Inefficient internalization of receptor-bound low density lipoprotein in human carcinoma A-431 cells.

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Abstract
Human epithelioid carcinoma A-431 cells express unusually large numbers of receptors for the polypeptide hormone epidermal growth factor. This cell line also expresses 5- to 10-fold more low density lipoprotein (LDL) receptors per cell than human fibroblasts or Chinese hamster ovary (CHO) cells. As visualized with an LDL-ferritin conjugate, the LDL receptors in A-431 cells appeared in clusters that were distributed uniformly over the cell surface, occurring over flat regions of the membrane as well as over the abundant surface extensions. Only 4% of the LDL receptors were located in coated pits. The LDL receptors in A-431 cells showed the same affinity and specificity as the LDL receptors in human fibroblasts and other cell types. They were subject to feedback regulation by sterols in the same manner as the LDL receptors in other cells. In contrast to other cell types in which the receptor-bound LDL is internalized with high efficiency, in the A-431 cells only a small fraction of the receptor-bound LDL entered the cell. In CHO cells .apprx. 66% of the LDL receptors were located over coated regions of membrane, and the efficiency of LDL internalization was correspondingly 10-fold higher than in A-431 cells. The rate of LDL internalization apparently is proportional to the number of LDL receptors in coated pits, and the inefficiency of internalization in the A-431 cells is caused by a limitation in the ability of these cells to incorporate their LDL receptors into coated pits.