Immunocytochemical analysis of O 6-alkylguanine shows tissue specific formation in and removal from esophageal and liver DNA in rats treated with methylbenzylnitrosamine, dimethylnitrosamine, diethylnitrosamine and ethylnitrosourea

Abstract

The formation and repair of carcinogen-DNA adducts in esophagus and liver of rats treated with a single i.p. dose of methylbenzylnitrosamine (MBN), dimethylnitrosamine (DMN), diethylnitrosamine (DEN) or ethylnitrosourea (ENU) has been studied using peroxidase immunocytochemistry to visualize O 6-alkylguanine in DNA of individual cells. After MBN O 6-methylguanine (O 6-MeG) specific nuclear staining was only present in the target tissue for tumor induction, the esophageal epithelium. Part of the adducts persisted for at least 72 h. No O 6-MeG could be detected in liver. DEN, a carcinogen in liver and esophagus, led to DNA modification of esophageal epithelial cells, and liver parenchymal and non-parenchymal (Kupffer and sinusoidal) cells of the centrlobular area. O 6-EtG was removed within 72 h from both liver cell populations. A similar distribution of adduct (O 6-MeG) formation was observed in liver after the hepatocarcinogen DMN, but this nitrosamine did not detectably modify esophageal cells. O 6-MeG persisted in Kupffer and especially sinusoidal lining cells of liver, consistent with the induction of sarcomas by DMN. The relatively unspecific, directly alkylating carcinogen ENU modified DNA of all cell types to a similar extent. A qualitative correlation was obtained between the tissue specific ability to induce tumors and the formation of O 6-alkylguanine (O 6-alkylG). Our experiments support the hypothesis that DNA modification is necessary for the initiation of carcinogenesis by chemical carcinogens, and that a low capacity to repair promutagenic lessions, like O 6-alkylG, potentiates this process.