Pregnane X receptor (PXR) is the major regulator of xenobiotic metabolism. PXR itself is controlled by various signaling molecules including glucocorticoids. Moreover, negative feed-back regulation has been proposed at the transcriptional level. We examined the involvement of the 3'-untranslated region (3'-UTR) of NR1I2 mRNA and microRNAs in PXR- and glucocorticoid receptor (GR)-mediated regulation of NR1I2 gene expression. PXR ligands were found to significantly downregulate NR1I2 mRNA expression in a set of 14 human hepatocyte cultures. Similarly, PXR was downregulated by PCN in the C57/BL6 mice liver. In mechanistic studies with the full-length 3'-UTR cloned into luciferase reporter or expression vectors, we showed that the 3'-UTR reduces PXR expression. From the miRNAs tested, miR-18a-5p inhibited both NR1I2 expression and CYP3A4 gene induction. Importantly, we observed significant upregulation of miR-18a-5p expression 6 h after treatment with the PXR ligand rifampicin, which indicates a putative mechanism underlying NR1I2 negative feed-back regulation in hepatic cells. Additionally, glucocorticoids upregulated NR1I2 expression not only through the promoter region but also via 3'-UTR regulation, which likely involves downregulation of miR-18a-5p. We conclude that miR-18a-5p is involved in the down-regulation of NR1I2 expression by its ligands and in the upregulation of NR1I2 mRNA expression by glucocorticoids in hepatic cells.

Department of Pharmacology and Toxicology Faculty of Pharmacy in Hradec Kralove Charles University Hradec Kralove CZ 500 05 Czech Republic

Department of Pharmacology Faculty of Medicine in Hradec Kralove Charles University Hradec Kralove CZ 500 03 Czech Republic

Institute of Clinical Biochemistry and Diagnostics University Hospital Hradec Kralove Hradec Kralove CZ 500 05 Czech Republic

IRMB INSERM University Montpellier Montpellier France

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