Dose-dependence of promotion of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline-induced rat hepatocarcinogenesis by ethanol: evidence for a threshold.

Although ethanol is thought to be a tumor-promoter, there are conflicting results concerning its effects on experimental hepatocarcinogenesis. Furthermore, the relationship between the amount of ethanol consumed and tumor promoting effects has hitherto not been investigated in detail. In the present study, 21-day-old F344/DuCrj rats were fed 200 p.p.m. 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in their diet for 8 weeks and thereafter received ethanol at doses of 0, 0.1, 0.3, 1, 3, 10 and 20% in drinking water ad libitum for 16 weeks. The incidences of hepatocellular adenoma and total tumors increased dose-dependently with statistical significance at doses of 10% and 20%, compared to the initiated control value. Similarly, dose dependence was observed for the incidence of hepatocellular carcinoma, which was elevated significantly at the dose of 20%. No alteration in development of preneoplastic glutathione-S-transferase placental form positive foci or tumors was observed with 0.1-1%. Cell proliferation also increased dose-dependently and CYP2E1 protein induction was recognized in centrilobular regions without alteration in mRNA levels, but no effects were evident on formation of 8-hydroxy-2'-deoxyguanosine, an oxidative DNA damage marker, or lipid peroxidation in any of the initiated groups. The mRNA expression of cyclin D1 increased dose dependently. The results demonstrated that ethanol dose-dependently promotes hepatocarcinogenesis induced by MeIQx, but with no adverse influence at doses of 1% or less, comparable to sensible drinking levels in humans.