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

Selenium inhibits 15-hydroperoxyoctadecadienoic acid-induced intracellular adhesion molecule expression in aortic endothelial cells

Increased intracellular adhesion molecule 1 (ICAM-1) expression and enhanced monocyte recruitment to the endothelium are critical steps in the early development of atherosclerosis. The 15-lipoxygenase 1 (15-LOX1) pathway can generate several proinflammatory eicosanoids that are known to enhance ICAM-1 expression within the vascular endothelium. Oxidative stress can exacerbate endothelial cell inflammatory responses by modifying arachidonic acid metabolism through the 15-LOX1 pathway. Because selenium (Se) influences the oxidant status of cells and can modify the expression of eicosanoids, we investigated the role of this micronutrient in modifying ICAM-1 expression as a consequence of enhanced 15-LOX1 activity. Se supplementation reduced ICAM-1 expression in bovine aortic endothelial cells, an effect that was reversed with 15-LOX1 overexpression or treatment with exogenous 15-hydroperoxyoctadecadienoic acid (15-HPETE). ICAM-1 expression increased proportionately when intracellular15-HPETE levels were allowed to accumulate. However, changes in intracellular 15-HETE levels did not seem to affect ICAM-1 expression regardless of Se status. Our results indicate that Se supplementation can reduce 15-HPETE-induced expression of ICAM-1 by controlling the intracellular accumulation of this fatty acid hydroperoxide in endothelial cells.

15-Lipoxygenase-2 expression in human macrophages induces chemokine secretion and T cell migration

BACKGROUND

We determined previously that hypoxia results in increased 15-lipoxygenase type 2 (15-LOX-2) expression and CXCL8 secretion in macrophages. This study sought to determine whether 15-LOX-2 expression links directly with the secretion of inflammatory molecules in macrophages and also investigated its subsequent effects on T cell migration.

METHODS

Adenovirus-mediated gene delivery caused overexpression of 15-LOX-2 in human macrophages. We used cytometric bead array to measure chemokine secretion, and assessed T cell migration by counting cells in chemotaxis chambers. Expression of chemokine receptors was determined by FACS analysis. Using siRNA, we reduced 15-LOX-2 expression in human macrophages. We used scrambled siRNA as control.

RESULTS

Macrophages that overexpress 15-LOX-2 showed increased secretion of chemokine CXCL10 after 24h incubation. In addition, preconditioned medium from 15-LOX-2-overexpressing cells increased T cell migration and surface expression of CXCR3, the CXCL10 receptor. Knockdown of 15-LOX-2 expression decreased CXCL10 secretion from hypoxic macrophages and also reduced T cell migration.

CONCLUSION

In macrophages, overexpression of 15-LOX-2 results in increased secretion of CXCL10 and CCL2. Products released in response to increased 15-LOX-2 activation lead to increased expression of CD69, the T cell activation marker as well as increased T cell migration. Therefore, increased expression of 15-LOX-2 induced by hypoxia may participate in T cell recruitment in diseases such as atherosclerosis.

The two faces of the 15-lipoxygenase in atherosclerosis

Chronic inflammation plays a major role in atherogenesis and understanding the role of inflammation and its resolution will offer novel approaches to interfere with atherogenesis. The 15(S)-lipoxygenase (15-LOX) plays a janus-role in inflammation with pro-inflammatory and anti-inflammatory effects in cell cultures and primary cells and even opposite effects on atherosclerosis in two different animal species. There is evidence for a pro-atherosclerotic effect of 15-LOX including the direct contribution to LDL oxidation and to the recruitment of monocytes to the vessel wall, its role in angiotensin II mediated mechanisms and in vascular smooth muscle cell proliferation. In contrast to the pro-atherosclerotic effects of 15-LOX, there is also a broad line of evidence that 15-LOX metabolites of arachidonic and linoleic acid have anti-inflammatory effects. The 15-LOX arachidonic acid metabolite 15-HETE inhibits superoxide production and polymorphonuclear neutrophil (PMN) migration across cytokine-activated endothelium and can be further metabolized to the anti-inflammatory lipoxins. These promote vasorelaxation in the aorta and counteract the action of most other pro-inflammatory factors like leukotrienes and prostanoids. Anti-atherogenic properties are also reported for the linoleic acid oxidation product 13-HODE through inhibition of adhesion of several blood cells to the endothelium. Furthermore, there is evidence that 15-LOX is involved in the metabolism of the long-chain omega-3 fatty acid docosahexaenoic acid (DHA) leading to a family of anti-inflammatory resolvins and protectins. From these cell culture and animal studies the role of the 15-LOX in human atherosclerosis cannot be predicted. However, recent genetic studies characterized the 15-LOX haplotypes in Caucasians and discovered a functional polymorphism in the human 15-LOX promoter. This will now allow large studies to investigate an association of 15-LOX with coronary artery disease and to answer the question whether 15-LOX is pro- or anti-atherogenic in humans.

Effect of conjugated linoleic acids on the activity and mRNA expression of 5- and 15-lipoxygenases in human macrophages

Lipoxygenases are a family of non-heme enzyme dioxygenases. The role of lipoxygenases is synthesis of hydroperoxides of fatty acids, which perform signaling functions in the body. Studies on conjugated linoleic acids (CLAs) as fatty acids with a potential anti-atherosclerotic function have recently been initiated. The aim of the study was to test the effect of CLAs and linoleic acid on 5- and 15-lipoxygenase (5-LO, 15-LO-1) enzyme activity, their mRNA expression, and concentration in the cells. It was also desired to determine whether the CLAs are substrates for the enzymes. For the experiments monocytic cell line (THP-1) and monocytes obtained from human venous blood were used. Monocytes were differentiated to macrophages: THP-1 (CD14+) by PMA administration (100 nM for 24 h) and monocytes from blood (CD14+) by 7-day cultivation with the autologous serum (10%). After differentiation, macrophages were cultured with 30 microM CLAs or linoleic acid for 48 h. The 15- and 5-lipoxygenase products were measured by HPLC method. mRNA expression and protein content were analyzed by real-time PCR and Western blot analysis. The in vitro studies proved that both CLA isomers are not substrates for 15-LO-1; in ex vivo studies hydroxydecadienoic acid (HODE) concentration was significantly reduced (p = 0.019). The trans-10,cis-12 CLA isomer reduced HODE concentration by 28% (p = 0.046) and the cis-9,trans-11 CLA isomer by 35% (p = 0.028). In macrophages obtained from THP-1 fatty acids did not change significantly mRNA expression of the majority of the investigated genes. CLAs did not change the content of 5-LO and 15-LO-1 proteins in macrophages obtained from peripheral blood. Linoleic acid induced 15-LO-1 expression (2.6 times, p < 0.05). CLAs may perform the function of an inhibitor of lipoxygenase 15-LO-1 activity in macrophages.

Levels of lipid peroxidation in human plasma and erythrocytes: comparison between fatty acids and cholesterol

Lipid peroxidation has gained renewed attention with increasing evidence showing its biological role in producing toxic compounds and cellular signaling mediators. The assessment of lipid peroxidation levels in vivo is difficult partly because lipids are oxidized by different oxidants by different mechanisms to give versatile types of products, which may undergo metabolism and secondary reactions. In the present study, total hydroxyoctadecadienoic acids (tHODE) and 7alpha- and 7beta-hydroxycholesterol (t7-OHCh) from 44 healthy human subjects were assessed as biomarkers after reduction with sodium borohydride followed by saponification with potassium hydroxide comparing with the prevailing standard 8-isoprostaglandin F(2alpha) (t8-iso-PGF(2alpha)). The average concentrations of tHODE, total 8-isoprostaglandin F(2alpha) (t8-iso-PGF(2alpha)), t7alpha-OHCh, and t7beta-OHCh were 203, 0.727, 87.1, and 156 nmol/l plasma and 1,917, 12.8, 1,372, and 3,854 nmol/l packed erythrocytes, respectively. The ratios of tHODE and t7-OHCh to the parent substrates were 194 and 3,519 micromol tHODE/mol linoleates and 40.9 and 686 micromol t7-OHCh/mol cholesterol in plasma and erythrocytes, respectively. It was found that (1) t7-OHCh in blood was unexpectedly high, as high as or even higher than tHODE, (2) the amounts of tHODE was more than 100 fold higher than t8-iso-PGF(2alpha) (3) the level of lipid oxidation products in erythrocytes was higher than that in plasma, and (4) lipid peroxidation products level tended to increase while antioxidant level decrease with age. These products may be used as potential biomarker for assessment of lipid peroxidation and oxidative stress in vivo.

Mechanisms by which diabetes increases cardiovascular disease

Diabetes mellitus is one of the major risk factors for cardiovascular disease which is the leading cause of death in the U.S. Increasing prevalence of diabetes and diabetic atherosclerosis makes identification of molecular mechanisms by which diabetes promotes atherogenesis an important task. Targeting common pathways may ameliorate both diseases. This review focuses on well known as well as newly discovered mechanisms which may represent promising therapeutic targets.

Functional polymorphism in ALOX15 results in increased allele-specific transcription in macrophages through binding of the transcription factor SPI1

The reticulocyte-type 15-lipoxygenase-1 (ALOX15) has antiinflammatory and inflammatory effects, and is implicated in the development of asthma, arthritis, and atherosclerosis. We screened the human ALOX15 gene for variations because genetic variability in ALOX15 may influence these diseases. We detected 11 variations, including five polymorphisms located in the ALOX15 promoter region. One of these polymorphisms, a C-to-T substitution at position c.-292, created a novel transcription factor binding site for SPI1. Transcription assays revealed that promoter variants with c.-292 T transcribe twice as efficiently as all the other promoter variants containing c.-292C. This was true in macrophages that constitutively express SPI1, but not in a lung epithelial cell line that does not express SPI1. Mutation of the core-binding site for SPI1 abolished the higher transcriptional activity, and electrophoretic mobility shift assays showed that SPI1 selectively binds to the mutant c.-292 T [corrected] promoter. These results were corroborated in primary human macrophages, in which macrophages from heterozygous c.-292CT carriers expressed three times more ALOX15 mRNA than macrophages from homozygous c.-292CC carriers. We conclude that the c.-292 T allele in the ALOX15 promoter generates a novel binding site for the transcription factor SPI1 that results in higher transcription of the gene in macrophages. This may lead to an increase in ALOX15-mediated lipid metabolites, which play a role in inflammation.

Monocyte 15-Lipoxygenase Expression Is Regulated by a Novel Cytosolic Signaling Complex with Protein Kinase C δ and Tyrosine-Phosphorylated Stat3

Our previous studies demonstrated that the IL-13-induced 15-lipoxygenase expression in primary human monocytes is regulated by the activation of both Stat1 and Stat3 and by protein kinase C (PKC)delta. IL-13 stimulated the phosphorylation of Stat3 on both Tyr705 and Ser727. In this study we show that IL-13 induces the association of PKCdelta with Stat3, not with Stat1, and is required for Stat3 Ser727 phosphorylation. We found a novel IL-13-dependent cytosolic signaling complex of PKCdelta and tyrosine-phosphorylated Stat3. A tyrosine kinase inhibitor blocked PKCdelta association with Stat3 as well as Stat3 Ser727 phosphorylation. We therefore hypothesized that tyrosine phosphorylation was required for Stat3 interaction with PKCdelta and subsequent PKCdelta-dependent phosphorylation of Stat3 Ser727. We developed an efficient transfection protocol for human monocytes. Expression of Stat3 containing a mutation in Tyr705 inhibited the association of PKCdelta with Stat3 and blocked Stat3 Ser727 phosphorylation, whereas transfection with wild-type Stat3 did not. Furthermore, by transfecting monocytes with Stat3 containing mutations in Tyr705 or Ser727 or with wild-type Stat3, we demonstrated that both Stat3 tyrosine and serine phosphorylations are required for optimal binding of Stat3 with DNA and maximal expression of 15-lipoxygenase, an important regulator of inflammation and apoptosis.

LDL oxidized with iron in the presence of homocysteine/cystine at acidic pH has low cytotoxicity despite high lipid peroxidation

Fe(III) can have a strong oxidizing effect in the presence of reductants at acidic pH, which may occur under anaerobic conditions or in regions of inflammation. Low density lipoprotein (LDL) oxidation with Fe(III) and homocysteine/cystine at acidic pH provoked mainly formation of lipid hydroperoxides and thiobarbituric acid reactive substances (TBARS) in the absence of significant protein modification. Even when oxidized to a high TBARS content, LDL oxidized at acidic pH was not cytotoxic when added to THP-1 monocytes in a concentration causing cell death when LDL was oxidized to a similar TBARS content at plasma pH with Fe(III) or Cu(II) in the presence or absence of homocysteine/cystine. Inducible nitric oxide production by RAW264.7 mouse macrophages was only weakly inhibited by LDL oxidized at acidic pH, even if acetylated before oxidation to increase uptake, as compared to LDL oxidized with Cu(II) at plasma pH to a similar TBARS content or anodic electrophoretic mobility. LDL oxidized at acidic pH may mainly induce protective mechanisms against oxidative stress while causing little acute damage of cells.

Biologic relevance of lipoxygenase isoforms in atherogenesis

Lipoxygenases (LOXs) form a heterogeneous family of lipid-peroxidizing enzymes, which have originally been implicated in cell differentiation and biosynthesis of inflammatory mediators. More recent studies suggested a role of various LOX-isoforms in the pathogenesis of human diseases, including bronchial asthma, osteoporosis and atherosclerosis. According to their phylogenetic relatedness, LOX-isoforms may be classified into four subfamilies, three of which (12/15-LOX, 5-LOX, platelet 12-LOX) have been related to atherogenesis. Several lines of experimental evidence suggest a role for LOXs in atherosclerosis, but the mechanisms remain a matter of discussion. This review will briefly summarize the current understanding on the molecular enzymology of the LOX family and the current status of knowledge on the role of different LOX isoforms in atherogenesis. The available literature data will be critically reviewed and a short perspective on future developments in the field will be provided.

Inflammation and immune regulation by 12/15-lipoxygenases

12/15-Lipoxygenases (12/15-LOX) are members of the LOX family, which are expressed in mammals by monocytes and macrophages following induction by the T helper type 2 cytokines, interleukins-4 and -13. They oxygenate free polyenoic fatty acids but also ester lipids and even complex lipid-protein assemblies such as biomembranes and lipoproteins. The primary oxidation products are either reduced by glutathione peroxidases to corresponding hydroxy derivatives or metabolized into secondary oxidized lipids including leukotrienes, lipoxins and hepoxilins, which act as lipid mediators. Examination of knockout and transgenic animals revealed important roles for 12/15-LOX in inflammatory diseases, including atherosclerosis, cancer, osteoporosis, angiotension II-dependent hypertension and diabetes. In vitro studies suggested 12/15-LOX products as coactivators of peroxisomal proliferator activating-receptors (PPAR), regulators of cytokine generation, and modulators of gene expression related to inflammation resolution. Despite much work in this area, the biochemical mechanisms by which 12/15-LOX regulates physiological and pathological immune cell function are not fully understood. This review will summarize the biochemistry and tissue expression of 12/15-LOX and will describe the current knowledge regarding its immunobiology and regulation of inflammation.

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

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

The role of lipoxygenase-isoforms in atherogenesis

Lipoxygenases (LOXs) form a heterogeneous family of lipid-peroxidizing enzymes, and several LOX-isoforms (12/15-LOX, 5-LOX) have been implicated in atherogenesis. However, the precise role of these enzymes is still a matter of discussion. 12/15-LOXs are capable of oxidizing lipoproteins (low-density lipoprotein (LDL), high-density lipoprotein (HDL)) to atherogenic forms, and functional inactivation of this enzyme in murine atherosclerosis models slows down lesion formation. In contrast, rabbits that overexpress this enzyme were protected from lesion formation when fed a lipid-rich diet. To contribute to this discussion, we recently investigated the impact of 12/15-LOX overexpression on in vitro foam cell formation. When 12/15-LOX-transfected J774 cells were incubated in culture with modified LDL, we found that intracellular lipid deposition was reduced in the transfected cells when compared with the corresponding control transfectants. This paper briefly summarizes the current status of knowledge on the biological activity of different LOX-isoforms in atherogenesis and will also provide novel experimental data characterizing the role of 12/15-LOX in cellular LDL modification and for in vitro foam cell formation.

Antioxidant properties of two novel 2-biphenylmorpholine compounds (EP2306 and EP2302) in vitro and in vivo

The oxidation of low-density lipoprotein (LDL) is an important event in the development of atherosclerosis. In the present study, the antioxidant properties of two novel 2-biphenylmorpholine compounds (EP2306 and EP2302) were studied. Both compounds inhibited dose-dependently the in vitro oxidation of LDL induced by copper ions. EP2306 and EP2302 increased significantly the lag phase of the oxidation reaction at 0.1 and 10 microM, respectively, whereas they reduced the rate of the reaction at 1 and 10 microM, respectively. This inhibitory effect was not due to a free radical scavenging or copper-chelating activity of EP2300 compounds. Moreover, EP2306 and EP2302 inhibited 12-lipoxygenase activity dose-dependently with IC50 values of 454 and 318 microM, respectively, but had no effect on 15-lipoxygenase activity. In hyperlipidaemic rabbits treated with EP2306 for 4 weeks, there was a decrease in thiobarbituric acid-reactive substance (TBARS) levels and a significant increase in total peroxyl radical-trapping potential (TRAP) levels as compared to control animals. The present data suggest that EP2300 compounds are effective inhibitors of copper-mediated LDL oxidation in vitro. Moreover, EP2306 acts as an antioxidant in hyperlipidaemic rabbits, a property which could be beneficial in reducing atherosclerosis.

Fatty acids liberated from low-density lipoprotein trigger endothelial apoptosis via mitogen-activated protein kinases

Enzymatic modification of low-density lipoprotein (LDL) as it probably occurs in the arterial intima drastically increases its cytotoxicity, which could be relevant for the progression of atherosclerotic lesions. LDL was treated with a protease and cholesterylesterase to generate a derivative similar to lesional LDL, with a high content of free cholesterol and fatty acids. Exposure of endothelial cells to the enzymatically modified lipoprotein (E-LDL), but not to native or oxidized LDL, resulted in programmed cell death. Apoptosis was triggered by apoptosis signal-regulating kinase 1 dependent phosphorylation of p38. Depletion and reconstitution experiments identified free fatty acids (FFA) as the triggers of this pathway. Levels of FFA in native LDL are low and the lipoprotein is therefore not cytotoxic; enzymatic cleavage of cholesterylesters liberates FFA that can rapidly trigger an apoptosis signaling cascade in neighboring cells. Blockade of this pathway can rescue cells from death.

Essential roles of lipoxygenases in LDL oxidation and development of atherosclerosis

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

Hypoxia Increases LDL Oxidation and Expression of 15-Lipoxygenase-2 in Human Macrophages

Objective— Macrophage-mediated oxidation of low-density lipoprotein (LDL) by enzymes, such as the lipoxygenases, is considered of major importance for the formation of oxidized LDL during atherogenesis. Macrophages have been identified in hypoxic areas in atherosclerotic plaques.

Methods and Results— To investigate the role of hypoxia in macrophage-mediated LDL oxidation, we incubated human monocyte-derived macrophages with LDL under normoxic (21% O 2 ) or hypoxic (0% O 2 ) conditions. The results showed that hypoxic macrophages oxidized LDL to a significantly higher extent than normoxic cells. Interestingly, the mRNA and protein expression of 15-lipoxygenase-2 (15-LOX-2) as well as the activity of this enzyme are elevated in macrophages incubated at hypoxia. Both the unspliced 15-LOX-2 and the spliced variant 15-LOX-2sv-a are found in macrophages. In addition, 15-LOX-2 was identified in carotid plaques in some macrophage-rich areas but was only expressed at low levels in nondiseased arteries.

Conclusions— In summary, these observations show for the first time that 15-LOX-2 is expressed in hypoxic macrophages and in atherosclerotic plaques and suggest that 15-LOX-2 may be one of the factors involved in macrophage-mediated LDL oxidation at hypoxia.

Critical role of macrophage 12/15-lipoxygenase for atherosclerosis in apolipoprotein E-deficient mice

BACKGROUND

Mice lacking leukocyte type 12/15-lipoxygenase (12/15-LO) show reduced atherosclerosis in several models. 12/15-LO is expressed in a variety of cells, including vascular cells, adipocytes, macrophages, and cardiomyocytes. The purpose of this study was to determine which cellular source of 12/15-LO is important for atherosclerosis.

METHODS AND RESULTS

Bone marrow from 12/15-LO-/-/apoE-/- mice was transplanted into apoE-/- mice and vice versa. Deficiency of 12/15-LO in bone marrow cells protected apoE-/- mice fed a Western diet from atherosclerosis to the same extent as complete absence of 12/15-LO, although plasma 8,12-iso-iPF2alpha-IV, a measure of lipid peroxidation, remained elevated. 12/15-LO-/-/apoE-/- mice regained the severity of atherosclerotic lesion typical of apoE-/- mice after replacement of their bone marrow cells with bone marrow from apoE-/- mice. Peritoneal macrophages obtained from wild-type but not 12/15-LO-/- mice caused endothelial activation in the presence of native LDL. Absence of 12/15-LO decreased the ability of macrophages to form foam cells when exposed to LDL.

CONCLUSIONS

We conclude that macrophage 12/15-LO plays a dominant role in the development of atherosclerosis by promoting endothelial inflammation and foam cell formation.

Lipoxygenase Pathways in Atherogenesis

15-lipoxygenase-1 (also known as 12/15-LO in mice) and 5-LO/5-LO-activating protein (FLAP) cascades play central roles in low-density lipoprotein oxidation and leukotriene biosynthesis, respectively. Several genetic and expression studies unraveling an association of the 5-LO/FLAP pathway to human cardiovascular disease have surfaced recently. Experimental studies in 12/15-LO knockout, 15-LO-1 transgenic, and 5-LO knockout mice on atherosclerotic backgrounds combined with gene expression data in human coronary artery disease have created compelling links that these pathways participate in the etiologic progression. However, a few conflicting studies and several unexplained mechanistic issues need to be resolved prior to assigning firm roles for LOs in cardiovascular disease. Development of novel pharmacologic tools to dissect the individual enzymes and receptors in the LO pathways should improve understanding of the individual components in the inflammatory aspects of atherosclerosis disease progression.

Urinary 8-iso-prostaglandin F2alpha as a risk marker in patients with coronary heart disease: a matched case-control study

BACKGROUND

Oxidative stress is involved in the pathophysiology of atherosclerosis, diabetes mellitus, hypertension, obesity, and cigarette smoking, all of these being risk factors for coronary heart disease (CHD). We tested the hypothesis that risk factors of CHD are associated with abundant systemic oxidative stress.

METHODS AND RESULTS

We conducted a case-control study with 93 CHD patients and 93 control subjects frequency-matched by age and sex. Urinary excretion of the F2-isoprostane 8-iso-prostaglandin (PG) F2alpha and its major urinary metabolite, 2,3-dinor-5,6-dihydro-8-iso-PGF2alpha, were measured by gas chromatography-tandem mass spectrometry. Body mass index, systolic blood pressure, and C-reactive protein were elevated in CHD patients (P<0.01). Urinary 8-iso-PGF2alpha and 2,3-dinor-5,6-dihydro-8-iso-PGF2alpha also differed, from 77 (interquartile range, 61-101) to 139 (93-231) pmol/mmol creatinine and from 120 (91-151) to 193 (140-275) pmol/mmol in control subjects and case subjects, respectively (P<0.001). 8-iso-PGF2alpha and its metabolite were highly correlated (Spearman's rho=0.664, P<0.001). HDL cholesterol was diminished in CHD patients (P<0.001). All of these characteristics predicted CHD in univariate analysis. In a multivariate model, the odds ratios were increased only for 8-iso-PGF2alpha (> or =131 pmol/mmol, P<0.001) and C-reactive protein (>3 mg/L, P<0.01), ie, by 30.8 (95% CI, 7.7-124) and 7.2 (1.9-27.6), respectively. 8-iso-PGF2alpha was found to be a novel marker in addition to known risk factors of CHD, ie, diabetes mellitus, hypercholesterolemia, hypertension, and smoking. Urinary excretion of 8-iso-PGF2alpha correlated with the number of risk factors for all subjects (P<0.001 for trend).

CONCLUSIONS

8-iso-PGF2alpha is a sensitive and independent risk marker of CHD.

Dihydrolipoic acid inhibits 15-lipoxygenase-dependent lipid peroxidation

The potential antioxidant effects of the hydrophobic therapeutic agent lipoic acid (LA) and of its reduced form dihydrolipoic acid (DHLA) on the peroxidation of either linoleic acid or human non-HDL fraction catalyzed by soybean 15-lipoxygenase (SLO) and rabbit reticulocyte 15-lipoxygenase (RR15-LOX) were investigated. DHLA, but not LA, did inhibit SLO-dependent lipid peroxidation, showing an IC(50) of 15 microM with linoleic acid and 5 microM with the non-HDL fraction. In specific experiments performed with linoleic acid, inhibition of SLO activity by DHLA was irreversible and of a complete, noncompetitive type. In comparison with DHLA, the well-known lipoxygenase inhibitor nordihydroguaiaretic acid and the nonspecific iron reductant sodium dithionite inhibited SLO-dependent linoleic acid peroxidation with an IC(50) of 4 and 100 microM, respectively, while the hydrophilic thiol N-acetylcysteine, albeit possessing iron-reducing and radical-scavenging properties, was ineffective. Remarkably, DHLA, but not LA, was also able to inhibit the peroxidation of linoleic acid and of the non-HDL fraction catalyzed by RR15-LOX with an IC(50) of, respectively, 10 and 5 microM. Finally, DHLA, but once again not LA, could readily reduce simple ferric ions and scavenge efficiently the stable free radical 1,1-diphenyl-2-pycrylhydrazyl in ethanol; DHLA was considerably less effective against 2,2'-azobis(2-amidinopropane) dihydrochloride-mediated, peroxyl radical-induced non-HDL peroxidation, showing an IC(50) of 850 microM. Thus, DHLA, at therapeutically relevant concentrations, can counteract 15-lipoxygenase-dependent lipid peroxidation; this antioxidant effect may stem primarily from reduction of the active ferric 15-lipoxygenase form to the inactive ferrous state after DHLA-enzyme hydrophobic interaction and, possibly, from scavenging of fatty acid peroxyl radicals formed during lipoperoxidative processes. Inhibition of 15-lipoxygenase oxidative activity by DHLA could occur in the clinical setting, eventually resulting in specific antioxidant and antiatherogenic effects.

Nonradiometric HPLC measurement of 13(S)-hydroxyoctadecadienoic acid from rat tissues

A major bioactive metabolite of linoleic acid formed by the action of 15-lipoxygenase-1 is 13(S)-hydroxy-cis-9, trans-11-octadecadienoic acid (13(S)-HODE). 13(S)-HODE is an important intracellular signal agent and is involved in cell proliferation and differentiation in various biological systems. Separation and quantification of 13(S)-HODE from biological materials has previously been achieved only by using radiolabeled linoleic acid as the substrate and two serially connected or two separate HPLC columns to achieve separation of 13(S)-HODE. In the current method, separation and quantification of 13(S)-HODE was achieved by use of a normal-phase HPLC and a solvent system containing hexane/isopropanol/acetonitrile/acetic acid (800/8/30/1, v/v) using isocratic elution with detection at 235 nm. With the currently described method, good separation from unreacted interfering compounds and quantification for 13(S)-HODE were achieved within 35 min with a minimum detection limit of 0.5 ng per injection.

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

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

Transient experimental anemia in cholesterol-fed rabbits induces systemic overexpression of the reticulocyte-type 15-lipoxygenase and protects from aortic lipid deposition

Oxidative modification of low-density lipoprotein has been implicated in atherogenesis and the lipid peroxidizing enzyme 12/15-lipoxygenase (12/15-LOX) was suggested to be involved. For this study, we induced a strong and long-lasting systemic overexpression of the 15-LOX, in female New Zealand White rabbits by transient experimental anemia. After the hematopoietic parameters had returned to normal, these animals and age-matched controls were fed a lipid-rich Western-type diet for 10 weeks. Analyzing the lipid deposition in the aortic wall, we found that the 15-LOX overexpressing rabbits deposited significantly (P<0.01) less cholesteryl linoleate in the thoracic aorta than the corresponding controls. Similar results were obtained when free cholesterol and cholesteryl oleate were quantified. However, in the aortic arch where lipid deposition was much more severe a similar trend was observed, but the effects were not significant any more. Comparative determination (lipoxygenase overexpressing vs. control animals) of various plasma parameters as well as histological inspections of major organs did not reveal any indications for major organ malfunction. These data suggest that transient experimental anemia, which is accompanied by a long-lasting overexpression of the reticulocyte-type 15-LOX protects cholesterol-fed rabbits from lipid deposition in the aortic wall.

Enzymatically modified nonoxidized low-density lipoprotein induces interleukin-8 in human endothelial cells: role of free fatty acids

BACKGROUND

Treatment of low-density lipoprotein (LDL) with a protease and cholesterolesterase transforms the lipoprotein to an entity that resembles lipoprotein particles in atherosclerotic lesions, which have a high content of free cholesterol, reflecting extensive de-esterification in the intima. Because de-esterification would occur beneath the endothelium, we examined the effects of enzymatically modified LDL (E-LDL) on cultured endothelial cells.

METHODS AND RESULTS

Incubation of endothelial cells with E-LDL provoked selective accumulation of interleukin (IL)-8 mRNA and production of the cytokine. Chemical analyses and depletion experiments indicated that the effect was caused by the presence of free fatty acids in the altered lipoprotein. Reconstitution studies demonstrated that the oleic and linoleic acids associated with E-LDL are particularly effective IL-8 inducers. The effects of E-LDL on endothelial cells could be abrogated with albumin.

CONCLUSION

IL-8 is required for rolling monocytes to adhere firmly to the endothelium; thus, the findings reveal a link between subendothelial entrapment of LDL, cleavage of cholesterol esters, and monocyte recruitment into the lesion.

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.

Three different pathways for human LDL oxidation are inhibited in vitro by water extracts of the medicinal herb Achyrocline satureoides

In this study we investigated the antioxidant properties of one herbal preparation widely used in complementary and alternative medicine in large areas of the world: Achyrocline satureoides (AS), popularly known as "marcela". Although rich in flavonoids, the ethnopharmacological uses of this plant do not include atherosclerosis prevention. Furthermore, no study had been conducted so far exploring the antioxidant activity of Achyrocline satureoides vis-à-vis human LDL oxidation, which is the compelling issue in pinpointing potential cardioprotective new uses for a traditional remedy. We explored the effects of AS extracts on human LDL oxidation, employing 3 different systems which are thought to play a role in oxidation of LDL in the arterial wall: copper, peroxynitrite, and lipoxygenase. Oxidation was monitored by conjugate dienes, TBARS formation and aggregation of apoB using SDS-PAGE. In copper-initiated oxidation a dose dependent inhibition of the initiation and propagation of lipid oxidation is shown by an increase in the lag phase for conjugate diene production which was 60 +/- 15 min in the absence and 120 +/- 20 min in the presence of 4 microg/ml AS extracts (p < 0.001). TBARS production was reduced by 95% after 3 h incubation at 5 microg/ml. Aggregation of apoB was abolished at the same concentrations. SIN-1 (3-morpholinosydnonimine) produces peroxynitrite via generation of NO and O2-. When LDL was incubated in its presence, a milder oxidation was observed as compared with Cu2+, and AS produced over 70% inhibition. Finally, we show a striking dose-dependent inhibitory effect of lipoxygenase conjugate diene production, which is over 95% at AS concentrations of 5 microg/ml. When compared with other antioxidants, AS effect is greater but in the same order of magnitude than that of ascorbic acid and similar to the popular herbal tea Ilex paraguariensis. In all three systems employed an effect is already substantiated at a concentration of the AS extract of 4 microg/ml, which corresponds to a 1/100 dilution of the preparations usually drunk.

Aggregation of LDL with chondroitin-4-sulfate makes LDL oxidizable in the presence of water-soluble antioxidants

The content of plasma and arterial interstitial fluid in water-soluble antioxidants makes it unlikely for low-density lipoprotein (LDL) to oxidize by the oxidation mechanisms most frequently discussed. By aggregation of LDL in the presence of chondroitin-4-sulfate (C-4-S), but not with chondroitin-6-sulfate or sphingomyelinase, a complex arises which can oxidize in the presence of 20 microM ascorbate and 300 microM urate. This oxidation sensitivity even persists after the gel-filtration of an LDL/C-4-S/Cu(2+) complex, indicating entrapment of Cu(2+) within. This corresponds well to the known ability of C-4-S to bind copper ions and is a potential mechanism by which LDL oxidation in the arterial intima is facilitated after prolonged retention by the extracellular matrix.

Clinicopathological significance of lipid peroxidation in carotid plaques

Several reports have suggested an association between lipid peroxidation and human carotid atherosclerosis, but few reports have demonstrated a link between lipid peroxidation and carotid plaques in humans. In this study, we investigated the relationship between clinical features, histopathological characteristics and lipid peroxidation in patients undergoing carotid endarterectomy (CEA). Forty-one carotid plaques were obtained. A portion of the most severe lesions was subjected to histopathologic examination, and the remainder of the plaques examined for lipid peroxidation. Thiobarbituric acid-reactive substances (TBARS) values were determined as a marker for lipid peroxidation. The lipid-rich core (LC) and macrophage infiltration (Mphi) component as a percentage of total plaque area were measured morphometrically. Based on the results, all plaques were classified into four groups. Group I (GI): LC <10%; Group IIa (GIIa): LC 10-30%, Mphi <5%; Group IIb (GIIb): LC 10-30%, Mphi < or = 5%, and Group III (GIII): LC < or =30%. The plaque TBARS values of GIII were significantly higher than those of GI, GIIa, and GIIb. The TBARS values of GIIb were one-and-a-half times higher than those of GIIa. Our results show that lipid peroxidation in carotid plaques is significantly associated with carotid atherosclerosis, especially plaque instability. These findings provide direct evidence of an association between lipid peroxidation and human atherosclerosis.

Dissociation of apoptosis induction and CD36 upregulation by enzymatically modified low-density lipoprotein in monocytic cells

Modified low-density lipoprotein (LDL) has been implicated as an initiating or amplifying factor in atherogenesis. Some of its biological activities, such as apoptosis induction and upregulation of the scavenger receptor CD36, appear to share common signaling pathways in cells of the cardiovascular system. Exposure of low-differentiated monocytic cells to LDL modified with 15-lipoxygenase and secretory phospholipase A(2) induced apoptosis and upregulated CD36. Cell treatment with constituents of modified LDL, such as 13-hydroxyoctadecadienoic acid (13-HODE), 25-hydroxycholesterol, and lysophosphatidyl choline, and with an unrelated apoptogen (TNF-related apoptosis-inducing ligand) induced apoptosis. In contrast, only 13-HODE caused upregulation of CD36 expression. Cotreatment with the pan-caspase inhibitor z.VAD-fmk resulted in suppression of apoptosis, but was without any effect on CD36 expression. These data indicate that in monocytic cells enzymatically modified LDL is capable of inducing both apoptosis and upregulation of CD36 expression. However, in our cellular model, the two induction processes appear to be causally unrelated.

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.

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.

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.

Nitric oxide regulation of free radical- and enzyme-mediated lipid and lipoprotein oxidation

The regulation of nonenzymatic and enzymatic lipid oxidation reactions by nitric oxide (.NO) is potent and pervasive and reveals novel non-cGMP-dependent reactivities for this free radical inflammatory and signal transduction mediator.NO and its metabolites stimulate and inhibit lipid peroxidation reactions, modulate enzymatically catalyzed lipid oxidation, complex with lipid-reactive metals, and alter proinflammatory gene expression. Through these mechanisms,.NO can regulate nonenzymatic lipid oxidation and the production of inflammatory and vasoactive eicosanoids by prostaglandin endoperoxide synthase and lipoxygenase. The accumulation of macrophages and oxidized low density lipoprotein within the vascular wall can also be modulated by.NO. A key determinant of the pro-oxidant versus oxidant-protective influences of.NO is the underlying oxidative status of tissue. When.NO is in excess of surrounding oxidants, lipid oxidation and monocyte margination into the vascular wall are attenuated, producing antiatherogenic effects. However, when endogenous tissue rates of oxidant production are accelerated or when tissue oxidant defenses become depleted,.NO gives rise to secondary oxidizing species that can increase membrane and lipoprotein lipid oxidation as well as foam cell formation in the vasculature, thus promoting proatherogenic effects. In summary,.NO is a multifaceted molecule capable of reacting via multiple pathways to modulate lipid oxidation reactions, thereby impacting on tissue inflammatory reactions.

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.

Regulation of human 12/15-lipoxygenase by Stat6-dependent transcription

Human 12/15-lipoxygenase is a lipid-peroxidating enzyme implicated in the pathophysiology of atherosclerosis and airway inflammation. Interleukin (IL)-4 specifically induces 12/15-lipoxygenase messenger RNA, protein, and enzymatic activity in primary cultures of human monocytes and airway epithelial cells. The induction of the human 12/15-lipoxygenase by IL-4 suggests that the signal transducer and activator of transcription (Stat)-6 protein is critical for its expression. Several putative Stat6 response elements are located in the proximal 1.8 kb of 12/15-lipoxygenase 5'-flanking region. In this study we use BEAS-2B human airway epithelial cells as a model to demonstrate the dependence of 12/15-lipoxygenase expression on the IL-4/Stat6 signal transduction pathway. Transient transfections of human 12/15-lipoxygenase promoter/luciferase reporter genes indicate that this induction occurs through direct transcriptional mechanisms mediated by a specific Stat6 response element located 952 base pairs upstream of the translational start codon. Using this Stat6 response element as a probe, electrophoretic mobility shift assays show an IL-4-dependent binding activity in nuclear extracts. Supershift assays confirm that Stat6 participates in this binding complex. These data indicate that the human 12/15-lipoxygenase gene is induced in airway epithelial cells through Stat6-dependent transcriptional mechanisms mediated by a specific Stat6 response element in the 5'-flanking region.

High density lipoprotein oxidation: in vitro susceptibility and potential in vivo consequences

Elevated levels of plasma high density lipoprotein (HDL) are strongly predictive of protection against atherosclerotic vascular disease. HDL particles likely have several beneficial actions in vivo, including the initiation of reverse cholesterol transport. The apparent importance of oxidative modification of low density lipoprotein in atherogenesis raises the question of how oxidative modification of HDL might affect its cardioprotective actions. HDL is readily oxidized using numerous models of lipoprotein oxidation. In vitro evidence suggests oxidation might impair some protective actions, but actually enhance other mechanisms induced by HDL that prevent the accumulation of cholesterol in the artery wall. This article reviews the current literature concerning the relative oxidizability of HDL, the structural changes induced in HDL by oxidation in vitro, and the potential consequences of oxidative modification on the protective actions of HDL in vivo.

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.

Essential involvement of 12-lipoxygenase in regiospecific andstereospecific oxidation of low density lipoprotein by macrophages

To establish a role of the 12-lipoxygenase on the generation of oxidized low density lipoprotein (LDL) in macrophages that leads to foam cell formation in atherosclerosis, we overexpressed 12-lipoxygenases in a macrophage-like cell line, J774A.1, that does not show intrinsic enzyme activity. When the 12-lipoxygenase-expressing cells were incubated with 400 microg.mL-1 LDL in Dulbecco's modified Eagle's medium at 37 degrees C for 12 h, LDL oxidation, as determined by thiobarbituric acid reactive substance, was markedly increased compared with the mock-transfected cells. Oxygenated products in the modified LDL were examined by HPLC before and after alkaline hydrolysis. Most of the oxygenated derivatives were of an esterified form, and the major product was identified as 13S-hydroxyoctadeca-9Z,11E-dienoic acid. These results clearly demonstrate that esterified fatty acids in LDL are oxygenated by the 12-lipoxygenases expressed in the J774A.1 cells. Furthermore, the oxidized LDL generated by intracellular 12-lipoxygenases was recognized by a scavenger receptor as assessed by macrophage degradation assay.

Differential localization of 5- and 15-lipoxygenases to the nuclear envelope in RAW macrophages

Leukotriene formation is initiated in myeloid cells by an increase in intracellular calcium and translocation of 5-lipoxygenase from the cytoplasm to the nuclear envelope where it can utilize arachidonic acid. Monocyte- macrophages and eosinophils also express 15-lipoxygenase, which converts arachidonic acid to 15(S)-hydroxyeicosatetraenoic acid. Enhanced green fluorescent 5-lipoxygenase (5-LO) and 15-lipoxygenase (15-LO) fusion proteins were expressed in the cytoplasm of RAW 264.7 macrophages. Only 5-lipoxygenase translocated to the nuclear envelope after cell stimulation, suggesting that differential subcellular compartmentalization can regulate the generation of leukotrienes versus 15(S)-hydroxyeicosatetraenoic acid in cells that possess both lipoxygenases. A series of truncation mutants of 5-LO were created to identify putative targeting domains; none of these mutants localized to the nuclear envelope. The lack of targeting of 15-LO was then exploited to search for specific targeting motifs in 5-LO, by creating 5-LO/15-LO chimeric molecules. The only chimera that could sustain nuclear envelope translocation was one which involved replacement of the N-terminal 237 amino acids with the corresponding segment of 15-LO. Significantly, no discrete targeting domain could be identified in 5-LO, suggesting that sequences throughout the molecule are required for nuclear envelope localization.

The arachidonate 12/15 lipoxygenases. A review of tissue expression and biologic function

12/15-Lipoxygenase is a highly regulated lipid-peroxidating enzyme whose expression and arachidonic acid metabolites are implicated in several important inflammatory conditions including airway and glomerular inflammation as well as atherosclerosis. Tissue expression of the original 12/15-lipoxygenase is well characterized in reticulocytes, eosinophils, airway epithelial cells, and monocytes/macrophages and is likely in other cell systems and tissues under specific conditions. The physiologic role of this family of enzymes is dependent on the context in which it is expressed. In general, the arachidonic acid metabolites antagonize inflammatory responses and counteract the proinflammatory effects of the 5-lipoxygenase pathway. However, certain diHETEs are associaled with pro-inflammatory effects, specifically neutrophilic and eosiniphilic chemotaxis. The direct action of these enzymes on complex lipids and cellular membranes also links them to such significant process as reticulocyte maturation, LDL oxidation in atherosclerosis and pulmonary host defenses. The availability of new specific inhibitors and murine lines that lack expression of the homologous 12-lipoxygenase will allow confirmation of many of these effects with in vivo models of inflammation.

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.

The diversity of the lipoxygenase family. Many sequence data but little information on biological significance

Lipoxygenases form a family of lipid peroxidising enzymes, which oxygenate free and esterified polyenoic fatty acids to the corresponding hydroperoxy derivatives. They are widely distributed in both the plant and animal kingdoms. During the last couple of years more and more lipoxygenase isoforms have been discovered but for most of them the biological significance remains unclear. This review attempts to classify the currently known mammalian lipoxygenase isoforms and critically reviews the concepts for their biological importance.