Requirement of Cell Proliferation for the Initiation of Liver Carcinogenesis as Assayed by Three Different Procedures¹

Experiments were designed to determine the role of cell proliferation in the initiation of liver carcinogenesis induced by chemicals. To investigate this, two methylating carcinogens, N-methyl-N-nitrosourea and 1,2-dimethylhydrazine, were used as the initiating carcinogens. The initiated hepatocytes were monitored by selectively stimulating them to grow into focal islands of presumptive preneoplastic hepatocytes. The experimental approach in brief consisted of the following. Rats received a nonnecrogenic dose of the carcinogen; at a time period when the carcinogen could no longer be detected in the system, they were subjected to either partial or sham hepatectomy. The initiated cells thus formed were selectively stimulated to grow into foci of preneoplastic hepatocytes using three different selection regimens: (a) feeding a diet containing 0.02% 2-acetylaminofluorene plus one administration of carbon tetrachloride (2 ml/kg body weight) intragastrically; (b) feeding a diet containing 0.05% phenobarbital; and (c) feeding a choline-deficient diet. The foci were quantitated by staining them for the presence of γ-glutamyltranspeptidase.

The results obtained indicated that irrespective of the type of selection procedure used foci of preneoplastic hepatocytes were seen only in rats that received the carcinogen coupled with a cell-proliferative stimulus such as partial hepatectomy. Very few or no foci were seen in rats that received the carcinogen plus sham hepatectomy. These results suggest that cell proliferation plays an important role in the initiation of liver carcinogenesis by chemicals.