Nitroglycerin-induced S-nitrosylation and desensitization of soluble guanylyl cyclase contribute to nitrate tolerance

Nitric oxide and thiols: Chemical biology, signalling paradigms and vascular therapeutic potential

Nitric oxide (• NO) interactions with biological thiols play crucial, but incompletely determined, roles in vascular signalling and other biological processes. Here, we highlight two recently proposed signalling paradigms: (1) the formation of a vasodilating labile nitrosyl ferrous haem (NO-ferrohaem) facilitated by thiols via thiyl radical generation and (2) polysulfides/persulfides and their interaction with • NO. We also describe the specific (bio)chemical routes in which • NO and thiols react to form S-nitrosothiols, a broad class of small molecules, and protein post-translational modifications that can influence protein function through catalytic site or allosteric structural changes. S-Nitrosothiol formation depends upon cellular conditions, but critically, an appropriate oxidant for either the thiol (yielding a thiyl radical) or • NO (yielding a nitrosonium [NO+ ]-donating species) is required. We examine the roles of these collective • NO/thiol species in vascular signalling and their cardiovascular therapeutic potential.

Modulation of the nitric oxide/cGMP pathway in cardiac contraction and relaxation: Potential role in heart failure treatment

Evidence exists that heart failure (HF) has an overall impact of 1-2 % in the global population being often associated with comorbidities that contribute to increased disease prevalence, hospitalization, and mortality. Recent advances in pharmacological approaches have significantly improved clinical outcomes for patients with vascular injury and HF. Nevertheless, there remains an unmet need to clarify the crucial role of nitric oxide/cyclic guanosine 3',5'-monophosphate (NO/cGMP) signalling in cardiac contraction and relaxation, to better identify the key mechanisms involved in the pathophysiology of myocardial dysfunction both with reduced (HFrEF) as well as preserved ejection fraction (HFpEF). Indeed, NO signalling plays a crucial role in cardiovascular homeostasis and its dysregulation induces a significant increase in oxidative and nitrosative stress, producing anatomical and physiological cardiac alterations that can lead to heart failure. The present review aims to examine the molecular mechanisms involved in the bioavailability of NO and its modulation of downstream pathways. In particular, we focus on the main therapeutic targets and emphasize the recent evidence of preclinical and clinical studies, describing the different emerging therapeutic strategies developed to counteract NO impaired signalling and cardiovascular disease (CVD) development.

Inhibitory Peptide of Soluble Guanylyl Cyclase/Trx1 Interface Blunts the Dual Redox Signaling Functions of the Complex

Soluble guanylyl cyclase (GC1) and oxido-reductase thioredoxin (Trx1) form a complex that mediates two NO signaling pathways as a function of the redox state of cells. Under physiological conditions, reduced Trx1 (rTrx1) supports the canonical NO-GC1-cGMP pathway by protecting GC1 activity from thiol oxidation. Under oxidative stress, the NO-cGMP pathway is disrupted by the S-nitrosation of GC1 (addition of a NO group to a cysteine). In turn, SNO-GC1 initiates transnitrosation cascades, using oxidized thioredoxin (oTrx1) as a nitrosothiol relay. We designed an inhibitory peptide that blocked the interaction between GC1 and Trx1. This inhibition resulted in the loss of a) the rTrx1 enhancing effect of GC1 cGMP-forming activity in vitro and in cells and its ability to reduce the multimeric oxidized GC1 and b) GC1's ability to fully reduce oTrx1, thus identifying GC1 novel reductase activity. Moreover, an inhibitory peptide blocked the transfer of S-nitrosothiols from SNO-GC1 to oTrx1. In Jurkat T cells, oTrx1 transnitrosates procaspase-3, thereby inhibiting caspase-3 activity. Using the inhibitory peptide, we demonstrated that S-nitrosation of caspase-3 is the result of a transnitrosation cascade initiated by SNO-GC1 and mediated by oTrx1. Consequently, the peptide significantly increased caspase-3 activity in Jurkat cells, providing a promising therapy for some cancers.

Nitric oxide augments signaling for contraction in hypoxic pulmonary arterial smooth muscle—Implications for hypoxic pulmonary hypertension

Introduction: Hypoxic persistent pulmonary hypertension in the newborn (PPHN) is usually treated with oxygen and inhaled nitric oxide (NO), both pulmonary arterial relaxants. But treatment failure with NO occurs in 25% of cases. We previously demonstrated that 72 h exposure to hypoxia, modeling PPHN, sensitized pulmonary artery smooth muscle cells (PASMC) to the contractile agonist thromboxane and inhibited relaxant adenylyl cyclase (AC) activity.

Methods: In this study, we examined the effects of sodium nitroprusside (SNP), as NO donor, on the thromboxane-mediated contraction and NO-independent relaxation pathways and on reactive oxygen species (ROS) accumulation in PASMC. In addition, we examined the effect of the peroxynitrite scavenger 5,10,15,20-Tetrakis (4-sulfonatophenyl)porphyrinato Iron (III) (FeTPPS) on these processes.

Results: Exposure of PASMC to 72 h hypoxia increased total intracellular ROS compared to normoxic control cells and this was mitigated by treatment of cells with either SNP or FeTPPS. Total protein nitrosylation was increased in hypoxic PASMC compared to controls. Both normoxic and hypoxic cells treated with SNP exhibited increased total protein nitrosylation and intracellular nitrite; this was reduced by treatment with FeTPPS. While cell viability and mitochondrial number were unchanged by hypoxia, mitochondrial activity was decreased compared to controls; addition of FeTPPS did not alter this. Basal and maximal mitochondrial metabolism and ATP turnover were reduced in hypoxic PASMC compared to controls. Hypoxic PASMC had higher basal Ca2+, and a heightened peak Ca2+ response to thromboxane challenge compared to controls. Addition of SNP further elevated the peak Ca2+ response, while addition of FeTPPS brought peak Ca2+ response down to control levels. AC mediated relaxation was impaired in hypoxic PASMC compared to controls but was normalized following treatment with FeTPPS. Addition of SNP inhibited adenylyl cyclase activity in both normoxic and hypoxic PASMC. Moreover, addition of the Ca2+ chelator BAPTA improved AC activity, but the effect was minimal.

Discussion: We conclude that NO independently augments contraction and inhibits relaxation pathways in hypoxic PASMC, in part by a mechanism involving nitrogen radical formation and protein nitrosylation. These observations may partially explain impaired effectiveness of NO when treating hypoxic pulmonary hypertension.

Cellular Factors That Shape the Activity or Function of Nitric Oxide-Stimulated Soluble Guanylyl Cyclase

NO-stimulated guanylyl cyclase (SGC) is a hemoprotein that plays key roles in various physiological functions. SGC is a typical enzyme-linked receptor that combines the functions of a sensor for NO gas and cGMP generator. SGC possesses exclusive selectivity for NO and exhibits a very fast binding of NO, which allows it to function as a sensitive NO receptor. This review describes the effect of various cellular factors, such as additional NO, cell thiols, cell-derived small molecules and proteins on the function of SGC as cellular NO receptor. Due to its vital physiological function SGC is an important drug target. An increasing number of synthetic compounds that affect SGC activity via different mechanisms are discovered and brought to clinical trials and clinics. Cellular factors modifying the activity of SGC constitute an opportunity for improving the effectiveness of existing SGC-directed drugs and/or the creation of new therapeutic strategies.

Emerging pharmacologic interventions for pre-eclampsia treatment

INTRODUCTION

Pre-eclampsia is a serious pregnancy complication and a major global concern for the mortality of both mother and fetus. Existing symptomatic treatments do not delay disease progression; thus, timely delivery of the baby is the most effective measure. However, the risk of various maternal and fetal injuries remains.

AREAS COVERED

In this review, we summarize the potential strategies for pharmacologic interventions in pre-eclamptic therapy. Specifically, we discuss the pathophysiological process of various effective candidate therapeutics that act on potential pathways and molecular targets to inhibit key stages of the disease. We refer to this pathogenesis-focused drug discovery model as a pathogenesis-target-drug (P-T-D) strategy. Finally, we discuss the introduction of nanotechnologies to improve the safety and efficacy of therapeutics via their specific placental targeting ability and placental retention effects.

EXPERT OPINION

Despite the active development of novel pharmacological treatments based on our current knowledge of pre-eclamptic pathogenesis, investigations are still in the early phase. Thus, further exploration of the pathological mechanisms, integrated with the P-T-D strategy and novel nanosystems, could encourage the development of more effective and safer strategies. Such advances could lead to a shift from expectant management to mechanistic-based therapy for pre-eclampsia.

Soluble guanylyl cyclase: Molecular basis for ligand selectivity and action in vitro and in vivo

Nitric oxide (NO), carbon monoxide (CO), oxygen (O₂), hydrogen sulfide (H₂S) are gaseous molecules that play important roles in the physiology and pathophysiology of eukaryotes. Tissue concentrations of these physiologically relevant gases vary remarkable from nM range for NO to high μM range of O₂. Various hemoproteins play a significant role in sensing and transducing cellular signals encoded by gaseous molecules or in transporting them. Soluble guanylyl cyclase (sGC) is a hemoprotein that plays vital roles in a wide range of physiological functions and combines the functions of gaseous sensor and signal transducer. sGC uniquely evolved to sense low non-toxic levels of NO and respond to elevated NO levels by increasing its catalytic ability to generate the secondary signaling messenger cyclic guanosine monophosphate (cGMP). This review discusses sGC's gaseous ligand selectivity and the molecular basis for sGC function as high-affinity and selectivity NO receptor. The effects of other gaseous molecules and small molecules of cellular origin on sGC's function are also discussed.

Soluble guanylyl cyclase mediates noncanonical nitric oxide signaling by nitrosothiol transfer under oxidative stress

Soluble guanylyl cyclase (GC1) is an α/β heterodimer producing cGMP when stimulated by nitric oxide (NO). The NO-GC1-cGMP pathway is essential for cardiovascular homeostasis but is disrupted by oxidative stress, which causes GC1 desensitization to NO by heme oxidation and S-nitrosation (SNO) of specific cysteines. We discovered that under these conditions, GC1-α subunit increases cellular S-nitrosation via transfer of nitrosothiols to other proteins (transnitrosation) in cardiac and smooth muscle cells. One of the GC1 SNO-targets was the oxidized form of Thioredoxin1 (oTrx1), which is unidirectionally transnitrosated by GC1 with αC610 as a SNO-donor. Because oTrx1 itself drives transnitrosation, we sought and identified SNO-proteins targeted by both GC1 and Trx1. We found that transnitrosation of the small GTPase RhoA by SNO-GC1 requires oTrx1 as a nitrosothiol relay, suggesting a SNO-GC1→oTrx1→RhoA cascade. The RhoA signaling pathway, which is antagonized by the canonical NO-cGMP pathway, was alternatively inhibited by GC1-α-dependent S-nitrosation under oxidative conditions. We propose that SNO-GC1, via transnitrosation, mediates adaptive responses triggered by oxidation of the canonical NO-cGMP pathway.

Nitric Oxide Is the Cause of Nitroglycerin Tolerance: Providing an Old Dog New Tricks for Acute Heart Failure

Our paper highlights the past 50 years of research focusing solely on tolerance involving nitroglycerin (glyceryl trinitrate, GTN). It also identifies and discusses inconsistencies in previous mechanistic explanations that have failed to provide a way to administer GTN continuously, free of limitations from tolerance and without the requirement of a nitrate-free interval. We illustrate, for the first time in 135 years, a mechanism whereby nitric oxide, the mediator of vasodilation by GTN, may also be the cause of tolerance. Based on targeting superoxide from mitochondrial complex I, uncoupled by glutathione depletion in response to nitric oxide from GTN, a novel unit dose GTN formulation in glutathione for use as a continuous i.v. infusion has been proposed. We hypothesize that this will reduce or eliminate tolerance seen currently with i.v. GTN. Finally, to evaluate the new formulation we suggest future studies of this new formulation for the treatment of acute decompensated heart failure.

Impairment of Anti-Aggregatory Responses to Nitric Oxide and Prostacyclin: Mechanisms and Clinical Implications in Cardiovascular Disease

The propensity towards platelet-rich thrombus formation increases substantially during normal ageing, and this trend is mediated by decreases in platelet responsiveness to the anti-aggregatory nitric oxide (NO) and prostacyclin (PGI₂) pathways. The impairment of soluble guanylate cyclase and adenylate cyclase-based signalling that is associated with oxidative stress represents the major mechanism of this loss of anti-aggregatory reactivity. Platelet desensitization to these autacoids represents an adverse prognostic marker in patients with ischemic heart disease and may contribute to increased thrombo-embolic risk in patients with heart failure. Patients with platelet resistance to PGI₂ also are unresponsive to ADP receptor antagonist therapy. Apart from ischemia, diabetes and aortic valve disease are also associated with impaired anti-aggregatory homeostasis. This review examines the association of impaired platelet cyclic nucleotide (i.e., cGMP and cAMP) signalling with the emerging evidence of thromboembolic risk in cardiovascular diseases, and discusses the potential therapeutic strategies targeting this abnormality.

Glyceryl Trinitrate: History, Mystery, and Alcohol Intolerance

Glyceryl trinitrate (GTN) is one of the earliest known treatments for angina with a fascinating history that bridges three centuries. However, despite its central role in the nitric oxide (NO) story as a NO-donating compound, establishing the precise mechanism of how GTN exerts its medicinal benefit has proven to be far more difficult. This review brings together the explosive and vasodilatory nature of this three-carbon molecule while providing an update on the likely in vivo pathways through which GTN, and the rest of the organic nitrate family, release NO, nitrite, or a combination of both, while also trying to explain nitrate tolerance. Over the last 20 years the alcohol detoxification enzyme, aldehyde dehydrogenase (ALDH), has undoubtedly emerged as the front runner to explaining GTN's bioactivation. This is best illustrated by reduced GTN efficacy in subjects carrying the single point mutation (Glu504Lys) in ALDH, which is also responsible for alcohol intolerance, as characterized by flushing. While these findings are significant for anyone following the GTN story, they appear particularly relevant for healthcare professionals, and especially so, if administering GTN to patients as an emergency treatment. In short, although the GTN puzzle has not been fully solved, clinical study data continue to cement the importance of ALDH, as uncovered in 2002, as a key GTN activator.

Higher susceptibility to heme oxidation and lower protein stability of the rare α1C517Yβ1 sGC variant associated with moyamoya syndrome

NO sensitive soluble guanylyl cyclase (sGC) plays a key role in mediating physiological functions of NO. Genetic alterations of the GUCY1A3 gene, coding for the α1 subunit of sGC, are associated with several cardiovascular dysfunctions. A rare sGC variant with Cys517 → Tyr substitution in the α1subunit, has been associated with moyamoya disease and achalasia. In this report we characterize the properties of this rare sGC variant. Purified α₁C517Yβ₁ sGC preserved only ~25% of its cGMP-forming activity and showed an elevated K_m for GTP substrate. However, the mutant enzyme retained a high affinity for and robust activation by NO, similar to wild type sGC. Purified α₁C517Yβ₁ enzyme was more sensitive to specific sGC heme oxidizers and less responsive to heme reducing agents. When expressed in COS7 cells, α₁C517Yβ₁ sGC showed a much stronger response to cinaciguat or gemfibrozil, which targets apo-sGC or sGC with ferric heme, as compared to its NO response or the relative response of the wild type sGC. A stronger response to cinaciguat was also observed for purified α₁C517Yβ₁ in the absence of reducing agents. In COS7 cells, αCys517β sGC was less stable than the wild type enzyme under normal conditions and exhibited accelerated degradation upon induction of cellular oxidative stress. We conclude that diminished cGMP-forming activity of this sGC variant is aggravated by its high susceptibility to oxidative stress and diminished protein stability. The combination of these deficiencies contributes to the severity of observed moyamoya and achalasia symptoms in human carriers of this rare α₁C517Yβ₁ sGC variant.

Tracing the path of inhaled nitric oxide: Biological consequences of protein nitrosylation

Nitric oxide (NO) is a comprehensive regulator of vascular and airway tone. Endogenous NO produced by nitric oxide synthases regulates multiple signaling cascades, including activation of soluble guanylate cyclase to generate cGMP, relaxing smooth muscle cells. Inhaled NO is an established therapy for pulmonary hypertension in neonates, and has been recently proposed for the treatment of hypoxic respiratory failure and acute respiratory distress syndrome due to COVID-19. In this review, we summarize the effects of endogenous and exogenous NO on protein S-nitrosylation, which is the selective and reversible covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine. This posttranslational modification targets specific cysteines based on the acid/base sequence of surrounding residues, with significant impacts on protein interactions and function. S-nitrosothiol (SNO) formation is tightly compartmentalized and enzymatically controlled, but also propagated by nonenzymatic transnitrosylation of downstream protein targets. Redox-based nitrosylation and denitrosylation pathways dynamically regulate the equilibrium of SNO-proteins. We review the physiological roles of SNO proteins, including nitrosohemoglobin and autoregulation of blood flow through hypoxic vasodilation, and pathological effects of nitrosylation including inhibition of critical vasodilator enzymes; and discuss the intersection of NO source and dose with redox environment, in determining the effects of protein nitrosylation.

Maturation, inactivation, and recovery mechanisms of soluble guanylyl cyclase

Soluble guanylate cyclase (sGC) is a heme-containing heterodimeric enzyme that generates many molecules of cGMP in response to its ligand nitric oxide (NO); sGC thereby acts as an amplifier in NO-driven biological signaling cascades. Because sGC helps regulate the cardiovascular, neuronal, and gastrointestinal systems through its cGMP production, boosting sGC activity and preventing or reversing sGC inactivation are important therapeutic and pharmacologic goals. Work over the last two decades is uncovering the processes by which sGC matures to become functional, how sGC is inactivated, and how sGC is rescued from damage. A diverse group of small molecules and proteins have been implicated in these processes, including NO itself, reactive oxygen species, cellular heme, cell chaperone Hsp90, and various redox enzymes as well as pharmacologic sGC agonists. This review highlights their participation and provides an update on the processes that enable sGC maturation, drive its inactivation, or assist in its recovery in various settings within the cell, in hopes of reaching a better understanding of how sGC function is regulated in health and disease.

Selective cysteines oxidation in soluble guanylyl cyclase catalytic domain is involved in NO activation

Nitric oxide (NO) binds to soluble guanylyl cyclase (GC1) and stimulates its catalytic activity to produce cGMP. Despite the key role of the NO-cGMP signaling in cardiovascular physiology, the mechanisms of GC1 activation remain ill-defined. It is believed that conserved cysteines (Cys) in GC1 modulate the enzyme's activity through thiol-redox modifications. We previously showed that GC1 activity is modulated via mixed-disulfide bond by protein disulfide isomerase and thioredoxin 1. Herein we investigated the novel concept that NO-stimulated GC1 activity is mediated by thiol/disulfide switches and aimed to map the specific Cys that are involved. First, we showed that the dithiol reducing agent Tris (2-carboxyethyl)-phosphine reduces GC1 response to NO, indicating the significance of Cys oxidation in NO activation. Second, using dibromobimane, which fluoresces when crosslinking two vicinal Cys thiols, we demonstrated decreased fluorescence in NO-stimulated GC1 compared to unstimulated conditions. This suggested that NO-stimulated GC1 contained more bound Cys, potentially disulfide bonds. Third, to identify NO-regulated Cys oxidation using mass spectrometry, we compared the redox status of all Cys identified in tryptic peptides, among which, ten were oxidized and two were reduced in NO-stimulated GC1. Fourth, we resorted to computational modeling to narrow down the Cys candidates potentially involved in disulfide bond and identified Cys489 and Cys571. Fifth, our mutational studies showed that Cys489 and Cys571 were involved in GC1'response to NO, potentially as a thiol/disulfide switch. These findings imply that specific GC1 Cys sensitivity to redox environment is critical for NO signaling in cardiovascular physiology.

Molecular Determinants for Nitric Oxide Regulation of the Murine Cationic Amino Acid Transporter CAT-2A

Cationic amino acid transporters (CATs) supply cells with essential and semiessential dibasic amino acids. Among them, l-arginine is the substrate for nitric oxide synthases (NOS) to produce nitric oxide (NO), a key signaling molecule and second messenger. In cardiac preparations, we showed that NO acutely and directly modulates transport activity by noncompetitively inhibiting these CATs. We hypothesize that this NO regulation occurs through modification of cysteine residues in CAT proteins. Homology modeling and a computational chemistry approach identified Cys³⁴⁷ as one of two putative targets for NO binding, of 15 Cys residues present in the low-affinity mouse CAT-2A (mCAT-2A). To test this prediction, mammalian cell lines overexpressing mCAT-2A were used for site-directed mutagenesis and uptake studies. When Cys³⁴⁷ was replaced with alanine (Cys³⁴⁷Ala), mCAT-2A became insensitive to inhibition by NO donors. In addition, the transport capacity of this variant decreased by >50% compared to that of the control, without affecting membrane expression levels or apparent affinities for the transported amino acids. Interestingly, replacing Cys³⁴⁷ with serine (Cys³⁴⁷Ser) restored uptake levels to those of the control while retaining NO insensitivity. Other Cys residues, when replaced with Ala, still produced a NO-sensitive CAT-2A. In cells co-expressing NOS and mCAT-2A, exposure to extracellular l-arginine inhibited the uptake activity of control mCAT-2A, via NO production, but not that of the Cys³⁴⁷Ser variant. Thus, the -SH moiety of Cys³⁴⁷ is largely responsible for mCAT-2A inhibition by NO. Because of the endogenous NO effect, this modulation is likely to be physiologically relevant and a potential intervention point for therapeutics.

S-Nitrosylation of Akt by organic nitrate delays revascularization and the recovery of cardiac function in mice following myocardial infarction

The effects of long‐term nitrate therapy are compromised due to protein S‐Nitrosylation, which is mediated by nitric oxide (NO). This study is to determine the role of Akt S‐Nitrosylation in the recovery of heart functions after ischaemia. In recombinant Akt protein and in HEK293 cells, NO donor decreased Akt activity and induced Akt S‐Nitrosylation, but was abolished if Akt protein was mutated by replacing cysteine 296/344 with alanine (Akt‐C296/344A). In endothelial cells, NO induced Akt S‐Nitrosylation, reduced Akt activity and damaged multiple cellular functions including proliferation, migration and tube formation. These alterations were ablated if cells expressed Akt‐C296/344A mutant. In Apoe^−/− mice, nitroglycerine infusion increased both Akt S‐Nitrosylation and infarct size, reduced Akt activity and capillary density, and delayed the recovery of cardiac function in ischaemic hearts, compared with mice infused with vehicle. Importantly, these in vivo effects of nitroglycerine in Apoe^−/− mice were remarkably prevented by adenovirus‐mediated enforced expression of Akt‐C296/344A mutant. In conclusion, long‐term usage of organic nitrate may inactivate Akt to delay ischaemia‐induced revascularization and the recovery of cardiac function through NO‐mediated S‐Nitrosylation.

Non-canonical chemical feedback self-limits nitric oxide-cyclic GMP signaling in health and disease

Nitric oxide (NO)-cyclic GMP (cGMP) signaling is a vasoprotective pathway therapeutically targeted, for example, in pulmonary hypertension. Its dysregulation in disease is incompletely understood. Here we show in pulmonary artery endothelial cells that feedback inhibition by NO of the NO receptor, the cGMP forming soluble guanylate cyclase (sGC), may contribute to this. Both endogenous NO from endothelial NO synthase and exogenous NO from NO donor compounds decreased sGC protein and activity. This effect was not mediated by cGMP as the NO-independent sGC stimulator, or direct activation of cGMP-dependent protein kinase did not mimic it. Thiol-sensitive mechanisms were also not involved as the thiol-reducing agent N-acetyl-L-cysteine did not prevent this feedback. Instead, both in-vitro and in-vivo and in health and acute respiratory lung disease, chronically elevated NO led to the inactivation and degradation of sGC while leaving the heme-free isoform, apo-sGC, intact or even increasing its levels. Thus, NO regulates sGC in a bimodal manner, acutely stimulating and chronically inhibiting, as part of self-limiting direct feedback that is cGMP independent. In high NO disease conditions, this is aggravated but can be functionally recovered in a mechanism-based manner by apo-sGC activators that re-establish cGMP formation.

Therapeutic Targeting of the Soluble Guanylate Cyclase

The soluble guanylate cyclase (sGC) is the physiological sensor for nitric oxide and alterations of its function are actively implicated in a wide variety of pathophysiological conditions. Intense research efforts over the past 20 years have provided significant information on its regulation, culminating in the rational development of approved drugs or investigational lead molecules, which target and interact with sGC through novel mechanisms. However, there are numerous questions that remain unanswered. Ongoing investigations, with the critical aid of structural chemistry studies, try to further elucidate the enzyme's structural characteristics that define the association of "stimulators" or "activators" of sGC in the presence or absence of the heme moiety, respectively, as well as the precise conformational attributes that will allow the design of more innovative and effective drugs. This review relates the progress achieved, particularly in the past 10 years, in understanding the function of this enzyme, and focusses on a) the rationale and results of its therapeutic targeting in disease situations, depending on the state of enzyme (oxidized or not, heme-carrying or not) and b) the most recent structural studies, which should permit improved design of future therapeutic molecules that aim to directly upregulate the activity of sGC.

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

S-nitrosylation of endothelial nitric oxide synthase impacts erectile function

Neuronal and endothelial nitric oxide synthases (nNOS and eNOS respectively) play major roles in generating the nitric oxide bioactivity necessary for erectile function. S-nitrosylation has been shown to regulate NOS activity. The presence of S-nitrosylated NOS in the penis and the impact of NOS S-nitrosylation/denitrosylation on erectile function were examined. S-nitrosylated forms of NOS were identified by biotin-switch assay followed by western blot analysis. Erectile function in S-nitrosoglutathione reductase deficient (GSNO^+/-) and null (GSNO^-/-) mice were assessed by continuous cavernous nerve electrical stimulation (CCNES). Glutathione ethyl ester (GSHee) was used to manipulate S-nitrosylated NOS levels. Immunohistological and immunofluorescence analyses were used to identify the location of eNOS and GSNO-R in corporal tissue. eNOS and nNOS were S-nitrosylated in unstimulated penises of the mice. CCNES resulted in a time-dependent increase in eNOS S-nitrosylation with peak eNOS S-nitrosylation observed during detumescence. S-nitrosylated nNOS levels were unchanged. Intracorporal injection of GSHee reduced S-nitrosylated eNOS levels, enhancing time to maximum intracorporal pressure (ICP). eNOS and GSNO-R co-localize to the endothelium of the corpus cavernosum in the mouse and the human. ICP measurements obtained during CCNES demonstrate GSNO-R^+/- and GSNO-R^-/- animals cannot maintain an elevated ICP. Results suggest eNOS S-nitrosylation/denitrosylation is an important mechanism regulating eNOS activity during erectile function. GSNO-R is a key enzyme involved in the eNOS denitrosylation. The increase in eNOS S-nitrosylation (inactivation) observed with tumescence may begin a cycle leading to detumescence. Clinically this may indicate that alterations in the balance of S-nitrosylation/denitrosylation either directly or indirectly contribute to erectile dysfunction.

S-Nitrosylation of Prostacyclin Synthase Instigates Nitrate Cross-Tolerance In Vivo

Development of nitrate tolerance is a major drawback to nitrate therapy. Prostacyclin (PGI2) is a powerful vasodilator produced from prostaglandin (PGH2) by prostacyclin synthase (PGIS) in endothelial cells. This study aimed to determine the role of PGIS S-nitrosylation in nitrate tolerance induced by nitroglycerin (GTN). In endothelial cells, GTN increased PGIS S-nitrosylation and disturbed PGH2 metabolism, which were normalized by mutants of PGIS cysteine 231/441 to alanine (C231/441A). Clearance of nitric oxide by carboxy-PTIO or inhibition of S-nitrosylation by N-acetyl-cysteine decreased GTN-induced PGIS S-nitrosylation. Enforced expression of mutated PGIS with C231/441A markedly abolished GTN-induced PGIS S-nitrosylation and nitrate cross-tolerance in Apoe^-/- mice. Inhibition of cyclooxygenase 1 by aspirin, supplementation of PGI2 by beraprost, and inhibition of PGIS S-nitrosylation by N-acetyl-cysteine improved GTN-induced nitrate cross-tolerance in rats. In patients, increased PGIS S-nitrosylation was associated with nitrate tolerance. In conclusion, GTN induces nitrate cross-tolerance through PGIS S-nitrosylation at cysteine 231/441.

Induction of microRNA-199 by Nitric Oxide in Endothelial Cells Is Required for Nitrovasodilator Resistance via Targeting of Prostaglandin I2 Synthase

Background:

Nitrates are widely used to treat coronary artery disease, but their therapeutic value is compromised by nitrate tolerance, because of the dysfunction of prostaglandin I2 synthase (PTGIS). MicroRNAs repress target gene expression and are recognized as important epigenetic regulators of endothelial function. The aim of this study was to determine whether nitrates induce nitrovasodilator resistance via microRNA-dependent repression of PTGIS gene expression.

Methods:

Nitrovasodilator resistance was induced by nitroglycerin (100 mg·kg –1 ·d –1 , 3 days) infusion in Apoe –/– mice. The responses of aortic arteries to nitric oxide donors were assessed in an organ chamber. The expression levels of microRNA-199 (miR-199)a/b were assayed by quantitative reverse transcription polymerase chain reaction or fluorescent in situ hybridization.

Results:

In cultured human umbilical vein endothelial cells, nitric oxide donors induced miR-199a/b endogenous expression and downregulated PTGIS gene expression, both of which were reversed by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt or silence of serum response factor. Evidence from computational and luciferase reporter gene analyses indicates that the seed sequence of 976 to 982 in the 3′-untranslated region of PTGIS mRNA is a target of miR-199a/b. Gain functions of miR-199a/b resulting from chemical mimics or adenovirus-mediated overexpression increased PTGIS mRNA degradation in HEK293 cells and human umbilical vein endothelial cells. Furthermore, nitroglycerin-decreased PTGIS gene expression was prevented by miR-199a/b antagomirs or was mirrored by the enforced expression of miR-199a/b in human umbilical vein endothelial cells. In Apoe –/– mice, nitroglycerin induced the ectopic expression of miR-199a/b in the carotid arterial endothelium, decreased PTGIS gene expression, and instigated nitrovasodilator resistance, all of which were abrogated by miR-199a/b antagomirs or LNA—anti–miR-199. It is important that the effects of miR-199a/b inhibitions were abolished by adenovirus-mediated PTGIS deficiency. Moreover, the enforced expression of miR-199a/b in vivo repressed PTGIS gene expression and impaired the responses of aortic arteries to nitroglycerin/sodium nitroprusside/acetylcholine/cinaciguat/riociguat, whereas the exogenous expression of the PTGIS gene prevented nitrovasodilator resistance in Apoe –/– mice subjected to nitroglycerin infusion or miR-199a/b overexpression. Finally, indomethacin, iloprost, and SQ29548 improved vasorelaxation in nitroglycerin-infused Apoe –/– mice, whereas U51605 induced nitrovasodilator resistance. In humans, the increased expressions of miR-199a/b were closely associated with nitrate tolerance.

Conclusions:

Nitric oxide–induced ectopic expression of miR-199a/b in endothelial cells is required for nitrovasodilator resistance via the repression of PTGIS gene expression. Clinically, miR-199a/b is a novel target for the treatment of nitrate tolerance.

S-nitrosylation of NOS pathway mediators in the penis contributes to cavernous nerve injury-induced erectile dysfunction

cGMP-independent nitric oxide (NO) signaling occurs via S-nitrosylation. We evaluated whether aberrant S-nitrosylation operates in the penis under conditions of cavernous nerve injury and targets proteins involved in regulating erectile function. Adult male Sprague-Dawley rats underwent bilateral cavernous nerve crush injury (BCNI) or sham surgery. Rats were given a denitrosylation agent N-acetylcysteine (NAC, 300 mg/kg/day) or vehicle in drinking water starting 2 days before BCNI and continuing for 2 weeks following surgery. After assessment of erectile function (intracavernous pressure), penes were collected for measurements of S-nitrosylation by Saville-Griess and TMT-switch assays and PKG-I function by immunoblotting of phospho (P)-VASP-Ser-239. Erectile function was decreased (P<0.05) after BCNI, and it was preserved (P<0.05) by NAC treatment. Total S-nitrosothiols and total S-nitrosylated proteins were increased (P<0.05) after BCNI, and these were partially prevented by NAC treatment. S-nitrosylation of sGC was increased (P<0.05) after BCNI, and it was prevented (P<0.05) by NAC treatment. S-nitrosylation of eNOS was increased (P<0.05) after BCNI, and showed a trend towards decrease by NAC treatment. Protein expression of P-VASP-Ser-239 was decreased (P<0.05) after BCNI, and showed a trend towards increase by NAC treatment. In conclusion, erectile dysfunction following BCNI is mediated in part by S-nitrosylation of eNOS and its downstream signaling mediator GC, while denitrosylation protects erectile function by preserving the NO/cGMP signaling pathway.

Nitric oxide signalling in cardiovascular health and disease

Nitric oxide (NO) signalling has pleiotropic roles in biology and a crucial function in cardiovascular homeostasis. Tremendous knowledge has been accumulated on the mechanisms of the nitric oxide synthase (NOS)-NO pathway, but how this highly reactive, free radical gas signals to specific targets for precise regulation of cardiovascular function remains the focus of much intense research. In this Review, we summarize the updated paradigms on NOS regulation, NO interaction with reactive oxidant species in specific subcellular compartments, and downstream effects of NO in target cardiovascular tissues, while emphasizing the latest developments of molecular tools and biomarkers to modulate and monitor NO production and bioavailability.

Poly (I:C) impairs NO donor-induced relaxation by overexposure to NO via the NF-kappa B/iNOS pathway in rat superior mesenteric arteries

Recent studies have suggested a link between vascular dysfunction and innate immune activation including toll-like receptors (TLRs), but the detailed mechanism remains unclear. Here we investigated whether poly (I:C) [a synthetic double-strand RNA recognized by TLR3, melanoma differentiation-associated gene 5 (MDA5), and retinoic acid-inducible gene I (RIG-I)] affected nitric oxide (NO)/cGMP-related vascular relaxation, one of the major cascades of relaxation, in rat superior mesenteric arteries. Using organ-cultured arteries, we found that poly (I:C) (30μg/mL for approximately 1 day) markedly reduced sodium nitroprusside (SNP)-induced relaxation (vs. vehicle); this was prevented by co-treatment with a TLR3 inhibitor. Relaxation induced by 8-Br cGMP (a phosphodiesterase (PDE)-resistant cGMP analogue) and the expression of proteins related to NO/cGMP signaling did not differ between vehicle- and poly (I:C)-treated groups. When PDEs were inhibited by IBMX (a nonselective PDE inhibitor), the SNP-induced relaxation was still greatly reduced in poly (I:C)-treated arteries (vs. vehicle). Poly (I:C) reduced SNP-stimulated cGMP production, but increased NO production and iNOS expression (vs. vehicle). The impairment of SNP-induced relaxation by poly (I:C) was prevented by co-treatment with either iNOS or a nuclear factor-kappa B (NF-κB) inhibitor. This effect induced by poly (I:C) appeared to be independent of oxidative stress. The SNP-induced relaxation was reduced in freshly isolated arteries by pre-incubation with SNP in a concentration-dependent manner. Poly (I:C) did not alter protein levels of TLR3, TRIF/TICAM-1, or phospho-IRF3/IRF3, whereas RIG-I and MDA5 were significantly upregulated (vs. vehicle). These results suggest that poly (I:C) impairs NO donor-induced relaxation in rat superior mesenteric arteries via overexposure to NO produced by the NF-κB/iNOS pathway.

H₂S-Mediated Protein S-Sulfhydration: A Prediction for Its Formation and Regulation

Protein S-sulfhydration is a newly discovered post-translational modification of specific cysteine residue(s) in target proteins, which is involved in a broad range of cellular functions and metabolic pathways. By changing local conformation and the final activity of target proteins, S-sulfhydration is believed to mediate most cellular responses initiated by H₂S, a novel gasotransmitter. In comparison to protein S-sulfhydration, nitric oxide-mediated protein S-nitrosylation has been extensively investigated, including its formation, regulation, transfer and metabolism. Although the investigation on the regulatory mechanisms associated with protein S-sulfhydration is still in its infancy, accumulated evidence suggested that protein S-sulfhydration may share similar chemical features with protein S-nitrosylation. Glutathione persulfide acts as a major donor for protein S-sulfhydration. Here, we review the present knowledge on protein S-sulfhydration, and also predict its formation and regulation mechanisms based on the knowledge from protein S-nitrosylation.

Guanylyl cyclase sensitivity to nitric oxide is protected by a thiol oxidation-driven interaction with thioredoxin-1

Nitric oxide (NO) modulates many physiological events through production of cGMP from its receptor, the NO-sensitive guanylyl cyclase (GC1). NO also appears to function in a cGMP-independent manner, via S-nitrosation (SNO), a redox-based modification of cysteine thiols. Previously, we have shown that S-nitrosated GC1 (SNO-GC1) is desensitized to NO stimulation following prolonged NO exposure or under oxidative/nitrosative stress. In animal models of nitrate tolerance and angiotensin II-induced hypertension, decreased vasodilation in response to NO correlates with GC1 thiol oxidation, but the physiological mechanism that resensitizes GC1 to NO and restores basal activity is unknown. Because GC1 interacts with the oxidoreductase protein-disulfide isomerase, we hypothesized that thioredoxin-1 (Trx1), a cytosolic oxidoreductase, could be involved in restoring GC1 basal activity and NO sensitivity because the Trx/thioredoxin reductase (TrxR) system maintains thiol redox homeostasis. Here, by manipulating activity and levels of the Trx1/TrxR system and by using a Trx1-Trap assay, we demonstrate that Trx1 modulates cGMP synthesis through an association between Trx1 and GC1 via a mixed disulfide. A proximity ligation assay confirmed the endogenous Trx1-GC1 complex in cells. Mutational analysis suggested that Cys⁶⁰⁹ in GC1 is involved in the Trx1-GC1 association and modulation of GC1 activity. Functionally, we established that Trx1 protects GC1 from S-nitrosocysteine-induced desensitization. A computational model of Trx1-GC1 interaction illustrates a possible mechanism for Trx1 to maintain basal GC1 activity and prevent/rescue GC1 desensitization to NO. The etiology of some oxidative vascular diseases may very well be explained by the dysfunction of the Trx1-GC1 association.

Transnitrosylation: A Factor in Nitric Oxide-Mediated Penile Erection

INTRODUCTION

Nitric oxide (NO) signaling can be mediated not only through classic 3',5'-cyclic guanosine monophosphate but also through S-nitrosylation. However, the impact of S-nitrosylation on erectile function and in NO regulation and oxidative stress in the penis remains poorly understood.

AIMS

To characterize the role of S-nitrosoglutathione reductase (GSNOR), a major regulator of S-nitrosylation homeostasis, on erection physiology and on endothelial NO synthase (eNOS) function and oxidative-nitrosative stress in the penis.

METHODS

Adult GSNOR-deficient and wild-type (WT) mice were used. Erectile function was assessed in response to electrical stimulation of the cavernous nerve. Total NO in penile homogenates was measured by Griess reaction. Protein S-nitrosylation, eNOS phosphorylation on Ser-1177 (positive regulatory site), eNOS uncoupling, and markers of oxidative stress (4-hydroxy-2-nonenal, malondialdehyde, and nitrotyrosine) in the penis were measured by western blot.

MAIN OUTCOME MEASURES

Erectile function, eNOS function, and oxidative stress in the penis of GSNOR-deficient mice.

RESULTS

Erectile function was intact in GSNOR-deficient mice. Total S-nitrosylated proteins were increased (P < .05) in the GSNOR(-/-) compared with WT mouse penis. Although eNOS phosphorylation on Ser-1177 did not differ between the GSNOR(-/-) and WT mouse penises at baseline, electrical stimulation of the cavernous nerve increased (P < .05) phosphorylated eNOS in the WT mouse penis but failed to increase phosphorylated eNOS in the GSNOR(-/-) mouse penis. Total NO production was decreased (P < .05), whereas eNOS uncoupling, 4-hydroxy-2-nonenal, malondialdehyde, and nitrotyrosine were increased (P < .05) in the GSNOR-deficient mouse penis compared with the WT mouse penis.

CONCLUSION

Transnitrosylation mechanisms play an important role in regulating NO bioactivity in the penis. Deficiency of GSNOR leads to eNOS dysfunction and increased oxidative damage, suggesting that homeostatic eNOS function in the penis is governed by transnitrosylation.

Crosstalk of mitochondria with NADPH oxidase via reactive oxygen and nitrogen species signalling and its role for vascular function

Cardiovascular diseases are associated with and/or caused by oxidative stress. This concept has been proven by using the approach of genetic deletion of reactive species producing (pro-oxidant) enzymes as well as by the overexpression of reactive species detoxifying (antioxidant) enzymes leading to a marked reduction of reactive oxygen and nitrogen species (RONS) and in parallel to an amelioration of the severity of diseases. Likewise, the development and progression of cardiovascular diseases is aggravated by overexpression of RONS producing enzymes as well as deletion of antioxidant RONS detoxifying enzymes. Thus, the consequences of the interaction (redox crosstalk) of superoxide/hydrogen peroxide produced by mitochondria with other ROS producing enzymes such as NADPH oxidases (Nox) are of outstanding importance and will be discussed including the consequences for endothelial nitric oxide synthase (eNOS) uncoupling as well as the redox regulation of the vascular function/tone in general (soluble guanylyl cyclase, endothelin-1, prostanoid synthesis). Pathways and potential mechanisms leading to this crosstalk will be analysed in detail and highlighted by selected examples from the current literature including hypoxia, angiotensin II-induced hypertension, nitrate tolerance, aging and others. The general concept of redox-based activation of RONS sources via "kindling radicals" and enzyme-specific "redox switches" will be discussed providing evidence that mitochondria represent key players and amplifiers of the burden of oxidative stress.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

New Developments in Platelet Cyclic Nucleotide Signalling: Therapeutic Implications

Altered platelet physiology may contribute to the emergence of thrombosis in patients with many forms of cardiovascular disease. Excess platelet activation may reflect increased stimulation of pro-aggregatory pathways. There is, however, increasing evidence that excessive platelet response, due to impaired efficacy of anti-aggregatory autacoids such as nitric oxide (NO) and prostacyclin (PGI₂), may be just as important. For example, diminished platelet response to NO has been documented in acute and chronic myocardial ischaemia, heart failure, aortic valve disease and in the presence of hyperglycaemia. This "NO resistance" has been shown to reflect both the scavenging of NO by reactive oxygen species and dysfunction of its intracellular "receptor", soluble guanylate cyclase. Importantly, these abnormalities of NO signalling are potentially reversible through judicious application of pharmacotherapy. The analogous condition of impaired PGI₂/adenylate cyclase (AC) signalling has received comparatively less attention to date. We have shown that platelet response to prostaglandin E₁ (PGE₁) is frequently impaired in patients with symptomatic myocardial ischaemia. Because the effects of ADP receptor antagonists such as clopidogrel and ticagrelor at the level of the P2Y₁₂ receptor are coupled with changes in activity of AC, impaired response to PGE₁ might imply both increased thrombotic risk and a reduced efficacy of anti-aggregatory drugs. Accordingly, patient response to treatment with clopidogrel is determined not only by variability of clopidogrel bio-activation, but also extensively by the integrity of platelet AC signalling. We here review these recent developments and their emerging therapeutic implications for thrombotic disorders.

GSNOR Deficiency Enhances In Situ Skeletal Muscle Strength, Fatigue Resistance, and RyR1 S-Nitrosylation Without Impacting Mitochondrial Content and Activity

AIM

Nitric oxide (NO) plays important, but incompletely defined roles in skeletal muscle. NO exerts its regulatory effects partly though S-nitrosylation, which is balanced by denitrosylation by enzymes such as S-nitrosoglutathione reductase (GSNOR), whose functions in skeletal muscle remain to be fully deciphered.

RESULTS

GSNOR null (GSNOR^-/-) tibialis anterior (TA) muscles showed normal growth and were stronger and more fatigue resistant than controls in situ. However, GSNOR^-/- lumbrical muscles showed normal contractility and Ca²⁺ handling in vitro, suggesting important differences in GSNOR function between muscles or between in vitro and in situ environments. GSNOR^-/- TA muscles exhibited normal mitochondrial content, and capillary densities, but reduced type IIA fiber content. GSNOR inhibition did not impact mitochondrial respiratory complex I, III, or IV activities. These findings argue that enhanced GSNOR^-/- TA contractility is not driven by changes in mitochondrial content or activity, fiber type, or blood vessel density. However, loss of GSNOR led to RyR1 hypernitrosylation, which is believed to increase muscle force output under physiological conditions. cGMP synthesis by soluble guanylate cyclase (sGC) was decreased in resting GSNOR^-/- muscle and was more responsive to agonist (DETANO, BAY 41, and BAY 58) stimulation, suggesting that GSNOR modulates cGMP production in skeletal muscle.

INNOVATION

GSNOR may act as a "brake" on skeletal muscle contractile performance under physiological conditions by modulating nitrosylation/denitrosylation balance.

CONCLUSIONS

GSNOR may play important roles in skeletal muscle contractility, RyR1 S-nitrosylation, fiber type specification, and sGC activity. Antioxid. Redox Signal. 26, 165-181.

The Role of Reactive Oxygen and Nitrogen Species in the Expression and Splicing of Nitric Oxide Receptor

SIGNIFICANCE

Nitric oxide (NO)-dependent signaling is critical to many cellular functions and physiological processes. Soluble guanylyl cyclase (sGC) acts as an NO receptor and mediates the majority of NO functions. The signaling between NO and sGC is strongly altered by reactive oxygen and nitrogen species. Recent Advances: Besides NO scavenging, sGC is affected by oxidation/loss of sGC heme, oxidation, or nitrosation of cysteine residues and phosphorylation. Apo-sGC or sGC containing oxidized heme is targeted for degradation. sGC transcription and the stability of sGC mRNA are also affected by oxidative stress.

CRITICAL ISSUES

Studies cited in this review suggest the existence of compensatory processes that adapt cellular processes to diminished sGC function under conditions of short-term or moderate oxidative stress. Alternative splicing of sGC transcripts is discussed as a mechanism with the potential to both enhance and reduce sGC function. The expression of α1 isoform B, a functional and stable splice variant of human α1 sGC subunit, is proposed as one of such compensatory mechanisms. The expression of dysfunctional splice isoforms is discussed as a contributor to decreased sGC function in vascular disease.

FUTURE DIRECTIONS

Targeting the process of sGC splicing may be an important approach to maintain the composition of sGC transcripts that are expressed in healthy tissues under normal conditions. Emerging new strategies that allow for targeted manipulations of RNA splicing offer opportunities to use this approach as a preventive measure and to control the composition of sGC splice isoforms. Rational management of expressed sGC splice forms may be a valuable complementary treatment strategy for existing sGC-directed therapies. Antioxid. Redox Signal. 26, 122-136.

Thiol-Based Redox Modulation of Soluble Guanylyl Cyclase, the Nitric Oxide Receptor

SIGNIFICANCE

Soluble guanylyl cyclase (sGC), which produces the second messenger cyclic guanosine 3', 5'-monophosphate (cGMP), is at the crossroads of nitric oxide (NO) signaling: sGC catalytic activity is both stimulated by NO binding to the heme and inhibited by NO modification of its cysteine (Cys) thiols (S-nitrosation). Modulation of sGC activity by thiol oxidation makes sGC a therapeutic target for pathologies originating from oxidative or nitrosative stress. sGC has an unusually high percentage of Cys for a cytosolic protein, the majority solvent exposed and therefore accessible modulatory targets for biological and pathophysiological signaling. Recent Advances: Thiol oxidation of sGC contributes to the development of cardiovascular diseases by decreasing NO-dependent cGMP production and thereby vascular reactivity. This thiol-based resistance to NO (e.g., increase in peripheral resistance) is observed in hypertension and hyperaldosteronism.

CRITICAL ISSUES

Some roles of specific Cys thiols have been identified in vitro. So far, it has not been possible to pinpoint the roles of specific Cys of sGC in vivo and to investigate the molecular mechanisms in an animal model.

FUTURE DIRECTIONS

The role of Cys as redox sensors, intermediates of activation, and mediators of change in sGC conformation, activity, and dimerization remains largely unexplored. To understand modulation of sGC activity, it is critical to investigate the roles of specific oxidative thiol modifications that are formed during these processes. Where the redox state of sGC thiols contribute to pathologies (vascular resistance and sGC desensitization by NO donors), it becomes crucial to design therapeutic strategies to restore sGC to its normal, physiological thiol redox state. Antioxid. Redox Signal. 26, 137-149.

S-nitrosothiols dilate the mesenteric artery more potently than the femoral artery by a cGMP and L-type calcium channel-dependent mechanism

S-nitrosothiols (SNOs) are metabolites of NO with potent vasodilatory activity. Our previous studies in sheep indicated that intra-arterially infused SNOs dilate the mesenteric vasculature more than the femoral vasculature. We hypothesized that the mesenteric artery is more responsive to SNO-mediated vasodilation, and investigated various steps along the NO/cGMP pathway to determine the mechanism for this difference. In anesthetized adult sheep, we monitored the conductance of mesenteric and femoral arteries during infusion of S-nitroso-l-cysteine (L-cysNO), and found mesenteric vascular conductance increased (137 ± 3%) significantly more than femoral conductance (26 ± 25%). Similar results were found in wire myography studies of isolated sheep mesenteric and femoral arteries. Vasodilation by SNOs was attenuated in both vessel types by the presence of ODQ (sGC inhibitor), and both YC-1 (sGC agonist) and 8-Br-cGMP (cGMP analog) mediated more potent relaxation in mesenteric arteries than femoral arteries. The vasodilatory difference between mesenteric and femoral arteries was eliminated by antagonists of either protein kinase G or L-type Ca(2+) channels. Western immunoblots showed a larger L-type Ca(2+)/sGC abundance ratio in mesenteric arteries than in femoral arteries. Fetal sheep mesenteric arteries were more responsive to SNOs than adult mesenteric arteries, and had a greater L-Ca(2+)/sGC ratio (p = 0.047 and r = -0.906 for correlation between Emax and L-Ca(2+)/sGC). These results suggest that mesenteric arteries, especially those in fetus, are more responsive to SNO-mediated vasodilation than femoral arteries due to a greater role of the L-type calcium channel in the NO/cGMP pathway.

cGMP becomes a drug target

Cyclic guanosine 3′,5′-monophosphate (cGMP) serves as a second messenger molecule, which regulates pleiotropic cellular functions in health and disease. cGMP is generated by particulate or soluble guanylyl cyclases upon stimulation with natriuretic peptides or nitric oxide, respectively. Furthermore, the cGMP concentration is modulated by cGMP-degrading phosphodiesterases. Several targets of cGMP are utilized to effect its various cellular functions. These effector molecules comprise cGMP-dependent protein kinases, ion channels, and phosphodiesterases. During the last decade, it emerged that cGMP is a novel drug target for the treatment of pulmonary and cardiovascular disorders. In this respect, several drugs were developed, which are now in clinical phase studies for, e.g., pulmonary hypertension or cardiovascular diseases. These new drugs act NO-independently with/without heme on soluble guanylyl cyclases or induce subtypes of particular guanylyl cyclases and thereby lead to new therapeutic concepts and horizons. In this regard, the fifth cGMP meeting held in June 2011 in Halle, Germany, comprised the new therapeutic challenges with the novel functional and structural concepts of cGMP generating and effector molecules. This report summarizes the new data on molecular mechanisms, (patho)physiological relevance, and therapeutic potentials of the cGMP signaling system that were presented at this meeting.

Effect of calcitonin gene-related peptide antagonist on the cardiovascular events, mortality, and prostaglandin E2 production by nitrate-induced tolerant rats with acute myocardial infarction

Background

Anti-ischemic effects of NO releasing by nitroglycerin (NTG) and the release of calcitonin gene-related peptide (CGRP) are involved in the decrease of vascular remodeling in different cardiovascular diseases. Using a nitrate-free period is still generally required to prevent nitrate tolerance and should be used as the first-line option to maintain adequate symptom control and on an individual basis. Personalized anti-ischemic concerns require the urgent change of paradigm from interventional measures to predictive, preventive, and personalized treatment with organic nitrates and its combination with drugs that may improve prognosis and drugs that can be added for patients who remain symptomatic despite therapy with the other classes of agents. The purpose of this study was to evaluate the influence of human calcitonin gene-related peptide antagonist (CGRP_8-37) on cardiohemodynamic events, prostaglandin E₂ (PGE₂) plasma concentration, the severity of ventricular arrhythmias, and mortality occurring during acute myocardial infarction (AMI) in NTG-tolerant and nontolerant rats.

Methods

In the pilot study of efficacy of calcitonin gene-related peptide antagonist (CGRP_8-37), 58 male Wistar rats were included. All procedures were performed according to protocols approved by the General Animal Care and Use Committee. Adult male rats underwent surgery to induce AMI by ligating the left anterior descending coronary artery or SHAM. ECG was used to confirm myocardial ischemia. In each experiment, a rat was maintained under anesthesia for the duration of the experiment. At the end of the experiment, the rat was killed by an overdose of pentobarbital. All animals in accordance with the received pharmacological agent were randomized into three groups: I—received only NTG, 50 mg/kg daily, s.c. injections b.i.d. 3 days prior to AMI; II—received NTG by the same dose, route, and frequency of administration + CGRP antagonist (CGRP_8-37), 10 μg/kg two times daily by a similar period of administration; and III—served as control (C) group without preliminary tolerance to NTG.

Results

Subcutaneous injections of NTG (50 mg/kg) 30 min prior to AMI in NTG-tolerant animals (group I) and in NTG-tolerant rats + CGRP antagonist (group II) caused minor changes in blood pressure and heart period that was accompanied before NTG s.c. administration with blunted baroreflex sensitivity in response to i.v. administration of sodium nitroprusside in these groups of rats (0.66 ± 0.05 and 0.56 ± 0.04 ms/mmHg, P < 0.05, respectively) in comparison to C (group III) animals (0.9 ± 0.1 ms/mmHg). AMI 1 h duration was associated with a high incidence of ventricular arrhythmia and significant mortality in group I (70 %) and especially in group II (90 %) animals at 72 h after reperfusion as compared with group III rats (56 %), that correlated to a decrease of PGE₂ plasma content in group II (2.2 ± 0.4 ng/ml, P < 0.001) and group I (3.6 ± 0.2 ng/ml, P < 0.01) vs. control group of rats (4.8 ± 0.3 ng/ml).

Conclusions

CGRP could be involved in the mechanism of nitrate tolerance via the inhibition of release of the potent vasodilator CGRP leading to exacerbation of acute myocardial ischemia. The influence of CGRP antagonist could enhance this condition.

Identification of novel S-nitrosation sites in soluble guanylyl cyclase, the nitric oxide receptor

Soluble Guanylyl Cyclase (sGC) is the main receptor for nitric oxide (NO). NO activates sGC to synthesize cGMP, triggering a plethora of signals. Recently, we discovered that NO covalently modifies select sGC cysteines via a post-translational modification termed S-nitrosation or S-nitrosylation. Earlier characterization was conducted on a purified sGC treated with S-nitrosoglutathione, and identified three S-nitrosated cysteines (SNO-Cys). Here we describe a more biologically relevant mapping of sGC SNO-Cys in cells to better understand the multi-faceted interactions between SNO and sGC. Since SNO-Cys are labile during LC/MS/MS, MS analysis of nitrosation typically occurs after a biotin switch reaction, in which a SNO-Cys is converted to a biotin-Cys. Here we report the identification of ten sGC SNO-Cys in rat neonatal cardiomyocytes using an Orbitrap MS. A majority of the SNO-Cys identified is located at the solvent-exposed surface of the sGC, and half of them in the conserved catalytic domain, suggesting biological significance. These findings provide a solid basis for future studies of the regulations and functions of diverse sGC S-nitrosation events in cells.

Organic Nitrate Therapy, Nitrate Tolerance, and Nitrate-Induced Endothelial Dysfunction: Emphasis on Redox Biology and Oxidative Stress

Organic nitrates, such as nitroglycerin (GTN), isosorbide-5-mononitrate and isosorbide dinitrate, and pentaerithrityl tetranitrate (PETN), when given acutely, have potent vasodilator effects improving symptoms in patients with acute and chronic congestive heart failure, stable coronary artery disease, acute coronary syndromes, or arterial hypertension. The mechanisms underlying vasodilation include the release of •NO or a related compound in response to intracellular bioactivation (for GTN, the mitochondrial aldehyde dehydrogenase [ALDH-2]) and activation of the enzyme, soluble guanylyl cyclase. Increasing cyclic guanosine-3',-5'-monophosphate (cGMP) levels lead to an activation of the cGMP-dependent kinase I, thereby causing the relaxation of the vascular smooth muscle by decreasing intracellular calcium concentrations. The hemodynamic and anti-ischemic effects of organic nitrates are rapidly lost upon long-term (low-dose) administration due to the rapid development of tolerance and endothelial dysfunction, which is in most cases linked to increased intracellular oxidative stress. Enzymatic sources of reactive oxygen species under nitrate therapy include mitochondria, NADPH oxidases, and an uncoupled •NO synthase. Acute high-dose challenges with organic nitrates cause a similar loss of potency (tachyphylaxis), but with distinct pathomechanism. The differences among organic nitrates are highlighted regarding their potency to induce oxidative stress and subsequent tolerance and endothelial dysfunction. We also address pleiotropic effects of organic nitrates, for example, their capacity to stimulate antioxidant pathways like those demonstrated for PETN, all of which may prevent adverse effects in response to long-term therapy. Based on these considerations, we will discuss and present some preclinical data on how the nitrate of the future should be designed.

S-nitrosylation of PDE5 increases its ubiquitin-proteasomal degradation

Phosphodiesterase type 5 (PDE5) expression is upregulated in human failing heart, and overexpression of PDE5 in transgenic mice exacerbates stress-induced left-ventricular dysfunction, suggesting that increased PDE5 expression might contribute to the development of congestive heart failure. However, the underlying mechanisms for increased PDE5 expression are not totally understood. In the present study, we found that PDE5 activity and expression were regulated by S-nitrosylation, a covalent modification of cysteine residues by nitric oxide (NO). S-nitrosylation of PDE5 occurs at Cys220, which is located in the GAFA domain. Upon S-nitrosylation, PDE5 exhibits reduced activity and degradation via the ubiquitin-proteasome system. The decrease in PDE5 expression induced by NO could be blunted by mutation of Cys220 or the phosphorylation site of PDE5 (S102), as well as by pretreatment with H2O2. Conversely, decreased NO bioavailability by nitric oxide synthase (NOS) inhibitors or knockout of NOS3 increased PDE5 expression in cardiomyocytes. Collectively, to the best of our knowledge, our data demonstrate for the first time that S-nitrosylation is one of the mechanisms for PDE5 degradation. This novel regulatory mechanism probably accounts for the increase in PDE5 in the failing heart and other diseases in which NO bioavailability is decreased by oxidative stress.

Mitochondrial Oxidative Stress, Mitochondrial DNA Damage and Their Role in Age-Related Vascular Dysfunction

The prevalence of cardiovascular diseases is significantly increased in the older population. Risk factors and predictors of future cardiovascular events such as hypertension, atherosclerosis, or diabetes are observed with higher frequency in elderly individuals. A major determinant of vascular aging is endothelial dysfunction, characterized by impaired endothelium-dependent signaling processes. Increased production of reactive oxygen species (ROS) leads to oxidative stress, loss of nitric oxide (^•NO) signaling, loss of endothelial barrier function and infiltration of leukocytes to the vascular wall, explaining the low-grade inflammation characteristic for the aged vasculature. We here discuss the importance of different sources of ROS for vascular aging and their contribution to the increased cardiovascular risk in the elderly population with special emphasis on mitochondrial ROS formation and oxidative damage of mitochondrial DNA. Also the interaction (crosstalk) of mitochondria with nicotinamide adenosine dinucleotide phosphate (NADPH) oxidases is highlighted. Current concepts of vascular aging, consequences for the development of cardiovascular events and the particular role of ROS are evaluated on the basis of cell culture experiments, animal studies and clinical trials. Present data point to a more important role of oxidative stress for the maximal healthspan (healthy aging) than for the maximal lifespan.

Tolerance to nitroglycerin through proteasomal down-regulation of aldehyde dehydrogenase-2 in a genetic mouse model of ascorbate deficiency

Background and Purpose

L-gulonolactone oxidase-deficient (Gulo^(-/-)) mice were used to study the effects of ascorbate deficiency on aortic relaxation by nitroglycerin (GTN) with focus on changes in the expression and activity of vascular aldehyde dehydrogenase-2 (ALDH2), which catalyses GTN bioactivation.

Experimental Approach

Ascorbate deficiency was induced in Gulo^(-/-) mice by ascorbate deprivation for 4 weeks. Some of the animals were concomitantly treated with the proteasome inhibitor bortezomib and effects compared with ascorbate-supplemented Gulo^(-/-), untreated or nitrate-tolerant wild-type mice. Aortic relaxation of the experimental groups to GTN, ACh and a NO donor was studied. Changes in mRNA and protein expression of vascular ALDH2 were quantified by qPCR and immunoblotting, respectively, and aortic GTN denitration rates determined.

Key Results

Like GTN treatment, ascorbate deprivation induced vascular tolerance to GTN that was associated with markedly decreased rates of GTN denitration. Ascorbate deficiency did not affect ALDH2 mRNA levels, but reduced ALDH2 protein expression and the total amount of ubiquitinated proteins to about 40% of wild-type controls. These effects were largely prevented by ascorbate supplementation or treating Gulo^(-/-) mice with the 26S proteasome inhibitor bortezomib.

Conclusions and Implications

Our data indicate that ascorbate deficiency results in vascular tolerance to GTN via proteasomal degradation of ALDH2. The results support the view that impaired ALDH2-catalysed metabolism of GTN contributes significantly to the development of vascular nitrate tolerance and reveal a hitherto unrecognized protective effect of ascorbate in the vasculature.

S-Nitrosoglutathione improves haemodynamics in early-onset pre-eclampsia

AIMS

To determine the effects of in vivo S-nitrosoglutathione (GSNO) infusion on cardiovascular function, platelet function, proteinuria and biomarker parameters in early-onset pre-eclampsia.

METHODS

We performed an open-label dose-ranging study of GSNO in early-onset pre-eclampsia. Six women underwent GSNO infusion whilst receiving standard therapy. The dose of GSNO was increased incrementally to 100 μg min(-1) whilst maintaining blood pressure of >140/80 mmHg. Aortic augmentation index, aortic pulse wave velocity, blood pressure and maternal-fetal Doppler parameters were measured at each dose. Platelet P-selectin, protein-to-creatinine ratio and soluble anti-angiogenic factors were measured pre- and postinfusion.

RESULTS

Augmentation index fell at 30 μg min(-1) S-nitrosoglutathione (-6%, 95% confidence interval 0.6 to 13%), a dose that did not affect blood pressure. Platelet P-selectin expression was reduced [mean (interquartile range), 6.3 (4.9-7.6) vs. 4.1 (3.1-5.7)% positive, P = 0.03]. Soluble endoglin levels showed borderline reduction (P = 0.06). There was a borderline significant change in pre-to-postinfusion protein-to-creatinine ratio [mean (interquartile range), 0.37 (0.09-0.82) vs. 0.23 (0.07-0.49) g mmol(-1) , P = 0.06]. Maternal uterine and fetal Doppler pulsatility indices were unchanged.

CONCLUSIONS

In early-onset pre-eclampsia, GSNO reduces augmentation index, a biomarker of small vessel tone and pulse wave reflection, prior to affecting blood pressure. Proteinuria and platelet activation are improved at doses that affect blood pressure minimally. These effects of GSNO may be of therapeutic potential in pre-eclampsia, a condition for which no specific treatment exists. Clinical studies of GSNO in early-onset pre-eclampsia will determine whether these findings translate to improvement in maternal and/or fetal outcome.

The nitric oxide pathway and possible therapeutic options in pre-eclampsia

Pre-eclampsia is a serious multisystem disorder with diverse clinical manifestations. Although not causal, endothelial dysfunction and reduced nitric oxide bioavailability are likely to play an important role in the maternal and fetal pathophysiology of this condition. Lack of treatment modalities that can target the underlying pathophysiological changes and reverse the endothelial dysfunction frequently leads to iatrogenic preterm delivery of the fetus, causing neonatal morbidity and mortality, and the condition itself is associated with short- and longer term maternal morbidity and mortality. Drugs that target various components of the nitric oxide-soluble guanylyl cyclase pathway can help to increase NO bioavailability. The purpose of this review is to outline the current status of clinical research involving these therapeutic modalities in the context of pre-eclampsia, with the focus being on the following: nitric oxide donors, including organic nitrates and S-nitrosothiols; l-arginine, the endogenous precursor of NO; inhibitors of cyclic guanosine 3',5'-monophosphate breakdown, including sildenafil; and other novel inhibitors of NO donor metabolism. The advantages and limitations of each modality are outlined, and scope for development into established therapeutic options for pre-eclampsia is explored.

Role of sGC-dependent NO signalling and myocardial infarction risk

The NO/cGMP pathway plays an important role in many physiological functions and pathophysiological conditions. In the last few years, several genetic and functional studies pointed to an underestimated role of this pathway in the development of atherosclerosis. Indeed, several genetic variants of key enzymes modulating the generation of NO and cGMP have been strongly associated with coronary artery disease and myocardial infarction risk. In this review, we aim to place the genomic findings on components of the NO/cGMP pathway, namely endothelial nitric oxide synthase, soluble guanylyl cyclase and phosphodiesterase 5A, in context of preventive and therapeutic strategies for treating atherosclerosis and its sequelae.

Extending the translational potential of targeting NO/cGMP-regulated pathways in the CVS

The discovery of NO as both an endogenous signalling molecule and as a mediator of the cardiovascular effects of organic nitrates was acknowledged in 1998 by the Nobel Prize in Physiology/Medicine. The characterization of its downstream signalling, mediated through stimulation of soluble GC (sGC) and cGMP generation, initiated significant translational interest, but until recently this was almost exclusively embodied by the use of PDE5 inhibitors in erectile dysfunction. Since then, research progress in two areas has contributed to an impressive expansion of the therapeutic targeting of the NO-sGC-cGMP axis: first, an increased understanding of the molecular events operating within this complex pathway and second, a better insight into its dys-regulation and uncoupling in human disease. Already-approved PDE5 inhibitors and novel, first-in-class molecules, which up-regulate the activity of sGC independently of NO and/or of the enzyme's haem prosthetic group, are undergoing clinical evaluation to treat pulmonary hypertension and myocardial failure. These molecules, as well as combinations or second-generation compounds, are also being assessed in additional experimental disease models and in patients in a wide spectrum of novel indications, such as endotoxic shock, diabetic cardiomyopathy and Becker's muscular dystrophy. There is well-founded optimism that the modulation of the NO-sGC-cGMP pathway will sustain the development of an increasing number of successful clinical candidates for years to come.