Effect of cholesterol feeding on the susceptibility of lipoproteins to oxidative modification

In previous studies we have shown that the liver endothelial and Kupffer cells in hypercholesterolemic rabbits are very active in endocytosis of low-density lipoprotein (LDL) and beta-very-low-density lipoprotein (beta-VLDL) (Nenseter et al. (1992) J. Lipid Res. 33, 867-877; Gudmundsen et al. (1993) J. Lipid Res. 34, 589-600). These data raised the question whether subfractions of LDL and beta-VLDL were modified in vivo to forms recognized by the scavenger/oxidized LDL receptors of the non-parenchymal liver cells. The purpose of the present study was to address this question by assessing the effect of cholesterol feeding on the susceptibility of the lipoproteins to oxidative modification in vitro. In addition, the effect of HDL on the lipid peroxidation of LDL was evaluated. LDL and beta-VLDL were isolated from rabbits given a diet supplemented with cholesterol (2% w/w) for 3 weeks. The extent of Cu(2+)-catalyzed oxidation of the lipoproteins was compared with that of LDL from control-fed rabbits. Extent of oxidation assessed by formation of conjugated dienes, lipid peroxides, thiobarbituric acid-reactive substances, relative electrophoretic mobility and uptake of lipoproteins by J774 macrophages suggested that LDL and beta-VLDL from the hypercholesterolemic rabbits were more susceptible to oxidation than LDL from normolipidemic rabbits. HDL protected LDL and beta-VLDL from lipid peroxidation in vitro. Taken together, the increased susceptibility of LDL and beta-VLDL to oxidative modification in vitro, combined with the low levels of alpha-tocopherol, and the reduced ratio of HDL to LDL cholesterol observed in the hypercholesterolemic rabbits, and the protective effect of HDL on the lipid peroxidation of LDL, support the probability that oxidative modification of LDL and beta-VLDL occur in vivo in the hypercholesterolemic rabbits.