title: Bile salt-dependent efflux of cellular phospholipids mediated by ATP binding cassette protein B4.
abstract: Human ABCB4 (multidrug resistance [MDR]3 P-glycoprotein) is expressed in the canalicular membrane of the hepatocyte. ABCB4 has been shown to be required for phosphatidylcholine (PC) secretion into the bile and to translocate PC across the plasma membrane. To further investigate the function of ABCB4, we established a cell line stably expressing ABCB4 (human embryonic kidney [HEK]/ABCB4). The efflux of phospholipids from HEK/ABCB4 cells was remarkably increased by the addition of taurocholate. In addition, the cholesterol efflux from HEK/ABCB4 cells was also enhanced in the presence of taurocholate. Light scattering measurements suggested that the taurocholate monomer plays an important role in ABCB4-mediated lipid secretion. On the other hand, the efflux of phospholipids and cholesterol was not mediated by ABCB1 (MDR1) even in the presence of taurocholate. Taurocholate promoted the efflux of phospholipids and cholesterol from HEK/ABCB4 cells more efficiently than glycocholate and cholate. ABCB4-K435M and ABCB4-K1075M, Walker A lysine mutants, did not mediate the phospholipid and cholesterol efflux in the presence of taurocholate, suggesting that ATP hydrolysis is essential for the efflux. Verapamil completely inhibited the taurocholate-dependent efflux of phospholipids and cholesterol from HEK/ABCB4 cells. Mass spectrometry revealed that, in the presence of taurocholate, HEK/ABCB4 cells preferentially secreted PC compared to sphingomyelin. PC vesicles induced cholesterol diffusion from cell membrane, but did not accept cholesterol from ABCB4.
  
  
  CONCLUSIONS
  ABCB4 mediates the efflux of phospholipids into the canalicular lumen in the presence of bile salts, and plays a crucial role in bile formation and lipid homeostasis.
authors.affiliation: Laboratory of Cellular Biochemistry, Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kyoto, Japan.
authors.forename: Shin-ya
authors.initials: SY
authors.lastname: Morita
authors.forename: Aya
authors.initials: A
authors.lastname: Kobayashi
authors.forename: Yasukazu
authors.initials: Y
authors.lastname: Takanezawa
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Kioka
authors.forename: Tetsurou
authors.initials: T
authors.lastname: Handa
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Arai
authors.forename: Michinori
authors.initials: M
authors.lastname: Matsuo
authors.forename: Kazumitsu
authors.initials: K
authors.lastname: Ueda
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000235:genetics / Q000378:metabolism*; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D001647:Bile Acids and Salts / Q000494:pharmacology* / Q000502:physiology; D002460:Cell Line; D006801:Humans; D007668:Kidney; D013058:Mass Spectrometry; D010743:Phospholipids / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D018528:ATP-Binding Cassette Transporters; D001647:Bile Acids and Salts; D010743:Phospholipids; D011994:Recombinant Proteins; C095122:multidrug resistance protein 3