The purpose of this study was to assess the roles of the promutagenic DNA lesion O6-methylguanine (O6MG) and de novo DNA synthesis in the cellular specificity of dimethylnitrosamine (DMN) hepatocarcinogenicity in C3H mice. Male and female mice were given 0 or 10 ppm DMN ad libitum in the drinking water for 2, 4, 8, 16, or 32 days. The average DMN dosage was 2 mg/kg/day. For estimations of de novo DNA synthesis, as determined by [3H]thymidine incorporation into DNA, mice were given i.p. injections of [3H]thymidine, 2.5 µCi/g, 1 hr before sacrifice. Livers were perfused in situ with collagenase and hepatocytes and nonparenchymal cells (NPC) separated by differential centrifugation. DNA was isolated by hydroxyapatite chromatography. Purine bases were separated by high-performance liquid chromatography and quantitated from ultraviolet light-absorbing and fluorescing peaks. No sex differences were demonstrated with regard to DNA alkylation or de novo DNA synthesis in C3H mice after exposure to DMN. Concentrations of 7-methylguanine (7MG) in the two liver cell populations increased between 2 and 4 days of exposure and were maximal by 16 days. No cell type differences in 7MG concentrations were observed. NPC accumulated O6MG over the time course of DMN administration, such that increasing O}su6}/suMG:7MG ratios were observed. In contrast, hepatocytes demonstrated removal of O6MG after exposure to DMN such that O6MG:7MG ratios were approximately one-tenth that of the NPC. The NPC exhibited a progressive increase in de novo DNA synthesis as a result of DMN exposure. In contrast, hepatocytes expressed marginal increases in de novo DNA synthesis over the course of DMN exposure. The data suggest that mispairing of DNA templates containing O6MG during ]periods of enhanced de novo DNA synthesis may be responsible for the high incidence of hemangiomas and angiosarcomas in DMN-exposed mice.


Presented at the 21st Annual Conference of the Society of Toxicology, Boston, Mass. February 22 to 26, 1982 (20).

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