DNA Adduct Formation, Nucleolar Segregation and Cell Proliferation in Rats Treated with 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

In order to validate early biomarkers for chemical carcinogenesis, alterations of DNA damage and subsequent cell replication induced by 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP) were sequentially investigated in the rat colon and liver. Male 6-week-old Sprague Dawley rats were singly administered by gavage 300 mg/kg bw PhIP and control rats received vehicle alone. All rats were euthanized after 4-72 hr, and the organs were removed for histopathological examination, immunohistochemistry and electron microscopic observation. Immunohistochemically, in the colon, PhIP-DNA adduct already appeared at 4 hr and the positive ratios peaked at 24 hr after the PhIP exposure. Nucleolar alteration, demonstrable by electron microscopy as segregation of nucleolar components into granular and fibrillar compartments, was evident in cells of the target organ colon. Sequential observation clarified that such alteration was highest in frequency after 48 hr in colon cells, suggesting that nucleolar segregation occurs subsequent to generation of DNA adduction. Following these events, Ki-67-labeling in the colon was significantly increased at 72 hr. No significant PhIP-DNA adduct formation, nucleolar alteration or cell proliferation were noted in colons of the control rats nor in livers regardless of the PhIP treatment. Our results thus indicate an identity between the target cells for PhIP- DNA adduct formation, nucleolar segregation and enhanced cell replication, which correlated with DNA damage. These biomarkers could be useful for predicting the target organs of chemical carcinogenesis. (J Toxicol Pathol 2007; 20: 39-47)