Chemical hepatocarcinogenesis in transgenic mice overexpressing mature TGF beta-1 in liver

Igf2bp2 knockdown improves CCl4-induced liver fibrosis and TGF-β-activated mouse hepatic stellate cells by regulating Tgfbr1

Progressive liver fibrosis is a dynamic process characterized by the net accumulation of extracellular matrix (ECM), which could eventually develop into cirrhosis, leading to malignant transformation. In this study, insulin-like growth factor 2 mRNA binding protein 2 (Igf2bp2) was found to be up-regulated in carbon tetrachloride (CCl₄)-induced liver fibrosis and transforming growth factor-beta 1 (TGF-β)-activated hepatic stellate cells (HSCs). Igf2bp2 knockdown in the CCl₄-induced hepatic fibrosis mice model significantly improved CCl₄-induced liver damage by decreasing necrosis and fibrotic septa, reducing hydroxyproline levels, and down-regulating fibrotic markers levels. In TGF-β-activated HSCs, Igf2bp2 knockdown partially attenuated TGF-β-induced cellular effects by suppressing HSCs viability and DNA synthesis and reducing the ECM-associated factors such as α-SMA, COLLAGEN I, and COLLAGEN III. Integrative network and signaling analysis revealed that the Igf2bp2 could bind to Tgfbr1. Transforming growth factor-beta receptor 1 (Tgfbr1) was found to be significantly up-regulated in the fibrotic liver and activated HSCs, and positively correlated with Igf2bp2. Tgfbr1 knockdown partially eliminated TGF-β-induced fibrotic changes and Igf2bp2 overexpression effects on TGF-β-activated HSCs in vitro. Moreover, Igf2bp2 overexpression promoted the phosphorylation of SMAD2/SMAD3, AKT, and PI3K, whereas Tgfbr1 knockdown exhibited the opposite effect; Tgfbr1 knockdown also partially attenuated the effects of Igf2bp2 overexpression on the phosphorylation of SMAD2/SMAD3, AKT, and PI3K. In closing, Igf2bp2 and Tgfbr1 are up-regulated in CCl₄-induced liver fibrosis and TGF-β-activated mHSCs. Igf2bp2 knockdown improved CCl₄-induced liver fibrosis and TGF-β-activated HSCs by targeting Tgfbr1, possibly through the PI3K/Akt pathway.

Proliferative polyploid cells give rise to tumors via ploidy reduction

Polyploidy is a hallmark of cancer, and closely related to chromosomal instability involved in cancer progression. Importantly, polyploid cells also exist in some normal tissues. Polyploid hepatocytes proliferate and dynamically reduce their ploidy during liver regeneration. This raises the question whether proliferating polyploids are prone to cancer via chromosome missegregation during mitosis and/or ploidy reduction. Conversely polyploids could be resistant to tumor development due to their redundant genomes. Therefore, the tumor-initiation risk of physiologic polyploidy and ploidy reduction is still unclear. Using in vivo lineage tracing we here show that polyploid hepatocytes readily form liver tumors via frequent ploidy reduction. Polyploid hepatocytes give rise to regenerative nodules with chromosome aberrations, which are enhanced by ploidy reduction. Although polyploidy should theoretically prevent tumor suppressor loss, the high frequency of ploidy reduction negates this protection. Importantly, polyploid hepatocytes that undergo multiple rounds of cell division become predominantly mononucleated and are resistant to ploidy reduction. Our results suggest that ploidy reduction is an early step in the initiation of carcinogenesis from polyploid hepatocytes.

In Vivo Lineage Tracing of Polyploid Hepatocytes Reveals Extensive Proliferation during Liver Regeneration

The identity of cellular populations that drive liver regeneration after injury is the subject of intense study, and the contributions of polyploid hepatocytes to organ regeneration and homeostasis have not been systematically assessed. Here, we developed a multicolor reporter allele system to genetically label and trace polyploid cells in situ. Multicolored polyploid hepatocytes undergo ploidy reduction and subsequent re-polyploidization after transplantation, providing direct evidence of the hepatocyte ploidy conveyor model. Marker segregation revealed that ploidy reduction rarely involves chromosome missegregation in vivo. We also traced polyploid hepatocytes in several different liver injury models and found robust proliferation in all settings. Importantly, ploidy reduction was seen in all injury models studied. We therefore conclude that polyploid hepatocytes have extensive regenerative capacity in situ and routinely undergo reductive mitoses during regenerative responses.

Hepatic stellate cells as key target in liver fibrosis

Progressive liver fibrosis, induced by chronic viral and metabolic disorders, leads to more than one million deaths annually via development of cirrhosis, although no antifibrotic therapy has been approved to date. Transdifferentiation (or "activation") of hepatic stellate cells is the major cellular source of matrix protein-secreting myofibroblasts, the major driver of liver fibrogenesis. Paracrine signals from injured epithelial cells, fibrotic tissue microenvironment, immune and systemic metabolic dysregulation, enteric dysbiosis, and hepatitis viral products can directly or indirectly induce stellate cell activation. Dysregulated intracellular signaling, epigenetic changes, and cellular stress response represent candidate targets to deactivate stellate cells by inducing reversion to inactivated state, cellular senescence, apoptosis, and/or clearance by immune cells. Cell type- and target-specific pharmacological intervention to therapeutically induce the deactivation will enable more effective and less toxic precision antifibrotic therapies.

Mouse models of liver cancer

Hepatocellular carcinoma (HCC) is the sixth most common cancer worldwide, and the third leading cause of cancer mortality. The great majority of patients are not eligible for curative therapies, and therapeutic approaches for advanced disease show only limited efficacy. Difficulties to treat HCC are due to the heterogenous genetic alterations of HCC, profound alterations in the hepatic microenvironment, and incomplete understanding of HCC biology. Mouse models of HCC will be helpful to improve our understanding of HCC biology, the contributions of the specific pathways and genetic alterations to carcinogenesis. In addition, mouse models of HCC may contribute to elucidate the role of the tumor microenvironment, and serve as models for preclinical studies. As no single mouse model is appropriate to study all of the above, we discuss key features and limitations of commonly used models. Furthermore, we provide detailed protocols for select models, in which HCC is induced genetically, chemically or by transplantation of tumor cells.

Leptin receptor somatic mutations are frequent in HCV-infected cirrhotic liver and associated with hepatocellular carcinoma

BACKGROUND & AIMS

Hepatocellular carcinoma develops in patients with chronic hepatitis or cirrhosis via a stepwise accumulation of various genetic alterations. To explore the genetic basis of development of hepatocellular carcinoma in hepatitis C virus (HCV)-associated chronic liver disease, we evaluated genetic variants that accumulate in nontumor cirrhotic liver.

METHODS

We determined the whole exome sequences of 7 tumors and background cirrhotic liver tissues from 4 patients with HCV infection. We then performed additional sequencing of selected exomes of mutated genes, identified by whole exome sequencing, and of representative tumor-related genes on samples from 22 cirrhotic livers with HCV infection. We performed in vitro and in vivo functional studies for one of the mutated genes.

RESULTS

Whole exome sequencing showed that somatic mutations accumulated in various genes in HCV-infected cirrhotic liver tissues. Among the identified genes, the leptin receptor gene (LEPR) was one of the most frequently mutated in tumor and nontumor cirrhotic liver tissue. Selected exome sequencing analyses detected LEPR mutations in 12 of 22 (54.5%) nontumorous cirrhotic livers. In vitro, 4 of 7 (57.1%) LEPR mutations found in cirrhotic livers reduced phosphorylation of STAT3 to inactivate LEPR-mediated signaling. Moreover, 40% of Lepr-deficient (C57BL/KsJ-db/db) mice developed liver tumors after administration of thioacetamide compared with none of the control mice.

CONCLUSIONS

Based on analysis of liver tissue samples from patients, somatic mutations accumulate in LEPR in cirrhotic liver with chronic HCV infection. These mutations could disrupt LEPR signaling and increase susceptibility to hepatocarcinogenesis.

Dissecting the effect of targeting the epidermal growth factor receptor on TGF-β-induced-apoptosis in human hepatocellular carcinoma cells

BACKGROUND & AIMS

Transforming growth factor-beta (TGF-β) induces apoptosis in hepatocytes, a process that is inhibited by the epidermal growth factor receptor (EGFR) pathway. The aim of this work was to ablate EGFR in hepatocellular carcinoma (HCC) cells to understand its role in impairing TGF-β-induced cell death.

METHODS

Response to TGF-β in terms of apoptosis was analyzed in different HCC cell lines and the effect of canceling EGFR expression was evaluated.

RESULTS

TGF-β induces apoptosis in some HCC cells (such as Hep3B, PLC/PRF/5, Huh7, or SNU449), but it also mediates survival signals, coincident with the up-regulation of EGFR ligands. Inhibition of the EGFR, either by targeted knock-down with specific siRNA or by pharmacological inhibition, significantly enhances apoptotic response. TGF-β treatment in EGFR targeted knock-down cells correlates with higher levels of the NADPH oxidase NOX4 and changes in the expression profile of BCL-2 and IAP families. However, other HCC cells, such as HepG2, which show over activation of the Ras/ERKs pathway, SK-Hep1, with an endothelial phenotype, or SNU398, where the TGF-β-Smad signaling is altered, show apoptosis resistance that is not restored through EGFR blockade.

CONCLUSIONS

The inhibition of EGFR in HCC may enhance TGF-β-induced pro-apoptotic signaling. However, this effect may only concern those tumors with an epithelial phenotype which do not bear alterations in TGF-β signaling nor exhibit an over-activation of the survival pathways downstream of the EGFR.

Comparison of angiotensin converting enzyme inhibitors and angiotensin II type 1 receptor blockade for the prevention of premalignant changes in the liver

AIM

We investigate and compare the possible antitumor activity of clinically used angiotensin converting enzyme (ACE) inhibitors; captopril, perindopril and angiotensin II type 1 receptor (AT1R) blocker, losartan against hepatocarcinogenesis initiated by diethylnitrosoamines (DENA) and promoted by carbon tetrachloride (CCl(4)).

MAIN METHODS

Diethylnitrosamine (DENA) (200mg/kgi.p.) initiated and carbon tetrachloride (CCl(4)) (2ml/kgi.p.) promoted hepatocarcinogenesis in male Wistar rats after 8weeks.

RESULTS

Hepatocarcinogenesis was manifested biochemically by elevation of serum hepatic tumor markers tested; α-feto protein (AFP) and carcinoembryonic antigen (CEA). In addition, hepatic carcinogenesis was further confirmed by a significant increase in hepatic tissue growth factors; vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (FGF). Moreover a marked increase in matrix metalloproteinase-2 and hydroxyproline content were also observed. Hepatocarcinogenesis was further confirmed by a significant decrease in hepatic endostatin and metallothonein level.

KEY FINDINGS

Long-term administration of the selected drugs for 2weeks before and throughout the experimental period produced a significant protection against hepatic carcinogenesis. The present results claimed that different doses of the selected drugs succeeded in normalization of serum tumor markers. Furthermore, the drugs reduced the elevated level in the hepatic growth factors, matrix metalloproteinase-2 and hydroxyproline induced by the hepatocarcinogen. Moreover, the amelioration was also accompanied by augmentation of hepatic content of metallothionein and endostatin. Histopathological examination of liver tissues of rats treated with DENA-CCl(4) correlated with the biochemical observations.

SIGNIFICANCE

These findings suggest a similar protective effect of ACE inhibitors; captopril; perindopril and AT1R blocker, losartan against premalignant stages of liver cancer in the DENA initiated and CCl(4) promoted hepatocarcinogenesis model in rats. Therefore, RAS especially angiotensin II (Ang II) and AT1R interaction plays a pivotal role hepatocarcinogenesis development.

Ginger ingredients inhibit the development of diethylnitrosoamine induced premalignant phenotype in rat chemical hepatocarcinogenesis model

To investigate the possible antitumor activity of ginger extract against hepatic carcinogenesis initiated by diethylnitrosoamines (DEN) and promoted by carbon tetrachloride (CCl(4) ). A total of 60 male Wistar albino rats were divided into four groups with 15 animals in each group. Rats in group 1 (control group) received a single intraperitoneal (i.p.) injection of normal saline. Animals in group 2 were given ginger (50 mg/kg/day) in drinking water for 8 weeks. Rats in group 3 (DEN group) were injected with a single dose of DEN (200 mg/kg, i.p.), 2 weeks later received a single dose of CCl(4) (2 mL/kg i.g) by gavage as 1:1 dilution in corn oil. Animals in group 4 (DEN-ginger group) received the same carcinogenesis induction protocol as in group 3 plus ginger (50 mg/kg/day) in drinking water for 2 weeks before induction of hepatocarcinogenesis and continued throughout the experimental period. DEN-initiated and CCl(4) -promoted hepatocarcinogenesis in male Wistar rats was manifested biochemically by elevation of serum hepatic tumor markers tested; α-fetoprotein and carcinoembryonic antigen. In addition, hepatocarcinogenesis was further confirmed by a significant increase in hepatic tissue growth factors; vascular endothelial growth factor, basic fibroblast growth factor, and hydroxyproline content. A marked decrease in endostatin and metallothonein were also observed. Long-term ginger extract administration 2 weeks before induction of hepatocarcinogenesis and throughout the experimental period prevented the decrease of the hepatic content of metallothionein and endostatin and the increase in the growth factors induced by the carcinogen. Moreover, ginger extract normalize serum hepatic tumor markers. Histopathological examination of liver tissue also correlated with the biochemical observations. These findings suggest a protective effect of ginger extract against premalignant stages of liver cancer in the DEN-initiated and CCl(4) -promoted hepatocarcinogenesis model in rats.

Mechanisms of liver fibrosis associated with experimental Fasciola hepatica infection: roles of Fas2 proteinase and hepatic stellate cell activation

We have evaluated the possible mechanisms of liver fibrosis caused by Fasciola hepatica in an animal model and in culture using immortalized human stellate cells. Liver biopsies of F. hepatica-infected rats were performed at wk 8 and 16. Serum-starved LX-2 cells, a human stellate cell line, were exposed to increasing concentrations of Fas2 antigen. The expression of key fibrosis-related genes was evaluated by qRT-PCR. There was a significant correlation between fibrogenic gene expression and both intensity and duration of infection. LX-2 cells exposed to Fas2 showed progressively increased expression of mRNAs for Collagen I, alpha-smooth muscle-actin, platelet-derived growth factor beta receptor, and tissue inhibitor of metalloproteinase II; inhibition of Fas2 cysteine proteinase activity by E-64 abrogated these increases, suggesting that the protease activity of Fas2 is involved in fibrogenic stimulation. In summary, F. hepatica infection is associated with up-regulation of mRNAs associated with hepatic fibrogenesis in vivo and in activated hepatic stellate cells.

Cytosolic phospholipase A2alpha and peroxisome proliferator-activated receptor gamma signaling pathway counteracts transforming growth factor beta-mediated inhibition of primary and transformed hepatocyte growth

Hepatocellular carcinoma often develops in the setting of abnormal hepatocyte growth associated with chronic hepatitis and liver cirrhosis. Transforming growth factor beta (TGF-beta) is a multifunctional cytokine pivotal in the regulation of hepatic cell growth, differentiation, migration, extracellular matrix production, stem cell homeostasis, and hepatocarcinogenesis. However, the mechanisms by which TGF-beta influences hepatic cell functions remain incompletely defined. We report herein that TGF-beta regulates the growth of primary and transformed hepatocytes through concurrent activation of Smad and phosphorylation of cytosolic phospholipase A(2)alpha (cPLA(2)alpha), a rate-limiting key enzyme that releases arachidonic acid for the production of bioactive eicosanoids. The interplays between TGF-beta and cPLA(2)alpha signaling pathways were examined in rat primary hepatocytes, human hepatocellular carcinoma cells, and hepatocytes isolated from newly developed cPLA(2)alpha transgenic mice.

CONCLUSION

Our data show that cPLA(2)alpha activates peroxisome proliferator-activated receptor gamma (PPAR-gamma) and thus counteracts Smad2/3-mediated inhibition of cell growth. Therefore, regulation of TGF-beta signaling by cPLA(2)alpha and PPAR-gamma may represent an important mechanism for control of hepatic cell growth and hepatocarcinogenesis.

Proliferative drive and liver carcinogenesis: too much of a good thing?

There have been innumerable studies published in the attempt to identify gene expression signatures in hepatocellular carcinoma (HCC). When all the regulators and targets of the differentially expressed genes are analyzed from larger studies, the most striking theme is upregulation of mitosis-promoting and cell proliferation genes in HCC compared with 'liver-specific gene clusters' in non-tumorous tissue. A major limitation of expression profiling is that it only provides a 'snapshot' of what is an evolving process and thus cannot distinguish the differences in gene expression that are primary effectors of dysregulated growth from those that represent downstream consequences. The development of HCC in a chronically diseased liver, often referred to as hepatocarcinogenesis, is a multistep process characterized by the progressive accumulation and interplay of genetic alterations causing aberrant growth, malignant transformation of liver parenchymal cells, followed by vascular invasion and metastasis. This review will discuss HCC precursor lesions, draw on the 'proliferation cluster' genes highlighted from HCC expression profiling studies, relate them to a selection of regulatory networks important in liver regeneration, cell cycle control and their potential significance in the pathogenesis of HCC or primary liver cancer.

Experimental models of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common and deadly cancer whose pathogenesis is incompletely understood. Comparative genomic studies from human HCC samples have classified HCCs into different molecular subgroups; yet, the unifying feature of this tumor is its propensity to arise upon a background of inflammation and fibrosis. This review seeks to analyze the available experimental models in HCC research and to correlate data from human populations with them in order to consolidate our efforts to date, as it is increasingly clear that different models will be required to mimic different subclasses of the neoplasm. These models will be instrumental in the evaluation of compounds targeting specific molecular pathways in future preclinical studies.

Differential intracellular signalling induced by TGF-beta in rat adult hepatocytes and hepatoma cells: implications in liver carcinogenesis

The transforming growth factor-beta (TGF-beta) regulates hepatocyte growth, inhibiting proliferation and inducing apoptosis. Indeed, escaping from the TGF-beta suppressor actions might be a prerequisite for liver tumour progression. In this work we show that TGF-beta plays a dual role in regulating apoptosis in FaO rat hepatoma cells, since, in addition to its pro-apoptotic effect, TGF-beta also activates survival signals, such as AKT, the epidermal growth factor receptor (EGFR) being required for its activation. TGF-beta induces the expression of the EGFR ligands transforming growth factor-alpha (TGF-alpha) and heparin-binding EGF-like growth factor (HB-EGF) and induces intracellular re-localization of the EGFR. Cells that overcome the apoptotic effects of TGF-beta undergo morphological changes reminiscent of an epithelial-mesenchymal transition (EMT) process. In contrast, TGF-beta does not activate AKT in adult hepatocytes, which correlates with lack of EGFR transactivation and no response to EGFR inhibitors. Although TGF-beta induces TGF-alpha and HB-EGF in adult hepatocytes, these cells show very low expression of TACE/ADAM 17 (TNF-alpha converting enzyme), which is required for EGFR ligand proteolysis and activation. Furthermore, adult hepatocytes do not undergo EMT processes in response to TGF-beta, which might be due, at least in part, to the fact that F-actin re-organization induced by TGF-beta in FaO cells require the EGFR pathway. Finally, results indicate that EGFR transactivation does not block TGF-beta-induced cell cycle arrest in FaO cells, but must be interfering with the pro-apoptotic signalling. In conclusion, TGF-beta is a suppressor factor for adult quiescent hepatocytes, but not for hepatoma cells, where it plays a dual role, both suppressing and promoting carcinogenesis.

Immunotherapy of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the third leading cause of cancer death, with over a million new cases annually. It is generally advanced upon detection due to underlying liver disease, which further complicates treatment. Most of the therapeutic strategies in current use (surgery, transplantation, irradiation or chemotherapy) are either palliative or only of benefit to a small percentage of patients. This article reviews the biology of HCC, including many of the molecular changes and mechanisms leading to HCC development. This article discusses the recent innovative strategies to interfere with the progression of HCC, including novel gene therapy strategies. The most recent data supporting the use of immunotherapy for hepatocellular cancer is reviewed in detail.

Biology of transforming growth factor beta in hepatocarcinogenesis

TGF-beta is an important factor in the regulation of liver growth. It is an inhibitor of hepatocyte DNA synthesis and may induce active cell death, e.g., to remove excessive tissue mass. Studies using transgenic mice suggest that expression in the resting liver has to be well balanced; either under- or overexpression appear to cause an increased turnover of hepatocytes and to predispose to hepatocarcinogenesis. TGF-beta overexpression is frequently observed in human hepatocellular carcinomas, probably as a late event in tumor development. In men and mice, TGF-beta overexpression appears to be associated with loss of TGF-beta responsiveness often by disruption of TGF-beta signaling. However, mechanisms as mutations in TGF-beta receptor II or Smad2 and 4 genes, frequently observed in other human cancers, have only rarely been observed in hepatocellular carcinomas. Further studies may clarify the mechanisms by which hepatocellular tumors escape TGF-beta growth control, as well as analyze possible roles of TGF-beta overexpression in immunosuppression and angiogenesis.

Role of transforming growth factor beta in cancer

Transforming growth factor beta (TGF-beta) is an effective and ubiquitous mediator of cell growth. The significance of this cytokine in cancer susceptibility, cancer development and progression has become apparent over the past few years. TGF-beta plays various roles in the process of malignant progression. It is a potent inhibitor of normal stromal, hematopoietic, and epithelial cell growth. However, at some point during cancer development the majority of transformed cells become either partly or completely resistant to TGF-beta growth inhibition. There is growing evidence that in the later stages of cancer development TGF-beta is actively secreted by tumor cells and not merely acts as a bystander but rather contributes to cell growth, invasion, and metastasis and decreases host-tumor immune responses. Subtle alteration of TGF-beta signaling may also contribute to the development of cancer. These various effects are tissue and tumor dependent. Identifying and understanding TGF-beta signaling pathway abnormalities in various malignancies is a promising avenue of study that may yield new modalities to both prevent and treat cancer. The nature, prevalence, and significance of TGF-beta signaling pathway alterations in various forms of human cancer as well as potential preventive and therapeutic interventions are discussed in this review.

Biology of transforming growth factor beta in hepatocarcinogenesis

TGF-beta is an important factor in the regulation of liver growth. It is an inhibitor of hepatocyte DNA synthesis and may induce active cell death, e.g., to remove excessive tissue mass. Studies using transgenic mice suggest that expression in the resting liver has to be well balanced; either under- or overexpression appear to cause an increased turnover of hepatocytes and to predispose to hepatocarcinogenesis. TGF-beta overexpression is frequently observed in human hepatocellular carcinomas, probably as a late event in tumor development. In men and mice, TGF-beta overexpression appears to be associated with loss of TGF-beta responsiveness often by disruption of TGF-beta signaling. However, mechanisms as mutations in TGF-beta receptor II or Smad2 and 4 genes, frequently observed in other human cancers, have only rarely been observed in hepatocellular carcinomas. Further studies may clarify the mechanisms by which hepatocellular tumors escape TGF-beta growth control, as well as analyze possible roles of TGF-beta overexpression in immunosuppression and angiogenesis.

Role of transforming growth factor beta in cancer

Transforming growth factor beta (TGF-beta) is an effective and ubiquitous mediator of cell growth. The significance of this cytokine in cancer susceptibility, cancer development and progression has become apparent over the past few years. TGF-beta plays various roles in the process of malignant progression. It is a potent inhibitor of normal stromal, hematopoietic, and epithelial cell growth. However, at some point during cancer development the majority of transformed cells become either partly or completely resistant to TGF-beta growth inhibition. There is growing evidence that in the later stages of cancer development TGF-beta is actively secreted by tumor cells and not merely acts as a bystander but rather contributes to cell growth, invasion, and metastasis and decreases host-tumor immune responses. Subtle alteration of TGF-beta signaling may also contribute to the development of cancer. These various effects are tissue and tumor dependent. Identifying and understanding TGF-beta signaling pathway abnormalities in various malignancies is a promising avenue of study that may yield new modalities to both prevent and treat cancer. The nature, prevalence, and significance of TGF-beta signaling pathway alterations in various forms of human cancer as well as potential preventive and therapeutic interventions are discussed in this review.

Hepatitis viruses and hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is among the most frequent malignancies worldwide. Hepatitis viruses, such as the hepatitis B virus (HBV) and hepatitis C virus (HCV) are undoubtedly listed in the etiology of HCC. Studies show that, in the near future, viral hepatitis will carry increasing weight in the etiology of HCC. This review briefly discusses the known carcinogenic effects of HBV and HCV in the light of experimental and human studies. The data show that viral proteins may directly interfere with gene products responsible for cell proliferation and cell growth. Many other signal transduction cascades may be affected as well. Direct integration of HBV viral sequences into the host genome increases the genomic instability. The genomic imbalance allows the development and survival of malignant clones bearing defected genomic information. HBV and HCV infection induces indirect and direct mechanisms through cellular damage, increased regeneration and cell proliferation, therefore enhancing the development of HCC.