DNA methylation in rat stomach and duodenum following chronic exposure to N-methyl-N'-nitro-N-nitrosoguanidine and the effect of dietary taurocholate

N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) induces a high incidenceof carcinomas in the glandular stomach of rats following chronicadministration in the drinking water. We determined the levelof 7-methylguanine and O⁶-mehtylguanine in gastric and duodenalDNA during chronic exposure to MNNG (80 p.p.m.). After considerablefluctuations during the initial 3 weeks, levels of methylpurinesreached a steady state which was approximately three times higherin the pylorus (i.e. the preferential site of tumor induction)thanin the fundus and duodenum, with 7-methylguanine and O⁶-methylguaninevalues in the rangeof 520 and 110 µmol/mol guanine, respectively.When rats were given MNNG in the drinking water at concentrationsranging from 10 to 80 p.p.m. for 3 weeks, levels of methylpurinesreached maximum values already at 10–20 p.p.m. At higherMNNG concentrations, there was no further increase in DNA alkylation.The reason for this lack of dose response remained unclear.Immunohistochemical analyses showed that DNA methylatlon byMNNG is restricted to epithelial cells bordering the luminalsurface. The possibility exists that in this target cell populationthe content of free thiols is a limiting factor for the decompositionof MNNG and its reaction with macromolecules in the gastricmucosa. Addition to the diet of sodium taurocholate, a bileacid previously shown to enhance MNNG-induced stomach carcinogenesis,did not influence the extent of DNA methylation, indicatingthat it acts as a promoter.