Adenovirus type 12-transformed rat embryo brain and rat liver epithelial cell lines: adenovirus type 12 genome content and viral protein expression

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Adenovirus 12-mediated down-regulation of the major histocompatibility complex (MHC) class I promoter: identification of a negative regulatory element responsive to Ad12 E1A

In highly oncogenic adenovirus (Ad) 12-transformed cells, major histocompatibility complex (MHC) class I gene expression is down-regulated by the products of the viral E1A oncogene at the level of initiation of transcription. However, class I gene expression is unaltered or elevated in non-oncogenic Ad2- or Ad5-transformed cells. These changes in class I expression may permit Ad12-transformed cells to escape host immune surveillance and elicit tumour formation. Here we show that the 2kb of 5' flanking region of the mouse H-2Kb class I gene is sufficient to mediate down-regulation of transcription driven from homologous or heterologous (HSV thymidine kinase) basal promoter elements in cells expressing Ad12 E1A, but not in Ad2 E1A-expressing cells. Deletion analysis of the 2kb region showed that sequences from -1.18 to -1.44kb (relative to the cap site) were a target for Ad12 E1A-mediated transcriptional down-regulation. Deletion of this entire region from the 2kb flanking sequence of the H-2Kb gene abolished Ad12 E1A-mediated down-regulation of transcription. Computer analysis of the -1.18 to -1.44kb sequence identified two 6/7bp matches with the AP-1 transcription factor consensus sequence and two matches with the pig MHC class I PD1 repressor element. Gel retardation analysis using overlapping DNA fragments derived from the -1.18 to -1.44kb sequence revealed several DNA:protein complexes formed using nuclear extract derived from Ad12-, but not from Ad2- or Ad5-transformed cells. Some of these DNA:protein complexes were also present, but at lower levels, in nuclear extracts from untransformed rat cells suggesting the possible involvement of cellular factors in the mechanism of down-regulation mediated by Ad12 E1A. A binding site for the AP-1 factor failed to compete for protein binding to fragments within the -1.18 to -1.44 sequence, while the PD1 site competed for binding only in the -1.15 to -1.23 region. These results indicate that novel factors (as well as a previously identified class I repressor, PD1) may be involved in Ad12 E1A-mediated down-regulation of MHC class I transcription.

Hypomethylation of host cell DNA synthesized after infection or transformation of cells by herpes simplex virus

Infection of rat embryo cells with herpes simplex virus type 2 caused undermethylation of host cell DNA synthesized during infection. DNA made prior to infection was not demethylated, but some of its degradation products, including methyl dCMP, were incorporated into viral DNA. The use of mutant virus showed that some viral DNA synthesis appears to be required for the inhibition of methylation. Inhibition of methylation cannot be explained by an absence of DNA methyltransferase as the activity of this enzyme did not change during the early period of infection. Inhibition of host cell DNA methylation may be an important step in the transformation of cells by herpesviruses, and various transformed cell lines tested showed reduced levels of DNA methylation.

Hypomethylation of host cell DNA synthesized after infection or transformation of cells by herpes simplex virus

Infection of rat embryo cells with herpes simplex virus type 2 caused undermethylation of host cell DNA synthesized during infection. DNA made prior to infection was not demethylated, but some of its degradation products, including methyl dCMP, were incorporated into viral DNA. The use of mutant virus showed that some viral DNA synthesis appears to be required for the inhibition of methylation. Inhibition of methylation cannot be explained by an absence of DNA methyltransferase as the activity of this enzyme did not change during the early period of infection. Inhibition of host cell DNA methylation may be an important step in the transformation of cells by herpesviruses, and various transformed cell lines tested showed reduced levels of DNA methylation.

Nuclear factor 1 (NF-1) binding sites in the genomes of human oncoviruses: a hypothetic role for reintegrated cellular origins of replication in malignant transformation

We have found that genomes of human T cell leukemia-lymphoma virus type I (HTLV-I), BK virus (BKV), and a hepatitis B virus (HBV) DNA sequence integrated into DNA of a hepatoma-derived cell line contain binding sites for nuclear factor 1 (NF-1), a cellular protein which binds to adenoviral and putative cellular origins of DNA replication. We suggest that cellular origins of DNA replication acquired by oncoviruses may play a role in malignant transformation after reintegration into the cellular genome by providing new targets for cellular factors initiating DNA replication and by perturbing the temporal order of replication.

Acylation of adenovirus type 12 early region 1b 18-kDa protein. Further evidence for its localisation in the cell membrane

The 18-kDa E1b protein in Ad 12-transformed rat cells and in Ad 12-infected human cells binds lipid strongly. The lipid is not removed by boiling in the presence of SDS or by extraction with methanol/chloroform. It is, however, dissociated from the protein by treatment with methanolic KOH suggesting that attachment is through an ester linkage. The acylated 18-kDa protein is detected only in the membrane fraction. Labelling cell surface proteins on Ad 12-transformed cells with [125I]iodosulphanilic acid shows that some of the Ad 12 18-kDa E1b protein is present on the outside of the cell. It is concluded that this protein is responsible for cell surface T-antigen activity.

Mutagenic effects of DNA-containing oncogenic viruses and malignant transformation of mammalian cells

It was discovered in the 1970s that oncogenic viruses could induce gene mutations in mammalian cells. The phenomenon seems to be widespread: it was observed with all groups of DNA-containing viruses and some retroviruses. The mutagenic effects of the tested viruses at gene level are not locus specific. The viruses induce point mutations, including base substitutions, as well as deletions and insertions. The mutagenic effect of SV40 is controlled by the activity of the early A gene, which encodes the T antigen. Presumably, the process of integration creates the possibility for occurrence of mutations early after infection. Mutagenesis seems to be induced by an integrated virus, though to a much smaller extent. Virus-induced mutagenesis may be connected with an activation of the cell error-prone repair systems. The sum total of the experimental data shows that virus-induced mutagenesis and transformation are interrelated: (A) viruses, like other carcinogenes, display mutagenic activity; (B) viruses that are far removed from each other systematically, whose only similarity lay in being oncogenic and capable of integration, simultaneously showed the ability to induce gene mutations; (C) agents changing the rate of transformation also changed the rate of gene mutations: (D) The function of mutagenicity was mapped in the oncogene of SV40 (gene A); and the DNA of (E) mouse mammary carcinoma virus (MMTV) and avian leukosis virus (ALLV) induced tumors has been found to contain nucleotide sequences that transform 3T3NIH cells but do not carry any viral genetic information. Mutagenesis induced by oncogenic viruses may play a part in the multistage process of malignant transformation, though its contribution may be different in various specific cases and for different groups of viruses. Further studies of the uncommon mutagens, which viruses seem to be, may greatly increase our knowledge of the virus-cell relationship. An understanding of the extent of genetic danger inherent in viruses and live viral vaccines is necessary for practical medicine.

Transformation by human adenoviruses

When, approximately 10 years ago, it was shown that the functions essential for cell transformation were localized in a small region of the adenovirus genome, a DNA segment which at that time was thought to be capable of encoding two or three average-sized proteins at most, it seemed reasonable to hope that an understanding of the mechanisms by which adenoviruses transform cells might be quickly achieved. While such optimism might be forgiven, it was quite clearly naive in the extreme. As a consequence of mRNA splicing and the use of overlapping reading frames the number of proteins encoded within E1 is 2-3-times greater than would have been predicted a decade ago, and post-translational modifications may add another dimension of complexity. In fact it has taken nearly all of the past decade just to identify the proteins encoded in E1 and to characterize them in the most rudimentary way. However, we have now entered a period in which new information is accumulating at an extremely rapid rate as a result of several major technical and fundamental advances. Chief among these are the use of recombinant DNA techniques, particularly site-directed mutagenesis, which combined with methods for introducing mutations made in cloned sequences back into infectious virus, clearly represents a powerful approach to studying the functions of transforming proteins. In addition, the ability to express transforming proteins in bacteria and to produce large amounts of highly purified proteins which previously were only just detectable in infected and transformed cells is a major breakthrough. Advances in immunological techniques, particularly the development of monoclonal antibodies and antisera against synthetic peptides, have enormously simplified the task of detecting and characterizing E1 proteins. Finally, recent results suggesting that adenovirus transforming proteins may be functionally and structurally similar to other oncogenes brings a new perspective to the study of oncogenic transformation. Have all the proteins involved in transformation by adenoviruses been identified? It seems probable that all those virally coded proteins which play a major role are now known but of course minor players in the cast could still be waiting in the wings. We have pointed out that viral functions encoded outside region E1 may have some importance at least in initiation of transformation by virions and have speculated on the possibility that one or more of these may be involved in the integration of viral DNA into the host cell chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenovirus type 12 specific cell surface antigen in transformed cells is a product of the E1b early region

Six syngeneic rat cell lines transformed with isolated or cloned left end fragments of adenovirus type 12 (Ad12) DNA were used in a study of the Ad12-specific cell surface antigen. Cells transformed with EcoRI-C, SalI-C, and HindIII-G fragments of Ad12 DNA fragment-transformed express the E1a and a part of or a complete E1b early regions. Two AccI-H DNA fragment-transformed cell lines contain and express only the E1a region. All these cells contain nuclear Ad12 antigen. Cytotoxic antibodies raised against syngeneic EcoRI-C DNA fragment-transformed cells kill EcoRI-C, SalI-C and HindIII-G fragment-transformed cells, but fail to kill cells transformed with AccI-H DNA fragment. Immunofluorescence analysis shows that such antibodies, which stain the surface of cells expressing the E1b region, do not stain the surface of AccI-H DNA fragment-transformed cells. Cells transformed with AccI-H fragment are also not killed by secondary cytolytic T cells, raised and effective against cells transformed with EcoRI-C, SalI-C, and HindIII-G DNA fragments. Cells transformed with AccI-H fragment do not elicit cytolytic T cells against any of the studied cell lines. The only Ad12-specific product shared by all cell lines killed in cytolytic assays which is absent from AccI-H fragment-transformed cells is the E1b 18K protein. Since it has also been shown in other studies that this protein is associated with the cellular membrane, the simplest interpretation of these data is that the Ad12-specific cell surface antigen in transformed rat cells is a product of the left end of the E1b region.

Human adenovirus type 2 but not adenovirus type 12 is mutagenic at the hypoxanthine phosphoribosyltransferase locus of cloned rat liver epithelial cells

Using resistance to the base analog 8-azaguanine as a genetic marker, we showed that adenovirus type 2, but not adenovirus type 12, is mutagenic at the hypoxanthine phosphoribosyltransferase locus of cloned diploid rat liver epithelial cells. Adenovirus type 2 increased the frequency of 8-azaguanine-resistant colonies by up to ninefold over the spontaneous frequency, depending on expression time and virus dose.