The induced lipoxygenase in atherosclerotic aorta converts linoleic acid to the platelet chemorepellant factor 13-HODE

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.

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.

Fluoride in low concentration modifies expression and activity of 15 lipoxygenase in human PBMC differentiated monocyte/macrophage

Epidemiological and experimental evidences demonstrate positive correlation between environmental and occupational fluoride exposure and risk to various cardio-respiratory disorders. That fore we decided to examine the effect of fluorides on activity and expression of 15LOX enzyme which is implicated in biosynthesis of inflammatory mediators. Expression of 15LOX-1 and -2 enzymes mRNA and protein was analyzed using RT PCT and immunoblotting methods respectively whereas HPLC method was used to measure the levels of 15 lipoxygenases end products. Additionally AA and LA concentration in cells was measured using GC method. We observed that fluoride in small concentration may significantly decrease activity of 15LOX-1 and -2 in human PBMC macrophages and then concentration of its end products: 15-HETE, 12-HETE and 9+13-HODE, what may cause development of inflammation through the cholesterol arrest into the macrophages and its differentiation to foam cell. Noted by our team overexpression of the 15LOX-1 enzyme in macrophages after addition of lowest fluoride concentrations (1 and 3 μM) may be aimed at fighting inflammation development and excessive intracellular lipid accumulation. But highest fluoride concentrations (6 and 10 μM) added to cell culture slowly declined expression of this enzyme probably because of developing inflammation. Additional 15LOX-2 expression in macrophages after fluoride addition was low in 1 and 3 μM concentrations, but increased significantly after 10 μM fluoride addition what may suggest developing acute inflammation, because 15LOX-2 is associated to increased local hypoxia. This study indicated that even in small concentrations fluorides changes the amounts and activity of 15 LOX-1 and -2 enzymes taking part in the development of inflammatory process.

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.

A festschrift for J. Martyn Bailey, a biochemist extraordinaire

A festschrift for Dr. John Martyn Bailey, Professor of Biochemistry and Molecular Biology was organized by the Biochemistry department of the George Washington University School of Medicine and Health Sciences on December 4-5, 2006 to honor his 48 years of contributions. He made important contributions in the areas of essential fatty acids, prostaglandins, thromboxanes and lipoxygenase metabolites.

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.

Free radical oxidation of coriolic acid (13-(S)-hydroxy-9Z,11E-octadecadienoic acid)

The reaction of (13S,9Z,11E)-13-hydroxy-9,11-octadecadienoic acid (1a), one of the major peroxidation products of linoleic acid and an important physiological mediator, with the Fenton reagent (Fe(2+)/EDTA/H(2)O(2)) was investigated. In phosphate buffer, pH 7.4, the reaction proceeded with >80% substrate consumption after 4h to give a defined pattern of products, the major of which were isolated as methyl esters and were subjected to complete spectral characterization. The less polar product was identified as (9Z,11E)-13-oxo-9,11-octadecadienoate (2) methyl ester (40% yield). Based on 2D NMR analysis the other two major products were formulated as (11E)-9,10-epoxy-13-hydroxy-11-octadecenoate (3) methyl ester (15% yield) and (10E)-9-hydroxy-13-oxo-10-octadecenoate (4) methyl ester (10% yield). Mechanistic experiments, including deuterium labeling, were consistent with a free radical oxidation pathway involving as the primary event H-atom abstraction at C-13, as inferred from loss of the original S configuration in the reaction products. Overall, these results provide the first insight into the products formed by oxidation of 1a with the Fenton reagent, and hint at novel formation pathways of the hydroxyepoxide 3 and hydroxyketone 4 of potential (patho)physiological relevance in settings of oxidative stress.

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.

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.

A sensitive gas chromatography-mass spectrometry assay reveals increased levels of monohydroxyeicosatetraenoic acid isomers in human plasma after extracorporeal photoimmunotherapy and under in vitro ultraviolet A exposure

Extracorporeal photoimmunotherapy (photopheresis) is a highly effective therapy in the treatment of various disorders. Although extracorporeal photoimmunotherapy has been successfully used for more than 10 y, its mechanism of action is still unclear. The formation of reactive oxygen species have been implicated in extracorporeal photoimmunotherapy, but malonyl dialdehyde as a marker of systemic lipid peroxidation did not increase significantly during treatment. To investigate further the involvement of reactive oxygen species in extracorporeal photoimmunotherapy, we have introduced a highly sensitive negative ion gas chromatography-mass spectrometry based method for quantitating oxygenated arachidonic acid isomers (hydroxyeicosatetraenoic acids) in plasma samples of patients treated with extracorporeal photoimmunotherapy. In the plasma of healthy volunteers pmole amounts of 2-, 3-, 5-, 8-12-, and 15-hydroxyeicosatetraenoic acid were detected and we observed a dose-dependent augmentation in these metabolites when the blood was irradiated with increasing doses of ultraviolet A in the presence of the photosensitizer 8-methoxypsoralen. Analysis of plasma samples obtained from patients before and after extracorporeal photoimmunotherapy revealed a characteristic increase in total hydroxyeicosatetraenoic acid levels, particularly of 5-hydroxyeicosatetraenoic acid which contributed 80% to the sum of all hydroxyeicosatetraenoic acid isomers. Chiral phase high-performance liquid chromatography indicated almost equal amounts of 5S- and 5R-hydroxyeicosatetraenoic acid suggesting that the majority of lipid peroxidation products are formed via nonenzymatic oxidation reactions.

Lipoxygenases and atherosclerosis: protection versus pathogenesis

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

Expression of leukocyte-type 12-lipoxygenase and reticulocyte-type 15-lipoxygenase in rabbits

From a rabbit reticulocyte library a full length cDNA was isolated which predicted a novel lipoxygenase (LOX) sharing 99% identical amino acids with the rabbit 15-lipoxygenase. HPLC product analysis of the bacterially expressed protein identified it as a leukocyte-type 12-lipoxygenase (1.12-LOX). This proves the co-expression of a 15-lipoxygenase and a 1.12-lipoxygenase in one mammalian species. Among the six amino acids that are different to rabbit 15-lipoxygenase, leucine 353 is shown to be the primary determinant for 12-positional specificity. In the 3'-untranslated region of the 12-LOX-mRNA a CU-rich, 20-fold repetitive element has been found, closely related to the differentiation control element (DICE) of the rabbit 15-LOX-mRNA which is organized by ten repeats of 19 bases. By genomic PCR the 3'-terminal part of the gene for the novel 12-lipoxygenase containing the introns 10-13 has been amplified and sequenced. The introns were very similar in length to the corresponding 15-lipoxygenase introns with 89% to 95% identical nucleotide sequences. By screening a rabbit reticulocyte library an alternative 15-lipoxygenase transcript of 3.6 kb has been detected containing a 1019 nucleotides longer 3'-untranslated region (UTR2) than the main 2.6 kb mRNA. The determination of the tissue distribution by Northern blotting showed that the 3.6 kb mRNA2 was only expressed in non-erythroid tissues, whereas the 2.6 kb mRNA1 was exclusively expressed in reticulocytes. The only cell type which has been found to express the 1.12-lipoxygenase abundantly are monocytes. The results indicate that the expression of 1.12-lipoxygenase and 15-lipoxygenase is highly regulated. The UTR2 of the 15-LOX-mRNA2 contained a novel eight-fold repetitive CU-rich motif of 23 bases length which is related but not identical to the DICE of 19 bases in the UTR1. The analysis of a genomic recombinant of the complete 9.0 kb Alox15 gene confirmed that UTR1 and UTR2 are not interrupted by an additional intron.

Simultaneous expression of leukocyte-type 12-lipoxygenase and reticulocyte-type 15-lipoxygenase in rabbits

In rabbit reticulocytes an arachidonic acid 15-lipoxygenase (15-LOX) is expressed at high yield. Rescreening a rabbit reticulocyte cDNA library for alternative 15-LOX transcripts, a full length cDNA which encodes a novel lipoxygenase was isolated. The predicted amino acid sequence of this enzyme shared a high degree (99%) of identity with the reticulocyte-type 15-lipoxygenase. Among the six amino acid residues different in both enzymes a Phe-Leu exchange was detected at position 353. Recently, site-directed mutagenesis studies have revealed that this amino acid exchange converts a 15-lipoxygenase to a 12-lipoxygenase. In fact, when the novel enzyme was expressed in Escherichia coli, mainly 12-lipoxygenation of arachidonic acid was observed. The recombinant enzyme exhibited a rather broad substrate specificity. Various C-18 and C-20 polyenoic fatty acids and even complex substrates such as biomembranes were effectively oxygenated. Thus, the novel enzyme may be classified as leukocyte-type 12-lipoxygenase. Genomic polymerase chain reaction of the 3' region of the leukocyte-type 12-lipoxygenase gene indicated that introns 10 to 13 differed to about 10% from the corresponding sequences of the 15-lipoxygenase gene although their size and the intron-exon organization were very similar. In the 3'-untranslated region of the novel mRNA a C+U-rich, 20-fold repetitive element was found which appears to be highly related to the differentiation control element of the 15-lipoxygenase mRNA. Activity assays with a variety of cells and tissues prepared from normal rabbits suggested that only peripheral monocytes abundantly express the enzyme, suggesting a tissue-specific regulation of gene expression. These data indicate for the first time the co-expression of two separate genes for a reticulocyte-type 15-lipoxygenase and for a leukocyte-type 12-lipoxygenase in one species. This is of importance for the implication of both enzymes in red blood cell development and atherogenesis.

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

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

The regulation of mitogenesis and apoptosis in response to the persistent stimulation of alpha1-adrenoceptors: a possible role of 15-lipoxygenase

1. Activation of alpha1-adrenoceptor stimulation regulates eicosanoid metabolism and growth in vascular smooth muscle cells (VSMCs). The purpose of this study was to investigate the functional implications of lipoxygenase pathway in alpha1-adrenoceptor-stimulated VSMCs growth through mutually exclusive biological functions, that is cell proliferation and cell death. 2. Phenylephrine (10 microM), a specific alpha1-adrenoceptor agonist, enhanced [3H]-thymidine incorporation by 300% above basal. Nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, caused 36 and 50% decrease in phenylephrine (10 microM)-stimulated [3H]-thymidine incorporation at concentrations of 1 microM and 10 microM respectively. 3. Inversely, treatment of phenylephrine (10 microM)-stimulated VSMCs with NDGA induced DNA fragmentation in a dose-dependent fashion. The level of induction of DNA fragmentation by NDGA was 225, 319 and 406% above the phenylephrine (10 microM)-level at concentrations of 0.1 microM, 1 microM and 10 microM, respectively. This induction of DNA fragmentation was partially prevented by exogenous 15-hydroxyeicosatetraenoic acid (15-HETE). The inhibition of apoptosis was 53 and 63% at concentrations of 5 microM and 10 microM of 15HETE, respectively, as compared with phenylephrine (10 microM) in the presence of NDGA (10 microM). 4. Furthermore, we performed the time-course analysis of Bcl-2 protein expression in phenylephrine (10 microM)-stimulated VSMCs. The expression of Bcl-2 protein disappeared after a 2 h incubation in the presence of NDGA (10 microM), but remained stable after a 2 h incubation period in the absence of NDGA (10 microM). 5, These results suggest that the lipoxygenase pathway is involved in cell proliferation by preventing apoptosis through the level of Bcl-2 protein expression.

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

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

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

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

Macrophage-mediated 15-lipoxygenase expression protects against atherosclerosis development

Oxidative modification of LDL increases its atherogenicity, and 15-lipoxygenase (15-LO) has been implicated in the process. To address this issue, we generated transgenic rabbits that expressed 15-LO in a macrophage-specific manner and studied their susceptibility to atherosclerosis development when they were fed a high-fat, high-cholesterol (HFHC) diet (Teklad 0533 rabbit diet 7009 with 10% corn oil and 0.25% cholesterol) for 13.5 wk. Transgenic and nontransgenic rabbits developed similar degrees of hypercholesterolemia and had similar levels of triglyceride, VLDL, LDL, and HDL. Quantitative morphometric analysis of the aortic atherosclerosis indicated that the transgenic animals (n = 19) had significantly smaller lesion areas (9.8+/-6.5%, mean+/-SD) than their littermate controls (n = 14, 17.8+/-15.0%) (P < 0.05). In a subgroup (n = 9) of transgenic rabbits that received the HFHC diet plus the antioxidant N',N '-diphenyl-phenylenediamine (1%), the extent of lesion involvement (9.8+/-7.5%) did not differ from the subgroup (n = 10) that received the regular HFHC diet (9.7+/-5.9%). Since the results were unexpected, we repeated the experiments. Again, we found that the nontransgenic littermates (n = 12) had more extensive lesions (11.6+/-10.6%) than the transgenic rabbits (n = 13; 9.5+/-7.8%), although the difference was not significant. In a third set of experiments, we crossed 15-LO transgenic rabbits with Watanabe heritable hyperlipidemic (WHHL) rabbits and found that the lesion area in the 15-LO transgenic/heterozygous WHHL rabbits (n = 14) was only about one third (7.7+/-5.7%) that found in nontransgenic heterozygous WHHL littermate controls (n = 11, 20.7+/-19.4%) (P < 0.05). These data suggest that overexpression of 15-LO in monocytes/macrophages protects against lipid deposition in the vessel wall during early atherogenesis in these rabbit models of atherosclerosis.

Enhanced levels of lipoperoxides in low density lipoprotein incubated with murine fibroblast expressing high levels of human 15-lipoxygenase

There is strong experimental evidence that oxidized low density lipoprotein (Ox-LDL) plays an important role in atherosclerosis. However, the mechanisms by which Ox-LDL is formed in vivo are unknown. To test whether 15-lipoxygenase (15-LO) could play a role in oxidation of LDL by cells, we expressed 15-LO activity in murine fibroblasts, which do not normally have 15-LO activity, and tested their ability to modify LDL. Using a retroviral vector, we prepared fibroblasts that expressed 2- to 20-fold more 15-LO activity than control fibroblasts infected with a vector containing beta-galactosidase (lacZ). Compared with LDL incubated with lacZ cells, LDL incubated with 15-LO-containing cells were enriched with lipid hydroperoxides. When these LDL samples were subsequently subjected to oxidative stress, they were more susceptible to further oxidative modification, as judged by increased conjugated diene formation and by increased ability to compete with 125I-Ox-LDL for uptake by macrophages. These findings establish that cellular 15-LO can contribute to oxidative modification of LDL, but the quantitative significance of these findings to the in vivo oxidation of LDL remains to be established.

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.

15-Lipoxygenase expression in smooth muscle cells from atherosclerotic rabbit aortas

To determine the extent and origin of the stimulation of 15-lipoxygenase activity in atherosclerotic aortas, formation of hydroxy-derivatives from arachidonic acid was measured by HPLC-analysis and 15-lipoxygenase mRNA expression was investigated by RNA blot and in situ hybridization in atherosclerotic and normal rabbit aortic tissues. The synthesis of hydroxy-eicosatetraenoic acids (HETE) from exogenously added [14C]arachidonic acid was unchanged in atherosclerotic aortas in comparison with healthy aortas, but pretreatment with indomethacin demonstrated that 15-HETE production resulted essentially (75%) from cyclooxygenase activity in healthy aorta and from lipoxygenase activity in atherosclerotic aorta. The RNA blot and in situ hybridization with radiolabelled oligonucleotide probe demonstrated that 15-lipoxygenase mRNA was strictly localized in intimal thickening of atherosclerotic aortas. The immunostaining using anti-alpha smooth muscle actin, revealed that smooth muscle cell rich areas of the intimal thickening expressed 15-lipoxygenase mRNA. In addition, RNA blot hybridization indicated that cultured smooth muscle cells from atherosclerotic aortas expressed strongly 15-lipoxygenase mRNA. These results demonstrate that augmentation of 15-lipoxygenase activity in atherosclerotic aortas is correlated with 15-lipoxygenase mRNA expression in atherosclerotic plaque, and that intimal smooth muscle cells were involved, in addition to macrophages, in the expression of 15-lipoxygenase.

Involvement of 15-lipoxygenase in early stages of atherogenesis

The arachidonate 15-lipoxygenase which is expressed in atherosclerotic lesions is implicated in the oxidative modification of low density lipoproteins during atherogenesis. To obtain experimental in vivo evidence for this hypothesis, we analyzed the structure of oxygenated lipids isolated from the aorta of rabbits fed with a cholesterol-rich diet for different time periods and compared the pattern of oxygenation products with that isolated from low density lipoproteins treated in vitro with the pure rabbit 15-lipoxygenase and with oxygenated lipids isolated from advanced human atherosclerotic lesions. In early atherosclerotic lesions (12-wk cholesterol feeding), specific lipoxygenase products were detected whose structure was similar to those isolated from lipoxygenase-treated low density lipoproteins. The appearance of these products did coincide with the lipid deposition in the vessel wall. In later stages of atherogenesis (26-wk cholesterol feeding) the degree of oxidative modification of the tissue lipids did increase but the share of specific lipoxygenase products was significantly lower, suggesting an increasing overlay of the specific lipoxygenase products by nonenzymatic lipid peroxidation. In advanced human atherosclerotic lesions, large amounts of oxygenation products were detected whose structure suggests a nonenzymatic origin. These data suggest that the arachidonate 15-lipoxygenase is of pathophysiological importance during the early stages of atherogenesis. In later stages of plaque development nonenzymatic lipid peroxidation becomes more relevant.

Isradipine decreases arterial thrombogenicity in rabbits. A morphometric and radioisotopic study

The effect of isradipine, a calcium antagonist, on aortic and iliac wall thrombogenicity was examined in rabbits. After one week of dosing, the abdominal aortic and iliac artery endothelium was abraded using a Fogarthy catheter. One group of animals (n = 8) was dosed for one week with isradipine 0.3 mg/kg. A second group of animals received 10 mg acetylsalicylic acid (ASA)/kg daily in addition, while a third group received the vehicle only. Finally, a fourth group of animals (n = 8) was treated with ASA only. The percentage denuded surface covered with contact (unspread) platelets decreased significantly (p < 0.01) from 14.7 +/- 2.0 to 9.3 +/- 2.1 (6.2 +/- 0.8 to 3.7 +/- 0.4). The amount of contact and spread platelets was diminished from 84.9 +/- 5.6 to 71.4 +/- 4.4 (91.8 +/- 5.3 to 75.2 +/- 4.6). Platelet thrombi decreased from 7.4 +/- 0.9 to 4.6 +/- 1.4 (9.4 +/- 1.9 to 5.2 +/- 0.7) in the aortic and the iliac artery, respectively. In-platelet deposition decreased by 39.9 and 41.9%. Concomitant ASA therapy not only abolished the effect of isradipine but enhanced thrombogenicity, probably as a result of almost complete blockade of vascular PGI2-production.

The role of endothelial cells in the relaxations induced by 13-hydroxy- and 13-hydroperoxylinoleic acid in canine arteries

1. One of the major fatty acids in the arterial wall is linoleic acid. It has been shown that its 13-hydroxy metabolite (13-HODE) is generated in significant amounts by cultured endothelial cells. The aim of the present study was to investigate the relaxations to 13-HODE and its hydroperoxyprecursor (13-HPODE) and to examine the role of the endothelial cells. 2. Ring segments of canine circumflex and splenic artery were mounted in organ chambers for isometric tension recording. During contractions induced by prostaglandin F2 alpha or noradrenaline, 13-HODE and 13-HPODE evoked dose-dependent relaxations. Removal of the endothelial cells reduced the relaxations to 13-HODE, but had no effect on those elicited by 13-HPODE. 3. Indomethacin and meclofenamate (0.3 microM to 30 microM) blocked the relaxations evoked by 13-HODE and 13-HPODE in endothelium-denuded rings. In segments with endothelium, both cyclo-oxygenase inhibitors again abolished the relaxations to 13-HODE, but only diminished those to 13-HPODE. 4. Prostacyclin biosynthesis, as measured by radioimmunoassay, increased upon incubation with 13-HODE and 13-HPODE (10 microM). Bioassay of the release of nitric oxide (NO) indicated that NO was not involved in the relaxations elicited by either metabolite. Moreover, L-NG-nitroarginine (100 microM), a specific inhibitor of NO synthesis, did not influence the relaxations to 13-HODE and 13-HPODE. The responses to 13-HPODE were also not altered by superoxide dismutase. 5. In the splenic artery 13-HPODE and 13-HODE induced contractions above 3 microM which were blocked by the thromboxane receptor antagonist, daltroban.In the circumflex artery contractile responses to high concentrations of 13-HODE could be observed only after inhibition of cyclo-oxygenase.6. We conclude that the vasodilatation induced by 13-HODE and 13-HPODE was due to stimulation of prostacyclin biosynthesis both in the endothelium and smooth muscle cells or other subendothelial structures. An additional, unidentified intermediate, which was neither NO nor a cyclo-oxygenase product nor superoxide anion, contributed to the relaxations to 13-HPODE in arteries with endothelium.

1-Deamino-8 D-arginine vasopressin decreases the production of 13-hydroxyoctadecadienoic acid by endothelial cells

1-Deamino-8 D-arginine vasopressin (DDAVP) has been used effectively to normalize the bleeding time in various hemostatic disorders. In von Willebrand disease the reduction in bleeding time is due to the preferential release of large multimers of von Willebrand factor from endothelial cells. However, since the bleeding time correction in patients with uremia and liver disease is independent of the release of von Willebrand antigen and activity, other mechanisms of action of DDAVP need to be considered. Endothelial cells generate several thromborepellant factors including 13-hydroxyoctadecadienoic acid (13-HODE), an inhibitor of platelet adhesion to subendothelium. Using cultured fetal bovine aortic endothelial cells (FBAECs), we have investigated whether DDAVP modulates the production of 13-HODE. We have demonstrated that 14C-linoleic acid labeled FBAECs release several oxygenated derivatives of linoleic acid following a 120 min incubation in the presence of serum. One of these products was identified by chromatographic procedures as 13-HODE. The production of 13-HODE was decreased significantly by DDAVP (1-100 ng/ml) with maximal reduction (approx. 25%) seen at 1 ng/ml of DDAVP. While vehicle treated control FBAECs generated 6780 +/- 690 cpm of 13-HODE per 10(6) cells (mean +/- SE, n = 8), DDAVP treated FBAECs produced 4950 +/- 310 (P < 0.01), 5390 +/- 390 (P < 0.01), and 5720 +/- 410 cpm (P < 0.05) of 13-HODE at 1, 10, and 100 ng/ml DDAVP respectively. Our findings of a decrease in 13-HODE would explain the previously observed morphologic changes of increased platelet adhesion to subendothelium following DDAVP infusion and contributes to our understanding of the mode of action of this therapeutic agent in hemostatic disorders.

Eicosanoids, other fatty acid metabolites and the cardiovascular system: are the present antithrombotic approaches rational?

Antiplatelet +/- anticoagulant drugs are currently used as the standard treatment to prevent and treat thrombosis. While this approach is beneficial, it is not optimal. Recent evidence suggests that constituents of the vascular endothelium play an important role in regulating vessel wall thrombogenecity, thereby inhibiting thrombogenesis. These include constituents such as PGI2, tissue plasminogen activator, thrombomodulin and the lipoxygenase fatty acid metabolite derived from linoleic acid, 13-hydroxyoctadecadienoic acid (13-HODE). Consequently, new strategies have been developed to maximize the use of these agents for antithrombotic therapy. We will review these different approaches, discuss their rationale, and based upon recent experimental data, introduce an alternative approach for antithrombotic therapy which may circumvent a number of limitations and side-effect of the currently used drugs.

Early atherosclerosis is accompanied by a decreased rather than an increased accumulation of fatty acid hydroxyderivatives

The content of 13-hydroxylinoleic acid (13-HODE) and 15-hydroxyarachidonic acid (15-HETE) in the rabbit thoracic aorta was measured using high performance liquid chromatography after chronic exposure to cholesterol and a high dose of molsidomine, a donor of nitric oxide (NO). Cholesterol-induced fatty streak formation was accompanied by a decrease in the amounts of esterified 13-HODE and 15-HETE. The reduction of the esterified 13-HODE content correlated significantly with the severity of the lesions. These results do not support the hypothesis that fatty acid hydroperoxides accumulate in the arterial wall during atherosclerosis. On the other hand, the quantity of esterified 13-HODE and 15-HETE was increased markedly after exposure to molsidomine. The high dose of this agent could have initiated radical reactions (via liberation of NO and production of superoxide anions) thereby leading to a raise of the 13-HODE and 15-HETE content of the vessel.

Relationship of vascular 15-lipoxygenase induction to atherosclerotic plaque formation in rabbits

Aortas from atherosclerotic rabbits have increased levels of 15-lipoxygenase, but the relationship between induction of this enzyme and the atherosclerotic process has not been defined. We found that dietary administration of cortisone acetate significantly suppressed atherosclerotic plaque formation in both Watanabe Heritable Hyperlipidemic (WHHL) and cholesterol-fed WHHL/NZW heterozygous rabbits. There was, however, no corresponding decrease in the elevated 15-lipoxygenase activity. In addition, the elevated 15-lipoxygenase activity in atherosclerotic rabbit aortas was uniformly distributed throughout the aorta, and was not preferentially localized in the lesions. These results indicate that induction of the 15-lipoxygenase is not necessarily causally related to plaque development, and that plaques are not the major source of the increased enzyme activity. However, the results confirm that hypercholesterolemia is a necessary condition for both atherosclerosis and 15-lipoxygenase induction, suggesting that perhaps the 15-lipoxygenase may represent a protective response to the hyperlipidemic stress. This possibility is supported by the finding that the induced 15-lipoxygenase converts linoleic acid, which is the predominant essential fatty acid in aorta, to 13-hydroxyoctadecadienoic acid (13-HODE). This compound is a chemorepellant factor for platelets, inhibits platelet thromboxane synthesis, and stimulates prostacyclin synthesis by endothelial cells.

Cyclic AMP regulation of endothelial cell triacylglycerol turnover, 13-hydroxyoctadecadienoic acid (13-HODE) synthesis and endothelial cell thrombogenicity

The 15-omega-lipoxygenase enzyme in endothelial cells metabolizes endogenous linoleic acid (18:2) into 13-hydroxyoctadecadienoic acid (13-HODE) under basal conditions, i.e., in unstimulated endothelial cells. 13-HODE is thought to regulate the non-adhesivity of the endothelium, contributing to vessel wall/blood cell biocompatibility. We performed experiments, therefore, to determine the relationship between basal levels of cAMP, 13-HODE synthesis, and platelet/endothelial cell adhesion. We found that 13-HODE synthesis increased with elevated cAMP levels and that the elevated 13-HODE levels correlated with increased 18:2 turnover in the triacylglycerol pool. In contrast, neither 18:2 nor arachidonic acid (20:4) turnover in the phospholipid nor prostacyclin (PGI2) production were changed with elevated cAMP levels. Platelet/endothelial cell adhesion was inversely proportional to 13-HODE synthesis. We conclude that intracellular 13-HODE influences platelet/vessel wall interactions, is synthesized from 18:2 released from the endogenous triacylglycerol pool, and that this pathway is modulated by intracellular cAMP levels.

Relationship between vessel wall 13-HODE synthesis and vessel wall thrombogenicity following injury: influence of salicylate and dipyridamole treatment

We performed studies to determine the relationship between injured vessel wall thrombogenicity, vessel wall 13-hydroxyoctadecadienoic acid (13-HODE) synthesis and cAMP levels in rabbit treated with salicylate or dipyridamole. Injured vessel wall thrombogenicity was measured as the number of 3H-adenine labelled platelets adhered to the subendothelial basement membrane exposed by air injury in carotid arteries of rabbits treated orally with salicylate or dipyridamole. Vessel wall 13-HODE was measured by HPLC and vessel wall cAMP was measured by RIA. Vessel wall thrombogenicity was increased two-fold in rabbits treated with salicylate and decreased by half in rabbits treated with dipyridamole. The levels of vessel wall cAMP levels were correlated both with the plasma dipyridamole levels and increases in 13-HODE synthesis. cAMP levels were unaffected by salicylate treatment, but 13-HODE synthesis was decreased. We conclude that there is a significant relationship between vessel wall cAMP levels and 13-HODE synthesis, which in turn, influences subsequent vessel wall thrombogenicity.