Abstract
Environmental carcinogens, such as polycyclic aromatic hydrocarbons (PAHs), require metabolic activation to DNA-reactive metabolites in order to exert their tumorigenic effects. Benzo[a]pyrene (B[a]P), a prototypic PAH, is metabolized by cytochrome P450 (P450) 1A1/1B1 and epoxide hydrolase to (-)-B[a]P-7,8-dihydro-7,8-diol (B[a]P-7,8-dihydrodiol). B[a]P-7,8-dihydrodiol then undergoes further P4501A1/1B1-mediated metabolism to the ultimate carcinogen, (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydro-B[a]P (B[a]PDE), which forms DNA-adducts primarily with 2'-deoxyguanosine (dGuo) to form (+)-anti-trans-B[a]PDE-N(2)-dGuo (B[a]PDE-dGuo) in DNA. Pretreatment of cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to induce P4501A1/1B1 mRNA expression through the aryl hydrocarbon receptor (AhR) pathway. This causes increased B[a]PDE-dGuo formation in liver cells. In contrast, TCDD induction of H358 lung cells surprisingly caused a decrease in (-)-B[a]P-7,8-dihydrodiol-mediated (+)-B[a]PDE-dGuo adduct formation when compared with the non-TCDD-induced cells. Furthermore, treatment of the TCDD-induced cells with (±)-B[a]PDE also resulted in decreased (+)-B[a]PDE-dGuo adduct formation when compared with the non-TCDD-induced cells. These data suggested that it was a detoxification pathway that had been up-regulated rather than an activation pathway that had been down-regulated. LC-MS was used to analyze B[a]PDE-dGuo and B[a]PDE-GSH-adducts in H358 lung and HepG2 liver cells. There was a significant increase in the (-)-B[a]PDE-GSH-adduct with high enantiomeric excess after treatment of the TCDD-induced H358 cells with (±)-B[a]PDE when compared with the noninduced cells. This could explain why increased activation of (-)-B[a]P-7,8-dihydrodiol through TCDD up-regulation of P4501A1/1B1 did not lead to increased (+)-B[a]PDE-dGuo adducts in the H358 lung cells. In addition, TCDD did not induce B[a]PDE-GSH-adduct formation in HepG2 liver cells. (±)-B[a]PDE-GSH-adducts were formed at much lower levels in both TCDD-induced and noninduced HepG2 cells when compared with (-)-B[a]PDE-GSH-adducts in the H358 lung cells. Therefore, our study has revealed that there is a subtle balance between activation and detoxification of B[a]P in lung-derived compared with liver-derived cells and that this determines how much DNA damage occurs.