The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates biological and toxicological effects by binding to its agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Previously we demonstrated that flavonoids suppressed the TCDD-induced DNA-binding activity of the AhR in a structure-dependent manner. In this study, we investigated the mechanisms by which flavonoids suppressed the AhR-mediated signal transduction in mouse hepatoma Hepa-1c1c7 cells. Flavones and flavonols suppressed the TCDD-induced nuclear translocation of the AhR and dissociation of its partner proteins, heat shock protein 90 and X-associated protein 2, whereas flavanones and catechins did not. Flavonoids of all these four subclasses suppressed the phosphorylation of both AhR and Arnt and the formation of a heterodimer consisting of these proteins. Since certain flavonoids are known to inhibit mitogen-activated protein kinases (MAPKs), we confirmed the contribution of MAPK/ERK kinase (MEK) to the AhR-mediated signal transduction by using U0126, an inhibitor of MEK1/2. U0126 suppressed TCDD-induced phosphorylation of the AhR and Arnt followed by the DNA-binding activity of the AhR. Flavanones and catechins suppressed the TCDD-induced phosphorylation of ERK1/2. The inhibition of MEK/ERK phosphorylation is one of the mechanisms by which flavanones and catechins suppress the AhR-mediated signal transduction in Hepa-1c1c7 cells.
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