Trans-activation of the human immunodeficiency virus long terminal repeat by the hepatitis B virus X protein

Identification of critical residues in the regulatory protein HBx for Smc5/6 interaction and hepatitis B virus production

The host structural maintenance of chromosomes 5/6 complex (Smc5/6) is a restriction factor of hepatitis B virus (HBV) that inhibits the transcription of viral ccDNA. HBV antagonizes this restriction by expressing the regulatory X protein (HBx) which targets Smc5/6 for degradation via DNA damage-binding protein 1 (DDB1) E3 ubiquitin ligase. However, the molecular insights into how Smc5/6 interacts with HBx remain elusive. In this study, we systematically investigated the interaction between Smc5/6 and HBx. Smc5/6 interacts with HBx through multiple sites in the absence of DDB1 in the pull-down assay. HBx C-terminal is sufficient for the interaction. Most importantly, residue Phe132, which is strictly conserved in all HBV subtypes, is critical for interaction with Smc5/6 both in vitro and in vivo. Mutation of this site (F132A) results in defect in Smc5/6 interaction, extrachromosomal reporter transcription, and HBV production both in cells and in mouse model. Collectively, our data identifies a key residue on HBx for Smc5/6 interaction and viral production. These results provide valuable information for both basic research and therapeutic drugs targeting HBx.

Identifying and Characterizing Interplay between Hepatitis B Virus X Protein and Smc5/6

Hepatitis B X protein (HBx) plays an essential role in the hepatitis B virus (HBV) replication cycle, but the function of HBx has been elusive until recently. It was recently shown that transcription from the HBV genome (covalently-closed circular DNA, cccDNA) is inhibited by the structural maintenance of chromosome 5/6 complex (Smc5/6), and that a key function of HBx is to redirect the DNA-damage binding protein 1 (DDB1) E3 ubiquitin ligase to target this complex for degradation. By doing so, HBx alleviates transcriptional repression by Smc5/6 and stimulates HBV gene expression. In this review, we discuss in detail how the interplay between HBx and Smc5/6 was identified and characterized. We also discuss what is known regarding the repression of cccDNA transcription by Smc5/6, the timing of HBx expression, and the potential role of HBx in promoting hepatocellular carcinoma (HCC).

Long-term responses to first-line antiretroviral therapy in HIV and hepatitis B co-infection in Ghana

OBJECTIVES

To observe the long term response to first-line antiretroviral therapy (ART) in HIV and hepatitis B virus (HBV) co-infected patients in Ghana and explore predictors of poor clinical outcomes.

METHODS

Retrospective cohort study of hepatitis B surface antigen (HBsAg) positive and negative patients receiving predominantly NNRTI-based ART with lamivudine plus either zidovudine or stavudine for up to seven years. Cox proportional hazards and Kaplan Meier survival analyses compared clinical outcomes and identified baseline characteristics predictive of poor outcomes. A mixed effects model compared changes in CD4 counts.

RESULTS

A total of 299 HBsAg-positive and 1869 HBsAg-negative patients started ART between 2004 and 2008. Over a median 35 months of follow-up, HBsAg-positive patients were more likely to die or default care than HBsAg-negative patients, aHR 1.36 (95% CI, 1.03-1.80). HBsAg-positive patients were also more likely to develop Grade 3/4 hepatotoxicity than HBsAg-negative patients, HR 1.99 (1.16-3.40) on survival analysis. There was no significant difference in CD4 responses between HBsAg-positive and HBsAg-negative patients.

CONCLUSIONS

HBsAg-positive patients are at significantly increased risk of adverse clinical outcomes after starting ART. Further studies are warranted to evaluate whether these risks remain now that tenofovir is becoming routinely available in Ghana.

The Role of Sexually Transmitted Infections in HIV-1 Progression: A Comprehensive Review of the Literature

Due to shared routes of infection, HIV-infected persons are frequently coinfected with other sexually transmitted infections (STIs). Studies have demonstrated the bidirectional relationships between HIV and several STIs, including herpes simplex virus-2 (HSV-2), hepatitis B and C viruses, human papilloma virus, syphilis, gonorrhea, chlamydia, and trichomonas. HIV-1 may affect the clinical presentation, treatment outcome, and progression of STIs, such as syphilis, HSV-2, and hepatitis B and C viruses. Likewise, the presence of an STI may increase both genital and plasma HIV-1 RNA levels, enhancing the transmissibility of HIV-1, with important public health implications. Regarding the effect of STIs on HIV-1 progression, the most studied interrelationship has been with HIV-1/HSV-2 coinfection, with recent studies showing that antiherpetic medications slow the time to CD4 <200 cells/µL and antiretroviral therapy among coinfected patients. The impact of other chronic STIs (hepatitis B and C) on HIV-1 progression requires further study, but some studies have shown increased mortality rates. Treatable, nonchronic STIs (i.e., syphilis, gonorrhea, chlamydia, and trichomonas) typically have no or transient impacts on plasma HIV RNA levels that resolve with antimicrobial therapy; no long-term effects on outcomes have been shown. Future studies are advocated to continue investigating the complex interplay between HIV-1 and other STIs.

Coinfection of hepatic cell lines with human immunodeficiency virus and hepatitis B virus leads to an increase in intracellular hepatitis B surface antigen

Liver-related mortality is increased in the setting of HIV-hepatitis B virus (HBV) coinfection. However, interactions between HIV and HBV to explain this observation have not been described. We hypothesized that HIV infection of hepatocytes directly affects the life cycle of HBV. We infected human hepatic cell lines expressing HBV (Hep3B and AD38 cells) or not expressing HBV (Huh7, HepG2, and AD43 cells) with laboratory strains of HIV (NL4-3 and AD8), as well as a vesicular stomatitis virus (VSV)-pseudotyped HIV expressing enhanced green fluorescent protein (EGFP). Following HIV infection with NL4-3 or AD8 in hepatic cell lines, we observed a significant increase in HIV reverse transcriptase activity which was infectious. Despite no detection of surface CD4, CCR5, and CXCR4 by flow cytometry, AD8 infection of AD38 cells was inhibited by maraviroc and NL4-3 was inhibited by AMD3100, demonstrating that HIV enters AD38 hepatic cell lines via CCR5 or CXCR4. High-level infection of AD38 cells (50%) was achieved using VSV-pseudotyped HIV. Coinfection of the AD38 cell line with HIV did not alter the HBV DNA amount or species as determined by Southern blotting or nucleic acid signal amplification. However, coinfection with HIV was associated with a significant increase in intracellular HBsAg when measured by Western blotting, quantitative HBsAg, and fluorescence microscopy. We conclude that HIV infection of HBV-infected hepatic cell lines significantly increased intracellular HBsAg but not HBV DNA synthesis and that increased intrahepatic HBsAg secondary to direct infection by HIV may contribute to accelerated liver disease in HIV-HBV-coinfected individuals.

Impact of hepatitis B virus infection on human immunodeficiency virus response to antiretroviral therapy in Nigeria

BACKGROUND

As highly active antiretroviral therapy (ART) is introduced into areas of the world in which hepatitis B virus (HBV) infection is highly endemic, it is important to determine the influence of HBV on persons with human immunodeficiency virus (HIV) and HBV coinfection who are receiving ART.

METHODS

We studied 1564 HIV-infected patients in Jos, Nigeria, who initiated ART. Participants with HIV-HBV coinfection had hepatitis B e antigen (HBeAg) and HBV DNA status determined. CD4+ T cell count and HIV load at ART initiation were compared between individuals with HIV monoinfection and those with HIV-HBV coinfection with use of univariate methods. Regression analyses were used to determine if HBeAg status or HBV DNA at ART initiation were associated with baseline HIV parameters or ART response.

RESULTS

The median CD4+ T cell count of the 262 participants with HIV-HBV coinfection (16.7%) was 107 cells/mL, compared with 130 cells/mL for participants with HIV monoinfection at ART initiation (P = .001). Participants with HIV-HBV coinfection also had higher HIV loads than did patients with HIV monoinfection (4.96 vs 4.75 log10 copies/mL; p = .02 ). Higher HBV DNA and detectable HBeAg levels were independently associated with lower CD4+ T cell counts at ART initiation but not with higher HIV loads. In a multivariable model, HBeAg-positive patients were less likely than HBeAg-negative patients to suppress HIV replication to <or= 400 copies/mL (odds ratio, 0.54; P = .03 ) at 24 weeks, but they had similar CD4+ T cell increases. At 48 weeks, there was no significant effect of HBeAg status on ART response.

CONCLUSIONS

Among HIV-infected Nigerian individuals, HBV coinfection, especially among those with high levels of HBV replication, was associated with lower CD4+ T cell counts at ART initiation, independent of HIV RNA level. Patients with HBeAg-positive status had a slower virological response to ART, compared with HBeAg-negative patients. Further work is needed to understand the effects of HBV on CD4+ T cells.

Hepatitis B virus X gene is implicated in liver carcinogenesis

Hepatitis B virus (HBV) is a small hepatotropic and highly species-specific enveloped DNA virus. The carcinogenicity of this virus has become focused on the X gene and its coded X protein. The X protein itself is unable to bind to DNA directly, but works as a potent transcriptional activator through multiple cis-acting elements and mediates several signal transduction cascades. Two regions of the X protein, aa.61-69 and aa.105-140, are found essential for the viral replication and expression as well. These functions interacting with transcription factors and signaling cascades are acting cooperatively to cause cell proliferation. Furthermore, the association of X protein with mitochondria causes loss of the mitochondrial membrane potential and subsequently causes cell death, the function of which is attributed to the aa.68-104 region of X protein. As a result, the X protein has two independent proliferative and cell death-promoting activities. Liver cancer has been shown to result from a series of mutations in specific oncogenes and tumor suppressor genes. In a recent study, X protein stimulates ROS generation in the mitochondria due to collapse of the membrane potential and increases the mutation frequency, that evokes malignant transformation. Inflammation as a result of HBV infection is concerned to cause DNA damage. In the past 10years, the possibility that several viral proteins directly engaged in the DNA damage has increased to some extent. From an evolutionary viewpoint, it is noteworthy that several arrangement proteins have been found in viruses. Thus, there is some clue that a small amount of X protein acts as an arrangement protein for HBV replication dependent upon cellular DNA damage due to generated ROS as an amplified signal.

The pathogenesis of liver disease in the setting of HIV–hepatitis B virus coinfection

There are many potential reasons for increased liver-related mortality in HIV–hepatitis B virus (HBV) coinfection compared with either infection alone. HIV infects multiple cells in the liver and might potentially alter the life cycle of HBV, although evidence to date is limited. Unique mutations in HBV have been defined in HIV–HBV-coinfected individuals and might directly alter pathogenesis. In addition, an impaired HBV- specific T-cell immune response is likely to be important. The roles of microbial translocation, immune activation and increased hepatic stellate cell activation will be important areas for future study.

Hepatocytes treated with HBV X protein as cytotoxic effectors kill primary hepatocytes by TNF-alpha-related apoptosis-induced ligand-mediated mechanism

OBJECTIVE

To investigate the effect of HBV X protein (HBX) on TNF-alpha-related apoptosis-induced ligand (TRAIL) expression in HepG2 cells, and observe death of primary hepatocytes induced by HBX-transfected HepG2 cells.

METHODS

Western blot was used to detect the TRAIL expression in HepG2 cells transfected with mammalian expression plasmid pSG5UTPL-HBX. The reverse transcription-PCR was used to observe TRAIL mRNA transcription stimulated by HBX protein, and chromium release assay was used to detect death of primary hepatocyte induced by HBX-transfected HepG2 cells.

RESULTS

HBX could increase TRAIL expression and mRNA transcription in HepG2 cells in a dose-dependent manner. The C-terminal truncated version of HBX (HBXD1) is responsible for inducing TRAIL expression in HepG2 cells. Chromium release assay results showed that HBX-transfected HepG2 cells kill primary human hepatocytes by a TRAIL-mediated mechanism. Neutralizing anti-TRAIL inhibits the HepG2 killing.

CONCLUSION

HBX protein increases TRAIL expression in HepG2 cells which induced death of primary hepatocytes. HBX protein may play an important role in mechanisms of hepatic cell death and hepatic inflammation caused by HBV infection.

Effects of hepatitis B virus X protein on human telomerase reverse transcriptase expression and activity in hepatoma cells

In subjects with hepatitis B, carcinogenesis has been associated with the hepatitis B virus (HBV) X protein (HBX) and human telomerase reverse transcriptase (hTERT). In the experiments reported here, we used immunohistochemical methods to study the expression of hTERT and HBV antigens (HBsAg, HBcAg and HBxAg) in 34 cases of HCC and corresponding paratumor tissues, 30 cases of liver cirrhosis, and 6 normal livers. To examine the effect of HBX on hTERT expression and activity in hepatoma cells, we transiently and stably transfected the pCMV-X plasmid cloned HBx gene into H7402 hepatoma cells, then measured the expression of c-Myc and hTERT in these cells with the use of Western-blot analysis. Telomerase activity was detected with the use of the telomerase repeat amplification protocol (TRAP) in transiently and stably transfected cells. We found that hTERT expression was 67.6%, 73.5%, and 100% in tumor, paratumor, and cirrhosis samples, respectively, but found no hTERT positivity in samples of normal liver. HBsAg, HBcAg, and HBxAg were expressed in 58.8%, 26.5%, and 76.5% of tumor tissues, respectively; in 64.7%, 41.2%, and 85.3% of the corresponding paratumor tissues; and in 76.7%, 66.7%, and 100% of cirrhotic tissues. The chi 2 test revealed no significant difference between the expression of hTERT and HBxAg in these tissues. Western-blot analysis revealed that expression of c-Myc and hTERT in the transiently transfected cells was much greater than that in the control cells. We elicited a similar result when we used the TRAP method to measure telomerase activity. Our data collectively demonstrate that HBX up-regulates the expression and activity of hTERT in hepatoma cells, suggesting that hTERT is associated with tumor development.

Feline foamy virus Tas protein is a DNA-binding transactivator

Foamy viruses (FVs) harbour a transcriptional transactivator (Tas) and two Tas-responsive promoter regions, one in the 5′ long terminal repeat (LTR) and the other an internal promoter (IP) in the envelope gene. To analyse the mechanism of transactivation of the FVs, the specificity of feline FV (FFV) Tas protein, which is more distantly related to the respective proteins of non-human primate origin, were investigated. FFV Tas has been shown specifically to activate gene expression from the cognate promoters. No cross-transactivation was noted of the prototype foamy virus and human immunodeficiency virus type 1 LTR. The putative transactivation response element of FFV Tas was mapped to the 5′ LTR U3 region (approximately nt −228 to −195). FFV Tas binds to this element in addition to a previously described sequence (position −66 to −51). It is therefore concluded that FFV Tas is a DNA-binding transactivator that interacts with at least two regions in the virus LTR.

The absence of up-regulation of telomerase activity during regeneration after partial hepatectomy in hepatitis B virus X gene transgenic mice

BACKGROUND/AIMS

Transgenic mice that express HBV X protein (HBx) have increased sensitivity to hepatocarcinogens. In the present study, we hypothesized that HBx interferes with the DNA protective increases in telomerase activity that occur in proliferating hepatocytes.

METHODS

Male CD-1 mice (4-6/grp) were killed and hepatic telomerase activity measured at 0, 6, 12, 24, 36, 48 h post partial hepatectomy (PHx). Four HBx transgenic mice were killed at 12 h post-PHx when maximum telomerase activity was observed in CD-1 non-transgenic mice. mRNA of the telomerase catalytic subunit; murine telomerase reverse transcriptase (mTERT), was measured by reverse transcription-polymerase chain reaction. Telomerase activity and human TERT (hTERT) were also measured in Chang and PLC/PRF/5 cells following transient transfection with HBx cDNA.

RESULTS

Telomerase activity peaked at 12 h post-PHx in normal mice, however, in HBx transgenic mice, telomerase activity was significantly lower, both at baseline (P<0.05) and 12 h post-PHx (P<0.01). Following PHx, mTERT mRNA expression remained constant in normal mice but decreased significantly (P<0.01) in HBx transgenic mice. Transfection of HBx in Chang and PLC/PRF/5 cells had no effect on telomerase activity or hTERT mRNA expression.

CONCLUSIONS

The results of this study suggest that HBx expression may play a role in hepatocellular carcinogenesis by interfering with telomerase activity during hepatocyte proliferation.

Absence of HBV and HCV, HTLV-I and -II, and human herpes virus-8 activation after allogeneic RBC transfusion in patients with advanced HIV-1 infection

BACKGROUND

The Viral Activation Transfusion Study was a prospective, randomized, double-blind comparison of transfusion with WBC-reduced versus non-WBC-reduced RBCs to HIV+ patients. The primary study characterized the effect of transfusion on HIV and CMV activation by monitoring viral load changes. The present study analyzed HBV, HCV, HTLV-I and -II, and human herpes virus-8 (HHV-8) viral load before and after transfusion to evaluate the further hypothesis that global immune stimulation following allogeneic RBC transfusion results in activation and increased viral proliferation of chronic viral infections other than HIV and CMV.

STUDY DESIGN AND METHODS

Baseline samples from 519 to 523 subjects were screened for HBV, HCV, HTLV-I and -II, and HHV-8 infection, and baseline, serial weekly, and quarterly blood samples from infected subjects in the non-WBC-reduced arm were evaluated for changes from baseline in viral nucleic acid and ALT levels.

RESULTS

Seroprevalence of HBV, HCV, HTLV-I and -II, and HHV-8 was 68, 25, 5, and 30 percent, respectively. No significant induction of HBV, HCV, HHV-8, or HTLV-I and -II viral replication following allogeneic transfusion of non-WBC-reduced blood was observed. A significant, albeit small, association was observed between transfusion and ALT.

CONCLUSIONS

Based on these results and our previous finding that no adverse effect on HIV and CMV viral load and disease progression results from allogeneic transfusion, no evidence is found to support the selective use of WBC-reduced blood components for HIV-infected patients.

Regulation of hepatocyte nuclear factor 1 activity by wild-type and mutant hepatitis B virus X proteins

The hepatitis B virus (HBV) core promoter regulates the transcription of two related RNA products named precore RNA and core RNA. Previous studies indicate that a double-nucleotide mutation that occurs frequently during chronic HBV infection converts a nuclear receptor binding site in the core promoter to the binding site of the transcription factor hepatocyte nuclear factor-1 (HNF-1) and specifically suppresses the transcription of the precore RNA. This mutation also changes two codons in the overlapping X protein coding sequence. In this report, we demonstrate that the X protein and its mutant X(mt) can physically bind to HNF-1 both in vitro and in vivo. Further analyses indicate that both X and X(mt) can enhance the gene transactivation and the DNA binding activities of HNF-1. This finding demonstrates for the first time that the X protein can stimulate the DNA binding activity of a homeodomain transcription factor. Interestingly, while both X and X(mt) can stimulate the HNF-1 activities, they differ in their effects: a smaller amount of X(mt) is needed to generate greater transactivation and DNA binding activities of HNF-1. This functional difference between X and X(mt) may have important implications in HBV pathogenesis and is apparently why they have different effects on the core promoter bearing the HNF-1 binding site.

Enhancement of hepatitis B virus replication by its X protein in transgenic mice

Hepatitis B virus (HBV) X gene encodes a multifunctional protein that can regulate cellular signaling pathways, interact with cellular transcription factors, and induce hepatocellular oncogenesis. In spite of its diverse activities, the precise role of the X protein in the viral life cycle of HBV remains unclear. To investigate this question, we have produced transgenic mice that carry either the wild-type HBV genome or a mutated HBV genome incapable of expressing the 16.5-kDa X protein. Our results indicate that while the X protein is not absolutely essential for HBV replication or its maturation in transgenic mice, it can enhance viral replication, apparently by activating viral gene expression. These results demonstrate a transactivation role of the X protein in HBV replication in transgenic mice.

Contribution of immune activation to the pathogenesis and transmission of human immunodeficiency virus type 1 infection

The life cycle of human immunodeficiency virus type 1 (HIV-1) is intricately related to the activation state of the host cells supporting viral replication. Although cellular activation is essential to mount an effective host immune response to invading pathogens, paradoxically the marked systemic immune activation that accompanies HIV-1 infection in vivo may play an important role in sustaining phenomenal rates of HIV-1 replication in infected persons. Moreover, by inducing CD4+ cell loss by apoptosis, immune activation may further be central to the increased rate of CD4+ cell turnover and eventual development of CD4+ lymphocytopenia. In addition to HIV-1-induced immune activation, exogenous immune stimuli such as opportunistic infections may further impact the rate of HIV-1 replication systemically or at localized anatomical sites. Such stimuli may also lead to genotypic and phenotypic changes in the virus pool. Together, these various immunological effects on the biology of HIV-1 may potentially enhance disease progression in HIV-infected persons and may ultimately outweigh the beneficial aspects of antiviral immune responses. This may be particularly important for those living in developing countries, where there is little or no access to antiretroviral drugs and where frequent exposure to pathogenic organisms sustains a chronically heightened state of immune activation. Moreover, immune activation associated with sexually transmitted diseases, chorioamnionitis, and mastitis may have important local effects on HIV-1 replication that may increase the risk of sexual or mother-to-child transmission of HIV-1. The aim of this paper is to provide a broad review of the interrelationship between immune activation and the immunopathogenesis, transmission, progression, and treatment of HIV-1 infection in vivo.

The hepatitis B virus X protein induces HIV-1 replication and transcription in synergy with T-cell activation signals: functional roles of NF-kappaB/NF-AT and SP1-binding sites in the HIV-1 long terminal repeat promoter

Co-infection with hepatitis B virus (HBV) and human immunodeficiency virus type-1 (HIV-1) is relatively common. However, the impact of this co-infection on the clinical outcome of HIV infection has not been elucidated. We herein demonstrate that the HBV X protein (HBx) superinduces ongoing HIV-1 replication and HIV-1 long terminal repeat (LTR) transcription by synergizing with Tat protein and with T-cell activation signals. Although HBx cooperated with mitogenic stimuli in the induction of reporter plasmids harboring the HIV-1 kappaB enhancer, in both a NF-kappaB-dependent manner and a NF-AT-dependent manner, deletion of this element from the LTR did not affect the HBx-mediated up-regulation in the presence of Tat and/or mitogens. In contrast, mutation of the proximal LTR Sp1-binding sites abolished the HBx-mediated synergistic activation, but only when it was accompanied by deletion of the kappaB enhancer. When HBx was targeted to the nucleus, its ability to synergize with cellular activation stimuli was maintained. Furthermore, mutations of HBx affecting its interaction with the basal transcription machinery abrogated the synergistic activation by HBx, suggesting that this protein exerts its function by acting as a nuclear co-activator. These results indicate that HBx could contribute to a faster progression to AIDS in HBV-HIV co-infected individuals.

State of HBV DNA in HBsAg-negative, anti-HCV-positive hepatocellular carcinoma: existence of HBV DNA possibly as nonintegrated form with analysis by Alu-HBV DNA PCR and conventional HBV PCR

The role of hepatitis B virus (HBV) in carcinogenesis of hepatitis B surface antigen (HBsAg)-negative, anti-hepatitis C virus (anti-HCV)-positive hepatocellular carcinoma (HCC) remains unknown. To investigate the state of HBV DNA in such HCC, HBV DNA was examined by polymerase chain reaction (PCR) between HBV DNA and human Alu sequence (HBV-Alu PCR), which could detect integrated form of HBV DNA only, and by conventional HBV PCR, which could detect both integrated and episomal forms of HBV DNA. In all the 17 HBsAg-positive HCC, HBV DNA was detected by both HBV-Alu PCR method and conventional HBV PCR method. By contrast, in HBsAg-negative, anti-HCV-positive cases, HBV DNA was detected in 10 of 21 (47.6%) by conventional HBV PCR and in none of 21 (0%) by HBV-Alu PCR method. Thus, integrated form of HBV DNA was not found in most HbsAg-negative, anti-HCV-positive HCC in the current study. The role of episomal form of HBV DNA requires further investigation of its involvement in the process of the development of HBsAg-negative, anti-HCV-positive HCC.

Microinjection technique used to study functional interaction between p53 and hepatitis B virus X gene in apoptosis

Microinjection of expression vectors into cultured cells has been utilized to study functional interaction of p53 and the hepatitis B virus HBx gene in apoptosis. This approach allows us to determine protein-protein interactions in primary cultured human cells at a single cell level, including fibroblasts, mammary epithelial cells, renal epithelial cells, and hepatocytes. In principle, this approach can be used to study functional interaction of p53 and any gene that is either pro- or anti-apoptotic. The use of primary cultured human cells minimizes ambiguous results associated with immortalized or tumorigenic cell lines. Moreover, it is an easy and effective way to introduce genes of interests into primary human cells with defined genetic defects, thereby facilitating the delineation of genetic pathways.

Role of NF-kappaB and myc proteins in apoptosis induced by hepatitis B virus HBx protein

Chronic infection with hepatitis B virus (HBV) promotes a high level of liver disease and cancer in humans. The HBV HBx gene encodes a small regulatory protein that is essential for viral replication and is suspected to play a role in viral pathogenesis. HBx stimulates cytoplasmic signal transduction pathways, moderately stimulates a number of transcription factors, including several nuclear factors, and in certain settings sensitizes cells to apoptosis by proapoptotic stimuli, including tumor necrosis factor alpha (TNF-alpha) and etopocide. Paradoxically, HBx activates members of the NF-kappaB transcription factor family, some of which are antiapoptotic in function. HBx induces expression of Myc protein family members in certain settings, and Myc can sensitize cells to killing by TNF-alpha. We therefore examined the roles of NF-kappaB, c-Myc, and TNF-alpha in apoptotic killing of cells by HBx. RelA/NF-kappaB is shown to be induced by HBx and to suppress HBx-mediated apoptosis. HBx also induces c-Rel/NF-kappaB, which can promote apoptotic cell death in some contexts or block it in others. Induction of c-Rel by HBx was found to inhibit its ability to directly mediate apoptotic killing of cells. Thus, HBx induction of NF-kappaB family members masks its ability to directly mediate apoptosis, whereas ablation of NF-kappaB reveals it. Investigation of the role of Myc protein demonstrates that overexpression of Myc is essential for acute sensitization of cells to killing by HBx plus TNF-alpha. This study therefore defines a specific set of parameters which must be met for HBx to possibly contribute to HBV pathogenesis.

Hepatitis B virus surface (S) transactivator with DNA-binding properties

Chronic infection with hepatitis B virus (HBV) in humans is strongly linked to the development of hepatocellular carcinoma (HCC). Activation of growth-regulatory genes may play a crucial role in carcinogenesis. Proto-oncogene expression has been shown to be higher in HCC tissue with integrated HBV DNA than in the normal liver. Earlier, we showed that the 3' end of the HBV major surface gene (S) (426-855 nucleotides of the S region) is a transactivator of the X promoter-enhancer regulatory element in co-transfection experiments. This region expresses a truncated carboxy terminal S protein extending from amino acid residues 102 to 226. In this study, the truncated S protein (trc-S) was examined for its enhancing activity on several viral and cellular regulatory elements. The results indicate that trc-S activates rous sarcoma virus long terminal repeat (LTR), human T-lymphotropic virus 2 LTR, human immunodeficiency virus 1 LTR, and the c-jun and c-fos promoters. Electrophoretic mobility shift assays carried out to investigate its DNA-binding properties established that trc-S binds to HBV X promoter and oligonucleotides representing binding sites for the AP1 and TFIID transcription factors. The specificity of this interaction was confirmed by using competition experiments and supershift assays. These experiments suggest that trc-S is a transactivator of several cellular and viral promoters and that this activity is mediated by direct interaction with DNA.

Different regions of hepatitis B virus X protein are required for enhancement of bZip-mediated transactivation versus transrepression

The hepatitis B virus X protein (pX) interacts directly with the bZip transactivator CREB and the bZip repressors ICERIIgamma and ATF3, increasing their DNA-binding affinity in vitro and their transcriptional efficacy in vivo. However, the mechanism of bZip-pX interaction and of the pX-mediated increase in the bZip transcriptional efficacy remains to be understood. In this study with deletion mutants of pX, we delineated a 67-amino-acid region spanning residues 49 to 115 required for direct CREB, ATF3, and ICER IIgamma interaction in vitro and in vivo and increased bZip/CRE binding in vitro. Transient transfections of the pX deletion mutants in AML12 hepatocytes demonstrate that pX(49-115) is as effective as the full-length pX in enhancing the ATF3- and ICERIIgamma-mediated transrepression. However, this pX region is inactive in increasing the transactivation efficacy of CREB; additional amino acid residues present in pX(49-140) are required to mediate the increased transactivation efficacy of CREB in vivo. This requirement for different regions of pX in affecting CREB transactivation suggests that amino acid residues 115 to 140 integrate additional events in effecting pX-mediated transactivation, such as concomitant interactions with select components of the basal transcriptional apparatus.

Repression of hepatitis B virus X gene expression by hammerhead ribozymes

The X protein (HBx) of human hepatitis B virus (HBV) is a transcriptional activator protein. The HBx protein plays an important role in viral replication in HBV infected cells and the liver diseases including hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Therefore, the repression of HBx gene expression by hammerhead ribozymes may be a good way to inhibit HBV replication and cure HBV-related liver diseases. We designed two hammerhead ribozymes, RzA and RzB, to cleave target sites at nucleotides 114 and 309 in the HBx open reading frame (ORF), respectively. In vitro, RzA and RzB cleaved HBx RNAs at their target sites up to 52 and 75%, respectively; however, the disabled ribozymes (dRzs) which have mutations in the catalytic site did not cleave the target RNAs at all. When each of the ribozymes were cotransfected into HepG2 cells with HBx expression plasmid, RzA and RzB reduced the level of HBx mRNA to 40 and 57%, respectively. The transactivation activity of HBx protein was also reduced dramatically by the ribozymes. These results suggest that the hammerhead ribozymes, RzA and RzB, can be used for the gene therapy of liver diseases caused by HBV.

Differential immediate early gene expression in conditional hepatitis B virus pX-transforming versus nontransforming hepatocyte cell lines

We report construction and characterization of tetracycline-controlled hepatitis B virus pX-expressing hepatocyte (AML12) cell lines. These cell lines were constructed in AML12 clonal isolates (clones 3 and 4), which express constitutively the tetracycline-controlled transactivator. Since pX is implicated in HCC, this immortalized hepatocyte model system was used to investigate the mechanism of pX in transformation. Clonal isolates of 3pX and 4pX lineages display conditional synthesis of pX mRNA and protein and a 2-fold increase in growth saturation density following tetracycline removal, implicating pX in monolayer overgrowth. Interestingly, only 3pX clones display pX-dependent anchorage independence. Clone 3 lineages express hepatocyte nuclear factor-1alpha and hepatocyte-specific marker genes; clone 4 lineages express hepatocyte nuclear factor-1beta and reduced levels of hepatocyte-specific marker genes, suggesting the importance of the differentiated hepatocyte in pX-mediated oncogenic transformation. Importantly, 3pX and 4pX lineages display differential expression of immediate early genes c-fos and ATF3. The pX-transforming 3pX lineage displays early, pX-dependent induction of ATF3 and prolonged induction of c-fos. The nontransforming 4pX cells display an absence of pX-dependent ATF3 induction and transient induction of c-fos. Our results support the direct link of pX expression to oncogenic transformation in 3pX lineage clones and underscore the advantage of this conditional cellular model system for studying mechanisms of pX-mediated oncogenesis.

Transactivation by expression of the hepatitis B virus X protein with an inducible system

We describe here the conditional expression of the hepatitis B virus X protein using the inducible system controlled by a tet-responsive promoter. Induction of the X protein in Rat-2 fibroblasts activated transcription from a heterologous gene promoter and stimulated the DNA-binding activity of NFkB. The ability to produce this biologically active X protein in a stable cell line will accelerate the elucidation of the function and mechanisms of X and eventually help us understand the role of X in natural viral infection and carcinogenesis.

The hepatitis B virus X protein is a co-activator of activated transcription that modulates the transcription machinery and distal binding activators

Hepatitis B virus X protein (HBx) transactivates viral and cellular genes through a wide variety of cis-elements, but the mechanism has not been well elucidated. Evidence for nuclear events in HBx transactivation has been reported. Here we examine the role of HBx in modulation of transcription with a transient transfection system and an in vitro transcription assay. Reporters bearing Gal4-binding sites were applied to avoid the effects of endogenous transcription factors with or without signaling processes. The Gal4-DNA binding domain fused form of HBx exhibited no effect on Gal4-responsive reporters. However, HBx augmented activated transcription by transcriptional activators, suggesting HBx retains a co-activator but not a transcriptional activator function. The functional domain for co-activation was the same as that for HBx transactivation, and the transcription factor IIB- and RNA polymerase II subunit 5-interacting sites of HBx, which were critical for HBx transactivation, were shown to be crucial for the co-activation function. Importantly, HBx stimulated transcription on templates bearing the X responsive elements in vitro with endogenous activators. These results imply that HBx acts as a co-activator that modulates transcriptional machinery and distal-binding activators, which may explain one of the mechanisms of transactivation by HBx when localized in nuclei.

Retrospective analysis of the impact of HIV infection and alcohol use on chronic hepatitis C in a large cohort of drug users

BACKGROUND/AIM

This retrospective study aimed to better define the respective biological and pathological impact of human immunodeficiency virus infection and chronic alcohol consumption on the course of hepatitis C virus infection in intravenous drug users.

METHODS

Two hundred and ten consecutive anti-HCV positive intravenous drug users, among whom 60 were also anti-HIV positive, took part in the study at the University Hospital, Paris, France.

RESULTS

The activity of aspartate aminotransferase and gamma-glutamyl transpeptidase was significantly increased in serum from anti-HIV positive patients. The mean hepatitis activity index was significantly higher in anti-HIV positive patients (p<0.05), among whom there was also a higher proportion of patients with cirrhosis as compared to anti-HIV negative patients (30.0 vs 15.3%, p<0.0001). Excessive alcohol drinking (recorded in around 35% of the patients, whatever their HIV status), as compared to non-excessive drinking, was more often associated with cirrhosis in anti-HIV negative (24.5 vs 11.3%, p<0.05) than in anti-HIV positive patients (30.4 vs 29.7%, not significant). In a multivariate analysis, HIV infection (relative risk 2.2, confidence interval 1.1-4.5) and excessive alcohol drinking (relative risk 1.9, confidence interval 1.0-3.9) were the variables independently associated with the risk of cirrhosis.

CONCLUSION

Human immunodeficiency virus infection worsens the course of chronic hepatitis C in intravenous drug users. Excessive alcohol drinking also appears to be a crucial negative cofactor, and therefore alcohol withdrawal should be proposed as an integral part of the therapy.

Tremblay M, Olivier M. The Lipophosphoglycan of Leishmania donovani Up-Regulates HIV-1 Transcription in T Cells Through the Nuclear Factor-κB Elements

We have recently demonstrated that the parasite Leishmania donovani and its surface molecule, lipophosphoglycan (LPG), can activate HIV-1 replication in monocytoid cells. Our present interest was to determine whether LPG could also up-regulate HIV-1 transcription in T cells. Using a CD4-positive human lymphoid T cell line (1G5) containing a stably integrated HIV-1 long terminal repeat (LTR)-luciferase construct, we found that LPG is a potent inducer of HIV-1 LTR activity. Treatment of 1G5 cells with signaling antagonists revealed that protein tyrosine kinase- and protein kinase A-dependent pathways were actively participating in the LPG-induced enhancement of HIV-1 LTR-driven activity. Transfection of Jurkat E6.1 cells with plasmids containing wild-type and nuclear factor-κB (NF-κB)-mutated HIV-1 LTR-luciferase constructs has suggested a role for NF-κB binding sites in the LPG-mediated induction of HIV-1 LTR activity. An LPG-induced binding factor specific to the NF-κB consensus sequences could be observed using electrophoretic mobility shift assay. Finally, transfection experiments performed with a vector containing HIV-1 κB binding sites only showed similar LPG-mediated induction, which was abrogated by sodium salicylate, a known NF-κB inhibitor. We thus demonstrate that the LPG-mediated induction of HIV-1 LTR activity in T cells involves several second messengers culminating in activation of HIV-1 LTR-driven transcription via NF-κB-binding consensus sequences. In conclusion, these results reinforce the idea that L. donovani is a putative cofactor in HIV-1 pathogenesis.

Establishment and characterization of cell lines constitutively expressing hepatitis B virus X-protein

We prepared human hepatoma cell lines, which expressed the human hepatitis B virus-X gene product. The plasmid pMAMneo-X, containing an HBV-X gene promoter, an enhancer and a structural gene was constructed. Transfected HBV-X gene integration and expression were detected by Southern and Northern blotting, as well as by chloramphenicol acetylase transferase (CAT) assay using various kinds of promoter-CAT reporter systems. HBV-X protein expression in stable transfectants was confirmed by immunofluorescence microscopy. Transfected cell lines showed permanent expression of HBV-X proteins. The HBV-X transfectant activated its target promoters in promoter-CAT constructs as reporters. The HBV-X transfectant enhanced AP-1 transcription factor binding to its target DNA. Therefore, X-transfectants are not only stable, but also have specific biological functions. Cell cycle analysis by flow cytometry showed that the majority of the transfectant cells are arrested in the G1 or G2 phase of the cell cycle. These cell lines may be useful in analyzing the biological functions of HBV-X and its functional role in the formation of hepatocellular carcinomas.

The hepatitis B virus X protein enhances the DNA binding potential and transcription efficacy of bZip transcription factors

The hepatitis B virus X protein interacts with the basic-region, leucine zipper protein (bZip) domain of cAMP response element-binding protein increasing its affinity for the cAMP response element site in vitro and its transcriptional efficacy in vivo (Williams, J. S., and Andrisani, O. M. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 3819-3823). Here we examine pX interactions with bZip transcription factors ATF3, gadd153/Chop10, ICER IIgamma, and NF-IL6. We demonstrate direct interactions in vitro between pX and the bZip proteins tested. In contrast MyoD and Gal4(1-147) fail to interact with pX. We also demonstrate by the mammalian two-hybrid assay the direct interaction of pX with cAMP response element- binding protein, ICER IIgamma, ATF3, and NF-IL6 in hepatocytes. In addition, pX increases the DNA binding potential of bZip proteins for their cognate DNA-binding site in vitro. In transient transfections in hepatocytes (AML12 cell line), pX increases the transcriptional efficacy of the bZip transcription factors. NF-IL6-mediated transcriptional activation is enhanced 3-fold by pX. Most interestingly, pX augments the repression mediated by bZip repressors ATF3 and ICER IIgamma, by 6- and 7-fold, respectively, demonstrating for the first time the involvement of pX in gene repression. We conclude that pX is an enhancer of the DNA binding potential of bZip transcription factors, thereby increasing the transactivation or repression efficacy of bZip-responsive genes.

Deletion variants within the NF-kappaB activation domain of the LMP1 oncogene in acquired immunodeficiency syndrome-related large cell lymphomas, in prelymphomas and atypical lymphoproliferations

The latent membrane protein 1 (LMP1) oncogene of Epstein Barr virus (EBV) is expressed in tumor cells of acquired immunodeficiency syndrome (AIDS) related lymphomas, HIV-negative, EBV-associated malignant lymphoproliferations, nasopharyngeal carcinoma, as well as in reactive immunoblasts of infectious mononucleosis. Naturally occurring LMP1 deletion variants (LMP1-del), characterized by clustered mutations and a distinct 30 base pair deletion within the carboxy terminal domain of LMP1, essential for maximal NF-kappaB stimulation, have been identified in the same conditions. These variants prevail in AIDS-related lymphomas, and are associated with clinically aggressive behaviour in HIV-negative lymphomas, and are frequent in prelymphomatous and reactive states. Functional studies showing a growth advantage of cells infected by these variants may explain the accumulation of LMP1-del in these entities. In the carboxy terminal NF-kappaB activation domain of LMP1, evidence of a hypervariable region close to the highly conserved 23 outermost amino acids essential for malignant transformation, may reflect the natural selection of growth promoting variants involved in signalling pathways. The prevalence of the same mutational pattern in AIDS-related lymphoma as well as in hyperplastic reactive states and prelymphomas supports the hypothesis that these variants confer a growth advantage manifested under impaired cellular immunity.

The hepatitis B virus X-associated protein, XAP3, is a protein kinase C-binding protein

The hepatitis B virus X protein induces transcriptional activation of a wide variety of viral and cellular genes. In addition to its ability to interact directly with many nuclear transcription factors, several reports indicate that the X protein stimulates different cytoplasmic kinase signal cascades. Using the yeast two-hybrid screen, we have isolated a clone designated X-associated protein 3 (XAP3) that encodes a human homolog of the rat protein kinase C-binding protein. One of the activation domains of X (amino acids 90-122) is required for binding to XAP3, while the NH2-terminal part of XAP3 is necessary for binding to X. Both X and XAP3 bound specifically to the eta PKC isoenzyme synthesized in rabbit reticulocyte lysates. Overexpression of XAP3 enhanced X transactivation activity. These results support earlier findings that one of the mechanisms of transactivation by X is through involvement with the cellular protein kinase C pathway.

Diagnosis and treatment of hepatic disease in patients with HIV

Liver involvement with opportunistic infections and neoplasms is a well-recognized component of AIDS, affecting most patients. The cause of hepatic disease in these patients may be divided into hepatitis, granulomatous disease, mass lesions, vascular lesions, hepatotoxic drugs, and nonspecific findings. With a rational approach, most patients with AIDS and liver disease can be diagnosed and treated in a cost-effective manner with low morbidity.

Hepatitis B virus X protein is a transcriptional modulator that communicates with transcription factor IIB and the RNA polymerase II subunit 5

Hepatitis B virus X protein (HBx) transactivates viral and cellular genes through a wide variety of cis-elements. However, the mechanism is still obscure. Our finding that HBx directly interacts with RNA polymerase II subunit 5 (RPB5), a common subunit of RNA polymerases, implies that HBx directly modulates the function of RNA polymerase (Cheong, J. H., Yi, M., Lin, Y., and Murakami, S. (1995) EMBO J. 14, 142-150). In this context, we examined the possibility that HBx and RPB5 interact with other general transcription factors. HBx and RPB5 specifically bound to transcription factor IIB (TFIIB) in vitro, both of which were detected by either far-Western blotting or the glutathione S-transferase-resin pull-down assay. Delineation of the binding regions of these three proteins revealed that HBx, RPB5, and TFIIB each has two binding regions for the other two proteins. Co-immunoprecipitation using HepG2 cell lysates that express HBx demonstrated trimeric interaction in vivo. Some HBx substitution mutants, which had severely impaired transacting activity, exhibited reduced binding affinity with either TFIIB or RPB5 in a mutually exclusive manner, suggesting that HBx transactivation requires the interactions of both RPB5 and TFIIB. These results indicated that HBx is a novel virus modulator that facilitates transcriptional initiation by stabilizing the association between RNA polymerase and TFIIB through communication with RPB5 and TFIIB.

Coinfection and superinfection of hepatitis B virus in patients infected with human immunodeficiency virus: no evidence of faster progression to AIDS

The influence of hepatitis B virus (HBV) on the natural history of human immunodeficiency virus (HIV) infection was evaluated in a prospective study of 347 HIV-positive, AIDS-free individuals infected through injecting drug use and sex and with known seroconversion dates. End points were CD4+ cell count < 200 x 10(6) cell/L and AIDS diagnosis. At entry, 229 had seromarkers to HBV; during the study, 107 had a CD4+ cell count < 200 x 10(6) cells/L and 66 developed AIDS. HBsAg chronic carriers, HBV infection-free subjects and those with baseline evidence of prior HBV infection did not differ in rates of progression to end points. Sexual transmission of HIV was significant predictor of CD4+ cell decline to < 200 x 10(6) cells/l [Hazard ratio (HZ): 1.56, 95% confidence interval (CI): 1.06-2.29, p = 0.0232] and progression to AIDS (HZ: 1.91, CI: 1.17-3.11, p = 0.0091). 15 HIV-positive and HBV infection-free patients had HBV seroconversion. They did not differ from those who remained HBV infection-free in rates of progression to end points, but 40% of them became HBsAg chronic carriers. These results suggest that HBV has no influence on progression of HIV disease, but that patients who have HIV before their HBV infection are more likely to become HBsAg chronic carriers than those who are infected with HBV before HIV.

XAP2, a novel hepatitis B virus X-associated protein that inhibits X transactivation

The hepatitis B virus X protein is a promiscuous transcriptional transactivator. Transactivation by the X protein is most likely mediated through binding to different cellular factors. Using the yeast two-hybrid method, we have isolated a clone that encodes a novel X-associated cellular protein: XAP2. X and XAP2 interactions also occur in vitro. Antiserum raised against XAP2 recognizes a cytoplasmic protein with an apparent molecular mass of 36 kDa. The interaction between X and XAP2 requires a small region on X containing amino acids 13-26. From Northern blot analyses, XAP2 is ubiquitously expressed in both liver-derived and non-liver-derived cell lines as well as in normal non-liver tissues. In contrast, XAP2 is expressed in very low level in the normal human liver. In transfection assays, overexpression of XAP2 abolishes transactivation by the X protein. Based on these results, we suggest that XAP2 is an important cellular negative regulator of the X protein, and that X-XAP2 interaction may play a role in HBV pathology.

Effects of a naturally occurring mutation in the hepatitis B virus basal core promoter on precore gene expression and viral replication

The basal core promoter (BCP) of hepatitis B virus (HBV) controls the transcription of both the precore RNA and the core RNA. The precore RNA codes for the secreted e antigen, while the core RNA codes for the major core protein and the DNA polymerase and also is the pregenomic RNA. The double mutation of nucleotides 1762 and 1764 in the BCP from A and G to T and A, respectively, is frequently observed in HBV sequences isolated from chronic patients. Several papers have reported conflicting results regarding whether this double mutation is important for e antigen expression. In order to address this issue, we have introduced this double mutation into the HBV genome and studied its effects on HBV gene expression and replication. Our results indicate that the mutated BCP can no longer bind a liver-enriched transcription factor(s) and that the transcription of only precore RNA and, consequently, the expression of e antigen were reduced. The reduction of precore gene expression was accompanied by an increase in progeny virus production. This increase was found to occur at or immediately prior to the encapsidation of the pregenomic RNA. Thus, the results of our in vitro study resolve the discrepancy of previous clinical observations and indicate that this double mutation suppresses but does not abolish the e antigen phenotype. The implications of these findings in the pathogenesis of HBV are discussed.

Replication protein A is a component of a complex that binds the human metallothionein IIA gene transcription start site

Previous studies revealed that sequences surrounding the initiation sites in many mammalian and viral gene promoters, called initiator (Inr) elements, may be essential for promoter strength and for determining the actual transcription start sites. DNA sequences in the vicinity of the human metallothionein IIA (hMTIIA) gene transcription start site share homology with some of the previously identified Inr elements. However, in the present study we have found by in vitro transcription assays that the hMTIIA promoter does not contain a typical Inr. Electrophoretic mobility shift assays identified several DNA-protein complexes at the hMTIIA gene transcription start site. A partially purified protein fraction containing replication protein A (RPA) binds to the hMTIIA gene transcription start site and represses transcription from the hMTIIA promoter in vitro. In addition, overexpression of the human 70-kDa RPA-1 protein represses transcription of a reporter gene controlled by the hMTIIA promoter in vivo. These findings suggest that hMTIIA transcription initiation is controlled by a mechanism different from most mammalian and viral promoters and that the previously identified RPA may also be involved in transcription regulation.

Hepatitis B virus HBx protein induces transcription factor AP-1 by activation of extracellular signal-regulated and c-Jun N-terminal mitogen-activated protein kinases

The HBx protein of hepatitis B virus is a dual-specificity activator of transcription, stimulating signal transduction pathways in the cytoplasm and transcription factors in the nucleus, when expressed in cell lines in culture. In the cytoplasm, HBx was shown to stimulate the Ras-Raf-mitogen-activated protein kinase (MAP kinase) cascade, which is essential for activation of transcription factor AP-1. Here we show that HBx protein stimulates two independently regulated members of the MAP kinase family when expressed transiently in cells. HBx protein stimulates the extracellular signal-regulated kinases (ERKs) and the c-Jun N-terminal kinases (JNKs). HBx activation of ERKs and JNKs leads to induction and activation of AP-1 DNA binding activity involving transient de novo synthesis of c-Fos protein and prolonged synthesis of c-Jun, mediated by N-terminal phosphorylation of c-Jun carried out by HBx-activated JNK. New c-Jun synthesis was blocked by coexpression with a dominant-negative MAP kinase kinase (MEK kinase, MEKK-1), confirming that HBx stimulates the prolonged synthesis of c-Jun by activating JNK signalling pathways. Activation of the c-fos gene was blocked by coexpression with a Raf-C4 catalytic mutant, confirming that HBx induces c-Fos by acting on Ras-Raf linked pathways. HBx activation of ERK and JNK pathways resulted in prolonged accumulation of AP-1-c-Jun dimer complexes. HBx activation of JNK and sustained activation of c-jun, should they occur in the context of hepatitis B virus infection, might play a role in viral transformation and pathogenesis.

Woodchuck hepatitis virus X protein is present in chronically infected woodchuck liver and woodchuck hepatocellular carcinomas which are permissive for viral replication

The woodchuck hepatitis virus (WHV) X gene (WHx) is required for infectivity of WHV in woodchucks, and the gene encodes a broadly acting transcription factor. Several lines of evidence from cell culture and transgenic mice suggest that X proteins can promote hepatocarcinogenesis. To determine whether WHx-encoded proteins are present during persistent infection and hepatocellular carcinoma (HCC) in woodchucks, we surveyed livers and HCCs from a panel of WHV carrier woodchucks for the presence of WHx by utilizing an immunoprecipitation-Western blot (immunoblot) procedure. We detected a single 15.5-kDa WHx gene product in 100% of the persistently infected livers but not in livers from animals which had recovered from acute infection or in those of uninfected woodchucks. Analysis of HCCs revealed that all of the tumors which contained WHV replication intermediates were also positive for WHx. In contrast, WHx was undetectable in HCCs which did not contain replicative intermediates. Subcellular localization studies detected WHx in the cytoplasm but not in the nuclei of primary woodchuck hepatocytes. Comparative immunoprecipitation experiments revealed that there were 4 X 10(4) to 8 X 10(4) molecules of WHx per primary woodchuck hepatocyte. Four lines of WHx transgenic mice did not develop HCC spontaneously. However, when one line was treated with diethylnitrosamine, the occurrence of precancerous lesions was enhanced compared with that in diethylnitrosamine-treated nontransgenic controls. The apparent absence of WHx in some woodchuck HCCs indicates that WHx may not be required to maintain the tumor phenotype, whereas its presence in all persistently infected livers leaves open the possibility that it plays a role in hepatocarcinogenesis.

Hepatitis B virus HBx protein activates transcription factor NF-kappaB by acting on multiple cytoplasmic inhibitors of rel-related proteins

The HBx protein is a small polypeptide encoded by mammalian hepadnaviruses that is essential for viral infectivity and is thought to play a role in development of hepatocellular carcinoma during chronic hepatitis B virus infection. HBx is a transactivator that stimulates Ras signal transduction pathways in the cytoplasm and certain transcription elements in the nucleus. To better understand the activities of HBx protein and its mechanism of action, we have explored the manner by which HBx activates the transcription factor NF-kappaB during transient expression. We show that HBx induces prolonged formation, in a Ras-dependent manner, of transcriptionally active NF-kappaB DNA-binding complexes, which make up the family of Rel-related proteins, p50, p52, RelA, and c-Rel. HBx was found to activate NF-kappaB through two distinct cytoplasmic pathways by acting on both the 37-kDa IkappaBalpha inhibitor and the 105-kappaDa NF-kappaB1 precursor inhibitor protein, known as p105. HBx induces phosphorylation of IkappaBalpha, a three- to fourfold reduction in IKBalpha stability, and concomitant nuclear accumulation of NF-kappaB DNA-binding complexes, similar to that reported for human T-cell leukemia virus type 1 Tax protein. In addition, HBx mediates a striking reduction in cytoplasmic p105 NF-kappaB1 inhibitor and p50 protein levels and release of RelA protein that was sequestered by the p105 inhibitor, concomitant with nuclear accumulation of NF-kappaB complexes. HBx mediated only a slight reduction in the cytoplasmic levels of NF-kappaB2 p100 protein, an additional precursor inhibitor of NF-kappaB, which is thought to be less efficiently processed or less responsive to release of NF-kappaB. No evidence was found for HBx activation of NF-kappaB by targeting acidic sphingomyelinase- controlled pathways. Studies also suggest that stimulation of NF-kappaB by HBx does not involve activation of Ras via the neutral sphingomyelin-ceramide pathway. Thus, HBx protein is shown to activate the NF-kappaB family of Rel-related proteins by acting on two distinct NF-kappaB cytoplasmic inhibitors.

A truncated mutant (residues 58-140) of the hepatitis B virus X protein retains transactivation function

The hepatitis B virus X protein (HBx) sequence (154 aa) has been divided into six regions (A-F) based on its sequence homology with X proteins of other mammalian hepadnaviruses. Regions A, C, and E are more conserved and include all the four conserved cysteines (C7, C61, C69, and C137). To localize the regions of HBx important for transactivation, a panel of 10 deletion mutants (X5-X14) and 4 single point mutants (X1-X4), each corresponding to a conserved cysteine residue, was constructed by site-directed mutagenesis. A HBx-specific monoclonal antibody was developed and used to confirm the expression of mutants by Western blot. Transactivation property of the HBx mutants was studied on Rous sarcoma virus-long terminal repeat (RSV-LTR) in transient transfection assays. We observed that deletion of the most conserved region A or substitution of the N-terminal cysteine (C7) had no effect on transactivation. Deletion of the nonconserved regions B or F also had no deleterious effects. Deletions of regions C and D resulted in a significant loss of function. Substitution of both C61 and C69 present in region C, caused almost 90% loss of activity that could be partially overcome by transfecting more expression plasmid. The fully conserved 9 amino acid segment (residues 132 to 140) within region E including C137 appeared to be crucial for its activity. Finally, a truncated mutant X15 incorporating only regions C to E (amino acids 58-140) was able to stimulate the RSV-LTR quite efficiently, suggesting a crucial role played by this domain in transactivation function.

Hepatitis B virus transcriptional activators: mechanisms and possible role in oncogenesis

The hepatitis B virus (HBV) genome encodes a 154 amino acid protein termed X (HBx, hepatitis B x protein), which is a promiscuous transcriptional activator of polymerase II and III promoters. HBx upregulates a wide range of cellular and viral genes and is thought to facilitate viral pregenome and mRNA transcription; however, its precise role in the viral replication cycle remains to be elucidated. The functional mechanisms of HBx appear very complex. It was shown to activate transcription factors AP-1 and NF-kappa B vis cytoplasmic pathways including ras-MAP kinase. In contrast, nuclear HBx is thought to activate the transcriptional machinery directly. A second transcriptional activator protein (Mst, middle s transactivator) is encoded by 3'-truncated preS2/S sequences of integrated HBV DNA, but not by the intact viral gene. HBx and Mst may contribute to the pathogenicity of chronic hepatitis B and are suggested to promote hepatocyte transformation via upregulation of cellular proto-oncogenes. Further, HBx may enhance HBV related carcinogenesis by inactivation of the tumour suppressor gene product p53.

Pathogenesis of hepatocellular carcinoma: a review from the viewpoint of molecular analysis

Chronic liver inflammation and hepatic regeneration by infection with hepatitis B (HBV) or C virus (HBC) seem to be important risk factors for hepatocellular carcinoma (HCC). Regarding the hepatocarcinogenesis of HBV DNA integration, it has been variously hypothesized that mechanisms such as the alteration of host chromosomal DNA and transcriptional trans-acting activity of the X gene are activated. On the other hand, integration of HCV virus into chromosomal DNA has not been reported. It is suggested that HCV could replicate more efficiently in noncancerous than in cancerous tissues. Therefore, it might affect some oncogenes or cause an inactivation of tumor suppressor genes in the early stage of HCC. Genetic alterations such as a point mutation and loss of heterozygosity are considered to be late events occurring after tumorigenesis. Regeneration of liver cells through chronic hepatitis increases the incidence of genetic alterations in hepatic cells and/or HCCs in both HBV- and HCV-infected patients.

Hepatitis B virus pX activates NF-kappa B-dependent transcription through a Raf-independent pathway

In this study, we characterized the molecular events involved in the activation of the ubiquitous transcription factor NF-kappa B by the viral transactivator pX. pX expression in HeLa cells determines a manyfold increase in NF-kappa B-dependent transcription, which is associated with an increase in p50/p65 heterodimer DNA-binding activity. Since the I kappa B-alpha inhibitory subunit proteolytic degradation, which follows its phosphorylation/modification, is a key event in NF-kappa B activation by different stimuli (such as growth factors, phorbol esters, tumor necrosis factor, UV irradiation, and oxygen radicals), we investigated pX effects on I kappa B-alpha, as well as the possible involvement of known signalling pathways in pX-induced NF-kappa B-dependent transcription. We observed that although pX had no direct effect on p50 or p65, it was able to restore the I kappa B-alpha-suppressed p50/p65 activity. More directly, the stable expression of pX in HeLa cells resulted in reduced levels of I kappa B-alpha in the cytoplasm. Pretreatment of the cells with H7, calphostin C, tyrphostin 25, or N-acetylcysteine did not impair the effects of pX on NF-kappa B, thus ruling out the involvement of protein kinase C, tyrosine kinases, and oxygen radicals. Finally, while most of the known NF-kappa B-activating agents converge on Raf-1 protein kinase, when Raf-1 activity is blocked by overexpression of a dominant negative mutant, the effects of pX on NF-kappa B are not impaired. Thus, we suggest that although pX is able to activate the Ras/Raf-1-signalling pathway, it triggers NF-kappa B activation by an as yet unidentified Raf-1-independent pathway.

Hepatitis B virus transgenic mice: models of viral immunobiology and pathogenesis

It should be apparent from the foregoing that the transgenic mouse model system has contributed substantially to our understanding of many aspects of HBV biology, immunobiology and pathogenesis in the past several years. We have learned that HBV can replicate within the mouse hepatocyte, as well as other mouse cell types, suggesting that there are probably no strong tissue or species specific constraints to viral replication once the viral genome enters the cell. However, the failure thus far to detect viral cccDNA in the hepatocyte nucleus in several independently derived transgenic lineages suggests that other, currently undefined, constraints on host range and tissue specificity may also be operative. Thanks to the transgenic mouse model we now understand the pathophysiological basis for HBsAg filament formation and ground glass cell production, and we have learned that at least this viral gene product can be toxic for the hepatocyte, first by compromising its ability to survive the hepatocytopathic effects of LPS and IFN alpha and eventually by causing it to die in the absence of any obvious exogenous stimulus. In recent studies, it has been shown that preformed nucleocapsid particles do not cross the nuclear membrane in either direction at least in the mouse hepatocyte. If this is confirmed, it will have two important implications: first, that nucleocapsid disassembly must occur in the cytoplasm before the nascent viral genome can enter the nucleus; second, that the intranuclear nucleocapsid particles are empty, and therefore serve no currently defined purpose in the viral life cycle. This should stimulate new interest in the analysis of the function of these particles that are a prominent feature of mammalian hepadnavirus infection. The transgenic mouse model has also established definitively that HBV-induced liver disease has an immunological basis, and that the class I-restricted CTL response plays a central role in this process. Additionally, the mouse studies have taught us that when the CTL recognize their target antigen on the hepatocytes they cause them to undergo apoptosis, forming the acidophilic, Councilman bodies that are characteristic of viral hepatitis. Further, we have learned that although the CTL initiate the liver disease, they actually contribute more to disease severity indirectly by recruiting antigen nonspecific effector cells into the liver than by directly killing the hepatocytes themselves. In addition, by releasing IFN gamma when they recognize antigen, the CTL can destroy enough of the liver to cause fulminant hepatitis in mice whose hepatocytes overproduce the large envelope protein and are hypersensitive to the cytopathic effects of this cytokine. We have also learned that the CTL are unable to recognize HBV-positive parenchymal cells outside of the liver, apparently because they cannot traverse the microvascular barriers that exist at most extrahepatic tissue sites. This important new discovery may permit the virus to survive a vigorous CTL response and contribute not only to the maintenance of memory T cells following acute hepatitis but also to serve as a reservoir to reseed the liver in patients with chronic hepatitis. The transgenic mouse model has also revealed that activated CTL and the cytokines they secrete can down-regulate HBV gene expression, and possibly even control viral replication, by noncytotoxic intracellular inactivation mechanisms involving the degradation of viral RNA and, perhaps, the degradation of viral nucleocapsids and replicative DNA intermediates without killing the cell. If HBV replication is indeed interrupted by this previously unsuspected activity, it could contribute substantially to viral clearance during acute infection when the immune response to HBV is vigorous. Alternatively, it could also contribute to viral persistence, by only partially down-regulating the virus during chronic infection when the immune response is weak.