Brown M S, Goldstein J L
Cell. 1976 Dec;9(4 PT 2):663-74. doi: 10.1016/0092-8674(76)90130-6.
A new type of mutation that affects a discrete step in the process of adsorptive endocytosis has been identified in one of 22 strains of fibroblasts derived from subjects with the clinical phenotype of homozygous familial hypercholesterolemia (FH). In this unique mutant strain (derived from subject J.D.), the cell surface receptor for plasma low density lipoprotein (LDL) was able to bind 125I-labeled LDL normally, but internalization of the receptor-bound lipoprotein failed to occur. Thus the defect in this strain differed from the defects found in the fibroblasts from the other 21 FH homozygote strains in which the binding of 125I-LDL to the receptor was either absent (receptor-negative) or markedly reduced (receptor-defective). The LDL receptor in the J.D. cells exhibited first, normal kinetics of high affinity binding at 4 degrees C and 37 degrees C; second, normal specificity (affinity for LDL more than 200 fold greater than for HDL); third, normal susceptibility to feed-back suppression by 25-hydroxycholesterol plus cholesterol; and fourth, a normal rate of turnover when the cells were treated with cycloheximide. Despite its normal ability to bind 125I-LDL, the LDL receptor in the J.D. cells failed to transport its LDL into the cell, and degradation of the lipoprotein in cellular lysosomes therefore did not occur. As a result, the lipoprotein did not suppress 3-hydroxy-3-methylglutaryl coenzyme A reductase activity; nor did it activate cholesteryl ester formation. A phenocopy of the internalization defect in the J.D. cells could be created by incubation of normal fibroblasts with N-ethyl maleimide, an agent that did not affect 125I-LDL binding to the receptor but blocked its subsequent internalization by the cell. The current data indicate that at least two gene products are necessary for the adsorptive endocytosis of LDL: one that is required for the binding of the lipoprotein, and one that promotes the internalization of the receptor-bound ligand.
在来自纯合子家族性高胆固醇血症（FH）临床表型个体的22株成纤维细胞系中，发现了一种新型突变，该突变影响吸附性胞吞作用过程中的一个离散步骤。在这个独特的突变细胞系（来自个体J.D.）中，血浆低密度脂蛋白（LDL）的细胞表面受体能够正常结合125I标记的LDL，但受体结合的脂蛋白未能发生内化。因此，该细胞系中的缺陷不同于其他21株FH纯合子细胞系中发现的缺陷，在其他细胞系中，125I-LDL与受体的结合要么不存在（受体阴性），要么显著降低（受体缺陷）。J.D.细胞中的LDL受体首先在4℃和37℃时表现出正常的高亲和力结合动力学；其次，具有正常的特异性（对LDL的亲和力比对HDL大200多倍）；第三，对25-羟基胆固醇加胆固醇的反馈抑制具有正常的敏感性；第四，当细胞用环己酰亚胺处理时，具有正常的周转速率。尽管J.D.细胞中的LDL受体具有正常结合125I-LDL的能力，但它未能将其LDL转运到细胞内，因此脂蛋白在细胞溶酶体中的降解也未发生。结果，脂蛋白没有抑制3-羟基-3-甲基戊二酰辅酶A还原酶活性；也没有激活胆固醇酯的形成。通过用N-乙基马来酰亚胺孵育正常成纤维细胞，可以产生J.D.细胞内化缺陷的拟表型，该试剂不影响125I-LDL与受体的结合，但会阻止细胞随后对其进行内化。目前的数据表明，LDL的吸附性胞吞作用至少需要两种基因产物：一种是脂蛋白结合所必需的，另一种是促进受体结合配体内化的。
