Sulfhydryl-disulfide modulation and the role of disulfide oxidoreductases in regulation of the catalytic activity of nitric oxide synthase in pulmonary artery endothelial cells

Peptide-stimulated angiogenesis: Role of lung endothelial caveolar signaling and nitric oxide

Endothelial nitric oxide (NO) synthase (eNOS)-derived NO plays a critical role in the modulation of angiogenesis in the pulmonary vasculature. We recently reported that an eleven amino acid (SSWRRKRKESS) cell penetrating synthetic peptide (P1) activates caveolar signaling, caveloae/eNOS dissociation, and enhance NO production in lung endothelial cells (EC). This study examines whether P1 promote angiogenesis via modulation of caveolar signaling and the level of NO generation in EC and pulmonary artery (PA) segments. P1-enhanced tube formation and cell sprouting were abolished by caveolae disruptor Filipin (FIL) in EC and PA, respectively. P1 enhanced eNOS activity and angiogenesis were attenuated by inhibition of eNOS as well as PLCγ-1, PKC-α but not PI3K-mediated caveolar signaling in intact EC and/or PA. P1 failed to enhance the catalytic activity of eNOS and angiogenesis in caveolae disrupted EC by FIL. Lower (0.01 mM) concentration of NOC-18 enhanced angiogenesis without inhibition of eNOS activity whereas higher concentration of NOC-18 (1.0 mM) inhibited eNOS activity and angiogenesis in EC. Inhibition of eNOS by l-NAME in the presence of P1 resulted in near total loss of tube formation in EC. Although P1 enhanced angiogenesis mimicked only by lower concentrations of NO generated by NOC-18, this response is independent of caveolar signaling/integrity. These results suggest that P1-enhanced angiogenesis is regulated by dynamic process involving caveolar signaling-mediated increased eNOS/NO activity or by the direct exposure to NOC-18 generating only physiologic range of NO independent of caveolae in lung EC and PA segments.

Possible mechanisms for induction of oxidative stress and suppression of systemic nitric oxide production caused by exposure to environmental chemicals

The cytotoxic effects evoked by exposure to environmental chemicals having electrophilic properties are often attributable to covalent attachment to intracellular macromolecules through sulfhydryl groups or enzyme-mediated redox cycling, leading to the generation of reactive oxygen species (ROS). When huge amounts of ROS form they overwhelm antioxidant defenses resulting in the induction of oxidative stress. Nitric oxide (NO) which plays a crucial role in vascular tone, is formed by endothelial NO synthase (eNOS). Since a decrease in systemic NO production is implicated in the pathophysiological actions of vascular diseases, dysfunction of eNOS by environmental chemicals is associated with cardiopulmonary-related diseases and mortality. In this review, we introduce the mechanism-based toxicities (covalent attachment and redox cycling) of electrophiles. Therefore, this review will focus on the possible mechanisms for the induction of oxidative stress and impairment of NO production caused by environmental chemicals.

Peptide-stimulation enhances compartmentalization and the catalytic activity of lung endothelial NOS

We reported that an 11 amino acid synthetic peptide (P1) activates lung endothelial cell nitric oxide synthase (eNOS) independent of its change in expression and/or phosphorylation. Since caveolae/eNOS dissociation is known to enhance the catalytic activity of eNOS, we examined whether P1-mediated increase of eNOS activity is associated with caveolae/cholesterol modulation, increased caveolin-1 phosphorylation, and intracellular compartmentalization of eNOS in pulmonary artery endothelial cells (PAEC). PAEC were incubated with or without (control) P1 or cholesterol modulators/caveolae disruptors, cholesterol oxidase (CHOX) and methyl-beta-cyclodextrin (CD), for 1 h at 37 degrees C. After incubation cells were used for: i) immunoprecipitation, ii) isolation of plasma membrane (PM)-, Golgi complex (GC)-, and non-Golgi complex (NGC)-enriched fractions, iii) immunofluorescence confocal imaging, and iv) electron microscopy for localization and/or eNOS activity. P1, CHOX, and CD-stimulation caused dissociation of eNOS from PM with increased localization to GC and/or NGC. P1 and CHOX significantly increased eNOS activity in PM and GC and CD-stimulation increased eNOS activity localized only in GC. P1 increased phosphorylation of caveolin-1 in intact cells and GC fraction. Immunofluorescence and/or immunogold labeled imaging/electron microscopy analysis of P1-, CHOX-, and CD-stimulated intact cells confirmed eNOS/caveolae dissociation and translocation of eNOS to GC. These results suggest that: i) P1-stimulation translocates eNOS to GC and enhances the catalytic activity of eNOS in both the PM and GC fractions of PAEC, ii) CHOX- but not CD-mediated caveolae and/or cholesterol modulation mimics the effect of P1-stimulated compartmentalization and activation of eNOS in PAEC, and iii) P1-stimulated caveolae/cholesterol modulation, phosphorylation of caveolin-1, and activation of eNOS is physiologically relevant since P1 is known to enhance NO/cGMP-dependent vasorelaxation in the pulmonary circulation.

Nitric oxide and dihydrolipoic acid modulate the activity of caspase 3 in HepG2 cells

Herein, we report that dihydrolipoic acid and lipoic acid (LA) plus lipoamide dehydrogenase and NADH denitrosate S-nitrosocaspase 3 (CASP-SNO). In HepG2 cells, S-nitroso-L-cysteine ethyl ester (SNCEE) impeded the activity of caspase 3 (CASP-SH), while a subsequent incubation of the cells in SNCEE-free medium resulted in endogenous denitrosation and reactivation of CASP-SH. The latter process was inhibited in thioredoxin reductase-deficient HepG2 cells, in which, however, LA markedly reactivated CASP-SH. The data obtained are discussed with focus on low molecular mass dithiols that mimic the activity of thioredoxin in reactions of protein S-denitrosation.

Peptides modified by myristoylation activate eNOS in endothelial cells through Akt phosphorylation

1. Myristoylated pseudosubstrate of PKCzeta (mPS) - a synthetic myristoylated peptide with a sequence (13 amino acids) mimicking the endogenous PKCzeta pseudosubstrate region -- is considered a selective cell-permeable inhibitor of PKCzeta. We present strong evidence that in endothelial cells the action of mPS is not limited to inhibition of PKC activity and that myristoylation of certain peptides can activate eNOS (endothelial nitric oxide synthase) through Akt phosphorylation. 2. mPS at micromolar concentrations (1-10 microM) induced profound phosphorylation of eNOS, Akt, ERK 1/2, and p38 MAPK in cultured pulmonary artery endothelial cells (PAEC). The same changes were observed after treatment of PAEC with a myristoylated scrambled version of mPS (mScr), whereas a cell-permeable version of PKCzeta pseudosubstrate fused to the HIV-TAT membrane-translocating peptide did not induce analogous changes, suggesting that myristoylation confers new properties on the peptides consisting of activation of different signaling pathways in endothelial cells. 3. In addition to mPS and mScr, a number of other myristoylated peptides induced phosphorylation of eNOS suggesting that myristoylation of peptides can activate eNOS by mechanisms unrelated to inhibition of PKC. All active myristoylated peptides contained basic amino acids motif and were longer than six amino acids. 4. Activation of eNOS by myristoylated peptides was dependent on the PI3K/Akt pathway and the rise of intracellular calcium and was associated with an elevation of cGMP levels in PAEC and with relaxation of precontracted isolated pulmonary artery segments. 5. Myristoylated peptides can be considered a new class of activators of NO production in endothelial cells and that using mPS as a specific inhibitor of PKC should be done with caution, especially in endothelial cells.

Low ethanol intake prevents salt-induced hypertension in WKY rats

Low alcohol intake in humans lowers the risk of coronary heart disease and may lower blood pressure. In hypertension, insulin resistance with altered glucose metabolism leads to increased formation of aldehydes. We have shown that chronic low alcohol intake decreased systolic blood pressure (SBP) and tissue aldehyde conjugates in spontaneously hypertensive rats and demonstrated a strong link between elevated tissue aldehyde conjugates and hypertension in salt-induced hypertensive Wistar-Kyoto (WKY) rats. This study investigated the antihypertensive effect of chronic low alcohol consumption in high salt-treated WKY rats and its effect on tissue aldehyde conjugates, platelet cytosolic free calcium ([Ca2+]i, and renal vascular changes. Animals, aged 7 weeks, were divided into three groups of six animals each. The control group was given normal salt diet (0.7% NaCl) and regular drinking water; the high salt group was given a high salt diet (8% NaCl) and regular drinking water; the high salt + ethanol group was given a high salt diet and 0.25% ethanol in drinking water. After 10 weeks, SBP, platelet [Ca2+]i, and tissue aldehyde conjugates were significantly higher in rats in the high salt group as compared with controls. Animals on high salt diets also showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidney. Ethanol supplementation prevented the increase in SBP and platelet [Ca2+]i and aldehyde conjugates in liver and aorta. Kidney aldehyde conjugates and renal vascular changes were attenuated. These results suggest that chronic low ethanol intake prevents salt-induced hypertension and attenuates renal vascular changes in WKY rats by preventing an increase in tissue aldehyde conjugates and cytosolic [Ca2+]i.

2,4,6-Trinitrotoluene inhibits endothelial nitric oxide synthase activity and elevates blood pressure in rats

2,4,6-Trinitrotoluene (TNT), which is widely used in explosives, is an important occupational and environmental pollutant. Human exposure to TNT has been reported to be associated with cardiovascular dysfunction, but the mechanism is not well understood. In this study, we examine the endothelial nitric oxide synthase (eNOS) activity and blood pressure value following TNT exposure. With a crude enzyme preparation, we found that TNT inhibited the enzyme activity of eNOS in a concentration-dependent manner (IC50 value = 49.4 microM). With an intraperitoneal administration of TNT (10 and 30 mg/kg) to rats, systolic blood pressure was significantly elevated 1 h after TNT exposure (1.2- and 1.3-fold of that of the control, respectively). Under the conditions, however, experiments with the inducible NOS inhibitor aminoguanidine revealed that an adaptive response against hypertension caused by TNT occurs. These results suggest that TNT is an environmental chemical that acts as an uncoupler of constitutive NOS isozymes, resulting in decreased nitric oxide formation associated with hypertension in rats.

Thioredoxin restores nitric oxide-induced inhibition of protein kinase C activity in lung endothelial cells

We previously reported that exposure to exogenous nitric oxide (NO) causes diminished expression of thioredoxin/thioredoxin reductase, a critical component of the redox system that regulates the functions of redox-sensitive enzymes, receptors, and transcription factors. Here we examined the role of thioredoxin in NO-induced inhibition of protein kinase C (PKC) isoform(s) and potential interaction of PKC and thioredoxin in pulmonary artery endothelial cells (PAEC) in culture. Exposure to NO gas (8 ppm) significantly diminished the catalytic activity of the representative isoforms of the conventional, novel, and atypical PKCs alpha, epsilon, and zeta, respectively, in PAEC. Further examination of NO's effect on PKC-zeta revealed that NO-induced inhibition of the catalytic activity of PKC-zeta was time-dependent and regulated by a posttranscriptional mechanism. NO-induced loss of the catalytic activity of PKC-zeta was restored by incubation with the disulfide reducing agent dithiothreitol (DTT) as well as by purified thioredoxin or thioredoxin reductase. Confocal imaging studies revealed co-localization of PKC and thioredoxin in PAEC. These results indicate that: (1) NO-induced inhibition of PKC isoforms is associated with S-nitrosylation-mediated disulfide formation of active site thiols in PKC-zeta as the disulfide reducing agent DTT and/or the thioredoxin enzyme system restore PKC-zeta catalytic activity and (2) NO causes oxidation of endogenous thioredoxin as exogenous reduced thioredoxin or thioredoxin reductase are required to reduce thioredoxin and to restore the catalytic activity of PKC-zeta in PAEC.

Thioredoxin system in premature and newborn biology

Thioredoxin is an important redox protein that is ubiquitously distributed. Thioredoxin exists in dynamic equilibrium between the oxidized and reduced forms, making it an ideal redox-regulatory protein. Thioredoxin, together with thioredoxin reductase and peroxiredoxins, forms a complete redox system that is similar to the glutathione system, but with distinct and divergent functions. This review provides a brief general summary of the thioredoxin system with particular emphasis on its role in premature birth and newborn physiology and disease states. Although extensive studies have examined the role of the thioredoxin system in antioxidant defense, cell proliferation, and signal transduction, further studies are needed to understand its role in embryogenesis and development. Such studies will facilitate our understanding of how thioredoxin may modulate newborn diseases via redox regulation.

Autoinhibitory domain fragment of endothelial NOS enhances pulmonary artery vasorelaxation by the NO-cGMP pathway

Catalytic activity of eNOS is regulated by multiple posttranscriptional mechanisms, including a 40-amino acid (604-643) autoinhibitory domain (AID) located in the reductase domain of the eNOS protein. We examined whether an exogenous synthetic AID, an 11-amino acid (626-636) fragment of AID (AAF), or scrambled AAF (AAF-SR), enhanced eNOS activity and NO-cGMP-mediated vasorelaxation using pulmonary artery (PA) endothelial/smooth muscle cell (PAEC/PASM) coculture, isolated PA segment, and isolated lung perfusion models. Incubation of isolated total membrane fraction of PAEC with AID or AAF resulted in concentration-dependent loss of eNOS activity. In contrast, incubation of intact PAEC with AID or AAF but not AAF-SR caused concentration- and time-dependent activation of eNOS. Because AID and AAF had similar effects on activation of eNOS, AAF and AAF-SR were used for further evaluation. Although AAF stimulation increased catalytic activity of PKC-alpha in PAEC, AAF-mediated activation of eNOS was independent of phosphorylation of Ser1177 or Thr495 and/or expression of eNOS protein. AAF stimulation of PAEC increased NO and cGMP production, which were attenuated by pretreatment with the eNOS inhibitor l-NAME. AAF caused time-dependent vasodilation of U-46619-precontracted endothelium-intact but not endothelium-denuded PA segments, and this response was attenuated by l-NAME. AAF, but not AAF-SR, also caused vasorelaxation in an ex vivo isolated mouse lung perfusion model precontracted with U-46619. Incubation with fluorescence-labeled AAF demonstrated translocation of AAF in PAEC in culture, isolated PA, and isolated intact lungs. These results demonstrate that AAF-stimulated vasodilation is mediated via activation of eNOS and enhanced NO-cGMP production in PA and intact lung.

Role of ascorbate in the regulation of nitric oxide generation by polymorphonuclear leukocytes

We have recently demonstrated that NO-mediated polymorphonuclear (PMN)-dependent inhibition of rat platelet aggregation is significantly enhanced in the presence of ascorbate. Consequently, the present study was undertaken to elucidate the underlying mechanisms involved in ascorbate-mediated potentiation of NO synthesis in PMNs. We observed that ascorbate or its oxidized product, dehydroascorbate (DHA), enhanced NOS activity, as measured by nitrite content, diaminofluorescein fluorescence or conversion of L-[3H]arginine to L-[3H]citrulline in rat, monkey, and human PMNs. The increase in NO generation following ascorbate treatment was due to the intracellular ascorbate as iodoacetamide-mediated inhibition of DHA to ascorbate conversion attenuated the DHA-mediated increase in NO synthesis. The augmentation of NOS activity in the PMN homogenate by tetrahydrobiopterin was significantly enhanced by ascorbate, while ascorbate alone did not influence the NOS activity. Ascorbate-mediated enhancement of NOS activity in the cultured PMNs was significantly reduced in the presence of biopterin synthesis inhibitors. Ascorbate, thus, seems to regulate the NOS activity in the PMNs through tetrahydrobiopterin.

NO and the vasculature: where does it come from and what does it do?

Nitric oxide (NO) is involved in a large number of cellular processes and dysfunctions in NO production have been implicated in many different disease states. In the vasculature NO is released by endothelial cells where it modulates the underlying smooth muscle to regulate vascular tone. Due to the unique chemistry of NO, such as its reactive and free radical nature, it can interact with many different cellular constituents such as thiols and transition metal ions, which determine its cellular actions. In this review we also discuss many of the useful pharmacological tools that have been developed and used extensively to establish the involvement of NO in endothelium-derived relaxations. In addition, the recent literature identifying a potential source of NO in endothelial cells, which is not directly derived from endothelial nitric oxide synthase is examined. Finally, the photorelaxation phenomena, which mediates the release of NO from a vascular smooth muscle NO store, is discussed.

Thioredoxin-related regulation of NO/NOS activities

The role of regulation of nitric oxide synthase (NOS) activity in mitigating oxidative stress in neonatal lungs and contributing to pulmonary vasodilation at birth is still unclear. Furthermore, it is known that, depending on interactions between the individual components of the mitogen-activated protein kinase (MAPK) signaling cascades, many biological consequences, including apoptosis, are initiated. Although the importance of nitric oxide (NO) in apoptosis is controversial and likely depends on NO concentrations and cell types, this highly reactive free radical can activate the p38 MAPK signal cascade. Recent studies have suggested that thioredoxin may play an important role as an effector for some of these functions. Thioredoxin is a major redox protein for many enzymes/transcription factors and is involved in cellular functions, such as viability, activation, and proliferation. In addition to its redox regulation, thioredoxin binds directly to the apoptosis signal-regulating kinase 1 (ASK1), thus inhibiting the activation of stress-induced MAPK signaling cascades that lead to apoptosis. Furthermore, NO produced from newly induced neuronal NOS was reported to induce expression of thioredoxin and several other genes for preconditioning-induced neuroprotection. Moreover, although exposure of endothelial cells to NO decreases NOS activity, this inhibition was shown to be reversed by thioredoxin. Finally, the correlation of expression of thioredoxin with endothelial NOS activity seems to suggest an important role played by this protein in perinatal changes of pulmonary artery functions. Therefore, thioredoxin may participate in the regulation of NOS activity and be involved in NO functions via multiple mechanisms.

Oxidation of proximal protein sulfhydryls by phenanthraquinone, a component of diesel exhaust particles

Diesel exhaust particles (DEP) contain quinones that are capable of catalyzing the generation of reactive oxygen species in biological systems, resulting in induction of oxidative stress. In the present study, we explored sulfhydryl oxidation by phenanthraquinone, a component of DEP, using thiol compounds and protein preparations. Phenanthraquinone reacted readily with dithiol compounds such as dithiothreitol (DTT), 2,3-dimercapto-1-propanol (BAL), and 2,3-dimercapto-1-propanesulfonic acid (DMPS), resulting in modification of the thiol groups, whereas minimal reactivities of this quinone with monothiol compounds such as GSH, 2-mercaptoethanol, and N-acetyl-L-cysteine were seen. The modification of DTT dithiol caused by phenanthraquinone proceeded under anaerobic conditions but was accelerated by molecular oxygen. Phenanthraquinone was also capable of modifying thiol groups in pulmonary microsomes from rats and total membrane preparation isolated from bovine aortic endothelial cells (BAEC), but not bovine serum albumin (BSA), which has a Cys34 as a reactive monothiol group. A comparison of the thiol alkylating agent N-ethylmaleimide (NEM) with that of phenanthraquinone indicates that the two mechanisms of thiol modification are distinct. Studies revealed that thiyl radical intermediates and reactive oxygen species were generated during interaction of phenanthraquinone with DTT. From these findings, it is suggested that phenanthraquinone-mediated destruction of protein sulfhydryls appears to involve the oxidation of presumably proximal thiols and the reduction of molecular oxygen.

Role for endothelin-1-induced superoxide and peroxynitrite production in rebound pulmonary hypertension associated with inhaled nitric oxide therapy

Our previous studies have demonstrated that inhaled nitric oxide (NO) decreases nitric oxide synthase (NOS) activity in vivo and that this inhibition is associated with rebound pulmonary hypertension upon acute withdrawal of inhaled NO. We have also demonstrated that inhaled NO elevates plasma endothelin-1 (ET-1) levels and that pretreatment with PD156707, an ETA receptor antagonist, blocks the rebound hypertension. The objectives of this study were to further elucidate the role of ET-1 in the rebound pulmonary hypertension upon acute withdrawal of inhaled NO. Inhaled NO (40 ppm) delivered to thirteen 4-week-old lambs decreased NOS activity by 36.2% in control lambs (P<0.05), whereas NOS activity was preserved in PD156707-treated lambs. When primary cultures of pulmonary artery smooth muscle cells were exposed to ET-1, superoxide production increased by 33% (P<0.05). This increase was blocked by a preincubation with PD156707. Furthermore, cotreatment of cells with ET-1 and NO increased peroxynitrite levels by 26% (P<0.05), whereas preincubation of purified human endothelial nitric oxide synthase (eNOS) protein with peroxynitrite generated a nitrated enzyme with 50% activity relative to control (P<0.05). Western blot analysis of peripheral lung extracts obtained after 24 hours of inhaled NO revealed a 90% reduction in 3-nitrotyrosine residues (P<0.05) in PD156707-treated lambs. The nitration of eNOS was also reduced by 40% in PD156707-treated lambs (P<0.05). These data suggest that the reduction of NOS activity associated with inhaled NO therapy may involve ETA receptor-mediated superoxide production. ETA receptor antagonists may prevent rebound pulmonary hypertension by protecting endogenous eNOS activity during inhaled NO therapy.

Contrasting effects of thiol-modulating agents on endothelial NO bioactivity

The bioactivity of endothelium-derived nitric oxide (NO) is an important component of vascular homeostasis that is sensitive to intracellular redox status. Because glutathione (GSH) is a major determinant of intracellular redox state, we sought to define its role in modulating endothelial NO bioactivity. In porcine aortic endothelial cells (PAECs), we depleted intracellular GSH (>70%) using 1) buthionine-(S,R)-sulfoximine (BSO), which inhibits GSH synthesis; 2) diamide, which oxidizes thiols; or 3) 1-chloro-2,4-dinitrobenzene (CDNB), which putatively depletes GSH through glutathione S-transferase activity. Cellular GSH depletion with BSO had no effect on endothelial NO bioactivity measured as A-23187-induced cGMP accumulation. In contrast, oxidation of intracellular thiols with diamide inhibited both A-23187-induced cGMP accumulation and the cGMP response to exogenous NO. Diamide treatment of either PAECs, PAEC membrane fractions, or purified endothelial nitric oxide synthase (eNOS) resulted in significant inhibition (approximately 75%) of eNOS catalytic activity measured as L-[(3)H]arginine-to-L-[(3)H]citrulline conversion. This effect appeared related to oxidation of eNOS thiols as it was completely reversed by dithiothreitol. Glutathione depletion with CDNB inhibited A-23187-stimulated cGMP accumulation but not the cGMP response to exogenous NO. Rather, CDNB treatment impaired eNOS catalytic activity in intact PAECs, and this effect was reversed by excess NADPH in isolated purified eNOS assays. Consistent with these results, we found spectral evidence that CDNB reacts with NADPH and renders it inactive as a cofactor for either eNOS or glutathione reductase. Thus thiol-modulating agents exert pleiotropic effects on endothelial NO bioactivity, and these data may help to resolve a number of conflicting previous studies linking GSH status with endothelial cell NO bioactivity.

Phenanthraquinone inhibits eNOS activity and suppresses vasorelaxation

Diesel exhaust particles cause an impairment of endothelium-dependent vasorelaxation and are associated with cardiopulmonary-related diseases and mortality, but the mechanistic details are poorly understood. Since we reported previously that phenanthraquinone, an environmental chemical contained in diesel exhaust particles, suppresses neuronal nitric oxide synthase (nNOS) activity by shunting electrons away from the normal catalytic pathway, it was hypothesized that phenanthraquinone inhibits endothelial NOS (eNOS) activity and affects vascular tone. Therefore, the effects of phenanthraquinone on eNOS activity, endothelium-dependent relaxation, and blood pressure were examined in the present study. Phenanthraquinone inhibited NO formation evaluated by citrulline formed by total membrane fraction of bovine aortic endothelial cells with an IC(50) value of 0.6 microM. A kinetic study revealed that phenanthraquinone is a competitive inhibitor with respect to NADPH and a noncompetitive inhibitor with respect to L-arginine. Endothelium-dependent relaxation of rat aortic rings by ACh was significantly inhibited by phenanthraquinone (5 microM), whereas the endothelium-independent relaxation by nitroglycerin was not. Furthermore, an intraperitoneal injection of phenanthraquinone (0.36 mmol/kg) to rats resulted in an elevation of blood pressure (1.4-fold, P < 0.01); under this condition, plasma levels of stable NO metabolites, nitrite/nitrate, in phenanthraquinone-treated rats was reduced to 68% of control levels. The present findings suggest that phenanthraquinone has a potent inhibitory action on eNOS activity via a similar mechanism reported for nNOS, thereby causing the suppression of NO-mediated vasorelaxation and elevation of blood pressure.

Role of Nitric Oxide Synthase in Collagen–Platelet Interaction: Involvement of Platelet Nonintegrin Collagen Receptor Nitrotyrosylation

Platelets possess the endothelial isoform of nitric oxide synthase (eNOS), which plays an important role in platelet function. Other laboratories, including ours, have reported that nitric oxide (NO) is released upon exposure of platelets to collagen, but the mechanism of the interaction is not yet established. The objective of this study is to examine the possible role of nonintegrin receptor nitrotyrosylation on collagen-induced platelet aggregation. Results of the study show that two platelet proteins with M(r) of 65- and 23-kDa proteins are nitrotyrosylated in a time-dependent manner after the addition of type I collagen. The M(r) 65-kDa protein is identified as the platelet receptor for type I collagen. The recombinant protein of the platelet receptor for type I collagen can also be nitrotyrosylated. The nitrotyrosylated recombinant protein loses its ability to inhibit type I collagen-induced platelet aggregation. In addition, the polyclonal anti-65 kDa immunoprecipitates eNOS suggesting that the platelet nonintegrin receptor for type I collagen is closely linked to the eNOS. These results demonstrate that the inhibitory effect of NO on collagen-induced platelet aggregation may be mediated by the nitrotyrosylation of the 65-kDa receptor.

Physiology of nitric oxide in skeletal muscle

In the past five years, skeletal muscle has emerged as a paradigm of "nitric oxide" (NO) function and redox-related signaling in biology. All major nitric oxide synthase (NOS) isoforms, including a muscle-specific splice variant of neuronal-type (n) NOS, are expressed in skeletal muscles of all mammals. Expression and localization of NOS isoforms are dependent on age and developmental stage, innervation and activity, history of exposure to cytokines and growth factors, and muscle fiber type and species. nNOS in particular may show a fast-twitch muscle predominance. Muscle NOS localization and activity are regulated by a number of protein-protein interactions and co- and/or posttranslational modifications. Subcellular compartmentalization of the NOSs enables distinct functions that are mediated by increases in cGMP and by S-nitrosylation of proteins such as the ryanodine receptor-calcium release channel. Skeletal muscle functions regulated by NO or related molecules include force production (excitation-contraction coupling), autoregulation of blood flow, myocyte differentiation, respiration, and glucose homeostasis. These studies provide new insights into fundamental aspects of muscle physiology, cell biology, ion channel physiology, calcium homeostasis, signal transduction, and the biochemistry of redox-related systems.

How does ascorbic acid prevent endothelial dysfunction?

Human coronary and peripheral arteries show endothelial dysfunction in a variety of conditions, including atherosclerosis, hypercholesterolemia, smoking, and hypertension. This dysfunction manifests as a loss of endothelium-dependent vasodilation to acetylcholine infusion or sheer stress, and is typically associated with decreased generation of nitric oxide (NO) by the endothelium. Vitamin C, or ascorbic acid, when acutely infused or chronically ingested, improves the defective endothelium-dependent vasodilation present in these clinical conditions. The mechanism of the ascorbic acid effect is unknown, although it has been attributed to an antioxidant function of the vitamin to enhance the synthesis or prevent the breakdown of NO. In this review, multiple mechanisms are considered that might account for the ability of ascorbate to preserve NO. These include ascorbate-induced decreases in low-density lipoprotein (LDL) oxidation, scavenging of intracellular superoxide, release of NO from circulating or tissue S-nitrosothiols, direct reduction of nitrite to NO, and activation of either endothelial NO synthase or smooth muscle guanylate cyclase. The ability of ascorbic acid supplements to enhance defective endothelial function in human diseases provides a rationale for use of such supplements in these conditions. However, it is first necessary to determine which of the many plausible mechanisms account for the effect, and to ensure that undesirable toxic effects are not present.

Phenylarsine oxide inhibits nitric oxide synthase in pulmonary artery endothelial cells

The role of protein tyrosine phosphorylation during regulation of NO synthase (eNOS) activity in endothelial cells is poorly understood. Studies to define this role have used inhibitors of tyrosine kinase or tyrosine phosphatase (TP). Phenylarsine oxide (PAO), an inhibitor of TP, has been reported to bind thiol groups, and recent work from our laboratory demonstrates that eNOS activity depends on thiol groups at its catalytic site. Therefore, we hypothesized that PAO may have a direct effect on eNOS activity. To test this, we measured (i) TP and eNOS activities both in total membrane fractions and in purified eNOS prepared from porcine pulmonary artery endothelial cells and (ii) sulfhydryl content and eNOS activity in purified bovine aortic eNOS expressed in Escherichia coli. High TP activity was detected in total membrane fractions, but no TP activity was detected in purified eNOS fractions. PAO caused a dose-dependent decrease in eNOS activity in total membrane and in purified eNOS fractions from porcine pulmonary artery endothelial cells, even though the latter had no detectable TP activity. PAO also caused a decrease in sulfhydryl content and eNOS activity in purified bovine eNOS. The reduction in eNOS sulfhydryl content and the inhibitory effect of PAO on eNOS activity were prevented by dithiothreitol, a disulfide-reducing agent. These results indicate that (i) PAO directly inhibits eNOS activity in endothelial cells by binding to thiol groups in the eNOS protein and (ii) results of studies using PAO to assess the role of protein tyrosine phosphorylation in regulating eNOS activity must be interpreted with great caution.

Nitric oxide-induced reduction of lung cell and whole lung thioredoxin expression is regulated by NF-kappaB

We examined whether nitric oxide (NO)-induced inhibition of thioredoxin (Thx) expression is regulated by a mechanism mediated by a transcription factor, i.e., nuclear factor-kappaB (NF-kappaB), in cultured porcine pulmonary artery endothelial cells (PAEC) and in mouse lungs. Western blot analysis revealed that IkappaB-alpha content was reduced by 20 and 60% in PAEC exposed to 8.5 ppm NO for 2 and 24 h, respectively. NO exposure also caused significant reductions of cytosol fraction p65 and p52 content in PAEC. The nuclear fraction p65 and p52 contents were significantly reduced only in PAEC exposed to NO for 24 h. Exposure to NO resulted in a 50% reduction of p52 mRNA but not of the IkappaB-alpha subunit. DNA binding activity of the oligonucleotide encoding the NF-kappaB sequence in the Thx gene was significantly reduced in PAEC exposed to NO for 24 h. Exposure of mice to 10 ppm NO for 24 h resulted in a significant reduction of lung Thx and IkappaB-alpha mRNA and protein expression and in the oligonucleotide encoding Thx and NF-kappaB/DNA binding. These results 1) demonstrate that the effects of NO exposure on Thx expression in PAEC are comparable to those observed in intact lung and 2) suggest that reduced expression of the NF-kappaB subunit, leading to reduced NF-kappaB/DNA binding, is associated with the loss of Thx expression in PAEC and in intact mouse lungs.

Increased expression of calreticulin is linked to ANG IV-mediated activation of lung endothelial NOS

This study demonstrates that ANG IV-induced activation of lung endothelial cell nitric oxide synthase (ecNOS) is mediated through mobilization of Ca(2+) concentration and by increased expression and release of the Ca(2+) binding protein calreticulin in pulmonary artery endothelial cells (PAEC). In Ca(2+)-free medium and in the presence of the ANG II AT(1) and AT(2) receptor antagonists losartan and PD-123319 (1 microM each), respectively, ANG IV (5, 50, and 500 nM) significantly increased intracellular Ca(2+) release in PAEC (P < 0.05 for all concentrations). In contrast, ANG IV-mediated activation of ecNOS was abolished by the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM. ANG IV stimulation resulted in significantly increased expression of calreticulin in cells as well as release of calreticulin into the medium of cells as early as 2 h after ANG IV stimulation (P < 0.05). Catalytic activity of purified ecNOS in the absence of calmodulin was increased in a concentration-dependent fashion by calreticulin. Immunocoprecipitation studies revealed that ecNOS and calreticulin were coprecipitated in ANG IV-stimulated PAEC. These results demonstrate that ANG IV-mediated activation of ecNOS is regulated by intracellular Ca(2+) mobilization and by increased expression of calreticulin, which appears to involve interaction of ecNOS and calreticulin proteins in PAEC.

Endothelial membrane potential regulates production of both nitric oxide and superoxide--a fundamental determinant of vascular health

There is recent evidence that the membrane potential of vascular endothelium regulates not only nitric oxide (NO) synthesis, but also superoxide generation, such that hyperpolarization stimulates NO production while suppressing that of superoxide. Given that NO works in a variety of ways to inhibit atherothrombotic disease and hypertension, whereas superoxide not only vetoes the benefits of NO but also disrupts endothelial metabolism and promotes LDL oxidation through its oxidant activity, it is thus evident that endothelium membrane potential is a crucial determinant of cardiovascular risk. Membrane polarization can be enhanced by measures which increase the synthesis or availability of the Na+-K+-ATPase, moderately enhance serum K+ and increase the conductance of membrane K+ channels. Such measures may include high-K+/low-Na+ natural diets, insulin sensitizing modalities, 'euthyroid replacement therapy' and ACE inhibitors. Epidemiological correlations of insulin resistance with hypertension and cardiovascular risk may reflect the low membrane potential of insulin-resistant vascular endothelium. Adjunctive measures for suppressing the generation or half-life of endothelial superoxide are suggested.

Oxidants downstream from superoxide inhibit nitric oxide production by vascular endothelium--a key role for selenium-dependent enzymes in vascular health

Although superoxide can directly quench endothelium-generated nitric oxide (NO), there is considerable evidence that oxidants derived from superoxide--notably peroxides and their further derivatives--can also impair NO bioactivity. In part, this reflects inhibition of NO synthase activity, perhaps mediated by the oxidation of labile sulfhydryl groups, as well as the activation of protein kinase C. Selenium deficiency exacerbates these effects, presumably owing to the crucial role of selenium-dependent thioredoxin reductase and glutathione peroxidases in preventing and reversing oxidant damage to proteins. High-normal homocyst(e)ine levels may induce an 'effective selenium deficiency' by suppressing glutathione peroxidase transcription in endothelial cells. Considerable epidemiology, primarily of European origin, points to mediocre selenium nutrition as a significant vascular risk factor; the risk associated with elevated plasma homocyst(e)ine levels is now well established. In addition to preventing LDL oxidation, vitamin E can be expected to minimize the contribution of lipid peroxides to endothelial dysfunction. Lipoic acid, which can function in vivo as a versatile antioxidant and sulfhydryl reductant, may have particular value for protecting endothelium from oxidants; its clinical utility in diabetic neuropathy may reflect this benefit. Good selenium status, as well as supra-nutritional intakes of lipoic acid, may down-regulate cytokine-mediated endothelial activation by helping to maintain the proper structure of oxidant-labile proteins--such as tyrosine phosphatases--that modulate this signaling. It can be concluded that a number of supplemental nutrients--including selenium, vitamin E, lipoic acid, and the vitamins that promote catabolism of homocysteine--have the potential to promote vascular health by mitigating the adverse impact of superoxide-derived oxidants on endothelial function.

The reported clinical utility of taurine in ischemic disorders may reflect a down-regulation of neutrophil activation and adhesion

The first publications regarding clinical use of taurine were Italian reports claiming therapeutic efficacy in angina, intermittent claudication and symptomatic cerebral arteriosclerosis. A down-regulation of neutrophil activation and endothelial adhesion might plausibly account for these observations. Endothelial platelet-activating factor (PAF) is a crucial stimulus to neutrophil adhesion and activation, whereas endothelial nitric oxide (NO) suppresses PAF production and acts in various other ways to antagonize binding and activation of neutrophils. Hypochlorous acid (HOCl), a neutrophil product which avidly oxidizes many sulfhydryl-dependent proteins, can be expected to inhibit NO synthase while up-regulating PAF generation; thus, a vicious circle can be postulated whereby HOCl released by marginating neutrophils acts on capillary or venular endothelium to promote further neutrophil adhesion and activation. Taurine is the natural detoxicant of HOCl, and thus has the potential to intervene in this vicious circle, promoting a less adhesive endothelium and restraining excessive neutrophil activation. Agents which inhibit the action of PAF on neutrophils, such as ginkgolides and pentoxifylline, have documented utility in ischemic disorders and presumably would complement the efficacy of taurine in this regard. Fish oil, which inhibits endothelial expression of various adhesion factors and probably PAF as well, and which suppresses neutrophil leukotriene production, may likewise be useful in ischemia. These agents may additionally constitute a non-toxic strategy for treating inflammatory disorders in which activated neutrophils play a prominent pathogenic role. Double-blind studies to confirm the efficacy of taurine in symptomatic chronic ischemia are needed.

Hemodynamic effects of basal and stimulated release of endogenous nitric oxide in isolated human lungs

Background-We compared the hemodynamic responses to inhibition or stimulation of endothelial nitric oxide (NO) release of isolated explanted lungs from transplantation recipients with pulmonary hypertension and in normotensive unallocated donor lungs. Methods and Results-Lungs from 10 patients with severe pulmonary hypertension (SPH) and from 16 patients with severe chronic obstructive lung disease (COLD) were studied. Fourteen normotensive lungs were studied as controls. The lungs were perfused at a constant flow. In protocol 1 N(G)-nitro-L-arginine methyl ester caused a similar rise in baseline pulmonary artery pressure (PAP) that was similar in SPH (+17.1+/-4.2 mm Hg; n=5), COLD (+15.5+/-4.8 mm Hg; n=8), and control lungs (+14.5+/-1.5 mm Hg; n=7). Arterial occlusion demonstrated that most of the changes with N(G)-nitro-L-arginine methyl ester were precapillary. The response to sodium nitroprusside (10(-8) to 10(-4) mol/L) was similar in all groups. In protocol 2, the lungs were preconstricted, and acetylcholine (10(-9) to 10(-5) mol/L) caused a lesser fall in PAP in both COLD and SPH lungs compared with control (-41.9+/-8.6%, -55. 7+/-7.6%, and -73.2+/-2.5%, respectively; P<0.05), whereas sodium nitroprusside (10(-5) mol/L) decreased PAP to initial levels in all lungs. Conclusions-Stimulated release of NO is impaired in arteries of lungs with plexogenic or hypoxemic pulmonary hypertension. In contrast, basal release of NO appears to be maintained.

Induction of thioredoxin and thioredoxin reductase gene expression in lungs of newborn primates by oxygen

Thioredoxin (TRX) is a potent protein disulfide oxidoreductase important in antioxidant defense and regulation of cell growth and signal transduction processes, among them the production of nitric oxide. We report that lung TRX and its reductase, TR, are specifically upregulated at birth by O2. Throughout the third trimester, mRNAs for TRX and TR were expressed constitutively at low levels in fetal baboon lungs. However, after premature birth (125 or 140 of 185 days gestation), lung TRX and TR mRNAs increased rapidly with the onset of O2 or air breathing. Lung TRX mRNA also increased in lungs of term newborns with air breathing. Premature animals (140 days) breathing 100% O2 develop chronic lung disease within 7-14 days. These animals had greater TRX and TR mRNAs after 1, 6, or 10 days of life than fetal control animals. In 140-day animals given lesser O2 concentrations (as needed) who do not develop chronic lung disease, lung TRX and TR mRNAs were also increased on days 1 and 6 but not significantly on day 10. In fetal distal lung explant culture, mRNAs for TRX and TR were elevated within 4 h in 95% O2 relative to 1% O2, and the response was similar at various gestations. In contrast, TRX protein did not increase in lung explants from premature animals (125 or 140 days) but did in those from near-term (175-day) fetal baboons after exposure to hyperoxia. However, lung TRX protein and activity, as well as TR activity, eventually did increase in vivo in response to hyperoxia (6 days). Increases in TRX and TR mRNAs in response to 95% O2 also were observed in adult baboon lung explants. When TRX redox status was determined, increased O2 tension shifted TRX to its oxidized form. Treatment of lung explants with actinomycin D inhibited TRX and TR mRNA increases in 95% O2, indicating transcriptional regulation by O2. The acute increase in gene expression for both TRX and TR in response to O2 suggests an important role for these proteins during the transition from relatively anaerobic fetal life to O2 breathing at birth.

Angiotensin II-stimulated nitric oxide release from porcine pulmonary endothelium is mediated by angiotensin IV

In this study, a nitric oxide (NO) sensor was used to examine the ability of angiotensin II (AngII), AngIV, and bradykinin (Bk) to stimulate NO release from porcine pulmonary artery (PPAE) and porcine aortic endothelial (PAE) cells and to explore the mechanism of the AngII-stimulated NO release. Physiologic concentrations of AngII, but not Bk, caused release of NO from PPAE cells. In contrast, Bk, but not AngII, stimulated NO release from PAE cells. AngIII-stimulated NO release from PPAE cells required extracellular L-arginine and was inhibited by L-nitro-arginine methyl ester. AT1 and AT2 receptor inhibition had no affect on AngII-mediated NO release or activation of NO synthase (NOS). AngIV, an AngII metabolite with binding sites that are pharmacologically distinct from the classic AngII receptors, stimulated considerably greater NO release and greater endothelial-type constitutive NOS activity than the same amount of AngII. The AngIV receptor antagonist, divalinal AngIV, blocked both AngII- and AngIV-mediated NO release as well as NOS activation. The results demonstrate that AngIV and the AngIV receptor are responsible, at least in part, for AngII-stimulated NO release and the associated endothelium-dependent vasorelaxation. Furthermore, these results suggest that differences exist in both AngII- and Bk-mediated NO release between PPAE and PAE cells, which may reflect important differences in response to these hormones between vascular beds.

Angiotensin IV receptor-mediated activation of lung endothelial NOS is associated with vasorelaxation

The hexapeptide angiotensin (ANG) IV, a metabolic product of ANG II, has been reported to play a functional role in the regulation of blood flow in extrapulmonary tissues. Here, we demonstrate that ANG IV-specific (AT4) receptors are present in porcine pulmonary arterial endothelial cells (PAECs) and that the binding of ANG IV to AT4 receptors can be blocked by its antagonist divalinal ANG IV but not by the ANG II-, AT1-, and AT2-receptor blockers [Sar1,Ile8]ANG II, losartan, and PD-123177, respectively. ANG IV significantly increased endothelial cell constitutive nitric oxide synthase (ecNOS) activity (P < 0.05) as well as cellular cGMP content (P < 0. 001). Western blot analysis revealed that ecNOS protein expression was comparable in control and ANG IV-stimulated cells. Divalinal ANG IV but not [Sar1,Ile8]ANG II, losartan, or PD-123177 inhibited the ANG II- and ANG IV-stimulated increases in ecNOS activity and cGMP content in PAECs. Incubation in the presence of N-nitro-L-arginine methyl ester (L-NAME) or methylene blue but not of indomethacin significantly diminished ANG IV-stimulated as well as basal levels of cGMP (P < 0.001). Similarly, in situ studies with precontracted porcine pulmonary arterial rings showed that ANG IV caused an endothelium-dependent relaxation that was blocked by L-NAME or methylene blue. Collectively, these results demonstrate that ANG IV binds to AT4 receptors, activates ecNOS by posttranscriptional modulation, stimulates cGMP accumulation in PAECs, and causes pulmonary arterial vasodilation, suggesting that ANG IV plays a role in the regulation of blood flow in the pulmonary circulation.

DNA binding activity of the aryl hydrocarbon receptor is sensitive to redox changes in intact cells

The potential involvement of vicinal dithiols in the transformation of the aryl hydrocarbon (Ah) receptor from its ligand binding to DNA binding form in Hepa-1 cells was explored through the use of diamide and phenylarsine oxide (PAO), which have been shown to specifically form a stable ring complex with vicinal sulfhydryl groups in selected proteins. Pretreatment with diamide and PAO rapidly prevented the inducer-dependent formation of the Ah receptor/xenobiotic response element complex detected by electrophoretic mobility shift assays and suppressed Ah receptor-mediated transcription. Diamide and PAO also inhibited DNA binding activity of the nuclear Ah receptor subsequent to its translocation to the nucleus but to a lesser extent than that observed with pretreatment conditions. The Ah receptor exhibited much higher sensitivity to cellular redox changes than Sp1, a transcription factor previously shown to be very sensitive to redox regulation. Diamide added to nuclear extracts inhibited Ah receptor DNA binding more than when it was added in intact cells. In contrast, Ah receptor DNA binding activity was more sensitive to PAO when it was added to intact cells than when it was added to nuclear extracts. Finally, dithiol 2,3-dimercaptopropanol was over 100 times more effective than monothiol 2-mercaptoethanol in reversing the PAO-dependent inhibition of Ah receptor DNA binding activity. This suggests that vicinal sulfhydryl residues may be involved in DNA binding of the Ah receptor.

Thioredoxin overexpression prevents NO-induced reduction of NO synthase activity in lung endothelial cells

We recently reported that nitric oxide (NO) induces posttranscriptional modulation of lung endothelial cell NO synthase (ecNOS) that results in loss of activity. The loss of activity can be reversed by the redox regulatory proteins thioredoxin (Thx)/thioredoxin reductase (Thx-R). The present study was designed to examine whether diminished expression of endogenous Thx and Thx-R may account for regulation of ecNOS activity in NO-exposed cells and whether overexpression of Thx can prevent NO-induced reduction of ecNOS activity in cultured porcine pulmonary artery endothelial cells (PAEC). Exposure to 8.5 ppm NO gas for 24 h resulted in an 80% decrease of Thx and a 27% decrease of Thx-R mRNA expression. Similarly, NO exposure caused 30 and 50% reductions in Thx and Thx-R protein mass, respectively. This NO-induced decrease in the expression of Thx-R mRNA and protein was accompanied by a significant (P < 0.05) decrease in the catalytic activity of Thx-R but not of glutaredoxin or the cellular levels of reduced glutathione and oxidized glutathione. Overexpression of Thx gene in PAEC was achieved by transient transfection of these cells with pcDNA 3.1 vector inserted in sense or antisense (native) orientation in a human Thx cDNA. Thx mRNA and protein contents in transfected cells were four- and threefold higher, respectively, than those in native PAEC. Exposure of native cells to 10 microM NO solution for 30 min resulted in a significant (P < 0.01) loss of ecNOS activity, whereas ecNOS activity was comparable in Thx-overexpressed cells with or without NO exposure. These results demonstrate that NO exposure results in diminished expression of Thx and Thx-R in PAEC. Endogenous levels of Thx are critical to restoring the NO-induced loss of ecNOS activity because overexpression of Thx prevented the NO-induced loss of ecNOS catalytic activity. These results also demonstrate that NO modulation of ecNOS and Thx proteins is regulated by a physiologically relevant redox mechanism.

Differential changes in rat brain nitric oxide synthase in vivo and in vitro by methylmercury

Alterations in mRNA level, protein content and enzyme activity for nitric oxide synthase (NOS) in the cerebrum and cerebellum during a continuous exposure of neurotoxic metal, methylmercury, were examined in Wistar rats. Subcutaneous (s.c.) administration of methylmercuric chloride (MMC, 10 mg kg-1 day-1, 8 days) resulted in significant increases with time of NOS activities in the cerebrum (1. 6-1.9-fold, 5-8 days) and cerebellum (1.4-fold, 8 days). RT-PCR and immunoblot analyses indicated that the increase in the enzyme activity caused by this metal appears to be due to increase in protein levels of neuronal NOS (nNOS), but not inducible NOS (iNOS) because little appreciable mRNA and protein for iNOS were seen during MMC exposure. The direct effect of mercuric compounds on nNOS activity in vitro was evaluated using 20,000xg supernatant from rat cerebellum homogenate. In contrast to the in vivo observation, inorganic-, alkyl-, and aryl-mercuric compound showed potent inhibition of nNOS activity with IC50 values of 11-43 microM, whereas dimethylmercury (DMM) was without effect on the enzyme activity. Further experiments indicated that the inhibition of nNOS by organomercurial occurred via thiol modification.

H2O2 causes endothelial barrier dysfunction without disrupting the arginine-nitric oxide pathway

We have previously demonstrated that nitric oxide (.NO) donors attenuate and that inhibition of endogenous nitric oxide synthase (NOS) enhances hydrogen peroxide (H2O2)-mediated porcine pulmonary artery endothelial cell (PAEC) injury. The current study investigates the hypothesis that oxidant-mediated inhibition of NOS contributes to PAEC injury. PAEC barrier function, measured as the transmonolayer clearance of albumin, was significantly impaired by H2O2 (10-100 microM) in the absence of cytotoxicity. Treatment with H2O2 did not alter NOS activity, measured as the conversion of [3H]arginine to [3H]citrulline in PAEC lysates, either immediately after treatment with 0-250 microM H2O2 for 30 min or for up to 120 min after treatment with 100 microM H2O2. H2O2 had little effect on NOS activity in intact PAECs, measured as 1) the formation of [3H]citrulline in [3H]arginine-loaded PAECs, 2) PAEC guanosine 3',5'-cyclic monophosphate content, and 3) PAEC.NO release to the culture media. These results indicate that the arginine-.NO pathway remains intact after exposure to oxidant conditions sufficient to promote functional derangements of vascular endothelial cells.

Dithiothreitol improves recovery from in vitro diaphragm fatigue

There is increasing evidence that reactive oxygen species are produced during strenuous skeletal muscle work and that they contribute to the development of muscle fatigue. Although the precise cellular mechanisms underlying such a phenomenon remain obscure, it has been hypothesized that endogenously produced reactive oxygen species may down-regulate force production during fatigue by oxidizing critical sulfhydryl groups on important contractile proteins. To test this hypothesis, we fatigued rat diaphragm strips in vitro for 4 min at 20 Hz stimulation and a duty cycle of 0.33. Following fatigue, the tissue baths were drained and randomly replaced with either physiologic saline or physiologic saline containing the disulfide reducing agent, dithiothreitol (DTT) at varying doses (0.1-5.0 mM). Force-frequency characteristics were then measured over a 90-min recovery period. At the 0.5 and 1.0 mM doses, DTT treatment was associated with significantly greater force production in the recovery period. DTT's effects were observed at most frequencies tested, but appeared more prominent at the higher frequencies. The beneficial effects of DTT were not evident at the 0.1 or 5.0 mM doses and appeared to be specific for fatigued muscle. These recovery-enhancing effects of a potent disulfide reducing agent suggest that important contractile proteins may be oxidized during fatigue; such changes may be readily reversible.

Elevation of manganese superoxide dismutase gene expression by thioredoxin

Manganese superoxide dismutase (MnSOD) is a mitochondrial enzyme that dismutates potentially toxic superoxide radical into hydrogen peroxide and dioxygen. This enzyme is critical for protection against cellular injury due to elevated partial pressures of oxygen. Thioredoxin (TRX) is a potent protein disulfide reductase found in most organisms that participates in many thiol-dependent cellular reductive processes and plays an important role in antioxidant defense, signal transduction, and regulation of cell growth and proliferation. Here we describe induction of manganese superoxide dismutase by thioredoxin. MnSOD mRNA and activity were increased dramatically by low concentrations of TRX (28 microM). Elevation of MnSOD mRNA by TRX was inhibited by actinomycin D, but not cycloheximide, occurring both in cell lines and primary human lung microvascular endothelial cells. mRNAs for other antioxidant enzymes including copper-zinc superoxide dismutase and catalase were not elevated, demonstrating specificity of induction of MnSOD by TRX. Thiol oxidation by diamide or alkylation by chlorodinitrobenzene inhibited MnSOD induction, further indicating a requirement for reduced TRX. Because both oxidized and reduced thioredoxin (28 microM) induced MnSOD mRNA, the intracellular redox status of externally added Escherichia coli oxidized TRX was determined. About 45% of internalized E. coli TRX was reduced, with 8% in fully reduced form and about 37% in partially reduced form. However, when TRX reductase and nicotinamide adenine dinucleotide (NADPH) were added to the extracellular medium with TRX, more than 80% of E. coli TRX was found to be in a fully reduced state in human adenocarcinoma (A549) cells. Although lower concentrations of oxidized TRX (7 microM) did not induce MnSOD mRNA, this concentration of TRX, when reduced by NADPH and TRX reductase, increased MnSOD mRNA six-fold. In additional studies, MCF-7 cells stably transfected with the human TRX gene had elevated expression of MnSOD mRNA relative to vector-transfected controls. Thus, both endogenously produced and exogenously added TRX elevate MnSOD gene expression. These findings suggest a novel mechanism involving reduced TRX in regulation of MnSOD.

Molecular cloning, characterization and expression of a nitric oxide synthase from porcine pulmonary artery endothelial cells

The lack of sequence information and clones of porcine pulmonary artery endothelial cell (PAEC) constitutive nitric oxide synthase (ecNOS) cDNA limits comparative analysis between porcine and human PAEC. Therefore, we cloned, characterized and expressed the ecNOS cDNA from porcine PAEC. Two oligonucleotide primers were designed based on the published human ecNOS cDNA sequence and used to clone porcine PAEC ecNOS using 5' and 3' rapid amplification of cDNA ends reverse transcriptase polymerase chain reaction technique. A full-length ecNOS cDNA was cloned and sequenced, representing a protein of 1205 amino acids with a molecular mass of 134 kDa. A mammalian expression vector (pcDNA3) containing this cDNA was transfected into COS-7 cells, and ecNOS activity was detected by monitoring the formation of [3H]-citrulline from [3H]-L-arginine. Expression of ecNOS activity was predominantly associated (> 90%) with the total membrane fraction of these transfected cells. The deduced amino acid sequence of porcine ecNOS cDNA, containing binding sites for NADPH, flavin adenine dinucleotide and bound flavin mononucleotide, shows 94% identity to human ecNOS. The molecular weight of porcine ecNOS mRNA was estimated to be 4.7 kb by Northern blot analysis, similar to human ecNOS mRNA. This suggests that porcine ecNOS is similar to human ecNOS in deduced amino acid sequence and structure.

Inhibition of peroxynitrite-mediated oxidation of glutathione by carbon dioxide

Peroxynitrite reacts with CO2 to from an adduct containing a weak O--O bond that can undergo homolytic and/or heterolytic cleavage to give other reactive intermediates. Because the peroxynitrite/CO2 reaction is fast and physiological concentrations of CO2 are relatively high, peroxynitrite-mediated oxidations of biological species probably involve the peroxynitrite-CO2 adduct and its subsequent reactive intermediates. We have examined the reaction of glutathione with peroxynitrite in the presence and absence of added bicarbonate. In the presence of added bicarbonate, CO2 competes with glutathione for peroxynitrite, resulting in a markedly decreased consumption of glutathione compared with that observed in the absence of added bicarbonate. However, the consumption of glutathione still is much higher than predicted from the assumption that the glutathione-peroxynitrite reaction is the only reaction that can consume glutathione in this system. These results suggest that glutathione partially, but not completely, traps intermediate(s) derived from the peroxynitrite and CO2 reaction. Some rate constants for the trapping of the intermediates are estimated by simulating the reactions, and possible mechanisms for the reaction of peroxynitrite with glutathione in the presence of added bicarbonate are discussed.

Peroxynitrite inhibition of nitric oxide synthases

Peroxynitrite (PN) can be formed under mainly pathophysiological conditions from nitric oxide (NO) and superoxide anion and may be responsible for oxidative modifications of biomolecules. Preparations of nitric oxide synthases from porcine cerebellum (nNOS), bovine aortic endothelium (eNOS) and cytokine-treated murine macrophages (iNOS) were inhibited by PN in their ability to transform arginine to citrulline and nitric oxide with IC50 values of 15, 28, and 10 microM, respectively. Glutathione, bovine serum albumin and tyrosine provided varying degrees of protection in the three preparations. Intact endothelial cells, upon exposure to PN, rapidly lost their glutathione content but protein-SH groups and eNOS activity remained largely unaffected. Destruction of the heme-thiolate catalytic site was observed when nNOS was exposed to PN suggesting that the irreversible oxidation of this bond may be the common mechanism of NOS inhibition.