The effects of putative DNA repair inhibitors on DNA adduct levels and unscheduled DNA synthesis in rat hepatocytes exposed to 2-acetylaminofluorene

Detection of excision repaired DNA damage in the comet assay by using Ara-C and hydroxyurea in three different cell types

Because of its characteristics, the comet assay has been used to evaluate the ability of virtually any type of eukaryotic cell to repair different kinds of DNA damage, including double and single strand breaks and base damage. The ability to detect excision repair sites using the alkaline version can be enhanced by the inclusion of repair inhibitors, DNA synthesis inhibitors, or chain terminators. In this sense, we evaluated the ability of hydroxyurea (HU) and cytosine arabinoside (Ara-C), for detecting lesions produced by the alkylating agents ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS) in three different cell systems. Two hundred cells for experimental point were analyzed in the alkaline version of the comet assay, and the results are evidences of the utility of the assay to detect alkylation of bases in the cells lines MRC-5 and TK-6, as the treatment with HU +Ara-C significantly increases both the basal and induced frequency of DNA damage. The use of whole blood, although it detected the effects of MMS, with and without repair inhibitors, failed to detect the effect of the selected dose of EMS and does not permit detection increases in the background level.

Modulation of DNA repair by various inhibitors of DNA synthesis following 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) induced DNA damage

The tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is present in tobacco smoke and is hepatocarcinogenic in rats. Its bioactivation in rat hepatocytes leads to methylation and pyridyloxobutylation of DNA. Rat hepatocytes were cultured in serum-free William medium E on collagen-coated dishes. We demonstrated that some enzymes of the base and/or excision-repair pathways were involved in repair of NNK-induced DNA damage, measured by [methyl-3H] thymidine incorporation. Unscheduled DNA synthesis (UDS) induced by N-methyl-N-nitrosourea (MNU), NNK, N'-nitrosonornicotine (NNN) and 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc) increased 2.9-, 2.8-, 1.5- and 3.5-fold, respectively, suggesting that methylated and/or pyridyloxobutylated-DNA by these four nitroso compounds is repaired by the excision pathway. Moreover, levels of NNK-induced UDS were dose (1-3 mM) and time (1-18 h) dependent. Enzymes involved in the excision repair pathways were selectively inhibited. Inhibitors of DNA topoisomerase I (camptothecin) and topoisomerase II (etoposide, nalidixic acid) did not decrease the induction of UDS, suggesting that topoisomerases are not involved in the repair of NNK-induced damage. While aphidicolin and arabinocytidine (DNA polymerase alpha, delta, epsilon inhibitors) totally inhibited NNK- and NNKOAc-induced UDS, dideoxythymidine (DNA polymerase beta inhibitor) inhibited NNK- and NNKOAc-induced UDS by 40 and 33%, respectively. We conclude that DNA polymerase alpha, delta or epsilon and to a lesser degree polymerase beta are involved in the repair of pyridyloxobutylated DNA. Previous studies showed that inhibition of poly(ADP-ribosyl) polymerase (PARP) by 3-aminobenzamide (3-ab) facilitated DNA ligation. Our results demonstrate that 3-ab increased NNK-induced UDS, but does not affect NNKOAc-induced UDS. These observations suggest that the ligation step is rate limiting in the repair of methylated DNA but not of pyridyloxobutylated DNA.

Simultaneous measurement of unscheduled and replicating DNA synthesis by means of a new cell culture insert DNA retention method: rapid induction of replicating DNA synthesis in response to genotoxic carcinogens

In order to measure simultaneously replicating DNA synthesis (RDS) and unscheduled DNA synthesis (UDS) in rat hepatocytes responding to exposure to carcinogens, a new method, namely the "cell culture insert DNA retention (CDR)" method, was developed. All CDR procedures for cell culture, digestion of cytoplasm and retention of DNA were performed on membranes attached to cell culture containers. Four subgroups of primary cultures of hepatocytes prepared from rats were exposed to a genotoxic or non-genotoxic carcinogen with or without 10 mM hydroxyurea and incubated for 4 h with 10 microCi/ml [3H]thymidine. The membranes were then processed for both liquid scintillation and autoradiography. Among seven tested chemicals, three genotoxic agents, 3,2'-dimethyl-4-aminobiphenyl, 2-acetylaminofluorene and diethylnitrosamine, and two non-genotoxic carcinogens, nafenopin and phenobarbital, induced RDS within 4 h after the exposure, indicating that these carcinogenic agents induce cell proliferation is non-proliferating rat hepatocytes prior to the emergence of genotoxic changes. Several indices were devised to characterize the genotoxicity of the tested chemicals. The induction patterns obtained showed a wide variation in the individual characteristics of carcinogen-induced genotoxicity and mitogenicity in the early phase of initiation. This is the first report of simultaneous measurement, by using a combination of autoradiography and liquid scintillation, of UDS and RDS induced in rat hepatocytes. The described CDR approach will be useful for risk assessment and characterization of carcinogenic and tumor-promoting agents.

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.

DNA repair induced by various mutagens in rat hepatocyte primary cultures measured in the presence of hydroxyurea, guanazole or aphidicolin

Guanazole and aphidicolin were chosen as candidates in the search for a selective, non-genotoxic inhibitor of DNA replication which could be used instead of hydroxyurea to measure DNA repair synthesis in rat hepatocyte primary cultures by liquid scintillation counting. The genotoxicity of these 3 chemicals was studied using the Salmonella/liver homogenate assay and the autoradiographic UDS test in hepatocytes. Hydroxyurea was positive in both of these assays. Guanazole and aphidicolin did not induce DNA repair in hepatocytes. Aphidicolin was not mutagenic for Salmonella typhimurium, whereas guanazole increased the revertant numbers of strain TA102 slightly. The incorporation of [3H]thymidine was measured by liquid scintillation to determine DNA repair induced by 2-acetylaminofluorene (2-AAF), aflatoxin B1, benzo[a]pyrene, cyclophosphamide, H2O2, 6-hydroxydopamine, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methylnitrosourea (MNU), 4-nitroquinoline-N-oxide and UV irradiation in the presence of either 10 mM hydroxyurea, 15 mM guanazole or 0.015 mM aphidicolin. Aphidicolin had an inhibitory effect on DNA repair. Except for the 3 chemicals mentioned below, the sensitivity of the DNA repair measurement was the same, no matter whether hydroxyurea or guanazole was used to inhibit replicative DNA synthesis. In the presence of hydroxyurea, DNA repair synthesis was found at lower concentrations in the case of aflatoxin B1, due to differences in the solvent control values, and in the case of H2O2, possibly due to a synergistic effect between hydroxyurea and H2O2. Guanazole allowed the detection of DNA repair induced by MNNG at lower concentrations, probably because of an antagonistic effect between hydroxyurea and MNNG. Based on these results, it was concluded that guanazole, but not aphidicolin, could be used instead of hydroxyurea to measure DNA repair synthesis by liquid scintillation in rat hepatocyte primary cultures. Although guanazole does not completely fulfill the criteria for an ideal DNA replication inhibitor, it has the advantage of being less genotoxic than hydroxyurea, and also appears to have a smaller potential to falsify the results by interacting with the test compounds.