ABSENCE OF DNA ADDUCT FORMATION BY PHENOBARBITAL, POLYCHLORINATED-BIPHENYLS, AND CHLORDANE IN MOUSE-LIVER USING THE P-32 POSTLABELING ASSAY

Phenobarbital (PB), polychlorinated biphenyls ls (PCBs), and chlordane (CLD) increase liver tumor incidences in rodents, and all are tumor p romoters. Most indirect tests for DNA reactivity, including mutagenici ty and chromosomal damage, have been negative with these agents. Conse quently, the modes of action for tumorigenesis by these compounds are not believed to involve direct DNA reactivity; however, only limited i nformation from direct tests is available for the lack of DNA adduct f ormation. PB, PCBs, and CLD were tested for DNA adduct formation in th e liver of male and female B6C3F(1) mice after either single or 2-week dietary exposures. Single gavage dose levels were as follows : PB, 20 0 mg/kg; PCBs, 50 mg/kg: and CLD, 50 mg/kg. Dietary dose levels were a s follows: PB. 1000 ppm; PCBs. 200 ppm and CLD, 200 ppm. Animals were killed 24 h following the end of test-substance administration. DNA wa s extracted from the liver, and DNA adduct concentrations were enriche d using either 1-butanol extraction of adducted nucleotides or nucleas e P-1 digestion of unadducted nucleotides. Using this protocol, none o f the three test compounds produced DNA adducts detected by P-32-postl abeling. Similar negative results were obtained for DNA from the liver s of both male and female mice receiving either single or 2-week expos ures. The two positive controls, benzidine for the 1-butanol extractio n procedure and 2-acetylaminofluorene for the nuclease P-1 procedure, showed the expected patterns of DNA adducts, These results support the conclusion that the carcinogenicity of PB. PCBs, and CLD in experimen tal animals is not the result of direct DNA reactivity, but involves e pigenetic mechanisms. (C) 1998 Academic Press.