Aberrant epigenetic modifications in hepatocarcinogenesis induced by hepatitis B virus X protein

Epigenetic regulation of signaling pathways in cancer: role of the histone methyltransferase EZH2

EZH2 is the histone H3 lysine 27 methyltransferase of polycomb-repressive complex 2. It transcriptionally silences cohorts of developmental regulators in stem/progenitors and cancer cells. EZH2 is essential in maintaining stem cell identity by globally repressing differentiation programs. Analogously, it plays a key role in oncogenesis by targeting signaling molecules that control cell differentiation. Emerging data indicate that EZH2 promotes cancer formation and progression through epigenetic activation of oncogenic signaling cascades and inhibition of pro-differentiation pathways. Genome-wide mapping analysis has been expanding the repertoire of target genes and the associated signaling pathways regulated by EZH2. Better understanding of the molecular basis of such regulations in various cancer types will help establish EZH2-mediated epigenetic silencing as a therapeutic target.

IL-32: a host proinflammatory factor against influenza viral replication is upregulated by aberrant epigenetic modifications during influenza A virus infection

Our previous studies with clinical data analysis have shown that the proinflammatory factor IL-32 is activated in response to influenza virus infection. However, little is known about how influenza virus induces IL-32 production, and the role of IL-32 in the host immune responses during viral infection remains unclear. In this study, we show that IL-32 production is stimulated by influenza A virus or dsRNA in human PBMCs from healthy volunteers. We demonstrate that the NF-κB and CREB pathways play key roles in the activation of IL-32 production in response to influenza virus infection in A549 human lung epithelial cells. We then show that aberrant epigenetic modifications in the IL32 promoter are important in the transcriptional regulation of IL-32 expression. Interestingly, one CpG demethylation within the CREB binding site increases the binding of CREB to the promoter, which is followed by IL32 transcriptional activation in influenza A virus-infected cells. Overexpression assays combined with RNA interference show that DNA methyltransferases DNMT1 and DNMT3b are critical for IL32 promoter methylation and gene silencing before viral infection. We have demonstrated the anti-influenza virus function of IL-32. Assays for each of the six IL-32 isoforms (α, β, γ, δ, ε, and ζ) during influenza virus infection indicated that all the isoforms have antiviral activity, with different inhibitory rates, and that the effect of IL-32γ is strongest. Our results indicate that the elevated IL-32 levels triggered by influenza virus infection in turn hamper viral replication.

DLEC1 Expression Is Modulated by Epigenetic Modifications in Hepatocelluar Carcinoma Cells: Role of HBx Genotypes

Deleted in Lung and Esophageal Cancer 1 (DLEC1) is a functional tumor suppressor gene (TSG). It has been found to be silenced in a variety of human cancers including hepatocellular carcinoma (HCC). The silencing of DLEC1 can be modulated by epigenetic modifications, such as DNA hypermethylation and histone hypoacetylation. In the case of HCC, hepatitis B virus X protein (HBx) has been implicated in methylation of target promoters resulting in the down-regulation of tumor suppressor genes, which in turn contributes to the development of HCC. In the present study, we first established a cell system in which epigenetic modifications can be modulated using inhibitors of either DNA methylation or histone deacetylation. The cell system was used to reveal that the expression of DLEC1 was upregulated by HBx in a genotype-dependent manner. In particular, HBx genotype A was found to decrease DNA methylation of the DLEC1 promoter. Our results have provided new insights on the impact of HBx in HCC development by epigenetic modifications.

Array-based profiling of the differential methylation status of CpG islands in hepatocellular carcinoma cell lines

Alterations in the DNA methylation status particularly in CpG islands are involved in the initiation and progression of many types of human cancer. A number of DNA methylation alterations have been reported in hepatocellular carcinoma (HCC). However, a systematic analysis is required to elucidate the relationship between differential DNA methylation status and the characteristics and progression of HCC. In the present study, a global analysis of DNA methylation using a human CpG-island 12K array was performed on a number of HCC cell lines of different origin and metastatic potential. Based on a standard methylation alteration ratio of ≥2 or ≤0.5, 58 CpG island sites and 66 tumor-related genes upstream, downstream or within were identified. This study showed a series of CpG island methylation alterations in the HCC cell lines. The expression of various oncogenes, tumor suppressor genes and other key genes were up- or downregulated, respectively, resulting in CpG island hypomethylation or hypermethylation accordingly. To conclude, a foundation has been provided for screening CpG island methylation profiles as HCC biological markers.

Viral hit and run-oncogenesis: genetic and epigenetic scenarios

It is well documented that viral genomes either inserted into the cellular DNA or co-replicating with it in episomal form can be lost from neoplastic cells. Therefore, "hit and run"-mechanisms have been a topic of longstanding interest in tumor virology. The basic idea is that the transient acquisition of a complete or incomplete viral genome may be sufficient to induce malignant conversion of host cells in vivo, resulting in neoplastic development. After eliciting a heritable change in the gene expression pattern of the host cell (initiation), the genomes of tumor viruses may be completely lost, i.e. in a hit and run-scenario they are not necessary for the maintenance of the malignant state. The expression of viral oncoproteins and RNAs may interfere not only with regulators of cell proliferation, but also with DNA repair mechanisms. DNA recombinogenic activities induced by tumor viruses or activated by other mechanisms may contribute to the secondary loss of viral genomes from neoplastic cells. Viral oncoproteins can also cause epigenetic dysregulation, thereby reprogramming cellular gene expression in a heritable manner. Thus, we expect that epigenetic scenarios of viral hit and run-tumorigenesis may facilitate new, innovative experiments and clinical studies in spite of the fact that the regular presence of a suspected human tumor virus in an early phase of neoplastic development and its subsequent regular loss have not been demonstrated yet. We propose that virus-specific "epigenetic signatures", i.e. alterations of the host cell epigenome, especially altered DNA methylation patterns, may help to identify viral hit and run-oncogenic events, even after the complete loss of tumor viruses from neoplastic cells.

Epigenetic reprogramming of host genes in viral and microbial pathogenesis

One of the key questions in the study of mammalian gene regulation is how epigenetic methylation patterns on histones and DNA are initiated and established. These stable, heritable, covalent modifications are largely associated with the repression or silencing of gene transcription, and when deregulated can be involved in the development of human diseases such as cancer. This article reviews examples of viruses and bacteria known or thought to induce epigenetic changes in host cells, and how this might contribute to disease. The heritable nature of these processes in gene regulation suggests that they could play important roles in chronic diseases associated with microbial persistence; they might also explain so-called ‘hit-and-run’ phenomena in infectious disease pathogenesis.

[Hepatitis B virus (HBV) infection and hepatocarcinogenesis]

Hepatitis B virus is one of the most important etiologic factors of hepatocellular carcinoma. The present review discusses the molecular mechanisms of virus-induced carcinogenesis, indirect and direct effects of the infection. The cell damage-evoked regeneration and proliferation, as well as the viral proteins that induce chromosomal, genetic and epigenetic changes, play a key role in the multistep process leading to malignant cell transformation. Integration of HBV DNA in to the host DNA, activation of oncogenes and inactivation of tumor suppressor genes are of basic significance. The hepatitis B virus related complications such as cirrhosis and hepatocellular carcinoma can be prevented by vaccination or eradication of the virus with antiviral therapy.

Hepatitis B virus X protein promotes hypermethylation of p16(INK4A) promoter through upregulation of DNA methyltransferases in hepatocarcinogenesis

The hepatitis B virus×protein (HBx) has been implicated as a potential trigger of the epigenetic deregulation of some genes, but the underlying mechanism remains unknown. The aim of this study is to identify underlying mechanisms involved in HBx-mediated epigenetic modification in the process of HBx induced p16(INK4A) promoter hypermethylation. Liver cell lines were stably transfected with HBx-expressing vector. The methylation status of p16(INK4A) was examined by methyl-specific polymerase chain reaction (MSP) and bisulfite sequencing. Reverse transcription and real-time polymerase chain reaction (real-time RT-PCR), Western blot and immunohistochemistry were used to analyze the expression of HBx, HBx-mediated DNA methylation abnormalities and p16(INK4A). Some cases of HCC and corresponding noncancerous liver tissues were studied. HBx up-regulates DNMT1 and DNMT3A expression in both mRNA level and protein level, and HBx represses p16(INK4A) expression through inducing hypermethylation of p16(INK4A) promoter. Moreover, HBx induces hypermethylation of p16(INK4A) promoter through DNMT1 and DNMT3A. Regulation of DNMT1 and DNMT3A by HBx promoted hypermethylation of p16(INK4A) promoter region. HBx-DNMTs-p16(INK4A) promoter hypermethylation may suggest a mechanism for tumorigenesis during hepatocarcinogenesis.

Down-regulated microRNA-152 induces aberrant DNA methylation in hepatitis B virus-related hepatocellular carcinoma by targeting DNA methyltransferase 1

The hepatitis B virus (HBV) X protein has been implicated as a potential trigger of the epigenetic modifications of some genes during hepatocarcinogenesis, but the underlying mechanisms remain unknown. MicroRNAs (miRNAs), which are noncoding RNAs that regulate gene expression, are involved in diverse biological functions and in carcinogenesis. In this study, we investigated whether some miRNAs are aberrantly expressed and involved in the regulation of the abnormal DNA methylation status in HBV-related hepatocellular carcinoma (HCC). Our results showed that the expression of microRNA-152 (miR-152) was frequently down-regulated in HBV-related HCC tissues in comparison with adjacent noncancerous hepatic tissues and was inversely correlated to DNA methyltransferase 1 (DNMT1) messenger RNA (mRNA) expression in HBV-related HCCs. The forced expression of miR-152 in liver cell lines resulted in a marked reduction of the expression of DNMT1 at both the mRNA and protein levels by directly targeting the 3' untranslated regions of DNMT1. This in turn led to a decrease in global DNA methylation, whereas inhibition of miR-152 caused global DNA hypermethylation and increased the methylation levels of two tumor suppressor genes, glutathione S-transferase pi 1 (GSTP1) and E-cadherin 1 (CDH1).

CONCLUSION

Our findings suggest that miR-152 is frequently down-regulated and regulates DNMT1 in HBV-related HCC. These findings support a tumor-suppressive role of miR-152 in the epigenetic aberration of HBV-related HCC and the potential development of miRNA-based targeted approaches for the treatment of HBV-related HCC.

DNA cytosine methylation in the bovine leukemia virus promoter is associated with latency in a lymphoma-derived B-cell line: potential involvement of direct inhibition of cAMP-responsive element (CRE)-binding protein/CRE modulator/activation transcription factor binding

Bovine leukemia virus (BLV) proviral latency represents a viral strategy to escape the host immune system and allow tumor development. Besides the previously demonstrated role of histone deacetylation in the epigenetic repression of BLV expression, we showed here that BLV promoter activity was induced by several DNA methylation inhibitors (such as 5-aza-2'-deoxycytidine) and that overexpressed DNMT1 and DNMT3A, but not DNMT3B, down-regulated BLV promoter activity. Importantly, cytosine hypermethylation in the 5'-long terminal repeat (LTR) U3 and R regions was associated with true latency in the lymphoma-derived B-cell line L267 but not with defective latency in YR2 cells. Moreover, the virus-encoded transactivator Tax(BLV) decreased DNA methyltransferase expression levels, which could explain the lower level of cytosine methylation observed in the L267(LTaxSN) 5'-LTR compared with the L267 5'-LTR. Interestingly, DNA methylation inhibitors and Tax(BLV) synergistically activated BLV promoter transcriptional activity in a cAMP-responsive element (CRE)-dependent manner. Mechanistically, methylation at the -154 or -129 CpG position (relative to the transcription start site) impaired in vitro binding of CRE-binding protein (CREB) transcription factors to their respective CRE sites. Methylation at -129 CpG alone was sufficient to decrease BLV promoter-driven reporter gene expression by 2-fold. We demonstrated in vivo the recruitment of CREB/CRE modulator (CREM) and to a lesser extent activating transcription factor-1 (ATF-1) to the hypomethylated CRE region of the YR2 5'-LTR, whereas we detected no CREB/CREM/ATF recruitment to the hypermethylated corresponding region in the L267 cells. Altogether, these findings suggest that site-specific DNA methylation of the BLV promoter represses viral transcription by directly inhibiting transcription factor binding, thereby contributing to true proviral latency.

Epigenetic activation of E-cadherin is a candidate therapeutic target in human hepatocellular carcinoma

E-cadherin is a key cell adhesion molecule implicated in tumor suppression that is frequently altered in hepatocellular carcinoma (HCC), particularly in hepatitis B virus-related tumors. Here, we report that the epigenetic drugs 5-azacytidine and trichostatin A up-regulated E-cadherin expression in HCC cells. The depletion of DNMT1 restored E-cadherin expression via demethylation, whereas the depletion of DNMT3A or DNMT3B did not. Activated E-cadherin suppressed HCC cell colony formation. However, E-cadherin expression was repressed by HBx transfection due to the DNA methylation induced by the elevation of DNMT1 in the HCC cell lines. The present study indicates that E-cadherin expression is regulated by epigenetic agents in HCC cells, which suggests a schema for restoring E-cadherin by targeting its epigenetic mechanism.

Viral hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is the fifth most common cancer and the third leading cause of cancer death worldwide. Despite recent advances in the diagnosis and treatment of HCC, its prognosis remains dismal. Infection with hepatitis B virus (HBV) and hepatitis C virus (HCV) are the major risk factors for HCC. Although both are hepatotropic viral infections, there are important differences between the oncogenic mechanisms of these two viruses. In addition to the oncogenic potential of its viral proteins, HBV, as a DNA virus, can integrate into host DNA and directly transform hepatocytes. In contrast, HCV, an RNA virus, is unable to integrate into the host genome, and viral protein expression has a more critical function in hepatocarcinogenesis. Both HBV and HCV proteins have been implicated in disrupting cellular signal transduction pathways that lead to unchecked cell growth. Most HCC develops in the cirrhotic liver, but the linkage between cirrhosis and HCC is likely multifactorial. In this review, we summarize current knowledge regarding the pathogenetic mechanisms of viral HCC.

The preclinical activity of the histone deacetylase inhibitor PXD101 (belinostat) in hepatocellular carcinoma cell lines

The activity of the histone deacetylase inhibitor PXD101 was investigated in three hepatocellular carcinoma (HCC) cell lines. PXD101 was found to inhibit cell growth at a dose-dependent manner and induce histone acetylation in PLC/PRF/5, Hep3B and HepG2 cells. In PLC/PRF/5 and Hep3B cells which express hepatitis B-related genes (HBx, HBc and HBc), treatment with PXD101 resulted in apoptosis without a significant effect on viral gene expression. Exposure to PXD101 for up to 48 h had varying effects on the expression of 12 cellular genes with tumor suppressor functions, including p21, SOCS1, CMTM5, RASAL1, DLEC1, SFRP (-1, -2, -4 and -5), ADAMTS (-8 and -9). This study provided the basis for a phase II clinical trial of PXD101 in inoperable hepatitis-B associated HCC.

Expression of pituitary tumor-transforming gene 1 (PTTG1)/securin in hepatitis B virus (HBV)-associated liver diseases: evidence for an HBV X protein-mediated inhibition of PTTG1 ubiquitination and degradation

Chronic infection with hepatitis B virus (HBV) is strongly associated with hepatocellular carcinoma (HCC), and the viral HBx protein plays a crucial role in the pathogenesis of liver tumors. Because the protooncogene pituitary tumor-transforming gene 1 (PTTG1) is overexpressed in HCC, we investigated the regulation of this protein by HBx. We analyzed PTTG1 expression levels in liver biopsies from patients chronically infected with HBV, presenting different disease stages, and from HBx transgenic mice. PTTG1 was undetectable in biopsies from chronic hepatitis B patients or from normal mouse livers. In contrast, hyperplastic livers from transgenic mice and biopsies from patients with cirrhosis, presented PTTG1 expression which was found mainly in HBx-expressing hepatocytes. PTTG1 staining was further increased in HCC specimens. Experiments in vitro revealed that HBx induced a marked accumulation of PTTG1 protein without affecting its messenger RNA levels. HBx expression promoted the inhibition of PTTG1 ubiquitination, which in turn impaired its degradation by the proteasome. Glutathione S-transferase pull-down and co-immunoprecipitation experiments demonstrated that the interaction between PTTG1 and the Skp1-Cul1-F-box ubiquitin ligase complex (SCF) was partially disrupted, possibly through a mechanism involving protein-protein interactions of HBx with PTTG1 and/or SCF. Furthermore, confocal analysis revealed that HBx colocalized with PTTG1 and Cul1. We propose that HBx promotes an abnormal accumulation of PTTG1, which may provide new insights into the molecular mechanisms of HBV-related pathogenesis of progressive liver disease leading to HCC development.

Proteomic analysis of HBV-associated HCC: insights on mechanisms of disease onset and biomarker discovery

The development of hepatocellular carcinoma (HCC) can be considered as an end-stage outcome of chronic hepatitis B virus (HBV) infection. Early prognostic markers are needed to allow effective treatments and prevent HCC from developing. Proteomics analysis has been used to identify markers from clinical samples from HCC patients. This approach can be further improved by identifying early biomarkers before the onset of HCC. One way would be to use the cell-based HBV replication system, which is reflective of the early stage of virus infection and thus secreted proteins identified at this stage may have relevance in HCC prognosis. In this review, we focus the discussion on the current status of proteomics analysis of cellular proteins and HCC biomarker identification, with a special highlight on the potential of the cell-based HBV replication system for the identification of prognostic HCC biomarkers.

Hepatitis B virus replication induces methylation of both host and viral DNA

Control of viral replication is a major therapeutic goal to reduce morbidity and mortality from chronic hepatitis B virus (HBV) infection. Recently, methylation has been identified as a novel host defense mechanism, and methylation of viral DNA leads to downregulation of HBV gene expression. To better understand the mechanisms of HBV methylation, cell lines were exposed to HBV using a model system that mimics natural infection and the expression of host DNA methyltransferase genes (DNMTs) was measured. DNMT1, DNMT2, and DNMT3 were all significantly upregulated in response to HBV. DNMT3 was further studied because of its known role in the de novo methylation of DNA. Cotransfection experiments with full-length HBV and DNMT3 led to the downregulation of viral protein and pregenomic RNA production. To investigate whether the upregulation of DNMTs could also have an effect on the methylation of host DNA, cell lines were exposed to HBV in two independent model systems, one that mimics natural infection and a second model with temporary transfection. Host DNA methylation was measured by DNA microarray analysis. Increased methylation of host CpG islands was detected in both experimental systems. Two CpG islands, corresponding to genes SUFU and TIRAP, were selected, and the downregulation of these genes in hepatocellular carcinomas was confirmed. In conclusion, hepatocytes respond to HBV infection by upregulating DNMTs. The DNMTs methylate viral DNA, leading to decreased viral gene expression and decreased viral replication. However, virus-induced overexpression of DNMTs also leads to methylation of host CpG islands.

Hepatitis B virus X protein induces hypermethylation of p16(INK4A) promoter via DNA methyltransferases in the early stage of HBV-associated hepatocarcinogenesis

The aim of the present study was to authenticate the involvement of DNA methyltransferases (DNMTs) and methyl-CpG binding domain protein 2 (MBD2) in the process of HBx induced p16(INK4A) promoter hypermethylation in HBV-related hepatocellular carcinoma (HCC) and their corresponding noncancerous liver tissues. Eighty-eight fresh tissue specimens of surgically resected HBV-associated HCC and their corresponding noncancerous liver tissues were studied. The methylation status of the p16(INK4A) promoter was determined by methylation-specific polymerase chain reaction (MSP). Reverse transcription and real-time polymerase chain reaction (RT-PCR) showed the expression of DNMTs, MBD2 and HBx. Western blot and immunohistochemistry were used for the protein analysis of HBx, DNMT1, DNMT3A and P16. Tissue HBV-DNA levels were determined by RT-PCR. HBV genotype was examined by nested PCR and restriction fragment length polymorphism (RFLP). In the corresponding noncancerous liver tissues, higher HBx expression was associated with the hypermethylation of the p16(INK4A) promoter. HBx was positively correlated with the DNMT1 and DNMT3A at both the mRNA and protein level. Furthermore, HBx, DNMT1 and DNMT3A protein expression were negatively correlated with p16 protein expression. In HCC tissues, HBx was positively correlated with DNMT1 and DNMT3A at both mRNA and protein level, but HBx expression did not correlate with hypermethylation of the p16(INK4A) promoter or p16 protein expression. The methylation status of the p16(INK4A) promoter did not correlate with clinicopathological characteristics. DNMT1 and DNMT3A may play important roles in the process of HBx inducing hypermethylation of the p16(INK4A) promoter in the early stages of HBV-associated HCC. HBx-DNMTs-p16(INK4A) promoter hypermethylation may constitute a mechanism for tumorigenesis during HBV-associated hepatocarcinogenesis.

Viral epigenomes in human tumorigenesis

Viruses are associated with 15-20% of human cancers worldwide. In the last century, many studies were directed towards elucidating the molecular mechanisms and genetic alterations by which viruses cause cancer. The importance of epigenetics in the regulation of gene expression has prompted the investigation of virus and host interactions not only at the genetic level but also at the epigenetic level. In this study, we summarize the published epigenetic information relating to the genomes of viruses directly or indirectly associated with the establishment of tumorigenic processes. We also review aspects such as viral replication and latency associated with epigenetic changes and summarize what is known about epigenetic alterations in host genomes and the implications of these for the tumoral process. The advances made in characterizing epigenetic features in cancer-causing viruses have improved our understanding of their functional mechanisms. Knowledge of the epigenetic changes that occur in the genome of these viruses should provide us with markers for following cancer progression, as well as new tools for cancer therapy.

Epigenetic silence of ankyrin-repeat-containing, SH3-domain-containing, and proline-rich-region- containing protein 1 (ASPP1) and ASPP2 genes promotes tumor growth in hepatitis B virus-positive hepatocellular carcinoma

The ankyrin-repeat-containing, SH3-domain-containing, and proline-rich-region-containing protein (ASPP) family of proteins regulates apoptosis through interaction with p53 and its family members. This study evaluated the epigenetic regulation of ASPP1 and ASPP2 in hepatitis B virus (HBV)-positive hepatocellular carcinoma (HCC) and explores the effects of down-regulation of ASPP1 and ASPP2 on the development of HCC. HCC cell lines and tissues from HCC patients were used to examine the expression and methylation of ASPP1 and ASPP2. The expression of ASPP1 and ASPP2 was diminished in HCC cells by epigenetic silence owing to hypermethylation of ASPP1 and ASPP2 promoters. Analyses of 51 paired HCC and surrounding nontumor tissues revealed that methylation of ASPP1 and ASPP2 was associated with the decreased expression of ASPP1 and ASPP2 in tumor tissues and the early development of HCC. Moreover, ASPP2 became methylated upon HBV x protein (HBx) expression. The suppressive effects on tumor growth by ASPP1 and ASPP2 were examined with RNA interference-mediated gene silence. Down-regulation of ASPP1 and ASPP2 promoted the growth of HCC cells in soft agar and in nude mice and decreased the sensitivity of HCC cells to apoptotic stimuli.

CONCLUSION

ASPP1 and ASPP2 genes are frequently down-regulated by DNA methylation in HBV-positive HCC, which may play important roles in the development of HCC. These findings provide new insight into the molecular mechanisms leading to hepatocarcinogenesis and may have potent therapeutic applications.

Genome-wide DNA methylation profiles in liver tissue at the precancerous stage and in hepatocellular carcinoma

To clarify genome-wide DNA methylation profiles during hepatocarcinogenesis, bacterial artificial chromosome (BAC) array-based methylated CpG island amplification was performed on 126 tissue samples. The average numbers of BAC clones showing DNA hypo- or hypermethylation increased from noncancerous liver tissue obtained from patients with hepatocellular carcinomas (HCCs) (N) to HCCs. N appeared to be at the precancerous stage, showing DNA methylation alterations that were correlated with the future development of HCC. Using Wilcoxon test, 25 BAC clones, whose DNA methylation status was inherited by HCCs from N and were able to discriminate 15 N samples from 10 samples of normal liver tissue obtained from patients without HCCs (C) with 100% sensitivity and specificity, were identified. The criteria using the 25 BAC clones were able to discriminate 24 additional N samples from 26 C samples in the validation set with 95.8% sensitivity and 96.2% specificity. Using Wilcoxon test, 41 BAC clones, whose DNA methylation status was able to discriminate patients who survived more than 4 years after hepatectomy from patients who suffered recurrence within 6 months and died within a year after hepatectomy, were identified. The DNA methylation status of the 41 BAC clones was correlated with the cancer-free and overall survival rates of patients with HCC. Multivariate analysis revealed that satisfying the criteria using the 41 BAC clones was an independent predictor of overall outcome. Genome-wide alterations of DNA methylation may participate in hepatocarcinogenesis from the precancerous stage, and DNA methylation profiling may provide optimal indicators for carcinogenetic risk estimation and prognostication.

Nuclear HBx binds the HBV minichromosome and modifies the epigenetic regulation of cccDNA function

HBV cccDNA, the template for transcription of all viral mRNAs, accumulates in the nucleus of infected cells as a stable episome organized into minichromosomes by histones and non-histone viral and cellular proteins. Using a cccDNA-specific chromatin immunoprecipitation (ChIP)-based quantitative assay, we have previously shown that transcription of the HBV minichromosome is regulated by epigenetic changes of cccDNA-bound histones and that modulation of the acetylation status of cccDNA-bound H3/H4 histones impacts on HBV replication. We now show that the cellular histone acetyltransferases CBP, p300, and PCAF/GCN5, and the histone deacetylases HDAC1 and hSirt1 are all recruited in vivo onto the cccDNA. We also found that the HBx regulatory protein produced in HBV replicating cells is recruited onto the cccDNA minichromosome, and the kinetics of HBx recruitment on the cccDNA parallels the HBV replication. As expected, an HBV mutant that does not express HBx is impaired in its replication, and exogenously expressed HBx transcomplements the replication defects. p300 recruitment is severely impaired, and cccDNA-bound histones are rapidly hypoacetylated in cells replicating the HBx mutant, whereas the recruitment of the histone deacetylases hSirt1 and HDAC1 is increased and occurs at earlier times. Finally, HBx mutant cccDNA transcribes significantly less pgRNA. Altogether our results further support the existence of a complex network of epigenetic events that influence cccDNA function and HBV replication and identify an epigenetic mechanism (i.e., to prevent cccDNA deacetylation) by which HBx controls HBV replication.

iTRAQ-coupled 2-D LC-MS/MS analysis of protein profile associated with HBV-modulated DNA methylation

The development of hepatocellular carcinoma (HCC) is believed to be associated with multiple risk factors, including the infection of hepatitis B virus (HBV). Based on the analysis of individual genes, evidence has indicated the association between HCC and HBV and has also been expanded to epigenetic regulation, with an involvement of HBV in the DNA methylation of the promoter of cellular target genes leading to changes in their expression. Proteomic study has been widely used to map a comprehensive protein profile, which in turn could provide a better understanding of underlying mechanisms of disease onset. In the present study, we performed a proteomic profiling by using iTRAQ-coupled 2-D LC/MS-MS analysis to identify cellular genes down-regulated in HBV-producing HepG2.2.15 cells compared with HepG2 cells. A total of 15 proteins including S100A6 and Annexin A2 were identified by our approach. The significance of these cellular proteins as target of HBV-mediated epigenetic regulation was supported by our validation assays, including their reactivation in cells treated with 5-aza-2'-deoxycytidine (a DNA methyltransferase inhibitor) by real-time RT-PCR and Western blot analysis, as well as the DNA methylation status analysis by bisulfite genome sequencing. Our approach provides a comprehensive analysis of cellular target proteins to HBV-mediated epigenetic regulation and further analysis should facilitate a better understanding of its involvement in HCC development.

Overexpression of DNA methyltransferase 1 and its biological significance in primary hepatocellular carcinoma

AIM: To explore the relationship between DNA methyltransferase 1 (DNMT1) and hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC) and its biological significance in primary HCC.

METHODS: We carried out an immunohistochemical examination of DNMT1 in both HCC and paired non-neoplastic liver tissues from Chinese subjects. DNMT1 mRNA was further examined in HCC cell lines by real-time PCR. We inhibited DNMT1 using siRNA and detected the effect of depletion of DNMT1 on cell proliferation ability and cell apoptosis in the HCC cell line SMMC-7721.

RESULTS: DNMT1 protein expression was increased in HCCs compared to histologically normal non-neoplastic liver tissues and the incidence of DNMT1 immunoreactivity in HCCs correlated significantly with poor tumor differentiation (P = 0.014). There were more cases with DNMT1 overexpression in HCC with HBV (42.85%) than in HCC without HBV (28.57%). However, no significant difference in DNMT1 expression was found in HBV-positive and HBV-negative cases in the Chinese HCC group. There was a trend that DNMT1 RNA expression increased more in HCC cell lines than in pericarcinoma cell lines and normal liver cell lines. In addition, we inhibited DNMT1 using siRNA in the SMMC-7721 HCC cell line and found depletion of DNMT1 suppressed cells growth independent of expression of proliferating cell nuclear antigen (PCNA), even in HCC cell lines where DNMT1 was stably decreased.

CONCLUSION: The findings implied that DNMT1 plays a key role in HBV-related hepatocellular tumorigenesis. Depletion of DNMT1 mediates growth suppression in SMMC-7721 cells.

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.

The dynamic DNA methylomes of double-stranded DNA viruses associated with human cancer

The natural history of cancers associated with virus exposure is intriguing, since only a minority of human tissues infected with these viruses inevitably progress to cancer. However, the molecular reasons why the infection is controlled or instead progresses to subsequent stages of tumorigenesis are largely unknown. In this article, we provide the first complete DNA methylomes of double-stranded DNA viruses associated with human cancer that might provide important clues to help us understand the described process. Using bisulfite genomic sequencing of multiple clones, we have obtained the DNA methylation status of every CpG dinucleotide in the genome of the Human Papilloma Viruses 16 and 18 and Human Hepatitis B Virus, and in all the transcription start sites of the Epstein-Barr Virus. These viruses are associated with infectious diseases (such as hepatitis B and infectious mononucleosis) and the development of human tumors (cervical, hepatic, and nasopharyngeal cancers, and lymphoma), and are responsible for 1 million deaths worldwide every year. The DNA methylomes presented provide evidence of the dynamic nature of the epigenome in contrast to the genome. We observed that the DNA methylome of these viruses evolves from an unmethylated to a highly methylated genome in association with the progression of the disease, from asymptomatic healthy carriers, through chronically infected tissues and pre-malignant lesions, to the full-blown invasive tumor. The observed DNA methylation changes have a major functional impact on the biological behavior of the viruses.

Hypomethylation of long interspersed nuclear element-1 in hepatocellular carcinomas

Recent studies have revealed the epigenetic alterations are involved in hepatocarcinogenesis. However, the function of long interspersed nuclear element-1 hypomethylation in hepatocellular carcinomas, and relationship among other clinicopathologic features, and genetic and epigenetic alterations, including CpG island hypermethylation, have not been studied. We determined long interspersed nuclear element-1 methylation, a marker of global methylation, in 57 tumor and nonneoplastic samples, including 24 from high-risk and 33 from low-risk countries. We compared methylation levels of long interspersed nuclear element-1 with eight CpG islands including p16, cyclooxygenase-2, T-type calcium channel, and estrogen receptor genes, and MINT31, MINT1, MINT2, and MINT27, as well as CpG island methylator phenotype and p53 gene mutation. Most hepatocellular carcinomas samples (88%) showed hypomethylation of long interspersed nuclear element-1, with a mean level of global methylation of 58+/-14 compared to 77+/-6 in nonneoplastic hepatic tissue (P<0.001). Levels of long interspersed nuclear element-1 hypomethylation differed depending on geographic location (P=0.02), status of hepatitis (P=0.01), hypermethylation of p16, estrogen receptor and MINT2 (P=0.01, 0.002, and 0.045, respectively), CpG island methylator phenotype-positive status (P=0.006), and p53 gene mutation (P=0.04). In conclusion, environmental factors such as geographic location and hepatitis status contribute to hepatocarcinogenesis through global hypomethylation. In hepatocellular carcinomas, hypermethylation of CpG islands, and CpG island methylator phenotype status seems to correlate with levels of long interspersed nuclear element-1 hypomethylation.

Microbe-induced epigenetic alterations in host cells: the coming era of patho-epigenetics of microbial infections. A review

It is well documented that the double-stranded DNA (dsDNA) genomes of certain viruses and the proviral genomes of retroviruses are regularly targeted by epigenetic regulatory mechanisms (DNA methylation, histone modifications, binding of regulatory proteins) in infected cells. In parallel, proteins encoded by viral genomes may affect the activity of a set of cellular promoters by interacting with the very same epigenetic regulatory machinery. This may result in epigenetic dysregulation and subsequent cellular dysfunctions that may manifest in or contribute to the development of pathological changes (e.g. initiation and progression of malignant neoplasms; immunodeficiency). Bacteria infecting mammals may cause diseases in a similar manner, by causing hypermethylation of key cellular promoters at CpG dinucleotides (promoter silencing, e.g. by Campylobacter rectus in the placenta or by Helicobacter pylori in gastric mucosa). I suggest that in addition to viruses and bacteria, other microparasites (protozoa) as well as macroparasites (helminths, arthropods, fungi) may induce pathological changes by epigenetic reprogramming of host cells they are interacting with. Elucidation of the epigenetic consequences of microbe-host interactions (the emerging new field of patho-epigenetics) may have important therapeutic implications because epigenetic processes can be reverted and elimination of microbes inducing patho-epigenetic changes may prevent disease development.

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.

Current progress in epigenetic research for hepatocarcinomagenesis

Hepatocellular carcinoma is the main type of primary liver cancer, and also one of the most malignant tumors. At present, the pathogenesis mechanisms of liver cancer are not entirely clear. It has been shown that inactivation of tumor suppressor genes and activation of oncogenes play a significant role in carcinogenesis, caused by the genetic and epigenetic aberrance. In the past, people generally thought that genetic mutation is a key event of tumor pathogenesis, and somatic mutation of tumor suppressor genes is in particular closely associated with oncogenesis. With deeper understanding of tumors in recent years, increasing evidence has shown that epigenetic silencing of those genes, as a result of aberrant hypermethylation of CpG islands in promoters and histone modification, is essential to carcinogenesis and metastasis. The term epigenetics refers to heritable changes in gene expression caused by regulation mechanisms, other than changes in the underlying DNA sequence. Specific epigenetic processes include DNA methylation, genome imprinting, chromotin remodeling, histone modification and microRNA regulations. This paper reviews recent epigenetics research progress in the hepatocellular carcinoma study, and tries to depict the relationships between hepatocellular carcinomagenesis and DNA methylation as well as microRNA regulation.

Methylation of Tip30 promoter is associated with poor prognosis in human hepatocellular carcinoma

PURPOSE

To investigate Tip30 promoter methylation status in human hepatocellular carcinoma (HCC) and the correlation with clinicopathologic features and prognosis.

EXPERIMENTAL DESIGN

The methylation status of CpG islands in Tip30 promoter was examined in 15 HCC cell lines as well as 59 paired HCC and adjacent nontumor tissues. The associations between Tip30 methylation status and the survival of patients were analyzed.

RESULTS

Tip30 promoter was hypermethylated in 6 of 10 HCC cell lines with reduced Tip30 mRNA. DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine, greatly enhanced TIP30 expression and sensitized HCC cells to cytotoxic drug-induced cell death. The promoter region of Tip30 was identified and the main promoter activity was located in the -135 to -45 region sited within a CpG island. The minimal promoter element contained four Sp1 binding sites, which were hypermethylated in HCC cell-derived promoters. Moreover, analyses of Tip30 promoter methylation status in 59 paired HCC tissues showed that 47% of the cases were hypermethylated. Recurrence rate (95% versus 67%; P = 0.011) and mortality (82% versus 53%; P = 0.033) were significantly higher in patients with methylated Tip30. Disease-free survival was significantly higher in patients with unmethylated Tip30 (33.3% versus 4.5%; P = 0.036).

CONCLUSIONS

Our results show that epigenetic silencing of Tip30 gene expression by CpG island DNA hypermethylation is associated with poor prognosis in patients with HCC.

Hepatitis B virus X protein upregulates expression of SMYD3 and C-MYC in HepG2 cells

The carcinogenic role of Hepatitis B X (HBX) in hepatocellular carcinoma (HCC) remains largely unknown. Histone H3 lysine 4 methyltransferase SMYD3 was found to be over-expressed and have a pro-carcinogenic effect in HCC. The role of HBX in regulating SMYD3 activity and the corresponding C-MYC gene in HCC carcinogenesis was investigated. SMYD3 and C-MYC expression in HBV-negative HepG2 and HBV-positive HepG2.2.15 were detected by real time PCR and Western blot. After transfection of HBX into HepG2, SMYD3 and C-MYC protein expression was detected and the apoptosis and proliferation of hepatoma cells were assayed. After SMYD3 expression in HepG2 with HBX transfection downregulated by siRNA, the corresponding C-MYC expression, cellular apoptosis, and proliferation were assayed by FACS. SMYD3 mRNA and protein and C-MYC protein were significantly higher in HepG2.2.15 than in HepG2. HBX transfection resulted in enhanced SMYD3 and C-MYC expressions, decreased cell apoptosis, and increased cell proliferation in HepG2 cells. Knocking down of SMYD3 in HepG2 with HBX transfection inhibited C-MYC expression and promoted apoptosis. These results suggest that HBX upregulates SMYD3 expression in HepG2, which may promote hepatoma development and progress. C-MYC may act as a down-stream gene in HBX-SMYD3-related hepatocarcinogenesis.

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

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

Hepatitis B virus-X protein recruits histone deacetylase 1 to repress insulin-like growth factor binding protein 3 transcription

Hepatitis B virus (HBV), a major causative agent of hepatocelluar carcinoma (HCC), encodes an oncogenic X-protein (HBx) which has been known as a transcriptional transactivator on multiple viral and celluar promoters. In the report, we verified that HBx transcriptionally repress insulin-like growth factor binding protein-3 (IGFBP-3) by promoting HBx/histone deacetylase 1 (HDAC1) complex formation. HBx recruited HDAC1 forms complex with Sp1 in a p53-independent manner) and deacetylates Sp1 which resulted in the diminished binding of Sp1 on targeted DNA during transcriptional repression. Deacetylation of Sp1 by HBx recruited HDAC1 likely to be a part of the mechanism that controls HBx induced IGFBP-3 repression and the modification of chromatin structure.

HBx genotype D represses GSTP1 expression and increases the oxidative level and apoptosis in HepG2 cells

Epigenetics has been implicated in human cancer development. Epigenetic factors include HBx protein, which is able to induce hypermethylation and suppresses tumor suppressor genes. One of such tumor suppressor genes, GSTP1, shows reduced expression in many human cancers. Hypermethylation of GSTP1 is the most studied mechanism of its silence. In the present study, we reported that GSTP1 expression was completely depleted in HBV integrated HepG2.2.15 cells due to the hypermethylation in its promoter region. And it was HBx, especially HBx genotype D, that played the key role in repressing GSTP1 expression. Further functional studies like ROS assay and apoptosis detection were also used to confirm this repression. Our findings should facilitate the understanding of HBV and their influences on the epigenetic modulations for epigenetic tumorigenesis during HBV-mediated hepatocellular carcinogenesis.

HBV X protein: elucidating a role in oncogenesis

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

Variable DNA methylation patterns associated with progression of disease in hepatocellular carcinomas

Hepatocellular carcinoma (HCC) most commonly arises from chronic inflammation due to viral infection, as a result of genetic and epigenetic abnormalities. A global picture of epigenetic changes in HCC is lacking. We used methylated CpG island amplification microarrays (MCAMs) to study 6458 CpG islands in HCC and adjacent preneoplastic tissues [chronic hepatitis (CH) or liver cirrhosis (LC)] in comparison with normal liver tissues where neither viral infection nor hepatitis has existed. MCAM identified 719 (11%) prominent genes of hypermethylation in HCCs. HCCs arising from LC had significantly more methylation than those arising from CH (1249 genes or 19% versus 444 genes or 7%, P < 0.05). There were four patterns of aberrant methylation: Type I (4%, e.g. matrix metalloproteinase 14) shows a substantially high methylation level in adjacent tissue and does not increase further in cancer. Type II (55%, e.g. RASSF1A) shows progressively increasing methylation from adjacent tissue to HCC. Type III (4%, e.g. GNA14) shows decreased methylation in adjacent tissue but either similar or increased methylation in HCC. Type IV (37%, e.g. CDKN2A) shows low levels of methylation in normal tissue and adjacent tissue but high levels in HCC. These DNA methylation changes were confirmed by quantitative pyrosequencing methylation analysis in representative 24 genes and were analyzed for correlation with clinicopathological parameters in 38 patients. Intriguingly, methylation in the Type IV genes is characteristic of moderately/poorly differentiated cancer. Our global epigenome analysis reveals distinct patterns of methylation that are probably to represent different pathophysiologic processes in HCCs.

Tumor suppressor and hepatocellular carcinoma

A few signaling pathways are driving the growth of hepatocellular carcinoma. Each of these pathways possesses negative regulators. These enzymes, which normally suppress unchecked cell proliferation, are circumvented in the oncogenic process, either the over-activity of oncogenes is sufficient to annihilate the activity of tumor suppressors or tumor suppressors have been rendered ineffective. The loss of several key tumor suppressors has been described in hepatocellular carcinoma. Here, we systematically review the evidence implicating tumor suppressors in the development of hepatocellular carcinoma.

Viral hepatocarcinogenesis: from infection to cancer

Hepatocellular carcinoma (HCC) is a worldwide health issue that has started receiving attention but is still poorly understood. However, the hepatitis B virus (HBV) and the hepatitis C virus (HCV) are known to be two major causative agents of HCC. They differ in their modes of infection, their treatment options, their genomes and their carcinogenic abilities. However, both share a link with HCC through alterations of the host genome. In order to continue in our search for the mechanisms behind viral hepatocarcinogenesis, the individual entities (HBV, HCV, HCC and host), their natural history, treatment options and genomic properties must be further understood. Additionally, an understanding of the genomics, the link between the entities, is crucial for the success of the ongoing search for therapeutic options for HCC. Similar to most types of cancer, hepatocarcinogenesis is a multistep process involving different genetic alterations that ultimately lead to malignant transformation of the hepatocyte. As technology advances and research continues, the genetic changes and influences among these entities will prove essential to improved diagnostic and therapeutic options. It remains a challenge to provide a clear picture of the connection between virus and cancer. We review (i) the epidemiological link between HBV/HCV infection to HCC; (ii) prevention and control of chronic hepatitis B or C in reducing HCC risk; and (iii) genetic characters of viruses and hosts and the mechanisms associated with HCC susceptibilities, with the intention of providing a direction for future research and treatment.

Molecular pathogenesis of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer and the third leading cause of cancer death worldwide. Hepatocarcinogenesis is a multistep process evolving from normal through chronic hepatitis/cirrhosis and dysplastic nodules to HCC. With advances in molecular methods, there is a growing understanding of the molecular mechanisms in hepatocarcinogenesis. Hepatocarcinogenesis is strongly linked to increases in allelic losses, chromosomal changes, gene mutations, epigenetic alterations and alterations in molecular cellular pathways. Some of these alterations are accompanied by a stepwise increase in the different pathological disease stages in hepatocarcinogenesis. Overall, a detailed understanding of the underlying molecular mechanisms involved in the progression of HCC is of fundamental importance to the development of effective prevention and treatment regimes for 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.

Hepatocellular carcinoma: epidemiology and clinical aspects

Liver cancer is one of the most frequent solid cancers that kills more than 650,000 people around the world each year. Though great improvements have been done in last 10 years on the understanding the molecular mechanisms involved in liver oncogenesis, the prognosis of patients affected by liver cancer is still poor for most of them. Even in those where a relatively early diagnosis is done, the course of the disease is often fatal due to the underlying liver cirrhosis. In this review authors report the most recent findings on the pathogenesis of liver cancer and on therapeutic approaches, included those emerging from the most recent literature.