Suppression of cytochrome P450 2C11 by aromatic hydrocarbons: mechanistic insights from studies of the 5'-flanking region of the CYP2C11 gene.

The aromatic hydrocarbon receptor (AHR) functions as a ligand-activated transcription factor that mediates responses to aromatic hydrocarbons (AHs). Induction of cytochrome p450 1A1 (CYP1A1) is the most fully characterized response and is mediated by binding of the activated AHR complex to dioxin-responsive elements (DREs) located in the 5'-flanking region of the gene. In contrast to CYP1A1 induction, several other genes including the rat male-specific constitutive hepatic CYP2C11 are suppressed by AHs. Our aim was to determine whether CYP2C11 suppression by AHs is mediated by the AHR via interaction with DRE-like sequences. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) suppressed CYP2C11 mRNA in primary rat hepatocytes without altering the mRNA half-life. We identified five regions in the CYP2C11 5'-flank containing the DRE invariant core; electrophoretic gel retardation assays showed that at least one of these DREs is a potential binding site for the AHR. To test the function of the CYP2C11-DREs, Hepa-1, BRL 5637, and HepG2 cells were transfected with reporter constructs containing regions of the CYP2C11 5'-flank and promoter. No decrease in luciferase activity was found following TCDD treatment. In primary rat hepatocytes, the luciferase reporter vectors were suppressed by interleukin-1 beta but not by TCDD. In vitro footprinting showed protein binding at several sites in the CYP2C11 5'-flank, but the pattern was not altered by in vivo 3-methylcholanthrene treatment. These studies imply that AHs down-regulate CYP2C11 by a negative transcriptional mechanism that is not simply due to AHR binding to an identified DRE-like sequence and that is distinct from that used by inflammatory cytokines.