Heme catalyzes tyrosine 385 nitration and inactivation of prostaglandin H2 synthase-1 by peroxynitrite

The importance of mechanosensitive cell mediated prostaglandin and nitric oxide synthesis in the pathogenesis of apical periodontitis: comparative with chronic periodontitis

OBJECTIVES

Mechano-sensitive odontoblast cells, which sense mechanical loading and various stresses in the tooth structure, synthesize early signaling molecules such as prostaglandin E2 (PGE2) and nitric oxide (NO) as an adaptive response. It is thought that these synthesized molecules can be used for the diagnosis and treatment of periodontal and periapical diseases. The aim of this study was to investigate the relationship between the severity of apical periodontitis (AP) and chronic periodontitis (CP) and serum (s) TNF-α, IL-10, PGE2 and NO levels, as well as PGE2 and NO levels in gingival crevicular fluid (GCF) samples.

MATERIALS & METHODS

A total of 185 subjects were divided into three categories: AP group (n = 85), CP group (n = 50) and healthy control group (n = 50). The AP group was divided into 3 subgroups according to abscess scoring (AS-PAI 1, 2 and 3) based on the periapical index. The CP group was divided into 4 subgroups according to the periodontitis staging system (PSS1, 2,3 and 4). After recording the demographic and clinical characteristics of all participants, serum (s) and gingival crevicular fluid (GCF) samples were taken. TNF-α, IL-10, PGE2 and NO levels were measured in these samples.

RESULTS

Unlike serum measurements (sTNF-α, sIL-10, sNO and sPGE2), GCF-NO and GCF-PGE levels of the AP group were significantly higher than the control group in relation to abscess formation (54.4 ± 56.3 vs. 22.5 ± 12.6 µmol/mL, p < 0.001 and 100 ± 98 vs. 41 ± 28 ng/L, p < 0.001, respectively). Confirming this, the GCF-NO and GCF-PGE levels of the AS-PAI 1 group, in which abscesses have not yet formed, were found to be lower than those in AS-PAI 2 and 3, which are characterized by abscess formation [(16.7(3.7-117.8), 32.9(11.8-212.8) and 36.9(4.3-251.6) µmol/mL, p = 0,0131; 46.0(31.4-120.0), 69.6(40.3-424.2) and 74.4(32.1-471.0) ng/L, p = 0,0020, respectively]. Consistent with the increase in PSS, the levels of sTNF [29.8 (8.2-105.5) vs. 16.7(6.3-37.9) pg/mL, p < 0.001], sIL-10 [542(106-1326) vs. 190(69-411) pg/mL, p < 0.001], sNO [182.1(36.3-437) vs. 57.0(15.9-196) µmol/mL, p < 0.001], sPGE2 [344(82-1298) vs. 100(35-1178) ng/L, p < 0.001], GCF-NO [58.9 ± 33.6 vs. 22.5 ± 12.6 ng/L, p < 0.001] and GCF-PGE2 [ 99(37-365) vs. 30(13-119), p < 0.001] in the CP group were higher than the control group. Comparison ROC analysis revealed that the GCF-PGE2 test had the best diagnostic value for both AP and CP (sensitivity: 94.1 and 88.0; specificity: 64.0 and 78.0, respectively; p < 0.001).

CONCLUSIONS

GCF-PE2 and GCF-NO have high diagnostic value in the determination of AP and CP, and can be selected as targets to guide treatment. In addition, the measurements of PGE2 and NO in GCF can be used as an important predictor of pulpal necrosis leading to abscess in patients with AP.

CLINICAL RELEVANCE

In this article, it is reported that syntheses of early signaling molecules such as PGE2 and NO can be used for the diagnosis and treatment target of periapical and periodontal infections.

Unveiling the human nitroproteome: Protein tyrosine nitration in cell signaling and cancer

Covalent amino acid modification significantly expands protein functional capability in regulating biological processes. Tyrosine residues can undergo phosphorylation, sulfation, adenylation, halogenation, and nitration. These posttranslational modifications (PTMs) result from the actions of specific enzymes: tyrosine kinases, tyrosyl-protein sulfotransferase(s), adenylate transferase(s), oxidoreductases, peroxidases, and metal-heme containing proteins. Whereas phosphorylation, sulfation, and adenylation modify the hydroxyl group of tyrosine, tyrosine halogenation and nitration target the adjacent carbon residues. Because aberrant tyrosine nitration has been associated with human disorders and with animal models of disease, we have created an updated and curated database of 908 human nitrated proteins. We have also analyzed this new resource to provide insight into the role of tyrosine nitration in cancer biology, an area that has not previously been considered in detail. Unexpectedly, we have found that 879 of the 1971 known sites of tyrosine nitration are also sites of phosphorylation suggesting an extensive role for nitration in cell signaling. Overall, the review offers several forward-looking opportunities for future research and new perspectives for understanding the role of tyrosine nitration in cancer biology.

Paracetamol - An old drug with new mechanisms of action

Paracetamol (acetaminophen) is the most commonly used over-the-counter (OTC) drug in the world. Despite its popularity and use for many years, the safety of its application and its mechanism of action are still unclear. Currently, it is believed that paracetamol is a multidirectional drug and at least several metabolic pathways are involved in its analgesic and antipyretic action. The mechanism of paracetamol action consists in inhibition of cyclooxygenases (COX-1, COX-2, and COX-3) and involvement in the endocannabinoid system and serotonergic pathways. Additionally, paracetamol influences transient receptor potential (TRP) channels and voltage-gated Kv7 potassium channels and inhibits T-type Cav3.2 calcium channels. It also exerts an impact on L-arginine in the nitric oxide (NO) synthesis pathway. However, not all of these effects have been clearly confirmed. Therefore, the aim of our paper was to summarize the current state of knowledge of the mechanism of paracetamol action with special attention to its safety concerns.

3-Nitrotyrosine and related derivatives in proteins: precursors, radical intermediates and impact in function

Oxidative post-translational modification of proteins by molecular oxygen (O2)- and nitric oxide (•NO)-derived reactive species is a usual process that occurs in mammalian tissues under both physiological and pathological conditions and can exert either regulatory or cytotoxic effects. Although the side chain of several amino acids is prone to experience oxidative modifications, tyrosine residues are one of the preferred targets of one-electron oxidants, given the ability of their phenolic side chain to undergo reversible one-electron oxidation to the relatively stable tyrosyl radical. Naturally occurring as reversible catalytic intermediates at the active site of a variety of enzymes, tyrosyl radicals can also lead to the formation of several stable oxidative products through radical-radical reactions, as is the case of 3-nitrotyrosine (NO2Tyr). The formation of NO2Tyr mainly occurs through the fast reaction between the tyrosyl radical and nitrogen dioxide (•NO2). One of the key endogenous nitrating agents is peroxynitrite (ONOO-), the product of the reaction of superoxide radical (O2•-) with •NO, but ONOO--independent mechanisms of nitration have been also disclosed. This chemical modification notably affects the physicochemical properties of tyrosine residues and because of this, it can have a remarkable impact on protein structure and function, both in vitro and in vivo. Although low amounts of NO2Tyr are detected under basal conditions, significantly increased levels are found at pathological states related with an overproduction of reactive species, such as cardiovascular and neurodegenerative diseases, inflammation and aging. While NO2Tyr is a well-established stable oxidative stress biomarker and a good predictor of disease progression, its role as a pathogenic mediator has been laboriously defined for just a small number of nitrated proteins and awaits further studies.

Tyrosine Nitration Contributes to Nitric Oxide-Stimulated Degradation of CYP2B6

Human cytochrome P450 (P450) CYP2B6 undergoes nitric oxide (NO)-dependent proteasomal degradation in response to the NO donor dipropylenetriamine NONOate (DPTA) and biologic NO in HeLa and HuH7 cell lines. CYP2B6 is also downregulated by NO in primary human hepatocytes. We hypothesized that NO or derivative reactive nitrogen species may generate adducts of tyrosine and/or cysteine residues, causing CYP2B6 downregulation, and selected Tyr and Cys residues for mutation based on predicted solvent accessibility. CYP2B6V5-Y317A, -Y380A, and -Y190A mutant proteins expressed in HuH7 cells were less sensitive than wild-type (WT) enzyme to degradation evoked by DPTA, suggesting that these tyrosines are targets for NO-dependent downregulation. The Y317A or Y380A mutants did not show increases in high molecular mass (HMM) species after treatment with DPTA or bortezomib + DPTA, in contrast to the WT enzyme. Carbon monoxide-releasing molecule 2 treatment caused rapid suppression of 2B6 enzyme activity, significant HMM species generation, and ubiquitination of CYP2B6 protein but did not stimulate CYP2B6 degradation. The CYP2B6 inhibitor 4-(4-chlorophenyl)imidazole blocked NO-dependent CYP2B6 degradation, suggesting that NO access to the active site is important. Molecular dynamics simulations predicted that tyrosine nitrations of CYP2B6 would cause significant destabilizing perturbations of secondary structure and remove correlated motions likely required for enzyme function. We propose that cumulative nitrations of Y190, Y317, and Y380 by reactive nitrogen species cause destabilization of CYP2B6, which may act synergistically with heme nitrosylation to target the enzyme for degradation. SIGNIFICANCE STATEMENT: This work provides novel insight into the mechanisms by which nitric oxide, which is produced in hepatocytes in response to inflammation, triggers the ubiquitin-dependent proteasomal degradation of the cytochrome P450 (P450) enzyme CYP2B6. Our data demonstrate that both nitration of specific tyrosine residues and interaction of nitric oxide (NO) with the P450 heme are necessary for NO to trigger ubiquitination and protein degradation.

Smoking-Associated Disordering of the Airway Basal Stem/Progenitor Cell Metabotype

The airway epithelium is a complex pseudostratified multicellular layer lining the tracheobronchial tree, functioning as the primary defense against inhaled environmental contaminants. The major cell types of the airway epithelium include basal, intermediate columnar, ciliated, and secretory. Basal cells (BCs) are the proliferating stem/progenitor population that differentiate into the other specialized cell types of the airway epithelium during normal turnover and repair. Given that cigarette smoke delivers thousands of xenobiotics and high levels of reactive molecules to the lung epithelial surface, we hypothesized that cigarette smoke broadly perturbs BC metabolism. To test this hypothesis, primary airway BCs were isolated from healthy nonsmokers (n = 11) and healthy smokers (n = 7) and assessed by global metabolic profiling by liquid chromatography-mass spectrometry. The analysis identified 52 significant metabolites in BCs differentially expressed between smokers and nonsmokers (P < 0.05). These changes included metabolites associated with redox pathways, energy production, and inflammatory processes. Notably, BCs from smokers exhibited altered levels of the key enzyme cofactors/substrates nicotinamide adenine dinucleotide, flavin adenine dinucleotide, acetyl coenzyme A, and membrane phospholipid levels. Consistent with the high burden of oxidants in cigarette smoke, glutathione levels were diminished, whereas 3-nitrotyrosine levels were increased, suggesting that protection of airway epithelial cells against oxidative and nitrosative stress is significantly compromised in smoker BCs. It is likely that this altered metabotype is a reflection of, and likely contributes to, the disordered biology of airway BCs consequent to the stress cigarette smoking puts on the airway epithelium.

Repair Mechanisms in Oxidant-Driven Chronic Inflammatory Disease

The interplay that governs chronic diseases through pathways specifically associated with chronic inflammation remains undefined. Many metabolic events have been identified during the injury and repair process. Nonetheless, the cellular events that control the pathogenesis of inflammation-induced disease have not been fully characterized. We and others reason that chronic inflammatory diseases associated with a cascade of complex network mediators, such as nitric oxide, arachidonic acid metabolites, cytokines, and reactive oxygen species, play a significant role in the governance of alterations in homeostasis, oxidative stress, and thromboatherosclerosis. In this context, we discuss lipid mediators associated with the maintenance of health, including the specialized proresolving mediators that help drive cellular repair. Emphasis is placed on the pathophysiology of chronic metabolic insults involving both the airways and the cardiovascular system during oxidant-driven inflammatory disease. In this review, we highlight new pathways of inquiry that show promise for the identification of those metabolic targets that can improve therapy for chronic inflammation.

Myeloperoxidase scavenges peroxynitrite: A novel anti-inflammatory action of the heme enzyme

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Ferric myeloperoxidase accelerates the decomposition of peroxynitrite.

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Compound I of myeloperoxidase does not promote this removal of peroxynitrite.

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Peroxynitrite removal is enhanced by substances reacting well with myeloperoxidase Compound II.

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In the presence of H₂O₂, peroxynitrite diminishes the chlorinating activity of myeloperoxidase.

Peroxynitrite, a potent pro-inflammatory and cytotoxic species, interacts with a variety of heme containing proteins. We addressed the question whether (i) the interaction of myeloperoxidase (MPO, an enzyme generating hypochlorous acid from hydrogen peroxide and chloride ions) with peroxynitrite affects the clearance of peroxynitrite, and (ii) if peroxynitrite could modulate the chlorinating activity of MPO. Our results show that this interaction promotes the decomposition of the highly reactive pro-inflammatory oxidant, whereby MPO Compound II (but not Compound I) is formed. The efficiency of MPO to remove peroxynitrite was enhanced by l-tyrosine, nitrite and (−)-epicatechin, substances known to reduce Compound II with high reaction rate. Next, peroxynitrite (added as reagent) diminished the chlorinating activity of MPO in the presence of hydrogen peroxide. Alternatively, SIN-1, a peroxynitrite donor, reduced hypochlorous acid formation by MPO, as measured by aminophenyl fluorescein oxidation (time kinetics) and taurine chloramine formation (end point measurement). At inflammatory loci, scavenging of peroxynitrite by MPO may overcome the uncontrolled peroxynitrite decomposition and formation of reactive species, which lead to cell/tissue damage.

Posttranslational modification of Sirt6 activity by peroxynitrite

The mammalian sirtuin 6 (Sirt6) is a site-specific histone deacetylase that regulates chromatin structure and many fundamental biological processes. It inhibits endothelial cell senescence and inflammation, prevents development of cardiac hypertrophy and heart failure, modulates glucose metabolism, and represses tumor growth. The basic molecular mechanisms underlying regulation of Sirt6 enzymatic function are largely unknown. Here we hypothesized that Sirt6 function can be regulated via posttranslational modification, focusing on the role of peroxynitrite, one of the major reactive nitrogen species formed by excessive nitric oxide and superoxide generated during disease processes. We found that incubation of purified recombinant Sirt6 protein with 3-morpholinosydnonimine (SIN-1; a peroxynitrite donor that generates nitric oxide and superoxide simultaneously) increased Sirt6 tyrosine nitration and decreased its intrinsic catalytic activity. Similar results were observed in SIN-1-treated Sirt6, which was overexpressed in HEK293 cells, and in endogenous Sirt6 when human retinal microvascular endothelial cells were treated with SIN-1. To further investigate whether Sirt6 nitration occurs under pathological conditions, we determined Sirt6 nitration and activity in retina using a model of endotoxin-induced retinal inflammation. Our data showed that Sirt6 nitration was increased, whereas its activity was decreased, in this model. With mass spectrometry, we identified that tyrosine 257 in Sirt6 was nitrated after SIN-1 treatment. Mutation of tyrosine 257 to phenylalanine caused loss of Sirt6 activity and abolished SIN-1-induced nitration and decrease in its activity. Mass spectrometry analysis also revealed oxidation of methionine and tryptophan in Sirt6 after SIN-1 treatment. Our results demonstrate a novel regulatory mechanism controlling Sirt6 activity through reactive nitrogen species-mediated posttranslational modification under oxidative and nitrosative stress.

Evaluation of the anti-nitrative effect of plant antioxidants using a cowpea Fe-superoxide dismutase as a target

Nitric oxide cytotoxicity arises from its rapid conversion to peroxynitrite (ONOO(-)) in the presence of superoxide, provoking functional changes in proteins by nitration of tyrosine residues. The physiological significance of this post-translational modification is associated to tissue injury in animals, but has not been yet clarified in plants. The objective of this study was to establish new approaches that could help to understand ONOO(-) reactivity in plants. A recombinant Fe-superoxide dismutase from cowpea (Vigna unguiculata (L.) Walp.), rVuFeSOD, was the target of the ONOO(-)-generator SIN-1, and the anti-nitrative effect of plant antioxidants and haemoglobins was tested in vitro. Nitration on rVuFeSOD was evaluated immunochemically or as the loss of its enzymatic activity. This assay proved to be useful to test a variety of plant compounds for anti-nitrative capacity. Experimental data confirmed that rice (Oryza sativa L.) haemoglobin-1 (rOsHbI) and cowpea leghaemoglobin-2 exerted a protective function against ONOO(-) by diminishing nitration on rVuFeSOD. Both plant haemoglobins were nitrated by SIN-1. The chelator desferrioxamine suppressed nitration in rOsHbI, indicating that Fe plays a key role in the reaction. The removal of the haem moiety in rOsHbI importantly suppressed nitration, evidencing that this reaction may be self-catalyzed. Among small antioxidants, ascorbate remarkably decreased nitration in all tests. The phenolic compounds caffeic acid, gallic acid, pyrogallol, 4-hydroxybenzoic acid and the flavonoid gossypin also diminished tyrosine nitration and protected rVuFeSOD to different extents. It is concluded that small plant antioxidants, especially ascorbate, and haemoglobins may well play key roles in ONOO(-) homeostasis in vivo.

Metal-catalyzed protein tyrosine nitration in biological systems

Protein tyrosine nitration is an oxidative postranslational modification that can affect protein structure and function. It is mediated in vivo by the production of nitric oxide-derived reactive nitrogen species (RNS), including peroxynitrite (ONOO(-)) and nitrogen dioxide ((•)NO₂). Redox-active transition metals such as iron (Fe), copper (Cu), and manganese (Mn) can actively participate in the processes of tyrosine nitration in biological systems, as they catalyze the production of both reactive oxygen species and RNS, enhance nitration yields and provide site-specificity to this process. Early after the discovery that protein tyrosine nitration can occur under biologically relevant conditions, it was shown that some low molecular weight transition-metal centers and metalloproteins could promote peroxynitrite-dependent nitration. Later studies showed that nitration could be achieved by peroxynitrite-independent routes as well, depending on the transition metal-catalyzed oxidation of nitrite (NO₂(-)) to (•)NO₂ in the presence of hydrogen peroxide. Processes like these can be achieved either by hemeperoxidase-dependent reactions or by ferrous and cuprous ions through Fenton-type chemistry. Besides the in vitro evidence, there are now several in vivo studies that support the close relationship between transition metal levels and protein tyrosine nitration. So, the contribution of transition metals to the levels of tyrosine nitrated proteins observed under basal conditions and, specially, in disease states related with high levels of these metal ions, seems to be quite clear. Altogether, current evidence unambiguously supports a central role of transition metals in determining the extent and selectivity of protein tyrosine nitration mediated both by peroxynitrite-dependent and independent mechanisms.

Characterization of a cellular denitrase activity that reverses nitration of cyclooxygenase

Protein 3-nitrotyrosine (3-NT) formation is frequently regarded as a simple biomarker of disease, an irreversible posttranslational modification that can disrupt protein structure and function. Nevertheless, evidence that protein 3-NT modifications may be site selective and reversible, thus allowing for physiological regulation of protein activity, has begun to emerge. We have previously reported that cyclooxygenase (COX)-1 undergoes heme-dependent nitration of Tyr(385), an internal and catalytically essential residue. In the present study, we demonstrate that nitrated COX-1 undergoes a rapid reversal of nitration by substrate-selective and biologically regulated denitrase activity. Using nitrated COX-1 as a substrate, denitrase activity was validated and quantified by analytic HPLC with electrochemical detection and determined to be constitutively active in murine and human endothelial cells, macrophages, and a variety of tissue samples. Smooth muscle cells, however, contained little denitrase activity. Further characterizing this denitrase activity, we found that it was inhibited by free 3-NT and may be enhanced by endogenous nitric oxide and exogenously administered carbon monoxide. Finally, we describe a purification protocol that results in significant enrichment of a discrete denitrase-containing fraction, which maintains activity throughout the purification process. These findings reveal that nitrated COX-1 is a substrate for a denitrase in cells and tissues, implying that the reciprocal processes of nitration and denitration may modulate bioactive lipid synthesis in the setting of inflammation. In addition, our data reveal that denitration is a controlled process that may have broad importance for regulating cell signaling events in nitric oxide-generating systems during oxidative/nitrosative stress.

Autocatalytic nitration of prostaglandin endoperoxide synthase-2 by nitrite inhibits prostanoid formation in rat alveolar macrophages

AIMS

Prostaglandin endoperoxide H(2) synthase (PGHS) is a well-known target for peroxynitrite-mediated nitration. In several experimental macrophage models, however, the relatively late onset of nitration failed to coincide with the early peak of endogenous peroxynitrite formation. In the present work, we aimed to identify an alternative, peroxynitrite-independent mechanism, responsible for the observed nitration and inactivation of PGHS-2 in an inflammatory cell model.

RESULTS

In primary rat alveolar macrophages stimulated with lipopolysaccharide (LPS), PGHS-2 activity was suppressed after 12 h, although the prostaglandin endoperoxide H(2) synthase (PGHS-2) protein was still present. This coincided with a nitration of the enzyme. Coincubation with a nitric oxide synthase-2 (NOS-2) inhibitor preserved PGHS-2 nitration and at the same time restored thromboxane A(2) (TxA(2)) synthesis in the cells. Formation of reactive oxygen species (ROS) was maximal at 4 h and then returned to baseline levels. Nitrite (NO(2)(-)) production occurred later than ROS generation. This rendered generation of peroxynitrite and the nitration of PGHS-2 unlikely. We found that the nitrating agent was formed from NO(2)(-), independent from superoxide ((•)O(2)(-)). Purified PGHS-2 treated with NO(2)(-) was selectively nitrated on the active site Tyr(371), as identified by mass spectrometry (MS). Exposure to peroxynitrite resulted in the nitration not only of Tyr(371), but also of other tyrosines (Tyr).

INNOVATION AND CONCLUSION

The data presented here point to an autocatalytic nitration of PGHS-2 by NO(2)(-), catalyzed by the enzyme's endogenous peroxidase activity and indicate a potential involvement of this mechanism in the termination of prostanoid formation under inflammatory conditions.

The role of nitric oxide in prostaglandin biology; update

The biosynthesis of nitric oxide (NO) and prostaglandin share many similarities. Two major forms of nitric oxide synthase (NOS) and cyclooxygenase (COX) have been identified: constitutive versus inducible. In general, the constitutive form functions in housekeeping and physiologic roles whereas the inducible form is up-regulated by mitogenic or inflammatory stimuli and is responsible for pathophysiological responses. The cross talk between the COX and NOS pathways was initially reported in 1993 and since then, numerous studies have been undertaken to delineate the functional consequences of this interaction as well as the potential mechanism by which each pathway interacts. This review will focus in particular on recent advances in this field that extend our understanding of these two pathways under various systems.

Protein nitrotryptophan: formation, significance and identification

Reactive nitrogen species are formed during a variety of disease states and have been shown to modify several amino acids on proteins. To date, the majority of research in this area has focused on the nitration of tyrosine residues to form 3-nitrotyrosine. However, emerging evidence suggests that another modification, nitration of tryptophan residues, to form nitrotryptophan (NO(2)-Trp), may also play a significant role in the biology of nitrosative stress. This review takes an in-depth look at NO(2)-Trp, presenting the current research about its formation, prevalence and biological significance, as well as the methods used to identify NO(2)-Trp-modified proteins. Although more research is needed to understand the full biological role of NO(2)-Trp, the data presented herein suggest a contribution to nitrosative stress-induced cell dysregulation and perhaps even in physiological cell processes.

Physical evidence for substrate binding in preventing cyclooxygenase inactivation under nitrative stress

Prostaglandin biosynthesis is catalyzed by two spatially and functionally distinct active sites in cyclooxygenase (COX) enzymes. Despite the crucial role of COXs in biology, molecular details regarding the function and regulation of these enzymes are incompletely defined. Reactive nitrogen species, formed during oxidative stress, produce modifications that alter COX functionalities and prostaglandin biosynthesis. We previously established that COX-1 undergoes selective nitration on Tyr385 via a mechanism that requires the presence of bound heme cofactor. As this is a critical residue for COX-1 catalysis, nitration at this site results in enzyme inactivation. We now show that occupancy of the COX-1 active site with substrate protects against Tyr385 nitration and redirects nitration to alternative Tyr residues on COX-1, preserving catalytic activity. This study reveals a novel role for the substrate in protecting COX-1 from inactivation by nitration in pathophysiological settings.

Inducible nitric oxide synthase gene deletion exaggerates MAPK-mediated cyclooxygenase-2 induction by inflammatory stimuli

Cyclooxygenase (COX)-2 and inducible nitric oxide (NO) synthase (iNOS) are responsive to a wide array of inflammatory stimuli, have been localized to vascular smooth muscle cells (SMCs), and are intimately linked to the progression of vascular disease, including atherosclerotic lesion formation. We and others have shown that the production and subsequent impact of COX products appear to be correlative with the status of NO synthesis. This study examined the impact of inflammation-driven NO production on COX-2 expression in SMCs. Concurrent stimulation of quiescent rat aortic SMCs with lipopolysaccharide (LPS) and interferon (IFN)-gamma increased COX-2, iNOS, and nitrite production. Pharmacological inhibition of NO synthase (N(G)-monomethyl-l-arginine) concentration- and time-dependently magnified LPS + IFN-gamma-mediated COX-2 mRNA and protein induction in a cGMP-independent manner. COX-2 induction was associated with activation of the ERK, p38, and JNK mitogen-activated protein kinase (MAPK) pathways. Interestingly, NO synthase inhibition enhanced ERK, p38, and to a lesser extent JNK phosphorylation but suppressed MAPK phosphatase (MKP)-1 induction in response to LPS + IFN-gamma. Similarly, the exposure of SMCs from iNOS(-/-) mice to LPS + IFN-gamma produced an enhancement of COX-2 induction, p38, and JNK phosphorylation and an attenuated upregulation of MKP-1 versus their wild-type counterparts. Taken together, our data indicate that NO, in part derived from iNOS, negatively regulates the immediate early induction of COX-2 in response to inflammatory stimuli.

Silent Partner in Blood Vessel Homeostasis? Pervasive Role of Nitric Oxide in Vascular Disease

The endothelium generates powerful mediators that regulate blood flow, temper inflammation and maintain a homeostatic environment to prevent both the initiation and progression of vascular disease. Nitric oxide (NO) is arguably the single most influential molecule in terms of dictating blood vessel homeostasis. In addition to direct effects associated with altered NO production (e.g. vasoconstriction, excessive inflammation, endothelial dysfunction), NO is a critical modulator of vaso-relevant pathways including cyclooxygenase (COX)-derived prostaglandin production and angiotensin II generation by the renin-angiotensin system. Furthermore, NO may influence the selectivity of COX-2 inhibitors and ultimately contribute to controversies associated with the use of these drugs. Consistent with a central role for NO in vascular disease, disruptions in the production and bioavailability of NO have been linked to hypertension, diabetes, hypercholesterolemia, obesity, aging, and smoking. The ability of the vessel wall to control disease-associated oxidative stress may be the most critical determinant in maintaining homeostatic levels of NO and subsequently the prospect of stroke, myocardial infarction and other CV abnormalities. To this end, investigation of mechanisms that alter the balance of protective mediators, including pathways that are indirectly modified by NO, is critical to the development of effective therapy in the treatment of CV disease.

Myeloperoxidase interaction with peroxynitrite: chloride deficiency and heme depletion

Myeloperoxidase (MPO) is a hemoprotein involved in the leukocyte-mediated defense mechanism and uses hydrogen peroxide (H2O2) and chloride (Cl(-)) to produce hypochlorous acid. In human saliva and in hypochloremic alkalosis syndrome occurring in breast-fed infants, the MPO-H2O2 system functions in a lower Cl(-) concentration (10-70 mM) compared to plasma levels (100 mM) as part of the antibacterial defense system. The impact of low Cl(-) concentration and exposure to high peroxynitrite (ONOO(-)) synthesized from cigarette smoke or oxidative stress on MPO function is still unexplored. Rapid mixing of ONOO(-) and MPO caused immediate formation of a transient intermediate MPO Compound II, which then decayed to MPO-Fe(III). Double mixing of MPO with ONOO(-) followed by H2O2 caused immediate formation of Compound II, followed by MPO heme depletion, a process that occurred independent of ONOO(-) concentration. Peroxynitrite/H2O2-mediated MPO heme depletion was confirmed by HPLC analysis, and in-gel heme staining showing 60-70% less heme content compared to the control. A nonreducing denaturing SDS-PAGE showed no fragmentation or degradation of protein. Myeloperoxidase heme loss was completely prevented by preincubation of MPO with saturating amounts of Cl(-). Chloride binding to the active site of MPO constrains ONOO(-) binding by filling the space directly above the heme moiety or by causing a protein conformational change that constricts the distal heme pocket, thus preventing ONOO(-) from binding to MPO heme iron. Peroxynitrite interaction with MPO may serve as a novel mechanism for modulating MPO catalytic activity, influencing the regulation of local inflammatory and infectious events in vivo.

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.

The role of nitrite ion in phagocyte function--perspectives and puzzles

Macrophages and neutrophils are essential elements of host cellular defense systems that function, at least in part, by generating respiration-driven oxidative toxins in response to external stimuli. In both cells, encapsulation by phagocytosis provides a mechanism to direct the toxins against the microbes. The toxic chemicals formed by these two phagocytic cells differ markedly, as do the enzymatic catalysts that generate them. Nitrite ion is microbicidal under certain conditions, is generated by activated macrophages, and is present at elevated concentration levels at infection sites. In this review, we consider potential roles that nitrite might play in cellular disinfection by these phagocytes within the context of available experimental information. Although the suggested roles are plausible, based upon the chemical and biochemical reactivity of NO2(-), studies to date provide little support for their implementation within phagosomes.

Atherosclerosis: A Link between Lipid Intake and Protein Tyrosine Nitration

Atherosclerosis, a disease characterized by plaque formation in the arterial wall that can lead to heart attack and stroke, is a principal cause of death in the world. Since the 1990's, protein nitrotyrosine formation has been known to occur in the atherosclerotic plaque. This potentially damaging reaction occurs as a result of tyrosine modification by reactive nitrogen species, such as nitrogen dioxide radical, which forms upon peroxynitrite decomposition or nitrite oxidation by hydrogen peroxide-activated peroxidase enzymes. The presence of protein-bound nitrotyrosine can be considered an indicator of a loss in the natural balance of oxidants and antioxidants, and as such, there is an emerging view that protein-bound nitrotyrosine may be a risk factor for cardiovascular disease. This review brings together evidence that the accumulation of protein nitrotyrosine during atherogenesis is more widespread than initially thought (as its presence can be detected not only in the lesion but also in the blood stream and other organs) and is closely linked to lipid intake.

Paracetamol (Acetaminophen): mechanisms of action

Paracetamol has a central analgesic effect that is mediated through activation of descending serotonergic pathways. Debate exists about its primary site of action, which may be inhibition of prostaglandin (PG) synthesis or through an active metabolite influencing cannabinoid receptors. Prostaglandin H(2) synthetase (PGHS) is the enzyme responsible for metabolism of arachidonic acid to the unstable PGH(2). The two major forms of this enzyme are the constitutive PGHS-1 and the inducible PGHS-2. PGHS comprises of two sites: a cyclooxygenase (COX) site and a peroxidase (POX) site. The conversion of arachidonic acid to PGG(2) is dependent on a tyrosine-385 radical at the COX site. Formation of a ferryl protoporphyrin IX radical cation from the reducing agent Fe(3+) at the POX site is essential for conversion of tyrosine-385 to its radical form. Paracetamol acts as a reducing cosubstrate on the POX site and lessens availability of the ferryl protoporphyrin IX radical cation. This effect can be reduced in the presence of hydroperoxide-generating lipoxygenase enzymes within the cell (peroxide tone) or by swamping the POX site with substrate such as PGG(2). Peroxide tone and swamping explain lack of peripheral analgesic effect, platelet effect, and anti-inflammatory effect by paracetamol. Alternatively, paracetamol effects may be mediated by an active metabolite (p-aminophenol). p-Aminophenol is conjugated with arachidonic acid by fatty acid amide hydrolase to form AM404. AM404 exerts effect through cannabinoid receptors. It may also work through PGHS, particularly in areas of the brain with high concentrations of fatty acid amide hydrolase.

SOD2 knockdown mouse model of early AMD

PURPOSE

To test the hypothesis that oxidative injury to the retinal pigment epithelium (RPE) may lead to retinal damage similar to that associated with the early stages of age-related macular degeneration (AMD).

METHODS

A ribozyme that targets the protective enzyme manganese superoxide dismutase (MnSOD) was expressed in RPE-J cells, and adeno-associated virus (AAV) expressing the ribozyme gene was injected beneath the retinas of adult C57BL/6 mice. The RPE/choroid complex was examined for SOD2 protein levels and protein markers of oxidative damage using immunoblot analysis and LC MS/MS-identification of proteins and nitration sites. Lipids were extracted from retinal tissue and analyzed for the bis-retinoid compounds A2E and iso-A2E. The mice were analyzed by full-field electroretinography (ERG) for light response. Light and electron microscopy were used to measure cytological changes in the retinas.

RESULTS

The treatment of RPE-J cells with Rz432 resulted in decreased MnSOD mRNA and protein as well as increased levels of superoxide anion and apoptotic cell death. When delivered by AAV, Rz432 reduced MnSOD protein and increased markers of oxidative damage, including nitrated and carboxyethylpyrrole-modified proteins in the RPE-choroid of mice. Ribozyme delivery caused a progressive loss of electroretinograph response, vacuolization, degeneration of the RPE, thickening of Bruch's membrane, and shortening and disorganization of the photoreceptor outer and inner segments. Progressive thinning of the photoreceptor outer nuclear layer resulted from apoptotic cell death. Similar to the eyes of patients with AMD, ribozyme-treated eyes exhibited increased autofluorescence and elevated levels of A2E and iso-A2E, major bis-retinoid pigments of lipofuscin.

CONCLUSIONS

These results support the hypothesis that oxidative damage to the RPE may play a role in some of the key features of AMD.

Cardiac proinflammatory pathways are altered with different dietary n-6 linoleic to n-3 alpha-linolenic acid ratios in normal, fat-fed pigs

Although dietary fat has been associated with inflammation and cardiovascular diseases (CVD), most studies have focused on individuals with preexisting diseases. However, the role of dietary fatty acids on inflammatory pathways before the onset of any abnormality may be more relevant for identifying initiating factors and interventions for CVD prevention. We fed young male pigs one of three diets differing in n-6 and n-3 polyunsaturated fatty acids (PUFA) linoleic acid (LA, 18:2n-6) and alpha-linolenic acid (ALA, 18:3n-3) for 30 days. Cardiac membrane phospholipid fatty acids, phospholipase A(2) (PLA(2)) isoform activities, and cyclooxygenase (COX)-1 and -2 and 5-lipoxygenase (5-LO) expression were measured. The low PUFA diet (% energy, 1.2% LA+0.06% ALA) increased arachidonic acid (AA) and decreased eicosapentaenoic acid (EPA) in heart membranes and increased Ca(2+)-independent iPLA(2) activity, COX-2 expression, and activation of 5-LO. Increasing dietary ALA while keeping LA constant (1.4% LA+1.2% ALA) decreased the heart membrane AA, increased EPA, and prevented proinflammatory enzyme activation. However, regardless of high ALA, high dietary LA (11.6% LA and 1.2% ALA) decreased EPA and led to a high heart membrane AA, and Ca(2+)-dependent cPLA(2) with a marked increase in nitrosative stress. Our results suggest that the potential cardiovascular benefit of ALA is achieved only when dietary LA is reduced concomitantly rather than fed with high LA diet. The increased nitrosative stress in the unstressed heart with high dietary LA suggests that biomarkers of nitrosative stress may offer a useful early marker of the effects of dietary fat on oxidative tissue stress.

Impact of protein acetylation in inflammatory lung diseases

Chronic inflammatory lung diseases are characterized by increased expression of multiple inflammatory genes following activation by proinflammatory transcription factors, such as nuclear factor kappaB (NF-kappaB) and AP-1. Gene expression is, at least in part, regulated by acetylation of core histones through the action of coactivators, such as CREB-binding protein (CBP), which have intrinsic histone acetyltransferase (HAT) activity. Conversely gene repression is mediated via a combination of histone deacetylases (HDAC) and other corepressors. In asthma, the level of HAT activity is elevated in bronchial biopsies, whereas HDAC activity levels are only partially reduced and inhaled corticosteroids are able to reduce the increased HAT activity back to those seen in normal subjects. In contrast, in chronic obstructive pulmonary disease (COPD), there is a greater reduction in HDAC activity and HDAC2 expression but no difference in HAT activity. HAT and HDAC are also reported to modify a large and expanding number of nonhistone proteins, including nuclear import proteins, chaperones, cytoskeletal proteins, and other transcriptional factors, such as NF-kappaB and signal transducer and activation of transcription (STAT). Acetylation regulates several aspects of protein function and stability leading to differing effects on inflammatory gene expression and cell recruitment involved in the pathogenesis of inflammatory diseases. This review will examine the impact of acetylation on the function of key proteins involved in airway inflammatory disease and the effects of current therapies on acetylation status of key proteins. Further appreciation of the role of these changes may lead to the development of novel therapeutic approaches to inflammatory lung diseases that are currently difficult to treat.

Nitration of a critical tyrosine residue in the allosteric inhibitor site of muscle glycogen phosphorylase impairs its catalytic activity

Muscle glycogen phosphorylase (GP) is a key enzyme in glucose metabolism, and its impairment can lead to muscle dysfunction. Tyrosine nitration of glycogen phosphorylase occurs during aging and has been suggested to be involved in progressive loss of muscle performance. Here, we show that GP (in its T and R form) is irreversibly impaired by exposure to peroxynitrite, a biological nitrogen species known to nitrate reactive tyrosine residues, and to be involved in physiological and pathological processes. Kinetic and biochemical analysis indicated that irreversible inactivation of GP by peroxynitrite is due to the fast (k(inact)=3 x 10(4) M(-1) s(-1)) nitration of a unique tyrosine residue of the enzyme. Endogenous GP was tyrosine nitrated and irreversibly inactivated in skeletal muscle cells upon exposure to peroxynitrite, with concomitant impairment of glycogen mobilization. Ligand protection assays and mass spectrometry analysis using purified GP suggested that the peroxynitrite-dependent inactivation of the enzyme could be due to the nitration of Tyr613, a key amino acid of the allosteric inhibitor site of the enzyme. Our findings suggest that GP functions may be regulated by tyrosine nitration.

Balancing reactivity against selectivity: the evolution of protein S-nitrosylation as an effector of cell signaling by nitric oxide

Produced by the action of lightning in the atmosphere of the pre-biotic earth, nitric oxide (NO) is a free radical molecule that provided the major nitrogen source for development of life. Remarkably, when atmospheric sources of NO became restrictive, organisms evolved the capacity for NO biosynthesis and NO took on bioregulatory roles. We now recognize NO as an ancestral regulator of diverse and important biological functions, acting throughout the phylogenetic tree. In mammals, NO has been implicated as a pivotal regulator of virtually every major physiological system. The bioactivities of NO, and reactive species derived from NO, arise predominantly from their covalent addition to proteins. Importantly, S-nitrosylation of protein cysteine (Cys) residues has emerged as a preeminent effector of NO bioactivity. How and why NO selectively adds to particular Cys residues in proteins is poorly understood, yet fundamental to how NO communicates its bioactivities. Also, evolutionary pressures that have shaped S-nitrosylation as a biosignaling modality are obscure. Considering recently recognized NO signaling paradigms, we speculate on the origin of NO signaling in biological systems and the molecular adaptations that have endowed NO with the ability to selectively target a subset of protein Cys residues that mediate biosignaling.

Reprint of “Oxidative alterations of cyclooxygenase during atherogenesis” [Prostag. Oth. Lipid. M. 80 (2006) 1–14]

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NOx) species modulate cyclooxygenase (COX; also known as prostaglandin H2 synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO-) has multiple effects on COX activity. ONOO- can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO- results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO- can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE-/- mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE-/- mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO- persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO- and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NOx species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NOx species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

The role of the cylooxygenase pathway in nociception and pain

Cycloxygenase (COX) pathways have long been targeted for the treatment of inflammatory pain, initially through the use of NSAIDs. With the demonstration of two major COX isoforms, COX-1 and COX-2, involved in the production of prostanoids, but with different distribution and regulation, selective COX-2 inhibitors have been developed. This review covers factors influencing COX enzyme activity, the role of their products in the development and maintenance of pain and discusses recent safety concerns of COX-2 inhibitors.

Oxidative alterations of cyclooxygenase during atherogenesis

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NO(x)) species modulate cyclooxygenase (COX; also known as prostaglandin H(2) synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO(-)) has multiple effects on COX activity. ONOO(-) can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO(-) results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO(-) can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE(-/-) mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE(-/-) mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO(-) persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO(-) and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NO(x) species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NO(x) species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.