X-region-specific transcript in mammalian hepatitis B virus-infected liver

The Regulation of HBV Transcription and Replication

Hepatitis B virus (HBV) is a major human pathogen lacking a reliable curative therapy. Current therapeutics target the viral reverse transcriptase/DNA polymerase to inhibit viral replication but generally fail to resolve chronic HBV infections. Due to the limited coding potential of the HBV genome, alternative approaches for the treatment of chronic infections are desperately needed. An alternative approach to the development of antiviral therapeutics is to target cellular gene products that are critical to the viral life cycle. As transcription of the viral genome is an essential step in the viral life cycle, the selective inhibition of viral RNA synthesis is a possible approach for the development of additional therapeutic modalities that might be used in combination with currently available therapies. To address this possibility, a molecular understanding of the relationship between viral transcription and replication is required. The first step is to identify the transcription factors that are the most critical in controlling the levels of HBV RNA synthesis and to determine their in vivo role in viral biosynthesis. Mapping studies in cell culture utilizing reporter gene constructs permitted the identification of both ubiquitous and liver-enriched transcription factors capable of modulating transcription from the four HBV promoters. However, it was challenging to determine their relative importance for viral biosynthesis in the available human hepatoma replication systems. This technical limitation was addressed, in part, by the development of non-hepatoma HBV replication systems where viral biosynthesis was dependent on complementation with exogenously expressed transcription factors. These systems revealed the importance of specific nuclear receptors and hepatocyte nuclear factor 3 (HNF3)/forkhead box A (FoxA) transcription factors for HBV biosynthesis. Furthermore, using the HBV transgenic mouse model of chronic viral infection, the importance of various nuclear receptors and FoxA isoforms could be established in vivo. The availability of this combination of systems now permits a rational approach toward the development of selective host transcription factor inhibitors. This might permit the development of a new class of therapeutics to aid in the treatment and resolution of chronic HBV infections, which currently affects approximately 1 in 30 individuals worldwide and kills up to a million people annually.

Inhibition of hepatitis B virus and induction of hepatoma cell apoptosis by ASGPR-directed delivery of shRNAs

Hepatitis B virus (HBV) infection is a worldwide liver disease and nearly 25% of chronic HBV infections terminate in hepatocellular carcinoma (HCC). Currently, there is no effective therapy to inhibit HBV replication and to eliminate hepatoma cells, making it highly desired to develop novel therapies for these two stages of the HBV-caused detrimental disease. Recently, short hairpin RNA (shRNA) has emerged as a potential therapy for virus-infected disease and cancer. Here, we have generated a shRNA, pGenesil-siHBV4, which effectively inhibits HBV replication in the human hepatoma cell line HepG2.2.15. The inhibitory effects of pGenesil-siHBV4 are manifested by the decrease of both the HBV mRNA level and the protein levels of the secreted HBV surface antigen (HBsAg) and HBV e antigen (HBeAg), and by the reduction of secreted HBV DNA. Using mouse hydrodynamic tail vein injection, we demonstrate that pGenesil-siHBV4 is effective in inhibiting HBV replication in vivo. Because survivin plays a key role in cancer cell escape from apoptosis, we further generated pGenesil-siSurvivin, a survivin-silencing shRNA, and showed its effect of triggering apoptosis of HBV-containing hepatoma cells. To develop targeted shRNA therapy, we have identified that as a specific binder of the asialoglycoprotein receptor (ASGPR), jetPEI-Hepatocyte delivers pGenesil-siHBV4 and pGenesil-siSurvivin specifically to hepatocytes, not other types of cells. Finally, co-transfection of pGenesil-siHBV4 and pGenesil-siSurvivin exerts synergistic effects in inducing hepatoma cell apoptosis, a novel approach to eliminate hepatoma by downregulating survivin via multiple mechanisms. The application of these novel shRNAs with the jetPEI-Hepatocyte targeting strategy demonstrates the proof-of-principle for a promising approach to inhibit HBV replication and eliminate hepatoma cells with high specificity.

Establishment and primary application of a mouse model with hepatitis B virus replication

AIM: To establish a rapid and convenient animal model with hepatitis B virus (HBV) replication.

METHODS: A naked DNA solution of HBV-replication-competent plasmid was transferred to BALB/C mice via the tail vein, using a hydrodynamic in vivo transfection procedure. After injection, these mice were sacrificed on d 1, 3, 4, 5, 7 and 10. HBV DNA replication intermediates in the liver were analyzed by Southern blot hybridization. The expression of hepatitis B core antigen (HBcAg) and hepatitis B surface antigen (HBsAg) in the liver was checked by immunohistochemistry. Serum HBsAg and hepatitis B e antigen (HBeAg) was detected by enzyme-linked immunosorbent assay (ELISA). Inhibition of HBV replication was compared in HBV replication model mice treated intraperitoneally with polyinosinic-polytidylin acid (polyIC) or phosphate-buffered saline (PBS).

RESULTS: After hydrodynamic in vivo transfection, HBV DNA replication intermediates in the mouse liver were detectable on d 1 and abundant on d 3 and 4, the levels were slightly decreased and remained relatively stable between d 5 and 7, and were almost undetectable on d 10. The expression patterns of HBcAg and HBsAg were similar to that of HBV replication intermediate DNA, except that they reached a peak on d 1 after injection. No obvious differences in HBV DNA replication intermediates were observed in the left, right and middle lobes of the liver. After treatment with polyIC, the level of HBV intermediate DNA in the liver was lower than that in the control mice injected with PBS.

CONCLUSION: A rapid and convenient mouse model with a high level of HBV replication was developed and used to investigate the inhibitory effect of polyIC on HBV replication, which provides a useful tool for future functional studies of the HBV genome.

Expression of hepatitis B virus X protein in transgenic mice

AIM: To establish a mice model harboring hepatitis B virus x gene (adr subtype) for studying the function of hepatitis B virus X protein, a transactivator of viral and cellular promoter/enhancer elements.

METHODS: Expression vector pcDNA3-HBx, containing CMV promoter and hepatitis B virus x gene open reading fragment, was constructed by recombination DNA technique. Hela cells were cultured in DMEM and transfected with pcDNA3-HBx or control pcDNA3 plasmids using FuGENE6 Transfection Reagent. Expression of pcDNA3-HBx vectors in the transfected Hela cells was confirmed by Western blotting. After restriction endonuclease digestion, the coding elements were microinjected into male pronuclei of mice zygotes. The pups were evaluated by multiplex polymerase chain reaction (PCR) at genomic DNA level. The x gene transgenic mice founders were confirmed at protein level by Western blotting, immunohistochemistry and immunogold transmission electron microscopy.

RESULTS: Expression vector pcDNA3-HBx was constructed by recombination DNA technique and identified right by restriction endonuclease digestion and DNA direct sequencing. With Western blotting, hepatitis X protein was detected in Hela cells transfected with pcDNA3-HBx plasmids, suggesting pcDNA3-HBx plasmids could express in eukaryotic cells. Following microinjection of coding sequence of pcDNA3-HBx, the embryos were transferred to oviducts of psedopregnant females. Four pups were born and survived. Two of them were verified to have the HBx gene integrated in their genomic DNA by multiplex PCR assay, and named C57-TgN (HBx)S MMU1 and C57-TgN (HB x) SMMU3 respectively. They expressed 17KD X protein in liver tissue by Western blotting assay. With the immunohistochemistry, X protein was detected mainly in hepatocytes cytoplasm of transgenic mice, which was furthermore confirmed by immunogold transmission electon microscopy.

CONCLUSION: We have constructed the expression vector pcDNA3-HBx that can be used to study the function of HBx gene in eukaryotic cells in vitro. We also established HBx gene (adr subtype) transgenic mice named C57-TgN (HBx) SMMU harboring HBx gene in their genome and express X protein in hepatocytes, Which might be a valuable animal system for studying the roles of HBx gene in hepatitis B virus life cycle and development of hepatocellular carcinoma in vivo.

Intracellular inhibition of hepatitis B virus S gene expression by chimeric DNA-RNA phosphorothioate minimized ribozyme

Chronic hepatitis B virus (HBV) infection is a major problem in Asia. Current therapies for chronic hepatitis B have limited efficacy. The successful use of ribozymes for intracellular inhibition of HBV gene expression was recently reported. As an alternative to ribozymes, the use of DNA-containing, phosphorothioate-modified, minimized hammerhead ribozymes (minizymes) to inhibit hepatitis B surface antigen (HBsAg) expression and viral replication was investigated. Such molecules can be synthesized and supplied exogenously. Two conserved sites within the HBsAg open reading frame (ORF) were targeted. PLC/PRF5 cells or 2.2.15 cells were treated with minizymes or antisense oligomers to assess the effects on cell viability, HBsAg expression, and viral DNA production. Treatment with the minizyme, MZPS1, resulted in >80% inhibition of HBsAg expression in PLC/PRF5 cells. MZPS1 had more inhibitory effect than the antisense oligonucletoide target at the same region, whereas the control minizyme had little effect. Another gene-specific minizyme, MZPS2, did not show any effect. Treated cells remained fully viable. Treatment of 2.2.15 cells with MZPS1 also led to decreased HBsAg expression. In addition, a 2.3-fold decrease in viral production was observed. Our data showed that minizymes can inhibit HBV gene expression and may potentially be useful for clinical therapy against chronic HBV infection.

Modulation of glutathione S-transferase alpha by hepatitis B virus and the chemopreventive drug oltipraz

Persistent infection by hepatitis B virus (HBV) and exposure to chemical carcinogens correlates with the prevalence of hepatocellular carcinoma in endemic areas. The precise nature of the interaction between these factors is not known. Glutathione S-transferases (GST) are responsible for the cellular metabolism and detoxification of a variety of cytotoxic and carcinogenic compounds by catalysis of their conjugation with glutathione. Diminished GST activity could enhance cellular sensitivity to chemical carcinogens. We have investigated GST isozyme expression in hepatocellular HepG2 cells and in an HBV-transfected subline. Total GST activity and selenium-independent glutathione peroxidase activity are significantly decreased in HBV transfected cells. On immunoblotting, HBV transfected cells demonstrate a significant decrease in the level of GST Alpha class. Cytotoxicity assays reveal that the HBV transfected cells are more sensitive to a wide range of compounds known to be detoxified by GST Alpha conjugation. Although no significant difference in protein half-life between the two cell lines was found, semi-quantitative reverse transcription-polymerase chain reaction shows a reduced amount of GST Alpha mRNA in the transfected cells. Because the HBV x protein (HBx) seems to play a role in HBV transfection, we also demonstrated that expression of the HBx gene into HepG2 cells decreased the amount of GST Alpha protein. Transient transfection experiments using both rat and human GST Alpha (rGSTA5 and hGSTA1) promoters in HepG2 cells show a decreased CAT activity upon HBx expression, supporting a transcriptional regulation of both genes by HBx. This effect is independent of HBx interaction with Sp1. Treatment with oltipraz, an inducer of GST Alpha, partially overcomes the effect of HBx on both promoters. Promoter deletion studies indicate that oltipraz works through responsive elements distinct from AP1 or NF-kappaB transcription factors. Thus, HBV infection alters phase II metabolizing enzymes via different mechanisms than those modulated by treatment with oltipraz.

A novel negative cis-regulatory element on the hepatitis B virus S-(+)-strand

Hepatitis B virus (HBV) has a double-stranded DNA genome. The minus-strand contains coding regions for all known HBV proteins and most of the cis-regulatory elements. Little is known about transcription from the S-(+)-strand and its regulation. Thus, the presence of regulatory elements located on the S-(+)-strand was investigated by inserting nt 1038-1783 of HBV in both orientations between the human cytomegalovirus (HCMV) promoter and a luciferase gene. Transfection experiments revealed that the plasmid containing this HBV DNA fragment in an orientation allowing expression from the S-(+)-strand (antisense) led to inhibition of luciferase gene expression compared to the plasmid containing this sequence in an orientation that allows gene expression from the L-(-)-strand (sense). Deletion analyses delimit the sequence essential for the inhibitory effect to a 150 bp region that also carries part of the enhancerII/core promoter complex. However, the possible influence of this regulatory element has been excluded in various experiments. The repressing HBV sequence acts in an orientation- and position-dependent manner; no inhibition was observed when this DNA element was inserted upstream of the HCMV promoter or downstream of the luciferase gene. Northern blot analyses revealed reduced luciferase mRNA steady-state levels in cells transfected with constructs containing the essential HBV sequence in antisense orientation compared to plasmids containing this sequence in sense orientation. Since nuclear run-on experiments showed similar transcription initiation rates with these plasmids, the diminished luciferase mRNA steady-state levels must be due to altered stabilities, suggesting that nt 1783-1638 of HBV encode an RNA-destabilizing element.

Evidence for a bidirectional promoter complex within the X gene of woodchuck hepatitis virus

The genetic organization of hepadnaviruses is unusual in that all cis-acting regulatory sequences are located within genes. Thus, in the mammalian hepadnavirus genome, the presurface, surface, and X transcript promoters reside within the polymerase gene while the pregenome transcript promoter is located within the X gene. In this study we have identified two additional promoters within the woodchuck hepatitis virus (WHV) X gene that stimulate production of transcripts in vitro. First, we cloned regions of the WHV X gene into a promoterless expression vector (pGL2) to examine their ability to promote expression of firefly luciferase and mapped a previously unidentified promoter to positions 1475-1625 of the WHV8 genome. Deletion analysis revealed that the essential domain of this promoter, termed the ORF5/deltaX transcript promoter, mapped to nucleotides 1525-1625. Analysis revealed that this transcript initiated at nucleotide 1572 in both human (HuH-7) and woodchuck (WLC-3) hepatoma cell lines. Consistent with this finding, DNA footprinting analysis revealed protection of nucleotides 1567-1578 on the positive strand of the WHV8 genome. The function of this transcript in vivo is unclear, however, it may be used to produce a truncated form of the X protein that initiates at an AUG codon at position 1743-1745 on the WHV8 genome. Next, a second promoter was identified at positions 1625-1975 that was responsible for production of an antisense transcript. The activity of this promoter was comparable to that of the previously characterized surface transcript promoter of WHV in the absence of an enhancer. The antisense transcript promoter resides immediately upstream of open reading frame (ORF) 6, a previously identified ORF on the strand opposite of the known WHV protein-encoding sequences, that is thought to represent a vestigial gene. Analysis indicates that the antisense transcript had multiple start sites: nucleotides 1683 and 1762 on the WHV8 genome when assayed in HuH-7 cells, and nucleotide 1786 when assayed in WLC-3 cells. These data are consistent with footprinting analysis of supercoiled WHV DNA that revealed that the regions encompassing nucleotides 1696-1685, 1781-1766, and 1801-1787 on the negative sense DNA strand were protected from nuclease degradation. It is possible that such a transcript was once used in protein expression in an ancestral virus and may now be used for genetic control of WHV replication and/or gene expression. Overall, these data are consistent with the presence of a bidirectional promoter complex within the WHV X gene.

Expression of hepatitis B virus X protein in HBV-infected human livers and hepatocellular carcinomas

Transactivation of cellular genes and functional inactivation of p53 by the hepatitis B virus (HBV) X gene-encoded protein (HBx) are proposed as alternative mechanisms for induction of hepatocellular carcinomas (HCCs) in chronic HBV infection. Using an immunohistochemical approach, we studied the expression of HBx in 39 explanted livers with HBV-associated disease. Because the data reported previously have been inconsistent, possibly due to the application of different antibodies, we compared results with 5 polyclonal and 6 monoclonal anti-HBx antibodies from five laboratories. Ten of the 11 antibodies reacted with recombinant HBx by Western blotting, but only 1 polyclonal and 2 monoclonal antibodies reacted specifically with HBx in tissue, and were thus suitable for immunohistochemistry. Three other polyclonal antibodies reacted with tissue components in addition to HBx. One polyclonal and 4 monoclonal antibodies did not recognize the HBx in the tissue. HBx was demonstrated in 16 of 30 (53.3%) cirrhotic livers and 10 of 18 (58.8%) HCCs by all specific antibodies. The expression of HBx, among three HBV antigens examined, was found to be preferentially maintained in HCC and the surrounding liver parenchyma, including focal or nodular preneoplastic lesions. However, the immunoreactivity was always limited to the cytoplasm of a small number of parenchymal and neoplastic cells. The role of X gene expression in HBV-associated human hepatocarcinogenesis remains to be established.

Major differences between WHV and HBV in the regulation of transcription

Studies were carried out to further characterize enhancer and promoter elements on the woodchuck hepatitis virus (WHV) genome. We were able to confirm the existence of WHV promoters analogous to the major promoters of the related human hepatitis B virus (HBV) and of an enhancer analogous to the recently described WHV E2 element (Ueda, K., Wei, Y., and Ganem, D., Virology 217, 413, 1996). However, we were unable to identity an enhancer analogous to the E1 element of (HBV), despite the fact that these two viruses share a high degree of sequence homology and genetic organization. Some factor binding sites in the E1 region appeared to be conserved between the two viruses and may be required for the activity of the overlapping X gene promoter of WHV. Others did not appear to be essential for WHV X gene promoter activity, and their functional activity, if any, was not revealed. Our failure to detect a functional enhancer element in the region of WHV homologous to the HBV E1 enhancer may indicate that (i) fundamental differences exist in transcriptional regulation of the small circular genomes of WHV and HBV; (ii) WHV contains an E1 element which is functional in the context of the intact viral genome, but which is unable to function in the context of the various expression constructs used in our experiments; or (iii) correct regulation of WHV transcription via an E1 element is dependent upon transcription factors which are not expressed in the liver-specific cell lines used in our experiments.

Members of the nuclear receptor superfamily regulate transcription from the hepatitis B virus nucleocapsid promoter

The role of members of the nuclear receptor superfamily of transcription factors in regulating hepatitis B virus (HBV) transcription was investigated. Hepatocyte nuclear factor 4 (HNF4), the retinoid X receptor (RXR), and the peroxisome proliferator-activated receptor (PPAR) were examined for their capacity to modulate the level of transcriptional activity from the four HBV promoters by transient-transfection analysis in the dedifferentiated hepatoma cell line, HepG2.1. It was found that the nucleocapsid and large surface antigen promoters were transactivated in the presence of HNF4 whereas the enhancer I/X gene, nucleocapsid, and large surface antigen promoters were transactivated in the presence of RXR and PPAR. Characterization of the nuclear receptors interacting with the nucleocapsid promoter region demonstrated that HNF4 is the primary transcription factor binding to the regulatory region spanning nucleotides -127 to -102 whereas HNF4, RXR-PPAR heterodimers, COUPTF1, and ARP1 bind the regulatory region spanning nucleotides -34 to -7. Transcriptional transactivation from the nucleocapsid promoter by HNF4 appears to be mediated through the two HNF4 binding sites in the promoter, whereas modulation of the level of transcription from the nucleocapsid promoter by RXR-PPAR appears to be regulated by the regulatory sequence element spanning nucleotides -34 to -7 and the HBV enhancer 1 region. These observations indicate that HBV transcription, and pregenomic RNA synthesis in particular, is regulated by ligand-dependent nuclear receptors. Agonists and antagonists capable of regulating the activity of these nuclear receptors may permit the modulation of HBV transcription and consequently replication during viral infection.

Long-term productive episomal hepatitis B virus replication in primary cultures of adult human hepatocytes infected in vitro

We have previously increased the efficiency of hepatitis B virus (HBV) infection of human hepatocytes in vitro by using polyethylene glycol. After further documenting by neutralization experiments, this in vitro infection, we used this model to define new culture conditions that would maintain stable episomal replication for several weeks. We found that in the presence of 10% porcine serum and 2% dimethyl sulphoxide (DMSO), high-density cultures survived more than 3 months, while addition of hydrocortisone instead of DMSO resulted in survival of less than 1 month. HBV episomal replication was maintained without any evidence of viral integration into the host genome. The maintenance of HBV replication was demonstrated by: first, stability of the covalently-closed-circular DNA in the nucleus and relaxed circular and single-stranded replicative intermediates in the cytoplasm; second, detection of two major transcripts of 3.5 and 2.1-2.4 kb corresponding to the pregenomic and surface genes respectively; and third, continuous secretion of mature viral particles in the supernatant of infected cells. We showed that under these culture conditions, hepatocytes were blocked in the G1 phase of the cell cycle and did not spontaneously proliferate. Upon hepatocyte growth factor (HGF) stimulation, however, the ability of hepatocytes to divide was demonstrated and was compared in infected and non-infected cells. No change in proliferative capacity and no variation in c-myc and c-jun levels could be found. Hepatocyte survival was not modified in infected cells, confirming that HBV is not cytopathic for normal human hepatocytes. These new culture conditions represent substantial progress in the study of HBV-host cell interactions.

Regulation of transcription from the hepatitis B virus large surface antigen promoter by hepatocyte nuclear factor 3

The influence of hepatocyte nuclear factor 3 (HNF3) on the level of transcriptional activity from the four hepatitis B virus promoters was investigated by transient-transfection analysis in the dedifferentiated hepatoma cell line, HepG2.1. It was found that the large surface antigen promoter and, to a much lesser extent, the nucleocapsid promoter were transactivated in the presence of HNF3. DNase I footprinting analysis demonstrated that purified recombinant HNF3 alpha protects one region of the large surface antigen promoter. Gel retardation analysis showed that a double-stranded oligonucleotide containing this HNF3-binding site formed a specific complex with DNA-binding proteins in the differentiated hepatoma cell lines, Huh7 and HepG2. The complex formed with Huh7 cell extract comigrated with exogenously expressed HNF3 beta in HeLa S3 extracts and was specifically inhibited from forming by the addition of HNF3 beta antiserum. The promoter element which appears to mediate the HNF3 transactivation was functionally mapped by mutational analysis to a region between nucleotides -65 and -54 relative to the transcriptional start site. This regulatory sequence is within the region protected from DNase I digestion by HNF3 alpha and contains 10 of 12 nucleotides homologous to the HNF3-binding-site consensus sequence. A synthetic promoter construct containing this HNF3-binding site was able to mediate transactivation by HNF3 beta. These and previous results suggest that the hepatitis B virus large surface antigen promoter is regulated by at least two liver-enriched transcription factors, HNF1 and HNF3, which together may contribute to the differentiated liver cell type specificity of this promoter.

Expression of insulin-like growth factor II in hepatitis B, cirrhosis and hepatocellular carcinoma: its relationship with hepatitis B virus antigen expression

Expression of insulin-like growth factor II in two human hepatocellular carcinoma cell lines and in hepatitis B, cirrhosis and hepatocellular carcinoma in 419 cases were investigated, and its relationship with the expression of hepatitis B virus X gene was studied by means of immunohistochemical and electron microscopic techniques. The results demonstrated that hepatocellular carcinoma cells (SMMC 7721 and QGY 7703) in culture could express insulin-like growth factor II. Expression seemed to be regulated by cell density, which was suggested as the molecular basis of the contact inhibition of cell proliferation. In tissue sections, cells with high expression of insulin-like growth factor II were observed not only in hepatocellular carcinoma (93%) but also in 95% of the pericancerous liver tissues, 72% of cirrhotic livers, 64% of chronic active hepatitis and 37% of chronic persistent hepatitis. In most cases of hepatocellular carcinoma, insulin-like growth factor II was localized in the cytoplasm of the cancer cells. In the benign liver disorders, four types of cells that highly expressed insulin-like growth factor II were observed: (a) a kind of small liver cell we named the small polygonal liver cell; (b) multinuclear giant hepatocytes; (c) hepatocytes in most of hyperplastic and neoplastic nodules, small hepatocyte nodules and some of regenerative nodules; and (d) some proliferating ductular cells. Even more interestingly, insulin-like growth factor II expression was shown to be closely related to the expression of hepatitis B virus X gene product. We suggest that the activation of insulin-like growth factor II gene and its overexpression may be a crucial step in the processes of hepatitis B virus-associated hepatocarcinogenesis and that the X gene product may activate the insulin-like growth factor II gene through a transactivation mechanism. In addition, we studied the characteristics of small polygonal liver cells, and the roles they may play in the regeneration and carcinogenesis of hepatitis B virus-infected liver are discussed.

Analysis of upstream region of hepatitis B virus core gene using in vitro transcription system

Transcription of the core (C) gene of hepatitis B virus DNA (HBV-DNA) was studied by an in vitro transcription system using nuclear extracts of human liver cell (HepG2) and non-liver cell (HeLa) origins. RNA polymerase II-dependent run-off transcription of 3.5-kb (C) mRNA was observed in both nuclear extracts; but the efficiency was much higher in the HepG2 nuclear extract. Analysis of run-off transcripts using upstream deletion mutants of HBV-DNA demonstrated that there are two transcription start sites located at approximately nucleotide numbers (nt) 1,797 +/- 5 and 1,815 +/- 5. This analysis also suggested that the minimum core promoter sequence and a cis-acting and liver-specific element for C mRNA transcription are located in the downstream region from -80 and -110 (HincII site) of transcription start sites, respectively. DNA-binding protein assays using synthetic double-stranded oligonucleotide probes corresponding to three regions in the upstream region (nt from 1,401 to 1,760) of transcription start sites revealed that there are some liver cell-specific and non-specific DNA-binding proteins in both nuclear extracts. The amount of those proteins was generally higher in the HepG2 nuclear extract. However, no obvious correlation was observed in the present study between the presence of DNA-binding proteins and transcription activity of nuclear extracts in our system. The possible causes of this discrepancy are discussed.

HBV and HDV replication in experimental models: effect of interferon

The use of HBV and HDV experimental models has significantly contributed to understand the viral life cycle and to systematically test antiviral effects of various drugs on a pre-clinical level. Similar replication strategies of related hepadna viruses permit the use of chimpanzees (Pan troglodytes), woodchucks (Marmota monax), ground and tree squirrels (Spermophilus beecheyi) or Pekin ducks (Anas domesticus) as appropriate animal models. Cell culture systems for in vitro infection or transfection using both primary cultures of human and non-human hepatocytes and non-hepatocytes and cell lines have recently been identified. The advantages and restrictions of these experimental models with respect to evaluation of interferon effects on viral and hepatocellular gene expression are discussed.

A novel hepatitis B virus (HBV) genetic element with Rev response element-like properties that is essential for expression of HBV gene products

Many viruses possess complex mechanisms involving multiple gene products and cis-regulatory elements in order to achieve a fine control of their gene expression at both transcriptional and posttranscriptional levels. Hepatitis B virus (HBV) and retroviruses share many structural and functional similarities. In this study, by genetic and biochemical analyses, we have demonstrated the existence of a novel genetic element within the HBV genome which is essential for high-level expression of viral gene products. This element is located 3' to the envelope coding region. We have shown that this genetic element is cis acting at the posttranscriptional level and that its function is exerted at the level of RNA processing as part of transcribed sequences. This RNA element is also functional in the context of a heterologous gene. Similar to the function of Rev-Rev response element interaction of human immunodeficiency virus type 1, this element appears to inhibit the splicing process and facilitate the transport and utilization of HBV transcripts.

A novel hepatitis B virus (HBV) genetic element with Rev response element-like properties that is essential for expression of HBV gene products

Many viruses possess complex mechanisms involving multiple gene products and cis-regulatory elements in order to achieve a fine control of their gene expression at both transcriptional and posttranscriptional levels. Hepatitis B virus (HBV) and retroviruses share many structural and functional similarities. In this study, by genetic and biochemical analyses, we have demonstrated the existence of a novel genetic element within the HBV genome which is essential for high-level expression of viral gene products. This element is located 3' to the envelope coding region. We have shown that this genetic element is cis acting at the posttranscriptional level and that its function is exerted at the level of RNA processing as part of transcribed sequences. This RNA element is also functional in the context of a heterologous gene. Similar to the function of Rev-Rev response element interaction of human immunodeficiency virus type 1, this element appears to inhibit the splicing process and facilitate the transport and utilization of HBV transcripts.

Characterization of functional Sp1 transcription factor binding sites in the hepatitis B virus nucleocapsid promoter

The hepatitis B virus nucleocapsid minimal promoter contains sequence elements which are similar to the Sp1 transcription factor binding site consensus sequence. The interaction of these regulatory elements with Sp1 was examined by DNase I footprinting with purified Sp1 protein and DNase I footprinting and gel retardation analysis with nuclear extracts from human cell lines and was examined functionally with transient transfection assays in human hepatoma and Drosophila melanogaster Schneider line-2 cells. DNase I footprinting identified two regions of the nucleocapsid promoter, representing three recognition elements, that bound purified Sp1. Gel retardation analysis with Huh7 nuclear extracts demonstrated that each of the three recognition elements bound the same or similar transcription factor(s) as that recognized by the Sp1 consensus sequence recognition element. The function of the nucleocapsid promoter elements was examined by transient transfection assays in D. melanogaster Schneider line-2 cells by using these binding sites cloned into a minimal promoter element. Each of these regulatory regions transactivated transcription from the minimal promoter element in response to exogenously expressed Sp1. In addition, the second Sp1 site was shown to be an essential element of the nucleocapsid promoter in human hepatoma cells. This demonstrates that the hepatitis B virus nucleocapsid promoter contains three functional Sp1 binding sites which may contribute to the level of transcription from this promoter during viral infection.

The woodchuck hepatitis virus X gene is important for establishment of virus infection in woodchucks

All mammalian hepadnaviruses possess a gene, termed X, that encodes a protein capable of transactivating virus gene expression. The X gene overlaps the polymerase and precore genes as well as two newly identified open reading frames (ORFs) termed ORF5 and ORF6. In this investigation, we examined whether ORF5, ORF6, and the X gene were important for the replication of woodchuck hepatitis virus (WHV) in susceptible woodchucks. First, we investigated whether proteins were produced from ORF5 and ORF6 by in vitro translation of appropriate viral transcripts, searched for antibodies against the putative proteins in the sera of animals infected with wild-type virus, and looked for an antisense WHV transcript, necessary for expression of a protein from ORF6, in the livers of acutely or chronically infected woodchucks. All such experiments yielded negative results. Next, we used oligonucleotide-directed mutagenesis to introduce termination codons into ORF5 and ORF6 at two locations within each ORF. Adult woodchucks in groups of three were transfected with one of the four mutant genomes. All of these woodchucks developed WHV infections that were indistinguishable from those of animals transfected with the wild-type WHV recombinant. Polymerase chain reaction amplification and direct DNA sequencing confirmed that reversion of the mutants to a wild-type genotype did not occur. Taken together, these data indicate that ORF5 and ORF6 are not essential for virus replication and are unlikely to represent authentic genes. Finally, we generated five WHV X-gene mutants that either removed the initiation codon for protein synthesis or truncated the carboxyl terminus of the protein by 3, 16, 31, or 52 amino acids. Groups of three adult woodchucks were transfected with one of the five X-gene mutants. Only the mutant that possessed an X gene lacking 3 amino acids from the carboxyl terminus was capable of replication within the 6-month time frame of the experiment. In contrast, all seven woodchucks transfected with wild-type WHV DNA developed markers consistent with viral infection. Thus, it is likely (P < 0.01) that the WHV X gene is important for virus replication in the natural host.

Regulation of transcription from the hepatitis B virus major surface antigen promoter by the Sp1 transcription factor

The DNA-binding proteins which recognize the regulatory sequence elements of the hepatitis B virus (HBV) major surface antigen promoter were examined by gel retardation analysis, using nuclear extracts from the human hepatoma cell line Huh7. Using this assay, we identified four regions (B, D, E, and F) of the promoter that interact with the same or similar transcription factor(s). In addition, the recognition sequence for the Sp1 transcription factor bound the same or similar transcription factor(s) present in Huh7 cell nuclear extracts, and this binding was inhibited by the four major surface antigen promoter elements, B, D, E, and F. Purified Sp1 transcription factor was shown to bind to three (B, D, and F) of the major surface antigen promoter regulatory sequence elements by DNase I footprinting. Using transient transfection assays with Drosophila Schneider line 2 cells, we found that transcription from the major surface antigen promoter was transactivated by exogenously expressed Sp1, whereas transcription from the other three HBV promoters was not. Deletion analysis of the major surface antigen promoter demonstrated that the promoter region between -35 and +157 was sufficient to confer Sp1 responsiveness. This promoter region includes one of the regulatory elements footprinted by the purified Sp1 transcription factor. The function of the B, D, E, and F promoter elements was further examined by using these binding sites cloned into a minimal promoter element. Each of these regulatory regions transactivated transcription from the minimal promoter element in response to exogenously expressed Sp1. This finding demonstrates that the HBV major surface antigen promoter contains four functional Sp1 binding sites which probably contribute to the level of expression from this promoter during viral infection.

Characterization of the hepatitis B virus X- and nucleocapsid gene transcriptional regulatory elements

The regulatory DNA sequence elements that control the expression of the hepatitis B virus X- and nucleocapsid genes in the differentiated human hepatoma cell lines, Huh7, Hep3B, PLC/PRF/5, and HepG2, the dedifferentiated human hepatoma cell line, HepG2.1, and the human cervical carcinoma cell line, HeLa S3, were analyzed using transient transfection assays. In this system, the hepatitis B virus enhancer I located between coordinates 1071 (-239) and 1238 (-72) increases transcription from the X-gene promoter located between coordinates 1239 (-71) and 1376 (+67) more than 30-fold in the differentiated hepatoma and the HeLa S3 cell lines. In the dedifferentiated hepatoma cell line, HepG2.1, the enhancer I sequence increases the level of transcription from the X-gene promoter approximately 10-fold. The enhancer I subregion between coordinates 1117 (-193) and 1204 (-106) appears to be important for enhancer function only in the differentiated hepatoma cell lines, whereas the enhancer I subregion between coordinates 1222 (-88) and 1238 (-72) is required for enhancer activity in each of the cell lines examined. In all of the cell lines, the X-gene minimal promoter element was within a 138-nucleotide sequence located between coordinates 1239 (-71) and 1376 (+67). The enhancer I sequence increases transcription from the nucleocapsid promoter approximately 3- to 10-fold in the Huh7, Hep3B, PLC/PRF/5, and HeLa S3 cell lines, whereas it had little influence on the level of transcription from this promoter in HepG2 and HepG2.1 cells. The minimal nucleocapsid promoter element was within a 105 nucleotide sequence located between coordinates 1700 (-85) and 1804 (+20). This indicates that the levels of transcription from the X- and nucleocapsid gene promoters are determined in a cell-type-specific manner, in part, by the hepatitis B virus enhancer I and the corresponding minimal promoter sequence.

Temporal aspects of major viral transcript expression in Hep G2 cells transfected with cloned hepatitis B virus DNA: with emphasis on the X transcript

The major transcripts of hepatitis B virus (HBV) and the kinetics of their expression were studied in a transient expression system by transfecting partially duplicated copies of HBV genome into Hep G2 cells. By Northern blotting, six species of HBV-specific transcripts could be identified. They were the pregenomic (3.6 kb), the preS1 (2.6 kb), the preS2/S (2.2 kb), the X (0.8 kb), and two spliced (2.2 kb) RNAs, respectively. The preS2/S RNA and the spliced RNAs could be distinguished when a core gene-specific probe, which could not hybridize with the former, was used. Kinetic analysis of the expression of these RNAs revealed that the X transcript exhibited a pattern different from that of other viral transcripts. Amounts of all RNAs peaked at 24-48 hr post-transfection and then gradually declined. However, the X transcript became undetectable on Day 4 post-transfection while other viral RNAs persisted for at least 10 days. The unique expression profile of the X transcript suggested that it probably behaves as an early gene and this is consistent with its proposed role as a transactivator. Nevertheless, frameshift mutations within the X ORF had no obvious effects on the activities and temporal pattern of HBV transcription in this transient expression system.

Promoter-specific transactivation of hepatitis B virus transcription by a glutamine- and proline-rich domain of hepatocyte nuclear factor 1

The cloned transcription factor hepatocyte nuclear factor 1 (HNF1) transactivates transcription from the hepatitis B virus (HBV) large surface antigen promoter but does not influence the transcriptional activities of the other three HBV promoters. This indicates that this transcription factor can differentially influence the activities of the HBV promoter. By using a transient-transfection system, the major domain of the HNF1 polypeptide involved in transcriptional activation of the large surface antigen promoter in the human hepatoma cell line HepG2.1 has been mapped to a region that is rich in glutamine and proline residues (9 of 18) and is different from the previously identified regions of this factor responsible for in vitro transcriptional activation of a promoter containing human albumin promoter HNF1 binding sites. The human albumin promoter HNF1 binding site mediates transcriptional activation through the same HNF1 polypeptide domain as the HBV large surface antigen promoter HNF1 binding site in transient-transfection assays with HepG2.1 cells, suggesting that HNF1 may possess multiple transcriptional activation domains.

In vitro transcription of the hepatitis B virus gene by nuclear extracts of human hepatoma cells

In vitro transcription of hepatitis B virus DNA (HBV DNA) was studied using nuclear extracts of human hepatoma cell lines. RNA polymerase II-dependent run-off transcription of pre-S mRNA under the control of pre-S1 promoter was observed in nuclear extracts obtained from HepG2 and PLC/PRF/5 cells, and the efficiencies in these extracts were significantly higher than those in nuclear extracts of non-liver cells such as HeLa, Molt-4, and Ehrlich. Analysis of run-off transcripts by the pre-S1 promoter, using deletion mutants of HBV DNA as templates and synthetic oligonucleotides as competitors, showed that hepatocyte nuclear factor 1 was necessary for initiation of in vitro transcription of pre-S mRNA. The run-off transcript of pregenome RNA was also detected and its initiation site was determined. Nuclear extracts of not only hepatoma cells but non-liver cells were active in transcription of pregenome RNA in vitro. However, run-off transcripts of S mRNA and X mRNA were not observed in this system. These results suggest that there were some differences between the mechanisms of HBV DNA transcription in vitro and in vivo. This in vitro transcription system will be useful for clarifying the mechanism regulating transcription of HBV DNA since the biochemical and functional characteristics of the nuclear factors can readily be analyzed.

HBx gene of hepatitis B virus induces liver cancer in transgenic mice

The exact role of hepatitis B virus in the development of liver cancer is not known. The recent identification of a viral regulatory gene HBx suggests a possible direct involvement of the virus whereby the HBx protein, acting as a transcriptional transactivator of viral genes, may alter host gene expression and lead to the development of hepatocellular carcinoma. We have tested this possibility of placing the entire HBx gene under its own regulatory elements directly into the germline of mice. Transgenic animals harbouring this viral gene succumbed to progressive histopathological changes specifically in the liver, beginning with multifocal areas of altered hepatocytes, followed by the appearance of benign adenomas, and proceeding to the development of malignant carcinomas. Male mice developed disease and died much earlier than females. This transgenic animal model appears ideal for defining the molecular events that follow the expression of the viral HBx gene and are responsible for the development of liver cancer.

Early and frequent detection of HBxAg and/or anti-HBx in hepatitis B virus infection

To clarify the significance of the X gene of hepatitis B virus, we have tested for anti-HBx in the serum and HBxAg in the liver at different stages of the natural history of hepatitis B virus infection. Sera were screened by enzyme-linked immunosorbent assay and positive results confirmed by immunoblot. Purified recombinant MS2 Pol-HBx fusion protein was used as target for both assays. Among serial sera of patients with nonfulminant acute hepatitis, 24 of 64 patients (37.5%) were positive for anti-HBx. In fulminant cases, 15 of 36 patients (42%) had anti-HBx. In chronic hepatitis patients with high rates of hepatitis B virus replication, we found a significantly (p less than 0.01) higher prevalence of anti-HBx, 14 of 25 patients (56%), than in those with low replication, 14 of 66 patients (21%), or among asymptomatic HBsAg carrier blood donors (20 of 126 = 16%) without detectable hepatitis B virus replication (p less than 0.0001). The highest prevalence of anti-HBx was found in HBsAg carriers with cirrhosis (41 of 54 patients = 76%) and/or with hepatocellular carcinoma (18 of 33 patients = 54%). The findings suggest that anti-HBx appears as a common and early marker of hepatitis B virus infection, transient in self-limited hepatitis but persisting with progression to chronicity. In chronic hepatitis, the prevalence of anti-HBx correlated with the intensity and duration of hepatitis B virus replication but neither with the severity of the liver disease nor with malignant transformation per se.(ABSTRACT TRUNCATED AT 250 WORDS)

The hepatitis B virus X-C fusion protein is unlikely to be produced by the mechanism of ribosomal frameshifting

A novel hepatitis B viral (HBV) protein of 35-40 kDa, characterized by antibodies and proposed as an X-C fusion protein, was previously described in core particles isolated from HBV-, WHV-, and GSHV-infected livers. The X and C genes are two adjacent genes in all mammalian hepadnaviruses but are not contiguous in WHV and GSHV. After examination of the X and preC/C junction sequences of 10 HBV, 4 WHV, and 1 GSHV, we found that the ORF of preC can be extended 7 more sense codons upstream so that X overlaps with the preC/C gene in all sequences. The number of overlapping base pairs (bp) is varied: 46 bp in HBV, 19 bp in WHV, and 10 bp in GSHV. In this region a conserved A-track was found to be followed by a pair of inverted repeats, suggesting that a ribosomal frameshift may occur for X-C fusion protein production. To assess this possibility, we have used an in vitro transcription and translation coupling system to identify X-C protein production. Two recombinant SP6 plasmids were used. One contained a full length of the X and preC/C gene of wild-type HBV-DNA and the other fused the X-preC/C gene by inserting a 10-bp HindIII linker at the junction of the X-preC/C region. No X-C fusion protein was detected from the wild-type plasmid. In contrast a large amount of X-C fusion protein was produced from the linker-inserted clone. It appears, therefore, that the X-C fusion protein is unlikely to be produced via the mechanism of ribosomal frameshifting.

Organization of the X gene region of the hepatitis B virus genome

In the hepatitis B virus (HBV) genome four long open reading frames (ORFs) have been found that encode the virus core, surface and polymerase proteins as well as a protein that appears to be involved in virus gene expression (X). However, all HBV genomes examined contain two addition ORFs designated ORF5 and ORF6. ORF5 is located on the same strand as the four known viral genes and is 70-100 codons in size. ORF6 is located on the DNA strand complementary to the one that encodes the other virus genes, and is approximately 210 codons in length. Both ORFs are located in the X gene region which corresponds to the 3' terminus of the linear RNA genome. Northern blot analysis identified an X region specific transcript of approximately 0.7 kb. Although this transcript may be the template for the translation of the X gene protein it may also be involved in the expression of the protein encoded by ORF5. The promoter for this transcript may consist, in part, of the 15 residue sequence GCYTGYYTTGCYCGC because this sequence is near the 5' end of the transcript, it is highly conserved among hepadnaviruses, and it contains sequences involved in RNA polymerase binding. Also, the nucleotides within this region of the hepadnavirus genome are capable of forming a stable (G = -18 kcal/mole) hairpin structure. Understanding the organization and gene expression of the X region may be crucial in expanding our knowledge on the biology of HBV.

Differentiation-specific transcriptional regulation of the hepatitis B virus large surface antigen gene in human hepatoma cell lines

The transcriptional activities of the four hepatitis B virus promoters were compared in three differentiated hepatoma cell lines, HepG2, Hep3B, and PLC/PRF/5; a dedifferentiated subline of HepG2, HepG2.1; a human cervical carcinoma cell line, HeLa S3; and a mouse fibroblast cell line, NIH 3T3. The plasmid constructs, which contain the complete hepatitis B virus genome directing the expression of the luciferase reporter gene, were analyzed by transient transfection assays. The relative orders of the levels of the transcriptional activities of the four promoters were similar in each of the cell lines. The major surface antigen and X-gene promoters displayed the highest activity levels, the core promoter activity level was less than or similar to the activity levels of these two promoters, and the large surface antigen promoter had the lowest activity level in all of the cell lines examined. The core promoter demonstrated an approximately 2- to 20-fold higher relative level of expression in the differentiated hepatoma cell lines, suggesting that this promoter might be preferentially active in these cells. The relative level of activity of the large surface antigen promoter in the differentiated hepatoma cell lines was approximately 5 to 90 times greater than that observed in the other cell lines, indicating that the activity of this promoter is highly specific for differentiation state and cell type. Deletion analysis of the large surface antigen promoter demonstrated that the sequence element responsible for the differentiation state-specific expression from this promoter is located between nucleotides 2719 and 2733 (-90 and -76). Within this sequence element is a binding site (GTTAATCATTACT) for the liver-specific transcription factor hepatocyte nuclear factor 1 (HNF1). This indicates that the preferential expression from the large surface antigen promoter in the differentiated hepatoma cell lines is probably mediated by HNF1 or an HNF1-related transcription factor.

Malignant transformation of immortalized transgenic hepatocytes after transfection with hepatitis B virus DNA

Persistent infection by hepatitis B virus (HBV) is epidemiologically correlated with the prevalence of hepatocellular carcinoma, but its role in tumor development is not yet understood. To study the putative oncogenic potential of HBV, a non-malignant immortal mouse hepatocyte line FMH202 harboring metallothionein promoter-driven simian virus 40 large tumor antigen was transfected with HBV DNA. All stably transfected clones which replicated HBV displayed malignant growth characteristics in soft agar and were tumorigenic upon inoculation in nude mice. The nude mice tumors were histologically classified as differentiated or anaplastic hepatocellular carcinomas. As with human liver carcinomas, rearrangements of in vitro integrated HBV sequences were observed in the nude mouse tumors, and in tumor-derived cell lines. In one case, expression of viral core and surface antigens was blocked in the tumors, correlating with hypermethylation of the HBV genome. However, the expression of X gene was maintained in most tumors and tumor-derived cell lines. X protein was detected in nuclei by immune fluorescence and by immune blot. These results provide the first demonstration that HBV displays oncogenic potential in an experimental system. This system could be useful to functionally identify HBV genes which convey a tumorigenic phenotype.

A trans-activator function is generated by integration of hepatitis B virus preS/S sequences in human hepatocellular carcinoma DNA

The X gene of wild-type hepatitis B virus or integrated DNA has recently been shown to stimulate transcription of a variety of enhancers and promoters. To further delineate the viral sequences responsible for trans-activation in hepatomas, we cloned the single hepatitis B virus insert from human hepatocellular carcinoma DNA M1. The plasmid pM1 contains 2004 base pairs of hepatitis B virus DNA subtype adr, including truncated preS/S sequences and the enhancer element. The X promoter and 422 nucleotides of the X coding region are present. The entire preC/C gene is deleted. In transient cotransfection assays using Chang liver cells (CCL 13), pM1 DNA exerts a 6- to 10-fold trans-activating effect on the expression of the pSV2CAT reporter plasmid. The trans-activation occurs by stimulation of transcription and is dependent on the simian virus 40 enhancer in the reporter plasmid. Deletion analysis of pM1 subclones reveals that the trans-activator is encoded by preS/S and not by X sequences. A frameshift mutation within the preS2 open reading frame shows that this portion is indispensable for the trans-activating function. Initiation of transcription has been mapped to the S1 promoter. A comparable trans-activating effect is also observed with cloned wild-type hepatitis B virus sequences similarly truncated. These results show that a transcriptional trans-activator function not present in the intact gene is generated by 3' truncation of integrated hepatitis B virus DNA preS/S sequences.

Detection of hepatitis B virus transcripts in patients with chronic liver disease

Hepatitis B virus (HBV) transcription was studied by Northern blot analysis on total cellular RNA purified from liver biopsies in 70 patients with chronic liver disease (24 HBsAg positive, 15 antiHBs and/or antiHBc positive, 31 HBV negative). No transcripts were found in the HBV negative and in the antiHBs and/or antiHBc positive patients. In the others, three major RNA species were identified: i. a 3.5 kb transcript corresponding to the RNA pregenome; ii. 2.4-2.1 kb transcript corresponding to the s and preS1 gene RNA; iii. lower molecular weight species. All three forms were present simultaneously only in patients with active viral replication, with a strict relation between the presence of the 3.5 kb RNA in the liver and serum HBV-DNA. In conclusion, Northern blot analysis can easily be performed to study viral replication and it can contribute to a better understanding of the molecular processes underlying HBV infection and leading to liver disease in man.

The preS2/S region of integrated hepatitis B virus DNA encodes a transcriptional transactivator

Hepatitis B virus (HBV) is regarded as the main aetiologic factor in the development of human hepatocellular carcinoma (HCC), one of the most frequent fatal malignancies worldwide. Detection of integrated HBV sequences in the cellular DNA of almost all HCCs studied, and the recent finding that the integrated HBV open reading frame (orf) X encodes a transactivating activity, supports the notion that integrated HBV DNA could contribute to liver carcinogenesis by activation of cellular genes in trans. But not all HCCs seem to harbour a functional orf X. We report here that 3'-truncated preS2/S sequences in integrated HBV DNA of liver cell carcinomas encode a so far unidentified transcriptional trans-activation activity. This activity is also produced by an artificially 3'-truncated preS2/S gene of the wild-type HBV genome. Besides the simian virus 40 promoter of the reporter plasmid pSV2CAT, the promoter of the human c-myc oncogene can also be activated. These results, taken together with the fact that preS/S is the only HBV gene found to be integrated in almost every HBV-related HCC analysed so far, indicate that trans-activation by integrated preS2/S sequences is a possible mechanism for HBV-associated oncogenesis.

The hepatitis B virus X-gene product trans-activates both RNA polymerase II and III promoters

The transcriptional regulatory activity of the human hepatitis B virus (HBV) X-gene product was investigated. We demonstrate a new property for the HBV X-gene, the strong transcriptional trans-activation of promoters for class III genes. The stimulation of RNA polymerase III (pol III) as well as pol II promoters is shown in cells transiently transfected with the X-gene, and after its stable integration into hepatocytes. We demonstrate that X-gene containing cells stimulate the frequency of pol III transcription initiation by 20- to 40-fold, and accelerate the rate of formation of stable pol III initiation complexes in a manner indistinguishable from that of adenovirus E1a protein. Since the transcription factor TFIIIC has been shown to be limiting in the formation of stable pol III initiation complexes, template commitment experiments were performed which titrate the level of this factor in extracts. We show that X-protein containing extracts are far more efficient in forming stable pol III preinitiation complexes that cannot be competed away upon addition of a second template, indicating that TFIIIC is very probably a target of the X-protein. Thus, the HBV X-protein is apparently a member of a family of trans-activators capable of stimulating both pol II and III promoters, which includes the adenovirus E1a-protein and SV40 t antigen.

Characterization of hepatitis B virus major surface antigen gene transcriptional regulatory elements in differentiated hepatoma cell lines

The regulatory DNA sequence elements that control the expression of the hepatitis B virus major surface antigen gene in the hepatoblastoma cell line HepG2 were analyzed by using transient transfection assays. In this system, the hepatitis B virus enhancer increases transcription from the surface antigen promoter approximately twofold. The promoter elements regulating the expression of this gene are within a 200-nucleotide sequence located immediately upstream of the transcription initiation sites. The promoter consists of an 85-nucleotide distal element which increases transcription from the surface antigen gene by two- to fourfold and a proximal element of approximately 115 nucleotides which is essential for transcriptional activity. The proximal and distal promoter elements were shown to bind factors present in HepG2 nuclear extracts, which is consistent with the regulatory role demonstrated for these sequences. The regulatory role of these promoter sequences in the hepatocellular carcinoma cell lines PLC/PRF/5 and Hep3B was also demonstrated, indicating similar transcriptional regulation of the surface antigen gene in each of these differentiated hepatoma cell lines.

Characterization of woodchuck hepatitis virus DNA and RNA in the hepatocellular carcinomas of woodchucks

Integration and transcription of woodchuck hepatitis virus DNA were studied by Southern and Northern blot analysis in 26 hepatocellular carcinomas and in adjacent nontumor tissue of woodchucks (Marmota monax). All liver tissue chronically infected with woodchuck hepatitis virus contained various amounts of episomal and replicative forms of woodchuck hepatitis virus DNA: episomal and replicative forms of woodchuck hepatitis virus DNA without integration were found in six tumors, episomal and integrated woodchuck hepatitis virus DNA was observed in 18 tumors and exclusively integrated woodchuck hepatitis virus DNA was found in two tumors. In most tumors and in all of the liver tissues chronically infected with woodchuck hepatitis virus, two major woodchuck hepatitis virus RNA species (3.7 and 2.1 kilobases) were detected. In tumors of two other animals (HW76 and HW89) with integrated woodchuck hepatitis virus DNA, only single major transcripts of 3.5 and 2.5 kilobases, respectively, were detected. Hybridization with subcloned woodchuck hepatitis virus DNA probes showed that both aberrant transcripts lacked the C gene and a part of the pre-S1 gene; characterization of corresponding integrated woodchuck hepatitis virus DNA sequences revealed that the C gene was deleted in one tumor, although not in the other. In agreement with the nucleic acid data, we found expression of core protein by Western blotting only in chronically infected liver tissue of these animals, but not in the corresponding tumors. Deletion of the C gene in mRNA may be due to deletion of this gene in the integrated sequences or due to transcriptional regulation.

Complete nucleotide sequence of a molecular clone of woodchuck hepatitis virus that is infectious in the natural host

Woodchuck hepatitis virus (WHV) DNA was cloned from viral particles obtained from the serum of a woodchuck with a naturally acquired infection. The complete nucleotide sequence of the virus genome was determined and found to be 3323 base pairs long. Transfection experiments demonstrated that the recombinant WHV DNA was infectious in each of 18 woodchucks tested and established a chronic carrier state in 1 of 13 neonates and 3 of 5 adult animals. WHV DNA from serum particles from the chronically infected neonate was cloned and the nucleotide sequence of three independent recombinants was compared directly with that of the input recombinant DNA. The consensus sequence of the three progeny genomes was identical to that of the parental DNA sequence. Therefore, transfection of woodchuck livers with recombinant WHV DNA induces active virus replication and gene expression and yields progeny genomes that are faithful copies of the input virus genome.