INTERACTION OF LPB, LPB-E, LPB-C-III, AND LPB-C-III-E LIPOPROTEINS WITH THE LOW-DENSITY-LIPOPROTEIN RECEPTOR OF HELA-CELLS

In this study we measured the binding parameters of different apolipoprotein (apo) B-containing lipoproteins to the low density lipoprotein (LDL) receptor of HeLa cells. Our goal was to determine the respective roles of the different apolipoproteins of these particles, with particular emphasis on apos B, E, and C-III. Very low density lipoprotein from hypertriglyceridemic subjects (B to E molar ratio = 1:16) bound to HeLa cells with an affinity higher than that of LDL, but the apparent number of binding sites per cell was lower. Because of the heterogeneity of these lipoproteins, which were isolated by ultracentrifugation, we used immunoaffinity chromatography to define these particles on the basis of their apolipoprotein content. Lipoprotein B (LpB) particles that contained apo B as their sole apolipoprotein had lower affinity for the LDL receptor than did total LDL but had an apparently higher number of binding sites. The presence of apo E of phenotype E3/E3 or E4/E4 on one particle increased the affinity of the apo B-containing lipoprotein for the LDL receptor. The apparent number of binding sites decreased, probably due to the fact that a lipoprotein particle containing multiple copies of apo E bound to more than one molecule of LDL receptor. Interaction with several LDL receptors would also explain the higher binding affinity that we observed. The calculated number of binding sites expressed for each apo E molecule is close to the number of binding sites for lipoproteins containing only apo B (LpB or LDL), indicating that each apo E can interact with one LDL receptor. When the apo E phenotype was E2/E2, the LpB:E lipoproteins did not bind to the LDL receptor. A similar result was obtained with the LpB:C-III particles. These data suggest that apos E and C-III impaired the interaction of apo B with the LDL receptor. It is likely that in LpB:E, only apo E (in the case of E3/E3 or E4/E4 phenotype) participates in the LDL receptor binding.