Aryl hydrocarbon receptor response to indigoids in vitro and in vivo

Indigo and indirubin have been reported to be present at low levels in human urine. The possibility that indigoids are physiological ligands of the aryl hydrocarbon receptor (AhR) has been suggested by initial studies in yeast, where indirubin was found to be 50 times more potent than 2,3,7,8-tetrachlorodibenzo[p]dioxin (TCDD), and indigo was found to be equipotent. To demonstrate that these indigoids are bona fide agonists in mammalian systems, we employed a number of in vitro and in vivo measures of AhR agonist potency. In a hepatoma cell reporter system, indigo yielded an EC50 of ∼5 × 10−6 M (indirubin 3′-oxime EC50 ∼5 × 10−7 M, indirubin EC50 ∼1 × 10−7 M). A comparison of these EC50 values with that of 2,3,7,8-tetrachlorodibenzofuran (TCDBF) (∼3 × 10−9 M) indicated that these compounds are less potent than classic halogenated-dibenzofurans or -dibenzo-p-dioxins. Competitive binding assays for AhR occupancy showed similar IC50 values for indirubin and TCDBF (∼2 × 10−9 and 5 × 10−9 M), with the IC50 values of indigo and indirubin 3′-oxime being ∼10-fold higher. When rats were treated with these indigoids in the range of 1.5–50 mg/kg, induction of hepatic cytochrome P450 1A1 was detected. Differences in the rank-order of potency observed in vivo and in vitro could, in part, be explained by metabolism. Although their biological potencies are not as high as has been previously suggested, collectively the results show that these indole-derived pigments are agonists of AhR in vivo. The in vivo results suggest that solubility, distribution, and metabolism influence the response to the compounds.