Influence of liver tumor promoters on apoptosis in rat hepatocytes induced by 2-acetylaminofluorene, ultraviolet light, or transforming growth factor beta 1.

DNA damage is recognized widely as a cause of programmed cell death (apoptosis), aimed at eliminating cells bearing genotoxic lesions. Therefore, inhibition of DNA damage-induced apoptosis may play an important role in carcinogenesis and has been suggested as a mechanism of action of tumor-promoting agents. In the present study, the effects of treatment with UV light or the carcinogenic aromatic amine 2-acetylaminofluorene (2-AAF) on apoptosis were studied in rat hepatocytes in primary culture. A significantly increased incidence of apoptotic nuclei, showing condensed or fragmented chromatin visualized with the fluorescent dye Hoechst 33258, was found after each type of treatment. After 48 h, the incidence of apoptosis had returned to the control level. When the liver tumor promoters 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and phenobarbital were added to the medium, apoptosis did not increase in UV- or 2-AAF-treated compared with untreated cultures. Furthermore, TCDD and phenobarbital suppressed internucleosomal DNA fragmentation elicited by UV irradiation. In contrast, the promoters did not suppress apoptosis induced by transforming growth factor beta 1. Immunoprecipitation of the tumor suppressor gene product p53 demonstrated that the increase in p53 observed after UV irradiation was abrogated almost completely by TCDD. Apoptosis induced in rat hepatocytes by DNA-damaging agents such as UV light or 2-AAF is suppressed by TCDD and phenobarbital. Inhibition of apoptosis allowing survival of hepatocytes bearing genotoxic lesions may be crucial for the tumor-promoting action of TCDD and phenobarbital in the liver.