Genes that are important for the detoxification of drugs and other xenobiotics show a high degree of interindividual variation attributable to regulation by diverse genetic, nongenetic, and epigenetic mechanisms including microRNAs (miRNAs). We selected a set of 56 miRNAs predicted to target the 39-untranslated region of absorption, distribution, metabolism, excretion (ADME) genes to assess their hepatic expression levels and interindividual variability in a well-documented human liver tissue cohort (n = 92), together with the well-known hepatic miRNAs miR-122, miR-21, miR-27b, and miR-148a. Quantification by stem-loop real-time reversetranscription polymerase chain reaction confirmed high expression for these microRNAs and revealed particularly strong variability of expression (>1000-fold) for miR-539, miR-200c, miR-31, miR-15a, and miR-22. Association analysis revealed a high degree of correlation among various miRNAs, suggesting coregulation. Statistical analysis considering liver donor meta-data including correction for multiple testing revealed strongly elevated levels of miR-21, miR-34a, miR-130b, and miR-132 in cholestatic liver and of miR-21 and miR-130b during inflammation, as indicated by elevated C-reactive protein levels in serum. Although none of the miRNAs was strongly associated with sex, several miRNAs, including miR-34a and miR-200a/b, were positively correlated with age. Association analysis with ADME gene expression profiles and with cytochrome P450 gene expression phenotypes (mRNA, protein, enzymatic activity) revealed numerous significant correlations. Negatively affected protein and/or activity levels were observed for CYP1A1 (e.g., miR-132, miR-142-3p, miR-21), CYP2A6 (miR-142-3p, miR-21), CYP2C19 (e.g., miR-130b, miR-185, miR-34a), and CYP2E1 (miR-10a, let-7g, miR-200c). These data should be useful to further elucidate regulatory functions of miRNAs in liver pathophysiology and regulation of ADME gene expression.
Of 57 functional human cytochrome P450 (CYP) isoforms, about a dozen liver‐expressed, highly variable forms are responsible for the biotransformation of most drugs. Owing to numerous genetic and nongenetic sources of variation, each individual possesses his/her own, rather unique, CYP profile. Here, we explore the potential of new technologies and developments in genetics, epigenetics, and the regulation of gene expression to increase our understanding of the mechanisms that lead to the enormous inter‐ and intraindividual variability of these enzymes.
In addition to its well-characterized role in the regulation of drug metabolism and transport by xenobiotics, pregnane X receptor (PXR) critically impacts on lipid homeostasis. In mice, both ligand-dependent activation and knockout of PXR were previously shown to promote hepatic steatosis. To elucidate the respective pathways in human liver, we generated clones of human hepatoma HepG2 cells exhibiting different PXR protein levels, and analyzed effects of PXR activation and knockdown on steatosis and expression of lipogenic genes. Ligand-dependent activation as well as knockdown of PXR resulted in increased steatosis in HepG2 cells. Activation of PXR induced the sterol regulatory element-binding protein (SREBP) 1-dependent lipogenic pathway via PXR-dependent induction of SREBP1a, which was confirmed in primary human hepatocytes. Inhibiting SREBP1 activity by blocking the cleavage-dependent maturation of SREBP1 protein impaired the induction of lipogenic SREBP1 target genes and triglyceride accumulation by PXR activation. On the other hand, PXR knockdown resulted in up-regulation of aldo-keto reductase (AKR) 1B10, which enhanced the acetyl-CoA carboxylase (ACC)-catalyzed reaction step of de novo lipogenesis. In a cohort of human liver samples histologically classified for non-alcoholic fatty liver disease, AKR1B10, SREBP1a and SREBP1 lipogenic target genes proved to be up-regulated in steatohepatitis, while PXR protein was reduced. In summary, our data suggest that activation and knockdown of PXR in human hepatic cells promote de novo lipogenesis and steatosis by induction of the SREBP1 pathway and AKR1B10-mediated increase of ACC activity, respectively, thus providing mechanistic explanations for a putative dual role of PXR in the pathogenesis of steatohepatitis.
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