Transcription of the hepatitis B surface antigen gene in mouse cells transformed with cloned viral DNA

Co-Transcriptional Regulation of HBV Replication: RNA Quality Also Matters

Chronic hepatitis B (CHB) virus infection is a major public health burden and the leading cause of hepatocellular carcinoma. Despite the efficacy of current treatments, hepatitis B virus (HBV) cannot be fully eradicated due to the persistence of its minichromosome, or covalently closed circular DNA (cccDNA). The HBV community is investing large human and financial resources to develop new therapeutic strategies that either silence or ideally degrade cccDNA, to cure HBV completely or functionally. cccDNA transcription is considered to be the key step for HBV replication. Transcription not only influences the levels of viral RNA produced, but also directly impacts their quality, generating multiple variants. Growing evidence advocates for the role of the co-transcriptional regulation of HBV RNAs during CHB and viral replication, paving the way for the development of novel therapies targeting these processes. This review focuses on the mechanisms controlling the different co-transcriptional processes that HBV RNAs undergo, and their contribution to both viral replication and HBV-induced liver pathogenesis.

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.

Synthetic RNAi triggers and their use in chronic hepatitis B therapies with curative intent

Current therapies for chronic hepatitis B virus infection (CHB) - nucleos(t)ide analogue reverse transcriptase inhibitors and interferons - result in low rates of functional cure defined as sustained off-therapy seroclearance of hepatitis B surface antigen (HBsAg). One likely reason is the inability of these therapies to consistently and substantially reduce the levels of viral antigen production. Accumulated evidence suggests that high serum levels of HBsAg result in exhaustion of the host immune system, rendering it unable to mount the effective antiviral response required for HBsAg clearance. New mechanistic approaches are required to produce high rates of HBsAg seroclearance in order to greatly reduce off-treatment disease progression. Already shown to be a clinically viable means of reducing gene expression in a number of other diseases, therapies based on RNA interference (RNAi) can directly target hepatitis B virus transcripts with high specificity, profoundly reducing the production of viral proteins. The fact that the viral RNA transcripts contain overlapping sequences means that a single RNAi trigger can result in the degradation of all viral transcripts, including all messenger RNAs and pregenomic RNA. Advances in the design of RNAi triggers have increased resistance to degradation and reduced nonspecific innate immune stimulation. Additionally, new methods to effectively deliver the trigger to liver hepatocytes, and specifically to the cytoplasmic compartment, have resulted in increased efficacy and tolerability. An RNAi-based drug currently in clinical trials is ARC-520, a dynamic polyconjugate in which the RNAi trigger is conjugated to cholesterol, which is coinjected with a hepatocyte-targeted, membrane-active peptide. Phase 2a clinical trial results indicate that ARC-520 was well tolerated and resulted in significant, dose-dependent reduction in HBsAg for up to 57days in CHB patients. RNAi-based therapies may play an important role in future therapeutic regimes aimed at improving HBsAg seroclearance and eliminating the need for lifelong therapy. This paper forms part of a symposium in Antiviral Research on "An unfinished story: from the discovery of the Australia antigen to the development of new curative therapies for hepatitis B."

Variations in the S and P regions of the hepatitis B virus genome under immunosuppression in vitro and in vivo

To provide a basis for improved prevention and treatment of hepatitis B virus (HBV) re-infection after liver transplantation, variations in the S and P genes of HBV under immunosuppression in vitro and their association with patient prognosis were investigated. For the in vitro study, HepG2.2.15 hepatocellular carcinoma cells stably producing HBV particles were treated with the immunosuppressants methylprednisolone (MP) and tacrolimus (FK506) at doses found to be non-toxic by the methylthiazolyl tetrazolium (MTT) cell viability assay. MP dose-dependently inhibited HBV DNA expression in HepG2.2.15 cells, while FK506 did not, as determined by quantitative real-time PCR (qRT-PCR). By gene sequencing, both MP and FK506 were found to cause variations in HBV S, P, and S/P overlapping regions. MP- but not FK506-induced mutations were common in the glucocorticoid response element of the P region, while both immunosuppressants caused mutations outside the nucleoside analogue resistance sites. For the in vivo study, 14 patients with HBV-related end-stage liver disease re-infected after liver transplantation, and 20 cases without HBV re-infection as controls, were studied. Seventy-five percent of re-infected recipients showed multi-loci amino acid mutations at different sites besides lamivudine (LAM)-resistant loci in the P region, including in the glucocorticoid response element. Fifty percent of re-infected recipients had mutations in the "a" determinant region and flanking sequences. Re-infection was associated with negative serum hepatitis B immunoglobulin (HBIG), as measured by a microparticle capture enzyme immunoassay. Nucleotide mutations in the S region caused missense or synonymous mutations, which caused synonymous mutations in the overlapping P region. These results showed that effects of immunosuppressants on HBV genes in vitro were different from those in clinical recipients. Positive HBV DNA and gene mutations pre-transplantation were factors affecting re-infection post-transplantation. Multiple mutations found in the P and S genes suggest that the formation of quasispecies contributes to HBV re-infection after liver transplantation.

Novel type of hepatitis B virus mutation: replacement mutation involving a hepatocyte nuclear factor 1 binding site tandem repeat in chronic hepatitis B virus genotype E

The genetic diversity of hepatitis B virus (HBV) strains has evolved through mutations such as point mutations, deletions or insertions, and recombination. We identified and characterized a novel type of mutation which is a complex of external insertion, deletion, and internal duplication in sequences from one of six patients with chronic hepatitis B virus genotype E (HBV/E). We provisionally named this mutation a "replacement mutation"; the core promoter upstream regulatory sequence/basic core promoter was replaced with a part of the S1 promoter covering the hepatocyte nuclear factor 1 (HNF1) binding site, followed by a tandem repeat of the HNF1 site. A longitudinal analysis of the HBV population over 6 years showed the clonal change from wild-type HBV/E to replacement-mutant type, resulting in a lower hepatitis B (HB) e antigen titer, a high HBV DNA level in serum, and progression of liver fibrosis. In an in vitro study using a replication model, the replacement-mutant HBV showed higher replication levels than the wild-type HBV/E replicon, probably mediated by altered transcription factor binding. Additionally, this HNF1 site replacement mutation was associated with excessive HB nucleocapsid protein expression in hepatocytes, in both in vivo and in vitro studies. This novel mutation may be specific to HBV genotype E, and its prevalence requires further investigation.

Late transient expression of human hepatitis B virus genes in monkey cells

The expression of human hepatitis B virus (HBV) surface (HBS) and e (HBe) antigens has been studied comparatively in monkey and mouse cell lines co-transfected with HBV DNA and the dominant selective marker aminoglycoside 3'-phosphotransferase gene. We have found that the kinetics and stability of expression of the HBS gene varies with the cell lines used. Only a late transient expression of both HBS and HBe is observed between 1 and 5 weeks after transfection in monkey kidney Vero cells transfected with the complete HBV genome, while a permanent expression of HBS and HBe is obtained in mouse cells. HBS and HBe are excreted into the cell culture medium. HBe is expressed in cells transfected with the complete HBV genome, but not with isolated HBS gene. In clones of Vero cells transformed with the HBS gene, HBV sequences were rearranged or lost.

Complete nucleotide sequences and the characteristics of two hepatitis B virus mutants causing serologically negative acute or chronic hepatitis B

Hepatitis B virus (HBV) DNA was amplified by the polymerase chain reaction from the sera of a patient with acute hepatitis and a patient with chronic hepatitis. Both patients were negative for serum hepatitis B surface antigen and hepatitis B core antibodies and had been previously diagnosed as non-A, non-B, non-C, non-D, non-E hepatitis. The nucleotide sequence revealed an 8-nucleotide deletion in the X-gene coding region creating a C-terminally truncated X protein, and probable mutation of the enhancer II/core promoter element. In addition, DR2 showed a T-to-C mutation at the extreme 5'-terminus. These mutations within the X-gene coding region must suppress replication and expression of HBV DNA, and this seems to be responsible for absence of serological markers despite the presence of HBV infection.

Inhibition of hepatitis B virus surface antigen gene expression in carcinogen-induced liver tumors from transgenic mice

We previously showed that hepatitis B surface antigen (HBsAg)-producing transgenic mice were more sensitive to hepatocarcinogens than their normal littermates were. We have now investigated the regulation of hepatitis B virus (HBV) gene expression in carcinogen-induced liver tumors of HBV-carrier transgenic mice and in three cell lines derived from tumor samples. Transcription of the S gene was repressed in 17 tumors even though they had normal levels of liver-specific mRNAs such as albumin and transferrin. Three hepatoma cell lines, derived from independent tumor samples, were analyzed for their capacity to express the S gene after transfection of cloned DNA. Although they no longer expressed the endogenous S gene, they were still able to express it from transfected viral DNA both transiently and stably. The loss of HBsAg expression in tumors and in the cell lines was accompanied by de novo methylation of the S region, which is a way to permanently repress gene expression. Our data confirm in an animal model previous observations of S-gene expression in human hepatocarcinoma and suggest a role for its downregulation in tumor progression.

Hepatitis B virus proteins expressed by recombinant vaccinia viruses: influence of preS2 sequence on expression surface and nucleocapsid proteins in human diploid cells

Fifteen vaccinia virus (VV) recombinants derived from VV strains Praha, LIVP and DD (i.e. Dryvax Wyeth vaccine-derived) and expressing genes for S, preS2-S or c antigens of hepatitis B virus (HBV) were tested in monkey CV-1 cells and human diploid LEP cells. The production of infectious virus was found to be alike in all the recombinants and parental viruses as well. However, several recombinants produced markedly lesser amounts of S and preS2 antigens in LEP cells than in CV-1 cells. This reduction was independent of the parental virus used. There was, however, a relationship between the production of preS2 in CV-1 cells and the production of S and preS2 antigens in LEP cells; in general, recombinants efficiently inducing preS2 antigen formation in CV-1 cells produced markedly reduced amounts of S and preS2 antigens in LEP cells. Reduction of HBV antigen production in LEP cells was not apparent in recombinants expressing only S or c antigens of HBV, and the production of c antigen by double recombinants was not influenced by simultaneous expression of preS2 and S. The various recombinants also differed in the ratio of S:preS2 antigen formation. This difference seemed to be associated with the length of the untranslated leader sequence preceding preS2 but not with the parental virus or cell type used. The titers of antibodies against S and preS2 antigens induced in mice immunized with different recombinants differed markedly. The differences in the ratio of S:preS2 antigen production in vitro were not reflected in vivo by S:preS2 antibody ratio.

Unconventional processing of the 3' termini of the Epstein-Barr virus DNA polymerase mRNA

Northern blot analysis of the Epstein-Barr virus DNA polymerase mRNA identified two discrete sizes of virally encoded polymerase transcripts, 5.08 kb detected in strains P3HR1, Raji, W-91, and FF-41 and 3.7 kb detected solely in the prototype B95-8 strain. 3' S1-nuclease mapping and analysis of cDNA sequence generated by RNA-based PCR demonstrated that the 3.7-kb polymerase mRNA from B95-8 terminates 484 base pairs downstream of the open reading frame in a region of the genome remarkable for its lack of an apparent polyadenylylation signal. Moreover, between the cleavage point and the poly(A) tract of the cDNAs are a series of inserted nucleotides, mostly adenosine and uridine residues of unknown origin. A similar analysis of the 3' terminus of the 5.0-kb mRNA from the other cell lines revealed that polyadenylylation occurs 1.4 kb downstream of the B95-8 terminus. This region is deleted in B95-8, which accounts for the alternate upstream terminus used in B95-8. Like the 3.7-kb terminus, the 5.0-kb terminus lacks a canonical polyadenylylation signal, but contains a rarely used UAUAAA sequence 32 bp upstream of the poly(A) tail. These results indicate that the mRNA encoded by the Epstein-Barr virus DNA polymerase gene is polyadenylylated at two different termini without the use of canonical signals, raising the possibility of involvement of a virus-encoded factor in 3' processing of this message.

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.

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.

Complex regulation of transcription from the hepatitis B virus major surface antigen promoter in human hepatoma cell lines

A detailed mutational analysis of the regulatory DNA sequence elements that control expression of the hepatitis B virus major surface antigen gene was performed in the human hepatoma cell lines HepG2.1 and Huh7, using transient transfection assays. Seven regions (A to G) of the major surface antigen promoter located within 200 nucleotides of the RNA initiation site have been identified which influence the level of transcription from this promoter. The three distal regions (A to C), located between -188 and -68, appear to possess a level of redundancy in their ability to influence the transcriptional activity from the major surface antigen promoter. The simultaneous deletion of regions A, B, and C resulted in an approximately fourfold reduction in transcription from the major surface antigen promoter. Region D, located between -67 and -49, is an essential element of the major surface antigen promoter. The three proximal regions (E to G) are located within 45 nucleotides of the major transcription initiation site. Region E prevents the negative influence of region F and can compensate for the effect of mutation of region G on transcription from the major surface antigen promoter. Region G can compensate for the effect of the loss of a functional region E sequence on the transcriptional activity of the major surface antigen promoter only in the absence of a functional region F sequence. These results imply that the level of expression of the major surface antigen gene is controlled by the complex interplay between a minimum of six transcription factors which activate and one transcription factor which represses transcription from this gene.

Diversity of hepatitis B virus X gene-related transcripts in hepatocellular carcinoma: a novel polyadenylation site on viral DNA

RNA was isolated from tissue of two patients with hepatocellular carcinoma developed on the background of a chronic hepatitis B virus infection. For identification and characterization of 3' ends of X gene open reading frame (ORF)-related transcripts, RNA was reverse transcribed into cDNA and subjected to polymerase chain reaction. Cloned amplification products from tumor tissue of one patient represented an approximately even distribution of transcripts terminating at the established poly(A) signal (standard transcripts) and of truncated transcripts terminating at a CATAAA poly(A) signal within the 3' end region of X gene ORF (truncated transcripts). Amplified cDNA from tumor tissue of the second patient could be attributed mainly to the standard type of transcripts, whereas cDNA from the nontumor tissue of the same patient could be assigned to four groups of transcripts: (i) standard transcripts, (ii) transcripts with internal deletions affecting the 3' end of the X gene, (iii) truncated transcripts, and (iv) hybrid transcripts displaying 5' sequences from the X gene ORF fused to cellular sequences.

Enhancement of gene expression by somatic hybridization with primary cells: high-level synthesis of the hepatitis B surface antigen in monkey Vero cells by fusion with primary hepatocytes

Vero cells transfected with the S gene encoding the surface antigen (HBsAg) of the hepatitis B virus (HBV) synthesize HBsAg at low levels. We have obtained a large increase in S gene expression by somatic hybridization of Vero cells with primary hepatocytes, which are the natural target cells for HBV infection. Fusion with cells other than hepatocytes did not enhance expression of the S gene. The Vero/hepatocyte hybrid clones analyzed are stable and have maintained a high level of HBsAg synthesis over prolonged periods. Hybrid cell lines may be of general interest for the high-level synthesis of proteins using cloned genes.

Synthesis and immunogenicity of hepatitis B virus envelope antigen expressed by recombinant vaccinia virus. Finding of retention signal in the C-terminal portion of the preS1 domain of subtype adyw

Five different recombinant vaccinia viruses expressing the envelope antigen of hepatitis B virus (HBsAg) under the control of the P7.5 promoter were constructed. Cell cultures infected with some of the recombinant viruses synthesized both middle (M) and major surface (S) protein of HBsAg. It was shown that the length of the nontranslated sequence preceding preS2-ATG influenced the extracellular or intracellular HBV antigen distribution and the preS2:S antigen ratio. Some recombinants synthesized an M protein that was enlarged by additional 35 amino acids of preS1 domain and was entirely retained within the infected cells. Antibody responses to the S and preS2 antigens in mice revealed significant differences in the immunogenicity of individual recombinants.

Expression pattern of the hepatitis B virus genome in transfected mouse fibroblasts

Permanent mouse fibroblast LTK- cells were transfected with dimeric hepatitis B virus (HBV) DNA linked to the simian virus 40 (SV40) early promoter/enhancer. Many clones stably expressed high levels of polyadenylated RNAs encoding hepatitis B surface (HBs) proteins (2.1 kb), HBe protein (3.6 kb), and HBx protein (0.6 kb). Although a chimeric RNA (4.0 kb) probably starting from the SV40 promoter was also synthesized, transcription of viral RNAs was predominantly directed by HBV promoters and its terminator. In contrast to HBV-transfected liver cells, the fibroblasts expressed only pregenomic 3.6-kb transcripts starting 5' to, but not within, the precore sequence. Thus, no normal core protein could be synthesized, but the cells expressed and secreted HBe protein of heterogeneous size. Small and middle HBs proteins were strongly expressed, while large HBs protein was almost absent. HBx mRNA expression was more efficient in mouse fibroblasts than in human hepatoma cells and 18-kDa HBx protein was exclusively detected in purified nuclei. Expression of HBe, small and middle HBs, and HBx proteins apparently does not require hepatic factors. Underexpression of HBc mRNA and large HBs mRNA suggests that activity of their promoters depends on cell-type-specific transcription factors.

Priming effect of recombinant vaccinia virus coding for the middle hepatitis B surface antigen

Recombinant vaccinia virus expressing the middle hepatitis B virus surface antigen was incapable of inducing marked antibody response against the S and pre-S2 antigenic specifities in mice. However, mice immunized with this virus produced antibodies to both these antigens after the following administration of subtreshold doses of plasmatic hepatitis B surface antigen.

A recombinant vaccinia virus expressing hepatitis B virus middle surface protein. Restricted expression of HBV antigens in human diploid cells

Several vaccinia virus recombinants inducing the synthesis of the middle surface (M) protein of hepatitis B virus (HBV) were constructed. One of them, denoted v137, was examined in some detail. The virus replicated nearly to the same extent in various cell lines, viz. human embryo diploid fibroblast LEP and MRC-5 cells, rabbit embryo fibroblast REF cells, TK- rat RAT-2 cells, and green monkey CV-1 cells. However, the production of M protein was found considerably lower in the human LEP and MRC-5 than in the other cells examined. In addition, the kinetics of M formation were different in these two cell systems, LEP cells lagging significantly behind CV-1 cells. The low-level production of M protein in LEP cells was not increased by repeated v137 passages in LEP cells, nor by a passage in a laboratory worker accidentally infected with the v137 virus, nor by shortening the leader sequence preceding the translation initiation codon. The greater part of the M antigen was found to be cell associated, more so in the cells of human than monkey origin. From the major HBV S antigen (HBsAg) isolated from the plasma of chronically infected subjects, the antigen released by cell destruction differed by binding to polymerized human albumin. This property was utilized in ELISA to detect anti-preS2 antibody. Rabbits inoculated intradermally with the v137 virus developed antibodies reactive in this assay as well as with a synthetic peptide corresponding in the amino acids 14-34 of the NH2 terminus of the HBsAg preS2 region.

In vitro regulation of human hepatitis B virus core gene transcription

In the present study we used a HeLa whole cell extract transcription system to map the transcription start sites and the minimal promoter of the hepatitis B virus core gene. Two initiation sites located at residues 1792 +/- 5 and 1817 +/- 5 were identified. The minimal upstream region essential and sufficient for transcription was defined to a 105-base pair DNA fragment. These results are identical to the in vivo mapping of the transcription start sites and the minimal core gene promoter. When in vitro transcription elongation was carried out in the presence of the anionic detergent Sarkosyl, known to enhance premature transcription termination (attenuation), two short transcripts (as well as two run-offs) were synthesized. Kinetic studies indicated that the short transcripts resulted from a block to transcription elongation and not from RNA processing. RNA mapping showed that the short attenuated transcripts indeed initiated at the two core gene initiation sites and both prematurely terminated at nucleotide 1966 +/- 5, defined as the attenuation site. This site is located in the attenuator RNA within a uridine-rich sequence preceded by a stable hairpin structure. Attenuation at the same site occurred when transcription of the core gene was directed by the Ad2 major late promoter (MLP) and when the poly(A) signal, which precedes the attenuation site, was mutated from TATAAA to TAGAAA. We suggest that the elongation block at nt 1966 +/- 5 in vivo exerts a dual function: first, it regulates the level of RNA by attenuation during the first cycle of transcription and, second, it acts as a termination site at the end of the primary RNA transcript.

Hepatitis B virus transcripts and surface antigen in human peripheral blood lymphocytes

The presence of hepatitis B virus (HBV) genome, transcripts, and antigens (HBsAg, HBcAg, HBeAg) was examined in peripheral blood lymphocytes (PBL) from 12 patients with HBsAg-positive (B) chronic active hepatitis (CAH) and 8 normal donors by Southern and Northern blot techniques and enzyme-linked immunoassays (ELISA). HBV DNA was detected in 5 patients with B-CAH as extrachromosomal, full-length monomers of 3.2 kb. In 3 of these patients Northern blot analysis revealed the presence of the 3.6-3.8 kb RNA species, which were accompanied in one case by the HBsAg-specific 2.4 kb transcript. An ELISA performed on cell lysate obtained from this patient showed low but detectable amounts of HBsAg as compared to control PBL incubated with up to 50 micrograms/ml of the viral antigen. Serum HBV DNA was found in 3 patients with B-CAH, whereas all individuals positive for cellular HBV DNA had circulating HBeAg. These data indicate that lymphocytes from some patients with hepatitis B can harbor a transcriptionally and translationally active HBV genome.

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.

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.

Effect of the preS1 RNA sequence on the efficiency of the hepatitis B virus preS2 and S protein translation

The gene coding for hepatitis B virus surface antigen consists of preS1, preS2, and S regions. Two species of mRNAs of this gene are transcribed. The larger species covers all three regions and is translated solely into preS1 protein, whereas the smaller one covers the preS2 and S regions and is translated into preS2 and S proteins. This study examines the influence of the 5' upstream sequence lying in the preS1 region on the synthesis of preS2 and S proteins. For this purpose, several expression plasmids were constructed by inserting various portions of the preS1 region between the retroviral LTR promoter and the preS2/S coding region, and preS2/S protein production was examined in the transfected CHL cells. All the transcripts were initiated in the LTR. A sequence located in the region between 102 and 38 nucleotides upstream from the preS2 initiation codon was found to reduce the production of preS2/S proteins probably at the level of translation. Expression of the heterologous chloramphenicol acetyltransferase gene was similarly inhibited when it was placed downstream of the preS1-102/-38 sequence.

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.

Analysis of hepatitis B virus transcripts in infected human livers

The hepatitis B virus transcripts in human hepatoma and its adjacent nontumorous liver were examined with probes specific to hepatitis B virus surface antigen, core antigen, X region and pre-S region. The study shows that the patterns of hepatitis B virus transcripts for tumorous tissue and the counterpart in nontumorous liver differ. In most of the tissues examined, the surface antigen gene is transcribed. Most of these transcripts, besides having surface antigen sequences, also have an X region; some also include a pre-S region. The transcripts that hybridized to a core-specific probe were a pair of poly(A+) RNA, 3.5 and 2.2 kilobases in size, present in two of the nontumorous hepatocytes where the virus was actively replicating. The 3.5-kilobase transcript not only hybridized to the core probe, but was able to be hybridized to other hepatitis B virus subgenomic probes and might represent the RNA pregenome involving hepatitis B virus DNA replication. Whereas most of the transcripts hybridizable to hepatitis B virus probe are in the size range of 2.1 to 2.7 kilobases, some transcripts other than the pregenomic RNA appear to be greater than 3.2 kilobases in size and may represent the hybrid RNAs of viral and host sequences.

Hepatitis B virus infection of adult human hepatocytes cultured in the presence of dimethyl sulfoxide

We investigated the possibility of infecting normal adult human hepatocytes maintained in pure cultures or in cocultures with hepatitis B virus (HBV). Several assays with different infectious sera and hepatocyte populations from various donors identified only limited HBV replication, with significant variations from one cell preparation to another. The addition of 1.5% dimethyl sulfoxide to the culture medium markedly enhanced the infection process. Indeed, hepatitis B e antigen secretion, the appearance of both HBV DNA replicative forms and major HBV transcripts, and the release of complete HBV particles into the medium were demonstrated. It is possible that the significant increase in intracellular HBV DNA in dimethyl sulfoxide-treated cells was related to enhanced adsorption of the virus. When viral particles produced by a transfected HepG2 cell line were used to infect normal hepatocytes, the same results were obtained. In addition, comparative assays with hepatocytes from three different donors showed that although high amounts of intracellular viral DNA were found in all cases, viral replicative intermediates were visualized in only one case. These findings suggest that this HBV-producing cell line could serve as a reproducible source of infectious virus and that primary culturing of human hepatocytes represents a unique tool for analyzing intracellular regulating factors which, in addition to the penetration step, modulate HBV replication.

Translational selection in the expression of the hepatitis B virus envelope proteins

The region of the hepatitis B virus (HBV) genome coding for the viral envelope proteins contains three inphase ATGs that are conserved among viral subtypes. Each of these ATGs can be used as mRNA initiation codons. The three translated proteins share a carboxy-terminal region (the S protein) and extend amino-terminally to include the pre-S2 region in the middle (M) protein, and the pre-S1 and pre-S2 regions in the large (L) protein. We have inserted the HBV DNA coding for the M protein into a baculovirus expression vector. Infected insect cells transcribe a mRNA that is initiated solely within a baculovirus promoter, and that contains the initiator codons for both M and S proteins. Although these cells primarily secrete the M protein, the major translational product is the S protein, which is not secreted. This preferential translation, the result of the use of an internal initiator codon, demonstrates that the regulation of HBV envelope protein production can occur at the translational level.

Upstream region of hepatitis B virus S gene responsible for transcriptional stimulation by dexamethasone

Transcriptional regulation of hepatitis B virus (HBV) surface antigen (HBs Ag) gene was studied in human hepatoma-derived cell lines. Treatment with dexamethasone (Dex; 1 microM) induced an increase in the smaller HBs-mRNA initiated within Pre-S region encoding S and Pre-S2 proteins, but not the larger HBs-mRNA initiated in the further upstream encoding Pre-S1 protein. The Bg1II-MstII fragment (map position 2425-3201) in the upstream of the S gene was used as a transcriptional promoter of chloramphenicol acetyltransferase (CAT) gene. The CAT activity brought about by this construct in the transient assay was elevated by 5-fold in the presence of Dex. Deletion analysis localized the sequence required for the full response to Dex within a 590-base pair fragment in the upstream of the transcriptional initiation site of the smaller HBs-mRNA. And this fragment contained the binding site for the nuclear factor I (NF-I), which might have some role in Dex-dependent transcriptional stimulation.

Production of hepatitis B virus-infected human B-cell hybridomas: transmission of the viral genome to normal lymphocytes in cocultures

The presence of hepatitis B virus (HBV) genome and transcripts in mononuclear cells from a patient with acute type B hepatitis offered the possibility of developing a cell line which could serve as a model for HBV replication in lymphocytes. A human B-cell hybridoma, KDG92, was then produced which carries HBV DNA in an episomal state and expresses the major virus transcripts as well as its surface (HBsAg), core (HBcAG), and e (HBeAg) antigens. KDG92 releases in the supernatant surface antigen particles but not core or Dane particles. However, in cocultures this hybridoma is able to transmit episomal HBV DNA to normal lymphocytes, both T and B cells. This in vitro system can therefore provide important indications as to the virus life cycle in lymphocytes and the mechanisms of virus propagation from cell to cell.

Binding of tissue-specific factors to the enhancer sequence of hepatitis B virus

We have identified tissue-specific factors, in human hepatoma cells, that bind specifically to the transcriptional enhancer sequence of the human hepatitis B virus (HBV). Two different types of protein factor were found in nuclear extracts of hepatoma cells by gel mobility shift assay. One factor was observed in human hepatoma cells but not in human kidney, lung, or vein cells, or in embryonic mouse cells. The other was discovered in both human hepatoma cells and human vein cells. DNase I footprint analysis, using the enhancer fragment (164 bp, AccI-SphI) from HBV, revealed that two specific sites are recognized by the nuclear factors. These sites contain consensus octamer sequences which have been found in many other enhancer elements. These results strongly suggest that the two nuclear factors found in hepatoma cells play key roles in the function of the HBV enhancer.

Regulation of hepatitis B virus S gene promoter in transfected cell lines

Hepatitis B virus (HBV) contains an enhancer element that activates the viral core and X gene promoters. To investigate the transcriptional regulation of the viral S gene promoter, we transfected SK-Hep1 cells with circularized forms of HBV DNAs and their enhancerless mutants. We have found that expression of the S gene, determined by measurement of the appearance of HBsAg in the media and by RNA analysis, is to a large extent enhancer-dependent. This observation was further confirmed by analysis of a series of plasmids containing the chloramphenicol acetyl-transferase (CAT) gene under the control of the S gene promoter and the HBV enhancer element. Interestingly, in contrast to its behavior in SK-Hep1 cells, the S gene promoter is highly active in Alexander cells, in the absence of the enhancer element. This implies that activity of the S gene promoter is cell-type specific.

trans-activation of viral enhancers by the hepatitis B virus X protein

We have used the chloramphenicol acetyltransferase (cat) gene expression system to study the effect of the X protein of hepatitis B virus (HBV) on viral enhancers. Plasmids containing the HBV enhancer and the core gene promoter linked to the cat gene were cotransfected with a plasmid containing the X gene into the human hepatoma cell line PLC/PRF/5. Our results indicate that the transfected X gene caused a trans-activation of the HBV enhancer. If a frameshift mutation or a deletion in the X structural gene was created, this trans-activation function was abolished. This result and the observation that the frameshift mutation did not alter the transcription of X mRNA suggest that the X protein is the trans-activating factor. Using similar techniques, we found that the X protein was also capable of trans-activating the simian virus 40 (SV40) and Rous sarcoma virus enhancers (pSV2cat and pRSVcat) in CV-1 cells. However, trans-activation of the SV40 enhancer by the X protein was not observed in COS-1 cells. By cotransfecting pSV2cat and the X gene with a plasmid containing either the intact SV40 genome, the SV40 genome devoid of the T-antigen (T-ag) gene, or only the T-ag gene, we demonstrated that SV40 T-ag can suppress trans-activation by the X protein. SV40 T-ag did not inhibit expression of the X gene or inactivate the X protein. The most probable mechanism of this inhibition is that T-ag competes with the X protein for a common target.

Replicative intermediate of hepatitis B virus in transfected murine fibroblasts

The NIH 3T3-derived cell line psi AM22b, which carries a defective Moloney murine leukemia virus, was transfected with a plasmid carrying the neo gene and two head-to-tail copies of the hepatitis B virus (HBV) genome positioned with opposing polarities. Both the two HBV dimers and the neo gene were located between two Moloney murine leukemia virus long terminal repeats. Poly(A)+ RNAs isolated from one clone that grew in the presence of G418 contained the two major classes of HBV-specific transcripts (3.5-kilobase pregenome and 2.1-kilobase mRNAs) in approximately equivalent amounts, which was reminiscent of the profiles of viral mRNAs from the livers of infected humans and chimpanzees.

Cell type specific expression of pre S 1 antigen and secretion of hepatitis B virus surface antigen. Brief Report

Production of the three hepatitis B surface (HBs) proteins was studied in a hepatoma cell line (PLC/PRF/5) and two HBs antigen secreting cell lines (HeLa and mouse L-cells), which had been transfected by a viral genome isolated by molecular cloning from PLC/PRF/5 chromosomal DNA. The DNA used for transfection contains the HBs-specific promoters and the enhancer which regulate the expression of HBs genes in the transfected cell lines. All three cell lines expressed well the small and middle HBs protein, but the larger pre S 1 containing protein was barely detectable in the L-cell. In vivo growth of the transfected HeLa cell as nude mouse tumour increased pre S 1 expression and suppressed secretion of HBsAg.

Multiple nuclear proteins in liver cells are bound to hepatitis B virus enhancer element and its upstream sequences

The transcriptional enhancer element in the hepatitis B virus (HBV) genome displays tissue-specific activity, suggesting that this element interacts with cellular specific factors. Using a nitrocellulose filter binding assay and DNase I footprinting, we have found that liver cell-specific nuclear proteins are bound to the HBV enhancer element (the E site) and its adjacent sequences. Four DNase I-protected sites were revealed, all contain a sequence motif resembling the sequence of the SV40 enhancer core element. Evidence is provided to show that: (i) these sites are protected by at least three distinct nuclear proteins and (ii) the presence of some of these proteins is dependent on the differentiation stage of the liver cells. Interestingly an octamer sequence found in the E site appears also in the promoter region of several liver-specific genes, which suggests that the E site and its corresponding binding protein(s) determine the tissue-specific expression of the HBV enhancer element.

Murine cells carrying integrated tandem genomes of hepatitis B virus DNA transcribe RNAs from endogenous promoters on both viral strands and express middle and major viral envelope proteins

Clone 4.10 cells were isolated as a methotrexate-resistant clone arising after cotransfection of mouse 3T3 cells with plasmid DNA containing a head-to-tail dimer of the hepatitis B virus (HBV) genome and DNA coding for methotrexate-resistant dihydrofolate reductase. The majority of methotrexate-resistant clones derived by this procedure have been found to contain multiple copies of the HBV genome, but the intact HBV dimer was rarely preserved. In contrast, 4.10 cells contained at least 40 copies of intact HBV dimer per cell. These cells produced large amounts of 22-nm hepatitis B surface antigen particles that included viral envelope proteins reactive with the pre-S2 region-specific antibody, indicating transcription and translation of the pre-S2 and S regions of the integrated viral genomes. The cells also synthesized viral e antigen, which was released into the culture medium. Characterization of polyadenylated viral RNAs transcribed from the long (minus) strand of the integrated HBV DNA demonstrated the presence of shorter-than-genome-length RNAs containing only X region sequences, shorter-than-genome-length RNAs containing both X and S region sequences, and longer-than-genome-length RNAs containing core, X, and S region sequences. Start sites for transcripts were mapped 5' to and within the pre-S region and 5' to and within the precore region at approximately the same sites as those utilized for HBV transcription during viral replication in infected livers. Polyadenylated RNA transcripts complementary to the short (plus) strand of HBV that initiated and terminated within the intact and integrated head-to-tail tandem viral genomes were also detected.

Selective synthesis and secretion of particles composed of the hepatitis B virus middle surface protein directed by a recombinant vaccinia virus: induction of antibodies to pre-S and S epitopes

Selective synthesis in mammalian cells of the hepatitis B virus middle surface (MS) protein, which is 55 amino acids longer than the major surface (S) protein, was achieved by using a recombinant vaccinia virus. The 33-kilodalton MS polypeptide was glycosylated and secreted as particles that resembled human hepatitis B surface antigen as well as particles composed solely of S protein with regard to antigenicity, buoyant density, size, and electron micrographic appearance. The MS particles differed from S particles, however, by binding to polymerized human albumin and inducing antibodies that reacted with a pre-S peptide and inhibited the binding of human plasma-derived hepatitis B surface antigen to polymerized human albumin.

The enhancer sequence of human hepatitis B virus can enhance the activity of its surface gene promoter

The transcriptional enhancer sequence has recently been demonstrated in the hepatitis B viral genome. This enhancer sequence has also been shown to increase the activity of HBcAg gene promoter. Using the chloramphenicol acetyltransferase gene expression system, we demonstrated that the intrinsic promoter activity of HBsAg gene promoter was stronger than that of HBcAg gene promoter in both human hepatoma cell lines, Hep3B and HuH-7. Furthermore, we showed that the HBV enhancer sequence not only stimulated the HBcAg gene promoter activity, but also stimulated the HBsAg gene promoter activity in both Hep3B and HuH-7 cells. The enhancer sequence increased the HBsAg gene promoter activity 20-fold in both Hep3B and HuH-7 cell lines, while the increase of the HBcAg gene promoter was 2- and 8-fold in Hep3B and HuH-7 cells, respectively.

Production of hepatitis B virus in vitro by transient expression of cloned HBV DNA in a hepatoma cell line

Transfection of human hepatoma cell lines with cloned HBV DNA resulted in the secretion of large amounts of hepatitis B surface antigen (HBsAg) and core-related antigens (HBc/HBeAg) if well-differentiated cell lines were employed. Synthesis of both viral antigens was the highest in cell line HuH-7 and continued for approximately 25 days. Particles resembling hepatitis B virions (Dane particles) by morphology, density and by the presence of the preS1 surface antigen were released from the transfected HuH-7 cells into the culture medium. These particles produced in vitro were also indistinguishable from the naturally occurring hepatitis B virions in containing the virus-associated DNA polymerase and mature HBV genomes. Restriction analysis of these DNA molecules was compatible with the nucleotide sequence of the transfecting HBV DNA sequence. Viral surface antigens and core proteins present in the culture medium were fractionated and characterized by immunoprecipitation and SDS--PAGE after labeling with [35S]methionine. Antisera specific for X-gene products identified in cell extracts two hitherto unknown HBV gene products. This system thus provides a new approach to open questions regarding HBV-related gene function and HBV replication.

Hepatitis B surface antigen gene expression is regulated by sex steroids and glucocorticoids in transgenic mice

We have investigated the basis for liver-specific and sex-linked expression of hepatitis B surface antigen (HBsAg) gene in transgenic mice by monitoring the level of liver HBsAg mRNA and serum HBsAg at different stages of development and in response to sex-hormone regulation. Transcription of the HBsAg gene starts at day 15 of development, together with that of the albumin gene, and reaches a comparable level at birth. HBsAg mRNA level and HBsAg production are parallel in males and females during prenatal development and until the first month of life, but HBsAg gene expression increases 5-10 times in males at puberty. After castration, the level of expression decreases dramatically in both males and females and is subsequently increased by injection of testosterone or estradiol. Glucocorticoids also regulated positively expression of the HBsAg gene. Our results suggest that sex hormones play a role in hepatitis B virus gene expression during natural infection and could explain the difference in incidence of chronic carriers between men and women.

Amplifiable expression vectors for mammalian cell lines

We have constructed a prototype of a general expression vector for mammalian cells, which carriers a strong promoter active in a variety of tissues and animal species for expression of the gene of interest and a truncated gene for amplification in selective medium. Expression of the S gene of HBV encoding the surface antigen was used to evaluate various versions of this vector. The human metallothionein IIA promoter was found to be particularly efficient for expression of this gene, both in mouse L and monkey Vero cells. Including a promoterless tk gene in the vector led to gene amplification in Ltk- cells by selection of TK+ variants in selective medium containing hypoxanthine, aminopterine and thymidine (HAT medium). Concomitant increases of the S gene expression levels were initially 50-100 fold. Although many clones were unstable, even in selective medium, some maintained the high expression levels for over one year.