Plant Occurring Flavonoids as Modulators of the Aryl Hydrocarbon Receptor

Abstract

The aryl hydrocarbon receptor (AhR) is a transcription factor deeply implicated in health and diseases. Historically identified as a sensor of xenobiotics and mainly toxic substances, AhR has recently become an emerging pharmacological target in cancer, immunology, inflammatory conditions, and aging. Multiple AhR ligands are recognized, with plant occurring flavonoids being the largest group of natural ligands of AhR in the human diet. The biological implications of the modulatory effects of flavonoids on AhR could be highlighted from a toxicological and environmental concern and for the possible pharmacological applicability. Overall, the possible AhR-mediated harmful and/or beneficial effects of flavonoids need to be further investigated, since in many cases they are contradictory. Similar to other AhR modulators, flavonoids commonly exhibit tissue, organ, and species-specific activities on AhR. Such cellular-context dependency could be probably beneficial in their pharmacotherapeutic use. Flavones, flavonols, flavanones, and isoflavones are the main subclasses of flavonoids reported as AhR modulators. Some of the structural features of these groups of flavonoids that could be influencing their AhR effects are herein summarized. However, limited generalizations, as well as few outright structure-activity relationships can be suggested on the AhR agonism and/or antagonism caused by flavonoids.

Keywords: Ah receptor; bioactive; dioxin receptor; flavonoids; functional food; phytochemicals; phytocompounds; polyphenols; transcription factor; xenobiotics.

Figure 1

Schematic domain organization of the aryl hydrocarbon receptor (AhR) and its binding sites.

Figure 2

Schematic representation of AhR-mediated pathways. Ligands bind to the cytoplasmic AhR, where it forms a complex with HSP90, p23, XAP2, and SRC. (I) Agonist ligands promote conformational changes and the exposure of NLS, allowing the nuclear translocation mediated by importin β through the NPC. (II) Once in the nucleus, the heterodimer AhR-ARNT can bind to different sequences in the DNA and trigger the synthesis of proteins (e.g., CYP1A1, UGT1A6), which is additionally regulated by coactivators. (III) Crosstalk interactions with other signaling pathways are acknowledged (e.g., ESRs, AR). Moreover, the Ah receptor can be repressed by AhRR, and its degradation is mainly mediated by 26S proteasome.

Figure 3

Examples of AhR endogenous and exogenous modulators (structures, common names, and CAS numbers are shown).

Figure 4

General classes of flavonoids and some relevant subclassifications.