Division of Clinical Pharmacology and Toxicology, University Hospital Zurich, Zurich, Switzerland.
J Hepatol. 2011 Dec;55(6):1368-76. doi: 10.1016/j.jhep.2011.04.014. Epub 2011 May 19.
BACKGROUND & AIMS: 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.
胆小管磷脂酰胆碱和胆固醇的分泌需要 ATP 结合盒转运蛋白的协调作用：胆汁盐输出泵（Bsep）用于胆汁盐（BS）和磷脂酰胆碱转位蛋白多药耐药蛋白 2（Mdr2）。分泌后，磷脂酰胆碱和 BS 形成混合胶束，作为胆小管胆固醇的受体。我们已经表明，胆小管肝质膜（cLPM）含有富含神经鞘磷脂和胆固醇的脂筏。由于 BS 具有去污剂的特性，并且其在胆小管中的浓度非常高，因此我们测试了胆小管膜中是否含有 BS 抗性微区的假设。
在 4°C 下用不同的 BS 或去污剂提取分离的 cLPM，并在蔗糖分步梯度中进行浮选，然后进行 Western 印迹和脂质组成分析。
用牛磺胆酸钠浓度递增孵育 cLPM 揭示了 BS 抗性微区的存在。这些微区在存在和不存在脂质的情况下都可以用不同的 BS 找到，并且包含筏标记蛋白 Reggie-1/-2 和小窝蛋白-1 和胆小管转运蛋白 Bsep、Mrp2 和 Abcg5，后者独立于脂质的存在。BS 抗性微区主要含有胆固醇、磷脂酰胆碱和磷脂酰乙醇胺。用类似于大鼠胆汁的不同 BS 的混合物提取 cLPM 显示出类似的微区组成。
cLPM 含有 BS 抗性微区，可能保护 cLPM 免受 BS 的去污剂作用。不同 BS 的组合对微区组成没有协同作用。
