Development of resistance during the early stages of experimental liver carcinogenesis.

The present study was designed to determine whether the resistant phenotype is acquired at the initiated cell stage itself or requires further exposure to a promoting regimen to express resistance. Male Fischer 344 rats were initiated with diethylnitrosamine (DENA) (200 mg/kg i.p.) and were subjected to either no further treatment or to the resistant hepatocyte (RH) model of liver tumor promotion. Six weeks later, the resistance of the focal lesions generated in these two groups to the mitoinhibitory effects of 2-acetylaminofluorene (2-AAF) was determined by subjecting the rats to two-thirds partial hepatectomy (PH) in the presence of a mitoinhibitory dose of 2-AAF (5 mg/kg i.p.) given at the time of PH. Labeling index was determined by administering multiple injections of [(3)H]thymidine. All rats were killed 48 h post-PH. While only a small percentage (23%) of the glutathione S-transferase-positive foci generated by DENA in the absence of an exogenous liver tumor promoting regimen were resistant to the mitoinhibitory effects of 2-AAF, a majority (85%) of the foci became resistant to 2-AAF following exposure to the RH model of liver tumor promotion. Further, initiated rats exposed to either 2-AAF or to CCl(4) alone, the two components of the RH model, resulted in 71% of the foci being resistant to the mitoinhibitory effects of 2-AAF. Similar patterns of results were obtained when the resistance of the foci to the mitoinhibitory effects of orotic acid, a liver tumor promoter and an inhibitor of DNA synthesis in normal hepatocytes, was monitored. These results suggest that the majority of initiated hepatocytes are not of resistant phenotype, however, they have acquired a unique ability to express resistance upon exposure to certain agents such as 2-AAF and CCl(4) or to a promoting regimen such as the RH model of liver tumor promotion.