title: The pregnane X receptor down-regulates organic cation transporter 1 (SLC22A1) in human hepatocytes by competing for ("squelching") SRC-1 coactivator.
abstract: The organic cation transporter 1 (OCT1) transports cationic drugs into hepatocytes. The high hepatic expression of OCT1 is controlled by the HNF4α and USF transcription factors. Pregnane X receptor (PXR) mediates induction of the principal xenobiotic metabolizing enzymes and transporters in the liver. Here, we have assessed the down-regulation of OCT1 expression by PXR activation.
  
  
  
  We used primary human hepatocytes and related cell lines to measure OCT1 expression and activity, by assaying MPP(+) accumulation. Western blotting, qRT-PCR, the OCT1 promoter gene reporter constructs and chromatin immunoprecipitation assays were also used.
  
  
  
  OCT1 mRNA in human hepatocytes was down-regulated along with reduced [(3) H]MPP(+) accumulation in differentiated HepaRG cells after treatment with rifampicin. Rifampicin and hyperforin as well as the constitutively active PXR mutant T248D suppressed activity of the 1.8 kb OCT1 promoter construct in gene reporter assays. Silencing of both PXR and HNF4α in HepaRG cells blocked the PXR ligand-mediated down-regulation of OCT1 expression. The mutation of HNF4α and USF1 (E-box) responsive elements reversed the PXR-mediated inhibition in gene reporter assays. Chromatin immunoprecipitation assays indicated that PXR activation sequestrates the SRC-1 coactivator from the HNF4α response element and E-box of the OCT1 promoter. Consistent with these findings, exogenous overexpression of the SRC-1, but not the PGC1α coactivator, relieved the PXR-mediated repression of OCT1 transactivation.
  
  
  
  PXR ligands reduced the HNF4α-mediated and USF-mediated transactivation of OCT1 gene expression by competing for SRC-1 and decreased delivery of a model OCT1 substrate into hepatocytes.
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove, Czech Republic.
authors.forename: Lucie
authors.initials: L
authors.lastname: Hyrsova
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove, Czech Republic.
authors.forename: Tomas
authors.initials: T
authors.lastname: Smutny
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove, Czech Republic.
authors.forename: Alejandro
authors.initials: A
authors.lastname: Carazo
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove, Czech Republic.
authors.forename: Stefan
authors.initials: S
authors.lastname: Moravcik
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove, Czech Republic.
authors.forename: Jana
authors.initials: J
authors.lastname: Mandikova
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove, Czech Republic.
authors.forename: Frantisek
authors.initials: F
authors.lastname: Trejtnar
authors.affiliation: INSERM, U1183, Institute for Regenerative Medicine and Biotherapy, Montpellier, France.|Université de Montpellier, Montpellier, France.
authors.forename: Sabine
authors.initials: S
authors.lastname: Gerbal-Chaloin
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove, Czech Republic.
authors.forename: Petr
authors.initials: P
authors.lastname: Pavek
mesh_terms: D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000378:metabolism*; D006801:Humans; D056920:Nuclear Receptor Coactivator 1 / Q000378:metabolism*; D050811:Octamer Transcription Factor-1 / Q000235:genetics / Q000378:metabolism*; D010696:Phloroglucinol / Q000031:analogs & derivatives / Q000494:pharmacology; D000077297:Pregnane X Receptor; D011987:Receptors, Steroid / Q000378:metabolism*; D012293:Rifampin / Q000494:pharmacology; D013329:Structure-Activity Relationship; D013729:Terpenes / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D050811:Octamer Transcription Factor-1; C494165:POU2F1 protein, human; D000077297:Pregnane X Receptor; D011987:Receptors, Steroid; D013729:Terpenes; D010696:Phloroglucinol; C538859:NCOA1 protein, human; D056920:Nuclear Receptor Coactivator 1; C001654:hyperforin; D012293:Rifampin