Pro-apoptotic effect of the hepatitis B virus X gene

Hepatitis B doubly spliced protein (HBDSP) promotes hepatocellular carcinoma cell apoptosis via ETS1/GATA2/YY1-mediated p53 transcription

IMPORTANCE

Hepatitis B virus (HBV) spliced variants are associated with viral persistence or pathogenicity. Hepatitis B doubly spliced protein (HBDSP), which has been previously reported as a pleiotropic transactivator protein, can potentially serve as an HBV virulence factor. However, the underlying mechanisms of HBDSP in HBV-associated liver diseases remain to be elucidated. In this study, we revealed that HBDSP promotes cellular apoptosis and induces wt-p53-dependent apoptotic signaling pathway in wt-p53 hepatocellular cells by transactivating p53 transcription, and increases the release of HBV progeny. Therefore, HBDSP may promote the HBV particles release through wt-p53-dependent hepatocellular apoptosis. Our findings suggest that blocking HBDSP-induced wt-p53-dependent apoptosis might have therapeutic values for chronic hepatitis B.

HBV infection increases the risk of macular degeneration: the roles of HBx-mediated sensitization of retinal pigment epithelial cells to UV and blue light irradiation

Background

Hepatitis B virus (HBV) infection is strongly associated with hepatocellular carcinoma due to the main pathogenic X protein of HBV (HBx). Whether HBV infection and the HBx protein could result in macular degeneration (MD) is not known. The aim of this study is to assess the association and underlying mechanisms between HBV infection and MD.

Methods

The National Health Research Institutes in Taiwan built a large database, the National Health Insurance Research Database (NHIRD), which includes the claims data from the Taiwan National Health Insurance (NHI) program. The Taiwan NHI is a single-payer, compulsory health insurance program for Taiwan citizens. The data for the present study were derived from the Longitudinal Health Insurance Database, which contains the claims data of 1 million insured people within the NHIRD, including beneficiary registration, inpatient and outpatient files, drug use, and other medical services. In this study, we first investigated the association of HBV infection and the risk of MD by a population-based cohorts study enrolling 39,796 HBV-infected patients and 159,184 non-HBV-infected patients.

Results

After adjustment of age, sex, and comorbidities, the risk of MD was significantly higher in the HBV-infected cohort than in the non-HBV-infected cohort (adjusted HR = 1.31; 95% CI = 1.17–1.46). In vitro, we provided evidence to demonstrate that overexpression of HBx in the human retinal pigment epithelial (RPE) cell line, ARPE19, significantly reduced cell viability and clonogenic survival upon UV and blue light irradiation. By gene microarray analysis, we further showed that almost all genes in DNA repair pathways including base excision repair, nucleotide excision repair, mismatch repair, and homologous recombination were significantly down-regulated in the UV-induced cell death of HBx-transfected ARPE19 cells.

Conclusions

The HBx protein may sensitize RPE cells to UV and blue light irradiation and increase the risk of HBV-infection-associated MD through down-regulation of multiple DNA repair pathways.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1594-4) contains supplementary material, which is available to authorized users.

The roles of hepatitis B virus-encoded X protein in virus replication and the pathogenesis of chronic liver disease

INTRODUCTION

Hepatitis B virus (HBV) is a major cause of chronic liver disease (CLD) and hepatocellular carcinoma (HCC) worldwide. More than 350 million people are at risk for HCC, and with few treatment options available, therapeutic approaches to targets other than the virus polymerase will be needed. This review suggests that the HBV-encoded X protein, HBx, would be an outstanding target because it contributes to the biology and pathogenesis of HBV in three fundamental ways.

AREAS COVERED

First, HBx is a trans-activating protein that stimulates virus gene expression and replication, thereby promoting the development and persistence of the carrier state. Second, HBx partially blocks the development of immune responses that would otherwise clear the virus, and protects infected hepatocytes from immune-mediated destruction. Thus, HBx contributes to the development of CLD without virus clearance. Third, HBx alters patterns of host gene expression that make possible the emergence of HCC. The selected literature cited is from the National Library of Medicine (Pubmed and Medline).

EXPERT OPINION

Understanding the mechanisms, whereby HBx supports virus replication and promotes pathogenesis, suggests that HBx will be an important therapeutic target against both virus replication and CLD aimed at the chemoprevention of HCC.

Hepatitis B virus X protein targets the Bcl-2 protein CED-9 to induce intracellular Ca2+ increase and cell death in Caenorhabditis elegans

HBx is a multifunctional hepatitis B virus (HBV) protein that is crucial for HBV infection and pathogenesis and a contributing cause of hepatocyte carcinogenesis. However, the host targets and mechanisms of action of HBx are poorly characterized. We show here that expression of HBx in Caenorhabditis elegans induces both necrotic and apoptotic cell death, mimicking an early event of liver infection by HBV. Genetic and biochemical analyses indicate that HBx interacts directly with the B-cell lymphoma 2 (Bcl-2) homolog CED-9 (cell death abnormal) through a Bcl-2 homology 3 (BH3)-like motif to trigger both cytosolic Ca(2+) increase and cell death. Importantly, Bcl-2 can substitute for CED-9 in mediating HBx-induced cell killing in C. elegans, suggesting that CED-9 and Bcl-2 are conserved cellular targets of HBx. A genetic suppressor screen of HBx-induced cell death has produced many mutations, including mutations in key regulators from both apoptosis and necrosis pathways, indicating that this screen can identify new apoptosis and necrosis genes. Our results suggest that C. elegans could serve as an animal model for identifying crucial host factors and signaling pathways of HBx and aid in development of strategies to treat HBV-induced liver disorders.

Hepatitis B virus X protein targets the Bcl-2 protein CED-9 to induce intracellular Ca2+ increase and cell death in Caenorhabditis elegans

HBx is a multifunctional hepatitis B virus (HBV) protein that is crucial for HBV infection and pathogenesis and a contributing cause of hepatocyte carcinogenesis. However, the host targets and mechanisms of action of HBx are poorly characterized. We show here that expression of HBx in Caenorhabditis elegans induces both necrotic and apoptotic cell death, mimicking an early event of liver infection by HBV. Genetic and biochemical analyses indicate that HBx interacts directly with the B-cell lymphoma 2 (Bcl-2) homolog CED-9 (cell death abnormal) through a Bcl-2 homology 3 (BH3)-like motif to trigger both cytosolic Ca(2+) increase and cell death. Importantly, Bcl-2 can substitute for CED-9 in mediating HBx-induced cell killing in C. elegans, suggesting that CED-9 and Bcl-2 are conserved cellular targets of HBx. A genetic suppressor screen of HBx-induced cell death has produced many mutations, including mutations in key regulators from both apoptosis and necrosis pathways, indicating that this screen can identify new apoptosis and necrosis genes. Our results suggest that C. elegans could serve as an animal model for identifying crucial host factors and signaling pathways of HBx and aid in development of strategies to treat HBV-induced liver disorders.

HBx activates FasL and mediates HepG2 cell apoptosis through MLK3-MKK7-JNKs signal module

AIM

To investigate the possible mechanism by which hepatitis B virus X protein (HBx) mediates apoptosis of HepG2 cells.

METHODS

HBx expression vector pcDNA3.1-X was transfected into HepG2 cells to establish an HBx high-expression cellular model as pcDNA3.1-X transfected group. The pcDNA3.1-X and pSilencer3.1-shHBX (HBx antagonist) were cotransfected into HepG2 cells to establish an HBx low-expression model as RNAi group. Untransfected HepG2 cells and HepG2 cells transfected with negative control plasmid were used as controls. Apoptosis rate, the expression of Fas/FasL signaling pathway-related proteins and the phosphorylation levels of MLK3, MKK7 and JNKs, which are upstream molecules of death receptor pathways and belong to the family of mitogen-activated protein kinases (MAPKs), were measured in each group.

RESULTS

Compared with HepG2 cell group and RNAi group, apoptosis rate, the expression of Fas and FasL proteins, and the activation of MLK3, MKK7 and JNKs were increased in the pcDNA3.1-X transfected group. The activation of JNKs and expression of FasL protein were inhibited in the pcDNA3.1-X transfected group when treated with a known JNK inhibitor, SP600125. When authors treated pcDNA3.1-X transfected group with K252a, a known MLK3 inhibitor, the activation of MLK3, MKK7 and JNKs as well as expression of FasL protein was inhibited. Furthermore, cell apoptosis rate was also significantly declined in the presence of K252a in the pcDNA3.1-X transfected group.

CONCLUSION

HBx can induce HepG2 cell apoptosis via a novel active MLK3-MKK7-JNKs signaling module to upregulate FasL protein expression.

Apoptosis of hepatitis B virus-infected hepatocytes prevents release of infectious virus

Apoptosis of infected cells is critically involved in antiviral defense. Apoptosis, however, may also support the release and spread of viruses. Although the elimination of infected hepatocytes is required to combat hepatitis B virus (HBV) infection, it is still unknown which consequences hepatocyte apoptosis has for the virus and whether or not it is advantageous to the virus. To study this, we designed a cell culture model consisting of both HBV-producing cell lines and primary human hepatocytes serving as an infection model. We showed that the release of mature, enveloped virions was 80% to 90% reduced 24 h after the induction of apoptosis in HBV-replicating hepatoma cells or HBV-infected hepatocytes. Importantly, HBV particles released from apoptotic hepatocytes were immature and nonenveloped and proved not to be infectious. We found an inverse correlation between the strength of an apoptotic stimulus and the infectivity of the virus particles released: the more potent the apoptotic stimulus, the higher the ratio of nonenveloped capsids to virions and the lower their infectivity. Furthermore, we demonstrated that HBV replication and, particularly, the expression of the HBx protein transcribed from the viral genome during replication do not sensitize cells to apoptosis. Our data clearly reject the hypothesis that the apoptosis of infected hepatocytes facilitates the propagation of HBV. Rather, these data indicate that HBV needs to prevent the apoptosis of its host hepatocyte to ensure the release of infectious progeny and, thus, virus spread in the liver.

The Molecular Diagnosis of Hepatitis B Virus-Associated Hepatocellular Carcinoma

Hepatitis B virus (HBV) infection is the major cause of hepatocellular carcinoma (HCC) worldwide. The pathogenesis of HBV-associated HCC has been studied extensively, and molecular changes during malignant transformation have been identified. It has been proposed that the insertion of HBV DNA into the human genome results in chromosomal instability and inactivation of tumor suppressor genes. Transactivation of oncogenes, inactivation of tumor suppressor genes, and alteration of the cell cycle by HBV proteins are also involved in the progression of hepatocellular carcinogenesis. Traditional clinical examinations of HCC, such as biopsy, computer tomography, ultrasonic imaging, and detection of such biomarkers as a-fetoprotein, are currently the "gold standard" in diagnosis. These tests diagnose HCC only in the late stages of disease. This limitation has greatly reduced the chance of survival of HCC patients. To resolve this problem, new biomarkers that can diagnose HCC in earlier stages are necessary. Based on recent molecular studies of the effects of HBV on cellular transformation, differentially expressed biomarkers of HBV infection have been elucidated. With the analyses of the HBV replication profile, the viral load (HBV DNA levels) of patients, and the viral protein expression, the severity of hepatitis in the preneoplastic stages can be assessed. In the future, with the molecular profiles identified by genomic and proteomic approaches, stage-specific biomarkers should be identified to monitor the progression and prognosis of HCC.

Effects of hepatitis B virus X protein (HBx) on cell-growth inhibition in a CCL13-HBx stable cell line

OBJECTIVE

The known function of hepatitis B virus X protein (HBx) is to determine the fate of cells by modulating various signaling pathways. In our previous study, we demonstrated that HBx inhibits tumor formation in nude mice injected with CCL13-HBx stable cell lines; however, the mechanism underlying this inhibition is unclear.

METHODS

To investigate the possible mechanisms underlying HBx involvement in CCL13-HBx cells, gene profiles were initially analyzed by DNA microarray technology and subsequently confirmed by performing semiquantitative RT-PCR and Western blotting. Furthermore, the phenomenon of cell death via apoptosis was detected via DNA fragmentation, TUNEL staining, caspase-3 activity assay, and propidium iodide (PI) staining.

RESULTS

The results indicated that HBx induction downregulated Wnt-3 and beta-catenin that are involved in cell proliferation. Moreover, HBx induction repressed cell growth and downregulated the expressions of cyclin D1, CDK4, cyclin E, CDK2, and cyclin B1. Furthermore, HBx induction triggered cell death via apoptosis, as determined by DNA fragmentation, TUNEL staining, caspase-3 activity assay, and PI staining.

CONCLUSION

Most importantly, our results indicated that HBx induction in the CCL13-HBx stable cell line downregulated Wnt-3/beta-catenin expression and suppressed cell growth by repressing cell proliferation or triggering cell apoptosis.

Deactivation of Akt and STAT3 signaling promotes apoptosis, inhibits proliferation, and enhances the sensitivity of hepatocellular carcinoma cells to an anticancer agent, Atiprimod

Atiprimod is a novel anticancer and antiangiogenic drug candidate which is currently being evaluated in patients with liver carcinoid and multiple myeloma. In this study, we report that atiprimod selectively inhibited proliferation and induced apoptosis in HCC cells that expressed either hepatitis B virus (HBV) or hepatitis C virus, through deactivation of protein kinase B (Akt) and signal transducers and activators of transcription 3 (STAT3) signaling. In HepG2 AD38 cells, which express HBV genome under the control of a tetracycline-off promoter, both Akt and STAT3 were constitutively activated in response to HBV expression. However, this constitutive activation was not sensitive to lamivudine, a drug that inhibits HBV replication without affecting its gene expression, suggesting that HBV replication per se might not be responsible for the activation. Interestingly, the electrophoretic mobility of p-STAT3 protein bands on immunoblot was slower when AD38 cells were cultured in the absence of tetracycline, suggesting a differential phosphorylation in response to HBV expression. In HCC cells, interleukin 6 stimulates the phosphorylation of STAT3 both at serine 727 and at tyrosine 705 positions. The interleukin 6-stimulated activation of STAT3 and Akt was inhibited not only by atiprimod but also by LY294002, a phosphoinositide-3-kinase-specific inhibitor, and by NS398, a cyclooxygenase-2-selective inhibitor. The combination of these compounds did not produce any additive effect, implying that the mechanisms by which HBV activates Akt and STAT3 might also involve phosphoinositide-3-kinase and cyclooxygenase-2. Collectively, these results suggest that atiprimod could be useful as a multifunctional drug candidate for the treatment of HCC in humans.

Intrahepatic activation of caspases in human fulminant hepatic failure

BACKGROUND/AIMS

Apoptosis has been implicated in the pathogenesis of fulminant hepatic failure (FHF) potentially involving caspases. Thus far, apoptosis in FHF has mainly been studied in animal models while human data are sparse.

METHODS

Caspases-3, -8 and -9 activities and Fas expression were analyzed in correlation to TdT-mediated dUTP nick end labelling (TUNEL) positive apoptotic cells in livers of patients with FHF (n=26), chronic liver disease (CLD) (n=60) and normal controls (NC) (n=10).

RESULTS

Numbers of TUNEL-positive cells were higher in FHF than in CLD and NC (P<0.001) correlating to the intrahepatic activities of caspase-3. The highest caspase-3 activities were found in fulminant hepatitis B, significantly surpassing those in FHF of any other etiology. In fulminant hepatitis B, caspase-9 activity was also higher than in controls, while caspase-8 activation was not higher than in NC. Unlike caspase-3, caspases -8 and -9 activities were not correlated to the numbers of TUNEL positive cells. Fas expression was also the highest in FHF but did not differ between hepatitis B virus-FHF and other FHF.

CONCLUSIONS

Our data indicate differential activation of intrahepatic caspases in FHF depending on the underlying etiology. Massive activation of caspases in fulminant hepatitis B confirms a pivotal role of apoptotic pathways in the pathogenesis of human fulminant hepatitis B.

Human hepatitis B virus X protein induces apoptosis in HepG2 cells: Role of BH3 domain

The smallest protein of hepatitis B virus, HBX, has been implicated in the development of liver diseases by interfering with normal cellular processes. Its role in cell proliferation has been unclear as both pro-apoptotic and anti-apoptotic activities have been reported. We showed molecular evidence that HBX induced apoptosis in HepG2 cells. A Bcl-2 Homology Domain 3 was identified in HBX, which interacted with anti-apoptotic but not pro-apoptotic members of the Bcl-2 family of proteins. HBX induced apoptosis when transfected into HepG2 cells, as demonstrated by both flow cytometry and caspase-3 activity. However, HBX protein may not be stable in apoptotic cells triggered by its own expression as only its mRNA or the fusion protein with the glutathione-S-transferase was detected in transfected cells. Our results suggested that HBX behaved as a pro-apoptotic protein and was able to induce apoptosis.

The hepatitis B virus X protein sensitizes HepG2 cells to UV light-induced DNA damage

Various reports have implicated the virally encoded HBx protein as a cofactor in hepatocarcinogenesis. However, direct evidence of the role of HBx as a promoter of oncogenesis in response to an initiating factor such as DNA damage remains inadequate. Here, we report the effects of HBx in HepG2 cells exposed to UV light-induced DNA damage. HBx expression was found not to affect the morphology, viability, and cell cycle/apoptotic profiles or DNA repair machinery of untreated cells. Nonetheless, upon UV treatment, HBx protein levels increased concomitantly with p53 levels. Both HBx and p53 proteins were found to interact and colocalize primarily in the nucleus. The binding of HBx to p53 modulated (but did not inhibit) the transcriptional activation function of p53. Notably, HBx-expressing cells exhibited increased sensitivity to UV damage, resulting in greater G2/M arrest and apoptosis of these cells. Additionally, these cells displayed a reduced DNA repair capacity in response to UV damage. In conclusion, this work suggests that DNA damage may be an initiating factor in hepatocarcinogenesis and that HBx may act as the promoting factor by inhibiting DNA repair. In hepatitis B virus-infected hepatocytes, a chronic infection may present the opportunity for such a DNA-damaging event to occur, and accumulated errors caused by the inhibition of DNA repair by HBx may result in oncogenesis.

Possible mechanism for hepatitis B virus X gene to induce apoptosis of hepatocytes

AIM: To investigate the possible mechanism for HBV X gene to induce apoptosis of hepatocyte HL-7702 cells.

METHODS: HBV X gene eukaryon expression vector pcDNA3-X was established and transfected into HL-7702 cells by lipid-mediated transfection, including transient and stable transfection. Positive clones were screened by incubating in the selective medium with 600 mg/mL G418 and named HL-7702/HBV-encoded X protein (HBx) cells. The expressions of Fas/FasL, Bax/Bcl-2, and c-myc mRNA were measured by semi-quantitative RT-PCR in HL-7702/HBx and control group, respectively.

RESULTS: RT-PCR analysis confirmed that HBV X gene was transfected into HL-7702 cells successfully. By semi-quantitative RT-PCR analysis, Bax and c-myc mRNA levels in HL-7702/HBx cells of transient transfection were significantly higher than those in control, FasL and c-myc mRNA levels in HL-7702/HBx cells of stable transfection were significantly higher than those in control, whereas the Bcl-2 mRNA levels in HL-7702/HBx cells of transient and stable transfection were significantly lower than those in control.

CONCLUSION: HBV X gene may promote the apoptosis of hepatocytes by regulating the expressions of Fas/FasL, Bax/Bcl-2, and c-myc gene in a dose-dependent manner.

Apoptosis and its pathway in X gene-transfected HepG2 cells

AIM: To investigate the effect of hepatitis B virus (HBV) X gene on apoptosis and expressions of apoptosis factors in X gene-transfected HepG₂ cells.

METHODS: The HBV X gene eukaryon expression vector pcDNA3-X was transiently transfected into HepG₂ cells by lipid-media transfection. Untransfected HepG₂ and HepG₂ transfected with pcDNA3 were used as controls. Expression of HBx in HepG₂ was identified by RT-PCR. MTT and TUNEL were employed to measure proliferation and apoptosis of cells in three groups. Semi-quantified RT-PCR was used to evaluate the expression levels of Fas/FasL, Bax/Bcl-xL, and c-myc in each group.

RESULTS: HBV X gene was transfected into HepG₂ cells successfully. RT-PCR showed that HBx was only expressed in HepG₂/pcDNA3-X cells, but not expressed in HepG₂ and HepG₂/pcDNA3 cells. Analyzed by MTT, cell proliferation capacity was obviously lower in HepG₂/pcDNA3-X cells (0.08910±0.003164) than in HepG₂ (0.14410±0.004927) and HepG₂/pcDNA3 cells (0.12150±0.007159) (P<0.05 and P<0.01). Analyzed by TUNEL, cell apoptosis was much more in HepG₂/pcDNA3-X cells (980/2 000) than HepG₂ (420/2 000), HepG₂/pcDNA3 cells (520/2 000) (P<0.05 and P<0.01). Evaluated by semi-quantified RT-PCR, the expression level of Fas/FasL was significantly higher in HepG₂ cells transfected with HBx than in HepG₂ and HepG₂/pcDNA3cells (P<0.05 and P<0.01). Bax/Bcl-xL expression level was also elevated in HepG₂/pcDNA3-X cells (P<0.05 and P<0.01). Expression of c-myc was markedly higher in HepG₂/pcDNA3-X cells than in HepG₂ and HepG₂/pcDNA3 cells (P<0.05 andP<0.01).

CONCLUSION: HBV X gene can impair cell proliferation capacity, improve cell apoptosis, and upregulate expression of apoptosis factors. The intervention of HBV X gene on the expression of apoptosis factors may be a possible mechanism responsible for the change in cell apoptosis and proliferation.

The cysteine residues of the hepatitis B virus onco-protein HBx are not required for its interaction with RNA or with human p53

The hepatitis B virus (HBV) protein HBx has been implicated to induce liver cancer in transgenic mice and transactivates a variety of viral and cellular promoters. The 17 kDa protein HBx consists of 154 amino acids, contains 10 cysteine residues and is translated during the viral infection. It has been shown previously that the HBx protein is able to bind to singlestranded DNA and RNA. This nucleic acid binding activity might be relevant for HBx oncogenic character. Furthermore, HBx has been reported to interact with a series of cellular proteins, especially with transcription factors, including the tumor suppressor protein p53. To evaluate the importance of the cysteine residues in HBx for its interaction with RNA and p53 we expressed full-length HBx-wt as well as several truncated mini-HBx(18-142) proteins with multiple cysteine to serine point mutations as 6xHis fusion proteins in Escherichia coli. Using UV cross-linking assays we demonstrate that all truncated mini-HBx proteins with cysteine/serine point mutations maintained the ability to bind to an AU-38 RNA oligonucleotide. Furthermore, we performed in vitro binding assays of selected HBx mutants with GST-p53, circular dichroism spectroscopic analysis of the mutant HBx protein secondary structure and a p53 based transcription activation assay in yeast cells. In summary, our data suggest that the cysteine residues in the HBx protein are of minor importance for its interaction with both RNA and the p53 protein.

Mitochondria as Functional Targets of Proteins Coded by Human Tumor Viruses

Molecular analyses of tumor virus-host cell interactions have provided key insights into the genes and pathways involved in neoplastic transformation. Recent studies have revealed that the human tumor viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-cell leukemia virus type 1 (HTLV-1) express proteins that are targeted to mitochondria. The list of these viral proteins includes BCL-2 homologues (BHRF1 of EBV; KSBCL-2 of KSHV), an inhibitor of apoptosis (IAP) resembling Survivin (KSHV K7), proteins that alter mitochondrial ion permeability and/or membrane potential (HBV HBx, HPV E[wedge]14, HCV p7, and HTLV-1 p13(II)), and K15 of KSHV, a protein with undefined function. Consistent with the central role of mitochondria in energy production, cell death, calcium homeostasis, and redox balance, experimental evidence indicates that these proteins have profound effects on host cell physiology. In particular, the viral BCL-2 homologues BHRF1 and KSBCL-2 inhibit apoptosis triggered by a variety of stimuli. HBx, p7, E1[wedge]4, and p13(II) exert powerful effects on mitochondria either directly due to their channel-forming activity or indirectly through interactions with endogenous channels. Further investigation of these proteins and their interactions with mitochondria will provide important insights into the mechanisms of viral replication and tumorigenesis and could aid in the discovery of new targets for anti-tumor therapy.

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Study of PTPC composition during apoptosis for identification of viral protein target

The permeability transition pore complex (PTPC), a mitochondrial polyprotein complex, has been previously described to be involved in the control of mitochondrial membrane permeabilization (MMP) during chemotherapy-induced apoptosis. PTPC may contain proteins from both mitochondrial membranes [e.g., voltage-dependent anion channel (VDAC), PRAX-1, peripheral benzodiazepine receptor (PBR), adenine nucleotide translocator (ANT)], from cytosol (e.g., hexokinase II, glycerol kinase), from matrix [e.g., cyclophilin D (CypD)], and from intermembrane space (e.g., creatine kinase). PTPC may also interact with tumor suppressor proteins (i.e., Bax and Bid), oncoprotein homologues of Bcl-2 and some viral proteins, which can regulate apoptosis induced by pore opening. ANT and VDAC are the target of numerous pro-apoptotic MMP inducers. However, the precise composition of PTPC as well as the respective role of each PTPC component represent major issues in the understanding MMP process. Using several experimental strategies that combine co-immunoprecipitation, proteomics, and functional tests with proteoliposomes, we and others have been able to characterize some of the intra/inter-PTPC protein interactions leading to a better understanding of the process of MMP. In addition, this approach could identify new putative members and regulators of PTPC pro-apoptotic function and new targets of viral protein involved in the modulation of apoptosis during infection.

Transfection and expression of hepatitis B virus x gene and its effect on apoptosis in HL-7702 cells

AIM: To investigate the effects of hepatitis B virus x gene and its protein product HBxAg on apoptosis in hepatocyte line HL-7702.

METHODS: The reconstituted plasmid pcDNA3-x was established through recombination DNA technique; pcDNA3-X was transfected into HL-7702 cells by lipid-mediated trasfection. Positive clones were screened by G418, and HL-7702/HBx cells were analysed by the RT-PCR to confirm the steady expression of X gene in HL-7702 cells. The apoptosis rate in HL-7702 cells was determined by flow cytometry, TUNEL technology, electronic microscope. At the mean time, pcDNA3-X was transfected transiently into HL-7702 cells, and total RNA from HL-7702 cells was extracted 24, 48, 72, 96 and 120 h after the transient transfection, and semi-quantitative analysis was performed by RT-PCR to detect the expression of HBV X gene. Furthermore, apoptosis rate in HL-7702 cells was determined by flow cytometry analysis at the different times.

RESULTS: RT-PCR analysis showed that HBV X gene could be expressed stably in HL-7702 cells. Both flow cytometry and TUNEL technology revealed that the apoptosis rates of HL-7702/HBx cells were much higher than those of HL-7702/ pcDNA3 and HL-7702 cells. Furthermore, the apoptotic phenomena and apoptotic body were observed in HL-7702/ HBx cells under electronic microscope, but not in HL-7702/ pcDNA3 and HL-7702 cells. In the experiment of transient transfection, RT-PCR reveals that X gene was expressed most at 72 h after transfection; and the apoptosis rate reached the highest at the same time. After that, the apoptosis rate was reduced with the decrease of the X gene expression.

CONCLUSION: HBV X gene and X protein can promote the apoptosis in hepatocyte. And there exist a quantity-effect relationship between the X gene expression and apoptosis rate in hepatocyte.

Overexpression of HBxAg in hepatocellular carcinoma and its relationship with Fas/FasL system

AIM: To study the expression and serum level of HBxAg, Fas and FasL in tissues of HCC patients, and to assess the relationship between HBxAg and Fas/FasL system.

METHODS: Tissues from 50 patients with HCC were tested for the expression of HBxAg, Fas and FasL by S-P immunohistochemistry. Serum levels of sFas/sFasL and HBsAg/HBeAg were measured by ELISA assay. HBV X gene was detected by PCR in serum and confirmed by automatic sequencing. Fifty cases of liver cirrhosis and 30 normal controls were involved in serum analysis.

RESULTS: The expression of HBxAg, Fas and FasL in carcinoma tissues was 96%, 84% and 98%, respectively. Staining of HBxAg, Fas and FasL was observed predominately in cytoplasms, no significant difference was found in intensity between HBxAg, Fas and FasL (P > 0.05). HBxAg, Fas and FasL might express in the same area of carcinoma tissues and this co-expression could be found in most patients with HCC. The mean levels of sFas in serum from HCC, cirrhosis and normal controls were 762.29 ± 391.56 μg·L^-1, 835.36 ± 407.33 μg·L^-1 and 238.27 ± 135.29 μg·L^-1. The mean levels of sFasL in serum from HCC, cirrhosis and normal controls were 156.36 ± 9.61 μg·L^-1, 173.63 ± 18.74 μg·L^-1 and 121.96 ± 7.83 μg·L^-1. Statistical analysis showed that both sFas and sFasL in HCC and cirrhosis patients were significantly higher than those in normal controls (P < 0.01). Serum HBV X gene was found in 32% of HCC patients and 46% of cirrhotic patients. There was no significant relationship between serum level of sFas/sFasL and serum X gene detection (P > 0.05). Eight percent of HCC patients with negative HBsAg and HBeAg in serum might have X gene in serum and HBxAg expression in carcinoma tissues.

CONCLUSION: Our data suggest that HBxAg and Fas/FasL system plays an important role in the development of human HCC. Expression of HBxAg can leads to expression of Fas/ FasL system which and reverse apoptosis of hepatocellular carcinoma induced by FasL.

HBX causes cyclin D1 overexpression and development of breast cancer in transgenic animals that are heterozygous for p53

Transgenic mice, which selectively express the WAP-HBX transgene in mammary gland epithelial cells (ME-cells), were established in order to elucidate the consequences of HBX gene expression on organ differentiation, cell death program and tumor development. Transgene expression was demonstrable by RT-PCR, Northern and Western blot analysis during pregnancy, lactation and after weaning. HBX synthesis neither affect mammary gland differentiation nor apoptosis in ME-cells. Although breast cancer formation was rare in WAP-HBX animals (<1%), WAP-HBX*p53+/- hybrid animals developed breast tumors at an increased rate (12/85) after a latency period of 8-18 months. We also show here for the first time that HBX can immortalize ME-cells generated from mammary gland tissue segments in a p53-independent fashion. HBX causes cyclin D1 gene overexpression during early pregnancy, and this is maintained in ME-cells isolated either from mammary gland or from breast tumors. Intranuclear cyclin D1 accumulation also occurs in the absence of external growth factors and the BrdU incorporation rate remains high under serum starvation conditions. Finally, both cyclin D1 induction and HBX mitotic activity are dependent on p38 and c-Jun N-terminal kinase, but not on MEK-1 kinase activity.

Hepatocellular carcinoma in a hepatitis B 'x' transgenic mouse model: A sequential pathological evaluation

BACKGROUND

The introduction of transgenic technology has made it possible to study the steps of carcinogenesis and directly establish the link between viral subgenomic fragments and specific types of cancer. Research directed at hepatitis B virus (HBV)-related carcinogenesis has benefited from this technology. We present a detailed pathological evaluation of the sequential steps of hepatocarcinogenesis in a hepatitis B 'x' (HBx) transgenic mouse model. In this model, the transgene incorporates the region encoding amino acids 58-154 of the HBV X protein and the murine c-myc gene. This model demonstrated changes in the liver from birth with foci of multicentric dysplasia evolving into nodules and overt hepatocellular carcinoma between 20 and 28 weeks.

METHODS AND RESULTS

The hepatocytes were mitotically active and showed increased proliferative capacity soon after birth, with exponential increase thereafter. This was accompanied by a high rate of apoptosis, which later declined as the tumors developed. Other functional and immunophenotypic characteristics included a high c-myc expression in the neoplastic lesions, no alteration in p53 expression, and no alteration in the expression of hepatic enzymes except for diffuse expression of succinic dehydrogenase.

CONCLUSION

The entire process illustrates the disturbances of cell growth and death because of the collaborative influence of HBx and c-myc genes that result in the development of hepatocellular carcinoma after a prolonged latent period.

Altered DNA mutation spectrum in aflatoxin b1-treated transgenic mice that express the hepatitis B virus x protein

Humans chronically infected with hepatitis B virus (HBV) are at further risk of liver cancer upon exposure to dietary aflatoxin B1 (AFB1), a carcinogenic product of the mold Aspergillus flavus. For the present study, we utilized double-transgenic mice (ATX mice) that express the HBV X protein (HBx) and possess a bacteriophage lambda transgene to evaluate the in vivo effect of HBx expression on AFB1-induced DNA mutations. The expression of HBx correlated with a 24% increase in mutation frequency overall and an approximately twofold increase in the incidence of G/C-to-T/A transversion mutations following AFB1 exposure. These results are consistent with a model in which expression of HBx during chronic HBV infection may contribute to the development of hepatocellular carcinoma following exposure to environmental carcinogens.

Multiparametric flow cytometric analysis of biochemical and functional events associated with apoptosis and oncosis using the 7-aminoactinomycin D assay

Apoptosis and primary necrosis are the two modes of cell death induced by a lethal injury. The majority of structural and biochemical events occurring during cell death can be analysed by flow cytometry. The 7-aminoactinomycin D (7-AAD) assay can be used to detect the loss of membrane integrity during apoptosis of murine thymocytes and human peripheral lymphocytes. We describe here new applications of the 7-AAD assay. It can be applied to a variety of cell lines of different origins, including adherent cell lines, and it allows the co-detection of lipidic antigens such as phosphatidylserine (PS) residues, and biochemical processes linked to apoptosis, such as the loss of mitochondrial transmembrane potential, cardiolipin peroxidation, the expression of the 7A6 mitochondrial antigen and DNA fragmentation. Thus, this assay is a noninvasive method particularly adapted to the analysis of biochemical events associated with cell death. Finally, we show that this assay is not specific for apoptosis since it detects oncosis, the early stage of primary necrosis.

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.

The PreS2 activator MHBs(t) of hepatitis B virus activates c-raf-1/Erk2 signaling in transgenic mice

The large hepatitis B virus (HBV) surface protein (LHBs) and C-terminally truncated middle size surface proteins (MHBs(t)) form the family of the PreS2 activator proteins of HBV. Their transcriptional activator function is based on the cytoplasmic orientation of the PreS2 domain. MHBs(t) activators are paradigmatic for this class of activators. Here we report that MHBs(t) is protein kinase C (PKC)-dependently phosphorylated at Ser28. The integrity of the phosphorylation site is essential for the activator function. MHBs(t) triggers PKC-dependent activation of c-Raf-1/Erk2 signaling that is a prerequisite for MHBs(t)-dependent activation of AP-1 and NF-kappaB. To analyze the pathophysiological relevance of these data in vivo, transgenic mice were established that produce the PreS2 activator MHBs(t) specifically in the liver. In these mice, a permanent PreS2-dependent specific activation of c-Raf-1/Erk2 signaling was observed, resulting in an increased hepatocyte proliferation rate. In transgenics older than 15 months, an increased incidence of liver tumors occurs. These data suggest that PreS2 activators LHBs and MHBs(t) exert a tumor promoter-like function by activation of key enzymes of proliferation control.

The hepatitis B virus X protein abrogates Bcl-2-mediated protection against Fas apoptosis in the liver

The role of the hepatitis B virus protein HBx in liver cell proliferation and apoptosis remains controversial. Using a transgenic mouse model, we have recently shown that HBx stimulates the apoptotic turnover of hepatocytes, independently of p53. In this paper, we tested whether the proapoptotic function of HBx can interfere with Bcl-2 during hepatic apoptosis in vivo. HBx transgenic mice were crossed with PK-hBcl-2 mice that are protected against Fas killing by constitutive overexpression of Bcl-2 in hepatocytes. In a lethal challenge with Fas antibodies, HBx expressed at low levels restored sensitivity to Fas-mediated apoptosis and fulminant hepatic failure in mice overexpressing Bcl-2. Furthermore, cytochrome c release from mitochondria and caspase 3 activation were restored to normal levels in HBx/Bcl-2 mice during transduction of the Fas signal. Thus, the proapoptotic activity of HBx overcomes or bypasses the inhibitory effect of Bcl-2 against Fas cytotoxicity. This effect was not apparently mediated through downregulation of the PK-hBcl-2 transgene or via delocalization of the Bcl-2 protein, and a direct interaction of HBx with Bcl-2, Bcl-X(L) or Bax could not be evidenced in yeast two-hybrid assays. We further show that apoptosis induced by ectopic expression of HBx is associated with mitochondrial membrane alterations and caspase 3 activation. Our data indicate that the dominant function of HBx upon Bcl-2-regulated control of apoptosis might play an important role in the pathogenesis of chronic hepatitis B.

DDB2 induces nuclear accumulation of the hepatitis B virus X protein independently of binding to DDB1

The hepatitis B virus (HBV) X protein (HBx) is critical for the life cycle of the virus. HBx associates with several host cell proteins including the DDB1 subunit of the damaged-DNA binding protein DDB. Recent studies on the X protein encoded by the woodchuck hepadnavirus have provided correlative evidence indicating that the interaction with DDB1 is important for establishment of infection by the virus. In addition, the interaction with DDB1 has been implicated in the nuclear localization of HBx. Because the DDB2 subunit of DDB is required for the nuclear accumulation of DDB1, we investigated the role of DDB2 in the nuclear accumulation of HBx. Here we show that expression of DDB2 increases the nuclear levels of HBx. Several C-terminal deletion mutants of DDB2 that fail to bind DDB1 are able to associate with HBx, suggesting that DDB2 may associate with HBx independently of binding to DDB1. We also show that DDB2 enhances the nuclear accumulation of HBx independently of binding to DDB1, since a mutant that does not bind DDB1 is able to enhance the nuclear accumulation of HBx. HBV infection is associated with liver pathogenesis. We show that the nuclear levels of DDB1 and DDB2 are tightly regulated in hepatocytes. Studies with regenerating mouse liver indicate that during late G1 phase the nuclear levels of both subunits of DDB are transiently increased, followed by a sharp decrease in S phase. Taken together, these results suggest that DDB1 and DDB2 would participate in the nuclear functions of HBx effectively only during the late-G1 phase of the cell cycle.

Hepatitis B virus and hepatocellular carcinoma

Chronic hepatitis B virus (HBV) infection is a major global cause of hepatocellular carcinoma (HCC). Individuals who are chronic carriers have a greater than 100-fold increased relative risk of developing the tumour. Several mechanisms of HBV-induced HCC have been proposed. Integration of HBV DNA into the genome of hepatocytes occurs commonly, although integration at cellular sites that are important for regulation of hepatocyte proliferation appears to be a rare event. Functions of the HBx protein are also potentially oncogenic. These include transcriptional activation of cellular growth regulatory genes, modulation of apoptosis and inhibition of nucleotide excision repair of damaged cellular DNA. The effects of HBx are mediated by interaction with cellular proteins and activation of cell signalling pathways. Variations in HBV genome sequences may be important in hepatocarcinogenesis, although their significance has not yet been completely elucidated. Necroinflammatory hepatic disease, which often accompanies chronic HBV infection, may contribute indirectly to hepatocyte transformation in a number of ways, including by facilitating HBV DNA integration, predisposing to the acquisition of cellular mutations and generating mutagenic oxygen reactive species. Although HCC is a malignancy with a poor prognosis, the availability of an effective vaccine against HBV infection, and its inclusion in the Expanded Programme of Immunization of many countries, augurs well for the eventual elimination of HBV-associated HCC.

Hepatitis B virus X protein acts as a tumor promoter in development of diethylnitrosamine-induced preneoplastic lesions

Chronic infection with hepatitis B virus (HBV) is one of the major etiological factors in the development of human hepatocellular carcinoma. Transgenic mice that express the HBV X protein (HBx) have previously been shown to be more sensitive to the effects of hepatocarcinogens. Although the mechanism for this cofactor role remains unknown, the ability of HBx to inhibit DNA repair and to influence cell cycle progression suggests two possible pathways. To investigate these possibilities in vivo, we treated double-transgenic mice that both express HBx (ATX mice) and possess a bacteriophage lambda transgene with the hepatocarcinogen diethylnitrosamine (DEN). Histological examination of liver tissue confirmed that DEN-treated ATX mice developed approximately twice as many focal lesions of basophilic hepatocytes as treated wild-type littermates. Treatment of mice with DEN resulted in a six- to eightfold increase in the mutation frequency (MF), as measured by a functional analysis of the lambda transgene. HBx expression was confirmed by immunoprecipitation and Western blotting and was associated with a modest 23% increase in the MF. Importantly, the extent of hepatocellular proliferation in 14-day-old mice, as measured by the detection of proliferating cell nuclear antigen and by the incorporation of 5-bromo-2'-deoxyuridine, was determined to be approximately twofold higher in ATX livers than in wild-type livers. These results are consistent with a model in which HBx expression contributes to the development of DEN-mediated carcinogenesis by promoting the proliferation of altered hepatocytes rather than by directly interfering with the repair of DNA lesions.

Hepatitis B virus and hepatocellular carcinoma

Chronic hepatitis B virus (HBV) infection is a major global cause of hepatocellular carcinoma (HCC). Individuals who are chronic carriers have a greater than 100-fold increased relative risk of developing the tumour. Several mechanisms of HBV-induced HCC have been proposed. Integration of HBV DNA into the genome of hepatocytes occurs commonly, although integration at cellular sites that are important for regulation of hepatocyte proliferation appears to be a rare event. Functions of the HBx protein are also potentially oncogenic. These include transcriptional activation of cellular growth regulatory genes, modulation of apoptosis and inhibition of nucleotide excision repair of damaged cellular DNA. The effects of HBx are mediated by interaction with cellular proteins and activation of cell signalling pathways. Variations in HBV genome sequences may be important in hepatocarcinogenesis, although their significance has not yet been completely elucidated. Necroinflammatory hepatic disease, which often accompanies chronic HBV infection, may contribute indirectly to hepatocyte transformation in a number of ways, including by facilitating HBV DNA integration, predisposing to the acquisition of cellular mutations and generating mutagenic oxygen reactive species. Although HCC is a malignancy with a poor prognosis, the availability of an effective vaccine against HBV infection, and its inclusion in the Expanded Programme of Immunization of many countries, augurs well for the eventual elimination of HBV-associated HCC.

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.

Interaction of Hepatitis B virus with cellular processes in liver carcinogenesis

Hepatitis B infection is strongly linked epidemiologically to hepatocellular carcinoma development. This article reviews the molecular mechanisms by which hepatitis B encoded proteins such as hepatitis B x and hepatitis B surface transactivators may interact with gene transcription, tumor suppression, apoptosis, and signalling pathways of the liver cell with the possible consequence of tumor induction. Data on the interaction between hepatitis B proteins and cellular processes are often conflicting indicating a non-specific simultaneous interaction with antagonistic cellular processes that result in the formation of escape mutants that are not subject to these selective pressures.

Expression of hepatitis B virus X protein does not alter the accumulation of spontaneous mutations in transgenic mice

Chronic infection with hepatitis B virus (HBV) is one of the major etiological factors in the development of human hepatocellular carcinoma. Transgenic mice that express the HBV X protein (HBx) have previously been shown to be more sensitive to the effects of hepatocarcinogens, although the mechanism for this cofactor role remains unknown. The ability of HBx to inhibit DNA repair in transiently transfected cell lines suggests one possible pathway. In the present study, primary hepatocytes isolated from transgenic mice that possess the HBV X gene under the control of the human alpha-1-antitrypsin regulatory region (ATX mice) were found to be deficient in their ability to conduct unscheduled DNA synthesis in response to UV-induced DNA damage. In order to measure the impact of HBx expression on DNA repair in vivo, double-transgenic mice that express HBx and possess a bacteriophage lambda transgene were sacrificed at 30, 90, and 240 days of age. Mutation frequency was determined for high-molecular-weight liver DNA of ATX and control mice by functional analysis of the lambda transgene. Expression of HBx did not significantly increase the accumulation of spontaneous mutations. These results are consistent with previous studies of HBx transgenic mice in which no effect of HBx on liver histology was apparent. This new animal model provides a powerful system in which to investigate the in vivo cooperation between HBx expression and environmental carcinogens.

Putative role of hepatitis B virus X protein in hepatocarcinogenesis: effects on apoptosis, DNA repair, mitogen-activated protein kinase and JAK/STAT pathways

Chronic infection with hepatitis B virus (HBV) is a major risk factor for the development of hepatocellular carcinoma (HCC). The pathogenesis of HBV-induced malignant transformation is, however, incompletely understood. HBx, the protein encoded by the X open reading frame, is a transcriptional activator that has been implicated in hepatocarcinogenesis. HBx inhibits the function of the tumour suppressor protein p53 in what is thought to be an early event in hepatocyte transformation before the later accumulation of inactivating p53 point mutations. HBx inhibits apoptosis but also exerts pro-apoptotic effects. The effects of HBx on apoptosis may be important not only for the development of HCC but also for the establishment of HBV infection. Further implication of HBx in hepatocyte transformation has been the demonstration that it inhibits the repair of damaged hepatocyte DNA. This effect may be mediated by interaction with p53 or through binding to the damaged DNA binding protein (DDB), which plays an accessory role in nucleotide excision repair. In addition, HBx activates cell signalling cascades involving mitogen-activated protein kinase (MAPK) and Janus family tyrosine kinases (JAK)/signal transducer and activators of transcription (STAT) pathways. The implications of these modulating effects of HBx are not fully understood, but they are likely to have wide-ranging effects on hepatocyte proliferation, apoptosis and the regulation of cell growth checkpoints. The cellular functions ascribed to HBx are unusually diverse, and defining the biologically important role of HBx during HBV replication will go some way to understanding the sequelae of chronic HBV infection.

Hepatitis B virus in hepatocarcinogenesis

Hepatitis B virus (HBV) is an important etiologic agent of chronic hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Although the mechanism whereby HBV causes HCC is not fully understood, it is likely that there are many relevant molecular pathways that contribute to the development of HBV-associated HCC. This review provides an overview of some of these proposed pathways and their relative importance. It also raises questions on basic and translational research that will signficantly contribute to the better understanding of underlying mechanisms, prevention, and treatment of this tumor type.