The Interaction of the High-Density Lipoprotein with Cultured Cells of Bovine Vascular Endothelium.

Previous studies have shown that high-density lipoprotein(HDL) is mitogenic for low-density vascular endothelial cell cultures. In contrast, low-density lipoprotein (LUL) at physiological concentration is cytotoxic for this cell type [Tauber et at. (1980) J. Clin. Invest. 66, 696-709]. In order to relate these observations to the physico-chemical events occurring after the interaction of these lipoproteins with the cells, we have studied the binding, internalization, and degradation of HDL as a function of cell density and organization and have compared the results to those obtained with LDL. Cultured vascular endothelial cells bind HDL to an extent which, on a molar basis, is similar to that of LDL. Since an 80% loss of the LDL receptor sites in cells that were preexposed to LDL was not associated with a detectable decrease in HDL binding, it is likely that HDL and LDL binding sites exist on the cell surface as distinct entities. A 5-10-fold molar excess of LDL over 125I-HDL was almost as effective as a similar excess of unlabeled HDL in reducing the binding of 125I-HDL, indicating that LDL can compete with HDL for HDL- binding sites. In contrast, 125I-LDL binding and uptake were reduced by less than 20% in the presence of a 50-fold molar excess of unlabeled HDL. Both uptake and degradation of HDL were 10-20-fold lower than those of LDL. HDL degradation was only slightly or not inhibited by chloroquine at a concentration which almost fully inhibited the degradation of LDL. Confluent and highly organized endothelial cell cultures bind HDL and LDL particles to an extent which is half of the amount bound to sparse or subconlluent cultures. However, the formation of a resting endothelial cell monolayer was associated with only a twofold decrease in the uptake of HDL, as compared to a 10-20-fold decrease in the uptake and degradation of LDL. In both sparse and confluent endothelial cultures, uptake via fluid pinocytosis and the associated non-specific adsorptive endocytosis could account for most of the HDL uptake. The different pathways of internalization of HDL venus that of LDL could explain the lack of toxicity of MDL when added at high concentration to sparse and actively growing cultures. [ABSTRACT FROM AUTHOR]