Polo-like kinase 1 inhibition suppresses hepatitis B virus X protein-induced transformation in an in vitro model of liver cancer progression

Proteasome activation is critical for cell death induced by inhibitors of polo-like kinase 1 (PLK1) in multiple cancers

Inhibitors of polo-like kinase (PLK) are currently being evaluated as anticancer drugs. However, the molecular mechanism of PLK inhibitor-induced cell death is not fully understood. In this study, we found that GW843682X and BI2536, two inhibitors of PLK1, significantly induced cell death in multiple type cells. The induction of cell death was related to the preferring expression of PLK1. However, in human umbilical vascular endothelial cells (HUVEC) and human colorectal carcinoma cells, which expressed higher levels of both PLK1 and PLK2, PLK1 inhibitors induced very low levels of cell death. Clinical analysis reveals PLK1 presence in 26 of 30 NPC tumor tissues. In in vivo NPC lung metastasis nude mouse models, PLK1 inhibitors decreased NPC progress. Mechanistically, the PLK1 inhibitor did not activate p53, and the cell death was not reversed by p53 inhibition. Moreover, PLK1 inhibitor-induced cell death was PARP- and caspase-independent. Although PLK1 inhibitors induced down-regulation of calpain inhibitor calpastatin and calpain was activated by PLK1 inhibition, calpain blocking did not reverse cell death induced by PLK1 inhibitors, suggesting the non-involvement of calpain. Surprisingly, we found that PLK1 inhibitors induced the activation of proteasome, and the treatment of cells with PLK1 inhibitors reduced the levels of ubiquitinated proteins. And proteasome inhibitors reversed cell death induced by PLK1 inhibitors in various cell types in which PLK1 was preferentially expressed. Moreover, PLK1 inhibition reversed the degradation of proteins including p53, caspase 8, PARP and calpastatin. These results suggest that the activation of proteasome is critical for cell death induced by PLK1 inhibition.

Polo-like kinase-1 mediates hepatitis C virus-induced cell migration, a drug target for liver cancer

Polo-like kinase 1 (PLK1) is a regulator of cell mitosis and cytoskeletal dynamics. PLK1 overexpression in liver cancer is associated with tumour progression, metastasis, and vascular invasion. Hepatitis C virus (HCV) NS5A protein stimulates PLK1-mediated phosphorylation of host proteins, so we hypothesised that HCV-PLK1 interactions might be a mechanism for HCV-induced liver cancer. We used a HCV cell-culture model (Jc1) to investigate the effects of virus infection on the cytoskeleton. In HCV-infected cells, a novel posttranslational modification in β-actin was observed with phosphorylation at Ser239. Using in silico and in vitro approaches, we identified PLK1 as the mediating kinase. In functional experiments with a phosphomimetic mutant form of β-actin, Ser239 phosphorylation influences β-actin polymerization and distribution, resulting in increased cell motility. The changes were prevented by treating cells with the PLK1 inhibitor volasertib. In HCV-infected hepatocytes, increased cell motility contributes to cancer cell migration, invasion, and metastasis. PLK1 is an important mediator of these effects and early treatment with PLK1 inhibitors may prevent or reduce HCC progression, particularly in people with HCV-induced HCC.

Chromosome hyperploidy induced by chronic hepatitis B virus infection and its targeted therapeutic strategy

Chronic hepatitis B virus (HBV) infection induces chromosomal hyperploidy (including aneuploidy and polyploidy) and chromosomal instability in hepatocytes, which is one of the main causes of primary hepatocellular carcinoma (HCC). Although hepatocytes can regulate polyploidization of chromosomes under normal conditions, it is difficult to regulate hyperploidization caused by HBV infection and thus carcinogenesis. Studies have shown that HBV can cause dysregulation of many signal pathways such as PLK1/PRC1, and induce chromosome hyperploidy and malignant transformation of hepatocytes. Herein we review the mechanism of HBV infection-induced chromosomal hyperploidy of hepatocytes to cuase hepatocarcinogenesis and the advances in research of drugs targeting chromosomal hyperploidy.

Polo-like Kinase 1 as an emerging drug target: structure, function and therapeutic implications

Polo-like kinase 1 (PLK1) is a conserved mitotic serine-threonine protein kinase, functions as a regulatory protein, and is involved in the progression of the mitotic cycle. It plays important roles in the regulation of cell division, maintenance of genome stability, in spindle assembly, mitosis, and DNA-damage response. PLK1 is consist of a N-terminal serine-threonine kinase domain, and a C-terminal Polo-box domain (regulatory site). The expression of PLK1 is controlled by transcription repressor in the G1 stage and transcription activators in the G2 stage of the cell cycle. Overexpression of PLK1 results in undermining of checkpoints causes excessive cellular division resulting in abnormal cell growth, leading to the development of cancer. Blocking the expression of PLK1 by an antibody, RNA interference, or kinase inhibitors, causes a subsequent reduction in the proliferation of tumour cells and induction of apoptosis in tumour cells without affecting the healthy cells, suggesting an attractive target for drug development. In this review, we discuss detailed information on expression, gene and protein structures, role in different diseases, and progress in the design and development of PLK1 inhibitors. We have performed an in-depth analysis of the PLK1 inhibitors and their therapeutic implications with special focus to the cancer therapeutics.

STK39 is a novel kinase contributing to the progression of hepatocellular carcinoma by the PLK1/ERK signaling pathway

Rationale: Protein kinases are critical therapeutic targets for curing hepatocellular carcinoma (HCC). As a serine/threonine kinase, the potential roles of serine/threonine kinase 39 (STK39) in HCC remain to be explored. Methods: The expression of STK39 was examined by RT-qPCR, western blotting and immunohistochemistry. Cell proliferation and apoptosis were detected by CCK8 and TUNEL kit. Cell migration and invasion assays were performed using a transwell system with or without Matrigel. RNA-seq, mass spectrometry and luciferase reporter assays were used to identify STK39 binding proteins. Results: Here, we firstly report that STK39 was highly overexpressed in clinical HCC tissues compared with adjacent tissues, high expression of STK39 was induced by transcription factor SP1 and correlated with poor patient survival. Gain and loss of function assays revealed that overexpression of STK39 promoted HCC cell proliferation, migration and invasion. In contrast, the depletion of STK39 attenuated the growth and metastasis of HCC cells. Moreover, knockdown of STK39 induced the HCC cell cycle arrested in the G2/M phase and promoted apoptosis. In mechanistic studies, RNA-seq revealed that STK39 positively regulated the ERK signaling pathway. Mass spectrometry identified that STK39 bound to PLK1 and STK39 promoted HCC progression and activated ERK signaling pathway dependent on PLK1. Conclusions: Thus, our study uncovers a novel role of STK39/PLK1/ERK signaling axis in the progress of HCC and suggests STK39 as an indicator for prognosis and a potential drug target of HCC.

Polyploid Giant Cancer Cells, a Hallmark of Oncoviruses and a New Therapeutic Challenge

Tumors are renowned as intricate systems that harbor heterogeneous cancer cells with distinctly diverse molecular signatures, sizes and genomic contents. Among those various genomic clonal populations within the complex tumoral architecture are the polyploid giant cancer cells (PGCC). Although described for over a century, PGCC are increasingly being recognized for their prominent role in tumorigenesis, metastasis, therapy resistance and tumor repopulation after therapy. A shared characteristic among all tumors triggered by oncoviruses is the presence of polyploidy. Those include Human Papillomaviruses (HPV), Epstein Barr Virus (EBV), Hepatitis B and C viruses (HBV and HCV, respectively), Human T-cell lymphotropic virus-1 (HTLV-1), Kaposi's sarcoma herpesvirus (KSHV) and Merkel polyomavirus (MCPyV). Distinct viral proteins, for instance Tax for HTLV-1 or HBx for HBV have demonstrated their etiologic role in favoring the appearance of PGCC. Different intriguing biological mechanisms employed by oncogenic viruses, in addition to viruses with high oncogenic potential such as human cytomegalovirus, could support the generation of PGCC, including induction of endoreplication, inactivation of tumor suppressors, development of hypoxia, activation of cellular senescence and others. Interestingly, chemoresistance and radioresistance have been reported in the context of oncovirus-induced cancers, for example KSHV and EBV-associated lymphomas and high-risk HPV-related cervical cancer. This points toward a potential linkage between the previously mentioned players and highlights PGCC as keystone cancer cells in virally-induced tumors. Subsequently, although new therapeutic approaches are actively needed to fight PGCC, attention should also be drawn to reveal the relationship between PGCC and oncoviruses, with the ultimate goal of establishing effective therapeutic platforms for treatment of virus-associated cancers. This review discusses the presence of PGCCs in tumors induced by oncoviruses, biological mechanisms potentially favoring their appearance, as well as their consequent implication at the clinical and therapeutic level.

PLK1 regulates hepatic stellate cell activation and liver fibrosis through Wnt/β-catenin signalling pathway

As an outcome of chronic liver disease, liver fibrosis involves the activation of hepatic stellate cells (HSCs) caused by a variety of chronic liver injuries. It is important to explore approaches to inhibit the activation and proliferation of HSCs for the treatment of liver fibrosis. PLK1 is overexpressed in many human tumour cells and has become a popular drug target in tumour therapy. Therefore, further study of the function of PLK1 in the cell cycle is valid. In the present study, we found that PLK1 expression was elevated in primary HSCs isolated from CCl4 -induced liver fibrosis mice and LX-2 cells stimulated with TGF-β1. Knockdown of PLK1 inhibited α-SMA and Col1α1 expression and reduced the activation of HSCs in CCl4 -induced liver fibrosis mice and LX-2 cells stimulated with TGF-β1. We further showed that inhibiting the expression of PLK1 reduced the proliferation of HSCs and promoted HSCs apoptosis in vivo and in vitro. Furthermore, we found that the Wnt/β-catenin signalling pathway may be essential for PLK1-mediated HSCs activation. Together, blocking PLK1 effectively suppressed liver fibrosis by inhibiting HSC activation, which may provide a new treatment strategy for liver fibrosis.

PLK1/NF-κB feedforward circuit antagonizes the mono-ADP-ribosyltransferase activity of PARP10 and facilitates HCC progression

Dysregulation of PARP10 has been implicated in various tumor types and plays a vital role in delaying hepatocellular carcinoma (HCC) progression. However, the mechanisms controlling the expression and activity of PARP10 in HCC remain mostly unknown. The crosstalk between PLK1, PARP10, and NF-κB pathway in HCC was determined by performing different in vitro and in vivo assays, including mass spectrometry, kinase, MARylation, chromatin immunoprecipitation, and luciferase reporter measurements. Functional examination was performed by using small chemical drug, cell culture, and mice HCC models. Correlation between PLK1, NF-κB, and PARP10 expression was determined by analyzing clinical samples of HCC patients with using immunohistochemistry. PLK1, an important regulator for cell mitosis, directly interacts with and phosphorylates PARP10 at T601. PARP10 phosphorylation at T601 significantly decreases its binding to NEMO and disrupts its inhibition to NEMO ubiquitination, thereby enhancing the transcription activity of NF-κB toward multiple target genes and promoting HCC development. In turn, NF-κB transcriptionally inhibits the PARP10 promoter activity and leads to its downregulation in HCC. Interestingly, PLK1 is mono-ADP-ribosylated by PARP10 and the MARylation of PLK1 significantly inhibits its kinase activity and oncogenic function in HCC. Clinically, the expression levels of PLK1 and phosphor-p65 show an inverse correlation with PARP10 expression in human HCC tissues. These findings are the first to uncover a PLK1/PARP10/NF-κB signaling circuit that underlies tumorigenesis and validate PLK1 inhibitors, alone or with NF-κB antagonists, as potential effective therapeutics for PARP10-expressing HCC.

Antiviral activity of PLK1-targeting siRNA delivered by lipid nanoparticles in HBV-infected hepatocytes

BACKGROUND

A link between HBV and PLK1 was clearly evidenced in HBV-driven carcinogenesis, and we have also recently shown that PLK1 is a proviral factor in the early phases of HBV infection. Moreover, we have shown that BI-2536, a small molecule PLK1 inhibitor, was very efficient at inhibiting HBV DNA neosynthesis, notably by affecting nucleocapsid assembly as a result of the modulation of HBc phosphorylation. Yet, as small molecule kinase inhibitors often feature poor selectivity, a more specific and safer strategy to target PLK1 would be needed for a potential development against chronic HBV infections.

METHODS

Here, we analysed using both freshly isolated primary human hepatocytes and differentiated HepaRG, the anti-HBV properties of an LNP-encapsulated PLK1-targeting siRNA. Standard assays were used to monitor the effect of LNP siPLK1, or controls (LNP siHBV and LNP siNon-targeting), on HBV replication and cell viability.

RESULTS

A dose as low as 100 ng/ml of LNP-siPLK1 resulted in a >75% decrease in secreted HBV DNA (viral particles), which was comparable to that obtained with LNP siHBV or 10 µM of tenofovir (TFV), without affecting cell viability. Interestingly, and in contrast to that obtained with TFV, a strong inhibition of viral RNA and HBe/HBsAg secretions was also observed under LNP siPLK1 treatment. This correlated with a significant intracellular decrease of vRNA accumulation, which was independent of any change in cccDNA levels, thus suggesting a transcriptional or post-transcriptional modulation. Such an effect was not obtained with a biochemical approach of PLK1 inhibition, suggesting an enzymatic-independent role of PLK1.

CONCLUSIONS

This study emphasizes that a specific PLK1 inhibition could help in achieving an improved HBsAg loss in CHB patients, likely in combination with other HBsAg-targeting strategies.

NCAPG2 overexpression promotes hepatocellular carcinoma proliferation and metastasis through activating the STAT3 and NF-κB/miR-188-3p pathways

Background

Hepatocellular carcinoma (HCC) is a highly fatal malignant cancer worldwide. Elucidating the underlying molecular mechanism of HCC progression is critical for the identification of new therapeutic targets for HCC. This study aimed to determine the role of Non-SMC condensin II complex subunit G2 (NCAPG2) in HCC proliferation and metastasis.

Methods

We detected NCAPG2 expression in tissues using immunohistochemistry, western blotting and real-time PCR. The effects of NCAPG2 on cell proliferation and metastasis were evaluated both in vitro and in vivo. Immunocytochemistry, enzyme linked immunosorbent assay, co-immunoprecipitation and luciferase reporter assay were performed to uncover the underlying mechanisms.

Findings

We found that NCAPG2 is frequently upregulated in HCC tumour tissues and predicts a poor prognosis. NCAPG2 overexpression promotes HCC proliferation, migration, and invasion through activating STAT3 and NF-κB signalling pathways. Moreover, NCAPG2 is a direct target of miR-188-3p. We demonstrated the existence of a positive feedback loop between NCAPG2 and p-STAT3 and a negative feedback loop between NCAPG2 and miR-188-3p.

Interpretation

Our study indicates that NCAPG2 overexpression could drive HCC proliferation and metastasis through activation of the STAT3 and NF-κB/miR-188-3p pathways. These findings may contribute to the identification of novel biomarkers and therapeutic targets for HCC.

Fund

National Key Program for Science and Technology Research and Development (Grant No. 2016YFC0905902); the National Natural Scientific Foundation of China (Nos. 81772588, 81602058, 81773194); University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (Grant No. UNPYSCT-2016200); the Innovative Research Program for Graduate of Harbin Medical University (Grant Nos. YJSCX2017-38HYD, YJSCX2016-18HYD).

Hepatitis B virus X protein promotes DNA damage propagation through disruption of liver polyploidization and enhances hepatocellular carcinoma initiation

Hepatitis B virus X protein (HBx) contributes to Hepatitis B virus (HBV)-related liver cancer. However, its impact on hepatocyte proliferation and genomic stability remains elusive. We studied the role of HBx expression on the progression of cell cycle and liver polyploidization during proliferation and liver carcinogenesis. Full-length HBx transgenic mice (FL-HBx) were developed to investigate liver ploidy as well as hepatocyte proliferation, along normal liver maturation and during cancer initiation (chemical carcinogen treatment). Investigation of postnatal liver development in FL-HBx showed an aberrant G1/S and G2/M transitions, triggered (1) a delay of the formation of hepatocytes binucleation, (2) the early synthesis of polyploidy nuclei (≥4n) and (3) DNA damage appearance. Moreover, HBV infection during hepatocytes proliferation in a humanized liver mouse model led, to modifications in polyploidy of hepatocytes. In initiation of hepatocellular carcinoma, FL-HBx protein decreased ChK1 phosphorylation, Mre11 and Rad51 expression, upregulated IL-6 expression and impaired apoptosis. This was related to DNA damage accumulation in FL-HBx mice. At day 75 after initiation of hepatocellular carcinoma, FL-HBx mice revealed significant cell cycle changes related to the increased amount of 4n nuclei and of markers of cancer progenitor cells. Finally, PLK1 upregulation and p38/ERK activation in FL-HBx mice were implicated in aberrant polyploidization favoring DNA damage propagation and hepatocyte transformation. In conclusion, our data indicate that FL-HBx protein increases DNA damage through the hijack of hepatocyte polyploidization. That leads to enhancement of hepatocellular carcinoma initiation in an inflammatory context.

Detection of Viral RNA Splicing in Diagnostic Virology

This chapter is the first one to introduce the detection of viral RNA splicing as a new tool for clinical diagnosis of virus infections. These include various infections caused by influenza viruses, human immunodeficiency viruses (HIV), human T-cell leukemia viruses (HTLV), Torque teno viruses (TTV), parvoviruses, adenoviruses, hepatitis B virus, polyomaviruses, herpesviruses, and papillomaviruses. Detection of viral RNA splicing for active viral gene expression in a clinical sample is a nucleic acid-based detection. The interpretation of the detected viral RNA splicing results is straightforward without concern for carry-over DNA contamination, because the spliced RNA is smaller than its corresponding DNA template.

Although many methods can be used, a simple method to detect viral RNA splicing is reverse transcription-polymerase chain reaction (RT-PCR). In principle, the detection of spliced RNA transcripts by RT-PCR depends on amplicon selection and primer design. The most common approach is the amplification over the intron regions by a set of primers in flanking exons. A larger product than the predicted size of smaller, spliced RNA is in general an unspliced RNA or contaminating viral genomic DNA. A spliced mRNA always gives a smaller RT-PCR product than its unspliced RNA due to removal of intron sequences by RNA splicing. The contaminating viral DNA can be determined by a minus RT amplification (PCR). Alternatively, specific amplification of a spliced RNA can be obtained by using an exon-exon junction primer because the sequence at exon-exon junction is not present in the unspliced RNA nor in viral genomic DNA.

Hepatitis B Virus-Associated Hepatocellular Carcinoma and Hepatic Cancer Stem Cells

Chronic Hepatitis B Virus (HBV) infection is linked to hepatocellular carcinoma (HCC) pathogenesis. Despite the availability of a HBV vaccine, current treatments for HCC are inadequate. Globally, 257 million people are chronic HBV carriers, and children born from HBV-infected mothers become chronic carriers, destined to develop liver cancer. Thus, new therapeutic approaches are needed to target essential pathways involved in HCC pathogenesis. Accumulating evidence supports existence of hepatic cancer stem cells (hCSCs), which contribute to chemotherapy resistance and cancer recurrence after treatment or surgery. Understanding how hCSCs form will enable development of therapeutic strategies to prevent their formation. Recent studies have identified an epigenetic mechanism involving the downregulation of the chromatin modifying Polycomb Repressive Complex 2 (PRC2) during HBV infection, which results in re-expression of hCSC marker genes in infected hepatocytes and HBV-associated liver tumors. However, the genesis of hCSCs requires, in addition to the expression of hCSC markers cellular changes, rewiring of metabolism, cell survival, escape from programmed cell death, and immune evasion. How these changes occur in chronically HBV-infected hepatocytes is not yet understood. In this review, we will present the basics about HBV infection and hepatocarcinogenesis. Next, we will discuss studies describing the mutational landscape of liver cancers and how epigenetic mechanisms likely orchestrate cellular reprograming of hepatocytes to enable formation of hCSCs.

Aedes aegypti microRNA miR-2b regulates ubiquitin-related modifier to control chikungunya virus replication

Arboviruses that replicate in mosquitoes activate innate immune response within mosquitoes. Regulatory non-coding microRNAs (miRNA) are known to be modulated in mosquitoes during chikungunya infection. However, information about targets of these miRNAs is scant. The present study was aimed to identify and analyze targets of miRNAs that are regulated during chikungunya virus (CHIKV) replication in Aedes aegypti cells and in the mosquito. Employing next-generation sequencing technologies, we identified a total of 126 miRNAs from the Ae. aegypti cell line Aag2. Of these, 13 miRNAs were found to be regulated during CHIKV infection. Putative targets of three of the most significantly regulated miRNAs- miR-100, miR-2b and miR-989 were also analyzed using quantitative PCRs, in cell lines and in mosquitoes, to validate whether they were the targets of the miRNAs. Our study expanded the list of miRNAs known in Ae. aegypti and predicted targets for the significantly regulated miRNAs. Further analysis of some of these targets revealed that ubiquitin-related modifier is a target of miRNA miR-2b and plays a significant role in chikungunya replication.

Polo-like-kinase 1 is a proviral host factor for hepatitis B virus replication

Chronic hepatitis B virus (HBV) infection is a major risk factor for hepatocellular carcinoma (HCC) and current treatments for chronic hepatitis B and HCC are suboptimal. Herein, we identified cellular serine/threonine Polo-like-kinase 1 (PLK1) as a positive effector of HBV replication. The aim of this study was to demonstrate the proviral role of PLK1 in HBV biosynthesis and validate PLK1 inhibition a potential antiviral strategy. To this end, we employed physiologically relevant HBV infection models of primary human hepatocytes (PHHs) and differentiated HepaRG cells in conjunction with pharmacologic PLK1 inhibitors, small interfering RNA (siRNA)-mediated knockdown, and overexpression of constitutively active PLK1 (PLK1CA ). In addition, a humanized liver Fah-/- /Rag2-/- /Il2rg-/- (FRG) mouse model was used to determine the antiviral effect of PLK1 inhibitor BI-2536 on HBV infection in vivo. Finally, in vitro PLK1 kinase assays and site-directed mutagenesis were employed to demonstrate that HBV core protein (HBc) is a PLK1 substrate. We demonstrated that HBV infection activated cellular PLK1 in PHHs and differentiated HepaRG cells. PLK1 inhibition by BI-2536 or siRNA-mediated knockdown suppressed HBV DNA biosynthesis, whereas overexpression of PLK1CA increased it, suggesting that the PLK1 effects on viral biosynthesis are specific and that PLK1 is a proviral cellular factor. Significantly, BI-2536 administration to HBV-infected humanized liver FRG mice strongly inhibited HBV infection, validating PLK1 as an antiviral target in vivo. The proviral action of PLK1 is associated with the biogenesis of the nucleocapsid, as BI-2536 leads to its decreased intracellular formation/accumulation. In this respect, our studies identified HBc as a PLK1 substrate in vitro, and mapped PLK1 phosphorylation sites on this protein.

CONCLUSION

PLK1 is a proviral host factor that could be envisaged as a target for combined antiviral and antitumoral strategies against HBV infection and HBV-mediated carcinogenesis. (Hepatology 2017;66:1750-1765).

Regulation of PLK1 through competition between hnRNPK, miR-149-3p and miR-193b-5p

Polo-like kinase 1 (PLK1) is a critical regulator of cell cycle progression and apoptosis. However, its regulation remains poorly understood. In the present study, we investigated the molecular mechanism underlying the post-transcriptional regulation of PLK1. We observed that heterogeneous nuclear ribonucleoprotein K (hnRNPK) and PLK1 were positively associated in several different cancers and high expression levels of them correlated with poor prognosis in patients with cancer. Knockdown of hnRNPK resulted in reduced expression of PLK1, whereas conversely, PLK1 expression was increased in hnRNPK-overexpressing cells. We found that hnRNPK regulated PLK1 expression through KH1- and KH2-dependent interactions with the 3'UTR of PLK1 mRNA. In addition, microRNA-149-3p (miR-149-3p) and miR-193b-5p suppressed PLK1 expression by targeting the 3'UTR of PLK1 mRNA. MicroRNA-elicited enrichment of PLK1 mRNA in Ago2 immunoprecipitation was altered by the presence or absence of hnRNPK. Furthermore, the deletion of the cytosine (C)-rich sequences of the 3'UTR of PLK1 mRNA abolished the decreased PLK1 expression observed via hnRNPK silencing and administration of miRNAs, a finding that suggests that hnRNPK shares this C-rich motif with miR-149-3p and miR-193b-5p. We also found that downregulation of PLK1 by either silencing hnRNPK or overexpression of miR-149-3p and miR-193b-5p decreased clonogenicity and induced apoptosis. Our findings from this study demonstrate that hnRNPK regulates PLK1 expression by competing with the PLK1-targeting miRNAs, miR-149-3p and miR-193b-5p.

Volasertib suppresses the growth of human hepatocellular carcinoma in vitro and in vivo

Hepatocellular carcinoma (HCC) is the sixth most frequent malignant tumor with poor prognosis, and its clinical therapeutic outcome is poor. Volasertib, a potent small molecular inhibitor of polo-like kinase 1 (PLK1), is currently tested for treatment of multiple cancers in the clinical trials. However, the antitumor effect of volasertib on HCC is still unknown. In this study, our data show that volasertib is able to induce cell growth inhibition, cell cycle arrest at G2/M phase and apoptosis with the spindle abnormalities in human HCC cells. Furthermore, volasertib also increases the intracellular reactive oxidative species (ROS) levels, and pretreated with ROS scavenger N-acety-L-cysteine partly reverses volasertib-induced apoptosis. Moreover, volasertib markedly inhibits the subcutaneous xenograft growth of HCC in nude mice. Overall, our study provides new therapeutic potential of volasertib on hepatocellular carcinoma.

PLK1 and HOTAIR Accelerate Proteasomal Degradation of SUZ12 and ZNF198 during Hepatitis B Virus-Induced Liver Carcinogenesis

Elucidating mechanisms of hepatitis B virus (HBV)-mediated hepatocarcinogenesis is needed to gain insights into the etiology and treatment of liver cancer. Cells where HBV is replicating exhibit increased expression of Plk1 kinase and reduced levels of two transcription repression factors, SUZ12 and ZNF198. SUZ12 is an essential subunit of the transcription repressive complex PRC2. ZNF198 stabilizes the transcription repressive complex composed of LSD1, Co-REST, and HDAC1. These two transcription repressive complexes are held together by binding the long noncoding RNA HOTAIR. In this study, we linked these regulatory events mechanistically by showing that Plk1 induces proteasomal degradation of SUZ12 and ZNF198 by site-specific phosphorylation. Plk1-dependent ubiquitination of SUZ12 and ZNF198 was enhanced by expression of HOTAIR, significantly reducing SUZ12 and ZNF198 stability. In cells expressing the HBV X protein (HBx), downregulation of SUZ12 and ZNF198 mediated global changes in histone modifications. In turn, HBx-expressing cells propagated an altered chromatin landscape after cell division, as exemplified by changes in histone modifications of the EpCAM promoter, a target of PRC2 and LSD1/Co-REST/HDAC1 complexes. Notably, liver tumors from X/c-myc bitransgenic mice exhibited downregulation of SUZ12 and ZNF198 along with elevated expression of Plk1, HOTAIR, and EpCAM. Clinically, similar effects were documented in a set of HBV-related liver tumors consistent with the likelihood that downregulation of SUZ12 and ZNF198 leads to epigenetic reprogramming of infected hepatocytes. Because both Plk1 and HOTAIR are elevated in many human cancers, we propose that their combined effects are involved in epigenetic reprogramming associated broadly with oncogenic transformation.

Hepatitis B virus X protein induces EpCAM expression via active DNA demethylation directed by RelA in complex with EZH2 and TET2

Chronic hepatitis B virus (HBV) infection is a major risk factor for developing hepatocellular carcinoma (HCC), and HBV X protein (HBx) acts as cofactor in hepatocarcinogenesis. In liver tumors from animals modeling HBx- and HBV-mediated hepatocarcinogenesis, downregulation of chromatin regulating proteins SUZ12 and ZNF198 induces expression of several genes, including epithelial cell adhesion molecule (EpCAM). EpCAM upregulation occurs in HBV-mediated HCCs and hepatic cancer stem cells, by a mechanism not understood. Herein we demonstrate HBx induces EpCAM expression via active DNA demethylation. In hepatocytes, EpCAM is silenced by polycomb repressive complex 2 (PRC2) and ZNF198/LSD1/Co-REST/HDAC1 chromatin-modifying complexes. Cells with stable knockdown of SUZ12, an essential PRC2 subunit, upon HBx expression demethylate a CpG dinucleotide located adjacent to NF-κB/RelA half-site. This NF-κB/RelA site is in a CpG island downstream from EpCAM transcriptional start site (TSS). Chromatin immunoprecipitation (ChIP) assays demonstrate HBx-dependent RelA occupancy of NF-κB half-site, whereas RelA knockdown suppresses CpG demethylation and EpCAM expression. Tumor necrosis factor-α activates RelA, propagating demethylation to nearby CpG sites, shown by sodium bisulfite sequencing. RelA-dependent demethylation occurring upon HBx expression requires methyltrasferase EZH2, TET2 a key factor in cytosine demethylation and inactive DNMT3L, shown by knockdown assays and sodium bisulfite sequencing. Co-immunoprecipitations and sequential ChIP assays demonstrate that RelA in the presence of HBx forms a complex with EZH2, TET2 and DNMT3L, although the role of DNMT3L remains to be understood. Interestingly, the human EpCAM gene also has a CpG island downstream from its TSS, and a NF-κB-binding site flanked by CpGs. HepG2 cells derived from human HCC exhibit demethylation of these NF-κB-flanking CpG sites, and HBV replication propagates demethylation to nearby CpG sites. DLK1, another PRC2 target gene, also upregulated in HBV-mediated HCCs, is demethylated in liver tumors at CpG dinucleotides flanking the NF-κB-binding sequence, supporting that this active DNA demethylation mechanism functions during oncogenic transformation.

The oncogenic role of hepatitis B virus

The hepatitis B virus (HBV) is a small enveloped DNA virus that causes acute and chronic hepatitis. HBV infection is a world health problem, with 350 million chronically infected people at increased risk of developing liver disease and hepatocellular carcinoma (HCC). HBV has been classified among human tumor viruses by virtue of a robust epidemiologic association between chronic HBV carriage and HCC occurrence. In the absence of cytopathic effect in infected hepatocytes, the oncogenic role of HBV might involve a combination of direct and indirect effects of the virus during the multistep process of liver carcinogenesis. Liver inflammation and hepatocyte proliferation driven by host immune responses are recognized driving forces of liver cell transformation. Genetic and epigenetic alterations can also result from viral DNA integration into host chromosomes and from prolonged expression of viral gene products. Notably, the transcriptional regulatory protein HBx encoded by the X gene is endowed with tumor promoter activity. HBx has pleiotropic activities and plays a major role in HBV pathogenesis and in liver carcinogenesis. Because hepatic tumors carry a dismal prognosis, there is urgent need to develop early diagnostic markers of HCC and effective therapies against chronic hepatitis B. Deciphering the oncogenic mechanisms that underlie HBV-related tumorigenesis might help developing adapted therapeutic strategies.

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.

Deregulation of epigenetic mechanisms by the hepatitis B virus X protein in hepatocarcinogenesis

This review focuses on the significance of deregulation of epigenetic mechanisms by the hepatitis B virus (HBV) X protein in hepatocarcinogenesis and HBV replication. Epigenetic mechanisms, DNA methylation, and specific histone modifications, e.g., trimethylation of H3 on lysine-27 or lysine-4, maintain ‘cellular memory’ by silencing expression of lineage-inducing factors in stem cells and conversely, of pluripotency factors in differentiated cells. The X protein has been reported to induce expression of DNA methyltransferases (DNMTs), likely promoting epigenetic changes during hepatocarcinogenesis. Furthermore, in cellular and animal models of X-mediated oncogenic transformation, protein levels of chromatin modifying proteins Suz12 and Znf198 are down-regulated. Suz12 is essential for the Polycomb Repressive Complex 2 (PRC2) mediating the repressive trimethylation of H3 on lysine-27 (H3K27me3). Znf198, stabilizes the LSD1-CoREST-HDAC complex that removes, via lysine demethylase1 (LSD1), the activating trimethylation of H3 on lysine-4 (H3K4me3). Down-regulation of Suz12 also occurs in liver tumors of woodchucks chronically infected by woodchuck hepatitis virus, an animal model recapitulating HBV-mediated hepatocarcinogenesis in humans. Significantly, subgroups of HBV-induced liver cancer re-express hepatoblast and fetal markers, and imprinted genes, suggesting hepatocyte reprogramming during oncogenic transformation. Lastly, down-regulation of Suz12 and Znf198 enhances HBV replication. Collectively, these observations suggest deregulation of epigenetic mechanisms by HBV X protein influences both the viral cycle and the host cell.

Overexpression of hnRNPC2 induces multinucleation by repression of Aurora B in hepatocellular carcinoma cells

Heterogeneous ribonuclear protein C2 (hnRNPC2), an RNA binding protein, is a component of hnRNPC which is upregulated in many tumors. Multinucleation exists in many tumors and is positively correlated with tumor grade. To uncover the correlation between hnRNPC2 and multi-nucleation in hepatocellular carcinoma SMMC-7721 cells, we constructed a pEGFP-hnRNPC2 vector and transfected it into cancer cells. Our results revealed that overexpression of hnRNPC2 induced multinucleation in SMMC-7721 cells. Tracking tests indicated that the induced multinucleated cells were unable to recover to mononuclear cells and finally died as a result of defects in cell division. Furthermore, Aurora B, which was localized at the midbody and plays a role in cytokinesis, was repressed in hnRNPC2-overexpressing cells, whose knockdown by RNA interference also induced multinucleation in SMMC-7721 cells. Quantitative polymerase chain reaction (qPCR) and mRNA-protein co-immunoprecipitation results revealed that Aurora B mRNA did not decrease in hnRNPC2-overexpressing cells, instead it bound more hnRNPC2 and less eIF4E, an mRNA cap binding protein and translational initiation factor. Moreover, hnRNPC2 bound more eIF4E in hnRNPC2-overexpressing cells. These results indicate that hnRNPC2 repressed Aurora B binding with eIF4F, which must bind with Aurora B mRNA in order to initiate its translation. This induced multinucleation in hepatocellular carcinoma cells. In addition, hnRNPC2 accelerated hepatocellular carcinoma cell proliferation. Collectively, these data suggest that hnRNPC2 may be a potential target for hepatocellular carcinoma cell diagnosis and treatment.

Subset of Suz12/PRC2 target genes is activated during hepatitis B virus replication and liver carcinogenesis associated with HBV X protein

Chronic hepatitis B virus (HBV) infection is a major risk factor for developing liver cancer, and the HBV X protein (pX) has been implicated as a cofactor in hepatocyte transformation. We have shown that HBV replication as well as in vitro transformation by pX are associated with induction of the mitotic polo-like kinase 1 (Plk1) and down-regulation of the chromatin remodeling components Suz12 and Znf198. Herein, we demonstrate the same inverse relationship between Plk1 and Suz12/Znf198 in liver tumors from X/c-myc bitransgenic mice and woodchuck hepatitis virus (WHV)-infected woodchucks. Employing these animal models and the HBV replicating HepAD38 cells we examined the effect of Suz12/Znf198 down-regulation on gene expression. Genes analyzed include hepatic cancer stem cell markers BAMBI, DKK1,2, DLK1, EpCAM, MYC, and proliferation genes CCNA1, CCND2, IGFII, MCM4-6, PLK1, RPA2, and TYMS. Suz12 occupancy at the promoters of BAMBI, CCND2, DKK2, DLK1, EpCAM, and IGFII was demonstrated by chromatin immunoprecipitation in untransformed hepatocytes, but was markedly reduced in pX-transformed and Suz12 knockdown cells. Accordingly, we refer to these genes as "Suz12 repressed" genes in untransformed hepatocytes. The Suz12 repressed genes and proliferation genes were induced in HBV-replicating HepAD38 cells and, interestingly, they exhibited distinct expression profiles during hepatocellular carcinoma (HCC) progression in X/c-myc bitransgenics. Specifically, CCND2, EpCAM, and IGFII expression was elevated at the proliferative and preneoplastic stages in X/c-myc bitransgenic livers, whereas BAMBI and PLK1 were overexpressed in hepatic tumors from X/c-myc bitransgenics and WHV-infected woodchucks. Importantly, most of these genes were selectively up-regulated in HBV-induced HCCs.

CONCLUSION

The distinct expression profile of the identified Suz12 repressed genes in combination with the proliferation genes hold promise as biomarkers for progression of chronic HBV infection to HCC.

Polo-like kinase 1, a new therapeutic target in hepatocellular carcinoma

AIM

To investigate the role of polo-like kinase 1 (PLK1) as a therapeutic target for hepatocellular carcinoma (HCC).

METHODS

PLK1 gene expression was evaluated in HCC tissue and HCC cell lines. Gene knockdown with short-interfering RNA (siRNA) was used to study PLK1 gene and protein expression using real-time reverse transcription polymerase chain reaction (RT-PCR) and Western blotting, and cell proliferation using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2(4-sulfophenyl)-2H-tetrazolium (MTS) and bromodeoxyuridine (BrdU) assays. Apoptosis was evaluated using the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, and caspase-inhibition assay. Huh-7 cells were transplanted into nude mice and co-cultured with PLK1 siRNA or control siRNA, and tumor progression was compared with controls.

RESULTS

RT-PCR showed that PLK1 was overexpressed 12-fold in tumor samples compared with controls, and also was overexpressed in Huh-7 cells. siRNA against PLK1 showed a reduction in PLK1 gene and protein expression of up to 96% in Huh-7 cells, and a reduction in cell proliferation by 68% and 92% in MTS and BrdU cell proliferation assays, respectively. There was a 3-fold increase in apoptosis events, and TUNEL staining and caspase-3 assays suggested that this was caspase-independent. The pan-caspase inhibitor Z-VAD-FMK was unable to rescue the apoptotic cells. Immnofluorescence co-localized endonuclease-G to fragmented chromosomes, implicating it in apoptosis. Huh-7 cells transplanted subcutaneously into nude mice showed tumor regression in siPLK1-treated mice, but not in controls.

CONCLUSION

Knockdown of PLK1 overexpression in HCC was shown to be a potential therapeutic target, leading to apoptosis through the endonuclease-G pathway.

Diverse roles of hepatitis B virus in liver cancer

Hepatitis B virus (HBV) is a widespread human pathogen responsible for acute and chronic liver diseases. The hepatitis B burden is particularly heavy in endemic countries, where liver cirrhosis and hepatocellular carcinoma are leading causes of death. However, the oncogenic role of HBV remains enigmatic. As the virus has no cytopathic effect, liver damage is attributed to immune responses that induce inflammation, apoptosis and regeneration, fostering the accumulation of genetic and epigenetic alterations. In a more direct action, frequent integration of HBV DNA into host chromosomes may lead to insertional mutagenesis of cancer-related genes and chromosomal instability. HBV proteins, notably the HBx transactivator, participate as co-factors in oncogenesis. Better understanding of hepatitis B pathogenesis is mandatory for improving disease management.

Reduced efficacy of the Plk1 inhibitor BI 2536 on the progression of hepatocellular carcinoma due to low intratumoral drug levels

Highly promising preclinical data obtained in cultured cells and in nude mice bearing xenografts contrast with the rather modest clinical efficacy of Polo-like kinase 1 (Plk1) inhibitors. In the present study, we investigated if Plk1 might be a suitable target in hepatocellular carcinoma (HCC) and if a genetically engineered mouse tumor model that well reflects the tumor cell and micro-environmental features of naturally occurring cancers might be suitable to study anti-Plk1 therapy. Analysis of Plk1 expression in human HCC samples confirmed that HCC express much higher Plk1 levels than the adjacent normal liver tissue. Inhibition of Plk1 by an adenovirus encoding for a short hairpin RNA against Plk1 or by the small-molecule inhibitor BI 2536 reduced the viability of HCC cell lines and inhibited HCC xenograft progression in nude mice. Treatment of transforming growth factor (TGF) α/c-myc bitransgenic mice with BI 2536 during hepatocarcinogenesis reduced the number of dysplastic foci and of Ki-67-positive cells within the foci, indicating diminished tumorigenesis. In contrast, BI 2536 had no significant effect on HCC progression in the transgenic mouse HCC model as revealed by magnetic resonance imaging. Measurement of BI 2536 by mass spectrometry revealed considerably lower BI 2536 levels in HCC compared with the adjacent normal liver tissue. In conclusion, low intratumoral levels are a novel mechanism of resistance to the Plk1 inhibitor BI 2536. Plk1 inhibitors achieving sufficient intratumoral levels are highly promising in HCC treatment.

PLK1 as an oncology target: current status and future potential

The Polo-like kinases (PLKs) have been investigated as oncology targets for several years; however, only recently have potent inhibitors been described. Here, we report on progress in the clinical validation of the PLKs as antitumor drug targets as well as recent understanding gained regarding their synergistic roles in the context of other molecular defects occurring in tumors. Also relevant to the development of PLK inhibitors as therapeutics are the putative roles of other members of this family as tumor suppressors. The resulting potential drawbacks of non-isoform selective compounds are presented. As an alternative approach to achieving PLK1 specificity, we discuss prospects for developing small molecule inhibitors of the crucial regulatory and subcellular targeting domain containing the Polo-boxes.

Proteins ZNF198 and SUZ12 are down-regulated in hepatitis B virus (HBV) X protein-mediated hepatocyte transformation and in HBV replication

Chronic hepatitis B virus (HBV) infection is a major etiologic factor in hepatocellular carcinoma (HCC) pathogenesis, involving effects of chronic liver inflammation and of the weakly oncogenic HBV X protein (pX). pX-mediated hepatocyte transformation requires Polo-like kinase1 (Plk1) activity, but the mechanism is not fully understood. We identified by a genome-wide short hairpin RNA (shRNA) library screen the genes zinc finger, MYM-type 2 (ZNF198) and suppressor of zeste 12 homolog (Drosophila) (SUZ12) whose protein depletion rescues pX-expressing cells from DNA damage-induced apoptosis. ZNF198 and SUZ12 are components of chromatin remodeling complexes and associate with promyelocytic leukemia (PML) nuclear bodies. Knockdown of ZNF198 and SUZ12 by small interfering RNA (siRNA) reduced p53 stability and DNA repair, rescued pX-expressing hepatocytes from DNA damage-induced apoptosis, and increased pX-induced polyploidy and oncogenic transformation, suggesting ZNF198 and SUZ12 have a role in pX-mediated transformation. Interestingly, during pX-mediated transformation the protein but not messenger RNA (mRNA) levels of ZNF198 and SUZ12 progressively decreased, whereas Plk1 levels increased. Inhibition of Plk1 activity restored protein levels of ZNF198 and SUZ12. In addition, transfected Polo-box-domain (PBD) of Plk1 coimmunoprecipitated with ZNF198 and SUZ12, suggesting that these proteins are Plk1 substrates. Elevated Plk1 and reduced protein levels of ZNF198 and SUZ12 were also observed in human liver cancer cell lines derived from HBV-related tumors and in the presence of HBV replication. Importantly, knockdown by siRNA of ZNF198 and SUZ12 enhanced HBV replication.

CONCLUSION

Reduced protein levels of ZNF198 and SUZ12 and elevated Plk1 occur during pX-mediated hepatocyte transformation in human liver cancer cell lines, as well as during HBV replication, underscoring the significance of these genes both in HBV-mediated HCC pathogenesis and HBV replication. We propose Plk1 activity down-regulates ZNF198 and SUZ12, thereby enhancing both HBV replication and pX-mediated oncogenic transformation.

Gene signatures in hepatocellular carcinoma (HCC)

Primary hepatocellular carcinoma (HCC) is a significant human cancer globally, with poor prognosis. New and efficacious therapy strategies are needed as well as new biomarkers for early detection of at-risk patients. In this review, we discuss select microarray studies of human HCCs, and propose a gene signature that has promise for clinical/translational application. This gene signature combines the proliferation cluster of genes and the hepatic cancer initiating/stem cell gene cluster for identification of HCCs with poor prognosis. Evidence from cell-based assays identifies the existence of a mechanistic link between these two gene clusters, involving the proliferation cluster gene polo-like kinase 1 (PLK1). We propose that PLK1 is a promising therapy target for HCC.

Chronic inflammation and hepatocellular carcinoma

Hepatocellular carcinoma (HCC) invariably develops within a setting of chronic inflammation caused by either hepatotropic viruses, toxins, metabolic liver disease or autoimmunity. Mechanisms that link these two processes are not completely understood, but transcription factors of the NF-κB family and signal transducer and activator of transcription 3 (STAT3), cytokines such as IL-6 and IL-1α and ligands of the epidermal growth factor receptor (EGFR) family are clearly pivotal players. HCC may have its origins in either hepatocytes or hepatic progenitor cells (HPCs), and HCCs, like other solid tumours appear to be sustained by a minority population of cancer stem cells.

Polo-like kinase 1 activated by the hepatitis B virus X protein attenuates both the DNA damage checkpoint and DNA repair resulting in partial polyploidy

Hepatitis B virus X protein (pX), implicated in hepatocarcinogenesis, induces DNA damage because of re-replication and allows propagation of damaged DNA, resulting in partial polyploidy and oncogenic transformation. The mechanism by which pX allows cells with DNA damage to continue proliferating is unknown. Herein, we show pX activates Polo-like kinase 1 (Plk1) in the G(2) phase, thereby attenuating the DNA damage checkpoint. Specifically, in the G(2) phase of pX-expressing cells, the checkpoint kinase Chk1 was inactive despite DNA damage, and protein levels of claspin, an adaptor of ataxia telangiectasia-mutated and Rad3-related protein-mediated Chk1 phosphorylation, were reduced. Pharmacologic inhibition or knockdown of Plk1 restored claspin protein levels, Chk1 activation, and p53 stabilization. Also, protein levels of DNA repair protein Mre11 were decreased in the G(2) phase of pX-expressing cells but not with Plk1 knockdown. Interestingly, in pX-expressing cells, Mre11 co-immunoprecipitated with transfected Plk1 Polo-box domain, and inhibition of Plk1 increased Mre11 stability in cycloheximide-treated cells. These results suggest that pX-activated Plk1 by down-regulating Mre11 attenuates DNA repair. Importantly, concurrent inhibition of Plk1, p53, and Mre11 increased the number of pX-expressing cells with DNA damage entering mitosis, relative to Plk1 inhibition alone. By contrast, inhibition or knockdown of Plk1 reduced pX-induced polyploidy while increasing apoptosis. We conclude Plk1, activated by pX, allows propagation of DNA damage by concurrently attenuating the DNA damage checkpoint and DNA repair, resulting in polyploidy. We propose this novel Plk1 mechanism initiates pX-mediated hepatocyte transformation.

Molecular biology of the hepatitis B virus and role of the X gene

The hepatitis B virus (HBV) is a widespread human pathogen and a major health problem in many countries. Molecular cloning and sequencing of the viral DNA genome has demonstrated a small and compact structure organized into four overlapping reading frames that encode the viral proteins. Besides structural proteins of the core and the envelope, HBV encodes a DNA polymerase with reverse transcriptase activity, a secreted antigen of unknown function, and a transcriptional activator that is essential for viral replication. Major steps of the viral life cycle have been unraveled, including transcription of all viral RNAs from nuclear covalently closed circular DNA (cccDNA), followed by encapsidation of pregenomic RNA, a more-than-genome length transcript, and reverse transcription of pregenomic RNA leading to asymmetric synthesis of the DNA strands. Although HBV has been recognized as a human tumor virus, no direct transforming activity could be evidenced in different cellular and animal models. However, the transcriptional regulatory protein HBx encoded by the X gene is endowed with weak oncogenic activity. HBx harbors pleiotropic activities and plays a major role in HBV pathogenesis and in liver carcinogenesis.

Liver cell transformation in chronic HBV infection

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