Promotion of hepatocarcinogenesis in vivo and in vitro

Various chemicals including phenobarbital (PB) have been shown to act as promoters in hepatocarcinogenesis in rodents. The effect of initiation and/or promotion of chemicals can be quantitatively measured by scoring the number and size of enzyme-altered islands (EAI). The effect of promotion is proportional to the period of treatment and is much greater when the promoter is given earlier after initiation. A unique aspect of hepatic promoters is their inhibitory action against carcinogenesis when given simultaneously with a carcinogen, reducing both the initiating and promoting activities of the carcinogen. With the aid of the promoting action of PB, carcinogenic activity of pure initiators is demonstrated. Promoting effect of PB was found at as low a dose as 5 ppm, which corresponds to 20 mg per 50 kg person per day, a dose several times less than the common therapeutic dose. In an attempt to analyze the mechanism of promotion and also to develop a model system for detecting environmental promoters in vitro, hepatic cells obtained from animals during carcinogenic treatment were transferred to a culture system. Cells comprising EAI after a certain critical stage gave rise to proliferative epithelial cell foci. Some hepatic promoters showed a growth enhancing effect on these foci while others did not, suggesting both the existence of direct action by some promoters on initiated cells and the presence of different promoter mechanisms. Chronic treatment with PB selectively induced gamma-glutamyl transpeptidase activity in "spontaneous" hepatomas of C3H mice, suggesting that PB may enhance the expression of altered gene functions of initiated cells.