NONGENOTOXIC HEPATOCARCINOGENS STIMULATE DNA-SYNTHESIS AND THEIR WITHDRAWAL INDUCES APOPTOSIS, BUT IN DIFFERENT HEPATOCYTE POPULATIONS

Non-genotoxic hepatocarcinogenesis may involve suppression of the hepa tocyte apoptosis that would normally remove damaged or initiated cells , These protected hepatocytes could then remain as preferential target s for promotion by this class of compounds. Here we demonstrate clearl y that the non-genotoxic liver carcinogens and hepatomitogens cyproter one acetate (CPA) and nafenopin, a peroxisome proliferator, both suppr ess the basal level of rat liver apoptosis in vivo. After 10 days of d osing 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 r espectively, compared with 0.031 +/- 0.008 per 100 in controls, Concom itant with this suppression of apoptosis, bromodeoxyuridine (BrdU) lab elling 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 elevatio n in apoptosis. This was inhibited by the re-administration of either compound, To investigate if cells protected from apoptosis by non-geno toxic carcinogens are targets for replication, we examined the replica tive history of the apoptotic bodies generated upon withdrawal of CPA or nafenopin, Rats were administered BrdU during the hyperplastic phas e of compound administration (0-10 days). Livers were examined 5 days after compound withdrawal. With both CPA and nafenopin, apoptotic bodi es and S phase were predominantly in the periportal region, However, d espite this zonal co-localization, very few (<10%) of the apoptotic bo dies were labelled with BrdU, Overall, our data provide in vivo eviden ce to support the hypothesis that non-genotoxic hepatocarcinogens such as CPA and the peroxisome proliferators suppress apoptosis, Surprisin gly, the majority of the hepatocytes generated during compound-induced hyperplasia were protected from apoptosis during liver regression, Th ese data contribute to our understanding of clonal selection and promo tion during non-genotoxic hepatocarcinogenesis.