Non-genotoxic hepatocarcinogenesis stimulate DNA synthesis and their withdrawal induces apoptosis, but in different hepatocyte populations

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Abstract
Non-genotoxic hepatocarcinogenesis may involve suppression of the hepatocyte apoptosis that would normally remove damaged or initiated cells. These protected hepato-cytes could then remain as preferential targets for promotion by this class of compounds. Here we demonstrate clearly that the non-genotoxic liver carcinogens and hepato-mitogens cyproterone acetate (CPA) and nafenopin, a peroxisome proliferator, both suppress the basal level of rat liver apoptosis in vivo. After 10 days of dosing with CPA (120 mg/kg/day ) or nafenopin (25 mg/kg/day) there were 0.005 ± 0.010 and 0.002 ± 0.021 apoptotic bodies/ 100 hepatocytes respectively, compared with 0.031 ± 0.008 per 100 in controls. Concomitant with this suppression of apoptosis, bromodeoxyuridine (BrdU) labelling indices and mitotic figures rose, confirming a perturbation of both sides of the growth equation between cell death and replication. Withdrawal of CPA or nafenopin resulted in a 100- to 200-fold elevation in apoptosis. This was inhibited by the re-administration of either compound. To investigate if cells protected from apoptosis by non-genotoxic carcinogens are targets for replication, we examined the replicative history of the apoptotic bodies generated upon withdrawal of CPA or nafenopin. Rats were administered BrdU during the hyperplastic phase of compound administration (0–10 days). Livers were examined 5 days after compound withdrawal. With both CPA and nafenopin, apoptotic bodies and S phase were predominantly in the periportal region. However, despite this zonal co-localization, very few (<10%) of the apoptotic bodies were labelled with BrdU. Overall, our data provide in vivo evidence to support the hypothesis that non-genotoxic hepatocarcinogens such as CPA and the peroxisome proliferators suppress apoptosis. Surprisingly, the majority of the hepatocytes generated during compound-induced hyperplasia were protected from apoptosis during liver regression. These data contribute to our understanding of clonal selection and promotion during non-genotoxic hepatocarcinogenesis.

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