Analysis of a mutant strain of human fibroblasts with a defect in the internalization of receptor-bound low density lipoprotein.

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.