Evidence for an Aryl Hydrocarbon Receptor-Mediated Cytochrome P450 Autoregulatory 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. An essential role for the AhR in cellular biology has been established previously, 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 cytochromes P450 1A1, 1A2, or 1B1 reduced AhR-mediated luciferase reporter activity, whereas cytochrome P450 2E1 exhibited no significant effect. Studies with 2,4,3Ј,5Ј-tetramethoxystilbene, 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 cytochromes P450 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 assay analyses, 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 4-fold increase in AhR-mediated reporter activity in cells. Quantitative polymerase chain reaction 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.

Aryl hydrocarbon receptor polymorphism modulates DMBA‐induced inflammation and carcinogenesis in phenotypically selected mice

“…Therefore, our analysis of Ahr allele segregation in the AIR lines revealed complete LD between the mouse lines for the functional A375V polymorphism, affecting the capability of the receptor to bind and, in turns, to induce Cyp1a1 and Cyp1b1 enzymes that metabolize DMBA to active carcinogen species 4,13 . Indeed, AIRmax mice, which carry the Ahr d allele, were resistant, whereas AIRmin mice, which carry the Ahr b1 allele, were susceptible to contact hypersensitivity reaction and skin and lung tumors following DMBA treatment.…”

“…However, for other inflammatory stimuli such as Biogel induced subcutaneous inflammation 10 , in arthritic joints induced by pristane treatment, 11 or in colon after Dextran induced acute colitis, 12 the production of inflammatory cytokines was significantly higher in AIRmax in comparison with AIRmin. Cyp1b1 is transcriptionally regulated by Ahr and is primarily involved in metabolism of foreign chemicals 13 . The treatment did not mediate the induction of Cyp1a1 and this could be explained by the activation of IL-1b which can down-regulate the transcription of this Ahr responsive gene.…”

Hepatic Aryl Hydrocarbon Receptor Attenuates Fibroblast Growth Factor 21 Expression

“…Electromobility Shift Assay-EMSA was performed as previously described (31), using in vitro translated (Promega) mouse AHR, ARNT, ChREBP, and MLX. As a positive control for AHR, we used a 32 P-labeled Cyp1a1 oligonucleotide as previously described (32). In addition to the oligonucleotide described in Fig.…”

Inhibition of cytochrome P4501-dependent clearance of the endogenous agonist FICZ as a mechanism for activation of the aryl hydrocarbon receptor

“…3 and references therein). FICZ also is bound to the AHR with the highest affinity known to date and thereby demonstrates the characteristics of an endogenous signaling molecule; i.e., it stimulates an efficient receptor-mediated pathway for transcriptional activation of metabolic enzymes that, among other things, catalyze the clearance of FICZ itself, thereby generating negative feedback regulation of its action (3,26,27). The physiological effects of FICZ reported to date include alterations in circadian rhythms (28), adaptive responses to UV light (4,29), induced retrotransposition of long interspersed nucleotide element-1 (30), and stimulated production of IL-22, a member of the IL-10 cytokine family that helps regulate inflammatory responses associated with many autoimmune diseases (17,18,31).…”