Oncogene expression in hepatocytes of the fish Rivulus ocellatus marmoratus during the necrotic and regenerative phases of diethylnitrosamine toxicity.

Livers of mangrove rivulus (Rivulus ocellatus marmoratus) were examined after an acutely necrogenic dose of diethyl-nitrosamine (DEN). Immunohistochemical detection of oncoproteins and bromodeoxyuridine (BrdU), enzyme histochemical detection of gamma-glutamyltranspeptidase, and histological stains were used in an attempt to separate changes in protooncogene expression related to hepatic regeneration from those changes that were putatively preneoplastic. Perivenous hepatocytes were rounded and shrunken within 3 days of the beginning of DEN exposure, and widespread necrosis and hepatocyte proliferation occurred by 21 days (the last day of DEN exposure). Twenty-four days after the end of DEN exposure, livers were primarily composed of nodules of regenerated hepatocytes. Epidermal growth factor receptor expression in hepatocytes increased in inflamed areas and then returned to control levels as inflammation subsided. Increased expression of Fos, Ras and Myc occurred prior to necrosis in a zonal and chronological progression consistent with regeneration of hepatocytes. Fos, Ras, Myc and p53 expression persisted in scattered cells and foci for 24 days after the end of DEN exposure, and this expression was at levels higher than during normal cell-cycle progression. The spatial pattern and persistence of cells expressing Fos, Ras, Myc and p53 at high levels may have represented preneoplastic changes.