Chemical carcinogen alteration of SV40 virus induced transformation of normal human cell populations in vitro

Increased frequency of specific locus mutation following human cytomegalovirus infection

The effect of human cytomegalovirus (HCMV) infection on the frequency of mutations at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus was studied in Chinese hamster lung V79 cells. When V79 cells were infected with HCMV (strain AD169) at multiplicities of 0.1 to 50 plaque forming units (PFU) per cell the presumptive mutation frequency, as determined by the number of 6-thioguanine-resistant (TGr) colonies, was increased up to 16.8-fold (P < 0.005), depending on the multiplicity of infection. Increases in the mutation frequency at the hprt locus were also observed for other laboratory-adapted HCMV strains (C-87, Davis) and for low passage clinical isolates (82-1, 84-2). The expression time required for the maximum increase in TGr colonies was 3 days and was consistent among the HCMV strains evaluated in this study. UV-irradiation of HCMV stock up to a dose of 9.6 x 10(4) ergs/mm2 increased the mutation frequency, but further exposure to UV light or to heat (56 degrees for 30 min) significantly decreased the frequency of TGr-resistant colonies, suggesting that expression of HCMV genes was involved in the mutation process. HCMV-induced TGr cells demonstrated substantially reduced (> 96%) incorporation of [3H]hypoxanthine. PCR analysis of the hprt locus demonstrated deletions in 9 of 19 HCMV-induced TGr colonies randomly selected for further study, while 2 of 17 spontaneously developed TGr colonies demonstrated deletions. Although insertions were not detected in spontaneously developed clones, 3 of 19 HCMV-induced TGr clones had insertions in the hprt gene. Neither HCMV-specific DNA sequences nor HCMV-specific proteins were detected in the TGr clones obtained after HCMV infection. Infection of V79 cells with HCMV also increased their sensitivity to mutation with N-methyl-N'-nitro-N-nitrosoguanidine, giving a synergistic enhancement of the mutation frequency. These results indicate that HCMV infection has the capacity to induce mutations in the cellular genome and increase the sensitivity of infected cells to mutation by genotoxic chemicals. Although inactivated HCMV particles are responsible for a modest increase in the mutation frequency, expression of HCMV genes is associated with a substantial enhancement of the mutation frequency.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Effect of propane sultone pretreatment on Friend virus leukemogenesis in mice

Propane sultone (PS) injected i.p. 24 or more hours before Friend leukemia virus increased the incidence of lymphoma in SJL/J mice and at a higher dose increased the incidence of erythroleukemia in B10SJF1 mice. PS at the same time also decreased hematopoietic stem cell clonogenicity.

Cell transformation by chemical agents--a review and analysis of the literature. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The literature on cell transformation by chemical carcinogens has been critically reviewed. This subject is highly relevant to carcinogenesis in vivo, because the phenotypic changes that are collectively referred to as cell transformation usually involve the acquisition of tumorigenicity on inoculation into suitable rodent hosts. The systems chosen for review fall into 3 categories: cell strains (cells with a limited lifespan); cell lines (cells with an unlimited lifespan); and oncogenic viral-chemical interactions involving cells (Fischer rat embryo cells expressing an endogenous retrovirus, mouse embryo cells expressing the AKR leukemia virus, chemical enhancement of a simian adenovirus, SA7 transformation of Syrian hamster or rat embryo cells). Of the entire literature reviewed, 117 papers have been accepted for data abstraction by pre-defined criteria; these include 41 references to cell strains, 40 in cell lines, and 38 in viral-chemical interactions including cells. Because different systems have been reviewed, it would be meaningless to group all the compounds. The overall summary of the systems is as follows (many compounds have been tested in more than one system and, hence, are duplicated in these totals). (Chart: see text) In general, there is a reasonably good correlation between the results of the cell transformation systems and in vivo carcinogenesis. However, the many deficiencies of the EPA Merged Carcinogen List preclude definitive comparisons. Moreover, a number of 'false negatives' were obtained in systems that did not employ external metabolic activation. Further validation of all systems is required, but it seems very probable that several cell transformation systems will become valuable in assaying (with reasonable time and cost) the carcinogenic potential of environmental chemicals.

Virus-induced gene mutations of eukaryotic cells

Most animal viruses studied so far induce chromosomal aberrations. In addition, adenoviruses, papovaviruses, and retroviruses are known to induce gene mutations like mutagenic bacteriophages. At least in one case studied retrovirus induced mutagenesis involves gene and/or scripton splitting analogous to the mutagenic mechanism of action of mutatorphage Mu and other movable DNA elements. On the contrary, several results obtained by independent means indicate that Simian virus 40, a papovavirus, does not act by splitting the affected gene but presumably by generation of base pair substitutions or of other minor DNA damages leading to amino acid substitutions. The mechanisms involved are still unknown. There a some hints, however, that these mechanisms might have some step(s) in common with processes leading to malignancy. In fact those viruses proved unequivocally so far to be capable of inducing gene mutations are oncogenic viruses.

Chemical interactions with herpes simplex type 2 virus: enhancement of transformation by hydrazine and 1,2-dimethylhydrazine

Quantitative assays for the morphological transformation of 3T3 Swiss mouse cells by herpes simplex type 2 virus (HSV-2) were employed to examine the effect on cell transformation of chemical carcinogens and suspected carcinogens. Exposure of the cells to the chemical compound, followed by virus infection, resulted in enhancement of transformation when compared to that observed with chemical or virus alone. Enhancement occurred in tests utilizing either UV light-inactivated HSV-2 (strain 333) or a temperature-sensitive (ts) mutant of HSV-2 (A8(293)]. A series of seven ts-mutants were tested and exhibited varying degrees of transformation. Enhancement of transformation occurred in cells treated with hydrazine (HZ) and 1,2-dimethylhydrazine (SDMH). No enhancement occurred when cells were treated with monomethylhydrazine, 1,1-dimethylhydrazine and the jet fuels JP-5, JP-10, RJ-4 and RJ-5. A strong time dependence after treatment was demonstrated with some enhancement seen at 6 h after chemical treatment but the greatest enhancement appeared when virus infection began after 24 h of chemical exposure.

Potential identification of chemical carcinogens in a viral transformation system

Chemical carcinogens from several diverse chemical classes i.e.; aromatic amines, polycyclic hydrocarbons, nitrosamines, hormonal derivatives, metals and direct alkylating agents cause a 6.2-60.5-fold increase in the frequency of murine sarcoma virus (MSV)-induced transformation in a normal rat kidney (NRK) cell system. Exogenous metabolic activation with a rat liver S-9 homogenate is required for expression of this activity by procarcinogens. Non-carcinogenic analogs of these compounds fail to cause significant increases in the transformation frequency either with or without prior metabolic activation. Iododeoxyuridine, a mutagen also does not cause enhancement of transformation. This system may serve as the basis for a rapid and quantifiable means of identifying chemical carcinogens while introducing a new model for the understanding of the interactions between oncornaviruses and chemical carcinogens.

Interactions between initiating chemical carcinogens, tumor promoters, and adenovirus in cell transformation

Cell culture systems that respond to the combined action of initiating chemical carcinogens, tumor promoters, and transforming viruses represent useful model systems for studying the complex multifactor nature of the carcinogenic process. We have utilized both secondary rat embryo (2 degrees RE) and a clonal population of established Fischer rat embryo (CREF) cells to study the effect of multiple agents on the process of adenovirus transformation. In the present review we summarize our investigations on the effects of polycyclic aromatic hydrocarbons, tumor promoters, a bee venom polypeptide-melittin (MEL), and the polypeptide hormone epidermal growth factor (EGF) on transformation of rat embryo cells by a temperature-sensitive mutant of human adenovirus type 5 (H5ts125). We also describe the effect of tumor promoters on the progression of the transformed phenotype, ie the temporal acquisition of anchorage-independent growth in adenovirus-transformed clones and the ability of MEL and EGF to elicit effects similar to those of tumor promoters in adenovirus-transformed cells.

Benzo[a]pyrene metabolism in cells productively infected with simian virus 40

The metabolism of benzo[a]pyrene (BP) by cell cultures and cell-free extracts of the monkey kidney cell line CV-1 was studied in uninfected and Simian virus 40 (SV40) infected cells. Metabolites formed were separated by high-pressure liquid chromatography (HPLC) and quantified by liquid scintillation techniques. The profiles metabolites formed by uninfected and SV40 infected cells were similar except that SV40 infected cell cultures metabolized BP at an increased rate relative to uninfected cells. In addition, SV40 infected cell cultures and cell-free extracts produced an unknown compound which eluted between the 3-hydroxybenzo[a]pyrene and BP fractions. This material does not have a retention time characteristic of any of the known metabolites of BP. Labelled BP and/or its metabolites were bound to the viral DNA and histone components of intracellular viral minichromosomes as well as the viral DNA and proteins of mature virions.

DNA of simian virus 40 mutates Chinese hamster cells

Infection of Chinese hamster cells with SV 40 DNA gives rise to mutants resistant both to S-axaguanine (AG) and aminopterin (AP). This mutagenic effect can be raised when facilitating DNA uptake of cells by a helper agent. The extent of muyagenic action depends further on the concentration of DNA applied to the cells, with 2 micrograms/ml being more effective than 10 micrograms/ml, as well as on the period of incubation of infected cells before onset of mutant selection (mutation expression time). Using the AG resistance marker the mutation frequency can be increased more than 8-fold compared with the spontaneous mutation frequency. Reconstituted SV 40 minichromosomes show a mutagenic action which is similar to the DNA-mediated mutagensis whereas non-viral DNA from mammalian cells fails to induce mutations significantly. A major part of isolated clones of SV 40-induced mutants tested so far does express SV 40 T-antigen, suggesting the persistence of SV 40 genetic material in these clones. The possible existence of relations between mutagenic and transforming capacities of SV 40 is discussed.

Analysis of intracellular distribution and binding of benzo[alpha]pyrene in human diploid fibroblasts

Previous work with low passage synchronized human foreskin fibroblast cell populations has indicated that benzo[alpha]pyrene (BP) can induce a carcinogenic event [3]. BP additionally has shown to damage DNA in log-arithmically growing low passage cultures [9]. High passage cells, on the other hand, seem to be refractory to transformation by BP, even though this agent can induce DNA damage, similar to that seen in low passage cells. When low passage cells were treated with BP, the initial binding of the hydrocarbon was primarily to a cytoplasmic protein complex of molecular weight 12,500 while in high passage cells, a major portion of BP was bound to protein complex of molecular weight 200,000. High-pressure liquid chromatography (HPLC) profiles of ethyl acetate extractable fractions from the BP-cytoplasmic protein complexes of low and high passage cells demonstrated that the majority of the BP remained unmetabolized. When nuclei were isolated from low and high passage cells prior to the HPLC analysis, the major component (90%) was again unmetabolized BP. The results suggest selective attachment of BP to different cytoplasmic protein complexes of logarithmically growing human diploid fibroblast cells dependent on the passage level of the cells.