Comparison of different models of rat liver carcinogenesis: conclusions from a systemic analysis.

Different protocols of chemically induced hepatocarcinogenesis were applied to Wistar rats under identical experimental conditions. The following conclusions may be drawn after an analytic comparison of these results. Various chemical carcinogens show different carcinogenic capacities. Diethylnitrosamine is more potent than N-nitrosomorpholine which is more active than 2-acetylaminofluorene. A short-term exposure to such carcinogens is sufficient to initiate but not necessarily to complete the carcinogenic process. It can be promoted to completion by either a noncarcinogenic promoter or a carcinogen. From a systemic point of view, it appears that, as in the skin models, two-step protocols are not always equivalent to protocols using the same agent during the whole treatment. Moreover, the results observed with a multistep protocol indicate that during the initiating phase the carcinogen plays a selective role distinct both from a pure initiating role and from the promoting effect. The results obtained lead to the conclusion that the distinction between initiation and promotion remains purely operational as it still does not correspond to the nature of well-established biologic processes.