Nongenotoxic carcinogens shift cultured rat hepatocytes into G1 cell cycle phase: influence of tissue oxygen tension on cells with different ploidy.

The transition of quiescent freshly isolated rat hepatocytes (G0 phase) into a prereplicative stage of the cell cycle (G1 phase) has been visualized by a decrease in fluorescence of quinacrine dihydrochloride (QDH)-stained nuclei. This transition might be used as an early detectable and sensitive marker for the identification of chemicals with a mitogenic potential. This was tested with three presumedly nongenotoxic carcinogens in the rodent, namely cyproterone acetate (CPA), thioacetamide (TA) and phenobarbital (PB). Freshly isolated hepatocytes were cultured under 13% or 4% O2, representing the tissue oxygen tension in the periportal (13% O2) and the pericentral area (4% O2) in the liver lobules. Fluorescence intensities of QDH-stained hepatocyte nuclei of different ploidy levels (2N, 4N or 8N) were quantified by image analysis. The epidermal growth factor induced G0-G1 shift was affected by the oxygen tension and was reversible as shown after exposure to retinoic acid (RA). At subtoxic concentrations, all three nongenotoxic carcinogens induced a shift of quiescent hepatocytes of all ploidy levels into the G1 cell cycle phase within a 6-h period. CPA was the most effective compound, followed by PB and TA. The strongest induction of the G0-G1 shift was observed in most cases in 2N nuclei. The oxygen tensions applied and the individual compounds tested, differentially affected the response of the individual ploidy classes. The results correspond well with the site-specific mitogenic response within liver lobules as observed after exposure in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)