title: Bile acid transport in sister of P-glycoprotein (ABCB11) knockout mice.
abstract: In vertebrates, bile flow is essential for movement of water and solutes across liver canalicular membranes. In recent years, the molecular motor of canalicular bile acid secretion has been identified as a member of the ATP binding cassette transporter (ABC) superfamily, known as sister of P-glycoprotein (Spgp) or bile salt export pump (Bsep, ABCB11). In humans, mutations in the BSEP gene are associated with a very low level of bile acid secretion and severe cholestasis. However, as reported previously, because the spgp(-)(/)(-) knockout mice do not express severe cholestasis and have substantial bile acid secretion, we investigated the "alternative transport system" that allows these mice to be physiologically relatively normal. We examined the expression levels of several ABC transporters in spgp(-)(/)(-) mice and found that the level of multidrug resistance Mdr1 (P-glycoprotein) was strikingly increased while those of Mdr2, Mrp2, and Mrp3 were increased to only a moderate extent. We hypothesize that an elevated level of Mdr1 in the spgp(-)(/)(-) knockout mice functions as an alternative pathway to transport bile acids and protects hepatocytes from bile acid-induced cholestasis. In support of this hypothesis, we showed that plasma membrane vesicles isolated from a drug resistant cell line expressing high levels of P-glycoprotein were capable of transporting bile acids, albeit with a 5-fold lower affinity compared to Spgp. This finding is the first direct evidence that P-glycoprotein (Mdr1) is capable of transporting bile acids.
authors.affiliation: British Columbia Cancer Research Center, British Columbia Cancer Agency, Vancouver, British Columbia V5Z 1L3, Canada.
authors.forename: Ping
authors.initials: P
authors.lastname: Lam
authors.forename: Renxue
authors.initials: R
authors.lastname: Wang
authors.forename: Victor
authors.initials: V
authors.lastname: Ling
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000378:metabolism; D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters / Q000235:genetics* / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001646:Bile / Q000378:metabolism; D001647:Bile Acids and Salts / Q000378:metabolism*; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D013656:Taurocholic Acid / Q000378:metabolism
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters; C490549:Abcb11 protein, mouse; D001647:Bile Acids and Salts; D013656:Taurocholic Acid; D000255:Adenosine Triphosphate