The transcriptional transactivation function of HBx protein is important for its augmentation role in hepatitis B virus replication

HBxAPα/Rsf-1-mediated HBx-hBubR1 interactions regulate the mitotic spindle checkpoint and chromosome instability

Hepatitis B virus (HBV) X protein (HBx), encoded by the HBV genome, is involved in the development of HBV-mediated liver cancer, whose frequency is highly correlated with chromosomal instability (CIN). We reported previously that HBx induces mitotic checkpoint dysfunction by targeting the human serine/threonine kinase BubR1 (hBubR1). However, the underlying mechanism remained unresolved. Here, we show that HBx protein-associated protein α (HBxAPα)/Rsf-1 associates with hBubR1 and HBx in the chromatin fraction during mitosis. Depletion of HBxAPα/Rsf-1 abolished the interaction between HBx and hBubR1, indicating that HBxAPα/Rsf-1 mediates these interactions. Knockdown of HBxAPα/Rsf-1 with small interfering RNA did not affect the recruitment of hBubR1 to kinetochores; however, it did significantly impair HBx targeting to kinetochores. A deletion mutant analysis revealed that two Kunitz domains of HBx, the Cdc20-binding domain of hBubR1 and full-length of HBxAPα/Rsf-1 were essential for these interactions. Thus, binding of HBx to hBubR1, stabilized by HBxAPα/Rsf-1, significantly attenuated hBubR1 binding to Cdc20 and consequently increased the rate of mitotic aberrations. Collectively, our data show that the HBx impairs hBubR1 function and induces CIN through HBxAPα/Rsf-1, providing a novel mechanism for induction of genomic instability by a viral pathogen in hepatocarcinogenesis.

Methyltransferase PRMT1 is a binding partner of HBx and a negative regulator of hepatitis B virus transcription

The hepatitis B virus X protein (HBx) is essential for virus replication and has been implicated in the development of liver cancer. HBx is recruited to viral and cellular promoters and activates transcription by interacting with transcription factors and coactivators. Here, we purified HBx-associated factors in nuclear extracts from HepG2 hepatoma cells and identified protein arginine methyltransferase 1 (PRMT1) as a novel HBx-interacting protein. We showed that PRMT1 overexpression reduced the transcription of hepatitis B virus (HBV), and this inhibition was dependent on the methyltransferase function of PRMT1. Conversely, depletion of PRMT1 correlated with increased HBV transcription. Using a quantitative chromatin immunoprecipitation assay, we found that PRMT1 is recruited to HBV DNA, suggesting a direct effect of PRMT1 on the regulation of HBV transcription. Finally, we showed that HBx expression inhibited PRMT1-mediated protein methylation. Downregulation of PRMT1 activity was further observed in HBV-replicating cells in an in vivo animal model. Altogether, our results support the notion that the binding of HBx to PRMT1 might benefit viral replication by relieving the inhibitory activity of PRMT1 on HBV transcription.

A novel stop codon mutation within the hepatitis B surface gene is detected in the liver but not in the peripheral blood mononuclear cells of HIV-infected individuals with occult HBV infection

To characterize occult HBV infection (OHB) in different compartments of HIV+ individuals. This retrospective study involved 38 consecutive HIV+ patients; 24 HBsAg negative (HBV-) and 14 HBsAg positive (HBV+). OHB was assessed in serum samples, liver tissue (LT) and peripheral blood mononuclear cells (PBMC) by genomic amplification of the partial S, X and precore/core regions. HBV genomic analysis was inferred by direct sequencing of PCR products. The intracellular HBV-DNA was measured by a quantitative real-time PCR. HBV+ patients were used as a control for HBV replication and genomic profile. In HBV- patients, HBV-DNA was undetectable in all serum samples, while it was found positive in 7/24 (29%) LT in which genotype D prevailed (57%). HBV-DNA was found in 6/7 (86%) PBMC of occult-positive and none of occult-negative LT. Significantly lower HBV-DNA load was present in both compartments in OHB+ with respect to the HBV+ group (LT: P = 0.002; PBMC: P = 0.026). In the occult-positive cases, HBV replication was significantly higher in LT than in PBMC (P = 0.028). A hyper-mutated S gene in PBMC and a nucleotide mutation at position C695 in LT that produces a translational stop codon at amino acid 181 of the HBs gene characterized OHB. In this group of HIV+ persons, OHB is frequent and exhibits lower replication levels than chronic HBV in the different compartments examined. HBV-DNA detection in PBMC may offer a useful tool to identify OHB in serum-negative cases. The novel HBs gene stop codon found in LT could be responsible for reduced production leading to undetectability of HBsAg.

Cloning and functions of the HBxAg-binding protein XBP1

In the present study the hepatitis B virus X antigen binding protein 1 (XBP1) was cloned by inducing its expression, and its subcellular localization and function were examined. Total RNA was extracted from HepG2 cells and XBP1 was amplified using reverse transcription polymerase chain reaction (RT-PCR), followed by restriction enzyme digestion of the pGBKT7 yeast plasmid and identification by enzyme digestion. The plasmid was transformed into AH109 yeast via the lithium acetate method and protein extracts were prepared. XBP1 protein expression in the eukaryotic cells was determined using polyacrylamide gel electrophoresis and western blot analysis. The gene encoding the XBP1-binding protein was screened in liver cells using yeast two-hybrid technology. We transfected a human hepatocellular carcinoma cell line and observed the intracellular localization of the gene expression protein using a fluorescence microscope, followed by prokaryotic expression and XBP1 gene identification. A 921-bp XBP1 gene fragment was obtained via RT-PCR amplification and 20 proteins with known functions that interact with XBP1 were screened, including metallothionein, smooth muscle cell-related protein, asialoglycoprotein receptor, pyruvate dehydrogenase kinase 1 and a sequence with unknown functions. A green fluorescent protein expression plasmid pEGFP-C1-XBP1 of XBP1 was constructed successfully and its expression protein was localized in the cytoplasm. A 56-kDa recombinant protein was successfully obtained via prokaryotic expression and was demonstrated to have good specificity using western blot analysis. The XBP1 gene, which expresses the XBP1 protein, is located in the cytoplasm and plays a role in the intracellular structure, cell growth, intracellular metabolism and signal transduction pathway, as well as DNA duplication, transcription, recombination and repair.

Hepatitis B virus X protein stimulates gene expression selectively from extrachromosomal DNA templates

Chronic hepatitis B virus (HBV) infection is a major risk factor for liver cancer development. HBV encodes the hepatitis B virus X (HBx) protein that promotes transcription of the viral episomal DNA genome by the host cell RNA polymerase II. Here we provide evidence that HBx accomplishes this task by a conserved and unusual mechanism. Thus, HBx strongly stimulates expression of transiently transfected reporter constructs, regardless of the enhancer and promoter sequences. This activity invariably requires HBx binding to the cellular UV-damaged DDB1 E3 ubiquitin ligase, suggesting a common mechanism. Unexpectedly, none of the reporters tested is stimulated by HBx when integrated into the chromosome, despite remaining responsive to their cognate activators. Likewise, HBx promotes gene expression from the natural HBV episomal template but not from a chromosomally integrated HBV construct. The same was observed with the HBx protein of woodchuck HBV. HBx does not affect nuclear plasmid copy number and functions independently of CpG dinucleotide methylation.

CONCLUSION

We propose that HBx supports HBV gene expression by a conserved mechanism that acts specifically on episomal DNA templates independently of the nature of the cis-regulatory sequences. Because of its uncommon property and key role in viral transcription, HBx represents an attractive target for new antiviral therapies.

Differential functions of C- and N-terminal hepatitis B x protein in liver cells treated with doxorubicin in normoxic or hypoxic condition

Hepatitis viral B x protein (HBx), a hepatocarcinogen, is frequently mutated. Hypoxia influences the growth of HCC and also the sensitivity of tumor cells to treatments. We aimed to test the role of HBx and acute hypoxia in the efficacy of chemotherapy. In this study, we established 4 Chang liver cell lines with the full-length HBx (HBx), the first 50 amino acids of N-terminal HBx (HBx/50), the last 104 amino acids of C-terminal HBx (HBx/51) and empty vector (CL), respectively. MTT and TNUEL assays were used to assess cell viability and apoptosis respectively. Western blot was used to determine the expression of relevant proteins. Results showed that among 4 cell lines, doxorubicin was most effective in decreasing the viability and enhancing apoptosis in HBx/51 cells, while HBx/50 cells were most resistant to the treatment. Cells in hypoxia were more susceptible to doxorubicin than cells in normoxia. Hypoxia facilitated the Bid cleavage especially in HBx/51 cells via phosphorylating p38 MAPK. p38 MAPK inhibitor significantly reduced the tBid level and increased cell viability. In conclusion, N-terminal HBx and C-terminal HBx function differentially in their ability to regulate cell growth, with the former being promotive but the latter being inhibitory. The acute hypoxia may overcome the HBx-induced resistance and facilitate the chemotherapy.

Biological characteristics of the rtA181T/sW172* mutant strain of Hepatitis B virus in animal model

Background

The effects of Hepatitis B virus (HBV) rtA181T/sW172* mutation on viral replication and pathogenicity was concerned recently. This study aimed to investigate the biological characteristics of rtA181T/sW172* mutant strain of HBV in animal model.

Methods

The rtA181T/sW172* mutant plasmid was constructed using the pHBV4.1 (wild type HBV) as a template. The wild and mutant HBV replication mouse models were established utilizing a hydrodynamic technique. The titers of hepatitis B surface antigen (HBsAg), hepatitis B e antigen, and HBV DNA in serum, and the levels of HBsAg, hepatitis B core antigen(HBcAg), HBV DNA replication intermediates (HBV DNA RI) and HBV RNA in liver were measured after 1, 3, 5, 7, 10, 12 and 15 days of plasmid injection.

Results

In wild-type HBV replication mouse model, serum HBsAg was high on day 1, 3, and 5, but became lower since day 7; while in mutant HBV mouse model, serum HBsAg was always at very low level. In liver tissues, HBV DNA RI of wild type HBV was detected on day 1 after transfection. The level subsequently peaked on day 3, gradually declined after day 5, and was almost undetectable on day 10. However, the HBV DNA RI levels of the mutant strain were always higher and lasted longer until day 15. Consistently, the expression levels of HBsAg and HBcAg in liver of the mutant group were significantly increased.

Conclusions

In the case of the HBV rtA181T/sW172* mutation, the secretion of serum HBsAg was impaired, whereas HBV DNA replication and HBsAg/HBcAg expression were increased in liver. These results suggest that the mutation can impair HBsAg secretion, and may cause the accumulation of viral core particles in liver.

Modulation of apoptotic signaling by the hepatitis B virus X protein

Worldwide, an estimated 350 million people are chronically infected with the Hepatitis B Virus (HBV); chronic infection with HBV is associated with the development of severe liver diseases including hepatitis and cirrhosis. Individuals who are chronically infected with HBV also have a significantly higher risk of developing hepatocellular carcinoma (HCC) than uninfected individuals. The HBV X protein (HBx) is a key regulatory HBV protein that is important for HBV replication, and likely plays a cofactor role in the development of HCC in chronically HBV-infected individuals. Although some of the functions of HBx that may contribute to the development of HCC have been characterized, many HBx activities, and their putative roles during the development of HBV-associated HCC, remain incompletely understood. HBx is a multifunctional protein that localizes to the cytoplasm, nucleus, and mitochondria of HBV‑infected hepatocytes. HBx regulates numerous cellular signal transduction pathways and transcription factors as well as cell cycle progression and apoptosis. In this review, we will summarize reports in which the impact of HBx expression on cellular apoptotic pathways has been analyzed. Although various effects of HBx on apoptotic pathways have been observed in different model systems, studies of HBx activities in biologically relevant hepatocyte systems have begun to clarify apoptotic effects of HBx and suggest mechanisms that could link HBx modulation of apoptotic pathways to the development of HBV-associated HCC.

Hepatitis B virus X protein modulates remodelling of minichromosomes related to hepatitis B virus replication in HepG2 cells

Hepatitis B virus (HBV) covalently closed circular DNA (cccDNA) is organised into minichromosomes by histone and non-histone proteins. Remodelling of minichromosomes is crucial for the regulation of HBV replication, which is dependent on the presence of the hepatitis B virus X protein (HBx). However, the mechanisms of HBx-dependent HBV replication remain obscure. The objective of this study was to investigate the mechanism of HBx-dependent HBV replication through the pathway of chromatin remodelling. The role of HBx was investigated by transfecting human HepG2 cells with the linear full-length HBV genome (wild-type) or HBx-deficient mutant HBV DNA (HBx mutant). Our results showed that although the formation of cccDNA was not affected by HBx, HBV replication, transcription and antigen secretion were all significantly reduced, resulting from the absence of HBx. The acetylation, mono-methylation and phosphorylation of cccDNA-bound histone H3 were associated with HBV replication. In addition, the levels of cccDNA-bound methylated, phosphorylated and acetylated histone H3 decreased sharply in HBx mutant HBV DNA. HBx modulated not only the status of acetylation but also the methylation and phosphorylation of histone H3 bound to the cccDNA during HBV replication in HepG2 cells. These findings suggest that HBx plays an important role in modulating the remodelling of minichromosomes related to HBV replication and it may regulate viral replication through the pathway of chromatin remodelling.

Functional Characterization of Interferon Regulation Element of Hepatitis B virus Genome In Vivo

The roles of interferon regulatory element (IRE) in Hepatitis B virus (HBV) genome on inhibitory effect of interferon against HBV are controversial in vitro. This study aimed to determine the functional characterization of HBV-IRE sequence in vivo. Wild-type or IRE-mutant HBV replication-competent mice were firstly established, and mice were subquently treated with polyinosinic-polytidylin acid (polyI.C) or phosphate-buffered saline via intraperitoneal. Results showed that PolyI.C inhibited viral replication, and increased the level of 2',5'-oligoadenylate synthase mRNA transcripts, a marker of INF-α/β induction. Between wild-type and IRE-mutant HBV replication-competent mice, the levels of HBV-RNA and HBV-DNA replication intermediates were similar. After PolyI.C treatment, the decreasing of HBV-RNA was similar between two groups, but HBV-DNA replication intermediates decreased significantly less in IRE-mutant than wild-type HBV replication-competent mice. These findings suggested that IRE mutant reduced the inhibitory effect of interferon on HBV replication, which played a role in antiviral effect of interferon against HBV.

Design, synthesis, and molecular hybrids of caudatin and cinnamic acids as novel anti-hepatitis B virus agents

Forty-six conjugated derivatives of caudatin with substituted cinnamic acids were synthesized, and their anti-hepatitis B virus (HBV) activity was evaluated in HepG 2.2.15 cells. Most of the derivatives exhibited potent anti-HBV activity, especially inhibiting the HBV DNA replication with the IC(50) values from 2.44 to 22.89 μΜ. Compound 18 showed significant activity against the secretion of HBsAg, HBeAg, and HBV DNA replication with IC(50) values of 5.52, 5.52, 2.44 μΜ, respectively, and had good safety (LD(50) > 1250 mg/kg) according to the acute toxicity study. Preliminary mechanism investigation suggested that compound 18 exerted antivirus effects via interfering HBV X promoter and enhancer I to influence HBV transcriptions.

Inhibition of hepatitis B Virus replication by hepatocyte nuclear factor 4-alpha specific short hairpin RNA

BACKGROUND

Previous studies showed that hepatocyte nuclear factor 4α (HNF4α) may play a critical role in hepatitis B virus (HBV) replication.

AIMS

This study aimed to investigate the effect of knocking down of HNF4α with RNA interference technique on HBV replication in a HBV replication mouse model.

METHODS

Four HNF4α, specific short hairpin RNA (shRNA)-producing plasmids were constructed. HBV mRNA and DNA replication intermediates were analysed using Northern and Southern blot respectively. The expression of HNF4α and HBV core antigen (HBcAg) was detected using immunohistochemistry technique.

RESULTS

One of the HNF4α shRNAs, HNF4α shRNA1, efficiently inhibited the expression of HNF4α in HepG2 cells and mice liver. HBV RNA transcripts and DNA replication intermediates in HNF4α shRNA1 group were decreased 67.3 and 76%, respectively, in HepG2 cells, and 68.1 and 70.6% in mice liver respectively. The expression level of HBcAg in the liver was also decreased with the inhibition of HNF4α expression.

CONCLUSIONS

These results suggested that decreasing of HNF4α expression was associated with the reduced level of HBV replication in HepG2 cells and mice liver. These data indicated that HNF4α played a critical role in HBV replication in vivo, and HNF4α shRNA could inhibit HBV replication in vivo.

The C-terminal region of the hepatitis B virus X protein is required for its stimulation of HBV replication in primary mouse hepatocytes

Hepatitis B virus (HBV) infection is an important risk factor for hepatocellular carcinoma (HCC). The hepatitis B virus X protein (HBx), a multifunctional regulatory protein encoded by HBV, is known to be involved in stimulation of viral replication by modulating cell cycle status. HBx is required for maximal virus replication in plasmid-based replication assays in immortalized human liver HepG2 cells and in primary rat hepatocytes. Moreover, the C-terminal region of HBx is important for HBV replication in HepG2 cells. However, in normal hepatocytes, the region of HBx that is responsible for its effect on cell cycle regulation and HBV replication is unclear. We have demonstrated that HBx is similarly required for maximal HBV replication in primary mouse hepatocytes and that the C-terminus of HBx is essential for its ability to stimulate HBV replication by inducing quiescent hepatocytes to exit G0 phase of the cell cycle but stall in G1 phase. Our studies establish that primary mouse hepatocytes support HBx-dependent HBV replication, and provide further evidence for the effect of the C-terminal region of HBx on HBV infection and replication.

Hepatitis B virus regulatory HBx protein binding to DDB1 is required but is not sufficient for maximal HBV replication

Robust hepatitis B virus (HBV) replication is stimulated by the regulatory HBx protein. HBx binds the cellular protein DDB1; however, the importance of this interaction for HBV replication remains unknown. We tested whether HBx binding to DDB1 was required for HBV replication using a plasmid based replication assay in HepG2 cells. Three DDB1 binding-deficient HBx point mutants (HBx(69), HBx(90/91), HBx(R96E)) failed to restore wildtype levels of replication from an HBx-deficient plasmid, which established the importance of the HBx-DDB1 interaction for maximal HBV replication. Analysis of overlapping HBx truncation mutants revealed that both the HBx-DDB1 binding domain and the carboxyl region are required for maximal HBV replication both in vitro and in vivo, suggesting the HBx-DDB1 interaction recruits regulatory functions critical for replication. Finally we demonstrate that HBx localizes to the Cul4A-DDB1 complex, and discuss the possible implications for models of HBV replication.

Role of HBx and truncated HBx in HBV replication

As one of the principal causes of liver diseases, such as chronic hepatitis B, hepatic cirrhosis and hepatocellular carcinoma (HCC), hepatitis B virus (HBV) infection has been a major health problem worldwide. It is estimated that more than 500 million individuals have been infected with HBV worldwide and 1 million die of HBV infection-associated diseases annually. HBV X protein (HBx) is a multifunctional protein that can modulate various cellular processes and plays a crucial role in the pathogenesis of HCC. In recent years, the role of HBx in HBV replication has been more or less confirmed. In addition, more and more natural HBx truncated mutants and their roles in HBV replication have been found. This review aims to elucidate the roles of HBx and truncated HBx in HBV replication.

Hepatitis B virus X protein is essential to initiate and maintain virus replication after infection

BACKGROUND & AIMS

The molecular biology of hepatitis B virus (HBV) has been extensively studied but the exact role of the hepatitis B X protein (HBx) in the context of natural HBV infections remains unknown.

METHODS

Primary human hepatocytes and differentiated HepaRG cells allowing conditional trans complementation of HBx were infected with wild type (HBV(wt)) or HBx deficient (HBV(x-)) HBV particles and establishment of HBV replication was followed.

RESULTS

We observed that cells inoculated with HBx-deficient HBV particles (HBV(x-)) did not lead to productive HBV infection contrary to cells inoculated with wild type HBV particles (HBV(wt)). Although equal amounts of nuclear covalently closed circular HBV-DNA (cccDNA) demonstrated comparable uptake and nuclear import, active transcription was only observed from HBV(wt) genomes. Trans-complementation of HBx was able to rescue transcription from the HBV(x-) genome and led to antigen and virion secretion, even weeks after infection. Constant expression of HBx was necessary to maintain HBV antigen expression and replication. Finally, we demonstrated that HBx is not packaged into virions during assembly but is expressed after infection within the new host cell to allow epigenetic control of HBV transcription from cccDNA.

CONCLUSIONS

Our results demonstrate that HBx is required to initiate and maintain HBV replication and highlight HBx as the key regulator during the natural infection process.

Hepatitis B virus X gene and hepatocarcinogenesis

Chronic hepatitis B virus (HBV) infection has been identified as a major risk factor in hepatocellular carcinoma (HCC), which is one of the most common cancers worldwide. The pathogenesis of HBV-mediated hepatocarcinogenesis is, however, incompletely understood. Evidence suggests that the HBV X protein (HBx) plays a crucial role in HCC development. HBx is a multifunctional regulator that modulates transcription, signal transduction, cell cycle progression, apoptosis, protein degradation pathways, and genetic stability through interaction with host factors. This review describes the current state of knowledge of the molecular pathogenesis of HBV-induced HCC, with a focus on the role of HBx in hepatocarcinogenesis.

The molecular and pathophysiological implications of hepatitis B X antigen in chronic hepatitis B virus infection

Hepatitis B virus is considered one of the most significant environmental carcinogens in humans. Because the mechanisms of HBV replication and the development of hepatocellular carcinoma (HCC) are partially known, HBV-associated pathogenesis remains a challenge to increase its understanding. Evidence suggests that the regulatory protein hepatitis B virus X (HBx) mediates the establishment and maintenance of the chronic carrier state. HBx is a multifunctional and potentially oncogenic protein that is conserved among mammalian hepadnaviruses; it is produced very early after infection and throughout the chronic phase. HBx exerts its effects by interacting with cellular proteins and activating various signaling pathways. HBx stimulates the transcription of genes that regulate cell growth, apoptosis, and DNA repair. It also interacts with proteasome subunits and affects mitochondrial stability. Moreover, HBx participates in processes that are associated with the progression of chronic liver disease, including angiogenesis and fibrosis. This review discusses the function of HBx in the life cycle of HBV and its contribution to the pathogenesis of HCC.

Characterization of occult hepatitis B virus infection from blood donors in China

Prevalence and characteristics of occult hepatitis B virus (HBV) infection (OBI) of genotypes B and C prevalent in China have not been extensively explored. Characterization of OBI strains obtained from Chinese blood donors was based on clinical and serological analyses, follow-up testing, and sequence analyses. Twenty-eight samples from 165,371 HBV surface antigen (HBsAg)-negative plasmas were confirmed HBsAg negative and DNA positive(HBsAg(-)/DNA(+)), of which 22 were classified as OBIs and 6 as window period infections. The OBI incidence was 1:7,517 in blood donors, whose ages ranged between 20 and 45 years (median, 28 years). OBI donors had normal alanine aminotransferase (ALT) levels and low viral loads ranging between unquantifiable amounts and 178 IU/ml (median, 14 IU/ml). Sequences from 21 basic core promoter/precore (BCP/PC) regions, five whole genomes, and two additional pre-S/S regions from OBI strains were compared to genotypes B and C HBsAg(+) reference strains. Eighty-six percent (6/7) of OBI strains were genotype C. Deletions, insertions, stop codons, and substitutions were detected in 15/21 (71%) core regulatory elements of OBI strains. Critical mutations were found in the core proteins of 5/5 OBI strains in parallel with random substitutions in pre-S/S proteins from 6/7 (86%) OBI strains. Critical mutations in core regulatory elements and core proteins might affect OBI genotype B and C strain replication. That there were few S protein substitutions suggests a minor role of the host immune defenses in OBI occurrence.

Naturally occurring hepatitis B virus X deletions and insertions among Korean chronic patients

Deletions and insertions in the hepatitis B virus (HBV) X region have been associated with severe forms of liver disease, including hepatocellular carcinoma (HCC). However, the molecular epidemiologic features of this virus have been described rarely. Deletions and insertions in the X region were determined by direct sequencing in a Korean cohort of 267 patients with different clinical statuses. Deletions and insertions were observed in two sets of six patients each (2.2%, 6/267). The prevalence of deletions or insertions was significantly higher in patients with severe liver disease, HCC, or cirrhosis of the liver (7.2%, 10/132) compared to patients who were carriers or had chronic hepatitis (1.5%, 2/135) (P = 0.017). All deletions in six strains were concentrated at the C terminal end of HBx, encompassing the 113th to 154th codons. A total of four novel types of insertions (PKLL, GM, FFN, and tt) were observed in six patients. Of particular interest, all six strains with insertions were accompanied by double mutations in the basal core promoter (BCP). In conclusion, these results suggest that deletions or insertions in the X region may contribute to disease progression in Korean patients with genotype C infection.

Mechanism of inhibiting type I interferon induction by hepatitis B virus X protein

Hepatitis B virus (HBV) is regarded as a stealth virus, invading and replicating efficiently in human liver undetected by host innate antiviral immunity. Here, we show that type I interferon (IFN) induction but not its downstream signaling is blocked by HBV replication in HepG2.2.15 cells. This effect may be partially due to HBV X protein (HBx), which impairs IFNβ promoter activation by both Sendai virus (SeV) and components implicated in signaling by viral sensors. As a deubiquitinating enzyme (DUB), HBx cleaves Lys63-linked polyubiquitin chains from many proteins except TANK-binding kinase 1 (TBK1). It binds and deconjugates retinoic acid-inducible gene I (RIG I) and TNF receptor-associated factor 3 (TRAF3), causing their dissociation from the downstream adaptor CARDIF or TBK1 kinase. In addition to RIG I and TRAF3, HBx also interacts with CARDIF, TRIF, NEMO, TBK1, inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase epsilon (IKKi) and interferon regulatory factor 3 (IRF3). Our data indicate that multiple points of signaling pathways can be targeted by HBx to negatively regulate production of type I IFN.

Hepatitis B virus regulatory HBx protein binds to adaptor protein IPS-1 and inhibits the activation of beta interferon

Hepatitis B virus (HBV) encodes the regulatory HBx protein, which is required for virus replication, although its specific role(s) in the replication cycle remains under investigation. An immunoprecipitation/mass spectrometry approach was used to identify four novel HBx binding proteins from the cytoplasmic fraction of HBx transgenic mouse livers. One of these HBx binding partners is beta interferon promoter stimulator 1 (IPS-1), an adaptor protein that plays a critical role in mediating retinoic acid-inducible gene I (RIG-I) signaling, which leads to the activation of beta interferon (IFN-β). The HBx-IPS-1 protein interaction was confirmed in plasmid-transfected HepG2 cells by reciprocal coimmunoprecipitation and Western blotting. We hypothesized that HBx might alter IPS-1 function since proteins of hepatitis C virus and hepatitis A virus similarly bind IPS-1 and target it for inactivation. The effect of HBx on IPS-1-mediated IFN-β signaling was tested in transfected 293T and HepG2 cells, and we show that HBx inhibits double-stranded DNA (dsDNA)-mediated IFN-β activation in a dose-dependent manner when expressed either alone or within the context of HBV replication. However, HBx does not inhibit poly(I:C)-activated IFN-β signaling. These results demonstrate that HBx interferes with the RIG-I pathway of innate immunity. Hepatitis B virus now joins hepatitis C virus and hepatitis A virus in targeting the same innate immune response pathway, presumably as a shared strategy to benefit replication of these viruses in the liver.

The C-terminal region of the hepatitis B virus X protein is essential for its stability and function

More than 350 million people worldwide are chronically infected with the human hepatitis B virus (HBV). Chronic HBV infections are associated with the development of hepatocellular carcinoma. While the mechanism of HBV-associated carcinoma remains undefined, it is thought to involve a combination of a continuous inflammatory response to HBV-infected hepatocytes and activities of HBV proteins such as the HBV X protein (HBx). HBx stimulates HBV replication; however, the mechanism by which HBx stimulates HBV replication remains incompletely understood. Studies performed with the woodchuck hepatitis virus (WHV) in woodchucks demonstrated that a C-terminally truncated mutant of the WHV X protein could not stimulate WHV replication. However, whether the C-terminus of HBx is important for HBx-stimulation of HBV replication is unclear. We have constructed C-terminal truncation mutants of HBx and have demonstrated that the C-terminus of HBx impacts HBx stability, HBx activation of transcription, and HBx stimulation of HBV replication. These observations highlight the impact of the HBx C-terminus on HBx activities and the importance of directly analyzing HBx expression and functions in HBV-associated tumors that contain chromosomal integrants of HBV with truncations of the HBx gene.

Hepatitis B virus X protein suppresses virus-triggered IRF3 activation and IFN-beta induction by disrupting the VISA-associated complex

Viral RNAs produced during viral infection are recognized by the cytoplasmic RNA helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). A central adapter protein downstream of RIG-I and MDA5 is the mitochondrial membrane protein virus-induced signaling adaptor (VISA), which mediates the induction of type I interferons (IFNs) through the activation of transcription factors such as nuclear factor-kappaB (NF-kappaB) and IFN-regulatory factor-3 (IRF3). Here we found that hepatitis B virus (HBV)-encoded X protein (HBx) acts as an inhibitor of virus-triggered IRF3 activation and IFN-beta induction. Reporter and plaque assays indicate that HBx inhibits signaling by components upstream but not downstream of VISA. Immunoprecipitation experiments indicate that HBx interacts with VISA and disrupts the association of VISA with its upstream and downstream components. These findings suggest that HBx acts as a suppressor of virus-triggered induction of type I IFNs, which explains the observation that HBV causes transient and chronic infection in hepatocytes but fails to activate the pattern recognition receptor-mediated IFN induction pathways.

Replication of the hepatitis B virus requires a calcium-dependent HBx-induced G1 phase arrest of hepatocytes

Chronic HBV infections cause hepatocellular carcinoma (HCC). Activities of the HBV HBx protein regulate HBV replication and may contribute to the development of HCC. We previously reported that HBx causes primary rat hepatocytes to exit G0 but stall in G1 phase of the cell cycle; entry into G1 stimulated HBV replication. We now report that the activity of the mitochondria permeability transition pore is required for HBx regulation of cell cycle proteins and HBV replication in primary rat hepatocytes, that progression from G0 to G1 stimulates HBV polymerase activity, and that HBV replication is facilitated by the HBx-induced G1 arrest. HBx stimulation of HBV replication was linked to elevation of the R2 subunit of ribonucleotide reductase. Our studies suggest that HBx uses mitochondrial-dependent calcium signaling to cause hepatocytes to exit G0 but stall in G1 and that this HBx activity alters the cellular environment and stimulates HBV replication.

Molecular biology of the hepatitis B virus and role of the X gene

The hepatitis B virus (HBV) is a widespread human pathogen and a major health problem in many countries. Molecular cloning and sequencing of the viral DNA genome has demonstrated a small and compact structure organized into four overlapping reading frames that encode the viral proteins. Besides structural proteins of the core and the envelope, HBV encodes a DNA polymerase with reverse transcriptase activity, a secreted antigen of unknown function, and a transcriptional activator that is essential for viral replication. Major steps of the viral life cycle have been unraveled, including transcription of all viral RNAs from nuclear covalently closed circular DNA (cccDNA), followed by encapsidation of pregenomic RNA, a more-than-genome length transcript, and reverse transcription of pregenomic RNA leading to asymmetric synthesis of the DNA strands. Although HBV has been recognized as a human tumor virus, no direct transforming activity could be evidenced in different cellular and animal models. However, the transcriptional regulatory protein HBx encoded by the X gene is endowed with weak oncogenic activity. HBx harbors pleiotropic activities and plays a major role in HBV pathogenesis and in liver carcinogenesis.

Application of hepatitis B virus replication mouse model

AIM: To evaluate the value of the hepatitis B virus (HBV) replication mouse model with regard to several aspects of the study of HBV biology.

METHODS: To evaluate the HBV replication mouse model in detecting the efficacy of anti-HBV agents, the interferon inducer polyinosinic-polytidylin acid (polyIC) and nucleotide analogues adefovir and entecavir were administered to mice injected with wild type pHBV4.1, and the inhibiting effect of these agents on HBV DNA replication was evaluated. To identify the model’s value in a replication ability study of HBV drug-resistant mutants and a HBx-minus mutant, telbivudine resistance mutants (rtM204I, ayw subtype), adefovir resistance mutants (rtA181V + rtN236T, ayw subtype) and HBx-minus mutants were injected respectively, and their corresponding HBV DNA replication intermediates in mouse liver were assessed.

RESULTS: Compared with the wild type HBV replication mouse model without antiviral agent treatment, the HBV DNA replication intermediates of the polyIC-treated group were decreased 1-fold; while in the entecavir- and adefovir-treated groups, the levels of HBV DNA replication intermediates were inhibited 13.6-fold and 1.4-fold, respectively. For the mouse models injected with telbivudine resistance mutant, adefovir resistance mutant and HBx-minus mutant, HBV DNA replication intermediates could still be detected, but the levels of HBV DNA replication intermediates of these mutants decreased 4.5-fold, 5.6-fold and 2.9-fold respectively, compared with the mouse model with wild type HBV plasmid.

CONCLUSION: The HBV replication mouse model we established was a useful and convenient tool to detect the efficacy of antiviral agents and to study the replication ability of HBV mutants in vivo.

Mechanisms of HBV-related hepatocarcinogenesis

The hepatitis B virus (HBV) is a small enveloped DNA virus, which primarily infects hepatocytes and causes acute and persistent liver disease. Epidemiological studies have provided overwhelming evidence for a causal role of chronic HBV infection in the development of hepatocellular carcinoma, but the molecular mechanisms underlying virally-induced tumourigenesis remain largely debated. In the absence of a dominant oncogene encoded by the HBV genome, indirect roles have been proposed, including insertional activation of cellular cancer-related genes by HBV DNA integration, induction of genetic instability by viral integration or by the regulatory protein HBx, and long-term effects of viral proteins in enhancing immune-mediated liver disease. Recent genetic studies indicate that HBV-related tumours display a distinctive profile with a high rate of chromosomal alterations and low frequency of beta-catenin mutations. This review will discuss the evidence implicating chronic HBV infection as a causal risk factor of primary liver cancer. It will also discuss the molecular mechanisms that are critical for the tumourigenic process due to long lasting infection with HBV.

HBx protein is indispensable for development of viraemia in human hepatocyte chimeric mice

The non-structural X protein, HBx, of hepatitis B virus (HBV) is assumed to play an important role in HBV replication. Woodchuck hepatitis virus X protein is indispensable for virus replication, but the duck hepatitis B virus X protein is not. In this study, we investigated whether the HBx protein is indispensable for HBV replication in vivo using human hepatocyte chimeric mice. HBx-deficient (HBx-def) HBV was generated in HepG2 cells by transfection with an overlength HBV genome. Human hepatocyte chimeric mice were infected with HBx-def HBV with or without hepatic HBx expression by hydrodynamic injection of HBx expression plasmids. Serum virus levels and HBV sequences were determined with mice sera. The generated HBx-def HBV peaked in the sucrose density gradient at points equivalent to the generated HBV wild type and the virus in a patient's serum. HBx-def HBV-injected mice developed measurable viraemia only in continuously HBx-expressed liver. HBV DNA in the mouse serum increased up to 9 log(10) copies ml(-1) and the viraemia persisted for more than 2 months. Strikingly, all revertant viruses had nucleotide substitutions that enabled the virus to produce the HBx protein. It was concluded that the HBx protein is indispensable for HBV replication and could be a target for antiviral therapy.

The hepatitis B virus X protein modulates hepatocyte proliferation pathways to stimulate viral replication

Worldwide, there are over 350 million people who are chronically infected with the human hepatitis B virus (HBV); chronic HBV infections are associated with the development of hepatocellular carcinoma (HCC). The results of various studies suggest that the HBV X protein (HBx) has a role in the development of HBV-associated HCC. HBx can regulate numerous cellular signal transduction pathways, including those that modulate cell proliferation. Many previous studies that analyzed the impact of HBx on cell proliferation pathways were conducted using established or immortalized cell lines, and when HBx was expressed in the absence of HBV replication, and the precise effect of HBx on these pathways has often differed depending on experimental conditions. We have studied the effect of HBx on cell proliferation in cultured primary rat hepatocytes, a biologically relevant system. We demonstrate that HBx, both by itself and in the context of HBV replication, affected the levels and activities of various cell cycle-regulatory proteins to induce normally quiescent hepatocytes to enter the G(1) phase of the cell cycle but not to proceed to S phase. We linked HBx regulation of cell proliferation to cytosolic calcium signaling and HBx stimulation of HBV replication. Cumulatively, our studies suggest that HBx induces normally quiescent hepatocytes to enter the G(1) phase of the cell cycle and that this calcium-dependent HBx activity is required for HBV replication. These studies identify an essential function of HBx during HBV replication and a mechanism that may connect HBV infections to the development of HCC.

Nuclear HBx binds the HBV minichromosome and modifies the epigenetic regulation of cccDNA function

HBV cccDNA, the template for transcription of all viral mRNAs, accumulates in the nucleus of infected cells as a stable episome organized into minichromosomes by histones and non-histone viral and cellular proteins. Using a cccDNA-specific chromatin immunoprecipitation (ChIP)-based quantitative assay, we have previously shown that transcription of the HBV minichromosome is regulated by epigenetic changes of cccDNA-bound histones and that modulation of the acetylation status of cccDNA-bound H3/H4 histones impacts on HBV replication. We now show that the cellular histone acetyltransferases CBP, p300, and PCAF/GCN5, and the histone deacetylases HDAC1 and hSirt1 are all recruited in vivo onto the cccDNA. We also found that the HBx regulatory protein produced in HBV replicating cells is recruited onto the cccDNA minichromosome, and the kinetics of HBx recruitment on the cccDNA parallels the HBV replication. As expected, an HBV mutant that does not express HBx is impaired in its replication, and exogenously expressed HBx transcomplements the replication defects. p300 recruitment is severely impaired, and cccDNA-bound histones are rapidly hypoacetylated in cells replicating the HBx mutant, whereas the recruitment of the histone deacetylases hSirt1 and HDAC1 is increased and occurs at earlier times. Finally, HBx mutant cccDNA transcribes significantly less pgRNA. Altogether our results further support the existence of a complex network of epigenetic events that influence cccDNA function and HBV replication and identify an epigenetic mechanism (i.e., to prevent cccDNA deacetylation) by which HBx controls HBV replication.

Identification of androgen response elements in the enhancer I of hepatitis B virus: a mechanism for sex disparity in chronic hepatitis B

Hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC) occurs more often in men than in women. Male HBV carriers usually have higher viral loads, which is a well-known risk factor for HCC. Whether and how the male androgen axis regulates HBV transcription and replication is investigated here. We used HBV transgenic mice to evaluate any sex disparity of serum hepatitis B surface antigen and HBV titers as well as the castration effect on this disparity. Compared to females, HBV transgenic male mice showed higher hepatitis B surface antigen and viral titers, which were lessened by castration of the males. In a cell culture system, HepG2 cells transfected with HBV and androgen receptor (AR) constructs were used to study the effect of the androgen pathway on viral transcription and replication. We found the ligand-stimulated AR could increase the transcription of HBV RNAs through its transcription activation domain. A genomic region within HBV enhancer I was identified that is responsible for the transcriptional activation of AR. The results from chromatin immunoprecipitation and in vitro binding assays further demonstrated a direct binding of AR to this region, in a ligand-dependent manner. Two androgen-responsive element motifs in this region were identified, and their mutations can significantly abolish the AR effects.

CONCLUSION

This study demonstrated that the androgen pathway can increase the transcription of HBV through direct binding to the androgen-responsive element sites in viral enhancer I. This may explain a higher HBV titer in male carriers and an increased risk of HCC.

Tripartite motif-containing 22 inhibits the activity of hepatitis B virus core promoter, which is dependent on nuclear-located RING domain

Members of the tripartite motif (TRIM) family are a part of the innate immune system to counter intracellular pathogens. TRIM22 has been reported to possess antiretroviral activity. Here we report that TRIM22 is involved in antiviral immunity against hepatitis B virus (HBV). Our results showed that TRIM22, being a strongly induced gene by interferons in human hepatoma HepG2 cells, could inhibit HBV gene expression and replication in a cell culture system as well as in a mouse model system. Importantly, it was found that TRIM22 could inhibit the activity of HBV core promoter (CP) in a dose-dependent manner. However, TRIM22 lacking the C terminal SPRY domain lost this activity. Further study showed that the SPRY domain deletion mutant was localized exclusively to the cytoplasm of HepG2 cells. In contrast, the wild-type TRIM22 was localized to the nucleus, as expected for a transcriptional suppressor. Interestingly, although RING domain mutants of TRIM22 were localized to the nucleus, they could not inhibit HBV CP activity, indicating that TRIM22-mediated anti-HBV activity was dependent on the nuclear-located RING domain.

CONCLUSION

These findings suggest that TRIM22, which exhibits anti-HBV activity by acting as a transcriptional suppressor, may play an important role in the clearance of HBV.

The correlation of hepatocyte nuclear factor 4 alpha and 3 beta with hepatitis B virus replication in the liver of chronic hepatitis B patients

Hepatocyte nuclear factors 4 alpha (HNF4alpha) and 3 beta (HNF3beta) are members of a group of liver-enriched transcription factors (LETFs) that play important roles in regulating the replication of hepatitis B virus (HBV). Using cell culture and animal models, we showed that HNF4alpha supports HBV replication in nonhepatic cells and HNF3beta inhibits HBV replication. However, the expression of HNF4alpha and HNF3beta in the liver tissue of chronic HBV-infected patients and the relationship between the levels of HNF4alpha and HNF3beta and HBV replication are unclear. In this study, liver biopsy specimens from 86 chronic HBV-infected patients were collected. The expression levels of HNF4alpha, HNF3beta, hepatitis B surface antigen (HBsAg) and hepatitis B core antigen (HBcAg) were detected by an immunohistochemical technique and the level of HBV DNA was checked by in situ hybridization with serial sections from liver biopsy tissue samples. We show here that samples with higher levels of HNF4alpha expression also have higher levels of HBsAg, HBcAg and HBV DNA. In contrast, in samples with higher levels of HNF3beta expression, levels of HBsAg, HBcAg and HBV DNA were lower. There was a positive correlation between HNF4alpha expression and HBV replication, and a negative correlation between HNF3beta expression and HBV replication, in the liver of chronic HBV-infected patients. This suggests that HNF4alpha and HNF3beta likely participate in HBV replication in patients with HBV infection, or that HBV replication may somehow influence the expression of HNF4alpha and HNF3beta in the liver.

Characterization of occult hepatitis B virus strains in South African blood donors

Since October 2005, all blood units collected in South Africa were screened individually for human immunodeficiency virus (HIV)-1, hepatitis B and C virus (HBV, HCV) genomes uncovering preseroconversion window period (WP) infections for each virus and occult HBV infections (OBIs) defined as persistent HBV DNA without detectable hepatitis B surface antigen (HBsAg). Samples identified as HBsAg-negative/DNA-positive were confirmed by combining real-time quantitative polymerase chain reaction, nested amplification, anti-HBc and anti-HBs. Amplified basic core promoter/precore, pre-S/S, and whole genome were sequenced, analyzed, and compared to 73 HBsAg+ strains. Genotype was determined by phylogenetic analysis. From 109 samples examined, 54 were classified as OBI, 14 as WP, 20 as false-positive, five as other classification, and 16 as undetermined due to lack of serological or follow-up data. OBI donors were predominantly males (67%), median age 31 years, black (54%), with normal alanine aminotransferase levels. Viral load ranged between unquantifiable and 518 IU/mL (median 5 IU/mL). Genotype A1 was more frequent (23 strains) than genotype D (seven strains). Genotype A1 strains were little mutated. In the major hydrophilic region, 56.5% strains were wild type or with few amino acid substitutions. Most important, all 13 full genome sequences presented 1 to 7 mutations known to or assumed to negatively impact viral replication. In particular, 6/13 sequences had a stop codon in the HBx gene translated into deletion of 117 or 19-25 C-terminus amino acids not found in 15 HBeAg+ HBsAg+ strains. One WP sequence with an HBx stop codon suggested infectivity.

CONCLUSION

Genotype A1 OBIs are different from genotype A2 and D OBIs in that there is little evidence of immune pressure as a major factor involved in OBI genesis. Limited replication appears mostly related to genetic viral defects.

A new strategy for constructing in vitro replication-competent 1.3 copies of hepatitis B virus genome

In the absence of a robust infectable cell culture system, assays related to replication of clinical HBV isolates are based on the transfection of replication-competent HBV DNA into hepatoma cell lines that are able to replicate and secrete HBV virions. Current methods for constructing HBV 1.1 genomes work well for drug susceptibility assays, but are not very suitable for research on HBV replication capacity or regulation since a heterogeneous promoter is required to drive pgRNA transcription. A new strategy for constructing HBV 1.3 genomes that contain HBV intrinsic promoter necessary for pgRNA transcription is reported in this paper. Using this strategy, three HBV 1.3 genomes from isolates of three patients were constructed. When the three HBV 1.3 genomes were transfected into the HepG2 cell line, replicative intermediates were detectable by Southern blotting with digoxigenin-labeled DNA probe in two of the three constructs. Using overlap extension PCR and avoiding as much as possible the digestion-and-ligation process, this strategy could be applied to constructing longer-than-genome units for most genotypes of HBV strains.

Hepatitis B virus HBx protein localized to the nucleus restores HBx-deficient virus replication in HepG2 cells and in vivo in hydrodynamically-injected mice

Identifying the requirements for the regulatory HBx protein in hepatitis B virus (HBV) replication is an important goal. A plasmid-based HBV replication assay was used to evaluate whether HBx subcellular localization influences its ability to promote virus replication, as measured by real time PCR quantitation of viral capsid-associated DNA. HBx targeted to the nucleus by a nuclear localization signal (NLS-HBx) was able to restore HBx-deficient HBV replication, while HBx containing a nuclear export signal (NES-HBx) was not. Both NLS-HBx and NES-HBx were expressed at similar levels (by immunoprecipitation and Western blotting), and proper localization of the signal sequence-tagged proteins was confirmed by deconvolution microscopy using HBx, NLS-HBx, and NES-HBx proteins fused to GFP. Importantly, these findings were confirmed in vivo by hydrodynamic injection into mice. Our results demonstrate that in these HBV replication assays, at least one function of HBx requires its localization to the nucleus.

Proteomic profiling identifies aberrant epigenetic modifications induced by hepatitis B virus X protein

The hepatitis B virus-encoded X (HBx) protein coactivates transcription of a variety of viral and cellular genes and it is believed to play essential roles in viral replication and hepatocarcinogenesis. To examine the pleiotropic effects of HBx protein on host cell protein expression, we utilized 2-DE and MS analysis to compare and identify differentially expressed proteins between a stable HBx-transfected cell line (HepG2-HBx), constitutively expressing HBx, and vector control cells. Of the 60 spots identified as differentially expressed (+/- over 2-fold, p < 0.05) between the two cell lines, 54 spots were positively identified by MS/MS analysis. Several recent studies suggested that HBx was involved in regional hypermethylation of tumor suppressor genes and global hypomethylation of satellite 2 repeats during hepatocarcinogenesis; however, no specific gene has been reported as hypomethylated by HBx. Promoter methylation analysis was examined for those protein spots showing significant alterations, and our results revealed that specific genes, such as aldehyde dehydrogenase 1 (ALDH1), can be hypomethylated by HBx, and two calcium ion-binding proteins, S100A6 and S100A4, were hypermethylated by HBx and could be re-expressed by AZA (DNA methylase inhibitor) treatment. Moreover, via cluster and pathway analysis, we proposed a hypothetical model for the HBx regulatory circuit involving aberrant methylation of retinol metabolism-related genes and calcium homeostasis-related genes. In summary, we profiled proteome alterations between HepG2-HBx and control cells, and found that HBx not only induces regional hypermethylation but also specific hypomethylation of host cell genes.

Stimulation of hepatitis B virus genome replication by HBx is linked to both nuclear and cytoplasmic HBx expression

HBx, a small regulatory protein of hepatitis B virus, plays an important role in stimulating viral genome replication. HBx was shown to be associated with diverse subcellular locations, such as the nucleus, cytoplasm and mitochondria. Some studies have linked the stimulation of genome replication by HBx to its cytoplasmic function, while other reports have attributed this function to its nuclear component. To clarify this discrepancy, we measured viral genome replication by complementing an HBx-null replicon in two different ways: by (i) co-transfecting with an increasing amount of HBx expression plasmid and (ii) co-transfecting with re-targeted variants of HBx that are confined to either the nucleus or the cytoplasm due to either the nuclear localization signal (NLS) or the nuclear export signal (NES) tags, respectively. Intriguingly, immunostaining analysis indicated that the subcellular localization of HBx is primarily influenced by its abundance; HBx is confined to the nucleus at low levels but is usually detected in the cytoplasm at high levels. Importantly, HBx, whether re-targeted by either the NLS or NES tag, stimulates viral genome replication to a level comparable to that of the wild-type. Furthermore, similar to the wild-type, the stimulation of viral genome replication by the re-targeted HBx occurred at the transcription level. Thus, we concluded that the stimulation of viral genome replication by HBx is linked to both nuclear and cytoplasmic HBx, although the underlying mechanism of stimulation most likely differs.

A double-spliced defective hepatitis B virus genome derived from hepatocellular carcinoma tissue enhanced replication of full-length virus

In hepatitis B virus (HBV) replication cycle, pregenomic RNA undergoes splicing and the reverse transcribed defective genomes can be packaged and released. Various types of spliced defective HBV genomes have been isolated from the sera and liver tissues of viral hepatitis B patients. To explore the functions of a 2.2 kb double spliced HBV variant, a 3.2 kb full-length HBV isolate (#97-34) and its 2.2 kb double-spliced HBV variant (#AP-12) from the tumor tissue of a patient with hepatocellular carcinoma (HCC) were amplified and cloned. Sequencing results showed that #AP12 had deletions in pre-S2, part of pre-S1, S genes, part of the spacer, and part of the reverse transcriptase gene, while the X gene was intact. When this defective double-spliced genome and its full-length counterpart genome were co-transfected into HepG2 cells, the former was shown to enhance the replication of the latter, both by real-time PCR and Southern blotting. When a replication incompetent clone 97-34G1881A was used to co-transfect with #AP12, #AP12 DNA was increased, indicating that replication of the wild-type virus was not the only factor involved in this observation. However, the replication enhancing competency of #AP12 was shown to require an intact HBV X expression cassette. The double-spliced defective variant might contribute to persistent HBV replication in a subpopulation of HCC patients.

Hepatitis B virus X mutations occurring naturally associated with clinical severity of liver disease among Korean patients with chronic genotype C infection

Few reports have detailed mutation frequencies and mutation patterns in the entire X region according to clinical status. The aims of this study were to elucidate the relationships between mutation patterns and their frequencies in the X region and clinical status in a Korean cohort and determine specific X mutation types, related closely with liver disease progression. All X mutations were determined by direct sequencing in 184 patients with different clinical features. Mutation rates in the X region in patients with more severe liver disease, hepatocellular carcinoma (HCC) (3.6%) or liver cirrhosis (4%) were always significantly higher than in patients with corresponding less severe forms, chronic hepatitis (2.9%) or asymptomatic carriers (2.1%), but no significant difference in mutation rates was found in terms of HBeAg serostatus. All five mutation types (V5M/L, P38S, H94Y, I127T/N, and K130M and V131I) affecting the six codons were found to be related significantly to clinical severity. Among these, two mutation types (V5M/L and K130M and V131I) were observed more frequently in HBeAg negative patients than in HBeAg positive patients. In conclusion, the results suggest that an accumulation of mutations in the X region contributes to disease progression in chronic patients, at least Korean patients with genotype C. Specific mutation types appears to be related more to severe liver diseases such as HCC or liver cirrhosis. In particular, a novel mutation type (V5M/L) discovered firstly during the present study was found to be associated significantly with HCC.

A new mutation pattern of hepatitis B virus X gene

AIM: To identify a novel X gene mutation pattern of hepatitis B virus (HBV) in patients with chronic HBV infection.

METHODS: A pair of primers was designed on the basis of nucleotide sequences of X gene. Polymerase chain reaction (PCR) was used to amplify the target region from HBV DNA samples extracted from chronic hepatitis B patients in Xiamen city. After electrophoresis of the PCR products in 9 g/L agarose gel, the target regions were cut, re-purified and TA-cloned into pMD19 T vector. The inserted regions in positive clones were sequenced. Sequence comparison with HBV genome submitted in GenBank was made to find the mutation sites.

RESULTS: Totally 74 strains from 21 patients with chronic HBV infection were sequenced, and the results showed that there was a characteristic deletion region, with a length of 234 nt (nt 1601-1834) in 54 clones, and a length of 245 nt in 1 clone. There were 3 replacement mutations bounding to deletion mutation: G/A1515C, G1518C and A1585T, which caused substitutions in the 44th and 45th amino acid site to LL. These mutant strains only coded 76 aa of up-stream HBx.

CONCLUSION: A novel deletion mutation in HBV X gene is observed in patients with chronic HBV infection. The deletion mutants encode 76-aa X factor instead of X protein.

Hepatitis B virus X protein induces apoptosis by enhancing translocation of Bax to mitochondria

Hepatitis B virus X protein (HBx) is essential for viral replication and plays an important role in viral pathogenesis. HBx transactivates many viral and cellular genes and participates in cellular signal transduction pathways, proliferation, and apoptosis. In the present study, we report that HBx induces apoptosis by enhancing the translocation of Bax to mitochondria, followed by inducing the loss of mitochondrial membrane potential and release of cytochrome C. In addition, Bcl-2, inhibitor of Bax, rescues the disruption of mitochondrial membrane potential and DNA fragmentation induced by serum starvation in HepG2-X cells expressing HBx. We also found that HBx binds directly to Bax and interferes with the interaction between Bax and 14-3-3epsilon to enhance the translocation of Bax to mitochondria. Taken together, our data suggest that HBx induces apoptosis by interacting with Bax and enhancing its translocation to mitochondria.

Hepatitis B virus HBx protein localizes to mitochondria in primary rat hepatocytes and modulates mitochondrial membrane potential

Over 350 million people are chronically infected with hepatitis B virus (HBV), and a significant number of chronically infected individuals develop primary liver cancer. HBV encodes seven viral proteins, including the nonstructural X (HBx) protein. The results of studies with immortalized or transformed cells and with HBx-transgenic mice demonstrated that HBx can interact with mitochondria. However, no studies with normal hepatocytes have characterized the precise mitochondrial localization of HBx or the effect of HBx on mitochondrial physiology. We have used cultured primary rat hepatocytes as a model system to characterize the mitochondrial localization of HBx and the effect of HBx expression on mitochondrial physiology. We now show that a fraction of HBx colocalizes with density-gradient-purified mitochondria and associates with the outer mitochondrial membrane. We also demonstrate that HBx regulates mitochondrial membrane potential in hepatocytes and that this function of HBx varies depending on the status of NF-kappaB activity. In primary rat hepatocytes, HBx activation of NF-kappaB prevented mitochondrial membrane depolarization; however, when NF-kappaB activity was inhibited, HBx induced membrane depolarization through modulation of the mitochondrial permeability transition pore. Collectively, these results define potential pathways through which HBx may act in order to modulate mitochondrial physiology, thereby altering many cellular activities and ultimately contributing to the development of HBV-associated liver cancer.