Reactions of prostaglandin endoperoxide synthase with nitric oxide and peroxynitrite

Free radical-dependent inhibition of prostaglandin endoperoxide H Synthase-2 by nitro-arachidonic acid

Prostaglandin endoperoxide H synthase (PGHS) is a heme-enzyme responsible for the conversion of arachidonic acid (AA) to prostaglandin H₂ (PGH₂). PGHS have both oxygenase (COX) and peroxidase (POX) activities and is present in two isoforms (PGHS-1 and -2) expressed in different tissues and cell conditions. It has been reported that PGHS activity is inhibited by the nitrated form of AA, nitro-arachidonic acid (NO₂AA), which in turn could be synthesized by PGHS under nitro-oxidative conditions. Specifically, NO₂AA inhibits COX in PGHS-1 as well as POX in both PGHS-1 and -2, in a dose and time-dependent manner. NO₂AA inhibition involves lowering the binding stability and displacing the heme group from the active site. However, the complete mechanism remains to be understood. This review describes the interactions of PGHS with NO₂AA, focusing on mechanisms of inhibition and nitration. In addition, using a novel approach combining EPR-spin trapping and mass spectrometry, we described possible intermediates formed during PGHS-2 catalysis and inhibition. This literature revision as well as the results presented here strongly suggest a free radical-dependent inhibitory mechanism of PGHS-2 by NO₂AA. This is of relevance towards understanding the underlying mechanism of inhibition of PGHS by NO₂AA and its anti-inflammatory potential.

Neuroprotective effect of cyclooxygenase inhibitors in ICV-STZ induced sporadic Alzheimer's disease in rats

Sporadic Alzheimer's disease is an age-related neurological and psychiatric disorder characterized by impaired energy metabolism. Oxidative stress and neuroinflammation have been implicated in pathophysiology of sporadic type of dementia. The central streptozotocin administration induces behavioral and biochemical alterations resembling those in sporadic type of Alzheimer's patients. The present study was designed to investigate the effects of chronic pretreatment with cyclooxygenase-1 or cyclooxygenase-2 or cyclooxygenase-3 selective inhibitors on cognitive dysfunction and oxidative stress markers in intracerebroventricular streptozotocin-treated rats. Chronic treatment with valeryl salicylate (5 and 10 mg/kg, i.p.) and etoricoxib (5 and 10 mg/kg, i.p.) on a daily basis for a period of 21 days, beginning 1 h prior to first intracerebroventricular streptozotocin injection, significantly improved streptozotocin-induced cognitive impairment. However, phenacetin (20 and 40 mg/kg, i.p.) failed to restore the cognitive performances of streptozotocin-treated rats. Besides, improving cognitive dysfunction, chronic administration of highly selective cyclooxygenase-1 and/or cyclooxygenase-2 inhibitors (valeryl salicylate and etoricoxib, respectively), but not cyclooxygenase-3 inhibitor (phenacetin), significantly reduced elevated malondialdehyde, nitrite levels, and restored reduced glutathione and superoxide dismutase levels. Furthermore, cyclooxygenase-1 and/or cyclooxygenase-2 inhibitors significantly increased the survival of pyramidal neurons. In summary, we demonstrate for the first time that both cyclooxygenase-1 and cyclooxygenase-2 isoforms, but not cyclooxygenase-3, are involved in the progression of neuronal damage in intracerebroventricular streptozotocin-treated rats.

The biology of reactive intermediates in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune syndrome marked by autoantibody production. Innate immunity is essential to transform humoral autoimmunity into the clinical lupus phenotype. Nitric oxide (NO) is a membrane- permeable signaling molecule involved in a broad array of biologic processes through its ability to modify proteins, lipids, and DNA and alter their function and immunogenicity. The literature regarding mechanisms through which NO regulates inflammation and cell survival is filled with contradictory findings. However, the effects of NO on cellular processes depend on its concentration and its interaction with reactive oxygen. Understanding this interaction will be essential to determine mechanisms through which reactive intermediates induce cellular autoimmunity and contribute to a sustained innate immune response and organ damage in SLE.

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.

Nitro-linoleic acid inhibits vascular smooth muscle cell proliferation via the Keap1/Nrf2 signaling pathway

Nitroalkenes, the nitration products of unsaturated fatty acids formed via NO-dependent oxidative reactions, have been demonstrated to exert strong biological actions in endothelial cells and monocytes/macrophages; however, little is known about their effects on vascular smooth muscle cells (VSMCs). The present study examined the role of nitro-linoleic acid (LNO(2)) in the regulation of VSMC proliferation. We observed that LNO(2) inhibited VSMC proliferation in a dose-dependent manner. In addition, LNO(2) induced growth arrest of VSMCs in the G(1)/S phase of the cell cycle with an upregulation of the cyclin-dependent kinase inhibitor p27(kip1). Furthermore, LNO(2) triggered nuclear factor-erythroid 2-related factor 2 (Nrf2) nuclear translocation and activation of the antioxidant-responsive element-driven transcriptional activity via impairing Kelch-like ECH-associating protein 1 (Keap1)-mediated negative control of Nrf2 activity in VSMCs. LNO(2) upregulated the expression of Nrf2 protein levels, but not mRNA levels, in VSMCs. A forced activation of Nrf2 led to an upregulation of p27(kip1) and growth inhibition of VSMCs. In contrast, knock down of Nrf2 using an Nrf2 siRNA approach reversed the LNO(2)-induced upregulation of p27(kip1) and inhibition of cellular proliferation in VSMCs. These studies provide the first evidence that nitroalkene LNO(2) inhibits VSMC proliferation through activation of the Keap1/Nrf2 signaling pathway, suggesting an important role of nitroalkenes in vascular biology.

Differential effects of selective and non-selective inhibition of nitric oxide synthase on the expression and activity of cyclooxygenase-2 during gastric ulcer healing

Nitric oxide synthases (NOS) and cyclooxygenase-2 (COX-2) are important enzymes involved in ulcer healing but interactions between them have not been clearly defined. The aim of this study was to investigate the effects of selective or non-selective inhibition of NOS on the expression and activity of COX-2 during healing of acetic acid-induced gastric ulcers in rats. N-[3-(aminomethyl)benzyl] acetamidine (1400 W), a potent selective inhibitor of inducible nitric oxide synthase (iNOS), at a dose of 0.1 mg/kg/day, was found to reduce the ulcer sizes at day 3 and 7 post-ulcer induction. On the other hand, 15 mg/kg/day of NG-nitro-L-arginine methyl ester (L-NAME), a non-selective NOS inhibitor that suppresses both iNOS and endothelial nitric oxide synthase (eNOS), enlarged the ulcer sizes over the same time periods. The expression of COX-2 and COX activity, together with NF-kappaB activation in the ulcer tissues were down-regulated by L-NAME but not 1400 W. It is concluded that iNOS may contribute to ulcer formation while COX-2 and eNOS promote ulcer healing. eNOS enhances COX-2 expression possibly through the activation of NF-kappaB.

Vasodilatory mechanisms in contracting skeletal muscle

Skeletal muscle blood flow is closely coupled to metabolic demand, and its regulation is believed to be mainly the result of the interplay of neural vasoconstrictor activity and locally derived vasoactive substances. Muscle blood flow is increased within the first second after a single contraction and stabilizes within ∼30 s during dynamic exercise under normal conditions. Vasodilator substances may be released from contracting skeletal muscle, vascular endothelium, or red blood cells. The importance of specific vasodilators is likely to vary over the time course of flow, from the initial rapid rise to the sustained elevation during steady-state exercise. Exercise hyperemia is therefore thought to be the result of an integrated response of more than one vasodilator mechanism. To date, the identity of vasoactive substances involved in the regulation of exercise hyperemia remains uncertain. Numerous vasodilators such as adenosine, ATP, potassium, hypoxia, hydrogen ion, nitric oxide, prostanoids, and endothelium-derived hyperpolarizing factor have been proposed to be of importance; however, there is little support for any single vasodilator being essential for exercise hyperemia. Because elevated blood flow cannot be explained by the failure of any single vasodilator, a consensus is beginning to emerge for redundancy among vasodilators, where one vasoactive compound may take over when the formation of another is compromised. Conducted vasodilation or flow-mediated vasodilation may explain dilation in vessels (i.e., feed arteries) not directly exposed to vasodilator substances in the interstitium. Future investigations should focus on identifying novel vasodilators and the interaction between vasodilators by simultaneous inhibition of multiple vasodilator pathways.

Inhibitory effects of nardostachin on nitric oxide, prostaglandin E2, and tumor necrosis factor-alpha production in lipopolysaccharide activated macrophages

Nardostachin, which is an iridoid isolated from Patrinia saniculaefolia, was examined by assessing its effect on the production of tumor necrosis factor-alpha (TNF-alpha) and expression of 2 enzymes, inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), in lipopolysaccharide (LPS)-stimulated Raw264.7 macrophages. This compound consistently inhibited the production of nitric oxide (NO) and TNF-alpha production in a dose-dependent manner, with respective IC(50) values of 12.3 and 16.2 microM. The decrease in quantity of NO products was accompanied by a decrease in the iNOS protein level, as assessed by Western blotting probed with specific anti-iNOS antibodies. In addition, this compound also reduced the COX-2 protein expression level and the attendant PGE(2) production in LPS-stimulated macrophages. These results suggest that nardostachin may be useful for inhibiting the production of inflammatory mediators such as TNF-alpha, NO and PGE(2) in inflammatory diseases.

Effects of aminoguanidine and cyclooxygenase inhibitors on nitric oxide and prostaglandin production, and nitric oxide synthase and cyclooxygenase expression induced by lipopolysaccharide in the estrogenized rat uterus

BACKGROUND/OBJECTIVE

The aim of our study was first to investigate if there exists an interaction between nitric oxide (NO) and prostaglandin (PG) generation in the estrogenized rat uterus challenged by lipopolysaccharide (LPS), and, secondly, which isoforms of nitric oxide synthase (NOS) and cyclooxygenase (COX) participate in this process.

METHODS

To study the effect of LPS and to characterize the isoenzymes involved in the process, specific inhibitors of iNOS (aminoguanidine) and COX-II (meloxicam, nimesulide) and non-specific of COX (indomethacin) were injected intraperitoneally to determine their effect on NO and PG production, and on NOS and COX expression induced by LPS in estrogenized rat uterus. NO production was measured by arginine-citrulline conversion assay and PGE(2)/PGF(2alpha,)by radioconversion. Enzyme expression was evaluated by Western blot analysis.

RESULTS

The present work shows that iNOS inhibitor, aminoguanidine, reduced NO and PGE(2)/PGF(2alpha) production induced by LPS injection. Aminoguanidine exerts its effect over the PG metabolism by inhibiting COX-II activity and expression. On the other hand, both indomethacin, a non-selective PG inhibitor, and meloxicam, a COX-II inhibitor, stimulated NO production and reduced PGE(2)/PGF(2alpha) generation. Indomethacin also reduced COX-II and iNOS expression.

CONCLUSION

These results indicate that in the estrogenized rat uterus challenged with LPS, PG and NO interact affecting each other's metabolic pathways. The above findings indicate that the interaction between NOS and COX might be important in the regulation of physiopathologic events during pregnancy.

NO-NSAIDs and cancer: promising novel agents

Three potential applications of NO-donating NSAIDs in human cancer include their use: as chemopreventive agents; against already developed cancers (chemotherapy); and for the control of cancer symptoms, notably cancer pain. The evidence to date of greater safety and enhanced efficacy of NO-donating NSAIDs underscores their potential to prevent colon cancer and overcome the limitations of traditional NSAIDs. NO-donating NSAIDs affect several pathways critical to colon carcinogenesis and this may explain in part their greater efficacy in colon cancer prevention as assessed in preclinical models.

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.

Role of nitric oxide in the metabolism of arachidonic acid in the rat anterior pituitary gland

Nitric oxide (NO) affects cyclooxygenase (COX) and lipooxygenase (LOX) activities in several tissues. The aim of this study was to investigate the effect of NO on the AA metabolism in the anterior pituitary. LOX and COX products from anterior pituitaries of Wistar male rats were determined by [14C]-AA radioconversion method. Sodium nitroprusside (NP, 0.5 mM) and DETA NONOate (1 mM), NO donors, decreased 5-hydroxy-5,8,11,14-eicosatetraenoic acid (5-HETE) synthesis (P<0.05), effects that were reversed by hemoglobin. L-arginine also inhibited LOX activity. To the contrary, the inhibition of NO synthase by L-NAME (0.5 mM) or aminoguanidine (0.5 mM) increased 5-HETE production (P<0.05). COX activity was slightly stimulated by NP and L-arginine. However, DETA NONOate induced a stimulation of the synthesis of all prostanoids (P<0.05), this effect being reversed by hemoglobin. Neither NOS inhibitors nor hemoglobin modified basal prostanoids synthesis. These results indicate that NO inhibits LOX activity and stimulates COX activity in the anterior pituitary gland. The inhibition of LOX by NO may be another mechanism involved in the effects of NO on hormone release in the anterior pituitary.

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

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

Cell signaling by reactive nitrogen and oxygen species in atherosclerosis

The production of reactive oxygen and nitrogen species has been implicated in atherosclerosis principally as means of damaging low-density lipoprotein that in turn initiates the accumulation of cholesterol in macrophages. The diversity of novel oxidative modifications to lipids and proteins recently identified in atherosclerotic lesions has revealed surprising complexity in the mechanisms of oxidative damage and their potential role in atherosclerosis. Oxidative or nitrosative stress does not completely consume intracellular antioxidants leading to cell death as previously thought. Rather, oxidative and nitrosative stress have a more subtle impact on the atherogenic process by modulating intracellular signaling pathways in vascular tissues to affect inflammatory cell adhesion, migration, proliferation, and differentiation. Furthermore, cellular responses can affect the production of nitric oxide, which in turn can strongly influence the nature of oxidative modifications occurring in atherosclerosis. The dynamic interactions between endogenous low concentrations of oxidants or reactive nitrogen species with intracellular signaling pathways may have a general role in processes affecting wound healing to apoptosis, which can provide novel insights into the pathogenesis of atherosclerosis.

Regulation of prostaglandin biosynthesis by nitric oxide is revealed by targeted deletion of inducible nitric-oxide synthase

We investigated the effects of targeted deletion of the inducible NO synthase (iNOS) gene on the formation of prostaglandins in vivo and ex vivo. Peritoneal macrophages were obtained from control and iNOS-deficient mice, and prostaglandin E(2) (PGE(2)) was quantified after stimulation with gamma-interferon and lipopolysaccharide to induce COX-2. Total nitrate and nitrite production was completely abolished in cells from iNOS-deficient animals compared with control cells. PGE(2) formation by cells from iNOS-deficient animals was decreased compared with cells from control animals 80% at 12 h (0.85 +/- 0.90 ng/10(6) cells versus 15.4 +/- 2.1 ng/10(6) cells, p < 0.01) and 74% at 24 h (9.4 +/- 4.3 ng/10(6) cells versus 36.8 +/- 4.1 ng/10(6) cells, p < 0.01). COX-2 protein expression was not significantly different in cells from control or knockout animals. Levels of PGE(2) in the urine of iNOS-deficient mice were decreased 78% (0.24 +/- 0.14 ng/mg of creatinine versus 1.09 +/- 0.66 ng/mg of creatinine, p < 0.01) compared with control animals. In addition, the levels of urinary F(2)-isoprostanes, an index of endogenous oxidant stress, were significantly decreased in iNOS-deficient animals. In contrast, the levels of thromboxane B(2) derived from platelets allowed to aggregate ex vivo were significantly increased in iNOS-deficient mice compared with wild-type mice. These studies support the hypothesis that NO and/or NO-derived species modulate cyclooxygenase activity and eicosanoid production in vivo.