Received for publication June 13, 2007.
Revised August 23, 2007.
Accepted for publication August 24, 2007.

Evidence for an Ah receptor-mediated cytochrome P450 auto-regulatory pathway

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor responsible for mediating the cellular response to the toxic compound 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD). An essential role for the AhR in cellular biology has been previously established, but no high affinity endogenous ligand has yet been identified. We have confirmed the presence of a putative endogenous ligand(s) in CV-1 cells through transient transfection with various cytochrome P450 isoforms. Expression of cytochrome P450s 1A1, 1A2 or 1B1 reduced AhR-mediated luciferase reporter activity, while cytochrome P450 2E1 exhibited no significant effect. Studies with 2,4,3',5'-tetramethoxystilbene (TMS), a potent and specific inhibitor of cytochrome P450 1B1, was able to partially block cytochrome P450 1B1-mediated reduction in reporter gene activity. These results provide evidence of the existence of a possible feedback mechanism in which AhR regulated cytochrome P450s from the CYP1A and CYP1B families are able to metabolically alter putative endogenous ligand(s). Several experiments were performed to provide initial characterization of these putative endogenous ligands. Including electrophoretic mobility shift assays (EMSA) analysis which demonstrated that these ligands directly activate the AhR. Soluble extracts from various C57BL/6J and Ahr-null mouse tissues were also analyzed for the presence of AhR activators. Studies revealed that Ahr-null mouse lung tissue had a four-fold increase in AhR-mediated reporter activity in cells. Quantitative PCR analysis revealed that lung tissue exhibits relatively high constitutive CYP1A1 mRNA levels. These results suggest that there is an autoregulatory feedback loop between the AhR and cytochrome P450 1A1 in mouse lung.

Key words: Regulation of gene expression, Cytochrome P450, Ah receptor

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