DNA methylation in the digestive tract of F344 rats during chronic exposure toN-methyl-N-nitrosourea

Summary

The formation ofO ⁶-methyldeoxyguanosine (O ⁶-MedGuo) was determined by an immuno-slot-blot assay in DNA of various tissues of F344 rats exposed toN-methyl-N-nitrosourea (MNU) in the drinking water at 400 ppm for 2 weeks. Although the pyloric region of the glandular stomach is a target organ under these experimental conditions, the extent of DNA methylation was highest in the forestomach (185 μmolO ⁶-MedGuo/mol guanine). Fundus (91 μmol/mol guanine) and pylorus (105 μmol/mol guanine) of the glandular stomach, oesophagus (124 μmol/mol guanine) and duodenum (109 μmol/mol guanine) showed lower levels ofO ⁶-MedGuo but differed little between each other. Thus, no correlation was observed between target organ specificity and the extent of DNA methylation. This is in contrast to the gastric carcinogen,N-methyl-N′-nitro-N-nitrosoguanidine (MNNG), which preferentially alkylates DNA of the pylorus, the main site of induction of gastric carcinomas by this chemical. In contrast to MNU, the non-enzymic decomposition of MNNG is accelerated by thiol compounds (reduced glutathione,l-cysteine), which are present at much higher concentrations in the glandular stomach than in the forestomach and oesophagus. During chronic exposure to MNNG (80 ppm), mucosal cells immunoreactive toO ⁶-MedGuo are limited to the luminal surface [Kobori et al. (1988) Carcinogenesis 9:2271–2274]. Although MNU (400 ppm) produced similar levels ofO ⁶-MedGuo in the pylorus, no cells containing methylpurines were detectable by immunohistochemistry, suggesting a more uniform methylation of mucosal cells by MNU than by MNNG. After a single oral dose of MNU (90 mg/kg) cells containing methylpurines were unequivocally identified using antibodies toO ⁶-MedGuo and the imidazole-ring-opened product of 7-methyldeoxyguanosine. In the gastric fundus, their distribution was similar to those methylated by exposure to MNNG, whereas the pyloric region contained immunoreactive cells also in the deeper mucosal layers. After a 2-week MNU treatment, the rate of cell proliferation, as determined by bromodeoxyuridine immunoreactivity, was only slightly enhanced in the oesophagus and in the fundus, but markedly in the forestomach and the pyloric region of the glandular stomach. It is concluded that the overall extent of DNA methylation, the distribution of alkylated cells within the mucosa and the proliferative response all contribute to the organ-specific carcinogenicity of MNU.