Variations in the Cell Cycle During Azo Dye Hepatocarcinogenesis2

Abstract

Radioautographic and cytophotometric studies were undertaken to determine whether variations in the duration of the G₂ phase of the cell cycle occur in rat hepatocytes during 4-dimethylaminoazobenzene (DAB) carcinogenesis. In hepatic nodules of DAB-fed rats, a decrease in the number of labeled mitoses was accompanied by an increase in the level of ploidy, indicating a delay or an arrest of cells in the G₂ phase of the cell cycle. In hyperbasophilic foci considered as the sites of neoplastic transformation, higher percentages of labeled mitoses were obtained and ploidy was reduced, indicating a shortening of the G₂ phase. Thus it seems that neoplastic transformation is associated with an elimination or inhibition of both the G₁ and G₂ blocks of the cell cycle. Both changes contribute to the acceleration of cell proliferation in sites of tumor formation.