MODULATION OF LIPOPROTEIN B BINDING TO THE LDL RECEPTOR BY EXOGENOUS LIPIDS AND APOLIPOPROTEIN-CI, APOLIPOPROTEIN-CII, APOLIPOPROTEIN-CIII,AND APOLIPOPROTEIN-E

We have recently shown that apo B-containing lipoproteins isolated by immunoaffinity chromatography bind to the LDL receptor with an affinit y dependent on their apo E or apo CIII content. However, these lipopro teins-LpB:E, LpB:CIII, and LpB:CIII:E-isolated from whole plasma have variable lipid acid apolipoprotein contents, and it is difficult to co nsider each parameter separately, particularly because an increase in the apo CIII content is always associated with an increase in the cont ent of other C apolipoproteins. Therefore, we used affinity-purified L pB free of other apolipoproteins. Lipid content of LpB was increased b y 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 adde d to LpB and TG-LpB and their associations to the lipoprotein were ass essed by gel filtration, nondenaturing electrophoresis, and immunoblot ting. Molar ratios of 6 (ape E), 30 (ape CII), 20 (ape CIII), and 30 ( apo CI) for 1 apo B were obtained. The association of apo CII to LpB a nd TG-LpB induced modifications to the LpB structure and a redistribut ion of lipids and apolipoproteins on the lipoprotein particles. The bi nding of these LpBs and TG-LpBs with and without added apo CI, CII, CI II, and E was tested at 4 degrees C on the LDL receptors of HeLa cells . The increased content of lipids reduced TG-LpB binding to the LDL re ceptor. Addition of apo CIII to LpB decreased its affinity, although t his 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 an d apo CIII. The apo CIII effect was specific and cannot be obtained wi th apo CI. With apo CII, an inhibitory effect can also be obtained but to a lesser extent than with apo CIII. At 37 degrees C the C apolipop roteins 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 degrees C but markedly increased their catabolism at 37 degrees C.