Organ and Cell Specificity of DNA Methylation by N-Nitrosomethylamylamine in Rats¹

N-Nitrosomethylamylamine (NMAA) is a potent carcinogen in rodents with the esophagus as the principal target organ. The present study aims at an assessment of DNA methylation by NMAA in various rat tissues and an identification of cell populations actively involved in its bioactivation. Adult male F344 rats received a single i.p. dose of N-nitroso[methyl-¹⁴C]amylamine (0.1 mmol/kg). After 6 h organs were removed and the DNA was extracted, hydrolyzed in 0.1 m HCl, and subjected to radiochromatography on Sephasorb-HP. Highest levels of DNA alkylation were found in esophagus (798 µmol 7-methylguanine/mol guanine), followed by nasal epithelium (672 µmol) and liver (624 µmol). Trachea, lung, forestomach, and kidney had considerably lower levels of alkylation and in glandular stomach, spleen, and duodenum, values were close to the limit of detection. Specific target cell populations were identified autoradiographically and by immunohistochemistry using a rabbit antiserum to O⁶-methyldeoxyguanosine. In the esophagus, NMAA was selectively metabolized by the basal cells of the mucosa. In the respiratory tract, O⁶-methyldeoxyguanosine was almost exclusively present in the tracheal and bronchiolar epithelia. In the nasal cavity, labeled nuclei were found in both the olfactory and the respiratory epithelium and in the serous glands. Our studies indicate that NMAA and related asymmetrical nitrosamines are, in addition to liver, preferentially metabolized in tissues derived from the ventral entoderm, including the upper respiratory and gastrointestinal tract.