Enhanced Cholesterol Efflux by Tyrosyl Radical-oxidized High Density Lipoprotein Is Mediated by Apolipoprotein AI-AII Heterodimers

Abstract

Myeloperoxidase secreted by phagocytes in the artery wall may be a catalyst for lipoprotein oxidation. High density lipoprotein (HDL) oxidized by peroxidase-generated tyrosyl radical has a markedly enhanced ability to deplete cultured cells of cholesterol. We have investigated the structural modifications in tyrosylated HDL responsible for this effect. Spherical reconstituted HDL (rHDL) containing the whole apolipoprotein (apo) fraction of tyrosylated HDL reproduced the ability of intact tyrosylated HDL to enhance cholesterol efflux from cholesterol-loaded human fibroblasts when reconstituted with the whole lipid fraction of either HDL or tyrosylated HDL. Free apoAI or apoAII showed no increased capacity to induce cholesterol efflux from cholesterolloaded fibroblasts following oxidation by tyrosyl radical, either in their lipid-free forms or in rHDL. The product of oxidation of a mixture of apoAI and apoAII (1:1 molar ratio) by tyrosyl radical, however, reproduced the enhanced ability of tyrosylated HDL to induce cholesterol efflux when reconstituted with the whole lipid fraction of HDL. HDL containing only apoAI or apoAII showed no enhanced ability to promote cholesterol efflux following oxidation by tyrosyl radical, whereas HDL containing both apoAI and apoAII did. rHDL containing apoAI-apoAII monomer and apoAI-(apoAII) 2 heterodimers showed a markedly increased ability to prevent the accumulation of LDL-derived cholesterol mass by sterol-depleted fibroblasts compared with other apolipoprotein species of tyrosylated HDL. These results indicate a novel product of HDL oxidation, apoAIapoAII heterodimers, with a markedly enhanced capacity to deplete cells of the regulatory pool of free cholesterol and total cholesterol mass. The recent observation of tyrosyl radical-oxidized LDL in vivo suggests that a similar modification of HDL would significantly enhance its ability to deplete peripheral cells of cholesterol in the first step of reverse cholesterol transport.Lipoprotein oxidation is felt to be pivotal for the formation of macrophage foam cells in the developing atherosclerotic lesion (1). Low density lipoprotein (LDL) 1 particles containing modifications consistent with in vivo oxidation have been isolated from human atheromatous lesions (2-5). Numerous potential mechanisms for the oxidation of lipoproteins in vivo have been proposed (6). One such pathway is the release of myeloperoxidase and oxygen radicals by activated phagocytes at sites of inflammation, such as the intima of the atherosclerotic artery (7). Myeloperoxidase employs hydrogen peroxide to catalyze the oxidation of substrates, including chloride ion and L-tyrosine, in the extracellular fluid (8). The likelihood of oxidation of lipoproteins by myeloperoxidase-generated tyrosyl radical in vivo is suggested by the presence of the active enzyme (9), micromolar concentrations of L-tyrosine (10), and tyrosyl radical-modified LDL (5) in the artery wall.High density lipoprotein (HDL) is felt to protect against atherosclerosis in part by stimulating the ...

Effects of Protein Oxidation on the Structure and Stability of Model Discoidal High-Density Lipoproteins

“…To test this notion, we analyzed thermal denaturation of A-I:DMPC disks that were oxidized to various stages. Intact and ox(A-I:DMPC) complexes were heated and cooled from 10 to 98 °C at a rate of 11 °C/h, and thermal denaturation and reconstitution were monitored at 222 nm by CD and by 90° light scattering for changes in the α-helical structure and in the particle size, respectively (35). The results show that oxidation leads to progressive low-temperature shifts in the apparent temperature of disk denaturation, from T m =82 °C in intact disks to 60 °C in ox(A-I:DMPC) disks that were modified using 50:1 oxidant:protein ratio (Fig.…”

“…Oxidative crosslinking of apoA-I and apoA-II in HDL, which occurs via the tyrosyl radicals, reportedly alters apoA-I conformation, leading to increased solvent exposure of the protein α-helices (33,34) and to enhanced ability of HDL to promote cholesterol efflux from fibroblasts (35). Here, we analyze the effects of these oxidative modifications on thermal stability and remodeling of discoidal rHDLs.…”

“…In fact, oxidation of Met 112 and Met 148 to MetO reportedly impairs apoA-I ability to activate LCAT (29); it also leads to partial unfolding and destabilization of the α-helices in lipid-free and in lipidbound apoA-I and apoA-II, as well as to reduced protein affinity for lipid that is attributed to reduced hydrophobicity of the Met-containing apolar helical faces (29,30,32). Oxidative crosslinking of apoA-I and apoA-II in HDL, which occurs via the tyrosyl radicals, reportedly alters apoA-I conformation, leading to increased solvent exposure of the protein α-helices (33,34) and to enhanced ability of HDL to promote cholesterol efflux from fibroblasts (35). Here, we analyze the effects of these oxidative modifications on thermal stability and remodeling of discoidal rHDLs.…”

Oxidized HDL

“…22 Moreover, cross-linked heterodimers of apo A-I and apo A-II in tyrosylated HDL appear to be responsible for its enhanced ability to remove cholesterol from lipid-laden cells. 19,24 These heterodimers act more like lipid-free apolipoproteins, perhaps because of conformational changes in apo A-I that expose more amphipathic ␣-helices to ABCA1. It will be of great interest to determine whether tyrosylated HDL interacts with ABCA1 and to investigate the underlying mechanisms for enhanced cholesterol transport by this pathway.…”

Plasma Levels of Extracellular Superoxide Dismutase in an Australian Population

“…39,40 Although in vivo apoA-I oxidation remains to be demonstrated, a recent study suggests that compared with native apoA-I, oxidized apoA-I may promote more efficient reverse cholesterol transport. 41 Therefore, a possible functional link between apoA-I and NO and between apoA-I and EC-SOD is a scenario casting apoA-I as an antioxidant or ROS recipient whereby overproduction of NO might necessitate increased apoA-I production to counter the untoward oxidation effect.…”