DNA synthesis inhibition as an indirect mechanism of chromosome aberrations: comparison of DNA-reactive and non-DNA-reactive clastogens

Positive results in the in vitro assay for chromosome aberrations sometimes occur with test chemicals that apparently do not react with DNA, being negative in tests for mutation in bacteria, for DNA strand breaks, and for covalent binding to DNA. These chromosome aberrations typically occur over a narrow concentration range at toxic doses, and with mitotic inhibition. Indirect mechanisms, including oxidative damage, cytotoxicity and inhibition of DNA synthesis induced by chemical exposure, may be involved. Understanding when such mechanisms are operating is important in evaluating potential mutagenic hazards, since the effects may occur only above a certain threshold dose. Here, we used two-parameter flow cytometry to assess DNA synthesis inhibition (uptake of bromodeoxyuridine [BrdUrd]) associated with the induction of aberrations in CHO cells by DNA-reactive and non-reactive chemicals, and to follow cell cycle progression. Aphidicolin (APC), a DNA polymerase inhibitor, induces aberrations without reacting with DNA; 50 microM APC suppressed BrdUrd uptake during a 3-h treatment to <10% of control levels. Several new drug candidates induced aberrations concomitant with marked reductions in cell counts at 20 h (to 50-60% of controls) and suppression of BrdUrd uptake (<15% of control). Several non-mutagenic chemicals and a metabolic poison, which induce DNA double strand breaks and chromosome aberrations at toxic dose levels, also suppressed DNA synthesis. In contrast, the alkylating agents 4-nitroquinoline-1-oxide, mitomycin C, methylnitrosourea, ethylnitrosourea, methylmethane sulfonate and ethylmethane sulfonate, and a topoisomerase II inhibitor, etoposide, produced many aberrations at concentrations that were less toxic (cell counts >/=73% of controls) and gave little inhibition of DNA synthesis during treatment (BrdUrd uptake >/=85% of controls), although cell cycle delay was seen following the 3-h treatment. Thus, inhibition of DNA synthesis at the time of treatment is supporting evidence for an indirect mechanism of aberrations, when there is no direct DNA reactivity.

A role for mismatch repair in production of chromosome aberrations by methylating agents in human cells

We have shown previously that certain alkylation products, or alkylation derived lesions, which induce chromosome aberrations (abs) persist for at least two cell cycles in Chinese hamster ovary cells. The increase in abs in the second cycle after treatment contrasts with the classical observation of reduction in ab yield with successive mitoses following ionizing radiation. Here we present evidence that processing of lesions by mismatch repair is a mechanism for ab induction by methylating agents. Our previous studies implicated O6-methylguanine (O6MeG) as an important lesion in induction of abs, particularly in the second cell cycle after treatment. In the absence of repair of O6MeG by alkylguanine DNA alkyltransferase (AGT), new abs were induced in the second cycle after treatment with e.g. methylnitronitrosoguanidine (MNNG) and methylnitrosourea (MNU). Thus, we hypothesized that abs were produced not by O6MeG or its repair in the first S phase, but by subsequent processing of the lesions. We suggested that after replication proceeded past the O6MeG lesion in the first S phase, inserting an incorrect base on the newly synthesized strand, recognition and repair by mismatch repair in the second S phase led to a chromosome ab. Here we used MT1 cells, a human lymphoblastoid cell line that has a defect in strand-specific mismatch repair. MT1 cells are alkylation tolerant and have a mutator phenotype, compared with their parent line, TK6; both MT1 and TK6 cells lack AGT so do not remove the methyl group from O6MeG. While the initial levels of abs at the first metaphase were similar in MT1 and TK6 cells, ab levels in MT1 cells were greatly reduced in the second and third cell cycles following treatment with MNNG, dimethylnitrosamine and MNU, in contrast with the parent TK6 cells, which had more abs in the second cell cycle than in the first. This supports the hypothesis that repair of mismatched base pairs involving O6MeG is one mechanism for induction of chromosome abs. In contrast to the difference in response to methylating agents between TK6 cells and mismatch repair-deficient MT1 cells, the profile of ab induction by an ethylating agent, ethylnitronitrosourea, was similar in MT1 cells to those for TK6 cells and CHO cells.

Chromosome aberrations: persistence of alkylation damage and modulation by O6-alkylguanine-DNA alkyltransferase

Alkylating agents produce a spectrum of DNA lesions alkylated at different sites on the molecule. These lesions differ in their propensities to cause effects such as cytotoxicity, mutations and sister-chromatid exchanges. We have used our observations that some methylating agents produce increasing levels of chromosome aberrations (abs) through successive cell cycles in Chinese hamster ovary cells, but not in normal human cells, to begin a study of which alkylated products are most likely to lead to chromosome abs, and in particular which adducts persist in DNA and cause abs after the first cell cycle. We previously observed increasing yields of abs with successive cell cycles in CHO-WBL cells treated with dimethyl nitrosamine (DMN), e.g., at 10 mM DMN, 8.8% cells with abs at first metaphase (M1) and 26.0% at third metaphase (M3) after treatment. Here we tested 4 methylating agents and their ethyl analogs in CHO cells, normal human fibroblasts (L136), and human lymphocytes. We sampled cells at several times after treating for 3 h (CHO and lymphocytes) or 4.5 h (L136). S9 metabolic activation was used for DMN and diethyl nitrosamine. BrdUrd labeling was used to identify cells in M1, M2 and M3. The methylating agents were more potent aberration (ab) inducers than ethylating agents, on a molar basis. In CHO cells, yields of abs were maintained or increased through up to 3 cell cycles after treatment with DMN, methyl methanesulfonate, methyl nitrosourea and 1-methyl-3-nitro-1-nitrosoguanidine (MNNG). With ethylating agents the ab yields in CHO cells were similar or lower in second and third cycles. In contrast, there was no evidence for persistence of lesions leading to abs in either human cell type; ab yields were markedly decreased with subsequent cell cycles for all agents. Normal human cells are proficient in repair of alkylation at the O6 site of guanine by O6-alkylguanine-DNA alkyltransferase (AGT), whereas CHO cells lack AGT activity. To explore the role of repair by AGT on the lesions involved in production of abs, we studied L136 cells, with and without O6-benzylguanine (BZG), a specific inhibitor of AGT. With MNNG, inhibition of AGT resulted in higher ab yield and production of abs through later cell cycles, so that human fibroblasts now behaved similarly to CHO cells. Preliminary data from the reciprocal experiment in CHO cells engineered to express high levels of AGT revealed a greatly decreased ab response to MNNG. In addition, the low ab yields observed were similar through later cycles or increased only slightly.(ABSTRACT TRUNCATED AT 400 WORDS)

Evaluation of the need for a late harvest time in the assay for chromosome aberrations in Chinese hamster ovary cells

Harvest time is one of the most important variables in the assessment of whether a compound is clastogenic and in establishing a dose relation. In CHO cells we have found that for a variety of chemicals one harvest time near 20 h is optimal following a 3-h treatment (Bean et al., 1992). However, some guidelines for testing for regulatory purposes recommend an additional late harvest time 24 h after the first. We tested 10 diverse chemicals in CHO-WBL cells harvested 20-21 h and 42-44 h from the beginning of a 3-hr treatment. We added BrdUrd after treatment and recorded the total% of aberrant cells, and the proportions of aberrations (abs) in first (M1), second (M2) or later metaphases. The chemicals fell into 3 categories: ab yield greatly decreased at 44 h: benzo[a]pyrene, cadmium sulfate, chlorambucil, 2,6-diaminotoluene, 4-nitroquinoline N-oxide and mitomycin C (e.g., 37.0% cells with abs at 20 h and 1.0% at 44 h); ab yields similar at 20 and 44 h: 2-aminobiphenyl, eugenol and 8-hydroxyquinoline (e.g., 8.5% at 20 h and 7.0% at 44 h); and one, dimethylnitrosamine (DMN), which was detected at both times but gave a stronger response at 44 h than at 20 h (e.g., at 10 mM: 6.2% at 20 h and 25.0% at 44 h). This DMN effect was not seen in normal diploid human cells. For DMN the higher ab levels at 44 h than at 20 h were contributed by abs in M3 cells. Thus, while for some chemicals ab yields decrease with successive division, further increases can be seen in CHO in later metaphases, notably for DMN. Overall, however, after a 3-h pulse treatment of CHO cells a positive ab result could be obtained at the early harvest time (20 h) for all 10 chemicals.

A quantitative assessment of the cytotoxicity associated with chromosomal aberration detection in Chinese hamster ovary cells

Regulatory guidelines suggest testing chemicals up to cytotoxic doses in chromosomal-aberration assays. To investigate the utility and limitations of various cytotoxicity indicators we used Chinese hamster ovary (CHO) cells to test 8 chemicals with differing ratios of cytotoxicity to clastogenicity. We measured immediate or delayed cell killing and growth inhibition (ATP levels, cell counts, colony-forming efficiency, CFE) and cell-cycle perturbations (mitotic index, MI; average generation time, AGT). Aberrations (abs) were scored 10 and 24 h from the beginning of the 3-h treatment. All 8 compounds induced abs at concentrations that reduced cell growth at 24 h by 50% or less. Concentrations of each chemical which induced at least 15% cells with abs, gave little loss of CFE (0-20%) for mitomycin C, adriamycin, cadmium sulfate and 2,6-diaminotoluene in contrast to the marked loss of CFE (70-80%) for eugenol (EUG), 2-aminobiphenyl and 8-hydroxyquinoline (8-HQ). 2,4-Diaminotoluene (2,4-DAT) was intermediate. Higher aberration yields were found at 24 h than at 10 h, even when minimal cell-cycle delay was detected by AGT estimates from BrdUrd-labeled cells. Cells with multiple abs were seen at 24 but not at 10 h, and often confirmed clastogenicity when there was only a weak increase in the percentage of cells with aberrations. Total ATP per culture did not always correlate with cell number, especially at later times after treatment. This is likely due to metabolic perturbations or altered cell biomass that are known to affect cell ATP content. MI suppression often did not correlate with AGT, e.g., only small increases in AGT were seen for 8-HQ, 2,4-DAT and EUG despite severe mitotic suppression at 10 h. By 24 h the MI for all chemicals had recovered, sometimes exceeding control levels. Marked mitotic accumulation was seen at 10 h for 2,4-DAT, indicating cell synchrony. Thus, the MI has limited value for dose selection. In conclusion, even weakly active chemicals were detected at a single time without exceeding a 50% growth reduction at 24 h.

Effect of sampling time on chromosome aberration yield for 7 chemicals in Chinese hamster ovary cells

Choice of harvest time is one of the most important variables in the assessment of whether a compound is clastogenic and in establishing a dose relation. We examined the effects of sampling time on aberration yield for 7 diverse chemicals in CHO-WBL cells by harvesting at intervals from 9 to 30 h after treatment for 3 h with or without S9 metabolic activation. We observed both the percentage of aberrant cells and the total number of aberrations. Our data suggest that for most compounds a single harvest time approximately 17-21 h after the beginning of a 3-h treatment is optimal for aberration detection in CHO cells. Maximal aberration yields were observed for 2,4-diaminotoluene, 2,6-diaminotoluene and cytosine beta-D-arabinofuranoside from 17 to 21 h, eugenol from 15 to 21 h, cadmium sulfate from 15 to 24 h and 2-aminobiphenyl, from 17 to 24 h. For adriamycin at 1 microM, the % aberrant cells remained elevated throughout the period from 9 to 29 h, while small increases at 0.1 microM ADR were found only at 13 and at 25 h. For most chemicals the maximal aberration yield occurred at a different time for each concentration tested. However, the use of 3 or more closely spaced concentrations, carefully selected to yield up to 50% toxicity, allowed detection of a positive response at a single harvest time for all 7 chemicals.

Effects of high osmotic strength on chromosome aberrations, sister-chromatid exchanges and DNA strand breaks, and the relation to toxicity

Substantial increases in chromosome aberrations were induced in Chinese hamster ovary cells by medium made hyperosmotic with NaCl, KCl, sucrose, sorbitol or dimethyl methylphosphonate. The increases were associated with cytotoxicity but occurred in the range (e.g., 70% survival) commonly included in in vitro tests for 'genotoxicity'. The relation between increased osmotic pressure and chromosome aberrations is compound-dependent, e.g., some compounds may have a direct effect in addition to an effect mediated by osmotic pressure/ionic strength. Also, glycerol at high osmolality was not toxic and did not induce aberrations, probably because rapid equilibration across the cell membrane precluded severe osmotic stress to the cells. Weak increases in DNA single-strand breaks (NaCl and KCl) and double-strand breaks (NaCl) were also detectable, at higher concentrations and more toxic levels than those required to produce aberrations. Slight elevations in sister-chromatid exchange frequencies caused by hyperosmotic medium were found in the presence of toxicity and severe cell cycle delay. Our data on cell growth inhibition suggest that this is the result of increased incorporation of bromodeoxyuridine per cell due to decreased numbers of growing cells, although other mechanisms cannot be ruled out. The observations on chromosome aberrations demonstrate the need for keeping in vitro test conditions in the physiological range, and provide a means for investigation of indirect DNA damage.

Evaluation of the alkaline elution/rat hepatocyte assay as a predictor of carcinogenic/mutagenic potential

We have recently developed an alkaline elution/rat hepatocyte assay to sensitively measure DNA single-strand breaks induced by xenobiotics in non-radiolabeled rat hepatocytes. Here we have evaluated this assay as a predictor of carcinogenic/mutagenic activity by testing 91 compounds (64 carcinogens and 27 non-carcinogens) from more than 25 diverse chemical classes. Hepatocytes were isolated from uninduced rats by collagenase perfusion, exposed to chemicals for 3 h, harvested, and analyzed for DNA single-strand breaks by alkaline elution. DNA determinations were done fluorimetrically. Cytotoxicity was estimated by glutamate-oxaloacetate transaminase release or by trypan blue dye exclusion. The assay correctly predicted the reported carcinogenic/non-carcinogenic potential of 92% of the carcinogens tested and 85% of non-carcinogens tested. The assay detected a number of compounds, including inorganics, certain pesticides, and steroids, which give false-negative results in other short-term tests. Only 2 rat liver carcinogens were incorrectly identified; the other carcinogens incorrectly identified are weakly or questionably carcinogenic (i.e., they cause tumors only in one species, after lifetime exposure, or at high doses). Some chemicals cause DNA damage only at cytotoxic concentrations; of 16 such compounds in this study, 12 are weak carcinogens suggesting a link between DNA damage caused by cytotoxicity and carcinogenesis. Our data indicate that this assay rapidly, reproducibly, sensitively, and accurately detects DNA single-strand breaks in rat hepatocytes and that the production of these breaks correlates well with carcinogenic and mutagenic activity.

Optimal phenotypic expression times for HPRT mutants induced in foreskin-, skin-, and lung-derived human diploid fibroblasts

A series of paired lung- and skin-derived fibroblast cultures has been established from human embryonic tissues under carefully controlled, identical conditions, providing the unique opportunity to study differences between normal diploid fibroblast populations from skin and lung without the confusion of genetic differences between donors. To reliably assess differences in the induced mutation frequencies observed among different cell populations, optimal phenotypic expression times in the HPRT mutagenesis system were determined for neonatal foreskin, fetal skin, and fetal lung cultures. Cell populations were mutagenized with several doses of N-methyl-N'-nitro-N-nitrosoguanidine and were replated in 6-thioguanine selective medium at intervals over 14 days. Survivals following MNNG exposure ranged from 1.6% to 45.5%. For all doses and survivals tested a 7-day expression period was the optimal value for cultures from the three different tissue sources in six independent experiments. Mutant frequency data derived from untreated control populations confirmed that spontaneous mutations during the expression period contributed negligibly to the final mutant frequency. Differences between the mutation frequencies obtained using an in situ and a replating protocol were approximately twofold for lung-, skin-, and foreskin-derived cultures.