Nitric oxide activates guanylate cyclase and increases guanosine 3':5'-cyclic monophosphate levels in various tissue preparations

Vascular nitric oxide: Beyond eNOS

As the first discovered gaseous signaling molecule, nitric oxide (NO) affects a number of cellular processes, including those involving vascular cells. This brief review summarizes the contribution of NO to the regulation of vascular tone and its sources in the blood vessel wall. NO regulates the degree of contraction of vascular smooth muscle cells mainly by stimulating soluble guanylyl cyclase (sGC) to produce cyclic guanosine monophosphate (cGMP), although cGMP-independent signaling [S-nitrosylation of target proteins, activation of sarco/endoplasmic reticulum calcium ATPase (SERCA) or production of cyclic inosine monophosphate (cIMP)] also can be involved. In the blood vessel wall, NO is produced mainly from l-arginine by the enzyme endothelial nitric oxide synthase (eNOS) but it can also be released non-enzymatically from S-nitrosothiols or from nitrate/nitrite. Dysfunction in the production and/or the bioavailability of NO characterizes endothelial dysfunction, which is associated with cardiovascular diseases such as hypertension and atherosclerosis.

Hemoglobin interaction with GP1bα induces platelet activation and apoptosis: a novel mechanism associated with intravascular hemolysis

Intravascular hemolysis increases the risk of hypercoagulation and thrombosis in hemolytic disorders. Our study shows a novel mechanism by which extracellular hemoglobin directly affects platelet activation. The binding of Hb to glycoprotein1bα activates platelets. Lower concentrations of Hb (0.37-3 μM) significantly increase the phosphorylation of signaling adapter proteins, such as Lyn, PI3K, AKT, and ERK, and promote platelet aggregation in vitro. Higher concentrations of Hb (3-6 μM) activate the pro-apoptotic proteins Bak, Bax, cytochrome c, caspase-9 and caspase-3, and increase platelet clot formation. Increased plasma Hb activates platelets and promotes their apoptosis, and plays a crucial role in the pathogenesis of aggregation and development of the procoagulant state in hemolytic disorders. Furthermore, we show that in patients with paroxysmal nocturnal hemoglobinuria, a chronic hemolytic disease characterized by recurrent events of intravascular thrombosis and thromboembolism, it is the elevated plasma Hb or platelet surface bound Hb that positively correlates with platelet activation.

Strategies to increase nitric oxide signalling in cardiovascular disease

Nitric oxide (NO) is a key signalling molecule in the cardiovascular, immune and central nervous systems, and crucial steps in the regulation of NO bioavailability in health and disease are well characterized. Although early approaches to therapeutically modulate NO bioavailability failed in clinical trials, an enhanced understanding of fundamental subcellular signalling has enabled a range of novel therapeutic approaches to be identified. These include the identification of: new pathways for enhancing NO synthase activity; ways to amplify the nitrate-nitrite-NO pathway; novel classes of NO-donating drugs; drugs that limit NO metabolism through effects on reactive oxygen species; and ways to modulate downstream phosphodiesterases and soluble guanylyl cyclases. In this Review, we discuss these latest developments, with a focus on cardiovascular disease.

Nitric oxide in liver diseases

Nitric oxide (NO) and its derivatives play important roles in the physiology and pathophysiology of the liver. Despite its diverse and complicated roles, certain patterns of the effect of NO on the pathogenesis and progression of liver diseases are observed. In general, NO derived from endothelial NO synthase (eNOS) in liver sinusoidal endothelial cells (LSECs) is protective against disease development, while inducible NOS (iNOS)-derived NO contributes to pathological processes. This review addresses the roles of NO in the development of various liver diseases with a focus on recently published articles. We present here two recent advances in understanding NO-mediated signaling - nitrated fatty acids (NO2-FAs) and S-guanylation - and conclude with suggestions for future directions in NO-related studies on the liver.

The molecular targets of approved treatments for pulmonary arterial hypertension

Until recently, three classes of medical therapy were available for the treatment of pulmonary arterial hypertension (PAH)--prostanoids, endothelin receptor antagonists and phosphodiesterase type 5 (PDE5) inhibitors. With the approval of the soluble guanylate cyclase stimulator riociguat, an additional drug class has become available targeting a distinct molecular target in the same pathway as PDE5 inhibitors. Treatment recommendations currently include the use of all four drug classes to treat PAH, but there is a lack of comparative data for these therapies. Therefore, an understanding of the mechanistic differences between these agents is critical when making treatment decisions. Combination therapy is often used to treat PAH and it is therefore important that physicians understand how the modes of action of these drugs may interact to work as complementary partners, or potentially with unwanted consequences. Furthermore, different patient phenotypes mean that patients respond differently to treatment; while a certain monotherapy may be adequate for some patients, for others it will be important to consider alternating or combining compounds with different molecular targets. This review describes how the four currently approved drug classes target the complex pathobiology of PAH and will consider the distinct target molecules of each drug class, their modes of action, and review the pivotal clinical trial data supporting their use. It will also discuss the rationale for combining drugs (or not) from the different classes, and review the clinical data from studies on combination therapy.

Acute effects of riociguat in borderline or manifest pulmonary hypertension associated with chronic obstructive pulmonary disease

Riociguat is the first oral soluble guanylate cyclase stimulator shown to improve pulmonary hemodynamics in patients with pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension (PH). This pilot study assessed the impact of a single dose of riociguat on hemodynamics, gas exchange, and lung function in patients with PH associated with chronic obstructive pulmonary disease (COPD). Adults with COPD-associated borderline or manifest PH (pulmonary vascular resistance > 270 dyn·s·cm(-5), mean pulmonary artery pressure ≥ 23 mmHg, ratio of forced expiratory volume in 1 second [FEV1] to forced vital capacity < 70%, and partial pressure of oxygen and carbon dioxide in arterial blood > 50 and ≤ 55 mmHg, respectively) received riociguat 1 or 2.5 mg during right heart catheterization. Twenty-two patients completed the study (11 men, 11 women, aged 56-82 years; 1-mg group: n = 10 [mean FEV1: 43.1%]; 2.5-mg group: n = 12 [mean FEV1: 41.2%]). Riociguat caused significant improvements (P < 0.01) from baseline in mean pulmonary artery pressure (1 mg: -3.60 mmHg [-11.44%]; 2.5 mg: -4.83 mmHg [-14.76%]) and pulmonary vascular resistance (1 mg: -58.32 dyn·s·cm(-5) [-15.35%]; 2.5 mg: -123.8 dyn·s·cm(-5) [-32.96%]). No relevant changes in lung function or gas exchange were observed. Single doses of riociguat were well tolerated and showed promising hemodynamic effects without untoward effects on gas exchange or lung function in patients with COPD-associated PH. Placebo-controlled studies of chronic treatment with riociguat are warranted.

The effect of cyclic nucleotide analog drugs on the mediators release from basophils

Background:

The cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), are intracellular second messengers that play an important role in modulating inflammatory cells involved in allergic diseases. In general, cAMP suppresses the activity of immune and inflammatory cells. We aim to evaluate the roles of cAMP and cGMP in regulating basophil activity.

Materials and Methods:

Basophil-enriched preparations were incubated with analogs and then challenged with anti-IgE or IL-3 (4 or 24 hours). Supernatants were assayed for histamine, IL-4, and IL-13 release. The effects of Sp-8-CPT-cAMPS and Sp-8-CPT-cGMPS on IL-3-dependent mediator release from basophils were determined. The cells were pre-incubated with an analog and then incubated with IL-3 for 24 hours.

Results:

Sp-8-CPT-cAMPS was an effective (P < 0.05) inhibitor of IL-4, IL-13, and histamine release from basophils. However, paradoxically, Sp-8-CPT-cGMPS enhanced histamine release and IL-13 generation, but by contrast, had little effect on IL-4 generation. Sp-8-CPT-cGMPS inhibited cytokine generation, but enhanced the release of histamine release to a modest extent.

Conclusion:

This study shows that the cAMP/protein kinase A (PKA) pathway may be inhibitory to the IgE- and non-IgE-dependent release of mediators from basophils.

Chloroquine-induced scratching is mediated by NO/cGMP pathway in mice

Chloroquine (CQ), a 4-aminoquinoline drug, has long been used in the treatment and prevention of malaria. However its side effect generalized pruritus contributes to treatment failures, and consequently results in the development of chloroquine resistant strains of Plasmodium falciparum. It was proposed that the administration of CQ correlated with increase in nitric oxide (NO) production. Nitric oxide is involved in some pruritic disorders such as atopic dermatitis, psoriasis and scratching behavior evoked by pruritogens like substance P. Therefore, the aim of this study was to investigate the involvement of NO/cGMP pathway in CQ-induced scratching in mice. Scratching behaviors were recorded by a camera after intradermal (ID) injection of CQ in the shaved rostral back of the mice. The results obtained show that CQ elicited scratching in a dose-dependent manner with a peak effective dose of 400μg/site. Injection of non-specific NOS inhibitor, N-nitro-l-arginine methyl ester or neuronal NOS selective inhibitor and 7-nitroindazole, reduced CQ-induced scratching significantly. On the other hand, administration of aminoguanidine as inducible NOS inhibitor has no inhibitory effect on this behavior. Also, injection of l-arginine as a precursor of NO significantly increased this response. Conversely, accumulation of cGMP by sildenafil as a selective phosphodiesterase type 5 inhibitor, potentiated the scratching behavior by CQ. This study therefore shows that CQ-induced scratching behavior is mediated by the NO/cGMP pathway.

Nitric oxide is a ubiquitous signal for maintaining redox balance in plant cells: regulation of ascorbate peroxidase as a case study

Oxidative and nitrosative stresses and their respective antioxidant responses are common metabolic adjustments operating in all biological systems. These stresses result from an increase in reactive oxygen species (ROS) and reactive nitrogen species (RNS) and an imbalance in the antioxidant response. Plants respond to ROS and RNS accumulation by increasing the level of the antioxidant molecules glutathione and ascorbate and by activating specific antioxidant enzymes. Nitric oxide (NO) is a free radical considered to be toxic or protective depending on its concentration, combination with ROS compounds, and subcellular localization. In this review we focus on the mechanisms of NO action in combination with ROS on the regulation of the antioxidant system in plants. In particular, we describe the redox post-translational modifications of cytosolic ascorbate peroxidase and its influence on enzyme activity. The regulation of ascorbate peroxidase activity by NO as a redox sensor of acute oxidative stress or as part of a hormone-induced signalling pathway leading to lateral root development is presented and discussed.

Elevation of soluble guanylate cyclase suppresses proliferation and survival of human breast cancer cells

Nitric oxide (NO) is an essential signaling molecule in biological systems. Soluble guanylate cyclase (sGC), composing of α1 and β1 subunit, is the receptor for NO. Using radioimmunoassay, we discovered that activation of sGC by treatment with bradykinin or sodium nitroprusside (SNP) is impaired in MCF-7 and MDA-MB-231 breast cancer cells as compared to normal breast epithelial 184A1 cells. The 184A1 cells expressed both sGC α1 and sGCβ1 mRNAs. However, levels of sGCβ1 mRNAs were relatively lower in MCF-7 cells while both mRNA of sGC subunits were absent in MDA-MB-231 cells. Treatment with DNA methyltransferase inhibitor 5-aza-2’-deoxycytidine (5-aza-dC) increased mRNA levels of both sGCα1 and sGCβ1 in MDA-MB-231 cells but only sGCβ1 mRNAs in MCF-7 cells. The 5-aza-dC treatment increased the SNP-induced cGMP production in MCF-7 and MDA-MB-231, but not in 184A1 cells. Bisulfite sequencing revealed that the promoter of sGCα1 in MDA-MB-231 cells and promoter of sGCβ1 in MCF-7 cells were methylated. Promoter hypermethylation of sGCα1 and sGCβ1 was found in 1 out of 10 breast cancer patients. Over-expression of both sGC subunits in MDA-MB-231 cells induced apoptosis and growth inhibition in vitro as well as reduced tumor incidence and tumor growth rate of MDA-MB-231 xenografts in nude mice. Elevation of sGC reduced protein abundance of Bcl-2, Bcl-xL, Cdc2, Cdc25A, Cyclin B1, Cyclin D1, Cdk6, c-Myc, and Skp2 while increased protein expression of p53. Our study demonstrated that down-regulation of sGC, partially due to promoter methylation, provides growth and survival advantage in human breast cancer cells.

Genetic differences and aberrant methylation in the apelin system predict the risk of high-altitude pulmonary edema

Hypoxia-inducible factor stimulates the expression of apelin, a potent vasodilator, in response to reduced blood arterial oxygen saturation. However, aberrations in the apelin system impair pulmonary vascular function, potentially resulting in the development of high-altitude (HA)-related disorders. This study aimed to elucidate the genetic and epigenetic regulation of apelin, apelin receptor (APLNR), and endothelial nitric oxide synthase (NOS3) in HA adaptation and HA pulmonary edema (HAPE). A genome-wide association study and sequencing identified variants of apelin, APLNR, and NOS3 that were validated in a larger sample size of HAPE-patients (HAPE-p), HAPE-free controls (HAPE-f), and healthy highland natives (HLs). Apelin-13 and nitrite levels and apelin and NOS3 expression were down-regulated in HAPE-p (P < 0.001). Among the several studied polymorphisms, apelin rs3761581, rs2235312, and rs3115757; APLNR rs11544374 and rs2282623; and NOS3 4b/4a, rs1799983, and rs7830 were associated with HAPE (P < 0.03). The risk allele rs3761581G was associated with a 58.6% reduction in gene expression (P = 0.017), and the risk alleles rs3761581G and rs2235312T were associated with low levels of apelin-13 and nitrite (P < 0.05). The latter two levels decreased further when both of these risk alleles were present in the patients (P < 0.05). Methylation of the apelin CpG island was significantly higher in HAPE-p at 11.92% than in HAPE-f and HLs at ≤ 7.1% (P < 0.05). Moreover, the methylation effect was 9% stronger in the 5' UTR and was associated with decreased apelin expression and apelin-13 levels. The rs3761581 and rs2235312 polymorphisms and methylation of the CpG island influence the expression of apelin in HAPE.

Protein kinase G-regulated production of H2S governs oxygen sensing

Reflexes initiated by the carotid body, the principal O2-sensing organ, are critical for maintaining cardiorespiratory homeostasis during hypoxia. O2 sensing by the carotid body requires carbon monoxide (CO) generation by heme oxygenase-2 (HO-2) and hydrogen sulfide (H2S) synthesis by cystathionine-γ-lyase (CSE). We report that O2 stimulated the generation of CO, but not that of H2S, and required two cysteine residues in the heme regulatory motif (Cys(265) and Cys(282)) of HO-2. CO stimulated protein kinase G (PKG)-dependent phosphorylation of Ser(377) of CSE, inhibiting the production of H2S. Hypoxia decreased the inhibition of CSE by reducing CO generation resulting in increased H2S, which stimulated carotid body neural activity. In carotid bodies from mice lacking HO-2, compensatory increased abundance of nNOS (neuronal nitric oxide synthase) mediated O2 sensing through PKG-dependent regulation of H2S by nitric oxide. These results provide a mechanism for how three gases work in concert in the carotid body to regulate breathing.

The safety and effectiveness of riociguat to treat chronic thromboembolic pulmonary hypertension

Chronic thromboembolic pulmonary hypertension (CTEPH) is an insidious, progressive disease with a poor prognosis. The treatment of choice is pulmonary thromboendarterectomy, although not all patients benefit from surgery at a specialized center. Riociguat, an oral soluble guanylate cyclase (sGC) stimulator is the first pharmacotherapeutic agent that has been shown to improve exercise capacity and hemodynamics in a large multicenter, double-blind, randomized placebo-controlled trial for the treatment of patients with inoperable or persistent CTEPH. Riociguat stimulates sGC directly in a nitric oxide (NO)-independent manner, thereby increasing the sensitivity of sGC to NO, and also in synergy with NO, leading to increased production of cyclic guanosine monophosphate, an intracellular messenger involved in regulating vascular tone, smooth muscle cell proliferation, fibrosis and inflammation. This review will summarize the pharmacodynamics, pharmacokinetics as well as safety and efficacy data of riociguat in inoperable or persistent CTEPH.

Neuronal nitric oxide synthase is dislocated in type I fibers of myalgic muscle but can recover with physical exercise training

Trapezius myalgia is the most common type of chronic neck pain. While physical exercise reduces pain and improves muscle function, the underlying mechanisms remain unclear. Nitric oxide (NO) signaling is important in modulating cellular function, and a dysfunctional neuronal NO synthase (nNOS) may contribute to an ineffective muscle function. This study investigated nNOS expression and localization in chronically painful muscle. Forty-one women clinically diagnosed with trapezius myalgia (MYA) and 18 healthy controls (CON) were included in the case-control study. Subsequently, MYA were randomly assigned to either 10 weeks of specific strength training (SST, n = 18), general fitness training (GFT, n = 15), or health information (REF, n = 8). Distribution of fiber type, cross-sectional area, and sarcolemmal nNOS expression did not differ between MYA and CON. However, MYA showed increased sarcoplasmic nNOS localization (18.8 ± 12 versus 12.8 ± 8%, P = 0.049) compared with CON. SST resulted in a decrease of sarcoplasm-localized nNOS following training (before 18.1 ± 12 versus after 12.0 ± 12%; P = 0,027). We demonstrate that myalgic muscle displays altered nNOS localization and that 10 weeks of strength training normalize these disruptions, which supports previous findings of impaired muscle oxygenation during work tasks and reduced pain following exercise.

Strategies for correcting very long chain acyl-CoA dehydrogenase deficiency

Very long acyl-CoA dehydrogenase (VLCAD) deficiency is a genetic pediatric disorder presenting with a spectrum of phenotypes that remains for the most part untreatable. Here, we present a novel strategy for the correction of VLCAD deficiency by increasing mutant VLCAD enzymatic activity. Treatment of VLCAD-deficient fibroblasts, which express distinct mutant VLCAD protein and exhibit deficient fatty acid β-oxidation, with S-nitroso-N-acetylcysteine induced site-specific S-nitrosylation of VLCAD mutants at cysteine residue 237. Cysteine 237 S-nitrosylation was associated with an 8-17-fold increase in VLCAD-specific activity and concomitant correction of acylcarnitine profile and β-oxidation capacity, two hallmarks of the disorder. Overall, this study provides biochemical evidence for a potential therapeutic modality to correct β-oxidation deficiencies.

Nitric oxide and reactive oxygen species in the pathogenesis of preeclampsia

Preeclampsia (PE) is characterized by disturbed extravillous trophoblast migration toward uterine spiral arteries leading to increased uteroplacental vascular resistance and by vascular dysfunction resulting in reduced systemic vasodilatory properties. Its pathogenesis is mediated by an altered bioavailability of nitric oxide (NO) and tissue damage caused by increased levels of reactive oxygen species (ROS). Furthermore, superoxide (O₂⁻) rapidly inactivates NO and forms peroxynitrite (ONOO⁻). It is known that ONOO⁻ accumulates in the placental tissues and injures the placental function in PE. In addition, ROS could stimulate platelet adhesion and aggregation leading to intravascular coagulopathy. ROS-induced coagulopathy causes placental infarction and impairs the uteroplacental blood flow in PE. The disorders could lead to the reduction of oxygen and nutrients required for normal fetal development resulting in fetal growth restriction. On the other hand, several antioxidants scavenge ROS and protect tissues against oxidative damage. Placental antioxidants including catalase, superoxide dismutase (SOD), and glutathione peroxidase (GPx) protect the vasculature from ROS and maintain the vascular function. However, placental ischemia in PE decreases the antioxidant activity resulting in further elevated oxidative stress, which leads to the appearance of the pathological conditions of PE including hypertension and proteinuria. Oxidative stress is defined as an imbalance between ROS and antioxidant activity. This review provides new insights about roles of oxidative stress in the pathophysiology of PE.

The neuronal nitric oxide synthase (nNOS) gene and neuroprotection against alcohol toxicity

When a mother abuses alcohol during pregnancy, the offspring can suffer a myriad of abnormalities, collectively known as fetal alcohol spectrum disorder (FASD). Foremost among these abnormalities is central nervous system dysfunction, which commonly manifests itself as mental retardation, clumsiness, hyperactivity, and poor attention span. These behavior problems are due, in large part, to alcohol-induced neuronal losses in the developing fetal brain. However, not all fetuses are equally affected by maternal alcohol consumption during pregnancy. While some fetuses are severely affected and develop hallmarks of FASD later in life, others exhibit no evident neuropathology or behavioral abnormalities. This variation is likely due, at least in part, to differences in fetal genetics. This review focuses on one particular gene, neuronal nitric oxide synthase, whose mutation worsens alcohol-induced neuronal death, both in vitro and in vivo. In addition, ectopic expression of the neuronal nitric oxide synthase gene protects neurons against alcohol toxicity. The gene encodes an enzyme that produces nitric oxide (NO), which facilitates the protective effects of neuronal growth factors and which underlies the ability of neurons to resist alcohol toxicity as they mature. Nitric oxide exerts its protective effects against alcohol via a specific signaling pathway, the NO-cGMP-PKG pathway. Pharmacologic manipulation of this pathway could be of therapeutic use in preventing or ameliorating FASD.

Endothelial nitric oxide synthase uncoupling: a novel pathway in OSA induced vascular endothelial dysfunction

The mechanism of vascular endothelial dysfunction (VED) and cardiovascular disease in obstructive sleep apnea (OSA) is unknown. We performed a comprehensive evaluation of endothelial nitric oxide synthase (eNOS) function directly in the microcirculatory endothelial tissue of OSA patients who have very low cardiovascular risk status. Nineteen OSA patients underwent gluteal biopsies before, and after effective treatment of OSA. We measured superoxide (O2(•-)) and nitric oxide (NO) in the microcirculatory endothelium using confocal microscopy. We evaluated the effect of the NOS inhibitor l-Nitroarginine-Methyl-Ester (l-NAME) and the NOS cofactor tetrahydrobiopterin (BH4) on endothelial O2(•-) and NO in patient endothelial tissue before and after treatment. We found that eNOS is dysfunctional in OSA patients pre-treatment, and is a source of endothelial O2(•-) overproduction. eNOS dysfunction was reversible with the addition of BH4. These findings provide a new mechanism of endothelial dysfunction in OSA patients and a potentially targetable pathway for treatment of cardiovascular risk in OSA.

S-nitrosylation of cofilin-1 mediates estradiol-17β-stimulated endothelial cytoskeleton remodeling

Rapid nitric oxide (NO) production via endothelial NO synthase (eNOS) activation represents a major signaling pathway for the cardiovascular protective effects of estrogens; however, the pathways after NO biosynthesis that estrogens use to function remain largely unknown. Covalent adduction of a NO moiety to cysteines, termed S-nitrosylation (SNO), has emerged as a key route for NO to directly regulate protein function. Cofilin-1 (CFL1) is a small actin-binding protein essential for actin dynamics and cytoskeleton remodeling. Despite being identified as a major SNO protein in endothelial cells, whether SNO regulates CFL-1 function is unknown. We hypothesized that estradiol-17β (E2β) stimulates SNO of CFL1 via eNOS-derived NO and that E2β-induced SNO-CFL1 mediates cytoskeleton remodeling in endothelial cells. Point mutation studies determined Cys80 as the primary SNO site among the 4 cysteines (Cys39/80/139/147) in CFL1. Substitutions of Cys80 with Ala or Ser were used to prepare the SNO-mimetic/deficient (C80A/S) CFL1 mutants. Recombinant wild-type (wt) and mutant CFL1 proteins were prepared; their actin-severing activity was determined by real-time fluorescence imaging analysis. The activity of C80A CFL1 was enhanced to that of the constitutively active S3/A CFL1, whereas the other mutants had no effects. C80A/S mutations lowered Ser3 phosphorylation. Treatment with E2β increased filamentous (F)-actin and filopodium formation in endothelial cells, which were significantly reduced in cells overexpressing wt-CFL. Overexpression of C80A, but not C80S, CFL1 decreased basal F-actin and further suppressed E2β-induced F-actin and filopodium formation compared with wt-CFL1 overexpression. Thus, SNO(Cys80) of cofilin-1 via eNOS-derived NO provides a novel pathway for mediating estrogen-induced endothelial cell cytoskeleton remodeling.

Unraveling the mechanisms of action of lactoferrin-derived antihypertensive peptides: ACE inhibition and beyond

The characterization of lactoferrin-derived antihypertensive peptides shows that they might act on several molecular targets.

Optimal management of pulmonary arterial hypertension: prognostic indicators to determine treatment course

Pulmonary arterial hypertension (PAH) is a rapidly progressive pulmonary vascular disease with a multifactorial etiopathogenesis that can result in right-sided heart failure and death. A number of studies indicate that an early therapeutic intervention yields better results on disease progression as compared to delayed treatment. In this review, we will analyze treatment strategies that may be used for monitoring disease progression and for guiding treatment decisions. Several factors (ie, symptoms, functional class, exercise capacity as assessed by a walking test and cardiopulmonary stress testing, hemodynamic parameters, cardiac magnetic resonance imaging, and plasma levels of biochemical markers) have been prognostic of survival. These indicators may be used both at the time of diagnosis and during treatment follow-up. No resolutive therapy is currently available for PAH; however, in the last decade, the advent of specific pharmacological treatments has given new hope to patients suffering from this debilitating disease with a poor prognosis. Combination drug therapies offer increased benefits over monotherapy, and current guidelines recommend a sequential “add on” design approach for patients in functional class II–IV. The goal-oriented “treat to target” therapy sets the timing for treatment escalation in case of inadequate response to currently known prognostic indicators. To date, further longitudinal studies should be urgently conducted to identify new goals that may improve therapeutic strategies in order to optimize personalized treatment in PAH patients.

Organic nitrates: past, present and future

Nitric oxide (NO) is one of the most important vasodilator molecules produced by the endothelium. It has already been established that NO/cGMP signaling pathway deficiencies are involved in the pathophysiological mechanisms of many cardiovascular diseases. In this context, the development of NO-releasing drugs for therapeutic use appears to be an effective alternative to replace the deficient endogenous NO and mimic the role of this molecule in the body. Organic nitrates represent the oldest class of NO donors that have been clinically used. Considering that tolerance can occur when these drugs are applied chronically, the search for new compounds of this class with lower tolerance potential is increasing. Here, we briefly discuss the mechanisms involved in nitrate tolerance and highlight some achievements from our group in the development of new organic nitrates and their preclinical application in cardiovascular disorders.

Crosstalk between nitrite, myoglobin and reactive oxygen species to regulate vasodilation under hypoxia

The systemic response to decreasing oxygen levels is hypoxic vasodilation. While this mechanism has been known for more than a century, the underlying cellular events have remained incompletely understood. Nitrite signaling is critically involved in vessel relaxation under hypoxia. This can be attributed to the presence of myoglobin in the vessel wall together with other potential nitrite reductases, which generate nitric oxide, one of the most potent vasodilatory signaling molecules. Questions remain relating to the precise concentration of nitrite and the exact dose-response relations between nitrite and myoglobin under hypoxia. It is furthermore unclear whether regulatory mechanisms exist which balance this interaction. Nitrite tissue levels were similar across all species investigated. We then investigated the exact fractional myoglobin desaturation in an ex vivo approach when gassing with 1% oxygen. Within a short time frame myoglobin desaturated to 58±12%. Given that myoglobin significantly contributes to nitrite reduction under hypoxia, dose-response experiments using physiological to pharmacological nitrite concentrations were conducted. Along all concentrations, abrogation of myoglobin in mice impaired vasodilation. As reactive oxygen species may counteract the vasodilatory response, we used superoxide dismutase and its mimic tempol as well as catalase and ebselen to reduce the levels of reactive oxygen species during hypoxic vasodilation. Incubation of tempol in conjunction with catalase alone and catalase/ebselen increased the vasodilatory response to nitrite. Our study shows that modest hypoxia leads to a significant nitrite-dependent vessel relaxation. This requires the presence of vascular myoglobin for both physiological and pharmacological nitrite levels. Reactive oxygen species, in turn, modulate this vasodilation response.

Mitochondrial ion channels/transporters as sensors and regulators of cellular redox signaling

SIGNIFICANCE

Mitochondrial ion channels/transporters and the electron transport chain (ETC) serve as key sensors and regulators for cellular redox signaling, the production of reactive oxygen species (ROS) and nitrogen species (RNS) in mitochondria, and balancing cell survival and death. Although the functional and pharmacological characteristics of mitochondrial ion transport mechanisms have been extensively studied for several decades, the majority of the molecular identities that are responsible for these channels/transporters have remained a mystery until very recently.

RECENT ADVANCES

Recent breakthrough studies uncovered the molecular identities of the diverse array of major mitochondrial ion channels/transporters, including the mitochondrial Ca2+ uniporter pore, mitochondrial permeability transition pore, and mitochondrial ATP-sensitive K+ channel. This new information enables us to form detailed molecular and functional characterizations of mitochondrial ion channels/transporters and their roles in mitochondrial redox signaling.

CRITICAL ISSUES

Redox-mediated post-translational modifications of mitochondrial ion channels/transporters and ETC serve as key mechanisms for the spatiotemporal control of mitochondrial ROS/RNS generation.

FUTURE DIRECTIONS

Identification of detailed molecular mechanisms for redox-mediated regulation of mitochondrial ion channels will enable us to find novel therapeutic targets for many diseases that are associated with cellular redox signaling and mitochondrial ion channels/transporters.

What is next in nitric oxide research? From cardiovascular system to cancer biology

The broad role of nitric oxide (NO) and cyclic GMP in biochemistry and biology as important messenger molecules is evident from the numerous publications in this research field. NO and cGMP have been known as components of the key signaling pathway in regulating numerous processes such as vascular dilation, blood pressure, neurotransmission, cardiovascular function, and renal function. In spite of almost 150,000 publications with nitric oxide and cyclic GMP, there are few publications regarding the effects of these messenger molecules on gene regulation, cell differentiation and cell proliferation. Our research data with embryonic stem cells and several cancer cell lines suggest that nitric oxide, its receptor soluble guanylyl cyclase (sGC) and sGC's product cyclic GMP can regulate the processes of proliferation and differentiation. Furthermore, we have found that undifferentiated stem cells and some malignant tumors such as human glioma have decreased levels of sGC and translocation of the sGCβ1 subunit to the nucleus. We propose that sGC and cyclic GMP function as tumor suppressors. An understanding of the mechanisms of the translocation of the sGCβ1 subunit into the nucleus and the possible regulation of gene expression of NO and/or cyclic CMP could lead to novel and innovative approaches to cancer therapy and stem cell proliferation and differentiation.

The role of cGMP in the physiological and molecular responses of the right ventricle to pressure overload

Pulmonary arterial hypertension (PAH) is a progressive disease that is associated with a poor prognosis and results in right heart dysfunction. While pulmonary vascular disease is the obvious primary pathological focus, right ventricular hypertrophy (RVH) and right ventricular (RV) dysfunction are major determinants of prognosis in PAH. Our knowledge about the molecular physiology and pathophysiology of RV hypertrophy and failure in response to pressure overload is still limited, and most data are derived from left heart research. However, the molecular mechanisms of left ventricular remodelling cannot be generalized to the RV, because the right and left ventricles differ greatly in their size, shape, architecture and function. Despite the recent advances in diagnosis and treatment of PAH, little is known about the molecular and cellular mechanisms that underlie the transition from compensatory to maladaptive RV remodelling. The cGMP-phosphodiesterase 5 (PDE5) pathway is one of the extensively studied pathways in PAH, but our knowledge about cGMP-PDE5 signalling in RV pathophysiology is still limited. For this purpose, there is need for animal models that can represent changes in the RV that closely mimic the human situation. The availability of an animal model of pressure-overload-induced RVH (e.g. pulmonary artery banding model) provides us with a valuable tool to understand the differences between adaptive and maladaptive RVH and to explore the direct effects of current PAH therapy on the heart. In this report, we discuss myocardial regulatory effects of cGMP-PDE5 signalling in preclinical models of RV pressure overload for understanding the physiological/pathophysiological mechanisms involved in maladaptive RVH.

Interactions between N-ethylmaleimide-sensitive factor and GluR2 in the nucleus accumbens contribute to the expression of locomotor sensitization to cocaine

Many studies have reported a withdrawal-dependent increase in synaptic AMPA receptor (AMPAR) levels in the nucleus accumbens (NAc) of cocaine-sensitized rats; however, the exact relationship between the expression of sensitization and altered AMPAR surface expression in the NAc has not yet been investigated. We demonstrated that the expression of behavioral sensitization was negatively controlled by N-ethylmaleimide-sensitive factor (NSF)-GluR2 interactions in the NAc. The upregulation of NSF-GluR2 interactions, which may be resulted by the increase in NSF S-nitrosylation after withdrawal from cocaine, was associated with the changes in the expression of behavioral sensitization. Disruption of NSF-GluR2 interactions in the NAc with a specific peptide, TAT-pep-R845A, increased the locomotor response of rats to cocaine by decreasing GluR2 surface insertion. In contrast, prevention of GluR2-containing AMPARs removal from synapses with Pep2-EVKI attenuated the expression of behavioral sensitization. Similarly, treatment with the nitric oxide donor, S-Nitroso-N-acetyl-DL-penicillamine (SNAP), attenuated the expression of locomotor sensitization by promoting GluR2 surface expression. This effect was mediated by the binding of S-nitrosylated NSF to GluR2, which promoted the surface expression of AMPARs. Noticeably, exogenous injection of SNAP into NAc also attenuated the expression of cocaine-induced conditioned place preference. Thus, these results indicate that increased NSF-GluR2 interactions in the NAc after withdrawal from cocaine attenuated the expression of behavioral sensitization and serves as a negative regulatory mechanism in drug-exposed individuals.

The current approach into signaling pathways in pulmonary arterial hypertension and their implication in novel therapeutic strategies

Many mediators and signaling pathways, with their downstream effectors, have been implicated in the pathogenesis of pulmonary hypertension. Currently approved drugs, representing an option of specific therapy, target NO, prostacyclin or ET-1 pathways and provide a significant improvement in the symptomatic status of patients and a slower rate of clinical deterioration. However, despite such improvements in the treatment, PAH remains a chronic disease without a cure, the mortality associated with PAH remains high and effective therapeutic regimens are still required. Knowledge about the role of the pathways involved in PAH and their interactions provides a better understanding of the pathogenesis of the disease and may highlight directions for novel therapeutic strategies for PAH. This paper reviews some novel, promising PAH-associated signaling pathways, such as RAAS, RhoA/ROCK, PDGF, PPAR, and TGF, focusing also on their possible interactions with well-established ones such as NO, ET-1 and prostacyclin pathways.

Molecular mechanisms of neuronal nitric oxide synthase in cardiac function and pathophysiology

Neuronal nitric oxide synthase (nNOS or NOS1) is the major endogenous source of myocardial nitric oxide (NO), which facilitates cardiac relaxation and modulates contraction. In the healthy heart it regulates intracellular Ca(2+), signalling pathways and oxidative homeostasis and is upregulated from early phases upon pathogenic insult. nNOS plays pivotal roles in protecting the myocardium from increased oxidative stress, systolic/diastolic dysfunction, adverse structural remodelling and arrhythmias in the failing heart. Here, we show that the downstream target proteins of nNOS and underlying post-transcriptional modifications are shifted during disease progression from Ca(2+)-handling proteins [e.g. PKA-dependent phospholamban phosphorylation (PLN-Ser(16))] in the healthy heart to cGMP/PKG-dependent PLN-Ser(16) with acute angiotensin II (Ang II) treatment. In early hypertension, nNOS-derived NO is involved in increases of cGMP/PKG-dependent troponin I (TnI-Ser(23/24)) and cardiac myosin binding protein C (cMBP-C-Ser(273)). However, nNOS-derived NO is shown to increase S-nitrosylation of various Ca(2+)-handling proteins in failing myocardium. The spatial compartmentation of nNOS and its translocation for diverse binding partners in the diseased heart or various nNOS splicing variants and regulation in response to pathological stress may be responsible for varied underlying mechanisms and functions. In this review, we endeavour to outline recent advances in knowledge of the molecular mechanisms mediating the functions of nNOS in the myocardium in both normal and diseased hearts. Insights into nNOS gene regulation in various tissues are discussed. Overall, nNOS is an important cardiac protector in the diseased heart. The dynamic localization and various mediating mechanisms of nNOS ensure that it is able to regulate functions effectively in the heart under stress.

HIV-1 protein Tat produces biphasic changes in NMDA-evoked increases in intracellular Ca2+ concentration via activation of Src kinase and nitric oxide signaling pathways

HIV-associated neurocognitive disorders afflict about half of HIV-infected patients. HIV-infected cells shed viral proteins, such as the transactivator of transcription (Tat), which can cause neurotoxicity by over activation of NMDA receptors. Here, we show that Tat causes a time-dependent, biphasic change in NMDA-evoked increases in intracellular Ca(2+) concentration ([Ca(2+)]i). NMDA-evoked responses were potentiated following 2-h exposure to Tat (50 ng/mL). Tat-induced potentiation of NMDA-evoked increases in [Ca(2+)]i peaked by 8 h and then adapted by gradually reversing to baseline by 24 h and eventually dropping below control by 48 h. Tat-induced potentiation of NMDA-evoked responses was blocked by inhibition of lipoprotein receptor-related protein (LRP) or Src tyrosine kinase. Potentiation was unaffected by inhibition of nitric oxide synthase (NOS). However, NOS activity was required for adaptation. Adaptation was also prevented by inhibition of soluble guanylate cyclase (sGC) and cyclic guanosine monophosphate-dependent protein kinase G (PKG). Together, these findings indicate that Tat potentiates NMDA-evoked increases in [Ca(2+)]i via LRP-dependent activation of Src and that this potentiation adapts via activation of the NOS/sGC/PKG pathway. Adaptation may protect neurons from excessive Ca(2+) influx and could reveal targets for the treatment of HIV-associated neurocognitive disorders. HIV-associated neurocognitive disorders (HAND) afflict about half of HIV-infected patients. HIV-infected cells shed viral proteins, such as the transactivator of transcription (Tat), which can cause neurotoxicity by over activation of NMDA receptors (NMDARs). We show that HIV-1 Tat evoked biphasic changes in NMDA-evoked [Ca(2+) ]i responses. Initially, Tat potentiated NMDA-evoked responses following LRP-mediated activation of Src kinase. Subsequently, Tat-induced NMDAR potentiation adapted by activation of a NOS/sGC/PKG pathway that attenuated NMDA-evoked increases in [Ca(2+)]i . Adaptation may be a novel neuroprotective mechanism to prevent excessive Ca(2+) influx. Solid and dashed arrows represent direct and potentially indirect connections, respectively.

Regulation of soluble guanylate cyclase by matricellular thrombospondins: implications for blood flow

Nitric oxide (NO) maintains cardiovascular health by activating soluble guanylate cyclase (sGC) to increase cellular cGMP levels. Cardiovascular disease is characterized by decreased NO-sGC-cGMP signaling. Pharmacological activators and stimulators of sGC are being actively pursued as therapies for acute heart failure and pulmonary hypertension. Here we review molecular mechanisms that modulate sGC activity while emphasizing a novel biochemical pathway in which binding of the matricellular protein thrombospondin-1 (TSP1) to the cell surface receptor CD47 causes inhibition of sGC. We discuss the therapeutic implications of this pathway for blood flow, tissue perfusion, and cell survival under physiologic and disease conditions.

Vasodilator responses to acetylcholine are not mediated by the activation of soluble guanylate cyclase or TRPV4 channels in the rat

The effects of 1H-[1,2,4]-oxadizaolo[4,3-]quinoxaline-1-one (ODQ), an inhibitor of the activation of soluble guanylate cyclase (sGC) on responses to NO donors acetylcholine (ACh) and bradykinin (BK) were investigated in the pulmonary and systemic vascular beds of the rat. In these studies the administration of ODQ in a dose of 5 mg/kg iv attenuated vasodilator responses to five different NO donors without inhibiting responses to ACh and BK in the systemic and pulmonary vascular beds of the rat. Vasodilator responses to ACh were not inhibited by l-NAME or the transient receptor vanilloid type 4 (TRPV4) antagonist GSK-2193874, which attenuated vasodilator responses to the TRPV4 agonist GSK-1016790A. ODQ did not inhibit vasodilator responses to agents reported to act in an NO-independent manner or to vasoconstrictor agents, and ODQ did not increase blood methemoglobin levels, suggesting that off target effects were minimal. These results show that ODQ in a dose that inhibited NO donor-mediated responses did not alter vasodilator responses to ACh in the pulmonary and systemic vascular beds and did not alter systemic vasodilator responses to BK. The present results indicate that decreases in pulmonary and systemic arterial pressures in response to ACh are not mediated by the activation of sGC or TRPV4 channels and that ODQ can be used to study the role of the activation of sGC in mediating vasodilator responses in the rat.

Bang-bang model for regulation of local blood flow

The classical model of metabolic regulation of blood flow in muscle tissue implies the maintenance of basal tone in arterioles of resting muscle and their dilation in response to exercise and/or tissue hypoxia via the evoked production of vasodilator metabolites by myocytes. A century-long effort to identify specific metabolites responsible for explaining active and reactive hyperemia has not been successful. Furthermore, the metabolic theory is not compatible with new knowledge on the role of physiological radicals (e.g., nitric oxide, NO, and superoxide anion, O2 (-) ) in the regulation of microvascular tone. We propose a model of regulation in which muscle contraction and active hyperemia are considered the physiologically normal state. We employ the "bang-bang" or "on/off" regulatory model which makes use of a threshold and hysteresis; a float valve to control the water level in a tank is a common example of this type of regulation. Active bang-bang regulation comes into effect when the supply of oxygen and glucose exceeds the demand, leading to activation of membrane NADPH oxidase, release of O2 (-) into the interstitial space and subsequent neutralization of the interstitial NO. Switching arterioles on/off when local blood flow crosses the threshold is realized by a local cell circuit with the properties of a bang-bang controller, determined by its threshold, hysteresis, and dead-band. This model provides a clear and unambiguous interpretation of the mechanism to balance tissue demand with a sufficient supply of nutrients and oxygen.

Hemodynamic regulation of reactive oxygen species: implications for vascular diseases

SIGNIFICANCE

Arterial blood vessels functionally and structurally adapt to altering hemodynamic forces in order to accommodate changing needs and to provide stress homeostasis. This ability is achieved at the cellular level by converting mechanical stimulation into biochemical signals (i.e., mechanotransduction). Physiological mechanical stress helps maintain vascular structure and function, whereas pathologic or aberrant stress may impair cellular mechano-signaling, and initiate or augment cellular processes that drive disease.

RECENT ADVANCES

Reactive oxygen species (ROS) may represent an intriguing class of mechanically regulated second messengers. Chronically enhanced ROS generation may be induced by adverse mechanical stresses, and is associated with a multitude of vascular diseases. Although a causal relationship has clearly been demonstrated in large numbers of animal studies, an effective ROS-modulating therapy still remains to be established by clinical studies.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This review article focuses on the role of various mechanical forces (in the form of laminar shear stress, oscillatory shear stress, or cyclic stretch) as modulators of ROS-driven signaling, and their subsequent effects on vascular biology and homeostasis, as well as on specific diseases such as arteriosclerosis, hypertension, and abdominal aortic aneurysms. Specifically, it highlights the significance of the various NADPH oxidase (NOX) isoforms as critical ROS generators in the vasculature. Directed targeting of defined components in the complex network of ROS (mechano-)signaling may represent a key for successful translation of experimental findings into clinical practice.