Schouten D, Kleinherenbrink-Stins M, Brouwer A, Knook D L, Van Berkel T J
Center for Bio-Pharmaceutical Sciences, University of Leiden, Sylvius Laboratories, The Netherlands.
Biochem J. 1988 Dec 1;256(2):615-21. doi: 10.1042/bj2560615.
The interaction of apolipoprotein (apo) E-free high-density lipoprotein (HDL) with parenchymal, endothelial and Kupffer cells from liver was characterized. At 10 min after injection of radiolabelled HDL into rats, 1.0 +/- 0.1% of the radioactivity was associated with the liver. Subfractionation of the liver into parenchymal, endothelial and Kupffer cells, by a low-temperature cell-isolation procedure, indicated that 77.8 +/- 2.4% of the total liver-associated radioactivity was recovered with parenchymal cells, 10.8 +/- 0.8% with endothelial cells and 11.3 +/- 1.7% with Kupffer cells. It can be concluded that inside the liver a substantial part of HDL becomes associated with endothelial and Kupffer cells in addition to parenchymal cells. With freshly isolated parenchymal, endothelial and Kupffer cells the binding properties for apo E-free HDL were determined. For parenchymal, endothelial and Kupffer cells, evidence was obtained for a saturable, specific, high-affinity binding site with Kd and Bmax. values respectively in the ranges 10-20 micrograms of HDL/ml and 25-50 ng of HDL/mg of cell protein. In all three cell types nitrosylated HDL and low-density lipoproteins did not compete for the binding of native HDL, indicating that lipids and apo B are not involved in specific apo E-free HDL binding. Very-low-density lipoproteins (VLDL), however, did compete for HDL binding. The competition of VLDL with apo E-free HDL could not be explained by label exchange or by transfer of radioactive lipids or apolipoproteins between HDL and VLDL, and it is therefore suggested that competition is exerted by the presence of apo Cs in VLDL. The results presented here provide evidence for a high-affinity recognition site for HDL on parenchymal, liver endothelial and Kupffer cells, with identical recognition properties on the three cell types. HDL is expected to deliver cholesterol from peripheral cells, including endothelial and Kupffer cells, to the liver hepatocytes, where cholesterol can be converted into bile acids and thereby irreversibly removed from the circulation. The observed identical recognition properties of the HDL high-affinity site on liver parenchymal, endothelial and Kupffer cells suggest that one receptor may mediate both cholesterol efflux and cholesterol influx, and that the regulation of this bidirectional cholesterol (ester) flux lies beyond the initial binding of HDL to the receptor.
对无载脂蛋白（apo）E的高密度脂蛋白（HDL）与肝脏实质细胞、内皮细胞和库普弗细胞之间的相互作用进行了表征。给大鼠注射放射性标记的HDL后10分钟，1.0±0.1%的放射性与肝脏相关。通过低温细胞分离程序将肝脏亚分成实质细胞、内皮细胞和库普弗细胞，结果表明，与肝脏相关的总放射性中，77.8±2.4%存在于实质细胞中，10.8±0.8%存在于内皮细胞中，11.3±1.7%存在于库普弗细胞中。可以得出结论，在肝脏内部，除了实质细胞外，HDL的很大一部分还与内皮细胞和库普弗细胞相关。利用新鲜分离的实质细胞、内皮细胞和库普弗细胞，测定了它们对无apo E的HDL的结合特性。对于实质细胞、内皮细胞和库普弗细胞，均获得了存在可饱和、特异性、高亲和力结合位点的证据，其解离常数（Kd）和最大结合量（Bmax）值分别在10 - 20微克HDL/毫升和25 - 50纳克HDL/毫克细胞蛋白范围内。在所有三种细胞类型中，亚硝基化HDL和低密度脂蛋白不竞争天然HDL的结合，这表明脂质和载脂蛋白B不参与无apo E的HDL特异性结合。然而，极低密度脂蛋白（VLDL）确实竞争HDL的结合。VLDL与无apo E的HDL之间的竞争无法用标记交换或HDL与VLDL之间放射性脂质或载脂蛋白的转移来解释，因此提示竞争是由VLDL中apo Cs的存在所施加的。本文给出的结果为HDL在肝脏实质细胞、肝内皮细胞和库普弗细胞上存在高亲和力识别位点提供了证据，这三种细胞类型具有相同的识别特性。HDL有望将胆固醇从包括内皮细胞和库普弗细胞在内的外周细胞转运至肝脏肝细胞，在肝细胞中胆固醇可转化为胆汁酸，从而不可逆地从循环中清除。在肝脏实质细胞、内皮细胞和库普弗细胞上观察到的HDL高亲和力位点相同的识别特性表明，一种受体可能介导胆固醇流出和胆固醇流入，并且这种双向胆固醇（酯）通量的调节超出了HDL与受体的初始结合。
