Indirubin and indigo are potent aryl hydrocarbon receptor ligands present in human urine

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates genes involved in xenobiotic metabolism, cellular proliferat ion, and differentiation. Numerous xenobiotic and biological compounds are known to interact with AhR, but it remains an orphan receptor, because its physiological ligand is unknown. We identified AhR ligands in human urine u sing a yeast AhR signaling assay and then characterized their properties. T wo ligands, indirubin and indigo, were both present at average concentratio ns of similar to0.2 ml in the urine of normal donors. Indirubin was also de tected in fetal bovine serum and contributed half of the total AhR ligand a ctivity. The activities of indirubin and indigo were comparable with or mor e potent than that of the archetypal ligand, 2,3,7,8-tetrachlorodibenzo-p-d ioxin, in yeast AhR activation assays. We suggest that the endogenous level s and potencies of indirubin and indigo are such that they activate AhR-med iated signaling mechanisms in vivo.