Treatment of rats with the carcinogen 2-acetylaminofluorene (2-AAF) during liver regeneration (Solt-Farber protocol) induced a selective outgrowth of diploid, gamma-glutamyltranspeptidase (GGT)-positive hepatocytes (3-4 times increase) as well as of nonparenchymal (oval) liver cells. After cessation of treatment the oval cells rapidly disappeared, while the population of diploid, GGT-positive hepatocytes declined more slowly over the subsequent ten weeks. In animals pretreated with the initiating carcinogen diethylnitrosamine (DEN) a large fraction of the diploid, GGT-positive hepatocytes persisted. The results differ from those obtained with our standard, sequential treatment protocol (2-AAF given after completed regeneration), where there is no hyperproliferation of oval cells and where GGT-positive hepatocytes are found only in DEN-pretreated animals (Saeter et al, Carcinogenesis 9: 581-587, 1988). Different experimental models of liver carcinogenesis may thus present different patterns of liver cell proliferation, which should be taken into account when general hypotheses on the cellular origin of liver cancer are proposed.

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