Possible Functional Interactions of Apolipoprotein B-100 Segments That Associate With Cell Proteoglycans and the ApoB/E Receptor

The interaction of apoE lipoproteins with cells appears to be mediated by an association with basic sequences of proteoglycans and the apoB/E receptor. ApoB-100 has basic sequences, homologous with those of apoE, that form part of the apoB/E receptor-binding domain. These sequences of apoB-100 also interact with proteoglycans. We investigated whether such segments, in analogy with apoE, could act cooperatively on LDL interactions with proteoglycans and the receptor. As a model we used the two most basic regions of apoB-100, 3147 through 3157 and 3359 through 3367, connected by three glycines (3145-3157-GGG-3359-3367). Such segments may be proximal in LDL by the presence of a disulfide bridge between Cys(3167) and Cys(3297). The apoB heterodimer but not the separated monomers inhibited 125I-LDL degradation in fibroblasts and THP-1 cells by 50% at approximately 11 mumol/L. The heterodimer affinity with arterial proteoglycans was closer to that of LDL and higher than that of the individual peptides. The heterodimer appears to bind specifically to THP-1 cells, with a Kd of 6.2 x 10(-8) mol/L and a Bmax of 1.3 x 10(6) molecules/cell. Monoclonal antibody C-7, which recognizes the apoB receptor, inhibited the binding to cells. Treatment of fibroblasts with chondroitinase ABC or chlorate decreased 125I-LDL degradation markedly. Hydrolysis of pericellular proteoglycans of fibroblasts by chondroitinases reduced mostly the low-affinity, high-capacity component of LDL binding. This compartment appears to hold 70% of the cell-associated LDL when internalization is inhibited at 4 degrees C. Therefore, cell-surface chondroitin sulfate/dermatan sulfate proteoglycans appear to modulate binding and receptor-mediated internalization of LDL. This may be caused, at least in part, by the association of proteoglycans with the apoB-100 segments 3145 through 3157 and 3359 through 3367.