Effects of reagent and enzymatically generated hypochlorite on physicochemical and metabolic properties of high density lipoproteins

Myeloperoxidase as a Promising Therapeutic Target after Myocardial Infarction

Coronary artery disease (CAD) and myocardial infarction (MI) remain leading causes of death and disability worldwide. CAD begins with the formation of atherosclerotic plaques within the intimal layer of the coronary arteries, a process driven by persistent arterial inflammation and oxidation. Myeloperoxidase (MPO), a mammalian haem peroxidase enzyme primarily expressed within neutrophils and monocytes, has been increasingly recognised as a key pro-inflammatory and oxidative enzyme promoting the development of vulnerable coronary atherosclerotic plaques that are prone to rupture, and can precipitate a MI. Mounting evidence also implicates a pathogenic role for MPO in the inflammatory process that follows a MI, which is characterised by the rapid infiltration of activated neutrophils into the damaged myocardium and the release of MPO. Excessive and persistent cardiac inflammation impairs normal cardiac healing post-MI, resulting in adverse cardiac outcomes and poorer long-term cardiac function, and eventually heart failure. This review summarises the evidence for MPO as a significant oxidative enzyme contributing to the inappropriate inflammatory responses driving the progression of CAD and poor cardiac healing after a MI. It also details the proposed mechanisms underlying MPO's pathogenic actions and explores MPO as a novel therapeutic target for the treatment of unstable CAD and cardiac damage post-MI.

Scavenging dicarbonyls with 5'-O-pentyl-pyridoxamine increases HDL net cholesterol efflux capacity and attenuates atherosclerosis and insulin resistance

OBJECTIVE

Oxidative stress contributes to the development of insulin resistance (IR) and atherosclerosis. Peroxidation of lipids produces reactive dicarbonyls such as Isolevuglandins (IsoLG) and malondialdehyde (MDA) that covalently bind plasma/cellular proteins, phospholipids, and DNA leading to altered function and toxicity. We examined whether scavenging reactive dicarbonyls with 5'-O-pentyl-pyridoxamine (PPM) protects against the development of IR and atherosclerosis in Ldlr^-/- mice.

METHODS

Male or female Ldlr^-/- mice were fed a western diet (WD) for 16 weeks and treated with PPM versus vehicle alone. Plaque extent, dicarbonyl-lysyl adducts, efferocytosis, apoptosis, macrophage inflammation, and necrotic area were measured. Plasma MDA-LDL adducts and the in vivo and in vitro effects of PPM on the ability of HDL to reduce macrophage cholesterol were measured. Blood Ly6C^hi monocytes and ex vivo 5-ethynyl-2'-deoxyuridine (EdU) incorporation into bone marrow CD11b+ monocytes and CD34+ hematopoietic stem and progenitor cells (HSPC) were also examined. IR was examined by measuring fasting glucose/insulin levels and tolerance to insulin/glucose challenge.

RESULTS

PPM reduced the proximal aortic atherosclerosis by 48% and by 46% in female and male Ldlr^-/- mice, respectively. PPM also decreased IR and hepatic fat and inflammation in male Ldlr^-/- mice. Importantly, PPM decreased plasma MDA-LDL adducts and prevented the accumulation of plaque MDA- and IsoLG-lysyl adducts in Ldlr^-/- mice. In addition, PPM increased the net cholesterol efflux capacity of HDL from Ldlr^-/- mice and prevented both the in vitro impairment of HDL net cholesterol efflux capacity and apoAI crosslinking by MPO generated hypochlorous acid. Moreover, PPM decreased features of plaque instability including decreased proinflammatory M1-like macrophages, IL-1β expression, myeloperoxidase, apoptosis, and necrotic core. In contrast, PPM increased M2-like macrophages, Tregs, fibrous cap thickness, and efferocytosis. Furthermore, PPM reduced inflammatory monocytosis as evidenced by decreased blood Ly6C^hi monocytes and proliferation of bone marrow monocytes and HSPC from Ldlr^-/- mice.

CONCLUSIONS

PPM has pleotropic atheroprotective effects in a murine model of familial hypercholesterolemia, supporting the therapeutic potential of reactive dicarbonyl scavenging in the treatment of IR and atherosclerotic cardiovascular disease.

HDL and Oxidation

In this chapter, we will focus on HDLs' activity of inhibiting LDL oxidation and neutralizing some other oxidants. ApoA-I was known as the main antioxidant component in HDLs. The regulation of antioxidant capacity of HDL is mainly exhibited in regulation of apoA-I and alterations at the level of the HDL lipidome and the modifications of the proteome, especially MPO and PON1. HDL oxidation will influence the processes of inflammation and cholesterol transport, which are important processes in atherosclerosis, metabolic diseases, and many other diseases. In a word, HDL oxidation might be an effective antioxidant target in treatment of many diseases.

Role of myeloperoxidase and oxidant formation in the extracellular environment in inflammation-induced tissue damage

The heme peroxidase family generates a battery of oxidants both for synthetic purposes, and in the innate immune defence against pathogens. Myeloperoxidase (MPO) is the most promiscuous family member, generating powerful oxidizing species including hypochlorous acid (HOCl). Whilst HOCl formation is important in pathogen removal, this species is also implicated in host tissue damage and multiple inflammatory diseases. Significant oxidant formation and damage occurs extracellularly as a result of MPO release via phagolysosomal leakage, cell lysis, extracellular trap formation, and inappropriate trafficking. MPO binds strongly to extracellular biomolecules including polyanionic glycosaminoglycans, proteoglycans, proteins, and DNA. This localizes MPO and subsequent damage, at least partly, to specific sites and species, including extracellular matrix (ECM) components and plasma proteins/lipoproteins. Biopolymer-bound MPO retains, or has enhanced, catalytic activity, though evidence is also available for non-catalytic effects. These interactions, particularly at cell surfaces and with the ECM/glycocalyx induce cellular dysfunction and altered gene expression. MPO binds with higher affinity to some damaged ECM components, rationalizing its accumulation at sites of inflammation. MPO-damaged biomolecules and fragments act as chemo-attractants and cell activators, and can modulate gene and protein expression in naïve cells, consistent with an increasing cycle of MPO adhesion, activity, damage, and altered cell function at sites of leukocyte infiltration and activation, with subsequent tissue damage and dysfunction. MPO levels are used clinically both diagnostically and prognostically, and there is increasing interest in strategies to prevent MPO-mediated damage; therapeutic aspects are not discussed as these have been reviewed elsewhere.

Dysfunctional High-density Lipoprotein: The Role of Myeloperoxidase and Paraoxonase-1

Low circulating high-density lipoproteins (HDL) are not only defining criteria for metabolic syndrome, but are more generally associated with atherosclerotic cardiovascular disease (ASCVD) and other chronic diseases. Oxidative stress, a hallmark of cardio-metabolic disease, further influences HDL activity by suppressing their function. Especially the leukocyte- derived enzyme myeloperoxidase (MPO) has recently attracted great interest as it catalyzes the formation of oxidizing reactive species that modify the structure and function of HDL, ultimately increasing cardiovascular risk. Contrariwise, paraoxonase-1 (PON1) is an HDL-associated enzyme that protects HDL from lipid oxidation and then acts as a protective factor against ASCVD. It is noteworthy that recent studies have demonstrated how MPO, PON1 and HDL form a functional complex in which PON1 partially inhibits the MPO activity, while MPO in turn partially inactivates PON1.In line with that, a high MPO/PON1 ratio characterizes patients with ASCVD and metabolic syndrome and has been suggested as a potential marker of dysfunctional HDL as well as a predictor of ASCVD. In this review, we summarize the evidence on the interactions between MPO and PON1 with regard to their structure, function and interaction with HDL activity. We also provide an overview of in vitro and experimental animal models, finally focusing on clinical evidence from a cohort of patients with ASCVD and metabolic syndrome.

High-Density Lipoprotein Modifications: A Pathological Consequence or Cause of Disease Progression?

High-density lipoprotein (HDL) is well-known for its cardioprotective effects, as it possesses anti-inflammatory, anti-oxidative, anti-thrombotic, and cytoprotective properties. Traditionally, studies and therapeutic approaches have focused on raising HDL cholesterol levels. Recently, it became evident that, not HDL cholesterol, but HDL composition and functionality, is probably a more fruitful target. In disorders, such as chronic kidney disease or cardiovascular diseases, it has been observed that HDL is modified and becomes dysfunctional. There are different modification that can occur, such as serum amyloid, an enrichment and oxidation, carbamylation, and glycation of key proteins. Additionally, the composition of HDL can be affected by changes to enzymes such as cholesterol ester transfer protein (CETP), lecithin-cholesterol acyltransferase (LCAT), and phospholipid transfer protein (PLTP) or by modification to other important components. This review will highlight some main modifications to HDL and discuss whether these modifications are purely a consequential result of pathology or are actually involved in the pathology itself and have a causal role. Therefore, HDL composition may present a molecular target for the amelioration of certain diseases, but more information is needed to determine to what extent HDL modifications play a causal role in disease development.

Interaction between high-density lipoproteins and inflammation: Function matters more than concentration!

High-density lipoprotein (HDL) plays an important role in lipid metabolism and especially contributes to the reverse cholesterol transport pathway. Over recent years it has become clear that the effect of HDL on immune-modulation is not only dependent on HDL concentration but also and perhaps even more so on HDL function. This review will provide a concise general introduction to HDL followed by an overview of post-translational modifications of HDL and a detailed overview of the role of HDL in inflammatory diseases. The clinical potential of HDL and its main apolipoprotein constituent, apoA-I, is also addressed in this context. Finally, some conclusions and remarks that are important for future HDL-based research and further development of HDL-focused therapies are discussed.

Role of the oxidative stress index, myeloperoxidase, catalase activity for cardiac allograft vasculopathy in heart transplant recipients

BACKGROUND

The aim of this study was to explore the role of oxidative stress index (OSI), myeloperoxidase (MPO), and catalase (CAT) activity in cardiac allograft vasculopathy (CAV) in heart transplant recipients (HTRs).

METHODS

The study enrolled a median age of 41 ± 9 years 47 recipients. The HTx patients were divided into two groups based on the presence CAV as follows: CAV⁽⁺⁾ and CAV^(-) group. Also, CAV⁽⁺⁾ group were divided into two groups as mild/moderate to severe CAV. The OSI, MPO, and CAT activity were analyzed in both groups.

RESULTS

The mean total antioxidant capacity (0.79 ± 0.46 vs 1.03 ± 0.33 μmol H₂ O₂ equiv/L) P = .043 was significantly lower and OSI, MPO, CAT activity were significantly higher in CAV⁽⁺⁾ group (63 ± 38 vs 20 ± 16 arbitrary unit, P = .001; 398 ± 242 vs 139  ± 112 μg/L, P = .001; 51 ± 42 vs 26 ± 23 pmol/mg protein, P = .013, respectively). Also, mean OSI (38 ± 41 vs 93 ± 75, P = .05) were significantly higher in severe CAV⁽⁺⁾ group. Recipient age, male gender, and low density lipoprotein-cholesterol were significantly higher in CAV⁽⁺⁾ group. There was a moderate correlation between the CAV grade and OSI, MPO, and CAT levels in univariate analysis (r = .560, P = .002; r = .643, P = .007; r = .681, P = .001, respectively).

CONCLUSION

An increase in the serum level of OSI, MPO, and CAT was associated with CAV in HTRs.

The impact of myeloperoxidase on HDL function in myocardial infarction

PURPOSE OF REVIEW

The focus in cardiovascular research is shifting from determining mass HDL cholesterol levels toward investigating HDL functionalities as biomarker for cardiovascular disease. Myeloperoxidase (MPO), a main effector enzyme of the innate immune system, is increasingly implicated to negatively impact HDL function by various chemical modifications of HDL-associated proteins. This review summarizes recent insights how MPO affects HDL function in the setting of acute myocardial infarction (MI), mainly focusing on human data.

RECENT FINDINGS

First the mechanisms how MPO renders HDL particles dysfunctional and the usefulness of MPO as prospective biomarker for MI incidence and outcomes are described. Then the evidence for MPO causing specific HDL function impairments in MI and the clinical value of these observations is discussed in the context of the different HDL function assays employed.

SUMMARY

MPO modification of HDL in acute MI generates dysfunctional HDL. Features of HDL dysfunction can be used to stratify MI patients and seem associated with outcomes. More prospective studies are warranted to explore, if MPO-modified HDL is causally linked to severity and outcomes of MI. If this could be established, MPO would represent an attractive target to improve HDL dysfunction in MI and provide clinical benefit for patients.

Characterization of covalent modifications of HDL apoproteins by endogenous oxidized phospholipids

High density lipoprotein (HDL) is cardioprotective, unless it is pathologically modified under oxidative stress. Covalent modifications of lipid-free apoA-I, the most abundant apoprotein in HDL, compromise its atheroprotective functions. HDL is enriched in oxidized phospholipids (oxPL) in vivo in oxidative stress. Furthermore, oxidized phospholipids can covalently modify HDL apoproteins. We have now carried out a systematic analysis of modifications of HDL apoproteins by endogenous oxPL. Human HDL or plasma were oxidized using a physiologically relevant MPO-H₂O₂-NO₂^- system or AIPH, or were exposed to synthetic oxPL. Protein adduction by oxPL was assessed using LC-MS/MS and MALDI-TOF MS. The pattern of HDL apoprotein modification by oxPL was independent of the oxidation systems used. ApoA-I and apoA-II were the major modification targets. OxPL with a γ-hydroxy (or oxo)-alkenal were mostly responsible for modifications, and the Michael adduct was the most abundant adduct. Histidines and lysines in helices 5-8 of apoA-I were highly susceptible to oxPL modifications, while lysines in helices 1, 2, 4 and 10 were resistant to modification by oxPL. In plasma exposed to oxidation or synthetic oxPL, oxPL modification was highly selective, and four histidines (H155, H162, H193 and H199) in helices 6-8 of apoA-I were the main modification target. H710 and H3613 in apoB-100 of LDL and K190 of human serum albumin were also modified by oxPL but to a lesser extent. Comparison of oxPL with short chain aldehyde HNE using MALDI-TOF MS demonstrated high selectivity and efficiency of oxPL in the modification of HDL apoproteins. These findings provide a novel insight into a potential mechanism of the loss of atheroprotective function of HDL in conditions of oxidative stress.

Oxidation of C-reactive protein by hypochlorous acid leads to the formation of potent platelet activator

We examined the structural and functional consequences of oxidative modification of C-reactive protein (CRP) by hypochlorous acid (HOCl), which can be generated in vivo via the myeloperoxidase/H₂O₂/Cl^- system. HOCl exposure resulted in the oxidation and chlorination of CRP amino acid residues, leading to protein unfolding, greater surface hydrophobicity and the formation of aggregates. After treatment of isolated platelets with 50μg/ml HOCl-CRP, the modified CRP significantly stimulated platelet activation (over 10-fold increase in the fraction of CD62-positive platelets compared to controls, P<0.008), enhanced deposition of platelets onto immobilized fibrinogen (two-fold rise in platelet adhesion compared to controls, P<0.0001), and induced platelet aggregation by up to 79.5%. The ability of HOCl-CRP to interact with several platelet receptors (TLR-4, GPIIbIIIa) and plasma proteins (C1q, IgG) strongly indicates that HOCl-modification leads to structural changes of CRP resulting in the formation of new ligand binding sites, which is characteristic of the monomeric form of CRP exerting pro-inflammatory effects on a variety of cells. Overall, the oxidation of native CRP by HOCl seems to represent an alternative mechanism of CRP modification, by which CRP reveals its pro-inflammatory and pro-thrombotic properties, and as such, it might be of causal relevance in the pathogenesis of atherosclerosis.

PRISMA-combined Myeloperoxidase -463G/A gene polymorphism and coronary artery disease: A meta-analysis of 4744 subjects

BACKGROUND

Myeloperoxidase (MPO) -463G/A gene polymorphism may be associated with an increased risk of developing coronary artery disease (CAD). Studies on the subject, however, do not provide a clear consensus. This meta-analysis was performed to explore the relationship between MPO gene -463G/A polymorphism and CAD risk.

METHODS

This meta-analysis combines data from 4744 subjects from 9 independent studies. By using fixed or random effect models, the pooled odds ratios (ORs) and their corresponding 95% confidence intervals (CIs) were assessed.

RESULTS

Our analysis found a significant association between MPO gene -463G/A polymorphism and CAD in the whole population under all genetic models: allelic (OR: 0.68, 95% CI: 0.54-0.85, P = 0.0009), recessive (OR: 0.41, 95% CI: 0.22-0.76, P = 0.005), dominant (OR: 0.682, 95% CI: 0.534-0.871, P = 0.002), homozygous (OR: 0.36, 95% CI: 0.16-0.79, P = 0.01), heterozygous genetic model (OR: 0.832, 95% CI: 0.733-0.945, P = 0.004), and additive (OR: 0.64, 95% CI: 0.46-0.90, P = 0.01), especially in the Chinese subgroup (P < 0.05). On the contrary, we found no such relationship in the non-Chinese subgroup (P > 0.05).

CONCLUSION

The MPO gene -463G/A polymorphism is associated with CAD risk, especially within the Chinese population. The A allele of MPO gene -463G/A polymorphism might protect the people from suffering the CAD risk.

High density lipoprotein (HDL) particles from end-stage renal disease patients are defective in promoting reverse cholesterol transport

Atherosclerotic cardiovascular disease (CVD) represents the largest cause of mortality in end-stage renal disease (ESRD). CVD in ESRD is not explained by classical CVD risk factors such as HDL cholesterol mass levels making functional alterations of lipoproteins conceivable. HDL functions in atheroprotection by promoting reverse cholesterol transport (RCT), comprising cholesterol efflux from macrophage foam cells, uptake into hepatocytes and final excretion into the feces. ESRD-HDL (n = 15) were compared to healthy control HDL (n = 15) for their capacity to promote in vitro (i) cholesterol efflux from THP-1 macrophage foam cells and (ii) SR-BI-mediated selective uptake into ldla[SR-BI] cells as well as (iii) in vivo RCT. Compared with HDL from controls, ESRD-HDL displayed a significant reduction in mediating cholesterol efflux (p < 0.001) and SR-BI-mediated selective uptake (p < 0.01), two key steps in RCT. Consistently, also the in vivo capacity of ESRD-HDL to promote RCT when infused into wild-type mice was significantly impaired (p < 0.01). In vitro oxidation of HDL from healthy controls with hypochloric acid was able to fully mimic the impaired biological activities of ESRD-HDL. In conclusion, we demonstrate that HDL from ESRD patients is dysfunctional in key steps as well as overall RCT, likely due to oxidative modification.

The effect of reagents mimicking oxidative stress on fibrinogen function

Fibrinogen is one of the plasma proteins most susceptible to oxidative modification. It has been suggested that modification of fibrinogen may cause thrombotic/bleeding complications associated with many pathophysiological states of organism. We exposed fibrinogen molecules to three different modification reagents-malondialdehyde, sodium hypochlorite, and peroxynitrite-that are presented to various degrees in different stages of oxidative stress. We studied the changes in fibrin network formation and platelet interactions with modified fibrinogens under flow conditions. The fastest modification of fibrinogen was caused by hypochlorite. Fibers from fibrinogen modified with either reagent were thinner in comparison with control fibers. We found that platelet dynamic adhesion was significantly lower on fibrinogen modified with malondialdehyde and significantly higher on fibrinogen modified either with hypochlorite or peroxynitrite reflecting different prothrombotic/antithrombotic properties of oxidatively modified fibrinogens. It seems that, in the complex reactions ongoing in living organisms at conditions of oxidation stress, hypochlorite modifies proteins (e.g., fibrinogen) faster and more preferentially than malondialdehyde. It suggests that the prothrombotic effects of prior fibrinogen modifications may outweigh the antithrombotic effect of malondialdehyde-modified fibrinogen in real living systems.

N-acetyl lysyltyrosylcysteine amide inhibits myeloperoxidase, a novel tripeptide inhibitor

Myeloperoxidase (MPO) plays important roles in disease by increasing oxidative and nitrosative stress and oxidizing lipoproteins. Here we report N-acetyl lysyltyrosylcysteine amide (KYC) is an effective inhibitor of MPO activity. We show KYC inhibits MPO-mediated hypochlorous acid (HOCl) formation and nitration/oxidation of LDL. Disulfide is the major product of MPO-mediated KYC oxidation. KYC (≤4,000 μM) does not induce cytotoxicity in bovine aortic endothelial cells (BAECs). KYC inhibits HOCl generation by phorbol myristate acetate (PMA)-stimulated neutrophils and human promyelocytic leukemia (HL-60) cells but not superoxide generation by PMA-stimulated HL-60 cells. KYC inhibits MPO-mediated HOCl formation in BAEC culture and protects BAECs from MPO-induced injury. KYC inhibits MPO-mediated lipid peroxidation of LDL whereas tyrosine (Tyr) and tryptophan (Trp) enhance oxidation. KYC is unique as its isomers do not inhibit MPO activity, or are much less effective. Ultraviolet-visible spectral studies indicate KYC binds to the active site of MPO and reacts with compounds I and II. Docking studies show the Tyr of KYC rests just above the heme of MPO. Interestingly, KYC increases MPO-dependent H₂O₂ consumption. These data indicate KYC is a novel and specific inhibitor of MPO activity that is nontoxic to endothelial cell cultures. Accordingly, KYC may be useful for treating MPO-mediated vascular disease.

VPO1 mediates ApoE oxidation and impairs the clearance of plasma lipids

OBJECTIVE

ApoE is an abundant component of chylomicron, VLDL, IDL, and HDL. It binds to multiple types of lipids and is implicated in cholesterol and triglyceride homeostasis. Oxidation of ApoE plays a crucial role in the genesis of atherosclerosis. It is proposed that heme-containing peroxidases (hPx) are major mediators of lipoprotein oxidization. Vascular peroxidase 1 (VPO1) is a recently-discovered hPx, which is expressed in cardiovascular system, lung, liver etc. and secreted into plasma. Its plasma concentration is three orders of magnitude of that of myeloperoxidase. If VPO1 mediates ApoE oxidation and affects the lipid metabolism remains to be elucidated.

METHODS

Recombinant ApoE and VPO1 were expressed and purified from stably-expressing cell lines deriving from HEK293 cells. ApoE oxidation was carried out by VPO1 in the presence of H2O2 and chloride. ApoE oxidation was verified by a variety of approaches including immunoblot and amino acid analyses. To evaluate the functional changes in VPO1-oxidized ApoE, lipid emulsion particle binding assays were employed.

RESULTS

Oxidized ApoE binds weaker to lipid emulsion particles, which mimic the large lipid complexes in vivo. In lipid efflux assay, oxidized ApoE showed reduced capability in efflux of lipids from foam cells. Mice administrated with oxidized ApoE via blood exhibited weaker clearance ability of plasma lipids.

CONCLUSIONS

Our data suggest that VPO1 is a new mediator regulating lipid homeostasis, implying a role in genesis and development of atherosclerosis.

Myeloperoxidase-derived oxidants modify apolipoprotein A-I and generate dysfunctional high-density lipoproteins: comparison of hypothiocyanous acid (HOSCN) with hypochlorous acid (HOCl)

Oxidative modification of HDLs (high-density lipoproteins) by MPO (myeloperoxidase) compromises its anti-atherogenic properties, which may contribute to the development of atherosclerosis. Although it has been established that HOCl (hypochlorous acid) produced by MPO targets apoA-I (apolipoprotein A-I), the major apolipoprotein of HDLs, the role of the other major oxidant generated by MPO, HOSCN (hypothiocyanous acid), in the generation of dysfunctional HDLs has not been examined. In the present study, we characterize the structural and functional modifications of lipid-free apoA-I and rHDL (reconstituted discoidal HDL) containing apoA-I complexed with phospholipid, induced by HOSCN and its decomposition product, OCN− (cyanate). Treatment of apoA-I with HOSCN resulted in the oxidation of tryptophan residues, whereas OCN− induced carbamylation of lysine residues to yield homocitrulline. Tryptophan residues were more readily oxidized on apoA-I contained in rHDLs. Exposure of lipid-free apoA-I to HOSCN and OCN− significantly reduced the extent of cholesterol efflux from cholesterol-loaded macrophages when compared with unmodified apoA-I. In contrast, HOSCN did not affect the anti-inflammatory properties of rHDL. The ability of HOSCN to impair apoA-I-mediated cholesterol efflux may contribute to the development of atherosclerosis, particularly in smokers who have high plasma levels of SCN− (thiocyanate).

Preservation of biological function despite oxidative modification of the apolipoprotein A-I mimetic peptide 4F

Myeloperoxidase (MPO)-derived hypochlorous acid induces changes in HDL function via redox modifications at the level of apolipoprotein A-I (apoA-I). As 4F and apoA-I share structural and functional properties, we tested the hypothesis that 4F acts as a reactive substrate for hypochlorous acid (HOCl). 4F reduced the HOCl-mediated oxidation of the fluorescent substrate APF in a concentration-dependent manner (ED(50) ∼ 56 ± 3 μM). This reaction induced changes in the physical properties of 4F. Addition of HOCl to 4F at molar ratios ranging from 1:1 to 3:1 reduced 4F band intensity on SDS-PAGE gels and was accompanied by the formation of a higher molecular weight species. Chromatographic studies showed a reduction in 4F peak area with increasing HOCl and the formation of new products. Mass spectral analyses of collected fractions revealed oxidation of the sole tryptophan (Trp) residue in 4F. 4F was equally susceptible to oxidation in the lipid-free and lipid-bound states. To determine whether Trp oxidation influenced its apoA-I mimetic properties, we monitored effects of HOCl on 4F-mediated lipid binding and ABCA1-dependent cholesterol efflux. Neither property was altered by HOCl. These results suggest that 4F serves as a reactive substrate for HOCl, an antioxidant response that does not influence the lipid binding and cholesterol effluxing capacities of the peptide.

Site-specific oxidation of apolipoprotein A-I impairs cholesterol export by ABCA1, a key cardioprotective function of HDL

The mechanisms that deprive HDL of its cardioprotective properties are poorly understood. One potential pathway involves oxidative damage of HDL proteins by myeloperoxidase (MPO) a heme enzyme secreted by human artery wall macrophages. Mass spectrometric analysis demonstrated that levels of 3-chlorotyrosine and 3-nitrotyrosine - two characteristic products of MPO - are elevated in HDL isolated from patients with established cardiovascular disease. When apolipoprotein A-I (apoA-I), the major HDL protein, is oxidized by MPO, its ability to promote cellular cholesterol efflux by the membrane-associated ATP-binding cassette transporter A1 (ABCA1) pathway is diminished. Biochemical studies revealed that oxidation of specific tyrosine and methionine residues in apoA-I contributes to this loss of ABCA1 activity. Another potential mechanism for generating dysfunctional HDL involves covalent modification of apoA-I by reactive carbonyls, which have been implicated in atherogenesis and diabetic vascular disease. Indeed, modification of apoA-I by malondialdehyde (MDA) or acrolein also markedly impaired the lipoprotein's ability to promote cellular cholesterol efflux by the ABCA1 pathway. Tandem mass spectrometric analyses revealed that these reactive carbonyls target specific Lys residues in the C-terminus of apoA-I. Importantly, immunochemical analyses showed that levels of MDA-protein adducts are elevated in HDL isolated from human atherosclerotic lesions. Also, apoA-I co-localized with acrolein adducts in such lesions. Thus, lipid peroxidation products might specifically modify HDL in vivo. Our observations support the hypotheses that MPO and reactive carbonyls might generate dysfunctional HDL in humans. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Aminobenzoic acid hydrazide, a myeloperoxidase inhibitor, alters the adhesive properties of neutrophils isolated from acute myocardial infarction patients

Acute myocardial infarction (AMI) is associated with vascular inflammation, including activation and adherence of neutrophils to vascular endothelial cells via CD11b/CD18 intercellular adhesion molecule interactions. Myeloperoxidase (MPO) induces CD11b surface expression in polymorphonuclear neutrophils (PMNs); however, its role in regulating adhesion in AMI is not well characterized. This study investigates the effects of aminobenzoic acid hydrazide (ABAH), an inhibitor of MPO, antibodies specific for CD11b, on the adhesion of PMNs isolated from AMI patients to endothelial cells. Human neutrophils were isolated from the peripheral blood of 20 patients with AMI or 20 healthy participants as control using Percoll density gradient centrifugation. The major biochemical indicators were detected with different biochemical analyses. The effects of ABAH and anti-CD11b antibodies on neutrophil adhesion to endothelial cell were measured using adhesion assays in vitro. The adhesion rate was significantly higher for neutrophils isolated from AMI patients than healthy individuals (P < 0.001). ABAH significantly inhibited MPO activity in PMNs isolated from AMI patients. Neutrophil adhesion was significantly reduced upon treatment with 10 and 20 μM ABAH in a dose-dependent manner. Treatment with anti-CD11b antibodies also significantly reduced neutrophil adhesion in comparison with the untreated control group (P < 0.001). Thus, both ABAH and anti-CD11b antibodies reduced PMN adhesion. Further studies are necessary to determine whether MPO enhances neutrophil adhesion to endothelial cells in AMI patients through the upregulation of CD11b expression on the surface of neutrophils, which is abrogated by ABAH.

Impact of HDL oxidation by the myeloperoxidase system on sterol efflux by the ABCA1 pathway

Protein oxidation by phagocytic white blood cells is implicated in tissue injury during inflammation. One important target might be high-density lipoprotein (HDL), which protects against atherosclerosis by removing excess cholesterol from artery wall macrophages. In the human artery wall, cholesterol-laden macrophages are a rich source of myeloperoxidase (MPO), which uses hydrogen peroxide for oxidative reactions in the extracellular milieu. Levels of two characteristic products of MPO-chlorotyrosine and nitrotyrosine-are markedly elevated in HDL from human atherosclerotic lesions. Here, we describe how MPO-dependent chlorination impairs the ability of apolipoprotein A-I (apoA-I), HDL's major protein, to transport cholesterol by the ATP-binding cassette transporter A1 (ABCA1) pathway. Faulty interactions between apoA-I and ABCA1 are involved. Tandem mass spectrometry and investigations of mutated forms of apoA-I demonstrate that tyrosine residues in apoA-I are chlorinated in a site-specific manner by chloramine intermediates on suitably juxtaposed lysine residues. Plasma HDL isolated from subjects with coronary artery disease (CAD) also contains higher levels of chlorinated and nitrated tyrosine residues than HDL from healthy subjects. Thus, the presence of chlorinated HDL might serve as a marker of CAD risk. Because HDL damaged by MPO in vitro becomes dysfunctional, inhibiting MPO in vivo might be cardioprotective.

Hypochlorite-modified high-density lipoprotein promotes induction of HO-1 in endothelial cells via activation of p42/44 MAPK and zinc finger transcription factor Egr-1

Modification/chlorination of high-density lipoprotein (HDL) by hypochlorous acid (HOCl), formed by the myeloperoxidase-H₂O₂-chloride system of activated phagocytes, converts an anti-atherogenic lipoprotein into a pro-inflammatory lipoprotein particle. Chlorinated HDL is present in human lesion material, binds to and is internalized by endothelial cells and impairs expression and activity of endothelial nitric oxide synthase. The present study aimed at clarifying whether exposure of endothelial cells to pro-inflammatory HOCl-HDL impacts on expression of heme oxygenase-1, a potential rescue pathway against endothelial dysfunction. Our findings revealed that HDL modified by HOCl, added as reagent or generated enzymatically, induced phosphorylation of p42/44 mitogen-activated protein kinase, expression of transcription factor early growth response-1 (Egr-1) and enhanced expression of heme oxygenase-1 in human endothelial cells. Upregulation of heme oxygenase-1 could be blocked by an inhibitor upstream of p42/44 mitogen-activated protein kinase and/or knockdown of Egr-1 by RNA-interference. Electrophoretic mobility shift assays demonstrated HOCl-HDL-mediated induction of the Egr-1 DNA binding activity. Immunocytochemical and immunoblotting experiments demonstrated HOCl-HDL-induced translocation of Egr-1 to the nucleus. The present study demonstrates a novel compensatory pathway against adverse effects of HOCl-HDL, providing cytoprotection in a number of pathological conditions including cardiovascular disease.

Taurine in cardiovascular disease

PURPOSE OF REVIEW

The shift of modern dietary regimens from 'Mediterranean' to 'western' style is believed to be responsible, in part, for the increase in cardiovascular disease, obesity, type II diabetes and cancer. A classic 'Mediterranean' diet consists of adequate intake of seafood, vegetables, fruit, whole grain and nonpurified monounsaturated vegetable oil. Thus, in humans, dietary intake of seafood is the major source of taurine, as the level of endogenously produced taurine is low.

RECENT FINDINGS

Taurine has been shown to affect coronary artery disease, blood pressure, plasma cholesterol and myocardial function in animal models of human disease. A major role of taurine is to act as an antioxidant and absorb hypochlorous acid but not the oxidative radical. It seems that this beneficial effect of taurine in antioxidant therapy has not been well promoted.

SUMMARY

This review will focus on determining whether taurine could be a factor contributing to the further prevention of heart disease.

Sequence conservation of apolipoprotein A-I affords novel insights into HDL structure-function

We performed alignment of apolipoprotein A-I (apoA-I) sequences from 31 species of animals. We found there is specific conservation of salt bridge-forming residues in the first 30 residues of apoA-I and general conservation of a variety of residue types in the central domain, helix 2/3 to helix 7/8. In the lipid-associating domain, helix 7 and helix 10 are the most and least conserved helixes, respectively. Furthermore, eight residues are completely conserved: P66, R83, P121, E191, and P220, and three of seven Tyr residues in human apoA-I, Y18, Y115, and Y192, are conserved. Residue Y18 appears to be important for assembly of HDL. E191-Y192 represents the only completely conserved pair of adjacent residues in apoA-I; Y192 is a preferred target for site-specific oxidative modification within atheroma, and molecular dynamic simulations suggest that the conserved pair E191-Y192 is in a solvent-exposed loop-helix-loop. Molecular dynamics testing of human apoA-I showed that M112 and M148 interact with Y115, a microenvironment unique to human apoA-I. Finally, conservation of Arg residues in the α11/3 helical wheel position 7 supports several possibilities: interactions with adjacent phospholipid molecules and/or oxidized lipids and/or binding of antioxidant enzymes through cation-π orbital interactions. We conclude that sequence alignment of apoA-I provides unique insights into apoA-I structure-function relationship.

Myeloperoxidase: an oxidative pathway for generating dysfunctional high-density lipoprotein

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by artery wall macrophages triggers atherosclerosis, the leading cause of cardiovascular disease. Conversely, high-density lipoprotein (HDL) retards atherosclerosis by promoting cholesterol efflux from macrophages by the membrane-associated ATP-binding cassette transporter A1 (ABCA1) pathway. HDL has been proposed to lose its cardioprotective effects in subjects with atherosclerosis, but the underlying mechanisms are poorly understood. One potential pathway involves oxidative damage by myeloperoxidase (MPO), a heme enzyme secreted by human artery wall macrophages. We used mass spectrometry to demonstrate that HDL isolated from patients with established cardiovascular disease contains elevated levels of 3-chlorotyrosine and 3-nitrotyrosine, two characteristic products of MPO. When apolipoprotein A-I (apoA-I), the major HDL protein, was oxidized by MPO, its ability to promote cellular cholesterol efflux by ABCA1 was impaired. Moreover, oxidized apoA-I was unable to activate lecithin:cholesterol acyltransferase (LCAT), which rapidly converts free cholesterol to cholesteryl ester, a critical step in HDL maturation. Biochemical studies implicated tyrosine chlorination and methionine oxygenation in the loss of ABCA1 and LCAT activity by oxidized apoA-I. Oxidation of specific residues in apoA-I inhibited two key steps in cholesterol efflux from macrophages, raising the possibility that MPO initiates a pathway for generating dysfunctional HDL in humans.

Specific binding of hypochlorite-oxidized HDL to platelet CD36 triggers proinflammatory and procoagulant effects

OBJECTIVE

Oxidative stress and systemic inflammation negatively affect several protective functions of high density lipoproteins (HDL) and oxidative modification of HDL by the inflammation-derived oxidant hypochlorite converts HDL into a potent platelet agonist. Therefore it was the aim of this work to clarify if these platelet-activating effects result from specific binding of hypochlorite-oxidized HDL (hyp-OxHDL) to the platelet surface and to identify responsible receptors.

METHODS

Binding and functional studies were performed with hyp-OxHDL in absence and presence of (potential) competitors in normal and CD36-deficient human platelets. Platelet aggregation was quantified by light transmission aggregometry. Surface expression of CD62P, phosphatidylserine and CD40L was quantified by flow cytometry.

RESULTS

Binding studies reveal that hyp-OxHDL show specific and saturable high-affinity binding to the platelet surface. Hyp-OxHDL trigger platelet aggregation and in a dose dependent way provoke the release of significant amounts of CD40L as well as phosphatidylserine on the platelet surface. Blocking specific binding of hyp-OxHDL to the platelet surface interferes with the ability of hyp-OxHDL to stimulate human platelets. CD36-deficient human platelets show markedly reduced binding of hyp-OxHDL. Upon addition of hypochlorite-oxidized HDL, CD36-deficient platelets do not aggregate and completely fail to release CD40L or phosphatidylserine.

CONCLUSIONS

From these results we conclude that specific binding of hyp-OxHDL to platelet CD36 is essential for the proinflammatory and procoagulant effects of hyp-OxHDL shown within this work. The contribution of other receptors besides CD36 to specific binding of hyp-OxHDL to the platelet membrane appears to be minimal, at best.

Oxidative risk for atherothrombotic cardiovascular disease

In the vasculature, reactive oxidant species, including reactive oxygen, nitrogen, or halogenating species, and thiyl, tyrosyl, or protein radicals may oxidatively modify lipids and proteins with deleterious consequences for vascular function. These biologically active free radical and nonradical species may be produced by increased activation of oxidant-generating sources and/or decreased cellular antioxidant capacity. Once formed, these species may engage in reactions to yield more potent oxidants that promote transition of the homeostatic vascular phenotype to a pathobiological state that is permissive for atherothrombogenesis. This dysfunctional vasculature is characterized by lipid peroxidation and aberrant lipid deposition, inflammation, immune cell activation, platelet activation, thrombus formation, and disturbed hemodynamic flow. Each of these pathobiological states is associated with an increase in the vascular burden of free radical species-derived oxidation products and, thereby, implicates increased oxidant stress in the pathogenesis of atherothrombotic vascular disease.

In vivo administration of BL-3050: highly stable engineered PON1-HDL complexes

Background

Serum paraoxonase (PON1) is a high density lipoprotein (HDL)-associated enzyme involved in organophosphate (OP) degradation and prevention of atherosclerosis. PON1 comprises a potential candidate for in vivo therapeutics, as an anti-atherogenic agent, and for detoxification of pesticides and nerve agents. Because human PON1 exhibits limited stability, engineered, recombinant PON1 (rePON1) variants that were designed for higher reactivity, solubility, stability, and bacterial expression, are candidates for treatment. This work addresses the feasibility of in vivo administration of rePON1, and its HDL complex, as a potentially therapeutic agent dubbed BL-3050.

Methods

For stability studies we applied different challenges related to the in vivo disfunctionalization of HDL and PON1 and tested for inactivation of PON1's activity. We applied acute, repetitive administrations of BL-3050 in mice to assess its toxicity and adverse immune responses. The in vivo efficacy of recombinant PON1 and BL-3050 were tested with an animal model of chlorpyrifos-oxon poisoning.

Results

Inactivation studies show significantly improved in vitro lifespan of the engineered rePON1 relative to human PON1. Significant sequence changes relative to human PON1 might hamper the in vivo applicability of BL-3050 due to adverse immune responses. However, we observed no toxic effects in mice subjected to repetitive administration of BL-3050, suggesting that BL-3050 could be safely used. To further evaluate the activity of BL-3050 in vivo, we applied an animal model that mimics human organophosphate poisoning. In these studies, a significant advantages of rePON1 and BL-3050 (>87.5% survival versus <37.5% in the control groups) was observed. Furthermore, BL-3050 and rePON1 were superior to the conventional treatment of atropine-2-PAM as a prophylactic treatment for OP poisoning.

Conclusion

In vitro and in vivo data described here demonstrate the potential advantages of rePON1 and BL-3050 for treatment of OP toxicity and chronic cardiovascular diseases like atherosclerosis. The in vivo data also suggest that rePON1 and BL-3050 are stable and safe, and could be used for acute, and possibly repeated treatments, with no adverse effects.

Dysfunctional HDL as a diagnostic and therapeutic target

The atheroprotective effects of HDL are mediated by several mechanisms, including its role in reverse cholesterol transport and via its antiinflammatory properties. However, not all HDL is functionally similar. HDL and apolipoprotein A-I may become dysfunctional or even proinflammatory and thus promote atherosclerosis. ApoAI posttranslational modification can have a large impact on its function. Myeloperoxidase modification of apoAI impairs its function as a cholesterol acceptor, and the molecular changes induced by myeloperoxidase have been studied in detail. These studies provide the basis for the development of an oxidant-resistant form of apoAI and clinical measures of HDL modification and dysfunction, which may be useful as a treatment criterion.

Myeloperoxidase: a useful biomarker for cardiovascular disease risk stratification?

BACKGROUND

Inflammation and oxidative stress are associated with atherosclerosis. Myeloperoxidase (MPO) is linked to both inflammation and oxidative stress by its location in leukocytes and its role in catalyzing the formation of oxidizing agents. Recent evidence suggests that MPO activity precipitates atherogenesis. Measurement of MPO in plasma may therefore contribute to cardiovascular disease (CVD) risk stratification.

CONTENT

Cross-sectional studies, case-control studies, and prospective-cohort studies investigating the relation between MPO and CVD have been evaluated. Differences in study populations, sample materials, sample handling, and assays were ascertained. Potential causal mechanisms linking MPO to accelerated atherosclerosis are discussed here. A majority of studies indicate that measurement of MPO in plasma was associated with improved CVD risk stratification above and beyond risk stratification results obtained with markers used in routine clinical practice. However, comparison of these epidemiological studies with regard to MPO and outcome is hampered because the reported MPO concentration depends on the assay method, sampling material, and preanalytical and analytical procedures. The link between MPO and CVD can, at least partly, be explained by MPO-dependent oxidation of LDL and HDL, subsequently leading to cholesterol accumulation in the arterial wall. Furthermore, MPO may reduce the bioavailability of nitric oxide, resulting in endothelial dysfunction. Finally, MPO destabilizes atherosclerotic plaques.

SUMMARY

Increasing evidence suggests that MPO is causally linked to atherosclerosis and its measurement may improve CVD risk estimation. Before MPO can be used in routine clinical practice, however, standardization of sampling and laboratory procedures is needed.

Vitamin C attenuates hypochlorite-mediated loss of paraoxonase-1 activity from human plasma

Paraoxonase 1 (PON1) is a cardioprotective enzyme associated with high-density lipoprotein (HDL). We tested the hypothesis that vitamin C protects HDL and PON1 from deleterious effects of hypochlorous acid, a proinflammatory oxidant. In our experiments, HDL (from human plasma) or diluted human plasma was incubated with hypochlorite in either the absence (control) or presence of vitamin C before measuring chemical modification and PON1 activities. Vitamin C minimized chemical modification of HDL, as assessed by lysine modification and accumulation of chloramines. In the absence of vitamin C, chloramines accumulated to 114 +/- 4 micromol/L in HDL incubated with a 200-fold molar excess of hypochlorite; but addition of vitamin C (200 micromol/L) limited formation to 36 +/- 6 micromol/L (P < .001). In plasma exposed to hypochlorite, IC(50) values of 1.2 +/- 0.1, 9.5 +/- 1.0, and 5.0 +/- 0.6 mmol/L were determined for PON1's phosphotriesterase, arylesterase, and (physiologic) lactonase activities, respectively. Vitamin C lessened this inhibitory effect of hypochlorite on PON1 activities. In plasma supplemented with vitamin C (400 micromol/L), PON1 phosphotriesterase activity was 72% +/- 17% of normal after incubation with hypochlorite (2 mmol/L), compared with 42% +/- 6% for unsupplemented plasma (P < .05). Similar effects were seen for other PON1 activities. In some experiments, vitamin C also appeared to reverse hypochlorite-mediated loss of PON1 phosphotriesterase activity; but this effect was not observed for the other PON1 activities. In conclusion, vitamin C attenuated hypochlorite-mediated loss of PON1 activity in vitro and may, therefore, preserve cardioprotective properties of HDL during inflammation.

A new insight into resveratrol as an atheroprotective compound: inhibition of lipid peroxidation and enhancement of cholesterol efflux

Resveratrol, a polyphenolic constituent of red wine, is known for its anti-atherogenic properties and is thought to be beneficial in reducing the incidence of cardiovascular diseases (CVD). However, the mechanism of action by which it exerts its anti-atherogenic effect remains unclear. In this study, we investigated the relationship between the antioxidant effects of resveratrol and its ability to promote cholesterol efflux. We measured the formation of conjugated dienes and the rate of lipid peroxidation, and observed that resveratrol inhibited copper- and irradiation-induced LDL and HDL oxidation as observed by a reduction in oxidation rate and an increase in the lag phase (p<0.05). We used DPPH screening to measure free radical scavenging activity and observed that resveratrol (0-50microM) significantly reduced the content of free radicals (p<0.001). Respect to its effect on cholesterol homeostasis, resveratrol also enhanced apoA-1-mediated cholesterol efflux (r(2)=0.907, p<0.05, linear regression) by up-regulating ABCA-1 receptors, and reduced cholesterol influx or uptake in J774 macrophages (r(2)=0.89, p<0.05, linear regression). Incubation of macrophages (J774, THP-1 and MPM) with Fe/ascorbate ion, attenuated apoA-1 and HDL(3)-mediated cholesterol efflux whereas resveratrol (0-25microM) significantly redressed this attenuation in a dose-dependent manner (p<0.001). Resveratrol thus appears to be a natural antioxidant that enhances cholesterol efflux. These properties make it a potential natural antioxidant that could be used to prevent and treat CVD.

Distinct HDL subclasses present similar intrinsic susceptibility to oxidation by HOCl

The heme protein myeloperoxidase (MPO) functions as a catalyst for lipoprotein oxidation. Hypochlorous acid (HOCl), a potent two-electron oxidant formed by the MPO-H(2)O(2)-chloride system of activated phagocytes, modifies antiatherogenic high-density lipoprotein (HDL). The structural heterogeneity and oxidative susceptibility of HDL particle subfractions were probed with HOCl. All distinct five HDL subfraction were modified by HOCl as demonstrated by the consumption of tryptophan residues and free amino groups, cross-linking of apolipoprotein AI, formation of HOCl-modified epitopes, increased electrophoretic mobility and altered content of unsaturated fatty acids in HDL subclasses. Small, dense HDL3 were less susceptible to oxidative modification than large, light HDL2 on a total mass basis at a fixed HOCl:HDL mass ratio of 1:32, but in contrast not on a particle number basis at a fixed HOCl:HDL molar ratio of 97:1. We conclude that structural and physicochemical differences between HDL subclasses do not influence their intrinsic susceptibility to oxidative attack by HOCl.

High dietary taurine reduces apoptosis and atherosclerosis in the left main coronary artery: association with reduced CCAAT/enhancer binding protein homologous protein and total plasma homocysteine but not lipidemia

We sought to determine whether taurine could specifically protect against coronary artery disease during an atherogenic diet and whether taurine affects the lipid profile, metabolites of methionine, and endothelial atherogenic systems. Rabbits were fed one of the following diets for 4 weeks: (1) control diet; (2) 0.5% cholesterol+1.0% methionine; or (3) 0.5% cholesterol+1.0% methionine+2.5% taurine. Endothelial function was examined, and the left main coronary artery atherosclerosis was quantified by stereology and semiquantitative immunohistochemistry to determine the endothelial expression of proteins related to the NO, renin-angiotensin, endoplasmic reticulum, and oxidative stress systems, as well as apoptosis. Taurine normalized hyperhomocysteinemia (P<0.05) and significantly reduced hypermethioninemia (P<0.05) but not lipidemia. The intima:media ratio was reduced by 28% (P=0.034), and atherosclerosis was reduced by 64% (P=0.012) and endothelial cell apoptosis by 30% (P<0.01). Endothelial cell CCAAT/enhancer binding protein homologous protein was normalized (P<0.05). Taurine failed to improve hyperlipidemia, endothelial function, or endothelial proteins related to the NO, renin-angiotensin, and oxidative stress systems. Taurine reduces left main coronary artery wall pathology associated with decreased plasma total homocysteine, methionine, apoptosis, and normalization of CCAAT/enhancer binding protein homologous protein. These results elucidate the antiapoptotic and antiatherogenic properties of taurine, possibly via normalization of endoplasmic reticulum stress.

Myeloperoxidase and inflammatory proteins: pathways for generating dysfunctional high-density lipoprotein in humans

High-density lipoprotein (HDL) inhibits atherosclerosis by removing cholesterol from artery wall macrophages. Additionally, HDL is anti-inflammatory in animal studies, suggesting that this property might also be important for its cardioprotective effects. Recent studies in subjects with established cardiovascular disease (CVD) demonstrate that myeloperoxidase targets HDL for oxidation and blocks the lipoprotein's ability to remove excess cholesterol from cells, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Shotgun proteomic analysis of HDL identified multiple complement regulatory proteins, protease inhibitors, and acute-phase response proteins, supporting a central role for HDL in inflammation. Mass spectrometry and biochemical analyses demonstrated that HDL(3) from CVD subjects was selectively enriched in apolipoprotein E, suggesting that it carries a unique cargo of proteins in humans with clinically significant CVD. Thus, oxidative modifications to HDL and changes in its protein composition might be useful biomarkers-and perhaps mediators-of CVD.

Effects of protein oxidation on the structure and stability of model discoidal high-density lipoproteins

High-density lipoproteins (HDLs) prevent atherosclerosis by removing cholesterol from macrophages and by providing antioxidants for low-density lipoproteins. Oxidation of HDLs affects their functions via the complex mechanisms that involve multiple protein and lipid modifications. To differentiate between the roles of oxidative modifications in HDL proteins and lipids, we analyzed the effects of selective protein oxidation by hypochlorite (HOCl) on the structure, stability, and remodeling of discoidal HDLs reconstituted from human apolipoproteins (A-I, A-II, or C-I) and phosphatidylcholines. Gel electrophoresis and electron microscopy revealed that, at ambient temperatures, protein oxidation in discoidal complexes promotes their remodeling into larger and smaller particles. Thermal denaturation monitored by far-UV circular dichroism and light scattering in melting and kinetic experiments shows that protein oxidation destabilizes discoidal lipoproteins and accelerates protein unfolding, dissociation, and lipoprotein fusion. This is likely due to the reduced affinity of the protein for lipid resulting from oxidation of Met and aromatic residues in the lipid-binding faces of amphipathic alpha-helices and to apolipoprotein cross-linking into dimers and trimers on the particle surface. We conclude that protein oxidation destabilizes HDL disk assembly and accelerates its remodeling and fusion. This result, which is not limited to model discoidal but also extends to plasma spherical HDL, helps explain the complex effects of oxidation on plasma lipoproteins.

Hypochlorite-modified high-density lipoprotein acts as a sink for myeloperoxidase in vitro

AIMS

Myeloperoxidase (MPO), a cardiovascular risk factor in humans, is an in vivo catalyst for lipoprotein modification via intermediate formation of reactive chlorinating species. Among the different lipoprotein classes, anti-atherogenic high-density lipoprotein (HDL) represents a major target for modification by hypochlorous acid (HOCl), generated from H2O2 by MPO in the presence of physiological chloride concentrations. As MPO was identified as an HDL-associated protein that could facilitate selective oxidative modification of its physiological carrier, the aim of the present study was to investigate whether and to what extent modification of HDL by HOCl affects the binding affinity of MPO in vitro.

METHODS AND RESULTS

We show that binding affinity of 125I-labelled MPO to HDL markedly increases as a function of increasing extent of HOCl modification of HDL. In contrast to native HDL, HOCl-HDL potently inhibits MPO binding/uptake by endothelial cells and effectively attenuates metabolism of MPO by macrophages. Reduction of HDL-associated chloramines with methionine strongly impaired binding affinity of MPO towards HOCl-HDL. This indicates that N-chloramines generated by HOCl are regulators of the high-affinity interaction between HOCl-HDL and positively charged MPO. Most importantly, the presence of HOCl-HDL is almost without effect on the halogenating activity of MPO.

CONCLUSION

We propose that MPO-dependent modification of HDL and concomitant increase in the binding affinity for MPO could generate a vicious cycle of MPO transport to and MPO-dependent modification at sites of chronic inflammation.

Oxidation by hypochlorite converts protective HDL into a potent platelet agonist

High density lipoproteins (HDL) represent a protective factor of central importance that counteracts the development of cardiovascular disease, in part by normalizing platelet (hyper)reactivity. As HDL represent an efficient scavenger of the naturally occurring oxidant hypochlorite, this work was intended to investigate the influence of hypochlorite-oxidized HDL on platelet function. Addition of hypochlorite-oxidized HDL to human platelets results in an immediate and transient raise in intracellular calcium, surface expression of P-selectin and platelet aggregation. The observed effects are dose dependent and can be blocked by an antibody directed against the lipoprotein-binding domain of platelet thrombospondin- and scavenger receptor CD36.

Myeloperoxidase: an inflammatory enzyme for generating dysfunctional high density lipoprotein

PURPOSE OF REVIEW

Evidence indicates that high density lipoprotein (HDL) is cardioprotective and that several mechanisms are involved. One important pathway is a membrane-associated ATP-binding cassette transporter, ABCA1, that clears cholesterol from macrophage foam cells. Anti-inflammatory and antioxidant properties also might contribute to HDL's ability to inhibit atherosclerosis.

RECENT FINDINGS

Myeloperoxidase targets HDL for oxidation, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Myeloperoxidase-dependent oxidation of apolipoprotein A-I, the major protein in HDL, blocks HDL's ability to remove excess cholesterol from cells by the ABCA1 pathway. Analysis of mutated forms of apoA-I and oxidized apoA-I treated with methionine sulfoxide reductase implicate oxidation of specific tyrosine and methionine residues in impairing the ABCA1 transport activity of apoA-I. The crystal structure of lipid-free apoA-I suggests that such oxidative damage might disrupt negatively charged regions on the protein's surface or alter its remodeling, resulting in conformations that fail to interact with ABCA1.

SUMMARY

Oxidation of HDL by myeloperoxidase may represent a specific molecular mechanism for converting the cardioprotective lipoprotein into a dysfunctional form, raising the possibility that the enzyme represents a potential therapeutic target for preventing vascular disease in humans. Moreover, oxidized HDL might prove useful as a blood marker for clinically significant cardiovascular disease in humans.

Mild oxidation promotes and advanced oxidation impairs remodeling of human high-density lipoprotein in vitro

High-density lipoproteins (HDLs) prevent atherosclerosis by removing cholesterol from macrophages and by exerting antioxidant and anti-inflammatory effects. Oxidation is thought to impair HDL functions, yet certain oxidative modifications may be advantageous; thus, mild oxidation reportedly enhances cell cholesterol uptake by HDL whereas extensive oxidation impairs it. To elucidate the underlying energetic and structural basis, we analyzed the effects of copper and hypochlorite (which preferentially oxidize lipids and proteins, respectively) on thermal stability of plasma spherical HDL. Circular dichroism, light scattering, calorimetry, gel electrophoresis, and electron microscopy showed that mild oxidation destabilizes HDL and accelerates protein dissociation and lipoprotein fusion, while extensive oxidation inhibits these reactions; this inhibition correlates with massive protein cross-linking and with lipolysis. We propose that mild oxidation lowers kinetic barriers for HDL remodeling due to diminished apolipoprotein affinity for lipids resulting from oxidation of methionine and aromatic residues in apolipoproteins A-I and A-II followed by protein cross-linking into dimers and/or trimers. In contrast, advanced oxidation inhibits protein dissociation and HDL fusion due to lipid redistribution from core to surface upon lipolysis and to massive protein cross-linking. Our results help reconcile the apparent controversy in the studies of oxidized HDL and suggest that mild oxidation may benefit HDL functions.

Vitamin C preserves the cardio-protective paraoxonase activity of high-density lipoprotein during oxidant stress

HDL-associated paraoxonase (PON) antioxidant enzyme activity is cardio-protective. We investigated whether vitamin C prevented loss of PON activity from HDL during oxidant stress. HDL was incubated with either hydrophilic or lipophilic peroxyl radical initiators in the absence (control) or presence of vitamin C (50 and 100 micromol/L). Regardless of the type of radical, accumulation of lipid oxidation products in HDL was similar in incubations lacking vitamin C. Loss of PON activity was greater in HDL exposed to hydrophilic, in contrast to lipophilic, radicals, but addition of vitamin C maintained enzyme activity. Vitamin C's capacity to attenuate loss of the HDL ability to prevent atherogenic modification of LDL (assessed as electrophoretic mobility) was, however, modest, and appeared limited only to those incubations in which HDL was exposed to lipophilic radicals. Our results indicate that vitamin C may, under some conditions, prevent loss of cardio-protective function from HDL during oxidant stress.

The effect of atorvastatin on serum myeloperoxidase and CRP levels in patients with acute coronary syndrome

BACKGROUND

Inflammation is involved in the atherogenesis and pathogenesis of acute coronary syndrome (ACS). As the acute-phase reaction proteins in ACS, myeloperoxidase (MPO) and C-reactive protein (CRP) may play critical roles. Anti-inflammation may be one of benefits of statin drugs in ACS. Studies have showed that statins can suppress serum CRP concentrations. However, whether statins also reduce serum MPO concentrations in patients with ACS is unknown.

METHODS

Seventy-eight patients with ACS were randomly separated into Group A and Group B, the patients in Group A receiving conventional therapy, which include no cholesterol-lowering drugs, +atorvastatin (10 mg/day, n=40), the patients in Group B receiving conventional therapy (n=38). The serum concentrations of MPO were measured by enzyme-linked immunosorbent assay (ELISA) and CRP were measured by turbidimetric immunoassay.

RESULTS

Serum concentrations of MPO were significantly lower after 1-week therapy in both groups of patients [Group A from 590+/-168 to 496+/-154 microg/l, Group B from 570+/-165 to 521+/-153 microg/l; P<0.01, respectively]. Serum concentrations of CRP also were markedly lower than pretreatment [Group A from 6.56+/-1.87 to 5.14+/-2.07 mg/l; Group B from 6.36+/-1.94 to 5.45+/-1.90 mg/l, P<0.05, respectively]. Compared with conventional therapy alone, atorvastatin significantly further reduced serum MPO [P=0.014] and CRP concentrations [P=0.032]. There were no correlations detected between the reduction of MPO and CRP (r=0.124, P=0.068).

CONCLUSIONS

Atorvastatin reduced serum MPO and CRP concentrations in patients with ACS. These effects may explain some clinical benefits of statins in the treatment of these patients.

Modification of low-density lipoprotein by myeloperoxidase-derived oxidants and reagent hypochlorous acid

Substantial evidence supports the notion that oxidative processes contribute to the pathogenesis of atherosclerosis and coronary heart disease. The nature of the oxidants that give rise to the elevated levels of oxidised lipids and proteins, and decreased levels of antioxidants, detected in human atherosclerotic lesions are, however, unclear, with multiple species having been invoked. Over the last few years, considerable data have been obtained in support of the hypothesis that oxidants generated by the heme enzyme myeloperoxidase play a key role in oxidation reactions in the artery wall. In this article, the evidence for a role of myeloperoxidase, and oxidants generated therefrom, in the modification of low-density lipoprotein, the major source of lipids in atherosclerotic lesions, is reviewed. Particular emphasis is placed on the reactions of the reactive species generated by this enzyme, the mechanisms and sites of damage, the role of modification of the different components of low-density lipoprotein, and the biological consequences of such oxidation on cell types present in the artery wall and in the circulation, respectively.

Structural modifications of HDL and functional consequences

High density lipoproteins (HDL) are susceptible to structural modifications mediated by various mechanisms including oxidation, glycation, homocysteinylation or enzymatic degradation. Structural alterations of HDL may affect their functional and atheroprotective properties. Oxidants, such as hypochlorous acid, peroxyl radicals, metal ions, peroxynitrite, lipoxygenases and smoke extracts, can alter both surface and core components of HDL. The formation of lipid peroxidation derivatives, such as thiobarbituric acid reactive substances, conjugated dienes, lipid hydroperoxides and aldehydes, is associated with changes of physical properties (fluidity, molecular order) and of apoprotein conformation. Non-enzymatic glycation, generally associated with lipoxidation, leads to form irreversible complexes called advanced glycation end products. These HDL modifications are accompanied with altered biological activities of HDL and associated enzymes, including paraoxonase, CETP and LCAT. Homocysteine-induced modification of HDL is mediated by homocysteine-thiolactone, and can be prevented by a calcium-dependent thiolactonase/paraoxonase. Tyrosylation of HDL induces the formation of dimers and trimers of apo AI, and alters cholesterol efflux. Phospholipases and proteolytic enzymes can also modify HDL lipid and apoprotein structure. HDL modification induces generally the loss of their anti-inflammatory and cytoprotective properties. This could play a role in the pathogenesis of atherosclerosis and neurodegenerative diseases such as Alzheimer's disease.

Myeloperoxidase-mediated oxidation of high-density lipoproteins: Fingerprints of newly recognized potential proatherogenic lipoproteins

Substantial evidence supports the notion that oxidative processes participate in the pathogenesis of atherosclerotic heart disease. Major evidence for myeloperoxidase (MPO) as enzymatic catalyst for oxidative modification of lipoproteins in the artery wall has been suggested in numerous studies performed with low-density lipoprotein. In contrast to low-density lipoprotein, plasma levels of high-density lipoprotein (HDL)-cholesterol and apoAI, the major apolipoprotein of HDL, inversely correlate with the risk of developing coronary artery disease. These antiatherosclerotic effects are attributed mainly to HDL's capacity to transport excess cholesterol from arterial wall cells to the liver during 'reverse cholesterol transport'. There is now strong evidence that HDL is a selective in vivo target for MPO-catalyzed oxidation impairing the cardioprotective and antiinflammatory capacity of this antiatherogenic lipoprotein. MPO is enzymatically active in human lesion material and was found to be associated with HDL extracted from human atheroma. MPO-catalyzed oxidation products are highly enriched in circulating HDL from individuals with cardiovascular disease where MPO concentrations are also increased. The oxidative potential of MPO involves an array of intermediate-generated reactive oxygen and reactive nitrogen species and the ability of MPO to generate chlorinating oxidants-in particular hypochlorous acid/hypochlorite-under physiological conditions is a unique and defining activity for this enzyme. All these MPO-generated reactive products may affect structure and function of HDL as well as the activity of HDL-associated enzymes involved in conversion and remodeling of the lipoprotein particle, and represent clinically useful markers for atherosclerosis.

Tyrosine modification is not required for myeloperoxidase-induced loss of apolipoprotein A-I functional activities

Apolipoprotein A-I (apoAI), the major protein of high density lipoprotein, plays an important role in reverse cholesterol transport via its activity as an ABCA1-dependent acceptor of cellular cholesterol. We reported recently that myeloperoxidase (MPO) modification of apoAI inhibits its ABCA1-dependent cholesterol acceptor activity (Zheng, L., Nukuna, B., Brennan, M. L., Sun, M., Goormastic, M., Settle, M., Schmitt, D., Fu, X., Thomson, L., Fox, P. L., Ischiropoulos, H., Smith, J. D., Kinter, M., and Hazen, S. L. (2004) J. Clin. Invest. 114, 529-541). We also reported that MPO-mediated chlorination preferentially modifies two of the seven tyrosines in apoAI, and loss of parent peptides containing these residues dose-dependently correlates with loss in ABCA1-mediated cholesterol acceptor activity (Zheng, L., Settle, M., Brubaker, G., Schmitt, D., Hazen, S. L., Smith, J. D., and Kinter, M. (2005) J. Biol. Chem. 280, 38-47). To determine whether oxidative modification of apoA-I tyrosine residues was responsible for the MPO-mediated inactivation of cholesterol acceptor activity, we made recombinant apoAI with site-specific substitutions of all seven tyrosine residues to phenylalanine. ApoAI and the tyrosine-free apoAI were equally susceptible to dose-dependent MPO-mediated loss of ABCA1-dependent cholesterol acceptor activity, as well as lipid binding activity. MPO modification altered the migration of apoAI on SDS gels and decreased its alpha-helix content. MPO-induced modification also targeted apoAI tryptophan and lysine residues. Specifically, we detected apoAI tryptophan oxidation to mono- and dihydroxytryptophan and apoAI lysine modification to chlorolysine and 2-aminoadipic acid. Thus, tyrosine modification of apoAI is not required for its MPO-mediated inhibition of cholesterol acceptor activity.

Formation of Dysfunctional High-Density Lipoprotein by Myeloperoxidase

Recent studies identify the presence of high-density lipoprotein (HDL) particles in patients with cardiovascular disease, which are "dysfunctional," lacking in typical atheroprotective properties, and promoting proinflammatory effects. The mechanisms for generating dysfunctional HDL have been unclear. New evidence points to a role for myeloperoxidase (MPO)-generated oxidants as participants in rendering HDL dysfunctional within human atherosclerotic plaque. Myeloperoxidase was recently shown to bind to HDL within human atherosclerotic lesions, and biophysical studies reveal MPO binding occurs via specific interactions with apolipoprotein (apo) A-I, the predominant protein of HDL. This likely facilitates the observed selective targeting of apoA-I for site-specific chlorination and nitration by MPO-generated reactive oxidants in vivo. One apparent consequence of MPO-catalyzed apoA-I oxidation includes the functional impairment of the ability of HDL to promote cellular cholesterol efflux via the adenosine triphosphate binding cassette-1 transport system. Myeloperoxidase-mediated loss of the atheroprotective functional properties of HDL may thus provide a novel mechanism linking inflammation and oxidative stress to the pathogenesis of atherosclerosis.