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 As3+-, Cd2+-, and Cr6+-induced oxidative stress on the expression of Cyplal, Nqol, and Gst ya in Hepa lclc7 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 Cyplal activity by TCDD, Cyplal mRNA levels were potentiated. When cellular GSH was depleted with buthionine-(SR)sulfoximine (BSO), Cyplal mRNA expression was further potentiated whereas Cyplal activity was further inhibited. In parallel, pretreatment with the antioxidant N-acetyleysteine (NAC) did not alter Cyplal mRNA expression but completely abrogated the inhibition of Cyp I a I activity induction by all three metals. On the other hand, all three metals, alone or in the presence of TCDD, enhanced Nqo I and Gst ya mRNA levels and NqoI 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 Cyplal at transcriptional and posuranscriptional levels but induces Nqol and Gst ya at the transcriptional level. (c) 2005 Elsevier Inc. All rights reserved.