Differential activation of human pregnane X receptor PXR by isomeric mono-methylated indoles in intestinal and hepatic in vitro models

Dietary and microbial indoles can act as ligands and activators of pregnane X receptor (PXR), with implications in human intestinal health. In the current study, we examined the effects of simple mono-methylated indoles (MMIs) on the activity and function of PXR, using a series of human hepatic and intestinal cell models. Indoles 1-MMI and 2-MMI strongly induced CYP3A4 and MDR1 mRNAs in human intestinal adenocarcinoma cells LS180, but not in primary human hepatocytes. The levels of CYP3A4 mRNA were increased by 1-MMI and 2-MMI in wild type, but not in PXR-knock-out human hepatic progenitor HepaRG cells, implying the involvement of PXR in CYP3A4 induction by MMIs. Utilizing reporter gene assay, we observed dose-dependent activation of PXR by all MMIs, and their efficacies and potencies were comparable. Tested MMIs also displayed moderate antagonist effects on PXR, revealing about partial agonist effects of these compounds. As demonstrated using the Chromatin immunoprecipitation assay (ChIP),1-MMI increased PXR occupancy of the CYP3A4 promoter. Time-Resolved Fluorescence Resonance Energy Transfer revealed that MMIs are weak ligands of human PXR. Collectively, we show that MMIs are ligands and partial agonists of human PXR, which induce PXR-regulated genes in human intestinal cells.