Correlation of O4-ethyldeoxythymidine accumulation, hepatic initiation and hepatocellular carcinoma induction in rats...

Recent experiments have demonstrated that O6-ethyldeoxyguanosine (O6-EtdG) is efficiently repaired while O4-ethyldeoxythymidine (O4-EtdT) accumulates in hepatocyte DNA of 8-week-old F-344 rats during continuous diethylnitrosamine (DEN) administration. To determine if O4-EtdT accumulation correlates with hepatic initiation, we have quantitated O4-EtdT concentrations, and the incidence of gamma-glutamyl transferase positive (GGT+) foci and hepatocellular carcinoma induced by increasing duration of exposure to DEN in the drinking water (40 p.p.m.). In 8-week-old F-344 rats the number of GGT+ foci increased non-linearly with duration of exposure and reached a maximum of approximately 500 foci/cm3 after 10 weeks. Administration of DEN to 8-week-old F-344 rats for 6 weeks followed by a 15-week administration of 0.05% phenobarbital (PB) in the diet did not result in the induction of hepatocellular carcinoma. Exposure of 4-week-old F-344 rats to DEN for up to 10 weeks produced an O4-EtdT steady-state concentration (approximately 7-10 X 10(-6) mol O4-EtdT/mol dT) similar to that previously observed after administration of DEN to 8-week-old F-344 rats. However, the maximal concentration of O4-EtdT was detectable after a shorter period of DEN administration in the younger rats. The incidence of GGT+ foci also increased more rapidly in 4-week-old rats, but again plateaued at approximately 500 foci/cm3 after 4, 6 or 8 weeks of DEN administration. A 100% incidence of hepatocellular carcinoma occurred in 4-week-old rats administered DEN for 6, 8 or 10 weeks, followed by promotion with 0.05% PB in the diet until week 22 of the study. Lower incidences of hepato-cellular carcinoma (89 and 6%) were observed following PB-promotion of rats administered DEN for 4 and 2 weeks, respectively. The influence of age on DEN-induced hepatic initiation was examined further by quantitating GGT+ foci induced by 4 weeks of DEN administration in groups of rats which were 4-14 weeks old at the start of the carcinogen exposure. The results demonstrated that the younger rats were 15-fold more susceptible than the older rats to the initiating effects of DEN. This growth-dependent effect on hepatic initiation in the presence of nearly equivalent amounts of pro-mutagenic DNA damage further implicates the necessity of replication for hepatic initiation.