Hepatitis B virus X protein induces cell death by causing loss of mitochondrial membrane potential

The viral oncoprotein HBx of Hepatitis B virus promotes the growth of hepatocellular carcinoma through cooperating with the cellular oncoprotein RMP

The smallest gene HBx of Hepatitis B virus (HBV) is recognized as an important viral oncogene (V-oncogene) in the hepatocarcinogenesis. Our previous work demonstrated that RMP is a cellular oncogene (C-oncogene) required for the proliferation of hepatocellular carcinoma (HCC) cells. Here we presented the collaboration between V-oncogene HBx and C-oncogene RMP in the development of HCC. The coexpression of HBx and RMP resulted in the cooperative effect of antiapoptosis and proliferation of HCC cells. In vivo, overexpression of RMP accelerated the growth of HBx-induced xenograft tumors in nude mice and vice versa HBx promoted the growth of RMP-driven xenograft tumors. Although HBx didn't regulate the expression of RMP, HBx and RMP interact with each other and collocalized in the cytoplasm of HCC cells. HBx and RMP collaboratively inhibited the expression of apoptotic factors and promoted the expression of antiapoptotic factors. This finding suggests that HBV may induce, or at least partially contributes to the carcinogenesis of HCC, through its V-oncoprotein HBx interacting with the C-oncoprotein RMP.

Functional implications of mitochondrial reactive oxygen species generated by oncogenic viruses

Between 15-20% of human cancers are associated with infection by oncogenic viruses. Oncogenic viruses, including HPV, HBV, HCV and HTLV-1, target mitochondria to influence cell proliferation and survival. Oncogenic viral gene products also trigger the production of reactive oxygen species which can elicit oxidative DNA damage and potentiate oncogenic host signaling pathways. Viral oncogenes may also subvert mitochondria quality control mechanisms such as mitophagy and metabolic adaptation pathways to promote virus replication. Here, we will review recent progress on viral regulation of mitophagy and metabolic adaptation and their roles in viral oncogenesis.

Design of a PROTAC that antagonizes and destroys the cancer-forming X-protein of the hepatitis B virus

The X-protein of the hepatitis B virus (HBV) is essential for virus infection and contributes to the development of HBV-induced hepatocellular carcinoma (HCC), a disease which causes more than one million deaths each year. Here we describe the design of a novel PROTAC (proteolysis targeting chimeric molecule) capable of simultaneously inducing the degradation of the X-protein, and antagonizing its function. The PROTAC was constructed by fusing the N-terminal oligomerization and C-terminal instability domains of the X-protein to each other, and rendering them cell-permeable by the inclusion of a polyarginine cell-penetrating peptide (CPP). It was predicted that the oligomerization domain would bind the X-protein, and that the instability domain would cause the X-protein to be targeted for proteasomal degradation. Addition of the PROTAC to HepG2 liver cancer cells, engineered to express full-length and C-terminally truncated forms of the X-protein, resulted in the degradation of both forms of the X-protein. A cell-permeable stand-alone form of the oligomerization domain was taken up by HepG2 cells, and acted as a dominant-negative inhibitor, causing inhibition of X-protein-induced apoptosis. In summary, the PROTAC described here induces the degradation of the X-protein, and antagonizes its function, and warrants investigation in a preclinical study for its ability to prevent or treat HBV infection and/or the development of HCC.

Proteomic identification, characterization and expression analysis of Ctenopharyngodon idella VDAC1 upregulated by grass carp reovirus infection

Voltage-dependent anion channels (VDACs) located in the mitochondrial outer membrane are mitochondrial porins that play central roles in regulating cell life and death. In this present report, the VDAC protein 1 from grass carp Ctenopharyngodon idella (designated as CiVDAC1) was found to be upregulated by grass carp reovirus (GCRV) infection through two-dimensional gel electrophoresis and protein analysis of infected C. idella kidney (CIK) cells. The full-length cDNA of CiVDAC1 was 995 bp with an open reading frame (ORF) of 852 bp that encodes a putative 283-amino acid protein. Phylogenic analysis revealed that the complete ORF of CiVDAC1 demonstrated high identity with well characterized mammalian homologs. The deduced CiVDAC1 protein contains an α-helix at the amino terminal, 19 membrane-spanning β-strands, and one eukaryotic mitochondrial porin signature motif. Tissue tropism analysis indicated that CiVDAC1 is abundant in muscle, heart, skin, swim bladder, trunk kidney and spleen. Transcriptional expression profiles indicated that the CiVDAC1 gene was upregulated upon viral challenge in a manner similar to the Mx2 gene, which is a marker gene used to indicate activation of innate antiviral immunity. Similar expression patterns of the CiVDAC1 gene were observed in CIK cells stimulated with poly (I:C), as well as grass carp kidney tissue challenged with GCRV in vivo. CiVDAC1 silencing in CIK cells had no impact on progeny virus production, but over-expression of CiVDAC1 in vivo showed strongly protect against challenge with live virus. To interpret the role of other VDAC proteins in viral pathogenesis, CiVDAC2 was characterized and showed to respond positively to GCRV challenge, which suggested that CiVDAC2 might functionally complement CiVDAC1 in C. idella. The present data did demonstrate that CiVDAC1 might be mediated grass carp antiviral immune response.

'Liver let die': oxidative DNA damage and hepatotropic viruses

Chronic infections by the hepatotropic viruses hepatitis B virus (HBV) and hepatitis C virus (HCV) are major risk factors for the development of hepatocellular carcinoma (HCC). It is estimated that more than 700,000 individuals per year die from HCC, and around 80 % of HCC is attributable to HBV or HCV infection. Despite the clear clinical importance of virus-associated HCC, the underlying molecular mechanisms remain largely elusive. Oxidative stress, in particular DNA lesions associated with oxidative damage, play a major contributory role in carcinogenesis, and are strongly linked to the development of many cancers, including HCC. A large body of evidence demonstrates that both HBV and HCV induce hepatic oxidative stress, with increased oxidative DNA damage being observed both in infected individuals and in murine models of infection. Here, we review the impact of HBV and HCV on the incidence and repair of oxidative DNA damage. We begin by giving a brief overview of oxidative stress and the repair of DNA lesions induced by oxidative stress. We then review in detail the evidence surrounding the mechanisms by which both viruses stimulate oxidative stress, before focusing on how the viral proteins themselves may perturb the cellular response to oxidative DNA damage, impacting upon genome stability and thus hepatocarcinogenesis.

Epigenetic silencing of NAD(P)H:quinone oxidoreductase 1 by hepatitis B virus X protein increases mitochondrial injury and cellular susceptibility to oxidative stress in hepatoma cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a phase II enzyme that participates in the detoxification of dopamine-derived quinone molecules and reactive oxygen species. Our prior work using a proteomic approach found that NQO1 protein levels were significantly decreased in stable hepatitis B virus (HBV)-producing hepatoma cells relative to the empty-vector-transfected controls. However, the mechanism and biological significance of the NQO1 suppression remain elusive. In this study we demonstrate that HBV X protein (HBx) induces epigenetic silencing of NQO1 in hepatoma cells through promoter hypermethylation via recruitment of DNA methyltransferase DNMT3A to the promoter region of the NQO1 gene. In HBV-related hepatocellular carcinoma (HCC) specimens, HBx expression was correlated negatively to NQO1 transcripts but positively to NQO1 promoter hypermethylation. Downregulation of NQO1 by HBx reduced intracellular glutathione levels, impaired mitochondrial function, and increased susceptibility of hepatoma cells to oxidative stress-induced cell injury. These results suggest a novel mechanism for HBV-mediated pathogenesis of chronic liver diseases, including HCC.

Hepatitis B virus X (HBx) play an anti-apoptosis role in hepatic progenitor cells by activating Wnt/β-catenin pathway

Increasing evidence has shown that normal stem cells may act as cancer-initiating cells and contribute to the development and progression of cancer. HBx has a close relationship with hepatocellular carcinoma, however, the role of HBx in hepatic progenitor cells (HPCs) is poorly understood. In this study, we sought to determine the role of HBx in regulating HPCs apoptosis and the underlying molecular mechanism(s) using HPCs derived from mouse fetal liver. The apoptotic ratio of HPCs infected with adenovirus-expressing HBx (Ad-HBx) was examined using flow cytometry. Results showed that the Ad-HBx treatment led to substantially decreased apoptotic ratio of HPCs, as confirmed by the Hoechst 33342 staining and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL). Possible alterations of relative proteins were examined using Western blot and real-time PCR assays. The HBx expression in HPCs increased the expression levels of Bcl2 and Mcl1 while decreasing the expression levels of Bax and cleaved caspase-9 and -3. In addition, the mRNA and protein expression levels of β-catenin were both increased. The β-catenin protein were mainly accumulated in cytoplasm and tended to transfer into cell nucleus after Ad-HBx treatment. The over-expression of β-catenin decreased the apoptotic ratio of HPCs and inhibited the expression of cleaved caspase-9 and -3 while blocking β-catenin expression resulted in the opposite results. Taken together, our results strongly suggested that the HBx protein may inhibits apoptosis of hepatic progenitor cells, at least in part by activating the WNT/β-catenin pathway. This provided a new insight into the molecular mechanism of HBx-mediated live carcinogenesis.

Hepatitis B Virus-Encoded X Protein Downregulates EGFR Expression via Inducing MicroRNA-7 in Hepatocellular Carcinoma Cells

Hepatitis B virus (HBV) infection accounts for over a half of cases of hepatocellular carcinoma (HCC), the most frequent malignant tumor of the liver. HBV-encoded X (HBx) plays critical roles in HBV-associated hepatocarcinogenesis. However, it is unclear whether and how HBx regulates the expression of epidermal growth factor receptor (EGFR), an important gene for cell growth. Therefore, the study aimed to investigate the association between HBx and EGFR expression. In this study, we found that HBx upregulates miR-7 expression to target 3′UTR of EGFR mRNA, which in turn results in the reduction of EGFR protein expression in HCC cells. HBx-mediated EGFR suppression renders HCC cells a slow-growth behavior. Deprivation of HBx or miR-7 expression or restoration of EGFR expression can increase the growth rate of HCC cells. Our data showed the miR-7-dependent EGFR suppression by HBx, supporting an inhibitory role of HBx in the cell growth of HCC. These findings not only identify miR-7 as a novel regulatory target of HBx, but also suggest HBx-miR-7-EGFR as a critical signaling in controlling the growth rate of HCC cells.

[Effects of hepatitis B virus X protein on chronic hepatitis B pathophysiology]

The infection by hepatitis B virus often promotes chronic liver inflammation which progresses to cirrhosis and hepatocellular carcinoma in a high percentage of patients. The persistent activation of the immune system causes an incessant liver damage, which fosters a disorganized stimulation of tissue repair and remodelling phenomena. In turn, the viral protein X (HBx) is essential for virus replication and therefore for the maintenance of chronic infection. However, the important oncogenic potential of HBx seems to reside, on one hand, in its ability to integrate into cellular DNA and, additionally, in the transactivation of different cellular signaling pathways involved in cell growth regulation, apoptosis and DNA repair. HBx also interacts with proteasome subunits and notably affects mitochondrial electric potential, thus altering cellular calcium homeostasis. Finally, this review discusses the pathogenic role of HBx in the progression of chronic hepatitis B through its effects on angiogenic, fibrogenic and oncogenic processes.

C-terminal region of HBx is crucial for mitochondrial DNA damage

HBx is strongly associated with hepatocellular carcinoma development through transcription factor activation and reactive oxygen species (ROSs) production. However, the exact role of HBx during hepatocellular carcinogenesis is not fully understood. Recently, it was reported that C-terminal truncated HBx is associated with tumor metastasis. In the present study, we confirmed that the C-terminal region of HBx is required for ROS production and 8-oxoguanine (8-oxoG) formation, which is considered as a reliable biomarker of oxidative stress. These results suggest ROS production induced by the C-terminal region of HBx leads to mitochondrial DNA damage, which may play a role in HCC development.

VDAC1: from structure to cancer therapy

Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to cancer. Found at the outer mitochondrial membrane, VDAC1 assumes a crucial position in the cell, controlling the metabolic cross-talk between mitochondria and the rest of the cell. Moreover, its location at the boundary between the mitochondria and the cytosol enables VDAC1 to interact with proteins that mediate and regulate the integration of mitochondrial functions with other cellular activities. As a metabolite transporter, VDAC1 contributes to the metabolic phenotype of cancer cells. This is reflected by VDAC1 over-expression in many cancer types, and by inhibition of tumor development upon silencing VDAC1 expression. Along with regulating cellular energy production and metabolism, VDAC1 is also a key protein in mitochondria-mediated apoptosis, participating in the release of apoptotic proteins and interacting with anti-apoptotic proteins. The involvement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space is discussed, as is VDAC1 oligomerization as an important step in apoptosis induction. VDAC also serves as an anchor point for mitochondria-interacting proteins, some of which are also highly expressed in many cancers, such as hexokinase (HK), Bcl2, and Bcl-xL. By binding to VDAC, HK provides both metabolic benefit and apoptosis-suppressive capacity that offers the cell a proliferative advantage and increases its resistance to chemotherapy. VDAC1-based peptides that bind specifically to HK, Bcl2, or Bcl-xL abolished the cell’s abilities to bypass the apoptotic pathway. Moreover, these peptides promote cell death in a panel of genetically characterized cell lines derived from different human cancers. These and other functions point to VDAC1 as a rational target for the development of a new generation of therapeutics.

EBV up-regulates cytochrome c through VDAC1 regulations and decreases the release of cytoplasmic Ca2+ in the NPC cell line

EBV (Epstein-Barr virus) is considered to be a major factor that causes NPC (nasopharyngeal carcinoma), which is one of the sneakiest cancers frequently occurring in Southeast Asia and Southern China. Apoptosis and pro-apoptotic signals have been studied for decades; however, few have extended the prevailing view of EBV to its impact on NPC in perspective of apoptosis. One of the important proteins named VDAC1 (voltage-dependent anion protein 1) on the mitochondrial outer membrane controls the pro-apoptotic signals in mammalian cells. The impact of EBV infection on VDAC1 and related apoptotic signals remains unclear. In order to study the VDAC1's role in EBV-infected NPC cells, we employ siRNA (small interfering RNA) inhibition to analyse the release of Ca2+ and Cyto c (cytochrome c) signals in the cytoplasm, as they are important pro-apoptotic signals. The results show a decrease of Ca2+ release and up-regulation of Cyto c with EBV infection. After siRNA transfection, the dysregulation of Cyto c is neutralized, which is evidence that the level of Cyto c release in virus-infected NPC cells is the as same as that of non-infected NPC cells. This result indicates that EBV infection changes the cytoplasmic level of Cyto c through regulating VDAC1. In summary, this study reports that EBV changes the release of Ca2+ and Cyto c in the cytoplasm of NPC cells, and that Cyto c changes are mediated by VDAC1 regulation.

Modulation of apoptotic signaling by the hepatitis B virus X protein

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

Hepatitis B virus X protein 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.

Role of hepatitis B virus X protein in regulating LIM and SH3 protein 1 (LASP-1) expression to mediate proliferation and migration of hepatoma cells

BACKGROUND

Hepatitis B virus X protein (HBx) has been shown to be responsible for the development of hepatocellular carcinoma (HCC) caused by Hepatitis B virus infection. However, its potential effect on the progression of hepatocellular carcinoma remains yet unclear. LIM and SH3 protein 1 (LASP-1), a focal adhesion protein, is expressed in an up-regulation manner in the HCC tissues. LASP-1 plays an important role in the regulation of proliferation and migration of HCC. In this study, we investigated the effect of LASP-1 involved in HBx-related tumor progression.

METHODS

LASP-1 levels in the HBx stable transfected HepG2 and Huh-7 cells were detected by RT-PCR and western blot analysis. The cellular localization of LASP-1 was assessed by immunofluorescence analysis. The activity of phosphatidylinositol 3-kinase (PI3-K) pathway was demonstrated by western blot assay. The HBx-expressing cells were transfected with specific small interference RNA (siRNA) against LASP-1. The proliferation and migration ability of cells were evaluated by cell viability assay and plate clone formation assay. The migration ability of cells was detected by transwell assay and wound healing assay.

RESULTS

RT-PCR and western blot analysis indicated the expression of LASP-1 was increased in the stable HBx-expressing cells compared with the control cells. Immunofluorescence study revealed that the distributions of LASP-1 in HepG2-HBX cells were mainly in pseudopods and the cytoplasm while they were mainly localized in the cytoplasm of HepG2-Mock cells. The cellular localizations of LASP-1 in Huh-7-HBX cells were in the perinuclear fractions while they were mainly localized in the cytoplasm of Huh-7-Mock cells. The upregulation of LASP-1 was inhibited after treatment with LY294002, PI3-K pathway inhibitor. Overexpression of LASP-1 in the stable HBx-expressing cells enhanced the proliferation and migration ability of hepatocellular cells. siRNA-mediated LASP-1 knowdown in the stable HBx-expressing cells significantly suppressed hepatocellular cells proliferation and migration.

CONCLUSIONS

These results demonstrated that HBx could upregulate LASP-1 through PI3-K pathway to promote the proliferation and migration of hepatoma cells.

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.

The X protein of hepatitis B virus activates hepatoma cell proliferation through repressing melanoma inhibitory activity 2 gene

Hepatocellular carcinoma (HCC) is the fourth leading cause of cancer deaths globally. Chronic hepatitis B virus (HBV) infection accounts for over 75% of all HCC cases; however, the molecular pathogenesis of HCC is not well understood. In this study, we found that the expression of the newly identified gene melanoma inhibitory activity 2 (MIA2) was reduced by HBV infection in vitro and in vivo, and that HBV X protein (HBx) plays a major role in this regulation. Recent studies have revealed that MIA2 is a potential tumor suppressor, and that, in most HCCs, MIA2 expression is down-regulated or lost. We found that the knock-down of MIA2 in HepG2 cells activated cell growth and proliferation, suggesting that MIA2 inhibits HCC cell growth and proliferation. In addition, the over-expression of HBx alone induced cell proliferation, whereas MIA2 over-expression impaired the HBx-mediated induction of proliferation. Taken together, our results suggest that HBx activates hepatoma cell growth and proliferation through repression of the potential tumor suppressor MIA2.

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

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

Hepatitis B and C virus hepatocarcinogenesis: lessons learned and future challenges

Worldwide, hepatocellular carcinoma (HCC) is one of the most common cancers. It is thought that 80% of hepatocellular carcinomas are linked to chronic infections with the hepatitis B (HBV) or hepatitis C (HCV) viruses. Chronic HBV and HCV infections can alter hepatocyte physiology in similar ways and may utilize similar mechanisms to influence the development of HCC. There has been significant progress towards understanding the molecular biology of HBV and HCV and identifying the cellular signal transduction pathways that are altered by HBV and HCV infections. Although the precise molecular mechanisms that link HBV and HCV infections to the development of HCC are not entirely understood, there is considerable evidence that both inflammatory responses to infections with these viruses, and associated destruction and regeneration of hepatocytes, as well as activities of HBV- or HCV-encoded proteins, contribute to hepatocyte transformation. In this review, we summarize progress in defining mechanisms that may link HBV and HCV infections to the development of HCC, discuss the challenges of directly defining the processes that underlie HBV- and HCV-associated HCC, and describe areas that remain to be explored.

HBx sensitizes cells to oxidative stress-induced apoptosis by accelerating the loss of Mcl-1 protein via caspase-3 cascade

BACKGROUND

Oxidative stress has been implicated in the pathogenesis of a wide spectrum of human diseases, including Hepatitis B virus (HBV)-related liver disease. Hepatitis B virus X protein (HBx) is a key regulator of HBV that exerts pleiotropic activity on cellular functions. Recent studies showed that HBx alters mitochondrial membrane potential, thereby sensitizing cells to pro-apoptotic signals. However, it remains largely unknown whether susceptibility of hepatocytes could be disturbed by HBx under oxidative stress conditions. The purpose of this study is to determine the apoptotic susceptibility of HBx-expressing hepatocytes upon exposure to pro-oxidant stimuli in vitro and in vivo and explore its underlying mechanism.

RESULTS

Although expression of HBx itself did not activate apoptotic signaling, it significantly enhanced oxidative stress-induced cell death both in vitro and in vivo. Interestingly, this phenomenon was associated with a pronounced reduction of protein levels of Mcl-1, but not other anti-apoptotic Bcl-2 members. Importantly, enforced expression of Mcl-1 prevented HBx-triggered cell apoptosis; conversely, specific knockdown of Mcl-1 exacerbated HBx-induced apoptosis upon exposure to oxidative stress. Furthermore, inhibition of caspase-3 not only abrogated HBx-triggered apoptotic killing but also blocked HBx-induced Mcl-1 loss. Additionally, expression of HBx and Mcl-1 was found to be inversely correlated in HBV-related hepatocellular carcinogenesis (HCC) tissues.

CONCLUSIONS

Our findings indicate that HBx exerts pro-apoptotic effect upon exposure to oxidative stress probably through accelerating the loss of Mcl-1 protein via caspase-3 cascade, which may shed a new light on the molecular mechanism of HBV-related hepatocarcinogenesis.

Association between Hepatitis B Virus X Gene Mutations and Clinical Status in Patients with Chronic Hepatitis B Infection

BACKGROUND/AIMS

Few reports have described the association between mutations in the entire X gene of the hepatitis B virus (HBV) and the clinical status of HBV-infected patients. We studied the association between HBV X gene mutations and the disease status of patients infected with HBV genotype C.

METHODS

Mutations in the HBV X genes of 194 patients were determined by direct sequencing. The subject population consisted of patients with chronic hepatitis (n=60), liver cirrhosis (n=65), and hepatocellular carcinoma (HCC) (n=69). The sequencing results of these 3 groups were compared.

RESULTS

Each of the mutations G1386M, C1485T, C1653T, T1753V, A1762T, and G1764A was significantly associated with the patient's clinical status. The T1753V (p<0.001) and A1762T/G1764A (p<0.001) mutations were found more frequently in Hepatitis B e antigen (HBeAg)-negative than in HBeAg-positive patients. Specific X gene mutations (G1386M, C1653T, and A1762T/G1764A) were more prevalent in patients with liver cirrhosis and HCC than in chronic hepatitis patients (p<0.005 for all). In addition, the T1753V (p<0.001) and C1485T (p<0.001) mutations were significantly more prevalent in HCC patients than in chronic hepatitis patients. Only the prevalence of the T1753V mutation increased as the HBV infection progressed from liver cirrhosis to HCC (p=0.023).

CONCLUSIONS

Our findings show a difference in the pattern of X gene mutations that were associated with the clinical status of patients with chronic HBV infection.

Hepatitis B virus X protein enhances cisplatin-induced hepatotoxicity via a mechanism involving degradation of Mcl-1

Hepatitis B virus X protein (HBx) is implicated in the pathogenesis of hepatitis B virus (HBV)-associated liver diseases. However, whether HBx has the ability to disturb the susceptibility of hepatocytes to common chemotherapeutic agents remains incompletely understood. Here we demonstrate that HBx enhances cisplatin-induced hepatotoxicity by a mechanism involving degradation of Mcl-1, an antiapoptotic member of the Bcl-2 family. Ectopic expression of HBx sensitized hepatocytes to cisplatin-induced apoptosis, which was accompanied by a marked downregulation of Mcl-1 but not of Bcl-2 or Bcl-xL. Overexpression of Mcl-1 prevented HBx-induced proapoptotic and proinflammatory effects during cisplatin treatment both in vitro and in vivo. HBx-induced dysregulation of Mcl-1 resulted mainly from posttranslational degradation rather than transcription repression. Moreover, a caspase-3 inhibitor effectively abrogated HBx-enhanced Mcl-1 degradation and cell death. Importantly, antioxidants blocked activation of caspase-3 and acceleration of Mcl-1 loss, as well as cell death, in HBx-expressing hepatocytes upon cisplatin exposure in vitro and in vivo. Collectively, these data implicate oxidative stress-dependent caspase-3-mediated degradation of Mcl-1 as a mechanism contributing to HBx-mediated sensitization of cisplatin-induced hepatotoxicity. A combination of cisplatin and antioxidants might provide more advantage than cisplatin alone in the treatment of cancer patients with chronic HBV infection.

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.

[Pro-apoptotic function of hepatitis B virus X protein]

Infection of hepatitis B virus (HBV) is a main cause of liver diseases including hepatitis, cirrhosis and hepatocellular carcinoma (HCC). Among the HBV-encoded proteins, the HBV X protein (HBx) has been suspected to be strongly involved in HBV-associated liver pathogenesis. HBx, a virally encoded multifunctional regulator, has been shown to induce apoptosis, anti-apoptosis, proliferation, and transformation of cells depending on the cell lines, model systems used, assay protocols, and research groups. Among the several activities of HBx, the pro-apoptotic function of HBx will be discussed in this review. Given that the disruption of apoptosis pathway by HBx contributes to the liver pathogenesis, a better understanding of the molecular interference in the cellular pro-apoptotic networks by HBx will provide useful clues for the intervention in HBV-mediated liver diseases.

Redox regulation of T-cell turnover by the p13 protein of human T-cell leukemia virus type 1: distinct effects in primary versus transformed cells

The present study investigated the function of p13, a mitochondrial protein of human T-cell leukemia virus type 1 (HTLV-1). Although necessary for viral propagation in vivo, the mechanism of function of p13 is incompletely understood. Drawing from studies in isolated mitochondria, we analyzed the effects of p13 on mitochondrial reactive oxygen species (ROS) in transformed and primary T cells. In transformed cells (Jurkat, HeLa), p13 did not affect ROS unless the cells were subjected to glucose deprivation, which led to a p13-dependent increase in ROS and cell death. Using RNA interference we confirmed that expression of p13 also influences glucose starvation-induced cell death in the context of HTLV-1–infected cells. ROS measurements showed an increasing gradient from resting to mitogen-activated primary T cells to transformed T cells (Jurkat). Expression of p13 in primary T cells resulted in their activation, an effect that was abrogated by ROS scavengers. These findings suggest that p13 may have a distinct impact on cell turnover depending on the inherent ROS levels; in the context of the HTLV-1 propagation strategy, p13 could increase the pool of “normal” infected cells while culling cells acquiring a transformed phenotype, thus favoring lifelong persistence of the virus in the host.

The Marsupenaeus japonicus voltage-dependent anion channel (MjVDAC) protein is involved in white spot syndrome virus (WSSV) pathogenesis

Voltage-dependent anion channel (VDAC) proteins abound in the outer membrane of mitochondria. They play an important role in mitochondrial membrane permeabilization (MMP), which can lead to stress-induced cellular apoptosis and necrosis. Several pathogens regulate this MMP in their host cells to benefit their replication cycle, while in other cases, the host can use the same mechanism to combat pathogenesis. In this study, the first shrimp VDAC gene was identified and characterized from Marsupenaeus japonicus (MjVDAC). Its open reading frame (ORF) contained 849 bp encoding 282 amino acids. The deduced MjVDAC protein includes the 4-element eukaryotic porin signature motif, the conserved ATP binding motif (the GLK motif) and a VKAKV-like sequence known in other organisms to be involved in the protein's incorporation in the mitochondrial outer membrane. Tissue tropism analysis indicated that MjVDAC is abundant in the heart, muscle, stomach and pleopod. MjVDAC proteins colocalized with mitochondria in transiently transfected Sf9 cells and in shrimp hemocytes. dsRNA silencing of shrimp VDAC delayed white spot syndrome virus (WSSV) infection by 1 day in different shrimp organs. Taken together, these findings suggest that MjVDAC is likely to be involved in WSSV pathogenesis.

Increase of hepatic fat accumulation by liver specific expression of Hepatitis B virus X protein in zebrafish

The pathogenesis of fatty liver disease remains largely unknown. Here, we assessed the importance of hepatic fat accumulation on the progression of hepatitis in zebrafish by liver specific expression of Hepatitis B virus X protein (HBx). Transgenic zebrafish lines, GBXs, which selectively express the GBx transgene (GFP-fused HBx gene) in liver, were established. GBX Liver phenotypes were evaluated by histopathology and molecular analysis of fatty acid (FA) metabolism-related genes expression. Most GBXs (66-81%) displayed obvious emaciation starting at 4 months old. Over 99% of the emaciated GBXs developed hepatic steatosis or steatohepatitis, which in turn led to liver hypoplasia. The liver histology of GBXs displayed steatosis, lobular inflammation, and balloon degeneration, similar to non-alcoholic steatohepatitis (NASH). Oil red O stain detected the accumulation of fatty droplets in GBXs. RT-PCR and Q-rt-PCR analysis revealed that GBx induced hepatic steatosis had significant increases in the expression of lipogenic genes, C/EBP-alpha, SREBP1, ChREBP and PPAR-gamma, which then activate key enzymes of the de novo FA synthesis, ACC1, FAS, SCD1, AGAPT, PAP and DGAT2. In addition, the steatohepatitic GBX liver progressed to liver degeneration and exhibited significant differential gene expression in apoptosis and stress. The GBX models exhibited both the genetic and functional factors involved in lipid accumulation and steatosis-associated liver injury. In addition, GBXs with transmissible NASH-like phenotypes provide a promising model for studying liver disease.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

TNF-alpha-induced ROS production triggering apoptosis is directly linked to Romo1 and Bcl-X(L)

Reactive oxygen species (ROS) produced by tumor necrosis factor-alpha (TNF-alpha) have an important function in cell death by activating c-Jun N-terminal kinase. However, the exact mechanism of mitochondrial ROS production, after TNF-alpha stimulation, is not clearly understood. In this study, we determined that ROS modulator 1 (Romo1) and B-cell lymphoma-extra large (Bcl-X(L)) are directly associated with TNF-alpha-induced ROS production. In response to TNF-alpha, TNF complex II, which consists of receptor-interacting protein 1, TNF receptor-associated protein with death domain, TNF receptor-associated factor 2, Fas-associated death domain protein, and pro-caspase-8, binds to the C-terminus of Romo1 located in the mitochondria. Concurrently, Romo1 recruits Bcl-X(L) to reduce the mitochondrial membrane potential, resulting in ROS production and apoptotic cell death. On the basis of these results, we suggest that Romo1 is a molecular bridge between TNF-alpha signaling and the mitochondria for ROS production that triggers TNF-alpha-mediated apoptosis, as well as a novel target in the development of anti-inflammatory agents that block the origin of ROS production.

HBx targeting to mitochondria and ROS generation are necessary but insufficient for HBV-induced cyclooxygenase-2 expression

Chronic inflammation can be a major risk factor for cancer development and may contribute to the high worldwide incidence of hepatocellular carcinoma (HCC). Cyclooxygenase-2 (COX-2) is known to be an important mediator of inflammatory responses; however, its link to hepatitis B virus (HBV)-mediated inflammatory responses has not been established. Here, we demonstrate that the expression of COX-2 mRNA and protein was significantly elevated in cells transfected by HBV replicon but not in cells transfected by HBV genome lacking the HBx gene. Notably, COX-2 induction was correlated with HBx's ability to increase reactive oxygen species (ROS) levels. Consistently with this, antioxidant treatment and ectopic expression of manganese superoxide dismutase or catalase completely abolished COX-2 induction. Interestingly, a mitochondria localization-defective mutant of HBx (HBx(Delta 68-117)) neither increased intracellular ROS levels nor induced COX-2 expression. HBx(68-117), which encodes only amino acids 68-117 and is sufficient for mitochondria localization, increased ROS levels but did not induce COX-2 expression. Similarly, HBx targeting to the outer membrane of mitochondria (Mito-HBx) increased ROS but also failed to increase COX-2 expression, suggesting that other cytoplasmic signaling pathways are involved in HBx-mediated COX-2 induction. Indeed, inhibition of cytoplasmic calcium signaling by cyclosporine A, blocking mitochondrial permeability transition pore, and herbimycin, and inhibition of calcium-dependent tyrosine kinase suppressed HBV-mediated COX-2 induction. Thus, the data indicate that both mitochondrial ROS and cytoplasmic calcium signaling are necessary for the COX-2 induction. Our studies revealed a pathophysiological link between HBV infection and hepatic inflammation, and this chain of events might contribute to early steps in HBV-associated liver carcinogenesis.

Involvement of mitochondrial permeability transition in hepatitis B virus replication

The HBx protein of human hepatitis B virus (HBV) activates a calcium-dependent kinase pathway which is essential for the viral replication. In this study, we found that HBx expression in the absence of other HBV proteins and in the context of HBV replication decreased the mitochondrial calcein-AM/CoCl(2) signals by 10% and 14% in HepG2 cells and by 15% and 10% in Huh7 cells, respectively. This indicates that HBx can induce mitochondrial permeability transition (MPT) and cause calcium effusion into the plasma. In addition, RNA interference of Cylophilin D decreased HBx-induced MPT and suppressed HBV DNA replication by 41% in HepG2 cells. Our results suggest that HBx expression can induce MPT and facilitate HBV DNA replication.

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.

Hepatitis B virus X protein modulates apoptosis in primary rat hepatocytes by regulating both NF-kappaB and the mitochondrial permeability transition pore

The hepatitis B virus (HBV) X protein (HBx) is a multifunctional protein that regulates numerous cellular signal transduction pathways, including those that modulate apoptosis. However, different HBx-dependent effects on apoptosis have been reported; these differences are likely the consequence of the exact conditions and cell types used in a study. Many of the previously reported studies that analyzed HBx regulation of apoptosis were conducted in immortalized or transformed cells, and the alterations that have transformed or immortalized these cells likely impact apoptotic pathways. We examined the effects of HBx on apoptotic pathways in cultured primary rat hepatocytes, a biologically relevant system that mimics normal hepatocytes in the liver. We analyzed the effects of HBx on apoptosis both when HBx was expressed in the absence of other HBV proteins and in the context of HBV replication. HBx stimulation of NF-kappaB inhibited the activation of apoptotic pathways in cultured primary rat hepatocytes. However, when HBx-induced activation of NF-kappaB was blocked, HBx stimulated apoptosis; blocking the activity of the mitochondrial permeability transition pore inhibited HBx activation of apoptosis. These results suggest that HBx can be either proapoptotic or antiapoptotic in hepatocytes, depending on the status of NF-kappaB, and confirm previous studies that link some HBx activities to modulation of the mitochondrial permeability transition pore. Overall, our studies define apoptotic pathways that are regulated by HBx in cultured primary hepatocytes and provide potential mechanisms for the development of HBV-associated liver cancer.

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

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

[Molecular mechanisms of hepatitis B virus-associated hepatocellular carcinoma]

Hepatocellular carcinoma (HCC) is one of the most common malignant diseases in the world. The hepatitis B virus (HBV) replicates non-cytopathically in hepatocytes, and most of the liver injury associated with this infection reflects the immune response. Epidemiological studies have clearly demonstrated that a chronic HBV infection is a major etiological factor in the development of HCC. The pathogenesis of HBV-associated HCC has been studied extensively, and the molecular changes during the malignant transformation have been identified. The main carcinogenic mechanism of HBV-associated HCC is related to the long term-inflammatory changes caused by a chronic hepatitis B infection, which might involve the integration of the HBV. Integration of the HBV DNA into the host genome occurs at the early steps of clonal tumorous expansion. The hepatitis B x protein (HBx) is a multifunctional regulatory protein that communicates directly or indirectly with a variety of host targets, and mediates many opposing cellular functions, including its function in cell cycle regulation, transcriptional regulation, signaling, encoding of the cytoskeleton and cell adhesion molecules, as well as oncogenes and tumor suppressor genes. Continued study of the mechanisms of hepatocarcinogenesis will refine our current understanding of the molecular and cellular basis for neoplastic transformations in the liver. This review summarizes the current knowledge of the mechanisms involved in HBV-associated hepatocarcinogenesis.

Identification of functionally important amino acid residues in the mitochondria targeting sequence of hepatitis B virus X protein

Chronic hepatitis B virus (HBV) infection has been strongly associated with hepatocellular carcinoma (HCC) and the X protein (HBx) is thought to mediate the cellular changes associated with carcinogenesis. Recently, isolation of the hepatitis B virus integrants from HCC tissue by others have established the fact that the X gene is often truncated at its C-terminus. Expression of the GFP fusion proteins of HBx and its truncation mutants with a GFP tag in human liver cell-lines in this study revealed that the C-terminus of HBx is indispensable for its specific localization in the mitochondria. A crucial region of seven amino acids at the C-terminus has been mapped out in which the cysteine residue at position 115 serves as the most important residue for the subcellular localization. When cysteine 115 of HBx is mutated to alanine the mitochondria targeting property of HBx is abrogated.

HBV X protein: elucidating a role in oncogenesis

Chronic HBV infection is associated with the development of hepatocellular carcinoma (HCC). HBV contributes to tumorigenesis by encoding hepatitis B x antigen (HBxAg), which is a trans-regulatory protein that appears to contribute to HCC by altering patterns of host gene expression. In this review, recent data is presented that outlines some of the putative mechanisms whereby HBxAg contributes to HCC. With the development of animal models of HBxAg-mediated HCC, the relevance and temporal order of putative steps in this process can now be dissected to elucidate what is rate limiting and when. This will have a profound impact on the design of novel and specific therapeutics for HCC.

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

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

HTLV-1 and apoptosis: role in cellular transformation and recent advances in therapeutic approaches

A universal cellular defense mechanism against viral invasion is the elimination of infected cells through apoptotic cell death. To counteract host defenses many viruses have evolved complex apoptosis evasion strategies. The oncogenic human retrovirus HTLV-1 is the etiological agent of adult-T-cell leukemia/lymphoma (ATLL) and the neurodegenerative disease known as HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The poor prognosis in HTLV-1-induced ATLL is linked to the resistance of neoplastic T cells against conventional therapies and the immuno-compromised state of patients. Nevertheless, several studies have shown that the apoptotic pathway is largely intact and can be reactivated in ATLL tumor cells to induce specific killing. A better understanding of the molecular mechanisms employed by HTLV-1 to counteract cellular death pathways remains an important challenge for future therapies and the treatment of HTLV-1-associated diseases.

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

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

Mitochondrial dysfunction in lyssavirus-induced apoptosis

Lyssaviruses are highly neurotropic viruses associated with neuronal apoptosis. Previous observations have indicated that the matrix proteins (M) of some lyssaviruses induce strong neuronal apoptosis. However, the molecular mechanism(s) involved in this phenomenon is still unknown. We show that for Mokola virus (MOK), a lyssavirus of low pathogenicity, the M (M-MOK) targets mitochondria, disrupts the mitochondrial morphology, and induces apoptosis. Our analysis of truncated M-MOK mutants suggests that the information required for efficient mitochondrial targeting and dysfunction, as well as caspase-9 activation and apoptosis, is held between residues 46 and 110 of M-MOK. We used a yeast two-hybrid approach, a coimmunoprecipitation assay, and confocal microscopy to demonstrate that M-MOK physically associates with the subunit I of the cytochrome c (cyt-c) oxidase (CcO) of the mitochondrial respiratory chain; this is in contrast to the M of the highly pathogenic Thailand lyssavirus (M-THA). M-MOK expression induces a significant decrease in CcO activity, which is not the case with M-THA. M-MOK mutations (K77R and N81E) resulting in a similar sequence to M-THA at positions 77 and 81 annul cyt-c release and apoptosis and restore CcO activity. As expected, the reverse mutations, R77K and E81N, introduced in M-THA induce a phenotype similar to that due to M-MOK. These features indicate a novel mechanism for energy depletion during lyssavirus-induced apoptosis.

Progressive changes in hepatoma cells stably transfected with hepatitis B virus X gene

OBJECTIVE

The aim of this study is to investigate the molecular mechanism of hepatocellular carcinoma (HCC) development induced by hepatitis B virus X protein (HBx).

METHODS

We previously established a H7402-X cell line that constitutively expresses HBx protein. In the present study, H7402-X gene expression profiles and proteins were examined using cDNA microarrays and Western blot analysis. Apoptosis was induced by adriamycin in H7402-X cells. The transcriptional activities of NF-kappaB and AP-1 were examined using a luciferase reporter gene.

RESULTS

The DNA expression profiles identified candidate genes showing aberrant expression in cells overexpressing HBx. Western blot analysis showed that cyclin D, cyclin E, survivin, Bcl-2, and PCNA were up-regulated, whereas p27 was down-regulated in H7402-X cells. Treatment with RNAi targeting HBx mRNA led to the down-regulation of these genes. H7402-X cells were resistant to adriamycin-induced apoptosis. Luciferase reporter gene analysis revealed that HBx induces the transcriptional activities of NF-kappaB and AP-1.

CONCLUSION

Our data provide additional insight into cellular targets of HBx, which allows a better understanding of HBx function and the progressive changes during HBx-mediated hepatocarcinogenesis.

HBx modulates iron regulatory protein 1-mediated iron metabolism via reactive oxygen species

Hepatitis B virus X protein (HBx) is involved in viral metabolism and progression of liver disease. Iron metabolism plays a significant role in liver disease. In this report, to elucidate the relationship between iron metabolism and HBx, we established the Huh7 cell lines in which HBx was stably expressed (Huh7-HBx). In Huh7-HBx, we observed that transferrin receptor 1 (TfR1) expression decreased and ferritin heavy chain (FtH) expression increased as well as reactive oxygen species (ROS) level increased. We also found that these modulations were caused by the downregulation of iron regulatory protein 1 (IRP1). Furthermore, the levels of total iron and labile iron pool (LIP) were altered in Huh7-HBx. In addition, antioxidant N-acetylcystein (NaC) increased IRP1 expression by depleting HBx-induced ROS. We also confirmed these alterations of TfR1 and FtH in the primary hepatocytes of HBx transgenic mice and in HepG2.2.15 cells that constitutively replicate the intact HBV genome. In conclusion, these results suggest that HBx modulates iron metabolism via ROS leading to pathological status in liver diseases.

Mitochondrial Ca2+ homeostasis in lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a class of genetic disorders in which proteins responsible for digestion or absorption of endocytosed material do not function or do not localize properly. The resulting cellular "indigestion" causes buildup of intracellular storage inclusions that contain unprocessed lipids and proteins that form macromolecular complexes. The buildup of storage material is associated with degenerative processes that are observed in all LSDs, albeit the correlation between the amount of storage inclusions and the severity of the degenerative processes is not always evident. The latter suggests that a specific mechanism set in motion by aberrant lysosomal function drives the degenerative processes in LSDs. It is becoming increasingly clear that in addition to their function in degrading endocytosed material, lysosomes are essential housekeeping organelles responsible for maintaining healthy population of intracellular organelles, in particular mitochondria. The present review surveys the current knowledge on the lysosomal-mitochondrial axis and its possible role as a contributing factor to mitochondrial Ca(2+) homeostasis and to cell death in LSDs.

Molecular Pathogenesis of Hepatitis-B-virus-associated Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most frequent and malignant diseases worldwide. Epidemiological studies have clearly demonstrated that chronic hepatitis B virus (HBV) infection is a major etiological factor in the development of HCC. The pathogenesis of HBV-associated HCC has been studied extensively, and the molecular changes associated with malignant transformation have been identified. The predominant carcinogenic mechanisms of HBV-associated HCC are chronic inflammation and the effects of cytokines in the development of fibrosis and liver cell proliferation. An important role is also played by the integration of HBV DNA into host cellular DNA, which disrupts or promotes the expression of cellular genes that are important in cell growth and differentiation. Especially, HBx protein is a transactivating protein that promotes cell growth, survival, and the development of HCC. Continued investigation of the mechanisms underlying hepatocarcinogenesis will refine our current understanding of the molecular and cellular basis for neoplastic transformation in the liver. Prevention of HBV infections and effective treatments for chronic hepatitis B are still needed for the global control of HBV-associated HCC. This review summarizes the current knowledge on the mechanisms involved in HBV-associated hepatocarcinogenesis.

Hepatitis B virus X protein induces apoptosis and cell cycle deregulation through interfering with DNA repair and checkpoint responses

AIM

To investigate the effects of hepatitis B virus X (HBx) gene on apoptosis and cell cycle in hepatocyte line HL-7702 and to discuss the possible mechanisms in the pathway.

METHODS

The recombinant plasmid pcDNA3-X and vector pcDNA3 were transfected into HL-7702 cells and selected by G418 to construct two new cell lines, which were named HL-7702-HBx and HL-7702-con, respectively. Reverse transcription polymerase chain reaction (RT-PCR) and western blot analysis were used to confirm that HBx gene was expressed steadily in the HL-7702-HBx cells. Then apoptosis and cell cycle of the two cells were detected by DNA ladder, flow cytometric analysis, and electronic microscope observation. Apoptosis and cell cycle gene expressions in the two cells were subsequently evaluated by using gene arrays. Some of results were further confirmed by real-time PCR and western blot analysis.

RESULTS

RT-PCR and the western blot analysis showed that HL-7702-HBx expressed the HBx gene steadily. Comparedwith the HL-7702-con cells, there was increased apoptosis and accumulation of the S phase in the HL-7702-HBx cells. The gene array analysis indicated that some DNA repair genes (XRCC1, DDB1, etc.) and DNA damage checkpoint-related genes (Cdc47, RAD17, etc.) played roles in the HBx-mediated imbalance of apoptosis and cell cycle. Both cDNA array analysis and real-time RT-PCR showed that mRNA of XRCC1, Cdc47 and RAD17 were upregulated by HBx. Unexpectedly, the western blot analysis revealed that HBx inhibited their protein expression.

CONCLUSION

The expression of HBx in HL-7702 cells promoted apoptosis and accumulation of the S phase through the inhibition of DNA repair and checkpoints via post-transcriptional mechanisms.