Epidemiological evidence suggests that aflatoxin B1 (AFB1), a potent animal hepatotoxin and hepatocarcinogen may be an etiologic agent in the induction of primary liver cancer in humans. It undergoes activation via epoxidation and the interaction of its epoxide with DNA is believed to be responsible for its initiation of carcinogenesis. There is a wide different in carcinogenicity of AFB1 between rats and hamsters and AFB1-DNA binding in vivo and in vitro. Recent work with hepatosubcellular fractions and isolated hepatocytes of these two species from my laboratory has shown that the binding of DNA with AFB1 is strongly modulated by a competitive detoxication reaction with reduced glutathione (GSH), catalyzed by one or more of the cytosolic GSH S-transferases. The purpose of this study is to explore and delineate further a possible role of GSH and its S-transferases in AFB1 carcinogenesis.
The specific aims of this project are to extend and amplify our current studies by (1) investigating whether one or more of the known specific cytosolic GSH S-transferases are responsible for conversion of AFB1-epoxide to the AFB1-GSH conjugate. These studies would entail purification of various GSH S-transferases from rat and hamster livers, and examining their catalytic and kinetic properties in a reconstituted system using either microsome-mediated AFB1-epoxide or synthetic AFB1-epoxide as a substrate (2) investigate the kinetics of hepatic AFB1-DNA binding in rats and hamsters over a wide range of AFB1 dose and by modulation of GSH and its transferases. Pretreatment of animals with buthionine sulfoximine (BSO) will lower hepatic GSH levels and administration of phenobarbital (PB) will increase GSH S-transferases activities and (3) investigate the biological significance of GSH and its S-transferases in modulation of hepatic AFB1-DNA binding and hepatocarcinogenesis by examining production of a population of single GSH S-transferase placental form positive hepatocytes and of preneoplastic enzyme altered foci of hepatocytes as well as ultimate hepatocarcinomas in rats and hamsters in which GSH is reduced by pretreatment with BSO before the administration of AFB1. In similar experiments the effect of increased GSH S-transferases will be examined by administration of PB with and without BSO treatment. The long term objectives of this program are aimed at understanding the molecular mechanisms in AFB1 carcinogenesis.
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