Inhibitory effects of naturally occurring coumarins on the metabolic activation of benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene in cultured mouse keratinocytes.

Several naturally occurring coumarins to which humans are routinely exposed have been previously found to be potent inhibitors and inactivators of cytochrome P450 (P450) 1A1-mediated monooxygenase in both murine hepatic microsomes and in a reconstituted system using purified human P450 1A1 [Cai et al. (1993) Chem. Res. Toxicol., 6, 872-879 and Cai et al. (1996) Chem. Res. Toxicol., 9, 729-736]. In the present study, several of these coumarins were investigated for their inhibitory effects on the metabolism and metabolic activation of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA) in cultured mouse keratinocytes. Initial analysis of B[a]P metabolism in cultured keratinocytes showed that imperatorin, isoimperatorin, coriandrin, and bergamottin, at concentrations of 2 nM equal with B[a]P, reduced the formation of water-soluble metabolites of B[a]P by 33% to 57%. Bergamottin and coriandrin were the most potent inhibitors of the compounds examined. HPLC analysis of organic solvent-soluble metabolites of B[a]P indicated that all the coumarins tested significantly reduced the formation of individual B[a]P metabolites (including phenols, diols and tetraols). However, the greatest effect was on the formation of B[a]P tetraols. Additional experiments determined the ability of selected coumarins to block covalent binding of B[a]P and DMBA to DNA in keratinocytes. Bergamottin preferentially inhibited the binding of B[a]P to DNA by 56%, while coriandrin preferentially inhibited the binding of DMBA to DNA by 48%. Notably, analysis of individual DNA adducts formed from B[a]P and DMBA indicated that both bergamottin and coriandrin specifically inhibited the formation of anti diol-epoxide DNA adducts derived from both hydrocarbons. The preferential inhibitory effect of bergamottin and coriandrin on the formation of anti diol-epoxide adducts derived from DMBA was further confirmed by separation of anti- and syn-diol-epoxide-DNA adducts using immobilized boronate chromatography. The current study demonstrates that certain naturally occurring coumarins inhibited metabolic activation of B[a]P and DMBA in cultured mouse keratinocytes and specifically inhibited the formation of DNA adducts derived from the anti diol-epoxide diastereomers from either hydrocarbon. The current data also suggest that certain naturally occurring coumarins may possess anticarcinogenic activity toward polycyclic aromatic hydrocarbons.