Reduction of phosphatidylcholine hydroperoxide by apolipoprotein A-I: purification of the hydroperoxide-reducing proteins from human blood plasma

The role of high-density lipoprotein cholesterol, apolipoprotein A and paraoxonase-1 in the pathophysiology of neuroprogressive disorders

Lowered high-density lipoprotein (HDL) cholesterol has been reported in major depressive disorder, bipolar disorder, first episode of psychosis, and schizophrenia. HDL, its major apolipoprotein component, ApoA1, and the antioxidant enzyme paraoxonase (PON)1 (which is normally bound to ApoA1) all have anti-atherogenic, antioxidant, anti-inflammatory, and immunomodulatory roles, which are discussed in this paper. The paper details the pathways mediating the anti-inflammatory effects of HDL, ApoA1 and PON1 and describes the mechanisms leading to compromised HDL and PON1 levels and function in an environment of chronic inflammation. The molecular mechanisms by which changes in HDL, ApoA1 and PON1 might contribute to the pathophysiology of the neuroprogressive disorders are explained. Moreover, the anti-inflammatory actions of ApoM-mediated sphingosine 1-phosphate (S1P) signalling are reviewed as well as the deleterious effects of chronic inflammation and oxidative stress on ApoM/S1P signalling. Finally, therapeutic interventions specifically aimed at improving the levels and function of HDL and PON1 while reducing levels of inflammation and oxidative stress are considered. These include the so-called Mediterranean diet, extra virgin olive oil, polyphenols, flavonoids, isoflavones, pomegranate juice, melatonin and the Mediterranean diet combined with the ketogenic diet.

Long-Chain Base (LCB)-Targeted Lipidomics Study Uncovering the Presence of a Variety of LCBs in Mammalian Blood

Globotriaosylsphingosine (LysoGb3) is a biomarker for Fabry disease (OMIM 301500) that contains long-chain bases (LCBs) as a building block. There have been several studies proposing that LysoGb3 forms with distinct LCBs could be putative disease subtype-related biomarkers for this congenital disorder; however, there have been no detailed multiple reaction monitoring-based studies examining the LCB distribution in this lysosphingolipid. To achieve this, we established an assay procedure that aimed at elucidating the LCB-targeted lipidome using liquid chromatography–tandem mass spectrometry. Consistent with previous studies, we found d18:1 to be the major LCB species of the LysoGb3 in pooled human plasma, while some atypical LCBs, such as d18:2, d18:0, t18:1, d16:1, and d17:1, were detected as minor fractions. When the same methodology was applied to fetal bovine serum (FBS) as a positive control, we identified additional unique LCB species, such as t18:0, d20:1, t19:1, and t21:1, in herbivore LysoGb3. Furthermore, we found an elevation of sphingosine and LysoGb3, which are N-deacylated forms of ceramide and Gb3, respectively, in FBS, suggesting that ceramidase activity may be involved in this process. Thus, our LCB-targeted lipidomics data revealed that mammalian LCBs in glycosphingolipids have a greater variety of molecular species than previously expected.

Modified lipoproteins in periodontitis: a link to cardiovascular disease?

There is a strong association between periodontal disease and atherosclerotic cardiovascular disorders. A key event in the development of atherosclerosis is accumulation of modified lipoproteins within the arterial wall. We hypothesise that patients with periodontitis have an altered lipoprotein profile towards an atherogenic form. Therefore, the present study aims at identifying modifications of plasma lipoproteins in periodontitis. Lipoproteins from ten female patients with periodontitis and gender- and age-matched healthy controls were isolated by density-gradient ultracentrifugation. Proteins were separated by 2D gel-electrophoresis and identified by map-matching or by nano-LC followed by MS. Apolipoprotein (Apo) A-I (ApoA-I) methionine oxidation, Oxyblot, total antioxidant capacity and a multiplex of 71 inflammation-related plasma proteins were assessed. Reduced levels of apoJ, phospholipid transfer protein, apoF, complement C3, paraoxonase 3 and increased levels of α-1-antichymotrypsin, apoA-II, apoC-III were found in high-density lipoprotein (HDL) from the patients. In low-density lipoprotein (LDL)/very LDL (VLDL), the levels of apoL-1 and platelet-activating factor acetylhydrolase (PAF-AH) as well as apo-B fragments were increased. Methionine oxidation of apoA-I was increased in HDL and showed a relationship with periodontal parameters. α-1 antitrypsin and α-2-HS glycoprotein were oxidised in LDL/VLDL and antioxidant capacity was increased in the patient group. A total of 17 inflammation-related proteins were important for group separation with the highest discriminating proteins identified as IL-21, Fractalkine, IL-17F, IL-7, IL-1RA and IL-2. Patients with periodontitis have an altered plasma lipoprotein profile, defined by altered protein levels as well as post-translational and other structural modifications towards an atherogenic form, which supports a role of modified plasma lipoproteins as central in the link between periodontal and cardiovascular disease (CVD).

Inhibition of Triggering Receptor Expressed on Myeloid Cells 1 Ameliorates Inflammation and Macrophage and Neutrophil Activation in Alcoholic Liver Disease in Mice

Alcoholic liver disease (ALD) is characterized by macrophage and neutrophil leukocyte recruitment and activation in the liver. Damage‐ and pathogen‐associated molecular patterns contribute to a self‐perpetuating proinflammatory state in ALD. Triggering receptor expressed on myeloid cells 1 (TREM‐1) is a surface receptor that amplifies inflammation induced by toll‐like receptors (TLRs) and is expressed on neutrophils and monocytes/macrophages. We hypothesized that TREM‐1 signaling contributes to proinflammatory pathway activation in ALD. Using an in vivo ALD model in mice, we tested the effects of ligand‐independent TREM‐1 inhibitory peptides that were formulated into human high‐density lipoprotein (HDL)‐mimicking complexes GF9‐HDL and GA/E31‐HDL. As revealed in vitro, macrophages endocytosed these rationally designed complexes through scavenger receptors. A 5‐week alcohol feeding with the Lieber‐DeCarli diet in mice resulted in increased serum alanine aminotransferase (ALT), liver steatosis, and increased proinflammatory cytokines in the liver. TREM‐1 messenger RNA (mRNA) expression was significantly increased in alcohol‐fed mice, and TREM‐1 inhibitors significantly reduced this increase. TREM‐1 inhibition significantly attenuated alcohol‐induced spleen tyrosine kinase (SYK) activation, an early event in both TLR4 and TREM‐1 signaling. The TREM‐1 inhibitors significantly inhibited macrophage (epidermal growth factor‐like module‐containing mucin‐like hormone receptor‐like 1 [F4/80], clusters of differentiation [CD]68) and neutrophil (lymphocyte antigen 6 complex, locus G [Ly6G] and myeloperoxidase [MPO]) markers and proinflammatory cytokines (monocyte chemoattractant protein 1 [MCP‐1], tumor necrosis factor α [TNF‐α], interleukin‐1β [IL‐1β], macrophage inflammatory protein 1α [MIP‐1α]) at the mRNA level compared to the HDL vehicle. Administration of TREM‐1 inhibitors ameliorated liver steatosis and early fibrosis markers (α‐smooth muscle actin [αSMA] and procollagen1α [Pro‐Col1α]) at the mRNA level in alcohol‐fed mice. However, the HDL vehicle also reduced serum ALT and some cytokine protein levels in alcohol‐fed mice, indicating HDL‐related effects. Conclusion: HDL‐delivered novel TREM‐1 peptide inhibitors ameliorate early phases of inflammation and neutrophil and macrophage recruitment and activation in the liver and attenuate hepatocyte damage and liver steatosis. TREM‐1 inhibition represents a promising therapeutic approach for further investigations in ALD.

Feasibility of a plasma bioassay to assess oxidative protection of low-density lipoproteins by high-density lipoproteins

BACKGROUND

Traditionally, the impact of lipoproteins on vascular disease has been evaluated in light of their quantity, that is, cholesterol content, in plasma. However, recent studies of high-density lipoproteins (HDLs) have focused on functionality with regard to atheroprotection. For example, bioassays have emerged to assess the ability of HDL, in its near native plasma environment, to promote cholesterol removal (efflux) from cells. As a result, attention has focused on developing plasma-based assays for other putative HDL protective functions including protecting low-density lipoproteins (LDLs) from oxidative damage.

OBJECTIVE

To determine the feasibility of such an assay in a complex sample such as plasma, we evaluated the contribution of HDL vs other plasma factors in preventing LDL oxidation.

METHODS

We separated normolipidemic human plasma by gel filtration chromatography and assessed each fraction for its ability to prevent LDL modification by water soluble radical and copper-initiated oxidation mechanisms.

RESULTS

Using proteomics and selective precipitation methods, we identified major antioxidative contributions for fibrinogen, immunoglobulin G, albumin, and small soluble molecules like uric acid and ascorbate, with albumin being especially dominant in copper-initiated mechanisms. HDL particles were minor contributors (∼1%-2%) to the antioxidant capacity of plasma, irrespective of oxidation mechanism.

CONCLUSIONS

Given the overwhelming background of antioxidant capacity inherent to highly abundant plasma proteins, specific bioassays of HDL antioxidative function will likely require its complete separation from plasma.

Elevated plasma albumin and apolipoprotein A-I oxidation under suboptimal specimen storage conditions

S-cysteinylated albumin and methionine-oxidized apolipoprotein A-I (apoA-I) have been posed as candidate markers of diseases associated with oxidative stress. Here, a dilute-and-shoot form of LC-electrospray ionization-MS requiring half a microliter of blood plasma was employed to simultaneously quantify the relative abundance of these oxidized proteoforms in samples stored at -80 °C, -20 °C, and room temperature and exposed to multiple freeze-thaw cycles and other adverse conditions in order to assess the possibility that protein oxidation may occur as a result of poor sample storage or handling. Samples from a healthy donor and a participant with poorly controlled type 2 diabetes started at the same low level of protein oxidation and behaved similarly; significant increases in albumin oxidation via S-cysteinylation were found to occur within hours at room temperature and days at -20 °C. Methionine oxidation of apoA-I took place on a longer time scale, setting in after albumin oxidation reached a plateau. Freeze-thaw cycles had a minimal effect on protein oxidation. In matched collections, protein oxidation in serum was the same as that in plasma. Albumin and apoA-I oxidation were not affected by sample headspace or the degree to which vials were sealed. ApoA-I, however, was unexpectedly found to oxidize faster in samples with lower surface-area-to-volume ratios. An initial survey of samples from patients with inflammatory conditions normally associated with elevated oxidative stress-including acute myocardial infarction and prostate cancer-demonstrated a lack of detectable apoA-I oxidation. Albumin S-cysteinylation in these samples was consistent with known but relatively brief exposures to temperatures above -30 °C (the freezing point of blood plasma). Given their properties and ease of analysis, these oxidized proteoforms, once fully validated, may represent the first markers of blood plasma specimen integrity based on direct measurement of oxidative molecular damage that can occur under suboptimal storage conditions.

Building multidimensional biomarker views of type 2 diabetes on the basis of protein microheterogeneity

BACKGROUND

In 2008, the US Food and Drug Administration (FDA) issued a Guidance for Industry statement formally recognizing (during drug development) the conjoined nature of type 2 diabetes (T2D) and cardiovascular disease (CVD), which has precipitated an urgent need for panels of markers (and means of analysis) that are able to differentiate subtypes of CVD in the context of T2D. Here, we explore the possibility of creating such panels using the working hypothesis that proteins, in addition to carrying time-cumulative marks of hyperglycemia (e.g., protein glycation in the form of Hb A(₁c)), may carry analogous information with regard to systemic oxidative stress and aberrant enzymatic signaling related to underlying pathobiologies involved in T2D and/or CVD.

METHODS

We used mass spectrometric immunoassay to quantify, in targeted fashion, relative differences in the glycation, oxidation, and truncation of 11 specific proteins.

RESULTS

Protein oxidation and truncation (owing to modified enzymatic activity) are able to distinguish between subsets of diabetic patients with or without a history of myocardial infarction and/or congestive heart failure where markers of glycation alone cannot.

CONCLUSION

Markers based on protein modifications aligned with the known pathobiologies of T2D represent a reservoir of potential cardiovascular markers that are needed to develop the next generation of antidiabetes medications.

Antiatherogenic function of HDL particle subpopulations: focus on antioxidative activities

Oxidative stress, an emerging risk factor for premature atherosclerosis and cardiovascular disease, mediates the formation of proinflammatory, pro-atherogenic oxidized low-density lipoprotein (oxLDL) in the arterial intima. Circulating HDL particles, and particularly small, dense, protein-rich HDL3, may provide potent protection of LDL in vivo from oxidative damage by free radicals in the arterial intima, resulting in the inhibition of the generation of proinflammatory oxidized lipids, primarily lipid hydroperoxides (LOOH) but also short-chain oxidized phospholipids (oxPL). HDL-mediated inactivation of LOOH involves initial transfer of phospholipid hydroperoxides (PLOOH) from LDL to HDL3, which is governed by the rigidity of the surface monolayer of HDL, and subsequent reduction of PLOOH by redox-active Met residues of apolipoprotein A-I (apoA-I) with the formation of phospholipid hydroxides (PLOH) and methionine sulphoxides. HDL-associated enzymes may in turn contribute to the hydrolytic inactivation of short-chain oxPL. Mounting evidence suggests that the integrated antioxidative activity of HDL appear to be defective in atherogenic dyslipidaemias involving low HDL-cholesterol levels; anomalies in the proteome and lipidome of HDL particles in dyslipidaemic patients may underlie such functional deficiency. Pharmacological normalization of HDL metabolism concomitantly with correction of circulating levels, composition and biological activities of HDL particles, with enrichment in apoA-I and reduction in HDL surface rigidity, may constitute an efficacious therapeutic approach to attenuate atherosclerosis in dyslipidaemic patients at high cardiovascular risk.

HDL3-Mediated Inactivation of LDL-Associated Phospholipid Hydroperoxides Is Determined by the Redox Status of Apolipoprotein A-I and HDL Particle Surface Lipid Rigidity

Objectives— Small dense HDL3 particles of defined lipidome and proteome potently protect atherogenic LDL against free radical-induced oxidation; the molecular determinants of such antioxidative activity in these atheroprotective, antiinflammatory particles remain indeterminate.

Methods and Results— Formation of redox-active phosphatidylcholine hydroperoxides (PCOOH) and redox-inactive phosphatidylcholine hydroxides (PCOH) was initiated in LDL by free radical-induced oxidation. Human HDL3 inactivated LDL-derived PCOOH (−62%, P <0.01) and enhanced accumulation of PCOH (2.1-fold, P <0.05); in parallel, HDL3 accumulated minor amounts of PCOOH. Enzyme-deficient reconstituted dense HDL potently inactivated PCOOH (−43%, P <0.01). HDL3-mediated reduction of PCOOH to PCOH occurred concomitantly with oxidation of methionine residues in HDL3-apolipoprotein AI (apoAI). Preoxidation of methionine residues by chloramine T markedly attenuated PCOOH inactivation (−35%); by contrast, inhibition of HDL3-associated enzymes was without effect. PCOOH transfer rates from oxidized LDL to phospholipid liposomes progressively decreased with increment in the rigidity of the phospholipid monolayer.

Conclusions— The redox status of apoAI and surface lipid rigidity represent major determinants of the potent HDL3-mediated protection of LDL against free radical-induced oxidation. Initial transfer of PCOOH to HDL3 is modulated by the surface rigidity of HDL3 particles with subsequent reduction of PCOOH to PCOH by methionine residues of apoAI.

A sensitive and specific ELISA detects methionine sulfoxide-containing apolipoprotein A-I in HDL

Oxidized HDL has been proposed to play a key role in atherogenesis. A wide range of reactive intermediates oxidizes methionine residues to methionine sulfoxide (MetO) in apolipoprotein A-I (apoA-I), the major HDL protein. These reactive species include those produced by myeloperoxidase, an enzyme implicated in atherogenesis. The aim of the present study was to develop a sensitive and specific ELISA for detecting MetO residues in HDL. We therefore immunized mice with HPLC-purified human apoA-I containing MetO(86) and MetO(112) (termed apoA-I(+32)) to generate a monoclonal antibody termed MOA-I. An ELISA using MOA-I detected lipid-free apoA-I(+32), apoA-I modified by 2e-oxidants (hydrogen peroxide, hypochlorous acid, peroxynitrite), and HDL oxidized by 1e- or 2e-oxidants and present in buffer or human plasma. Detection was concentration dependent, reproducible, and exhibited a linear response over a physiologically plausible range of concentrations of oxidized HDL. In contrast, MOA-I failed to recognize native apoA-I, native apoA-II, apoA-I modified by hydroxyl radical or metal ions, or LDL and methionine-containing proteins other than apoA-I modified by 2e-oxidants. Because the ELISA we have developed specifically detects apoA-I containing MetO in HDL and plasma, it should provide a useful tool for investigating the relationship between oxidized HDL and coronary artery disease.

Increased methionine sulfoxide content of apoA-I in type 1 diabetes

Cardiovascular disease is a major cause of morbidity and premature mortality in diabetes. HDL plays an important role in limiting vascular damage by removing cholesterol and cholesteryl ester hydroperoxides from oxidized low density lipoprotein and foam cells. Methionine (Met) residues in apolipoprotein A-I (apoA-I), the major apolipoprotein of HDL, reduce peroxides in HDL lipids, forming methionine sulfoxide [Met(O)]. We examined the extent and sites of Met(O) formation in apoA-I of HDL isolated from plasma of healthy control and type 1 diabetic subjects to assess apoA-I exposure to lipid peroxides and the status of oxidative stress in the vascular compartment in diabetes. Three tryptic peptides of apoA-I contain Met residues: Q(84)-M(86)-K(88), W(108)-M(112)-R(116), and L(144)-M(148)-R(149). These peptides and their Met(O) analogs were identified and quantified by mass spectrometry. Relative to controls, Met(O) formation was significantly increased at all three locations (Met(86), Met(112), and Met(148)) in diabetic patients. The increase in Met(O) in the diabetic group did not correlate with other biomarkers of oxidative stress, such as N(epsilon)-malondialdehyde-lysine or N(epsilon)-(carboxymethyl)lysine, in plasma or lipoproteins. The higher Met(O) content in apoA-I from diabetic patients is consistent with increased levels of lipid peroxidation products in plasma in diabetes. Using the methods developed here, future studies can address the relationship between Met(O) in apoA-I and the risk, development, or progression of the vascular complications of diabetes.

Detection of specifically oxidized apolipoproteins in oxidized HDL

Atherosclerosis is associated with dysfunctional HDL, and oxidation of HDL is thought to give rise to HDL becoming dysfunctional. Lipoprotein oxidation represents a complex series of processes that can be assessed by various methods. In general, oxidation mediated by 1-electron or radical oxidants gives rise to lipid hydroperoxides (LOOHs) as the primary product. These LOOHs may then undergo further reactions giving rise to secondary lipid oxidation products and/or oxidation of lipoprotein-associated proteins. Thus, LOOHs specifically oxidize Met residues of apolipoprotein (apo) A-I and A-II (the major proteins of HDL) to MetO. Here we describe an HPLC-based method to detect oxidized HDL containing specifically oxidized forms of apoA-I and apoA-II. This method may be useful to assess the early stages of HDL oxidation in biological samples.

Establishment and evaluation of 2 monoclonal antibodies against oxidized apolipoprotein A-I (apoA-I) and its application to determine blood oxidized apoA-I levels

BACKGROUND

Apolipoprotein A-I (apoA-I) is the major lipoprotein component of high-density lipoprotein(HDL), and plays an important role in reverse cholesterol transport. Its function is known to be influenced by oxidation.

METHODS

Using H2O2-or chloramine T-oxidized apoA-I as antigen, we prepared 2 kinds of monoclonal antibodies, and established an ELISA system for the measurement of oxidized apoA-I.

RESULTS

The 2 monoclonal antibodies obtained, 7D3 and 98A7, exhibited different reactivity characteristics. The serum level of oxidized apoA-I was higher in patients with either inflammatory disease or diabetes than in healthy individuals, and suggested a diversity of oxidized apoA-I.

CONCLUSION

The 2 monoclonal antibodies are useful for the determination of oxidized apoA-I and study of diverse oxidized HDLs.

A proteomic analysis of murine bone marrow and its response to ionizing radiation

To characterize the mouse bone marrow tissue proteome and investigate the response to radiation damage we took bone marrow before and after 4-Gy gamma-irradiation from mouse strains (C57BL/6 and CBA/Ca) that differ in their short-term and long-term radiation responses and analyzed extracellular proteins by high-resolution 2-DE. Twenty proteins were identified from 71 protein spots in both C57BL/6 and CBA/Ca. We detected significant differences between control and irradiated bone marrow and between genotypes and identified many of the changed proteins by MS. In C57BL/6, 27 spots were significantly different between control and irradiated samples. In CBA/Ca, 18 spots showed significant changes following irradiation. Proteins such as serum albumin, apolipoprotein A-I, ferritin, haptoglobin (Hp) and alpha-1-antitrypsin were changed in irradiated bone marrow of both mouse strains, reflecting an ongoing acute-phase reaction. Several other proteins including serotransferrin, neutrophil collagenase, peroxiredoxin 2 and creatine kinase M chain were changed specifically in an individual mouse strain. The proteomic approach makes an important contribution to characterizing bone marrow proteome and investigating the tissue response of bone marrow to radiation, assists in identifying genotype-dependent responses and provides support for the importance of microenvironmental factors contributing to the overall response.

Detection of phosphatidylcholine oxidation products in rat heart using quadrupole time-of-flight mass spectrometry

An improved technique for the analysis of phosphatidylcholine (PC) and lyso-phosphatidylcholine (lyso-PC) oxidation products was developed using quadrupole time of flight (Q-TOF) mass spectrometry with electrospray ionization. We separated these products using an HPLC C(8) column with a gradient of methanol and 10mM aqueous ammonium acetate. Monohydroxides, oxo derivatives, and trihydroxides of palmitoyl-linoleoyl (C16:0/C18:2) PC, stearoyl-linoleoyl (C18:0/C18:2) PC, and oleoyl-linoleoyl (C18:1/C18:2) PC were detected mainly as MH(+) and [M+Na](+) ions in the heart of the intact rat. Using standard synthetic PC-OH (C16:0/C18:2-OH), the lipid extract component was identified as (C16:0/C18:2-OH) PC based on the product ions of ESI-MS-MS and, the PC-OH concentration was quantitated. Four oxidatively modified 1-lyso-phosphatidylcholines (lyso-PCs) were also detected. This is the first report showing the presence of monohydroxides, oxo derivatives, and trihydroxides of (C16:0/C18:2)PC, (C18:0/C18:2)PC, and (C18:1/C18:2) PC in the rat heart.

Phosphatidylcholine hydroperoxide levels in human plasma are lower than previously reported

The quantification of PC hydroperoxide (PCOOH) in human plasma was studied by HPLC with chemiluminescence detection (HPLC-CL). We identified for the first time the monohydroperoxide of 1-palmitoyl-2-linoleoyl-PC hydroperoxide (PC 16:0/18:2-OOH) in plasma by LC-MS and HPLC-CL. The standard compound, PC 16:0/18:2-OOH (synthetic PCOOH), as well as PCOOH from egg yolk, was used. Comparison of the PCOOH concentration in each participant's plasma as determined by use of a Finepak SIL NH2 column with 2-propanol/methanol/water as the mobile phase (system A, the conventional method) gave a higher concentration than did an LC-18-DB column with methanol containing 0.01% triethylamine (system B). The mean PCOOH concentration for the 43 healthy volunteers was 55.1+/-30.4 pmol/mL (mean+/-SD) for system A and 16.3+/-9.9 pmol/mL for system B. Moreover, the main peak of the plasma extract appeared at a different time from that of synthetic PCOOH or egg yolk PCOOH in system A, whereas in system B plasma sample retention time practically corresponded to that of standard PCOOH. These findings confirm that the PCOOH plasma concentration is not so high as previously reported.

Characterization of specifically oxidized apolipoproteins in mildly oxidized high density lipoprotein

Atherosclerosis is a state of heightened oxidative stress. Oxidized LDL is present in atherosclerotic lesions and used as marker for coronary artery disease, although in human lesions lipids associated with HDL are as oxidized as those of LDL. Here we investigated specific changes occurring to apolipoprotein A-I (apoA-I) and apoA-II, as isolated HDL and human plasma undergo mild, chemically induced oxidation, or autoxidation. During such oxidation, Met residues in apoA-I and apoA-II become selectively and consecutively oxidized to their respective Met sulfoxide (MetO) forms that can be separated by HPLC. Placing plasma at -20 degrees C prevents autoxidation, whereas metal chelators and butylated hydroxytoluene offer partial protection. Independent of the oxidation conditions, apoA-I and apoA-II (dimer) with two MetO residues accumulate as relatively stable oxidation products. Compared to controls, serum samples from subjects with the endothelial cell nitric oxide synthase a/b genotype that is associated with increased coronary artery disease contain increased concentrations of apoA-I with two MetO residues. Our results show that during the early stages, oxidation of HDL gives rise to specifically oxidized forms of apoA-I and apoA-II, some of which may be useful markers of in vivo HDL oxidation, and hence potentially atherosclerosis.

Detection of oxidized high-density lipoprotein

This paper reviews working procedures for the separation and detection of oxidized high-density lipoproteins (ox-HDL) and their constituents. It begins with an introductory overview of structural alterations of the HDL particle and its constituents generated during oxidation. The main body of the review delineates various procedures for the isolation and detection of ox-HDL as well as the purification and separation of phosphatidylcholine metabolites and denatured apolipoproteins in the particle. The useful methods published more recently are picked up and the utility of the separation techniques is described. The last section covers a clinical evaluation of changes in these factors in ox-HDL as well as future directions of ox-HDL research.

Plasmodium falciparum histidine-rich protein-2 (PfHRP2) modulates the redox activity of ferri-protoporphyrin IX (FePPIX): peroxidase-like activity of the PfHRP2-FePPIX complex

Histidine-rich protein-2 from Plasmodium falciparum (PfHRP2) binds up to 50 molecules of ferri-protoporphyrin IX (FePPIX) (Choi, C. Y., Cerda, J. F., Chu, H. A., Babcock, G. T., and Marletta, M. A. (1999) Biochemistry 38, 16916-16924). We reasoned that the PfHRP2-FePPIX complex has antioxidant properties that could be beneficial to the parasite. Therefore, we examined whether binding to PfHRP2 modulated the redox properties of FePPIX. We observed that PfHRP2 completely inhibited the auto-oxidation of ascorbate mediated by free FePPIX. We also investigated the peroxidase activity of PfHRP2-FePPIX using 13-hydroperoxy-9,11-octadienoate (18:2-OOH) as substrate. Reaction of PfHRP2-FePPIX with 18:2-OOH in the presence of added reducing agents gave 13-hydroxy-9,11-octadienoate (18:2-OH) as a major product and 13-keto-9,11-octadienoate (18:2=O) and 9,12,13-trihydroxy-10-octadecaenoate as minor products. Binding of FePPIX to PfHRP2 lowered the rate of decomposition of 18:2-OOH and increased the 18:2-OH to 18:2=O ratio. Similar to other authentic peroxidases, phenols, amines, and biological reductants like ascorbate promoted 18:2-OH production, and NaCN inhibited 18:2-OH production. Thioanisole also acted as a reductant and was converted to thioanisole sulfoxide, suggesting formation of compound I during the reaction. These data show that PfHRP2 modulates the redox activity of FePPIX and that the PfHRP2-FePPIX complex may have previously unrecognized antioxidant properties.

Apolipoprotein A-I(Milano) and apolipoprotein A-I(Paris) exhibit an antioxidant activity distinct from that of wild-type apolipoprotein A-I

Apolipoprotein A-I(Milano) (apoA-I(Milano)) and apoA-I(Paris) are rare cysteine variants of apoA-I that produce a HDL deficiency in the absence of cardiovascular disease in humans. This paradox provides the basis for the hypothesis that the cysteine variants possess a beneficial activity not associated with wild-type apoA-I (apoA-I(WT)). In this study, a unique antioxidant activity of apoA-I(Milano) and apoA-I(Paris) is described. ApoA-I(Milano) was twice as effective as apoA-I(Paris) in preventing lipoxygenase-mediated oxidation of phospholipids, whereas apoA-I(WT) was poorly active. Antioxidant activity was observed using the monomeric form of the variants and was equally effective before and after initiation of oxidative events. ApoA-I(Milano) protected phospholipid from reactive oxygen species (ROS) generated via xanthine/xanthine oxidase (X/Xo) but failed to inhibit X/Xo-induced reduction of cytochrome c. These results indicate that apoA-I(Milano) was unable to directly quench ROS in the aqueous phase. There were no differences between lipid-free apoA-I(Milano,) apoA-I(Paris), and apoA-I(WT) in mediating the efflux of cholesterol from macrophages, indicating that the cysteine variants interacted normally with the ABCA1 efflux pathway. The results indicate that incorporation of a free thiol within an amphipathic alpha helix of apoA-I confers an antioxidant activity distinct from that of apoA-I(WT). These studies are the first to relate gain of function to rare cysteine mutations in the apoA-I primary sequence.

Serum paraoxonase activity and the extent of lipid peroxidation are not affected by increased levels of human apolipoprotein A-I: studies in transgenic mice

The present study analyzed the effect of increased concentrations of human apolipoprotein (apo) A-I in transgenic mice serum on paraoxonase activity and on lipid peroxidation. In the transgenic mice serum, in comparison to control (non-transgenic) C57BL/6 mice, we found high concentrations of human apoA-I and high-density lipoprotein (HDL)-cholesterol, but serum lipid peroxidation (basal and free radical-induced) and serum paraoxonase activity were similar in the two mouse groups. Comparing the individual results, no significant correlation was found between free radical-induced serum lipid peroxidation and apoA-I concentrations. Serum paraoxonase activity also did not correlate with serum concentrations of human apoA-I. However, a significant inverse relationship (R2=0.75) was observed between the individual values of paraoxonase activity and free radical-induced lipid peroxidation in both mouse groups. Direct analysis of the effect of pure human apoA-I and paraoxonase (using the specific paraoxonase inhibitor PD-92770) on lipid peroxidation also revealed that paraoxonase, but not apoA-I, protects serum lipids from oxidation. We thus conclude that the increased human apoA-I concentration in the mouse serum neither affect serum paraoxonase activity, nor protects against lipid peroxidation, whereas paraoxonase significantly inhibits serum lipid peroxidation.

Oxidation of methionine residues affects the structure and stability of apolipoprotein A-I in reconstituted high density lipoprotein particles

To determine the effect of oxidative damage to lipid-bound apolipoprotein A-I (apo A-I) on its structure and stability that might be related to previously observed functional disorders of oxidized apo A-I in high density lipoproteins (HDL), we prepared homogeneous reconstituted HDL (rHDL) particles containing unoxidized apo A-I and its commonly occurring oxidized form (Met-112, 148 bis-sulfoxide). The size of the obtained discoidal rHDL particles ranged from 9.0 to 10.0 nm and did not depend upon the content of the oxidized protein. Using circular dichroism methods, no change in the secondary structure of lipid-bound oxidized apo A-I was found. Isothermal and thermal denaturation experiments showed a significant destabilization of the oxidized protein to denaturation by guanidine hydrochloride or heat. This effect was observed with and without co-reconstituted apolipoprotein A-II. Limited tryptic digestion indicated that the central region of oxidatively damaged apo A-I becomes exposed to proteolysis in the rHDL particles. Implications of these data for apolipoprotein function are discussed.

Oxidants and antioxidants in atherosclerosis

The oxidative theory suggests that LDL oxidation contributes to atherogenesis, implying that attenuation of this process by antioxidants should decrease atherosclerosis. However, a causative link between LDL oxidation and atherogenesis is not firmly established. It requires the identification of the oxidants that are responsible for the initiation of LDL oxidation, and an understanding of the modified moieties that are responsible for the proatherogenic activities of oxidized LDL. The present review summarizes recent data on potential biological oxidants for LDL in the vessel wall, and discusses the antiatherogenic role(s) of selected antioxidants.

Apolipoprotein A-I promotes the formation of phosphatidylcholine core aldehydes that are hydrolyzed by paraoxonase (PON-1) during high density lipoprotein oxidation with a peroxynitrite donor

High density lipoprotein (HDL) is rich in polyunsaturated phospholipids that are sensitive to oxidation. However, the effect of apolipoprotein A-I and paraoxonase-1 (PON-1) on phosphatidylcholine oxidation products has not been identified. We subjected native HDL, trypsinized HDL, and HDL lipid suspensions to oxidation by the peroxynitrite donor, 3-morpholinosydnonimine. HDL had a basal level of phosphatidylcholine mono- and di-hydroperoxides that increased to a greater extent in HDL, compared with either trypsinized HDL or HDL lipid alone. Phosphatidylcholine core aldehydes, which were present in small amounts, increased 10-fold during oxidation of native HDL, compared with trypsinized HDL (p = 0.004), and 4-fold compared with HDL lipid suspensions (p = 0.0021). In addition, the content of lysophosphatidylcholine increased 300% during oxidation of native HDL, but only 80 and 25%, respectively, during oxidation of trypsinized HDL and HDL lipid suspensions. Phosphatidylcholine isoprostanes accumulated in comparable amounts during the oxidation of all three preparations. Incubation of apolipoprotein A-I with 1-palmitoyl-2-linoleoyl glycerophosphocholine proteoliposomes in the presence of 3-morpholinosydnonimine or apoAI with phosphatidylcholine hydroperoxides resulted in a significant increase in phosphatidylcholine core aldehydes with no formation of lysophosphatidylcholine. We propose that apolipoprotein A-I catalyzes a one-electron oxidation of alkoxyl radicals. Purified PON-1 hydrolyzed phosphatidylcholine core aldehydes to lysophosphatidylcholine. We conclude that, upon HDL oxidation with peroxynitrite, apolipoprotein AI increases the formation of phosphatidylcholine core aldehydes that are subsequently hydrolyzed by PON1.

Fate of lipid hydroperoxides in blood plasma

Cholesteryl ester hydroperoxide (CE-OOH) and phosphatidylcholine hydroperoxide (PC-OOH) are the major primary oxidation products of lipoproteins. CE-OOH is present in human and rat plasmas while PC-OOH is undetectable. This is likely due to the enzymatic (plasma glutathione peroxidase) and the nonenzymatic (apolipoproteins A and B-100) reducing activities of PC-OOH in plasma, and to the enzymatic conversion of PC-OOH to CE-OOH by lecithin:cholesterol acyltransferase in high density lipoproteins. The regioisomeric distribution of CE-O(O)H in human plasma indicates that free radical-mediated chain oxidation is an ongoing process, even in healthy young individuals.

Anti-atherogenic effect of coenzyme Q10 in apolipoprotein E gene knockout mice

Oxidation of low-density lipoprotein (LDL) lipid is implicated in atherogenesis and certain antioxidants inhibit atherosclerosis. Ubiquinol-10 (CoQ10H2) inhibits LDL lipid peroxidation in vitro although it is not known whether such activity occurs in vivo, and, if so, whether this is anti-atherogenic. We therefore tested the effect of ubiquinone-10 (CoQ10) supplemented at 1% (w/w) on aortic lipoprotein lipid peroxidation and atherosclerosis in apolipoprotein E-deficient (apoE-/-) mice fed a high-fat diet. Hydroperoxides of cholesteryl esters and triacylglycerols (together referred to as LOOH) and their corresponding alcohols were used as the marker for lipoprotein lipid oxidation. Atherosclerosis was assessed by morphometry at the aortic root, proximal and distal arch, and the descending thoracic and abdominal aorta. Compared to controls, CoQ10-treatment increased plasma coenzyme Q, ascorbate, and the CoQ10H2:CoQ10 + CoQ10H2 ratio, decreased plasma alpha-tocopherol (alpha-TOH), and had no effect on cholesterol and cholesterylester alcohols (CE-OH). Plasma from CoQ10-supplemented mice was more resistant to ex vivo lipid peroxidation. CoQ10 treatment increased aortic coenzyme Q and alpha-TOH and decreased the absolute concentration of LOOH, whereas tissue cholesterol, cholesteryl esters, CE-OH, and LOOH expressed per bisallylic hydrogen-containing lipids were not significantly different. CoQ10-treatment significantly decreased lesion size in the aortic root and the ascending and the descending aorta. Together these data show that CoQ10 decreases the absolute concentration of aortic LOOH and atherosclerosis in apoE-/- mice.

Oxidation of methionine residues to methionine sulfoxides does not decrease potential antiatherogenic properties of apolipoprotein A-I

The initial stage of oxidation of high density lipoproteins (HDL) is accompanied by the lipid hydroperoxide-dependent, selective oxidation of two of the three Met residues of apolipoprotein A-I (apoA-I) to Met sulfoxides (Met(O)). Formation of such selectively oxidized apoA-I (i.e. apoA-I(+32)) may affect the antiatherogenic properties of HDL, because it has been suggested that Met(86) and Met(112) are important for cholesterol efflux and Met(148) is involved in the activation of lecithin:cholesterol acyl transferase (LCAT). We therefore determined which Met residues were oxidized in apoA-I(+32) and how such oxidation of apoA-I affects its secondary structure, the affinity for lipids, and its ability to remove lipids from human macrophages. We also assessed the capacity of discoidal reconstituted HDL containing apoA-I(+32) to act as substrate for LCAT, and the dissociation of apoA-I and apoA-I(+32) from reconstituted HDL. Met(86) and Met(112) were present as Met(O), as determined by amino acid sequencing and mass spectrometry of isolated peptides derived from apoA-I(+32). Selective oxidation did not alter the alpha-helicity of lipid-free and lipid-associated apoA-I as assessed by circular dichroism, and the affinity for LCAT was comparable for reconstituted HDL containing apoA-I or apoA-I(+32). Cholesteryl ester transfer protein mediated the dissociation of apoA-I more readily from reconstituted HDL containing apoA-I(+32) than unoxidized apoA-I. Also, compared with native apoA-I, apoA-I(+32) had a 2- to 3-fold greater affinity for lipid (as determined by the rate of clearance of multilamellar phospholipid vesicles) and its ability to cause efflux of [(3)H]cholesterol, [(3)H]phospholipid, and [(14)C]alpha-tocopherol from lipid-laden human monocyte-derived macrophages was significantly enhanced. By contrast, no difference was observed for cholesterol and alpha-tocopherol efflux to lipid-associated apolipoproteins. Together, these results suggest that selective oxidation of Met residues enhances rather than diminishes known antiatherogenic activities of apoA-I, consistent with the overall hypothesis that detoxification of lipid hydroperoxides by HDL is potentially antiatherogenic.

Dietary and pharmacological antioxidants in atherosclerosis

Antioxidants that inhibit LDL oxidation are thought to be potential anti-atherogenic compounds. The results of major human randomized trials with antioxidants have, however, been disappointing, except for probucol, which consistently inhibits restenosis. Similarly, animal intervention studies show that antioxidants do not generally inhibit atherosclerosis, although some compounds provide protection. Direct evidence for the oxidation of LDL causing atherosclerosis is needed. This article summarizes results from antioxidant intervention studies, and highlights some of the key issues that need to be addressed to link biochemical changes in the arterial wall more directly to the oxidation theory of atherosclerosis.

High density lipoproteins (HDL) and the oxidative hypothesis of atherosclerosis

The oxidative hypothesis of atherosclerosis classically implies a central role for low density lipoprotein (LDL) oxidation. However, new antiatherogenic properties have been recognized for high density lipoproteins (HDL), apart from their ability to reverse cholesterol transport. Indeed, native HDL could protect LDL from oxidation, thereby minimizing the deleterious consequences of this process. Several mechanisms have been suggested to explain this protective role. Two HDL-associated enzymes, paraoxonase and PAF-acetylhydrolase, detoxify oxidized phospholipids produced by lipid peroxidation. In addition, HDL could reduce hydroperoxides to their corresponding hydroxides. It has also been suggested that HDL could inhibit oxidized LDL-induced transduction signals. However, in vivo HDL oxidation in the subendothelial space would favor the atherosclerotic process. Indeed, atherogenic properties of these oxidized HDL partly result from some loss of their cholesterol effluxing capacity and from an inactivation of the lecithin-cholesterol acyltransferase, which is a HDL-associated enzyme involved in reverse cholesterol transport. Finally, oxidized HDL could induce cholesterol accumulation in macrophages. Further in-depth investigation is needed to assess these antagonistic effects and their consequences for the atherosclerotic process.

Effect of peroxisome proliferator on extracellular glutathione peroxidase in rat

Glutathione peroxidase (GPX) activity measured using tert-butyl hydroperoxide as a substrate detects solely cellular/classical GPX (cGPX) in rat liver and kidney, and extracellular/plasma glutathione peroxidase (EC-GPX) in rat serum. To investigate the effect of peroxisome proliferator on EC-GPX, we measured activities of GPX and catalase in rat liver, kidney and serum, and then we performed immunoblot and Northern blot analyses in the kidney. Rats were fed on a diet containing either 2% (w/w) di-2-ethylhexyl phthalate (DEHP) or 0.25% (w/w) clofibrate for two or three weeks, respectively. Catalase activity was increased 1.4-fold (p < 0.001) in the treated liver, but not in the kidney. GPX activity was decreased to 59.2% (DEHP) and 70.4% (clofibrate) of the control (p < 0.001) in the serum but was unaltered in the liver and kidney. The immunoreactivity for EC-GPX was also significantly decreased in the DEHP-treated kidney compared with the control. The mRNA levels of EC-GPX and cGPX were unaltered. The immunostaining for 4-hydroxy-2-nonenal, a maker of lipid peroxide, was more intense in the treated kidney compared with the control. These results suggest that EC-GPX is post-transcriptionally decreased by peroxisome proliferator through the oxidative stress in the renal tubules. This may be a new deleterious effect of an endocrine disruptor DEHP.

Reduction of lipid hydroperoxides by apolipoprotein B-100

We have previously isolated two proteins which can reduce phosphatidylcholine hydroperoxide (PC-OOH) from human blood plasma and identified one of the proteins as apolipoprotein A-I (Mashima, R. , et al. (1998) J. Lipid Res. 39, 1133-1140). In the present study we have identified the other protein as apolipoprotein B-100 (apo B-100) by amino acid sequence analysis of its tryptic peptides. The reactivity of lipid hydroperoxides with apo B-100 decreased in the order of PC-OOH > linoleic acid hydroperoxide > cholesteryl ester hydroperoxide under our experimental conditions. Pretreatment of apo B-100 with chloramine T, an oxidant of methionine, diminished the PC-OOH-reducing activity, indicating that some of 78 methionines are responsible for the reduction of PC-OOH. Despite the presence of 6 methionines in albumin, albumin was inactive to reduce PC-OOH. Free methionine was also inactive. These data suggest that the accessibility and binding of lipid hydroperoxides to the protein methionine residues are crucial for reduction of lipid hydroperoxides.