Solubilization of apolipoprotein B and its specific binding by the cellular receptor for low density lipoprotein

Abstract

Low density lipoprotein (LDL) and very low density lipoprotein (VLDL) bind specifically to a receptor on fibroblasts, and it was postulated that the apoprotein of LDL (apo B) confers the specificity of cellular binding. This hypothesis was tested in the present study with a water-soluble apo B-bovine serum albumin complex. The binding of 125I-labeled apo B to cultured fibroblasts was temperature-dependent. Specific binding ranged between 183-859 ng/mg of cell protein at a concentration of 5 .mu.g/ml; at 37.degree. C, 750-2199 ng/mg was bound and internalized. The binding of apo B greatly exceeded the amount of 125I-labeled LDL bound at 4 and 37.degree. C in the same experiment. Fibroblasts from a subject homozygous for hyper-.beta.-lipoproteinemia showed minimal binding of 125I-labeled LDL, consistent with the absence of the cellular LDL receptor. Such cells also had depressed binding of 125I-labeled apo B. Lymphocytes grown in lipoprotein-deficient medium demonstrated specific binding of LDL; freshly isolated lymphocytes did not show such binding. The binding of 125I-labeled apo B to lymphocytes paralleled the binding of 125I-labeled LDL. Unlabeled LDL and apo B-albumin complex both competitively inhibited the binding of 125I-labeled apo B and 125I-labeled LDL to fibroblasts. When labeled LDL was incubated with fibroblasts for 6 h at 37.degree. C, it underwent cellular internalization and degradation, as measured by the release of 125I-labeled fragments into the medium. This degradation was inhibited by unlabeled apo B. Conversely, 125I-labeled apo B also was internalized and degraded by fibroblasts, and this process was inhibited by LDL. These findings demonstrate that apo B binds specifically to the LDL receptor and that the cellular binding of LDL is determined by this apoprotein.