Two-stage liver carcinogenesis in the mouse.

A two-stage protocol for studying liver carcinogenesis was applied to the mouse. The protocol includes the treatment of 7-day-old mice with a single low dose of an initiating agent (diethylnitrosamine, NDEA), promotion starting after weaning and lasting about 20 weeks, and histologic analysis, at 30 weeks of age, of hepatocellular nodules on H&E stained sections. A stereologic analysis of results allows the evaluation of nodule frequency and size. Using this protocol in B6C3 and B6C mice, we have identified the promoting activity of the phenobarbital-like enzyme inducer, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), which was found to be a strong hyperplaseogenic agent for mouse liver. These studies also indicated that the different susceptibility to hepatocarcinogenesis in B6C3 and B6C mice may be related to a higher susceptibility of B6C3 than B6C initiated liver cells to growth stimulation. A long-term study showed that B6C mice have a low incidence of spontaneous liver tumors but are susceptible to chemical hepatocarcinogenesis and, therefore, they may be an alternative model to B6C3 mice in carcinogenesis bioassays.