The oxidation of lipoproteins by monocytes-macrophages. Biochemical and biological mechanisms

In vivo antiatherogenic properties of olive oil and its constituent lipid classes in hyperlipidemic rabbits

BACKGROUND AND AIM

The consumption of olive oil has been associated with lower incidence of cardiovascular disease in the Mediterranean countries. This may be due in part to the action of platelet-activating factor (PAF) antagonists which we have previously demonstrated to be present in olive oil. In order to assess the in vivo effects of olive oil lipids and PAF in the development of atherosclerosis, the effects of diet supplementation with olive oil (OO), olive oil polar lipid extract (OOPLE) and olive oil neutral lipid extract (OONLE) were studied in rabbits fed a cholesterol-enriched diet.

METHODS AND RESULTS

Rabbits were fed for 45 days with atherogenic diet (Group A) supplemented with OO (Group B), OOPLE (Group C) or OONLE (Group D). Lipoprotein profiles, plasma in vitro oxidation, blood PAF levels, PAF-induced platelet aggregation and PAF-acetylhydrolase (PAF-AH) activity, were measured on day 0 and 45. Atherosclerotic lesions formed in the aortic wall and wall elasticity were assessed on day 45. Changes in lipid profile were in accordance with previous studies. Blood PAF levels were higher in group A and decreased in group D on day 45. In rabbits fed an atherogenic diet (Group A) blood PAF and PAF-AH increased, atherosclerotic lesions formed and the elasticity of vessel walls declined. In animals fed olive oil (Group B) or OOPLE (Group C) blood PAF-AH increased, platelet aggregation was attenuated, less oxidation occurred in plasma, lesion thickness was reduced and vessel walls retained elasticity. Most of these beneficial changes were not seen in animals fed OONLE (Group D) although blood PAF and plasma oxidation were lower.

CONCLUSIONS

The antiatherogenic effects of OO result from OOPLE. The beneficial effect of these factors is linked to PAF metabolism and proaggregant activity.

Selective uptake and efflux of cholesteryl linoleate in LDL by macrophages expressing 12/15-lipoxygenase

Oxidation of low density lipoprotein (LDL) is a critical step for atherogenesis, and the role of the 12/15-lipoxygenase (12/15-LOX) as well as LDL receptor-related protein (LRP) expressed in macrophages in this process has been suggested. The oxygenation of cholesteryl linoleate in LDL by mouse macrophage-like J774A.1 cells overexpressing 12/15-LOX was inhibited by an anti-LRP antibody but not by an anti-LDL receptor antibody. When the cells were incubated with LDL double-labeled by [3H]cholesteryl linoleate and [125I]apoB, association with the cells of [3H]cholesteryl linoleate expressed as LDL protein equivalent exceeded that of [125I]apoB, indicating selective uptake of [3H]cholesteryl linoleate from LDL to these cells. An anti-LRP antibody inhibited the selective uptake of [3H]cholesteryl ester by 62% and 81% with the 12/15-LOX-expressing cells and macrophages, respectively. Furthermore, addition of LDL to the culture medium of the [3H]cholesteryl linoleate-labeled 12/15-LOX-expressing cells increased the release of [3H]cholesteryl linoleate to the medium in LDL concentration- and time-dependent manners. The transport of [3H]cholesteryl linoleate from the cells to LDL was also inhibited by an anti-LRP antibody by 75%. These results strongly suggest that LRP contributes to the LDL oxidation by 12/15-LOX in macrophages by selective uptake and efflux of cholesteryl ester in the LDL particle.

Cell-mediated LDL oxidation: The impact of transition metals and transferrin

Activated monocytes release oxygen radicals by respiratory burst and oxidative damage can be accelerated by transition metals. We investigated the cell-mediated and metal-catalysed in vitro oxidation of low-density lipoproteins (LDL), as well as the impact of the metal-binding protein transferrin (Tf). LDL oxidation was measured by monitoring the increase in fluorescence (350/440 nm excitation/emission). Maximal respiratory burst by U937 cells was achieved after 96 h differentiation with retinoic acid and dihydroxyvitamin D3 followed by stimulation with opsonised zymosan. Addition of activated cells resulted in the LDL oxidation, even in the absence of transition metals. Moreover, activated cells greatly enhanced metal-catalysed oxidative modifications, especially in the presence of copper. By binding metals, Tf was able to strongly impair this process. In conclusion, by generating oxygen radicals, activated U937 cells were able to oxidise LDL. The oxidising process was most pronounced in the presence of copper and could be blocked by Tf.

Effects of vitamin E on the toxicity of oxidized LDL on endothelial cells in vitro in smokers vs nonsmokers on diets rich in fish

OBJECTIVE

To clarify whether supplementation of vitamin E can alter the low density lipoprotein (LDL) oxidation properties and thereby affect endothelial cell function and prostacyclin production in smokers compared to nonsmokers on diets rich in fish in a pilot study.

DESIGN

The LDL of six smokers and six nonsmokers on habitual high fish diet was isolated before and after an 8-week supplementation of vitamin E (800 IU/day). LDL was oxidized by incubation with CuSO4. Cytotoxicity of LDL oxidized to different degrees on endothelial cells was investigated in vitro in these two groups.

SETTING

Helsinki University Central Hospital; Institute of Biomedicine, Pharmacology, University of Helsinki.

RESULTS

At baseline, the rate of oxidation was higher in nonsmokers than in smokers. The lag phase increased significantly after the supplementation of vitamin E both in smokers and nonsmokers. Native LDL dose dependently tended to reduce the viability of endothelial cells in vitro more markedly when isolated from smokers than from nonsmokers. Vitamin E supplementation had no beneficial effect on the cytotoxicity of oxidized LDLs in endothelial cell culture. On the other hand, simultaneous administration of Trolox, the water-soluble analogue of vitamin E, attenuated the LDL cytotoxicity on endothelial cells. The vitamin E supplementation to LDL donors attenuated the increase in prostacyclin production both in smokers and nonsmokers.

CONCLUSION

Supplementation of LDL donors (healthy male volunteers on habitual fish diet) with vitamin E increased the lag phase of LDL oxidation, but, on the other hand, did not influence in vitro cytotoxicity of LDL, or prostacyclin production.

Dyslipidemia and inflammation: an evolutionary conserved mechanism

Inflammation leads to changes in lipid metabolism aimed at decreasing the toxicity of a variety of harmful agents and tissue repair by redistributing nutrients to cells involved in host defence. Acute phase response, mediated by cytokines, preserves the host from acute injury. When this inflammation becomes chronic, it might lead to chronic disorders as atherosclerosis and the metabolic syndrome. The activation of the inflammatory cascade will induce a decrease in HDL-cholesterol (HDL-C), with impairment in reverse cholesterol transport, and parallel changes in apolipoproteins, enzymes, anti-oxidant capacity and ATP binding cassette A1-dependent efflux. This decrease in HDL-C and phospholipids could stimulate compensatory changes, as synthesis and accumulation of phospholipid-rich VLDL which binds bacterial products and other toxic substances, resulting in hypertriglyceridemia. The final consequence is an increased accumulation of cholesterol in cells. When the compensatory response (inflammation) is not able to repair injury, it turns into a harmful reaction, and the lipid changes will become chronic, either by repeated or overwhelming stimulus, enhancing the formation of atherosclerotic lesions. Thus, the classical lipid changes associated with the metabolic syndrome (increased triglycerides and decreased HDL-C) may be envisioned as a highly conserved evolutionary response aimed at tissue repair. Under this assumption, the problem is not the response but the persistence of the stimulus.

Essential roles of lipoxygenases in LDL oxidation and development of atherosclerosis

Oxidative modification of low-density lipoprotein (LDL) is one of the critical steps for the development of atherosclerosis. Accumulating studies have indicated that 12/15-lipoxygenase highly expressed in macrophages plays an essential role in the oxidation of circulating LDL. It has been demonstrated that LDL needs to bind the LDL receptor-related protein (LRP), a cell-surface receptor, prior to its oxidation by 12/15-lipoxygenase expressed in macrophages. LRP is suggested to mediate the selective transfer of cholesteryl ester in LDL to the plasma membrane of macrophages without endocytosis and degradation of the LDL particle. At the same time, binding of LDL to LRP translocates the 12/15-lipoxygenase from the cytosol to the plasma membrane. It is also demonstrated that 5-lipoxygenase localized in macrophages generates leukotrienes, which exhibit strong proinflammatory activities in cardiovascular tissues and contribute to lesion development. Therefore, the inhibition of these lipoxygenases may be effective in the prevention and treatment of the inflammatory diseases.

Translocation of phospholipase A2 to membranes by oxidized LDL and hydroxyoctadecadienoic acid to contribute to cholesteryl ester formation

We examined the mechanisms underlying the activation of group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) contributing to the supply of fatty acids required for the formation of cholesteryl ester in oxidized low-density lipoprotein (oxLDL)-stimulated macrophages. The possible involvement of oxidized lipids was also examined. In [(3)H]arachidonic acid-labeled mouse peritoneal macrophages, oxLDL stimulated the release of arachidonic acid, which was suppressed by methyl arachidonyl fluorophosphonate (MAFP), a cPLA(2)alpha inhibitor. oxLDL induced an increase in PLA(2)alpha levels in the membrane fraction without affecting those in whole cells or the activity in the lysate. Among 13-hydroxyoctadecadienoic acid (13-HODE), 7-ketocholesterol, and 25-hydroxycholesterol, oxidized lipids present in oxLDL particles, only 13-HODE induced the release of arachidonic acid, which was also sensitive to MAFP. Under conditions where addition of Ca(2+) to the cell lysate induced an increase in cPLA(2)alpha protein in the membrane fraction, preincubation with 13-HODE facilitated the Ca(2+)-dependent translocation of cPLA(2)alpha. Furthermore, 13-HODE increased cholesteryl ester formation in the presence of [(3)H]cholesterol. These results suggest that 13-HODE mediates the oxLDL-induced activation of cPLA(2)alpha through an increase in cPLA(2)alpha protein in the membranes, thus contributing, in part, to the supply of fatty acids required for the esterification of cholesterol in macrophages.

Myeloperoxidase: friend and foe

Neutrophilic polymorphonuclear leukocytes (neutrophils) are highly specialized for their primary function, the phagocytosis and destruction of microorganisms. When coated with opsonins (generally complement and/or antibody), microorganisms bind to specific receptors on the surface of the phagocyte and invagination of the cell membrane occurs with the incorporation of the microorganism into an intracellular phagosome. There follows a burst of oxygen consumption, and much, if not all, of the extra oxygen consumed is converted to highly reactive oxygen species. In addition, the cytoplasmic granules discharge their contents into the phagosome, and death of the ingested microorganism soon follows. Among the antimicrobial systems formed in the phagosome is one consisting of myeloperoxidase (MPO), released into the phagosome during the degranulation process, hydrogen peroxide (H2O2), formed by the respiratory burst and a halide, particularly chloride. The initial product of the MPO-H2O2-chloride system is hypochlorous acid, and subsequent formation of chlorine, chloramines, hydroxyl radicals, singlet oxygen, and ozone has been proposed. These same toxic agents can be released to the outside of the cell, where they may attack normal tissue and thus contribute to the pathogenesis of disease. This review will consider the potential sources of H2O2 for the MPO-H2O2-halide system; the toxic products of the MPO system; the evidence for MPO involvement in the microbicidal activity of neutrophils; the involvement of MPO-independent antimicrobial systems; and the role of the MPO system in tissue injury. It is concluded that the MPO system plays an important role in the microbicidal activity of phagocytes.

Oxidized linoleic acid regulates expression and shedding of syndecan-4

Syndecan-4, a heparan sulfate proteoglycan that is widely expressed in the vascular wall and as a cell surface receptor, modulates events relevant to acute tissue repair, including cell migration and proliferation, cell-substrate interactions, and matrix remodeling. While syndecan-4 expression is regulated in response to acute vascular wall injury, its regulation under chronic proatherogenic conditions such as those characterized by prolonged exposure to oxidized lipids has not been defined. In this investigation, arterial smooth muscle cells were treated with 13-hydroperoxy-9,11-octadecadienoic acid (HPODE) and 13-hydroperoxy-10,12-octadecadienoic acid, oxidized products of linoleic acid, which is the major oxidizable fatty acid in LDL. Both oxidized fatty acids induced a dose-dependent, rapid upregulation of syndecan-4 mRNA expression that was not attenuated by cycloheximide. This response was inhibited by pretreatment with N-acetylcysteine, catalase, or MEK1/2 inhibitors, but not by curcumin or lactacystin, known inhibitors of NF-κB. These data suggest that oxidized linoleic acid induces syndecan-4 mRNA expression through the initial generation of intracellular hydrogen peroxide with subsequent activation of the extracellular signal-regulated kinase signaling pathway via MEK1/2. Notably, the HPODE-induced enhancement of syndecan-4 mRNA was accompanied by accelerated shedding of syndecan-4. In principle, alterations in both the cell surface expression and shedding of syndecan-4 may augment a variety of proatherogenic events that occur in response to oxidized lipids.

Protein kinase Cdelta regulates p67phox phosphorylation in human monocytes

Phosphorylation of the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase components p67phox and p47phox accompanies the assembly and activation of this enzyme complex. We have previously reported that activation of human monocytes with opsonized zymosan (ZOP), a potent stimulator of NADPH oxidase activity, results in the phosphorylation of p67phox and p47phox. In this study, we investigated the regulation of p67phox phosphorylation. Although protein kinase C (PKC)alpha has previously been shown to regulate NADPH oxidase activity, we found that inhibition of PKCalpha had no effect on p67phox phosphorylation. Our studies demonstrate that pretreatment of monocytes with antisense oligodeoxyribonucleotides specific for PKCdelta or rottlerin, a selective inhibitor for PKCdelta, inhibited the phosphorylation of p67phox in monocytes, and Go6976, a specific inhibitor for conventional PKCs, PKCalpha and PKCbeta, had no such inhibitory effect. Additional studies indicate that ZOP stimulation of monocytes induces PKCdelta and p67phox to form a complex. We also demonstrate that lysates from activated monocytes as well as PKCdelta immunoprecipitates from activated monocytes can phosphorylate p67phox in vitro and that pretreatment of monocytes with rottlerin blocked the phosphorylation in each case. We further show that recombinant PKCdelta can phosphorylate p67phox in vitro. Finally, we show that PKCdelta-deficient monocytes produce significantly less superoxide anion in response to ZOP stimulation, thus emphasizing the functional significance of the PKCdelta regulation of p67phox phosphorylation. Taken together, this is the first report to describe the requirement of PKCdelta in regulating the phosphorylation of p67phox and the related NADPH oxidase activity in primary human monocytes.

Paraoxonases 1, 2, and 3, oxidative stress, and macrophage foam cell formation during atherosclerosis development

Paraoxonases PON1 and PON3, which are both associated in serum with HDL, protect the serum lipids from oxidation, probably as a result of their ability to hydrolyze specific oxidized lipids. The activity of HDL-associated PON1 seems to involve an activity (phospholipase A2-like activity, peroxidase-like activity, lactonase activity) which produces LPC. To study the possible role of PON1 in macrophage foam cell formation and atherogenesis we used macrophages from control mice, from PON1 knockout mice, and from PON1 transgenic mice. Furthermore, we analyzed PON1-treated macrophages and PON1-transfected cells to demonstrate the contribution of PON1 to the attenuation of macrophage cholesterol and oxidized lipid accumulation and foam cell formation. PON1 was shown to inhibit cholesterol influx [by reducing the formation of oxidized LDL (Ox-LDL), increasing the breakdown of specific oxidized lipids in Ox-LDL, and decreasing macrophage uptake of Ox-LDL]. PON1 also inhibits cholesterol biosynthesis and stimulates HDL-mediated cholesterol efflux from macrophages. PON2 and PON3 protect against oxidative stress, with PON2 acting mainly at the cellular level. Whereas serum PON1 and PON3 were inactivated under oxidative stress, macrophage PON2 expression and activity were increased under oxidative stress, probably as a compensatory mechanism against oxidative stress. Intervention to increase the paraoxonases (cellular and humoral) by dietary or pharmacological means can reduce macrophage foam cell formation and attenuate atherosclerosis development.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

Inflammation and atherosclerosis

Inflammation plays a pivotal role in all stages of atherogenesis, from foam cell to plaque formation to rupture and ultimately to thrombosis. Insight gained from recent basic and clinical data linking inflammation to atherosclerosis has yielded important diagnostic and prognostic information. Low-grade chronic inflammation as measured by high sensitivity C-reactive protein predicts future risk of acute coronary syndrome independent of traditional cardiovascular risk factors. In addition, individuals with higher "inflammatory burden" gain the largest absolute risk reduction with aggressive risk-lowering therapy. The link between inflammation and atherosclerosis provides a new venue for future pharmacologic agents that may slow the progression of atherosclerosis by inhibiting inflammation.

Aldose reductase-catalyzed reduction of aldehyde phospholipids

Oxidation of unsaturated phospholipids results in the generation of aldehyde side chains that remain esterified to the phospholipid backbone. Such "core" aldehydes elicit immune responses and promote inflammation. However, the biochemical mechanisms by which phospholipid aldehydes are metabolized or detoxified are not well understood. In the studies reported here, we examined whether aldose reductase (AR), which reduces hydrophobic aldehydes, metabolizes phospholipid aldehydes. Incubation with AR led to the reduction of 5-oxovaleroyl, 7-oxo-5-heptenoyl, 5-hydroxy-6-oxo-caproyl, and 5-hydroxy-8-oxo-6-octenoyl phospholipids generated upon oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC). The enzyme also catalyzed the reduction of phospholipid aldehydes generated from the oxidation of 1-alkyl, and 1-alkenyl analogs of PAPC, and 1-palmitoyl-2-arachidonoyl phosphatidic acid or phosphoglycerol. Aldose reductase catalyzed the reduction of chemically synthesized 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphatidylcholine (POVPC) with a K(m) of 10 mum. Addition of POVPC to the culture medium led to incorporation and reduction of the aldehyde in COS-7 and THP-1 cells. Reduction of POVPC in these cells was prevented by the AR inhibitors sorbinil and tolrestat and was increased in COS-7 cells overexpressing AR. Together, these observations suggest that AR may be a significant participant in the metabolism of several structurally diverse phospholipid aldehydes. This metabolism may be a critical regulator of the pro-inflammatory and immunogenic effects of oxidized phospholipids.

Modulation of LDL oxidation by 7,8-dihydroneopterin

Human macrophages stimulated with interferon-gamma generate neopterin and 7,8-dihydroneopterin which interfere with reactive species involved in LDL oxidation. While neopterin was found to have pro-oxidative effects on copper-mediated LDL oxidation, the influence of 7,8-dihydroneopterin is more complex. This study provides detailed information that 7,8-dihydroneopterin reveals both pro-oxidative and anti-oxidative effects on copper mediated LDL oxidation. 7,8-dihydroneopterin inhibited the oxidation of native LDL effectively monitored by (i) formation of conjugated dienes, (ii) relative electrophoretic mobility (EM) and (iii) specific oxidized epitopes. Using minimally oxidized LDL (mi-LDL) or moderately oxidized LDL (mo-LDL) 7,8-dihydroneopterin changed its antioxidative behavior to a strongly pro-oxidative. Incubation of 7,8-dihydroneopterin with native LDL, mi-LDL or mo-LDL in the absence of copper ions showed that formation of conjugated dienes was more increased in mo-LDL than in mi-LDL while no diene formation was observed with native LDL. We suggest that 7,8-dihydroneopterin is a modulator for LDL oxidation in the presence of copper ions depending on the "oxidative status" of this lipoprotein.

Oxidized low-density lipoprotein, iron stores, and haptoglobin polymorphism

BACKGROUND

In vitro experimental studies demonstrated that iron promotes free radical-induced low-density lipoprotein (LDL) oxidation.

OBJECTIVE

To test the hypothesis that circulating oxidized LDL (oxLDL) levels might be associated with body iron stores (serum ferritin) and iron-related genetic markers (hemochromatosis gene C282Y mutation, haptoglobin polymorphism).

METHODS

We investigated 381 (176 males, 205 females, age 45 +/- 6 years) healthy Caucasians. Serum oxLDL, assayed by a mAb-4E6-based enzyme-linked immunosorbent assay (ELISA), was expressed as oxLDL/LDL ratio to adjust for serum LDL-cholesterol concentration. Hemochromatosis gene C282Y mutation analysis was performed by a Taqman-based polymerase chain reaction (PCR) assay. Haptoglobin (Hp) phenotypes (Hp 1-1, Hp 2-1, Hp 2-2) were determined by starch gel electrophoresis.

RESULTS

In stepwise multivariate regression analysis, gender (P < 0.0001), current smoking (P < 0.0001), HDL-cholesterol (P = 0.0001), ferritin (P = 0.0051), body mass index (BMI) (P = 0.0063), and Hp phenotype (P = 0.0331) independently predicted oxLDL/LDL ratio in the total group. In men, smoking (P < 0.0001), ferritin (P = 0.0052), Hp phenotype (P = 0.0063), and HDL-cholesterol (P = 0.0127) were independent determinants of oxLDL/LDL ratio. In women, only body mass index (P < 0.0001), HDL-cholesterol (P = 0.0005), and smoking (P = 0.0025) were significantly associated with oxLDL/LDL ratio. The C282Y mutation (wild-type versus C282Y heterozygotes) was not associated with oxLDL/LDL ratio in both sexes.

CONCLUSION

Serum ferritin concentration and Hp polymorphism are independently associated with circulating oxLDL levels in males.

Cell-induced copper ion-mediated low density lipoprotein oxidation increases during in vivo monocyte-to-macrophage differentiation

Macrophage activation is associated with the production and release of reactive oxygen species (ROS), which are capable of mediating oxidative modification of low-density lipoprotein (LDL). In the present study we questioned whether cellular capacity to oxidize LDL increases during in vivo monocyte/macrophage maturation. We developed a novel model for macrophage maturation in vivo using mouse peritoneal macrophages (MPMs) harvested at increasing intervals after intraperitoneal thioglycollate injection. Macrophage maturation was evidenced by a progressive increase in cellular size, density, granulation, and expression of cell surface markers CD11b and CD36, and by a gradual decrement in myeloperoxidase activity. Cellular capacity to stimulate copper ion-mediated oxidation of LDL increased gradually by up to 2-fold during in vivo macrophage maturation in Balb/C mice, similar to the pattern observed during 1,25-dihydroxyvitamin D3-induced in vitro differentiation of the PLB-985 cell line. These effects were attributed to a gradual increase in production of ROS by up to 9-fold. The mechanism for the increase in cellular oxidative stress during macrophage maturation could be related, at least in part, to NADPH oxidase activation, as demonstrated by a gradual increase over time in p47phox expression (mRNA and protein) and in its translocation to the plasma membrane. In conclusion, in vivo monocyte-to-macrophage differentiation is associated with increased cell capacity to oxidize LDL, which may represent a protective mechanism for rapid removal of atherogenic LDL from extracellular spaces in the arterial wall.

[Taurine is a possible anti-atherosclerotic agent]

Atherosclerosis-related ischemic heart diseases are the principal cause of death in the last few years. Recently, several reports implicated that taurine, sulfur-containing beta-amino acid, prevented the progression of atherosclerosis through various anti-pathogenetic modifications. Firstly, taurine treatment inhibited lipid peroxidation and/or lowered serum LDL/VLDL cholesterol and elevated HDL, and as a result, it prevented lipid accumulation on the aortic valve in hypercholesterolaemic animals. Secondly, taurine administration prevented endothelial dysfunction, one of the initial events in the formation of lesions of atherosclerosis, through the amelioration of the impairment of monocyte function. Thirdly, while it is well known that taurine scavenges hypochlorous acid (HOCl) produced by myeloperoxidase in neutrophils and macrophages, recent studies revealed that HOCl was one of the major factors oxidizing LDL, implying that the anti-oxidative role of taurine contributes to the anti-atherosclerotic effect. Additionally, TauCl, produced by the reaction of taurine with HOCl, inhibits the activation of NF-kappaB followed by the inhibition of the production of the pro-inflammatory mediators.

Synergistic inhibition of low-density lipoprotein oxidation by rutin, gamma-terpinene, and ascorbic acid

Low-density lipoprotein (LDL) oxidation may play a significant role in atherogenesis. Flavonoids are well-known for their excellent antioxidative capacity in various model systems, therefore we examined the behaviour of rutin, a quercetin-3-rutinosid, in the copper-mediated LDL oxidation. Rutin alone has been shown to protect LDL against oxidation. Furthermore we investigated the combination of rutin with a hydrophilic (ascorbate) and a lipophilic antioxidant (gamma-terpinene) in copper-mediated LDL oxidation. In both cases we found a synergistic effect on lag phase prolongation. To elucidate whether this effect mainly depends on the copper chelating ability of rutin we examined its reaction in more detail. Although inhibiting the oxidation of alpha-linolenic acid in the "rose bengal system" no direct influence of a copper-rutin-complex was determined. We conclude that a redox active copper-rutin-complex is still able to initiate the LDL oxidation but may prevent copper from a reaction at the binding sites of apoB-100. The synergistic effect in preventing LDL oxidation is due to this trapping of copper in a complex in the case of ascorbate. The synergistic action of rutin and gamma-terpinene can be explained by different distribution of rutin and gamma-terpinene in, and around the LDL-particle, respectively.

Regulation of Superoxide Anion Production by NADPH Oxidase in Monocytes/Macrophages

Monocyte extravasation into the vessel wall has been shown to be a critical step in the development of atherosclerosis. Upon activation, monocytes produce a burst of superoxide anion due to activation of the NADPH oxidase enzyme complex. Monocyte-derived superoxide anion contributes to oxidant stress in inflammatory sites, is required for monocyte-mediated LDL oxidation, and alters basic cell functions such as adhesion and proliferation. We hypothesize that monocyte-derived superoxide anion production contributes to atherosclerotic lesion formation. In this brief review, we summarize our current understanding of the signal transduction pathways regulating NADPH oxidase activation and related superoxide anion production in activated human monocytes. Novel pathways are identified that may serve as future targets for therapeutic intervention in this pathogenic process. The contributions of superoxide anion and NADPH oxidase to atherogenesis are discussed. Future experiments are needed to clarify the exact role of NADPH oxidase-derived superoxide anion in atherogenesis, particularly that derived from monocytes.

Isolevuglandins, a novel class of isoprostenoid derivatives, function as integrated sensors of oxidant stress and are generated by myeloperoxidase in vivo

Isolevuglandins (isoLGs) are a family of reactive gamma-ketoaldehydes generated by free radical oxidation of arachidonate-containing lipids through the isoprostane pathway. Elevated plasma levels of isoLG protein adducts are observed in subjects with atherosclerosis compared with age/gender-matched controls. However, mechanisms for the generation of isoLGs in vivo are not established. Here we show that free radical-induced peroxidation promoted by the myeloperoxidase (MPO)/H2O2 system of leukocytes serves as one mechanism for the generation of isoLGs in vivo. Using a Candida sepsis model of inflammation, we demonstrate 3.5- and 2.7-fold increases in iso[4]LGE2 and isoLGE2 adducts of plasma proteins after pathogen exposure in wild-type mice. Plasma levels of F2 isoprostanes were not significantly increased after pathogen challenge in this model. MPO knockout mice demonstrated significant reductions (34%, P=0.003) in plasma levels of iso[4]LGE2 protein adducts after pathogen challenge compared with wild-type mice. Mass spectrometry and immunochemical methods demonstrate MPO-dependent formation of iso[4]LGE2 and isoLGE2 phospholipids and their corresponding isoLG protein adducts in model systems. The present studies thus identify MPO as one pathway for generation of isoLGs in vivo. They also suggest that long-lived protein isoLG adducts may serve as an alternative integrated sensor of oxidant stress in vivo.

Class B scavenger receptors CD36 and SR-BI are receptors for hypochlorite-modified low density lipoprotein

The presence of HOCl-modified epitopes inside and outside monocytes/macrophages and the presence of HOCl-modified apolipoprotein B in atherosclerotic lesions has initiated the present study to identify scavenger receptors that bind and internalize HOCl-low density lipoprotein (LDL). The uptake of HOCl-LDL by THP-1 macrophages was not saturable and led to cholesterol/cholesteryl ester accumulation. HOCl-LDL is not aggregated in culture medium, as measured by dynamic light scattering experiments, but internalization of HOCl-LDL could be inhibited in part by cytochalasin D, a microfilament disrupting agent. This indicates that HOCl-LDL is partially internalized by a pathway resembling phagocytosis-like internalization (in part by fluid-phase endocytosis) as measured with [14C]sucrose uptake. In contrast to uptake studies, binding of HOCl-LDL to THP-1 cells at 4 degrees C was specific and saturable, indicating that binding proteins and/or receptors are involved. Competition studies on THP-1 macrophages showed that HOCl-LDL does not compete for the uptake of acetylated LDL (a ligand to scavenger receptor class A) but strongly inhibits the uptake of copper-oxidized LDL (a ligand to CD36 and SR-BI). The binding specificity of HOCl-LDL to class B scavenger receptors could be demonstrated by Chinese hamster ovary cells overexpressing CD36 and SR-BI and specific blocking antibodies. The lipid moiety isolated from the HOCl-LDL particle did not compete for cell association of labeled HOCl-LDL to CD36 or SR-BI, suggesting that the protein moiety of HOCl-LDL is responsible for receptor recognition. Experiments with Chinese hamster ovary cells overexpressing scavenger receptor class A, type I, confirmed that LDL modified at physiologically relevant HOCl concentrations is not recognized by this receptor.

Serum oxidized low-density lipoproteins in rheumatoid arthritis

OBJECTIVE

The aim of this study was to measure serum oxidized low-density lipoprotein (Ox-LDL) levels in patients with rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE) and to identify any association with clinical variables.

METHODS

We studied 126 female patients (27 with active RA, 27 with inactive RA, 72 with SLE). One hundred fifteen age-matched healthy women (76 for RA, 39 for SLE) with no clinical or laboratory evidence of disease served as normal controls. Serum Ox-LDL levels were measured with a commercial enzyme-linked immunosorbent assay kit (Mercodia, Sweden).

RESULTS

The serum Ox-LDL levels were significantly higher in patients with active RA (P<0.05) or SLE (P<0.01) than age-matched controls and significantly higher in patients with active RA than with inactive RA (P<0.01). The levels of serum total cholesterol and LDL were significantly lower in patients with RA than in age-matched controls (P<0.01). There was no correlation among serum Ox-LDL levels and inflammatory markers (erythrocyte sedimentation rate, C-reactive protein) or rheumatoid factors in patients with RA.

CONCLUSIONS

Compared with healthy women, those with SLE or active RA had increased serum Ox-LDL levels, which may contribute to the increased risk of cardiovascular disease in this patient group.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

Oxidized LDL activates phospholipase A2 to supply fatty acids required for cholesterol esterification

We examined the roles of phospholipase A2 (PLA2) in oxidized LDL (oxLDL)-induced cholesteryl ester formation in macrophages. In [3H]oleic acid-labeled RAW264.7 cells and mouse peritoneal macrophages, oxLDL induced [3H]cholesteryl oleate formation with an increase in free [3H]oleic acid and a decrease in [3H]phosphatidylcholine. The changes in these lipids were suppressed by methyl arachidonyl fluorophosphonate (MAFP), a cytosolic PLA2 (cPLA2) inhibitor. However, MAFP had no effect on the ACAT activity or the binding and/or uptake of oxLDL. Stimulation with oxLDL in the presence of [3H]cholesterol increased [3H]cholesteryl ester bearing fatty acyl chains derived from cellular and/or exogenous (oxLDL) lipids. The formation of cholesteryl ester under this condition was also inhibited by MAFP, and the inhibitory effect was reversed by adding oleic acid. While oxLDL did not affect the activity or amounts of cPLA2, preincubation with oxLDL enhanced the release of oleic acid and arachidonic acid induced by ionomycin in RAW264.7 cells. 13(S)-hydroxyoctadecadienoic acid, but not 7-ketocholesterol, also enhanced ionomycin-induced oleic acid release. These results suggest that oxLDL induces cPLA2 activation, which contributes, at least in part, to the supply of fatty acids required for the cholesteryl esterification, probably through the acceleration by oxidized lipids of the catalytic action of cPLA2 in macrophages.

Perfusion-Perls and -Turnbull methods supplemented by DAB intensification for nonheme iron histochemistry: demonstration of the superior sensitivity of the methods in the liver, spleen, and stomach of the rat

Perfusion-Perls and -Turnbull methods supplemented by the intensification with 3,3'-diaminobenzidine (+ DAB) enabled stronger and more extensive staining of nonheme iron than the Perls + and Turnbull + DAB methods carried out on tissue sections fixed with 10% formalin in 0.9% saline or PBS. The section- and perfusion-Perls + DAB methods are not specific for the demonstration of nonheme ferric iron but also stain nonheme ferrous iron. However, owing to its high sensitivity, the perfusion-Perls + DAB method would provide useful information about nonheme iron deposition regardless of oxidation states in normal and pathological conditions. The perfusion-Turnbull + DAB method is specifically demonstrable of nonheme ferrous iron and the results from this method showed significant stores of nonheme ferrous iron in the hepatocytes, Kupffer cells, splenic macrophages, and gastric parietal cells of the rat. Since nonheme ferrous iron is considered to be critically involved in free radical generation, the perfusion-Turnbull + DAB method would visualize such populations of cells that are at risk from free radical damage.

Inhibition of Akt phosphorylation by thrombin, histamine and lysophosphatidylcholine in endothelial cells. Differential role of protein kinase C

The protein kinase Akt is involved in embryonic vascular development and neoangiogenesis as well as in several endothelial cell functions, including activation of endothelial NO-synthase (eNOS) and promotion of endothelial cell survival. We have examined the effects of G-protein activators thrombin and histamine as well as lysophosphatidylcholine (LPC) on Akt phosphorylation in cultured human umbilical vein endothelial cells (HUVEC). Akt phosphorylation was analyzed with the phosphospecific Akt (Ser473) antibody by Western blotting. While epidermal growth factor (EGF) was a potent stimulator of Akt phosphorylation histamine, thrombin and LPC blocked its activation when used in cotreatment with EGF. Following inhibition or downregulation of protein kinase C (PKC), the inhibitory effect of both histamine and thrombin on the endothelial response to EGF was prevented. Furthermore, stimulation of PKC, using short-term 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment, markedly inhibited the stimulatory effects of EGF on Akt phosphorylation. Rottlerin, an inhibitor of the PKCdelta, but not Gö6976, which is an inhibitor of alpha, beta, gamma and isoforms, reversed the inhibitory effects of histamine. Conversely, inhibition or downregulation of PKC did not prevent the inhibitory effect of LPC. Akt phosphorylation was also increased by sphingosine 1-phosphate (S1P) treatment and this activity was influenced by the various cotreatments in the same way as the activation by EGF. Overall, this study demonstrated that the G-protein activators thrombin and histamine inhibited both EGF- and S1P-mediated Akt phosphorylation in HUVEC by activation of PKCdelta, while the inhibitory effects of LPC were independent of PKCdelta.

Low density lipoproteins interact with acidic fibroblast growth factor and modify its function

OBJECTIVE

Oxidized LDL (oxLDL) was shown to trigger the release of acidic fibroblast growth factor (FGF-1). Because these components are likely to be present simultaneously in the atherosclerotic milieu, we investigated whether oxLDL interacts with FGF-1 and whether this interaction affects FGF-1 functioning.

METHODS AND RESULTS

Using molecular sieve and electrophoretic mobility shift assays, we found that FGF-1 forms a complex with oxLDL in vitro, in contrast to its low affinity for nonatherogenic, native LDL. The FGF-1/oxLDL complex had a dramatically decreased ability to bind heparin and was nonmitogenic on cultured smooth muscle cells. In human atherosclerotic lesions, the highest FGF-1 immunoreactivity was found in macrophages. With respect to oxLDL accumulation, 2 patterns were distinguished: (1) moderate, intracellular in matrix-rich regions containing viable cells and (2) massive, both cell-associated and extracellular oxLDL deposits in foam cell-rich regions with necrotic areas. The proliferating cell nuclear antigen readings for proliferating cells reflected that the mitogenic activity of FGF-1 was confined to the regions where oxLDL was strictly intracellular and was inhibited in the regions with extracellular oxLDL deposition.

CONCLUSIONS

oxLDL, besides being a bulky component of the atherosclerotic lesion, possibly manifests its pathogenicity by complexing FGF-1 and inhibiting its growth-promoting function during atherogenesis.

Low-density lipoprotein modification by normal, myeloperoxidase-deficient and NADPH oxidase-deficient granulocytes and the impact of redox active transition metal ions

The modification of low-density lipoprotein (LDL) by normal, myeloperoxidase (MPO)-deficient and NADPH oxidase-deficient granulocytes was investigated using the monoclonal antibody (mAb) OB/04, which was originally generated against copper-oxidized LDL. Incubation of LDL with normal granulocytes increased the reactivity of LDL with mAb OB/04. These effects were even more pronounced using MPO-deficient granulocytes. Inhibitors of oxidative reactions (the NADPH oxidase inhibitor diphenyleneiodonium chloride [DPI], catalase, superoxide dismutase [SOD]) did not significantly reduce LDL oxidation by normal granulocytes. Furthermore, granulocytes of a patient with NADPH oxidase deficiency were almost equally effective as normal granulocytes, indicating that oxidative burst-derived reactive oxygen species are of only minor importance in the generation of mAb OB/04-detectable new epitopes on LDL in vitro. In contrast, incubation of LDL with iron and copper prior to and during incubation with normal granulocytes markedly enhanced the generation of OB/04-detectable epitopes. It is supposed that, besides superoxide (in normal and MPO-deficient granulocytes) or instead of superoxide (in NADPH oxidase-deficient granulocytes), lytic enzymes released by activated granulocytes may enhance the availability of transition metals for oxidation of LDL. Our results support the concept that transition-metal-dependent pathways of LDL oxidation in combination with degranulation products of granulocytes are important.

Expanding expression of the 5-lipoxygenase pathway within the arterial wall during human atherogenesis

Oxidation products of low-density lipoproteins have been suggested to promote inflammation during atherogenesis, and reticulocyte-type 15-lipoxygenase has been implicated to mediate this oxidation. In addition, the 5-lipoxygenase cascade leads to formation of leukotrienes, which exhibit strong proinflammatory activities in cardiovascular tissues. Here, we studied both lipoxygenase pathways in human atherosclerosis. The 5-lipoxygenase pathway was abundantly expressed in arterial walls of patients afflicted with various lesion stages of atherosclerosis of the aorta and of coronary and carotid arteries. 5-lipoxygenase localized to macrophages, dendritic cells, foam cells, mast cells, and neutrophilic granulocytes, and the number of 5-lipoxygenase expressing cells markedly increased in advanced lesions. By contrast, reticulocyte-type 15-lipoxygenase was expressed at levels that were several orders of magnitude lower than 5-lipoxygenase in both normal and diseased arteries, and its expression could not be related to lesion pathology. Our data support a model of atherogenesis in which 5-lipoxygenase cascade-dependent inflammatory circuits consisting of several leukocyte lineages and arterial wall cells evolve within the blood vessel wall during critical stages of lesion development. They raise the possibility that antileukotriene drugs may be an effective treatment regimen in late-stage disease.

Myeloperoxidase functions as a major enzymatic catalyst for initiation of lipid peroxidation at sites of inflammation

Initiation of lipid peroxidation and the formation of bioactive eicosanoids are pivotal processes in inflammation and atherosclerosis. Currently, lipoxygenases, cyclooxygenases, and cytochrome P450 monooxygenases are considered the primary enzymatic participants in these events. Myeloperoxidase (MPO), a heme protein secreted by activated leukocytes, generates reactive intermediates that promote lipid peroxidation in vitro. For example, MPO catalyzes oxidation of tyrosine and nitrite to form tyrosyl radical and nitrogen dioxide ((.)NO(2)), respectively, reactive intermediates capable of initiating oxidation of lipids in plasma. Neither the ability of MPO to initiate lipid peroxidation in vivo nor its role in generating bioactive eicosanoids during inflammation has been reported. Using a model of inflammation (peritonitis) with MPO knockout mice (MPO(-/-)), we examined the role for MPO in the formation of bioactive lipid oxidation products and promoting oxidant stress in vivo. Electrospray ionization tandem mass spectrometry was used to simultaneously quantify individual molecular species of hydroxy- and hydroperoxy-eicosatetraenoic acids (H(P)ETEs), F(2)-isoprostanes, hydroxy- and hydroperoxy-octadecadienoic acids (H(P)ODEs), and their precursors, arachidonic acid and linoleic acid. Peritonitis-triggered formation of F(2)-isoprostanes, a marker of oxidant stress in vivo, was reduced by 85% in the MPO(-/-) mice. Similarly, formation of all molecular species of H(P)ETEs and H(P)ODEs monitored were significantly reduced (by at least 50%) in the MPO(-/-) group during inflammation. Parallel analyses of peritoneal lavage proteins for protein dityrosine and nitrotyrosine, molecular markers for oxidative modification by tyrosyl radical and (.)NO(2), respectively, revealed marked reductions in the content of nitrotyrosine, but not dityrosine, in MPO(-/-) samples. Thus, MPO serves as a major enzymatic catalyst of lipid peroxidation at sites of inflammation. Moreover, MPO-dependent formation of (.)NO-derived oxidants, and not tyrosyl radical, appears to serve as a preferred pathway for initiating lipid peroxidation and promoting oxidant stress in vivo.

Cytosolic phospholipase A2 (cPLA2) regulation of human monocyte NADPH oxidase activity. cPLA2 affects translocation but not phosphorylation of p67(phox) and p47(phox)

The NADPH oxidase of human monocytes is activated upon exposure to opsonized zymosan and a variety of other stimuli to catalyze the formation of superoxide anion. Assembly of the NADPH oxidase complex is believed to be a highly regulated process, and molecular mechanisms responsible for this regulation have yet to be fully elucidated. We have previously reported that cytosolic phospholipase A(2) (cPLA(2)) expression and activity are essential for superoxide anion production in activated human monocytes. In this study, we investigated the mechanisms involved in cPLA(2) regulation of NADPH oxidase activation by evaluating the effects of cPLA(2) on translocation and phosphorylation of p67(phox) and p47(phox). We report that translocation and phosphorylation of p67(phox), as well as p47(phox), occur upon activation of human monocytes and that decreased cPLA(2) protein expression, mediated by antisense oligodeoxyribonucleotides (AS-ODN) specific for cPLA(2) mRNA, blocked the stimulation-induced translocation of p47(phox) and p67(phox) from the cytosol to the membrane fraction. Inhibition of translocation of both p47(phox) and p67(phox) by cPLA(2) AS-ODN was above 85%. Arachidonic acid (AA), a product of cPLA(2) enzymatic activity, completely restored translocation of both of these oxidase components in the AS-ODN-treated, cPLA(2)-deficient human monocytes. These results represent the first report that cPLA(2) activity or AA is required for p67(phox) and p47(phox) translocation in human monocytes. Although cPLA(2) was required for translocation of p47(phox) and p67(phox), it did not influence phosphorylation of these components. These results suggest that one mechanism of cPLA(2) regulation of NADPH oxidase activity is to control the arachidonate-sensitive assembly of the complete oxidase complex through modulating the translocation of both p47(phox) and p67(phox). These studies provide insight into the mechanisms by which activation signals are transduced to allow the induction of superoxide anion production in human monocytes.

Increased macrophage glutathione content reduces cell-mediated oxidation of LDL and atherosclerosis in apolipoprotein E-deficient mice

We used the apolipoprotein E deficient (apo e-/-) mice to analyze the role of macrophage reduced glutathione (GSH) content in cell-mediated oxidation of LDL and in atherosclerotic lesion development. Apo e-/- mice were supplemented with L-2-oxo-4-thiazolidin carboxylate (OTC, which supplies cysteine residues, 500 mg/kg/day), or with buthionine sulfoximine (BSO, a specific inhibitor of GSH synthesis, 400 mg/kg/day) for 6 weeks. Then mouse peritoneal macrophages (MPM) and the mice aortas were collected. MPM from apo e-/- mice contained decreased GSH levels (by 58%), and a four-fold increased lipid peroxides content compared to control macrophages from C57BL6 mice. These MPM demonstrated increased capability to release superoxide anions and to oxidize LDL in comparison to control MPM. OTC supplementation resulted in a 26% increase in macrophage GSH, paralleled by a 25% reduction in cellular lipid peroxides content. Decrement by 30% in superoxide anion release and LDL oxidation by MPM, and also in the atherosclerotic lesion size by 25%, was found in the OTC-treated mice, compared to placebo-treated apo e-/- mice. In contrast, in BSO-treated mice MPM a further depletion of cellular GSH by 22% was found, paralleled by a two-fold increase in lipid peroxides content, and a 41% increased superoxide anion release and cell-mediated LDL oxidation, compared to placebo-treated apo e-/- mice MPM. Most important, BSO supplementation to apo e-/- mice caused a 59% increase in the atherosclerotic lesion area. An additional way to increase cellular GSH content was the use of dietary antioxidants. Vitamin E (40 mg/kg/day) or the isoflavan glabridin (25 microg/kg/day) administration for 2 months to apo e-/- mice resulted in the accumulation of these antioxidants in their MPM, and increased MPM GSH content by 24 and 80%, respectively. MPM lipid peroxides content was reduced by 31 or 60% upon vitamin E or glabridin supplementation, paralleled by a 30 or 60% decrease in cell-mediated oxidation of LDL, respectively. Finally, a significant inverse correlation (R=0.83) was found between macrophage GSH content and cell-mediated oxidation of LDL. We conclude that enrichment in vivo of macrophages with GSH, significantly decreases cellular oxidative stress, leading to reduced capability of the macrophages to oxidize LDL, and hence GSH may attenuate the development of atherosclerosis.

Identification of oxidized plasma proteins in Alzheimer's disease

The modification of proteins by reactive oxygen species is central to the pathology of Alzheimer's disease (AD). Previously, we have observed specific oxidized proteins in blood plasma of AD subjects [Biochem. Biophys. Res. Commun. 275 (2000) 678]. Plasma from AD subjects and age-matched controls was subjected to two-dimensional gel electrophoresis (2-DE). Oxidized proteins with new carbonyl groups were detected by reaction with 2,4-dinitrophenylhydrazine, followed by Western blotting with anti-DNP antibody. Seven principal oxidized protein spots (isoelectric point=4.7-5.5; molecular mass=45-65 kDa) were observed, with varying levels of oxidation in plasma samples from both AD and non-AD subjects. Matrix-assisted laser desorption mass spectroscopy (MALDI-TOF/MS) revealed that these oxidized proteins were isoforms of fibrinogen gamma-chain precursor protein and of alpha-1-antitrypsin precursor. These proteins exhibited a two- to sixfold greater specific oxidation index in plasma from AD subjects when compared to controls. Both these proteins have been previously implicated in the pathology of the disease. It is possible that oxidized isoforms of these proteins may serve as biomarkers for AD.

Role of oxidized low-density lipoprotein in renal disease

Accelerated atherosclerosis is often observed in patients with chronic renal failure. In the present review we summarize and discuss the recent literature on the pathogenic role of low-density lipoproteins modified by oxidative processes in atherosclerosis and the possible role in renal diseases. Pathogenetically, the oxidation of low-density lipoproteins is considered to be a key event in the development of atherosclerosis, in part by causing enhanced uptake of lipids by macrophages. In addition, oxidation of low-density lipoproteins exerts cytotoxic, proinflammatory and immunogenic properties, all of which could potentially contribute to the development and progression of atherosclerosis.

Antioxidants prevent the lymphocyte DNA damage induced by PMA-stimulated monocytes

Oxidative stress has been related to several degenerative diseases such as cancer and coronary heart disease. Reactive oxygen species can damage different cellular macromolecules, including DNA, which is directly responsible for mutation and carcinogenesis. In this study, monocytes that were activated by phorbol 12-myristate 13-acetate (PMA) were coincubated with lymphocytes, and the DNA damage was measured by single-cell gel electrophoresis (comet) assay. Stimulation of monocytes with PMA activates the "respiratory burst," which evokes DNA damage in lymphocytes. The extent of the damage is related to the concentration of monocytes and the exposure time. Exogenous addition of superoxide dismutase did not prevent the DNA damage, which suggests that superoxide ions are not directly responsible for the damage. Partial protection was observed when catalase was included (60% protection), which indicates that other reactive species, in addition to H2O2, are responsible for the damage. In this system, the protective activity of natural antioxidants at different concentrations was also investigated. After coincubation of PMA-activated monocytes with lymphocytes in the presence of each antioxidant for one hour at 37 degrees C, the lymphocyte DNA damage was determined. All the compounds protected the lymphocytes to a certain degree, with a maximum effect at different concentrations: 41% protection with 1 microM ascorbic acid, 55% protection with 40 microM alpha-tocopherol, 50% protection with 3 microM beta-carotene, and 56% protection with 5 microM quercetin. On the basis of these results, we maintain that this "ex vivo model," more closely related to physiological conditions, could be used to test the antioxidant activity of different compounds.

Stat1-dependent, p53-independent expression of p21(waf1) modulates oxysterol-induced apoptosis

7-Ketocholesterol (7kchol) is prominent in atherosclerotic lesions where apoptosis occurs. Using mouse fibroblasts lacking p53, p21(waf1), or Stat1, we found that optimal 7kchol-induced apoptosis requires p21(waf1) and Stat1 but not p53. Findings were analogous in a human cell system. Apoptosis was restored in Stat1-null human cells when wild-type Stat1 was restored. Phosphorylation of Stat1 on Ser(727) but not Tyr(701) was essential for optimum apoptosis. A neutralizing antibody against beta interferon (IFN-beta) blunted Ser(727) phosphorylation and apoptosis after 7kchol treatment; cells deficient in an IFN-beta receptor subunit exhibited blunted apoptosis. IFN-beta alone did not induce apoptosis; thus, 7kchol-induced release of IFN-beta was necessary but not sufficient for optimal apoptosis. In Stat1-null cells, expression of p21(waf1) was much less than in wild-type cells; introducing transient expression of p21(waf1) restored apoptosis. Stat1 and p21(waf1) were essential for downstream apoptotic events, including cytochrome c release from mitochondria and activation of caspases 9 and 3. Our data reveal key elements of the cellular pathway through which an important oxysterol induces apoptosis. Identification of the essential signaling events that may pertain in vivo could suggest targets for therapeutic intervention.

Human neutrophils oxidize low-density lipoprotein by a hypochlorous acid-dependent mechanism: the role of vitamin C

Oxidatively modified low-density lipoprotein (LDL) has been strongly implicated in the pathogenesis of atherosclerosis. Peripheral blood leukocytes, such as neutrophils, can oxidize LDL by processes requiring superoxide and redox-active transition metal ions; however, it is uncertain whether such catalytic metal ions are available in the artery wall. Stimulated leukocytes also produce the reactive oxidant hypochlorous acid (HOCl) via the heme enzyme myeloperoxidase. Since myeloperoxidase-derived HOCl may be a physiologically relevant oxidant in atherogenesis, we investigated the mechanisms of neutrophil-mediated LDL modification and its possible prevention by the antioxidant ascorbate (vitamin C). As a sensitive marker of LDL oxidation, we measured LDL thiol groups. Stimulated human neutrophils (5x10(6) cells/ml) incubated with human LDL (0.25 mg protein/ml) time-dependently oxidized LDL thiols (33% and 79% oxidized after 10 and 30 min, respectively). Supernatants from stimulated neutrophils also oxidized LDL thiols (33% oxidized after 30 min), implicating long-lived oxidants such as N-chloramines. Experiments using specific enzyme inhibitors and oxidant scavengers showed that HOCl, but not hydrogen peroxide nor superoxide, plays a critical role in LDL thiol oxidation by neutrophils. Ascorbate (200 microM) protected against neutrophil-mediated LDL thiol oxidation for up to 15 min of incubation, after which LDL thiols became rapidly oxidized. Although stimulated neutrophils accumulated ascorbate during oxidation of LDL, pre-loading of neutrophils with ascorbate did not attenuate oxidant production by the cells. Thus, activated neutrophils oxidize LDL thiols by HOCl- and N-chloramine-dependent mechanisms and physiological concentrations of vitamin C delay this process, most likely due to scavenging of extracellular oxidants, rather than by attenuating neutrophil oxidant production.

Defects in leukocyte-mediated initiation of lipid peroxidation in plasma as studied in myeloperoxidase-deficient subjects: systematic identification of multiple endogenous diffusible substrates for myeloperoxidase in plasma

More than a decade ago it was demonstrated that neutrophil activation in plasma results in the time-dependent formation of lipid hydroperoxides through an unknown, ascorbate-sensitive pathway. It is now shown that the mechanism involves myeloperoxidase (MPO)-dependent use of multiple low-molecular–weight substrates in plasma, generating diffusible oxidant species. Addition of activated human neutrophils (from healthy subjects) to plasma (50%, vol/vol) resulted in the peroxidation of endogenous plasma lipids by catalase-, heme poison-, and ascorbate-sensitive pathways, as assessed by high-performance liquid chromatography (HPLC) with on-line electrospray ionization tandem mass spectrometric analysis of free and lipid-bound 9-HETE and 9-HODE. In marked contrast, neutrophils isolated from multiple subjects with MPO deficiency failed to initiate peroxidation of plasma lipids, but they did so after supplementation with isolated human MPO. MPO-dependent use of a low-molecular–weight substrate(s) in plasma for initiating lipid peroxidation was illustrated by demonstrating that the filtrate of plasma (10-kd MWt cutoff) could supply components required for low-density lipoprotein lipid peroxidation in the presence of MPO and H2O2. Subsequent HPLC fractionation of plasma filtrate (10-kd MWt cutoff) by sequential column chromatography identified nitrite, tyrosine, and thiocyanate as major endogenous substrates and 17β-estradiol as a novel minor endogenous substrate in plasma for MPO in promoting peroxidation of plasma lipids. These results strongly suggest that the MPO–H2O2system of human leukocytes serves as a physiological mechanism for initiating lipid peroxidation in vivo.

Determination of DNA damage induced by oxidative stress in hyperlipidemic patients

In the present paper, we report data on the genotoxic properties of hydrogen peroxide in polymorphonuclear neutrophils (PMNLs) separated from normolipidemic and type II/a hyperlipidemic patients. In all, 15 hyperlipidemic patients (11 female, 4 male, mean age 54.6+/-10.25 years) were involved in the study, and 7 normolipidemic patients (5 female, 2 male, mean age 53.4+/-8.07 years) served as controls. Using the comet assay, there was a significant difference in the degree of DNA damage between the two groups. The visual score characteristic of the degree of DNA damage was 350.97+/-31.31 in the hyperlipidemic group, while it was 289.5+/-29.49 in the control group (P<0.001). In the hyperlipidemic patients, a positive correlation was found between the degree of DNA damage and the basic oxidation of PMNLs (r=0.517), and the superoxide anion production of the cells stimulated with phorbolmiristate acetate (PMA) (r=0.326) and formyl-Met-Leu-Phe (FMLP) (r=0.525) as well. There was a negative correlation between DNA damage and HDL-associated antioxidant paraoxonase (PON) activity (r=-0.469), and the PON/HDL ratio (r=-0.631). No correlation was found between the degree of DNA damage and the plasma concentration of nitric oxide (NO) (r=0.098) and thiobarbituric acid-reactive substances (TBARS) (r=0.061) in hyperlipidemic patients. Our results show that in hyperlipidemic patients there is an increase in lymphocyte DNA damage caused by oxidative stress when compared to normolipidemic individuals as demonstrated by comet assay. Decreased antioxidant capacity in hyperlipidemic patients may play a significant role in this process.

Oxysterol-induced activation of macrophage NADPH-oxidase enhances cell-mediated oxidation of LDL in the atherosclerotic apolipoprotein E deficient mouse: inhibitory role for vitamin E

In the present study we provide evidence, both direct and circumstantial, that macrophage oxysterols induce translocation of p47phox from the cytosol to the cell's plasma membrane, forming an active NADPH-oxidase complex which produces superoxide anion and facilitates cell-mediated oxidation of LDL. The study was performed on macrophages from atherosclerotic apolipoprotein E deficient (E(0)) mice, which are under oxidative stress. The oxysterol content in peritoneal macrophages (MPM) from E(0) mice was significantly higher (by 50-80%) than that observed in MPM from control (C57BL6) mice. E(0) MPM release 2-fold more superoxide anions and oxidize LDL by 2.5-fold more than control MPM. Furthermore, macrophage protein kinase C (PKC) activity and arachidonic acid (AA) release (which are both involved in NADPH-oxidase activation) were elevated by 60 and 70%, respectively, in E(0) MPM compared with control MPM. Dietary supplementation of vitamin E (40 mg/kg per day for 2 months) to E(0) mice resulted in a reduction in MPM total oxysterols content (-27%) and this effect was associated with a reduction in PKC activity (-36%), AA release (-39%), cytosolic p47phox translocation to the plasma membrane (-30%), superoxide anion release (-25%) and MPM-mediated LDL oxidation (-28%), compared with unsupplemented E(0) mice. Enrichment of MPM from control mice with the major oxysterols found in E(0) MPM (7-ketocholesterol, beta-epoxycholesterol and 7beta-hydroxycholesterol) resulted in a dose-dependent increase (60-80%) in PKC activity, AA release, p47phox translocation, superoxide anion release and cell-mediated oxidation of LDL. These data clearly demonstrate for the first time that under oxidative stress, cellular lipids are oxidized, and that macrophage enrichment with oxysterols (as exists in E(0) mice) activates the NADPH-oxidase system and enhances cell-mediated oxidation of LDL, a key event during early atherogenesis.

Analysis of steroidal lipids by gas and liquid chromatography

This article describes the most commonly used procedures and recent laboratory methodologies using gas and liquid chromatography developed for separation and quantitation of non-saponifiable steroidal lipids from clinical (human) studies, edible fats and oils or fatty foods.

Amyloid-beta: an antioxidant that becomes a pro-oxidant and critically contributes to Alzheimer's disease

Elevated production of amyloid-beta (A beta) as a preventive antioxidant for brain lipoproteins under the action of increased oxidative stress in aging is postulated to represent a major event in the development of Alzheimer's disease (AD). Increase in A beta production is followed by chelation of transition metal ions by A beta, accumulation of A beta-metal lipoprotein aggregates, production of reactive oxygen species and neurotoxicity. Chelation of copper by A beta is proposed to be a most important part of this pathway, because A beta binds copper stronger than other transition metals and because copper is a more efficient catalyst of oxidation than other metals. This amyloid-binds-copper (ABC) model does not remove A beta peptide from its central place in our current thinking of AD, but rather places additional factors in the center of discussion. Most importantly, they embrace pathological mechanisms known to develop in aging (which is the major risk factor for AD), such as increased production of reactive oxygen species by mitochondria, that are positioned upstream relative to the generation of A beta.

Effect of acetaminophen on atherosclerosis

OBJECTIVE

To evaluate the antioxidant effects of acetaminophen in atherosclerosis.

DATA SOURCES

Experimental literature and abstracts accessed through MEDLINE (1966-February 2001).

DATA SYNTHESIS

Atherosclerosis is an inflammatory disorder associated with coronary events. The oxidative stress burden resulting from excess pro-oxidant free radical formation contributes to oxidative modification of low-density lipoprotein (lipid peroxidation) and is associated with atherosclerosis. Acetaminophen (phenol-like compound) may limit these key processes that are involved. The findings of earlier experimental laboratory tests and abstracts are evaluated.

CONCLUSIONS

In vitro data suggest that acetaminophen may reduce lipid peroxidation, whereas animal data showed decreased progression of atherosclerosis. Further animal model and human studies are required to confirm these earlier findings.

A critical overview of the chemistry of copper-dependent low density lipoprotein oxidation: roles of lipid hydroperoxides, alpha-tocopherol, thiols, and ceruloplasmin

The mechanisms by which low-density lipoprotein (LDL) particles undergo oxidative modification to an atherogenic form that is taken up by the macrophage scavenger-receptor pathway have been the subject of extensive research for almost two decades. The most common method for the initiation of LDL oxidation in vitro involves incubation with Cu(II) ions. Although various mechanisms have been proposed to explain the ability of Cu(II) to promote LDL modification, the precise reactions involved in initiating the process remain a matter of contention in the literature. This review provides a critical overview and evaluation of the current theories describing the interactions of copper with the LDL particle. Following discussion of the thermodynamics of reactions dependent upon the decomposition of preexisting lipid hydroperoxides, which are present in all crude LDL preparations, attention is turned to the more difficult (but perhaps more physiologically-relevant) system of the hydroperoxide-free LDL particle. In both systems, the key role of alpha-tocopherol is discussed. In addition to its protective, radical-scavenging action, alpha-tocopherol can also behave as a prooxidant via its reduction of Cu(II) to Cu(I). Generation of Cu(I) greatly facilitates the decomposition of lipid hydroperoxides to chain-carrying radicals, but the mechanisms by which the vitamin promotes LDL oxidation in the absence of preformed hydroperoxides remain more speculative. In addition to the so-called tocopherol-mediated peroxidation model, in which polyunsaturated fatty acid oxidation is initiated by the alpha-tocopheroxyl radical (generated during the reduction of Cu(II) by alpha-tocopherol), an evaluation of the role of the hydroxyl radical is provided. Important interactions between copper ions and thiols are also discussed, particularly in the context of cell-mediated LDL oxidation. Finally, the mechanisms by which ceruloplasmin, a copper-containing plasma protein, can bring about LDL modification are discussed. Improved understanding of the mechanisms of LDL oxidation by copper ions should facilitate the establishment of any physiological role of the metal in LDL modification. It will also assist in the interpretation of studies in which copper systems of LDL oxidation are used in vitro to evaluate potential antioxidants.

Oxidized low-density lipoprotein promotes mature dendritic cell transition from differentiating monocyte

Proinflammatory oxidized phospholipids are generated during oxidative modification of low-density lipoproteins (LDL). The production of these proinflammatory oxidized phospholipids is controlled by secreted enzymes that circulate as proteins complexed with LDL and high-density lipoprotein. During the acute phase response to tissue injury, profound changes occur in lipoprotein enzymatic composition that alter their anti-inflammatory function. Monocytes may encounter oxidized phospholipids in vivo during their differentiation to macrophages or dendritic cells (DC). In this study we show that the presence of oxidized LDL (oxLDL) at the first day of monocyte differentiation to DC in vitro yielded phenotypically atypical cells with some functional characteristics of mature DC. Addition of oxLDL during the late stage of monocyte differentiation gave rise directly to phenotypically mature DC with reduced uptake capacity, secreting IL-12 but not IL-10, and supporting both syngeneic and allogeneic T cell stimulation. In contrast to known mediators of DC activation, oxLDL did not trigger maturation of immature DC. An intriguing possibility is that a burst of oxidized phospholipids is an endogenous activation signal for the immune system, which is tightly controlled by lipoproteins during the acute phase response.

Low density lipoprotein receptor-related protein is required for macrophage-mediated oxidation of low density lipoprotein by 12/15-lipoxygenase

The oxidative modification of low density lipoprotein (LDL) has been implicated in the early stage of atherosclerosis through multiple potential pathways, and 12/15-lipoxygenase is suggested to be involved in this oxidation process. We demonstrated previously that the 12/15-lipoxygenase overexpressed in mouse macrophage-like J774A.1 cells was required for the cell-mediated LDL oxidation. However, the mechanism of the oxidation of extracellular LDL by the intracellular 12/15-lipoxygenase has not yet been elucidated. In the present study, we found that not only the LDL receptor but also LDL receptor-related protein (LRP), both of which are cell surface native LDL-binding receptors, were down-regulated by the preincubation of the cells with cholesterol or LDL and up-regulated by lipoprotein-deficient serum. Moreover, 12/15-lipoxygenase-expressing cell-mediated LDL oxidation was decreased by the preincubation of the cells with LDL or cholesterol and increased by the preincubation with lipoprotein-deficient serum. Heparin-binding protein 44, an antagonist of the LDL receptor family, also suppressed the cell-mediated LDL oxidation in a dose-dependent manner. The cell-mediated LDL oxidation was dose-dependently blocked by an anti-LRP antibody but not by an anti-LDL receptor antibody. Furthermore, antisense oligodeoxyribonucleotides against LRP reduced the cell-mediated LDL oxidation under the conditions in which the expression of LRP was decreased. The results taken together indicate that LRP was involved essentially for the cell-mediated LDL oxidation by 12/15-lipoxygenase expressed in J774A.1 cells, suggesting an important pathophysiological role of this receptor-enzyme system as the initial trigger of the progression of atherosclerosis.

Is the emperor wearing clothes? Clinical trials of vitamin E and the LDL oxidation hypothesis

A wealth of evidence indicates that oxidized low density lipoprotein (LDL) may be of central importance in animal models of atherogenesis. In recent clinical trials, however, dietary vitamin E supplements have not consistently prevented cardiac events in humans with established coronary artery disease. Such mixed results have led many to question the role of LDL oxidation in human atherosclerosis, although this interpretation assumes that the doses of vitamin E used in the studies inhibited lipid oxidation in vivo. In fact, there is remarkably little evidence indicating that those particular regimens effectively inhibit lipid peroxidation in healthy humans. Moreover, evidence of increased oxidative stress was not a criterion for inclusion in the trials; therefore, vitamin E may have benefited only a subset of the participants. These uncertainties raise doubts about the ability of vitamin E to augment antioxidant defense mechanisms in vivo and leave many questions about LDL oxidation and atherosclerosis unanswered.

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.