title: The structure and function of Niemann-Pick C1-like 1 protein.
abstract: OBJECTIVE
  Intestinal absorption and biliary excretion of cholesterol represent two major pathways by which the body regulates cholesterol homeostasis. Niemann-Pick C1-like 1 (NPC1L1) is a polytopic transmembrane protein containing a sterol-sensing domain of unknown function. In 2004, NPC1L1 was identified to be essential for intestinal cholesterol absorption, a process that is sensitive to a cholesterol absorption inhibitor ezetimibe. This review summarizes recent studies on NPC1L1 function and proposes a model for NPC1L1-dependent cholesterol uptake.
  
  
  RESULTS
  Cell culture experiments have shown that NPC1L1 mediates cellular uptake of various sterols but seems to have lower affinity to plant sterols than cholesterol. Transgenic animal studies have demonstrated that hepatic NPC1L1 has the potential to regulate biliary cholesterol excretion. Cholesterol and many transcriptional factors appear to regulate NPC1L1 gene expression. NPC1L1 protein is enriched in the apical membrane of polarized cells and its intracellular itineraries are clearly regulated by cholesterol availability. Evidence suggests cholesterol-regulated clathrin-mediated endocytosis is likely the cellular basis for NPC1L1-dependent cholesterol uptake, which may reconcile disagreement regarding NPC1L1 subcellular localization.
  
  
  CONCLUSIONS
  NPC1L1 may have evolved at two sites (apical membrane of enterocytes and canalicular membrane of hepatocytes) to mediate cholesterol uptake through a clathrin-mediated endocytic process, protecting the body against fecal and biliary loss of cholesterol.
authors.affiliation: Department of Pathology Section on Lipid Sciences, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1040, USA. lyu@wfubmc.edu
authors.forename: Liqing
authors.initials: L
authors.lastname: Yu
mesh_terms: D000818:Animals; D001659:Biliary Tract / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D007408:Intestinal Absorption; D008099:Liver / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D013347:Subcellular Fractions / Q000378:metabolism
chemical_list: D008565:Membrane Proteins; D026901:Membrane Transport Proteins; C410800:NPC1L1 protein, human; C482445:Npc1l1 protein, mouse; D002784:Cholesterol