High sensitivity of Xeroderma pigmentosum cells to the carcinogen 4-nitroguinoline-1-oxide

The search for relevant short term bioassays for chemical carcinogens: the tribulation of a modern Sisyphus

Based on a good correlation between carcinogenicity and mutagenic activity several rapid microbial bioassays for chemical carcinogens have been recently developed. We would like to suggest, that these microbial tests should be followed by bioassays using cultured human cells of the "average" man, and of persons with elevated cancer risk or increased susceptibility to carcinogenic agents. The main objective of using DNA repair (unscheduled uptake of 3HTdR) and DNA fragmentation (shift in sedimentation profiles) of cultured human cells was to design a test system that can simulate conditions found in man and thus provide information relevant to the human population. A trial on 98 different carcinogens, precarcinogens and noncarcinogens showed the suitability of DNA repair synthesis as a rapid, economic and relevant assay for detection of chemical carcinogens. To check the adaptability of DNA repair synthesis of human cells as a bioassay for chemical carcinogens we examined carcinogenic nitrosation products which are formed from the interaction of nitrite and nitrosatable compounds, carcinogenic or mutagenic photosensitizing chemicals, and the effect of complex interactions. Organotropic carcinogens can be detected by measuring DNA fragmentation and DNA repair in various target organs following the in vivo application of chemical carcinogens. The pros and cons of several bioassays and their usefulness in judging a carcinogenic or mutagenic hazard to human populations is discussed.

DNA repair synthesis of cultured human cells as a rapid bioassay for chemical carcinogens

The feasibility of detection of carcinogenic chemicals using DNA repair synthesis of cultured human fibroblasts as measured by an unscheduled 3HTdR incorporation has been explored. Of 64 chemicals tested, 29 were proximate or ultimate carcinogens, 15 were precarcinogens that required metabolic activation, 16 were non-oncogenic compounds and 4 were of unknown carcinogenicity. All directly acting carcinogens triggered a DNA repair synthesis, whereas no unscheduled 3HTdR incorporation was observed following the application of the 16 non-oncogenic compounds. As a rule, the precarcinogens (without metabolic activation) do not elicit DNA repair synthesis. However, longer exposures and higher concentrations of the precarcinogens 2-AAF, aflatoxin B1 and sterigmatocystin gave unscheduled 3HTdR uptake. The results suggest the suitability of using repair synthesis as endpoint, and cultured human cells as subjects in a prescreening programme for chemical carcinogens.

The response of Xeroderma pigmentosum cells and controls to the activated mycotoxins, aflatoxins and sterigmatocystin

The activation of the mycotoxins aflatoxin B1, G1, B2, G2, aflatoxicol and sterigmatocystin by 9S fraction, microsomal preparation (105,000 times g) and supernatant (105,000 times g) of livers of several species was examined. DNA repair synthesis, chromosome aberrations and clone forming capacity were used as endpoints. Cultured fibroblasts of normal persons and DNA repair deficient Xeroderma pigmentosum patients were employed as test subjects. The activation mixtures significantly increase the chromosome breaking function, lethality and DNA damaging effect (measured as DNA repair synthesis) of aflatoxin B1, G1, aflatoxicol ans sterigmatocystin. The DNA repair-deficient XP cells respond to the activated mycotoxins with a low level of unscheduled 3HTdR incorporation as compared to that of control cells, but show a highly elevated sensitivity to the chromosome-damaging and lethal effect of aflatoxin B1 and sterigmatocystin.

Reduced DNA repair capacity and increased cytotoxicity following split doses of the mutagen 4-nitroquinoline-1-oxide in cultured human cells

Cultured human fibroblasts were exposed to single doses of 4-nitroquinoline-1-oxide(4NQO) and to two equimolar doses of 4NQO at intervals varying from 0.5 to 12 h. DNA repair synthesis as measured by an unscheduled uptake of tritium-labelled thymidine ([3-H]TdR), cell survival as estimated by the clone-forming capacity, and frequency of chromosome aberrations were used as endpoints. Cells respond with a reduced level of DNA repair synthesis when the second 4NQO dose (5 X 10 minus 7 or 1 X10-minus 7 M) is given within 3 h of the first 4NQO dose. If the interval between the two doses is 5 h or more, the level of DNA repair synthesis which is induced by the second 4NQO dose is comparable to that following a single 60-min 4NQO application. In this 3-h period the cultured cells show an increased sensitivity to the lethal effect and chromosome-damaging action of the second 4NQO dose. The reduced period of DNA repair capacity seems to increase the mutagenic effect of the chemical carcinogen.

Analyses of differential sensitivities of synchronized HeLa S3 cells to radiations and chemical carcinogens during the cell cycle. III. 4-Nitroquinoline 1-oxide and its derivatives

Sensitivity to the chemical carcinogens 4-nitroquinoline 1-oxide (4-NQO) and 4-hydroxyaminoquinoline I-oxide (4-HAQO), during the cell cycle of synchronized HeLa S3 cells, decreases from the late S to the early G2 phases. Cells in other phases are relatively sensitive to both carcinogens. [3-H]4-NQO and [ 3-H]4-HAQO seem to be bound preferably more with cellular DNA of the mitotic phase to the middle of the S phase than with that of the late S phase in which the cells are rather insensitive to these carcinogens. However, we found no significant difference in the excision rates of these carcinogens from the DNA of HeLa S3 cells through the cell cycle. These findings indicate that the cyclic variation of 4-NQO and 4-HAQO cell survivals during the cell cycle may be due to the differences in the amounts of 4-NQO and 4-HAQO bound with cellular DNA.

Excision-repair of 4-nitroquinolin-1-oxide damage responsible for killing, mutation, and cancer

Excision-repair of DNA base damage produced by 4-nitroquinoline-1-oxide (4NQO) was compared in Escherichia coli, human cells, and mouse cells. Paper chromotography of acid hydrolysates of DNA extracted from cells treated with 3H-labeled 4NQO revealed four peaks; two kinds of 4NQO-guanine adduct, one kind of 4NQO-adenine adduct, and free 4-aminoquinoline-1-oxide (4AQO). About 80% of the initially formed 4NQO-purine adducts were excised from DNA in E. coli uvrA+ cells during 60 min postincubation, but not at all in uvrA- (excisionless for uv damage) cells. Normal human cells excised about 60% of 4NQO-purine adducts during 24 hr postincubation, but xeroderma pigmentosum (excisionless) cells did not. A mouse cell line susceptible to repair of 4NQO-induced pretransformational damage also showed excision-repair ability for the 4NQO adducts. From these and other results, we conclude that the 4NQO-purine adducts and unstable 4NQO-guanine products (which release 4AQO) are, like pyrimidine dimers, repairable by excision-repair universal among E. coli, mouse, and human being, and that unexcised ones are probably the major cause of killing, mutation, and cancer.

Lack of chemically induced mutation in repair-deficient mutants of yeast

Two genes, rad6 and rad9, that confer radiation sensitivity in the yeast Saccharomyces cerevisiae also greatly reduce the frequency of chemically-induced reversions of a tester mutant cyc1-131, which is a chain initiation mutant in the structural gene determining iso-1-cytochrome c. Mutations induced by ethyl methanesulfonate (EMS), diethyl sulfate (DES), methyl methanesulfonate (MMS), dimethyl sulfate (DMS), nitroquinoline oxide (NQO), nitrosoguanidine (NTG), nitrogen mustard (HN2), beta-propiolactone, and tritiated uridine, as well as mutations induced by ultraviolet light (UV) and ionizing radiation were greatly diminished in strains homozygous for either the rad6 or rad9 gene. Nitrous acid and nitrosoimidazolidone (NIL), on the other hand, were highly mutagenic in these repair-deficient mutants, and at low doses, these mutagens acted with about the same efficiency as in the normal RAD strain. At high doses of either nitrous acid or NIL, however, reversion frequencies were significantly reduced in the two rad mutants compared to normal strains. Although both rad mutants are immutable to about the same extent, the rad9 strains tend to be less sensitive to the lethal effect of chemical mutagens than rad6 strains. It is concluded that yeast requires a functional repair system for mutation induction by chemical agents.

Carcinogens as frameshift mutagens: metabolites and derivatives of 2-acetylaminofluorene and other aromatic amine carcinogens

Several carcinogenic metabolites of the carcinogen 2-acetyl-aminofluorene, especially 2-nitrosofluorene and N-hydroxy-2-aminofluorene, are potent frameshift mutagens for Salmonella typhimurium. 2-Nitrosonaphthalene, 2-nitrosophenanthrene, 4-nitroso-trans-stilbene, 4-nitrosobiphenyl, and 4-nitrosoazobenzene, all of which are metabolites or likely metabolites of carcinogenic aromatic amines, are also potent frameshift mutagens. These compounds may be frameshift mutagens of the class that intercalates into DNA and then reacts covalently with the DNA; various ultimate carcinogens may be of this type. The utility of a set of bacterial strains for detecting carcinogens as mutagens is shown.