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Although N-acetylcysteine is an antioxidant which has been expected to be a cancer chemopreventive agent, its safety and risk assessment have not been evaluated. N-acetylcysteine increased the amount of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), a characteristic oxidative DNA lesion, in human leukemia cell line HL-60, whereas the amount of 8-oxodG in HP100, which is a hydrogen peroxide (H(2)O(2))-resistant cell line derived from HL-60, was not increased. To clarify the mechanism of cellular DNA damage, we investigated DNA damage and its site specificity induced by N-acetylcysteine, using (32)P-labeled DNA fragments obtained from the human p53 tumor suppressor gene and the c-Ha-ras-1 protooncogene. N-acetylcysteine induced extensive DNA damage in the presence of Cu(II). The DNA cleavage was enhanced by piperidine treatment, suggesting that N-acetylcysteine plus Cu(II) caused not only deoxyribose phosphate backbone breakage but also base modification. N-acetylcysteine plus Cu(II) frequently modified thymine and guanine residues. Bathocuproine, a specific Cu(I) chelator, and catalase inhibited the DNA damage, indicating the participation of Cu(I) and H(2)O(2) in the DNA damage. Typical hydroxyl radical scavengers did not inhibit N-acetylcysteine plus Cu(II)-induced DNA damage, whereas methional completely inhibited it. These results suggest that reactive species derived from the reaction of H(2)O(2) with Cu(I) participates in N-acetylcysteine plus Cu(II)-induced DNA damage. The content of 8-oxodG in calf thymus DNA was increased by N-acetylcysteine in the presence of Cu(II). The present study has demonstrated that N-acetylcysteine could induce metal-dependent H(2)O(2) generation and, subsequently, damage to cellular and isolated DNA. Therefore, it is reasonable to consider that N-acetylcysteine may have the dual function of carcinogenic and anti-carcinogenic potentials. This work requires further studies on safety and risk assessment of N-acetylcysteine.

Quercetin has been reported to have carcinogenic effects. However, both quercetin and luteolin have anti-cancer activity. To clarify the mechanism underlying the carcinogenic effects of quercetin, we compared DNA damage occurring during apoptosis induced by quercetin with that occuring during apoptosis induced by luteolin. Both quercetin and luteolin similarly induced DNA cleavage with subsequent DNA ladder formation, characteristics of apoptosis, in HL-60 cells. In HP 100 cells, an H2O2-resistant clone of HL-60 cells, the extent of DNA cleavage and DNA ladder formation induced by quercetin was less than that in HL-60 cells, whereas differences between the two cell types were minimal after treatment with luteolin. In addition, quercetin increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), an indicator of oxidative DNA damage, in HL-60 cells but not in HP 100 cells. Luteolin did not increase 8-oxodG formation, but inhibited topoisomerase II (topo II) activity of nuclear extract more strongly than quercetin and cleaved DNA by forming a luteolin-topo II-DNA ternary complex. These results suggest that quercetin induces H2O2-mediated DNA damage, resulting in apoptosis or mutations, whereas luteolin induces apoptosis via topo II-mediated DNA cleavage. The H2O2-mediated DNA damage may be related to the carcinogenic effects of quercetin.

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There is currently much interest in the possibility that dietary antioxidants may confer protection from certain diseases, such as atherosclerosis and cancer. The importance of alpha-tocopherol (vitamin E) as a biological antioxidant is widely recognized. However, pro-oxidant properties of alpha-tocopherol have been observed in chemical systems, and it has been reported that the vitamin can induce tumor formation and act as a complete tumor promotor in laboratory animals. In the present communication, we find that alpha-tocopherol can act as a potent DNA-damaging agent in the presence of copper(II) ions, using a simplified, in vitro model. alpha-Tocopherol was found to promote copper-dependent reactive oxygen species formation from molecular oxygen, resulting in DNA base oxidation and backbone cleavage. Neither alpha-tocopherol nor Cu(II) alone induced DNA damage. Bathocuproine, a Cu(I)-specific chelator, and catalase inhibited the DNA damage, whereas free hydroxyl radical scavengers did not. The order of DNA cleavage sites was thymine, cytosine > guanine residues. Examinations using an oxygen electrode and cytochrome c indicate that molecular oxygen was consumed in the reaction of alpha-tocopherol and Cu(II) and that superoxide was formed. Stoichiometry studies showed that two Cu(II) ions could be reduced by each alpha-tocopherol molecule. Electron spin resonance spin-trapping investigations were then used to demonstrate that hydrogen peroxide interacts with Cu(I) to generate the reactive species responsible for DNA damage, which is either the hydroxyl radical or a species of similar reactivity. These findings may be of relevance to the tumorigenic properties of the vitamin reported in the literature. However, further studies are required to establish the significance of these reactions under in vivo conditions.

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Estrogen-induced carcinogenesis involves enhanced cell proliferation (promotion) and genotoxic effects (initiation).To investigate the contribution of estrogens and their metabolites to tumor initiation, we examined DNA damage induced by estradiol and its metabolites, the catechol estrogens 2-hydroxyestradiol (2-OHE 2 ) and 4-hydroxyestradiol (4-OHE 2 ). In the presence of Cu(II), catechol estrogens formed piperidine-labile sites at thymine and cytosine residues in 32 P 5 -end-labeled DNA fragments and induced the formation of 8-oxo-7, Epidemiological studies and animal experiments have revealed that estrogens are carcinogenic. 1 In humans, elevated circulating estrogen levels, caused either by increased endogenous hormone production or by therapeutic doses of estrogen medications, increase breast and uterine cancer risk. 2 Prolonged exposure of women to high estrogen levels is associated with an elevated incidence of breast cancer. 3 Administration of estradiol to mice increased the incidence of mammary, pituitary, uterine, cervical, vaginal, testicular, lymphoid and bone tumors. 2,4 In rats, estrogen increased the incidence of mammary and/or pituitary tumors. 2,5 Estrogens can cause cancer by stimulating cell proliferation and causing genotoxic damage. 6,7 Estrogens are highly mitogenic in hormone-sensitive tissues, such as the uterus and breast. 8 Prolonged exposure of target tissues and cells to excessive mitogenic stimulation by estrogens has been considered an important etiological factor for the induction of estrogen-associated cancers in experimental animals and humans. 8 Thus, estrogens are considered to participate in tumor promotion.However, estrogens have been reported to cause cancer through their genotoxic effects. Estradiol caused sister chromatid exchanges and chromosomal aberrations in peripheral blood human lymphocyte culture, and metabolic activation enhanced these chromosomal lesions. 9 Catechol estrogens are also capable of causing chromosomal aberrations and gene mutations in cultured cells. 10 Thus, the genotoxic effects of estradiol could result from accumulation of potentially carcinogenic metabolites. Estradiol undergoes cytochrome P-450 -catalyzed hydroxylation to various catechol estrogens. 8 2-Hydroxylation is a major metabolic pathway in the liver, whereas 4-hydroxylation dominates in extrahepatic organs susceptible to estrogens. 8,11,12 Catechol estrogens show much more potent carcinogenic effects on mice than estradiol. 13 Therefore, catechol estrogens can be potent carcinogens and may participate in tumor initiation by causing DNA damage.To investigate the role of catechol estrogens in tumor initiation, we examined DNA damage induced by 2-hydroxyestradiol (2-OHE 2 ) and 4-hydroxyestradiol (4-OHE 2 ) using 32 P 5Ј-end-labeled DNA fragments obtained from the human p53 tumor-suppressor gene and the c-Ha-ras-1 proto-oncogene. We also measured the formation of 8-oxo-7,8-dihydro-2Ј-deoxyguanosine (8-oxodG), an indicator of oxidative damage to DNA, using an electrochemical detector coup...