Hepatitis B virus X protein increases expression of p21(Cip-1/WAF1/MDA6) and p27(Kip-1) in primary mouse hepatocytes, leading to reduced cell cycle progression

The N-linked glycosylation modifications in the hepatitis B surface protein impact cellular autophagy, HBV replication, and HBV secretion

N-linked glycosylation is a pivotal post-translational modification that significantly influences various aspects of protein biology. Autophagy, a critical cellular process, is instrumental in cell survival and maintenance. The hepatitis B virus (HBV) has evolved mechanisms to manipulate this process to ensure its survival within host cells. Significantly, post-translational N-linked glycosylation in the large surface protein of HBV (LHBs) influences virion assembly, infectivity, and immune evasion. This study investigated the role of N-linked glycosylation of LHBs in autophagy, and its subsequent effects on HBV replication and secretion. LHBs plasmids were constructed by incorporating single-, double-, and triple-mutated N-linked glycosylation sites through amino acid substitutions at N4, N112, and N309. In comparison to the wild-type LHBs, N-glycan mutants, including N309Q, N4-309Q, N112-309Q, and N4-112-309Q, induced autophagy gene expression and led to autophagosome accumulation in hepatoma cells. Acridine orange staining of cells expressing LHBs mutations revealed impaired lysosomal acidification, suggesting potential blockage of autophagic flux at later stages. Furthermore, N-glycan mutants increased the mRNA expression of HBV surface antigen (HBsAg). Notably, N309Q significantly elevated HBx oncogene level. The LHBs mutants, particularly N309Q and N112-309Q, significantly enhanced HBV replication, whereas N309Q, N4-309Q, and N4-112-309Q markedly increased HBV progeny secretion. Remarkably, our findings demonstrated that autophagy is indispensable for the impact of N-linked glycosylation mutations in LHBs on HBV secretion, as evidenced by experiments with a 3-methyladenine (3-MA) inhibitor. Our study provides pioneering insights into the interplay between N-linked glycosylation mutations in LHBs, host autophagy, and the HBV life cycle. Additionally, we offer a new clue for further investigation into carcinogenesis of hepatocellular carcinoma (HCC). These findings underscore the potential of targeting either N-linked glycosylation modifications or the autophagic pathway for the development of innovative therapies against HBV and/or HCC.

The oncogenic role of hepatitis B virus X gene in hepatocarcinogenesis: recent updates

Hepatocellular carcinoma (HCC) is the most prevalent form of primary liver cancers with high mortality rate. Among its various etiological factors, one of the major risk factors for HCC is a chronic infection of hepatitis B virus (HBV). HBV X protein (HBx) has been identified to play an important role in the HBV-induced HCC pathogenesis since it may interfere with several key regulators of many cellular processes. HBx localization within the cells may be beneficial to HBx multiple functions at different phases of HBV infection and associated hepatocarcinogenesis. HBx as a regulatory protein modulates cellular transcription, molecular signal transduction, cell cycle, apoptosis, autophagy, protein degradation pathways, and host genetic stability via interaction with various factors, including its association with various non-coding RNAs. A better understanding on the regulatory mechanism of HBx on various characteristics of HCC would provide an overall picture of HBV-associated HCC. This article addresses recent data on HBx role in the HBV-associated hepatocarcinogenesis.

The role of regulatory T cells and follicular T helper cells in HBV infection

Hepatitis B has become one of the major global health threats, especially in developing countries and regions. Hepatitis B virus infection greatly increases the risk for liver diseases such as cirrhosis and cancer. However, treatment for hepatitis B is limited when considering the huge base of infected people. The immune response against hepatitis B is mediated mainly by CD8+ T cells, which are key to fighting invading viruses, while regulatory T cells prevent overreaction of the immune response process. Additionally, follicular T helper cells play a key role in B-cell activation, proliferation, differentiation, and formation of germinal centers. The pathogenic process of hepatitis B virus is generally the result of a disorder or dysfunction of the immune system. Therefore, we present in this review the critical functions and related biological processes of regulatory T cells and follicular T helper cells during HBV infection.

A novel anti-HBV agent, E-CFCP, restores Hepatitis B virus (HBV)-induced senescence-associated cellular marker perturbation in human hepatocytes

Cellular senescence is a cellular state with a broad spectrum of age-related physiological conditions that can be affected by various infectious diseases and treatments. Therapy of hepatitis B virus (HBV) infection with nucleos(t)ide analogs [NA(s)] is well established and benefits many HBV-infected patients, but requires long-term, perhaps lifelong, medication. In addition to the effects of HBV infection, the effects of NA administration on hepatocellular senescence are still unclear. This study investigated how HBV infection and NA treatment influence cellular senescence in human hepatocytes and humanized-liver chimeric mice chronically infected with live HBV. HBV infection upregulates or downregulates multiple cellular markers including senescence-associated β-galactosidase (SA-β-Gal) activity and cell cycle regulatory proteins (e.g., p21^CIP1) expression level in hepatocellular nuclei and humanized-mice liver. A novel highly potent anti-HBV NA, E-CFCP, per se did not have significant disturbance on markers evaluated. Besides, E-CFCP treatment restored HBV-infected cells to their physiological phenotypes that are comparable to the HBV-uninfected cells. The results reported here demonstrate that, regardless of the mechanism(s), chronic HBV infection perturbates multiple senescence-associated markers in human hepatocytes and humanized-mice liver, but E-CFCP can restore this phenomenon.

Hepatitis B Viral Protein HBx and the Molecular Mechanisms Modulating the Hallmarks of Hepatocellular Carcinoma: A Comprehensive Review

With 296 million cases estimated worldwide, chronic hepatitis B virus (HBV) infection is the most common risk factor for hepatocellular carcinoma (HCC). HBV-encoded oncogene X protein (HBx), a key multifunctional regulatory protein, drives viral replication and interferes with several cellular signalling pathways that drive virus-associated hepatocarcinogenesis. This review article provides a comprehensive overview of the role of HBx in modulating the various hallmarks of HCC by supporting tumour initiation, progression, invasion and metastasis. Understanding HBx-mediated dimensions of complexity in driving liver malignancies could provide the key to unlocking novel and repurposed combinatorial therapies to combat HCC.

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

PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

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

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

Hepatitis B virus X 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.

HBV core promoter mutations and AKT upregulate S-phase kinase-associated protein 2 to promote postoperative hepatocellular carcinoma progression

Mutations in the hepatitis B virus (HBV) core promoter (CP) have been shown to be associated with hepatocellular carcinoma (HCC). The CP region overlaps HBV X gene, which activates AKT to regulate hepatocyte survival. However, the cooperation between these two cascades in HCC progression remains poorly understood. Here, we assayed virological factors and AKT expression in liver tissues from 56 HCC patients with better prognoses (BHCC, ≥5-year survival) and 58 with poor prognoses (PHCC, <5-year survival) after partial liver resection. Results showed double mutation A1762T/G1764A (TA) combined with other mutation(s) (TACO) in HBV genome and phosphorylated AKT (pAKT) were more common in PHCC than BHCC. TACO and pAKT levels correlated with proliferation and microvascularization but inversely correlated with apoptosis in HCC samples. These were more pronounced when TACO and pAKT co-expressed. Levels of p21 and p27 were decreased in TACO or pAKT overexpressing HCC due to SKP2 upregulation. Levels of E2F1 and both mRNA and protein of SKP2 were increased in TACO expressing HCC. Levels of 4EBP1/2 decreased and SKP2 mRNA level remained constant in pAKT-overexpressing HCC. Therefore, TACO and AKT are two independent predictors of postoperative survival in HCC. Their co-target, SKP2 may be a diagnostic or therapeutic marker.

EGFR and SYNE2 are associated with p21 expression and SYNE2 variants predict post-operative clinical outcomes in HBV-related hepatocellular carcinoma

This study was to explore the association between gene variants and p21 expression and investigate the TP53-independent p21 regulation in hepatitis B virus (HBV) related hepatocellular carcinoma (HCC) patients from Guangxi by genome-wide association study. 426 HBV-related HCC patients were enrolled. Results showed that, after quality control, a total of 21,643 SNPs were identified in 107 p21 positive and 298 p21 negative patients. The variants of epidermal growth factor receptor (EGFR; rs2227983 and rs6950826) and spectrin repeat containing, nuclear envelope 2 (SYNE2; rs8010699, rs4027405 and rs1890908) were associated with p21 expression. Moreover the haplotype block (rs2227983 and rs6950826, r² = 0.378) in EGFR and the haplotype block in SYNE2 (rs8010699 was in strong LD with rs4027405 and rs1890908 (r² = 0.91 and 0.70, respectively)) were identified, and the haplotype A-G of EGFR and haplotype G-A-A of SYNE2 were significantly associated with p21 expression (P < 0.01). rs4027405 and rs1890908 were significantly associated with overall survival, and patients with AG/GG genotypes of SYNE2 gene had a worse overall survival (P = 0.001, P = 0.002). Our findings indicate that variants of EGFR and SYNE2 play an important role in p21 regulation and are associated with the clinical outcome of HBV-related HCC in a TP53-indenpdent manner.

Hepatitis B virus molecular biology and pathogenesis

As obligate intracellular parasites, viruses need a host cell to provide a milieu favorable to viral replication. Consequently, viruses often adopt mechanisms to subvert host cellular signaling processes. While beneficial for the viral replication cycle, virus-induced deregulation of host cellular signaling processes can be detrimental to host cell physiology and can lead to virus-associated pathogenesis, including, for oncogenic viruses, cell transformation and cancer progression. Included among these oncogenic viruses is the hepatitis B virus (HBV). Despite the availability of an HBV vaccine, 350-500 million people worldwide are chronically infected with HBV, and a significant number of these chronically infected individuals will develop hepatocellular carcinoma (HCC). Epidemiological studies indicate that chronic infection with HBV is the leading risk factor for the development of HCC. Globally, HCC is the second highest cause of cancer-associated deaths, underscoring the need for understanding mechanisms that regulate HBV replication and the development of HBV-associated HCC. HBV is the prototype member of the Hepadnaviridae family; members of this family of viruses have a narrow host range and predominately infect hepatocytes in their respective hosts. The extremely small and compact hepadnaviral genome, the unique arrangement of open reading frames, and a replication strategy utilizing reverse transcription of an RNA intermediate to generate the DNA genome are distinguishing features of the Hepadnaviridae. In this review, we provide a comprehensive description of HBV biology, summarize the model systems used for studying HBV infections, and highlight potential mechanisms that link a chronic HBV-infection to the development of HCC. For example, the HBV X protein (HBx), a key regulatory HBV protein that is important for HBV replication, is thought to play a cofactor role in the development of HBV-induced HCC, and we highlight the functions of HBx that may contribute to the development of HBV-associated HCC.

Transcriptome-Wide Analysis of Hepatitis B Virus-Mediated Changes to Normal Hepatocyte Gene Expression

Globally, a chronic hepatitis B virus (HBV) infection remains the leading cause of primary liver cancer. The mechanisms leading to the development of HBV-associated liver cancer remain incompletely understood. In part, this is because studies have been limited by the lack of effective model systems that are both readily available and mimic the cellular environment of a normal hepatocyte. Additionally, many studies have focused on single, specific factors or pathways that may be affected by HBV, without addressing cell physiology as a whole. Here, we apply RNA-seq technology to investigate transcriptome-wide, HBV-mediated changes in gene expression to identify single factors and pathways as well as networks of genes and pathways that are affected in the context of HBV replication. Importantly, these studies were conducted in an ex vivo model of cultured primary hepatocytes, allowing for the transcriptomic characterization of this model system and an investigation of early HBV-mediated effects in a biologically relevant context. We analyzed differential gene expression within the context of time-mediated gene-expression changes and show that in the context of HBV replication a number of genes and cellular pathways are altered, including those associated with metabolism, cell cycle regulation, and lipid biosynthesis. Multiple analysis pipelines, as well as qRT-PCR and an independent, replicate RNA-seq analysis, were used to identify and confirm differentially expressed genes. HBV-mediated alterations to the transcriptome that we identified likely represent early changes to hepatocytes following an HBV infection, suggesting potential targets for early therapeutic intervention. Overall, these studies have produced a valuable resource that can be used to expand our understanding of the complex network of host-virus interactions and the impact of HBV-mediated changes to normal hepatocyte physiology on viral replication.

Oncogenic role of p21 in hepatocarcinogenesis suggests a new treatment strategy

A well-known tumor suppressor, p21, acts paradoxically by promoting tumor growth in some cellular conditions. These conflicting functions have been demonstrated in association with the HBx gene and in hepatocarcinogenesis. The molecular behavior of p21 depends on its subcellular localization. Nuclear p21 may inhibit cell proliferation and be proapoptotic, while cytoplasmic p21 may have oncogenic and anti-apoptotic functions. Because most typical tumor suppressive proteins also have different effects according to subcellular localization, elucidating the regulatory mechanisms underlying nucleo-cytoplasmic transport of these proteins would be significant and may lead to a new strategy for anti-hepatocellular carcinoma (HCC) therapy. Chromosome region maintenance 1 (CRM1) is a major nuclear export receptor involved in transport of tumor suppressors from nucleus to cytoplasm. Expression of CRM1 is enhanced in a variety of malignancies and in vitro studies have shown the efficacy of specific inhibition of CRM1 against cancer cell lines. Interestingly, interferon may keep p21 in the nucleus; this is one of the mechanisms of its anti-hepatocarcinogenic function. Here we review the oncogenic property of p21, which depends on its subcellular localization, and discuss the rationale underlying a new strategy for HCC treatment and prevention.

Doxorubicin Activates Hepatitis B Virus Replication by Elevation of p21 (Waf1/Cip1) and C/EBPα Expression

Hepatitis B virus reactivation is an important medical issue in cancer patients who undergo systemic chemotherapy. Up to half of CHB carriers receiving chemotherapy develop hepatitis and among these cases a notable proportion are associated with HBV reactivation. However, the molecular mechanism(s) through which various chemotherapeutic agents induce HBV reactivation is not yet fully understood. In this study, we investigated the role of the cell cycle regulator p21 (Waf1/Cip1) in the modulation of HBV replication when a common chemotherapeutic agent, doxorubicin, is present. We showed that p21 expression was increased by doxorubicin treatment. This elevation in p21 expression enhanced the expression of CCAAT/enhancer-binding protein α (C/EBPα); such an increase is likely to promote the binding of C/EBPα to the HBV promoter, which will contribute to the activation of HBV replication. Our current study thus reveals the mechanism underlying doxorubicin modulation of HBV replication and provides an increased understanding of HBV reactivation in CHB patients who are receiving systemic chemotherapy.

Hepatitis B Virus Core Promoter A1762T/G1764A (TA)/T1753A/T1768A Mutations Contribute to Hepatocarcinogenesis by Deregulating Skp2 and P53

BACKGROUND AND AIM

Hepatitis B virus core promoter (CP) mutations can increase risk of hepatocellular carcinoma. The CP region overlaps with the HBV X (HBx) gene, which has been associated with hepatocarcinogenesis. The cyclin kinase inhibitor P53 is an important regulator of cell cycle progression. We determined whether HBx mutants that result from mutations in the CP deregulate P53.

METHODS

A HBx combination (combo) mutant with changes in the CP region that corresponded to A1762T/G1764A (TA), T1753A, and T1768A was constructed and expressed in L-02 and Hep3B cells. The effects of CP mutations on expression and degradation of P53, and the effects on cell cycle progression and proliferation were analyzed.

RESULTS

The combo mutant decreased levels of P53 and increased cyclin D1 expression, accelerated P53 degradation in L-02 cells, accelerated cell cycle progression, and increased expression of S-phase kinase-associated protein 2 (Skp2) in L-02 and Hep3B cells. Silencing of Skp2 abrogated the effects of CP mutations on P53 expression. The kinetics of P53 expression correlated with changes in cell cycle distribution.

CONCLUSIONS

The HBx mutant with a combination of CP mutations can up-regulate Skp2, which then down-regulates P53 via ubiquitin-mediated proteasomal degradation, increasing the risk of hepatocellular carcinoma.

Tumor suppressor micro RNA miR-145 and onco micro RNAs miR-21 and miR-222 expressions are differentially modulated by hepatitis B virus X protein in malignant hepatocytes

BACKGROUND

Hepatitis B Virus (HBV) X protein (HBx) is known to be involved in the initiation and progression of hepatocellular carcinoma (HCC) through modulation of host gene response. Alterations in miRNA expressions are frequently noted in HCC. This study is aimed to examine the role of HBx protein in the modulation of oncogenic miRNA-21, miRNA-222 and tumor suppressor miRNA-145 in malignant hepatocytes.

METHODS

Expressions of miRNA-21, miRNA-222 and miRNA-145 were measured in HepG2 cells transfected with HBx-plasmid (genotype D) and with full length HBV genome (genotype D) and also in stably HBV producing HepG2.2.15 cells using real time PCR. Their target mRNAs and proteins - PTEN, p27 and MAP3K - were analyzed by real time PCR and western blot respectively. miRNA expressions were measured after HBx/D mRNA specific siRNA treatment. The expressions of these miRNAs were analyzed in liver cirrhosis and HCC patients also.

RESULTS

The study revealed a down-regulation of miRNA-21 and miRNA-222 expressions in HBx transfected HepG2 cells, pUC-HBV 1.3 plasmid transfected HepG2 cells as well as in HepG2.2.15 cells. Down regulation of miRNA-21 and miRNA-222 expression was observed in patient serum samples. Down regulation of miRNA-145 expression was observed in HepG2 cells transiently transfected with HBx and pUC-HBV1.3 plasmid as well as in patient samples but the expression of miRNA-145 was increased in HepG2.2.15 cells. Target mRNA and protein expressions were modulated in HepG2 cells and in HepG2.2.15 cell line consistent with the modulation of miRNA expressions.

CONCLUSION

Thus, HBx protein differentially modulated the expression of miRNAs. The study throws light into possible way by which HBx protein acts through microRNA and thereby regulates host functioning. It might suggest new therapeutic strategies against hepatic cancer.

HBx protein promotes oval cell proliferation by up-regulation of cyclin D1 via activation of the MEK/ERK and PI3K/Akt pathways

Growing evidence has shown that hepatic oval cells, also named liver progenitor cells, play an important role in the process of liver regeneration in various liver diseases. Oval cell proliferation has been reported in hepatitis B virus (HBV)-associated hepatocellular carcinoma (HCC) and chronic liver disease. Studies have found expression of HBV surface and core antigens in oval cells in the livers of patients with HCC, suggesting that HBV infection of oval cells could be a mechanism of human hepatocarcinogenesis. In addition, there is evidence of multiplication of HBV in oval cell culture. However, little research has been performed to explore the role of HBV-encoded proteins in the proliferation of hepatic oval cells. Previously, we successfully transfected the HBV x (HBx) gene, one of the four genes in the HBV genome, into a rat LE/6 oval cell line. In this study, we tested whether or not the transfected HBx gene could affect oval cell proliferation in vitro. Our results show that overexpression of HBx promotes the proliferation of oval cells and increases cyclin D1 expression, assessed at both the mRNA and protein levels. We also found that HBx activated the PI-3K/Akt and MEK/ERK1/2 pathways in HBx-transfected oval cells. Furthermore, the HBx-induced increases in cyclin D1 expression and oval cell proliferation were completely abolished by treatment with either MEK inhibitor PD184352 or PI-3K inhibitor LY294002. These results demonstrated that HBx has the ability to promote oval cell proliferation in vitro, and its stimulatory effects on cell proliferation and expression of cyclin D1 depend on the activation of the MEK/ERK and PI3K/Akt signaling pathways in cultured oval cells.

The Complex Relationship between Liver Cancer and the Cell Cycle: A Story of Multiple Regulations

The liver acts as a hub for metabolic reactions to keep a homeostatic balance during development and growth. The process of liver cancer development, although poorly understood, is related to different etiologic factors like toxins, alcohol, or viral infection. At the molecular level, liver cancer is characterized by a disruption of cell cycle regulation through many molecular mechanisms. In this review, we focus on the mechanisms underlying the lack of regulation of the cell cycle during liver cancer, focusing mainly on hepatocellular carcinoma (HCC). We also provide a brief summary of novel therapies connected to cell cycle regulation.

The roles of hepatitis B virus-encoded X protein in virus replication and the pathogenesis of chronic liver disease

INTRODUCTION

Hepatitis B virus (HBV) is a major cause of chronic liver disease (CLD) and hepatocellular carcinoma (HCC) worldwide. More than 350 million people are at risk for HCC, and with few treatment options available, therapeutic approaches to targets other than the virus polymerase will be needed. This review suggests that the HBV-encoded X protein, HBx, would be an outstanding target because it contributes to the biology and pathogenesis of HBV in three fundamental ways.

AREAS COVERED

First, HBx is a trans-activating protein that stimulates virus gene expression and replication, thereby promoting the development and persistence of the carrier state. Second, HBx partially blocks the development of immune responses that would otherwise clear the virus, and protects infected hepatocytes from immune-mediated destruction. Thus, HBx contributes to the development of CLD without virus clearance. Third, HBx alters patterns of host gene expression that make possible the emergence of HCC. The selected literature cited is from the National Library of Medicine (Pubmed and Medline).

EXPERT OPINION

Understanding the mechanisms, whereby HBx supports virus replication and promotes pathogenesis, suggests that HBx will be an important therapeutic target against both virus replication and CLD aimed at the chemoprevention of HCC.

Cell cycle regulation during viral infection

To replicate their genomes in cells and generate new progeny, viruses typically require factors provided by the cells that they have infected. Subversion of the cellular machinery that controls replication of the infected host cell is a common activity of many viruses. Viruses employ different strategies to deregulate cell cycle checkpoint controls and modulate cell proliferation pathways. A number of DNA and RNA viruses encode proteins that target critical cell cycle regulators to achieve cellular conditions that are beneficial for viral replication. Many DNA viruses induce quiescent cells to enter the cell cycle; this is thought to increase pools of deoxynucleotides and thus, facilitate viral replication. In contrast, some viruses can arrest cells in a particular phase of the cell cycle that is favorable for replication of the specific virus. Cell cycle arrest may inhibit early cell death of infected cells, allow the cells to evade immune defenses, or help promote virus assembly. Although beneficial for the viral life cycle, virus-mediated alterations in normal cell cycle control mechanisms could have detrimental effects on cellular physiology and may ultimately contribute to pathologies associated with the viral infection, including cell transformation and cancer progression and maintenance. In this chapter, we summarize various strategies employed by DNA and RNA viruses to modulate the replication cycle of the virus-infected cell. When known, we describe how these virus-associated effects influence replication of the virus and contribute to diseases associated with infection by that specific virus.

Hepatitis B virus HBx protein impairs liver regeneration through enhanced expression of IL-6 in transgenic mice

BACKGROUND & AIMS

Conflicting results have been reported regarding the impact of hepatitis B virus X protein (HBx) expression on liver regeneration triggered by partial hepatectomy (PH). In the present report we investigated the mechanisms by which HBx protein alters hepatocyte proliferation after PH.

METHODS

PH was performed on a transgenic mouse model in which HBx expression is under the control of viral regulatory elements and liver regeneration was monitored. LPS, IL-6 neutralizing antibody, and SB203580 were injected after PH to evaluate IL-6 participation during liver regeneration.

RESULTS

Cell cycle progression of hepatocytes was delayed in HBx transgenic mice compared to WT animals. Moreover, HBx induced higher secretion of IL-6 soon after PH. Upregulation of IL-6 was associated with an elevation of STAT3 phosphorylation, SOCS3 transcript accumulation and a decrease in ERK1/2 phosphorylation in the livers of HBx transgenic mice. The involvement of IL-6 overexpression in cell cycle deregulation was confirmed by the inhibition of liver regeneration in control mice after the upregulation of IL-6 expression using LPS. In addition, IL-6 neutralization with antibodies was able to restore liver regeneration in HBx mice. Finally, the direct role of p38 in IL-6 secretion after PH was demonstrated using SB203580, a pharmacological inhibitor.

CONCLUSIONS

HBx is able to induce delayed hepatocyte proliferation after PH, and HBx-induced IL-6 overexpression is involved in delayed liver regeneration. By modulating IL-6 expression during liver proliferation induced by stimulation of the cellular microenvironment, HBx may participate in cell cycle deregulation and progression of liver disease.

The C-terminal region of the hepatitis B virus X protein is required for its stimulation of HBV replication in primary mouse hepatocytes

Hepatitis B virus (HBV) infection is an important risk factor for hepatocellular carcinoma (HCC). The hepatitis B virus X protein (HBx), a multifunctional regulatory protein encoded by HBV, is known to be involved in stimulation of viral replication by modulating cell cycle status. HBx is required for maximal virus replication in plasmid-based replication assays in immortalized human liver HepG2 cells and in primary rat hepatocytes. Moreover, the C-terminal region of HBx is important for HBV replication in HepG2 cells. However, in normal hepatocytes, the region of HBx that is responsible for its effect on cell cycle regulation and HBV replication is unclear. We have demonstrated that HBx is similarly required for maximal HBV replication in primary mouse hepatocytes and that the C-terminus of HBx is essential for its ability to stimulate HBV replication by inducing quiescent hepatocytes to exit G0 phase of the cell cycle but stall in G1 phase. Our studies establish that primary mouse hepatocytes support HBx-dependent HBV replication, and provide further evidence for the effect of the C-terminal region of HBx on HBV infection and replication.

Hepatitis B virus core promoter mutations contribute to hepatocarcinogenesis by deregulating SKP2 and its target, p21

BACKGROUND & AIMS

Clinical studies have associated hepatitis B virus core promoter (CP) mutations with an increased risk of hepatocellular carcinoma. The CP region overlaps with the HBV X (HBx) gene, which has been implicated in hepatocarcinogenesis. The cyclin kinase inhibitor p21WAF1/CIP1 is an important regulator of cell cycle progression and proliferation. We determined whether HBx mutants that result from mutations in the CP deregulate p21 and these processes.

METHODS

We constructed a series of HBx mutants with changes in the CP region that correspond to A1762T/G1764A (TA), T1753A, T1768A, or a combination of these (combo) and expressed them, along with wild-type HBx under control of its endogenous promoter, in primary human hepatocytes (PHHs) and HepG2 cells. We then analyzed the effects of CP mutations on expression and degradation of p21 and the effects on cell cycle progression and proliferation.

RESULTS

The combo mutant decreased levels of p21 and increased cyclin E expression in PHHs and HepG2 cells. The combo mutant, but not HBx with single or double CP mutations, accelerated p21 degradation in HepG2 cells. The combo mutant increased expression of S-phase kinase-associated protein 2 (SKP2) in PHHs and Huh7 cells. Silencing of SKP2 abrogated the effects of CP mutations on p21 expression. The kinetics of p21 expression correlated with changes in cell cycle distribution. The combo mutant accelerated cell cycle progression; p21 overexpression restored G1 arrest.

CONCLUSIONS

HBx mutants with changes that correspond to a combination of CP mutations up-regulate SKP2, which then down-regulates p21 via ubiquitin-mediated proteasomal degradation. CP mutations might increase the risk of hepatocellular carcinoma via this pathway.

The molecular and pathophysiological implications of hepatitis B X antigen in chronic hepatitis B virus infection

Hepatitis B virus is considered one of the most significant environmental carcinogens in humans. Because the mechanisms of HBV replication and the development of hepatocellular carcinoma (HCC) are partially known, HBV-associated pathogenesis remains a challenge to increase its understanding. Evidence suggests that the regulatory protein hepatitis B virus X (HBx) mediates the establishment and maintenance of the chronic carrier state. HBx is a multifunctional and potentially oncogenic protein that is conserved among mammalian hepadnaviruses; it is produced very early after infection and throughout the chronic phase. HBx exerts its effects by interacting with cellular proteins and activating various signaling pathways. HBx stimulates the transcription of genes that regulate cell growth, apoptosis, and DNA repair. It also interacts with proteasome subunits and affects mitochondrial stability. Moreover, HBx participates in processes that are associated with the progression of chronic liver disease, including angiogenesis and fibrosis. This review discusses the function of HBx in the life cycle of HBV and its contribution to the pathogenesis of HCC.

Hepatitis B and C virus hepatocarcinogenesis: lessons learned and future challenges

Worldwide, hepatocellular carcinoma (HCC) is one of the most common cancers. It is thought that 80% of hepatocellular carcinomas are linked to chronic infections with the hepatitis B (HBV) or hepatitis C (HCV) viruses. Chronic HBV and HCV infections can alter hepatocyte physiology in similar ways and may utilize similar mechanisms to influence the development of HCC. There has been significant progress towards understanding the molecular biology of HBV and HCV and identifying the cellular signal transduction pathways that are altered by HBV and HCV infections. Although the precise molecular mechanisms that link HBV and HCV infections to the development of HCC are not entirely understood, there is considerable evidence that both inflammatory responses to infections with these viruses, and associated destruction and regeneration of hepatocytes, as well as activities of HBV- or HCV-encoded proteins, contribute to hepatocyte transformation. In this review, we summarize progress in defining mechanisms that may link HBV and HCV infections to the development of HCC, discuss the challenges of directly defining the processes that underlie HBV- and HCV-associated HCC, and describe areas that remain to be explored.

The role of oncogenic viruses in the pathogenesis of hepatocellular carcinoma

HBV and HCV have major roles in hepatocarcinogenesis. More than 500 million people are infected with hepatitis viruses and, therefore, HCC is highly prevalent, especially in those countries endemic for HBV and HCV. Viral and host factors contribute to the development of HCC. The main viral factors include the circulating load of HBV DNA or HCV RNA and specific genotypes. Various mechanisms are involved in the host-viral interactions that lead to HCC development, among which are genetic instability, self-sufficiency in growth signals, insensitivity to antigrowth signals, evasion of apoptosis, limitless replicative potential, sustained angiogenesis, and tissue invasiveness. Prevention of HBV by vaccination, as well as antiviral therapy against HBV and for HCV seem able to inhibit the development of HCC.

The hepatitis B virus HBx protein modulates cell cycle regulatory proteins in cultured primary human hepatocytes

There are over 350 million people chronically infected with the Hepatitis B virus (HBV); chronic HBV infections are associated with the development of hepatocellular carcinoma (HCC). While the precise mechanism of HBV-associated HCC remains undefined, it is believed to involve a combination of the host immune response to infection and activities of HBV proteins including the nonstructural X protein (HBx). HBx is a multifunctional protein that can modulate various cellular processes including cell proliferation. The exact effect of HBx on cell proliferation has varied depending on the cell line and exact conditions used in the study. Our previously published reports have demonstrated that HBx modulates the levels of cell cycle regulatory proteins in primary rat hepatocytes; however, the effect of HBx on cell cycle regulatory proteins in primary human hepatocytes, the natural host for HBV infection, has not been studied. Here we have examined the effect of HBx on cell cycle regulatory proteins in cultured, primary human hepatocytes. We demonstrate that HBx decreases the levels of cell cycle proteins that prevent progression into G1 phase and increases the levels of cell cycle proteins active in G1 phase. We have also shown that HBx modulation of cell cycle regulatory proteins requires cytosolic calcium, similar to the results we previously obtained in primary rat hepatocytes. Cumulatively, our results are the first demonstration that HBx modulates the levels of cell cycle regulatory proteins in a calcium-dependent manner in primary human hepatocytes.

Replication of the hepatitis B virus requires a calcium-dependent HBx-induced G1 phase arrest of hepatocytes

Chronic HBV infections cause hepatocellular carcinoma (HCC). Activities of the HBV HBx protein regulate HBV replication and may contribute to the development of HCC. We previously reported that HBx causes primary rat hepatocytes to exit G0 but stall in G1 phase of the cell cycle; entry into G1 stimulated HBV replication. We now report that the activity of the mitochondria permeability transition pore is required for HBx regulation of cell cycle proteins and HBV replication in primary rat hepatocytes, that progression from G0 to G1 stimulates HBV polymerase activity, and that HBV replication is facilitated by the HBx-induced G1 arrest. HBx stimulation of HBV replication was linked to elevation of the R2 subunit of ribonucleotide reductase. Our studies suggest that HBx uses mitochondrial-dependent calcium signaling to cause hepatocytes to exit G0 but stall in G1 and that this HBx activity alters the cellular environment and stimulates HBV replication.

Increase of hepatic fat accumulation by liver specific expression of Hepatitis B virus X protein in zebrafish

The pathogenesis of fatty liver disease remains largely unknown. Here, we assessed the importance of hepatic fat accumulation on the progression of hepatitis in zebrafish by liver specific expression of Hepatitis B virus X protein (HBx). Transgenic zebrafish lines, GBXs, which selectively express the GBx transgene (GFP-fused HBx gene) in liver, were established. GBX Liver phenotypes were evaluated by histopathology and molecular analysis of fatty acid (FA) metabolism-related genes expression. Most GBXs (66-81%) displayed obvious emaciation starting at 4 months old. Over 99% of the emaciated GBXs developed hepatic steatosis or steatohepatitis, which in turn led to liver hypoplasia. The liver histology of GBXs displayed steatosis, lobular inflammation, and balloon degeneration, similar to non-alcoholic steatohepatitis (NASH). Oil red O stain detected the accumulation of fatty droplets in GBXs. RT-PCR and Q-rt-PCR analysis revealed that GBx induced hepatic steatosis had significant increases in the expression of lipogenic genes, C/EBP-alpha, SREBP1, ChREBP and PPAR-gamma, which then activate key enzymes of the de novo FA synthesis, ACC1, FAS, SCD1, AGAPT, PAP and DGAT2. In addition, the steatohepatitic GBX liver progressed to liver degeneration and exhibited significant differential gene expression in apoptosis and stress. The GBX models exhibited both the genetic and functional factors involved in lipid accumulation and steatosis-associated liver injury. In addition, GBXs with transmissible NASH-like phenotypes provide a promising model for studying liver disease.

The hepatitis B virus X protein modulates hepatocyte proliferation pathways to stimulate viral replication

Worldwide, there are over 350 million people who are chronically infected with the human hepatitis B virus (HBV); chronic HBV infections are associated with the development of hepatocellular carcinoma (HCC). The results of various studies suggest that the HBV X protein (HBx) has a role in the development of HBV-associated HCC. HBx can regulate numerous cellular signal transduction pathways, including those that modulate cell proliferation. Many previous studies that analyzed the impact of HBx on cell proliferation pathways were conducted using established or immortalized cell lines, and when HBx was expressed in the absence of HBV replication, and the precise effect of HBx on these pathways has often differed depending on experimental conditions. We have studied the effect of HBx on cell proliferation in cultured primary rat hepatocytes, a biologically relevant system. We demonstrate that HBx, both by itself and in the context of HBV replication, affected the levels and activities of various cell cycle-regulatory proteins to induce normally quiescent hepatocytes to enter the G(1) phase of the cell cycle but not to proceed to S phase. We linked HBx regulation of cell proliferation to cytosolic calcium signaling and HBx stimulation of HBV replication. Cumulatively, our studies suggest that HBx induces normally quiescent hepatocytes to enter the G(1) phase of the cell cycle and that this calcium-dependent HBx activity is required for HBV replication. These studies identify an essential function of HBx during HBV replication and a mechanism that may connect HBV infections to the development of HCC.

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.

Peroxisome proliferator-activated receptor-gamma contributes to the inhibitory effects of Embelin on colon carcinogenesis

Down-regulation of XIAP (X-linked inhibitor of apoptosis protein) sensitizes colon cancer cells to the anticancer effect of peroxisome proliferator-activated receptor-gamma (PPARgamma) ligands in mice. The aims of this study were to evaluate the effect of embelin (2,5-dihydroxy-3-undecyl-1,4-benzoquinone), an antagonist of XIAP, on colon cancer, with a particular focus on whether PPARgamma is required for embelin to exert its effect. A dominant-negative PPARgamma was used to antagonize endogenous PPARgamma in HCT116 cells. Cells were treated with or without embelin. Cell proliferation, apoptosis, and nuclear factor-kappaB (NF-kappaB) activity were measured. For in vivo studies, 1,2-dimethylhydrazine dihydrochloride (DMH) was s.c. injected to induce colon cancer in PPARgamma(+/+) and PPARgamma(+/-) mice. Mice were fed embelin daily for 10 days before DMH injection, and continued for 30 more weeks. Embelin inhibited proliferation and induced apoptosis in HCT116 cells with marked up-regulation of PPARgamma. In addition, embelin significantly inhibited the expressions of survivin, cyclin D1, and c-Myc. These effects were partially dependent on PPARgamma. PPARgamma(+/-) mice were more susceptible to DMH-induced colon carcinogenesis than PPARgamma(+/+) mice, and embelin significantly reduced the incidence of colon cancer in PPARgamma(+/+) mice but not in PPARgamma(+/-) mice. Embelin inhibited NF-kappaB activity in PPARgamma(+/+) mice but marginally so in PPARgamma(+/-) mice. Thus, reduced expression of PPARgamma significantly sensitizes colonic tissues to the carcinogenic effect of DMH. Embelin inhibits chemical carcinogen-induced colon carcinogenesis, but this effect is partially dependent on the presence of functional PPARgamma, indicating that PPARgamma is a necessary signaling pathway involved in the antitumor activity of normal organisms.

Premature cell cycle entry induced by hepatitis B virus regulatory HBx protein during compensatory liver regeneration

The cycles of cell death and compensatory regeneration that occur during chronic hepatitis B virus (HBV) infection are central to viral pathogenesis and are a risk factor for the development of liver cancer. The HBV genome encodes one regulatory protein, HBx, which is required for virus replication, although its precise role in replication and pathogenesis is unclear. Because HBx can induce the G(0)-G(1) transition in cultured cells, the purpose of this study was to examine the effect of HBx during liver regeneration. Transgenic mice expressing HBx (ATX) and their wild-type (WT) littermates were used in the partial hepatectomy (PH) model for compensatory regeneration. Liver tissues collected from ATX and WT mice at varying sacrifice time points after PH were examined for markers of cell cycle progression. When compared with WT liver tissues, ATX livers had evidence of premature cell cycle entry as assessed by several variables (BrdUrd incorporation, proliferating cell nuclear antigen and mitotic indices, and reduced steady-state p21 protein levels). However, HBx did not affect apoptosis, glycogen storage, or PH-induced steatosis. Together, these results show that HBx expression can induce cell cycle progression within the regenerating liver. Our data are consistent with a model in which HBx expression contributes to liver disease and cancer formation by affecting early steps in liver regeneration.

Mitogen-activated protein kinase kinase 1/2 inhibitors and 17-allylamino-17-demethoxygeldanamycin synergize to kill human gastrointestinal tumor cells in vitro via suppression of c-FLIP-s levels and activation of CD95

Prior studies have noted that inhibitors of mitogen-activated protein kinase (MAPK) kinase 1/2 (MEK1/2) enhanced geldanamycin lethality in malignant hematopoietic cells by promoting mitochondrial dysfunction. The present studies focused on defining the mechanism(s) by which these agents altered survival in carcinoma cells. MEK1/2 inhibitors [PD184352; AZD6244 (ARRY-142886)] interacted in a synergistic manner with geldanamycins [17-allylamino-17-demethoxygeldanamycin (17AAG) and 17-dimethylaminoethylamino-17-demethoxy-geldanamycin] to kill hepatoma and pancreatic carcinoma cells that correlated with inactivation of extracellular signal-regulated kinase 1/2 and AKT and with activation of p38 MAPK; p38 MAPK activation was reactive oxygen species dependent. Treatment of cells with MEK1/2 inhibitors and 17AAG reduced expression of c-FLIP-s that was mechanistically connected to loss of MEK1/2 and AKT function; inhibition of caspase-8 or overexpression of c-FLIP-s abolished cell killing by MEK1/2 inhibitors and 17AAG. Treatment of cells with MEK1/2 inhibitors and 17AAG caused a p38 MAPK-dependent plasma membrane clustering of CD95 without altering the levels or cleavage of FAS ligand. In parallel, treatment of cells with MEK1/2 inhibitors and 17AAG caused a p38 MAPK-dependent association of caspase-8 with CD95. Inhibition of p38 MAPK or knockdown of BID, FAS-associated death domain, or CD95 expression suppressed MEK1/2 inhibitor and 17AAG lethality. Similar correlative data were obtained using a xenograft flank tumor model system. Our data show that treatment of tumor cells with MEK1/2 inhibitors and 17AAG induces activation of the extrinsic pathway and that suppression of c-FLIP-s expression is [Mol Cancer Ther 2008;7(9):2633-48].

Hepatitis B virus X protein mutants exhibit distinct biological activities in hepatoma Huh7 cells

The role of the hepatitis B virus X protein (HBx) in hepatocarcinogenesis remains controversial. To investigate the biological impact of hepatitis B virus x gene (HBx) mutation on hepatoma cells, plasmids expressing the full-length HBx or HBx deletion mutants were constructed. The biological activities in these transfectants were analyzed by a series of assays. Results showed that HBx3'-20 and HBx3'-40 amino acid deletion mutants exhibited an increase in cellular proliferation, focus formation, tumorigenicity, and invasive growth and metastasis through promotion of the cell cycle from G0/G1 to the S phase, when compared with the full-length HBx. In contrast, HBx3'-30 amino acid deletion mutant repressed cell proliferation by blocking in G1 phase. The expression of P53, p21(WAF1), p14(ARF), and MDM2 proteins was regulated by expression of HBx mutants. In conclusions, HBx variants showed different effects and functions on cell proliferation and invasion by regulation of the cell cycle progression and its associated proteins expression.

Mutations in the C-terminus of the X protein of hepatitis B virus regulate Wnt-5a expression in hepatoma Huh7 cells: cDNA microarray and proteomic analyses

Background: The hepatitis B virus x gene (HBx) is a promiscuous transactivator implicated in the development of hepatocellular carcinoma (HCC). The present study was designed to investigate the molecular events regulated by HBx. Methods: Genomic and proteomic expression profiling was performed in Huh7 HCC cells transfected with HBx mutants with a C-terminal deletion. The gene and protein expression of wingless-type murine-mammary-tumour virus (MMTV) integration site family, member 5A (Wnt-5a) was validated by analyses of reverse transcription–polymerase chain reaction (RT–PCR), real-time RT–PCR, western blot and immunohistochemistry. Results: Differentially expressed genes and proteins were found in the transfected Huh7 HCC cells; most of them were involved in transcriptional regulation, although others including oncogenes or tumor suppressor genes, and molecules involved in cell junctions, signal transduction pathways, metabolism or the immune response were also observed. The expression of the Wnt-5a gene was elevated >10-fold in Huh7 cells transfected with the HBx3′-30 amino acid deletion mutant. However, the expression was downregulated by the transfection with the HBx3′-40 amino acid deletion mutant. The changes in Wnt-5a expression were also observed in human HCC tissues, compared with corresponding non-cancerous liver tissues. A negative correlation was found between the expression of Wnt-5a and HBx COOH mutations in HCC tissues. Conclusions: HBx mutants may participate in the development and progression of HCC, at least in part through the Wnt-5a pathway.

Advances in research on p38MAPK-related hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common cancers in the world. Studies indicate that the development of HCC is related to signal transduction of Ras-MAPK.P38MAPK, an important member of the family of mitogen-activated protein kinases. P38MAPK participates in cell proliferation, apoptosis and differentiation and plays a key role in cell apoptosis. P38MAPK is closely related with carcinogenesis, rapid generation and infinite growth of liver cancer and plays a role in the occurrence and development of liver cancer induced by organics, HBV and HCV. Drugs exert their anti-tumor effects through p38MAPK which also takes part in the formation of drug resistance to HCC. This paper reviews the advances in studies on p38MAPK-related HCC.

Approaches for monitoring signal transduction changes in normal and cancer cells

This chapter will describe methods to assess the activities of protein kinases. Initial studies in the 1950s and 1960s in the field of glucose metabolism examined the activities of several highly specific protein and carbohydrate kinases in cell lysates or isolated cell fractions. As more protein kinases were discovered in the 1980s and 1990s, coupled with the development of immunoprecipitating antibodies, in vitro kinase assays of isolated kinase proteins using gamma-32P ATP became a standard procedure. In the 1990s, antibodies were developed that recognize specific sites of regulatory phosphorylation on a variety of protein kinases (phospho-specific antibodies), which have been used to assess kinase activity indirectly through immunoblotting. In this chapter, Methodologies to perform immune complex protein kinase assays and immunoblotting using phospho-specific antibodies against regulatory sites of phosphorylation in protein kinases will be described. The strengths and weaknesses of each approach in determining protein kinase activity will also be discussed.

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

BACKGROUND/AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Constitutive activation of NF-kappaB in human hepatocellular carcinoma: evidence of a cytoprotective role

Activation of nuclear factor-kappaB (NF-kappaB) can promote or inhibit apoptosis. Oxidative stress is an important mechanism by which certain anticancer drugs kill cancer cells, and is also one of the mechanisms that activate NF-kappaB. We therefore examined hepatic expression of the NF-kappaB monomer p65 in human hepatocellular carcinoma (HCC) tissue samples from eight patients and compared it with their respective samples of surrounding liver tissues. We also studied the effect of NF-kappaB inhibition in human HCC cells exposed to oxidative stress, by infecting HuH7 cells with a recombinant adenovirus carrying mutant IkappaBalpha (mIkappaBalpha). Cultured HuH7 cells were infected with mIkappaBalpha or beta-galactosidase (beta-Gal) for 24 hr followed by treatment with increasing concentrations of H2O2. Cytotoxicity, NF-kappaB translocation, NF-kappaB DNA binding, cell proliferation, and apoptosis were determined. The monomer p65 was overexpressed in six of eight human HCC tissues. In HuH7 cells, introduction of mIkappaBalpha potently inhibited the translocation, activation, and DNA binding of NF- kappaB. In control (beta-Gal-infected) HuH7 cells, exposure to H2O2 produced a dose-dependent increase in apoptosis, regardless of NF-kappaB status. mIkappaBalpha-mediated inhibition of NF-kappaB activation sensitized HuH7 cells to H2O2-induced inhibition of cell growth, and further promoted cell death. Addition of H2O2 (200-500 microM) to control or mIkappaBalpha-infected HuH7 cells enhanced caspase-3 activity and cleavage. Adenovirus-mediated transfer of mIkappaBalpha potently inhibits NF-kappaB activity in HuH7 cells, and this enhances oxidative stress-induced cell killing.

HBsAg and HBx knocked into the p21 locus causes hepatocellular carcinoma in mice

Hepatocellular carcinoma (HCC) affects males in a significantly higher proportion than females and is one of the human cancers etiologically related to viral factors. Many studies provide strong evidence of the direct role that hepatitis B virus (HBV) plays in hepatic carcinogenesis, but the functions of HBV surface antigen (HBsAg) and X protein (HBx) in hepatocarcinogenesis through direct or indirect mechanisms are still being debated. We generated two HBV gene knock-in transgenic mouse lines by homologous recombination. HBsAg and HBx genes were integrated into the mouse p21 locus. Both male and female p21-HBx transgenic mice developed HCC after the age of 18 months; however, male p21-HBsAg transgenic mice began to develop HCC 3 months earlier. The expression of a number of genes related to metabolism and genomic instability largely resembled the molecular changes during the development of HCC in humans. ER-beta (estrogen receptor-beta) was extremely up-regulated only in tumor tissues of male p21-HBsAg mice, providing genetic evidence that HBsAg might be the major risk factor affecting the gender difference in the causes of HCC. In conclusion, these mice might serve as good models for studying the different roles of HBsAg and HBx in early events of HBV-related hepatocarcinogenesis.

Gap junction-mediated intercellular communication in a long-term primary mouse hepatocyte culture system

Gap junction-mediated intercellular communication (GJIC) is critical for maintaining integral cellular processes including differentiation and growth control. The disruption of GJIC has been correlated with aberrant function in many cell types, including hepatocytes in vivo; therefore it is imperative that cellular model systems support intercellular communication to simulate normal cellular functions. Functional GJIC has been shown in long-term primary rat hepatocyte cultures, which have been implemented widely to study various aspects of hepatocellular function; however, the onset of transgenic technology in murine species has necessitated the development of a primary mouse hepatocyte system. In this report, we analyze GJIC in a dimethylsulfoxide (DMSO)-containing long-term primary mouse hepatocyte culture system. The cells retain morphologic and biochemical characteristics of differentiated hepatocytes through day 30 post plating, including liver-specific gene expression. We further show that connexin32 and connexin26 expression and gap junction plaque formation increase over time in culture concomitant with an increase in GJIC between adjoining primary mouse hepatocytes. In conclusion, the findings described in this study make it possible to maintain differentiated primary mouse hepatocytes that also show GJIC in long-term culture for 30 days. In addition, this system has the potential to be extended to study primary mouse hepatocytes isolated from genetically engineered mice.

Regulation of p21 and p27 expression by the hepatitis B virus X protein and the alternate initiation site X proteins, AUG2 and AUG3

BACKGROUND

The hepatitis B virus X gene has three in-frame start codons encoding the pX, AUG2 and AUG3 proteins. The AUG2 and AUG3 genes are 5'-truncated in respect to the full-length pX gene; however, all three genes terminate at the same stop codon. The activity of pX as an oncogene is well characterized; however, less is known about the AUG2 and AUG3 proteins.

METHODS

The effects of pX, AUG2 and AUG3 on p21Cip,1/WAF,1/MDA6 and p27Kip-1 cyclin kinase inhibitor (CKI) protein expression, and the impact they have on proliferation, were investigated in CHO K-1 cells. CHO K-1 cells were chosen because they can be transfected at 100% efficiency.

RESULTS

p21- and p27-luciferase reporter expression is modulated by increasing doses of the hepatitis B X proteins. At low concentrations of pX or AUG2, p21- and p27-luciferase activity was increased, and at high concentrations, p21- and p27-luciferase activity was decreased. Expression of the AUG3 gene showed a different profile: it was increasingly stimulatory with dose for both promoters. Western blot analyses demonstrated that p21 and p27 protein levels were modulated as predicted based on data generated in the promoter-luciferase experiments. Tritiated thymidine labeling of DNA showed biphasic kinetics of incorporation in the presence of varying pX and AUG2 concentrations, whereas labeling decreased with AUG3 concentration. The growth inhibitory effect of pX expression was reduced by antisense ablation of either p21 or p27.

CONCLUSIONS

The relative expression level of pX, AUG2, and AUG3 impacts on CKI expression and cell proliferation. Our findings may explain why divergent effects of pX expression on growth have been observed by different groups, which may be related to relative pX expression levels.

Expression of hepatitis B virus X protein in transgenic mice

AIM: To establish a mice model harboring hepatitis B virus x gene (adr subtype) for studying the function of hepatitis B virus X protein, a transactivator of viral and cellular promoter/enhancer elements.

METHODS: Expression vector pcDNA3-HBx, containing CMV promoter and hepatitis B virus x gene open reading fragment, was constructed by recombination DNA technique. Hela cells were cultured in DMEM and transfected with pcDNA3-HBx or control pcDNA3 plasmids using FuGENE6 Transfection Reagent. Expression of pcDNA3-HBx vectors in the transfected Hela cells was confirmed by Western blotting. After restriction endonuclease digestion, the coding elements were microinjected into male pronuclei of mice zygotes. The pups were evaluated by multiplex polymerase chain reaction (PCR) at genomic DNA level. The x gene transgenic mice founders were confirmed at protein level by Western blotting, immunohistochemistry and immunogold transmission electron microscopy.

RESULTS: Expression vector pcDNA3-HBx was constructed by recombination DNA technique and identified right by restriction endonuclease digestion and DNA direct sequencing. With Western blotting, hepatitis X protein was detected in Hela cells transfected with pcDNA3-HBx plasmids, suggesting pcDNA3-HBx plasmids could express in eukaryotic cells. Following microinjection of coding sequence of pcDNA3-HBx, the embryos were transferred to oviducts of psedopregnant females. Four pups were born and survived. Two of them were verified to have the HBx gene integrated in their genomic DNA by multiplex PCR assay, and named C57-TgN (HBx)S MMU1 and C57-TgN (HB x) SMMU3 respectively. They expressed 17KD X protein in liver tissue by Western blotting assay. With the immunohistochemistry, X protein was detected mainly in hepatocytes cytoplasm of transgenic mice, which was furthermore confirmed by immunogold transmission electon microscopy.

CONCLUSION: We have constructed the expression vector pcDNA3-HBx that can be used to study the function of HBx gene in eukaryotic cells in vitro. We also established HBx gene (adr subtype) transgenic mice named C57-TgN (HBx) SMMU harboring HBx gene in their genome and express X protein in hepatocytes, Which might be a valuable animal system for studying the roles of HBx gene in hepatitis B virus life cycle and development of hepatocellular carcinoma in vivo.