Inhibition of hepatitis-B-virus core promoter by p53: implications for carcinogenesis in hepatocytes

Tumor Suppressor p53 Inhibits Hepatitis B Virus Replication by Downregulating HBx via E6AP-Mediated Proteasomal Degradation in Human Hepatocellular Carcinoma Cell Lines

HBx, a multifunctional regulatory protein, plays an essential role in the replication and pathogenesis of the hepatitis B virus (HBV). In this study, we found that in human hepatoma cells, the tumor suppressor p53 downregulates HBx via ubiquitin-dependent proteasomal degradation. p53 transcriptional activity that results from HBV infection was not essential for this effect. This was shown by treatment with a potent p53 inhibitor, pifithrin-α. Instead, we found that p53 facilitated the binding of E6-associated protein (E6AP), which is an E3 ligase, to HBx and induced E6AP-mediated HBx ubiquitination in a ternary complex of p53, E6AP, and HBx. The ability of p53 to induce E6AP-mediated downregulation of HBx and inhibit HBV replication was demonstrated in an in vitro HBV infection system. This study may provide insights into the regulation of HBx and HBV replication, especially with respect to p53 status, which may also help in understanding HBV-associated tumorigenesis in patients.

Hepatitis B virus biology and life cycle

Hepatitis B virus (HBV) specifically infects hepatocytes and causes severe liver diseases. The HBV life cycle is unique in that the genomic DNA (relaxed-circular partially double-stranded DNA: rcDNA) is converted to a molecular template DNA (covalently closed circular DNA: cccDNA) to amplify a viral RNA intermediate, which is then reverse-transcribed back to viral DNA. The highly stable characteristics of cccDNA result in chronic infection and a poor rate of cure. This complex life cycle of HBV offers a variety of targets to develop antiviral agents. We provide here an update on the current knowledge of HBV biology and its life cycle, which may help to identify new antiviral targets.

Hepatitis B Virus X Protein Driven Alpha Fetoprotein Expression to Promote Malignant Behaviors of Normal Liver Cells and Hepatoma Cells

Background: The infection of Hepatitis B virus (HBV) is closely associated with the development of hepatocellular carcinoma(HCC), HBV-X protein(HBx) is able to induce expression of alpha-fetoprotein(AFP) in normal liver cells, and AFP harbors a function to promote malignant transformation of normal liver cells, but the role AFP playing in malignant behaviors of HCC cells is still unclear.

Methods: Fifty-six liver tissue samples were collected from the clinical patients through hepatectomy(include normal liver tissues, HBV-related hepatitis liver tissues and HBV-related HCC tissues), and diagnosis of these tissues by pathology section, expression of AFP, Ras and CXCR4 were evidenced by immunohisochemical staining and Western blotting; The proliferation of human normal liver cells line L-02 cells and human hepatoma cells line, HLE cells(non AFP-producing) were performed by MTT method; Repaired capacity of L-02 and HLE cells were compared by wound healing assay; Migration and invasion of these cells were analyzed by Transwell chamber assay; HBx expressed vectors(pcDNA3.1-HBx) were constructed and transfected into L-02 and HLE cells, effects of pcDNA3.1-HBx on the malignant behaviors were also detected by MTT, Transwell chamber assay and the expression of AFP, Ras and CXCR4 were evidenced by Western blotting.

Results: we found that expression of AFP, Ras and CXCR4 in HBV-related HCC and lymph nodes metastasis tissues were significantly elevated compared with HBV-related HCC, non metastasis tissues and HBV-related hepatitis tissues; Expression of AFP, Ras and CXCR4 in HBV-related hepatitis tissues were significantly enhanced compared with normal liver tissues; The growth ratio, migratory and invasive ability, expression of AFP, Ras and CXCR4 of the cells were outstanding promoted while L-02 and HLE cells were transfected with pcDNA3.1-HBx vectors. The proliferation ratio, migration and invasion ability, and expression of Ras and CXCR4 were significantly inhibited while L-02-X and HLE-X cells(stably transfected with pcDNA3.1-HBx) were silenced AFP expression by AFP-siRNA.

Conclusions: HBx through stimulating expression of AFP to promote malignant behaviors of human normal liver cells and HCC cells; AFP maybe used as a novel biotarget for therapeutics of HCC patients.

Latent Tuberculosis Infection and the Risk of Subsequent Cancer

The association of latent tuberculosis infection (LTBI) with subsequent cancer remains unclear. We investigated the risk of future cancer among tuberculosis (TB) contacts with or without subsequent TB activation. Using the Taiwan National Health Insurance Research Database, we conducted a nationwide population-based study. TB contacts during 1997 to 2012 were included as the study cohort. Patients with antecedent cancer and TB were excluded. Data from 11,522 TB contacts and 46,088 age-, sex-, and enrollment date-matched subjects during 1997 to 2012 were analyzed. The 2 cohorts were monitored until December 31, 2012 for incidence of cancer and TB infection. LTBI was defined as a TB contact with subsequent TB activation. The primary endpoint was occurrence of newly diagnosed cancer. There was no difference in cancer development between the TB contact cohort and comparison cohort (log-rank test, P = 0.714). After multivariate adjustment, the hazard ratio (HR) for cancer among the LTBI patients was 2.29 [95% confidence interval (CI), 1.26-4.17; P = 0.007]. There was increase in cancer incidences for several specific cancer types, including multiple myeloma (HR 340.28), lung (HR 2.69), kidney and bladder (HR 6.16), hepatobiliary (HR 2.36), and gastrointestinal (HR 2.99) cancers. None of the 136 TB contacts who received isoniazid prophylaxis developed cancer. LTBI patients had a higher risk of future cancer.

A Novel Microtubule-Disrupting Agent Induces Endoplasmic Reticular Stress-Mediated Cell Death in Human Hepatocellular Carcinoma Cells

Here, we present evidence of a novel microtubule-disrupting agent, N-deacetyl-N-(chromone-2-carbonyl)-thiocolchicine (TCD), exhibiting potent antitumor activity (with IC₅₀ values in the nanomolar range) against hepatocellular carcinoma cell lines. Cell cycle analysis revealed that TCD induced G₂/M cell-cycle arrest in a dose- and time-dependent manner in both Hep-J5 and Mahlavu HCC cell lines. TCD also induced a decrease in mitochondrial membrane potential (ΔΨm) and caused DNA damage. Mechanistically, TCD activated protein kinase RNA-like endoplasmic reticular kinase and several transcription factors, including activating transcription factor (ATF) 6, ATF4, ATF3, and the CCAAT-enhancer binding protein homologous protein. These data clearly demonstrate that the antitumor activity of TCD is mechanistically linked to its capacity to trigger both intrinsic and extrinsic apoptotic cell death via endoplasmic reticular stress pathway. The potent antitumor activity of TCD was similarly demonstrated in a hepatocellular carcinoma xenograft model, where 5 and 10 mg/kg doses of TCD significantly arrested Hep-J5 and Mahlavu tumor growth. Our finding suggests that TCD is a promising therapeutic agent against hepatocellular carcinoma; further translational assessment of its clinical usage is warranted.

Molecular mechanism of p73-mediated regulation of hepatitis B virus core promoter/enhancer II: implications for hepatocarcinogenesis

Hepatitis B virus (HBV) is a causative agent of chronic hepatitis and hepatocellular carcinoma. Recent findings demonstrating p73 and specifically N-terminally truncated p73 (DeltaTAp73) accumulation in hepatocellular carcinoma suggest that p73 plays a role in the malignant phenotype. Here, we investigated the mechanism of HBV pregenomic core promoter/enhancer II (cp/EII) regulation by full-length TAp73 and its oncogenic counterpart DeltaTAp73. Ectopic and endogenous expression of TAp73 leads to a significant downregulation of cp/EII activity in p53-deficient hepatoma cell lines. In contrast, overexpression of DeltaTAp73 results in significant cp/EII activation and increased HBV core (HBc) expression. TAp73-mediated repression of HBV transcription was substantially abolished by DeltaTAp73. We show that both TAp73 and DeltaTAp73 proteins directly bind to the Sp1 transcription factor, a key stimulator of HBV gene expression. However, only TAp73 abolishes Sp1 binding to cp/EII, whereas the DeltaTAp73-Sp1 complex further persists on the DNA. The inhibitory effect of p53/p73 on HBc expression is associated with the inhibition of viral replication, while DeltaTAp73 is not. These data strongly support the fact that the p73-isoform-related interaction with Sp1 is the underlying mechanism of the diverse outcome on HBc expression, suggesting a new mechanism by which oncogenic DeltaTAp73 could enhance the carcinogenic process in liver cells.

Differential effects on apoptosis induction in hepatocyte lines by stable expression of hepatitis B virus X protein

AIM: Hepatitis B virus protein X (HBx) has been shown to be weakly oncogenic in vitro. The transforming activities of HBx have been linked with the inhibition of several functions of the tumor suppressor p53. We have studied whether HBx may have different effects on p53 depending on the cell type.

METHODS: We used the human hepatoma cell line HepG2 and the immortalized murine hepatocyte line AML12 and analyzed stably transfected clones which expressed physiological amounts of HBx. P53 was induced by UV irradiation.

RESULTS: The p53 induction by UV irradiation was unaffected by stable expression of HBx. However, the expression of the cyclin kinase inhibitor p21^waf/cip/sdi which gets activated by p53 was affected in the HBx transformed cell line AML12-HBx9, but not in HepG2. In AML-HBx9 cells, p21^waf/cip/sdi-protein expression and p21^waf/cip/sdi transcription were deregulated. Furthermore, the process of apoptosis was affected in opposite ways in the two cell lines investigated. While stable expression of HBx enhanced apoptosis induced by UV irradiation in HepG2-cells, apoptosis was decreased in HBx transformed AML12-HBx9. P53 repressed transcription from the HBV enhancer I, when expressed from expression vectors or after induction of endogenous p53 by UV irradiation. Repression by endogenous p53 was partially reversible by stably expressed HBx in both cell lines.

CONCLUSION: Stable expression of HBx leads to deregulation of apoptosis induced by UV irradiation depending on the cell line used. In an immortalized hepatocyte line HBx acted anti-apoptotic whereas expression in a carcinoma derived hepatocyte line HBx enhanced apoptosis.

A direct intersection between p53 and transforming growth factor beta pathways targets chromatin modification and transcription repression of the alpha-fetoprotein gene

We purified the oncoprotein SnoN and found that it functions as a corepressor of the tumor suppressor p53 in the regulation of the hepatic alpha-fetoprotein (AFP) tumor marker gene. p53 promotes SnoN and histone deacetylase interaction at an overlapping Smad binding, p53 regulatory element (SBE/p53RE) in AFP. Comparison of wild-type and p53-null mouse liver tissue by using chromatin immunoprecipitation (ChIP) reveals that the absence of p53 protein correlates with the disappearance of SnoN at the SBE/p53RE and loss of AFP developmental repression. Treatment of AFP-expressing hepatoma cells with transforming growth factor-beta1 (TGF-beta1) induced SnoN transcription and Smad2 activation, concomitant with AFP repression. ChIP assays show that TGF-beta1 stimulates p53, Smad4, P-Smad2 binding, and histone H3K9 deacetylation and methylation, at the SBE/p53RE. Depletion, by small interfering RNA, of SnoN and/or p53 in hepatoma cells disrupted repression of AFP transcription. These findings support a model of cooperativity between p53 and TGF-beta effectors in chromatin modification and transcription repression of an oncodevelopmental tumor marker gene.

HCV NS5A interacts with p53 and inhibits p53-mediated apoptosis

Hepatitis C virus (HCV) causes a persistent infection, chronic hepatitis and hepatocellular carcinoma. HCV NS5A, one of non-structural proteins of HCV, was suggested to play a role in oncogenic transformation. Since the tumor suppressor p53 is important for preventing neoplastic transformation, we investigated the functional effects of HCV NS5A on p53. In vitro and in vivo coimmunoprecipitation and confocal microscopy were used to determine the interaction of NS5A and p53. HCV NS5A binds directly to p53 and colocalizes p53 in the perinuclear region. NS5A inhibits transcriptional transactivation by p53 in a dose-dependent manner by use of a reporter assay. Down regulation of endogenous p21/waf1 expression, which is activated by p53 in Hep3B cells, by NS5A was demonstrated by using FLAG- and FLAG-NS5A Hep3B stable cell lines. The effect of NS5A on p53-mediated apoptosis was determined by flow cytometry in both NS5A permanently and transiently transfected Hep3B cells with exogenous p53. The p53-induced apoptosis was abrogated by NS5A and the inhibition effect correlates well with the binding ability of NS5A to p53. In addition, HCV NS5A protein interacts with and colocalizes hTAF(II)32, a component of TFIID and an essential coactivator of p53, in vivo. These results suggest that HCV NS5A interacts with and partially sequestrates p53 and hTAF(II)32 in the cytoplasm and suppresses p53-mediated transcriptional transactivation and apoptosis during HCV infection, which may contribute to the hepatocarcinogenesis of HCV infection.

Hepatitis C virus core protein enhances p53 function through augmentation of DNA binding affinity and transcriptional ability

Hepatitis C virus (HCV) causes a persistent infection, chronic hepatitis, and hepatocellular carcinoma. Since there are several reports indicating that some viruses influence the tumor suppressor p53 function, we determined the effects of HCV proteins on p53 function and its mechanism determined by use of a reporter assay. Among seven HCV proteins investigated (core, NS2, NS3, NS4A, NS4B, NS5A, and NS5B), only core protein augmented the transcriptional activity of p53 and increased the expression of p21(waf1) protein, which is a major target of p53. Core protein increased both DNA-binding affinity of p53 in electrophoretic morbidity shift assay and transcriptional ability of p53 itself in a reporter assay. The direct interaction between core protein and C terminus of p53 was also shown by glutathione S-transferase fusion protein binding assay. In addition, core protein interacted with hTAF(II)28, a component of the transcriptional factor complex in vivo and in vitro. These results suggest that HCV core protein interacts with p53 and modulates p53-dependent promoter activities during HCV infection.

Interaction of Hepatitis B virus with cellular processes in liver carcinogenesis

Hepatitis B infection is strongly linked epidemiologically to hepatocellular carcinoma development. This article reviews the molecular mechanisms by which hepatitis B encoded proteins such as hepatitis B x and hepatitis B surface transactivators may interact with gene transcription, tumor suppression, apoptosis, and signalling pathways of the liver cell with the possible consequence of tumor induction. Data on the interaction between hepatitis B proteins and cellular processes are often conflicting indicating a non-specific simultaneous interaction with antagonistic cellular processes that result in the formation of escape mutants that are not subject to these selective pressures.

Infections as a major preventable cause of human cancer

Infections may be responsible for over 15% of all malignancies worldwide. Important mechanisms by which infectious agents may induce carcinogenesis include the production of chronic inflammation, the transformation of cells by insertion of oncogenes and inhibition of tumour suppressors, and the induction of immunosuppression. Common characteristics shared by infectious agents linked to malignancies are that they are persistent in the host, often highly prevalent in the host population and induce cancer after a long latency. The associations between a selection of infectious agents and malignancies are covered in detail.

Selective transcriptional regulations in the human liver cell by hepatitis B viral X protein

The hepatitis B viral X protein (HBx) is known as a transcription factor and potential oncogene. To gain a better view of the effect of HBx on the transcriptional regulation in the human liver cell, we constructed a HepG2 cell line stably expressing HBx (HepG2-HBx), and performed cDNA microarray analysis on 588 cellular cDNAs comparing with untransformed control cells. Two genes (IGFR-2, RhoA) of oncogenes, one gene (p55CDC) of cell cycle regulators, three genes (thrombin receptor, MLK-3, MacMARCKS) of intracellular transducers, one gene (HSP27) of stress response proteins, two genes (FAST kinase, Bak) of apoptosis response proteins, one gene (p21(WAF)) of transcription factors were highly up-regulated; one gene (transcription elongation factor SII) of transcription factors and two genes (monocyte chemotactic protein 1, T-lymphocyte-secreted protein I-309) of growth factors were highly down-regulated. These results showed selective transcriptional regulation by HBx in the human liver cell.

Sp1-mediated transcription of the Werner helicase gene is modulated by Rb and p53

The regulation of Werner's syndrome gene (WRN) expression was studied by characterizing the cis-regulatory elements in the promoter region and the trans-activating factors that bind to them. First, we defined the transcription initiation sites and the sequence of the 5' upstream region (2.8 kb) of WRN that contains a number of cis-regulatory elements, including 7 Sp1, 9 retinoblastoma control element (RCE), and 14 AP2 motifs. A region consisting of nucleotides -67 to +160 was identified as the principal promoter of WRN by reporter gene assays in HeLa cells, using a series of WRN promoter-luciferase reporter (WRN-Luc) plasmids that contained the 5'-truncated or mutated WRN upstream regions. In particular, two Sp1 elements proximal to the transcription initiation site are indispensable for WRN promoter activity and bind specifically to Sp1 proteins. The RCE enhances WRN promoter activity. Coexpression of the WRN-Luc plasmids with various dosages of plasmids expressing Rb or p53 in Saos2 cells lacking active Rb and p53 proteins showed that the introduced Rb upregulates WRN promoter activity a maximum of 2. 5-fold, while p53 downregulates it a maximum of 7-fold, both dose dependently. Consistently, the overexpressed Rb and p53 proteins also affected the endogenous WRN mRNA levels in Saos2 cells, resulting in an increase with Rb and a decrease with p53. These findings suggest that WRN expression, like that of other housekeeping genes, is directed mainly by the Sp1 transcriptional control system but is also further modulated by transcription factors, including Rb and p53, that are implicated in the cell cycle, cell senescence, and genomic instability.