Modulation of Lipoprotein B Binding to the LDL Receptor by Exogenous Lipids and Apolipoproteins CI, CII, CIII, and E

Abstract

Abstract We have recently shown that apo B–containing lipoproteins isolated by immunoaffinity chromatography bind to the LDL receptor with an affinity dependent on their apo E or apo CIII content. However, these lipoproteins—LpB:E, LpB:CIII, and LpB:CIII:E—isolated from whole plasma have variable lipid and apolipoprotein contents, and it is difficult to consider each parameter separately, particularly because an increase in the apo CIII content is always associated with an increase in the content of other C apolipoproteins. Therefore, we used affinity-purified LpB free of other apolipoproteins. Lipid content of LpB was increased by incubation with a lipid emulsion, and this triglyceride-enriched LpB was named TG-LpB. Free apo CI, apo CII, apo CIII, and apo E were added to LpB and TG-LpB and their associations to the lipoprotein were assessed by gel filtration, nondenaturing electrophoresis, and immunoblotting. Molar ratios of 6 (apo E), 30 (apo CII), 20 (apo CIII), and 30 (apo CI) for 1 apo B were obtained. The association of apo CII to LpB and TG-LpB induced modifications to the LpB structure and a redistribution of lipids and apolipoproteins on the lipoprotein particles. The binding of these LpBs and TG-LpBs with and without added apo CI, CII, CIII, and E was tested at 4°C on the LDL receptors of HeLa cells. The increased content of lipids reduced TG-LpB binding to the LDL receptor. Addition of apo CIII to LpB decreased its affinity, although this decrease was lower than that observed with LpB:CIII prepared from whole plasma. Apo CIII almost completely abolished the interaction of TG-LpB with the receptor, indicating a synergistic effect of lipids and apo CIII. The apo CIII effect was specific and cannot be obtained with apo CI. With apo CII, an inhibitory effect can also be obtained but to a lesser extent than with apo CIII. At 37°C the C apolipoproteins decreased the catabolism of LpB and TG-LpB by the LDL receptor of fibroblasts. Addition of apo E to either LpB or TG-LpB had a small effect on the binding of the enriched lipoproteins at 4°C but markedly increased their catabolism at 37°C.