Oxidative modification of human lipoproteins by lipoxygenases of different positional specificities

The reaction specificity of mammalian ALOX15B orthologs does not depend on the evolutionary ranking of the animals

Arachidonic acid lipoxygenases (ALOXs) play important roles in cell differentiation and in the pathogenesis of cardiovascular, hyperproliferative, neurodegenerative, and metabolic diseases. The human genome involves six intact ALOX genes and knockout studies of the corresponding mouse orthologs indicated that the coding multiplicity of ALOX isoforms is not an indication for functional redundancy. Despite their evolutionary relatedness human and mouse ALOX15 and ALOX15B orthologs exhibit different catalytic properties. Human ALOX15 oxygenates arachidonic acid mainly to 15S-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acid but 12S-hydroperoxy-5Z,8Z,10E,14Z-eicosatetraenoic acid is the dominant oxygenation product of mouse Alox15. This functional difference is the results of a targeted enzyme evolution but the driving forces for this process have not been well defined. For human and mouse ALOX15B orthologs similar functional differences have been reported but for the time being it was unclear whether these differences might also be a consequence of targeted enzyme evolution. To address this question, we systematically searched the public databases for ALOX15B genes, expressed selected enzymes, and characterized their functional properties. We found that functional ALOX15B genes frequently occur in Prototheria and Eutheria but orthologous genes are rare in Metatheria. The vast majority of mammalian ALOX15B orthologs constitute arachidonic acid 15-lipoxygenating enzymes and this property did not depend on the evolutionary ranking of the animals. Only several Muridae species including M. musculus, M. pahari, M. caroli, M. coucha, and A. niloticus express arachidonic acid 8-lipoxygenating ALOX15B orthologs. Consequently, the difference in the reaction specificity of mouse and human ALOX15B orthologs may not be considered a functional consequence of targeted enzyme evolution.

The Octadecanoids: Synthesis and Bioactivity of 18-Carbon Oxygenated Fatty Acids in Mammals, Bacteria, and Fungi

The octadecanoids are a broad class of lipids consisting of the oxygenated products of 18-carbon fatty acids. Originally referring to production of the phytohormone jasmonic acid, the octadecanoid pathway has been expanded to include products of all 18-carbon fatty acids. Octadecanoids are formed biosynthetically in mammals via cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) activity, as well as nonenzymatically by photo- and autoxidation mechanisms. While octadecanoids are well-known mediators in plants, their role in the regulation of mammalian biological processes has been generally neglected. However, there have been significant advancements in recognizing the importance of these compounds in mammals and their involvement in the mediation of inflammation, nociception, and cell proliferation, as well as in immuno- and tissue modulation, coagulation processes, hormone regulation, and skin barrier formation. More recently, the gut microbiome has been shown to be a significant source of octadecanoid biosynthesis, providing additional biosynthetic routes including hydratase activity (e.g., CLA-HY, FA-HY1, FA-HY2). In this review, we summarize the current field of octadecanoids, propose standardized nomenclature, provide details of octadecanoid preparation and measurement, summarize the phase-I metabolic pathway of octadecanoid formation in mammals, bacteria, and fungi, and describe their biological activity in relation to mammalian pathophysiology as well as their potential use as biomarkers of health and disease.

Functional Characterization of Mouse and Human Arachidonic Acid Lipoxygenase 15B (ALOX15B) Orthologs and of Their Mutants Exhibiting Humanized and Murinized Reaction Specificities

The arachidonic acid lipoxygenase 15B (ALOX15B) orthologs of men and mice form different reaction products when arachidonic acid is used as the substrate. Tyr603Asp+His604Val double mutation in mouse arachidonic acid lipoxygenase 15b humanized the product pattern and an inverse mutagenesis strategy murinized the specificity of the human enzyme. As the mechanistic basis for these functional differences, an inverse substrate binding at the active site of the enzymes has been suggested, but experimental proof for this hypothesis is still pending. Here we expressed wildtype mouse and human arachidonic acid lipoxygenase 15B orthologs as well as their humanized and murinized double mutants as recombinant proteins and analyzed the product patterns of these enzymes with different polyenoic fatty acids. In addition, in silico substrate docking studies and molecular dynamics simulation were performed to explore the mechanistic basis for the distinct reaction specificities of the different enzyme variants. Wildtype human arachidonic acid lipoxygenase 15B converted arachidonic acid and eicosapentaenoic acid to their 15-hydroperoxy derivatives but the Asp602Tyr+Val603His exchange murinized the product pattern. The inverse mutagenesis strategy in mouse arachidonic acid lipoxygenase 15b (Tyr603Asp+His604Val exchange) humanized the product pattern with these substrates, but the situation was different with docosahexaenoic acid. Here, Tyr603Asp+His604Val substitution in mouse arachidonic acid lipoxygenase 15b also humanized the specificity but the inverse mutagenesis (Asp602Tyr+Val603His) did not murinize the human enzyme. With linoleic acid Tyr603Asp+His604Val substitution in mouse arachidonic acid lipoxygenase 15b humanized the product pattern but the inverse mutagenesis in human arachidonic acid lipoxygenase 15B induced racemic product formation. Amino acid exchanges at critical positions of human and mouse arachidonic acid lipoxygenase 15B orthologs humanized/murinized the product pattern with C20 fatty acids, but this was not the case with fatty acid substrates of different chain lengths. Asp602Tyr+Val603His exchange murinized the product pattern of human arachidonic acid lipoxygenase 15B with arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid. An inverse mutagenesis strategy on mouse arachidonic acid lipoxygenase 15b (Tyr603Asp+His604Val exchange) did humanize the reaction products with arachidonic acid and eicosapentaenoic acid, but not with docosahexaenoic acid.

Arachidonate 15-lipoxygenase type B: Regulation, function, and its role in pathophysiology

As a lipoxygenase (LOX), arachidonate 15-lipoxygenase type B (ALOX15B) peroxidizes polyenoic fatty acids (PUFAs) including arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and linoleic acid (LA) to their corresponding fatty acid hydroperoxides. Distinctive to ALOX15B, fatty acid oxygenation occurs with positional specificity, catalyzed by the non-heme iron containing active site, and in addition to free PUFAs, membrane-esterified fatty acids serve as substrates for ALOX15B. Like other LOX enzymes, ALOX15B is linked to the formation of specialized pro-resolving lipid mediators (SPMs), and altered expression is apparent in various inflammatory diseases such as asthma, psoriasis, and atherosclerosis. In primary human macrophages, ALOX15B expression is associated with cellular cholesterol homeostasis and is induced by hypoxia. Like in inflammation, the role of ALOX15B in cancer is inconclusive. In prostate and breast carcinomas, ALOX15B is attributed a tumor-suppressive role, whereas in colorectal cancer, ALOX15B expression is associated with a poorer prognosis. As the biological function of ALOX15B remains an open question, this review aims to provide a comprehensive overview of the current state of research related to ALOX15B.

The Reaction Specificity of Mammalian ALOX15 Orthologs is Changed During Late Primate Evolution and These Alterations Might Offer Evolutionary Advantages for Hominidae

Arachidonic acid lipoxygenases (ALOXs) have been implicated in the immune response of mammals. The reaction specificity of these enzymes is decisive for their biological functions and ALOX classification is based on this enzyme property. Comparing the amino acid sequences and the functional properties of selected mammalian ALOX15 orthologs we previously hypothesized that the reaction specificity of these enzymes can be predicted based on their amino acid sequences (Triad Concept) and that mammals, which are ranked in evolution below gibbons, express arachidonic acid 12-lipoxygenating ALOX15 orthologs. In contrast, Hominidae involving the great apes and humans possess 15-lipoxygenating enzymes (Evolutionary Hypothesis). These two hypotheses were based on sequence data of some 60 mammalian ALOX15 orthologs and about half of them were functionally characterized. Here, we compared the ALOX15 sequences of 152 mammals representing all major mammalian subclades expressed 44 novel ALOX15 orthologs and performed extensive mutagenesis studies of their triad determinants. We found that ALOX15 genes are absent in extant Prototheria but that corresponding enzymes frequently occur in Metatheria and Eutheria. More than 90% of them catalyze arachidonic acid 12-lipoxygenation and the Triad Concept is applicable to all of them. Mammals ranked in evolution above gibbons express arachidonic acid 15-lipoxygenating ALOX15 orthologs but enzymes with similar specificity are only present in less than 5% of mammals ranked below gibbons. This data suggests that ALOX15 orthologs have been introduced during Prototheria-Metatheria transition and put the Triad Concept and the Evolutionary Hypothesis on a much broader and more reliable experimental basis.

The Reciprocal Relationship between LDL Metabolism and Type 2 Diabetes Mellitus

Type 2 diabetes mellitus and insulin resistance feature substantial modifications of the lipoprotein profile, including a higher proportion of smaller and denser low-density lipoprotein (LDL) particles. In addition, qualitative changes occur in the composition and structure of LDL, including changes in electrophoretic mobility, enrichment of LDL with triglycerides and ceramides, prolonged retention of modified LDL in plasma, increased uptake by macrophages, and the formation of foam cells. These modifications affect LDL functions and favor an increased risk of cardiovascular disease in diabetic individuals. In this review, we discuss the main findings regarding the structural and functional changes in LDL particles in diabetes pathophysiology and therapeutic strategies targeting LDL in patients with diabetes.

Structure and Dynamics of Oxidized Lipoproteins In Vivo: Roles of High-Density Lipoprotein

Oxidative modification of lipoproteins is implicated in the occurrence and development of atherosclerotic lesions. Earlier studies have elucidated on the mechanisms of foam cell formation and lipid accumulation in these lesions, which is mediated by scavenger receptor-mediated endocytosis of oxidized low-density lipoprotein (oxLDL). Mounting clinical evidence has supported the involvement of oxLDL in cardiovascular diseases. High-density lipoprotein (HDL) is known as anti-atherogenic; however, recent studies have shown circulating oxidized HDL (oxHDL) is related to cardiovascular diseases. A modified structure of oxLDL, which was increased in the plasma of patients with acute myocardial infarction, was characterized. It had two unique features: (1) a fraction of oxLDL accompanied oxHDL, and (2) apoA1 was heavily modified, while modification of apoB, and the accumulation of oxidized phosphatidylcholine (oxPC) and lysophosphatidylcholine (lysoPC) was less pronounced. When LDL and HDL were present at the same time, oxidized lipoproteins actively interacted with each other, and oxPC and lysoPC were transferred to another lipoprotein particle and enzymatically metabolized rapidly. This brief review provides a novel view on the dynamics of oxLDL and oxHDL in circulation.

Potential of Lipoprotein-Based Nanoparticulate Formulations for the Treatment of Eye Diseases

Lipoproteins are naturally occurring nanoparticles and their main physiological function is the promotion of lipid metabolism. They can be prepared in vitro for use as drug carriers, and these reconstituted lipoproteins show similar biological activity to their natural counterparts. Some lipoproteins can cross the blood-retinal barrier and are involved in intraocular lipid metabolism. Drug-loaded lipoproteins can be delivered to the retina for the treatment of posterior eye diseases. In this review, we have discussed the therapeutic applications of lipoproteins for eye diseases and introduced the emerging animal models used for the evaluation of their therapeutic effects.

Regulation and Functions of 15-Lipoxygenases in Human Macrophages

Lipoxygenases (LOXs) catalyze the stereo-specific peroxidation of polyunsaturated fatty acids (PUFAs) to their corresponding hydroperoxy derivatives. Human macrophages express two arachidonic acid (AA) 15-lipoxygenating enzymes classified as ALOX15 and ALOX15B. ALOX15, which was first described in 1975, has been extensively characterized and its biological functions have been investigated in a number of cellular systems and animal models. In macrophages, ALOX15 functions to generate specific phospholipid (PL) oxidation products crucial for orchestrating the nonimmunogenic removal of apoptotic cells (ACs) as well as synthesizing precursor lipids required for production of specialized pro-resolving mediators (SPMs) that facilitate inflammation resolution. The discovery of ALOX15B in 1997 was followed by comprehensive analyses of its structural properties and reaction specificities with PUFA substrates. Although its enzymatic properties are well described, the biological functions of ALOX15B are not fully understood. In contrast to ALOX15 whose expression in human monocyte-derived macrophages is strictly dependent on Th2 cytokines IL-4 and IL-13, ALOX15B is constitutively expressed. This review aims to summarize the current knowledge on the regulation and functions of ALOX15 and ALOX15B in human macrophages.

Emerging role of 12/15-Lipoxygenase (ALOX15) in human pathologies

12/15-lipoxygenase (12/15-LOX) is an enzyme, which oxidizes polyunsaturated fatty acids, particularly omega-6 and -3 fatty acids, to generate a number of bioactive lipid metabolites. A large number of studies have revealed the importance of 12/15-LOX role in oxidative and inflammatory responses. The in vitro studies have demonstrated the ability of 12/15-LOX metabolites in the expression of various genes and production of cytokine related to inflammation and resolution of inflammation. The studies with the use of knockout and transgenic animals for 12/15-LOX have further shown its involvement in the pathogenesis of a variety of human diseases, including cardiovascular, renal, neurological and metabolic disorders. This review summarizes our current knowledge on the role of 12/15-LOX in inflammation and various human diseases.

Oxidant-specific biomarkers of oxidative stress. Association with atherosclerosis and implication for antioxidant effects

The unregulated oxidative modification of lipids, proteins, and nucleic acids induced by multiple oxidants has been implicated in the pathogenesis of many diseases. Antioxidants with diverse functions exert their roles either directly or indirectly in the physiological defense network to inhibit such deleterious oxidative modification of biological molecules and resulting damage. The efficacy of antioxidants depends on the nature of oxidants. Therefore, it is important to identify the oxidants which are responsible for modification of biological molecules. Some oxidation products produced selectively by specific oxidant enable to identify the responsible oxidants, while other products are produced by several oxidants similarly. In this review article, several oxidant-specific products produced selectively by peroxyl radicals, peroxynitrite, hypochlorous acid, lipoxygenase, and singlet oxygen were summarized and their potential role as biomarker is discussed. It is shown that the levels of specific oxidation products including hydroxylinoleate isomers, nitrated and chlorinated products, and oxysterols produced by the above-mentioned oxidants are elevated in the human atherosclerotic lesions, suggesting that all these oxidants may contribute to the development of atherosclerosis. Further, it was shown that the reactivities of physiological antioxidants toward the above-mentioned oxidants vary extensively, suggesting that multiple antioxidants effective against these different oxidants are required, since no single antioxidant alone can cope with these multiple oxidants.

Female mice carrying a defective Alox15 gene are protected from experimental colitis via sustained maintenance of the intestinal epithelial barrier function

Lipoxygenases (ALOXs) are involved in the regulation of cellular redox homeostasis. They also have been implicated in the biosynthesis of pro- and anti-inflammatory lipid mediators and play a role in the pathogenesis of inflammatory diseases, which constitute a major health challenge owing to increasing incidence and prevalence in all industrialized countries around the world. To explore the pathophysiological role of Alox15 (leukocyte-type 12-LOX) in mouse experimental colitis we tested the impact of systemic inactivation of the Alox15 gene on the extent of dextrane sulfate sodium (DSS) colitis. We found that in wildtype mice expression of the Alox15 gene was augmented during DSS-colitis while expression of other Alox genes (Alox5, Alox15b) was hardly altered. Systemic Alox15 (leukocyte-type 12-LOX) deficiency induced less severe colitis symptoms and suppressed in vivo formation of 12-hydroxyeicosatetraenoic acid (12-HETE), the major Alox15 (leukocyte-type 12-LOX) product in mice. These alterations were paralleled by reduced expression of pro-inflammatory gene products, by sustained expression of the zonula occludens protein 1 (ZO-1) and by a less impaired intestinal epithelial barrier function. These results are consistent with in vitro incubations of colon epithelial cells, in which addition of 12S-HETE compromised enantioselectively transepithelial electric resistance. Consistent with these data transgenic overexpression of human ALOX15 intensified the inflammatory symptoms. In summary, our results indicate that systemic Alox15 (leukocyte-type 12-LOX) deficiency protects mice from DSS-colitis. Since exogenous 12-HETE compromises the expression of the tight junction protein ZO-1 the protective effect has been related to a less pronounced impairment of the intestinal epithelial barrier function.

Differential proteomic analysis of platelets suggested target-related proteins in rabbit platelets treated with Rhizoma Corydalis

CONTEXT

Corydalis yanhusuo W.T. Wang (Papaveraceae) (Rhizoma Corydalis) showed inhibitory effects on rabbit platelet aggregation induced by ADP, thrombin (THR) or arachidonic acid (AA).

OBJECTIVE

This study separates and identifies the possible target-related platelet proteins and suggests possible signal cascades of RC antiplatelet aggregation.

MATERIALS AND METHODS

Based on comparative proteomics, the differentially expressed platelet proteins treated before and after with 50 mg/mL RC 90% ethanol extract (for 15 min at 37 °C) were analyzed and identified by two dimensional gel electrophoresis (2-DE) and MALDI-TOF-MS/MS. To further verify the possible signalling pathways of RC antiplatelet aggregation function, the concentration of calcium (Ca²⁺) was measured by Fura-2/AM fluorescence (Ex 340/380 nm, Em 500 nm) (RC final concentrations of 0.0156-0.1563 mg/mL), the levels of P-selectin and cyclic guanosine monophosphate (cGMP) were quantified by ELISA (OD. 450 nm) (RC final concentrations of 0.0156-1.5625 mg/mL), and the 5-hydroxytryptamine (5-HT) level was measured using ortho-phthalaldehyde (OPT) fluorescence (Ex 340 nm, Em 470 nm) (RC final concentrations of 0.3125-1.5625 mg/mL).

RESULTS

The expression of 52 proteins were altered in rabbit platelets after the treatment and the MALDI-TOF-MS analysis indicated that those proteins include 12 cytoskeleton proteins, 7 cell signalling proteins, 3 molecular chaperone proteins, 6 proteins related to platelet function, 16 enzymes and 7 other related proteins. Furthermore, RC extract could decrease the levels of 5-HT [inhibition rate of 96.80% (p < 0.05, vs. THR-activated group) treated with 0.7813 mg/mL of RC], Ca²⁺[172.73 ± 5.07 to 113.56 ± 5.46 nM (p < 0.001, vs. THR-activated group) treated with 0.0313 mg/mL of RC] and P-selectin [13.48 ± 0.96 ng/3 × 10⁸ to 11.64 ± 0.17 ng/3 × 10⁸ (p < 0.05, vs. THR-activated group) treated with 0.0156 mg/mL of RC], and increase in cGMP level [38.93 ± 0.57 to 50.26 ± 4.05 ng/3 × 10⁸ (p < 0.05, vs. THR-activated group) treated with 1.5165 mg/mL of RC] in ADP (10 μmol/L), THR (0.25 u/mL) or AA-(0.205 mmol/L) activated rabbit platelets.

DISCUSSION AND CONCLUSION

The present study indicated that P2Y12 receptor might be one of the direct target proteins of RC in platelets. The signal cascades network of RC after binding with P2Y12 receptor is mediating Gαi proteins to activate downstream signalling pathways (AC and/or PI3K signalling pathways) for the inhibition of platelet aggregation.

Targeting of 12/15-Lipoxygenase in retinal endothelial cells, but not in monocytes/macrophages, attenuates high glucose-induced retinal leukostasis

AIMS

Our previous studies have established a role for 12/15-lipoxygenase (LO) in mediating the inflammatory response in diabetic retinopathy (DR). However, the extent at which the local or systemic induction of 12/15-LO activity involved is unclear. Thus, the current study aimed to characterize the relative contribution of retinal endothelial versus monocytic/macrophagic 12/15-LO to inflammatory responses in DR.

MATERIALS & METHODS

We first generated a clustered heat map for circulating bioactive lipid metabolites in the plasma of streptozotocin (STZ)-induced diabetic mice using liquid chromatography coupled with mass-spectrometry (LC-MS) to evaluate changes in circulating 12/15-LO activity. This was followed by comparing the in vitro mouse endothelium-leukocytes interaction between leukocytes isolated from 12/15-LO knockout (KO) versus those isolated from wild type (WT) mice using the myeloperoxidase (MPO) assay. Finally, we examined the effects of knocking down or inhibiting endothelial 12/15-LO on diabetes-induced endothelial cell activation and ICAM-1 expression.

RESULTS

Analysis of plasma bioactive lipids' heat map revealed that the activity of circulating 12/15-LO was not altered by diabetes as evident by no significant changes in the plasma levels of major metabolites derived from 12/15-lipoxygenation of different PUFAs, including linoleic acid (13-HODE), arachidonic acid (12- and 15- HETEs), eicosapentaenoic acid (12- and 15- HEPEs), or docosahexaenoic acid (17-HDoHE). Moreover, leukocytes from 12/15-LO KO mice displayed a similar increase in adhesion to high glucose (HG)-activated endothelial cells as do leukocytes from WT mice. Furthermore, abundant proteins of 12-LO and 15-LO were detected in human retinal endothelial cells (HRECs), while it was undetected (15-LO) or hardly detectable (12-LO) in human monocyte-like U937 cells. Inhibition or knock down of endothelial 12/15-LO in HRECs blocked HG-induced expression of ICAM-1, a well-known identified important molecule for leukocyte adhesion in DR.

CONCLUSION

Our data support that endothelial, rather than monocytic/macrophagic, 12/15-LO has a critical role in hyperglycemia-induced ICAM-1 expression, leukocyte adhesion, and subsequent local retinal barrier dysfunction. This may facilitate the development of more precisely targeted treatment strategies for DR.

Effects of MβCD on Lipoxygenase-Induced LDL Oxidation

Beta-cyclodextrin (β-CD) has been applied as drug/food carriers or potential drugs for treating some diseases. Most recently, some evidence indicated that methyl-β-cyclodextrin (MβCD) and 2-hydroxypropyl-β-cyclodextrin (2-HPβCD), two major derivatives of β-CD, may inhibit atherogenesis, implying that cyclodextrins also can be potential drugs for treating atherosclerosis. It is well known that modification (e.g. oxidation) of low-density lipoprotein (LDL) is one of the most critical steps of atherogenesis. Lipoxygenase, an enzyme able to be expressed by atherosclerosis-related vascular cells, is generally regarded as a possible in vivo agent of LDL oxidation. In this study, the effects of MβCD on LDL oxidation induced by lipoxygenase were investigated by measuring the electrophoretic mobility, conjugated diene formation, malondialdehyde (MDA) production, and amino group blockage of LDL. We found that the lipids depleted from LDL by MβCD could be oxygenated more readily by lipoxygenase whereas the lipoxygenase-induced oxidation of the remaining lipid-depleted LDL decreased. The data imply that MβCD has an inhibitory effect on lipoxygenase-induced LDL oxidation and probably helps to inhibit atherogenesis.

Mice deficient in L-12/15 lipoxygenase show increased vulnerability to 3-nitropropionic acid neurotoxicity

Considerable evidence supports a contributory role for leukocyte-type 12/15 Lipoxygenase (L-12/15 LO) in mediating hippocampal and cortical neuronal injury in models of Alzheimer's disease and stroke. Whether L-12/15 LO contributes to neuronal injury in a model of Huntington's disease (HD) has yet to be determined. HD is characterized by marked striatal neuronal loss, which can be mimicked in humans and animals by inhibition of mitochondrial complex II using 3-Nitropropionic acid (3-NP). Herein, we compared histological and behavioral outcomes between mice that were wild-type or null for L-12/15 LO following systemic injection of 3NP. We found that mice deficient in L-12/15 LO had a higher incidence of striatal lesions coincident with an increase in morbidity as compared to their wild-type littermate controls. This could not be explained by differential metabolism of 3-NP as striatal succinate dehydrogenase activity was inhibited to the same extent in both genotypes. The present results show that deleting L-12/15 LO is detrimental to the striatum in the setting of chronic, systemic 3-NP exposure and are consistent with the overall conclusion that region-specific effects may determine the ultimate outcome of L-12/15 LO activation in the setting of brain injury.

Dietary polyunsaturated fatty acids and inflammation: the role of phospholipid biosynthesis

The composition of fatty acids in the diets of both human and domestic animal species can regulate inflammation through the biosynthesis of potent lipid mediators. The substrates for lipid mediator biosynthesis are derived primarily from membrane phospholipids and reflect dietary fatty acid intake. Inflammation can be exacerbated with intake of certain dietary fatty acids, such as some ω-6 polyunsaturated fatty acids (PUFA), and subsequent incorporation into membrane phospholipids. Inflammation, however, can be resolved with ingestion of other fatty acids, such as ω-3 PUFA. The influence of dietary PUFA on phospholipid composition is influenced by factors that control phospholipid biosynthesis within cellular membranes, such as preferential incorporation of some fatty acids, competition between newly ingested PUFA and fatty acids released from stores such as adipose, and the impacts of carbohydrate metabolism and physiological state. The objective of this review is to explain these factors as potential obstacles to manipulating PUFA composition of tissue phospholipids by specific dietary fatty acids. A better understanding of the factors that influence how dietary fatty acids can be incorporated into phospholipids may lead to nutritional intervention strategies that optimize health.

Asthmatics exhibit altered oxylipin profiles compared to healthy individuals after subway air exposure

BACKGROUND

Asthma is a chronic inflammatory lung disease that causes significant morbidity and mortality worldwide. Air pollutants such as particulate matter (PM) and oxidants are important factors in causing exacerbations in asthmatics, and the source and composition of pollutants greatly affects pathological implications.

OBJECTIVES

This randomized crossover study investigated responses of the respiratory system to Stockholm subway air in asthmatics and healthy individuals. Eicosanoids and other oxylipins were quantified in the distal lung to provide a measure of shifts in lipid mediators in association with exposure to subway air relative to ambient air.

METHODS

Sixty-four oxylipins representing the cyclooxygenase (COX), lipoxygenase (LOX) and cytochrome P450 (CYP) metabolic pathways were screened using liquid chromatography-tandem mass spectrometry (LC-MS/MS) of bronchoalveolar lavage (BAL)-fluid. Validations through immunocytochemistry staining of BAL-cells were performed for 15-LOX-1, COX-1, COX-2 and peroxisome proliferator-activated receptor gamma (PPARγ). Multivariate statistics were employed to interrogate acquired oxylipin and immunocytochemistry data in combination with patient clinical information.

RESULTS

Asthmatics and healthy individuals exhibited divergent oxylipin profiles following exposure to ambient and subway air. Significant changes were observed in 8 metabolites of linoleic- and α-linolenic acid synthesized via the 15-LOX pathway, and of the COX product prostaglandin E(2) (PGE(2)). Oxylipin levels were increased in healthy individuals following exposure to subway air, whereas asthmatics evidenced decreases or no change.

CONCLUSIONS

Several of the altered oxylipins have known or suspected bronchoprotective or anti-inflammatory effects, suggesting a possible reduced anti-inflammatory response in asthmatics following exposure to subway air. These observations may have ramifications for sensitive subpopulations in urban areas.

Oxidized low-density lipoprotein

Oxidized low-density lipoprotein (Ox-LDL) has been studied for over 25 years. Numerous pro- and anti-atherogenic properties have been attributed to Ox-LDL. Yet, Ox-LDL has neither been defined nor characterized, as its components and composition change depending on its source, method of preparation, storage, and use. It contains unoxidized and oxidized fatty acid derivatives both in the ester and free forms, their decomposition products, cholesterol and its oxidized products, proteins with oxidized amino acids and cross-links, and polypeptides with varying extents of covalent modification with lipid oxidation products, and many others. It seems to exist in vivo in some form not yet fully characterized. Until its pathophysiological significance, and how it is generated in vivo are determined, the nature of its true identity will be only of classical interest. In this review, its components, their biological actions and methods of preparation will be discussed.

15-Lipoxygenase-2 is expressed in macrophages in human carotid plaques and regulated by hypoxia-inducible factor-1alpha

BACKGROUND

Macrophages are prominent in hypoxic areas of atherosclerotic lesions and their secreted cytokines, growth factors and activity of enzymes are involved in atherogenesis. Previously, we showed that 15-lipoxygenase (LOX)-2 is expressed in human monocyte-derived macrophages and that hypoxia increases 15-LOX-2 expression and secretion of pro-inflammatory molecules. Here we investigated whether human carotid plaque macrophages express 15-LOX-2 and whether its expression in macrophages is regulated by hypoxia through hypoxia-inducible factor 1alpha (HIF-1alpha).

MATERIALS AND METHODS

Carotid plaques from 47 patients with high-grade symptomatic carotid artery stenosis were analysed using immunohistochemistry, and stained areas were quantified by digital image analysis. Carotid plaque macrophages were isolated with anti-CD14 immunobeads using an immunomagnetic bead technique. Primary macrophages were transfected with HIF-1alpha siRNA or control siRNA before extraction of RNA and medium analysis.

RESULTS

In paired tissue sections, the extent of staining for CD68 correlated with staining for 15-LOX-2 but not for 15-LOX-1. In carotid plaque macrophages isolated with anti-CD14 immunobeads, 15-LOX-2 mRNA was expressed at high levels. In primary macrophages, 15-LOX-2 expression was significantly increased by incubation with the HIF-1alpha stabilizer dimethyloxalylglycine. Knockdown of HIF-1alpha significantly decreased production of the 15-LOX-2 enzyme products 12- and 15-hydroxyeicosatetraenoic acid. In carotid plaques, HIF-1alpha staining correlated with staining for 15-LOX-2.

CONCLUSIONS

These results demonstrate that 15-LOX-2 is highly expressed in human plaques and is correlated with the presence of macrophages and HIF-1alpha. 15-LOX-2 enzyme activity can be modulated by HIF-1alpha. Thus, increased expression of 15-LOX-2 in macrophages in hypoxic atherosclerotic plaque may enhance inflammation and the recruitment of inflammatory cells.

Improved survival and reduced vascular permeability by eliminating or blocking 12/15-lipoxygenase in mouse models of acute lung injury (ALI)

Acute lung injury (ALI) is a prevalent disease associated with high mortality. 12/15-lipoxygenase (12/15-LO) is an enzyme producing 12-hydroxyeicosatetraenoic acid (HETE) and 15-HETE from arachidonic acid. To test whether 12/15-LO is involved in increasing vascular permeability in the lung, we investigated the role of 12/15-LO in murine models of LPS-induced pulmonary inflammation and clinically relevant acid-induced ALI. The vascular permeability increase upon LPS inhalation was abolished in Alox15(-/-) mice lacking 12/15-LO and in wild-type mice after pharmacological blockade of 12/15-LO. Alox15(-/-) mice also showed improved gas exchange, reduced permeability increase, and prolonged survival in the acid-induced ALI model. Bone marrow chimeras and reconstitution experiments revealed that 12-HETE produced by hematopoietic cells regulates vascular permeability through a CXCR2-dependent mechanism. Our findings suggest that 12/15-LO-derived 12-HETE is a key mediator of vascular permeability in acute lung injury.

Murine 12/15-lipoxygenase regulates ATP-binding cassette transporter G1 protein degradation through p38- and JNK2-dependent pathways

12/15-Lipoxygenase (12/15LO) plays a role in the pathogenesis of atherosclerosis and diabetes and has been implicated in low density lipoprotein oxidation. Murine macrophages express high levels of 12/15LO and are key cells involved in the accumulation and efflux of oxidized low density lipoprotein in the arterial wall. During this process, macrophages up-regulate scavenger receptors that regulate lipid uptake, and ATP-binding cassette (ABC) transporters, that regulate lipid efflux. We have previously demonstrated that 12/15LO enhances the turnover and serine phosphorylation of ABCG1. In the current study, we further elucidate the mechanisms by which 12/15LO regulates ABCG1. Proteasomal inhibitors blocked the down-regulation of ABCG1 expression and resulted in accumulation of phosphorylated ABCG1. Macrophages that lack 12/15LO have enhanced transporter expression, reduced ABCG1 phosphorylation, and increased cholesterol efflux. Conversely, macrophages that overexpress 12/15LO have reduced ABCG1 expression, increased transporter phosphorylation, and reduced cholesterol efflux. 12/15LO plays a key role in activating the MAPK pathway. Inhibition of the p38 or JNK pathways with pharmacological inhibitors or dominant negative constructs blocked 12S-hydroxyeicosatetranoic acid-mediated degradation of ABCG1. Moreover, we isolated macrophages from JNK1-, JNK2-, and MKK3-deficient mice to analyze the involvement of specific MAPK pathways. JNK2- and MKK3-, but not JNK1-deficient macrophages were resistant to the down-regulation of ABCG1 protein, reduction in efflux, and increase in serine phosphorylation by 12S-hydroxyeicosatetranoic acid. These findings provide evidence that 12/15LO regulates ABCG1 expression and function through p38- and JNK2-dependent mechanisms, and that targeting these pathways may provide novel approaches for regulating cholesterol homeostasis.

Atherosclerosis and osteoporosis: age-dependent degenerative processes or related entities?

Osteoporosis and atherosclerosis, two multifactorial and degenerative entities, are major public health problems. These diseases accompany the aging process and share common risk factors. Furthermore, several common pathophysiological factors have been suggested. These include similar molecular pathways involving bone and vascular mineralization, estrogen deficiency, parathyroid hormone, homocysteine, lipid oxidation products, inflammatory process, as well as vitamin D and K. Moreover, the use of statins, biphosphonates, beta-blockers and experimental dual-purpose therapies based on the biological linkage of the above entities may simultaneously benefit bone loss and vascular disease. This review considers a potential link between osteoporosis and atherosclerosis beyond aging. These common factors may lead to appropriate treatment strategies.

12/15-Lipoxygenase activity increases the degradation of macrophage ATP-binding cassette transporter G1

OBJECTIVE

The purpose of this study was to evaluate the effect of 12/15-lipoxygenase (12/15LO) in macrophage ABCG1 expression and function associated with cholesterol efflux.

METHODS AND RESULTS

12/15LO was stably overexpressed in J774 macrophages. 12/15LO-overexpressing macrophages had a 30% reduction in HDL-mediated cholesterol efflux, corresponding with significantly reduced ABCG1 protein expression. Treatment of 12/15LO-overexpressing macrophages with a 12/15LO ribozyme to reduce 12/15LO restored HDL-mediated efflux and ABCG1 protein expression. Treating macrophages with 12/15LO unsaturated fatty acid substrates or eicosanoid products also reduced HDL-mediated cholesterol efflux. Additionally, both 12/15LO overexpression in macrophages and incubation of macrophages with eicosanoids reduced ABCG1 protein, but not mRNA, expression. However, incubation of macrophages with linoleic or arachidonic acids significantly reduced both ABCG1 mRNA and protein expression, suggesting that 12/15LO substrates and eicosanoid products differentially regulate ABCG1 expression. 12/15LO fatty acids did not decrease ABCG1 translation; however, 12/15LO fatty acids increased ABCG1 degradation when blocked by cyclohexidmide. ABCG1 degradation may be regulated through posttranslational modifications. Treatment with the 12/15LO eicosanoid product 12SHETE increased serine phosphorylation of ABCG1.

CONCLUSIONS

We conclude that serine phosphorylation may increase the degradation rate of ABCG1, and as a result cause macrophage cholesterol accumulation. These findings provide evidence that 12/15LO activity in the vessel wall contributes to atherogenesis by impairing the macrophage ABCG1 cholesterol efflux pathway.

Oxygenation of arachidonoyl lysophospholipids by lipoxygenases from soybean, porcine leukocyte, or rabbit reticulocyte

Oxygenation of arachidonoyl lysophosphatidylcholine (lysoPC) or arachidonoyl lysophosphatidic acid (lysoPA) by lipoxygenase (LOX) was examined. The oxidized products were identified by HPLC/UV spectrophotometry/mass spectrometry analyses. Straight-phase and chiral-phase HPLC analyses indicated that soybean LOX-1 and rabbit reticulocyte LOX oxygenated arachidonoyl lysophospholipids mainly at C-15 with the S form as major enantiomer, whereas porcine leukocyte LOX oxygenated at C-12 with the S form. Next, the sequential exposure of arachidonoyl-lysoPC to soybean LOX-1 and porcine leukocyte LOX afforded two major isomers of dihydroxy derivatives with conjugated triene structure, suggesting that 15(S)-hydroperoxyeicosatetraenoyl derivatives were converted to 8,15(S)-dihydroxyeicosatetraenoyl derivatives. Separately, arachidonoyl-lysoPA, but not arachidonoyl-lysoPC, was found to be susceptible to double oxygenation by soybean LOX-1 to generate a dihydroperoxyeicosatetraenoyl derivative. Overall, arachidonoyl lysophospholipids were more efficient than arachidonic acid as LOX substrate. Moreover, the catalytic efficiency of arachidonoyl-lysoPC as substrate of three lipoxygenases was much greater than that of arachidonoyl-lysoPA or arachidonic acid. Taken together, it is proposed that arachidonoyl-lysoPC or arachidonoyl-lysoPA is efficiently oxygenated by plant or animal lipoxygenases, C12- or C15-specific, to generate oxidized products with conjugated diene or triene structure.

Oxygenation of 1-docosahexaenoyl lysophosphatidylcholine by lipoxygenases; conjugated hydroperoxydiene and dihydroxytriene derivatives

Oxygenation of 1-docosahexaenoyl lysophosphatidylcholine (docosahexaenoyl-lysoPC) by soybean lipoxygenase-1 (LOX-1) or porcine leukocyte LOX was examined. The oxidized products were identified to be hydroperoxydocosahexaenoyl-lysoPC by UV and LC/MS spectrometric analyses. In SP-HPLC and chiral phase-HPLC analyses, the products from the oxygenation of docosahexaenoyl-lysoPC by soybean LOX-1 and porcine leukocyte LOX were found to contain hydroperoxide group mainly at C-17 and C-14, respectively with the S form as a major enantiomer. Next, the sequential exposure of docosahexaenoyl-lysoPC to soybean LOX-1 and porcine leukocyte LOX led to the formation of conjugated triene derivatives possessing a maximal absorption at 271 nm with shoulders at 262 and 281 nm. Based on MS-MS analysis, the conjugated triene derivatives were identified to be 10,17- or 16,17-dihydroxydocosahexaenoyl-lysoPC analogues, suggesting that the diols were produced mainly from hydrolysis of 16,17(S)-epoxide intermediate. In kinetic studies, docosahexaenoyl-lysoPC was more favorable than docosahexaenoic acid as substrate for soybean LOX-1 or leukocyte LOX. Taken together, it is proposed that docosahexaenoyl-lysoPC can be oxygenated as substrates for some lipoxygenases to form conjugated diene and/or triene derivatives.

Linoleoyl lysophosphatidic acid and linoleoyl lysophosphatidylcholine are efficient substrates for mammalian lipoxygenases

Oxygenation of two lysophospholipids, 1-linoleoyl lysophosphatidylcholine (linoleoyl-lysoPC) and 1-linoleoyl lysophosphatidic acid (linoleoyl-lysoPA), by reticulocyte lipoxygenase (LOX) or porcine leukocyte LOX was measured by monitoring the formation of conjugated dienes. Consistent with the above, the formation of linoleoyl-lysophospholipid hydroperoxide as oxygenation product was confirmed by LC/MS analyses. In further study, the oxygenation products of linoleoyl-lysoPC or linoleoyl-lysoPA were found to contain hydroperoxide group predominantly at C-13 with the S enantiomer as a major one, in a good agreement with the positional-specificity and stereo-selectivity of reticulocyte LOX or leukocyte LOX in oxygenation of linoleic acid. The kinetic study indicates that linoleoyl-lysoPA and linoleoyl-lysoPC are no less efficient than linoleic acid as substrates of reticulocyte LOX as well as leukocyte LOX. In contrast, these lysophospholipids were not oxygenated efficiently by potato LOX. Thus, linoleoyl-lysophospholipids such as linoleoyl-lysoPA or linoleoyl-lysoPC could be utilized as efficient substrates for some mammalian lipoxygenases.

Activated Platelets and Monocytes Generate Four Hydroxyphosphatidylethanolamines via Lipoxygenase

12/15-Lipoxygenase (LOX) mediates immune-regulatory activities not accounted for by its known free acid eicosanoids, suggesting that additional lipids may be generated by activated cells. To characterize novel LOX-derived lipids, a lipidomic approach was utilized. Ionophore-activated interleukin-4-treated human peripheral monocytes generated up to 10-fold more esterified 15-hydroxyeicosatetraenoic acid (15-HETE) than free in a phosphatidylinositol 3-kinase- and protein kinase C-sensitive manner. Precursor scanning electrospray ionization/tandem spectroscopy for m/z 319 (HETE, [M-H](-)) showed 4 ions at m/z 738, 764, 766, and 782 that were identified using tandem spectroscopy and MS3 as specific diacyl and plasmalogen 15-HETE phosphatidylethanolamines. Using H (18)(2)O water, the compounds were shown to form by direct oxidation of endogenous phosphatidylethanolamine (PE) by 15-LOX, with PE being the preferred phospholipid pool containing 15-HETE. Similarly, human platelets generated 4 analogous PE lipids that contained 12-HETE and increased significantly in response to ionophore, collagen, or convulxin. These products were retained in the cells, in contrast to free acids, which are primarily secreted. Precursor scanning of platelet extracts for the major platelet-derived prostanoid, thromboxane B2 (m/z 369.2), did not reveal PE esters, indicating that this modification is restricted to the LOX pathway. In summary, we show formation of PE-esterified HETEs in immune cells that may contribute to LOX signaling in inflammation.

Paradoxical increase in LDL oxidation by endothelial cells from an atherosclerosis-resistant mouse strain

Oxidative modification of LDL accumulated in the subendothelial space is a critical step in atherogenesis. Mouse strains C57BL/6 (B6) and BALB/c differ markedly in atherosclerosis susceptibility. We sought to determine whether variation of endothelial cells in the capacity to oxidize LDL or in response to minimally modified LDL (MM-LDL) constitutes a genetic component in atherosclerosis. LDL oxidation was assessed by measuring thiobarbituric acid-reactive substance (TBARS) production. Responses to MM-LDL were evaluated by examining induction of monocyte chemotactic protein-1, macrophage-colony stimulating factor, and vascular cell adhesion molecule-1. Both strains exhibited comparable endothelial responses to MM-LDL, whereas BALB/c mice had an increased rate of oxidizing LDL compared with B6 mice. To examine whether endothelial nitric oxide synthase (eNOS) contributed to the difference in LDL oxidation, cells were incubated with native LDL in the presence or absence of N(Omega)-nitro-l-arginine methyl ester (l-NAME), a specific NOS inhibitor. Although l-NAME significantly inhibited endothelial cell-mediated LDL oxidation, it failed to abolish the difference between the strains. In contrast, Baicalein, a specific 12/15 lipoxygenase inhibitor, abolished the difference in LDL oxidation. Thus, the paradoxical increase in LDL oxidation by endothelial cells is attributable to higher oxidant activity of 12/15-lipoxygenase in BALB/c mice and endothelial cells appear unlikely to be a source of the resistance to atherosclerosis.

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.

Absence of 12/15 lipoxygenase reduces brain oxidative stress in apolipoprotein E-deficient mice

The enzyme 12/15 lipoxygenase (12/15LO) has been implicated in the oxidative modification of lipoproteins and phospholipids in vivo. In addition, mice deficient in apolipoprotein E (ApoE-/-) are characterized by spontaneous hypercholesterolemia and a systemic increase in oxidative stress. Whereas the absence of 12/15LO reduces lipid peroxidation in the plasma and urine of ApoE-/- mice, the relative contribution of this enzyme to oxidative stress in the central nervous system remains unknown. Here, we provide the first in vivo evidence that 12/15LO modulates brain oxidative stress reactions using ApoE-/- mice crossbred with 12/15LO-deficient (12/15LO-/-) mice (12/15LO-/-/ApoE-/-). In chow-fed 12-month-old 12/15LO-/-/ApoE-/- mice, the amount of brain isoprostane iPF2alpha-VI, a marker of lipid peroxidation, and carbonyls, markers of protein oxidation, were significantly reduced when compared with 12/15LO-expressing controls (12/15LO+/+/ApoE-/-). These results were observed despite the fact that cholesterol, triglyceride, and lipoprotein levels were similar to those of ApoE-/- mice. These data indicate a functional role for 12/15LO in the modulation of oxidative reactions in the central nervous system, supporting the hypothesis that inhibition of this enzymatic pathway may be a novel therapeutic target in clinical settings involving increased brain oxidative stress.

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.

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.

Regio- and stereo-chemical oxidation of linoleic acid by human myoglobin and hydrogen peroxide: Tyr(103) affects rate and product distribution

Mb (myoglobin) plus H2O2 catalyses the oxidation of various substrates via a peroxidase-like activity. A Y103F (Tyr103-->Phe) variant of human Mb has been constructed to assess the effect of exchanging an electron-rich oxidizable amino acid on the peroxidase activity of human Mb. Steady-state analyses of reaction mixtures containing Y103F Mb, purified linoleic acid and H2O2 revealed a lower total yield of lipid oxidation products than mixtures containing the wild-type protein, consistent with the reported decrease in the rate constant for reaction of Y103F Mb with H2O2 [Witting, Mauk and Lay (2002) Biochemistry 41, 11495-11503]. Irrespective of the Mb employed, lipid oxidation yielded 9(R/S)-HODE [9(R,S)-hydroxy-10E,12Z-octadecadienoic acid] in preference to 13(R/S)-HODE [13(R,S)-hydroxy-9Z,11E-octadecadienoic acid], while 9- and 13-keto-octadecadienoic acid were formed in trace amounts. However, lipid oxidation by the Y103F variant of Mb proceeded with a lower V(max) value and an increased K(m) value relative to the wild-type control. Consistent with the increased K(m), the product distribution from reactions with Y103F Mb showed decreased selectivity compared with the wild-type protein, as judged by the decreased yield of 9(S)-relative to 9(R)-HODE. Together, these data verify that Tyr103 plays a significant role in substrate binding and orientation in the haem pocket of human Mb. Also, the midpoint potential for the Fe(III)/(II) one-electron reduction was shifted slightly, but significantly, to a higher potential, confirming the importance of Tyr103 to the hydrogen-bonding network involving residues that line the haem crevice of human Mb.

LDL phospholipid hydrolysis produces modified electronegative particles with an unfolded apoB-100 protein

Electronegative low density lipoprotein (LDL(-)) formation that structurally resembles LDL(-) isolated from plasma was evaluated after LDL treatment with snake venom phospholipase A(2) (PLA(2)). PLA(2) treatment of LDL increased its electrophoretic mobility in proportion to the amount of LDL(-) formed without evidence of lipid peroxidation. These changes dose-dependently correlated with the degree of phospholipid hydrolysis. Strong immunoreactivity of LDL(-) subfraction from plasma and PLA(2)-treated LDL (PLA(2)-LDL) to amyloid oligomer-specific antibody was observed. Higher beta-strand structural content and unfolding proportionate to the loss of alpha-helical structure of apolipoprotein B-100 (apoB-100) of LDL(-) isolated from both native and PLA(2)-LDLs was demonstrated by circular dichroism (CD) spectropolarimetry. These structural changes resembled the characteristics of some oxidatively modified LDLs and soluble oligomeric aggregates of amyloidogenic proteins. PLA(2)-LDL was also more susceptible to nitration by peroxynitrite, likely because of exposure of otherwise inaccessible hydrophilic and hydrophobic domains arising from apoB-100 unfolding. This was also demonstrated for plasma LDL(-). In contrast, PLA(2)-LDL was more resistant to copper-mediated oxidation that was reversed upon the addition of small amounts of unsaturated fatty acids. The observed similarities between PLA(2)-LDL(-)-derived LDL(-) and plasma LDL(-) implicate a role for secretory PLA(2) in producing modified LDL(-) that is facilitated by unfolding of apoB-100.

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.

Cholesteryl ester oxidation products in atherosclerosis

Lipid oxidation products are formed at sites of increased oxidant stress and have been shown to accumulate in atherosclerotic lesions. Although recent studies have focused on the formation and metabolism of oxidized lipids, very little is known about their biological activities and possible (patho)physiological functions. Oxidation of cholesteryl esters containing unsaturated fatty acids leads to the formation of hydroperoxides that are either reduced to alcohols or degrade into biologically active "core-aldehydes". In this review, the mechanisms of formation and metabolic fate of oxidized cholesteryl esters, their occurrence, as well as possible biological activities are discussed. Based on the current knowledge, cholesteryl ester oxidation leads to the formation of biologically active substances, which could actively contribute to the progression of atherosclerotic lesions and their resulting complications.

Activation of NAD(P)H oxidase by lipid hydroperoxides: mechanism of oxidant-mediated smooth muscle cytotoxicity

Oxidized lipids, such as 13-hydroperoxyoctadecadienoic acid (13-HPODE), have been implicated in the pathogenesis of atherosclerosis. 13-HPODE, a constituent of oxidized low-density lipoproteins, can induce cytotoxicity of vascular smooth muscle cells (SMC), which may facilitate plaque destabilization and/or rupture. 13-HPODE-induced cytotoxicity has been linked to oxidative stress, although the mechanisms by which this occurs are unknown. In the present study, we show that 13-HPODE and 9-HPODE (10-30 microM) increased superoxide (O2*-) production and induced cytotoxicity in SMC. The 13-HPODE-induced increase in O2*- was blocked by transfecting the cells with antisense oligonucleotides against p22phox, suggesting that the O2*- was produced by NAD(P)H oxidase. Similar concentrations of the corresponding HPODE reduction products, 13-hydroxyoctadecadienoic acid (13-HODE) and 9-HODE, neither increased O2*- production nor induced cytotoxicity, while 4-hydroxy nonenal (4-HNE), an unsaturated aldehyde lipid peroxidation product, induced cytotoxicity without increasing O2*- production. Treatment with superoxide dismutase or Tiron to scavenge O2*-, or transfection with p22phox antisense oligonucleotides to inhibit O2*- production, attenuated 13-HPODE-induced cytotoxicity, but not that induced by 4-HNE. These findings suggest that activation of NAD(P)H oxidase, and production of O2*-, play an important role in lipid hydroperoxide-induced smooth muscle cytotoxicity.

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.

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.

Hepoxilin B3 and its enzymatically formed derivative trioxilin B3 are incorporated into phospholipids in psoriatic lesions

In previous studies we observed that normal human epidermis forms 12-oxo-eicosatetraenoic acid (12-oxo-ETE) and hepoxilin B3 (HxB3) as major eicosanoids, both being elevated in psoriasis. We also observed that normal epidermis, in a reaction probably catalyzed by 12-lipoxygenase, only synthesize one of the two possible 10-hydroxy epimers of HxB3. We have now extended these previous studies investigating further transformation of HxB3 into trioxilin B3 (TrXB3) and esterification of both into phospholipids. Phospholipids were extracted from normal epidermis and from psoriatic scales. A combination of high performance liquid chromatography and gas chromatography-mass spectrometry analysis demonstrated the occurrence of HxB3 and TrXB3 in the phospholipids of psoriatic lesions. Alkaline- and phospholipase-A2-mediated hydrolysis of the phospholipids yielded similar quantities of both HxB3 and TrXB3 indicating their preference for the sn-2 position of glycerophospholipids. The thin layer chromatography analysis of the phospholipid classes after incubation of epidermal cells with [14C]-labeled HxB3, TrXB3, 12-hydroxy-eicosatetraenoic acid (12-HETE), 12-oxo-ETE, or 15-HETE showed that 12-HETE was the most esterified (12-HETE >15-HETE > TrXB3 > 12-oxo-ETE > HxB3). HxB3 and TrXB3 were mainly esterified in phosphatidyl-choline and phosphatidyl-ethanolamine. HxB3 was also enzymatically converted into TrXB3 in vitro. HxB3 epoxide hydrolase-like activity was not observed when boiled tissue was incubated with [14C]-HxB3, this activity being located in the cytosol fraction (100,000 x g supernatant) of fresh tissue. These findings suggest that in vivo some part of HxB3 is transformed into TrXB3 and both compounds are partially incorporated into the phospholipids.

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.

Effect of vitamin E on aortic lipid oxidation and intimal proliferation after arterial injury in cholesterol-fed rabbits

Oxidized low-density lipoproteins (LDL) are implicated in atherosclerosis. However, large-scale intervention studies designed to test whether antioxidants, such as vitamin E, can ameliorate cardiovascular disease have generated ambivalent results. This may relate to the fact that the mechanism whereby lipid oxidation is initiated in vivo is unknown and the lack of direct evidence for a deficiency of antioxidants in atherosclerotic lesions. Further, there is little evidence to suggest that vitamin E acts as an antioxidant for lipid peroxidation in vivo. Here we tested the antioxidant effect of dietary vitamin E (alpha-tocopherol) supplementation on intimal proliferation and lipid oxidation in balloon-injured, hypercholesterolemic rabbits. alpha-Tocopherol supplementation increased vascular content of alpha-tocopherol over 30-fold compared to nonsupplemented and alpha-tocopherol-deficient chows. Balloon injury resulted in oxidized lipid deposition in the aorta. Maximum levels of primary lipid oxidation products, measured as hydroperoxides of esterified lipid (LOOH) and oxidized linoleate (HODE), were 0.22 and 1.10 nmol/mg, representing 0.21 and 0.39% of the precursor molecule, respectively. Secondary lipid oxidation products, measured as oxysterols, were maximal at 5.60 nmol/mg or 1.48% of the precursor compound. Vascular HODE and oxysterols were significantly reduced by vitamin E supplementation. However, the intima/media ratio of aortic vessels increased with vitamin E supplementation, suggesting that the antioxidant promoted intimal proliferation. Thus, the study demonstrates a dissociation of aortic lipid oxidation and lesion development, and suggests that vitamin E does not prevent lesion development in this animal model.

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.