title: NPC2 regulates biliary cholesterol secretion via stimulation of ABCG5/G8-mediated cholesterol transport.
abstract: OBJECTIVE
  Biliary cholesterol secretion helps maintain cholesterol homeostasis; it is regulated by the cholesterol exporter adenosine triphosphate-binding cassettes G5 and G8 (ABCG5/G8) and the cholesterol importer Niemann-Pick C1-like 1 (NPC1L1). We studied another putative regulator of cholesterol secretion into bile, Niemann-Pick C2 (NPC2)--a cholesterol-binding protein secreted by the biliary system--and determined its effects on transporter-mediated biliary secretion of cholesterol.
  
  
  METHODS
  Mice with hepatic knockdown of Npc2 or that overexpressed NPC2 were created using adenovirus-mediated gene transfer; biliary lipids were characterized. The effects of secreted NPC2 on cholesterol transporter activity were examined in vitro using cells that overexpressed ABCG5/G8 or NPC1L1.
  
  
  RESULTS
  Studies of mice with altered hepatic expression of NPC2 revealed that this expression positively regulates the biliary secretion of cholesterol, supported by the correlation between levels of NPC2 protein and cholesterol in human bile. In vitro analysis showed that secreted NPC2 stimulated ABCG5/G8-mediated cholesterol efflux but not NPC1L1-mediated cholesterol uptake. Consistent with these observations, no significant changes in biliary cholesterol secretion were observed on hepatic overexpression of NPC2 in ABCG5/G8-null mice, indicating that NPC2 requires ABCG5/G8 to stimulate cholesterol secretion. Analyses of NPC2 mutants showed that the stimulatory effect of biliary NPC2 was independent of the function of lysosomal NPC2 as a regulator of intracellular cholesterol trafficking.
  
  
  CONCLUSIONS
  NPC2 is a positive regulator of biliary cholesterol secretion via stimulation of ABCG5/G8-mediated cholesterol transport.
authors.affiliation: Department of Pharmacy, The University of Tokyo Hospital, Faculty of Medicine, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
authors.forename: Yoshihide
authors.initials: Y
authors.lastname: Yamanashi
authors.forename: Tappei
authors.initials: T
authors.lastname: Takada
authors.forename: Takashi
authors.initials: T
authors.lastname: Yoshikado
authors.forename: Jun-Ichi
authors.initials: J
authors.lastname: Shoda
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Suzuki
mesh_terms: D000071018:ATP Binding Cassette Transporter, Subfamily G, Member 5; D000071019:ATP Binding Cassette Transporter, Subfamily G, Member 8; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000818:Animals; D001646:Bile / Q000737:chemistry*; D001692:Biological Transport; D002784:Cholesterol / Q000378:metabolism*; D004195:Disease Models, Animal; D005786:Gene Expression Regulation; D006801:Humans; D008074:Lipoproteins / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger; D033921:Vesicular Transport Proteins / Q000096:biosynthesis / Q000235:genetics*
chemical_list: C462600:ABCG5 protein, mouse; C462601:ABCG8 protein, mouse; D000071018:ATP Binding Cassette Transporter, Subfamily G, Member 5; D000071019:ATP Binding Cassette Transporter, Subfamily G, Member 8; D018528:ATP-Binding Cassette Transporters; D008074:Lipoproteins; C497045:Npc2 protein, mouse; D012333:RNA, Messenger; D033921:Vesicular Transport Proteins; D002784:Cholesterol