The liver consists not only of hepatocytes and supporting nonparenchymal cells, but it also contains a population of progenitor cells, which in case of serious liver injury and inhibition of hepatocyte regenerative capacity, can give rise to oval cells. The oval cells can develop into both hepatocytes and biliary epithelial cells, and they might play a significant role in hepatocarcinogenesis. There is little information about formation of DNA adducts and other genotoxic events in these cells. In order to determine the capability of polycyclic aromatic hydrocarbons (PAHs) to elicit genotoxic damage, we investigated the DNA adduct formation in rat liver WB-F344 cells, a well established in vitro model of oval cells for studies on nongenotoxic mechanisms of carcinogenesis, exposed to carcinogenic PAHs by 32P-postlabelling technique. The amount of adducts was compared with the potency of individual compounds to induce CYP1A1 and CYP1B1 mRNA expression, phosphorylation of p53 protein in response to genotoxic damage, and induction of apoptosis. Although all PAHs under study elicited maximum induction of CYP1A1 and CYP1B1 mRNA at the 1 μM concentration (with exception of dibenzo[al]pyrene (DB[al]P)), distinct genotoxic and cytotoxic potencies were observed. DB[al]P, and to a lesser extend also benzo[g]chrysene (B[g]Chry), formed high levels of DNA adducts. This observation was accompanied by accumulation of phosphorylated form of p53 protein and induction of apoptosis. Benzo[a]pyrene (B[a]P) exposure induced lower, but significant DNA adduct levels and cytotoxicity only at micromolar concentrations, whereas benz[a]anthracene (B[a]A), chrysene (Chry), benzo[b]fluoranthene (B[b]F) and dibenzo[a,h]anthracene (DB[ah]A) induced only low amount of DNA adducts, without significant effects on p53 phosphorylation or apoptosis. We have observed a signicant formation of DNA adducts by strongly genotoxic PAHs in WB-F344 cells, which led to further toxic effects, whereas weakly genotoxic PAHs produced a low amount of DNA damage in this model of liver progenitor cells. This suggests that this cell line can efficiently metabolize PAHs to ultimate genotoxic metabolites and it can be used for studies on both genotoxic and nongentoxic effects of organic contaminants. In conclusion, we have found significant PAH-DNA adduct formation in liver immature epithelial cells, which can be a target of chemical hepatocarcinogenesis. Damage to DNA might be an important mode of action of PAHs such as DB[al]P, B[g]Chry and B[a]P in this type of cells, however, the exact role in hepatocarcinogenesis of low levels of DNA adducts produced by those PAHs which apparently operate mostly by nongenotoxic modes of action (DB[ah]A, B[b]F, B[a]A) has to be elucidated. Support: Grant Agency of the ASCR (B6004407), Czech Ministry of Agriculture (MZE00002716201), Czech Ministry of Environment (grant No.VaV-SL/5/160/05).
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA