Pleiotropic drug resistance in hepatocytes induced by carcinogens administered to rats

The effect of hepatocarcinogen administration in vivo on the induction of pleiotropic drug resistance was studied in primary monolayer cultures of adult rat hepatocytes using a cytotoxicity assay in vitro. Dietary 2-acetylaminofluorene, 3'-methyl-4-dimethylaminoazobenzene, aflatoxin B1, ethionine, and diethylnitrosamine rapidly induced resistance to doses of Adriamycin, methotrexate, cycloheximide, and aflatoxin B1 which were cytocidal to normal hepatocytes from untreated rats. Up to 95% of some hepatocyte preparations became drug resistant before any new hepatocyte phenotypes could proliferate. Drug resistance was measured at 24 h after initiation of 2-acetylaminofluorene feeding and remained stable throughout the 16 wk of carcinogen exposure. When limited carcinogen exposure was followed by a return to a basal non-carcinogen-containing diet for many months, the hepatocytes in the resultant hepatocellular carcinomas also displayed pleiotropic drug resistance, and the cells of the peritumorous liver did so to a lesser extent. Drug resistance was not induced by chronic administration of the tumor promoters phenobarbital, choline-deficient diet, phorbol, nor with 2,3,7,8-tetrachlorodibenzo-p-dioxin, but was induced to a variable extent by three hepatotoxins (ethanol, methotrexate, carbon tetrachloride). Whereas the early appearing drug resistance appears to be an adaptation of the liver to the presence of a toxic carcinogen, the late resistance which does not disappear after withdrawal of the inducing carcinogen may be a constitutive characteristic of chemically induced hepatocellular carcinomas.