Suppression of apoptosis has been implicated as a mechanism for the hepatocarcinogenicity of the peroxisome proliferator class of non-genotoxic carcinogens. The ability of the peroxisome proliferator nafenopin to suppress or delay the onset of liver apoptosis was investigated using primary cultures of rat hepatocytes and the Reuber hepatoma cell line FaO. 50 microM nafenopin reversibly maintained the viability of primary rat hepatocyte cultures which otherwise degenerated within 8 d of establishment. The maintenance of viability of hepatocyte monolayers was associated with a significant decrease in the number of cells exhibiting chromatin condensation patterns typical of apoptosis. Apoptosis could be induced in hepatocytes by administration of 5 ng/ml TGF beta 1. Co-addition of 50 microM nafenopin significantly reduced TGF beta 1-induced apoptosis by 50-60%. TGF beta 1 (1-5 ng/ml) also induced apoptosis in the FaO rat hepatoma cell line. Cell death was accompanied by detachment of FaO cells from the monolayer and detached cells exhibited chromatin condensation and non-random DNA fragmentation patterns typical of apoptosis. Co-addition of 50 microM nafenopin to TGF beta 1-treated FaO cultures significantly reduced the number of apoptotic cells detaching from the monolayer at 24 h. In contrast, nafenopin had no significant effect on FaO apoptosis induced by the DNA damaging agents etoposide and hydroxyurea. We conclude that suppression of liver cell death by apoptosis may play a role in the hepatocarcinogenicity of the peroxisome proliferators, although the extent of this protection is dependent on the nature of the apoptotic stimulus.

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