Distinct responses of the constitutive androstane receptor NR1I3 to indole-containing metabolites of bacterial origin

The constitutive androstane receptor (CAR, NR1I3) has been recognised as a nuclear receptor for various xenobiotics, such as barbiturates and dietary polyphenols. In this study, the CAR responded to internal metabolites produced by intestinal bacteria from the dietary amino acid tryptophan. We screened 15 indole-containing compounds in HepG2 cells using a luciferase reporter assay and found that three of them, tryptamine, indole-3-pyruvic acid, and indole-3-ethanol, significantly increased transcriptional activity of both mouse and human CAR. The estimated EC₅₀ values of these compounds at the micromolar concentration order, which were close to those found in the host sera and tissues. Importantly, 3-methyl indole (skatole) inhibited mouse CAR activity to a lesser extent than androstanol, an inverse agonist of mouse CAR. Considering this, we investigated the transactivation mechanisms of these compounds in terms of their nuclear translocation. Indole-3-pyruvic acid and diindolylmethane slightly but not significantly increased the nuclear translocation of mouse CAR, whereas skatole significantly increased nuclear translocation. This is in contrast to the observation that androstanol does not induce nuclear translocation. Tryptamine is produced by Ruminococcus gnavus and skatole by Lactobacillus spp. Our findings suggest that the CAR can be positively or negatively regulated by indole-containing metabolites, depending on the composition of the gut microbiota.