title: Molecular mechanism of nuclear translocation of an orphan nuclear receptor, SXR.
abstract: The steroid and xenobiotic receptor (SXR) is an orphan nuclear receptor that plays a key role in the regulation of xenobiotic response by controlling the expression of drug metabolizing and clearance enzymes. We observed that pregnane X receptor (PXR), the mouse ortholog of SXR, was retained in the cytoplasm of hepatic cells of untreated mice, whereas PXR was translocated to the nucleus after administration of a ligand, pregnenolone 16 alpha-carbonitrile. To understand the molecular mechanisms underlying the xenochemical-dependent nuclear translocation of SXR, we identified the signal sequence of SXR that regulates its nuclear translocation; using an in vitro expression system, we allocated the nuclear localization signal (NLS) to amino acid residues 66 to 92 within the DNA binding domain of SXR. The NLS of SXR is characterized as the bipartite type, and is recognized by the three molecular species of importin alpha: Rch1 (PTAC58), NPI1, and Qip1, in the presence of PTAC97 of importin beta to target the nuclear pore. The nuclear translocation of SXR was observed as an essential regulatory event for transcription of its target genes such as CYP3A4. These results strongly suggest that the molecular mechanism of the nuclear import of SXR was different from that of another xenosensor, the constitutively active receptor, whose translocation into the nucleus is mediated by a leucine-rich xenochemical response signal in its ligand binding domain.
authors.affiliation: Research Institute, Saitama Cancer Center, Ina-machi, Saitama, Japan.
authors.forename: Katsuyoshi
authors.initials: K
authors.lastname: Kawana
authors.forename: Togo
authors.initials: T
authors.lastname: Ikuta
authors.forename: Yasuhito
authors.initials: Y
authors.lastname: Kobayashi
authors.forename: Osamu
authors.initials: O
authors.lastname: Gotoh
authors.forename: Ken
authors.initials: K
authors.lastname: Takeda
authors.forename: Kaname
authors.initials: K
authors.lastname: Kawajiri
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects; D006367:HeLa Cells; D006801:Humans; D008099:Liver / Q000378:metabolism*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008845:Microinjections; D010446:Peptide Fragments / Q000378:metabolism; D000077297:Pregnane X Receptor; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D011987:Receptors, Steroid / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000494:pharmacology; D013347:Subcellular Fractions; D014158:Transcription, Genetic / Q000187:drug effects; D014162:Transfection; D014407:Tumor Cells, Cultured; D028901:alpha Karyopherins / Q000378:metabolism
chemical_list: D010446:Peptide Fragments; D000077297:Pregnane X Receptor; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; D011993:Recombinant Fusion Proteins; D028901:alpha Karyopherins; D003577:Cytochrome P-450 Enzyme System; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human