The role of different P-glycoproteins in hepatobiliary secretion of fluorescently labeled short-chain phospholipids

Class III P-glycoproteins (Pgps) mediate biliary phosphatidylcholine (PC) s ecretion. Recent findings that class I P-glycoproteins are able to transpor t several short-chain phospholipid analogues raises questions about the rol e of these Pgps in physiological lipid transport. We investigated the bilia ry secretion of C6-7-nitro-2,1,3-benzoxadiazol-4-yl (NBD)-labeled ceramide and its metabolites in Mdr1a/b and Mdr2 knockout mice compared to control m ice. Biliary secretion of these NBD-Lipids was unaffected in Mdr1a/b -/- mi ce. Thus neither Mdr1a nor Mdr1b Pgp mediates biliary secretion of these li pids, In contrast, secretion of all three NBD-labeled short-chain phospholi pids was significantly reduced in Mdr2 -/- mice. As in vitro studies reveal ed that Mdr2 Pgp is not able to translocate these lipid analogues, we hypot hesized that Mdr2 -/- mice had a reduced PC content of the exoplasmic canal icular membrane leaflet so that extraction of the short-chain lipid probes from this membrane by canalicular bile salts was impaired. To investigate t his possibility we studied the bile salt-mediated extraction of natural sph ingomyelin (SM) and NBD-labeled short-chain SM from small unilamellar vesic les of different lipid composition. Natural SM could be extracted by the bi le salt tauroursodeoxycholate from vesicles con taining PC, cholesterol (CH OL), and SM (1:2:2) but not from vesicles containing only SM and CHOL (3:2) , NBD-labeled short-chain SM could be extracted from vesicles containing PC while its extraction from pure SM:CHOL vesicles was reduced by 65%. These data confirm that the efficiency of NBD-SM extraction depends on the lipid composition and suggest that the canalicular membrane outer leaflet of Mdr2 -/- mice has a reduced PC content.-Frijters, C. M. G., C. J. Tuijn, R. Ott enhoff, B. N. Zegers, A. K Green, and R. P. J. Oude Elferink. Role of diffe rent P-glycoproteins in hepatobiliary secretion of fluorescently labeled sh ort-chain phospholipids.