title: Cellular Cholesterol Accumulation Facilitates Ubiquitination and Lysosomal Degradation of Cell Surface-Resident ABCA1.
abstract: OBJECTIVE
  By excreting cellular cholesterol to apolipoprotein A-I, ATP-binding cassette transporter A1 (ABCA1) mediates the biogenesis of high-density lipoprotein in hepatocytes and prevents foam cell formation from macrophages. We recently showed that cell surface-resident ABCA1 (csABCA1) undergoes ubiquitination and later lysosomal degradation through the endosomal sorting complex required for transport system. Herein, we investigated the relevance of this degradation pathway to the turnover of csABCA1 in hypercholesterolemia.
  
  
  RESULTS
  Immunoprecipitation and cell surface-biotinylation studies with HepG2 cells and mouse peritoneal macrophages showed that the ubiquitination level and degradation of csABCA1 were facilitated by treatment with a liver X receptor (LXR) agonist and acetylated low-density lipoprotein. The effects of an LXR agonist and acetylated low-density lipoprotein on the degradation of csABCA1 were repressed completely by treatment with bafilomycin, an inhibitor of lysosomal degradation, and by depletion of tumor susceptibility gene 101, a major component of endosomal sorting complex required for transport-I. RNAi analysis indicated that LXRβ inhibited the accelerated lysosomal degradation of csABCA1 by the LXR agonist, regardless of its transcriptional activity. Cell surface coimmunoprecipitation with COS1 cells expressing extracellularly hemagglutinin-tagged ABCA1 showed that LXRβ interacted with csABCA1 and inhibited the ubiquitination of csABCA1. Immunoprecipitates with anti-ABCA1 antibodies from the liver plasma membranes showed less LXRβ and a higher ubiquitination level of ABCA1 in high-fat diet-fed mice than in normal chow-fed mice.
  
  
  CONCLUSIONS
  Under conditions of high cellular cholesterol content, csABCA1 became susceptible to ubiquitination by dissociation of LXRβ from csABCA1, which facilitated the lysosomal degradation of csABCA1 through the endosomal sorting complex required for transport system.
authors.affiliation: From the Laboratory of Molecular Pharmacokinetics, Department of Pharmacy, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
authors.forename: Tadahaya
authors.initials: T
authors.lastname: Mizuno
authors.affiliation: From the Laboratory of Molecular Pharmacokinetics, Department of Pharmacy, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Bunkyo-ku, Tokyo, Japan. hayapi@mol.f.u-tokyo.ac.jp.
authors.forename: Hisamitsu
authors.initials: H
authors.lastname: Hayashi
authors.affiliation: From the Laboratory of Molecular Pharmacokinetics, Department of Pharmacy, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Kusuhara
mesh_terms: D064286:ATP Binding Cassette Transporter 1 / Q000378:metabolism*; D000818:Animals; D016632:Apolipoprotein A-I / Q000378:metabolism; D050197:Atherosclerosis / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism*; D059305:Diet, High-Fat; D018450:Disease Progression; D056827:Endosomal Sorting Complexes Required for Transport / Q000378:metabolism; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000378:metabolism; D006801:Humans; D006937:Hypercholesterolemia / Q000378:metabolism*; D008075:Lipoproteins, HDL / Q000096:biosynthesis; D008077:Lipoproteins, LDL / Q000008:administration & dosage; D000071518:Liver X Receptors; D008247:Lysosomes / Q000378:metabolism; D017737:Macrophages, Peritoneal / Q000378:metabolism; D008297:Male; D008810:Mice, Inbred C57BL; D057093:Orphan Nuclear Receptors / Q000819:agonists; D054875:Ubiquitination
chemical_list: D064286:ATP Binding Cassette Transporter 1; D016632:Apolipoprotein A-I; D056827:Endosomal Sorting Complexes Required for Transport; D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D002784:Cholesterol
keywords: ABCA1; LXRβ; atherosclerosis; protein degradation; ubiquitin