Formation of 15-HETE as a major hydroxyeicosatetraenoic acid in the atherosclerotic vessel wall

Characterization of lipoproteins and associated lipidome in very preterm infants: a pilot study

BACKGROUND

Preterm birth is associated with higher risks of suboptimal neurodevelopment and cardiometabolic disease later in life. Altered maternal-fetal lipid supply could play a role in such risks. Our hypothesis was that very preterm infants born with very low birth weight (VLBW) have altered lipidome and apolipoprotein profiles, compared with term infants.

METHODS

Seven mothers of VLBW infants born at <32 GA and 8 full-term mother-infant dyads were included. Cholesterol and triglycerides in lipoproteins were determined in maternal plasma and in the two blood vessels of the umbilical cord (vein (UV) and artery (UA)) following FPLC isolation. Apolipoprotein concentrations in lipoproteins and plasma lipidomic analysis were performed by LC-MS/MS.

RESULTS

We found higher cholesterol and VLDL-cholesterol in UV and UA and lower apolipoprotein A-I in HDL2 in UV in preterm neonates. Phosphatidylcholine (PC) containing saturated and monounsaturated fatty acids and specific sphingomyelin species were increased in UV and UA, whereas PC containing docosahexaenoic acid (DHA) was reduced in UV of VLBW neonates.

CONCLUSIONS

Lower DHA-PC suggests a lower DHA bioavailability and may contribute to the impaired neurodevelopment. Altered HDL-2, VLDL, and sphingomyelin profile reflect an atherogenic risk and increased metabolic risk at adulthood in infants born prematurely.

IMPACT

Lower ApoA-I in HDL2, and increased specific sphingomyelin and phosphatidylcholine containing saturated and monounsaturated fatty acid could explain the accumulation of cholesterol in umbilical vein in VLBW preterm neonates. Decreased phosphatidylcholine containing DHA suggest a reduced DHA availability for brain development in VLBW preterm infants. Characterization of alterations in fetal lipid plasma and lipoprotein profiles may help to explain at least in part the causes of the elevated cardiovascular risk known in people born prematurely and may suggest that a targeted nutritional strategy based on the composition of fatty acids carried by phosphatidylcholine may be promising in infants born very early.

An Overview of Circulating Pulmonary Arterial Hypertension Biomarkers

Pulmonary arterial hypertension (PAH), also known as Group 1 Pulmonary Hypertension (PH), is a PH subset characterized by pulmonary vascular remodeling and pulmonary arterial obstruction. PAH has an estimated incidence of 15-50 people per million in the United States and Europe, and is associated with high mortality and morbidity, with patients' survival time after diagnosis being only 2.8 years. According to current guidelines, right heart catheterization is the gold standard for diagnostic and prognostic evaluation of PAH patients. However, this technique is highly invasive, so it is not used in routine clinical practice or patient follow-up. Thereby, it is essential to find new non-invasive strategies for evaluating disease progression. Biomarkers can be an effective solution for determining PAH patient prognosis and response to therapy, and aiding in diagnostic efforts, so long as their detection is non-invasive, easy, and objective. This review aims to clarify and describe some of the potential new candidates as circulating biomarkers of PAH.

Suppression of inflammatory arthritis in human serum paraoxonase 1 transgenic mice

Paraoxonase 1(PON1) is an HDL-associated protein, which metabolizes inflammatory, oxidized lipids associated with atherosclerotic plaque development. Because oxidized lipid mediators have also been implicated in the pathogenesis of rheumatoid arthritis (RA), we evaluated the role of PON1 in murine inflammatory arthritis. K/BxN serum transfer (STIA) or collagen antibody transfer (CAIA) was used for arthritis induction in B6 mice homozygous for the PON1 human transgene [PON1Tg], PON1 knock-out mice [PON1KO], and wild type littermate control mice [WT]. Experiments were also performed in K/BxN mice with chronic arthritis, and in RA patients and healthy controls. Arthritis activity in K/BxN mice was associated with a marked dyslipidemia, lower PON1 activity and higher bioactive lipid mediators (BLM), as well as a dysregulated hepatic lipid gene expression profile. Higher serum PON1 activity correlated with lower BLM and lower arthritis activity in both K/BxN mice and RA patients. Overexpression of the human PON1 transgene was associated with reduced inflammatory arthritis, which correlated strongly with higher circulating PON1 activity, upregulation of the hepatic glutathione pathway, and reduction of circulating BLM. These results implicate PON1 as a potential novel therapeutic target for joint disease in RA with potential for vascular benefit, which warrants further investigation.

Oral 15-Hydroxyeicosatetraenoic Acid Induces Pulmonary Hypertension in Mice by Triggering T Cell-Dependent Endothelial Cell Apoptosis

Pulmonary arterial hypertension (PAH) is a fatal disease characterized by increased mean pulmonary arterial pressure. Elevated plasma and lung concentrations of oxidized lipids, including 15-hydroxyeicosatetraenoic acid (15-HETE), have been demonstrated in patients with PAH and animal models. We previously demonstrated that feeding mice with 15-HETE is sufficient to induce pulmonary hypertension, but the mechanisms remain unknown. RNA sequencing data from the mouse lungs on 15-HETE diet revealed significant activation of pathways involved in both antigen processing and presentation and T cell-mediated cytotoxicity. Analysis of human microarray from patients with PAH also identified activation of identical pathways compared with controls. We show that in both 15-HETE-fed mice and patients with PAH, expression of the immunoproteasome subunit 5 is significantly increased, which was concomitant with an increase in the number of CD8/CD69 (cluster of differentiation 8 / cluster of differentiation 69) double-positive cells, as well as pulmonary arterial endothelial cell apoptosis in mice. Human pulmonary arterial endothelial cells cultured with 15-HETE were more prone to apoptosis when exposed to CD8 cells. Cultured intestinal epithelial cells secreted more oxidized lipids in response to 15-HETE, which is consistent with accumulation of circulating oxidized lipids in 15-HETE-fed mice. Administration of an apoA-I (apolipoprotein A-I) mimetic peptide, Tg6F (transgenic 6F), which is known to prevent accumulation of circulating oxidized lipids, not only inhibited pulmonary arterial endothelial cell apoptosis but also prevented and rescued 15-HETE-induced pulmonary hypertension in mice. In conclusion, our results suggest that (1) 15-HETE diet induces pulmonary hypertension by a mechanism that involves oxidized lipid-mediated T cell-dependent pulmonary arterial endothelial cell apoptosis and (2) Tg6F administration may be a novel therapy for treating PAH.

Seeking Optimal Nutrition for Healthy Body Mass Reduction among Former Athletes

The aim of the study was to investigate the efficacy of 6 week Mediterranean diet or 30% calorie restriction on the fatty acid profile and eicosanoids (hydroxyoctadecadienoi acids and hydroxyeicosatetraenoic acids) concentration. Furthermore, basic biochemical variables such as insulin, glucose, HOMA-IR, and a lipid profile were estimated. The study enrolled 94 Caucasian former athletes aged 20-42, with body height of 179 ± 16.00 cm and body mass of 89.26 ± 13.25 kg who had not been active for at least 5 years. The subjects were randomly assigned to one of the three intervention groups: CR group – the 30% calorie restriction (n = 32), MD group - the Mediterranean diet (n = 34), and C group - a control group (n = 28). The pattern of nutrition was analysed before and after the experiment using the 72 h food diaries. In order to evaluate the effect of diet intervention, the following variables were measured: anthropometrics, basic biochemical variables (insulin, fasting glucose, HOMA-IR, lipid profile), fatty acids and their blood derivatives profiles. The CR group showed significantly lower levels of several biochemical variables, i.e., BMI, total cholesterol LDL, TG, total lipids, insulin and HOMA – IR (p < 0.05). Subjects consuming the MD diet significantly decreased their BMI and reduced the level of total lipids (p < 0.05). We did not find any significant changes in the C group. The analysis of the fatty acid profile revealed that the CR group had a significantly decreased EPA level (p < 0.05). The MD group showed a significantly increased level of the DHA (p < 0.05) and improvement in the omega - 3 index (p < 0.05). Subjects following the MD also showed significantly lower concentrations of 15 - hydroxyicosatetraenoic acid (15-HETE). We did not observe any significant differences between the CR and C groups. Within short time, calorie restriction helps to improve lipid variables and insulin resistance. The MD diet seems to be more advantageous in the decrease of inflammation, but does not improve basic biochemical variables. We can conclude that calorie restriction can be a good choice for former athletes, although EPA and DHA supplementation is needed.

Heterodimers of the transcriptional factors NFATc3 and FosB mediate tissue factor expression for 15(S)-hydroxyeicosatetraenoic acid-induced monocyte trafficking

Tissue factor (TF) is expressed in vascular and nonvascular tissues and functions in several pathways, including embryonic development, inflammation, and cell migration. Many risk factors for atherosclerosis, including hypertension, diabetes, obesity, and smoking, increase TF expression. To better understand the TF-related mechanisms in atherosclerosis, here we investigated the role of 12/15-lipoxygenase (12/15-LOX) in TF expression. 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), the major product of human 15-LOXs 1 and 2, induced TF expression and activity in a time-dependent manner in the human monocytic cell line THP1. Moreover, TF suppression with neutralizing antibodies blocked 15(S)-HETE-induced monocyte migration. We also found that NADPH- and xanthine oxidase-dependent reactive oxygen species (ROS) production, calcium/calmodulin-dependent protein kinase IV (CaMKIV) activation, and interactions between nuclear factor of activated T cells 3 (NFATc3) and FosB proto-oncogene, AP-1 transcription factor subunit (FosB) are involved in 15(S)-HETE-induced TF expression. Interestingly, NFATc3 first induced the expression of its interaction partner FosB before forming the heterodimeric NFATc3-FosB transcription factor complex, which bound the proximal AP-1 site in the TF gene promoter and activated TF expression. We also observed that macrophages from 12/15-LOX-/- mice exhibit diminished migratory response to monocyte chemotactic protein 1 (MCP-1) and lipopolysaccharide compared with WT mouse macrophages. Similarly, compared with WT macrophages, monocytes from 12/15-LOX-/- mice displayed diminished trafficking, which was rescued by prior treatment with 12(S)-HETE, in a peritonitis model. These observations indicate that 15(S)-HETE-induced monocyte/macrophage migration and trafficking require ROS-mediated CaMKIV activation leading to formation of NFATc3 and FosB heterodimer, which binds and activates the TF promoter.

Comparative systems toxicology analysis of cigarette smoke and aerosol from a candidate modified risk tobacco product in organotypic human gingival epithelial cultures: A 3-day repeated exposure study

Smoking is one of the major lifestyle-related risk factors for periodontal diseases. Modified risk tobacco products (MRTP) offer a promising alternative in the harm reduction strategy for adult smokers unable to quit. Using a systems toxicology approach, we investigated and compared the exposure effects of a reference cigarette (3R4F) and a heat-not-burn technology-based candidate MRTP, the Tobacco Heating System (THS) 2.2. Human gingival epithelial organotypic cultures were repeatedly exposed (3 days) for 28 min at two matching concentrations of cigarette smoke (CS) or THS2.2 aerosol. Results showed only minor histopathological alterations and minimal cytotoxicity upon THS2.2 aerosol exposure compared to CS (1% for THS2.2 aerosol vs. 30% for CS, at the high concentration). Among the 14 proinflammatory mediators analyzed, only 5 exhibited significant alterations with THS2.2 exposure compared with 11 upon CS exposure. Transcriptomic and metabolomic analysis indicated a general reduction of the impact in THS2.2 aerosol-exposed samples with respect to CS (∼79% lower biological impact for the high THS2.2 aerosol concentration compared to CS, and 13 metabolites significantly perturbed for THS2.2 vs. 181 for CS). This study indicates that exposure to THS2.2 aerosol had a lower impact on the pathophysiology of human gingival organotypic cultures than CS.

Alterations in Serum Polyunsaturated Fatty Acids and Eicosanoids in Patients with Mild to Moderate Chronic Obstructive Pulmonary Disease (COPD)

Smoking is a major risk factor for several diseases including chronic obstructive pulmonary disease (COPD). To better understand the systemic effects of cigarette smoke exposure and mild to moderate COPD-and to support future biomarker development-we profiled the serum lipidomes of healthy smokers, smokers with mild to moderate COPD (GOLD stages 1 and 2), former smokers, and never-smokers (n = 40 per group) (ClinicalTrials.gov registration: NCT01780298). Serum lipidome profiling was conducted with untargeted and targeted mass spectrometry-based lipidomics. Guided by weighted lipid co-expression network analysis, we identified three main trends comparing smokers, especially those with COPD, with non-smokers: a general increase in glycero(phospho)lipids, including triglycerols; changes in fatty acid desaturation (decrease in ω-3 polyunsaturated fatty acids, and an increase in monounsaturated fatty acids); and an imbalance in eicosanoids (increase in 11,12- and 14,15-DHETs (dihydroxyeicosatrienoic acids), and a decrease in 9- and 13-HODEs (hydroxyoctadecadienoic acids)). The lipidome profiles supported classification of study subjects as smokers or non-smokers, but were not sufficient to distinguish between smokers with and without COPD. Overall, our study yielded further insights into the complex interplay between smoke exposure, lung disease, and systemic alterations in serum lipid profiles.

Reactive Oxygen Species (ROS) Mediate p300-dependent STAT1 Protein Interaction with Peroxisome Proliferator-activated Receptor (PPAR)-γ in CD36 Protein Expression and Foam Cell Formation

Previously, we have demonstrated that 15(S)-hydroxyeicosatetranoic acid (15(S)-HETE) induces CD36 expression involving STAT1. Many studies have shown that peroxisome proliferator-activated receptor (PPAR)-γ mediates CD36 expression. Therefore, we asked the question whether these transcriptional factors interact with each other in the regulation of CD36 expression by 15(S)-HETE. Here, we show that STAT1 interacts with PPARγ in the induction of CD36 expression and foam cell formation by 15(S)-HETE. In addition, using molecular biological approaches such as EMSA, supershift EMSA, ChIP, re-ChIP, and promoter-reporter gene assays, we demonstrate that the STAT1 and PPARγ complex binds to the STAT-binding site at -107 nucleotides in the CD36 promoter and enhances its activity. Furthermore, the interaction of STAT1 with PPARγ depends on STAT1 acetylation, which is mediated by p300. In addition, our findings show that reactive oxygen species-dependent Syk and Pyk2 stimulation is required for p300 tyrosine phosphorylation and activation. Together, these results demonstrate that an interaction between STAT1, p300, and peroxisome proliferator-activated receptor-γ is required for 15(S)-HETE-induced CD36 expression, oxidized low density lipoprotein uptake, and foam cell formation, critical events underlying the pathogenesis of atherosclerosis.

12/15-Lipoxygenase-dependent ROS production is required for diet-induced endothelial barrier dysfunction

To understand the mechanisms of 15(S)-HETE-induced endothelial cell (EC) barrier dysfunction, we examined the role of xanthine oxidase (XO). 15(S)-HETE induced junction adhesion molecule A (JamA) phosphorylation on Y164, Y218, and Y280 involving XO-mediated reactive oxygen species production and Src and Pyk2 activation, resulting in its dissociation from occludin, thereby causing tight junction (TJ) disruption, increased vascular permeability, and enhanced leukocyte and monocyte transmigration in vitro using EC monolayer and ex vivo using arteries as models. The phosphorylation of JamA on Y164, Y218, and Y280 appears to be critical for its role in 15(S)-HETE-induced EC barrier dysfunction, as mutation of any one of these amino acid residues prevented its dissociation from occludin and restored TJ integrity and barrier function. In response to high-fat diet (HFD) feeding, WT, but not 12/15-lipoxygenase (LO)(-/-), mice showed enhanced XO expression and its activity in the artery, which was correlated with increased aortic TJ disruption and barrier permeability with enhanced leukocyte adhesion and these responses were inhibited by allopurinol. These observations provide novel insights on the role of XO in 12/15-LO-induced JamA tyrosine phosphorylation and TJ disruption leading to increased vascular permeability in response to HFD.

ROS-dependent Syk and Pyk2-mediated STAT1 activation is required for 15(S)-hydroxyeicosatetraenoic acid-induced CD36 expression and foam cell formation

15(S)-Hydroxyeicosatetraenoic acid (15(S)-HETE), the major 15-lipoxygenase 1/2 (15-LO1/2) metabolite of arachidonic acid (AA), induces CD36 expression through xanthine oxidase and NADPH oxidase-dependent ROS production and Syk and Pyk2-dependent STAT1 activation. In line with these observations, 15(S)-HETE also induced foam cell formation involving ROS, Syk, Pyk2, and STAT1-mediated CD36 expression. In addition, peritoneal macrophages from Western diet-fed ApoE(-/-) mice exhibited elevated levels of xanthine oxidase and NADPH oxidase activities, ROS production, Syk, Pyk2, and STAT1 phosphorylation, and CD36 expression compared to those from ApoE(-/-):12/15-LO(-/-) mice and these events correlated with increased lipid deposits, macrophage content, and lesion progression in the aortic roots. Human atherosclerotic arteries also showed increased 15-LO1 expression, STAT1 phosphorylation, and CD36 levels as compared to normal arteries. Together, these findings suggest that 12/15-LO metabolites of AA, particularly 12/15(S)-HETE, might play a crucial role in atherogenesis by enhancing foam cell formation.

Biosynthesis, biological effects, and receptors of hydroxyeicosatetraenoic acids (HETEs) and oxoeicosatetraenoic acids (oxo-ETEs) derived from arachidonic acid

Arachidonic acid can be oxygenated by a variety of different enzymes, including lipoxygenases, cyclooxygenases, and cytochrome P450s, and can be converted to a complex mixture of oxygenated products as a result of lipid peroxidation. The initial products in these reactions are hydroperoxyeicosatetraenoic acids (HpETEs) and hydroxyeicosatetraenoic acids (HETEs). Oxoeicosatetraenoic acids (oxo-ETEs) can be formed by the actions of various dehydrogenases on HETEs or by dehydration of HpETEs. Although a large number of different HETEs and oxo-ETEs have been identified, this review will focus principally on 5-oxo-ETE, 5S-HETE, 12S-HETE, and 15S-HETE. Other related arachidonic acid metabolites will also be discussed in less detail. 5-Oxo-ETE is synthesized by oxidation of the 5-lipoxygenase product 5S-HETE by the selective enzyme, 5-hydroxyeicosanoid dehydrogenase. It actions are mediated by the selective OXE receptor, which is highly expressed on eosinophils, suggesting that it may be important in eosinophilic diseases such as asthma. 5-Oxo-ETE also appears to stimulate tumor cell proliferation and may also be involved in cancer. Highly selective and potent OXE receptor antagonists have recently become available and could help to clarify its pathophysiological role. The 12-lipoxygenase product 12S-HETE acts by the GPR31 receptor and promotes tumor cell proliferation and metastasis and could therefore be a promising target in cancer therapy. It may also be involved as a proinflammatory mediator in diabetes. In contrast, 15S-HETE may have a protective effect in cancer. In addition to GPCRs, higher concentration of HETEs and oxo-ETEs can activate peroxisome proliferator-activated receptors (PPARs) and could potentially regulate a variety of processes by this mechanism. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Vascular endothelial tight junctions and barrier function are disrupted by 15(S)-hydroxyeicosatetraenoic acid partly via protein kinase C ε-mediated zona occludens-1 phosphorylation at threonine 770/772

Disruption of tight junctions (TJs) perturbs endothelial barrier function and promotes inflammation. Previously, we have shown that 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), the major 15-lipoxygenase 1 (15-LO1) metabolite of arachidonic acid, by stimulating zona occludens (ZO)-2 tyrosine phosphorylation and its dissociation from claudins 1/5, induces endothelial TJ disruption and its barrier dysfunction. Here, we have studied the role of serine/threonine phosphorylation of TJ proteins in 15(S)-HETE-induced endothelial TJ disruption and its barrier dysfunction. We found that 15(S)-HETE enhances ZO-1 phosphorylation at Thr-770/772 residues via PKCε-mediated MEK1-ERK1/2 activation, causing ZO-1 dissociation from occludin, disrupting endothelial TJs and its barrier function, and promoting monocyte transmigration; these effects were reversed by T770A/T772A mutations. In the arteries of WT mice ex vivo, 15(S)-HETE also induced ZO-1 phosphorylation and endothelial TJ disruption in a PKCε and MEK1-ERK1/2-dependent manner. In line with these observations, in WT mice high fat diet feeding induced 12/15-lipoxygenase (12/15-LO) expression in the endothelium and caused disruption of its TJs and barrier function. However, in 12/15-LO(-/-) mice, high fat diet feeding did not cause disruption of endothelial TJs and barrier function. These observations suggest that the 12/15-LO-12/15(S)-HETE axis, in addition to tyrosine phosphorylation of ZO-2, also stimulates threonine phosphorylation of ZO-1 in the mediation of endothelial TJ disruption and its barrier dysfunction.

The transcription factor CREB enhances interleukin-17A production and inflammation in a mouse model of atherosclerosis

The enzyme 15-lipoxygenase (15-LO) plays a role in atherogenesis (also known as atherosclerosis), but the underlying mechanisms are unclear. We found that 15(S)-hydroxyeicosatetraenoic acid [15(S)-HETE], the major 15-LO-dependent metabolite of arachidonic acid, stimulated the production of reactive oxygen species (ROS) by monocytes through the xanthine oxidase-mediated activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. ROS production led to the Syk-, Pyk2-, and mitogen-activated protein kinase (MAPK)-dependent production of the proinflammatory cytokine interleukin-17A (IL-17A) in a manner that required the transcription factor CREB (cyclic adenosine monophosphate response element-binding protein). In addition, this pathway was required for the 15(S)-HETE-dependent migration and adhesion of monocytes to endothelial cells. Consistent with these observations, we found that peritoneal macrophages from apolipoprotein E-deficient (ApoE-/-) mice fed a high-fat diet (a mouse model of atherosclerosis) exhibited increased xanthine oxidase and NADPH oxidase activities; ROS production; phosphorylation of Syk, Pyk2, MAPK, and CREB; and IL-17A production compared to those from similarly fed ApoE-/-:12/15-LO-/- mice. These events correlated with increased lipid deposits and numbers of monocytes and macrophages in the aortic arches of ApoE-/- mice, which resulted in atherosclerotic plaque formation. Together, these observations suggest that 15(S)-HETE exacerbates atherogenesis by enhancing CREB-dependent IL-17A production and inflammation.

Role of 15-hydroxyeicosatetraenoic acid in hemozoin-induced lysozyme release from human adherent monocytes

Natural hemozoin (nHZ), a lipid-bound ferriprotoporphyrin IX crystal produced by Plasmodium parasites after hemoglobin catabolism, seriously compromises the functions of human monocytes, and 15-hydroxyeicosatetraenoic acid (15-HETE) and 4-hydroxynonenal (4-HNE), two nHZ lipoperoxidation products, have been related to such a functional impairment. nHZ was recently shown to promote inflammation-mediated lysozyme release from human monocytes through p38 mitogen-activated protein kinase- (MAPK)- and nuclear factor (NF)-κB-dependent mechanisms. This study aimed at identifying the molecule of nHZ lipid moiety that was responsible for these effects. Results showed that 15-HETE mimicked nHZ effects on lysozyme release, whereas 4-HNE did not. 15-HETE-enhanced lysozyme release was abrogated by anti-TNF-α and anti-IL-1β-blocking antibodies and mimicked by recombinant cytokines; on the contrary, MIP-1α/CCL3 was not involved as a soluble mediator of 15-HETE effects. Moreover, 15-HETE early activated p38 MAPK and NF-κB pathways by inducing p38 MAPK phosphorylation; cytosolic I-κBα phosphorylation and degradation; NF-κB nuclear translocation and DNA-binding. Inhibition of both routes through chemical inhibitors (SB203580, quercetin, artemisinin, and parthenolide) prevented 15-HETE-dependent lysozyme release. Collectively, these data suggest that 15-HETE plays a major role in nHZ-enhanced monocyte degranulation.

12/15-Lipoxygenase mediates high-fat diet-induced endothelial tight junction disruption and monocyte transmigration: a new role for 15(S)-hydroxyeicosatetraenoic acid in endothelial cell dysfunction

A convincing body of evidence suggests that 12/15-lipoxygenase (12/15-LO) plays a role in atherosclerosis. However, the mechanisms of its involvement in the pathogenesis of this disease are not clear. Therefore, the purpose of this study is to understand the mechanisms by which 12/15-LO mediates endothelial dysfunction. 15(S)-Hydroxyeicosatetraenoic acid (15(S)-HETE), the major 12/15-LO metabolite of arachidonic acid (AA), induced endothelial barrier permeability via Src and Pyk2-dependent zonula occluden (ZO)-2 tyrosine phosphorylation and its dissociation from the tight junction complexes. 15(S)-HETE also stimulated macrophage adhesion to the endothelial monolayer in Src and Pyk2-dependent manner. Ex vivo studies revealed that exposure of arteries from WT mice to AA or 15(S)-HETE led to Src-Pyk2-dependent ZO-2 tyrosine phosphorylation, tight junction disruption, and macrophage adhesion, whereas the arteries from 12/15-LO knock-out mice are protected from these effects of AA. Feeding WT mice with a high-fat diet induced the expression of 12/15-LO in the arteries leading to tight junction disruption and macrophage adhesion and deletion of the 12/15-LO gene disallowed these effects. Thus, the findings of this study provide the first evidence of the role of 12/15-LO and its AA metabolite, 15(S)-HETE, in high-fat diet-induced endothelial tight junction disruption and macrophage adhesion, the crucial events underlying the pathogenesis of atherosclerosis.

A possible role for 15-lipoxygenase in atherogenesis

It is well accepted that high levels of low-density lipoprotein (LDL) cholesterol in the plasma are associated with increased risk of atherosclerosis. The cellular and molecular mechanisms linking the two however, have not been fully resolved. One of the processes involved in atherogensis that has been intensively studied in this regard is the oxidation of LDL. Oxidation may convert LDL into an atherogenic form, which incites an inflammatory and proliferative response characteristic of the atherosclerotic lesion. One of the potential mediators in this process is the lipid peroxidating enzyme 15-lipoxygenase, which has been shown to be induced in the atherosclerotic lesion and is capable of oxidizing LDL. In this article, we review the motivation for looking at mechanisms of LDL oxidation and the proposed involvement of 15-lipoxygenase in the pathogenesis of the disease.

Macrophage 12/15 lipoxygenase expression increases plasma and hepatic lipid levels and exacerbates atherosclerosis

12/15 lipoxygenase (12/15LO) oxidizes polyunsaturated fatty acids (PUFAs) to form bioactive lipid mediators. The role of 12/15LO in atherosclerosis development remains controversial. We evaluated atherosclerosis development and lipid metabolism in 12/15LO-LDL receptor (LDLr) double knockout (DK) vs. LDLr knockout (SK) mice fed a PUFA-enriched diet to enhance production of 12/15LO products. Compared with SK controls, DK mice fed a PUFA-enriched diet had decreased plasma and liver lipid levels, hepatic lipogenic gene expression, VLDL secretion, and aortic atherosclerosis and increased VLDL turnover. Bone marrow transplantation and Kupffer cell ablation studies suggested both circulating leukocytes and Kupffer cells contributed to the lipid phenotype in 12/15LO-deficient mice. Conditioned medium from in vitro incubation of DK vs. SK macrophages reduced triglyceride secretion in McArdle 7777 hepatoma cells. Our results suggest that, in the context of dietary PUFA enrichment, macrophage 12/15LO expression adversely affects plasma and hepatic lipid metabolism, resulting in exacerbated atherosclerosis.

12/15-Lipoxygenase gene knockout severely impairs ischemia-induced angiogenesis due to lack of Rac1 farnesylation

To understand the mechanisms by which 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE) activates Rac1 in the induction of angiogenesis, we studied the role of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and αPix. 15(S)-HETE stimulated Rac1 in a sustained manner in human dermal microvascular endothelial cells (HDMVECs). Simvastatin, a potent inhibitor of HMG-CoA reductase, suppressed 15(S)-HETE-induced Rac1 activation in HDMVECs affecting their migration and tube formation. 15(S)-HETE by inducing HMG-CoA reductase expression caused increased farnesylation and membrane translocation of Rac1 where it became activated by Src-dependent αPix stimulation. Mevalonate rescued 15(S)-HETE-induced Rac1 farnesylation and membrane translocation in HDMVECs and the migration and tube formation of these cells from inhibition by simvastatin. Down-regulation of αPix inhibited 15(S)-HETE-induced HDMVEC migration and tube formation. Hind-limb ischemia induced Rac1 farnesylation and activation leading to increased angiogenesis and these effects were blocked by simvastatin and rescued by mevalonate in WT mice. In contrast, hind-limb ischemia failed to induce Rac1 farnesylation and activation as well as angiogenic response in 12/15-Lox(-/-) mice. Activation of Src and αPix were also compromised at least to some extent in 12/15-Lox(-/-) mice compared with WT mice in response to hind-limb ischemia. Together, these findings demonstrate for the first time that HMG-CoA reductase plays a determinant role in 12/15-Lox-induced angiogenesis.

15-Lipoxygenase-1-enhanced Src-Janus kinase 2-signal transducer and activator of transcription 3 stimulation and monocyte chemoattractant protein-1 expression require redox-sensitive activation of epidermal growth factor receptor in vascular wall remodeling

To understand the mechanisms by which 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE) activates signal transducer and activator of transcription 3 (STAT3), we studied the role of epidermal growth factor receptor (EGFR). 15(S)-HETE stimulated tyrosine phosphorylation of EGFR in a time-dependent manner in vascular smooth muscle cells (VSMCs). Interference with EGFR activation blocked 15(S)-HETE-induced Src and STAT3 tyrosine phosphorylation, monocyte chemoattractant protein-1 (MCP-1) expression and VSMC migration. 15(S)-HETE also induced tyrosine phosphorylation of Janus kinase 2 (Jak2) in VSMCs, and its inhibition substantially reduced STAT3 phosphorylation, MCP-1 expression, and VSMC migration. In addition, Src formed a complex with EGFR and Jak2, and its inhibition completely blocked Jak2 and STAT3 phosphorylation, MCP-1 expression, and VSMC migration. 15(S)-HETE induced the production of H(2)O(2) via an NADPH oxidase-dependent manner and its scavengers, N-acetyl cysteine (NAC) and catalase suppressed 15(S)-HETE-stimulated EGFR, Src, Jak2, and STAT3 phosphorylation and MCP-1 expression. Balloon injury (BI) induced EGFR, Src, Jak2, and STAT3 phosphorylation, and inhibition of these signaling molecules attenuated BI-induced MCP-1 expression and smooth muscle cell migration from the medial to the luminal surface resulting in reduced neointima formation. In addition, inhibition of EGFR blocked BI-induced Src, Jak2, and STAT3 phosphorylation. Similarly, interference with Src activation suppressed BI-induced Jak2 and STAT3 phosphorylation. Furthermore, adenovirus-mediated expression of dnJak2 also blocked BI-induced STAT3 phosphorylation. Consistent with the effects of 15(S)-HETE on the activation of EGFR-Src-Jak2-STAT3 signaling in VSMCs in vitro, adenovirus-mediated expression of 15-lipoxygenase 1 (15-Lox1) enhanced BI-induced EGFR, Src, Jak2, and STAT3 phosphorylation leading to enhanced MCP-1 expression in vivo. Blockade of Src or Jak2 suppressed BI-induced 15-Lox1-enhanced STAT3 phosphorylation, MCP-1 expression, and neointima formation. In addition, whereas dominant negative Src blocked BI-induced 15-Lox1-enhanced Jak2 phosphorylation, dnJak2 had no effect on Src phosphorylation. Together, these observations demonstrate for the first time that the 15-Lox1-15(S)-HETE axis activates EGFR via redox-sensitive manner, which in turn mediates Src-Jak2-STAT3-dependent MCP-1 expression leading to vascular wall remodeling.

AP-1 (Fra-1/c-Jun)-mediated induction of expression of matrix metalloproteinase-2 is required for 15S-hydroxyeicosatetraenoic acid-induced angiogenesis

To understand the involvement of matrix metalloproteinases (MMPs) in 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE)-induced angiogenesis, we have studied the role of MMP-2. 15(S)-HETE induced MMP-2 expression and activity in a time-dependent manner in human dermal microvascular endothelial cells (HDMVECs). Inhibition of MMP-2 activity or depletion of its levels attenuated 15(S)-HETE-induced HDMVEC migration, tube formation, and Matrigel plug angiogenesis. 15(S)-HETE also induced Fra-1 and c-Jun expression in a Rac1-MEK1-JNK1-dependent manner. In addition, 15(S)-HETE-induced MMP-2 expression and activity were mediated by Rac1-MEK1-JNK1-dependent activation of AP-1 (Fra-1/c-Jun). Cloning and site-directed mutagenesis of MMP-2 promoter revealed that AP-1 site proximal to the transcriptional start site is required for 15(S)-HETE-induced MMP-2 expression, and Fra-1 and c-Jun are the essential components of AP-1 that bind to MMP-2 promoter in response to 15(S)-HETE. Hind limb ischemia led to an increase in MEK1 and JNK1 activation and Fra-1, c-Jun, and MMP-2 expression resulting in enhanced neovascularization and recovery of blood perfusion in wild-type mice as compared with 12/15-Lox(-/-) mice. Together, these results provide the first direct evidence for a role of 12/15-Lox-12/15(S)-HETE axis in the regulation of ischemia-induced angiogenesis.

15(S)-hydroxyeicosatetraenoic acid-induced angiogenesis requires Src-mediated Egr-1-dependent rapid induction of FGF-2 expression

To understand the mechanisms underlying 15(S)-hydroxyeicosatetraenoic acid [15(S)-HETE]-induced angiogenesis, we studied the role of Egr-1. 15(S)-HETE induced Egr-1 expression in a time-dependent manner in human dermal microvascular endothelial cells (HDMVECs). Blockade of Egr-1 via forced expression of its dominant-negative mutant attenuated 15(S)-HETE-induced HDMVEC migration and tube formation as well as Matrigel plug angiogenesis. 15(S)-HETE-induced Egr-1 expression requires Src activation. In addition, adenovirus-mediated expression of dominant-negative mutant of Src blocked 15(S)-HETE's effects on migration and tube formation of HDMVECs and Matrigel plug angiogenesis. 15(S)-HETE induced fibroblast growth factor-2 (FGF-2) expression rapidly via Src-mediated production of Egr-1. Cloning and mutational analysis of FGF-2 promoter revealed that Egr-1 binding site proximal to transcription start site is required for 15(S)-HETE-induced FGF-2 expression. Neutralizing antibody-mediated suppression of FGF-2 function also attenuated the effects of 15(S)-HETE on HDMVEC migration and tube formation as well as Matrigel plug angiogenesis. Furthermore, in contrast to wild-type mice, 12/15-LOX(-/-) mice exhibited decreased Matrigel plug angiogenesis in response to AA, which was rescued by 15(S)-HETE. On the basis of these observations, we conclude that 15(S)-HETE-induced angiogenesis requires Src-mediated Egr-1-dependent rapid induction of FGF-2. These findings may suggest that 15(S)-HETE could be a potential endogenous regulator of pathologic angiogenesis associated with atherosclerosis and restenosis.

Src-dependent STAT-3-mediated expression of monocyte chemoattractant protein-1 is required for 15(S)-hydroxyeicosatetraenoic acid-induced vascular smooth muscle cell migration

To understand the role of human 15-lipoxygenase 1 (15-LOX1) in vascular wall remodeling, we have studied the effect of the major 15-LOX1 metabolite of arachidonic acid, 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), on vascular smooth muscle cell (VSMC) migration both in vitro and in vivo. Among 5(S)-HETE, 12(S)-HETE, and 15(S)-HETE, 15(S)-HETE potentially stimulated more vascular smooth muscle cell (VSMC) migration. In addition, 15(S)-HETE-induced VSMC migration was dependent on Src-mediated activation of signal transducer and activator of transcription-3 (STAT-3). 15(S)-HETE also induced monocyte chemoattractant protein-1 (MCP-1) expression via Src-STAT-3 signaling, and neutralizing anti-MCP-1 antibodies completely negated 15(S)-HETE-induced VSMC migration. Cloning and characterization of a 2.6-kb MCP-1 promoter revealed the presence of four putative STAT-binding sites, and the site that is proximal to the transcription start site was found to be essential for 15(S)-HETE-induced Src-STAT-3-mediated MCP-1 expression. Rat carotid arteries that were subjected to balloon injury and transduced with Ad-15-LOX1 upon exposure to [(3)H]arachidonic acid ex vivo produced 15-HETE as a major eicosanoid and enhanced balloon injury-induced expression of MCP-1 in smooth muscle cells in Src and STAT-3-dependent manner in vivo. Adenovirus-mediated delivery of 15-LOX1 into rat carotid artery also led to recruitment and homing of macrophages to medial region in response to injury. In addition, transduction of Ad-15-LOX1 into arteries enhanced balloon injury-induced smooth muscle cell migration from media to intima and neointima formation. These results show for the first time that 15-LOX1-15(S)-HETE axis plays a major role in vascular wall remodeling after balloon angioplasty.

The 15(S)-hydroxyeicosatetraenoic acid-induced angiogenesis requires Janus kinase 2-signal transducer and activator of transcription-5B-dependent expression of interleukin-8

To understand the molecular basis underlying 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE)-induced angiogenesis, we have studied the role of the Janus kinase-signal transducer and activator of transcription (Jak-STAT) signaling. The 15(S)-HETE stimulated tyrosine phosphorylation of Jak2 in a time-dependent manner in human retinal microvascular endothelial cells (HRMVECs). Inhibition of Jak2 activation via adenovirus-mediated expression of its dominant-negative mutant attenuated 15(S)-HETE-induced HRMVEC migration and tube formation and Matrigel plug angiogenesis. Similarly, 15(S)-HETE activated tyrosine phosphorylation of STAT-5B in a time-dependent manner. Dominant-negative mutant-mediated interference of STAT-5B activation suppressed 15(S)-HETE-induced HRMVEC migration and tube formation and Matrigel plug angiogenesis. The 15(S)-HETE induced interleukin-8 (IL-8) expression in Jak2-STAT-5B-dependent manner in HRMVECs. In addition, neutralizing anti-IL-8 antibodies reduced 15(S)-HETE-induced HRMVEC migration and tube formation and Matrigel plug angiogenesis. Cloning and Transfac analysis of IL-8 promoter revealed the presence of 1 putative STAT-binding sequence at -476 nt, and electrophoretic mobility shift assay and chromatin immunoprecipitation analysis showed the binding of STAT-5B to this site in response to 15(S)-HETE. Mutational analysis showed that STAT binding site is essential for 15(S)-HETE-induced IL-8 promoter activity. Together, these observations suggest that 15(S)-HETE-induced angiogenesis requires Jak2-STAT-5B-dependent expression of IL-8.

A role for TRPV1 in agonist-evoked activation of human platelets

BACKGROUND

Platelets play a role in a number of inflammatory conditions including atherosclerosis; however, the mechanisms of platelet activation under these conditions are unclear.

OBJECTIVES

To investigate the presence of the vanilloid receptor, TRPV1, which is stimulated by noxious stimuli and by inflammatory mediators, in human platelets.

METHODS

Platelets loaded with fura-2 or sodium-binding benzofuran isophalate acetoxymethyl ester (SBFI) were used to monitor cytosolic calcium or sodium concentrations. 5-HT secretion was determined by fluorescence assay after conjugation with o-phthaldialdehyde. ATP secretion was determined using luciferin-luciferase.

RESULTS

TRPV1 was identified by Western blotting using a specific anti-hTRPV1 antibody. The TRPV1 agonist, capsaicin, evoked both Ca(2+) influx and Ca(2+) release from intracellular stores, responses that were blocked in a dose-dependent manner by the TRPV1 antagonists, 5'-Iodo-resiniferatoxin (5'-Iodo-RTX) and AMG 9810. Capsaicin also increased platelet cytosolic [Na(+)]. Capsaicin-evoked Ca(2+) release was abolished in the absence of extracellular Na(+) or by the 5-HT(2A) receptor antagonist, ketanserin. Capsaicin evoked 5-HT release from platelets, a response abolished in the absence of extracellular Na(+) or by 5'-Iodo-RTX. Thus capsaicin-evoked Ca(2+) release appeared to be mediated by Na(+)-dependent 5-HT release. TRPV1-dependent 5-HT release also contributed to ADP- and thrombin-evoked Ca(2+) entry and release. 5'-Iodo-RTX reduced ADP- and thrombin-evoked Ca(2+) signals, effects not additive with those of ketanserin, and 5'-Iodo-RTX inhibited agonist-evoked 5-HT and ATP release.

CONCLUSION

These results indicate that TRPV1 is present and functionally important in human platelets. The presence of this receptor may provide a link between inflammatory mediators and platelet activation in conditions such as atherosclerosis.

A novel role for activating transcription factor-2 in 15(S)-hydroxyeicosatetraenoic acid-induced angiogenesis

To investigate the mechanisms underlying 15(S)-HETE-induced angiogenesis, we have studied the role of the small GTPase, Rac1. We find that 15(S)-HETE activated Rac1 in human retinal microvascular endothelial cells (HRMVEC) in a time-dependent manner. Blockade of Rac1 by adenovirus-mediated expression of its dominant negative mutant suppressed HRMVEC migration as well as tube formation and Matrigel plug angiogenesis. 15(S)-HETE stimulated Src in HRMVEC in a time-dependent manner and blockade of its activation inhibited 15(S)-HETE-induced Rac1 stimulation in HRMVEC and the migration and tube formation of these cells as well as Matrigel plug angiogenesis. 15(S)-HETE stimulated JNK1 in Src-Rac1-dependent manner in HRMVEC and adenovirus-mediated expression of its dominant negative mutant suppressed the migration and tube formation of these cells and Matrigel plug angiogenesis. 15(S)-HETE activated ATF-2 in HRMVEC in Src-Rac1-JNK1-dependent manner and interference with its activation via adenovirus-mediated expression of its dominant negative mutant abrogated migration and tube formation of HRMVEC and Matrigel plug angiogenesis. In addition, 15(S)-HETE-induced MEK1 stimulation was found to be dependent on Src-Rac1 activation. Blockade of MEK1 activation inhibited 15(S)-HETE-induced JNK1 activity and ATF-2 phosphorylation. Together, these findings show that 15(S)-HETE activates ATF-2 via the Src-Rac1-MEK1-JNK1 signaling axis in HRMVEC leading to their angiogenic differentiation.

15(S)-hydroxyeicosatetraenoic acid-induced angiogenesis requires STAT3-dependent expression of VEGF

15(S)-Hydroxyeicosatetraenoic acid [15(S)-HETE] activated signal transducer and activator of transcription 3 (STAT3) as measured by its tyrosine phosphorylation, translocation from the cytoplasm to the nucleus, DNA binding, and reporter gene activity in human dermal microvascular endothelial cells (HDMVEC). Inhibition of STAT3 activation via adenovirus-mediated expression of its dominant-negative mutant suppressed 15(S)-HETE-induced HDMVEC migration and tube formation in vitro and aortic ring and Matrigel plug angiogenesis in vivo. 15(S)-HETE induced the expression of vascular endothelial growth factor (VEGF) in a time- and STAT3-dependent manner in HDMVEC. In addition, neutralizing anti-VEGF antibodies blocked 15(S)-HETE-induced HDMVEC migration and tube formation in vitro and aortic ring and Matrigel plug angiogenesis in vivo. Together, these results show for the first time that 15(S)-HETE-induced angiogenesis requires STAT3-dependent expression of VEGF. In view of these findings, it is suggested that eicosanoids, particularly 15(S)-HETE, via its capacity to stimulate angiogenesis, may influence the progression of cancer and vascular disease.

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.

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.

15(S)-hydroxyeicosatetraenoic acid induces angiogenesis via activation of PI3K-Akt-mTOR-S6K1 signaling

To determine whether the lipoxygenase metabolites of arachidonic acid, 5(S)-, 12(S)-, and 15(S)-hydroxyeicosatetraenoic acids [5(S)-HETE, 12(S)-HETE, and 15(S)-HETE, respectively] are angiogenic, we have studied their effects on human dermal microvascular endothelial cell (HDMVEC) tube formation and migration. All three HETEs stimulated HDMVEC tube formation and migration. Because 15(S)-HETE was found to be more potent than 5(S)-HETE and 12(S)-HETE in HDMVEC tube formation, we next focused on elucidation of the signaling mechanisms underlying its angiogenic activity. 15(S)-HETE stimulated Akt and S6K1 phosphorylation in HDMVEC in a time-dependent manner. Wortmannin and LY294002, two specific inhibitors of phosphatidylinositol 3-kinase (PI3K), blocked both Akt and S6K1 phosphorylation, whereas rapamycin, a specific inhibitor of Akt downstream effector, mammalian target of rapamycin (mTOR), suppressed only S6K1 phosphorylation induced by 15(S)-HETE suggesting that this eicosanoid activates the PI3K-Akt-mTOR-S6K1 signaling in HDMVEC. Wortmannin, LY294002, and rapamycin also inhibited 15(S)-HETE-induced HDMVEC tube formation and migration. In addition, all three HETEs stimulated angiogenesis as measured by in vivo Matrigel plug assay with 15(S)-HETE being more potent. Pharmacologic inhibition of PI3K-Akt-mTOR-S6K1 signaling completely suppressed 15(S)-HETE-induced in vivo angiogenesis. Consistent with these observations, adenoviral-mediated expression of dominant-negative Akt also blocked 15(S)-HETE-induced HDMVEC tube formation and migration and in vivo angiogenesis. Together, these results show for the first time that 15(S)-HETE stimulates angiogenesis via activation of PI3K-Akt-mTOR-S6K1 signaling.

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.

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.

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

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

Oxidative stress as a regulator of gene expression in the vasculature

Oxidative stress and the production of intracellular reactive oxygen species (ROS) have been implicated in the pathogenesis of a variety of diseases. In excess, ROS and their byproducts that are capable of causing oxidative damage may be cytotoxic to cells. However, it is now well established that moderate amounts of ROS play a role in signal transduction processes such as cell growth and posttranslational modification of proteins. Oxidants, antioxidants, and other determinants of the intracellular reduction-oxidation (redox) state play an important role in the regulation of gene expression. Recent insights into the etiology and pathogenesis of atherosclerosis suggest that this disease may be viewed as an inflammatory disease linked to an abnormality in oxidation-mediated signals in the vasculature. In this review, we summarize the evidence supporting the notion that oxidative stress and the production of ROS function as physiological regulators of vascular gene expression mediated via specific redox-sensitive signal transduction pathways and transcriptional regulatory networks. Elucidating, at the molecular level, the regulatory processes involved in redox-sensitive vascular gene expression represents a foundation not only for understanding the pathogenesis of atherosclerosis and other inflammatory diseases but also for the development of novel therapeutic treatment strategies.

Identification of the 11,14,15- and 11,12, 15-trihydroxyeicosatrienoic acids as endothelium-derived relaxing factors of rabbit aorta

A number of endothelium-derived relaxing factors have been identified including nitric oxide, prostacyclin, and the epoxyeicosatrienoic acids. Previous work showed that in rabbit aortic endothelial cells, arachidonic acid was metabolized by a lipoxygenase to vasodilatory eicosanoids. The identity was determined by the present study. Aortic homogenates were incubated in the presence of [U-14C]arachidonic acid, [U-14C]arachidonic acid plus 15-lipoxygenase (soybean lipoxidase), or [U-14C]15-hydroxyeicosatetraenoic acid (15-HPETE) and analyzed by reverse phase high pressure liquid chromatography (RP-HPLC). Under both experimental conditions, there was a radioactive metabolite that migrated at 17.5-18.5 min on RP-HPLC. When the metabolite was isolated from aortic homogenates, it relaxed precontracted aortas in a concentration-dependent manner. Gas chromatography/mass spectrometry (GC/MS) of the derivatized metabolite indicated the presence of two products; 11,12,15-trihydroxyeicosatrienoic acid (THETA) and 11,14,15-THETA. A variety of chemical modifications of the metabolite supported these structures and confirmed the presence of a carboxyl group, double bonds, and hydroxyl groups. With the combination of 15-lipoxygenase, arachidonic acid, and aortic homogenate, an additional major radioactive peak was observed. This fraction was analyzed by GC/MS. The mass spectrum was consistent with this peak, containing both the 11-hydroxy-14, 15-epoxyeicosatrienoic acid (11-H-14,15-EETA) and 15-H-11,12-EETA. The hydroxyepoxyeicosatrienoic acid (HEETA) fraction also relaxed precontracted rabbit aorta. Microsomes derived from rabbit aortas also synthesized 11,12,15- and 11,14,15-THETAs from 15-HPETE, and pretreatment with the cyctochrome P450 inhibitor, miconazole, blocked the formation of these products. The present studies suggest that arachidonic acid is metabolized by 15-lipoxygenase to 15-HPETE, which undergoes an enzymatic rearrangement to 11-H-14,15-EETA and 15-H-11,12-EETA. Hydrolysis of the epoxy group results in the formation of 11,14,15- and 11,12,15-THETA, which relaxed rabbit aorta. Thus, the 15-series THETAs join prostacyclin, nitric oxide, and epoxyeicosatrienoic acids as new members of the family of endothelium-derived relaxing factors.

Is there a role for 15-lipoxygenase in atherogenesis?

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

Effects of advanced glycation endproducts on the generation of macrophage-mediated oxidized low-density lipoprotein

Macrophages have a receptor that recognizes advanced glycation endproducts (AGE). In this study, we evaluated the effect of AGEs on the generation of macrophage-mediated oxidized low-density lipoprotein (LDL) by measuring the electrophoretic mobilities and lipid hydroperoxide (LHPO) levels of LDL. In the absence of the macrophage monolayer, the differences of the electrophoretic mobilities or LHPO levels of native (n) LDL did not differ significantly between control (c) bovine serum albumin (BSA) and those with AGE-BSA. In the presence of the macrophage monolayer, however, the difference was significant with higher levels after the incubation with AGE-BSA than with c-BSA. In the case of cLDL and glycated (g) LDL, the electrophoretic mobilities and LHPO levels of LDL after 20 h incubation with AGE-BSA in the presence of the macrophage monolayer were significantly higher than those with c-BSA. There were no significant differences, however between the electrophoretic mobilities and LHPO levels of cLDL and of gLDL. These results suggest that AGEs stimulate the generation of macrophage-mediated oxidized LDL, but do not directly stimulate the oxidative modification of gLDL.

Leukotriene C4 from vascular endothelium enhances neutrophil adhesiveness

We have examined the synthesis of leukotriene C4 from bovine aortic and pulmonary artery endothelium. Under basal conditions, neither aortic nor pulmonary artery endothelium revealed significant amounts of hydroxy fatty acids. Following incubation with ionophore A23187, several peaks including one which co-migrated with authentic LTC4 could be demonstrated from both aortic and pulmonary endothelium. LTC4 production was maximal after 30 min incubation, was inhibitable by the lipoxygenase inhibitor nordihydroguairetic acid, and was synthesized by bovine endothelium from tritiated arachidonic acid substrate. The putative LTC4 from endothelium was shown to be identical to authentic LTC4 by chromatography and scanning UV spectroscopy. Endothelial-derived LTC4 increased the adherence of bovine aortic endothelium for neutrophils in a concentration dependent pattern similar to authentic LTC4. These data suggest that vascular endothelium may influence leukocyte-endothelial interactions through synthesis of biologically active arachidonic acid metabolites such as LTC4.

Contribution of arachidonic acid metabolites to reduced norepinephrine-induced contractions in hypercholesterolemic rabbit aortas

Because alterations in the aortic metabolism of arachidonic acid and in vascular responsiveness occur in hypercholesterolemic rabbits, we hypothesized that an arachidonic acid metabolite may contribute to the regulation of vascular tone. Aortic contractions to norepinephrine were investigated in rabbits fed either standard chow or chow containing 2% cholesterol. In normal rabbits, norepinephrine (10(-6) M) elicited a 126 +/- 2% contraction compared with a 95 +/- 2% contraction in cholesterol-fed rabbits. The factor mediating the depressed response was endothelium-dependent because removal of the endothelium blocked the decrease in norepinephrine-induced contractions observed in the cholesterol-fed rabbits. The endothelium-derived factor was not nitric oxide, because blockade of nitric oxide synthase with nitro-L-arginine did not abolish the decreased response in the cholesterol-fed rabbits. Pretreatment of aortas with a cyclooxygenase inhibitor, indomethacin (10(-5) M) caused a slight decrease in the norepinephrine-induced contractions, suggesting that the factor could be a vasoconstrictor cyclooxygenase metabolite or a vasodilatory lipoxygenase or cytochrome P450 epoxygenase metabolite. Pretreatment with the thromboxane A2/prostaglandin H2-receptor antagonist, SQ 29458, had no effect on norepinephrine-induced contractions. Whereas the lipoxygenase inhibitor, nordihydroguaiaretic acid (5 x 10(-5) M), caused a slight increase in the contractions to norepinephrine in cholesterol-fed rabbits compared with normal rabbits, the cytochrome P450 epoxygenase inhibitor, metyrapone (10(-4) M), produced a greater enhancement of norepinephrine-induced contractions in cholesterol-fed rabbits but had no effect on responses in the normal rabbits. Characterization of [3H]arachidonic acid metabolism in cholesterol-fed aortic tissue indicated that norepinephrine stimulated the synthesis of both lipoxygenase and epoxygenase metabolites in an endothelium-dependent manner. This study demonstrated that (a) an endothelium-derived metabolite of arachidonic acid regulates vascular tone, (b) this metabolite appears to be a lipoxygenase or cytochrome P450 product or both, and (c) the activity or synthesis of the factor is enhanced by hypercholesterolemia.

Role of hydroperoxyeicosatetraenoic acids in oxidative stress-induced activating protein 1 (AP-1) activity

We have previously reported that hydrogen peroxide, an active oxygen species and a cellular oxidant, induces c-Fos and c-Jun mRNA expression and DNA synthesis in vascular smooth muscle cells and that these events require arachidonic acid release and metabolism through the lipoxygenase pathway. Here we have identified the eicosanoids that mediate the hydrogen peroxide-induced growth-related events in these cells. Hydrogen peroxide stimulated the production of 12- and 15-hydroperoxyeicosatetraenoic acids in vascular smooth muscle cells. Both 12- and 15-hydroperoxyeicosatetraenoic acids induced the expression of c-Fos and c-Jun protein and increased activating protein 1 (AP-1) activity, as measured by AP-1-DNA binding and AP-1-dependent human collagenase promoter-driven chloramphenicol acetyltransferase reporter gene transcription. Hydrogen peroxide and arachidonic acid also induced the expression of c-Fos and c-Jun protein and AP-1 activity. Nordihydroguaiaretic acid, an inhibitor of the lipoxygenase pathway, significantly inhibited both hydrogen peroxide and arachidonic acid-stimulated c-Fos and c-Jun protein expression and AP-1 activity. Together, these findings suggest that hydrogen peroxide induces the production of eicosanoids and that the eicosanoids are potential mediators of the oxidative stress-stimulated growth-related events in vascular smooth muscle cells.

Influence of hypercholesterolaemia on the reactivity of isolated rabbit arteries to 15-lipoxygenase metabolites of arachidonic acid: comparison with platelet-derived agents and vasodilators

The lipoxygenase product 15-hydroxyeicosatetraenoic acid (15-HETE) was shown to be the most important eicosanoid formed in the atherosclerotic rabbit aorta. The aim of the present study was to compare the effects of 15-HETE and its hydroperoxy precursor 15-HpETE with those of other vasoconstrictor and vasodilator agents in arteries from rabbits fed either a control or a cholesterol-rich diet for 16 and 30 weeks. 5-Hydroxytryptamine (5-HT) aggregated platelets and thrombin caused contractions of isolated rabbit aortas. The contractile responses elicited by platelets from control animals were similar to those evoked by platelets from atherosclerotic rabbits. After 16 weeks of hypercholesterolemia, the contractile responses were either augmented (5-HT), unchanged (platelets) or reduced (thrombin). After 30 weeks of hypercholesterolemia, the responses to all contractile agents used had decreased. In both aortas and pulmonary arteries the endothelium-dependent relaxations to the calcium ionophore, A23167, and to acetylcholine were progressively lost and the endothelium-independent relaxations to nitroglycerin were reduced by the progressing hypercholesterolemia. The 15-lipoxygenase metabolites contracted the isolated thoracic aorta and pulmonary artery from control rabbits and to a lesser extent those of the cholesterol-fed rabbits. After raising the tone in these vessels with prostaglandin F2 alpha PGF2 alpha) or noradrenaline, 15-HpETE induced relaxations which were not significantly influenced by the development of fatty streaks. Our data illustrate that the contractions of the blood vessel wall to 15-HETE, like those to other vasoconstrictors, are markedly reduced by developing atherosclerosis. In contrast, the relaxations to 15-HpETE in the rabbit arteries remain unaltered after 16 to 30 weeks of hypercholesterolemia. This is unlike the reactions to other vasodilators, which are markedly reduced.

Systemic activation of 15-lipoxygenase in heart, lung, and vascular tissues by hypercholesterolemia: relationship to lipoprotein oxidation and atherogenesis

There is evidence that oxidized lipoproteins are a major contributing factor in atherosclerosis. 15-Lipoxygenase is the principal mammalian enzyme that can oxidize polysaturated fatty acids present in intact lipoproteins, and in membrane phospholipids in situ. We, and others, have reported previously that levels of the enzyme are increased in aortas of cholesterol-fed and spontaneously atherosclerotic WHHL rabbits. In the present study, rabbits were fed an atherogenic diet containing 1% cholesterol for 14 weeks, and levels of [14C]arachidonate metabolizing enzymes in the excised tissues were measured by HPLC analysis. 15-Lipoxygenase levels in heart, aortic adventitia, and lung, but not in liver, were increased up to 100-fold above controls, without major significant changes in prostaglandin endoperoxide synthases or the 5- and 12-lipoxygenases. The induced 15-lipoxygenase activity in the aortic adventitia was approximately 15 times greater than that found in the vessel wall. Hypercholesterolemia and elevated 15-lipoxygenase were associated with a 40% lowering of blood hematocrit. The hemolytic agent phenylhydrazine duplicated the effects of hypercholesterolemia on hematocrit, and induced up to 100-fold increases in 15-lipoxygenase activity in tissues within 7 days. The induced 15-lipoxygenase activities in heart and lung were 4 and 8 times greater, respectively, than in reticulocytes, previously the richest known source of the enzyme. Direct measurements of hemoglobin content also demonstrated that contaminating reticulocytes were not the source of the tissue enzyme. A similar tissue-specific activation of 15-lipoxygenase was observed in rat heart and lung, but also not in liver. It is concluded that the elevated level of 15-lipoxygenase activity previously reported in atherosclerotic aorta is symptomatic of a generalized and massive induction of the enzyme in cardio-pulmonary tissues by hypercholesterolemia, which may be related to the membrane perturbation and increased hemolysis that is induced by cholesterol feeding.

Modified low density lipoprotein and its constituents augment cytokine-activated vascular cell adhesion molecule-1 gene expression in human vascular endothelial cells

Early features in the pathogenesis of atherosclerosis include accumulation of oxidized LDL (oxLDL) and endothelial expression of the vascular adhesion molecule VCAM-1. Because antioxidants inhibit endothelial VCAM-1 expression, we tested the hypothesis that oxLDL functions as a prooxidant signal in atherogenesis to augment VCAM-1 activation by inflammatory signals. Cultured human aortic endothelial cells (HAECs) or human umbilical vein endothelial cells (HUVECs) were incubated with unmodified LDL, oxLDL, or glycated LDL for 48 h. No change in VCAM-1, intercellular cell adhesion molecule-1 (ICAM-1), or E-selectin expression from control was observed by ELISA. However, dose-response and time course studies demonstrated that oxLDL enhanced VCAM-1 expression induced by the cytokin tumor necrosis factor alpha (TNF alpha) 63% in HAECs and 45% in HUVECs over unmodified LDL or control. Using flow cytometry analysis, oxLDL augmented TNF alpha-induced VCAM-1 expression in a uniform HAEC population. oxLDL had no effect on E-selection induction. oxLDL augmented TNF alpha-induced ICAM-1 expression 44% in HAECs but not in HUVECs. Glycated LDL augmented TNF alpha-induced VCAM-1 expression 35% in HAECs but not HUVECs. Similar results were obtained with 13-HPODE or lysophosphatidylcholine, significant components of oxLDL. 13-HPODE augmented TNF alpha-induced mRNA accumulation and transcriptional activation of VCAM-1 in HAECs. These results suggest that as long-term regulatory signals, specific oxidized fatty acid and phospholipid components of oxLDL augment the ability of vascular endothelial cells to express cytokine-mediated VCAM-1. These studies link oxidant signals conferred by oxLDL to oxidation-sensitive regulatory mechanisms controlling the expression of endothelial cell adhesion molecules involved in early atherosclerosis.