Induced drug resistance inhibits selection of initiated cells and cancer development

Compounds exerting a mitoinhibitory effect on normal hepatocytes are potentpromoters in the resistant hepatocyte model of chemical carcinogenesis incombination with stimulation of regenerative growth by partial hepatectomyor treatment with carbon tetrachloride. 2- Acetylaminofluorene (2-AAF)almost completely inhibits liver cell regeneration after partialhepatectomy, allowing only resistant cells to participate in regenerativegrowth. After initiation by diethylnitrosamine and promotion with 2-AAF andpartial hepatectomy (PH), focal growth of initiated cells generates liverlesions which occupy 40% of the hepatic volume three weeks after PH. Inthis work the mechanism for the anti promoting effects of phenobarbital and3- methylcholantrene were investigated as well as their effects on thedevelopment of malignant hepatocellular carcinoma in the resistanthepatocyte model. Treatment with phenobarbital or, especially, 3-methylcholanthrene rendered normal rat hepatocytes resistant to themitoinhibitory effect of 2-AAF. In combination with 2-AAF/PH, 3-methylcholanthrene shortened the regenerative growth period to less thanone week. In the Solt-Farber protocol for experimentalhepatocarcinogenesis, treatment with phenobarbital or 3- methylcholanthreneduring promotion with 2-AAF/PH permitted hepatocytes surrounding the focallesions to respond with regenerative growth. The foci and surrounding livergrew until the liver/body mass index reached the control value. Withphenobarbital treatment the total focal volume was 20% of the liver volumethree weeks after PH, whereas the corresponding value in the case of3-methylcholanthrene was only 1%. Labelling index data supported theconclusion that growth of the liver lesions in the resistant hepatocytemodel was dependent on differential inhibition of normal hepatocyte growthby the promoter and that the size of the foci obtained was related to thelength of time after PH required to complete liver regeneration.3-methylcholanthrene induced 2- AAF resistance prevented the development oflarge persistent nodules and hepatocellular carcinoma while phenobarbitaldelayed cancer development with several month. The data thus supports theidea that the degree of clonal expansion during promotion determines thesize of the population at risk for malignant transformation, as well as thefinal frequency of carcinomas.