Lipoxygenase contributes to the oxidation of lipids in human atherosclerotic plaques

Complementary roles for scavenger receptor A and CD36 of human monocyte-derived macrophages in adhesion to surfaces coated with oxidized low-density lipoproteins and in secretion of H2O2

Oxidized low-density lipoprotein (oxLDL) is considered one of the principal effectors of atherogenesis. To explore mechanisms by which oxLDL affects human mononuclear phagocytes, we incubated these cells in medium containing oxLDL, acetylated LDL (acLDL), or native LDL, or on surfaces coated with these native and modified lipoproteins. The presence of soluble oxLDL, acLDL, or native LDL in the medium did not stimulate H2O2 secretion by macrophages. In contrast, macrophages adherent to surfaces coated with oxLDL secreted three- to fourfold more H2O2 than macrophages adherent to surfaces coated with acLDL or native LDL. Freshly isolated blood monocytes secreted little H2O2 regardless of the substrate on which they were plated. H2O2 secretion was maximal in cells maintained for 4-6 d in culture before plating on oxLDL-coated surfaces. Fucoidan, a known ligand of class A macrophage scavenger receptors (MSR-A), significantly reduced macrophage adhesion to surfaces coated with oxLDL or acLDL. Monoclonal antibody SMO, which blocks oxLDL binding to CD36, did not inhibit adhesion of macrophages to oxLDL-coated surfaces but markedly reduced H2O2 secretion by these cells. These studies show that MSR-A is primarily responsible for adhesion of macrophages to oxLDL-coated surfaces, that CD36 signals H2O2 secretion by macrophages adherent to these surfaces, and that substrate-bound, but not soluble, oxLDL stimulates H2O2 secretion by macrophages.

Induction of 15-lipoxygenase expression by IL-13 requires tyrosine phosphorylation of Jak2 and Tyk2 in human monocytes

The enzyme 15-lipoxygenase (15-LO) participates in the dioxygenation of polyenoic fatty acids. This activity leads to the degradation of mitochondrial membranes during reticulocyte differentiation, the production of pro- and anti-inflammatory mediators by a variety of cell types, and the oxidation of lipids in atherosclerotic lesions. The cytokines, IL-4 and IL-13, are reported to induce the expression of 15-LO in human peripheral blood monocytes. In this report we explore the signaling mechanisms involved in the IL-13-mediated induction of 15-LO expression. First we demonstrate that the delayed induction of 15-LO requires continuous stimulation of monocytes for a minimum period of 12 h. We also found that tyrosine kinase inhibitors blocked the induction of 15-LO in a dose-dependent manner. By immunoprecipitation and antiphosphotyrosine blotting experiments, IL-13 was shown to induce tyrosine phosphorylation of Jak2 and Tyk2, but not Jak1 or Jak3, within 5 min of treatment in human monocytes. To investigate whether the early induction of tyrosine phosphorylation of both Jak2 and Tyk2 was ultimately involved in 15-LO expression, we generated antisense oligodeoxyribonucleotides (ODNs) against Tyk2 and Jak2. We employed a cationic lipid-mediated delivery technique to transfect the monocytes and found that both antisense ODNs inhibited expression of their target proteins by 75-85%. The treatments were specific and did not affect the expression of each other. Furthermore, the antisense ODNs to Jak2 and Tyk2 both inhibited the induction of expression of 15-LO in monocytes treated with IL-13. Parallel experiments with sense ODNs to Jak2 and Tyk2 did not affect their protein levels or the induction of 15-LO by IL-13, and down-regulation of Jak1 also did not affect expression of 15-LO. Our results suggest the novel finding that IL-13 can induce tyrosine phosphorylation of both Jak2 and Tyk2 in primary human monocytes. This occurs as an early and essential signal transduction event for the IL-13-mediated induction of 15-LO expression. These data represent the first characterization of upstream kinases involved in the induced expression of 15-LO.

Calcium is required for quasi-lipoxygenase activity of hemoproteins

Bovine and guinea pig heart homogenates, porcine leukocyte homogenate, and human hemolysate were found to vigorously oxidize linoleic acid, with lipoxygenase-like activity, to its hydroperoxy, epoxy, hydroxy-epoxy, and keto compounds in the presence of calcium chloride. In the absence of calcium, the reaction was significantly reduced. Attempts to characterize this quasi-lipoxygenase activity revealed that calcium potentiated the quasi-lipoxygenase activities of hemoproteins (hemoglobin, myoglobin, myeloperoxidase, catalase, cytochrome c) and hemin at the physiological pH of 7.5. Lipid peroxidation by hemoproteins was inhibited by albumin and erythrocyte membranes in blood, as well as by a low concentration of calcium in cells. However, it seems possible that in extracellular fluid, which contains a high concentration of calcium and a low concentration of albumin, hemoprotein released from damaged cells could oxidize unsaturated fatty acids derived by phospholipase-A2 from phospholipids of damaged cellular membranes. In a model of quasi-lipoxygenase activation under such conditions, lipids of erythrocyte membranes were oxidized by hemoglobin in the presence of phospholipase-A2 and calcium. The effect of nitrogen oxide, paraquat, and bleomycin on oxidation by hemoproteins and hemin was also discussed.

Oxidants and antioxidants in the pathogenesis of atherosclerosis: implications for the oxidized low density lipoprotein hypothesis

The oxidation hypothesis proposes that low density lipoprotein must be oxidatively modified to trigger the pathological events of atherosclerosis. In this article, we evaluate recent studies addressing the pathways that promote low density lipoprotein oxidation in vivo and the impact of antioxidants on atherogenesis in animals, paying particular attention to the clinical implications of these studies for the oxidation hypothesis.

Oxidized LDL and malondialdehyde-modified LDL in patients with acute coronary syndromes and stable coronary artery disease

BACKGROUND

The association between oxidative modifications of LDL and coronary artery disease (CAD) is suspected but not established. Therefore, the association between plasma levels of oxidized LDL and malondialdehyde (MDA)-modified LDL and acute coronary syndromes and stable CAD was investigated.

METHODS AND RESULTS

The study population contained 63 patients with acute coronary syndromes (45 with acute myocardial infarction and 18 with unstable angina pectoris), 35 nontransplanted patients with angiographically confirmed stable angina, 28 heart transplant patients with posttransplant CAD, 79 heart transplant patients without CAD, and 65 control subjects. After correction for age, sex, and LDL and HDL cholesterol, plasma levels of oxidized LDL and MDA-modified LDL were significantly higher in patients with CAD than in individuals without CAD (r2=0.57 and r2=0.26, respectively; both P=0.0001). Plasma levels of MDA-modified LDL were significantly higher in patients with acute coronary syndromes than in individuals with stable CAD (r2=0.65; P=0.0001) and were associated with increased levels of troponin I and C-reactive protein (r2=0.39 and r2=0.34, respectively; both P=0.0001). Plasma levels of oxidized LDL were not associated with increased levels of troponin I and C-reactive protein (r2=0.089 and r2=0.063, respectively).

CONCLUSIONS

Elevated plasma levels of oxidized LDL are associated with CAD. Elevated plasma levels of MDA-modified LDL suggest plaque instability and may be useful for the identification of patients with acute coronary syndromes.

Linoleic acid peroxidation--the dominant lipid peroxidation process in low density lipoprotein--and its relationship to chronic diseases

Modern separation and identification methods enable detailed insight in lipid peroxidation (LPO) processes. The following deductions can be made: (1) Cell injury activates enzymes: lipoxygenases generate lipid hydroperoxides (LOOHs), proteases liberate Fe ions--these two processes are prerequisites to produce radicals. (2) Radicals attack any activated CH2-group of polyunsaturated fatty acids (PUFAs) with about a similar probability. Since linoleic acid (LA) is the most abundant PUFA in mammals, its LPO products dominate. (3) LOOHs are easily reduced in biological surroundings to corresponding hydroxy acids (LOHs). LOHs derived from LA, hydroxyoctadecadienoic acids (HODEs), surmount other markers of LPO. HODEs are of high physiological relevance. (4) In some diseases characterized by inflammation or cell injury HODEs are present in low density lipoproteins (LDL) at 10-100 higher concentration, compared to LDL from healthy individuals.

The rabbit 15-lipoxygenase preferentially oxygenates LDL cholesterol esters, and this reaction does not require vitamin E

The oxidation of low density lipoprotein (LDL) by mammalian 15-lipoxygenases (15-LOX) was implicated in early atherogenesis. We investigated the molecular mechanism of 15-LOX/LDL interaction and found that during short term incubations, LDL cholesterol esters are oxygenated preferentially by the enzyme. Even when the LDL particle was loaded with free linoleic acid, cholesteryl linoleate constituted the major LOX substrate. In contrast, only small amounts of free oxygenated fatty acid isomers were detected, and re-esterification of oxidized fatty acids into the LDL ester lipid fraction was ruled out. When LDL was depleted from alpha-tocopherol, specific oxygenation of the cholesterol esters was not prevented, and the product pattern was not altered. Similar results were obtained at low (LDL/LOX ratio of 1:1) and high LOX loading (LDL/LOX ratio of 1:10) of the LDL particle. During long term incubations (up to 24 h), a less specific product pattern was observed. However, when the hydroperoxy lipids formed by the 15-LOX were immediately reduced by the phospholipid hydroperoxide glutathione peroxidase, when the reaction was carried out with vitamin E-depleted LDL, or when the assay sample was diluted, the specific pattern of oxygenation products was retained over a long period of time. These data suggest that mammalian 15-LOX preferentially oxidize LDL cholesterol esters, forming a specific pattern of oxygenation products. During long term incubations, free radical-mediated secondary reactions, which lead to a more unspecific product pattern, may become increasingly important. These secondary reactions appear to be suppressed when the hydroperoxy lipids formed are immediately reduced, when alpha-tocopherol-depleted LDL was used, or when the incubation sample was diluted. It may be concluded that 15-LOX-initiated LDL oxidation constitutes a dual-type oxygenase reaction with an initial enzymatic and a subsequent nonenzymatic phase. The biological relevance of this dual-type reaction for atherogenesis will be discussed.

Pathophysiology of nitric oxide and related species: free radical reactions and modification of biomolecules

Since its initial discovery as an endogenously produced bioactive mediator, nitric oxide (.NO) has been found to play a critical role in the cellular function of nearly all organ systems. Furthermore, aberrant production of .NO or reactive nitrogen species (RNS) derived from .NO, has been implicated in a number of pathological conditions, such as acute lung disease, atherosclerosis and septic shock. While .NO itself is fairly non-toxic, secondary RNS are oxidants and nitrating agents that can modify both the structure and function of numerous biomolecules both in vitro, and in vivo. The mechanisms by which RNS mediate toxicity are largely dictated by its unique reactivity. The study of how reactive nitrogen species (RNS) derived from .NO interact with biomolecules such as proteins, carbohydrates and lipids, to modify both their structure and function is an area of active research, which is lending major new insights into the mechanisms underlying their pathophysiological role in human disease. In the context of .NO-dependent pathophysiology, these biochemical reactions will play a major role since they: (i) lead to removal of .NO and decreased efficiency of .NO as an endothelial-derived relaxation factor (e.g. in hypertension, atherosclerosis) and (ii) lead to production of other intermediate species and covalently modified biomolecules that cause injury and cellular dysfunction during inflammation. Although the physical and chemical properties of .NO and .NO-derived RNS are well characterised, extrapolating this fundamental knowledge to a complicated biological environment is a current challenge for researchers in the field of .NO and free radical research. In this review, we describe the impact of .NO and .NO-derived RNS on biological processes primarily from a biochemical standpoint. In this way, it is our intention to outline the most pertinent and relevant reactions of RNS, as they apply to a diverse array of pathophysiological states. Since reactions of RNS in vivo are likely to be vast and complex, our aim in this review is threefold: (i) address the major sources and reactions of .NO-derived RNS in biological systems, (ii) describe current knowledge regarding the functional consequences underlying .NO-dependent covalent modification of specific biomolecules, and (iii) to summarise and critically evaluate the available evidence implicating these reactions in human pathology. To this end, three areas of special interest have been chosen for detailed description, namely, formation and role of S-nitrosothiols, modulation of lipid oxidation/nitration by RNS, and tyrosine nitration mechanisms and consequences.

Cholesteryl hydroperoxyoctadecadienoate from oxidized low density lipoprotein inactivates platelet-derived growth factor

Both oxidized low density lipoprotein (ox-LDL) and platelet-derived growth factor (PDGF) have been implicated in the genesis of various inflammatory responses, including atherosclerosis. We demonstrate here a novel interaction between specific oxidized lipids derived from ox-LDL and PDGF. The lipid moieties of ox-LDL caused concentration-dependent inactivation of PDGF as measured by loss of its mitogenic activity and its binding to high affinity receptors. Reverse-phase and normal-phase HPLC were used to purify the inactivating component in the lipid mixture. By fast atom bombardment mass spectrometry and infrared spectroscopy, we identified the inactivating lipids as the 9- and 13-hydroperoxy derivatives of cholesteryl linoleate, cholesteryl hydroperoxyoctadecadienoate. When a series of cholesteryl esters were subjected to oxidizing conditions, only those containing two or more double bonds caused inactivation of PDGF; the extent of inactivation increased with increased levels of oxidation. Exposing PDGF to cumene hydroperoxide, t-butyl hydroperoxide, or hydrogen peroxide did not affect the activity of the mitogen. The oxidized lipid had no effect on the mitogenic activity of epidermal growth factor but did abolish the mitogenic activity of basic fibroblast growth factor and the antiproliferative activity of transforming growth factor beta1. The inactivation of PDGF and other cytokines by lipid hydroperoxides may occur in such processes as vascular disease, inflammation, and wound healing.

Secretory phospholipase A2 and lipoprotein lipase enhance 15-lipoxygenase-induced enzymic and nonenzymic lipid peroxidation in low-density lipoproteins

The oxidation of low-density lipoprotein (LDL) is thought to contribute to atherogenesis. 15-Lipoxygenase (15LO) induces LDL oxidation, and phospholipase A2 enhances this process [Sparrow, C. P. , Parthasarathy, S., and Steinberg, D. (1988) J. LipidRes. 29, 745-753]. As the underlying mechanism of the enhancing effect has not been investigated previously, we here show that in the presence of soybean 15LO (SLO) or human 15LO (rhLO), the addition of lipoprotein lipase, porcine pancreatic, or human type IIa secretory phospholipase A2 (sPLA2) greatly enhanced the accumulation of hydro(pero)xides of all major classes of LDL's lipids. Hydroperoxides of free fatty acids accumulated exclusively as enzymic products with kinetics reflecting both the formation of free fatty acids and the initial 'build-up' of alpha-tocopheroxyl radical. In contrast, hydroperoxides of cholesteryl esters and phosphatidylcholine accumulated linearly over comparatively longer periods of time and, in the case of rhLO, well beyond inactivation of the oxygenase. With SLO, formation of oxidized esterified lipids occurred nonenzymically, independent of the presence of lipase and despite the oxygenase remaining active until the end of the incubation. Enhancement of rhLO-induced LDL lipid peroxidation by sPLA2 was eliminated by a neutralizing anti-sPLA2 antibody, indicating that lipolytic activity was required for this effect. LDL depleted of alpha-tocopherol was resistant to oxidation by 15LO alone, whereas lipase overcame this resistance, demonstrating that lipases enhance 15LO-induced enzymic and nonenzymic peroxidation of LDL lipids. This is likely due to provision of free fatty acid substrate, resulting in an enhanced rate of free radical formation which itself causes nonenzymic peroxidation of esterified lipids. As lipases and 15LO are present in atherosclerotic lesions, our findings could be of pathophysiological significance.

Colocalization of iron and ceroid in human atherosclerotic lesions

The presence of ceroid, a complex of protein associated with oxidized lipids, is commonly observed in human atherosclerotic lesions. When the human aortic walls were examined by Perls' staining, it was found that the iron deposits were evident in aortas with atherosclerosis. The extent of iron deposition was associated with the severity of the lesion. Furthermore, the iron deposits appeared to be colocalized with ceroids either extracellularly or intracellularly in foam cell-like macrophages or smooth muscle cells. Electron microscopy and X-ray microanalysis revealed that some of the extracellular iron aggregates were present within the ceroids. Likewise, some of the subcellular iron aggregates were found to be located near the lipid droplets or within the ceroids of foam cells. Collectively, these observations support the theory that the lipid oxidation occurring in lipid-laden cells of aortic lesions is facilitated by iron-overload in these cells.

Cellular thiol production and oxidation of low-density lipoprotein

Compelling evidence suggests that low-density lipoprotein (LDL) is oxidized by cells within the arterial intima and that, once oxidized, it is profoundly atherogenic. The precise mechanism(s) by which cells promote the oxidation of LDL in vivo are not known; in vitro, however, oxidation of LDL can be enhanced by a number of differing mechanisms, including reaction with free and protein-bound metal ions, thiols, reactive oxygen species, lipoxygenase, myeloperoxidase and peroxynitrite. This review is concerned with the mechanisms by which cells enhance the oxidation of LDL in the presence of transition metals; in particular, the regulation, pro- and anti-oxidant consequences, and mechanism of action of cellular thiol production are examined, and contrasted with thiol-independent oxidation of LDL in the presence of transition metals.

Glutathione-related antioxidant defenses in human atherosclerotic plaques

BACKGROUND

Oxidative stress, resulting from an antioxidant/prooxidant imbalance, seems to be crucial in atherogenesis. Recent evidence has emerged, however, of a surprisingly high content of low-molecular-weight antioxidants in human atherosclerotic plaques, although other antioxidant systems have not been investigated in these lesions.

METHODS AND RESULTS

We studied glutathione-related antioxidant defenses (which play a key role in tissue antioxidant protection) in carotid atherosclerotic plaques of 13 patients subjected to endarterectomy and in normal internal mammary arteries of 13 patients undergoing coronary artery bypass surgery. Selenium-dependent glutathione peroxidase activity was undetectable in the plaques of 7 patients; the other 6 patients with plaques showed a mean enzymatic activity approximately 3.5-fold lower than that of mammary arteries. Glutathione reductase activity was also markedly lower in the plaques than in the arteries. Glutathione transferase instead had comparable activity in the two tissues. Remarkably, 5 of the 7 patients with an undetectable selenium-dependent glutathione peroxidase activity but none of the 6 with a detectable one were characterized by multivascular atherosclerotic involvement (3 patients) or stenosis of the contralateral carotid artery (2 patients).

CONCLUSIONS

A weak glutathione-related enzymatic antioxidant shield is present in human atherosclerotic lesions. Although the cause of this phenomenon remains to be determined, the present data suggest that a specific antioxidant/prooxidant imbalance operative in the vascular wall may be involved in atherogenic processes in humans.

The iron ligand sphere geometry of mammalian 15-lipoxygenases

We investigated the geometry of the iron ligand sphere of the native rabbit 15-lipoxygenase (15-LOX) by X-ray absorption spectroscopy using synchrotron radiation. The soybean LOX-1 was used as a reference compound because its iron ligand sphere is well characterized. For structural information the X-ray absorption spectra were evaluated using the Excurve Program (CCLRC Daresbury Laboratory, Warrington, U.K.). From the positions of the absorption edges and from the intensities of the 1s-3d pre-edge transition peaks a six-coordinate ferrous iron was concluded for the rabbit 15-LOX. Evaluation of the extended region of the absorption spectra suggested six nitrogen and/or oxygen atoms as direct iron ligands, and the following binding distances were determined (means+/-S.D.; estimated accuracy is +/-0.001nm for bond distances, on the basis of more than 22 X-ray absorption spectra): 0.213+/-0.001nm, 0.213+/-0. 001 nm, 0.236+/-0.001 nm, 0.293+/-0.001 nm, 0.189+/-0.001 nm and 0. 242+/-0.001. Lyophilization of the LOX altered the binding distances but did not destroy the octahedral iron ligand sphere. For construction of a structural model of the iron ligand sphere the binding distances extracted from the X-ray spectra were assigned to specific amino acids (His-360, -365, -540, -544 and the C-terminal Ile-662) by molecular modelling using the crystal coordinates of the soybean LOX-1 and of a rabbit 15-LOX-inhibitor complex.

Oxidized LDL regulates macrophage gene expression through ligand activation of PPARgamma

Macrophage uptake of oxidized low-density lipoprotein (oxLDL) is thought to play a central role in foam cell formation and the pathogenesis of atherosclerosis. We demonstrate here that oxLDL activates PPARgamma-dependent transcription through a novel signaling pathway involving scavenger receptor-mediated particle uptake. Moreover, we identify two of the major oxidized lipid components of oxLDL, 9-HODE and 13-HODE, as endogenous activators and ligands of PPARgamma. Our data suggest that the biologic effects of oxLDL are coordinated by two sets of receptors, one on the cell surface, which binds and internalizes the particle, and one in the nucleus, which is transcriptionally activated by its component lipids. These results suggest that PPARgamma may be a key regulator of foam cell gene expression.

A specific 15-lipoxygenase inhibitor limits the progression and monocyte-macrophage enrichment of hypercholesterolemia-induced atherosclerosis in the rabbit

Oxidant signalling and lipoprotein oxidation may play important roles in atherosclerotic lesion development. Given coincident localization of 15-lipoxygenase (15-LO), stereospecific products of 15-LO and epitopes of modified LDL in atherosclerotic lesions, we hypothesized that inhibition of 15-LO by PD146176, an inhibitor of 15-LO with an IC50 in cells or isolated enzyme of 0.5-0.8 microM, may limit atherosclerotic lesion development through regulation of monocyte-macrophage enrichment. Rabbits exposed to chronic endothelial denudation of the iliac-femoral artery were meal-fed a 0.25% cholesterol (C), 3% peanut oil (PNO), 3% coconut oil (CNO) diet twice daily with and without 175 mg/kg PD146176 for 12 weeks. In a second study, atherosclerotic lesions were pre-established in rabbits through chronic endothelial denudation and meal-fed a 0.5% C, 3% PNO, 3% CNO diet for 9 weeks and a 0% C/fat diet for 6 weeks prior to an 8 week administration of PD146176 at 175 mg/kg, q.d. Plasma total and lipoprotein cholesterol exposure were similar in control and PD146176-treated animals in both studies but PD146176 increased plasma triglyceride exposure 2- to 4-fold. Plasma PD146176 concentrations ranged from 99 to 214 ng/ml at 2 h post-dose. In the progression study, the iliac-femoral monocyte-macrophage area was reduced 71%, cross-sectional lesion area was unchanged and cholesteryl ester (CE) content was reduced 63%. In the regression study, size and macrophage content of iliac-femoral, fibrous plaque-like lesions were decreased 34%, CE content was reduced 19% and gross extent of thoracic aortic lesions were reduced 41%. We conclude that PD146176 can limit monocyte macrophage enrichment of atherosclerotic lesions and can attenuate development of fibrofoamy and fibrous plaque lesions in the absence of changes in plasma total or lipoprotein cholesterol concentrations.

Inhibition of mammalian 15-lipoxygenase-dependent lipid peroxidation in low-density lipoprotein by quercetin and quercetin monoglucosides

Lipoxygenase is suggested to be involved in the early event of atherosclerosis by inducing plasma low-density lipoprotein (LDL) oxidation in the subendothelial space of the arterial wall. Since flavonoids such as quercetin are recognized as lipoxygenase inhibitors and they occur mainly in the glycoside form, we assessed the effect of quercetin and its glycosides (quercetin 3-O-beta-glucopyranoside, Q3G; quercetin 4'-O-beta-glucopyranoside, Q4'G; quercetin 7-O-beta-glucopyranoside, Q7G) on rabbit reticulocyte 15-lipoxygenase (15-LOX)-induced human LDL lipid peroxidation and compared it with the inhibition obtained by ascorbic acid and alpha-tocopherol, the main water-soluble and lipid-soluble antioxidants in blood plasma, respectively. Quercetin inhibited the formation of cholesteryl ester hydroperoxides (CE-OOH) and endogenous alpha-tocopherol consumption effectively throughout the incubation period of 6 h. Ascorbic acid exhibited an effective inhibition only in the initial stage and LDL preloaded with fivefold alpha-tocopherol did not affect the formation of CE-OOH compared with the native LDL. CE-OOH formation was inhibited by both quercetin and quercetin monoglucosides in a concentration-dependent manner. Quercetin, Q3G, and Q7G exhibited a higher inhibitory effect than Q4'G (IC50: 0.3-0.5 microM for quercetin, Q3G, and Q7G and 1.2 microM for Q4'G). While endogenous alpha-tocopherol was completely depleted after 2 h of LDL oxidation, quercetin, Q7G, and Q3G prevented the consumption of alpha-tocopherol. Quercetin and its monoglucosides were also exhausted during the LDL oxidation. These results indicate that quercetin glycosides as well as its aglycone are capable of inhibiting lipoxygenase-induced LDL oxidation more efficiently than ascorbic acid and alpha-tocopherol.

Fibroblasts that overexpress 15-lipoxygenase generate bioactive and minimally modified LDL

Several lines of evidence suggest that the cellular enzyme 15 lipoxygenase (15-LO) may be important in promoting the oxidation of lipoproteins in vivo. In previous studies we have shown that fibroblasts transfected with 15-LO "seed" LDL with lipoperoxides such that subsequent oxidation readily generates an LDL that is taken up by macrophages through scavenger receptors. We now demonstrate that LDL incubated with 15-LO cells is "minimally modified" and has bioactive properties. Characterization of LDL incubated with 15-LO cells reveals that lipid peroxidation is modest, with low levels of TBARS generated (12.6 +/- 4.7 nmole MDA per mg protein) and small amounts of 18:2 lost as a result of oxidation (7%, compared with extensive loss [82%] with copper oxidation). The 15-LO-conditioned LDL showed mildly increased electrophoretic mobility on agarose gels, and on polyacrylamide gels it showed only mild protein degradation compared with copper-oxidized LDL. Additionally 15-LO-conditioned LDL competed very well for the LDL receptor of fibroblasts but did not compete for macrophage uptake of 125I-acetylated LDL. Importantly, compared with LDL incubated on beta-galactosidase (lac Z)-transfected control cells, LDL incubated on 15-LO cells stimulated monocyte chemotaxis (15-LO-LDL, 6.9 +/- 1.2 monocytes per field versus lac Z-LDL, 0 +/- 0.9 monocytes per field) and when added to endothelial cells enhanced adhesion (15-LO-LDL, 31.1 +/- 5.0 monocytes per field versus lac Z-LDL, 0 +/- 2.0 monocytes per field). Preincubation of 15-LO cells with 15-LO inhibitors significantly inhibited the generation of bioactive LDL. Lipid extracts of LDL conditioned on 15-LO cells showed chemotactic activity not related to lysophosphatidylcholine levels. Preincubation of target endothelial cells with several different platelet-activating factor receptor antagonists prevented stimulation of monocyte adhesion by 15-LO-conditioned LDL. When probucol- or vitamin E-enriched LDL was incubated with 15-LO cells it was less oxidized and less bioactive, which suggests that these cells seed LDL with LOOH, which then requires further propagation of lipid peroxidation to yield bioactivity. These studies demonstrate that fibroblasts expressing 15-LO reliably produce a bioactive "minimally modified" LDL, which may explain in part how cellular 15-LO activity may generate atherogenic LDL in vivo.

Oxidation of free fatty acids in low density lipoprotein by 15-lipoxygenase stimulates nonenzymic, alpha-tocopherol-mediated peroxidation of cholesteryl esters

15-Lipoxygenase has been implicated in the in vivo oxidation of low density lipoprotein (LDL) a process thought to be important in the origin and/or progression of human atherogenesis. We have suggested previously that oxidation of LDL's cholesteryl esters (CE) and phospholipids by soybean (SLO) or human recombinant 15-lipoxygenase (rhLO) can be ascribed largely to alpha-tocopherol (alpha-TOH)-mediated peroxidation (TMP). In this study we demonstrate that addition to LDL of unesterified linoleate (18:2), other free fatty acid (FFA) substrates, or phospholipase A2 (PLA2) significantly enhanced the accumulation of CE hydro(pero)xides (CE-O(O)H) induced by rhLO, whereas the corresponding CE and nonsubstrate FFA were without effect. The enhanced CE-O(O)H accumulation showed a dependence on the concentration of free 18:2 in LDL. In contrast, addition of 18:2 had little effect on LDL oxidation induced by aqueous peroxyl radicals or Cu2+ ions. Analyses of the regio- and stereoisomers of oxidized 18:2 in SLO-treated native LDL demonstrated that the small amounts of 18:2 associated with the lipoprotein were oxidized enzymically and within minutes, whereas cholesteryl linoleate (Ch18:2) was oxidized nonenzymically and continuously over hours. alpha-Tocopheroxyl radical (alpha-TO.) formed in LDL exposed to SLO was enhanced by addition of 18:2 or PLA2. With rhLO and 18:2-supplemented LDL, oxidation of 18:2 was entirely enzymic, whereas that of Ch18:2 was largely, though not completely, nonenzymic. The small extent of enzymic Ch18:2 oxidation increased with increasing enzyme to LDL ratios. Ascorbate and the reduced form of coenzyme Q, ubiquinol-10, which are both capable of reducing alpha-TO. and thereby preventing TMP, inhibited nonenzymic Ch18:2 oxidation induced by rhLO. Trolox and ascorbyl palmitate, which also inhibit TMP, ameliorated both enzymic and nonenzymic oxidation of LDL's lipids, whereas probucol, a radical scavenger not capable of preventing TMP, was ineffective. These results demonstrate that rhLO-induced oxidation of CE is largely nonenzymic and increases with LDL's content of FFA substrates. We propose that conditions which increase LDL's FFA content, such as the presence of lipases, increase 15-LO-induced LDL lipid peroxidation and that this process requires only an initial, transient enzymic activity.

Prostaglandin F2-like compounds, F2-isoprostanes, are present in increased amounts in human atherosclerotic lesions

Oxidative modification of LDL is believed to play a major role in atherogenesis. As major lipid peroxidation products oxygenated linoleic acid derivatives and oxysterols have been described in human atherosclerotic lesions. Here we report that human lesions contain isoprostanes as peroxidation products of arachidonic acid at a level of 27.1 +/- 21.2 pg/mg wet weight (n = 10), which corresponds to 75.9 +/- 59.3 pg/mg dry weight, n contrast, human umbilical veins (n = 10), which were used as nonatherosclerotic control vessels, contain much smaller amounts of isoprostanes (1.4 +/- 0.7 pg/mg wet weight, which corresponds to 11.7 +/- 6.2 pg/mg dry weight), and there are significant differences between the two types of vessels. As major products of linoleic acid oxidation, racemic hydroxy linoleate isomers were detected in the lesional ester lipids. In human lesions, the hydroxy linoleic acid/linoleic acid ratio was about 0.5%, a result indicating that 5 out of 1000 linoleate residues are present as hydroxylated derivatives. In umbilical veins, no hydroxy linoleic acid could be detected. These data show that human atherosclerotic lesions contain increased amounts of hydroxy linoleic acid isomers and isoprostanes when compared with nonatherosclerotic vessel wall and suggest a link between local lipid peroxidation and progression of atherosclerosis. For evaluation of the degree of lipid peroxidation, the determination of the hydroxy linoleic acid/linoleic acid ratio appears to be more suitable than the isoprostane content.

Is there a role for 15-lipoxygenase in atherogenesis?

15-Lipoxygenase has been suggested to play a role in atherogenesis. The proposed action of this enzyme is to oxidize low density lipoprotein (LDL) to the extent that LDL becomes a ligand for the macrophage scavenger receptor. 15-Lipoxygenase and oxidized LDL are co-localized in atherosclerotic lesions; antioxidant drugs that block the lipoxygenase also block oxidation of LDL and the progression of experimental atherosclerosis. Biochemical experiments have demonstrated that the lipoxygenase can be induced by cytokines and/or another factor(s) associated with hypercholesterolemia. However, molecular biological work has shown that induction of the enzyme alone is not sufficient to induce lesion formation. Furthermore, the mechanism of action of 15-lipoxygenase in atherogenesis remains unclear. Predictions of the stereochemistry of enzyme-oxidized linoleate products appear to conflict with the available data. In fact, most studies have discovered substantial levels of racemic 13-hydroxyoctadecadienoic acid (13-HODE) in arterial lesions rather than the stereochemically pure 13(S)-HODE expected from purified enzyme. The possibility that the generation of products of 15-lipoxygenase metabolism must occur in a specific cellular location and during a brief time window in the development of the disease has been discussed. It is also possible that the true function of the linoleate metabolites is to modulate gene expression and regulate mitogenesis, and that oxidation of LDL may play a secondary role. The advent of transgenic species that both develop atherosclerosis and either fail to express or overexpress the lipoxygenase presents an opportunity to clarify some of these issues in the near future.

Low-density lipoprotein and oxidised low-density lipoprotein: their role in the development of atherosclerosis

Oxidation of low-density lipoprotein (LDL) may be implicated in the development of atherosclerotic disease. Oxidised LDL is taken up more readily by monocyte-derived macrophages than LDL. Antibodies to oxidised LDL are found in atherosclerotic lesions, Increased risk of ischaemic heart disease is associated with a preponderance of small dense LDL particles, which are more susceptible to oxidation. Proatherogenic alterations in cell biochemistry and signalling pathways occur in the presence of LDL and more markedly oxidised LDL. In vitro antioxidants inhibit changes in cell biochemistry, while in vivo, they have been shown to attenuate or reverse development of atherosclerosis.

Cytokine modulation of LDL oxidation by activated human monocytes

There is considerable evidence to suggest that cytokines modulate the pathological cellular events that occur in human atherosclerosis. We sought to determine the effects of T-helper-lymphocyte (TH)-1- and TH2-type cytokines on the ability of human monocytes to oxidize LDL, one of the pathological processes believed to occur in atherosclerosis. The ability of opsonized zymosan (ZOP)-activated human monocytes to oxidize LDL in a 24-hour period was significantly enhanced by pretreatment of the monocytes with the TH2 cytokines, interleukin (IL)-4, or IL-13 compared with untreated monocytes. In contrast, interferon (IFN)-gamma, a TH1 cytokine, inhibited LDL oxidation by activated monocytes. Treatment with IFN-gamma also prevented the IL-4- and IL-13-mediated enhancement of LDL oxidation by ZOP-activated monocytes. Untreated or cytokine-treated unactivated monocytes did not oxidize LDL. The enhancement of LDL oxidation mediated by IL-4 or IL-13 treatment was not due to a mitogenic effect of the cytokines on the monocytes, nor to modulation of superoxide anion (O2-) production. The cytokine regulation of 15-lipoxygenase (LO) in the monocytes was also examined. IL-4 and IL-13 induction of 15-LO mRNA and 15-LO activity in the monocytes was confirmed, as was the previously reported inhibition of induction by IFN-gamma. In summary, IL-4 and IL-13 enhance the ability of activated human monocytes to oxidize LDL, whereas IFN-gamma inhibits the cell-mediated oxidation. The up- and downregulation of activated monocyte-mediated LDL oxidation by these cytokines correlates with the expression of 15-LO activity. Considerable evidence suggests that the progression of atherosclerosis includes events that are immunologically mediated, lending potential physiological relevance to these in vitro observations.

Separation and characterization of cholesteryl oxo- and hydroxy-linoleate isolated from human atherosclerotic plaque

In previous work we demonstrated that up to 30% of cholesteryl linoleate in homogenates of advanced human plaque samples is present in oxidized forms. Here we show that the material from plaque hexane extracts which co-elutes with cholesteryl hydroxy-linoleate on reversed phase HPLC (Anal Biochem 1993;213:79), is composed of several isomers of cholesteryl hydroxy- and cholesteryl oxo-octadecadienoate. Enzymatic hydrolysis and measurement of liberated cholesterol and disappearance of the esters revealed that almost all of the material consisted of unoxidized cholesterol esterified to oxidized derivatives of octadecadienoate. Semi-preparative reversed-phase HPLC was used to obtain sufficient quantities of this co-eluting material to undertake normal phase HPLC separation of these components. The nature of such separated and isolated compounds was identified, by co-chromatography with authentic standards, UV spectroscopy and chemical ionization and electron impact mass spectrometry, as cholesteryl hydroxy- and cholesteryl oxo-octadecadienoate. These oxidized fatty acids have been observed previously in plaque, in agreement with our new unambiguous demonstration of their presence as cholesteryl esters. The application of the methods described for the separation of the various forms of oxidized cholesteryl octadecadienoate may aid mechanistic studies of in vitro and in vivo lipoprotein lipid oxidation.

Regulation of 12-lipoxygenase by cytokines in vascular smooth muscle cells

Increasing evidence suggests that cytokines such as interleukin-1beta (IL-1), IL-4, and IL-8 may play an important role in the chronic inflammation and cellular growth observed in cardiovascular diseases. The lipoxygenase (LO) pathway of arachidonate metabolism has also been related to the pathology of hypertension and atherosclerosis. LO products have chemotactic, hypertrophic, and mitogenic effects in vascular cells, and the LO enzyme has been implicated in the oxidation of LDL. Furthermore, earlier studies have shown that vascular smooth muscle cell (VSMC) growth factors such as angiotensin II and platelet-derived growth factor can increase LO activity and expression in VSMCs. In the present study, we have examined whether vasoactive and inflammatory cytokines such as IL-1, IL-4, and IL-8 can modulate 12-LO activity and expression in porcine VSMCs and also whether they have growth-promoting effects in these cells. Treatment of porcine VSMCs with these cytokines led to significant increases in the levels of a cell-associated 12-LO product, 12-hydroxyeicosatetraenoic acid, as well as intracellular 12-LO enzyme activity. Furthermore, each of these cytokines led to a dose-dependent increase in 12-LO mRNA expression (333-base pair PCR product) as well as 12-LO protein expression (72 kD). In addition, all three interleukins could induce significant increases in VSMC DNA synthesis as well as proliferation. These results suggest that these cytokines have mitogenic effects in VSMCs and are also potent positive regulators of the 12-LO pathway. Thus, enhanced 12-LO activity and expression may be a key mechanism for cytokine-induced VSMC migration and proliferation.

Angiotensin II increases macrophage-mediated modification of low density lipoprotein via a lipoxygenase-dependent pathway

The molecular and cellular mechanisms by which hypertension enhances atherosclerosis are poorly understood. Angiotensin II (Ang II) has been implicated in the regulation of cellular lipoxygenases (LO), which are thought to play a role in atherogenesis by inducing oxidative modification of low density lipoprotein (LDL). We sought to test the hypothesis that Ang II would stimulate murine macrophage LO activity (which has both 12- and 15-LO activity). Competitive binding studies revealed the presence of Ang II AT1 receptors on mouse peritoneal macrophages (MPM) and J-774 cells, but not on the RAW cell line. Valsartan, a specific AT1 receptor antagonist inhibited Ang II binding, whereas PD 123319, an AT2 receptor antagonist did not. Incubation of MPM or J-774 cells with Ang II (10 pM to 1 microM) for 24 h led to a 2.5-3.5-fold increase in LO activity, measured as generated 13-HODE or 12(S)-HETE. This stimulation was inhibited by valsartan, but not by PD 123319. In contrast, Ang II did not stimulate LO activity in RAW macrophages. Semiquantitative reverse transcriptase-polymerase chain reaction showed a 2-3-fold increase in LO mRNA in MPM, but not in RAW cells after treatment with Ang II. Ang II also induced an increase in 12-LO protein. In addition, pretreatment of J-774 cells with Ang II increased in a dose-dependent manner the ability of the cells to modify LDL, resulting in greater chemotactic activity for monocytes, typical of minimally modified LDL. This stimulation was inhibited by AT1 receptor blockade. In summary, these data suggest that Ang II increases macrophage LO activity via AT1 receptor-mediated mechanisms and this further increases the ability of the cells to generate minimally oxidized LDL. These studies provide a link between hypertension and the associated increased atherosclerosis observed in hypertensive patients.

Structural elucidation of oxygenated storage lipids in cucumber cotyledons. Implication of lipid body lipoxygenase in lipid mobilization during germination

At early stages of germination, a special lipoxygenase is expressed in cotyledons of cucumber and several other plants. This enzyme is localized at the lipid storage organelles and oxygenates their storage triacylglycerols. We have isolated this lipid body lipoxygenase from cucumber seedlings and found that it is capable of oxygenating in vitro di- and trilinolein to the corresponding mono-, di-, and trihydroperoxy derivatives. To investigate the in vivo activity of this enzyme during germination, lipid bodies were isolated from cucumber seedlings at different stages of germination, and the triacylglycerols were analyzed for oxygenated derivatives by a combination of high pressure liquid chromatography, gas chromatography/mass spectrometry, and nuclear magnetic resonance spectroscopy. We identified as major oxygenation products triacylglycerols that contained one, two, or three 13S-hydroperoxy-9(Z),11(E)-octadecadienoic acid residues. During germination, the amount of oxygenated lipids increased strongly, reaching a maximum after 72 h and declining afterward. The highly specific pattern of hydroperoxy lipids formed suggested the involvement of the lipid body lipoxygenase in their biosynthesis. These data suggest that this lipoxygenase may play an important role during the germination process of cucumber and other plants and support our previous hypothesis that the specific oxygenation of the storage lipids may initiate their mobilization as a carbon and energy source for the growing seedling.

In vivo action of 15-lipoxygenase in early stages of human atherogenesis

Oxidative modification of low density lipoprotein has been suggested as patho-physiologically relevant process in atherogenesis and the lipid peroxidizing enzyme 15-lipoxygenase may be involved. For experimental evidence on the in vivo action of this enzyme in the time course of plaque formation we analyzed the lipid extracts of lesional areas representing various stages of human atherogenesis for the occurrence of specific 15-lipoxygenase products. In advanced human lesions the degree of oxygenation of the lesion lipids measured as hydroxy linoleic acid/linoleic acid ratio varied between 0.2 and 3.2%. Here an unspecific pattern of oxygenated lipids that did not differ from the pattern formed during copper-catalyzed LDL oxidation was detected. In both cases an enantiomer ratio (S/R-ratio) of 13-hydroxy-9Z,11E-octadecadienoic acid (13-HODE) of approximately 1:1 was found. In young human lesions which were obtained from the collection of the pathological determinants of atherosclerosis in youth (PDAY) program the hydroxy linoleic acid/linoleic acid ratio was much smaller (variation between 0.05 and 0.6%), and a significant share of specific 15-lipoxygenase products was detected (S/R-ratio of 13-hydroxy linoleic acid of 54 +/- 3.1/46 +/- 3.1 [mean +/- SD]). These data suggest that the 15-lipoxygenase is enzymatically active on endogenous substrates in young human lesions and thus, may be of patho-physiological importance for early atherogenesis. In advanced human plaques the 15-lipoxygenase may be functionally silent and specific lipoxygenase products formed in earlier stages may be decomposed or superimposed by large amounts of nonenzymatic lipid peroxidation products.

Attenuation of diet-induced atherosclerosis in rabbits with a highly selective 15-lipoxygenase inhibitor lacking significant antioxidant properties

1. 15-Lipoxygenase (15-LO) has been implicated in the pathogenesis of atherosclerosis because of its localization in lesions and the many biological activities exhibited by its products. To provide further evidence for a role of 15-LO, the effects of PD 146176 on the development of atherosclerosis in cholesterol-fed rabbits were assessed. This novel drug is a specific inhibitor of the enzyme in vitro and lacks significant non specific antioxidant properties. 2. PD 146176 inhibited rabbit reticulocyte 15-LO through a mixed noncompetitive mode with a Ki of 197 nM. The drug had minimal effects on either copper or 2,2'-azobis(2-amidinopropane)hydrochloride (ABAP) induced oxidation of LDL except at concentrations 2 orders higher than the Ki. 3. Control New Zealand rabbits were fed a high-fat diet containing 0.25% wt./wt. cholesterol; treated animals received inhibitor in this diet (175 mg kg-1, b.i.d.). Plasma concentrations of inhibitor were similar to the estimated Ki (197 nM). During the 12 week study, there were no significant differences in weight gain haematocrit, plasma total cholesterol concentrations, or distribution of lipoprotein cholesterol. 4. The drug plasma concentrations achieved in vivo did not inhibit low-density lipoprotein (LDL) oxidation in vitro. Furthermore, LDL isolated from PD 146176-treated animals was as susceptible as that from controls to oxidation ex vivo by either copper or ABAP. 5. PD 146176 was very effective in suppressing atherogenesis, especially in the aortic arch where lesion coverage diminished from 15 +/- 4 to 0% (P < 0.02); esterified cholesterol content was reduced from 2.1 +/- 0.7 to 0 micrograms mg-1 (P < 0.02) in this region. Immunostainable lipid-laden macrophages present in aortic intima of control animals were totally absent in the drug-treated group. 6. Results of these studies are consistent with a role for 15-LO in atherogenesis.

Phenylalanine 353 is a primary determinant for the positional specificity of mammalian 15-lipoxygenases

Mammalian lipoxygenases are implicated in the biosynthesis of inflammatory mediators, in the pathogenesis of atherosclerosis and in the process of blood cell differentiation and maturation. With respect to their reaction specificity, three major types of mammalian lipoxygenases (15-lipoxygenases, 12-lipoxygenases and 5-lipoxygenases) may be classified. Although this nomenclature is commonly used, the mechanistic reasons for the positional specificity of lipoxygenases are not well understood. We investigated the structural reasons for lipoxygenase specificity by a combination of chimera formation and site-directed mutagenesis, and identified phenylalanine 353 as primary determinant for the positional specificity of rabbit reticulocyte 15-lipoxygenase. Modeling of the enzyme-substrate interaction suggested that the alignment of arachidonic acid at the active site appears to be influenced by this residue. According to the substrate orientation, the 15-lipoxygenase may be differentiated from two types of mammalian 12-lipoxygenases.

Platelet-derived growth factor BB mediated regulation of 12-lipoxygenase in porcine aortic smooth muscle cells

Platelet-derived growth factor BB (PDGF) is a potent mitogen and chemoattractant for vascular smooth muscle cells (VSMC). In the present study, we have examined the effect of PDGF on the 12-lipoxygenase (12-LO) pathway of arachidonate metabolism in porcine aortic VSMC (PVSMC). The rationale for this is previous studies showing that LO products have growth and chemotactic effects in VSMC and that another VSMC growth factor, angiotensin II, is a potent positive regulator of 12-LO activity and expression. We observed that PDGF causes a significant increase in the formation of the 12-LO product, 12-hydroxyeicosatetraenoic acid (12-HETE) in PVSMC. In addition, PDGF also markedly increased leukocyte-type 12-LO messenger RNA and protein expression. PDGF-induced PVSMC migration was inhibited significantly by two LO blockers but not by a cyclooxygenase blocker. Furthermore, although the proliferative effects of PDGF on PVSMC were not altered by cell culture under hyperglycemic conditions (25 mM glucose, HG), the chemotactic effects of PDGF as well as those of 10% fetal calf serum were significantly greater in cells cultured in HG as compared to normal glucose conditions (5.5 mM), thus indicating a potential new mechanism for the accelerated cardiovascular disease usually observed in diabetes. These results indicate a novel mechanism for the biological effects of PDGF in leading to cardiovascular disease.

Identification and quantification of regioisomeric cholesteryl linoleate hydroperoxides in oxidized human low density lipoprotein and high density lipoprotein

Oxidation of human LDL is implicated as an initiator of atherosclerosis. Isolated low density lipoprotein (LDL) and high density lipoprotein (HDL2) were exposed to aqueous radicals generated from the thermolabile azo compound 2,2'-azobis(2-amidinopropane) dihydrochloride. The primary nonpolar lipid products formed from the autoxidation of LDL and HDL were the regioisomeric cholesteryl linoleate hydroperoxides. In LDL oxidations, 9- and 13-hydroperoxides with trans,cis conjugated diene were formed as the major oxidation products if endogenous alpha-tocopheral was present in the LDL. After extended oxidation of LDL, at the time when endogenous alpha-tocopherol was consumed, the two trans,cis conjugated diene hydroperoxides began to disappear and the 9- and 13-hydroperoxides with trans,trans conjugated diene appeared. At very long oxidation times, none of the primary products, the conjugated diene hydroperoxides, were present. In HDL2, which has only very low levels of antioxidants, both the 9- and 13-hydroperoxides with trans,cis conjugated diene and the 9- and 13-hydroperoxides with trans,trans conjugated diene were formed at early stages of oxidation. The corresponding alcohols were also formed in the HDL2 oxidations. A mechanistic hypothesis consistent with these observations is presented.

Blood radicals: reactive nitrogen species, reactive oxygen species, transition metal ions, and the vascular system

Free radicals, such as superoxide, hydroxyl and nitric oxide, and other "reactive species", such as hydrogen peroxide, hypochlorous acid and peroxynitrite, are formed in vivo. Some of these molecules, e.g. superoxide and nitric oxide, can be physiologically useful, but they can also cause damage under certain circumstances. Excess production of reactive oxygen or nitrogen species (ROS, RNS), their production in inappropriate relative amounts (especially superoxide and NO) or deficiencies in antioxidant defences may result in pathological stress to cells and tissues. This oxidative stress can have multiple effects. It can induce defence systems, and render tissues more resistant to subsequent insult. If oxidative stress is excessive or if defence and repair responses are inadequate, cell injury can be caused by such mechanisms as oxidative damage to essential proteins, lipid peroxidation, DNA strand breakage and base modification, and rises in the concentration of intracellular "free" Ca(2+). Considerable evidence supports the view that oxidative damage involving both ROS and RNS is an important contributor to the development of atherosclerosis. Peroxynitrite (derived by reaction of superoxide with nitric oxide) and transition metal ions (perhaps released by injury to the vessel wall) may contribute to lipid peroxidation in atherosclerotic lesions.

Lipoxygenase treatment render low-density lipoprotein susceptible to Cu2+-catalysed oxidation

Oxidative modification of low-density lipoprotein (LDL) has been implicated in foam-cell formation at all stages of atherosclerosis. Since transition metals and mammalian 15-lipoxygenases are capable of oxidizing LDL to its atherogenic form, a concerted action of these two catalysts in atherogenesis has been suggested. Cu2+-catalysed LDL oxidation is characterized by a kinetic lag period in which the lipophilic antioxidants are decomposed and by a complex mixture of unspecific oxidation products. We investigated the kinetics of the 15-lipoxygenase-catalysed oxygenation of LDL and found that the enzyme is capable of oxidizing LDL in the presence of the endogenous lipophilic antioxidants. In contrast with the Cu2+-catalysed reaction, no kinetic lag phase was detected. The pattern of products formed during short-term incubations was highly specific, with cholesterol-esterified (13S)-hydroperoxy-(9Z,11E)-octadecadinoic acid being the major product. However, after long-term incubations the product pattern was less specific. Preincubation with 15-lipoxygenase rendered human LDL more susceptible to Cu2+-catalysed oxidation as indicated by a dramatic shortening of the lag period. Addition of Cu2+ to lipoxygenase-treated LDL led to a steep decline in its antioxidant content and to a greatly reduced lag period. Interestingly, if normalized to a comparable hydroperoxide content, autoxidation and addition of exogenous hydroperoxy fatty acids both failed to overcome the lag period. The local peroxide concentrations in various LDL subcompartments will be discussed as a possible reason for this unexpected behaviour.

The selenoenzyme phospholipid hydroperoxide glutathione peroxidase controls the activity of the 15-lipoxygenase with complex substrates and preserves the specificity of the oxygenation products

Mammalian 15-lipoxygenases have been suggested to be involved in cell differentiation and atherogenesis because of their capability of oxygenating polyenoic fatty acids esterified to biomembranes and lipoproteins. We investigated the interaction of the lipid-peroxidizing 15-lipoxygenase and the hydroperoxy lipid-reducing phospholipid hydroperoxide glutathione peroxidase during their reaction with biomembranes and lipoproteins and obtained the following results. 1) Lipoxygenase treatment of submitochondrial membranes led to the formation of hydroperoxyphosphatidylethanolamine and hydroperoxyphosphatidylcholine as indicated by high performance liquid chromatography with chemiluminescence detection. In 15-lipoxygenase-treated low density lipoprotein cholesteryl hydroperoxylinoleate was the major oxygenation product. 2) Phospholipid hydroperoxide glutathione peroxidase was capable of reducing the hydroperoxy lipids formed by the 15-lipoxygenase to their corresponding alcohols. 3) Preincubation of low density lipoprotein and submitochondrial membranes with the phospholipid hydroperoxide glutathione peroxidase completely prevented the lipoxygenase reaction. However, addition of exogenous hydroperoxy lipids restored the oxygenase activity. 4) Short-term incubations of the complex substrates with the 15-lipoxygenase led to a specific pattern of oxidation products which was rendered more unspecific at long-term incubation or at high substrate concentrations. If the phosholipid hydroperoxide glutathione peroxidase was present during the reaction, the specific product pattern was preserved. These data indicate that the phospholipid hydroperoxide glutathione peroxidase is capable of reducing hydroperoxy ester lipids formed by a 15-lipoxygenase, and that it may down-regulate the 15-lipoxygenase pathways in mammalian cells. The specificity of 15-lipoxygenase-derived hydroperoxy lipids depends on their immediate reduction to the corresponding alcohols preventing postcatalytic isomerization.