Importance of CYP1A1 and CYP1B1 in bioactivation of benzo[a]pyrene in human lung cell lines

Polycyclic aromatic hydrocarbons are ubiquitous environmental pollutants classified as carcinogens in humans and rodents. The cytochromes P4501A1 and 1B1 have both shown capacity to carry out bioactivation of the prototype PAH, benzoa]pyrene (Ba]P) to its ultimate carcinogenic Ba]P-diol-epoxide-I-1 form. The part played by each enzyme in human lung cells, however, has not been clarified. To get further insight into their individual role in the metabolic activation of Ba]P, RNA-interference was used to down-regulate CYP1A1 and/or CYP1B1 gene expression in the human lung cell lines BEP2D and NCIH2009. Fluorescence-HPLC analysis revealed that formation of Ba]P-tetrol-I-1 (hydrolyzed form of the corresponding diol-epoxide) was dependent primarily on CYP1A1. In cells without down-regulation of CYP1A1, the Ba]P-tetrol-I-1 was the major tested isomer formed. In contrast, the Ba]P-cis- and trans-7,8-dihydrodiol isomers were readily formed in cells expressing high levels of either CYP-gene. Simultaneous down-regulation of CYP1A1 and CYP1B1 mRNA resulted in low levels of metabolites overall. Residual unmetabolized Ba]P levels followed the expression of CYP1A1 in an inverse manner. In conclusion, these results indicate a major role of CYP1A1 in the bioactivation of Ba]P to carcinogenic Ba]P-diol-epoxides and in overall metabolism of Ba]P in human lung cell lines. In contrast, both CYP1A1 and CYP1B1 contribute significantly to the formation of the Ba]P-cis- and trans-7,8-dihydrodiol isomers.