A hepatitis B virus transgenic mouse model with a conditional, recombinant, episomal genome

Background & Aims

Development of new and more effective therapies against hepatitis B virus (HBV) is limited by the lack of suitable small animal models. The HBV transgenic mouse model containing an integrated overlength 1.3-mer construct has yielded crucial insights, but this model unfortunately lacks covalently closed circular DNA (cccDNA), the episomal HBV transcriptional template, and cannot be cured given that HBV is integrated in every cell.

Methods

To solve these 2 problems, we generated a novel transgenic mouse (HBV1.1X), which generates an excisable circular HBV genome using Cre/LoxP technology. This model possesses a HBV1.1-mer cassette knocked into the ROSA26 locus and is designed for stable expression of viral proteins from birth, like the current HBV transgenic mouse model, before genomic excision with the introduction of Cre recombinase.

Results

We demonstrated induction of recombinant cccDNA (rcccDNA) formation via viral or transgenic Cre expression in HBV1.1X mice, and the ability to regulate HBsAg and HBc expression with Cre in mice. Tamoxifen-inducible Cre could markedly downregulate baseline HBsAg levels from the integrated HBV genome. To demonstrate clearance of HBV from HBV1.1X mice, we administered adenovirus expressing Cre, which permanently and significantly reduced HBsAg and core antigen levels in the murine liver via rcccDNA excision and a subsequent immune response.

Conclusions

The HBV1.1X model is the first Cre-regulatable HBV transgenic mouse model and should be of value to mimic chronic HBV infection, with neonatal expression and tolerance of HBV antigens, and on-demand modulation of HBV expression.

Lay summary

Hepatitis B virus (HBV) can only naturally infect humans and chimpanzees. Mouse models have been developed with the HBV genome integrated into mouse chromosomes, but this prevents mice from being cured. We developed a new transgenic mouse model that allows for HBV to be excised from mouse chromosomes to form a recombinant circular DNA molecule resembling the natural circular HBV genome. HBV expression could be reduced in these mice, enabling curative therapies to be tested in this new mouse model.

BRIT1 dysfunction confers synergistic inhibition of hepatocellular carcinoma by targeting poly (ADP-ribose) polymerases and PI3K

BRIT1 has emerged as a novel key player in homologous recombination (HR). It is located in 8p23, a locus frequently deleted in hepatocellular carcinoma (HCC). Previously, we found that BRIT1-deficiency triggered genomic instability and tumor formation in our mouse model. Here we aim to determine whether BRIT1 aberrations are implicated in HCC and, if so, whether they can be used for targeted therapy with PARP inhibitors and other agents. We analyzed HCC samples for BRIT1 alterations at DNA, RNA and protein levels. BRIT1 was found deleted and/or downregulated in ~30% of HCC samples; BRIT1 mutant K659fsX10 identified in HCC abolished DNA repair function. Notably, BRIT1 deletion was correlated with poor survival and high recurrence of HCC. To determine the role of BRIT1 deficiency in potentiating the drug response, we subsequently generated BRIT1-deficient HCC cells, determined their HR defects, and assessed their response to the PARPi olaparib and PI3K inhibitor in vitro and in mice. BRIT1-deficient HCC cells were HR defective and hypersensitive to olaparib alone or in combination with PI3K inhibitor BEZ235, both in vitro and in vivo. The cytotoxicity of olaparib alone or in combination with BEZ235 was largely alleviated by ectopic BRIT1. We also found that BEZ235 markedly enhanced the production of poly (ADP-ribose) and the level of double-strand breaks (DSB) and single-strand breaks (SSB) in BRIT1-deficient cells. In summary, our results identify BRIT1 deficiency as a potential driver for HCC development, and BRIT1 status is critical to sensitivity to treatment with olaparib and/or BEZ235. PI3K inhibition induces substantial DNA damage and makes cells more dependent on PARP activity in the context of BRIT1 deficiency, thus, BRIT1 depletion facilitates enhancing synthetic lethality of PARP inhibitors and PI3K inhibitors in HCC. This study provides a new mechanistic foundation for significantly expanding the application of PARPi in HCC therapy.

Hepatitis B Virus HBx Protein Mediates the Degradation of Host Restriction Factors through the Cullin 4 DDB1 E3 Ubiquitin Ligase Complex

The hepatitis B virus (HBV) regulatory HBx protein is required for infection, and its binding to cellular damaged DNA binding protein 1 (DDB1) is critical for this function. DDB1 is an adaptor protein for the cullin 4A Really Interesting New Gene (RING) E3 ubiquitin ligase (CRL4) complex and functions by binding cellular DDB1 cullin associated factor (DCAF) receptor proteins that recruit substrates for ubiquitination and degradation. We compared the proteins found in the CRL4 complex immunoprecipitated from uninfected versus HBV-infected hepatocytes from human liver chimeric mice for insight into mechanisms by which HBV and the cell interact within the CRL4 complex. Consistent with its role as a viral DCAF, HBx was found in the HBV CRL4 complexes. In tissue culture transfection experiments, we showed that HBx expression led to decreased levels of known restriction factor structural maintenance of chromosomes protein 6 (SMC6) and putative restriction factors stromal interaction molecule 1 (STIM1, zinc finger E-box binding homeobox 2 (ZEB2), and proteasome activator subunit 4 (PSME4). Moreover, silencing of these proteins led to increased HBV replication in the HepG2-sodium taurocholate cotransporting polypeptide (NTCP) infection model. We also identified cellular DCAF receptors in CRL4 complexes from humanized mice. Increasing amounts of HBx did not reveal competitive DCAF binding to cullin4 (CUL4)-DDB1 in plasmid-transfected cells. Our results suggest a model in which HBx benefits virus replication by directly or indirectly degrading multiple cellular restriction factors.

Relative Abundance of Integrant-Derived Viral RNAs in Infected Tissues Harvested from Chronic Hepatitis B Virus Carriers

Five matching sets of nonmalignant liver tissues and hepatocellular carcinoma (HCC) samples from individuals chronically infected with hepatitis B virus (HBV) were examined. The HBV genomic sequences were determined by using overlapping PCR amplicons covering the entire viral genome. Four pairs of tissues were infected with HBV genotype C, while one pair was infected with HBV genotype B. HBV replication markers were found in all tissues. In the majority of HCC samples, the levels of pregenomic/precore RNA (pgRNA) and covalently closed circular DNA (cccDNA) were lower than those in liver tissue counterparts. Regardless of the presence of HBV replication markers, (i) integrant-derived HBV RNAs (id-RNAs) were found in all tissues by reverse transcription-PCR (RT-PCR) analysis and were considerably abundant or predominant in 6/10 tissue samples (2 liver and 4 HCC samples), (ii) RNAs that were polyadenylated using the cryptic HBV polyadenylation signal and therefore could be produced by HBV replication or derived from integrated HBV DNA were found in 5/10 samples (3 liver and 2 HCC samples) and were considerably abundant species in 3/10 tissues (2 livers and 1 HCC), and (iii) cccDNA-transcribed RNAs polyadenylated near position 1931 were not abundant in 7/10 tissues (2 liver and 5 HCC samples) and were predominant in only two liver samples. Subsequent RNA sequencing analysis of selected liver/HCC samples also showed relative abundance of id-RNAs in most of the examined tissues. Our findings suggesting that id-RNAs could represent a significant source of HBV envelope proteins, which is independent of viral replication, are discussed in the context of the possible contribution of id-RNAs to the HBV life cycle.IMPORTANCE The relative abundance of integrant-derived HBV RNAs (id-RNAs) in chronically infected tissues suggest that id-RNAs coding for the envelope proteins may facilitate the production of a considerable fraction of surface antigens (HBsAg) in infected cells bearing HBV integrants. If the same cells support HBV replication, then a significant fraction of assembled HBV virions could bear id-RNA-derived HBsAg as a major component of their envelopes. Therefore, the infectivity of these HBV virions and their ability to facilitate virus cell-to-cell spread could be determined mainly by the properties of id-RNA-derived envelope proteins and not by the properties of replication-derived HBsAg. These interpretations suggest that id-RNAs may play a role in the maintenance of chronic HBV infection and therefore contribute to the HBV life cycle. Furthermore, the production of HBsAg from id-RNAs independently of viral replication may explain at least in part why treatment with interferon or nucleos(t)ides in most cases fails to achieve a loss of serum HBsAg.

Farnesoid X receptor ablation sensitizes mice to hepatitis b virus X protein-induced hepatocarcinogenesis

Chronic hepatitis B virus infection is a major risk factor for hepatocellular carcinoma (HCC). Hepatitis B virus X protein (HBx) is a hepatitis B virus protein that has multiple cellular functions, but its role in HCC pathogenesis has been controversial. Farnesoid X receptor (FXR) is a nuclear receptor with activities in anti-inflammation and inhibition of hepatocarcinogenesis. However, whether or how FXR can impact hepatitis B virus/HBx-induced hepatocarcinogenesis remains unclear. In this study, we showed that HBx can interact with FXR and function as a coactivator of FXR. Expression of HBx in vivo enhanced FXR-responsive gene regulation. HBx also increased the transcriptional activity of FXR in a luciferase reporter gene assay. The HBx-FXR interaction was confirmed by coimmunoprecipitation and glutathione S-transferase pull-down assays, and the FXR activation function 1 domain was mapped to bind to the third α helix in the C terminus of HBx. We also found that the C-terminally truncated variants of HBx, which were found in clinical HCC, were not effective at transactivating FXR. Interestingly, recruitment of the full-length HBx, but not the C-terminally truncated HBx, enhanced the binding of FXR to its response element. In vivo, FXR ablation markedly sensitized mice to HBx-induced hepatocarcinogenesis.

CONCLUSIONS

We propose that transactivation of FXR by full-length HBx may represent a protective mechanism to inhibit HCC and that this inhibition may be compromised upon the appearance of C-terminally truncated HBx or when the expression and/or activity of FXR is decreased. (Hepatology 2017;65:893-906).

Hepatitis B Virus X and Regulation of Viral Gene Expression

The efficient replication of hepatitis B virus (HBV) requires the HBV regulatory hepatitis B virus X (HBx) protein. The exact contributions of HBx are not fully understood, in part because of the limitations of the assays used for its study. When HBV replication is driven from a plasmid DNA, the contribution of HBx is modest. However, there is an absolute requirement for HBx in assays that recapitulate the infectious virus life cycle. There is much evidence that HBx can contribute directly to HBV replication by acting on viral promoters embedded within protein coding sequences. In addition, HBx may also contribute indirectly by modulating cellular pathways to benefit virus replication. Understanding the mechanism(s) of HBx action during virus replication may provide insight into novel ways to disrupt chronic HBV replication.

Activation of signal transduction pathways during hepatic oncogenesis

BACKGROUND AND AIMS

Understanding the molecular pathogenesis of hepatocellular carcinoma (HCC) is essential to identify therapeutic targets. A hepatitis B virus (HBV) related double transgenic murine model was developed.

METHODS

Liver specific expression of HBV X protein (HBx) and insulin receptor substrate 1 (IRS1) was achieved and transgenic mice were followed from birth to age 21 months. Liver and tumor tissue were assessed for histologic changes as well as activation of signal transduction pathways by qRT-PCR and multiplex ELISA protein assays.

RESULTS

Overexpression of HBx and IRS1 stimulates liver cell proliferation in the double transgenic mice. Only the male mice developed HCC starting at age 15-18 months. The IN/IGF1/IRS1/MAPK/ERK and IN/IGF1/IRS1/PI3K/AKT/GSK3β cascades were activated early (6-9 months) in the liver followed by WNT/β-catenin and Notch signaling. Aspartate β-hydroxylase (ASPH) was found to link these upstream growth factor signaling pathways to downstream Notch activation in tumor tissues.

CONCLUSIONS

Sustained overexpression of HBx and IRS1 led to constitutive activation of a tripartite growth factor signal transduction cascade in the liver and was necessary and sufficient to promote HCC development and progression.

Technical standards for hepatitis B virus X protein (HBx) research

Chronic infection with hepatitis B virus (HBV) is a risk factor for developing hepatocellular carcinoma (HCC). The life cycle of HBV is complex and has been difficult to study because HBV does not infect cultured cells. The HBV regulatory X protein (HBx) controls the level of HBV replication and possesses an HCC cofactor role. Attempts to understand the mechanism(s) that underlie HBx effects on HBV replication and HBV-associated carcinogenesis have led to many reported HBx activities that are likely influenced by the assays used. This review summarizes experimental systems commonly used to study HBx functions, describes limitations of these experimental systems that should be considered, and suggests approaches for ensuring the biological relevance of HBx studies.

Abstract 2211: Viral subtypes in hepatocellular carcinoma are associated with different mechanisms of WNT/CTNNB1 alteration

Hepatocellular carcinoma (HCC) is one of the most prevalent human cancers, especially in Asia, and regions of endemic hepatitis infection. The primary risk factors for HCC are chronic hepatitis infection by hepatitis C (HCV) or hepatitis B (HBV). We performed whole exome sequencing on 549 matched tumor and normal samples from three centers (BCM HGSC, RCAST, NCC) to identify somatic changes in protein coding regions and changes in copy number. Here we describe variants associated with specific viral etiologies in HCC.

We found TP53, CTNNB1, ARID1A, ARID2, and AXIN1 to be among the most frequently mutated genes in the entire dataset, implicating DNA damage, WNT signaling, and chromatin remodeling as recurrently altered pathways in HCC. Our validation rate for mutations in significantly mutated genes was over 95%. We identified 18 genes which differed significantly (p < 0.05) in mutation frequency between viral subtypes. Of these, AXIN1 was mutated in over 15% of HBV-associated HCC but less than 5% of HCV and non-viral-associated HCC. Additionally, we performed GISTIC analysis based on read depth from our whole exome sequencing data. We found that WNT3A and MYC were significantly amplified and AXIN1 and TP53 were significantly deleted in HCV-associated HCC but not HBV or non-virus-associated HCC.

Our findings indicate that WNT/beta-catenin signaling is a frequently altered pathway in HCC. While aberrations in this pathway were present in all viral etiologies, HBV-associated HCC tended to mutate AXIN1 while HCV-associated HCC tended to have AXIN1 deletion. These differences may result from virus-specific oncogenic processes.

Citation Format: Kyle R. Covington, Lawrence A. Donehower, Chad Creighton, Betty L. Slagle, John A. Goss, Ronald T. Cotton, Marie-Claude Gingras, Eve Shinbrot, Jacfranz J. Guiteau, Thao N. Nguyen, Theresa R. Harring, Donna M. Muzny, Kimberly Walker, HarshaVardhan Doddapaneni, Richard A. Gibbs, Hiroyuki Aburatani, Tatsuhiro Shibata, David A. Wheeler, Japan ICGC HCC Project. Viral subtypes in hepatocellular carcinoma are associated with different mechanisms of WNT/CTNNB1 alteration. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2211. doi:10.1158/1538-7445.AM2014-2211

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 regulatory HBx protein binding to DDB1 is required but is not sufficient for maximal HBV replication

Robust hepatitis B virus (HBV) replication is stimulated by the regulatory HBx protein. HBx binds the cellular protein DDB1; however, the importance of this interaction for HBV replication remains unknown. We tested whether HBx binding to DDB1 was required for HBV replication using a plasmid based replication assay in HepG2 cells. Three DDB1 binding-deficient HBx point mutants (HBx(69), HBx(90/91), HBx(R96E)) failed to restore wildtype levels of replication from an HBx-deficient plasmid, which established the importance of the HBx-DDB1 interaction for maximal HBV replication. Analysis of overlapping HBx truncation mutants revealed that both the HBx-DDB1 binding domain and the carboxyl region are required for maximal HBV replication both in vitro and in vivo, suggesting the HBx-DDB1 interaction recruits regulatory functions critical for replication. Finally we demonstrate that HBx localizes to the Cul4A-DDB1 complex, and discuss the possible implications for models of HBV replication.

NF-kappaB signaling mediates the induction of MTA1 by hepatitis B virus transactivator protein HBx

Metastasis-associated protein 1 (MTA1), a master chromatin modifier, has been shown to regulate cancer progression and is widely upregulated in human cancer, including hepatitis B virus-associated hepatocellular carcinomas (HCCs). Here we provide evidence that hepatitis B virus transactivator protein HBx stimulates the expression of MTA1 but not of MTA2 or MTA3. The underlying mechanism of HBx stimulation of MTA1 involves HBx targeting of transcription factor nuclear factor (NF)-kappaB and the recruitment of HBx/p65 complex to the NF-kappaB consensus motif on the relaxed MTA1 gene chromatin. We also discovered that MTA1 depletion in HBx-expressing cells severely impairs the ability of HBx to stimulate NF-kappaB signaling and the expression of target proinflammatory molecules. Furthermore, the presence of HBx in HBx-infected HCCs correlated well with increased MTA1 and NF-kappaB-p65. Collectively, these findings revealed a previously unrecognized integral role of MTA1 in HBx stimulation of NF-kappaB signaling and consequently, the expression of NF-kappaB targets gene products with functions in inflammation and tumorigenesis.

Abstract B25: DDB1 is a cell-nonautonomous tumor suppressor targeted by HBx in the liver

Hepatocellular carcinoma (HCC) is the fifth most frequent cancer and the third leading cause of cancer death worldwide. A major aetiologic risk factor for HCC is chronic infection of hepatitis B virus (HBV), which encodes a small regulatory x protein (HBx) critical for HBV-induced hepatocarcinogenesis. How HBx contributes to HCC is not clear, but is likely dependent on its association with host factors to activate transcription and regulate mitogenic pathways.

Here we demonstrate that deletion of one such HBx-interacting protein, the Damaged DNA Binding Protein 1 (DDB1), in liver parenchymal hepatocytes leads to development of HCC in mice, after prolonged inflammation resulting from continuous loss of DDB1-deficient hepatocytes and regeneration of DDB1-proficient hepatocytes. Expression of HBx in this regenerating liver impairs the proliferation of regenerated DDB1-proficient hepatocytes, and significantly accelerates the onset of tumorigenesis. Surprisingly, tumors arising from these animals are made up of cancer cells displaying no disruption of the DDB1 gene. This cell-nonautonomous function of DDB1 inactivation in HCC initiation can be further enhanced by deletion of the tumor suppressor p53 or administration of chemical carcinogen.

Our results indicate that cancer initiating mutations are not necessarily present in cancer initiating cells, but can be exploited by oncogenic signals such as HBx expression and p53 mutation to promote cancer progression.

Citation Information: Cancer Res 2009;69(23 Suppl):B25.

Overexpression of insulin receptor substrate-1 and hepatitis Bx genes causes premalignant alterations in the liver

Activation of the insulin (IN)/insulin receptor substrate-1 (IRS-1)/mitogen-associated protein kinase (MAPK) and the Wnt/beta-catenin signaling cascades occurs frequently in hepatocellular carcinoma (HCC) associated with persistent viral infection. The aims of this study were to provide a chronic proliferative stimulus through IRS-1 in the context of hepatitis Bx (HBx) protein expression in transgenic mice and determine if constitutive expression of these genes is sufficient to cause hepatocyte dysplasia and cellular transformation. We generated transgenic mice in which the HBx (ATX), IRS-1, or both (ATX+/IRS-1) genes were expressed under a liver-specific promoter. We also assessed histology and oxidative damage as well as up-regulation of molecules related to these signal transduction cascades in the liver by quantitative reverse-transcriptase polymerase chain reaction. Whereas mice with a single transgene (ATX or IRS-1) did not develop tumors, ATX+/IRS-1+ double transgenic livers had increased frequency of hepatocellular dysplasia and developed HCC. All three transgenic lines had significantly increased insulin growth factor 1 (IGF-1), Wnt 1 and Wnt 3 mRNA levels, and evidence of DNA damage and oxidative stress. The ATX+/IRS+ double transgenic mice were distinguished by having the highest level of activation of Wnt 3 and Frizzled 7 and selectively increased expression of IGF-II, proliferating cell nuclear antigen, and aspartyl-(asparaginyl)-beta-hydroxylase, a gene associated with increased cell migration.

CONCLUSION

These results suggest that continued expression of the ATX or IRS-1 transgenes can contribute to hepatocyte transformation but are not sufficient to trigger neoplastic changes in the liver. However, dual expression that activates both the IN/IRS-1/MAPK and Wnt/beta-catenin cascades is sufficient to cause dysplasia and HCC in a previously normal liver.

Hepatitis B virus HBx protein localized to the nucleus restores HBx-deficient virus replication in HepG2 cells and in vivo in hydrodynamically-injected mice

Identifying the requirements for the regulatory HBx protein in hepatitis B virus (HBV) replication is an important goal. A plasmid-based HBV replication assay was used to evaluate whether HBx subcellular localization influences its ability to promote virus replication, as measured by real time PCR quantitation of viral capsid-associated DNA. HBx targeted to the nucleus by a nuclear localization signal (NLS-HBx) was able to restore HBx-deficient HBV replication, while HBx containing a nuclear export signal (NES-HBx) was not. Both NLS-HBx and NES-HBx were expressed at similar levels (by immunoprecipitation and Western blotting), and proper localization of the signal sequence-tagged proteins was confirmed by deconvolution microscopy using HBx, NLS-HBx, and NES-HBx proteins fused to GFP. Importantly, these findings were confirmed in vivo by hydrodynamic injection into mice. Our results demonstrate that in these HBV replication assays, at least one function of HBx requires its localization to the nucleus.

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.

Elevated expression of the miR-17-92 polycistron and miR-21 in hepadnavirus-associated hepatocellular carcinoma contributes to the malignant phenotype

Alterations in microRNA (miRNA) expression in both human and animal models have been linked to many forms of cancer. Such miRNAs, which act directly as repressors of gene expression, have been found to frequently reside in fragile sites and genomic regions associated with cancer. This study describes a miRNA signature for human primary hepatitis B virus-positive human hepatocellular carcinoma. Moreover, two known oncomiRs--miRNAs with known roles in cancer--the miR-17-92 polycistron and miR-21, exhibited increased expression in 100% of primary human and woodchuck hepatocellular carcinomas surveyed. To determine the importance of these miRNAs in tumorigenesis, an in vitro antisense oligonucleotide knockdown model was evaluated for its ability to reverse the malignant phenotype. Both in human and woodchuck HCC cell lines, separate treatments with antisense oligonucleotides specific for either the miR-17-92 polycistron (all six members) or miR-21 caused a 50% reduction in both hepatocyte proliferation and anchorage-independent growth. The combination of assays presented here supports a role for these miRNAs in the maintenance of the malignant transformation of hepatocytes.

Enhancement of hepatitis B virus replication by the regulatory X protein in vitro and in vivo

The 3.2-kb hepatitis B virus (HBV) genome encodes a single regulatory protein termed HBx. While multiple functions have been identified for HBx in cell culture, its role in virus replication remains undefined. In the present study, we combined an HBV plasmid-based replication assay with the hydrodynamic tail vein injection model to investigate the function(s) of HBx in vivo. Using a greater-than-unit-length HBV plasmid DNA construct (payw1.2) and a similar construct with a stop codon at position 7 of the HBx open reading frame (payw1.2*7), we showed that HBV replication in transfected HepG2 cells was reduced 65% in the absence of HBx. These plasmids were next introduced into the livers of outbred ICR mice via hydrodynamic tail vein injection. At the peak of virus replication, at 4 days postinjection, intrahepatic markers of HBV replication were reduced 72% to 83% in mice injected with HBx-deficient payw1.2*7 compared to those measured in mice receiving wild-type payw1.2. A second plasmid encoding HBx was able to restore virus replication from payw1.2*7 to wild-type levels. Finally, viremia was monitored over the course of acute virus replication, and at 4 days postinjection, it was reduced by nearly 2 logs in the absence of HBx. These studies establish that the role for HBx in virus replication previously shown in transfected HepG2 cells is also apparent in the mouse liver within the context of acute hepatitis. Importantly, the function of HBx can now be studied in an in vivo setting that more closely approximates the cellular environment for HBV replication.

Increased liver pathology in hepatitis C virus transgenic mice expressing the hepatitis B virus X protein

Transgenic mice expressing the full-length HCV coding sequence were crossed with mice that express the HBV X gene-encoded regulatory protein HBx (ATX mice) to test the hypothesis that HBx expression accelerates HCV-induced liver pathogenesis. At 16 months (mo) of age, hepatocellular carcinoma was identified in 21% of HCV/ATX mice, but in none of the single transgenic animals. Analysis of 8-mo animals revealed that, relative to HCV/WT mice, HCV/ATX mice had more severe steatosis, greater liver-to-body weight ratios, and a significant increase in the percentage of hepatocytes staining for proliferating cell nuclear antigen. Furthermore, primary hepatocytes from HCV, ATX, and HCV/ATX transgenic mice were more resistant to fas-mediated apoptosis than hepatocytes from nontransgenic littermates. These results indicate that HBx expression contributes to increased liver pathogenesis in HCV transgenic mice by a mechanism that involves an imbalance in hepatocyte death and regeneration within the context of severe steatosis.

Altered DNA mutation spectrum in aflatoxin b1-treated transgenic mice that express the hepatitis B virus x protein

Humans chronically infected with hepatitis B virus (HBV) are at further risk of liver cancer upon exposure to dietary aflatoxin B1 (AFB1), a carcinogenic product of the mold Aspergillus flavus. For the present study, we utilized double-transgenic mice (ATX mice) that express the HBV X protein (HBx) and possess a bacteriophage lambda transgene to evaluate the in vivo effect of HBx expression on AFB1-induced DNA mutations. The expression of HBx correlated with a 24% increase in mutation frequency overall and an approximately twofold increase in the incidence of G/C-to-T/A transversion mutations following AFB1 exposure. These results are consistent with a model in which expression of HBx during chronic HBV infection may contribute to the development of hepatocellular carcinoma following exposure to environmental carcinogens.

Enhancement of hepatitis B virus replication by its X protein in transgenic mice

Hepatitis B virus (HBV) X gene encodes a multifunctional protein that can regulate cellular signaling pathways, interact with cellular transcription factors, and induce hepatocellular oncogenesis. In spite of its diverse activities, the precise role of the X protein in the viral life cycle of HBV remains unclear. To investigate this question, we have produced transgenic mice that carry either the wild-type HBV genome or a mutated HBV genome incapable of expressing the 16.5-kDa X protein. Our results indicate that while the X protein is not absolutely essential for HBV replication or its maturation in transgenic mice, it can enhance viral replication, apparently by activating viral gene expression. These results demonstrate a transactivation role of the X protein in HBV replication in transgenic mice.

DDB2 induces nuclear accumulation of the hepatitis B virus X protein independently of binding to DDB1

The hepatitis B virus (HBV) X protein (HBx) is critical for the life cycle of the virus. HBx associates with several host cell proteins including the DDB1 subunit of the damaged-DNA binding protein DDB. Recent studies on the X protein encoded by the woodchuck hepadnavirus have provided correlative evidence indicating that the interaction with DDB1 is important for establishment of infection by the virus. In addition, the interaction with DDB1 has been implicated in the nuclear localization of HBx. Because the DDB2 subunit of DDB is required for the nuclear accumulation of DDB1, we investigated the role of DDB2 in the nuclear accumulation of HBx. Here we show that expression of DDB2 increases the nuclear levels of HBx. Several C-terminal deletion mutants of DDB2 that fail to bind DDB1 are able to associate with HBx, suggesting that DDB2 may associate with HBx independently of binding to DDB1. We also show that DDB2 enhances the nuclear accumulation of HBx independently of binding to DDB1, since a mutant that does not bind DDB1 is able to enhance the nuclear accumulation of HBx. HBV infection is associated with liver pathogenesis. We show that the nuclear levels of DDB1 and DDB2 are tightly regulated in hepatocytes. Studies with regenerating mouse liver indicate that during late G1 phase the nuclear levels of both subunits of DDB are transiently increased, followed by a sharp decrease in S phase. Taken together, these results suggest that DDB1 and DDB2 would participate in the nuclear functions of HBx effectively only during the late-G1 phase of the cell cycle.

Transcriptional regulation of the TFIIH transcription repair components XPB and XPD by the hepatitis B virus x protein in liver cells and transgenic liver tissue

Human hepatitis B virus is a risk factor for the development of hepatocellular carcinoma. The hepatitis B virus x protein (HBx) has been shown to inactivate the p53 tumor suppressor protein and impair DNA repair, cell cycle, and apoptosis mechanisms. Herein we report that HBx represses two components of the transcription-repair factor TFIIH, XPB (p89), and XPD (p80), both in p53-proficient and p53-deficient liver cells. This inhibition is observed while HBx maintains its transactivation function. Expression of HBx in liver cells results in down-regulation of endogenous XPB and XPD mRNAs and proteins; this inhibition is not observed with other TFIIH subunits, XPA or PCNA. In liver tissue from HBx transgenics, XPB and XPD proteins are down-regulated in comparison to matched normal liver tissue. HBx has been shown to interact with Sp1 transcription factor and affects its DNA binding activity. Sp1 is essential for the basal promoter activity of XPB in liver cells and Drosophila SL2 cells. In the Sp1-deficient SL2 cells, HBx-induced XPB and XPD inhibition is Sp1-dependent. In summary, our results provide evidence that HBx represses the expression of key TFIIH proteins at least in part through Sp1 elements; this repression may impair TFIIH function in DNA repair mechanisms.

Hepatitis B virus X protein acts as a tumor promoter in development of diethylnitrosamine-induced preneoplastic lesions

Chronic infection with hepatitis B virus (HBV) is one of the major etiological factors in the development of human hepatocellular carcinoma. Transgenic mice that express the HBV X protein (HBx) have previously been shown to be more sensitive to the effects of hepatocarcinogens. Although the mechanism for this cofactor role remains unknown, the ability of HBx to inhibit DNA repair and to influence cell cycle progression suggests two possible pathways. To investigate these possibilities in vivo, we treated double-transgenic mice that both express HBx (ATX mice) and possess a bacteriophage lambda transgene with the hepatocarcinogen diethylnitrosamine (DEN). Histological examination of liver tissue confirmed that DEN-treated ATX mice developed approximately twice as many focal lesions of basophilic hepatocytes as treated wild-type littermates. Treatment of mice with DEN resulted in a six- to eightfold increase in the mutation frequency (MF), as measured by a functional analysis of the lambda transgene. HBx expression was confirmed by immunoprecipitation and Western blotting and was associated with a modest 23% increase in the MF. Importantly, the extent of hepatocellular proliferation in 14-day-old mice, as measured by the detection of proliferating cell nuclear antigen and by the incorporation of 5-bromo-2'-deoxyuridine, was determined to be approximately twofold higher in ATX livers than in wild-type livers. These results are consistent with a model in which HBx expression contributes to the development of DEN-mediated carcinogenesis by promoting the proliferation of altered hepatocytes rather than by directly interfering with the repair of DNA lesions.

Stimulation of cellular proliferation by hepatitis B virus X protein

Chronic infection with the hepatitis B virus (HBV) is a known risk factor in the development of human hepatocellular carcinoma (HCC). The HBV-encoded X protein, HBx, has been investigated for properties that may explain its cancer cofactor role in transgenic mouse lines. We discuss here recent data showing that HBx is able to induce hepatocellular proliferation in vitro and in vivo. This property of HBx is predicted to sensitize hepatocytes to other HCC cofactors, including exposure to carcinogens and to other hepatitis viruses. Cellular proliferation is intimately linked to the mechanism(s) by which most tumor-associated viruses transform virus-infected cells. The HBx alteration of the cell cycle provides an additional mechanism by which chronic HBV infection may contribute to HCC.

Expression of hepatitis B virus X protein does not alter the accumulation of spontaneous mutations in transgenic mice

Chronic infection with hepatitis B virus (HBV) is one of the major etiological factors in the development of human hepatocellular carcinoma. Transgenic mice that express the HBV X protein (HBx) have previously been shown to be more sensitive to the effects of hepatocarcinogens, although the mechanism for this cofactor role remains unknown. The ability of HBx to inhibit DNA repair in transiently transfected cell lines suggests one possible pathway. In the present study, primary hepatocytes isolated from transgenic mice that possess the HBV X gene under the control of the human alpha-1-antitrypsin regulatory region (ATX mice) were found to be deficient in their ability to conduct unscheduled DNA synthesis in response to UV-induced DNA damage. In order to measure the impact of HBx expression on DNA repair in vivo, double-transgenic mice that express HBx and possess a bacteriophage lambda transgene were sacrificed at 30, 90, and 240 days of age. Mutation frequency was determined for high-molecular-weight liver DNA of ATX and control mice by functional analysis of the lambda transgene. Expression of HBx did not significantly increase the accumulation of spontaneous mutations. These results are consistent with previous studies of HBx transgenic mice in which no effect of HBx on liver histology was apparent. This new animal model provides a powerful system in which to investigate the in vivo cooperation between HBx expression and environmental carcinogens.

Dissociation of DDB1-binding and transactivation properties of the hepatitis B virus X protein

The hepatitis B virus (HBV) X protein (HBx) is a transactivator encoded by mammalian hepadnaviruses, and is thought to stimulate transcription by interacting with one or more host cell factors. Numerous cellular proteins have been reported to interact with HBx including a component of the nucleotide excision repair complex called ultraviolet damaged DNA binding (UV-DDB, or DDB1) protein. Recent studies have identified a role for DDB1 in transcription, raising the possibility that HBx may acquire its broad transcriptional properties by interacting with DDB1. A panel of HBx mutant proteins, some of which no longer bind to DDB1, was used to test this hypothesis. Plasmid DNAs encoding HBx wildtype and mutant derivatives were transfected into HepG2 cells, and their ability to transactivate a cotransfected reporter plasmid tested. Results from the transactivation assays in HepG2 cells were then compared with data obtained from HBx-DDB1 binding studies performed in yeast. Several HBx mutant proteins unable to bind DDB1 remained competent for transactivation, indicating that HBx binding to DDB1 is not required for HBx transactivation of the ETS1 promoter. It remains possible that a subset of HBx transactivation function targets an as yet undefined DDB1-specific pathway.

The naturally occurring mutants of DDB are impaired in stimulating nuclear import of the p125 subunit and E2F1-activated transcription

The human UV-damaged-DNA binding protein DDB has been linked to the repair deficiency disease xeroderma pigmentosum group E (XP-E), because a subset of XP-E patients lack the damaged-DNA binding function of DDB. Moreover, the microinjection of purified DDB complements the repair deficiency in XP-E cells lacking DDB. Two naturally occurring XP-E mutations of DDB, 82TO and 2RO, have been characterized. They have single amino acid substitutions (K244E and R273H) within the WD motif of the p48 subunit of DDB, and the mutated proteins lack the damaged-DNA binding activity. In this report, we describe a new function of the p48 subunit of DDB, which reveals additional defects in the function of the XP-E mutants. We show that when the subunits of DDB were expressed individually, p48 localized in the nucleus and p125 localized in the cytoplasm. The coexpression of p125 with p48 resulted in an increased accumulation of p125 in the nucleus, indicating that p48 plays a critical role in the nuclear localization of p125. The mutant forms of p48, 2RO and 82TO, are deficient in stimulating the nuclear accumulation of the p125 subunit of DDB. In addition, the mutant 2RO fails to form a stable complex with the p125 subunit of DDB. Our previous studies indicated that DDB can associate with the transcription factor E2F1 and can function as a transcriptional partner of E2F1. Here we show that the two mutants, while they associate with E2F1 as efficiently as wild-type p48, are severely impaired in stimulating E2F1-activated transcription. This is consistent with our observation that both subunits of DDB are required to stimulate E2F1-activated transcription. The results provide insights into the functions of the subunits of DDB and suggest a possible link between the role of DDB in E2F1-activated transcription and the repair deficiency disease XP-E.

Hepatitis B virus-induced liver injury and altered expression of carcinogen metabolising enzymes: the role of the HBx protein

Hepatitis B virus (HBV) and aflatoxins are major risk factors for hepatocellular carcinoma (HCC) exhibiting a synergistic interaction in the development of this disease. The molecular mechanisms of this interaction remain to be elucidated but an altered carcinogen metabolism in the presence of hepatitis-induced liver injury is one hypothesis. The availability of biomarkers of aflatoxin exposure and metabolism permits this hypothesis to be examined in human populations whilst animal models, such as HBV transgenic mice permit parallel studies in an experimental setting. The hepatitis B virus X protein (HBx) is suspected to play a role in the hepatocarcinogenic process by virtue of its capacity to transactivate oncogenes and several other cellular genes via cis-acting elements. In previous studies in HBV transgenic mice expressing the HB surface antigen and X genes we observed a marked induction of specific cytochrome P450s (CYP) (Kirby et al., 1994a). In the current study we investigated the status of CYP, glutathione S-transferases (GST) and antioxidant enzymes in mice carrying only the X gene under the control of the alpha-1 antitrypsin regulatory elements (ATX mice). Livers of ATX mice showed no major pathological alterations compared to age-matched non-transgenic control mice. Immunohistochemical staining for CYP1A, 2A5 and GST expression and determination of related enzymatic activities (7-ethoxyresorufin O-deethylation, 7-methoxyresorufin O-deethylation, coumarin 7-hydroxylation and GST activities) revealed no differences between control and ATX mice. In addition, no differences in antioxidant enzymes were observed. Overall, these results support the conclusion that HBx expression alone is insufficient to induce transactivation of CYP and GST genes or to alter the antioxidant system and that the induction in other HBV models is a result of inflammatory injury in the liver, a feature absent in ATX mice. These data are compared to biomarker studies of enzyme activities in aflatoxin-exposed human populations with and without HBV infection.

Expression of ras, c-myc, and p53 proteins in cervical intraepithelial neoplasia

BACKGROUND

The development of cervical carcinoma is influenced by multiple factors, including the presence of certain high risk types of human papillomavirus. The purpose of the current study was to investigate possible cooperating genetic changes by examining the expression of p53, p62 myc, and p21 ras in cervical biopsy specimens.

METHODS

Three hundred and ninety-five cervical biopsy specimens representing normal through high grade cervical intraepithelial neoplasia (CIN) were screened by immunohistochemistry for expression of p53, p62myc, and p21ras.

RESULTS

Neither the proportion of tissues staining positive for a given protein nor the staining patterns within the epithelial layers differed significantly among normal or CIN biopsy samples. However, grade specific nuclear staining of p21ras was found in the cells of 10 lesions that were classified as CIN I by histology.

CONCLUSIONS

These results established the normal distribution and expression patterns of p53, p62myc, and p21ras within 395 cervical biopsy samples representing normal through CIN III histology. The expression of these proteins (e.g., staining intensity and layer of epithelium staining positive) is similar in normal tissues and those demonstrating all grades of CIN.

Hepatitis B virus X protein interferes with cellular DNA repair

The hepatitis B virus X protein (HBx) is a broadly acting transactivator implicated in the development of liver cancer. Recently, HBx has been reported to interact with several different cellular proteins, including our report of its binding to XAP-1, the human homolog of the simian repair protein UVDDB. In the present study, several HBx mutants were used to localize the minimal domain of HBx required for binding to XAP-1/UVDDB to amino acids 55 to 101. The normal function of XAP-1/UVDDB is thought to involve binding to damaged DNA, the first step in nucleotide excision repair (NER); therefore, we hypothesized that this interaction may affect the cell's capacity to correct lesions in the genome. When tested in two independent assays that measure NER (unscheduled DNA synthesis and host cell reactivation), the expression of HBx significantly inhibited the ability of cells to repair damaged DNA. Under the assay conditions, HBx was expressed at a level similar to that previously observed during natural viral infection and was able to transactivate several target reporter genes. These results are consistent with a model in which HBx acts as a cofactor in hepatocarcinogenesis by preventing the cell from efficiently repairing damaged DNA, thus leading to an accumulation of DNA mutations and, eventually, cancer. An adverse effect on cellular DNA repair processes suggests a new mechanism by which a tumor-associated virus might contribute to carcinogenesis.

Frequent loss of chromosome 8p in hepatitis B virus-positive hepatocellular carcinomas from China

The majority of hepatocellular carcinomas (HCCs) from hepatitis B virus (HBV)-endemic areas contain integrated viral sequences. To better understand the role of HBV DNA insertion in tumorigenesis, we examined the integration site of a HCC harboring a single insert. Cellular DNAs flanking the viral sequences were mapped to chromosomes 17 and 8, indicating a translocation had occurred at the site of viral integration. Regional mapping of chromosome 17 demonstrated that HBV had integrated in 17p12-pter, a region that harbors the p53 tumor suppressor gene. Many studies have shown that chromosome 17p allele loss occurs frequently in HCCs from certain geographical areas. To investigate the chromosome 8 allele status in Chinese HCCs, a panel of 37 matched normal and HCC DNAs from Qidong, China was analyzed for tumor-specific allele loss with RFLP probes from both arms of chromosome 8. Tumor-specific loss of heterozygosity was highest on the short arm with 71.4% (10/14) and 85.0% (17/20) of the informative patients missing an allele for 8p23 (YNM3) or 8p21 (NEFL), respectively. Allele loss from the long arm of chromosome 8 was also observed with 30.0% (6/20) and 33.3% (7/21) of the samples informative for 8q22 (CA2) and 8q24 (MCT128.2), respectively. The high allele loss on 8p correlates with recent studies of other human cancers and is interpreted to indicate that a tumor suppressor gene(s) whose loss is important for carcinogenesis lies within this region. These findings also support a model in which HBV insertions associated with gross chromosomal changes can identify genomic regions where alteration is important for development of some HCCs.

Activation of the HTLV-I long terminal repeat by the hepatitis B virus X protein

The human T-cell leukemia virus type I (HTLV-I) Tax protein and the hepatitis B virus (HBV) X protein have each been shown to activate transcription of their respective viral promoters as well as a subset of cellular gene promoters. Here we show that the HTLV-I long terminal repeat (LTR) is responsive to HBV X transactivation. Maximum levels of X-mediated transactivation of the LTR were 8-fold. An X-responsive-region (XRR) of the LTR is located between nucleotides -355 and -276 and contains an AP-2 binding site, a previously recognized X-responsive element. We demonstrated that Tax and X synergize to activate transcription from the HTLV-I LTR, although the AP-2 binding site was not required for this synergy. These results raise the possibility that the HBV X protein may affect the level of HTLV-I gene expression in co-infected individuals.

Increased sensitivity to the hepatocarcinogen diethylnitrosamine in transgenic mice carrying the hepatitis B virus X gene

The role of the hepatitis B virus (HBV) X protein in liver tumorigenesis is unresolved. Transgenic mice harboring the X gene (nt 1376-1840 under the control of the human alpha-1-antitrypsin regulatory elements) (ATX mice) display only minor histopathologic alterations of the liver. To determine if ATX mice are more susceptible to the effects of hepatocarcinogens, 12- to 15-d-old male ATX and control littermate mice were injected with a single dose (2 microgram/g body weight) of diethylnitrosamine (DEN). The animals were killed 6-10 mo after exposure and were analyzed for histological changes in the liver. One hundred percent of the DEN-treated AXT mice developed abnormal liver lesions. Then their liver tissues were compared by stereological analysis with those of non-transgenic animals, the ATX mice had a relative twofold increase in the total number of focal lesion and a twofold increase in the incidence of hepatocellular carcinoma. Elevated levels of X protein and p53 protein were not detected in carcinogen-induced nodules or tumors. These results are consistent with a model in which the expression of the HBV X protein potentiates the induction of DEN-mediated liver disease.

Increased sensitivity to the hepatocarcinogen diethylnitrosamine in transgenic mice carrying the hepatitis B virus X gene

The role of the hepatitis B virus (HBV) X protein in liver tumorigenesis is unresolved. Transgenic mice harboring the X gene (nt 1376-1840 under the control of the human alpha-1-antitrypsin regulatory elements) (ATX mice) display only minor histopathologic alterations of the liver. To determine if ATX mice are more susceptible to the effects of hepatocarcinogens, 12- to 15-d-old male ATX and control littermate mice were injected with a single dose (2 microgram/g body weight) of diethylnitrosamine (DEN). The animals were killed 6-10 mo after exposure and were analyzed for histological changes in the liver. One hundred percent of the DEN-treated AXT mice developed abnormal liver lesions. Then their liver tissues were compared by stereological analysis with those of non-transgenic animals, the ATX mice had a relative twofold increase in the total number of focal lesion and a twofold increase in the incidence of hepatocellular carcinoma. Elevated levels of X protein and p53 protein were not detected in carcinogen-induced nodules or tumors. These results are consistent with a model in which the expression of the HBV X protein potentiates the induction of DEN-mediated liver disease.

Increased sensitivity to the hepatocarcinogen diethylnitrosamine in transgenic mice carrying the hepatitis B virus X gene

The role of the hepatitis B virus (HBV) X protein in liver tumorigenesis is unresolved. Transgenic mice harboring the X gene (nt 1376-1840 under the control of the human alpha-1-antitrypsin regulatory elements) (ATX mice) display only minor histopathologic alterations of the liver. To determine if ATX mice are more susceptible to the effects of hepatocarcinogens, 12- to 15-d-old male ATX and control littermate mice were injected with a single dose (2 microgram/g body weight) of diethylnitrosamine (DEN). The animals were killed 6-10 mo after exposure and were analyzed for histological changes in the liver. One hundred percent of the DEN-treated AXT mice developed abnormal liver lesions. Then their liver tissues were compared by stereological analysis with those of non-transgenic animals, the ATX mice had a relative twofold increase in the total number of focal lesion and a twofold increase in the incidence of hepatocellular carcinoma. Elevated levels of X protein and p53 protein were not detected in carcinogen-induced nodules or tumors. These results are consistent with a model in which the expression of the HBV X protein potentiates the induction of DEN-mediated liver disease.

Increased sensitivity to the hepatocarcinogen diethylnitrosamine in transgenic mice carrying the hepatitis B virus X gene

The role of the hepatitis B virus (HBV) X protein in liver tumorigenesis is unresolved. Transgenic mice harboring the X gene (nt 1376-1840 under the control of the human alpha-1-antitrypsin regulatory elements) (ATX mice) display only minor histopathologic alterations of the liver. To determine if ATX mice are more susceptible to the effects of hepatocarcinogens, 12- to 15-d-old male ATX and control littermate mice were injected with a single dose (2 microgram/g body weight) of diethylnitrosamine (DEN). The animals were killed 6-10 mo after exposure and were analyzed for histological changes in the liver. One hundred percent of the DEN-treated AXT mice developed abnormal liver lesions. Then their liver tissues were compared by stereological analysis with those of non-transgenic animals, the ATX mice had a relative twofold increase in the total number of focal lesion and a twofold increase in the incidence of hepatocellular carcinoma. Elevated levels of X protein and p53 protein were not detected in carcinogen-induced nodules or tumors. These results are consistent with a model in which the expression of the HBV X protein potentiates the induction of DEN-mediated liver disease.

Induction of senescent cell-derived inhibitor of DNA synthesis gene, SDI1, in hepatoblastoma (HepG2) cells arrested in the G2-phase of the cell cycle by 9-nitrocamptothecin

BACKGROUND

Recent studies have demonstrated that the plant-derived alkaloid camptothecin (CPT) and its derivative, 9-nitro-CPT (9NC), are cytotoxic in tumorigenic cells but cytostatic in nontumorigenic cells in vitro and in vivo. Also, CPT induces differentiation of human leukemia cells in vitro along specific lineages. In this study, we have investigated the effects of 9NC on nontumorigenic HepG2 cells derived from human hepatoblastoma. A newly discovered senescent cell-derived inhibitor (SDI1) plays a critical role in the cell cycle, so we evaluated the effect of 9NC on the expression of the SDI1 gene.

EXPERIMENTAL DESIGN

The effects of 9NC on HepG2 cells were evaluated by monitoring DNA synthesis, morphologic and ultrastructural changes of cells, and perturbation in the cell cycle and by assessing the levels of p53 protein and SDI1 mRNA.

RESULTS

Treatment of HepG2 cells with 9NC results in a dose-dependent inhibition of cell proliferation and DNA synthesis. Flow cytometric analysis of DNA content showed that 9NC-treated HepG2 cells are arrested in the G2-phase of the cell cycle. Light and electron microscopic examination revealed that 9NC at low concentrations induces morphologic and growth features that resemble properties highly differentiated or senescent cells, i.e., increased cell size and decreased nuclear/cytoplasmic ratio, as well as enlarged numbers of lysosomes, mitochondria, and lipid in the cytoplasm. No significant alteration in the p53 protein level was noted in 9NC-treated cells. In contrast to untreated, logarithmically grown HepG2 cells, 9NC-treated cells arrested at the G2-phase of the cell cycle and contained increased levels of SDI1 mRNA. Kinetic studies revealed gradual increases in SDI1 mRNA synthesis.

CONCLUSIONS

Induction of SDI1 mRNA by 9NC represents the first documentation that the SDI1 gene can be overexpressed in the G2-phase of the cell cycle and provides a valuable cell culture system to dissect the events controlling the G2 checkpoint. In addition, this finding corroborates the hypothesis that genes up-regulated in senescent cells can also be induced in tumor-derived immortalized cells.

Allelotype and loss of heterozygosity of p53 in primary and recurrent hepatocellular carcinomas. A study of 150 patients

BACKGROUND

The allelotype and loss of heterozygosity (LOH) of the p53 gene in human hepatocellular carcinoma (HCC) were studied in 150 patients with resected primary HCC and 18 with recurrent HCC.

METHODS

DNA samples of paired HCC and livers were cut with BanII enzyme for the study of p53 allelotype and allele loss. The medical records of the patients were carefully reviewed.

RESULTS

Sixty-four (42.7%) patients were heterozygous for the p53 gene, 69 (46%) were homozygous for the 1.5/1.4 kb small (S) allele, and 17 (11.3%) were homozygous for the 2.9 kb large (L) allele. The frequencies of the minor L allele (0.323) and of the major S allele (0.677) in this population of Chinese patients differed from the frequencies previously reported for North American Caucasians (0.13 and 0.87, respectively). The heterozygous patients tended to have lower serum hepatitis B surface- and e antigens (HBsAg and HBeAg) and higher diabetes mellitus (DM) than did homozygous patients (SS and LL). Thirty-seven (57.8%) of the 64 heterozygous patients had a tumor-specific p53 allele LOH, being two times more common in HCC tumors larger than 8 cm than in HCC tumors 2 cm or smaller. The frequency of DM was four times higher in the heterozygous patients who had p53 LOH than in those who retained both alleles. LOH of p53 did not correlate with tumor invasiveness or differentiation, hepatitis B or C virus infection, or prognosis.

CONCLUSION

The allelotype of p53 gene in HCC correlates with HBsAg and HBeAg seropositivities and DM. LOH of the p53 gene is a common event in HCC, correlates with DM, and occurs less often in familial HCC. LOH can identify the clonal origin of recurrent HCC but is not a critical prognostic factor.

Allelotype and loss of heterozygosity of p53 in primary and recurrent hepatocellular carcinomas. A study of 150 patients

BACKGROUND

The allelotype and loss of heterozygosity (LOH) of the p53 gene in human hepatocellular carcinoma (HCC) were studied in 150 patients with resected primary HCC and 18 with recurrent HCC.

METHODS

DNA samples of paired HCC and livers were cut with BanII enzyme for the study of p53 allelotype and allele loss. The medical records of the patients were carefully reviewed.

RESULTS

Sixty-four (42.7%) patients were heterozygous for the p53 gene, 69 (46%) were homozygous for the 1.5/1.4 kb small (S) allele, and 17 (11.3%) were homozygous for the 2.9 kb large (L) allele. The frequencies of the minor L allele (0.323) and of the major S allele (0.677) in this population of Chinese patients differed from the frequencies previously reported for North American Caucasians (0.13 and 0.87, respectively). The heterozygous patients tended to have lower serum hepatitis B surface- and e antigens (HBsAg and HBeAg) and higher diabetes mellitus (DM) than did homozygous patients (SS and LL). Thirty-seven (57.8%) of the 64 heterozygous patients had a tumor-specific p53 allele LOH, being two times more common in HCC tumors larger than 8 cm than in HCC tumors 2 cm or smaller. The frequency of DM was four times higher in the heterozygous patients who had p53 LOH than in those who retained both alleles. LOH of p53 did not correlate with tumor invasiveness or differentiation, hepatitis B or C virus infection, or prognosis.

CONCLUSION

The allelotype of p53 gene in HCC correlates with HBsAg and HBeAg seropositivities and DM. LOH of the p53 gene is a common event in HCC, correlates with DM, and occurs less often in familial HCC. LOH can identify the clonal origin of recurrent HCC but is not a critical prognostic factor.

Deletion of the E-cadherin gene in hepatitis B virus-positive Chinese hepatocellular carcinomas

Frequent allele loss from chromosome 16q was recently described for human tumors of the breast, prostate gland and liver, indicating the possible presence of a tumor-suppressor gene on that chromosome arm. In this study, the chromosome 16 allele status of 38 hepatocellular carcinomas in Chinese patients was determined with restriction-fragment-length polymorphism analysis. Tumor-specific allele loss was detected in 14 (74%) of 19 patients informative for 16p markers and in 22 (85%) of 26 patients informative for 16q markers. Quantitative densitometric analysis revealed reduction to hemizygosity of the E-cadherin cell adhesion gene (localized to 16q22.1) in 18 (64%) of the 28 patients for whom quantitative data were available. Reduced expression of E-cadherin has been associated with invasion and metastasis in several human cell lines and primary tumors, and our results suggest that one mechanism of reduced E-cadherin expression is the loss of one copy of the E-cadherin gene.

Mice deficient for p53 are developmentally normal but susceptible to spontaneous tumours

Mutations in the p53 tumour-suppressor gene are the most frequently observed genetic lesions in human cancers. To investigate the role of the p53 gene in mammalian development and tumorigenesis, a null mutation was introduced into the gene by homologous recombination in murine embryonic stem cells. Mice homozygous for the null allele appear normal but are prone to the spontaneous development of a variety of neoplasms by 6 months of age. These observations indicate that a normal p53 gene is dispensable for embryonic development, that its absence predisposes the animal to neoplastic disease, and that an oncogenic mutant form of p53 is not obligatory for the genesis of many types of tumours.

p53 mutations cluster at codon 249 in hepatitis B virus-positive hepatocellular carcinomas from China

DNA samples from 36 hepatocellular carcinoma (HCC) patients from China were screened for a specific mutation affecting codon 249 of the p53 gene, recently identified as a hotspot mutation in some HCCs. We detected the tumor-specific p53 codon 249 mutation in 21 (58%) of 36 HCCs examined. Thirteen patients with the specific codon 249 mutation had lost the remaining allele of p53, whereas the remaining eight patients appeared to have retained both copies of the gene. These results suggest that alterations of p53 may be important events in the genesis of HCCs and that point mutation may precede allele loss.

Hepatitis B virus integration event in human chromosome 17p near the p53 gene identifies the region of the chromosome commonly deleted in virus-positive hepatocellular carcinomas

The development of hepatocellular carcinoma (HCC) presumably occurs in multiple steps and is influenced by numerous factors. Hepatitis B virus (HBV) is strongly associated with the development of HCC in people chronically infected with the virus, but the mechanism of viral involvement remains unclear. One possibility is that the gross chromosomal alterations frequently observed in HCC DNA at the site of HBV integration may alter the expression of important nearby cellular genes. We previously reported the cloning and characterization of a HBV insert from a Chinese HCC. The viral insert mapped to chromosome 17p11.2-12, and cellular sequences were duplicated at the site of viral integration. In the present study a DNA probe derived from cellular DNA sequences adjacent to the previously characterized HBV insert was used to analyze a set of 19 matched normal liver and HBV-positive hepatoma samples obtained from the same region of China, near Shanghai. Tumor-specific DNA changes were detected in two additional HCCs, suggesting that the small region of chromosome 17p defined by the flanking cell DNA probe is commonly altered in hepatomas. Restriction fragment length polymorphism studies demonstrated that the loss of one copy of portions of chromosome 17 occurred in 10 (53%) of the 19 patients. The loss of one allele of the p53 gene (located on chromosome 17p13) occurred in at least 6 (60%) of the 10 patients who were heterozygous at the p53 locus. As the p53 gene is known to possess tumor suppressor activity, the functional loss of this gene may be a significant step in the development of a subset of HCCs. High levels of allele loss also were detected for chromosomes 8q (4 of 9; 44%) and 16p (5 of 6; 83%) and may indicate the presence of additional cellular genes whose functional loss is important in the development of HCCs.

Development of a transgenic mouse system for the analysis of stages in liver carcinogenesis using tissue-specific expression of SV40 large T-antigen controlled by regulatory elements of the human alpha-1-antitrypsin gene

alpha-1-Antitrypsin (AAT) is the major antiprotease in human plasma; it is synthesized primarily in hepatocytes and to a lesser extent in several nonhepatic tissues. Under the control of regulatory elements of the human AAT gene, expression of SV40-large tumor antigen (T-ag) in transgenic mice occurred in the liver, stomach, pancreas, and kidney. Among seven founder transgenic animals, six developed liver carcinoma, four showed gastric neoplasia, and one developed pancreatic carcinoma. In three animals the kidneys showed glomerular or tubular epithelial hyperplasia but no malignancy. A stable transgenic line, 1812, was established. Members of this line reproducibly develop liver tumors by 10 weeks of age but do not exhibit any phenotypic changes in other tissues. Histological changes leading to liver tumor formation occurred with predictable kinetics and could be classified into four distinct stages: (a) embryonal/fetal stage, no recognizable histological changes; (b) newborn to 2 weeks of age, hyperplastic hepatocytes with reduced amounts of cytoplasm but no nuclear alterations; (c) between 3 and 8 weeks of age, diffuse liver cell dysplasia without observable tumor nodules; and (d) 8 weeks of age and thereafter, hepatocellular carcinomas in a background of liver dysplasia. Embryonic and newborn liver tissue showed uniform, high level expression of T-ag in the majority of hepatocytes by immunohistochemistry, whereas the dysplastic and tumoral stages were characterized by considerable variation in both the intensity of T-ag staining and the proportion of T-ag-positive cells. Immunoprecipitation analyses showed that T-ag was complexed with cellular protein p53 in all tumor samples. This study showed that SV40 T-ag expression in the liver resulted in cellular hyperplasia and dysplasia; additional event(s) apparently were required for progression to neoplasia. Those cooperating events occurred with predictable kinetics. This transgenic mouse system displays several similarities with human liver disease and provides a practical model for the study of separate steps in hepatocarcinogenesis.

Identification of a conserved sequence in the non-coding regions of many human genes

We have analyzed a sequence of approximately 70 base pairs (bp) that shows a high degree of similarity to sequences present in the non-coding regions of a number of human and other mammalian genes. The sequence was discovered in a fragment of human genomic DNA adjacent to an integrated hepatitis B virus genome in cells derived from human hepatocellular carcinoma tissue. When one of the viral flanking sequences was compared to nucleotide sequences in GenBank, more than thirty human genes were identified that contained a similar sequence in their non-coding regions. The sequence element was usually found once or twice in a gene, either in an intron or in the 5' or 3' flanking regions. It did not share any similarities with known short interspersed nucleotide elements (SINEs) or presently known gene regulatory elements. This element was highly conserved at the same position within the corresponding human and mouse genes for myoglobin and N-myc, indicating evolutionary conservation and possible functional importance. Preliminary DNase I footprinting data suggested that the element or its adjacent sequences may bind nuclear factors to generate specific DNase I hypersensitive sites. The size, structure, and evolutionary conservation of this sequence indicates that it is distinct from other types of short interspersed repetitive elements. It is possible that the element may have a cis-acting functional role in the genome.

Structural analysis of a hepatitis B virus genome integrated into chromosome 17p of a human hepatocellular carcinoma

Hepatitis B virus (HBV) is clearly a factor in the development of hepatocellular carcinoma, but its mechanism of action remains obscure. One possibility is that the HBV integration event alters the expression of a nearby growth-regulatory cellular gene. A 9-kilobase (kb) DNA fragment containing an HBV insert plus flanking cellular sequences was cloned from a hepatoma specimen from Shanghai, People's Republic of China. Restriction mapping of the insert revealed a large inverted repeat structure consisting of both viral sequences (encompassing all of the core and pre-S regions and portions of the X and S genes) and at least 3 kb of unique cellular sequences. The virus-cell junction mapped 11 nucleotides from the DR1 region, in a position within the HBV X gene and included in the cohesive overlap region. A probe generated from 1.0 kb of the flanking cellular DNA mapped the viral insert to chromosome 17 in the region designated 17p11.2-17p12, which is near the human proto-oncogene p53. Sequence data from a portion of the flanking cellular DNA revealed a stretch of approximately 70 base pairs that showed highly significant homology with a conserved region of a number of functional mammalian DNAs, including the human autonomously replicating sequence 1 (ARS1).