title: The alternatively spliced murine pregnane X receptor isoform, mPXR(delta171-211) exhibits a repressive action.
abstract: The orphan nuclear receptor pregnane X receptor regulates enzymes and transport proteins involved in the detoxification and clearance of numerous endobiotic and xenobiotic compounds, including pharmaceutical agents. Multiple alternatively spliced pregnane X receptor isoforms have been identified which are significantly expressed in humans and mice (up to 30% of the total pregnane X receptor transcript), however, little is known about their biological action. We explored functional differences between the major mouse pregnane X receptor isoforms mPXR(431) and mPXR(Delta171-211) that lacks 41 amino acids adjacent to the ligand-binding pocket. Transient transfection assays showed that mPXR(Delta171-211) reduced the basal transcription of cytochrome P450 3A4 and the drug transporter P-glycoprotein/Multi Drug Resistance Protein 1 and directly repressed the regulatory effects of mPXR(431) on these genes. Replacement of the mPXR(Delta171-211) DNA-binding domain with that of GAL4 showed mPXR(Delta171-211) retained its repressive role independent of binding to PXR responsive elements located within the cytochrome P450 3A4 and Multi Drug Resistance Protein 1 regulatory regions. Use of the histone deacetylase inhibitor, trichostatin A, demonstrated that the repressive function of mPXR(Delta171-211) acts independently of histone acetylation state. Protein interaction assays revealed mPXR(Delta171-211) and mPXR(431) differentially bind the obligatory heterodimer partner retinoid X receptor. Furthermore, mPXR(431) and mPXR(Delta171-211) proteins could heterodimerize. These studies demonstrate that the variant mouse PXR isoform, mPXR(Delta171-211), has a distinct repressive function from mPXR(431) in regulating genes encoding important drug metabolizing enzymes and transport proteins.
authors.affiliation: Cancer Pharmacology Unit, ANZAC Research Institute, Hospital Road, Concord RG Hospital, NSW 2139, Australia. mmatic@med.usyd.edu.au
authors.forename: Marko
authors.initials: M
authors.lastname: Matic
authors.forename: Anthony P
authors.initials: AP
authors.lastname: Corradin
authors.forename: Maria
authors.initials: M
authors.lastname: Tsoli
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Clarke
authors.forename: Patsie
authors.initials: P
authors.lastname: Polly
authors.forename: Graham R
authors.initials: GR
authors.lastname: Robertson
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000235:genetics / Q000378:metabolism*; D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D002522:Chlorocebus aethiops; D051544:Cytochrome P-450 CYP3A / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins; D015536:Down-Regulation; D056572:Histone Deacetylase Inhibitors / Q000494:pharmacology; D006801:Humans; D008297:Male; D051379:Mice; D009928:Organ Specificity; D000077297:Pregnane X Receptor; D054730:Protein Interaction Domains and Motifs; D020033:Protein Isoforms / Q000235:genetics / Q000502:physiology; D055503:Protein Multimerization; D011987:Receptors, Steroid / Q000737:chemistry / Q000235:genetics* / Q000502:physiology*; D011993:Recombinant Fusion Proteins; D050436:Regulatory Elements, Transcriptional / Q000235:genetics; D047488:Retinoid X Receptors / Q000378:metabolism; D016415:Sequence Alignment; D017384:Sequence Deletion
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D004268:DNA-Binding Proteins; D056572:Histone Deacetylase Inhibitors; D000077297:Pregnane X Receptor; D020033:Protein Isoforms; D011987:Receptors, Steroid; D011993:Recombinant Fusion Proteins; D047488:Retinoid X Receptors; D051544:Cytochrome P-450 CYP3A