Functional expression of the canalicular bile salt export pump of human liver.
OBJECTIVE
Hepatic bile salt secretion is an essential function of vertebrate liver. Rat and mouse bile salt export pump (Bsep) are adenosine triphosphate (ATP)-dependent bile salt transporters. Mutations in human BSEP were identified as the cause of progressive familial intrahepatic cholestasis type 2. BSEP protein is highly identical with its rat and mouse orthologs and has not yet been functionally characterized; the effect of BSEP mutations on its function has also not been studied. Therefore, the aim of this study was to functionally characterize human BSEP.


METHODS
Complementary DNA for BSEP was isolated from human liver and expressed with the baculovirus system in Sf9 cells. ATP-dependent bile salt transport assays were performed with Sf9 cell vesicles expressing BSEP and a rapid filtration assay.


RESULTS
Cloning of human BSEP required the inactivation of a bacterial cryptic promoter motif within its coding region. BSEP expressed in Sf9 cells transports different bile salts in an ATP-dependent manner with Michaelis constant values as follows: taurocholate, 7.9 +/- 2.1 micromol/L; glycocholate, 11.1 +/- 3.3 micromol/L; taurochenodeoxycholate, 4.8 +/- 1.7 micromol/L; tauroursodeoxycholate, 11.9 +/- 1.8 micromol/L. The rank order of the intrinsic clearance of bile salts was taurochenodeoxycholate > taurocholate > tauroursodeoxycholate > glycocholate.


CONCLUSIONS
This study characterizes human BSEP as an ATP-dependent bile salt export pump with transport properties similar to its rat and mouse orthologs. Expression of BSEP in Sf9 cells will enable functional characterization of the consequences of mutations in the human BSEP gene.
Noé
Johannes
J
Division of Clinical Pharmacology and Toxicology, Department of Medicine, University Hospital, Zurich, Switzerland.
Stieger
Bruno
B
Meier
Peter J
PJ
D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000502:physiology; D000818:Animals; D016367:Baculoviridae / Q000235:genetics; D001483:Base Sequence / Q000235:genetics; D001647:Bile Acids and Salts / Q000378:metabolism; D001648:Bile Canaliculi / Q000378:metabolism*; D001692:Biological Transport / Q000502:physiology; D002460:Cell Line; D005260:Female; D018014:Gene Transfer Techniques; D005822:Genetic Vectors; D006801:Humans; D007313:Insecta / Q000166:cytology; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008875:Middle Aged; D008969:Molecular Sequence Data
C093491:ABCB11 protein, human; D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters; C490549:Abcb11 protein, mouse; C490548:Abcb11 protein, rat; D001647:Bile Acids and Salts; D000255:Adenosine Triphosphate