Cytotoxicity and genotoxicity of ({+/-})-benzo[a]pyrene-trans-7,8-dihydrodiol in CYP1A1-expressing human fibroblasts quantitatively correlate with CYP1A1 expression level

Cytochrome P450 1A1 (CYP1A1) activity is associated with increasedsusceptibility to lung cancer induced by polycyclic aromatichydrocarbons such as benzoa]pyrene (BP). In non-hepatic humantissues, CYP1A1 is the principal enzyme responsible for themetabolic activation of the proximate BP mutagenic metabolite,(–)-benzoa]pyrene-trans-7,8-dihydrodiol, to (+)-anti-benzoa]pyrene-trans-7,8-dihydrodiol-9,10-epoxide,the ultimate BP mutagen. We have genetically engineered bothDNA repair-deficient (xeroderma pigmentosum group A) and DNArepair-proficient human skin fibroblasts to express human CYP1A1under control of the inducible mouse metallothionein-I promoter.CYP1A1 activity was induced by CdSO₄ and monitored by followingthe O-deethylation of ethoxy fluorescein ethyl ester or of 7-ethoxyresorufin.Induced CYP1A1 activities were similar in both cell lines andwere dependent on CdSO₄ concentration and induction time. MaximalCYP1A1 activities were obtained in 4–6 h with 5–7µM CdSO₄. BPD-induced cytotoxicity and hypoxanthine phosphoribosyltransferase mutagenicity were both quantitatively correlatedwith the level of CYP1A1 activity and were greater in DNA repair-deficientcells than in DNA repair-proficient cells. The results suggestthat modestly induced CYP1A1 activity is a risk factor in polycyclicaromatic hydrocarbon-induced carcinogenesis.