B X gene, and a novel spliced transcript of HBV, referred to as the hepatitis B spliced protein. The proteins expressed from these integrated genes have been shown to have intracellular activities that may account for their association with HCC, including effects on cellular growth and apoptosis. Finally, some patients with HCC have no detectable hepatitis B surface antigen in serum but do have low levels of HBV DNA in serum and integrated molecules of HBV DNA in tissue. Occult HBV infection may account for a proportion of cases of HCC that occur in patients without serologic markers for hepatitis B and C and may be a cofactor in HCC in patients with chronic hepatitis C who have coexistent occult HBV infection.

Gene expression profile in liver of hB1F transgenic mice

AIM: To analyze the tissue morphologic phenotype and liver gene expression profile of hB1F transgenic mice.

METHODS: Transgene expression was analyzed with RT-PCR and Western blotting. For one of the transgenic mouse lines, tissue expression pattern of the transgene was also examined with immunochemical methods. Pathological analysis was used to examine the tissue morphologic phenotype of established transgenic mice. The liver gene expression profile of transgenic mice was analyzed with microchip, and some of the differentially expressed genes were verified with RT-PCR.

RESULTS: The expressions of hB1F were shown in livers from 6 of 7 transgenic mouse lines. The overexpression of hB1F transgene did not cause pathological changes. Expressions of three genes were up-regulated, while down-regulation was observed for 25 genes.

CONCLUSION: The overexpression of hB1F transgene may cause changes of gene expression profiles in the liver of transgenic mice.

Prospero-Related Homeobox (Prox1) Is a Corepressor of Human Liver Receptor Homolog-1 and Suppresses the Transcription of the Cholesterol 7-α-Hydroxylase Gene

Cholesterol 7-α-hydroxylase (CYP7A1) catalyzes a rate-limiting step in bile acid synthesis in liver, and its gene transcription is under complex regulation by multiple nuclear receptors in response to bile acids, cholesterol derivatives, and hormones. The liver receptor homolog-1 (LRH-1), a member of the fushi tarazu factor 1 subfamily of nuclear receptors, has emerged as an essential regulator for the expression of cyp7a1. In this report, we demonstrate Prox1, a prospero-related homeobox transcription factor, identified through a yeast two-hybrid screening, can directly interact with human LRH-1 (hLRH-1) and suppresses hLRH-1-mediated transcriptional activation of human cyp7a1 gene. Biochemical analysis demonstrates that Prox1 interacts with both the ligand binding domain (LBD) and the DNA binding domain (DBD) of hLRH-1. An LRKLL motif in Prox1 is important for the interaction with the LBD but not the DBD of hLRH-1. In hLRH-1 LBD, helices 2 and 10 are essential for Prox1 recruitment. The suppression by Prox1 on the transcriptional activity of hLRH-1 can be mediated through its interaction with the LBD or the DBD of hLRH-1. Gel shift assays reveal that Prox1 impairs the binding of hLRH-1 to the promoter of human cyp7a1 gene.

Differential expression of steroidogenic factor-1/adrenal 4 binding protein and liver receptor homolog-1 (LRH-1)/fetoprotein transcription factor in the rat testis: LRH-1 as a potential regulator of testicular aromatase expression

Aromatase converts testicular androgens to estrogens, which are essential for male fertility. Aromatase expression in testis occurs via transcription from promoter II, and requires the presence of a nuclear receptor half-site that binds the orphan receptor steroidogenic factor-1 [SF-1 (nuclear receptor 5A1)] to mediate basal and (in part) cAMP-induced transcription. We hypothesized that liver receptor homolog-1 (LRH-1) (nuclear receptor 5A2), a receptor closely related to SF-1, could also play a role in regulating aromatase expression in the testis. We demonstrate expression of LRH-1 in adult rat and immature mouse Leydig cells (LHR-1 > SF-1) as well as in pachytene spermatocytes and round spermatids but not in Sertoli cells, which in contrast, express high levels of SF-1. In transient transfection assays using TM3 Leydig cells and TM4 Sertoli cells, a rat promoter II luciferase reporter construct was stimulated by cotransfection of LRH-1 expression vector. Mutation analysis showed that induction by LRH-1 in TM3 and TM4 cells requires an AGGTCA motif at position -90, to which LRH-1 bound in gel shift analysis. We therefore provide evidence that LRH-1 plays an important role in the regulation of aromatase expression in Leydig cells. The colocalization of LRH-1 and aromatase to multiple testis cell types suggests that LRH-1 may have important effects on estrogen production, testis development, spermatogenesis, and testicular carcinogenesis.

LRH-1/hB1F and HNF1 synergistically up-regulate hepatitis B virus gene transcription and DNA replication

Enhancer II (ENII) is one of the critical cis-elements in the Hepatitis B Virus (HBV) genome for the hepatic viral gene transcription and DNA replication. The liver-specific activity of ENII is regulated by multiple liver-enriched transcription factors, including LRH-1/hB1F, HNF1, HNF3b, HNF4 and C/EBP. Knowledge on the interplay of these important factors is still limited. In this study, we demonstrate a functional synergism between the orphan nuclear receptor LRH-1/hB1F and the homeoprotein HNF1 in up-regulating the liver-specific activity of ENII. This synergism is sufficient for initiating the viral gene transcription and DNA replication in non-hepatic cells. We have defined the activation domains in hB1F and HNF1 that contribute to the synergism. We further show that hB1F and HNF1 can interact directly in vitro and have mapped the domains required for this interaction.

The role of the orphan nuclear receptor, liver receptor homologue-1, in the regulation of human corpus luteum 3beta-hydroxysteroid dehydrogenase type II

After ovulation, ovarian 3beta-hydroxysteroid dehydrogenase type II (HSD3B2) expression increases to enhance the shift of steroidogenesis toward progesterone biosynthesis. Steroidogenic factor-1 (SF-1) is a transcription factor for several genes encoding steroidogenic enzymes. However, the level of SF-1 expression decreases in the human corpus luteum (CL) after ovulation. Liver receptor homolog-1 (LRH-1) is another member of the orphan nuclear receptor family. We hypothesize that LRH-1, rather than SF-1, plays an essential role in the regulation of corpus luteum steroidogenesis. Semiquantitative RT-PCR and real-time PCR were performed to quantify the level of LRH-1 expression and correlate with HSD3B2 level. Cell transfection, mutation analysis, and EMSA were performed to examine the role of LRH-1 in the regulation of HSD3B2. LRH-1 expression was higher in CL, compared with mature ovarian follicles. Cotransfection of granulosa cells with HSD3B2 and LRH-1 resulted in a 10-fold increase of transcription. DAX-1 inhibited LRH-1-stimulated HSD3B2, which was maintained in the presence of dibutyryl cAMP. Mutation of the either of the two putative LRH-1 binding sites, which were confirmed by EMSA, in the HSD3B2 promoter decreased LRH-1 stimulation. Our findings suggest that LRH-1 is highly expressed in CL, and it plays an essential role in the regulation of HSD3B2.

Pancreatic-duodenal homeobox 1 regulates expression of liver receptor homolog 1 during pancreas development

Liver receptor homolog 1 (LRH-1) and pancreatic-duodenal homeobox 1 (PDX-1) are coexpressed in the pancreas during mouse embryonic development. Analysis of the regulatory region of the human LRH-1 gene demonstrated the presence of three functional binding sites for PDX-1. Electrophoretic mobility shift assays and chromatin immunoprecipitation analysis showed that PDX-1 bound to the LRH-1 promoter, both in cultured cells in vitro and during pancreatic development in vivo. Retroviral expression of PDX-1 in pancreatic cells induced the transcription of LRH-1, whereas reduced PDX-1 levels by RNA interference attenuated its expression. Consistent with direct regulation of LRH-1 expression by PDX-1, PDX-1(-/-) mice expressed smaller amounts of LRH-1 mRNA in the embryonic pancreas. Taken together, our data indicate that PDX-1 controls LRH-1 expression and identify LRH-1 as a novel downstream target in the PDX-1 regulatory cascade governing pancreatic development, differentiation, and function.

Liver receptor homolog 1 controls the expression of carboxyl ester lipase

The orphan nuclear receptor liver receptor homolog 1 (LRH-1) plays a central role in cholesterol homeostasis by regulating a number of hepatic and intestinal genes critical for reverse cholesterol transport and bile acid homeostasis. Herein, we describe the identification of carboxyl ester lipase (CEL) as a novel target of LRH-1 in pancreas, a tissue in which LRH-1 is abundantly expressed. In situ hybridization and gene expression studies demonstrate that both LRH-1 and CEL are co-expressed and confined to the exocrine pancreas. LRH-1 interacts with a consensus LRH-1 response element in the human CEL promoter, which is perfectly conserved in the rat gene, and induces CEL promoter activity in cotransfection assays. As reported for other LRH-1 target genes, the nuclear receptor short heterodimer partner represses LRH-1-induced CEL promoter activity. Chromatin immunoprecipitation demonstrates that binding of LRH-1 to the CEL promoter increases histone H4 acetylation corresponding with the activation of endogenous CEL gene transcription. Our data, identifying CEL as the first pancreatic LRH-1 target gene, indicate that LRH-1 is an important player in enterohepatic cholesterol homeostasis.

Expression and functional analysis of liver receptor homologue 1 as a potential steroidogenic factor in rat ovary

Liver receptor homologue 1 (LRH-1) is a member of the nuclear receptor superfamily originally found in liver cells. LRH-1 participates in regulation of cholesterol metabolism and bile acid synthesis. Recent studies have shown that LRH-1 is even more highly expressed in the ovary, and LRH-1 has been implicated as a key transcriptional regulator of cytochrome P450 aromatase (P450arom) in vitro. In the present study, we investigated the spatiotemporal expression patterns of LRH-1 using in situ hybridization and immunohistochemistry in ovaries from rats with a 4-day estrous cycle, from pregnant rats, from immature rats treated with eCG to stimulate follicular development, and from eCG-treated rats that were subsequently given hCG to stimulate ovulation and luteinization. To establish a potential connection between the expression of LRH-1 and that of the steroidogenic genes in vivo, we directly compared the localization patterns of LRH-1 and P450arom transcripts in consecutive ovarian sections from these animals. LRH-1 mRNA and protein were primarily localized to granulosa cells and luteinized follicles or newly formed corpora lutea (CLs) of immature and adult rats, and the levels of expression increased during eCG-hCG-induced follicular development and ovulation. In the functional CLs of pregnant rats, a biphasic change in LRH-1 mRNA content occurred throughout the gestation process, whereas LRH-1 protein was persistently detected during the entire pregnancy. In the consecutive ovarian sections, expression of LRH-1 was approximately colocalized with that of P450arom in both tertiary and Graafian follicles and the functional CLs of pregnant rats. LRH-1 mRNA and protein expression preceded those of P450arom during early follicular development. Stage-specific expression of LRH-1 in rat granulosa and luteal cells suggests a role for LRH-1 in the regulation of ovarian function. The overlapping but distinct expression patterns of LRH-1 and P450arom circumstantially support the recent finding that LRH-1 serves as a critical upstream regulator of P450arom gene expression in ovarian cells, but LRH-1 also may be a multifunctional steroidogenic factor in ovarian physiology.

Regulation of hepatitis B virus replication by the ras-mitogen-activated protein kinase signaling pathway

The replication of hepatitis B virus (HBV) can be regulated by a variety of factors, including hormones, growth factors, and cytokines. However, the molecular mechanisms of these regulations are largely unknown. Ras is a small GTPase that responds to many of these external stimuli. In this study, we investigated the possible effect of Ras on the replication of HBV. Our results indicated that activated Ras could suppress the replication of HBV in both Huh7 and HepG2 cells. This suppression was independent of the X protein and most likely occurred at the transcriptional level. Deletion-mapping analysis of the HBV core promoter and its upstream ENI and ENII enhancers revealed multiple elements responsive to activated Ras. This suppression of HBV replication by activated Ras was apparently mediated by the mitogen-activated protein (MAP) kinase pathway, as it was accompanied by activation of ERK1/2 and abolished by the MEK1/2 inhibitor U0126. Our results thus indicate that external stimuli may suppress HBV replication through the Ras-MAP kinase pathway.

Nuclear receptors and the control of metabolism

The metabolic nuclear receptors act as metabolic and toxicological sensors, enabling the organism to quickly adapt to environmental changes by inducing the appropriate metabolic genes and pathways. Ligands for these metabolic receptors are compounds from dietary origin, intermediates in metabolic pathways, drugs, or other environmental factors that, unlike classical nuclear receptor ligands, are present in high concentrations. Metabolic receptors are master regulators integrating the homeostatic control of (a) energy and glucose metabolism through peroxisome proliferator-activated receptor gamma (PPARgamma); (b) fatty acid, triglyceride, and lipoprotein metabolism via PPARalpha, beta/delta, and gamma; (c) reverse cholesterol transport and cholesterol absorption through the liver X receptors (LXRs) and liver receptor homolog-1 (LRH-1); (d) bile acid metabolism through the farnesol X receptor (FXR), LXRs, LRH-1; and (e) the defense against xeno- and endobiotics by the pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR). The transcriptional control of these metabolic circuits requires coordination between these metabolic receptors and other transcription factors and coregulators. Altered signaling by this subset of receptors, either through chronic ligand excess or genetic factors, may cause an imbalance in these homeostatic circuits and contribute to the pathogenesis of common metabolic diseases such as obesity, insulin resistance and type 2 diabetes, hyperlipidemia and atherosclerosis, and gallbladder disease. Further studies should exploit the fact that many of these nuclear receptors are designed to respond to small molecules and turn them into therapeutic targets for the treatment of these disorders.

Establishment of transgenic mice carrying the gene of human nuclear receptor NR5A2 (hB1F)

AIM: Human hepatitis B virus enhancer II B1 binding factor (hB1F) was cloned and characterized as a novel member of the Ftz-F1 (NR5A) nuclear receptor subfamily. Although progresses have recently been made, its biological function remains largely unidentified. The aim of this study was to establish an hB1F transgenic mouse model to promote the functional study of hB1F.

METHODS: Transgene fragments were microinjected into fertilized eggs of mice. The manipulated embryos were transferred into the oviducts of pseudopregnant female mice. The offsprings were identified by PCR and Southern blot analysis. Transgene expression was analyzed with RT-PCR and Western blot analysis. Transgenic founder mice were used to establish transgenic mouse lineages. The F1 and F2 mice were identified by PCR analysis.

RESULTS: Seven mice were identified as carrying copies of transgene. RT-PCR and Western blotting results showed that the transgene was expressed in heart, liver, lung, kidney and stomach in one of the transgenic mouse lineages. Genetic analysis of the transgenic mice demonstrated that the transgene was integrated into the chromosome at a single site, and was transmitted stably.

CONCLUSION: In this study we established an hB1F transgenic mouse model, which will facilitate the investigation of the biological function of hB1F in vivo.

Bile acid regulation of gene expression: roles of nuclear hormone receptors

Bile acids derived from cholesterol and oxysterols derived from cholesterol and bile acid synthesis pathways are signaling molecules that regulate cholesterol homeostasis in mammals. Many nuclear receptors play pivotal roles in the regulation of bile acid and cholesterol metabolism. Bile acids activate the farnesoid X receptor (FXR) to inhibit transcription of the gene for cholesterol 7alpha-hydroxylase, and stimulate excretion and transport of bile acids. Therefore, FXR is a bile acid sensor that protects liver from accumulation of toxic bile acids and xenobiotics. Oxysterols activate the liver orphan receptors (LXR) to induce cholesterol 7alpha-hydroxylase and ATP-binding cassette family of transporters and thus promote reverse cholesterol transport from the peripheral tissues to the liver for degradation to bile acids. LXR also induces the sterol response element binding protein-1c that regulates lipogenesis. Therefore, FXR and LXR play critical roles in coordinate control of bile acid, cholesterol, and triglyceride metabolism to maintain lipid homeostasis. Nuclear receptors and bile acid/oxysterol-regulated genes are potential targets for developing drug therapies for lowering serum cholesterol and triglycerides and treating cardiovascular and liver diseases.

Liver receptor homologue-1 (LRH-1) regulates expression of aromatase in preadipocytes

Estrogen biosynthesis from C(19) steroids is catalyzed by aromatase cytochrome P450. Aromatase is expressed in breast adipose tissue through the use of a distal, cytokine-responsive promoter (promoter I.4). Breast tumors, however, secrete soluble factors that stimulate aromatase expression through an alternative proximal promoter, promoter II. In other estrogenic tissues such as ovaries, transcription from promoter II requires the presence of the Ftz-F1 homologue steroidogenic factor-1 (SF-1); adipose tissue, however, does not express SF-1. We have explored the hypothesis that in adipose tissue, an alternative Ftz-F1 family member, liver receptor homologue-1 (LRH-1), substitutes for SF-1 in driving transcription from promoter II. In transient transfection assays using 3T3-L1 preadipocytes, promoter II reporter constructs were modestly (2-3-fold) stimulated by either treatment with activators of protein kinases A or C (PKA/C) or by cotransfection with LRH-1. In combination, these treatments synergistically activated promoter II (>30-fold). Induction by LRH-1 (but not by PKA/C) required an AGGTCA motif at -130 base pairs, to which LRH-1 bound in gel shift assays. Activity of GAL4-LRH-1 fusion proteins was not altered by activators of PKA or PKC. Quantitative real-time PCR revealed that LRH-1 (but not SF-1) is expressed in the preadipocyte fraction of human adipose tissue at levels comparable with that of liver. Differentiation of cultured human preadipocytes into mature adipocytes was associated with a time-dependent induction of peroxisome proliferator-activated receptor-gamma (PPARgamma), and rapid loss of LRH-1 and aromatase expression. We conclude that LRH-1 is a preadipocyte-specific nuclear receptor that regulates expression of aromatase in adipose tissue. Alterations in LRH-1 expression and/or activity in adipose tissue could therefore have considerable effects on local estrogen production and breast cancer development.

Dual mechanisms for repression of the monomeric orphan receptor liver receptor homologous protein-1 by the orphan small heterodimer partner

The orphan nuclear hormone receptor liver receptor homologous protein-1 (LRH-1; NR5A2, also known as FTF), an unusual receptor that binds DNA as a monomer, is an essential regulator of expression of a rate-limiting enzyme in bile acid formation, cholesterol 7-alpha-hydroxylase. In a classic negative feedback loop that is a crucial component of the complex regulation of cholesterol metabolism, cholesterol 7-alpha-hydroxylase expression is decreased when bile acid levels are high. This repression is thought to be based on the bile acid-dependent induction of expression of the orphan receptor small heterodimer partner (SHP) NR0B2, which inhibits the activity of LRH-1. We have explored the molecular basis for this important regulatory effect by characterizing the mechanisms by which mouse and human SHP inhibit LRH-1-mediated transactivation. Both SHP proteins specifically interact with the AF-2 transactivation domain of LRH-1 both in vivo and in vitro. This domain is a common target for coactivator interaction, and the SHP proteins can compete with p160 coactivators for binding to LRH-1. In addition to the N-terminal receptor interaction domain, SHP includes a C-terminal domain with autonomous repression function. Neither a deletion nor a point mutation specifically affecting this domain blocked the ability to interact with LRH-1 to compete for coactivator binding or to repress LRH-1 transactivation. However, the relative ability of these mutants to inhibit LRH-1-mediated transactivation was markedly decreased. We conclude that the proposed central role of SHP in cholesterol metabolism is based on a two-step mechanism that is dependent on both coactivator competition and direct transcriptional repression.

Transcriptional regulation of the human sterol 12alpha-hydroxylase gene (CYP8B1): roles of heaptocyte nuclear factor 4alpha in mediating bile acid repression

Sterol 12alpha-hydroxylase catalyzes the synthesis of cholic acid and controls the ratio of cholic acid over chenodeoxycholic acid in the bile. Transcription of CYP8B1 is inhibited by bile acids, cholesterol, and insulin. To study the mechanism of CYP8B1 transcription by bile acids, we have cloned and determined 3389 base pairs of the 5'-upstream nucleotide sequences of the human CYP8B1. Deletion analysis of CYP8B1/luciferase reporter activity in HepG2 cells revealed that the sequences from -57 to +300 were important for basal and liver-specific promoter activities. Hepatocyte nuclear factor 4alpha (HNF4alpha) strongly activated human CYP8B1 promoter activities, whereas cholesterol 7alpha-hydroxylase promoter factor (CPF), an NR5A2 family of nuclear receptors, had much less effect. Electrophoretic mobility shift assay identified an overlapping HNF4alpha- and CPF-binding site in the +198/+227 region. The human CYP8B1 promoter activities were strongly repressed by bile acids, and the bile acid response element was localized between +137 and +220. Site-directed mutagenesis of the HNF4alpha-binding site markedly reduced promoter activity and its response to bile acid repression. On the other hand, mutation of the CPF-binding site had little effect on promoter activity and bile acid inhibition. A negative nuclear receptor, small heterodimer partner markedly inhibited transactivation of CYP8B1 by HNF4alpha. Mammalian two-hybrid assay confirmed that HNF4alpha interacted with small heterodimer partner. Furthermore, bile acids and farnesoid X receptor reduced the expression of nuclear HNF4alpha in HepG2 cells and rat livers and its binding to DNA. Bile acids and farnesoid X receptor also inhibited mouse HNF4alpha gene transcription. In summary, our data revealed the critical roles HNF4alpha play on CYP8B1 transcription and its repression by bile acids. Bile acids repress human CYP8B1 transcription by reducing the transactivation activity of HNF4alpha through interaction of HNF4alpha with SHP and reduction of HNF4alpha expression in the liver.

HBV integrants of hepatocellular carcinoma cell lines contain an active enhancer

Hepatitis B virus (HBV) infection is a major risk factor worldwide for the development of hepatocellular carcinoma (HCC). Integrated HBV DNA fragments, often highly rearranged, are frequently detected in HCC. In woodchuck, the viral enhancer plays a central role in hepatocarcinogenesis, but in humans the mechanism of HBV oncogenesis has not been established. In this study we investigated the status of the viral enhancer in two human HCC cell lines, Hep3B and PLC/PRF/5 each containing one or more integrated HBV DNA fragments. Active enhancer was defined by virtue of its protein occupancy as determined by genomic in vivo DMS footprinting. In PLC/PRF/5 cells, the HBV DNA was integrated in a cellular gene at chromosome 11q13, at a locus reported to be amplified in many tumors. We show here that in both cell lines, the integrated HBV DNA fragments contain an active enhancer-I. In particular, the occupation of the two previously defined basic enhancer elements, E and EP, was prominent. While in both cell lines the same protein binds to the EP elements, the E element, however, is occupied in a cell-line specific manner. In PLC/PRF/5 but not Hep3B, the prominent binding of an undefined protein was detected. Our data suggest that this protein is likely to be the fetoprotein transcription factor (FTF). The finding that enhancer sequences are conserved and functional in different cell lines suggests a selection pressure for their long-term maintenance. We therefore propose that the HBV enhancer-I might play a role in hepatocellular carcinogenesis.

Characterization of the genomic structure and tissue-specific promoter of the human nuclear receptor NR5A2 (hB1F) gene

The human homologue of the Drosophila melanogaster orphan nuclear receptor fushi tarazu factor 1 (Ftz-F1), NR5A2 (hB1F), was initially identified as a regulatory factor that binds and activates enhancer II of hepatitis B virus. NR5A2 (hB1F) is expressed specifically in pancreas and liver, playing important roles in the regulation of several liver-specific genes. A detailed analysis on the genomic structure and promoter activity will greatly promote future studies on the function of the NR5A2 (hB1F) gene. In this report, a bacterial artificial chromosome clone and several phage clones covering the NR5A2 (hB1F) gene were isolated and the complete genomic sequence was obtained. Alignment of different cDNAs of the NR5A2 (hB1F) gene with the genomic sequence facilitated the delineation of its structural organization, which spans over 150 kb and consists of eight exons interrupted by seven introns. RT-PCR and 3'-RACE revealed that utilization of two polyadenylation signals results in the 3.8 and 5.2 kb transcripts that were observed previously. The transcription start site of the NR5A2 (hB1F) gene was mapped downstream of a canonical TATA box. An upstream fragment containing binding sites for several liver-specific and ubiquitous transcription factors exhibits hepatocyte-specific promoter activity. Transient transfections indicated that hepatocyte nuclear factors HNF1 and HNF3beta could activate NR5A2 (hB1F) promoter.

Transcription repression of human hepatitis B virus genes by negative regulatory element-binding protein/SON

A negative regulatory element (NRE) is located immediately upstream of the upstream regulatory sequence of core promoter and second enhancer of human hepatitis B virus (HBV). NRE represses the transcription activation function of the upstream regulatory sequence of core promoter and the second enhancer. In this study, we described the cloning and characterization of an NRE-binding protein (NREBP) through expression cloning. NREBP cDNA is 8266 nucleotides in size and encodes a protein of 2386 amino acids with a predicted molecular mass of 262 kDa. Three previously described cDNAs, DBP-5, SONB, and SONA, are partial sequence and/or alternatively spliced forms of NREBP. The genomic locus of the NREBP/SON gene is composed of 13 exons and 12 introns. The endogenous NREBP protein is localized in the nucleus of human hepatoma HuH-7 cells. Antibody against NREBP protein can specifically block the NRE binding activity present in fractionated nuclear extracts in gel shifting assays, indicating that NREBP is the endogenous nuclear protein that binds to NRE sequence. By polymerase chain reaction-assisted binding site selection assay, we determined that the consensus sequence for NREBP binding is GA(G/T)AN(C/G)(A/G)CC. Overexpression of NREBP enhances the repression of the HBV core promoter activity via NRE. Overexpression of NREBP can also repress the transcription of HBV genes and the production of HBV virions in a transient transfection system that mimics the viral infection in vivo.

The mouse fetoprotein transcription factor (FTF) gene promoter is regulated by three GATA elements with tandem E box and Nkx motifs, and FTF in turn activates the Hnf3beta, Hnf4alpha, and Hnf1alpha gene promoters

Fetoprotein transcription factor (FTF) is an orphan nuclear receptor that activates the alpha(1)-fetoprotein gene during early liver developmental growth. Here we sought to define better the position of FTF in transcriptional cascades leading to hepatic differentiation. The mouse FTF gene was isolated and assigned to chromosome 1 band E4 (one mFTF pseudogene was also found). Exon/intron mapping shows an mFTF gene structure similar to that of its close homologue SF1, with two more N-terminal exons in the mFTF gene; exon mapping also delimits several FTF mRNA 5'- and 3'-splice variants. The mFTF transcription initiation site was located in adult liver at 238 nucleotides from the first translation initiator codon, with six canonical GATA, E box, and Nkx motifs clustered between -50/-140 base pairs (bp) from the cap site; DNA/protein binding assays also pinpointed an HNF4-binding element at +36 bp and an FTF-binding element at -257 bp. Transfection assays and point mutations showed that the mFTF promoter is activated by GATA, HNF4alpha, FTF, Nkx, and basic helix-loop-helix factors, with marked cooperativity between GATA and HNF4alpha. A tandem GATA/E box activatory motif in the proximal mFTF promoter is strikingly similar to a composite motif coactivated by differentiation inducers in the hematopoietic lineage; a tandem GATA-Nkx motif in the distal mFTF promoter is also similar to a composite motif transducing differentiation signals from transforming growth factor-beta-like receptors in the cardiogenic lineage. Three genes encoding transcription factors critical to early hepatic differentiation, Hnf3beta, Hnf4alpha, and Hnf1alpha, each contain dual FTF-binding elements in their proximal promoters, and all three promoters are activated by FTF in transfection assays. Direct DNA binding action and cooperativity was demonstrated between FTF and HNF3beta on the Hnf3beta promoter and between FTF and HNF4alpha on the Hnf1alpha promoter. These combined results suggest that FTF is an early intermediary between endodermal specification signals and downstream genes that establish and amplify the hepatic phenotype.

Site-specific mutation of the hepatitis B virus enhancer II B1 element: effect on virus transcription and replication

The hepatitis B virus (HBV) enhancer II (EII) is highly liver-specific and plays an important role in regulating the transcription of all HBV genes. In this report, mutational analysis on the B1F-binding site in the major functional unit of HBV EII is described. The activity of HBV EII in EII-CAT reporter plasmids was significantly decreased when the sequence of the B1F-binding site in EII was mutated. Furthermore, a single point mutation in the B1 element that aborted the binding of B1F caused a dramatic decrease in viral gene transcription initiated from the HBV core promoter, which resulted in a reduction of the production of the HBV e antigen and pregenomic RNA, the template for viral DNA replication. In conclusion, the interaction of B1F with its target binding sequence in the EII region is crucial for liver-specific transcription and DNA replication of the virus.

Expression and regulation of transcripts encoding two members of the NR5A nuclear receptor subfamily of orphan nuclear receptors, steroidogenic factor-1 and NR5A2, in equine ovarian cells during the ovulatory process

Steroidogenic factor-1 (SF-1, NR5A1a) is a member of the NR5A nuclear receptor subfamily and has been implicated as a key transcriptional regulator of all ovarian steroidogenic genes in vitro. To establish links between the expression of SF-1 and that of the steroidogenic genes in vivo, the objectives of this study were to clone equine SF-1 and examine the regulation of its messenger RNA (mRNA) in follicular cells during human CG (hCG)-induced ovulation. The equine SF-1 primary transcript was cloned by a combination of RT-PCR techniques. Results showed that the transcript was composed of a 5'-untranslated region (UTR) of 161 bp, an open reading frame (ORF) of 1386 bp that encodes a highly-conserved 461-amino acid protein, and a 3'-UTR of 518 bp. The cloning of SF-1 also led to the unexpected and serendipitous isolation of the highly-related orphan nuclear receptor NR5A2, which was shown to include a 5'-UTR of 243 bp, an ORF of 1488 bp, and a 3'-UTR of 1358 bp. The NR5A2 ORF encodes a 495-amino acid protein that is 60% identical to SF-1, including 99%-similar DNA-binding domains. Northern blot analysis revealed that SF-1 and NR5A2 were expressed in all major steroidogenic tissues, with the exception that NR5A2 was not present in the adrenal. Interestingly, NR5A2 was found to be, by far, the major NR5A subfamily member expressed in the preovulatory follicle and the corpus luteum. Using a semiquantitative RT-PCR/Southern blotting approach, the regulation of SF-1 and NR5A2 mRNAs in vivo was studied in equine follicular cells obtained from preovulatory follicles isolated between 0 and 39 h post hCG. Results showed that the theca interna was the predominant site of SF-1 mRNA expression in the follicle, and that hCG caused a significant decrease in SF-1 levels between 12-39 h in theca interna and between 24-39 h post hCG in granulosa cells (P < 0.05). In contrast, the granulosa cell layer was the predominant, if not the sole, site of NR5A2 mRNA expression in the follicle. Importantly, NR5A2 was much more highly expressed in granulosa cells than SF-1. The administration of hCG caused a significant decrease in NR5A2 transcripts in granulosa cells at 30, 36, and 39 h post hCG (P < 0.05). Thus, this study is the first to report the concomitant regulation of SF-1 in theca interna and granulosa cells throughout the ovulation/luteinization process, and to demonstrate the novel expression and hormonal regulation of NR5A2 in ovarian cells. Based on the marked expression of NR5A2 in equine granulosa and luteal cells and on mounting evidence of a functional redundancy between SF-1 and NR5A2 in other species, it is proposed that NR5A2 may play a key role in the regulation of gonadal steroidogenic gene expression.

The conserved nuclear receptor Ftz-F1 is required for embryogenesis, moulting and reproduction in Caenorhabditis elegans

BACKGROUND

Nuclear receptors are essential players in the development of all metazoans. The nematode Caenorhabditis elegans possesses more than 200 putative nuclear receptor genes, several times more than the number known in any other organism. Very few of these transcription factors are conserved with components of the steroid response pathways in vertebrates and arthropods. Ftz-F1, one of the evolutionarily oldest nuclear receptor types, is required for steroidogenesis and sexual differentiation in mice and for segmentation and metamorphosis in Drosophila.

RESULTS

We employed two complementary approaches, direct mutagenesis and RNA interference, to explore the role of nhr-25, a C. elegans ortholog of Ftz-F1. Deletion mutants show that nhr-25 is essential for embryogenesis. RNA interference reveals additional requirements throughout the postembryonic life, namely in moulting and differentiation of the gonad and vulva. All these defects are consistent with the nhr-25 expression pattern, determined by in situ hybridization and GFP reporter activity.

CONCLUSIONS

Our data link the C. elegans Ftz-F1 ortholog with a number of developmental processes. Significantly, its role in the periodical replacement of cuticle (moulting) appears to be evolutionarily shared with insects and thus supports the monophyletic origin of moulting.

Transcriptional stimulation by hepatocyte nuclear factor-6. Target-specific recruitment of either CREB-binding protein (CBP) or p300/CBP-associated factor (p/CAF)

Transcription factors of the ONECUT class, whose prototype is HNF-6, contain a single cut domain and a divergent homeodomain characterized by a phenylalanine at position 48 and a methionine at position 50. The cut domain is required for DNA binding. The homeodomain is required either for DNA binding or for transcriptional stimulation, depending on the target gene. Transcriptional stimulation by the homeodomain involves the F48M50 dyad. We investigate here how HNF-6 stimulates transcription. We identify transcriptionally active domains of HNF-6 that are conserved among members of the ONECUT class and show that the cut domain of HNF-6 participates to DNA binding and, via a LXXLL motif, to transcriptional stimulation. We also demonstrate that, on a target gene to which HNF-6 binds without requirement for the homeodomain, transcriptional stimulation involves an interaction of HNF-6 with the coactivator CREB-binding protein (CBP). This interaction depends both on the LXXLL motif of the cut domain and on the F48M50 dyad of the homeodomain. On a target gene for which the homeodomain is required for DNA binding, but not for transcriptional stimulation, HNF-6 interacts with the coactivator p300/CBP-associated factor but not with CBP. These data show that a transcription factor can act via different, sequence-specific, mechanisms that combine distinct modes of DNA binding with the use of different coactivators.

The hepatitis B virus core promoter is strongly activated by the liver nuclear receptor fetoprotein transcription factor or by ectopically expressed steroidogenic factor 1

Orphan nuclear receptor fetoprotein transcription factor (FTF) was previously identified as a specific regulator of the alpha(1)-fetoprotein gene during early liver development and in response to hormonal signals (L. Galarneau, J.-F. Paré, D. Allard, D. Hamel, L. Lévesque, J. D. Tugwood, S. Green, and L. Bélanger, Mol. Cell. Biol. 16:3853-3865, 1996). Here we report a functional analysis of FTF interactions with the hepatitis B virus (HBV) nucleocapsid promoter. DNA-protein-binding assays show that the HBV core promoter contains two high-affinity FTF-binding sites and a third, lower-affinity site shared with other receptors. Transfections in HepG2, Hep3B, and PLC/PRF/5 hepatoma cells using chloramphenicol acetyltransferase reporter genes with the nucleocapsid promoter linked or not linked to enhancer I indicate that FTF is a potent activator of the HBV core promoter, more efficient than HNF4alpha, HNF3alpha, HNF3beta, or C/EBPalpha. Steroidogenic factor 1, a close FTF homolog which binds to the same DNA motif and is expressed ectopically in HepG2 cells, seems to be an even stronger inducer than FTF. Point mutations of the FTF-binding sites indicate direct FTF activatory effects on the core promoter and the use of both high-affinity sites for productive interaction between the core promoter and enhancer I. Coexpression assays further indicate that FTF and HNF4alpha are the most efficient partners for coactivation of the pregenomic core promoter, which may largely account for the hepatic tropism and the early amplification of HBV infection. Carboxy terminus-truncated FTF behaves as a dominant negative mutant to compete all three FTF sites and strongly deactivate core promoter interactions with enhancer I; this suggests possible new ways to interfere with HBV infection.

Identification of multiple transcription factors, HLF, FTF, and E4BP4, controlling hepatitis B virus enhancer II

Hepatitis B virus (HBV) enhancer II (EnII) is a hepatotropic cis element which is responsible for the hepatocyte-specific gene expression of HBV. Multiple transcription factors have been demonstrated to interact with this region. In this study, the region from HBV nucleotides (nt) 1640 to 1663 in EnII was demonstrated to be essential for enhancer activity and to be another target sequence of putative transcription factors. To elucidate the factors which bind to this region, we used a yeast one-hybrid screening system and cloned three transcription factors, HLF, FTF, and E4BP4, from a human adult liver cDNA library. All of these factors had binding affinity to the sequence from nt 1640 to 1663. Investigation of the effects of these factors on transcriptional regulation revealed that HLF and FTF had stimulatory activity on nt 1640 to 1663, whereas E4BP4 had a suppressing effect. FTF coordinately activated both 3. 5-kb RNA and 2.4/2.1-kb RNA transcription in a transient transfection assay with an HBV expression vector. HLF, however, activated only 3.5-kb RNA transcription, and in primer extension analysis, HLF strongly stimulated the synthesis of pregenome RNA compared to precore RNA. Thus, FTF stimulated the activity of the second enhancer, while HLF stimulated the activity of the core upstream regulatory sequence, which affects only the core promoter, and had a dominant effect on the pregenome RNA synthesis.

CPF: an orphan nuclear receptor that regulates liver-specific expression of the human cholesterol 7alpha-hydroxylase gene

Cholesterol 7alpha-hydroxylase is the first and rate-limiting enzyme in a pathway through which cholesterol is metabolized to bile acids. The gene encoding cholesterol 7alpha-hydroxylase, CYP7A, is expressed exclusively in the liver. Overexpression of CYP7A in hamsters results in a reduction of serum cholesterol levels, suggesting that the enzyme plays a central role in cholesterol homeostasis. Here, we report the identification of a hepatic-specific transcription factor that binds to the promoter of the human CYP7A gene. We designate this factor CPF, for CYP7A promoter binding factor. Mutation of the CPF binding site within the CYP7A promoter abolished hepatic-specific expression of the gene in transient transfection assays. A cDNA encoding CPF was cloned and identified as a human homolog of the Drosophila orphan nuclear receptor fushi tarazu F1 (Ftz-F1). Cotransfection of a CPF expression plasmid and a CYP7A reporter gene resulted in specific induction of CYP7A-directed transcription. These observations suggest that CPF is a key regulator of human CYP7A gene expression in the liver.