Lipoxygenase contributes to the oxidation of lipids in human atherosclerotic plaques

Mammalian arachidonate 15-lipoxygenases structure, function, and biological implications

Lipoxygenases (LOXs) constitute a heterogeneous family of lipid peroxidizing enzymes capable of oxygenating polyunsaturated fatty acids to their corresponding hydroperoxy derivatives. In mammals, LOXs are classified with respect to their positional specificity of arachidonic acid oxygenation into 5-, 8-, 12-, and 15-LOXs. Arachidonate 15-LOXs may be sub-classified into a reticulocyte-type (type-1) and an epidermis-type (type-2) enzyme. Since the leukocyte-type 12-LOXs are very similar to the reticulocyte-type 15-LOXs, these enzymes are designated 12/15-LOXs. Several LOX isoforms, in particular the reticulocyte-type 15-LOX and the human 5-LOX, are well characterized with respect to their structural and functional properties On the other hand, the biological role of most LOX-isozymes including the reticulocyte-type 15-LOC is far from clear. This review is intended to summarize the recent developments in 15-LOX research with particular emphasis to molecular enzymology and regulation of gene expression. In addition, the major hypotheses on the physiological and patho-physiological roles of 15-LOXs will be discussed briefly.

Do changes in the cell membrane structure induce the generation of lipid peroxidation products which serve as first signalling molecules in cell to cell communication?

Evidence is presented that mammalian and plant cells respond equally to any event which changes their cell membrane structure. Proliferation, wounding or aging induces generation of lipidhydroperoxides from cell wall phospholipids. These are transformed to signalling compounds, some of these induce apoptosis. If the exerted impact exceeds a certain level, the original enzymic reaction switches to a non-enzymic one which produces peroxylradicals. The latter are not liberated enzymically. Peroxylradicals generate a second set of signalling compounds, but cause also severe damage: they epoxidize double bonds, and oxidize proteins, sugars and nucleic acids. Such reactions occur in all inflammatory diseases. Lipidhydoperoxides and their degradation products are incorporated in fat. Apparently, these compounds are transferred partly to LDL. Such LDL is still recognized by the cell LDL receptor. Toxic lipid peroxidation products are therefore introduced into cells and might be able to damage cells from inside long before the typical signs of atherosclerosis and other chronic diseases become visible.

Oxidized lipid accumulates in the presence of alpha-tocopherol in atherosclerosis

Oxidative modification of low-density lipoproteins in the arterial wall is a key feature of atherogenesis and widely believed to cause and/or accelerate lesion development. Linked to this is the expectation that vascular antioxidants are depleted during oxidation in vivo. However, whether alpha-tocopherol (vitamin E), an important lipid-soluble antioxidant, is depleted early in atherogenesis and can prevent lipid peroxidation in vivo is unresolved. To address this we examined the content of specific configurational isomers (cis/trans) of lipid hydro(pero)xides in lesions, which represent the major non-enzymic oxidation products, as formation and accumulation of cis/trans isomers is influenced by alpha-tocopherol in studies in vitro. Concordant with our previous findings that large amounts of oxidized lipid co-exist with relatively normal alpha-tocopherol levels in human lesions, we now show that cis/trans isomers predominate over other products in human carotid and aortic lesions and in lesion lipoproteins. Further, dietary vitamin E supplementation of rabbits after arterial injury significantly increases both the aortic levels of alpha-tocopherol and the overall content of cis/trans isomers. These data are fully consistent with alpha-tocopherol acting as a hydrogen donor during lipid oxidation in vivo and suggest that alpha-tocopherol does not prevent lipoprotein lipid oxidation in the diseased vessel wall.

Are changes of the cell membrane structure causally involved in the aging process?

Lipid peroxidation is recognized by proliferation, wounding, and aging. The connecting link between these different events is a change in cell wall structure, which activates membrane bound phospholipases. These cleave phospholipids. Thus liberated polyunsaturated fatty acids (PUFAs) are substrates for lipoxygenases, which accept equally well linoleic acid and arachidonic acid and generate lipid hydroperoxides (LOOHs). If the amount of free PUFAs exceeds a certain amount, lipoxygenases commit suicide. The consequence is liberation of free iron ions that react with LOOHs by formation of radicals. These start a chain reaction. LOO* radicals produced in the course of this process attack proteins, nucleic acids, and also double bonds of all unsaturated compounds by epoxidation. Morever LOOHs are decomposed to toxic epoxy acids and alphabetagammadelta-unsaturated aldehydes. Both species react with glutathione. The resulting products seem to induce apoptosis. Since the products generated by wounding or aging are formed by decomposition of LOOHs the investigation of the aging processes can be simplified by studying the physiological action of artificially generated lipid peroxidation products derived from pure PUFAs. Degradation products of LOOHs are generated by thermal decomposition of fat-containing PUFAs. These products are induced into the body by adsorption in the intestine. They are at least partly incorporated in low density lipoproteins (LDLs). Primarily investigations seem to indicate that an overload of a diet rich in PUFAs induces only after two days an increase in oxidized LDL/PUFAs for a factor up to two in young people and for a factor of more than two in old individuals.

The specificity of lipoxygenase-catalyzed lipid peroxidation and the effects of radical-scavenging antioxidants

The oxidation of low density lipoprotein (LDL) by lipoxygenase has been implicated in the pathogenesis of atherosclerosis. It has been known that lipoxygenase-mediated lipid peroxidation proceeds in general via regio-, stereo- and enantio-specific mechanisms, but that it is sometimes accompanied by a share of random hydroperoxides as side reaction products. In this study we investigated the oxidation of various substrates (linoleic acid, methyl linoleate, phosphatidylcholine, isolated LDL, and human plasma) by the arachidonate 15-lipoxygenases from rabbit reticulocytes and soybeans aiming at elucidating the effects of substrate, lipoxygenase and reaction milieu on the contribution and mechanism of random oxidation and also the effect of antioxidant. The specific character of the rabbit 15-lipoxygenase reaction was confirmed under all conditions employed here. However, the specificity by soybean lipoxygenase was markedly dependent on the conditions. When phosphatidylcholine liposomes and LDL were oxygenated by soybean lipoxygenase, the product pattern was found to be exclusively regio-, stereo-, and enantio-random. When free linoleic acid was incorporated into PC liposomes and oxidized by soybean lipoxygenase, the free acid was specifically oxygenated, whereas esterified linoleate gave random oxidation products exclusively. Radical-scavenging antioxidants such as alpha-tocopherol, ascorbic acid and 2-carboxy-2,5,7,8-tetramethyl-6-chromanol selectively inhibited the random oxidation but did not influence specific product formation. It is assumed that the random reaction products originate from free radical intermediates, which have escaped the active site of the enzyme and thus may be accessible to radical scavengers. These data indicate that the specificity of lipoxygenase-catalyzed lipid oxidation and the inhibitory effects of antioxidants depend on the physico-chemical state of the substrate and type of lipoxygenase and that they may change completely depending on the conditions.

Myeloperoxidase and hypochlorite, but not copper ions, oxidize heparin-bound LDL particles and release them from heparin

A key factor in atherosclerosis is the retention of low density lipoprotein (LDL) in the extracellular matrix of the arterial intima, where it binds to the negatively charged glycosaminoglycan chains of proteoglycans. Oxidation may lead to modification of the lysine residues of apolipoprotein B-100 of LDL, which normally mediate the binding of LDL to glycosaminoglycans. Here, we studied whether various modes of oxidation can release LDL from heparin, a glycosaminoglycan with a strong negative charge, in vitro. We found that copper ions were unable to oxidize heparin-bound LDL particles because of their redox inactivation by the glycosaminoglycans. In contrast, myeloperoxidase and hypochlorite, a product of myeloperoxidase, were able to oxidize heparin-bound LDL, and this oxidation led to the release of the oxidized particles from heparin. When the released LDL particles were compared with the residual heparin-bound LDL particles, the released particles were more electronegative and contained more modified lysine residues than did the particles that remained bound. Because human atherosclerotic lesions contain catalytically active myeloperoxidase and (lipo)proteins modified by hypochlorite, the results suggest that myeloperoxidase-secreting monocytes/macrophages in the arterial intima can oxidize and extract LDL from the extracellular matrix with ensuing uptake by the macrophages of the oxidized and released LDL, with eventual formation of foam cells.

Interactions of nitric oxide-derived reactive nitrogen species with peroxidases and lipoxygenases

Nitric oxide (NO) is a major free radical modulator of smooth muscle tone, which under basal conditions acts to preserve vascular homeostasis through its anti-inflammatory properties. The biochemistry of NO, in particular, its rapid conversion in vivo into secondary reactive nitrogen species (RNS), its chemical nature as a free radical and its high diffusibility and hydrophobicity dictate that this species will interact with numerous biomolecules and enzymes. In this review, we consider the interactions of a number of enzymes found in the vasculature with NO and NO-derived RNS. All these enzymes are either homeostatic or promote the development of atherosclerosis and hypertension. Therefore their interactions with NO and NO-derived RNS will be of central importance in the initiation and progression of vascular disease. In some examples, (e.g. lipoxygenase, LOX), such interactions provide catalytic 'sinks' for NO, but for others, in particular peroxidases and prostaglandin H synthase (PGHS), reactions with NO may be detrimental. Nitric oxide and NO-derived RNS directly modulate the activity of vascular peroxidases and LOXs through a combination of effects, including transcriptional regulation, altering substrate availability, and direct reaction with enzyme turnover intermediates. Therefore, these interactions will have two major consequences: (i) depletion of NO levels available to cause vasorelaxation and prevent leukocyte/platelet adhesion and (ii) modulation of activity of the target enzymes, thereby altering the generation of bioactive signaling molecules involved in maintenance of vascular homeostasis, including prostaglandins and leukotrienes.

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.

The content of lipoperoxidation products in normal and atherosclerotic human aorta

To evaluate the role of lipid oxidation in atherogenesis the levels of lipid- and protein-bound products of peroxidation in normal and atherosclerotic areas of human aorta were investigated. The level of fluorescent (360/430 nm) lipid products was measured in chloroform-methanol extracts of aortic tissue. Normal intima, initial lesions and fatty streaks had a similar content of fluorescent substances. On the other hand, high level of fluorescent products was found in atherosclerotic plaques. Cholesterol covalently bound to proteins, which serve as a marker of lipoperoxidation, was measured by high performance liquid chromatography after mild alkaline hydrolysis of delipidated tissue protein samples. The levels of protein-bound cholesterol in initial lesions and fatty streaks were close to its content in uninvolved intima (59 +/- 18 and 92 +/- 18 vs. 70 +/- 13 nmol/g protein). The content of covalently bound cholesterol in atherosclerotic plaques was dramatically higher (90-fold) than in the normal tissue. In addition to protein-bound cholesterol, considerable amount of lipofuscin was revealed in the cells of atherosclerotic plaques, but not in the cells of normal intima, initial lesions or fatty streaks. Thus, the contents of all investigated lipid- and protein-bound products of lipoperoxidation in earlier atherosclerotic lesions were similar to their levels in normal tissue. It can be due to a low rate of oxidized product formation and/or high rate of its degradation in or elimination from the vessel wall.

Lipid peroxidation in aging and age-dependent diseases

Aging is related with an increase in oxidation products derived from nucleic acids, sugars, sterols and lipids. Evidence will be presented that these different oxidation products are generated by processes induced by changes in the cell membrane structure (CMS), and not by superoxide, as commonly assumed. CMS activate apparently membrane bound phospholipases A2 in mammals and plants. Such changes occur by proliferation, aging and especially by wounding. After activation of phospholipases, influx of Ca2+ ions and activation of lipoxygenases (LOX) is induced. The LOX transform polyunsaturated fatty acids (PUFAs) into lipid hydroperoxides (LOOHs), which seem to be decomposed by action of enzymes to signalling compounds. Following severe cell injury, LOX commit suicide. Their suicide liberates iron ions that induce nonenzymic lipid peroxidation (LPO) processes by generation of radicals. Radicals attack all compounds with the structural element -CH=CH-CH(2)-CH=CH-. Thus, they act on all PUFAs independently either in free or conjugated form. The most abundant LPO products are derived from linoleic acid. Radicals induce generation of peroxyl radicals, which oxidise a great variety of biological compounds including proteins and nucleic acids. Nonenzymic LPO processes are induced artificially by the treatment of pure PUFAs with bivalent metal ions. The products are separable after appropriate derivatisation by gas chromatography (GC). They are identified by electron impact mass spectrometry (EI/MS). The complete spectrum of LPO products obtained by artificial LPO of linoleic acid is detectable after wounding of tissue, in aged individuals and in patients suffering from age-dependent diseases. Genesis of different LPO products derived from linoleic acid will be discussed in detail. Some of the LPO products are of high chemical reactivity and therefore escape detection in biological surrounding. For instance, epoxides and highly unsaturated aldehydic compounds that apparently induce apoptosis.

The role of vitamin E in atherogenesis: linking the chemical, biological and clinical aspects of the disease

Atherosclerosis is a disease involving both oxidative modifications and disbalance of the immune system. Vitamin E, an endogenous redox-active component of circulating lipoproteins and (sub)cellular membranes whose levels can be manipulated by supplementation, has been shown to play a role in the initiation and progression of the disease. Recent data reveal that the activities of vitamin E go beyond its redox function. Moreover, it has been shown that vitamin E can exacerbate certain processes associated with atherogenesis. In this essay we review the role of biology of atherosclerosis, and suggest that these two facets decide the clinical manifestation and outcome of the disease.

Catalytic consumption of nitric oxide by 12/15- lipoxygenase: inhibition of monocyte soluble guanylate cyclase activation

12/15-Lipoxygenase (LOX) activity is elevated in vascular diseases associated with impaired nitric oxide (( small middle dot)NO) bioactivity, such as hypertension and atherosclerosis. In this study, primary porcine monocytes expressing 12/15-LOX, rat A10 smooth muscle cells transfected with murine 12/15-LOX, and purified porcine 12/15-LOX all consumed *NO in the presence of lipid substrate. Suppression of LOX diene conjugation by *NO was also found, although the lipid product profile was unchanged. *NO consumption by porcine monocytes was inhibited by the LOX inhibitor, eicosatetraynoic acid. Rates of arachidonate (AA)- or linoleate (LA)-dependent *NO depletion by porcine monocytes (2.68 +/- 0.03 nmol x min(-1) x 10(6) cells(-1) and 1.5 +/- 0.25 nmol x min(-1) x 10(6) cells(-1), respectively) were several-fold greater than rates of *NO generation by cytokine-activated macrophages (0.1-0.2 nmol x min(-1) x 10(6) cells(-1)) and LA-dependent *NO consumption by primary porcine monocytes inhibited *NO activation of soluble guanylate cyclase. These data indicate that catalytic *NO consumption by 12/15-LOX modulates monocyte *NO signaling and suggest that LOXs may contribute to vascular dysfunction not only by the bioactivity of their lipid products, but also by serving as catalytic sinks for *NO in the vasculature.

12/15-lipoxygenase translocation enhances site-specific actin polymerization in macrophages phagocytosing apoptotic cells

The enzyme 12/15-lipoxygenase (12/15-LO) introduces peroxyl groups in a position-specific manner into unsaturated fatty acids in certain cells, but the role of such enzymatic lipid peroxidation remains poorly defined. Here we report a novel function for 12/15-LO in mouse peritoneal macrophages. When macrophages were coincubated with apoptotic cells, the enzyme translocated from cytosol to the plasma membrane and was more extensively concentrated at sites where macrophages bound apoptotic cells, colocalizing with polymerized actin of emerging filopodia. Disruption of F-actin did not prevent the 12/15-LO translocation. In contrast, inhibition of the 12/15-LO activity, or utilization of genetically engineered macrophages in which the 12/15-LO gene has been disrupted, greatly reduced actin polymerization in phagocytosing macrophages. Lysates of 12/15-LO-deficient macrophages had significantly lower ability to promote in vitro actin polymerization than the lysates of wild type macrophages. These studies suggest that the 12/15-LO enzyme plays a major role in local control of actin polymerization in macrophages in response to interaction with apoptotic cells.

Inhibition of oxidized low-density lipoprotein-induced apoptosis in endothelial cells by nitric oxide. Peroxyl radical scavenging as an antiapoptotic mechanism

Proatherogenic oxidized low-density lipoprotein (oxLDL) induces endothelial apoptosis. We investigated the anti-apoptotic effects of intracellular and extracellular nitric oxide (*NO) donors, iron chelators, cell-permeable superoxide dismutase (SOD), glutathione peroxidase mimetics, and nitrone spin traps. Peroxynitrite (ONOO-)-modified oxLDL induced endothelial apoptosis was measured by DNA fragmentation, TUNEL assay, and caspase-3 activation. Results indicated the following: (i) the lipid fraction of oxLDL was primarily responsible for endothelial apoptosis. (ii) Endothelial apoptosis was potently inhibited by *NO donors and lipophilic phenolic antioxidants. OxLDL severely depleted Bcl-2 levels in endothelial cells and *NO donors restored Bcl-2 protein in oxLDL-treated cells. (iii) The pretreatment of a lipid fraction derived from oxLDL with sodium borohydride or potassium iodide completely abrogated apoptosis in endothelial cells, suggesting that lipid hydroperoxides induce apoptosis. (iv) Metalloporphyrins dramatically inhibited oxLDL-induced apoptosis in endothelial cells. Neither S-nitrosation of caspase-3 nor induction of Hsp70 appeared to play a significant role in the antiapoptotic mechanism of *NO in oxLDL-induced endothelial apoptosis. We propose that cellular lipid peroxyl radicals or lipid hydroperoxides induce an apoptotic signaling cascade in endothelial cells exposed to oxLDL, and that *NO inhibits apoptosis by scavenging cellular lipid peroxyl radicals.

Absence of 12/15-lipoxygenase expression decreases lipid peroxidation and atherogenesis in apolipoprotein e-deficient mice

BACKGROUND

The enzyme 12/15-lipoxygenase (12/15-LO) has been implicated in the oxidative modification of LDL. In a murine model, we tested the hypothesis that deletion of 12/15-LO decreases atherogenesis by reducing oxidant stress, as measured by 2 indices of lipid peroxidation: isoprostane generation and autoantibody formation to malondialdehyde (MDA)-LDL, an epitope of LDL formed as a result of oxidative modification.

METHODS AND RESULTS

12/15-LO-deficient (12/15-LO(-/-)) mice were crossed with apolipoprotein E-deficient (apoE(-/-)) mice. At 10 weeks of age, atherosclerotic lesion initiation was significantly delayed in the double-knockout mice. The rate of lesion progression was diminished at 8 and 12 months, and even at 15 months, lesion size was reduced 50% (P<0.0005) compared with control apoE(-/-) mice. The urinary and plasma levels of the specific isoprostane 8,12-iso-iPF(2alpha)-VI, as well as IgG autoantibodies against MDA-LDL, were significantly reduced in the double-deficient mice in parallel with decreased atherosclerosis at all time points from 10 weeks to 15 months of age compared with apoE(-/-) controls.

CONCLUSIONS

Enzymatic action of 12/15-LO contributes significantly to atherosclerotic lesion initiation and propagation in this murine model. Strong positive correlations exist between lesion size, isoprostane levels, and MDA-LDL autoantibodies, providing in vivo evidence for an enzymatic (12/15-LO) component to lipid peroxidation and atherogenesis.

Antioxidants in health and disease

Free radical production occurs continuously in all cells as part of normal cellular function. However, excess free radical production originating from endogenous or exogenous sources might play a role in many diseases. Antioxidants prevent free radical induced tissue damage by preventing the formation of radicals, scavenging them, or by promoting their decomposition. This article reviews the basic chemistry of free radical formation in the body, the consequences of free radical induced tissue damage, and the function of antioxidant defence systems, with particular reference to the development of atherosclerosis.

Role of oxidized low-density lipoprotein in the atherosclerosis of uremia

Lipoprotein oxidation is involved in the genesis of atherosclerosis. In chronic renal failure (CRF), oxidative stress is enhanced because of an imbalance between pro-oxidant and antioxidant systems. Oxidative modifications of low-density lipoproteins (LDLs) occur not only at the level of lipid moiety, but also of protein moiety. We have shown that oxidation of LDL by hypochlorous acid (HOCl) in vitro, reflecting increased myeloperoxidase activity in vivo, leads to modifications of apoliproteins such that the latter in turn are capable of triggering macrophage nicotinamide adenine dinucleotide phosphate-oxidase activation. These oxidative changes of LDL protein moiety, if shown to occur to a significant extent in uremic patients in vivo, may represent an important alternative pathway in the pathogenesis of atheromatous lesions.

Interactions between nitric oxide and lipid oxidation pathways: implications for vascular disease

Nitric oxide ((.)NO) signaling pathways and lipid oxidation reactions are of central importance in both the maintenance of vascular homeostasis and the progression of vascular disease. Because both of these pathways involve free radical species that can also react together at extremely fast rates, convergent interactions between these pathways are expected. Biochemical and cell biology studies have defined multiple interactions of (.)NO with oxidizing lipids that could lead to either vascular protection or potentiation of inflammatory vascular injury. For example, low levels of (.)NO generated by endothelial nitric oxide synthase can terminate propagating lipid radicals and inhibit lipoxygenases, reactions that would be protective. Alternatively, if generated at elevated levels, for example, after inducible nitric oxide synthase expression in inflammation, (.)NO can be converted to prooxidant species, such as peroxynitrite (ONOO(-)) and nitrogen dioxide ((.)NO(2)), that can potentiate inflammatory injury to vascular cells. Finally, both enzymatic and nonenzymatic lipid oxidation reactions can influence (.)NO bioactivity by directly scavenging (.)NO or altering the induction and catalytic activity of nitric oxide synthase enzymes. In this review, we summarize the biochemical interactions between (.)NO and lipid oxidation reactions and discuss the recognized and potential roles of these reactions in the vasculature.

Hypochlorite modified LDL are a stronger agonist for platelets than copper oxidized LDL

Experimental low density lipoprotein (LDL) oxidation is usually performed using trace copper, although the in vivo relevance of this method has been called into question. Such LDL augment adenosine 5'-diphosphate (ADP) induced platelet aggregation, presumably by the action of lipid derived compounds. In striking contrast, we find that LDL oxidized to a comparable extent by hypochlorite, an in vivo occurring oxidant, reveal themselves to be potent promoters of platelet aggregation. Interestingly, hypochlorite modified LDL seem to mediate their influence on human platelets by means of the modified apolipoprotein B-100 (apoB) moiety. Also, the finding that hypochlorite modified albumin is able to trigger platelet aggregation suggests an essential role for hypochlorite modified protein(s) in the process of platelet activation.

Oxidized LDL reduces monocyte CCR2 expression through pathways involving peroxisome proliferator-activated receptor gamma

The CCR2-mediated recruitment of monocytes into the vessel wall plays an important role in all stages of atherosclerosis. In recent studies, we have shown that lipoproteins can modulate CCR2 expression and have identified native LDL as a positive regulator. In contrast, oxidized LDL (OxLDL), which is mainly formed in the aortic intima, reduces CCR2 expression, promotes monocyte retention, and may cause pathological accumulation of monocytes in the vessel wall. We now provide evidence that OxLDL reduces monocyte CCR2 expression by activating intracellular signaling pathways that may involve peroxisome proliferator-activated receptor gamma (PPARgamma). Receptor-mediated uptake of the lipoprotein particle was required and allows for delivery of the exogenous ligand to the nuclear receptor. The suppression of CCR2 expression by OxLDL was mediated by lipid components of OxLDL, such as the oxidized linoleic acid metabolites 9-HODE and 13-HODE, known activators of PPARgamma. Modified apoB had no such effect. Consistent with a participation of the PPARgamma signaling pathway, BRL49653 reduced CCR2 expression in freshly isolated human monocytes ex vivo and in circulating mouse monocytes in vivo. These results implicate PPARgamma in the inhibition of CCR2 gene expression by oxidized lipids, which may help retain monocytes at sites of inflammation, such as the atherosclerotic lesion.

Overexpression of 15-lipoxygenase in vascular endothelium accelerates early atherosclerosis in LDL receptor-deficient mice

To study the possible role of the human lipid-oxidizing enzyme 15-lipoxygenase (15-LO) in atherosclerosis, we overexpressed it specifically in the vascular wall of C57B6/SJL mice by using the murine preproendothelin-1 promoter. The mice overexpressing 15-LO were crossbred with low density lipoprotein (LDL) receptor-deficient mice to investigate atherogenesis. High levels of 15-LO were expressed in the atherosclerotic lesion in the double-transgenic mice as assessed by immunohistochemistry. The double-transgenic, 15-LO-overexpressing, LDL receptor-deficient mice (LDLR-/-/15LO) developed significantly larger atherosclerotic lesions at the aortic sinus compared with lesions in the LDL receptor-deficient (LDLR-/-) mice after 3 and 6 weeks (107,000 versus 28,000 microm(2) [P:<0.001] and 121,000 versus 87,000 microm(2) [P:<0.05], respectively) of an atherogenic diet. LDL from the LDLR-/-/15LO mice was more susceptible to oxidation than was the LDL from the control LDLR-/- mice, as shown by a shorter lag period for copper-induced conjugated diene formation. On the other hand, no differences were found in the levels of serum anti-oxidized LDL antibodies between the study groups. There were also no differences with respect to the density of macrophages and T lymphocytes infiltrating the lesions in both experimental groups. Taken together, these results support the hypothesis that 15-LO overexpression in the vessel wall is associated with enhanced atherogenesis.

Oxidation of LDL by myeloperoxidase and reactive nitrogen species: reaction pathways and antioxidant protection

Oxidative modification of low density lipoprotein (LDL) appears to play an important role in atherogenesis. Although the precise mechanisms of LDL oxidation in vivo are unknown, several lines of evidence implicate myeloperoxidase and reactive nitrogen species, in addition to ceruloplasmin and 15-lipoxygenase. Myeloperoxidase generates a number of reactive species, including hypochlorous acid, chloramines, tyrosyl radicals, and nitrogen dioxide. These reactive species oxidize the protein, lipid, and antioxidant components of LDL. Modification of apolipoprotein B results in enhanced uptake of LDL by macrophages with subsequent formation of lipid-laden foam cells. Nitric oxide synthases produce nitric oxide and, under certain conditions, superoxide radicals. Numerous other sources of superoxide radicals have been identified in the arterial wall, including NAD(P)H oxidases and xanthine oxidase. Nitric oxide and superoxide readily combine to form peroxynitrite, a reactive nitrogen species capable of modifying LDL. In this review, we examine the reaction pathways involved in LDL oxidation by myeloperoxidase and reactive nitrogen species and the potential protective effects of the antioxidant vitamins C and E.

Lipoxygenases and atherosclerosis: protection versus pathogenesis

15 lipoxygenase (15LO) is a lipid-oxidizing enzyme that is considered to contribute to the formation of oxidized lipids in atherosclerotic lesions. Monocyte-macrophages are the key cells that express 15LO in atherosclerotic lesions. In this review, we examine the evidence for 15LO involvement in atherogenesis and explore and evaluate the potential mechanisms whereby 15LO may contribute to the oxidation of LDL by monocyte-macrophages. We also describe several possible pro- versus anti-atherogenic functions that may be mediated by various products of 15LO lipid oxidation. Central pathways involved in regulating 15LO expression and activity that may serve as future targets for intervention and regulation of this enzyme are also reviewed.

Infection and inflammation induce LDL oxidation in vivo

Epidemiological studies have shown an increased incidence of coronary artery disease in patients with chronic infections and inflammatory disorders. Because oxidative modification of lipoproteins plays a major role in atherosclerosis, the present study was designed to test the hypothesis that the host response to infection and inflammation induces lipoprotein oxidation in vivo. Lipoprotein oxidation was measured in 3 distinct models of infection and inflammation. Syrian hamsters were injected with bacterial lipopolysaccharide (LPS), zymosan, or turpentine to mimic acute infection, acute systemic inflammation, and acute localized inflammation, respectively. Levels of oxidized fatty acids in serum and lipoprotein fractions were measured by determining levels of conjugated dienes, thiobarbituric acid-reactive substances, and lipid hydroperoxides. Our results demonstrate a significant increase in conjugated dienes and thiobarbituric acid-reactive substances in serum in all 3 models. Moreover, LPS and zymosan produced a 4-fold to 6-fold increase in conjugated diene and lipid hydroperoxide levels in LDL fraction. LPS also produced a 17-fold increase in LDL content of lysophosphatidylcholine that is formed during the oxidative modification of LDL. Finally, LDL isolated from animals treated with LPS was significantly more susceptible to ex vivo oxidation with copper than LDL isolated from saline-treated animals, and a 3-fold decrease occurred in the lag phase of oxidation. These results demonstrate that the host response to infection and inflammation increases oxidized lipids in serum and induces LDL oxidation in vivo. Increased LDL oxidation during infection and inflammation may promote atherogenesis and could be a mechanism for increased incidence of coronary artery disease in patients with chronic infections and inflammatory disorders.

Inhibitory effect of Chinese green tea on endothelial cell-induced LDL oxidation

Green tea has been shown to inhibit Cu(2+)-induced LDL oxidation and suppress lipoxygenase activity. Since LDL oxidation is a characteristic feature of atherogenesis and lipoxygenase is involved in the disease process, the effect of Lung Chen Tea, a non-fermented Chinese green tea, on LDL oxidation induced by human umbilical cord vascular endothelial cell was investigated in the present study. Lung Chen Tea was extracted with methanol and the dried powder was redissolved in water before extraction with chloroform and then ethyl acetate. Lung Chen Tea, chloroform and ethyl acetate fractions dose-dependently reduced LDL oxidation and decreased its relative electrophoretic mobility (P<0.001) when compared to the oxidized LDL. The lipid peroxidation products, thiobarbituric acid reactive substances, and cellular cholesterol were also significantly lowered by 5 and 10 microg/ml Lung Chen Tea (P<0.001) in a dose-dependent manner. The remaining aqueous layer, which was devoid of catechins after chloroform and ethyl acetate extractions, did not prevent LDL oxidation. The results of this study demonstrated that Lung Chen Tea and catechin-rich fractions significantly prevented endothelial cell induced LDL oxidation. The consumption of Lung Chen Tea may therefore lower the risk of coronary heart diseases.

Leukocytes utilize myeloperoxidase-generated nitrating intermediates as physiological catalysts for the generation of biologically active oxidized lipids and sterols in serum

The initiation of lipid peroxidation and the concomitant formation of biologically active oxidized lipids and sterols is believed to play a central role in the pathogenesis of inflammatory and vascular disorders. Here we explore the role of neutrophil- and myeloperoxidase (MPO)-generated nitrating intermediates as a physiological catalyst for the initiation of lipid peroxidation and the formation of biologically active oxidized lipids and sterols. Activation of human neutrophils in media containing physiologically relevant levels of nitrite (NO(2)(-)), a major end product of nitric oxide (nitrogen monoxide, NO) metabolism, generated an oxidant capable of initiating peroxidation of lipids. Formation of hydroxy- and hydroperoxyoctadecadienoic acids [H(P)ODEs], hydroxy- and hydroperoxyeicosatetraenoic acids [H(P)ETEs], F(2)-isoprostanes, and a variety of oxysterols was confirmed using on-line reverse phase HPLC tandem mass spectrometry (LC/MS/MS). Lipid oxidation by neutrophils required cell activation and NO(2)(-), occurred in the presence of metal chelators and superoxide dismutase, and was inhibited by catalase, heme poisons, and free radical scavengers. LC/MS/MS studies demonstrated formation of additional biologically active lipid and sterol oxidation products known to be enriched in vascular lesions, such as 1-hexadecanoyl-2-oxovalaryl-sn-glycero-3-phosphocholine, which induces upregulation of endothelial cell adhesion and chemoattractant proteins, and 5-cholesten-3beta-ol 7beta-hydroperoxide, a potent cytotoxic oxysterol. In contrast to the oxidant formed during free metal ion-catalyzed reactions, the oxidant formed during MPO-catalyzed oxidation of NO(2)(-) readily promoted lipid peroxidation in the presence of serum constituents. Collectively, these results suggest that phagocytes may employ MPO-generated reactive nitrogen intermediates as a physiological pathway for initiating lipid peroxidation and forming biologically active lipid and sterol oxidation products in vivo.

Reduction of transition metals by human (THP-1) monocytes is enhanced by activators of protein kinase C

Macrophages oxidize low density lipoprotein (LDL) by enzymatic and non-enzymatic mechanisms; however, it is evident that macrophage reduction of transition metals can accelerate LDL oxidation in vitro, and possibly in vivo. Distinct cellular pathways contribute to this process, including trans-plasma membrane electron transport (TPMET), and production of free thiols or superoxide. Here, we explore the role of protein kinase C (PKC) in regulating transition metal reduction by each of these redox-active pathways, in human (THP-1) monocytes. We demonstrate that PKC agonists and/or inhibitors modulate reduction of transition metals by monocytes: both thiol-independent (direct) and thiol-dependent (indirect) pathways for transition metal reduction are enhanced by PKC activation, suggesting a potential strategy for therapeutic intervention.

Inhibition of peroxyl radical-mediated lipid oxidation by plasmalogen phospholipids and alpha-tocopherol

The recently discovered peroxyl radical scavenging properties of plasmalogen phospholipids led us to evaluate their potential interactions with alpha-tocopherol. The oxidative decay of plasmalogen phospholipids and of polyunsaturated fatty acids as induced by peroxyl radicals (generated from 2,2'-azobis-2-amidinopropane hydrochloride; AAPH) was studied in micelles using 1H-NMR and chemical analyses. In comparison with alpha-tocopherol, a 20- to 25-fold higher concentration of plasmalogen phospholipids was needed to induce a similar inhibition of peroxyl radical-mediated oxidation of polyunsaturated fatty acids. Plasmalogen phospholipids and alpha-tocopherol protected each other from oxidative degradation. In low-density lipoproteins (LDL) and micelles supplemented with plasmalogen phospholipids plus alpha-tocopherol, the peroxyl radical-promoted oxidation was additively diminished. The differences in the capacities to inhibit oxidation processes induced by peroxyl radicals between the plasmalogen phospholipids and alpha-tocopherol were less pronounced in the LDL particles than in the micelles. In conclusion, plasmalogen phospholipids and alpha-tocopherol apparently compete for the interaction with the peroxyl radicals. Oxidation processes induced by peroxyl radicals are inhibited in an additive manner in the presence of the two radical scavengers. The contribution of the plasmalogen phospholipids to the protection against peroxyl radical promoted oxidation in vivo is expected to be at least as important as that of alpha-tocopherol.

Overexpression of human catalase gene decreases oxidized lipid-induced cytotoxicity in vascular smooth muscle cells

Reactive oxygen metabolites such as hydrogen peroxide (H(2)O(2)) and oxidized fatty acids are proinflammatory and are involved in the pathophysiology of various diseases including atherosclerosis. The effects of these oxidants could be inhibited by the external addition of an antioxidant, suggesting the promotion or propagation of further oxidation. In this study, we describe the stable overexpression of human catalase in smooth muscle cells and the resistance of these cells to cytotoxicity induced not only by the addition of H(2)O(2) but also by the addition of 13-hydroperoxyoctadecadienoic acid (13-HPODE). The results pose an intriguing possibility of the generation of H(2)O(2) from a peroxidized fatty acid. Accordingly, incubation of cells with both 13-HPODE and 13-hydroxyoctadecadienoic acid resulted in the generation of intracellular H(2)O(2). To explain the observed results by which catalase could overcome the effects of 13-HPODE, we propose that oxidized fatty acids are degraded in the cellular peroxisomes, resulting in the generation of H(2)O(2). In other words, the cellular effects of peroxidized fatty acids could be attributed to the generation of H(2)O(2).

15-Lipoxygenase catalytically consumes nitric oxide and impairs activation of guanylate cyclase

Analysis of purified soybean and rabbit reticulocyte 15-lipoxygenase (15-LOX) and PA317 cells transfected with human 15-LOX revealed a rapid rate of linoleate-dependent nitric oxide (.NO) uptake that coincided with reversible inhibition of product ((13S)-hydroperoxyoctadecadienoic acid, or (13S)-HPODE) formation. No reaction of .NO (up to 2 microM) with either native (Ered) or ferric LOXs (0.2 microM) metal centers to form nitrosyl complexes occurred at these .NO concentrations. During HPODE-dependent activation of 15-LOX, there was consumption of 2 mol of .NO/mol of 15-LOX. Stopped flow fluorescence spectroscopy showed that.NO (2.2 microM) did not alter the rate or extent of (13S)-HPODE-induced tryptophan fluorescence quenching associated with 15-LOX activation. Additionally, .NO does not inhibit the anaerobic peroxidase activity of 15-LOX, inferring that the inhibitory actions of .NO are due to reaction with the enzyme-bound lipid peroxyl radical, rather than impairment of (13S)-HPODE-dependent enzyme activation. From this, a mechanism of 15-LOX inhibition by .NO is proposed whereby reaction of .NO with EredLOO. generates Ered and LOONO, which hydrolyzes to (13S)-HPODE and nitrite (NO2-). Reactivation of Ered, considerably slower than dioxygenase activity, is then required to complete the catalytic cycle and leads to a net inhibition of rates of (13S)-HPODE formation. This reaction of .NO with 15-LOX inhibited. NO-dependent activation of soluble guanylate cyclase and consequent cGMP production. Since accelerated .NO production, enhanced 15-LOX gene expression, and 15-LOX product formation occurs in diverse inflammatory conditions, these observations indicate that reactions of .NO with lipoxygenase peroxyl radical intermediates will result in modulation of both .NO bioavailability and rates of production of lipid signaling mediators.

Myeloperoxidase-generated reactive nitrogen species convert LDL into an atherogenic form in vitro

Oxidized LDL is implicated in atherosclerosis; however, the pathways that convert LDL into an atherogenic form in vivo are not established. Production of reactive nitrogen species may be one important pathway, since LDL recovered from human atherosclerotic aorta is enriched in nitrotyrosine. We now report that reactive nitrogen species generated by the MPO-H2O2-NO2- system of monocytes convert LDL into a form (NO2-LDL) that is avidly taken up and degraded by macrophages, leading to massive cholesterol deposition and foam cell formation, essential steps in lesion development. Incubation of LDL with isolated MPO, an H2O2-generating system, and nitrite (NO2-)-- a major end-product of NO metabolism--resulted in nitration of apolipoprotein B 100 tyrosyl residues and initiation of LDL lipid peroxidation. The time course of LDL protein nitration and lipid peroxidation paralleled the acquisition of high-affinity, concentration-dependent, and saturable binding of NO2-LDL to human monocyte-derived macrophages and mouse peritoneal macrophages. LDL modification and conversion into a high-uptake form occurred in the absence of free metal ions, required NO2-, occurred at physiological levels of Cl-, and was inhibited by heme poisons, catalase, and BHT. Macrophage binding of NO2-LDL was specific and mediated by neither the LDL receptor nor the scavenger receptor class A type I. Exposure of macrophages to NO2-LDL promoted cholesteryl ester synthesis, intracellular cholesterol and cholesteryl ester accumulation, and foam cell formation. Collectively, these results identify MPO-generated reactive nitrogen species as a physiologically plausible pathway for converting LDL into an atherogenic form.

Disruption of the 12/15-lipoxygenase gene diminishes atherosclerosis in apo E-deficient mice

Atherosclerosis may be viewed as an inflammatory disease process that includes early oxidative modification of LDLs, leading to foam cell formation. This "oxidation hypothesis" has gained general acceptance in recent years, and evidence for the role of lipoxygenases in initiation of, or participation in, the oxidative process is accumulating. However, the relative contribution of macrophage-expressed lipoxygenases to atherogenesis in vivo remains unknown. Here, we provide in vivo evidence for the role of 12/15-lipoxygenase in atherogenesis and demonstrate diminished plasma IgG autoantibodies to oxidized LDL epitopes in 12/15-lipoxygenase knockout mice crossbred with atherosclerosis-prone apo E-deficient mice (apo E-/-/L-12LO-/-). In chow-fed 15-week-old apo E-/-/L-12LO-/- mice, the extent of lesions in whole-aorta en face preparations (198 +/- 60 microm2) was strongly reduced (P < 0.001, n = 12) when compared with 12/15-lipoxygenase-expressing controls (apo E-/-/L-12LO+/+), which showed areas of lipid deposition (15,700 +/- 2,688 microm2) in the lesser curvature of the aortic arch, branch points, and in the abdominal aorta. These results were observed despite cholesterol, triglyceride, and lipoprotein levels that were similar to those in apo E-deficient mice. Evidence for reduced lesion development was observed even at 1 year of age in apo E-/-/L-12LO-/- mice. The combined data indicate a role for 12/15-lipoxygenase in the pathogenesis of atherosclerosis and suggest that inhibition of this enzyme may decrease disease progression.

Secondary radicals derived from chloramines of apolipoprotein B-100 contribute to HOCl-induced lipid peroxidation of low-density lipoproteins

Oxidation of low-density lipoproteins (LDL) is thought to contribute to atherogenesis. Although there is increasing evidence for a role of myeloperoxidase-derived oxidants such as hypochlorite (HOCl), the mechanism by which HOCl modifies LDL remains controversial. Some studies report the protein component to be the major site of attack, whereas others describe extensive lipid peroxidation. The present study addresses this controversy. The results obtained are consistent with the hypothesis that radical-induced oxidation of LDL's lipids by HOCl is a secondary reaction, with most HOCl consumed via rapid, non-radical reaction with apolipoprotein B-100. Subsequent incubation of HOCl-treated LDL gives rise to lipid peroxidation and antioxidant consumption in a time-dependent manner. Similarly, with myeloperoxidase/H2O2/Cl- (the source of HOCl in vivo), protein oxidation is rapid and followed by an extended period of lipid peroxidation during which further protein oxidation does not occur. The secondary lipid peroxidation process involves EPR-detectable radicals, is attenuated by a radical trap or treatment of HOCl-oxidized LDL with methionine, and occurs less rapidly when the lipoprotein was depleted of alpha-tocopherol. The initial reaction of low concentrations of HOCl (400-fold or 800-fold molar excess) with LDL therefore seems to occur primarily by two-electron reactions with side-chain sites on apolipoprotein B-100. Some of the initial reaction products, identified as lysine-residue-derived chloramines, subsequently undergo homolytic (one-electron) reactions to give radicals that initiate antioxidant consumption and lipid oxidation via tocopherol-mediated peroxidation. The identification of these chloramines, and the radicals derived from them, as initiating agents in LDL lipid peroxidation offers potential new targets for antioxidative therapy in atherogenesis.

Uptake of 13-hydroperoxylinoleic acid by cultured cells

Oxidized free fatty acids have profound effects on cultured cells. However, little is known about whether these effects depend on their uptake and metabolism by cells or primarily involve their interaction with cell-surface components. We determined the uptake and metabolism of unoxidized (linoleic or oleic acid) and oxidized linoleic acid (13-hydroperoxyoctadecadienoic acid, 13-HPODE) by endothelial cells, smooth muscle cells, and macrophages. We show that 13-HPODE is poorly taken up by cells. The levels of uptake were dependent on the cell type but were independent of the expression of CD36. 13-HPODE was also poorly used by microsomal lysophosphatidylcholine acyltransferase that is involved in the formation of phosphatidylcholine. Based on these results, we suggest that most of the biological effects of 13-HPODE and other oxidized free fatty acids on cells might involve a direct interaction with cell-surface components. Alternatively, very small amounts of oxidized free fatty acids that enter the cell may have effects, analogous to those of hormones or prostanoids.

Regulation of 15-lipoxygenase expression and mucus secretion by IL-4 in human bronchial epithelial cells

Our laboratory has recently shown that mucus differentiation of cultured normal human tracheobronchial epithelial (NHTBE) cells is accompanied by the increased expression of 15-lipoxygenase (15-LO). We used differentiated NHTBE cells to investigate the regulation of 15-LO expression and mucus secretion by inflammatory cytokines. Interleukin (IL)-4 and IL-13 dramatically enhanced the expression of 15-LO, whereas tumor necrosis factor-alpha, IL-1beta, and interferon (IFN)-gamma had no effect. These cytokines did not increase the expression of cyclooxygenase-2, with the exception of a modest induction by IL-1beta. The IL-4-induced 15-LO expression was concentration dependent, and mRNA and protein expression increased within 3 and 6 h, respectively, after IL-4 treatment. In metabolism studies with intact cells, 15-hydroxyeicosatetraenoic acid (15-HETE) and 13-hydroxyoctadecadienoic acid (13-HODE) were the major metabolites formed from exogenous arachidonic acid and linoleic acid. No prostaglandins were detected. IL-4 treatment dramatically increased the formation of 13-HODE and 15-HETE compared with that in untreated NHTBE cells, and several additional 15-LO metabolites were observed. Pretreatment of NHTBE cells with IFN-gamma or dexamethasone did not inhibit the IL-4-induced expression of 15-LO except at high concentrations (100 ng/ml of IFN-gamma and 10 microM dexamethasone). IL-4 treatment inhibited mucus secretion and attenuated the expression of the mucin genes MUC5AC and MUC5B at 12-24 h after treatment. Addition of 15-HETE precursor and 13-HODE precursor to the cultures did not alter mucin secretion or mucin gene expression. On the basis of the data presented, we conclude that the increase in 15-LO expression by IL-4 and attenuation of mucus secretion may be independent biological events.

The role of oxidized low-density lipoprotein in the activation of peroxisome proliferator-activated receptor gamma: implications for atherosclerosis

A molecular mechanism has been discovered that helps explain the formation of foam cells, which are components of atherosclerotic plaque. It involves the stimulation by oxidized low-density lipoprotein of its own uptake into macrophages through activation of the peroxisome proliferator-activated receptor gamma.

Lipid hydroperoxides inhibit nitric oxide production in RAW264.7 macrophages

The effects of oxidatively modified low density lipoprotein (oxLDL) on atherogenesis may be partly mediated by alterations in the production of nitric oxide (NO) by vascular cells. Lipid hydroperoxides (LOOH) and lysophosphatidylcholine (lysoPC) are the major primary products of LDL oxidation. The purpose of this study was to characterize the effects of oxLDL, LOOH and lysoPC on NO production and the expression of inducible nitric oxide synthase (iNOS) gene in lipopolysaccharide (LPS) stimulated macrophages. LDL was oxidized using an azo-initiator 2,2'-azobis (2-amidinopropane) HCl (ABAP) and octadecadienoic acid was oxidized by lipoxygenase to generate 13-hydroperoxyl octadecadienoic acid (13-HPODE). Our study showed that oxLDL markedly decreased the production of NO, the levels of iNOS protein and iNOS mRNA in LPS stimulated macrophages. The inhibition potential of oxLDL on NO production and iNOS gene expression depended on the levels of LOOH formed in oxLDL and was not due to oxLDL cytotoxicity. Furthermore, 13-HPODE markedly reduced NO production and iNOS protein levels, whereas lysoPC showed only slight reduction. The effects of 13-HPODE and lysoPC did not require an acetylated LDL carrier. Our results suggest that 13-HPODE is a much more potent inhibitor of NO production and iNOS gene expression than lysoPC in LPS stimulated RAW264.7 macrophages.

Oxidation of low density lipoprotein and plasma by 15-lipoxygenase and free radicals

It is generally accepted that the oxidation of pentadiene structures of polyunsaturated lipids by lipoxygenase (LOX) is regio- and enantio-specific, while the free radical-mediated lipid peroxidation gives stereo-random racemic products. It was confirmed that the oxidation of human low density lipoprotein (LDL) by 15-LOX from rabbit reticulocytes gave phosphatidylcholine (PC) and cholesteryl ester (CE) hydroperoxides regio-, stereo- and enantio-specifically. 15-LOX also oxidized human plasma to give specific PC and CE hydroperoxides in spite of the presence of high concentrations of antioxidants. More CE hydroperoxides were formed than PC hydroperoxides from LDL, but the reverse order was observed for plasma oxidation. The S/R ratio of the hydroperoxides decreased during long time incubation but remained significantly larger than one, while free radical-mediated oxidation of LDL and plasma gave racemic products.

The relationship of hydroxyeicosatetraenoic acids and F2-isoprostanes to plaque instability in human carotid atherosclerosis

Evidence for increased oxidant stress has been reported in human atherosclerosis. However, no information is available about the importance of in situ oxidant stress in relation to plaque stability. This information is relevant because the morbidity and mortality of atherosclerosis are essentially the consequences of acute ischemic syndromes due to unstable plaques. We studied 30 carotid atherosclerotic plaques retrieved by endarterectomy from 18 asymptomatic (stable plaques) and 12 symptomatic patients (unstable plaques). Four normal arteries served as controls. After lipid extraction and ester hydrolysis, quantitation of different indices of oxidant stress were analyzed, including hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatetraenoic acids (EETs), ketoeicosatetraenoic acids (oxo-ETEs), and F2-isoprostanes using online reverse-phase high-performance liquid chromatography tandem mass spectrometry (LC/MS/MS). All measurements were carried out in a strictly double-blind procedure. We found elevated levels of the different compounds in atherosclerotic plaques. Levels of HETEs were 24 times higher than EETs, oxo-ETEs, or F2-isoprostanes. Levels of HETEs, but not those of EETs, oxo-ETEs or F2-isoprostanes, were significantly elevated in plaques retrieved from symptomatic patients compared with those retrieved from asymptomatic patients (1, 738 +/- 274 vs. 1,002 +/- 107 pmol/ micromol lipid phosphorous, respectively; P < 0.01). One monooxygenated arachidonate species, 9-HETE, which cannot be derived from known enzymatic reactions, was the most abundant and significant compound observed in plaques, suggesting that nonenzymatic lipid peroxidation predominates in advanced atherosclerosis and may promote plaque instability.

A sensitive chemiluminescence method to measure the lipoxygenase catalyzed oxygenation of complex substrates

Oxidative modification of low-density lipoprotein (LDL) has been implicated as a patho-physiological process in early atherogenesis and 15-lipoxygenases (15-LOX) may be involved. While studying the in vitro kinetics of the 15-LOX/LDL interaction, we found that the conventional spectrophotometric assays failed in the range of substrate saturation owing to the high optical density of concentrated LDL solutions. Therefore, we developed a much more sensitive assay system which was based on peroxide induced isoluminol enhanced chemiluminescence. With this method reliable kinetic data were obtained at LDL concentrations of up to 1 mg/ml. To validate this luminometric method the kinetic parameters of 15-LOX catalyzed oxygenation of linoleic acid (Km=3.7 microM, kcat=17 s-1) were determined and we observed a good agreement with previously published data obtained with a spectrophotometric assay. Moreover, we found that the kinetic constants of 15-LOX catalyzed LDL oxidation (Km=0.64 microM, kcat=0.15 s-1) are quite different from those of free fatty acid oxygenation and that the cholesterol esters are preferentially oxidized during 15-LOX/LDL interaction. Vitamin E depletion does not reduce the rate of LDL oxidation and analysis of the structure of the oxygenation products suggests that the majority of the products were formed via direct LOX catalyzed oxidation of LDL ester lipids. The luminometric method described here is not restricted to the measurement of LOX catalyzed LDL oxidation, but may also be used to determine kinetic constants for the oxidation of other complex substrates such as biomembranes or liposomes.