title: Bile salt-stimulated phospholipid efflux mediated by ABCB4 localized in nonraft membranes.
abstract: ABCB4 is necessary for the secretion of phospholipids from hepatocytes into bile and for the protection of cell membranes against bile salts. Lipid rafts are plasma membrane microdomains containing high contents of cholesterol and sphingolipids, which are separated by Triton X-100 extraction or OptiPrep gradient centrifugation. In this study, we investigated the relationship between the function of ABCB4 and lipid rafts using mouse canalicular membranes and HEK293 cells stably expressing ABCB4. ABCB4 and ABCB1 were mainly distributed in nonraft membranes. The expression of ABCB4, but not ABCB1, led to significant increases in the phosphatidylcholine (PC), phosphatidylethanolamine (PE), and sphingomyelin (SM) contents in nonraft membranes and further enrichment of SM and cholesterol in raft membranes. The ABCB4-mediated efflux of PC, PE, and SM was significantly stimulated by taurocholate, while the efflux of PE and SM was much less than that of PC. This ABCB4-mediated efflux was completely abolished by BODIPY-verapamil, which hardly partitioned into raft membranes. In addition, ABCB1 and ABCB4 mediated the efflux of rhodamine 123 and rhodamine 6G from nonraft membranes, which was not affected by taurocholate. We conclude that ABCB4 located in nonrafts, but not in rafts, is predominantly involved in the efflux of phospholipids and other substrates.
authors.affiliation: Department of Pharmacy, Shiga University of Medical Science Hospital, Otsu City, Shiga 520-2192, Japan.
authors.forename: Shin-ya
authors.initials: SY
authors.lastname: Morita
authors.forename: Tadanori
authors.initials: T
authors.lastname: Tsuda
authors.forename: Manami
authors.initials: M
authors.lastname: Horikami
authors.forename: Reiko
authors.initials: R
authors.lastname: Teraoka
authors.forename: Shuji
authors.initials: S
authors.lastname: Kitagawa
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Terada
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000235:genetics / Q000378:metabolism*; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D005786:Gene Expression Regulation; D057809:HEK293 Cells; D022781:Hepatocytes; D006801:Humans; D021962:Membrane Microdomains / Q000378:metabolism; D051379:Mice; D010743:Phospholipids / Q000378:metabolism*; D020112:Rhodamine 123 / Q000378:metabolism; D012235:Rhodamines / Q000378:metabolism
chemical_list: C513055:ABCB1 protein, human; D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D001647:Bile Acids and Salts; D010743:Phospholipids; D012235:Rhodamines; C026188:rhodamine 6G; D020112:Rhodamine 123; C095122:multidrug resistance protein 3