Polymerase chain reaction-directed DNA sequencing of bleomycin-induced "nondeletion"-type, 6-thioguanine-resistant mutants in Chinese hamster ovary cell derivative AS52: effects of an inhibitor and a mimic of superoxide dismutase

Effects of 2.45 GHz electromagnetic fields with a wide range of SARs on bacterial and HPRT gene mutations

Present day use of mobile phones is ubiquitous. This causes some concern for human health due to exposure to high-frequency electromagnetic fields (HFEMF) from mobile phones. Consequently, we have examined the effects of 2.45 GHz electromagnetic fields on bacterial mutations and the hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene mutations. Using the Ames test, bacteria were exposed to HFEMF for 30 min at specific absorption rates (SARs) from 5 to 200 W/kg. In all strains, there was no significant difference in the frequency of revertant colonies between sham exposure and HFEMF-exposed groups. In examination of mutations of the HPRT gene, Chinese hamster ovary (CHO)-K1 cells were exposed to HFEMF for 2 h at SARs from 5 to 200 W/kg. We detected a combination effect of simultaneous exposure to HFEMF and bleomycin at the respective SARs. A statistically significant difference was observed between the cells exposed to HFEMF at the SAR of 200 W/kg. Cells treated with the combination of HFEMF at SARs from 50 to 200 W/kg and bleomycin exhibited increased HPRT mutations. As the exposure to HFEMF induced an increase in temperature, these increases of mutation frequency may be a result of activation of bleomycin by heat. We consider that the increase of mutation frequency may be due to a thermal effect.

Mutational spectrum of bleomycin in lacZ mouse kidney: a possible model for mutational spectrum of reactive oxygen species

The mutational spectrum of bleomycin was compared with the spontaneous mutational spectrum in lacZ mouse kidney. Mice were treated with four 20 mg/kg of doses of bleomycin over a two-week period, leading to a mutant fraction several times greater than that of controls. The major class of bleomycin-induced mutations consisted of small deletions, in particular -1 deletions at AT base pairs and hot spots for deletions at 5'-GTC-3' sequences. Smaller, but significant fractions of GC > AT followed by GC > TA substitutions were also observed. In untreated mice, the major class of mutations consisted of GC > AT substitutions followed by GC > TA mutations, and a much smaller fraction of deletions. Other than the specificity of bleomycin for AT base pairs and the 5'-GTC-3' hotspots, the mutational spectrum of bleomycin in mice is similar to that reported for ionizing radiation. However, bleomycin initially mediates the formation of oxidized DNA via reduction of molecular oxygen, as opposed to the radiolysis of water. In this respect mutagenesis induced by bleomycin may be more similar to that induced by endogenous reactive oxygen species (ROS) than mutagenesis induced by ionizing radiation. If bleomycin-induced mutagenesis is an appropriate model for mutagenesis induced by ROS, then, based on the difference between the mutational spectrum of bleomycin and spontaneous mutagenesis, the latter appears not to result predominantly from ROS, at least in mouse kidney.

Influence of dietary antioxidants on the mutagenicity of 7,12-dimethylbenz[a]anthracene and bleomycin in female rats

Studies on agents that modulate carcinogen-induced genotoxic effects in experimental animals provide end points that can be used for assessing the antimutagenic or anticarcinogenic properties of putative chemopreventive compounds and for predicting their protective efficacy in humans. In this study, we investigated the ability of the dietary antioxidant Vitamins C, E, beta-carotene and the mineral selenium to inhibit the mutant frequency (MF) induced by treatment of rats with 7,12-dimethylbenz[a]anthracene (DMBA), a mammary carcinogen and bleomycin (BLM), an anti-tumor agent that can damage DNA by free radical mechanisms. Both chemicals have been previously shown to be mutagenic in the rat lymphocyte Hprt assay. Adult female Fischer 344 rats were given the antioxidants singly or in a combination 2 weeks prior to mutagen treatment. Antioxidant intake continued for an additional 4 weeks post-mutagen treatment. At sacrifice, spleens were aseptically removed for the isolation of lymphocytes to conduct the mutagenesis assay at the Hprt locus. The DMBA and BLM treatment induced a marked increase in MF, 52.8 x 10(-6) and 19.2 x 10(-6), respectively, over the controls. The MFs seen in the individual antioxidants alone (single or mixture) were relatively similar to the controls, with the exception of Vitamins C and E, that had 1.7- and 1.5-fold increase, respectively. The degree of inhibitory response was dependent on the type of mutagen and the particular antioxidant. BLM/antioxidant combination had inhibitions ranging from 44 to 80%, while DMBA/antioxidant system ranged from 60 to 93%, with Vitamins C and E achieving the highest inhibition in both systems. The mixture displayed low inhibitory responses, 44.6% for BLM/mix and 47% DMBA/mix. On the whole, the results indicate that the dietary constituents tested are antimutagenic; however, because of the gradations seen with the responses, the protective efficacy of these antioxidants may depend on the type of mutagen/carcinogen they encounter. Pending molecular analysis of mitochondrial DNA mutations will also indicate whether there is a shift in the mutational spectra produced by the carcinogens in the presence of antioxidants.

Folk medicine Terminalia catappa and its major tannin component, punicalagin, are effective against bleomycin-induced genotoxicity in Chinese hamster ovary cells

Terminalia catappa L. is a popular folk medicine for preventing hepatoma and treating hepatitis in Taiwan. In this paper, we examined the protective effects of T. catappa leaf water extract (TCE) and its major tannin component, punicalagin, on bleomycin-induced genotoxicity in cultured Chinese hamster ovary cells. Pre-treatment with TCE or punicalagin prevented bleomycin-induced hgprt gene mutations and DNA strand breaks. TCE and punicalagin suppressed the generation of bleomycin-induced intracellular free radicals, identified as superoxides and hydrogen peroxides. The effectiveness of TCE and punicalagin against bleomycin-induced genotoxicity could be, at least in part, due to their antioxidative potentials.

Attenuation of bleomycin-induced Hprt mutant frequency in female and male rats by calorie restriction

Calorie restriction modulates spontaneous and chemically induced tumors and increases maximal life span in experimental animals; however, the mechanism by which calorie restriction exerts its ameliorating effects is not fully elucidated, although reduced levels of reactive oxygen species (ROS) by calorie restriction has generated much interest. In the present study, we have determined whether or not calorie restriction would affect the mutagenic response in rats treated with bleomycin (BLM) a radiomimetic drug that is associated with DNA damage by a free radical mechanism. Fourteen weeks after weaning, the rats were divided into two groups; ad libitum (AL)-fed and 40% calorie restriction. Both AL and calorie-restricted animals were injected with 2.5, 5.0 and 10.0 mg BLM/kg, or with phosphate-buffered saline (PBS), and they were killed 4 weeks post drug treatment. Lymphocytes from the spleens were seeded in 96-well microtiter plates to determine mutant frequency in the hypoxantine guanine phosphoribosyl transferase (Hprt) gene. The mutant frequency in the BLM-treated rats was higher in AL males (P=0.001), and AL females (P=0.0174) than in their calorie-restricted counterparts. The difference in mutagenic response relative to AL males and AL females appeared unrelated to a low percent cloning efficiency seen in the males, since the mean absolute number of Hprt mutant clones was higher in the AL males compared to the females. A reduction in animal weight by calorie restriction was significant in both sexes (P<0.001), but the dose effect appeared non-significant. The results indicate that calorie intake of 60% reduced the mutagenic response of BLM, a compound known to induce oxidative DNA damage, and suggest a possible decrease in ROS as a function of calorie restriction.

A role for apoptosis in the toxicity and mutagenicity of bleomycin in AHH-1 tk+/- human lymphoblastoid cells

The chromosomal mutagen, bleomycin, is also noted for its toxic properties, although the mechanism of cell death is not fully understood. In order to determine if cell death occurred by apoptosis or necrosis, AHH-1 tk+/- cells were exposed to bleomycin and the percentage of viable, apoptotic and necrotic cells quantified by flow cytometry. Logistic regression analysis indicated that the primary manner of cell death was through the apoptosis pathways, that apoptosis was delayed, and that apoptosis was accompanied by an arrest in the G2 phase of the cell cycle. Once apoptosis was established as a mechanism for cell death, the efficiency with which these pathways removed damaged cells from the population was evaluated with the use of specific-locus mutation assays (tk and hprt) as indicators of cells with DNA damage that maintained viability and clonogenicity. Linear regression analysis detected a significant, concentration-dependent increase in the numbers of TFTr clones with the slow-growth phenotype. This suggests that a proportion of cells with bleomycin-induced DNA damage did not undergo cell death by apoptosis and that apoptosis, a mechanism for the destruction of damaged cells, is not fully efficient in the AHH-1 tk +/- cell line.

Mutagenic potential of peripheral blood leukocytes: in vivo exposure to the carcinogen 7,12-dimethylbenz[a]anthracene, and the tumor promoter 12-O-tetradecanoylphorbol acetate followed by in vitro co-culture with AS52 cells

Co-culture of AS52 cells with peripheral blood leukocytes (PBLs), obtained from SENCAR mice topically treated with either tetradecanoyl-phorbol-13-acetate (TPA) or 7,12-dimethylbenz[a]anthracene (DMBA)-TPA, resulted in a 7-160-fold increase in the mutation frequency of the gpt gene in AS52 cells when compared to that induced by PBLs isolated from mice treated with either acetone or DMBA. This increase in mutation frequency was inhibited by the anti-oxidant (-)epigallocatechin gallate (EGCG). These results demonstrate that the AS52 cell line can be used as a mammalian mutagenesis model for the study of in vivo mechanism(s) of mutagenesis by leukocytes and also as a model for in vivo chemoprevention studies.

DNA damage and mutagenesis by radiomimetic DNA-cleaving agents: bleomycin, neocarzinostatin and other enediynes

Bleomycin and the enediyne antibiotics effect concerted, simultaneous site-specific free radical attack on sugar moieties in both strands of DNA, resulting in double-strand breaks of defined geometry and chemical structure, as well as abasic sites with closely opposed strand breaks. The hypersensitivity of several mammalian double-strand break repair-deficient mutants to these agents confirms the role of these double-strand breaks in mediating cytotoxicity. In bacteria, mutagenesis by both bleomycin and neocarzinostatin appears to result from replicative bypass of abasic sites, the repair of which is blocked by the presence of closely opposed strand breaks. However, in mammalian cells, such abasic sites decompose to form double-strand breaks, and mutagenesis consists primarily of small deletions, large deletions, and gene rearrangements, all of which probably result from errors in double-strand break repair by a nonhomologous end-joining mechanism. Studies with the radiomimetic antibiotics emphasize the importance of this end-joining repair pathway, and these agents provide useful probes of its mechanistic details, particularly the effects of chemically modified DNA termini on repair.

Spectrum of mutations in kidney, stomach, and liver from lacI transgenic mice recovered after treatment with tris(2,3-dibromopropyl)phosphate

The flame retardant tris(2,3-dibromopropyl)phosphate (TDBP), once used in cotton sleep wear for children, is presently banned from commerce. It produces tumors in rodents in both a sex- and tissue-specific manner. The kidney is the main target for tumor formation in male and female rats, as well as in male mice. In contrast, tumors are formed in the liver of female animals. We have used lacI transgenic male B6C3F1 mice (Big Blue) to examine the induction of mutation in kidney, liver, and stomach after exposure to 150 mg/kg (2 days), 300 mg/kg (4 days), and 600 mg/kg (4 days) of TDBP. At the highest dose, the mutant frequency was approximately 50% above control values in the kidney (P < 0.01). A smaller increase was observed in the liver (P = 0.07), while no increase was seen in the stomach (P = 0.28). Sequence analysis of the recovered mutants showed a TDBP-specific change in mutation spectrum in kidney, which was not observed in liver and stomach. In kidney, a dose-dependent decrease in G:C-->A:T transitions, including at 5'-CpG-3' sites, was observed. This was accompanied by an increase in the loss of single G:C base pairs from approximately 3% to 15%. These results illustrate both the sensitivity and specificity of the lacI transgenic system in the analysis of tissue-specific mutation. This study also reinforces the importance of examining mutational spectra when mutant induction levels are low.

Induction of deletion mutations by methoxyacetaldehyde in Chinese hamster ovary (CHO)-AS52 cells

We have reported previously that methoxyacetaldehyde (MALD), a metabolite of 2-methoxyethanol, induces gpt gene mutations in Chinese hamster ovary (CHO)-AS52 cells but not hprt gene mutations in the standard CHO-K1-BH4 cells. In addition, MALD induces chromosome aberrations in both CHO cell lines. The data presented suggest that MALD induces deletion-type mutations. In this study, we analyzed MALD-induced CHO-AS52 mutants for deletion-type mutations using the nested-polymerase chain reaction (nested-PCR) assay. Spontaneous CHO-AS52 mutants are used as untreated control. Ethylnitrosourea (ENU)-induced CHO-AS52 mutants are used as negative control for multilocus deletions since ENU is a potent inducer of point mutations. The results show that the frequency of MALD-induced mutants containing total deletion of the gpt gene is 42.4% which is 2.3-fold higher than that from spontaneous mutants (18.6%). The frequency of ENU-induced deletion mutation is 3%. The data substantiate our hypothesis that MALD induces major deletion mutations.