The role of oxidative stress in the modulation of aryl hydrocarbon receptor-regulated genes by As3+, Cd2+, and Cr6+

Heavy metals alter the capacity of AhR ligands to induce the bioactivating phase I and the detoxifying phase II xenobiotic-metabolizing enzymes but the mechanism(s) remain unknown. In the present study, we evaluated the role of As3+-, Cd2+-, and Cr6+-induced oxidative stress on the expression of Cyp1a1, Nqo1, and Gst ya in Hepa 1c1c7 cells. Both Cd2+ and Cr6+, but not As3+, increased the production of ROS. However, all metals increased cellular GSH content and heme oxygenase-1 mRNA levels. Although all three metals inhibited the induction of Cyp1a1 activity by TCDD, Cyp1a1 mRNA levels were potentiated. When cellular GSH was depleted with buthionine-(S,R)-sulfoximine (BSO), Cyp1a1 mRNA expression was further potentiated whereas Cyp1a1 activity was further inhibited. In parallel, pretreatment with the antioxidant N-acetylcysteine (NAC) did not alter Cyp1a1 mRNA expression but completely abrogated the inhibition of Cyp1a1 activity induction by all three metals. On the other hand, all three metals, alone or in the presence of TCDD, enhanced Nqo1 and Gst ya mRNA levels and Nqo1 activity. These effects were potentiated in the presence of BSO and abrogated with NAC. Our data clearly show that As3+-, Cd2+-, and Cr6+-induced oxidative stress modulates Cyp1a1 at transcriptional and posttranscriptional levels but induces Nqo1 and Gst ya at the transcriptional level.