Chromosomal site of hepatitis B virus (HBV) integration in a human hepatocellular carcinoma-derived cell line

Transcriptional regulation of mitotic checkpoint gene MAD1 by p53

p53 regulates a number of genes through transcriptional activation and repression. p53-dependent mitotic checkpoint has been described, but the underlying mechanism is still obscure. Here we examined the effect of p53 on the expression of a human mitotic checkpoint protein, Mitosis Arrest Deficiency 1 (MAD1), in cultured human cells. The expression of MAD1 was reduced when the cells were overexpressing exogenously introduced wild-type p53. The same reduction was also observed when the cells were treated with anticancer agents 5-fluorouracil and cisplatin or were irradiated with UV. Consistently, MAD1 promoter activity diminished in a dose-dependent manner when induced by p53, indicating that p53 repressed MAD1 at a transcriptional level. Intriguingly, several tumor hot spot mutations in p53 (V143A, R175H, R248W, and R273H) did not abolish the ability of p53 to repress MAD1 expression. By serial truncation of the MAD1 promoter, we confined the p53-responsive element to a 38-bp region that represents a novel sequence distinct from the known p53 consensus binding site. Trichostatin A, a histone deacetylase inhibitor, relieved the p53 transrepression activity on MAD1. Chromatin immunoprecipitation assay revealed that p53, histone deacetylase 1, and co-repressor mSin3a associated with the MAD1 promoter in vivo. Taken together, our findings suggest a regulatory mechanism for the mitotic checkpoint in which MAD1 is inhibited by p53.

Increased chromosomal alterations and micronuclei formation in human hepatoma HepG2 cells transfected with the hepatitis B virus HBX gene

The protein encoded by the hepatitis B virus (HBV)-X gene, HBX, has been implicated to be involved in the development of HBV-associated liver cancer. HBX is a multifunctional regulatory protein that has been identified as a potential oncogene but its exact function remains unclear. HBX was documented to interact with several factors involved in cellular DNA repair as well as compromise the cell's ability to repair damaged DNA. We previously documented an accumulation of genetic alterations in two HepG2 cell lines independently transfected with HBV. In this report, we investigate the effect of the HBV-X gene (HBX) on the stability of the host genome using HepG2 stable transfectants (HepG2-HBX) and vector controls (HepG2-neo). We document that all HepG2-HBX clones analyzed contain HBX gene integrated and HBX transcript. Our data demonstrate that HepG2-HBX cells have an increased number of chromosome alterations and micronuclei formation compared to vector controls. A total of 10 de novo chromosomal rearrangements involving nine different chromosomes were detected in the HepG2-HBX clones, while no new rearrangements were found in vector controls. Each HepG2-HBX clone contained independently occurring de novo alterations not found in other HBX or vector clones. A three-fold increase of micronuclei formation was detected in HepG2-HBX cells compared to vector controls. Micronuclei originated from all chromosomes, however, preliminary data indicated that micronuclei originating from chromosomes 2, 3, 7, 18 and 20 were found in a greater amount in cells expressing the HBX gene. Interestingly, chromosomes 2, 18 and 20 were three of the chromosomes found rearranged in HepG2-HBX clones. These data provide evidence that genomic integrity was affected in cells expressing the HBX gene. De novo cytogenetic alterations identified in HepG2-HBX clones implicate the involvement of HBX in the process and support the hypothesis that HBX may interfere with normal cellular processes responsible for genomic integrity, increasing the risk for acquiring genetic mutations in infected hepatocytes.

Multiple genetic alterations, 4q28, a new suppressor region, and potential gender differences in human hepatocellular carcinoma

Primary hepatocellular carcinomas (HCCs) of different etiologies were studied to determine the rate of alteration of several genetic regions previously associated with the HCC phenotype. The focus of our study was to identify the frequency of genetic alterations within individual HCCs and their distribution among male and female cases. Genetic differences were evaluated between DNA isolated from tumor (T) and corresponding non-tumor (N) tissue using short tandem repeat (STR)-microsatellites and restriction fragment length polymorphism (RFLP) analyses. Twenty-eight HCC cases were studied with polymorphic markers from different parts of the genome. Three or more loci were identified with genetic alterations from 28 loci tested in 63% of HCC cases. The highest frequency of alteration occurred in the chromosome regions 1p36, 13q14, 17p13, and the 4q28 region identified in HCCs for the first time. High loss of heterozygosity (LOH) in 1p36, 4q28, 13q14, and 17p13 regions indicates important HCC suppressors within the regions. The study documents no association between cause and specific genetic change or their frequency in HCCs analyzed. The data document a significant gender distortion for genetic alteration in chromosome 13q14 and 17p13 regions and a concordant gender alteration rate in the 1p36 and 4q28 regions. An overall higher frequency of genetic alterations was identified in male cases. Future study with an extended number of HCC cases should substantiate the frequency of alteration between genders and identify tentative suppressors in the 1p36 and 4q28 regions.

Accumulation of genetic alterations in a human hepatoma cell line transfected with hepatitis B virus

Chromosome and molecular analyses of the hepatitis B virus (HBV)-transfected HepG2T14.1 variant of the HepG2 cell line was conducted. In HepG2T14.1 cells several genetic alterations such as de novo aberrations of chromosomes 9, 14, 15, and 20 were identified that are not present in the parental HepG2 cell line. Furthermore, HepG2T14.1 cells showed loss of heterozygosity (LOH) in the q region of chromosome 14. The single HBV integration site in HepG2T14.1 cells mapped to the 2q35-36 region of one copy of chromosome 2 by fluorescence in situ hybridization (FISH). No genetic changes were identified at or near the HBV integration site at the level of these analyses. In addition, growth rates in vivo and in vitro were dramatically accelerated in HepG2T14.1 cells. These results document that a HBV-transfected hepatoma cell line has de novo genetic mutations at several sites of the host genome, one HBV integration site in an non-rearranged chromosome and an altered phenotype. These findings support our hypothesis that HBV might play a role in cellular transformation by interfering with cellular processes responsible for the stability of the genome.

Expression of insulin-like growth factor II in hepatitis B, cirrhosis and hepatocellular carcinoma: its relationship with hepatitis B virus antigen expression

Expression of insulin-like growth factor II in two human hepatocellular carcinoma cell lines and in hepatitis B, cirrhosis and hepatocellular carcinoma in 419 cases were investigated, and its relationship with the expression of hepatitis B virus X gene was studied by means of immunohistochemical and electron microscopic techniques. The results demonstrated that hepatocellular carcinoma cells (SMMC 7721 and QGY 7703) in culture could express insulin-like growth factor II. Expression seemed to be regulated by cell density, which was suggested as the molecular basis of the contact inhibition of cell proliferation. In tissue sections, cells with high expression of insulin-like growth factor II were observed not only in hepatocellular carcinoma (93%) but also in 95% of the pericancerous liver tissues, 72% of cirrhotic livers, 64% of chronic active hepatitis and 37% of chronic persistent hepatitis. In most cases of hepatocellular carcinoma, insulin-like growth factor II was localized in the cytoplasm of the cancer cells. In the benign liver disorders, four types of cells that highly expressed insulin-like growth factor II were observed: (a) a kind of small liver cell we named the small polygonal liver cell; (b) multinuclear giant hepatocytes; (c) hepatocytes in most of hyperplastic and neoplastic nodules, small hepatocyte nodules and some of regenerative nodules; and (d) some proliferating ductular cells. Even more interestingly, insulin-like growth factor II expression was shown to be closely related to the expression of hepatitis B virus X gene product. We suggest that the activation of insulin-like growth factor II gene and its overexpression may be a crucial step in the processes of hepatitis B virus-associated hepatocarcinogenesis and that the X gene product may activate the insulin-like growth factor II gene through a transactivation mechanism. In addition, we studied the characteristics of small polygonal liver cells, and the roles they may play in the regeneration and carcinogenesis of hepatitis B virus-infected liver are discussed.

Cytogenetic studies on human liver cancer cell lines

Karyotyping of human liver cancer cell lines and chromosome in situ hybridization of hepatitis B virus (HBV) DNA was performed in order to elucidate the possible mechanism of hepatocarcinogenesis from evidence of chromosomal changes and HBV integration patterns. HepG2-2 and HepG2-5 cell lines were HepG2 cells experimentally transfected with HBV DNA; the HCC36 cell line was derived from a hepatocellular carcinoma containing integrated HBV DNA from a Taiwanese patient. HepG2 cells, a hepatoblastoma cell line without HBV DNA integration, served as negative control. In HepG2-2, HepG2-5, and HCC36 cells, multiple integrations of HBV DNA were observed by in situ hybridization and hybridization signals occurred preferentially on certain chromosomes: 2, 5, 10, 11, 18; 7, 10, 13, 17, 18; and 4, 6, 11, 12q+, 18; respectively. In addition, a strong correlation between chromosomal changes and HBV integration was noticed in HCC36 cells, especially at chromosome 12q+.

Chromosomal sites for hepatitis B virus integration in human hepatocellular carcinoma

The discovery that hepatitis B virus (HBV) integrates into host chromosomes raises the question of whether such viral DNA integration correlates directly with the activation of specific oncogenes or the inactivation of anti-oncogenes. To obtain insight into this problem, we randomly collected HBV integrant samples from different human hepatocellular carcinomas and identified the site of chromosomal integration by using in situ hybridization and/or linkage analysis with the flanking cellular DNAs as probes. Our findings did not specifically identify particular HBV DNA integration sites in chromosomes, although chromosomes 11 and 17 seemed to have more than the average number of integrants.

Chromosomal rearrangements in a primary hepatocellular carcinoma

Chromosome analysis of cells obtained at biopsy from a 65-year-old man with primary hepatocellular carcinoma revealed characteristic abnormalities of chromosomes 1, 5, 6, 9, 13, 16, and 22 in each cell and maintenance of a pseudodiploid chromosome number (46,XY). Five of the chromosomal sites involved in these rearrangements are either in fragile site regions or in regions containing genes that encode cellular oncogenes. Some of the tumor cells manifest mitotic deviations in the form of asynchronies, spiralization, premature centromere division, and non-sister chromatid associations. The significance of these findings to hepatocellular carcinogenesis is discussed.

No evidence for loss of alleles at 11p in HBV negative hepatocellular carcinomas

The aim of the present study was to determine whether loss of DNA sequences at the chromosome arm 11p, where presumed tumor suppressor genes are located, might occur in hepatocellular carcinomas (HCCs) of hepatitis B virus (HBV) negative patients. Normal liver and HCC genotypes were compared at 6 loci on 11p with 7 polymorphic probes detecting 10 restriction length polymorphisms (RFLPs). Each of the 8 paired normal-tumor tissue samples was informative for at least three different loci on 11p13-pter. None of them showed loss of constitutional heterozygosity for those markers. The retainment of 11p alleles suggests that mechanisms other than loss of tumor suppressor genes on llp are involved in hepatocarcinogenesis of HBV negative patients.

Abnormal structure and expression of p53 gene in human hepatocellular carcinoma

There is little information regarding the molecular mechanisms of hepatocarcinogenesis. We studied the p53 gene at the DNA, RNA, and protein level in seven human hepatocellular carcinoma (HCC)-derived cell lines; six of seven showed p53 abnormalities. By Southern blotting, the p53 gene was found to be partially deleted in Hep 3B and rearranged in SK-HEP-1 cells. Transcripts of the p53 gene were undetectable in Hep 3B as well as in FOCUS cells that had no apparent deletion or rearrangement of the p53 gene. Immunoprecipitation after [35S]methionine labeling of HCC cells demonstrated that p53 protein was absent in Hep 3B and FOCUS and reduced in concentration in PLC/PRF/5 cells. p53 synthesized by Mahlavu cells showed a slower migration on SDS/polyacrylamide gels suggesting it was an abnormal protein. In Huh7 cells, p53 protein had a prolonged half-life leading to its accumulation in the nuclei; increased levels of p53 protein were also found by immunoblotting. The p53 gene and its expression appeared to be unaltered in the hepatoblastoma-derived Hep G2 cell line. We found that the loss of p53 expression did not occur as a late in vitro event in the FOCUS cell line because p53 protein was also nondetectable at an early passage. We conclude that the loss of p53 expression or the presence of abnormal forms of the protein are frequently associated with HCC cell lines. These observations suggest that alterations in p53 may be important events in the transformation of hepatocytes to the malignant phenotype.

Chromosomal assignment of woodchuck hepatitis virus (WHV) DNA integration sites in a woodchuck hepatocellular carcinoma-derived cell line (WH257GE10)

The chromosomal sites of woodchuck hepatitis virus (WHV) DNA integration were identified in a woodchuck hepatocellular carcinoma-derived cell line (WH257GE10) by the in situ hybridization technique using 3H-labelled WHV whole genome (WHV 2) as a probe. The G-banded chromosome spreads from WH257GE10 were identified and diagrammed schematically according to their band patterns. WHV DNA was integrated into 2 sites: 33 region of the long arm of chromosome 6 (6q33) and 31 region of the long arm of chromosome 8 (8q31). Chromosomal sites of WHV DNA integration were stable during successive passage periods analyzed in vitro.

Expression and biosynthetic variation of the epidermal growth factor receptor in human hepatocellular carcinoma-derived cell lines

Expression of the epidermal growth factor (EGF) was analyzed in six human hepatocellular carcinoma-derived and one human hepatoblastoma-derived cell line, each of which retained the differentiated phenotype and functions of the parenchymal hepatocyte. The level of receptor expression of each hepatoma cell line was similar to that of the normal human fibroblast, approximately 10(5) molecules per cell. However, NPLC/PRF/5, a subline of the PLC/PRF/5 cell line obtained following reestablishment of a xenograft tumor in vitro, was found to express 4 x 10(6) high-affinity EGF receptor molecules per cell. Proliferation of the NPLC/PRF/5 cell line was inhibited in the presence of nanomolar quantities of ligand. Receptor overexpression was found to result from EGF receptor gene amplification without apparent rearrangement of the EGF receptor coding sequences. Although cell-specific variability in posttranslational processing of EGF receptor N-linked oligosaccharides in the hepatoma cell lines was found, no difference between the receptors in PLC/PRF/5 and NPLC/PRF/5 was observed and no aberrant receptor-related species were detected. EGF receptor gene amplification in the NPLC/PRF/5 cell line is probably a reflection of genome instability and selection of variants with augmented growth potential in limiting concentrations of EGF in vivo. When viewed in this light, EGF receptor overexpression could represent a manifestation of tumor progression in the EGF-responsive hepatocyte.

Expression and biosynthetic variation of the epidermal growth factor receptor in human hepatocellular carcinoma-derived cell lines

Expression of the epidermal growth factor (EGF) was analyzed in six human hepatocellular carcinoma-derived and one human hepatoblastoma-derived cell line, each of which retained the differentiated phenotype and functions of the parenchymal hepatocyte. The level of receptor expression of each hepatoma cell line was similar to that of the normal human fibroblast, approximately 10(5) molecules per cell. However, NPLC/PRF/5, a subline of the PLC/PRF/5 cell line obtained following reestablishment of a xenograft tumor in vitro, was found to express 4 x 10(6) high-affinity EGF receptor molecules per cell. Proliferation of the NPLC/PRF/5 cell line was inhibited in the presence of nanomolar quantities of ligand. Receptor overexpression was found to result from EGF receptor gene amplification without apparent rearrangement of the EGF receptor coding sequences. Although cell-specific variability in posttranslational processing of EGF receptor N-linked oligosaccharides in the hepatoma cell lines was found, no difference between the receptors in PLC/PRF/5 and NPLC/PRF/5 was observed and no aberrant receptor-related species were detected. EGF receptor gene amplification in the NPLC/PRF/5 cell line is probably a reflection of genome instability and selection of variants with augmented growth potential in limiting concentrations of EGF in vivo. When viewed in this light, EGF receptor overexpression could represent a manifestation of tumor progression in the EGF-responsive hepatocyte.

Analysis of the chromosomal abnormalities of a woodchuck hepatocellular carcinoma-derived cell line (WH257GE10)

The normal woodchuck karyotype was determined by the G-banding technique, and the chromosomal abnormalities of a woodchuck hepatocellular carcinoma-derived cell line (WH257GE10) were analyzed and compared with it. The normal woodchuck karyotype was 38,XY in a male, and 38,XX in a female. Autosomes were classified into 2 groups (A and B). Group A consisted of 13 pairs (1-13) of metacentric or sub-metacentric chromosomes, and group B consisted of 5 pairs (14-18) of telocentric or sub-telocentric ones. The X chromosome was sub-metacentric, and the Y chromosome was telocentric. In WH257GE10, the modal number was 36. Deletion of one of the chromosome 5 pair, of the chromosome 13 pair, and of the Y chromosome was observed. Two marker chromosomes (M1 and M2) were found. These markers showed the same G-banded patterns except for a small chromosome fragment at the distal part of the long arm of M1. The chromosome aberrations were very stable through successive passages in vitro. These findings suggest that the chromosomal abnormalities found in WH257GE10 might be associated with the pathogenesis of woodchuck hepatocellular carcinoma.

Aetiological factors in hepatocellular cancer

The major risk factors for HCC are outlined in Table 2. Each factor may contribute to the multistep process of hepatocarcinogenesis. Hepatitis B virus and aflatoxins are the principal aetiological candidates and may be considered as initiators of the malignant state (see Figure 1). The majority of HCC arises via the cirrhotic pathway; the associated changes in the hormonal milieu may alter the handling of environmental carcinogens and the rounds of cell proliferation may increase sensitivity to sub-threshold doses of carcinogens. Exogenous androgens and oestrogens may operate through a similar mechanism to promote clonal expansion of preneoplastic cells. The importance of identifying the major aetiological factors lies in the possibility of prevention. The prognosis of HCC is dismal and it represents a major scourge in developing Third World countries. It is encouraging to think that the majority of cases could be prevented by the widespread use of hepatitis B vaccines and the development of intervention programmes against aflatoxin contamination of foodstuffs.