Absence of DNA adduct formation by phenobarbital, polychlorinated biphenyls, and chlordane in mouse liver using the 32P-postlabeling assay.

Phenobarbital (PB), polychlorinated biphenyls (PCBs), and chlordane (CLD) increase liver tumor incidences in rodents, and all are tumor promoters. Most indirect tests for DNA reactivity, including mutagenicity and chromosomal damage, have been negative with these agents. Consequently, the modes of action for tumorigenesis by these compounds are not believed to involve direct DNA reactivity; however, only limited information from direct tests is available for the lack of DNA adduct formation. PB, PCBs, and CLD were tested for DNA adduct formation in the liver of male and female B6C3F1 mice after either single or 2-week dietary exposures. Single gavage dose levels were as follows: PB, 200 mg/kg; PCBs, 50 mg/kg; and CLD, 50 mg/kg. Dietary dose levels were as follows: PB, 1000 ppm; PCBs, 200 ppm and CLD, 200 ppm. Animals were killed 24 h following the end of test-substance administration. DNA was extracted from the liver, and DNA adduct concentrations were enriched using either 1-butanol extraction of adducted nucleotides or nuclease P1 digestion of unadducted nucleotides. Using this protocol, none of the three test compounds produced DNA adducts detected by 32P-postlabeling. Similar negative results were obtained for DNA from the livers of both male and female mice receiving either single or 2-week exposures. The two positive controls, benzidine for the 1-butanol extraction procedure and 2-acetylaminofluorene for the nuclease P1 procedure, showed the expected patterns of DNA adducts. These results support the conclusion that the carcinogenicity of PB, PCBs, and CLD in experimental animals is not the result of direct DNA reactivity, but involves epigenetic mechanisms.