The complete liver
carcinogen 2-acetylaminofluorene (2-AAF) promoted the outgrowth of large neoplastic liver nodules and
hepatocellular carcinomas in
diethylnitrosamine-treated rats.
2-AAF did not alter the overall proliferative activity of normal hepatocytes, but suppressed binucleation and induced, on a long-term basis, an increase in proliferative activity and in the fraction of diploid hepatocytes relative to control animals. The analogue
4-acetylaminofluorene (4-AAF) was much less effective than
2-AAF as a promoter of large nodules and
carcinomas, but promoted the outgrowth of medium-sized nodules (1 < 2.5 mm). While
2-AAF specifically stimulated the growth of cells in
enzyme-altered foci, the cells responding to
4-AAF were more randomly distributed throughout the liver tissue. In contrast to
2-AAF,
4-AAF strongly stimulated the growth (
DNA synthesis) of normal hepatocytes, but like
2-AAF it suppressed binucleation and caused a long-term increase in the proliferative activity and in the fraction of diploid hepatocytes. Other liver tumour promoters (
cyproterone acetate,
alpha-hexachlorocyclohexane,
methylclofenapate) likewise stimulated the growth and suppressed the binucleation of normal hepatocytes. All hepatocellular ploidy classes were affected virtually equally by mitogenic stimulation, but at low proliferation rates the mononuclear cells were more proliferative than the binuclear cells. Since this difference could be eliminated by increasing the
mitogen dose, it would seem that mononuclear cells may be somewhat more sensitive towards
mitogens than binuclear cells. In contrast to previously reported results [Styles et al. (1990)
Carcinogenesis, 11, 1149-1152],
methylclofenapate was not found to specifically stimulate binuclear hepatocytes. Our results indicate that liver tumour promoters in general tend to induce a non-binucleating, non-polyploidizing hepatocellular growth pattern, similar to that observed during liver regeneration.
4-AAF is confirmed to be, at best, a very weak promoter of liver
carcinogenesis, but appears to be an effective promoter of benign tumours.