Evidence for oxidative metabolism in the genotoxicity of the atmospheric reaction product 2-nitronaphthalene in human lymphoblastoid cell lines.

2-Nitronaphthalene (2NN) has been identified as a mutagenic atmospheric reaction product of naphthalene in the Ames bacterial reversion assay. Recent experiments have shown this nitroarene to be genotoxic in a human lymphoblastoid cell line (MCL-5) transfected with plasmids encoding epoxide hydrolase and four cytochrome P450 monooxygenase activities. The present study investigated the genotoxicity of 2NN in two related human B-lymphoblastoid cell lines, h1A1v2 containing a single P450 isozyme (cytochrome P450 1A1) and L3 cells which are isogenic with MCL-5 cells and are distinguished only by the absence of transfected plasmids. The results indicate that 2NN-induced mutagenesis at the heterozygous thymidine kinase (tk) locus was dependent on metabolic activities provided by the transfected plasmids in MCL-5; no significant induction of mutants was observed in L3 cells studied in parallel. A similar induction of mutation was observed in h1A1v2 and MCL-5 cell lines at the tk locus and no induction was observed at the hemizygous hypoxanthine phosphoribosyl transferase (hprt) locus. The induction of mutations in h1A1v2 cells suggests that cytochrome P450 1A1 alone can activate 2NN to a mutagenic species, however, this interpretation may be confounded by differences between the h1A1v2 and MCL-5 cell lines. The observed genotoxic activity induced by 2NN prompted testing of the amino analogue, beta-naphthylamine (betaNA), to investigate potential similarities in the metabolic activation pathways of the two compounds. The negative response of betaNA in all cell lines suggests that 2NN and betaNA are not activated in these human cells by similar metabolic pathways.

Evaluation of the potential health effects of the atmospheric reaction products of polycyclic aromatic hydrocarbons.

The genotoxic risks from exposure to polycyclic aromatic hydrocarbons (PAHs) have long been recognized. Less well understood are the potential genotoxic risks of the atmospheric reaction products of this class of compounds. In this investigation, we have utilized several human cell assays to evaluate the genotoxicity of naphthalene, phenanthrene, and their atmospheric reaction products 1-nitronaphthalene, 2-nitronaphthalene (2NN), 1-hydroxy-2NN, 2-hydroxy-1-nitronaphthalene, 1,4-naphthoquinone, and 2-nitrodibenzopyranone (2NDBP). In addition, simulated atmospheric reaction products of naphthalene were generated in a 6,700 liter (L) Teflon environmental chamber, collected on a solid adsorbent, extracted, and fractionated by normal-phase high-performance liquid chromatography (HPLC). Individual fractions were then analyzed using gas chromatography/mass spectrometry (GC/MS), and tested for genotoxic effects. Genotoxicity was primarily determined using the human B-lymphoblastoid cell line, MCL-5, which expresses several transfected P450 and epoxide hydrolase genes. Mutagenicity was evaluated at both the heterozygous thymidine kinase (tk) locus and the hemizygous hypoxanthine phosphoribosyl transferase (hprt) locus, permitting detection of both intragenic and chromosomal scale mutational events. Test compounds were also screened using the CREST modified micronucleus assay. The results indicate that 2NN and 2NDBP possess greater mutagenic potency than their parent compounds, and, interestingly, both compounds induced significant increases in mutation frequency at the tk but not the hprt locus. These findings suggest a mechanistic difference in human cell response to 2NN and 2NDBP as compared to bacteria, where both compounds were previously shown to induce point mutations in the Salmonella typhimurium reversion assay. The genotoxicity of 2NN and 2NDBP in human cells, together with their high concentrations in ambient air relative to nitro-PAHs directly emitted from combustion sources, emphasizes the need to consider atmospheric reaction products of PAHs in assessments of the genotoxicity of air pollutants. We also investigated whether transfected cytochrome P450 monooxygenase activities were required to activate 2NN and 2NDBP to genotoxic species, and whether a single enzyme could be sufficient for metabolic activation. Three directly related cell lines with multiple (MCL-5), single (AHH-1 1A1), or no (L3) transfected cytochrome P450 genes were used. AHH-1 is additionally distinguished by elevated mutagenic response at the tk locus, a heterozygous mutation in p53, and apoptosis capacity. The effect of these metabolic and genetic differences on genotoxicity of 2NN, 2NDBP, and beta-naphthylamine (beta NA) was also investigated. The results indicated that 2NN and 2NDBP were not activated to genotoxic species through nitroreduction pathways. Mutagenicity induced at the tk locus was dependent on oxidative metabolism, provided by transfected cytochrome P450 enzymes in MCL-5 and AHH-1 1A1. Mutagenicity was not observed in the L3 cell line, which does not carry transfected cytochrome P450 activities. The negative response of beta NA in all cell lines indicates that, contrary to previous hypotheses, 2NN and beta NA are not activated by similar metabolic pathways in these human cell lines. Taken as a whole, these results suggest that the genotoxicity of nitro-PAHs in human cells requires oxidative metabolism.

Targeted breakage of paracentromeric heterochromatin induces chromosomal instability.

Current models suggest that genomic instability is crucial in the accumulation of the multiple alterations required for tumorigenesis. However, the nature of the initial damage responsible for the origin of genomic instability remains poorly understood. In this investigation we demonstrate that the nucleotide analog 2,6-diaminopurine (DAP) can be used to induce highly focused damage to the large blocks of paracentromeric heterochromatin on chromosomes 1, 9 and 16. A large fraction of cells exposed to DAP exhibit undercondensation of alpha and classical heterochromatin which persists into metaphase. Subsequent chromosome breakage was observed for one of the target chromosomes by preferential exclusion of chromosome 16 fragments into micronuclei (P < 0.0001). The specificity of DAP-induced chromosomal breakage enabled us to utilize it as a reagent to demonstrate that paracentromeric heterochromatin is a sensitive target for the induction of persistent genomic instability. We observed a 100-fold increase in mutagenesis affecting a chromosome 16 marker (APRT) compared with marker loci on chromosomes 17 (TK) or X (HPRT). We previously reported that APRT- mutants were recovered at a high rate upon selection in DAP in a process involving recombinationally mediated loss of heterozygosity that extends from the telomere to the boundary region of the paracentromeric heterochromatin. Karyotypic analysis of DAP-resistant APRT- mutant clones demonstrated extensive genomic instability, particularly evidence of multiple and sequential events affecting chromosome 16. These data suggest that the heterochromatic breakage observed cytogenetically immediately following DAP exposure is also responsible for the initiation of persistent genomic instability.

Genotoxicity induced in human lymphoblasts by atmospheric reaction products of naphthalene and phenanthrene.

The genotoxic risks from exposure to polycyclic aromatic hydrocarbons (PAH) have long been recognized. Less well understood are the potential genotoxic risks of the atmospheric reaction products of this class of compounds. In this investigation, we have utilized several human cell genotoxicity assays to evaluate naphthalene, phenanthrene, and their atmospheric reaction products 1-nitronaphthalene, 2-nitronaphthalene, 1-hydroxy-2-nitronaphthalene, 2-hydroxy-1-nitronaphthalene, 1,4-naphthoquinone and 2-nitrodibenzopyranone. In addition, reaction products of naphthalene were generated in a 6700-1 Teflon environmental chamber, collected on a solid adsorbent, extracted and fractionated by normal-phase HPLC. Individual fractions were then analyzed using GC-MS, and tested for genotoxicity. Genotoxicity was determined using the human B-lymphoblastoid cell line, MCL-5, which expresses several transfected P450 and epoxide hydrolase genes. Mutagenicity was evaluated at both the heterozygous tk locus and the hemizygous hprt locus, permitting detection of both intragenic and chromosomal scale mutational events. Test compounds were also screened using the CREST modified micronucleus assay. Genotoxicity results indicate that 2-nitronaphthalene and 2-nitrodibenzopyranone possess greater mutagenic potency than their parent compounds, and interestingly, both compounds induced significant increases in mutation frequency at tk but not hprt. These results suggest a mechanistic difference in human cell response as compared to bacteria, where both compounds were previously shown to induce point mutations in the Salmonella reversion assay. The genotoxicity of 2-nitronaphthalene and 2-nitrodibenzopyranone in human cells, together with their high concentrations in ambient air relative to nitro-PAH directly emitted from combustion sources, emphasizes the need to consider atmospheric reaction products of PAH in genotoxicity assessments.

Clonal analysis of delayed karyotypic abnormalities and gene mutations in radiation-induced genetic instability.

Many tumors exhibit extensive chromosomal instability, but karyotypic alterations will be significant in carcinogenesis only by influencing specific oncogenes or tumor suppressor loci within the affected chromosomal segments. In this investigation, the specificity of chromosomal rearrangements attributable to radiation-induced genomic instability is detailed, and a qualitative and quantitative correspondence with mutagenesis is demonstrated. Chromosomal abnormalities preferentially occurred near the site of prior rearrangements, resulting in complex abnormalities, or near the centromere, resulting in deletion or translocation of the entire chromosome arm, but no case of an interstitial chromosomal deletion was observed. Evidence for chromosomal instability in the progeny of irradiated cells also included clonal karyotypic heterogeneity. The persistence of instability was demonstrated for at least 80 generations by elevated mutation rates at the heterozygous, autosomal marker locus tk. Among those TK- mutants that showed a loss of heterozygosity, a statistically significant increase in mutation rate was observed only for those in which the loss of heterozygosity encompasses the telomeric region. This mutational specificity corresponds with the prevalence of terminal deletions, additions, and translocations, and the absence of interstitial deletions, in karyotypic analysis. Surprisingly, the elevated rate of TK- mutations is also partially attributable to intragenic base substitutions and small deletions, and DNA sequence analysis of some of these mutations is presented. Complex chromosomal abnormalities appear to be the most significant indicators of a high rate of persistent genetic instability which correlates with increased rates of both intragenic and chromosomal-scale mutations at tk.

Ionizing radiation signature mutations in human cell mutants induced by low-dose exposures.

Investigation of mutational specificity at low doses has generally not been possible since the number of induced mutants may be similar or significantly lower than the spontaneous background. The use of a low-dose fractionated exposure protocol in TK6 human lymphoblasts results in an incremental accumulation of mutants induced by individual 20 cGy gamma-ray exposures. Therefore, the frequency of induced mutants within a population at the conclusion of a fractionated exposure regimen is sufficiently elevated to permit the recovery of a low-dose mutant collection. Statistical analysis of the data identified no significant differences between mutants induced by 20 or 200 cGy. However, deletions encompassing one or more Xq26 STS markers flanking the hprt locus represented only 1/107 (0.009) spontaneous HPRT- mutants but 34/170 (0.20) mutants induced by 20 or 200 cGy of ionizing radiation (P < 0.0001). The data presented here demonstrate that mutational fingerprints can be effectively defined using deletion mapping for clastogens such as ionizing radiation, and that the radiation-induced mutational spectrum is independent of dose.