The myeloperoxidase product hypochlorous acid oxidizes HDL in the human artery wall and impairs ABCA1-dependent cholesterol transport

Abstract

Although oxidatively damaged lipoproteins are implicated in vascular injury, there is little information regarding the role of highdensity lipoprotein (HDL) oxidation in atherogenesis. One potential pathway involves hypochlorous acid (HOCl) produced by myeloperoxidase (MPO), a heme protein secreted by phagocytes. We previously showed that 3-chlorotyrosine is a specific product of HOCl. Therefore, to explore the role of oxidized HDL in the pathogenesis of vascular disease, we used MS to quantify 3-chlorotyrosine in HDL isolated from plasma and atherosclerotic tissue. HDL from human aortic atherosclerotic intima had an 8-fold higher level of 3-chlorotyrosine than plasma HDL. Tandem MS analysis identified MPO as a component of lesion HDL, suggesting that the two interact in the artery wall. Moreover, immunohistochemical studies found that specific epitopes derived from HOCl colocalized with apolipoprotein A-I, the major protein of HDL. These observations strongly support the hypothesis that MPO promotes HDL oxidation in the human artery wall. Levels of 3-chlorotyrosine were elevated in HDL isolated from the blood of humans with established coronary artery disease, suggesting that circulating levels of oxidized HDL represent a unique marker for clinically significant atherosclerosis. HDL or lipid-free apolipoprotein A-I exposed to HOCl was less able to remove cholesterol from cultured cells by a pathway requiring the cell membrane transporter ATP-binding cassette transporter A1. The detection of 3-chlorotyrosine in HDL isolated from vascular lesions raises the possibility that MPO, by virtue of its ability to form HOCl, may promote atherogenesis by counteracting the established antiatherogenic effects of HDL and the ATP-binding cassette transporter A1 pathway.

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

“…The active form of MPO and the proteins modified by MPOgenerated HOCl have been found in human atherosclerotic lesions, supporting the notion that MPO mediates HDL oxidation in the artery wall (24,25). Physiological concentrations of HOCl have not been accurately determined, although some in-vivo-based reports suggest that, at sites of acute inflammation, the molar ratio of oxidant to HDL may be as high as 30:1 (26 and references therein).…”

“…This destabilization was attributed to reduced protein affinity for lipid upon MetO formation in the apolar lipid-binding faces of the amphipathic apolipoprotein α-helices (29). Such MetO formation occurs at early stages of oxidation and is followed by modifications of other protein groups, such as aromatic residues and Lys, and their cross-linking (24)(25)(26)(27)(28)(29)(30)(31). Correlation of rHDL destabilization (Fig.…”

“…Specifically, limited proteolysis and mass spectrometry studies of apoA-I in solution or on HDL suggest that, at a 5:1 HOCl:protein molar ratio, Met 86, Met 112 and Met 148 are converted to methionine sulfoxides (MetO); at a 10:1 ratio, Phe 37 and Phe 71 are the primary targets of oxidation; and at a 25:1 ratio, Tyr 192 is chlorinated (25,28). In addition, progressive oxidation of Trp residues in apoA-I has been detected by fluorescence (31).…”

“…Dose-dependent HOCl-induced modifications in the major HDL proteins, apoA-I and apoA-II, have been well-documented by using limited proteolysis, mass spectrometry and liquid chromatography; the general consensus is that oxidation of Met and Cys occurs first, followed by modifications of Lys and aromatic residues and, eventually, protein cross-linking into homoor heterodimers and trimers (24,(26)(27)(28)(29)(30). Specifically, limited proteolysis and mass spectrometry studies of apoA-I in solution or on HDL suggest that, at a 5:1 HOCl:protein molar ratio, Met 86, Met 112 and Met 148 are converted to methionine sulfoxides (MetO); at a 10:1 ratio, Phe 37 and Phe 71 are the primary targets of oxidation; and at a 25:1 ratio, Tyr 192 is chlorinated (25,28). In addition, progressive oxidation of Trp residues in apoA-I has been detected by fluorescence (31).…”

Mild Oxidation Promotes and Advanced Oxidation Impairs Remodeling of Human High-Density Lipoprotein In Vitro

“…Thus, cell cholesterol efflux to HDL, a key early step in reverse cholesterol transport mediated by ATP-binding cassette transporter A1, is reportedly impaired upon HDL oxidation. [14][15][16][17][18][19] This impairment has been ascribed to specific protein modifications such as oxidation of two-to-three Met in the major HDL protein, apolipoprotein A-I (apoA-I), that occurs at initial stages of HDL oxidation, 20-22 along with tyrosylation and protein cross-linking via the modified Tyr or Lys. 13,16,17,23 However, several studies report that oxidation enhances rather than impairs HDL functions in stimulating lipid efflux from cells 22,24-26 and protecting against atherosclerosis.…”