Induction of DNA repair synthesis in isolated rat hepatocytes by 5-diazouracil and other DNA damaging compounds

Mechanisms of the DNA breaking activity of mutagenic 5-diazouracil

5-Diazouracil in monohydrated form showed mutagenicity and cytotoxicity on Salmonella typhimurium TA98 and TA100 strains without metabolic activation, and induced mouse micronucleated peripheral reticulocytes. Incubation of a plasmid supercoiled DNA with the compound caused DNA single-strand breaking: the supercoiled form was transformed into an open circular relaxed form and then into a linear form. The breaking was similarly caused in the absence of molecular oxygen. The breaking was not inhibited by superoxide dismutase and catalase, but inhibited by ethanol, butyl hydroxyanisole and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), suggesting the involvement of radical species other than oxygen-derived radical species. Sequencing analysis of the singly 5'-end-labeled DNA fragment showed that the phosphodiester breaking was not site-specific. When Escherichia coli cells were incubated with the compound, the intracellular double-strand DNA was fragmented. The fragmentation was inhibited by ethanol, DMPO, N-tert.-butyl-alpha-phenylnitrone (PBN) and thiol compounds. Generation of the carbon-centered radical was confirmed by the electron spin resonance spin-trapping technique using DMPO and PBN. The mutagenicity and the DNA breaking activity of 5-diazouracil can be ascribed to the carbon-centered radical.

Structure-activity relationships in the rat hepatocyte DNA-repair test for 300 chemicals

312 chemicals/mixtures were tested for genotoxicity in the rat hepatocyte/DNA-repair test. A variety of structure-activity relationships was evident. Of the 309 pure chemicals, 142 were positive. Of these, 43 were judged by IARC to have sufficient or limited evidence of carcinogenicity and none of the remainder was a proven noncarcinogen. Among the 167 negative chemicals, 44 were carcinogens. Some of these are known to be genotoxic in other systems, but based on several lines of evidence, many are considered to be epigenetic carcinogens that lack the ability to react with DNA and rather lead to neoplasia by nongenotoxic mechanisms.

Genotoxicity of carcinogens in human hepatocytes: application in hazard assessment

Evaluation of chemical genotoxicity has been used in assessing human cancer hazard, based on the observation that most human carcinogens are known to be DNA-reactive. The availability of data on the DNA-reactivity of compounds in metabolically competent human cells would assist hazard assessment by providing direct information of human genotoxicity. To evaluate the reliability of human hepatocytes for this purpose, the induction of DNA repair by DNA-reactive carcinogens of several structural classes and related noncarcinogens was studied. All the carcinogens elicited DNA repair synthesis, whereas the noncarcinogens did not. These studies provide additional support for the use of human hepatocytes in a DNA repair test in the investigation of genotoxicity. The demonstration of genotoxicity in human cells is suggested to provide important information for hazard assessment.

Application, limitations and research requirements of in vitro test systems in toxicology

There is increasing application of in vitro-test systems for toxicological evaluation of chemicals, which became possible by increasing understanding of the biological endpoints present in such systems and their capability for metabolic activation and inactivation. This communication focuses on the capacities of metabolic activation and inactivation in mutagenicity test systems, using bacteria, mammalian cells in culture and isolated hepatocytes. Bacterial test procedures with S-9-fraction are specific metabolic activation systems with low inactivation capacity. Mammalian cells are either deficient in metabolic activities or contain only limited activation capacity although special cell lines derived from hepatoma cells express certain metabolic activation as well as inactivation reactions. Isolated hepatocytes contain enzymatic activities similar to those in the intact liver, which however decrease at different rates. It is the goal of present research to construct cell lines with defined and sufficient activities of these enzymes. In isolated hepatocytes chemically induced DNA repair can be readily detected when a clear discrimination between mitochondrial, semiconservative and repair synthesis is provided. In such systems genotoxicity of reactive oxygen species is decreased by physiological concentrations of alpha-keto-acids, pyruvate possessing the highest antioxidant activity. It is concluded that in vitro test systems provide a suitable tool for detecting genotoxic and toxic effects of chemicals. However, many biological parameters such as metabolic activity, degree of differentiation of the cells, cofactor requirement, and composition of the medium affect the reliability of the test system. Thus, only a detailed understanding of the biology and biochemistry of such test allow production of reliable and reproducible results.

Reliability of the hepatocyte primary culture/DNA repair test in testing of coded carcinogens and noncarcinogens

The hepatocyte primary culture/DNA repair test was evaluated for its reliability using a series of coded samples. Among the 30 chemicals tested, 15 were general reference compounds and 15 were chemicals that had been tested for carcinogenicity in the U.S. National Cancer Institute Bioassay Program. The latter group were from the same lot that had been used for the in vivo testing and had also been tested for mutagenicity in the Ames test. From the group of 15 reference compounds, 5 were positive for DNA repair and all 5 were carcinogens. Of the 10 samples scored as negative, 4 were noncarcinogens and 6 were carcinogens. Among the 6 carcinogens were 3 compounds whose carcinogenicity probably does not involve the production of DNA damage. From the 15 coded chemicals that were tested for carcinogenicity by the NCI in long-term animal studies, 7 were scored as positive. 5 of these were judged carcinogenic in the in vivo bioassays and the other 2, which were also mutagenic in Salmonella, showed some indication of carcinogenicity. Of the 8 compounds that were scored as negative, 5 were noncarcinogenic. Among the 3 carcinogens that were not detected, there was at least one whose carcinogenicity probably does not involve DNA damage. Thus, the results of this study indicate that positive results in the hepatocyte primary culture/DNA repair test are highly specific for carcinogens and that the test is also highly sensitive in the detection of DNA-damaging genotoxic carcinogens.