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.

中文翻译：

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3'非翻译区有助于PXR配体下调孕烷X受体（PXR）的表达和糖皮质激素上调。

孕烷X受体（PXR）是异生物代谢的主要调节剂。PXR本身受各种信号分子（包括糖皮质激素）的控制。此外，在转录水平上已经提出了负反馈调节。我们检查了NR1I2 mRNA和microRNA的3'-非翻译区（3'-UTR）在PXR和糖皮质激素受体（GR）介导的NR1I2基因表达调控中的参与。在一组14个人肝细胞培养物中，发现PXR配体显着下调NR1I2 mRNA表达。同样，PCN在C57 / BL6小鼠肝脏中下调PXR。在将全长3'-UTR克隆到荧光素酶报道基因或表达载体中的机制研究中，我们表明3'-UTR降低了PXR的表达。从测试的miRNA中，miR-18a-5p抑制NR1I2表达和CYP3A4基因诱导。重要的是，我们在用PXR配体利福平治疗6小时后观察到miR-18a-5p表达的显着上调，这表明肝细胞中NR1I2负反馈调节的潜在机制。此外，糖皮质激素不仅通过启动子区域还通过3'-UTR调节上调了NR1I2表达，这可能涉及miR-18a-5p的下调。我们得出的结论是，miR-18a-5p参与其配体对NR1I2表达的下调，并参与糖皮质激素在肝细胞中对NR1I2 mRNA表达的上调。这表明肝细胞中NR1I2负反馈调节的潜在推测机制。此外，糖皮质激素不仅通过启动子区域还通过3'-UTR调节上调了NR1I2表达，这可能涉及miR-18a-5p的下调。我们得出的结论是，miR-18a-5p参与其配体对NR1I2表达的下调，并参与糖皮质激素在肝细胞中对NR1I2 mRNA表达的上调。这表明肝细胞中NR1I2负反馈调节的潜在推测机制。此外，糖皮质激素不仅通过启动子区域还通过3'-UTR调节上调了NR1I2表达，这可能涉及miR-18a-5p的下调。我们得出的结论是，miR-18a-5p参与其配体对NR1I2表达的下调，并参与糖皮质激素在肝细胞中对NR1I2 mRNA表达的上调。