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

Endogenous chemicals guard health through inhibiting ferroptotic cell death

Ferroptosis is a new form of regulated cell death caused by iron-dependent accumulation of lethal polyunsaturated phospholipids peroxidation. It has received considerable attention owing to its putative involvement in a wide range of pathophysiological processes such as organ injury, cardiac ischemia/reperfusion, degenerative disease and its prevalence in plants, invertebrates, yeasts, bacteria, and archaea. To counter ferroptosis, living organisms have evolved a myriad of intrinsic efficient defense systems, such as cyst(e)ine-glutathione-glutathione peroxidase 4 system (cyst(e)ine-GPX4 system), guanosine triphosphate cyclohydrolase 1/tetrahydrobiopterin (BH4) system (GCH1/BH4 system), ferroptosis suppressor protein 1/coenzyme Q10 system (FSP1/CoQ10 system), and so forth. Among these, GPX4 serves as the only enzymatic protection system through the reduction of lipid hydroperoxides, while other defense systems ultimately rely on small compounds to scavenge lipid radicals and prevent ferroptotic cell death. In this article, we systematically summarize the chemical biology of lipid radical trapping process by endogenous chemicals, such as coenzyme Q10 (CoQ10), BH4, hydropersulfides, vitamin K, vitamin E, 7-dehydrocholesterol, with the aim of guiding the discovery of novel ferroptosis inhibitors.

The contribution of an imbalanced redox signalling to neurological and neurodegenerative conditions

Nitric oxide and other redox active molecules such as oxygen free radicals provide essential signalling in diverse neuronal functions, but their excess production and insufficient scavenging induces cytotoxic redox stress which is associated with numerous neurodegenerative and neurological conditions. A further component of redox signalling is mediated by a homeostatic regulation of divalent metal ions, the imbalance of which contributes to neuronal dysfunction. Additional antioxidant molecules such as glutathione and enzymes such as super oxide dismutase are involved in maintaining a physiological redox status within neurons. When cellular processes are perturbed and generation of free radicals overwhelms the antioxidants capacity of the neurons, a resulting redox damage leads to neuronal dysfunction and cell death. Cellular sources for production of redox-active molecules may include NADPH oxidases, mitochondria, cytochrome P450 and nitric oxide (NO)-generating enzymes, such as endothelial, neuronal and inducible NO synthases. Several neurodegenerative and developmental neurological conditions are associated with an imbalanced redox state as a result of neuroinflammatory processes leading to nitrosative and oxidative stress. Ongoing research aims at understanding the causes and consequences of such imbalanced redox homeostasis and its role in neuronal dysfunction.

15-Lipoxygenase and its metabolites in the pathogenesis of breast cancer: A double-edged sword

15-lipoxygenase is one of the key enzymes for the metabolism of unsaturated fatty acids that its manipulation has been proposed recently as a new molecular target for regulating cancer cell growth. Aberrant expression of 15-lipoxygenase enzyme seems to play an indicative role in the pathology of different cancer types, tumor progression, metastasis, or apoptosis. Based on the fact that breast cancer is one of the most common cancers that imposes a burden of mortality in women also, on the other hand, evidence in experimental models and human studies indicate the emerging role of the 15-lipoxygenase pathway in breast cancer pathogenesis, we present a review of recent findings related to the role of 15- lipoxygenase enzyme and metabolites in breast cancer growth, apoptosis, metastasis, and invasion as well as their local and circulating expression pattern in patients with breast cancer. Our review supports the emerging role of 15- lipoxygenase in molecular and cellular processes regulating breast tumor cell fate with both positive and negative effects.

Structural considerations on lipoxygenase function, inhibition and crosstalk with nitric oxide pathways

Lipoxygenases (LOX) are non-heme iron-containing enzymes that catalyze regio- and stereo-selective dioxygenation of polyunsaturated fatty acids (PUFA). Mammalian LOXs participate in the eicosanoid cascade during the inflammatory response, using preferentially arachidonic acid (AA) as substrate, for the synthesis of leukotrienes (LT) and other oxidized-lipid intermediaries. This review focus on lipoxygenases (LOX) structural and kinetic implications on both catalysis selectivity, as well as the basic and clinical implications of inhibition and interactions with nitric oxide (^•NO) and nitroalkenes pathways. During inflammation ^•NO levels are increasingly favoring the formation of reactive nitrogen species (RNS). ^•NO may act itself as an inhibitor of LOX-mediated lipid oxidation by reacting with lipid peroxyl radicals. Besides, ^•NO may act as an O₂ competitor in the LOX active site, thus displaying a protective role on lipid-peroxidation. Moreover, RNS such as nitrogen dioxide (^•NO₂) may react with lipid-derived species formed during LOX reaction, yielding nitroalkenes (NO₂FA). NO₂FA represents electrophilic compounds that could exert anti-inflammatory actions through the interaction with critical LOX nucleophilic amino acids. We will discuss how nitro-oxidative conditions may limit the availability of common LOX substrates, favoring alternative routes of PUFA metabolization to anti-inflammatory or pro-resolutive pathways.

Redox lipid reprogramming commands susceptibility of macrophages and microglia to ferroptotic death

Ferroptotic death is the penalty for losing control over three processes-iron metabolism, lipid peroxidation and thiol regulation-that are common in the pro-inflammatory environment where professional phagocytes fulfill their functions and yet survive. We hypothesized that redox reprogramming of 15-lipoxygenase (15-LOX) during the generation of pro-ferroptotic signal 15-hydroperoxy-eicosa-tetra-enoyl-phosphatidylethanolamine (15-HpETE-PE) modulates ferroptotic endurance. Here, we have discovered that inducible nitric oxide synthase (iNOS)/NO•-enrichment of activated M1 (but not alternatively activated M2) macrophages/microglia modulates susceptibility to ferroptosis. Genetic or pharmacologic depletion/inactivation of iNOS confers sensitivity on M1 cells, whereas NO• donors empower resistance of M2 cells to ferroptosis. In vivo, M1 phagocytes, in comparison to M2 phagocytes, exert higher resistance to pharmacologically induced ferroptosis. This resistance is diminished in iNOS-deficient cells in the pro-inflammatory conditions of brain trauma or the tumour microenvironment. The nitroxygenation of eicosatetraenoyl (ETE)-PE intermediates and oxidatively truncated species by NO• donors and/or suppression of NO• production by iNOS inhibitors represent a novel redox mechanism of regulation of ferroptosis in pro-inflammatory conditions.

The discovery of nitro-fatty acids as products of metabolic and inflammatory reactions and mediators of adaptive cell signaling

Foundational advances in eicosanoid signaling, the free radical biology of oxygen and nitric oxide and mass spectrometry all converged to enable the discovery of nitrated unsaturated fatty acids. Due to the unique biochemical characteristics of fatty acid nitroalkenes, these species undergo rapid and reversible Michael addition of biological nucleophiles such as cysteine, leading to the post-translational modification of low molecular weight and protein thiols. This capability has led to the present understanding that nitro-fatty acid reaction with the alkylation-sensitive cysteine proteome leads to physiologically-beneficial alterations in transcriptional regulatory protein function, gene expression and in vivo rodent model responses to metabolic and inflammatory stress. These findings motivated the preclinical and clinical development of nitro-fatty acids as new drug candidates for treating acute and chronic metabolic and inflammatory disorders.

Comparison of 12-lipoxygenase expression in vascular smooth muscle cells from old normotensive Wistar-Kyoto rats with spontaneously hypertensive rats

Vascular aging and essential hypertension cause similar structural and molecular modifications in the vasculature. The 12-lipoxygenase (LO) pathway of arachidonic acid metabolism is linked to cell growth and the pathology of hypertension. Thus, elevated expression of 12-LO has been observed in vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR). In the present study, we investigated the differences in 12-LO expression and activity between VSMCs from old normotensive Wistar-Kyoto rats (old WKY, 90-week old) and SHR (13-week old). The protein and mRNA expression of basal or angiotensin II (Ang II)-induced 12-LO in old WKY VSMCs were higher than those in SHR VSMCs. The degradation rate of 12-LO mRNA in old WKY VSMCs was slower than that in SHR VSMCs. However, basal or Ang II-induced 12-LO mRNAs in both old WKY and SHR VSMCs decayed more rapidly than that in young WKY (13-week old) VSMCs. Higher expression of 12-LO in old WKY VSMCs than in SHR VSMCs was correlated with the expression level of Ang II subtype 1 receptor (AT(1)R). The reduced levels of nitric oxide (NO) in old WKY and SHR VSMCs compared with young WKY VSMCs were similar, and there was no significant difference in NO production between old WKY and SHR VSMCs transfected with 12-LO siRNA. In addition, in contrast to the proliferation of SHR VSMCs, the proliferation of old WKY VSMCs was not dependent on 12-LO activation. These results suggest that the potential role of 12-LO in normotensive aging vasculature may be different from that in SHR vasculature.

Nitric oxide inactivation mechanisms in the brain: role in bioenergetics and neurodegeneration

During the last decades nitric oxide ((•)NO) has emerged as a critical physiological signaling molecule in mammalian tissues, notably in the brain. (•)NO may modify the activity of regulatory proteins via direct reaction with the heme moiety, or indirectly, via S-nitrosylation of thiol groups or nitration of tyrosine residues. However, a conceptual understanding of how (•)NO bioactivity is carried out in biological systems is hampered by the lack of knowledge on its dynamics in vivo. Key questions still lacking concrete and definitive answers include those related with quantitative issues of its concentration dynamics and diffusion, summarized in the how much, how long, and how far trilogy. For instance, a major problem is the lack of knowledge of what constitutes a physiological (•)NO concentration and what constitutes a pathological one and how is (•)NO concentration regulated. The ambient (•)NO concentration reflects the balance between the rate of synthesis and the rate of breakdown. Much has been learnt about the mechanism of (•)NO synthesis, but the inactivation pathways of (•)NO has been almost completely ignored. We have recently addressed these issues in vivo on basis of microelectrode technology that allows a fine-tuned spatial and temporal measurement (•)NO concentration dynamics in the brain.

Nitric oxide signaling in the microcirculation

Several apparent paradoxes are evident when one compares mathematical predictions from models of nitric oxide (NO) diffusion and convection in vasculature structures with experimental measurements of NO (or related metabolites) in animal and human studies. Values for NO predicted from mathematical models are generally much lower than in vivo NO values reported in the literature for experiments, specifically with NO microelectrodes positioned at perivascular locations next to different sizes of blood vessels in the microcirculation and NO electrodes inserted into a wide range of tissues supplied by the microcirculation of each specific organ system under investigation. There continues to be uncertainty about the roles of NO scavenging by hemoglobin versus a storage function that may conserve NO, and other signaling targets for NO need to be considered. This review describes model predictions and relevant experimental data with respect to several signaling pathways in the microcirculation that involve NO.

N-acetylcysteine impairs survival of luteal cells through mitochondrial dysfunction

N-acetylcysteine (NAC) is known as an antioxidant and used for mucus viscosity reduction. However, this drug prevents or induces cell death depending on the cell type. The response of steroidogenic luteal cells to NAC is unknown. Our data shows that NAC can behave as an antioxidant or prooxidant in dependency on the concentration and mitochondrial energization. NAC elevated the flowcytometric-measured portion of hypodiploid (dying) cells. This rise was completely abolished by aurintricarboxylic acid, an inhibitor of topoisomerase II. NAC increased the secretion of nitric oxide and cellular nitrotyrosine. An image analysis indicated that cells pretreated with NAC and loaded with DHR showed a fluorescent structure probably elicited by the oxidative product of DHR, rhodamine 123 that sequesters mitochondrially. Pretreating luteal cells with NAC or adding NAC directly to mitochondrial fractions followed by assessing the mitochondrial transmembrane potential difference (Deltapsi) by the JC-1 technique demonstrated a marked decrease in Deltapsi. A protonophore restored Deltapsi and rotenone (an inhibitor of respiratory chain complex I) inhibited mitochondrial recovering. Thus, in steroidogenic luteal cells from healthy mature corpus luteum, NAC impairs cellular survival by interfering with mitochondrial metabolism. The protonophore-induced recovering of NAC-provoked decrease in Deltapsi indicates that an ATP synthase-favored route of H(+) re-entry to the matrix is essentially switched off by NAC while other respiratory chain complexes remain intact. These data may be important for therapeutic timing of treatments with NAC. (c) 2010 International Society for Advancement of Cytometry.

Signaling actions of electrophiles: anti-inflammatory therapeutic candidates

Over the past several years, research on biologically relevant electrophiles has been replete with new insights, expanding our understanding of the roles electrophiles play in vivo. Importantly, many electrophiles can form reversible covalent adducts with both proteins and small-molecule thiols in cells. This post-translational protein modification has important ramifications, including changes in protein enzymatic activity, the transduction of signals within and between cells, and alterations in gene expression. Electrophiles modulate a variety of cellular signaling processes that are involved in several major diseases with inflammatory components. The electrophilic fatty-acid derivatives discussed in this work are naturally occurring products of redox reactions and enzymatic activity. Furthermore, several of these electrophilic species and their derivatives represent potential therapeutic candidates.

Electrophilic nitro-fatty acids: anti-inflammatory mediators in the vascular compartment

Vascular inflammatory disorders are often associated with both decreased NO bioavailability and a lack of responsiveness to NO, a consequence of impaired NO biosynthesis, dysregulated l-arginine metabolism, endothelial nitric oxide synthase (eNOS) uncoupling and NO consumption induced by redox reactions of NO. The latter is mediated via oxidative inflammatory conditions altering NO-dependent endothelial function, including vascular tone and cell proliferation. The redox reactions of NO and byproducts such as nitrite can react to yield electrophilic nitro-fatty acid derivatives (NO(2)-FAs) and exemplify a biochemical convergence of reactions participating in NO and lipid signaling. NO(2)-FAs represent a novel therapeutic strategy to treat vascular disorders by improving endothelial dysfunction through enhancing NO signaling and blocking vascular smooth muscle proliferation, inflammation, and maladaptive remodeling.

Nitro-fatty acid inhibition of neointima formation after endoluminal vessel injury

RATIONALE

Fatty acid nitroalkenes are endogenously generated electrophilic byproducts of nitric oxide and nitrite-dependent oxidative inflammatory reactions. Existing evidence indicates nitroalkenes support posttranslational protein modifications and transcriptional activation that promote the resolution of inflammation.

OBJECTIVE

The aim of this study was to assess whether in vivo administration of a synthetic nitroalkene could elicit antiinflammatory actions in vivo using a murine model of vascular injury.

METHODS AND RESULTS

The in vivo administration (21 days) of nitro-oleic acid (OA-NO(2)) inhibited neointimal hyperplasia after wire injury of the femoral artery in a murine model (OA-NO(2) treatment resulted in reduced intimal area and intima to media ratio versus vehicle- or oleic acid (OA)-treated animals,P<0.0001). Increased heme oxygenase (HO)-1 expression accounted for much of the vascular protection induced by OA-NO(2) in both cultured aortic smooth muscle cells and in vivo. Inhibition of HO by Sn(IV)-protoporphyrin or HO-1 small interfering RNA reversed OA-NO(2)-induced inhibition of platelet-derived growth factor-stimulated rat aortic smooth muscle cell migration. The upregulation of HO-1 expression also accounted for the antistenotic actions of OA-NO(2) in vivo, because inhibition of neointimal hyperplasia following femoral artery injury was abolished in HO-1(-/-) mice (OA-NO(2)-treated wild-type versus HO-1(-/-) mice, P=0.016).

CONCLUSIONS

In summary, electrophilic nitro-fatty acids induce salutary gene expression and cell functional responses that are manifested by a clinically significant outcome, inhibition of neointimal hyperplasia induced by arterial injury.

Cardiovascular consequences when nitric oxide and lipid signaling converge

The identification of nitric oxide ((*)NO) as an endogenously produced free radical mediator of endothelial-dependent relaxation and host defense has fundamentally changed concepts of cell signal transduction. Ligand-receptor oriented paradigms of cell signaling were originally centered on the concept of a high affinity and specific interaction between a ligand and its receptor, resulting in the activation of secondary signaling events such as gene expression or modulation of catalytic protein function. While (*)NO ligation of the heme iron of soluble guanylate cyclase is consistent with this perspective, the readily diffusible and broadly reactive (*)NO is increasingly appreciated to react with a vast array of target molecules that mediate paracrine vasodilator actions, inhibition of thrombosis and neointimal proliferation, and both pro- and antiinflammatory signaling reactions that are not affected by inhibitors of soluble guanylate cyclase. There is an expanding array of functionally significant "off target" collateral reactions mediated by (*)NO that are guanylate cyclase-independent and rather are dictated by anatomic distribution and the formation of secondary (*)NO-derived species. These reactions are a critical element of redox-regulated signaling and are addressed herein in the context of the oxidation of unsaturated fatty acids to vascular and inflammatory signaling mediators. Because of their abundance and the intrinsic reactivity of unsaturated lipid intermediates and eicosanoid metabolism enzymes with (*)NO and other oxides of nitrogen, lipid signaling mechanisms are a significant target for regulation by (*)NO in the vascular compartment. This convergence of (*)NO and lipid signaling pathways thus adds another level of regulation to physiological responses such as vasodilation, thrombosis, and inflammation. Herein, interactions between (*)NO and lipid signaling events are placed in the context of cardiovascular regulation.

What is the real physiological NO concentration in vivo?

Clarity about the nitric oxide (NO) concentrations existing physiologically is essential for developing a quantitative understanding of NO signalling, for performing experiments with NO that emulate reality, and for knowing whether or not NO concentrations become abnormal in disease states. A decade ago, a value of about 1 μM seemed reasonable based on early electrode measurements and a provisional estimate of the potency of NO for its guanylyl cyclase-coupled receptors, which mediate physiological NO signal transduction. Since then, numerous efforts to measure NO concentrations directly using electrodes in cells and tissues have yielded an irreconcilably large spread of values. In compensation, data from several alternative approaches have now converged to provide a more coherent picture. These approaches include the quantitative analysis of NO-activated guanylyl cyclase, computer modelling based on the type, activity and amount of NO synthase enzyme contained in cells, the use of novel biosensors to monitor NO release from single endothelial cells and neurones, and the use of guanylyl cyclase as an endogenous NO biosensor in tissue subjected to a variety of challenges. All these independent lines of evidence suggest the physiological NO concentration range to be 100 pM (or below) up to ∼5 nM, orders of magnitude lower than was once thought.

Convergence of nitric oxide and lipid signaling: anti-inflammatory nitro-fatty acids

The signaling mediators nitric oxide ( NO) and oxidized lipids, once viewed to transduce metabolic and inflammatory information via discrete and independent pathways, are now appreciated as interdependent regulators of immune response and metabolic homeostasis. The interactions between these two classes of mediators result in reciprocal control of mediator synthesis that is strongly influenced by the local chemical environment. The relationship between the two pathways extends beyond coregulation of NO and eicosanoid formation to converge via the nitration of unsaturated fatty acids to yield nitro derivatives (NO(2)-FA). These pluripotent signaling molecules are generated in vivo as an adaptive response to oxidative inflammatory conditions and manifest predominantly anti-inflammatory signaling reactions. These actions of NO(2)-FA are diverse, with these species serving as a potential chemical reserve of NO, reacting with cellular nucleophiles to posttranslationally modify protein structure, function, and localization. In this regard these species act as potent endogenous ligands for peroxisome proliferator-activated receptor gamma. Functional consequences of these signaling mechanisms have been shown in multiple model systems, including the inhibition of platelet and neutrophil functions, induction of heme oxygenase-1, inhibition of LPS-induced cytokine release in monocytes, increased insulin sensitivity and glucose uptake in adipocytes, and relaxation of preconstricted rat aortic segments. These observations have propelled further in vitro and in vivo studies of mechanisms of NO(2)-FA signaling and metabolism, highlighting the therapeutic potential of this class of molecules as anti-inflammatory drug candidates.

Cytochrome P450 oxidoreductase participates in nitric oxide consumption by rat brain

In low nanomolar concentrations, NO (nitric oxide) functions as a transmitter in brain and other tissues, whereas near-micromolar NO concentrations are associated with toxicity and cell death. Control of the NO concentration, therefore, is critical for proper brain function, but, although its synthesis pathway is well-characterized, the major route of breakdown of NO in brain is unclear. Previous observations indicate that brain cells actively consume NO at a high rate. The mechanism of this consumption was pursued in the present study. NO consumption by a preparation of central glial cells was abolished by cell lysis and recovered by addition of NADPH. NADPH-dependent consumption of NO localized to cell membranes and was inhibited by proteinase K, indicating the involvement of a membrane-bound protein. Purification of this activity yielded CYPOR (cytochrome P450 oxidoreductase). Antibodies against CYPOR inhibited NO consumption by brain membranes and the amount of CYPOR in several cell types correlated with their rate of NO consumption. NO was also consumed by purified CYPOR but this activity was found to depend on the presence of the vitamin E analogue Trolox (6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid), included in the buffer as a precaution against inadvertent NO consumption by lipid peroxidation. In contrast, NO consumption by brain membranes was independent of Trolox. Hence, it appears that, during the purification process, CYPOR becomes separated from a partner needed for NO consumption. Cytochrome P450 inhibitors inhibited NO consumption by brain membranes, making these proteins likely candidates.

Cytoglobin is expressed in the vasculature and regulates cell respiration and proliferation via nitric oxide dioxygenation

Disposition of the second messenger nitric oxide (NO) in mammalian tissues occurs through multiple pathways including dioxygenation by erythrocyte hemoglobin and red muscle myoglobin. Metabolism by a putative NO dioxygenase activity in non-striated tissues has also been postulated, but the exact nature of this activity is unknown. In the present study, we tested the hypothesis that cytoglobin, a newly discovered hexacoordinated globin, participates in cell-mediated NO consumption. Stable expression of small hairpin RNA targeting cytoglobin in fibroblasts resulted in decreased NO consumption and intracellular nitrate production. These cells were more sensitive to NO-induced inhibition of cell respiration and proliferation, which could be restored by re-expression of human cytoglobin. We also demonstrated cytoglobin expression in adventitial fibroblasts as well as vascular smooth muscle cells from various species including human and found that cytoglobin was expressed in the adventitia and media of intact rat aorta. These results indicate that cytoglobin contributes to cell-mediated NO dioxygenation and represents an important NO sink in the vascular wall.

Peroxynitrite-mediated lipid oxidation and nitration: mechanisms and consequences

Lipid oxidation and nitration represents a novel area of research of relevance in the understanding of inflammatory processes. Peroxynitrite, the product of the diffusion-limited reaction between nitric oxide and superoxide anion, mediates oxidative modifications in lipid systems including cell membranes and lipoproteins. In this review, we discuss the mechanisms of lipid oxidation and nitration by peroxynitrite as well as the influence of physiological molecules and cell targets to redirect peroxynitrite reactivity. We also provide evidence to support that oxidation/nitration of lipids results in the formation of novel signaling modulators of key lipid-metabolizing enzymes.

Redox control of endothelial function and dysfunction: molecular mechanisms and therapeutic opportunities

The endothelium is essential for the maintenance of vascular homeostasis. Central to this role is the production of endothelium-derived nitric oxide (EDNO), synthesized by the endothelial isoform of nitric oxide synthase (eNOS). Endothelial dysfunction, manifested as impaired EDNO bioactivity, is an important early event in the development of various vascular diseases, including hypertension, diabetes, and atherosclerosis. The degree of impairment of EDNO bioactivity is a determinant of future vascular complications. Accordingly, growing interest exists in defining the pathologic mechanisms involved. Considerable evidence supports a causal role for the enhanced production of reactive oxygen species (ROS) by vascular cells. ROS directly inactivate EDNO, act as cell-signaling molecules, and promote protein dysfunction, events that contribute to the initiation and progression of endothelial dysfunction. Increasing data indicate that strategies designed to limit vascular ROS production can restore endothelial function in humans with vascular complications. The purpose of this review is to outline the various ways in which ROS can influence endothelial function and dysfunction, describe the redox mechanisms involved, and discuss approaches for preventing endothelial dysfunction that may highlight future therapeutic opportunities in the treatment of cardiovascular disease.

15-Lipoxygenase-1 Prevents Vascular Endothelial Growth Factor A– and Placental Growth Factor–Induced Angiogenic Effects in Rabbit Skeletal Muscles via Reduction in Growth Factor mRNA Levels, NO Bioactivity, and Downregulation of VEGF Receptor 2 Expression

Human 15-lipoxygenase-1 (15-LO-1) is an oxidizing enzyme capable of producing reactive lipid hydroperoxides. 15-LO-1 and its products have been suggested to be involved in many pathological conditions, such as inflammation, atherogenesis, and carcinogenesis. We used adenovirus-mediated gene transfers to study the effects of 15-LO-1 on vascular endothelial growth factor (VEGF)-A 165 – and placental growth factor (PlGF)-induced angiogenesis in rabbit skeletal muscles. 15-LO-1 significantly decreased all angiogenic effects induced by these growth factors, including capillary perfusion, vascular permeability, vasodilatation, and an increase in capillary number. The effects are attributable to the reduction in the amount of VEGF-A 165 and PlGF transcripts by 15-LO-1, resulting in reduced protein expression. The most likely mediator of the VEGF family–induced capillary vasodilatation is nitric oxide (NO), which is produced by NO synthases. Endothelial NO synthase protein expression and NO synthase activity were significantly induced by VEGF-A 165 , and these inductions were reduced by 15-LO-1. VEGF-A 165 induces its angiogenic effects primarily via vascular endothelial growth factor receptor (VEGFR)2, and also PlGF mediates angiogenic signaling via VEGFR2, even though it binds to VEGFR1. VEGFR2 expression is induced by peroxisome proliferator-activating receptor γ. We showed by quantitative RT-PCR and immunohistochemistry that expression of endogenous rabbit peroxisome proliferator-activating receptor γ and VEGFR2 were significantly increased in the growth factor–transduced muscles, but these inductions were efficiently prevented by 15-LO-1. In conclusion, the results suggest that expression of 15-LO-1 has an efficient antiangiogenic effect in vivo via reduction in growth factor mRNA levels, NO bioactivity, and VEGFR2 expression.

Inactivation of nitric oxide by rat cerebellar slices

Nitric oxide (NO) functions as an intercellular messenger throughout the brain. For this role to be performed efficiently, there must be a mechanism for neutralizing NO, but whether an active biological process exists, or whether NO is lost mainly through diffusion is unclear. To investigate this issue, rat cerebellar slices were exposed to constant levels of NO and the cGMP generated within the slice used as an indicator of NO concentrations therein. NO was about 1000-fold less potent in slices (EC50, 1 microM) than in separated cells from the same tissue (EC50, 1.6 nM), consistent with access of NO to the slice interior being greatly hindered by inactivation. Supporting this interpretation, immunohistochemical analysis indicated a marked concentration gradient of cGMP across the thickness of slices exposed to subsaturating NO concentrations, signifying a marked NO gradient. Several known NO-degrading processes, including reaction with lipid peroxyl radicals, erythrocytes and superoxide ions, were eliminated as contributing factors, indicating a novel mechanism. A diffusion-inactivation model was used to estimate the kinetics of NO consumption by the slices. The best fits to experimental data indicated a Michaelis-Menten-type reaction having a Vmax of 1-2 microM s-1 and a Km of around 10 nM. The rates predict that inactivation would impose a very short half-life (<10 ms) on NO in physiological concentrations (up to 10 nM) and that it would play an important role in shaping the NO concentration profiles when it is synthesized by multiple nearby sites.

Nitric oxide consumption through lipid peroxidation in brain cell suspensions and homogenates

Mechanisms which inactivate NO (nitric oxide) are probably important in governing the physiological and pathological effects of this ubiquitous signalling molecule. Cells isolated from the cerebellum, a brain region rich in the NO signalling pathway, consume NO avidly. This property was preserved in brain homogenates and required both particulate and supernatant fractions. A purified fraction of the particulate component was rich in phospholipids, and NO consumption was inhibited by procedures that inhibited lipid peroxidation, namely a transition metal chelator, the vitamin E analogue Trolox and ascorbate oxidase. The requirement for the supernatant was accounted for by its content of ascorbate which catalyses metal-dependent lipid peroxidation. The NO-degrading activity of the homogenate was mimicked by a representative mixture of brain lipids together with ascorbate and, under these conditions, the lipids underwent peroxidation. In a suspension of cerebellar cells, there was a continuous low level of lipid peroxidation, and consumption of NO by the cells was decreased by approx. 50% by lipid-peroxidation inhibitors. Lipid peroxidation was also abolished when NO was supplied at a continuously low rate (approximately 100 nM/min), which explains why NO consumption by this process is saturable. Part of the activity remaining after the inhibition of lipid peroxidation was accounted for by contaminating red blood cells, but there was also another component whose activity was greatly enhanced when the cells were maintained under air-equilibrated conditions. A similar NO-consuming process was present in cerebellar glial cells grown in tissue culture but not in blood platelets or leucocytes, suggesting a specialized mechanism.

Inflammation and immune regulation by 12/15-lipoxygenases

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

A New Approach for Studying Fast Biological Reactions Involving Nitric Oxide: Generation of NO Using Photolabile Ruthenium and Manganese NO Donors

Nitric oxide (NO) is recognized as one of the major players in various biochemical processes, including blood pressure, neurotransmission and immune responses. However, experimental studies involving NO are often limited by difficulties associated with the use of NO gas, including its toxicity and precise control over NO concentration. Moreover, the reactions of NO with biological molecules, which frequently occur on time scales of microseconds or faster, are limited by the millisecond time scale of conventional stopped-flow techniques. Here we present a new approach for studying rapid biological reactions involving NO. The method is based on designed ruthenium and manganese nitrosyls, [Ru(PaPy3)(NO)](BF4)2 and [Mn(PaPy3)(NO)](ClO4) (PaPy3H = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide), which upon photolysis produce NO on a fast time scale. The kinetics of the binding of the photogenerated NO to reduced cytochrome c oxidase (CcO) and myoglobin (Mb) was investigated using time-resolved optical absorption spectroscopy. The NO was found to bind to reduced CcO with an apparent lifetime of 77 micros using the [Mn(PaPy3)(NO)]+ complex; the corresponding rate is 10-20 times faster than can be detected by conventional stopped-flow methods. Second-order rate constants of approximately 1 x 10(8) M(-1) s(-1) and approximately 3 x 10(7) M(-1) s(-1) were determined for NO binding to reduced CcO and Mb, respectively. The generation of NO by photolysis of these complexes circumvents the rate limitation of stopped-flow techniques and offers a novel alternative to study other fast biological reactions involving NO.

Tyrosine nitration by superoxide and nitric oxide fluxes in biological systems: modeling the impact of superoxide dismutase and nitric oxide diffusion

Tyrosine nitration is a posttranslational modification observed in many pathologic states that can be associated with peroxynitrite (ONOO(-)) formation. However, in vitro, peroxynitrite-dependent tyrosine nitration is inhibited when its precursors, superoxide (O(2)*(-)) and nitric oxide ((*)NO), are formed at ratios (O(2)*(-)/(*)NO) different from one, severely questioning the use of 3-nitrotyrosine as a biomarker of peroxynitrite-mediated oxidations. We herein hypothesize that in biological systems the presence of superoxide dismutase (SOD) and the facile transmembrane diffusion of (*)NO preclude accumulation of O(2)*(-) and (*)NO radicals under flux ratios different from one, preventing the secondary reactions that result in the inhibition of 3-nitrotyrosine formation. Using an array of reactions and kinetic constants, computer-assisted simulations were performed in order to assess the flux of 3-nitrotyrosine formation (J(NO(2(-))Y)) during exposure to simultaneous fluxes of superoxide (J(O(2)*(-))) and nitric oxide (J((*)NO)), varying the radical flux ratios (J(O(2)*(-))/ J((*)NO)), in the presence of carbon dioxide. With a basic set of reactions, J(NO(2(-))Y) as a function of radical flux ratios rendered a bell-shape profile, in complete agreement with previous reports. However, when superoxide dismutation by SOD and (*)NO decay due to diffusion out of the compartment were incorporated in the model, a quite different profile of J(NO(2(-))Y) as a function of the radical flux ratio was obtained: despite the fact that nitration yields were much lower, the bell-shape profile was lost and the extent of tyrosine nitration was responsive to increases in either O(2)*(-) or (*)NO, in agreement with in vivo observations. Thus, the model presented herein serves to reconcile the in vitro and in vivo evidence on the role of peroxynitrite in promoting tyrosine nitration.

Depletion of iNOS-derived nitric oxide by prostaglandin H synthase-2 in inflammation-activated J774.2 macrophages through lipohydroperoxidase turnover

PGHS-2 (prostaglandin H synthase-2) is induced in mammalian cells by pro-inflammatory cytokines in tandem with iNOS [high-output ('inducible') nitric oxide synthase], and is co-localized with iNOS and nitrotyrosine in human atheroma macrophages. Herein, murine J774.2 macrophages incubated with lipopolysaccharide and interferon gamma showed induction of PGHS-2 and generated NO using iNOS that could be completely depleted by 12(S)-HPETE [12(S)-hydroperoxyeicosatetraenoic acid; 2.4 muM] or hydrogen peroxide (500 microM) (0.42+/-0.084 and 0.38+/-0.02 nmol x min(-1) x 10(6) cells(-1) for HPETE and H2O2 respectively). COS-7 cells transiently transfected with human PGHS-2 also showed HPETE- or H2O2-dependent NO decay (0.44+/-0.016 and 0.20+/-0.04 nmol x min(-1) x 10(6) cells(-1) for 2.4 microM HPETE and 500 microM H2O2 respectively). Finally, purified PGHS-2 consumed NO in the presence of HPETE or H2O2 (168 and 140 microM x min(-1) x microM enzyme(-1) for HPETE and H2O2 respectively), in a haem-dependent manner, with 20 nM enzyme consuming up to 4 microM NO. K(m) (app) values for NO and 15(S)-HPETE were 1.7+/-0.2 and 0.45+/-0.16 microM respectively. These data indicate that PGHS-2 catalytically consumes NO during peroxidase turnover and that pro-inflammatory cytokines simultaneously upregulate NO synthesis and degradation pathways in murine macrophages. Catalytic NO consumption by PGHS-2 represents a novel interaction between NO and PGHS-2 that may impact on the biological effects of NO in vascular signalling and inflammation.

In vivo aspirin supplementation inhibits nitric oxide consumption by human platelets

Antiplatelet therapies improve endothelial function in atherosclerosis, suggesting that platelets regulate vascular nitric oxide (NO) bioactivity in vivo. Herein, washed platelets consumed NO on activation in an aspirin-sensitive manner, and aspirin enhanced platelet NO responses in vitro. To examine whether in vivo aspirin can inhibit platelet NO consumption, a double-blind placebo-controlled study was conducted. After a 2-week nonsteroidal anti-inflammatory drug (NSAID)-free period, healthy men were randomly assigned and administered aspirin (75 mg/d orally) or identical placebo for 14 days, then crossed over to the opposite arm. Following in vivo aspirin, NO consumption by platelets was inhibited 91%. Rate of onset and recovery following aspirin withdrawal was consistent with cyclooxygenase 1 (COX-1) inhibition. In a small substudy, NO consumption by platelets from postmenopausal women was faster in hypercholesterolemics and less sensitive to aspirin (ie, 39% versus 76% inhibition for hypercholesterolemics or normocholesterolemics, respectively). However, 150 mg aspirin/day increased inhibition of NO consumption by platelets of hypercholesterolemics to 80%. Comparisons of platelet COX-1 or -2 expression and urinary 11-dehydro-thromboxane B2 excretion suggested that aspirin was less able to block platelet activation in vivo in hypercholesterolemia. In conclusion, aspirin inhibits NO consumption by platelets from healthy subjects, but its beneficial effects on NO bioactivity may be compromised in some hypercholesterolemic patients.

Apolipoprotein E mimetic Peptide dramatically lowers plasma cholesterol and restores endothelial function in watanabe heritable hyperlipidemic rabbits

BACKGROUND

These studies were designed to determine whether the dual-domain peptide with a class A amphipathic helix linked to the receptor-binding domain of apolipoprotein (apo) E (Ac-hE-18A-NH2) possesses both antidyslipidemic and antiinflammatory properties.

METHODS AND RESULTS

A single bolus (15 mg/kg IV) of Ac-hE-18A-NH2 that contains LRKLRKRLLR (141- to 150-residue region of apo E) covalently linked to apo A-I mimetic peptide 18A not only reduced plasma cholesterol levels (baseline, 562+/-29.0 mg/dL versus 287.7+/-22.0 mg/dL at 18 hours, P<0.001) in the Watanabe heritable hyperlipidemic rabbit model but also significantly improved arterial endothelial function. This improvement was associated with a reduction in 2 markers of oxidative stress. First, the plasma lipid hydroperoxide content was reduced significantly, an effect associated with a 5-fold increase in HDL paraoxonase activity. Second, the formation of superoxide anion, a scavenger of nitric oxide, was also significantly reduced in arteries of these animals.

CONCLUSIONS

Because dyslipidemia and endothelial dysfunction are common features of the atherosclerotic disease process, this unique dual-domain peptide has ideal composite properties that ameliorate key contributory factors to atherosclerosis.

Elevated endothelial nitric oxide bioactivity and resistance to angiotensin-dependent hypertension in 12/15-lipoxygenase knockout mice

12/15-Lipoxygenase (12/15-LOX) plays a pathogenic role in atherosclerosis. To characterize whether 12/15-LOX also contributes to endothelial dysfunction and hypertension, regulation of vessel tone and angiotensin II (ang II) responses were characterized in mice deficient in 12/15-LOX. There was a twofold increase in the magnitude of l-nitroarginine-methyl ester-inhibitable, acetylcholine-dependent relaxation or phenylephrine-dependent constriction in aortic rings isolated from 12/15-LOX(-/-) mice. Plasma NO metabolites and aortic endothelial NO synthase (eNOS) expression were also elevated twofold. Angiotensin II failed to vasoconstrict 12/15-LOX(-/-) aortic rings in the absence of L-nitroarginine-methyl ester, and ang II impaired acetylcholine-induced relaxation in wild-type, but not 12/15-LOX(-/-) rings. In vivo, 12/15-LOX(-/-) mice had similar basal systolic blood pressure measurements to wild type, however, blood pressure elevations in response to ang II infusion (1.1 mg/kg/day) were significantly attenuated (maximal pressure, 143.4 +/- 4 mmHg versus 122.1 +/- 5.3 mmHg for wild type and 12/15-LOX(-/-), respectively). In contrast, vascular hypertrophic responses to ang II, and ang II type 1 receptor (AT1-R) expression were similar in both strains. This study shows that 12/15-LOX(-/-) mice have increased NO biosynthesis and impaired ang II-dependent vascular responses in vitro and in vivo, suggesting that 12/15-LOX signaling contributes to impaired NO bioactivity in vascular disease in vivo.

Fatty acid transduction of nitric oxide signaling. Nitrolinoleic acid is a hydrophobically stabilized nitric oxide donor

The aqueous decay and concomitant release of nitric oxide (*NO) by nitrolinoleic acid (10-nitro-9,12-octadecadienoic acid and 12-nitro-9,12-octadecadienoic acid; LNO2) are reported. Mass spectrometric analysis of reaction products supports a modified Nef reaction as the mechanism accounting for the generation of *NO by the aqueous reactions of fatty acid nitroalkene derivatives. Nitrolinoleic acid is stabilized by an aprotic milieu, with LNO2 decay and *NO release strongly inhibited by phosphatidylcholine/cholesterol liposome membranes and detergents when present at levels above their critical micellar concentrations. The release of *NO from LNO2 was induced by UV photolysis and triiodide-based ozone chemiluminescence reactions currently used to quantify putative protein nitrosothiol and N-nitrosamine derivatives. This reactivity of LNO2 complicates the qualitative and quantitative analysis of biological oxides of nitrogen when applying UV photolysis and triiodide-based analytical systems to biological preparations typically abundant in nitrated fatty acids. The results reveal that nitroalkene derivatives of linoleic acid are pluripotent signaling mediators that act not only via receptor-dependent mechanisms, but also by transducing the signaling actions of *NO via pathways subject to regulation by the relative distribution of LNO2 to hydrophobic versus aqueous microenvironments.

Consumption of nitric oxide by endothelial cells: evidence for the involvement of a NAD(P)H-, flavin- and heme-dependent dioxygenase reaction

In the present study, we investigated the mechanism of nitric oxide (NO) inactivation by endothelial cells. All experiments were performed in the presence of superoxide dismutase to minimize the peroxynitrite reaction. Incubation of the NO donor diethylamine/NO adduct with increasing amounts of intact cells led to a progressive decrease of the NO concentration, demonstrating a cell-dependent consumption of NO. In cell homogenates, consumption of NO critically depended on the presence of NADPH or NADH and resulted in the formation of nitrate. Both NO consumption and nitrate formation were largely inhibited by the heme poisons NaCN and phenylhydrazine as well as the flavoenzyme inhibitor diphenylene iodonium. Further characterization of this NO consumption pathway suggests that endothelial cells express a unique membrane-associated enzyme or enzyme system analogous to the bacterial NO dioxygenase that converts NO to nitrate in a NAD(P)H-, flavin- and heme-dependent manner.

Nitroxia: the pathological consequence of dysfunction in the nitric oxide-cytochrome c oxidase signaling pathway

It is now recognized that mitochondria play an integral role in orchestrating the response of the cell to a wide variety of metabolic and environmental stressors. Of particular interest are the interactions of reactive oxygen and nitrogen species with the organelle and their potential regulatory function. The best understood example is the O(2) sensitive binding of NO (nitric oxide) to the heme group in cytochrome c oxidase. We have proposed that this reversible process serves the function of both regulating the formation of hydrogen peroxide from the respiratory chain for the purposes of signal transduction and controlling O(2) gradients in complex organs such as the liver or heart. It now appears that maladaptation in this pathway leads to a mitochondrial dysfunction which has some of the characteristics of hypoxia, such as a deficit in ATP, but occurs in the presence of normal or enhanced levels of O(2). These are the optimal conditions for the formation of reactive nitrogen species (RNS), such as peroxynitrite which lead to the irreversible modification of proteins. We term this unique pathological condition Nitroxia and describe how it may contribute to the pathology of chronic inflammatory diseases using ethanol-dependent hepatotoxicity as an example.

Nitric oxide and redox signaling in allergic airway inflammation

A number of diseases of the respiratory tract, as exemplified in this review by asthma, are associated with increased amounts of nitric oxide (NO) in the expired breath. Asthma is furthermore characterized by increased production of reactive oxygen species that scavenge NO to form more reactive nitrogen species as demonstrated by the enhanced presence of nitrated proteins in the lungs of these patients. This increased oxidative metabolism leaves less bioavailable NO and coincides with lower amounts of S-nitrosothiols. In this review, we speculate on mechanisms responsible for the increased amounts of NO in inflammatory airway disease and discuss the apparent paradox of higher levels of NO as opposed to decreased amounts of S-nitrosothiols. We will furthermore give an overview of the regulation of NO production and biochemical events by which NO transduces signals into cellular responses, with a particular focus on modulation of inflammation by NO. Lastly, difficulties in studying NO signaling and possible therapeutic uses for NO will be highlighted.

Protective effect of N-acetylcysteine against oxygen radical-mediated coronary artery injury

The present study investigated the protective effect of N-acetylcysteine (NAC) against oxygen radical-mediated coronary artery injury. Vascular contraction and relaxation were determined in canine coronary arteries immersed in Kreb's solution (95% O2-5% CO2), incubated or not with NAC (10 mM), and exposed to free radicals (FR) generated by xanthine oxidase (100 mU/ml) plus xanthine (0.1 mM). Rings not exposed to FR or NAC were used as controls. The arteries were contracted with 2.5 microM prostaglandin F2alpha. Subsequently, concentration-response curves for acetylcholine, calcium ionophore and sodium fluoride were obtained in the presence of 20 microM indomethacin. Concentration-response curves for bradykinin, calcium ionophore, sodium nitroprusside, and pinacidil were obtained in the presence of indomethacin plus Nomega-nitro-L-arginine (0.2 mM). The oxidative stress reduced the vascular contraction of arteries not exposed to NAC (3.93 +/- 3.42 g), compared to control (8.56 +/- 3.16 g) and to NAC group (9.07 +/- 4.0 g). Additionally, in arteries not exposed to NAC the endothelium-dependent nitric oxide (NO)-dependent relaxation promoted by acetylcholine (1 nM to 10 microM) was also reduced (maximal relaxation of 52.1 +/- 43.2%), compared to control (100%) and NAC group (97.0 +/- 4.3%), as well as the NO/cyclooxygenase-independent receptor-dependent relaxation provoked by bradykinin (1 nM to 10 microM; maximal relaxation of 20.0 +/- 21.2%), compared to control (100%) and NAC group (70.8 +/- 20.0%). The endothelium-independent relaxation elicited by sodium nitroprusside (1 nM to 1 microM) and pinacidil (1 nM to 10 microM) was not affected. In conclusion, the vascular dysfunction caused by the oxidative stress, expressed as reduction of the endothelium-dependent relaxation and of the vascular smooth muscle contraction, was prevented by NAC.

Nitric oxide metabolism in mammalian cells: substrate and inhibitor profiles of a NADPH-cytochrome P450 oxidoreductase-coupled microsomal nitric oxide dioxygenase

Human intestinal Caco-2 cells metabolize and detoxify NO via a dioxygen- and NADPH-dependent, cyanide- and CO-sensitive pathway that yields nitrate. Enzymes catalyzing NO dioxygenation fractionate with membranes and are enriched in microsomes. Microsomal NO metabolism shows apparent KM values for NO, O2, and NADPH of 0.3, 9, and 2 microM, respectively, values similar to those determined for intact or digitonin-permeabilized cells. Similar to cellular NO metabolism, microsomal NO metabolism is superoxide-independent and sensitive to heme-enzyme inhibitors including CO, cyanide, imidazoles, quercetin, and allicin-enriched garlic extract. Selective inhibitors of several cytochrome P450s and heme oxygenase fail to inhibit the activity, indicating limited roles for a subset of microsomal heme enzymes in NO metabolism. Diphenyleneiodonium and cytochrome c(III) inhibit NO metabolism, suggesting a role for the NADPH-cytochrome P450 oxidoreductase (CYPOR). Involvement of CYPOR is demonstrated by the specific inhibition of the NO metabolic activity by inhibitory anti-CYPOR IgG. In toto, the results suggest roles for a microsomal CYPOR-coupled and heme-dependent NO dioxygenase in NO metabolism, detoxification, and signal attenuation in mammalian cells.

Oxidative stress and endothelial nitric oxide bioactivity

The endothelium plays an important role in the maintenance of vascular homeostasis. Central to this role is the endothelial production of nitric oxide (NO), synthesized by the constitutively expressed endothelial isoform of nitric oxide synthase. Vascular diseases, including hypertension, diabetes, and atherosclerosis, are characterized by impaired endothelium-derived NO bioactivity that may contribute to clinical cardiovascular events. Growing evidence indicates that impaired endothelium-derived NO bioactivity is due, in part, to excess vascular oxidative stress. This review outlines how different forms of oxidative stress can impact on NO bioactivity and discusses strategies to prevent oxidative stress-induced endothelial dysfunction.

Free radicals and lipid signaling in endothelial cells

Lipid mediators generated by oxidative pathways play essential roles in vascular homeostasis and disease through activating signal transduction pathways that control a variety of cellular functions, including vascular tone, gene expression, and leukocyte and platelet activation. Several enzyme families generate oxidized lipids, and a number of these are either constitutively expressed or inducible in the endothelium, including prostaglandin H synthases, lipoxygenases, and cytochrome P450 isoforms. Mediators generated by these enzymes are predominantly arachidonate-derived and include lipid hydroxides, epoxides, hydroperoxides, and prostanoids. These enzymes may also generate low levels of lipid-derived radicals in the vasculature following escape of substrate radicals from the active site. Lipid oxidation enzymes are often up-regulated in atherosclerosis and hypertension, with several lines of evidence suggesting that they play a central role in the pathogenesis of the disease process itself. This review will describe the isoforms of lipid oxidation enzymes present in endothelial cells focusing on their physiological functions and proposed roles in initiation and progression of vascular disease.

Nitrolinoleate, a nitric oxide-derived mediator of cell function: synthesis, characterization, and vasomotor activity

Nitric oxide (*NO) and *NO-derived reactive species rapidly react with lipids during both autocatalytic and enzymatic oxidation reactions to yield nitrated derivatives that serve as cell signaling molecules. Herein we report the synthesis, purification, characterization, and bioactivity of nitrolinoleate (LNO2). Nitroselenylation of linoleic acid yielded LNO2 that was purified by solvent extraction, silicic acid chromatography, and reverse-phase HPLC. Structural characterization was performed by IR spectroscopy, 15N-NMR, LC-negative ion electrospray mass spectroscopy (MS), and chemiluminescent nitrogen analysis. Quantitative MS analysis of cell and vessel LNO2 metabolism, using L[15N]O2 as an internal standard, revealed that LNO2 is rapidly metabolized by rat aortic smooth muscle (RASM) monolayers and rat thoracic aorta, resulting in nitrite production and up to 3-fold increases in cGMP (ED50 = 30 microM for RASM, 50 microM for aorta). LNO2 induced endothelium-independent relaxation of preconstricted rat aortic rings, which was unaffected by L(G)-nitro-l-arginine methyl ester addition and inhibited by the guanylate cyclase inhibitor 1H-[1,2,4] oxadiazole[4,3-a]quinoxalin-1-one and the *NO scavenger HbO2. These results reveal that synthetic LNO2, identical to lipid derivatives produced biologically by the reaction of *NO and *NO-derived species with oxidizing unsaturated fatty acids (e.g., linoleate), can transduce vascular signaling actions of *NO.

Characterization of nitric oxide consumption pathways by normal, chronic granulomatous disease and myeloperoxidase-deficient human neutrophils

The detailed mechanisms by which acutely activated leukocytes metabolize NO and regulate its bioactivity are unknown. Therefore, healthy, chronic granulomatous disease (CGD) or myeloperoxidase (MPO)-deficient human neutrophils were examined for their ability to consume NO and attenuate its signaling. fMLP or PMA activation of healthy neutrophils caused NO consumption that was fully blocked by NADPH oxidase inhibition, and was absent in CGD neutrophils. Studies using MPO-deficient neutrophils, enzyme inhibitors, and reconstituted NADPH oxidase ruled out additional potential NO-consuming pathways, including Fenton chemistry, PGH synthase, lipoxygenase, or MPO. In particular, the inability of MPO to consume NO resulted from lack of H(2)O(2) substrate since all superoxide (O(2)(-.) reacted to form peroxynitrite. For healthy or MPO-deficient cells, NO consumption rates were 2- to 4-fold greater than O(2)(-.) generation, significantly faster than expected from 1:1 termination of NO with O(2)(-.). Finally, fMLP or PMA-stimulated NO consumption fully blocked NO-dependent neutrophil cGMP synthesis. These data reveal NADPH oxidase as the central regulator of NO signaling in human leukocytes. In addition, they demonstrate an important functional difference between CGD and either normal or MPO-deficient human neutrophils, namely their inability to metabolize NO which will alter their ability to adhere and migrate in vivo.

Interaction between nitric oxide, reactive oxygen intermediates, and peroxynitrite in the regulation of 5-lipoxygenase metabolism

We have shown that overnight lipopolysaccharide (LPS) suppresses alveolar macrophage (AM) leukotriene (LT) synthesis mediated in part by induction of inducible nitric oxide synthase (iNOS) and NO production. Here we examined the possibility that reactive oxygen intermediates (ROI) generated by LPS pretreatment contribute to the suppression of 5-lipoxygenase (5-LO) metabolism. Pretreatment of AM with xanthine/xanthine oxidase, which generates high concentrations of ROI, resulted in suppression of LT synthetic capacity. Since NO and ROI reactive species are known to react and form peroxynitrite (ONOO(-)), we examined the effect of ONOO(-) on 5-LO metabolism. Exogenous ONOO(-) caused a dose-dependent suppression of recombinant 5-LO cell-free activity. ONOO(-) also suppressed LT synthesis in intact AM, which was reversed by the ONOO(-) scavenger tetrakis(4-benzoic acid)porphyrin. ONOO(-) treatment also resulted in dose-dependent nitrotyrosination and S-nitrosylation of the recombinant 5-LO enzyme. Since the direct 5-LO inhibitor zileuton prevents the LPS-induced suppression of LT synthesis, we examined if 5-LO itself was the source of ROI. Zileuton reduced ROI generation in LPS-treated cells. These studies identify an important role for ROI and ONOO(-) in the suppression of 5-LO metabolism by LPS.

The nitration of proteins in platelets: significance in platelet function

Exogenous peroxynitrite has been shown to inhibit or activate platelets according to the concentration added and, at the same time, nitrate platelet proteins. Here, recent evidence is discussed which indicates that nitration of proteins may also occur during normal platelet activation by collagen, by mechanical stimulation during isolation and by exposure to low levels of hydrogen peroxide. Furthermore, this nitration appears to be transient. The implications of these findings are discussed in terms of platelet biology and cell signaling processes.

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.

Regulation of enzymatic lipid peroxidation: the interplay of peroxidizing and peroxide reducing enzymes

For a long time lipid peroxidation has only been considered a deleterious process leading to disruption of biomembranes and thus, to cellular dysfunction. However, when restricted to a certain cellular compartment and tightly regulated, lipid peroxidation may have beneficial effects. Early on during evolution of living organisms special lipid peroxidizing enzymes, called lipoxygenases, appeared and they have been conserved during phylogenesis of plants and animals. In fact, a diverse family of lipoxygenase isoforms has evolved starting from a putative ancient precursor. As with other enzymes, lipoxygenases are regulated on various levels of gene expression and there are endogenous antagonists controlling their cellular activity. Among the currently known mammalian lipoxygenase isoforms only 12/15-lipoxygenases are capable of directly oxygenating ester lipids even when they are bound to membranes and lipoproteins. Thus, these enzymes represent the pro-oxidative part in the cellular metabolism of complex hydroperoxy ester lipids. Its metabolic counterplayer, representing the antioxidative part, appears to be the phospholipid hydroperoxide glutathione peroxidase. This enzyme is unique among glutathione peroxidases because of its capability of reducing ester lipid hydroperoxides. Thus, 12/15-lipoxygenase and phospholipid hydroperoxide glutathione peroxidase constitute a pair of antagonizing enzymes in the metabolism of hydroperoxy ester lipids, and a balanced regulation of the two proteins appears to be of major cell physiological importance. This review is aimed at summarizing the recent developments in the enzymology and molecular biology of 12/15-lipoxygenase and phospholipid hydroperoxide glutathione peroxidase, with emphasis on cytokine-dependent regulation and their regulatory interplay.