TISSUE AND CELL-SPECIFIC METHYLATION, REPAIR AND SYNTHESIS OF DNA IN THE UPPER GASTROINTESTINAL-TRACT OF WISTAR RATS TREATED WITH N-METHYL-N'-NITRO-N-NITROSOGUANIDINE VIA THE DRINKING-WATER

Several potential cancer risk factors have been monitored concurrently in the upper gastrointestinal tract of young male Wistar rats given N -methyl-N'-nitro-N-nitrosoguanidine (MNNG) via the drinking water, a r egimen that induces a high yield of tumours in the pylorus and to a le sser extent in the duodenum. Radioimmunoassay was used to determine th e amounts of O6-methyl-2'-deoxyguanosine (O6-MedG) formed in the tissu e DNA of rats given MNNG at doses of 40 or 80 mug/ml for periods of 3, 6 and 12 weeks. The highest adduct concentration was found in the pyl orus with progressively lower concentrations in the corpus and duodenu m, jejunum, forestomach and oesophagus. Between 3 and 12 weeks these a dduct levels decreased in all tissues and there was no evidence of a d ose dependent accumulation of O6-MedG. When analysed by immunohistoche mistry the distribution of cells with nuclei containing O6-MedG was se en to be heterogeneous in the various tissues. O6-Alkylguanine-DNA alk yltransferase activity increased during the 12 weeks of MNNG treatment in oesophagus and forestomach, but decreased to approximately 50% of the initial value in the corpus, pylorus, duodenum and jejunum. The ma jor changes in DNA synthesis and cell proliferation were the marked up ward expansion (i.e. towards the lumen) of the zone of replicating cel ls in the glands- of the pylorus and the greatly increased numbers of replicating damaged cells (i.e. cells that contained O6-MedG whilst un dergoing DNA synthesis) as determined by sequential immunohistochemica l analysis and autoradiography. Such cells are the probable target cel ls in this chronic dose carcinogenesis regime. Although similar change s also occurred in the glands of the corpus these were of lesser exten t and the changes of labelling index in the oesophagus and forestomach were relatively minor. In the duodenum, MNNG treatment led to erosion of the upper part of the glands so that the zone of cells containing O6-MedG overlapped with the zone of proliferating cells resulting in t he formation of many replicating damaged cells. Thus, as in the single dose study (see preceding paper) the distribution of replicating dama ged cells coincides with the tumour yield in the tissues of the upper gastrointestinal tract. As in the case of single doses of MNNG the ris k factors for carcinogenesis are, a significant level of DNA damage. a lower capacity for DNA repair and an increased DNA synthetic activity , again suggesting that carcinogenic risk cannot readily be determined by studying risk factors individually.