The Sister of P-glycoprotein Represents the Canalicular Bile Salt Export Pump of Mammalian Liver

doi:10.1074/jbc.273.16.10046

The Sister of P-glycoprotein Represents the Canalicular Bile Salt Export Pump of Mammalian Liver*

From the ^‡Division of Clinical Pharmacology and Toxicology, Department of Medicine, University Hospital, CH-8091 Zurich, Switzerland, the ^¶Institute of Anatomy, University of Basel, CH-4056 Basel, Switzerland, the ^‖Division of Cellular and Molecular Pathology, Department of Pathology, University Hospital, CH-8091 Zurich, Switzerland, and the ^**Division of Gastroenterology, Department of Medicine, University of California, San Diego, California 92093-0813

Abstract

Canalicular secretion of bile salts is a vital function of the vertebrate liver, yet the molecular identity of the involved ATP-dependent carrier protein has not been elucidated. We cloned the full-length cDNA of the sister of P-glycoprotein (spgp; M _r ∼160,000) of rat liver and demonstrated that it functions as an ATP-dependent bile salt transporter in cRNA injectedXenopus laevis oocytes and in vesicles isolated from transfected Sf9 cells. The latter demonstrated a 5-fold stimulation of ATP-dependent taurocholate transport as compared with controls. This spgp-mediated taurocholate transport was stimulated solely by ATP, was inhibited by vanadate, and exhibited saturability with increasing concentrations of taurocholate (K _m ≃ 5 μm). Furthermore, spgp-mediated transport rates of various bile salts followed the same order of magnitude as ATP-dependent transport in canalicular rat liver plasma membrane vesicles, i.e.taurochenodeoxycholate > tauroursodeoxycholate = taurocholate > glycocholate = cholate. Tissue distribution assessed by Northern blotting revealed predominant, if not exclusive, expression of spgp in the liver, where it was further localized to the canalicular microvilli and to subcanalicular vesicles of the hepatocytes by in situ immunofluorescence and immunogold labeling studies. These results indicate that the sister of P-glycoprotein is the major canalicular bile salt export pump of mammalian liver.

Footnotes

↵* This work was supported by grants from the Deutsche Forschungsgemeinschaft (to T. G.), the Swiss National Science Foundation (to B. H., B. S., L. L., J. R. and P. J. M.), and the Cloetta Foundation Zurich (to B. H.). Work in San Diego was supported in part by National Institutes of Health Grant Dk 21506 (to A. F. H.), as well as a grant in aid from the Falk Foundation e.V., Freiburg, Germany.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

This study has been presented in part at the 48th Annual Meeting of the American Association for the Study of Liver Diseases in Chicago, November 7–11, 1997, and published in abstract form (49).
↵§ To whom correspondence should be addressed. Tel.: 0041-1-255- 3652; Fax: 0041-1-255-4411; E-mail:bstieger{at}kpt.unizh.ch.
↵1 The abbreviations use are: BSEP, bile salt export pump of mammalian liver; ABC, ATP binding cassette; BAT1, bile acid transporter of S. cerevisiae; mdr1a/1b, rodent isoforms of the multidrug resistance P-glycoprotein; mdr2, phospholipid-transporting mdr isoform of rodent liver; MRP1, human multidrug resistance protein 1; MRP2/mrp2, human/rat canalicular multidrug resistance protein; spgp, sister of P-glycoprotein; BSEP/spgp, identity of spgp and BSEP; PFIC, progressive familial intrahepatic cholestasis; bp base pair(s); kb, kilobase pair(s).
↵2 BSEP rather than cBAT (9) or cBST (8) is chosen as abbreviation, since BAT has already been reserved for “basic amino acid transporter” (47) and BST for the sphingosine-phosphate lyase gene (48). Furthermore, the term “export pump” points to ATP as the primary driving force of the canalicular bile salt transporter.
↵3 The nucleotide sequence of the full-length spgp cDNA clone coding for the deduced amino acid sequence is deposited in the GenBank^TM under accession number U69487.
↵4 R. J. Thompson, personal communication.
- Received December 8, 1997.
- Revision received January 30, 1998.
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