Activated ras oncogene collaborates with HBx gene of hepatitis B virus to transform cells by suppressing HBx-mediated apoptosis

Viral Hepatitis and Hepatocellular Carcinoma: State of the Art

Viral hepatitis is one of the main causes leading to hepatocellular carcinoma (HCC). The continued rise in incidence of HCC suggests additional factors following infection may be involved. This review examines recent studies investigating the molecular mechanisms of chronic hepatitis and its association with hepatocarcinogenesis. Hepatitis B virus patients with genotype C display an aggressive disease course leading to HCC more than other genotypes. Furthermore, hepatitis B excretory antigen (HBeAg) seems to be a more sensitive predictive tumor marker exhibiting a six-fold higher relative risk in patients with positive HBsAg and HBeAg than those with HBsAg only. Single or combined mutations of viral genome can predict HCC development in up to 80% of patients. Several mutations in HBx-gene are related with higher HCC incidence. Overexpression of the core protein in HCV leads to hepatocellular lipid accumulation associated with oncogenesis. Reduced number and decreased functionality of natural killer cells in chronic HCV individuals dysregulate their surveillance function in tumor and viral cells resulting in HCC. Furthermore, high T-cell immunoglobulin and mucin 3 levels supress CD8+ T-cells, which lead to immunological dysregulation. Hepatitis D promotes HCC development indirectly via modifications to innate immunity, epigenetic alterations and production of reactive oxygen species with the LHDAg being the most highly associated with HCC development. Summarizing the results, HBV and HCV infection represent the most associated forms of viral hepatitis causing HCC. Further studies are warranted to further improve the prediction of high-risk patients and development of targeted therapeutics preventing the transition from hepatic inflammation–fibrosis to cancer.

C-terminal-truncated hepatitis B virus X protein promotes hepatocarcinogenesis by activating the MAPK pathway

PURPOSE

C-terminally truncated hepatitis B virus X (ctHBx) is frequently detected in hepatocellular carcinoma (HCC) patients with hepatitis B virus (HBV) integrated into their genomes, but the molecular mechanisms of ctHBx-related oncogenic signaling remain unclear. In this study, the effects of ctHBx on HepG2 cells were investigated by measuring ctHBx-induced changes in the cell cycle-related target proteins cell division cycle 25C (cdc25C) and p53 downstream of the mitogen-activated protein kinase (MAPK) pathway.

MATERIALS AND METHODS

ctHBx lentiviruses were constructed and transfected into HepG2 cells. Then, we investigated HepG2 cell line function by conducting the Cell Counting Kit-8 (CCK8) assay, clone formation assay, scratch wound testing, Transwell assays and flow cytometry to examine cell cycle and apoptosis. Western blotting (WB) was performed to detect proteins related to and downstream of the extracellular signal-regulated kinase(ERK)/c-Jun N-terminal kinase(JNK)/p38 MAPK pathway, including cdc25C and p53.

RESULTS

ctHBx significantly enhanced the proliferation, migration, invasion and colony-forming capability of HepG2 cells. In addition, ctHBx activated the ERK/JNK/p38 MAPK signaling pathway to regulate cell viability by affecting the expression of cyclin-related proteins, including cdc25C and p53.

CONCLUSION

The present study demonstrates that ctHBx promote the formation and development of HCC via regulating MAPK/cdc25C and p53 axis. ctHBx should be the driving factor of HBV-induced hepatocarcinogenesis.

Synergistic Effect of MC-LR and C-Terminal Truncated HBx on HepG2 Cells and Their Effects on PP2A Mediated Downstream Target of MAPK Signaling Pathway

C-terminally truncated hepatitis B virus (HBV) X (ctHBx) infection and exposure to microcystins-LR (MC-LR) can lead to human hepatitis and liver cancer, but the mechanism associated with their synergistically effects not been fully elucidated. The ctHBx (HBxΔ4 and HBxΔ32) lentivirus were constructed and transfected into the HepG2 cells. Then we investigated the function of MC-LR and ctHBx using the molecular biology approaches, including enzyme-linked immunosorbent assay, clone formation assay, scratch wound testing, transwell assays, carried out flow cytometry respectively to examine cell cycle and apoptosis in each group, and detected the related proteins of HBx, MEK/ERK/JNK/p38 in mitogen-activated protein kinase (MAPK) pathway and the downstream proteins such as cdc2, cdc25C, and p53 by western blotting. We found that the protein phosphorylase 2A (PP2A) enzyme activity in MC-LR and HBxΔ32/HBxΔ4 groups decreased more than in MC-LR and HBx group at the same time point and MC-LR concentration (P < 0.05). Meanwhile the proliferation, migration, invasion and colony formation capability of HepG2 cells were significantly enhanced in MC-LR and ctHBx groups (P < 0.05). In addition the proportion of S stage cells in the MC-LR-treated HBxΔ32/HBxΔ4 groups was significantly greater than that in the untreated groups (P < 0.05). Furthermore, the protein expression of MAPK signaling pathway including phospho-MEK1/2, ERKl/2, p38, and JNK were up-regulated by MC-LR and HBxΔ32, and the expression of cyclin-related proteins, including p53, cdc25C, and cdc2 were also activated (P < 0.05). Taken together, our findings revealed the essential significance of the MC-LR and ctHBx on the PP2A/MAPK/p53, cdc25C and cdc2 axis in the formation and development of HCC and identified MC-LR and ctHBx as potential causal cofactors of hepatocarcinogenesis.

Hepatitis B Virus-X Downregulates Expression of Selenium Binding Protein 1

Selenium binding protein 1 (SELENBP1) has been known to be reduced in various types cancer, and epigenetic change is shown to be likely to account for the reduction of SELNEBP1 expression. With cDNA microarray comparative analysis, we found that SELENBP1 is markedly decreased in hepatitis B virus-X (HBx)-expressing cells. To clarify the effect of HBx on SELENBP1 expression, we compared the expression levels of SELENBP1 mRNA and protein by semi-quantitative RT-PCR, Northern blot, and Western blot. As expected, SELENBP1 expression was shown to be reduced in cells expressing HBx, and reporter gene analysis showed that the SELENBP1 promoter is repressed by HBx. In addition, the stepwise deletion of 5′ flanking promoter sequences resulted in a gradual decrease in basal promoter activity and inhibition of SELENBP1 expression by HBx. Moreover, immunohistochemistry on tissue microarrays containing 60 pairs of human liver tissue showed decreased intensity of SELENBP1 in tumor tissues as compared with their matched non-tumor liver tissues. Taken together, our findings suggest that inhibition of SELENBP1 expression by HBx might act as one of the causes in the development of hepatocellular carcinoma caused by HBV infection.

Genetic profiling of hepatocellular carcinoma using next-generation sequencing

Hepatocellular carcinoma (HCC) is a highly heterogeneous disease, both clinically and from a molecular standpoint. The advent of next-generation sequencing technologies has provided new opportunities to extensively analyze molecular defects in HCC samples. This has uncovered major cancer driver genes and associated oncogenic pathways operating in HCC. More sophisticated analyses of sequencing data have linked specific nucleotide patterns to external toxic agents and defined so-called 'mutational signatures' in HCC. Molecular signatures, taking into account intra- and inter-tumor heterogeneity, and their functional validation could provide useful data to predict treatment response to molecular therapies. In this review we will focus on the current knowledge of deep sequencing in HCC and its foreseeable clinical impact.

Structural and biochemical analysis of Bcl-2 interaction with the hepatitis B virus protein HBx

HBx is a hepatitis B virus protein that is required for viral infectivity and replication. Anti-apoptotic Bcl-2 family members are thought to be among the important host targets of HBx. However, the structure and function of HBx are poorly understood and the molecular mechanism of HBx-induced carcinogenesis remains unknown. In this study, we report biochemical and structural characterization of HBx. The recombinant HBx protein contains metal ions, in particular iron and zinc. A BH3-like motif in HBx (residues 110-135) binds Bcl-2 with a dissociation constant of ∼193 μM, which is drastically lower than that for a canonical BH3 motif from Bim or Bad. Structural analysis reveals that, similar to other BH3 motifs, the BH3-like motif of HBx adopts an amphipathic α-helix and binds the conserved BH3-binding groove on Bcl-2. Unlike the helical Bim or Bad BH3 motif, the C-terminal portion of the bound HBx BH3-like motif has an extended conformation and makes considerably fewer interactions with Bcl-2. These observations suggest that HBx may modulate Bcl-2 function in a way that is different from that of the classical BH3-only proteins.

Oncogenic circuit constituted by Ser31-HBx and Akt increases risks of chronic hepatitis and hepatocellular carcinoma

The X protein of hepatitis B virus (HBx) has been specifically implicated in the development of hepatocellular carcinoma (HCC). Clinical associations of HBx isoforms with chronic hepatitis and HCC have not been well studied. HBx has two roles in liver cells, namely pro-apoptotic and anti-apoptotic. In this report, we examined the role of Ser31-HBx in HCC and chronic hepatitis. Using the case-control study, we determined risks of chronic hepatitis and HCC conferred by hepatitis B virus (HBV) containing Ser31-HBx that was phosphorylated by Akt. Ser31-HBx isoforms conferred 3.23-fold risk of HCC in male and 3.36-fold risk in female. Ser31 isoforms were associated with 3.12-fold risk of chronic hepatitis and 3.43-fold risk of cirrhosis and also associated with higher HBV viral load and replication efficiency and lower rate of HBe loss. To determine the mechanism, we found that Ser31-HBx constituted an oncogenic circuit with Akt and cooperated with ras to transform NIH3T3 cells in contrast to non-Ser31-HBxs that did not transduce oncogenic signals. Our results give a clue to account for an underlying cause of HBx-mediated hepatocarcinogenesis. It appears that Ser31 phosphorylation of HBx by Akt plays an important role. The current study provides an example of association of HBV genome variations with risks of HCC and chronic hepatitis.

The HBx oncoprotein of hepatitis B virus engages nucleophosmin to promote rDNA transcription and cellular proliferation

The pleiotropic HBx oncoprotein of hepatitis B virus is well known to promote the expression of ribosomal RNAs and several host proteins that are known to support the development and progression of hepatocellular carcinoma (HCC). While overexpression of the nucleolar phosphoprotein, nucleophosmin (NPM), correlates with HCC progression, its upregulation by viral HBx and the resulting impact on perturbed nucleolar functions remain enigmatic. The present study shows that HBx up-regulates NPM levels and hijacks its functions to promote cellular proliferation. We found that HBx expression stabilizes NPM through post-translational modifications. Enhanced CDK2-mediated phosphorylation of NPM at Thr199 upon HBx expression prevented its proteolytic cleavage and provided resistance to apoptosis. Further, HBx directly interacted with the C-terminal domain of NPM and got translocated into the nucleolus where it facilitated the recruitment of RNA polymerase I transcriptional machinery onto the rDNA promoter. Our results indicate that HBx enhances rDNA transcription via a novel regulatory mechanism involving acetylation of NPM and the subsequent depletion of histones from the rDNA promoter. Enhanced production of ribosomal RNA resulting from co-expression of HBx and NPM promoted ribosome biogenesis, cellular proliferation and transformation. Taken together, our study strongly suggests an important role of NPM in mediating the oncogenic effects of HBx and the corresponding nucleolar perturbations induced by this viral oncoprotein.

Pathogenesis of hepatocellular carcinoma according to aetiology

Hepatocellular carcinoma is related to various etiologies including hepatitis B, hepatitis C, high alcohol intake, aflatoxin B1 and metabolic syndrome. Most of the time HCC developed on cirrhosis. Consequently, the mechanisms of carcinogenesis of these different risk factors are difficult to separate from the events leading to cirrhosis. In contrast, aflatoxin B1 and hepatitis B have a clear direct oncogenic role through point mutations in the TP53 tumour suppressor gene and insertional mutagenesis respectively. Finally, next-generation sequencing and transcriptome analysis will refine our knowledge of the relationship between aetiology and the genetic events that draw the mutational landscape of hepatocellular carcinoma.

Hepatitis B virus X protein accelerates hepatocarcinogenesis with partner survivin through modulating miR-520b and HBXIP

BACKGROUND

Hepatitis B virus X protein (HBx) plays crucial roles in hepatocarcinogenesis. However, the underlying mechanism remains elusive. We have reported that HBx is able to up-regulate survivin in hepatocellular carcinoma tissues. The oncopreotein hepatitis B X-interacting protein (HBXIP), a target of miR-520b, is involved in the development of cancer. In this study, we focus on the investigation of hepatocarcinogenesis mediated by HBx.

METHODS

The expression of HBx and survivin was examined in the liver tissues of HBx-Tg mice. The effect of HBx/survivin on the growth of LO2-X-S cells was determined by colony formation and transplantation in nude mice. The effect of HBx/survivin on promoter of miR-520b was determined by Western blot analysis, luciferase reporter gene assay, co-immunoprecipitation (co-IP) and chromatin immunoprecipitation (ChIP), respectively. The expression of HBx, survivin and HBXIP was detected by immunohistochemistry and real-time PCR in clinical HCC tissues, respectively. The DNA demethylation of HBXIP promoter was examined. The functional influence of miR-520b and HBXIP on proliferation of hepatoma cells was analyzed by MTT, colony formation, EdU and transplantation in nude mice in vitro and in vivo.

RESULTS

In this study, we provided evidence that HBx up-regulated survivin in the liver cancer tissues of HBx-Tg mice aged 18 M. The engineered LO2 cell lines with survivin and/or HBx were successfully established, termed LO2-X-S. MiR-520b was down-regulated in LO2-X-S cells and clinical HCC tissues. Our data revealed that HBx survivin-dependently down-regulated miR-520b through interacting with Sp1 in the cells. HBXIP was highly expressed in LO2-X-S cells, liver cancer tissues of HBx-Tg mice aged 18 M and clinical HCC tissues (75.17%, 112/149). The expression level of HBXIP was positively associated with those of HBx or survivin in clinical HCC tissues. In addition, we showed that HBx survivin-dependently up-regulated HBXIP through inducing demethylation of HBXIP promoter in LO2-X-S cells and clinical HCC tissues. In function, low level miR-520b and high level HBXIP mediated by HBx with partner survivin contributed to the growth of LO2-X-S cells in vitro and in vivo.

CONCLUSION

HBx accelerates hepatocarcinogenesis with partner survivin through modulating tumor suppressor miR-520b and oncoprotein HBXIP.

Tackling hepatitis B virus-associated hepatocellular carcinoma--the future is now

Hepatocellular carcinoma (HCC) is one of the most lethal and prevalent cancers in many developing countries including India. Among the various etiological factors being implicated in the cause of HCC, the most important cause, however, is hepatitis B virus (HBV) infection. Among all HBV genes, HBx is the most critical carcinogenic component, the molecular mechanisms of which have not been completely elucidated. Despite its clinical significance, there exists a very elemental understanding of the molecular, cellular, and environmental mechanisms that drive disease pathogenesis in HCC infected with HBV. Furthermore, there are only limited therapeutic options, the clinical benefits of which are insignificant. Therefore, the quest for novel and effective therapeutic regimen against HBV-related HCC is of paramount importance. This review attempts to epitomize the current state of knowledge of this most common and dreaded liver neoplasm, highlighting the putative treatment avenues and therapeutic research strategies that need to be implemented with immediate effect for tackling HBV-related HCC that has plagued the medical and scientific fraternity for decades. Additionally, this review proposes a novel "five-point" management algorithm for HBV-related HCC apart from portraying the unmet needs, principal challenges, and scientific perspectives that are relevant to controlling this accelerating global health crisis.

Synergistic function of Kras mutation and HBx in initiation and progression of hepatocellular carcinoma in mice

Although the activation of Ras pathway is frequently observed in human hepatocellular carcinoma (HCC), the in vivo role of Ras activation in HCC initiation and progression is underdetermined. To test the consequence of Kras activation in hepatocyte, we generated a hepatocyte-specific Kras(G12D) transgenic mouse strain and observed spontaneous development of HCC in these mice. Remarkably, HBV X protein (HBx) expression significantly promotes the formation and malignant progression of Kras(G12D)-driven HCC as shown with the accelerated tumor onset, the increased tumor burden and the more poorly differentiated lesions. At the cellular level, concomitant expression of Kras(G12D) and HBx results in a robust increase in hepatocellular proliferation. We reveal that the Akt, MAPK, p53 and TGF-β pathways are deregulated in the Kras(G12D)-driven HCCs. Also, the dysregulation is more pronounced in the HCCs developed in Kras(G12D) and HBx double transgenic mice. In addition, the altered expressions of β-catenin, CD44 and E-cadherin are only observed in the Kras(G12D) and HBx double transgenic mice. These results demonstrate a crucial role of Ras activation in hepatocellular carcinogenesis and the functional synergy between Kras(G12D) and HBx in HCC initiation and progression. The novel genetic mouse models that closely recapitulate the histopathologic progression and molecular alterations of human HCC may potentially facilitate the future therapeutic studies.

[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.

Pro-apoptotic or anti-apoptotic property of X protein of hepatitis B virus is determined by phosphorylation at Ser31 by Akt

The X protein of hepatitis B virus (HBx) has been specifically implicated in either pro-apoptotic or anti-apoptotic activity in an experimental system, but the underlying mechanism is yet uncertain. Activations of survival and proliferation signaling pathways appear to account partly for its anti-apoptotic property. Change in mitochondrial membrane potential may be responsible for its apoptotic property. In this study, we isolated two HBx isoforms from an HBV carrier, one of which contains Akt phosphorylation site at Ser31 and functions as an anti-apoptotic protein (designated HBx-S31). The other does not contain Akt phosphorylation site and functions as an apoptotic protein (designated HBx-L31). HBx-S31 can activate Akt, whereas HBx-L31 cannot; the former enhances tumor growth, whereas the latter suppresses tumorigenesis. Our study provides evidence that HBx plays dual roles, namely pro-apoptotic and anti-apoptotic, through different isoforms in which HBx with Ser31 transduces survival signal.

Hepatitis B virus X protein and c-Myc cooperate in the upregulation of ribosome biogenesis and in cellular transformation

Viral and cellular oncogenes are well known to enhance rRNA synthesis, leading to increased ribosome biogenesis and cell proliferation. Our study on the molecular underpinnings of the interaction between viral HBx and c-Myc, which is implicated in the development of hepatocellular carcinoma, showed a marked increase in the biosynthesis of rRNA, ribosomes and protein in hepatoma cells. A profound alteration in the nucleolar morphology and biochemical content of these cells was also observed. Increased biosynthetic activity was associated with increased cell proliferation and transformation of immortalized human hepatocytes. Furthermore, inhibition of RNA polymerase III activity impaired the proliferative advantage of hepatoma cells and transformation of immortalized hepatocytes as effectively as cisplatin treatment. These findings were corroborated in a transgenic HBx-myc microenvironment, in which an elevated hepatic level of rRNA was associated with conspicuous morphological and biochemical changes in the hepatocytic nucleoli. Thus, HBx and c-Myc seem to work cooperatively to support ribosome biogenesis and cellular transformation.

Involvement of hepatitis B virus X gene (HBx) integration in hepatocarcinogenesis via a recombination of HBx/Alu core sequence/subtelomeric DNA

The significance of hepatitis B virus (HBV) DNA-based integration in hepatocarcinogenesis is poorly understood. In the present study, we investigated whether the integration of HBV X gene (HBx) is involved in the event. Our finding showed that the integration of HBx fragment (316-462 bp/262-462 bp) was able to transform human immortalized normal liver LO2 cells using a cell model of HBx-integration. We identified that the recombination, HBx/Alu core sequence/subtelomeric DNA, was required for the transformation, which could be detected in 5 out of 44 clinical HBx-positive hepatocellular carcinoma tissues. Thus, we conclude that HBx integration is involved in the hepatocarcinogenesis.

Establishment of a HepG2 cell line stably transduced with a lentivirus expressing the HBV X gene

AIM: To establish a HepG2 cell line stably transduced with a lentivirus expressing the HBV X (HBx) gene for studying the biological function of HBx and its role in hepatocarcinogenesis.

METHODS: The HBV X gene was amplified from plasmid pIERES2-EGFP-HBV by PCR. The purified HBx gene fragment was inserted into a lentivirus vector (pZac2.1), and the insertion was identified by PCR, restriction endonuclease analysis and DNA sequencing. HepG2 cells were then transfected with the packaged recombinant lentivirus, and resistant cell clones were selected with puromycin. The expression of HBx was examined using RT-PCR, immunohistochemistry, and Western blot.

RESULTS: Restriction enzyme digestion and DNA sequencing showed that the full-length HBx (489 bp) gene had been successfully subcloned into the lentiviral vector to result in the recombinant vector pZac2.1-HBx. The titre of purified recombinant lentivirus was 1×10⁸ TU/mL. Monoclonal cell line HepG2-HBx was produced 8-10 d after transfection with the recombinant lentivirus and selected with puromycin. HBx mRNA could be detected on days 3, 14, 30 and at 2 mo after cell colony formation. Meanwhile, stable expression of HBx protein was verified by immunohistochemistry and Western blot.

CONCLUSION: A HepG2 cell line stably transduced with a lentivirus expressing the HBx gene has been successfully generated.

Akt augments the oncogenic potential of the HBx protein of hepatitis B virus by phosphorylation

Hepatitis B virus X protein (HBx) is a putative viral oncoprotein that plays an important role in various cellular processes, including modulation of the phosphatidylinositol 3-kinase/Akt signalling pathway. However, the molecular mechanism of Akt activation remains elusive. Here we show that HBx interacts with Akt1 kinase and is phosphorylated at serine 31 as indicated by mutational analysis of the Akt recognition motif (creating the HBxS31A mutant) or immunoblotting of HBx immunoprecipitates using Akt motif-specific antibody. The Akt-dependent phosphorylation of HBx was abrogated in the presence of the phosphatidylinositol 3-kinase inhibitor LY294002 or Akt1 gene silencing by specific siRNA. Co-immunoprecipitation studies provided evidence for HBx-Akt interaction in a cellular environment. This interaction was also confirmed in hepatoma HepG2.2.15 cells in which HBx was expressed at physiological levels from the integrated hepatitis B viral genome. The HBx-Akt interaction was essential for Akt signalling, and involved displacement of the Akt-bound negative regulator 'C-terminal modulator protein' by HBx. HBx-activated Akt phosphorylated its downstream target glycogen synthase kinase 3β, leading to stabilization of β-catenin, while p65 phosphorylation resulted in enhanced promoter recruitment and expression of target genes encoding cyclin D1 and Bcl-XL. Further, the oncogenic potential of HBx was significantly augmented in the presence of Akt in a soft agar colony formation assay. Together, these results suggest that oncogenic co-operation between HBx and Akt may be important for cell proliferation, abrogation of apoptosis and tumorigenic transformation of cells.

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 virus x protein in the pathogenesis of hepatitis B virus-induced hepatocellular carcinoma

Currently available evidence supports a role for the hepatitis B virus (HBV) x gene and protein in the pathogenesis of HBV-induced hepatocellular carcinoma (HCC). HBx gene is often included, and remains functionally active, in the HBV DNA that is frequently integrated into cellular DNA during hepatocellular carcinogenesis. HBx protein promotes cell cycle progression, inactivates negative growth regulators, and binds to and inhibits the expression of p53 tumour suppressor gene and other tumour suppressor genes and senescence-related factors. However, the molecular mechanisms responsible for HBx protein-induced HCC remain uncertain. Only some of the more fully documented or more recently recognised mechanisms are reviewed. During recent years evidence has accumulated that HBx protein modulates transcription of methyl transferases, causing regional hypermethylation of DNA that results in silencing of tumour suppressor genes, or global hypomethylation that results in chromosomal instability, thereby playing a role in hepatocarcinogenesis. HBx protein has both anti-apoptotic and pro-apoptotic actions, apparently contradictory effects that have yet to be explained. Particularly important among the anti-apoptotic properties is inhibition of p53. Recent experimental observations suggest that HBx protein may increase the expression of TERT and telomerase activity, prolonging the life-span of hepatocytes and contributing to malignant transformation. The protein also interferes with nucleotide excision repair through both p53-dependent and p53- independent mechanisms. Carboxy-terminal truncated HBx protein loses its inhibitory effects on cell proliferation and pro-apoptotic properties, and it may enhance the protein's ability to transform oncogenes. Dysregulation of IGF-II enhances proliferation and anti-apoptotic effects of oncogenes, resulting in uncontrolled cell growth.

Cooperation of tumor-derived HBx mutants and p53-249(ser) mutant in regulating cell proliferation, anchorage-independent growth and aneuploidy in a telomerase-immortalized normal human hepatocyte-derived cell line

Hepatocellular carcinoma (HCC) is a common cancer, and hepatitis B virus (HBV) is a major etiological agent. Convincing epidemiological and experimental evidence also links HCC to aflatoxin, a naturally occurring mycotoxin that produces a signature p53-249(ser) mutation. Recently, we have reported that tumor-derived HBx variants encoded by HBV exhibited attenuated transactivation and proapoptotic functions but retained their ability to block p53-mediated apoptosis. These results indicate that mutations in HBx may contribute to the development of HCC. In this study, we determined whether tumor-derived HBx mutants along, or in cooperation with p53-249(ser), could alter cell proliferation and chromosome stability of normal human hepatocytes. To test this hypothesis, we established a telomerase immortalized normal human hepatocycte line HHT4 that exhibited a near diploid karyotype and expressed many hepatocyte-specific genes. We found that overexpression one of the tumor-derived HBx mutants, CT, significantly increased colony forming efficiency (CFE) while its corresponding wild-type allele CNT significantly decreased CFE in HHT4 cells. p53-249(ser) rescued CNT-mediated inhibition of colony formation. Although HHT4 cells lacked an anchorage independent growth capability as they did not form any colonies in soft agar, the CT-expressing HHT4 cells could form colonies, which could be significantly enhanced by p53-249(ser). Induction of aneuploidy could be observed in HHT4 cells expressing CT, but additionally recurring chromosome abnormalities could only be detected in cells coexpressing CT and p53-249(ser). Our results are consistent with the hypothesis that certain mutations in HBx and p53 at codon 249 may cooperate in contributing to liver carcinogenesis.

[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.

Mechanisms of HBV-related hepatocarcinogenesis

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

Liver cell transformation in chronic HBV infection

Epidemiological studies have provided overwhelming evidence for a causal role of chronic HBV infection in the development of hepatocellular carcinoma (HCC), but the molecular mechanisms underlying virally-induced tumorigenesis remain largely debated. In the absence of a dominant oncogene encoded by the HBV genome, indirect roles have been proposed, including insertional activation of cellular oncogenes by HBV DNA integration, induction of genetic instability by viral integration or by the regulatory protein HBx, and long term effects of viral proteins in enhancing immune-mediated liver disease. In this chapter, we discuss different models of HBV-mediated liver cell transformation based on animal systems of hepadnavirus infection as well as functional studies in hepatocyte and hepatoma cell lines. These studies might help identifying the cellular effectors connecting HBV infection and liver cell transformation.

Impact of hepatitis B virus X protein on the DNA damage response during hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is one of the most prevalent and lethal cancers worldwide. The main HCC-associated diseases are chronic infections with hepatitis B virus (HBV) and hepatitis C virus (HCV), and HBV-associated HCC is still prevalent in Asia. Many studies have suggested that HBV X protein (HBX), which is the most common ORF integrated into the host genome, plays a crucial role in hepatocarcinogenesis. However, the accumulated evidence regarding HBX-mediated signaling pathways is not concordant, and it is difficult to understand the mechanistic nature of HBX-associated hepatocarcinogenesis. For example, HBX was reported to inactivate the early responses to DNA damage via p53-dependent and -independent pathways by interacting with several DNA damage-binding proteins and was also reported to sensitize cells to p53-mediated apoptosis via ataxia-telangiectasia and Rad3-related (ATR)-dependent signaling. HBX also interferes with the centrosome replication process, resulting in rearrangement of chromosomes with micronuclei. Moreover, HBX was found to sensitize protein kinases such as Ras/Raf/mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), stress-activated protein kinase/NH2-terminal-Jun kinase (SAPK/JNK), protein kinase B (PKB/Akt), and Janus kinase/STAT (JAK/STAT), indicating that a variety of signaling pathways may be activated by HBX. In this review, we focus on the roles of HBX in DNA damage repair during HCC development, with a view to achieving a better understanding of the significance of HBX in the early steps of hepatocarcinogenesis.

Transformation of human liver L-O2 cells mediated by stable HBx transfection

AIM

To explore the mechanism of hepatocarcinogenesis associated with the hepatitis B virus X protein (HBx), we investigated the role of HBx in transformation using human liver L-O2 cells stably transfected with HBx as a model.

METHODS

Plasmids encoding HBx were stably transfected into immortalized human liver L-O2 cells and rodent fibroblast NIH/3T3 cells. The expression of alfa-fetoprotein (AFP), c-Myc, HBx, and survivin in the engineered cells was examined by Western blotting. The malignant phenotype of the cells was demonstrated by anchorage-independent colony formation and tumor formation in nude mice. RNA interference assays, Western blotting, luciferase reporter gene assays and flow cytometry analysis were performed. The number of centrosomes in the L-O2-X cells was determined by gamma-tubulin immunostaining. The effect of HBx on the transcriptional activity of human telomerase reverse transcriptase (hTERT) and hTERT activity in L-O2-X cells and/or 3T3-X cells was detected by the luciferase reporter gene assay and telomerase repeat amplification protocol (TRAP).

RESULTS

Stable HBx transfection resulted in a malignant phenotype in the engineered cells in vivo and in vitro. Meanwhile, HBx was able to increase the transcription of the NF-kappaB, AP-1, and survivin genes and to upregulate the expression levels of c-Myc and survivin. Abnormal centrosome duplication and activated hTERT were responsible for the transformation.

CONCLUSION

Stable HBx transfection leads to genomic instability of host cells, which is responsible for hepatocarcinogenesis; meanwhile, transactivation by the HBx protein contributes to the development of hepatocellular carcinoma (HCC). The L-O2-X cell line is an ideal model for investigating the mechanism of HBx-mediated transformation.

Dysregulation of apoptosis in hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is a major health problem, being the sixth most common cancer world-wide. Dysregulation of the balance between proliferation and cell death represents a pro-tumorigenic principle in human hepatocarcinogenesis. This review updates the recent relevant contributions reporting molecular alterations for HCC that induce an imbalance in the regulation of apoptosis. Alterations in the expression and/or activation of p53 are frequent in HCC cells, which confer on them resistance to chemotherapeutic drugs. Many HCCs are also insensitive to apoptosis induced either by death receptor ligands, such as FasL or TRAIL, or by transforming growth factor-beta (TGF-β). Although the expression of some pro-apoptotic genes is decreased, the balance between death and survival is dysregulated in HCC mainly due to overactivation of anti-apoptotic pathways. Indeed, some molecules involved in counteracting apoptosis, such as Bcl-X_L, Mcl-1, c-IAP1, XIAP or survivin are over-expressed in HCC cells. Furthermore, some growth factors that mediate cell survival are up-regulated in HCC, as well as the molecules involved in the machinery responsible for cleavage of their pro-forms to an active peptide. The expression and/or activation of the JAK/STAT, PI3K/AKT and RAS/ERKs pathways are enhanced in many HCC cells, conferring on them resistance to apoptotic stimuli. Finally, recent evidence indicates that inflammatory processes, as well as the epithelial-mesenchymal transitions that occur in HCC cells to facilitate their dissemination, are related to cell survival. Therefore, therapeutic strategies to selectively inhibit anti-apoptotic signals in liver tumor cells have the potential to provide powerful tools to treat HCC.

Human oncogenic viruses: hepatitis B and hepatitis C viruses and their role in hepatocarcinogenesis

Chronic infections caused by hepatitis B virus (HBV) and/or hepatitis C virus (HCV) are the main risk factors for the development of hepatocellular carcinoma (HCC) in humans. Both viruses cause a wide spectrum of clinical manifestations ranging from healthy carrier state to acute and chronic hepatitis, liver cirrhosis, and HCC. HBV and HCV belong to different viral families (Hepadnoviridae and Flaviviridae, respectively); they are characterized by different genetic structures. Clinical manifestations of these viral infections result from the interaction between these viruses and host hepatocytes (i.e. between viral and cell genomes). Proteins encoded by both viruses play an important role in processes responsible for immortalization and transformation of these cells. Chronic inflammation determined by host immune response to the viral infection, hepatocyte death and their compensatory proliferation, as well as modulation of expression of some regulatory proteins of the cell (growth factors, cytokines, etc.) are the processes that play the major role in liver cancer induced by HBV and HCV.

Biological effects of HBV X protein on hepatocellular carcinogenesis in association with cellular factors

The X protein (HBx) of Human hepatitis B virus (HBV) acts as an indirect transcriptional transactivator to regulate the expression of many viral and cellular genes, as well as playing a critical role in pathogenesis and the development of Hepatocellular carcinoma (HCC). Here we described the biological effects of HBx in association with four cellular factors, including inflammatory factors (COX-2 and iNOS), oncoprotein (Ras), and a newly identified tumor suppressor (YueF). The characteristics of these effectors, which might be associated with hepatocellular carcinoma, are also discussed.

Cloning and identification of human hepatocellular carcinoma-related genes

AIM: To clone and identify the differentially expressed gene fragments in hepatocellular carcinoma (HCC) and normal liver tissues and to explore the molecular pathogenesis of HCC.

METHODS: Fluorescent mRNA differential display (mRNA-DD) was used to compare the differential gene expression in HCC and normal liver tissue samples. The obtained gene fragments were cloned and sequenced. The sequences were submitted to the GenBank for homologue analysis using BLAST software. RT-PCR was performed to detect the expression of each differential gene fragment in HCC and normal tissue samples.

RESULTS: Gel imaging scan revealed a 500-600 bp differential gene fragment. The sequence of this gene fragment was compared with that in GenBank, showing that this gene fragment was homologous to EEG1 (99%). RT-PCR demonstrated that the expression of this gene fragment was significantly lower in HCC tissue samples than in normal tissue samples (P = 0.001).

CONCLUSION: EEG1 gene expression is much lower in HCC tissue than in normal tissue. Its down-regulated expression in HCC tissues is probably related to the carcinogenesis of HCC.

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 overcomes oncogenic RAS-induced senescence in human immortalized cells

Chronic infection with hepatitis B virus (HBV) is a major risk factor for hepatocellular carcinoma. The HBV X protein (HBx) is thought to have oncogenic potential, although the molecular mechanism remains obscure. Pathological roles of HBx in the carcinogenic process have been examined using rodent systems and no report is available on the oncogenic roles of HBx in human cells in vitro. We therefore examined the effect of HBx on immortalization and transformation in human primary cells. We found that HBx could overcome active RAS-induced senescence in human immortalized cells and that these cells could form colonies in soft agar and tumors in nude mice. HBx alone, however, could contribute to neither immortalization nor transformation of these cells. In a population doubling analysis, an N-terminal truncated mutant of HBx, HBx-D1 (amino acids 51-154), which harbors the coactivation domain, could overcome active RAS-induced cellular senescence, but these cells failed to exhibit colonigenic and tumorigenic abilities, probably due to the low expression level of the protein. By scanning a HBx expression library of the clustered-alanine substitution mutants, the N-terminal domain was found to be critical for overcoming active RAS-induced senescence by stabilizing full-length HBx. These results strongly suggest that HBx can contribute to carcinogenesis by overcoming active oncogene-induced senescence.

Modulation of MAPK pathways and cell cycle by replicating hepatitis B virus: factors contributing to hepatocarcinogenesis

BACKGROUND/AIMS

Chronic infection with the hepatitis B virus (HBV) is strongly associated with the development of hepatocellular carcinoma but the mechanism by which this occurs is unknown. Numerous studies have focused on the HBV X protein showing that it activates signal transduction pathways while few have investigated these changes in HBV-replicating hepatocytes.

METHODS

We utilized the recombinant adenovirus system to deliver a replication competent HBV genome into Huh7 and primary marmoset hepatocytes (PMH) to examine the effects of active viral replication on the regulation of Ras-ERK signal transduction and related pathways.

RESULTS

Huh7 cells and PMHs replicating HBV demonstrated significant upregulation in phosphorylated ERK, Akt, c-myc together with increased p53, cyclin B1 and p21(cip1) expression and cell cycle progression to G2 phase in the absence of increased cell proliferation. Phosphorylation of the key cell survival kinase, Akt, was significantly increased, resulting in increased serine phosphorylation of the downstream target, GSK3-beta.

CONCLUSIONS

These results demonstrated simultaneous activation of the MAP Kinase and Akt pathways in HBV-replicating hepatocytes that resulted in dysregulation in the control of cell cycle progression and which help explain the early pathogenic mechanisms that underlie malignant transformation associated with chronic hepatitis B infection.

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.

Survival and apoptosis: a dysregulated balance in liver cancer

BACKGROUND/AIMS

Dysregulation of the balance between proliferation and cell death represents a protumorigenic principle in human hepatocarcinogenesis. This article aims to provide a review of the current findings about how physiological hepatocyte apoptosis is regulated and whether or not its dysregulation might contribute to the progression towards a hepatocellular carcinoma (HCC) process.

RESULTS

Although some physiological proapoptotic molecules are downregulated or inactivated in HCC, such as Fas, p53, Bax or Bid, dysregulation of the balance between death and survival is mainly due to overactivation of antiapoptotic signals. Thus, some growth factors that mediate cell survival are upregulated in HCC, as well as the molecules involved in the machinery responsible for cleavage of their proforms to an active peptide. The expression of the pten gene is reduced or absent in almost half the HCCs and the Spred family of Ras/ERK inhibitors is also dysregulated in HCC, which consequently lead to the overactivation of relevant survival kinases: AKT and ERKs. Alterations in the expression and/or activity of molecules involved in counteracting apoptosis, such as NF-kappaB, Bcl-X(L), Mcl-1 or c-IAP1, have also been observed in HCC.

CONCLUSIONS

Therefore, therapeutic strategies to inhibit selectively antiapoptotic signals in tumour cells have the potential to provide powerful tools to treat liver cancer.

Hepatitis B virus x protein induces perinuclear mitochondrial clustering in microtubule- and Dynein-dependent manners

The hepatitis B virus (HBV) X protein (HBx) is thought to play a key role in HBV replication and the development of liver cancer. It became apparent that HBx induces mitochondrial clustering at the nuclear periphery, but the molecular basis for mitochondrial clustering is not understood. Since mitochondria move along the cytoskeleton as a cargo of motor proteins, we hypothesized that mitochondrial clustering induced by HBx occurs by an altered intracellular motility. Here, we demonstrated that the treatment of HBx-expressing cells with a microtubule-disrupting drug (nocodazole) abrogated mitochondrial clustering, while the removal of nocodazole restored clustering within 30 to 60 min, indicating that mitochondrial transport is occurring in a microtubule-dependent manner. The addition of a cytochalasin D-disrupting actin filament, however, did not measurably affect mitochondrial clustering. Mitochondrial clustering was further studied by observations of HBV-related hepatoma cells and HBV-replicating cells. Importantly, the abrogation of the dynein activity in HBx-expressing cells by microinjection of a neutralizing anti-dynein intermediate-chain antibody, dynamitin overexpression, or the addition of a dynein ATPase inhibitor significantly suppressed the mitochondrial clustering. In addition, HBx induced the activation of the p38 mitogen-activated protein kinase (MAPK) and inhibition of the p38 kinase activity by SB203580-attenuated HBx-induced mitochondrial clustering. Taken together, HBx activation of the p38 MAPK contributed to the increase in the microtubule-dependent dynein activity. The data suggest that HBx plays a novel regulatory role in subcellular transport systems, perhaps facilitating the process of maturation and/or assembly of progeny particles during HBV replication. Furthermore, mitochondrion aggregation induced by HBx may represent a cellular process that underlies disease progression during chronic viral infection.

Molecular functions and biological roles of hepatitis B virus x protein

Chronic infection of hepatitis B virus (HBV) is one of the major causes of hepatocellular carcinoma (HCC) in the world. Hepatitis B virus X protein (HBx) has been long suspected to be involved in hepatocarcinogenesis, although its oncogenic role remains controversial. HBx is a multifunctional regulator that modulates transcription, signal transduction, cell cycle progress, protein degradation pathways, apoptosis, and genetic stability by directly or indirectly interacting with host factors. This review focuses on the biological roles of HBx in HBV replication and cellular transformation in terms of the molecular functions of HBx. Using the transient HBV replication assay, ectopically expressed HBx could stimulate HBV transcription and replication with the X-defective replicon to the level of those with the wild one. The transcription coactivation is mainly contributing to the stimulatory role of HBx on HBV replication although the other functions may affect HBV replication. Effect of HBx on cellular transformation remains controversial and was never addressed with human primary or immortal cells. Using the human immortalized primary cells, HBx was found to retain the ability to overcome active oncogene RAS-induced senescence that requires full-length HBx. At least two functions of HBx, the coactivation function and the ability to overcome oncogene-induced senescence, may be cooperatively involved in HBV-related hepatocarcinogenesis.

The role of cytokines in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a frequent malignancy worldwide with a high rate of metastasis. The hepatitis B and C viruses are considered major etiological factors associated with the development of HCC, particularly as a result of their induction of chronic inflammation. There is increasing evidence that the inflammatory process is inherently associated with many different cancer types, including HCC. Specifically, this review aims to cover evidence for the potential roles of cytokines, an important component of the immune system, in promoting HCC carcinogenesis and progression. A global summary of cytokine levels, functions, polymorphisms, and therapies with regard to HCC is presented. In particular, the role of proinflammatory Th1 and anti-inflammatory Th2 cytokine imbalances in the microenvironment of HCC patients with metastasis and the possible clinical significance of these findings are addressed. Overall, multiple studies, spanning many decades, have begun to elucidate the important role of cytokines in HCC.

Differential effects on apoptosis induction in hepatocyte lines by stable expression of hepatitis B virus X protein

AIM: Hepatitis B virus protein X (HBx) has been shown to be weakly oncogenic in vitro. The transforming activities of HBx have been linked with the inhibition of several functions of the tumor suppressor p53. We have studied whether HBx may have different effects on p53 depending on the cell type.

METHODS: We used the human hepatoma cell line HepG2 and the immortalized murine hepatocyte line AML12 and analyzed stably transfected clones which expressed physiological amounts of HBx. P53 was induced by UV irradiation.

RESULTS: The p53 induction by UV irradiation was unaffected by stable expression of HBx. However, the expression of the cyclin kinase inhibitor p21^waf/cip/sdi which gets activated by p53 was affected in the HBx transformed cell line AML12-HBx9, but not in HepG2. In AML-HBx9 cells, p21^waf/cip/sdi-protein expression and p21^waf/cip/sdi transcription were deregulated. Furthermore, the process of apoptosis was affected in opposite ways in the two cell lines investigated. While stable expression of HBx enhanced apoptosis induced by UV irradiation in HepG2-cells, apoptosis was decreased in HBx transformed AML12-HBx9. P53 repressed transcription from the HBV enhancer I, when expressed from expression vectors or after induction of endogenous p53 by UV irradiation. Repression by endogenous p53 was partially reversible by stably expressed HBx in both cell lines.

CONCLUSION: Stable expression of HBx leads to deregulation of apoptosis induced by UV irradiation depending on the cell line used. In an immortalized hepatocyte line HBx acted anti-apoptotic whereas expression in a carcinoma derived hepatocyte line HBx enhanced apoptosis.

Chronic hepatitis B in hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is the fifth most common cancer in the world, and has a wide geographical variation. Eighty per cent of HCC is attributed to hepatitis B virus (HBV). The predominant carcinogenic mechanism of HBV associated HCC is through the process of liver cirrhosis, but direct oncogenic effects of HBV may also contribute. Prevention of HBV infections as well as effective treatment of chronic hepatitis B is still needed for the global control of HBV associated HCC. Continued investigation of the mechanisms of hepatocarcinogenesis will refine our current understanding of the molecular and cellular basis for neoplastic transformation in the liver.

Hepatocellular carcinoma cells containing hepatitis B virus X protein have enhanced invasive potential conditionally

AIMS

To establish a sustaining hepatitis B virus X protein expressed Chang liver cell line and to explore their biological behaviours of invasive potential induced by hepatitis B virus X protein.

METHODS

Polymerase chain reaction was used to amplify the HBx gene from the whole hepatitis B virus genome. The gene was then subcloned into the eukaryotic expression vector pcDNA3.1 to construct the pcDNA3.1-HBx plasmid. Gene transfection mediated by Lipofectamine was used to introduce the plasmid into the human liver cell line Chang, and stable expression of the HBx gene was detected.

RESULTS

HBx gene was cloned from the transfected Chang liver cells by reverse transcription-polymerase chain reaction, and confirmed by electrophoresis. The stably transfected Chang cells expressing HBx with malignant characteristics were verified and compared with control cells in terms of their growth curves, clonogenicity, wound healing abilities, migration and metastasis.

CONCLUSION

The stabilising human liver cell lines Chang liver containing HBx gene expression have been established successfully. The invasive potential of Chang cells was conditionally enhanced by HBx transfection.

Functional switch of viral protein HBx on cell apoptosis, transformation, and tumorigenesis in association with oncoprotein Ras

The X protein (HBx) of hepatitis B virus (HBV) plays important roles in hepatitis, cirrhosis, and hepatocellular carcinoma (HCC) during viral infection. In this study, we demonstrated that co-transfection of mouse embryo fibroblasts (STO) with HBx and activated Ras triggered apoptotic cell death, while HBx or activated Ras individually failed to induce apoptosis. In addition, STO cells were able to form colonies on soft agar after transfected with HBx or Ras, and cells co-transfected with both genes failed to transform. Moreover, nude mice injected with STO cells carrying either HBx or Ras could develop tumor, but tumor growth was inhibited by the injection of both STO cells harboring HBx and carrying Ras. These results suggested that HBx plays a role as a tumor inducer and stimulates neoplastic transformation of normal cells, but shifts its function to the induction of apoptosis in association with Ras.

The X protein of hepatitis B virus binds to the F box protein Skp2 and inhibits the ubiquitination and proteasomal degradation of c-Myc

The HBx protein of hepatitis B virus is involved in deregulation of cell cycle and development of hepatocellular carcinoma. Since c-Myc also plays an important role in cell proliferation and tumor development, we studied its regulation by HBx in a human hepatoma cell line. Co-expression of HBx and c-Myc resulted in increased stability of intracellular c-Myc. HBx blocked the ubiquitination of Myc through a direct interaction with the F box region of Skp2 and destabilization of the SCF(Skp2) complex. We suggest that sustained presence of c-Myc combined with mitogenic activity inherent to HBx may be associated with cell cycle deregulation and transformation.

Hepatitis B virus X protein is essential for the activation of Wnt/beta-catenin signaling in hepatoma cells

Wnt/beta-catenin signaling contributes to diverse cellular functions, such as Drosophila wing development and colon carcinogenesis. Recently, stabilizing mutations of beta-catenin, a hallmark of Wnt signaling, were documented in significant numbers of primary hepatocellular carcinomas (HCC). However, whether the beta-catenin mutation leads to the activation of Wnt/beta-catenin signaling in hepatoma cells has not been established. We found that Wnt/beta-catenin signaling could be activated by ectopic expression of Wnt-1 in some hepatoma cells, such as Hep3B and PLC/PRF/5 cells, but not in others, such as Huh7 and Chang cells. Importantly, we noted that the former were derived from hepatitis B virus (HBV)-infected livers, whereas the latter were derived from HBV-negative livers. It was then speculated that HBx, a viral regulatory protein of HBV, is involved in activating Wnt/beta-catenin signaling in hepatoma cells. In agreement with this notion, ectopic expression of HBx along with Wnt-1 activated Wnt/beta-catenin signaling in Huh7 cells by stabilizing cytoplasmic beta-catenin. Further, we showed that such stabilization of beta-catenin by HBx was achieved by suppressing glycogen synthase kinase 3 activity via the activation of Src kinase. In conclusion, the data suggest that Wnt-1 is necessary but insufficient to activate Wnt/beta-catenin signaling in hepatoma cells and the enhanced stabilization of beta-catenin by HBx, in addition to Wnt-1, is essential for the activation of Wnt/beta-catenin signaling in hepatoma cells.

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.