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

Hepatocellular Carcinoma in Chronic Viral Hepatitis: Where Do We Stand?

Hepatocellular carcinoma (HCC) is one of the major causes of cancer-related death. Although the burden of alcohol- and NASH-related HCC is growing, chronic viral hepatitis (HBV and HCV) remains a major cause of HCC development worldwide. The pathophysiology of viral-related HCC includes liver inflammation, oxidative stress, and deregulation of cell signaling pathways. HBV is particularly oncogenic because, contrary to HCV, integrates in the cell DNA and persists despite virological suppression by nucleotide analogues. Surveillance by six-month ultrasound is recommended in patients with cirrhosis and in "high-risk" patients with chronic HBV infection. Antiviral therapy reduces the risks of development and recurrence of HCC; however, patients with advanced chronic liver disease remain at risk of HCC despite virological suppression/cure and should therefore continue surveillance. Multiple scores have been developed in patients with chronic hepatitis B to predict the risk of HCC development and may be used to stratify individual patient's risk. In patients with HCV-related liver disease who achieve sustained virological response by direct acting antivirals, there is a strong need for markers/scores to predict long-term risk of HCC. In this review, we discuss the most recent advances regarding viral-related HCC.

HBV-related hepatocarcinogenesis: the role of signalling pathways and innovative ex vivo research models

BACKGROUND

Hepatitis B virus (HBV) is the leading cause of liver cancer, but the mechanisms by which HBV causes liver cancer are poorly understood and chemotherapeutic strategies to cure liver cancer are not available. A better understanding of how HBV requisitions cellular components in the liver will identify novel therapeutic targets for HBV associated hepatocellular carcinoma (HCC).

MAIN BODY

The development of HCC involves deregulation in several cellular signalling pathways including Wnt/FZD/β-catenin, PI3K/Akt/mTOR, IRS1/IGF, and Ras/Raf/MAPK. HBV is known to dysregulate several hepatocyte pathways and cell cycle regulation resulting in HCC development. A number of these HBV induced changes are also mediated through the Wnt/FZD/β-catenin pathway. The lack of a suitable human liver model for the study of HBV has hampered research into understanding pathogenesis of HBV. Primary human hepatocytes provide one option; however, these cells are prone to losing their hepatic functionality and their ability to support HBV replication. Another approach involves induced-pluripotent stem (iPS) cell-derived hepatocytes. However, iPS technology relies on retroviruses or lentiviruses for effective gene delivery and pose the risk of activating a range of oncogenes. Liver organoids developed from patient-derived liver tissues provide a significant advance in HCC research. Liver organoids retain the characteristics of their original tissue, undergo unlimited expansion, can be differentiated into mature hepatocytes and are susceptible to natural infection with HBV.

CONCLUSION

By utilizing new ex vivo techniques like liver organoids it will become possible to develop improved and personalized therapeutic approaches that will improve HCC outcomes and potentially lead to a cure for HBV.

The Antiresection Activity of the X Protein Encoded by Hepatitis Virus B

Chronic infection of hepatitis B virus (HBV) is associated with an increased incidence of hepatocellular carcinoma (HCC). HBV encodes an oncoprotein, hepatitis B x protein (HBx), that is crucial for viral replication and interferes with multiple cellular activities including gene expression, histone modifications, and genomic stability. To date, it remains unclear how disruption of these activities contributes to hepatocarcinogenesis. Here, we report that HBV exhibits antiresection activity by disrupting DNA end resection, thus impairing the initial steps of homologous recombination (HR). This antiresection activity occurs in primary human hepatocytes undergoing a natural viral infection-replication cycle as well as in cells with integrated HBV genomes. Among the seven HBV-encoded proteins, we identified HBx as the sole viral factor that inhibits resection. By disrupting an evolutionarily conserved Cullin4A-damage-specific DNA binding protein 1-RING type of E3 ligase, CRL4^WDR70 , through its H-box, we show that HBx inhibits H2B monoubiquitylation at lysine 120 at double-strand breaks, thus reducing the efficiency of long-range resection. We further show that directly impairing H2B monoubiquitylation elicited tumorigenesis upon engraftment of deficient cells in athymic mice, confirming that the impairment of CRL4^WDR70 function by HBx is sufficient to promote carcinogenesis. Finally, we demonstrate that lack of H2B monoubiquitylation is manifest in human HBV-associated HCC when compared with HBV-free HCC, implying corresponding defects of epigenetic regulation and end resection. Conclusion: The antiresection activity of HBx induces an HR defect and genomic instability and contributes to tumorigenesis of host hepatocytes.

Contribution of hepatitis B virus X protein-induced aberrant microRNA expression to hepatocellular carcinoma pathogenesis

The hepatitis B virus-encoded X (HBX) protein plays important roles in Hepatocellular carcinoma (HCC). Previous studies have demonstrated that HBX can induce alterations in the expression of numerous microRNAs (miRNAs) involved in the carcinogenesis of various tumors. However, the global profile of liver miRNA changes induced by HBX has not been characterized. In this study, we conducted a miRNA microarray analysis to investigate the influence of HBX on the expression of total miRNAs in liver in relation to HCC. Comparative analysis of the data from human normal liver cells (L02) and human HCC cells (HepG2), with or without HBX, identified 19 differentially expressed miRNAs, including 5 with known association to HBX. Target gene prediction for the aberrantly expressed miRNAs identified a total of 304 potential target genes, involved in sundry pathways. Finally, pathway analysis of the HBXinduced miRNAs pathway showed that 5 of the total miRNAs formed an internetwork, suggesting that HBX might exert its pathological effects on hepatic cells through functional synergy with miRNAs that regulated common pathways in liver cells. Therefore, this work provides new insights into the mechanisms of HCC as well as potential diagnostic markers or therapeutic targets for use in clinical management of HCC.

Control of viral infections by epigenetic-targeted therapy

Epigenetics is defined as the science that studies the modifications of gene expression that are not owed to mutations or changes in the genetic sequence. Recently, strong evidences are pinpointing toward a solid interplay between such epigenetic alterations and the outcome of human cytomegalovirus (HCMV) infection. Guided by the previous possibly promising experimental trials of human immunodeficiency virus (HIV) epigenetic reprogramming, the latter is paving the road toward two major approaches to control viral gene expression or latency. Reactivating HCMV from the latent phase ("shock and kill" paradigm) or alternatively repressing the virus lytic and reactivation phases ("block and lock" paradigm) by epigenetic-targeted therapy represent encouraging options to overcome latency and viral shedding or otherwise replication and infectivity, which could lead eventually to control the infection and its complications. Not limited to HIV and HCMV, this concept is similarly studied in the context of hepatitis B and C virus, herpes simplex virus, and Epstein-Barr virus. Therefore, epigenetic manipulations stand as a pioneering research area in modern biology and could constitute a curative methodology by potentially consenting the development of broad-spectrum antivirals to control viral infections in vivo.

Comprehensive review of Hepatitis B Virus-associated hepatocellular carcinoma research through text mining and big data analytics

PubMed was text mined to glean insights into the role of Hepatitis B virus (HBV) in hepatocellular carcinoma (HCC) from the massive number of publications (9249) available to date. Reports from ∼70 countries identified >1300 human genes associated with either the Core, Surface or X gene in HBV-associated HCC. One hundred and forty-three of these host genes, which can potentially yield 1180 biomolecular interactions, each were reported in at least three different publications to be associated with the same HBV. These 143 genes function in 137 pathways, involved mainly in the cell cycle, apoptosis, inflammation and signalling. Fourteen of these molecules, primarily transcriptional regulators or kinases, play roles in several pathways pertinent to the hallmarks of cancers. 'Chronic' was the most frequent word used across the 9249 abstracts. A key event in chronic HBV infection is the integration of HBV into the host genome. The advent of cost-effective, next-generation sequencing technology facilitated the employment of big-data analytics comprehensively to characterize HBV-host integration within HCC patients. A total of 5331 integration events were reported across seven publications, with most of these integrations observed between the Core/X gene and the introns of genes. Nearly one-quarter of the intergenic integrations are within repeats, especially long interspersed nuclear elements (LINE) repeats. Integrations within 13 genes were each reported by at least three different studies. The human gene with the most HBV integrations observed is the TERT gene where a total of 224 integrations, primarily at its promoter and within the tumour tissue, were reported by six of seven publications. This unique review, which employs state-of-the-art text-mining and data-analytics tools, represents the most complete, systematic and comprehensive review of nearly all the publications associated with HBV-associated HCC research. It provides important resources to either focus future research or develop therapeutic strategies to target key molecules reported to play important roles in key pathways of HCC, through the systematic analyses of the commonly reported molecules associated with the various HBV genes in HCC, including information about the interactions amongst these commonly reported molecules, the pathways in which they reside as well as detailed information regarding the viral and host genes associated with HBV integration in HCC patients. Hence this review, which highlights pathways and key human genes associated with HBV in HCC, may facilitate the deeper elucidation of the role of HBV in hepato-carcinogenesis, potentially leading to timely intervention against this deadly disease.

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

Background

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

The function of HBx in HBV-induced hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the second cause cancer death in the world. HCC is frequently diagnosed at advanced stages with intrahepatic metstasis or vascular invasion and has a poor prognosis with a high mortality rate. In the world, hepatitis B virus (HBV) caused over 50% HCC, making it the most common carcinogen after tobacco. Notably, accumulating evidence suggests HBV X gene (HBx) play an important role in tumorigenesis of HBV-related HCC. In this review, we will summarize the functions of HBx proteins in tumorigenesis and discuss their potential implications in cancer therapy.

Delta-like 3 is silenced by HBx via histone acetylation in HBV-associated HCCs

Hepatocellular carcinoma (HCC) is a common malignant tumor with poor prognosis. We previously showed that expression of Delta-like 3 (DLL3), a member of the family of Delta/Serrate/Lag2 ligands for the Notch receptor, is silenced by aberrant DNA methylation and that overexpression of DLL3 in an HCC cell line induces cellular apoptosis. However, how DLL3 expression is regulated during hepatocarcinogenesis is still unclear. Here, we show that silencing of DLL3 during hepatocarcinogenesis is closely related to viral infection, especially hepatitis B virus (HBV) infection (p = 0.005). HepG2.2.15 cells, which are stably transformed with the HBV genome, showed lower DLL3 expression than the parent cell line, HepG2 cells. Treatment with Hepatitis B virus X protein (HBx) small interfering RNA upregulated DLL3 expression in HepG2.2.15 cells, and overexpression of HBx in HepG2 cells downregulated DLL3 expression. Treatment of cells with a histone deacetylase inhibitor induced DLL3 expression in HepG2.2.15 cells. These data suggest that DLL3 expression is silenced during hepatocarcinogenesis in association with HBV infection via an epigenetic mechanism.

A comparative study of hepatitis B virus X protein mutants K130M, V131I and KV130/131MI to investigate their roles in fibrosis, cirrhosis and hepatocellular carcinoma

Hepatitis B virus (HBV) genomic mutations A1762T, G1764A and AG1762/1764TA cause production of HBV X protein (HBx) mutants, namely K130M, V131I and KV130/131MI. These mutations are important biomarkers for the development of cirrhosis and hepatocellular carcinoma (HCC) in chronic HBV patients. This study comparatively analyses the impact of intracellular expression of HBx mutants on HCC cell line Huh7. It was found that expression of KV130/131MI induced: cell proliferation, altered expression of cell cycle regulatory genes in favour of cell proliferation, intracellular reactive oxygen species (ROS) production and mitochondrial depolarization. KV130/131MI may be directly involved in host cell proliferation and hepatocarcinogenesis via altering expression of cell cycle regulatory genes. KV130/131MI may also play pivotal roles in fibrosis and cirrhosis via inducing ROS production and mitochondrial depolarization. Furthermore, these might be the possible reasons for higher occurrence of AG1762/1764TA as compared to A1762T and G1764A in cirrhosis and HCC patients.

HBX Protein-Induced Downregulation of microRNA-18a is Responsible for Upregulation of Connective Tissue Growth Factor in HBV Infection-Associated Hepatocarcinoma

Background

This study was designed to improve our understanding of the role of miR-18a and its target (connective tissue growth factor (CTGF), which are mediators in HBX-induced hepatocellular carcinoma (HCC).

Material/Methods

We first investigated the expression of several candidate microRNAs (miRNAs) reported to have been aberrantly expressed between HepG2 and HepG2.2.15, which is characterized by stable HBV infection, while the CTGF is identified as a target of miR-18a. Furthermore, the expression of CTGF evaluated in HepG2 was transfected with HBX, while the HepG2.2.15 was transfected with miR-18a and CTGF siRNA. We examined the cell cycle at the same time.

Results

We found that the expression of miR-18a was abnormally reduced in the HBV-positive HCC tissue samples compared with HBV-negative HCC samples. Through the use of a luciferase reporter system, we also identified CTGF 3′UTR (1046–1052 bp) as the exact binding site for miR-18a. We also observed a clear increase in CTGF mRNA and protein expression levels in HBV-positive HCC human tissue samples in comparison with the HBV-negative controls, indicating a possible negatively associated relationship between miR-18a and CTGF. Furthermore, we investigated the effect of HBX overexpression on miR-18a and CTGF, as well as the viability and cell cycle status of HepG2 cells. In addition, we found that HBX introduction downregulated miR-18a, upregulated CTGF, elevated the viability, and promoted cell cycle progression. We transfected HepG2.2.15 with miR-18a mimics and CTGF siRNA, finding that upregulated miR-18a and downregulated CTGF suppress the viability and cause cell cycle arrest.

Conclusions

Our study shows the role of the CTGF gene as a target of miR-18a, and identifies the function of HBV/HBX/miR-18a/CTGF as a key signaling pathway mediating HBV infection-induced HCC.

Global re-wiring of p53 transcription regulation by the hepatitis B virus X protein

BACKGROUND

The tumour suppressor p53 is a central player in transcription regulation and cell fate determination. By interacting with p53 and altering its sequence-specific binding to the response elements, the hepatitis B virus X protein (HBx) was reported to re-direct p53 regulation of some genes.

RESULTS

Coupling massively parallel deep sequencing with p53 chromatin immunoprecipitation, we demonstrate that HBx modulates global p53 site selection and that this was strongly influenced by altered interaction with transcription co-factors/co-regulators as well as post-translational modifications. Specifically, HBx attenuated p53-TBP-RB1 transcription complex recruitment and interaction and this was associated with hyper-phosphorylation of p53 at serine 315 by HBx. Concurrently, HBx enhanced p53 DNA occupancy to other response elements either alone by displacing specific transcription factors such as CEBPB and NFkB1, or in complex with distinct interacting co-factors Sp1, JUN and E2F1. Importantly, re-wiring of p53 transcription regulation by HBx was linked to the deregulation of genes involved in cell proliferation and death, suggesting a role of HBx in errant cell fate determination mediated by altered p53 site selection of target genes.

CONCLUSIONS

Our study thus presents first evidence of global modes of p53 transcription alteration by HBx and provides new insights to understand and potentially curtail the viral oncoprotein.

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.

Modulation of DNA damage and repair pathways by human tumour viruses

With between 10% and 15% of human cancers attributable to viral infection, there is great interest, from both a scientific and clinical viewpoint, as to how these pathogens modulate host cell functions. Seven human tumour viruses have been identified as being involved in the development of specific malignancies. It has long been known that the introduction of chromosomal aberrations is a common feature of viral infections. Intensive research over the past two decades has subsequently revealed that viruses specifically interact with cellular mechanisms responsible for the recognition and repair of DNA lesions, collectively known as the DNA damage response (DDR). These interactions can involve activation and deactivation of individual DDR pathways as well as the recruitment of specific proteins to sites of viral replication. Since the DDR has evolved to protect the genome from the accumulation of deleterious mutations, deregulation is inevitably associated with an increased risk of tumour formation. This review summarises the current literature regarding the complex relationship between known human tumour viruses and the DDR and aims to shed light on how these interactions can contribute to genomic instability and ultimately the development of human cancers.

Potential mechanisms of hepatitis B virus induced liver injury

Chronic active hepatitis (CAH) is acknowledged as an imperative risk factor for the development of liver injury and hepatocellular carcinoma. The histological end points of CAH are chronic inflammation, fibrosis and cirrhosis which are coupled with increased DNA synthesis in cirrhotic vs healthy normal livers. The potential mechanism involved in CAH includes a combination of processes leading to liver cell necrosis, inflammation and cytokine production and liver scaring (fibrosis). The severity of liver damage is regulated by Hepatitis B virus genotypes and viral components. The viral and cellular factors that contribute to liver injury are discussed in this article. Liver injury caused by the viral infection affects many cellular processes such as cell signaling, apoptosis, transcription, DNA repair which in turn induce radical effects on cell survival, growth, transformation and maintenance. The consequence of such perturbations is resulted in the alteration of bile secretion, gluconeogenesis, glycolysis, detoxification and metabolism of carbohydrates, proteins, fat and balance of nutrients. The identification and elucidation of the molecular pathways perturbed by the viral proteins are important in order to design effective strategy to minimize and/or restore the hepatocytes injury.

'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.

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.

Hepatitis B virus X protein-induced aberrant epigenetic modifications contributing to human hepatocellular carcinoma pathogenesis

Hepatocellular carcinoma (HCC) remains one of the most prevalent malignant diseases worldwide, and the majority of cases are related to hepatitis B virus (HBV) infection. Interactions between the HBV-encoded X (HBx) protein and host factors are known to play major roles in the onset and progression of HBV-related HCC. These dynamic molecular mechanisms are extremely complex and lead to prominent changes in the host genetic and epigenetic architecture. This review summarizes the current knowledge about HBx-induced epigenetic changes, including aberrations in DNA methylation, histone modifications, and microRNA expression, and their roles in HBV-infected liver cells and HBV-related HCC. Moreover, the HBx-mediated epigenetic control of HBV covalently closed circular DNA (cccDNA) is also discussed. Although this field of study is relatively new, the accumulated evidence has indicated that the epigenetic events induced by HBx play important roles in the development of HBV-related HCC. Ongoing research will help to identify practical applications of the HBV-related epigenetic signatures as biomarkers for early HCC detection or as potential targets to prevent and treat HBV-related HCC.

Deep sequencing of the hepatitis B virus in hepatocellular carcinoma patients reveals enriched integration events, structural alterations and sequence variations

Chronic hepatitis B virus (HBV) infection is epidemiologically associated with hepatocellular carcinoma (HCC), but its role in HCC remains poorly understood due to technological limitations. In this study, we systematically characterize HBV in HCC patients. HBV sequences were enriched from 48 HCC patients using an oligo-bead-based strategy, pooled together and sequenced using the FLX-Genome-Sequencer. In the tumors, preferential integration of HBV into promoters of genes (P < 0.001) and significant enrichment of integration into chromosome 10 (P < 0.01) were observed. Integration into chromosome 10 was significantly associated with poorly differentiated tumors (P < 0.05). Notably, in the tumors, recurrent integration into the promoter of the human telomerase reverse transcriptase (TERT) gene was found to correlate with increased TERT expression. The preferred region within the HBV genome involved in integration and viral structural alteration is at the 3'-end of hepatitis B virus X protein (HBx), where viral replication/transcription initiates. Upon integration, the 3'-end of the HBx is often deleted. HBx-human chimeric transcripts, the most common type of chimeric transcripts, can be expressed as chimeric proteins. Sequence variation resulting in non-conservative amino acid substitutions are commonly observed in HBV genome. This study highlights HBV as highly mutable in HCC patients with preferential regions within the host and virus genome for HBV integration/structural alterations.

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.

Altered binding site selection of p53 transcription cassettes by hepatitis B virus X protein

The key cellular regulator p53 is a common target of viral oncoproteins. However, the mechanism by which p53 transcription regulation is modulated by hepatitis B virus X protein (HBx), a transcription cofactor implicated in hepatitis B virus-associated hepatocellular carcinoma (HCC), is poorly understood. By integrating p53 chromatin immunoprecipitation (ChIP)-on-chip and expression profiling of an HBx-expressing cell culture system, we report that HBx alters p53 binding site selectivity in the regulatory regions of genes, and this is associated with their aberrant expression. Using an HBx-deregulated gene, p53AIP1, as a model, we show that HBx aberrantly increases p53AIP1 expression by conferring p53 selectivity for a more conserved binding site in its regulatory region. We further demonstrate that HBx-deregulated increased p53AIP1 expression is relevant in HCC livers and define a functional role for p53AIP1 in mediating HBx-induced apoptosis in vitro. Significantly, we provide evidence that specific p53-associated transcription cofactors and coregulators are differentially recruited in the presence of HBx, effecting a PCAF-mediated "p53 Lys320 acetylation switch" that results in altered binding site selection of distinct p53 transcription cassettes. The findings here clarify the role of HBx in modulating p53 transcription regulation and provide a novel mechanistic insight into this deregulation.

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.

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.

Carboxyl-terminal truncated HBx regulates a distinct microRNA transcription program in hepatocellular carcinoma development

BACKGROUND

The biological pathways and functional properties by which misexpressed microRNAs (miRNAs) contribute to liver carcinogenesis have been intensively investigated. However, little is known about the upstream mechanisms that deregulate miRNA expressions in this process. In hepatocellular carcinoma (HCC), hepatitis B virus (HBV) X protein (HBx), a transcriptional trans-activator, is frequently expressed in truncated form without carboxyl-terminus but its role in miRNA expression and HCC development is unclear.

METHODS

Human non-tumorigenic hepatocytes were infected with lentivirus-expressing full-length and carboxyl-terminal truncated HBx (Ct-HBx) for cell growth assay and miRNA profiling. Chromatin immunoprecipitation microarray was performed to identify the miRNA promoters directly associated with HBx. Direct transcriptional control was verified by luciferase reporter assay. The differential miRNA expressions were further validated in a cohort of HBV-associated HCC tissues using real-time PCR.

RESULTS

Hepatocytes expressing Ct-HBx grew significantly faster than the full-length HBx counterparts. Ct-HBx decreased while full-length HBx increased the expression of a set of miRNAs with growth-suppressive functions. Interestingly, Ct-HBx bound to and inhibited the transcriptional activity of some of these miRNA promoters. Notably, some of the examined repressed-miRNAs (miR-26a, -29c, -146a and -190) were also significantly down-regulated in a subset of HCC tissues with carboxyl-terminal HBx truncation compared to their matching non-tumor tissues, highlighting the clinical relevance of our data.

CONCLUSION

Our results suggest that Ct-HBx directly regulates miRNA transcription and in turn promotes hepatocellular proliferation, thus revealing a viral contribution of miRNA deregulation during hepatocarcinogenesis.

Hepatitis B virus X gene and hepatocarcinogenesis

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

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.

Interaction of mutant hepatitis B X protein with p53 tumor suppressor protein affects both transcription and cell survival

This study examines the differential activities between wild-type Hepatitis B virus X protein (WtHBx) and a mutant HBx (MutHBx), which bears a hotspot mutation at nucleotides 1,762 and 1,764, resulting in a lysine to methionine change at codon 130 and a valine to isoleucine change at codon 131. This mutation leads to hepatocellular carcinoma, and we evaluated how WtHBx and MutHBx proteins differ in their interactions with the p53 tumor suppressor protein. This was experimentally addressed through co-immunoprecipitation assays examining the interaction between WtHBx and MutHBx proteins with p53, reporter assays determining the impact of the HBx proteins on p53-mediated gene transcription, and clonogenic survival assays evaluating the effect of HBx on cell growth in lines of varying p53-expression status. Both WtHBx and MutHBx proteins physically interact with p53 protein, but have different impacts on p53-mediated gene transcription. WtHBx did not effect p53-mediated gene transcription, whereas MutHBx inhibited it (P < 0.01). MutHBx inhibited colony formation in p53-proficient cells (P < 0.01), but not p53-deficient lines. Although both HBx proteins interact with p53, they affect p53-mediated gene transcription differently. WtHBx has no effect, whereas MutHBx inhibits it. In clonogenic survival assays, MutHBx inhibited cell growth in p53-proficient cells rather than enhanced it. This suggests that for MutHBx to behave oncogenically, the p53 pathway must be crippled or absent. This study has identified some important novel ways in which WtHBx and MutHBx differentially interact with p53 and this could begin to form the cellular explanation for the association between this particular mutant and liver cancer.

Hepatitis B virus X protein impedes the DNA repair via its association with transcription factor, TFIIH

Background

Hepatitis B virus (HBV) infections play an important role in the development of hepatocellular carcinoma (HCC). HBV X protein (HBx) is a multifunctional protein that can modulate various cellular processes and plays a crucial role in the pathogenesis of HCC. HBx is known to interact with DNA helicase components of TFIIH, a basal transcriptional factor and an integral component of DNA excision repair.

Results

In this study, the functional relevance of this association was further investigated in the context to DNA repair. By site-directed mutagenesis HBx's critical residues for interaction with TFIIH were identified. Similarly, TFIIH mutants lacking ATPase domain and the conserved carboxyl-terminal domain failed to interact with HBx. Yeast and mammalian cells expressing HBx^wt conferred hypersensitivity to UV irradiation, which is interpreted as a basic deficiency in nucleotide excision repair. HBx^mut120 (Glu to Val) was defective in binding to TFIIH and failed to respond to UV.

Conclusions

We conclude that HBx may act as the promoting factor by inhibiting DNA repair causing DNA damage and accumulation of errors, thereby contributing to HCC development.

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.

Hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC): molecular mechanisms and novel paradigms

Chronic hepatitis B (HBV) infection is a major risk factor for hepatocellular carcinoma (HCC). Most HCCs complicate the evolution of an active or inactive cirrhosis. However, some tumors occur on livers with minimal histological changes; the prevalence of such cases varies from one geographical region to the other, being much higher in the Southern half of Africa (around 40% of HCCs) than in Asia, America and Europe, where at least 90% of HCCs are associated in the cirrhosis. This heterogeneity is probably a reflection of different environmental and genetic factors. This review will summarise the current knowledge on the mechanisms involved in HBV-related liver carcinogenesis. It will show in particular how viruses can be viewed as tools to discover and dissect new cellular pathways involved in cancer development and emphasize the potential synergistic effects between HBV and hepatitis C virus (HCV), as well as between viral infections and other environmental factors, such as alcohol.

Lethal-7 is down-regulated by the hepatitis B virus x protein and targets signal transducer and activator of transcription 3

BACKGROUND & AIMS

The pleiotropic hepatitis B virus (HBV) x protein (HBx), associated with hepatocellular carcinoma (HCC), has been implicated in the deregulation of cellular gene expression at the transcriptional level. To date, it remains unknown if HBx regulates the expression of miRNAs which play important roles in gene-regulation at the post-transcriptional and/or translational level.

METHODS

miRNA microarrays were employed to compare the expression of cellular miRNAs in HBx-versus control-HepG2 cells. Reverse-transcription Taqman realtime-PCR was used to examine let-7a expression in normal liver as well as paired HCC-tumor and adjacent non-tumorous liver. Let-7a miRNA was functionally characterized in cells with transiently altered let-7a expression. The direct target of let-7a was identified in silico and validated using 3'UTR-reporter assay.

RESULTS

HBx up-regulates 7 and down-regulates 11 miRNAs, including the let-7 family. HBx expression was found to have a significant inverse correlation with the expression of the highly-expressed members of the let-7 family in HCC patients, highlighting the clinical relevance of our observations. Further characterization of let-7a, the most highly expressed let-7 family member, revealed that it negatively regulates cellular proliferation partly through targeting signal transducer and activator of transcription 3 (STAT3). HBx-mediated down-regulation of let-7a and up-regulation of STAT3 supports cell proliferation in HBx cells.

CONCLUSION

This study thus represents the first demonstration of HBx's ability to deregulate cellular miRNA expression. The deregulation of the expression of the let-7 family of miRNAs by HBx may represent a potential novel pathway through which HBx acts to deregulate cell proliferation leading to hepatocarcinogenesis.

Dysregulation of hepatocyte cell cycle and cell viability by hepatitis B virus

BACKGROUND/AIMS

Dysregulation of the cell cycle is frequently associated with tumor development. Hepatitis B virus (HBV) is associated with a significant risk of developing hepatocellular carcinoma but the effects of HBV on cell cycle regulation are not completely understood.

METHODS

We have used a recombinant adeno-HBV model system to investigate the effect of infection with HBV and the replication defective lamivudine resistant mutant rtM204I mutant on hepatocyte cell cycle and cell viability.

RESULTS

Huh7 cells synchronised at the G1/S phase of the cell cycle were arrested at the G2/M following infection with rAdHBV-wt and rAdHBV-M204I. This was accompanied by increased levels of p21(cip1), p-cdc2, cyclins D, A and B. Cell viability was reduced and cleaved caspase 3 levels were increased in HBV- and rtM204I-infected cells. rAdHBV-M204I-infected Huh7 cells also demonstrated significant up-regulation of phospho-ERK, phospho-Akt, p53 and phospho-Mdm2 compared to mock-infected cells. These changes were comparable to those following infection of Huh7 cells with rAdHBV-wt.

CONCLUSION

Our results suggest that HBV, regardless of phenotype, produces cell cycle arrest and reduced hepatocyte viability. Perturbations in these cellular processes are likely to underlie HBV-associated liver oncogenic transformation and may help explain the ongoing risk of developing hepatocellular carcinoma in individuals in whom the lamivudine resistant rtM204I mutant emerges.

Deregulated direct targets of the hepatitis B virus (HBV) protein, HBx, identified through chromatin immunoprecipitation and expression microarray profiling

The hepatitis B-X (HBx) protein is strongly associated with hepatocellular carcinoma. It is implicated not to directly cause cancer but to play a role in hepatocellular carcinoma as a co-factor. The oncogenic potential of HBx primarily lies in its interaction with transcriptional regulators resulting in aberrant gene expression and deregulated cellular pathways. Utilizing ultraviolet irradiation to simulate a tumor-initiating event, we integrated chip-based chromatin immunoprecipitation (ChIP-chip) with expression microarray profiling and identified 184 gene targets directly deregulated by HBx. One-hundred forty-four transcription factors interacting with HBx were computationally inferred. We experimentally validated that HBx interacts with some of the predicted transcription factors (pTF) as well as the promoters of the deregulated target genes of these pTFs. Significantly, we demonstrated that the pTF interacts with the promoters of the deregulated HBx target genes and that deregulation by HBx of these HBx target genes carrying the pTF consensus sequences can be reversed using pTF small interfering RNAs. The roles of these deregulated direct HBx target genes and their relevance in cancer was inferred via querying against biogroup/cancer-related microarray databases using web-based NextBio(TM) software. Six pathways, including the Jak-STAT pathway, were predicted to be significantly deregulated when HBx binds indirectly to direct target gene promoters. In conclusion, this study represents the first ever demonstration of the utilization of ChIP-chip to identify deregulated direct gene targets from indirect protein-DNA binding as well as transcriptional factors directly interacting with HBx. Increased knowledge of the gene/transcriptional factor targets of HBx will enhance our understanding of the role of HBx in hepatocellular carcinogenesis and facilitate the design of better strategies in combating hepatitis B virus-associated hepatocellular carcinoma.

The effects of HBx gene on the expression of DNA repair enzymes hOGG1 and hMYHalpha mRNA in HepG2 cells

To observe the alteration in the expression of DNA repair enzymes hOGG1 and hMYHalpha and the change in 8-OHdG levels in the HBx gene-transfected cells HepG2/HBx and to explore the mechanisms of the HBV-associated hepatocellular carcinoma, the gene-transfected cells HepG2/HBx which stably expressed HBx was established, and the effect of HBx on the cell cycle and proliferation of HepG2 was examined. By using the beta-actin as the interior control, real-time polymerase chain reaction (Real-time qPCR) was employed to quantitatively detect the expression of DNA repair enzymes hOGG1 and hMYHalpha in the HepG2/HBx, the control cells HepG2 and HepG2 transfected with pcDNA3.1 vector (HepG2/pDNA3.1). The 8-OHdG levels were determined by HPLC/ECD in the established gene-transfected cells HepG2/HBx and the control cells HepG2 and HepG2/pcDNA3.1. Our results showed that the expression of DNA repair enzyme hMYHalpha in the HepG2/HBx (0.021+/-0.007) was significantly lower than that of HepG2 (0.099+/-0.041) (P<0.05) and HepG2/pDNA3.1 (0.121+/-0.005) (P<0.05). However, the no significant differences existed in the expression of DNA repair enzyme hOGG1 among the three cell strains (P>0.05). The 8-OHdG level in the HepG2/HBx was significantly higher than that in HepG2 and HepG2/pcDNA3.1 (P<0.05). It is concluded that HBx gene may inhibit the expression of DNA repair enzyme hMYHalpha mRNA to impair the ability to repair the intracellular DNA oxidative damage, to increase the oxidative DNA-adduct 8-OHdG and to affect the nucleotide excision repair function, thus participate in the occurrence and development of hepatocellular carcinoma.

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

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

Epigenetic modification induced by hepatitis B virus X protein via interaction with de novo DNA methyltransferase DNMT3A

BACKGROUND/AIMS

The hepatitis B virus X protein (HBx) has been implicated as a potential trigger of the epigenetic deregulation of some genes, but the underlying mechanisms remain unknown. The aim of this study was to identify underlying mechanisms involved in HBx-mediated epigenetic modification.

METHODS

Interactions between HBx and DNA methyltransferase (DNMT) or histone deacetylase-1 (HDAC1) were assessed by co-immunoprecipitation. DNA methylation of gene promoters was detected by bisulfite sequencing, and HBx-mediated protein binding to gene regulatory elements was evaluated by chromatin immunoprecipitation. Target gene transcriptional activity was measured by real-time polymerase chain reaction.

RESULTS

HBx can interact directly with DNMT3A and HDAC1. HBx recruited DNMT3A to the regulatory promoters of interleukin-4 receptor and metallothionein-1F and subsequently silenced their transcription via de novo DNA methylation. By contrast, the transcription of CDH6 and IGFBP3 was triggered by HBx through the deprivation of DNMT3A from their promoters. Transcriptional levels of target genes in hepatocellular carcinoma (HCC) specimens were strongly correlated with the occurrence of HBx.

CONCLUSIONS

The interaction of HBx and DNMT3A facilitates cellular epigenetic modification (via regional hypermethylation or hypomethylation) at distinct genomic loci, providing an alternative mechanism within HBx-mediated transcriptional regulation, and a profound understanding of hepatitis and HCC pathogenesis.

Ataxia telangiectasia-mutated-Rad3-related DNA damage checkpoint signaling pathway triggered by hepatitis B virus infection

AIM: To explore whether acute cellular DNA damage response is induced upon hepatitis B virus (HBV) infection and the effects of the HBV infection.

METHODS: We incubated HL7702 hepatocytes with HBV-positive serum, mimicking a natural HBV infection process. We used immunoblotting to evaluate protein expression levels in HBV-infected cells or in non-infected cells; immunofluorescence to show ATR foci ands Chk1 phosphorylation foci formation; flow cytometry to analyze the cell cycle and apoptosis; ultraviolet (UV) radiation and ionizing radiation (IR)-treated cells to mimic DNA damage; and Trypan blue staining to count the viable cells.

RESULTS: We found that HBV infection induced an increased steady state of ATR protein and increased phosphorylation of multiple downstream targets including Chk1, p53 and H2AX. In contrast to ATR and its target, the phosphorylated form of ATM at Ser-1981 and its downstream substrate Chk2 phosphorylation at Thr-68 did not visibly increase upon infection. However, the level of Mre11 and p21 were reduced beginning at 0.5 h after HBV-positive serum addition. Also, HBV infection led to transient cell cycle arrest in the S and the G2 phases without accompanying increased apoptosis. Research on cell survival changes upon radiation following HBV infection showed that survival of UV-treated host cells was greatly increased by HBV infection, owing to the reduced apoptosis. Meanwhile, survival of IR-treated host cells was reduced by HBV infection.

CONCLUSION: HBV infection activates ATR DNA damage response to replication stress and abrogates the checkpoint signaling controlled by DNA damage response.

Effects of down-regulation of p21 by HBx gene on HepG2 cell proliferation and apoptosis

AIM: To establish gene-transfected cell strain HepG2/HBx and study the effect of HBx on cell cycle, proliferation and apoptosis of HepG2 cells as well as the potential regulative role of p21.

METHODS: HBx was transfected into HepG2 cells and G418 selection was used to obtain the positive clones of HepG2/HBx cells. Then HBx mRNA expression and protein expression were detected using RT-PCR and western blot analysis respectively. MTT assay and flow cytometry were adopted to measure the proliferation, cell cycle and apoptosis of HepG2/HBx, HepG2 and HepG2/pcDNA3.1 (HepG2 cells transfected with pcDNA3.1) cells. Semi-quantified RT-PCR was used to evaluate the expression of p21 and p53 in three groups.

RESULTS: The expression of mRNA and protein of HBx in HepG2/HBx cells was confirmed by RT-PCR and western blot respectively. The proliferation of HepG2/HBx cells was accelerated. The proportion of HepG2/HBx cells decreased significantly in G₀/G₁ phase (43.34% ± 3.11% vs 57.69 ± 4.28%, P < 0.01), but increased remarkably in S phase (28.69% ± 1.17% vs 22.41% ± 1.99%, P < 0.05) and the apoptosis rate of HepG2/HBx cells was at a significantly lower level (1.19% ± 0.06% vs 5.43% ± 0.42%, P < 0.001). Compared with HepG2 and HepG2/pcDNA3.1 cells, the expression of p21 mRNA in HepG2/HBx was down-regulated (0.16 ± 0.05 vs 0.78 ± 0.15, P < 0.001), while there was no significant difference in the expression of p53 gene.

CONCLUSION: The HBx gene down-regulates the expression of p21 mRNA, which may play an important role in accelerating cell cycle, improving growth and inhibiting apoptosis of HepG2 cells.

Reactivation of p53 in cells expressing hepatitis B virus X-protein involves p53 phosphorylation and a reduction of Hdm2

Multifunctional activities of the hepatitis B virus X-protein (HBx) in cells have been largely implicated in the development of liver cancer; one of these activities is the loss of p53 function by sequestering p53 in the cytoplasm. We have previously found that doxorubicin increased the p53 levels in cells containing p53-binding HBx protein and restored the p53-mediated transcriptional activity that was suppressed by HBx. Here, we investigated the mechanism underlying p53 reactivation. We found that six phosphorylation sites of the Serine residues of p53 were efficiently phosphorylated in HBx-expressing ChangX-34 cells, suggesting that the binding of HBx to the p53 protein does not interfere with the phosphorylation of p53 by signaling kinases. In addition, doxorubicin caused a dramatic reduction of Hdm2 mRNA and protein levels in cells expressing HBx. Intriguingly, reactivation of p53 was accompanied with a nuclear accumulation of p53 and the phosphorylated p53 at Serine15 was only detected in nuclear fraction, but not in cytosolic fraction of doxorubicin-treated ChangX-34 cells. Functional restoration of the p53 protein in HBx-expressing cells occurs according to the dual effects of doxorubicin: a significant reduction of Hdm2 expression and a nuclear accumulation of the phosphorylated p53 protein. Thus, proper usage of doxorubicin as an effective antitumor agent may be reevaluated and can be extended to tumors primarily caused by infection of DNA tumor viruses.

Cellular apoptosis induced by replication of hepatitis B virus: possible link between viral genotype and clinical outcome

HBV remains one of the major pathogens of liver diseases but the outcomes as inflammation, cirrhosis and cancer of the liver are greatly related to different viral genotypes. The aim of this study was to assess the pro-apoptotic effect of HBSP from three HBV genotypes on liver derived cells. HepG2 cells were applied in our system and transfected by HBV genotype A, B, and C. Cells were observed under phase contrast microscope, stained by apoptosis marker and analyzed by flow cytometre. HBSP expression was detected by western blot assay. BH3 sequences were aligned and analyzed by Vector NTI. HBV genotypes A, B, and C transfected cells displayed evidence of cell death which was further proved as apoptosis. Natural expression of a pro-apoptotic protein HBSP was detected during genomes transfection. The different apoptotic effects were correlated to the HBSP expression from each genome. Alignment and analysis of the BH3 domains from the three genomes revealed slight variance which might also contribute to the result. Our results suggested that variant HBSP expression and BH3 sequence of HBV genotypes may be involved in differential apoptotic effect in transfected cells. Detailed analysis of the role of HBV genotypes in cellular apoptotic process should provide molecular information on the reported clinical outcome of infection by different HBV genotypes.

X protein of hepatitis B virus functions as a transcriptional corepressor on the human telomerase promoter

The X protein of hepatitis B virus (HBx) is essential for transactivation of hepatitis B viral and host cellular genes. It has been specifically implicated in the development of hepatocellular carcinoma; however, the molecular mechanism remains unknown. Telomeres, the DNA-protein complexes at the ends of eukaryotic chromosomes, protect chromosomes from degradation at the terminal regions, fusion with a broken DNA end, and inappropriate recombination. The shortening of telomeres that occurs during hepatocellular carcinogenesis has been well studied. In the present study, we isolated an HBx isoform that resulted in telomere shortening in hepatoma cell lines. We found that this HBx isoform down-regulated the expression of human telomerase by transcriptionally repressing its promoter. To further determine the molecular mechanism, we examined human telomerase promoter and identified myc-associated zinc finger protein (MAZ) as a transcriptional repressor of the promoter. We found that the HBx isoform achieved transcriptional suppression of human telomerase by enhancing MAZ binding to its consensus sequence in the promoter through physical association with MAZ.

CONCLUSION

The data suggest that HBx can induce telomere shortening by acting as a transcriptional corepressor of MAZ on the human telomerase promoter.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Using or abusing: viruses and the cellular DNA damage response

During infection, viruses attempt to hijack the cell while the host responds with various defense systems. Traditional defenses include the interferon response and apoptosis, but recent work suggests that this antiviral arsenal also includes the cellular DNA damage response machinery. The observation of interactions between viruses and cellular DNA repair proteins has not only uncovered new complexities of the virus-host interaction but is also reinforcing the view that viruses can reveal key regulators of cellular pathways through the proteins they target.

Hepatitis B virus promotes hepatocarcinogenesis in transgenic mice

HBV is a major risk factor for hepatocellular carcinoma (HCC). However, whether HBV can directly cause HCC or only indirectly via the induction of chronic liver inflammation has been controversial. By using transgenic mice carrying the entire HBV genome as a model, we now demonstrate that HBV by itself is an inefficient carcinogen. However, it can efficiently promote hepatocarcinogenesis initiated by the carcinogen diethylnitrosamine (DEN). This effect of HBV does not involve chronic liver inflammation, is apparently due to enhanced hepatocellular apoptosis and compensatory regeneration following DEN treatment, and does not require the HBV X protein.

CONCLUSION

Our results demonstrate a direct role of HBV in a hepatocarcinogenesis pathway that involves the interaction between this virus and a dietary carcinogen.

p53 mediates the negative regulation of MDM2 by orphan receptor TR3

MDM2 is an oncoprotein whose transforming potential is activated by overexpression. The expression level of MDM2 is negatively regulated by orphan receptor TR3 that mainly acts as a transcriptional factor to regulate gene expression. However, the underlying mechanism is largely unclear. Here, we present the first evidence that inhibition of TR3 on MDM2 is mediated by p53. We found that TR3 directly interacts with p53 but not MDM2, and such interaction is critical for TR3 to inhibit MDM2 expression. TR3 downregulates p53 transcriptional activity by blocking its acetylation, leading to a decrease on the transcription level of MDM2. Furthermore, TR3 binding to p53 obstructs its ubiquitination and degradation induced by MDM2, resulting in the MDM2 ubiquitination and degradation. In addition, TR3 could enhance p53-mediated apoptosis induced by UV irradiation. Taken together, our findings demonstrate that p53 mediates the suppression of TR3 on MDM2 at both transcriptional and post-transcriptional level and suggest TR3 as a potential target to develop new anticancer agents that restrict MDM2-induced tumor progression.

The HBSP gene is expressed during HBV replication, and its coded BH3-containing spliced viral protein induces apoptosis in HepG2 cells

The mechanisms of liver injury in hepatitis B virus (HBV) infection are defined to be due not to the direct cytopathic effects of viruses, but to the host immune response to viral proteins expressed by infected hepatocytes. We showed here that transfection of mammalian cells with a replicative HBV genome causes extensive cytopathic effects, leading to the death of infected cells. While either necrosis or apoptosis or both may contribute to the death of infected cells, results from flow cytometry suggest that apoptosis plays a major role in HBV-induced cell death. Data mining of the four HBV protein sequences reveals the presence of a Bcl-2 homology domain 3 (BH3) in HBSP, a spliced viral protein previously shown to be able to induce apoptosis and associated with HBV pathogenesis. HBSP is expressed at early stage of our cell-based HBV replication. When transfected into HepG2 cells, HBSP causes apoptosis in a caspase dependent manner. Taken together, our results suggested a direct involvement of HBV viral proteins in cellular apoptosis, which may contribute to liver pathogenesis.

Hepatitis B virus-related hepatocellular carcinoma: paradigms for viral-related human carcinogenesis

As discussed in detail in other chapters of this review, chronic hepatitis B (HBV) infection is a major risk factor for hepatocellular carcinoma (HCC). Most HCCs complicate the evolution of an active or inactive cirrhosis. However, some tumors occur on livers with minimal histological changes; the prevalence of such cases varies from one geographical region to the other, being much higher in the southern half of Africa (around 40% of HCCs) than in Asia, America and Europe, where at least 90% of HCCs are associated with the cirrhosis. This heterogeneity is probably a reflection of different environmental and genetic factors. This review will summarize the current knowledge on the mechanisms involved in HBV-related liver carcinogenesis. It will show in particular how viruses can be viewed as tools to discover and dissect new cellular pathways involved in cancer development and emphasize the potential synergistic effects between HBV and hepatitis C virus, as well as between viral infections and other environmental factors, such as alcohol.