Ji Z S, Fazio S, Mahley R W
Gladstone Institute of Cardiovascular Disease, Cardiovascular Research Institute, San Francisco, California 94141-9100.
J Biol Chem. 1994 May 6;269(18):13421-8.
The initial step in the clearance of apolipoprotein (apo) E-enriched remnant lipoproteins from the plasma appears to be sequestration within the liver mediated by their binding to heparan sulfate proteoglycans (HSPG). The surface-bound remnants are believed to be internalized by their interaction with the low density lipoprotein (LDL) receptor-related protein or by the LDL receptor. Cholesterol-induced rabbit beta-very low density lipoproteins (beta-VLDL) enriched in human apoE3 display 4-5-fold enhanced binding to cultured cells. The present study attempts to determine whether recessive versus dominant type III hyperlipoproteinemia might be explained, at least in part, by a variable interaction of the mutant forms of apoE with the HSPG and impaired uptake. The beta-VLDL+apoE2(Arg158-->Cys), which is associated with recessive type III hyperlipoproteinemia, bound more poorly than beta-VLDL+apoE3 but still possessed significant enhanced binding (approximately 2-2.5-fold compared with beta-VLDL without added apoE) to HepG2 and McA-RH7777 cells. In comparison, beta-VLDL+apoE(Arg142-->Cys), beta-VLDL+apoE(Arg145-->Cys), and beta-VLDL+apoE-Leiden, which are associated with dominant type III hyperlipoproteinemia, bound more poorly. This same hierarchy of binding and uptake was determined by [14C]oleate incorporation into cholesteryl esters in LDL receptor-negative cells and by secretion of apoE3 and the variant apoE forms from McA-RH7777 cells. Furthermore, the enhanced binding of the apoE-enriched beta-VLDL was almost totally inhibited by heparinase treatment of the cells, and the basal binding activity was inhibited by 80-90% following addition of an LDL receptor antibody capable of blocking receptor-ligand interaction. The beta-VLDL enriched in apoE or apoE-dimyristoylphosphatidylcholine complexes bound to isolated HSPG from McA-RH7777 cells or the rat liver to a very similar degree. Likewise, the binding of beta-VLDL plus the various forms of apoE to the LDL receptor-related protein on ligand blots paralleled the results of other studies. In conclusion, all of the type III hyperlipoproteinemic apoE variants are defective in displaying enhanced binding to HSPG and in the cellular uptake initiated by HSPG. However, apoE2(Arg158-->Cys) displayed more activity than the variants associated with the dominant forms of type III hyperlipoproteinemia. The hierarchy of binding and uptake was as follows: apoE3 > apoE2(Arg158-->Cys) > apoE(Arg145-->Cys) > apoE(Arg142-->Cys) approximately apoE-Leiden (the latter two usually displaying very little, if any, enhanced binding and uptake). Thus, a correlation exists between the mode of expression of type III hyperlipoproteinemia and the binding and uptake of the specific apoE mutation.
从血浆中清除富含载脂蛋白（apo）E的残余脂蛋白的初始步骤似乎是通过它们与硫酸乙酰肝素蛋白聚糖（HSPG）结合介导的肝脏内隔离。表面结合的残余物被认为通过与低密度脂蛋白（LDL）受体相关蛋白相互作用或通过LDL受体而被内化。富含人apoE3的胆固醇诱导的兔β-极低密度脂蛋白（β-VLDL）与培养细胞的结合增强了4-5倍。本研究试图确定隐性与显性III型高脂蛋白血症是否至少部分可以通过apoE突变形式与HSPG的可变相互作用和摄取受损来解释。与隐性III型高脂蛋白血症相关的β-VLDL + apoE2（Arg158→Cys）的结合比β-VLDL + apoE3差，但与未添加apoE的β-VLDL相比，与HepG2和McA-RH7777细胞的结合仍显著增强（约2-2.5倍）。相比之下，与显性III型高脂蛋白血症相关的β-VLDL + apoE（Arg142→Cys）、β-VLDL + apoE（Arg145→Cys）和β-VLDL + apoE-Leiden的结合较差。通过将[14C]油酸酯掺入LDL受体阴性细胞中的胆固醇酯以及通过McA-RH7777细胞分泌apoE3和变体apoE形式确定了相同的结合和摄取层次结构。此外，用肝素酶处理细胞几乎完全抑制了富含apoE的β-VLDL的增强结合，并且加入能够阻断受体-配体相互作用的LDL受体抗体后，基础结合活性被抑制80-90％。富含apoE或apoE-二肉豆蔻酰磷脂酰胆碱复合物的β-VLDL与从McA-RH7777细胞或大鼠肝脏分离的HSPG的结合程度非常相似。同样，β-VLDL加各种形式的apoE与配体印迹上的LDL受体相关蛋白的结合与其他研究结果平行。总之，所有III型高脂蛋白血症apoE变体在显示与HSPG的增强结合以及由HSPG启动的细胞摄取方面都存在缺陷。然而，apoE2（Arg158→Cys）比与显性形式的III型高脂蛋白血症相关的变体表现出更多活性。结合和摄取的层次结构如下：apoE3> apoE2（Arg158→Cys）> apoE（Arg145→Cys）> apoE（Arg142→Cys）≈apoE-Leiden（后两者通常显示很少，如果有的话，增强的结合和摄取）。因此，III型高脂蛋白血症的表达模式与特定apoE突变的结合和摄取之间存在相关性。
