Tissue and cell specific methylation, repair and synthesis of DNA in the upper gastrointestinal tract of Wistar rats treated with single doses of N-methyl-N'-nitro-N-nitrosoguanidine

Several potential cancer risk factors have been monitored concurrently in the upper gastrointestinal tract of young adult male Wistar rats given single (i.g.) doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) which readily induces forestomach tumours under these conditions. Radioimmunoassay was used to determine the formation of O6-methyl-2'-deoxyguanosine (O6-MedG) in DNA after MNNG doses of 1, 5, 25 or 50 mg/kg and was found to be highest in the pylorus, with progressively lower levels in the corpus, forestomach, duodenum, oesophagus and jejunum. Immunohistochemical procedures showed that cells with nuclei containing O6-MedG were heterogeneously distributed in these tissues. O6-Alkylguanine-DNA alkyltransferase activity in untreated animals was highest in the mucosae of the corpus, lower and relatively similar in that of the pylorus, duodenum and jejunum and lowest in the tissues of oesophagus and forestomach. Estimates of DNA synthesis and cell proliferation indicated a 5-fold increase in the DNA labelling index in the forestomach whereas perturbations of DNA synthetic activity in the other tissues of the upper gastrointestinal tract were much less marked. As a result of these changes, cells with nuclei that contained O6-MedG and were also undergoing DNA synthesis (determined by sequential immunohistochemical analysis and autoradiography) were found most commonly in the forestomach and to a lesser extent in the pylorus. This distribution of replicating damaged cells corresponds with the relative tumour yields in these upper gastrointestinal tract tissues and such cells are the probable targets in this single dose carcinogenesis regime. Thus, whilst the highest concentration of O6-MedG did not correlate tumour incidence, the overall risk for tumour induction did correlate with a significant level of DNA damage, a lower capacity for DNA repair and a marked increase in DNA synthesis over the constitutive level in the target cells. Carcinogenic risk in this system is therefore more readily determined by studying several risk factors simultaneously.