Suppression ofCYP1A1Transcription by H2O2Is Mediated by Xenobiotic-Response Element

We have previously demonstrated that H2O2downregulatesCYP1A1andCYP1A2transcription in isolated rat hepatocytes (C. W. Barker,et al., 1994,J. Biol. Chem.269, 3985–3990). In the present study, induction of chloramphenicol acetyltransferase (CAT) expression driven by 3.1 kb of ratCYP1A1upstream regulatory sequences was suppressed by 56% in Hepa-1 cells treated with H2O2. Similarly, H2O2inhibited CAT expression from vectors containing two copies of either xenobiotic-response element (XRE) 1 or XRE2. H2O2did not inhibit basal CAT expression in cells that were not treated with the inducer β-napthoflavone. Electrophoretic mobility shift assays demonstrated that the suppression of XRE-dependent transcription by H2O2was not due to changes in nuclear aryl hydrocarbon (Ah) receptor DNA binding activity. Several types of experiments indicated that modulation of XRE enhancer strength by various means could modify H2O2-dependent suppression of CAT expression. Conditions that increased the transactivation potential of the Ah receptor (increase in XRE copy number or shortening of the distance between XREs and the minimalCYP1A1promoter) attenuated the action of H2O2, while conditions that reduced XRE-mediated transactivation potential (decrease in XRE copy number, increase of the distance between the XRE and the promoter, or reduction of the number of bound Ah receptors by lowering the concentration of inducer) potentiated the inhibitory action of H2O2.