Stage dependent enhanced induction of hepatocellular carcinomas in rats administered a second dose of diethylnitrosamine.

An experiment was performed to investigate whether the incidence of chemical carcinogen-induced liver tumors in rats is increased by administration, during promotion, of a second dose of carcinogen. Seven groups of male Fischer-344 rats were administered a dose of diethylnitrosamine (DEN), and were placed, 1 month thereafter, on a liver-tumor promotion-regimen, consisting of a choline-devoid diet containing 0.06% phenobarbital. A second dose of the carcinogen was administered, at monthly intervals of promotion, to some of the groups of animals. The experiment was terminated after 7 months of promotion, and the incidence of hepatic tumors was determined histopathologically. In rats administered the second dose of carcinogen after 2 months of promotion, the incidence of hepatocellular carcinomas was 2-3 times greater than that in the other groups of rats. The results are consistent with the proposal, advanced by others, that development of hepatocellular carcinomas may be a multi-hit process, and indicate that, after the first induced at initiation, further genomic alteration(s) may have to occur at very discrete stage(s) in the evolution of initiated cells to tumors.