Neutral lipid transfer and lipolysis convert high molecular weight LDL from cholesterol-fed nonhuman primates towards normal: a molecular analysis

Abstract

In cynomolgus monkeys ( Macaca fascicularis) fed an atherogenic diet, large, cholesterol ester-rich LDL ( M r > 3.5 · 10 6) are found at the same time that the plasma triacylglycerol levels are low. We studied whether the presence of higher concentrations of triacylglycerol-rich lipoproteins (VLDL) during in vitro incubations would allows depletion from LDL of cholesterol ester and a decreased LDL molecular weight. Three high M r LDL ( M r = (3.7−4.8) · 10 6), rich in cholesterol ester (50 ± 1.4% by weight), were isolated from three animals by zonal ultracentrifugation, and were then incubated with human VLDL at 37°C for 18 h in lipoprotein-deficient human plasma containing neutral lipid transfer activity. After incubation, modified LDL (M-LDL) was isolated by zonal ultracentrifugation. M-LDL was triacylglycerol-rich (36 ± 5% by weight) and cholesterol ester-poor (20 ± 3%), and cholesterol ester had transferred into VLDL. Purified lipoprotein lipase was added to the M-LDL, and triacylglycerol was hydrolyzed. The size of the post-lipolysis M-LDL (Mp-LDL) particles became smaller (mean diameters of 253 Å and 228 Å for two native LDLs and 215 Å and 193 Å for Mp-LDL, respectively). Both analytical and zonal ultracentrifugation showed Mp-LDL to be more dense than native LDL. Estimated molecular weights for Mp-LDL were 40%–50% less than that of the original LDL, and fell within the molecular weight range for normal human and monkey LDL. Lipid exchanges, but not apoprotein transfers, were responsible for LDL remodelling, as supported by three separate methods of analysis. Cholesterol ester losses accounted for about two-thirds of the molecular weight decrease. These in vitro results suggest that cholesterol ester enrichment of apoprotein B lipoprotein particles can be reversed by providing adequate levels of VLDL in the presence of neutral lipid transfer processes and lipolytic activity.