The competitive miscoding of O6-methylguanine and O6-ethylguanine and the possible importance of cellular deoxynucleoside 5'-triphosphate pool sizes in mutagenesis and carcinogenesis

Suppression of N-nitrosomethylbenzylamine-induced rat esophageal tumorigenesis by dietary feeding of 1'-acetoxychavicol acetate

The modifying effects of 1'-acetoxychavicol acetate (ACA) on N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumorigenesis were investigated in male F344 rats. At 5 weeks of age, all test animals, except those given the test chemical alone, and the control rats received s.c. injections of NMBA (0.5 mg/kg body weight/injection, three times per week) for 5 weeks. At the termination of the study (20 weeks), 75% of rats treated with NMBA alone had esophageal neoplasms (papillomas). However, the groups given a dose of 500 ppm ACA during the initiation phase developed a significantly reduced incidence of tumors (29%; P<0.01). Exposure to ACA (500 ppm) during the post-initiation phase also decreased the frequency of the tumors (38%; P<0.05). A reduction of the incidence of preneoplastic lesions (hyperplasia or dysplasia) was obtained when ACA was administered in the initiation phase (P<0.01). Cell proliferation in the esophageal epithelium, determined by assay of proliferating cell nuclear antigen (PCNA), was lowered by ACA (P<0.05). Blood polyamine contents in rats given NMBA and the test compound were also smaller than those of rats given the carcinogen (P<0.05). These findings suggest that dietary ACA is effective in inhibiting the development of esophageal tumors by NMBA when given during the initiation or post-initiation phase, and such inhibition is related to suppression of cell proliferation in the esophageal epithelium.

Formation and persistence of N7- and O6-methyl-guanine in DNA of chick embryo brain cells in ovo following administration of N-nitroso-N-methylurea

Previously, O6-methyl-guanine-DNA alkyltransferase (AT) of the chicken embryo has been investigated in vitro. In the present studies, the effect of in vivo (in ovo) treatment with methylnitrosourea (MNU) was examined at a developmental stage of 15 days and doses of 1.25-20 mg/egg, yielding about 1-16 mmol MNU/kg embryo weight. At intervals of 1-24 h, DNA of the brain was prepared. N7-methylguanine and O6-methylguanine were quantified by combining a rapid method of DNA isolation, high-pressure-liquid-chromatography (HPLC) and electrochemical detection of the guanine-alkyl adducts. In parallel, the AT activity of brain homogenates was determined. Within the range of the detection limits (N7-methylguanine: 16 nM, O6-methylguanine: 2.5 nM), no repair of the guanine adducts, being about 500 nmol O6-methyl- and 1800 nmol N7-methyl-adducts per mmol guanine 1 h following administration of 10 mg MNU/egg, was evident. The rather low acute toxicity of MNU in the chicken embryo at the 15th day of development DL50/24 h being > 4 mg MNU/embryo, argues, therefore, for an additional repair mechanism, e.g. cell replacement repair.

Esophageal cancer and microelements

Epidemiological studies have identified dietary zinc deficiency, methylbenzylnitrosamine, and ethanol as factors strongly associated with an increased incidence of esophageal carcinoma in man. In addition, these studies have identified other trace elements which may also affect the incidence of esophageal carcinoma. Animal models have confirmed that dietary zinc deficiency increases the incidence of methylbenzylnitrosamine-induced esophageal carcinoma and that dietary zinc deficiency also increases the incidence of other dialkylnitrosamine-induced carcinomas. The dialkylnitrosamine carcinogens are activated by NADPH-dependent cytochrome P-450 enzymes in their target tissues. The activated methylbenzylnitrosamine methylates DNA, forming O6-methylguanine adducts. These O6-methylguanine adducts can lead to point mutations in DNA, and such mutations are known to be responsible for the induction of certain carcinogen-induced tumors. We have demonstrated that dietary zinc deficiency increased the cytochrome P-450-dependent microsomal metabolism of methylbenzylnitrosamine and dimethylnitrosamine, two members of this class of dialkylnitrosamine carcinogens, while the addition of zinc in vitro noncompetitively inhibits the microsomal metabolism of these carcinogens. We have also demonstrated that dietary zinc deficiency is associated with an increased formation of O6-methylguanine in the esophageal DNA of zinc-deficient animals treated with methylbenzylnitrosamine. This increased formation of the mutagenic DNA adduct O6-methylguanine may explain the increased incidence of dialkylnitrosamine-induced carcinomas observed with dietary zinc deficiency. Other trace elements, including molybdenum, selenium, and magnesium, may also alter the incidence of esophageal carcinoma, but studies of these elements are not as conclusive as the epidemiological and experimental studies linking dietary zinc deficiency with an increased incidence of human esophageal carcinoma.

Site-specific gap-misrepair mutagenesis by O4-ethylthymine

The mutagenicity of the DNA base O-alkylation adduct, O4-ethylthymine, specifically incorporated into the plasmid vector pUC8 at the unique SalI and HincII recognition sites, was studied in vivo. Escherichia coli, Micrococcus luteus and AMV DNA polymerases catalyze the incorporation of O4-ethylTMP against template adenine and guanine residues, resulting in DNA sequence alteration during subsequent replication in the host E. coli K-12 strain JM83. The greatest mutation frequency was observed with error-prone AMV DNA polymerase. High levels of cognate restriction endonuclease-resistant mutant plasmid isolates were obtained by gap replication repair in the presence of O4-ethylTTP. The yields of mutant isolates were dependent upon the relative concentration of the competing pyrimidine deoxynucleoside triphosphates, TTP and dCTP, in the misreplication reaction. Repair of incorporated O4-ethylTMP of plasmid DNA by in vitro treatment with specific alkyltransferase, prior to transformation in the host, effectively increases the mutagenic efficiency of the adduct. The results obtained are consistent with the high miscoding potential O4-ethylthymine observed in in vitro studies and its ability to base-pair with noncomplementary guanine residues in DNA.