Binding and Degradation of Low Density Lipoproteins by Cultured Human Fibroblasts

125I-Labeled low density lipoproteins were found to associate with monolayers of cultured normal fibroblasts by two processes—one of high affinity and one of low affinity. The high affinity association appeared to represent binding of the low density lipoprotein to specific receptor sites on the cell surface. This binding process exhibited saturation kinetics at low concentrations of the lipoprotein and competition by related molecules such as very low density lipoproteins. In addition, this process was stimulated by the presence of calcium in the culture medium and could be destroyed by limited treatment of the cells with pronase. The other process, designated low affinity uptake, may represent nonspecific endocytosis since the uptake was proportional to the lipoprotein concentration in the medium with no apparent saturation and because it showed no competition by very low density lipoproteins, no stimulation by calcium, and no destruction by pronase treatment. The 125I-labeled low density lipoproteins associated with normal cells by either the high or low affinity process were degraded by proteolysis to trichloroacetic acid-soluble material, most of which was 125I-tyrosine. In normal cells, binding of low density lipoproteins to the high affinity membrane receptor sites appears to serve two functions: (a) it results in suppression of the synthesis of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-controlling enzyme in cholesterol biosynthesis, and (b) it facilitates the degradation of low density lipoproteins when they are present in the culture medium at low concentrations (i.e. high affinity degradation). Cultured cells from subjects with the homozygous form of familial hypercholesterolemia, which were found to lack the high affinity binding process, were resistant to suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by low density lipoproteins and were also deficient in high affinity degradation. On the other hand, these mutant cells showed a normal low affinity uptake of low density lipoproteins and were able to degrade these lipoproteins when they were present in the culture medium at high concentrations. The possibility is raised, therefore, that a prerequisite for the regulation of cholestero-genesis in cultured fibroblasts is the initial binding of low density lipoproteins to the high affinity surface receptor sites and that a defect in this process represents the primary genetic abnormality in the disorder familial hypercholesterolemia.