Tectochrysin in propolis is a potent natural aryl hydrocarbon receptor ligand

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the toxic and biological actions of many aromatic environmental pollutants such as dioxins. As part of an investigation to clarify the interaction of foods with AhR, we previously reported that excessive intake of foods containing AhR-activators may be conductive to promote dioxin-like toxicity though there would not be a problem following normal intake. Additionally, it is discussed that the signal transduction of natural AhR ligands, which occurs after AhR activation, should differ from that of dioxins [1]. In this study, we examined the binding ability of 50 extracts prepared from kinds of commercial supplements and health foods to the AhR using reporter gene assay. Though most tested samples did not show any luciferase induction at a high concentration level, propolis product showed activation of luciferase at high concentration range. To characterize the AhR-activating substances in the propolis product, its extract was subjected to fractionation with n-hexane, ethyl acetate, and water followed by estimation their AhR activities. HPLC analysis of the n-hexane fraction which showed AhR activity detected eight compounds including flavonoids such as tectochrysin and pinocembrin. Among their compounds, tectochrysin showed remarkable AhR activation. Recently, several papers reported that AhR activation may be involved in various immune system [2–4]. Therefore it is suggested that natural AhR ligands characterized in the present study may play some beneficial regulatory role in human.