Enhanced Proliferation of Putative Preneoplastic Cells in Rat Liver following Treatment with the Tumor Promoters Phenobarbital, Hexachlorocyclohexane, Steroid Compounds, and Nafenopin¹

Putative preneoplastic islands were induced in rat liver by diethylnitrosamine or nitrosomorpholine administered either as single high doses or continuously for 40 days at low dose levels. Following recovery periods of 3 weeks to 11 months, islands were identified by means of a positive γ-glutamyl transferase reaction and/or altered morphology. DNA synthesis, by means of [³H]thymidine autoradiography, as well as mitotic activity were determined. Under all conditions studied, proliferation rates of island cells were significantly higher than those of normal unaltered hepatocytes. Single doses of liver mitogens known or assumed to promote liver tumor development (phenobarbital, α-hexachlorocyclohexane, cyproterone acetate, nafenopin, and pregnenolone-16α-carbonitrile) were administered. Twenty-four to 30 hr later, this treatment produced even higher proliferative activities in island cells and increased the DNA synthesis index up to 50%, while proliferation in normal liver cells increased slightly to moderately. Thus, cells of putative preneoplastic islands appear to possess an inherent defect of growth control rendering them more susceptible to endogenous and exogenous growth stimuli. These findings partially explain why the mitogens mentioned induce rapid enlargement of preneoplastic foci and may provide a clue for further studies on the mechanism of tumor promotion in the liver. In addition, the results may form the basis for a short-term test to detect promoting activity of chemical compounds.