Interaction of bile salts with rat canalicular membrane vesicles: Evidence for bile salt resistant microdomains

Abstract

Canalicular phosphatidylcholine and cholesterol secretion requires the coordinate action of the ATP binding cassette transporters: the bile salt export pump (Bsep) for bile salts (BS) and the phosphatidylcholine translocator multidrug resistance protein 2 (Mdr2). After their secretion, phosphatidylcholine and BS form mixed micelles acting as acceptors for canalicular cholesterol. We have shown that the canalicular liver plasma membrane (cLPM) contains lipid raft enriched in sphingomyelin and cholesterol. As BS have detergent properties and their concentration in the canaliculus is very high, we tested the hypothesis that the canalicular membrane contains BS resistant microdomains. Isolated cLPMs were extracted at 4°C with different BS or detergents and subjected to flotation in sucrose step gradients followed by Western blotting and lipid composition analysis. Incubating cLPMs with increasing taurocholate concentrations revealed the presence of BS resistant microdomains. These microdomains were found with different BS in the presence and absence of lipids and contained the raft markers reggie-1/-2 and caveolin-1 and canalicular transporters Bsep, Mrp2, and Abcg5, the latter independent of the presence of lipids. BS resistant microdomains contain mainly cholesterol, phosphatidylcholine, and phosphatidylethanolamine. Extraction of cLPMs with a mixture of different BS similar to rat bile revealed a comparable microdomain composition. cLPM contains BS resistant microdomains potentially protecting the cLPM against the detergent action of BS. Combination of different BS has no synergistic effect on microdomain composition.