Abstract
Tobacco smoke exposure stimulates the expression of genes that are likely to be involved in the metabolism of its combustion products such as polycyclic aromatic hydrocarbons (PAH). Four of the smoke induced genes are aldo-keto reductases (AKR), enzymes that metabolically activate PAH to PAH o-quinones. Alternatively, PAHs are metabolized to (±)-anti-diol epoxides, such as (±)-anti-benzo[a]pyrene diol epoxide ((±)-anti-BPDE)), by the combined action of P4501A1/1B1 and epoxide hydrolase. (±)-anti-BPDE forms DNA adducts directly, while PAH o-quinones cause DNA damage by oxidative stress through a futile redox cycle. To address the role of AKRs in PAH cytotoxicity, we compared the cytotoxicity of PAH metabolites and the effects of overexpressing AKR1A1 in lung cells. (±)-anti-BPDE and B[a]P-7,8-trans-dihydrodiol, an intermediate in (±)-anti-BPDE metabolism, are toxic to A549 cells at concentrations with an IC(50) of ∼2 μM. In contrast, the PAH o-quinone B[a]P-7,8-dione was about 10-fold less toxic to A549 cells with an IC(50) > 20 μM. Similar differences in cytoxicity were observed with two other PAH o-quinones (benz[a]anthracene-3,4-dione and 7,12-dimethylbenz[a]anthracene-3,4-dione) compared with their respective diol-epoxide counterparts (BA-3,4-diol-1,2-epoxide and DMBA-3,4-diol-1,2-epoxide). In addition, both anti-BPDE and B[a]P-7,8-trans-dihydrodiol induced p53 expression ∼6 h post-treatment at concentrations as low as 1 μM consistent with extensive DNA damage. B[a]P-7,8-dione treatment did not induce p53 but generated reactive oxygen species (ROS) in A549 cells and induced the expression of oxidative response genes in H358 cells. We also observed that overexpression of AKR1A1 in H358 cells, which otherwise have low levels of AKR expression, protected cells 2-10-fold from the toxic effects of B[a]P-7,8-trans-dihydrodiol. These data suggest that overexpression of AKRs may protect lung cancer cells from the acute toxic effects of PAH.