Histochemical characterization of low density lipoprotein receptors in internalization-defective familial hypercholesterolemia.

We studied the localization of low density lipoprotein (LDL) bound to the receptors on the cultured fibroblasts from a patient (MN) with homozygous familial hypercholesterolemia and a defect in internalization of LDL and compared the localization with normal fibroblasts and with those from another internalization-defective cell, GM2408A. Monolayers of cells were cultured with lipoprotein-deficient human serum, and the cells were incubated with 125I- or ferritin-labeled LDL. The LDL binding was observed by autoradiography or by an electron microscope. Autoradiographs of bound 125I-LDL confirmed that MN's cells could not internalize LDL inside the cell at 37 degrees C. In these cells, ferritin LDL was found mainly in noncoated regions at 4 degrees C; it was not found in endocytic vesicles after incubation at 37 degrees C. Ferritin cores that bound on the surface of the cells from the normal subject, from GM2408A, from MN, and from MN's parents, were counted and quantitatively analyzed at 4 degrees C. In normal cells, 62% of the ferritin cores were bound in the coated pits; in MN's cells, only 11% of the ferritin LDL was found in the coated pits; in the GM2408A cells, 12% of the ferritin LDL was found in the coated pits; in the cells from MN's parents, 40% of the ferritin LDL was found on the coated pits. The results indicate that the internalization defect in MN's cells is the same as that in the GM2408A cells; neither can localize the LDL and LDL receptor complex in coated pits.