Localization of Low Density Lipoprotein Receptors on Plasma Membrane of Normal Human Fibroblasts and Their Absence in Cells from a Familial Hypercholesterolemia Homozygote

Monolayers of normal human fibroblasts were observed to bind ferritin-labeled low density lipoprotein (LDL-ferritin) at specific receptor sites on the cell surface membrane. When fibroblasts were incubated with LDL-ferritin at 4 degrees, more than 70% of the surface-bound ferritin cores were localized by electron microscopy to short segments of the plasma membrane where the membrane appeared indented and coated on both of its sides by a fuzzy material. These membrane segments corresponded to "coated regions" previously described in other cell types. Unver the conditions of these experiments, an average of 55 LDL-ferritin particles were bound to each millimeter of plasma membrane in normal cells. In the presence of a 15-fold excess of native LDL, the number of bound ferritin cores was reduced by 75%, suggesting that the LDL-ferritin was binding to specific LDL receptor sites. Although fibroblasts from a patient with the homozygous form of familial hypercholesterolemia contained the same number of indented, coated membrane regions per millimeter of cell surface as did normal cells, no LDL-ferritin was observed to bind to the cell membrane in these mutant cells. The present ultrastructural data are consistent with previous biochemical and genetic evidence indicating that LDL exerts its regulatory action on cellular cholesterol metabolism in fibroblasts through an interaction with a specific cell surface receptor and that this receptor is defective in homozygous familial hypercholesterolemia fibroblasts. Moreover, the data suggest that the LDL receptor is localized to indented, coated regions of the plasma membrane that appear to participate in the adsorptive endocytosis of proteins.