de Beer M C, Durbin D M, Cai L, Mirocha N, Jonas A, Webb N R, de Beer F C, van Der Westhuyzen D R
Department of Internal Medicine, University of Kentucky Medical Center, Lexington, Kentucky 40536, USA.
J Biol Chem. 2001 May 11;276(19):15832-9. doi: 10.1074/jbc.M100228200. Epub 2001 Feb 9.
High density lipoprotein (HDL) represents a mixture of particles containing either apoA-I and apoA-II (LpA-I/A-II) or apoA-I without apoA-II (LpA-I). Differences in the function and metabolism of LpA-I and LpA-I/A-II have been reported, and studies in transgenic mice have suggested that apoA-II is pro-atherogenic in contrast to anti-atherogenic apoA-I. The molecular basis for these observations is unclear. The scavenger receptor BI (SR-BI) is an HDL receptor that plays a key role in HDL metabolism. In this study we investigated the abilities of apoA-I and apoA-II to mediate SR-BI-specific binding and selective uptake of cholesterol ester using reconstituted HDLs (rHDLs) that were homogeneous in size and apolipoprotein content. Particles were labeled in the protein (with (125)I) and in the lipid (with [(3)H]cholesterol ether) components and SR-BI-specific events were analyzed in SR-BI-transfected Chinese hamster ovary cells. At 1 microg/ml apolipoprotein, SR-BI-mediated cell association of palmitoyloleoylphosphatidylcholine-containing AI-rHDL was significantly greater (3-fold) than that of AI/AII-rHDL, with a lower K(d) and a higher B(max) for AI-rHDL as compared with AI/AII-rHDL. Unexpectedly, selective cholesterol ester uptake from AI/AII-rHDL was not compromised compared with AI-rHDL, despite decreased binding. The efficiency of selective cholesterol ester uptake in terms of SR-BI-associated rHDL was 4-5-fold greater for AI/AII-rHDL than AI-rHDL. These results are consistent with a two-step mechanism in which SR-BI binds ligand and then mediates selective cholesterol ester uptake with an efficiency dependent on the composition of the ligand. ApoA-II decreases binding but increases selective uptake. These findings show that apoA-II can exert a significant influence on selective cholesterol ester uptake by SR-BI and may consequently influence the metabolism and function of HDL, as well as the pathway of reverse cholesterol transport.
高密度脂蛋白（HDL）是一种颗粒混合物，包含载脂蛋白A-I和载脂蛋白A-II（LpA-I/A-II）或不含载脂蛋白A-II的载脂蛋白A-I（LpA-I）。已有报道称LpA-I和LpA-I/A-II在功能和代谢上存在差异，对转基因小鼠的研究表明，与具有抗动脉粥样硬化作用的载脂蛋白A-I相比，载脂蛋白A-II具有促动脉粥样硬化作用。这些观察结果的分子基础尚不清楚。清道夫受体BI（SR-BI）是一种HDL受体，在HDL代谢中起关键作用。在本研究中，我们使用大小和载脂蛋白含量均一的重组HDL（rHDL），研究了载脂蛋白A-I和载脂蛋白A-II介导SR-BI特异性结合以及选择性摄取胆固醇酯的能力。对颗粒的蛋白质成分（用（125）I标记）和脂质成分（用[（3）H]胆固醇醚标记）进行标记，并在转染了SR-BI的中国仓鼠卵巢细胞中分析SR-BI特异性事件。在载脂蛋白浓度为1μg/ml时，SR-BI介导的含棕榈酰油酰磷脂酰胆碱的AI-rHDL与细胞的结合显著高于（3倍）AI/AII-rHDL，与AI/AII-rHDL相比，AI-rHDL的解离常数（K(d)）更低，最大结合量（B(max)）更高。出乎意料的是，尽管AI/AII-rHDL与细胞的结合减少，但其从AI/AII-rHDL中选择性摄取胆固醇酯的能力并未受到影响。就与SR-BI相关的rHDL而言，AI/AII-rHDL选择性摄取胆固醇酯的效率比AI-rHDL高4至5倍。这些结果与一个两步机制一致，即SR-BI先结合配体，然后介导选择性胆固醇酯摄取，其效率取决于配体的组成。载脂蛋白A-II降低结合但增加选择性摄取。这些发现表明，载脂蛋白A-II可对SR-BI选择性摄取胆固醇酯产生显著影响，进而可能影响HDL的代谢和功能，以及逆向胆固醇转运途径。
