A recent history of nitroxyl chemistry, pharmacology and therapeutic potential

Exploring PKG signaling as a therapeutic avenue for pressure overload, ischemia, and HFpEF

INTRODUCTION

Heart failure (HF) is a complex and heterogeneous syndrome resulting from any diastolic or systolic dysfunction of the cardiac muscle. In addition to comorbid conditions, pressure overload, and myocardial ischemia are associated with cardiac remodeling which manifests as extracellular matrix (ECM) perturbations, impaired cellular responses, and subsequent ventricular dysfunction.

AREAS COVERED

The current review discusses the main aspects of the cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG) pathway (cGMP-PKG) pathway modulators and highlights the promising outcomes of its novel pharmacological boosters.

EXPERT OPINION

Among several signaling pathways involved in the pathogenesis of pressure overload, ischemia and HF with preserved ejection fraction (HFpEF) is cGMP-PKG pathway. This pathway plays a pivotal role in the regulation of cardiac contractility, and modulation of cGMP-PKG signaling, contributing to the development of the diseases. Ventricular cardiomyocytes of HF patients and animal models are known to exhibit reduced cGMP levels and disturbed cGMP signaling including hypophosphorylation of PKG downstream targets. However, restoration of cGMP-PKG signaling improves cardiomyocyte function and promotes cardioprotective effects.

Nitroxyl donating and visualization with a coumarin-based fluorescence probe

Nitroxyl (HNO), the single-electron reduction product of nitric oxide (NO), has attracted great interest in the treatment of congestive heart failure in clinical trials. In this paper, we describe the first coumarin-based compound N-hydroxy-2-oxo-2H-chromene-6-sulfonamide (CD1) as a dualfunctional HNO donor, which can release both an HNO signaling molecule and a fluorescent reporter. Under physiological conditions (pH 7.4 and 37 °C), the CD1 HNO donor can readily decompose with a half-life of ∼90 min. The corresponding stoichiometry HNO from the CD1 donor was confirmed using both Vitamin B12 and phosphine compound traps. In addition to HNO releasing, specifically, the degradation product 2-oxo-2H-chromene-6-sulfinate (CS1) was generated as a fluorescent marker during the decomposition. Therefore, the HNO amount released in situ can be accurately monitored through fluorescence generation. As compared to the CD1 donor, the fluorescence intensity increased by about 4.9-fold. The concentration limit of detection of HNO releasing was determined to be ∼0.13 μM according to the fluorescence generation of CS1 at physiological conditions. Moreover, the bioimaging of the CD1 donor was demonstrated in the cell culture of HeLa cells, where the intracellular fluorescence signals were observed, inferring the site of HNO release. Finally, we anticipate that this novel coumarin-based CD1 donor opens a new platform for exploring the biology of HNO.

A high-throughput approach to identify BRCA1-downregulating compounds to enhance PARP inhibitor sensitivity

PARP inhibitors (PARPi) are efficacious in BRCA1-null tumors; however, their utility is limited in tumors with functional BRCA1. We hypothesized that pharmacologically reducing BRCA1 protein levels could enhance PARPi effectiveness in BRCA1 wild-type tumors. To identify BRCA1 downregulating agents, we generated reporter cell lines using CRISPR-mediated editing to tag endogenous BRCA1 protein with HiBiT. These reporter lines enable the sensitive measurement of BRCA1 protein levels by luminescence. Validated reporter cells were used in a pilot screen of epigenetic-modifying probes and a larger screen of more than 6,000 compounds. We identified 7 compounds that could downregulate BRCA1-HiBiT expression and synergize with olaparib. Three compounds, N-acetyl-N-acetoxy chlorobenzenesulfonamide (NANAC), A-443654, and CHIR-124, were validated to reduce BRCA1 protein levels and sensitize breast cancer cells to the toxic effects of olaparib. These results suggest that BRCA1-HiBiT reporter cells hold promise in developing agents to improve the clinical utility of PARPi.

Reusable HNO Sensors Derived from Cu Cyclam: A DFT Study on the Mechanistic Origin of High Reactivity and Favorable Conformation Changes and Potential Improvements

Nitroxyl (HNO) exhibits unique favorable properties in regulating biological and pharmacological activities. However, currently, there is only one Cu-based HNO sensor that can be recycled for reusable detection, which is a Cu cyclam derivative with a mixed thia/aza ligand. To elucidate the missing mechanistic origin of its high HNO reactivity and subsequent favorable conformation change toward a stable CuI product that is critical to be oxidized back by the physiological O2 level for HNO detection again, a density functional theory (DFT) computational study was performed. It not only reproduced experimental structural and reaction properties but also, more importantly, revealed an unknown role of the coordination atom in high reactivity. Its conformation change mechanism was found to not follow the previously proposed one but involve a novel favorable rotation pathway. Several newly designed complexes incorporating beneficial effects of coordination atoms and substituents to further enhance HNO reactivity while maintaining or even improving favorable conformation changes for reusable HNO detection were computationally validated. These novel results will facilitate the future development of reusable HNO sensors for true spatiotemporal resolution and repeated detection.

Endogenous plant nitroxyl, a new component of nitric oxide biology

Nitroxyl (HNO), a one-electron reduced and protonated congener of nitric oxide (•NO), was recently discovered in Arabidopsis thaliana. Due to its distinct chemical properties, we believe HNO must be further studied to determine how many physiological processes it impacts.

Synthesis and nitroxyl (HNO) donating properties of benzoxadiazole-based Piloty's acids

Developing functional nitroxyl (HNO) donors play a significant role in the further exploration of endogenous HNO in biochemistry and pharmacology. In this work, two novel Piloty's acids (SBD-D1 and SBD-D2) were proposed by incorporating benzoxadiazole-based fluorophores, in order to achieve the dual-function of releasing both HNO and a fluorophore in situ. Under physiological conditions, both SBD-D1 and SBD-D2 efficiently donated HNO (t_1/2 = 10.96 and 8.18 min, respectively). The stoichiometric generation of HNO was determined by both vitamin B₁₂ and phosphine compound traps. Interestingly, due to the different substitution groups on the aromatic ring, SBD-D1 with the chlorine showed no fluorescence emission, but SBD-D2 was strongly fluorescent due to the presence of the dimethylamine group. Specifically, the fluorescent signal would decrease during the release process of HNO. Moreover, theoretical calculations were performed to understand the emission difference. A strong radiation derived from benzoxadiazole with dimethylamine group due to the large transition dipole moment (∼4.3 Debye), while the presence of intramolecular charge transfer process in the donor with chlorine group caused a small transition dipole moment (<0.1 Debye). Finally, these studies would contribute to the future design and application of novel functional HNO donors for the exploration of HNO biochemistry and pharmacology.

Transformation of hydroxylamine to nitrosated and nitrated products during advanced oxidation process

Recently, hydroxylamine (HAm) was introduced to drive advanced oxidation processes (AOPs) for removing organic contaminants. However, we found that HAm-driven Cu(II)/peroxymonosulfate oxidation of phenol produced p-nitrosophenol, 2-nitrophenol and 4-nitrophenol. The total nitro(so) products accounted for approximately 25.0 % of the phenol transformation at certain condition. SO₄^•- and •OH were identified as the primary and second significant oxidants, respectively. Reactive nitrogen species (RNS) were involved in phenol transformation. The pathway and mechanism of HAm transformation in HAm-driven transition metal ion-catalyzed AOPs were proposed for the first time in this study. The product of HAm via twice single-electron oxidation by Cu(II) is nitroxyl (HNO/NO^-), which is a critical oxidation intermediate of HAm. Further oxidation of HNO by SO₄^•- or •OH is the initial step in propagating radical chain reactions, leading to nitric oxide radical (•NO) and nitrogen dioxide radical (•NO₂) as the primary RNS. HAm is a critical intermediate in natural nitrogen cycle, suggesting that HAm can drive the oxidation processes of pollutants in natural environments. Nitro(so) products will be readily produced when AOPs are applied for ecological remediation. This study highlights the formation of toxic nitrosated and nitrated products in HAm-driven AOPs, and the requirement of risk assessments to evaluate the possible health and ecological impacts.

Discovery of endogenous nitroxyl as a new redox player in Arabidopsis thaliana

Nitroxyl (HNO) is the one-electron reduced and protonated congener of nitric oxide (•NO), owning a distinct chemical profile. Based on real-time detection, we demonstrate that HNO is endogenously formed in Arabidopsis. Senescence and hypoxia induce shifts in the redox balance, triggering HNO decay or formation mediated by non-enzymatic •NO/HNO interconversion with cellular reductants. The stimuli-dependent HNO generation supports or competes with •NO signalling, depending on the local redox environment.

Antiglycation and Antioxidant Effect of Nitroxyl towards Hemoglobin

Donors of nitroxyl and nitroxyl anion (HNO/NO⁻) are considered to be promising pharmacological treatments with a wide range of applications. Remarkable chemical properties allow nitroxyl to function as a classic antioxidant. We assume that HNO/NO⁻ can level down the non-enzymatic glycation of biomolecules. Since erythrocyte hemoglobin (Hb) is highly susceptible to non-enzymatic glycation, we studied the effect of a nitroxyl donor, Angeli’s salt, on Hb modification with methylglyoxal (MG) and organic peroxide―tert-butyl hydroperoxide (t-BOOH). Nitroxyl dose-dependently decreased the amount of protein carbonyls and advanced glycation end products (AGEs) that were formed in the case of Hb incubation with MG. Likewise, nitroxyl effectively protected Hb against oxidative modification with t-BOOH. It slowed down the destruction of heme, formation of carbonyl derivatives and inter-subunit cross-linking. The protective effect of nitroxyl on Hb in this system is primarily associated with nitrosylation of oxidized Hb and reduction of its ferryl form, which lowers the yield of free radical products. We suppose that the dual (antioxidant and antiglycation) effect of nitroxyl makes its application possible as part of an additional treatment strategy for oxidative and carbonyl stress-associated diseases.

The Chemistry of HNO: Mechanisms and Reaction Kinetics

Azanone (HNO, also known as nitroxyl) is the protonated form of the product of one-electron reduction of nitric oxide (^•NO), and an elusive electrophilic reactive nitrogen species of increasing pharmacological significance. Over the past 20 years, the interest in the biological chemistry of HNO has increased significantly due to the numerous beneficial pharmacological effects of its donors. Increased availability of various HNO donors was accompanied by great progress in the understanding of HNO chemistry and chemical biology. This review is focused on the chemistry of HNO, with emphasis on reaction kinetics and mechanisms in aqueous solutions.

HNO Protects the Myocardium against Reperfusion Injury, Inhibiting the mPTP Opening via PKCε Activation

Donors of nitroxyl (HNO), the one electron-reduction product of nitric oxide (NO^.), positively modulate cardiac contractility/relaxation while limiting ischemia-reperfusion (I/R) injury. The mechanisms underpinning HNO anti-ischemic effects remain poorly understood. Using isolated perfused rat hearts subjected to 30 min global ischemia/1 or 2 h reperfusion, here we tested whether, in analogy to NO^., HNO protection requires PKCε translocation to mitochondria and K_ATP channels activation. To this end, we compared the benefits afforded by ischemic preconditioning (IPC; 3 cycles of I/R) with those eventually granted by the NO^. donor, diethylamine/NO, DEA/NO, and two chemically unrelated HNO donors: Angeli’s salt (AS, a prototypic donor) and isopropylamine/NO (IPA/NO, a new HNO releaser). All donors were given for 19 min before I/R injury. In control I/R hearts (1 h reperfusion), infarct size (IS) measured via tetrazolium salt staining was 66 ± 5.5% of the area at risk. Both AS and IPA/NO were as effective as IPC in reducing IS [30.7 ± 2.2 (AS), 31 ± 2.9 (IPA/NO), and 31 ± 0.8 (IPC), respectively)], whereas DEA/NO was significantly less so (36.2 ± 2.6%, p < 0.001 vs. AS, IPA/NO, or IPC). IPA/NO protection was still present after 120 min of reperfusion, and the co-infusion with the PKCε inhibitor (PKCV1-2500 nM) prevented it (IS = 30 ± 0.5 vs. 61 ± 1.8% with IPA/NO alone, p < 0.01). Irrespective of the donor, HNO anti-ischemic effects were insensitive to the K_ATP channel inhibitor, 5-OH decanoate (5HD, 100 μM), that, in contrast, abrogated DEA/NO protection. Finally, both HNO donors markedly enhanced the mitochondrial permeability transition pore (mPTP) ROS threshold over control levels (≅35–40%), an action again insensitive to 5HD. Our study shows that HNO donors inhibit mPTP opening, thus limiting myocyte loss at reperfusion, a beneficial effect that requires PKCε translocation to the mitochondria but not mitochondrial K⁺ channels activation.

Chemiluminescence Measurement of Reactive Sulfur and Nitrogen Species

Significance: Reactive sulfur and nitrogen species such as hydrogen sulfide (H₂S) and nitric oxide (NO^•) are ubiquitous cellular signaling molecules that play central roles in physiology and pathophysiology. A deeper understanding of these signaling pathways will offer new opportunities for therapeutic treatments and disease management. Recent Advances: Chemiluminescence methods have been fundamental in detecting and measuring biological reactive sulfur and nitrogen species, and new approaches are emerging for imaging these analytes in living intact specimens. Ozone-based and luminol-based chemiluminescence methods have been optimized for quantitative analysis of hydrogen sulfide and nitric oxide in biological samples and tissue homogenates, and caged luciferin and 1,2-dioxetanes are emerging as a versatile approach for monitoring and imaging reactive sulfur and nitrogen species in living cells and animal models. Critical Issues: This review article will cover the major chemiluminescence approaches for detecting, measuring, and imaging reactive sulfur and nitrogen species in biological systems, including a brief history of the development of the most established approaches and highlights of the opportunities provided by emerging approaches. Future Directions: Emerging chemiluminescence approaches offer new opportunities for monitoring and imaging reactive sulfur and nitrogen species in living cells, animals, and human clinical samples. Widespread adoption and translation of these approaches, however, requires an emphasis on rigorous quantitative methods, reproducibility, and effective technology transfer. Antioxid. Redox Signal. 36, 337-353.

Hydropersulfides (RSSH) and Nitric Oxide (NO) Signaling: Possible Effects on S-Nitrosothiols (RS-NO)

S-Nitrosothiol (RS-NO) formation in proteins and peptides have been implicated as factors in the etiology of many diseases and as possible regulators of thiol protein function. They have also been proposed as possible storage forms of nitric oxide (NO). However, despite their proposed functions/roles, there appears to be little consensus regarding the physiological mechanisms of RS-NO formation and degradation. Hydropersulfides (RSSH) have recently been discovered as endogenously generated species with unique reactivity. One important reaction of RSSH is with RS-NO, which leads to the degradation of RS-NO as well as the release of NO. Thus, it can be speculated that RSSH can be a factor in the regulation of steady-state RS-NO levels, and therefore may be important in RS-NO (patho)physiology. Moreover, RSSH-mediated NO release from RS-NO may be a possible mechanism allowing RS-NO to serve as a storage form of NO.

Exploring the Potential of 2-(2-Nitrophenyl)ethyl-Caged N-Hydroxysulfonamides for the Photoactivated Release of Nitroxyl (HNO)

The emergence of nitroxyl (HNO) as a biological signaling molecule is attracting increasing attention. HNO-based prodrugs show considerable potential in treating congestive heart failure, with HNO reacting rapidly with metal centers and protein-bound and free thiols. A new class of 2-(2-nitrophenyl)ethyl (2-NPE)-photocaged N-hydroxysulfonamides has been developed, and the mechanisms of photodecomposition have been investigated. Three photodecomposition pathways are observed: the desired concomitant C-O/N-S bond cleavage to generate HNO, sulfinate, and 2-nitrostyrene, C-O bond cleavage to give the parent sulfohydroxamic acid and 2-nitrostyrene, and O-N bond cleavage to release a sulfonamide and 2-nitrophenylacetaldehyde. Laser flash photolysis experiments provide support for a Norrish type II mechanism involving 1,5-hydrogen atom abstraction to generate an aci-nitro species. A mechanism is proposed in which the (Z)-aci-nitro intermediate undergoes either C-O bond cleavage to release RSO₂NHO(H), concerted C-O/N-S bond cleavage to generate sulfinate and HNO, or isomerization to the (E)-isomer prior to O-N bond cleavage. The pK_a of the N(H) of the N-hydroxysulfonamide plays a key role in determining whether C-O or concerted C-O/N-S bond cleavage occurs. Deprotonating this site favors the desired C-O/N-S bond cleavage at the expense of an increased level of undesired O-N bond cleavage. Triplet state quenchers have no effect on the observed photoproducts.

Vasoprotective Actions of Nitroxyl (HNO): A Story of Sibling Rivalry

ABSTRACT

Nitroxyl (HNO), the 1 electron-reduced and protonated form of nitric oxide (NO•), has emerged as a nitrogen oxide with a suite of vasoprotective properties and therapeutic advantages over its redox sibling. Although HNO has garnered much attention due to its cardioprotective actions in heart failure, its ability to modulate vascular function, without the limitations of tolerance development and NO• resistance, is desirable in the treatment of vascular disease. HNO serves as a potent vasodilator and antiaggregatory agent and has an ability to limit vascular inflammation and reactive oxygen species generation. In addition, its resistance to scavenging by reactive oxygen species and ability to target distinct vascular signaling pathways (Kv, KATP, and calcitonin gene-related peptide) contribute to its preserved efficacy in hypertension, diabetes, and hypercholesterolemia. In this review, the vasoprotective actions of HNO will be compared with those of NO•, and the therapeutic utility of HNO donors in the treatment of angina, acute cardiovascular emergencies, and chronic vascular disease are discussed.

Kinetic Study on the Reactivity of Azanone (HNO) toward Cyclic C-Nucleophiles

Azanone (HNO) is an elusive electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present a comparative kinetic study of HNO reactivity toward selected cyclic C-nucleophiles under aqueous conditions at pH 7.4. We applied the competition kinetics method, which is based on the use of a fluorescein-derived boronate probe FlBA and two parallel HNO reactions: with the studied scavenger or with O₂ (k = 1.8 × 10⁴ M⁻¹s⁻¹). We determined the second-order rate constants of HNO reactions with 13 structurally diverse C-nucleophiles (k = 33–20,000 M⁻¹s⁻¹). The results show that the reactivity of HNO toward C-nucleophiles depends strongly on the structure of the scavenger. The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with C-nucleophiles is provided.

Nitric Oxide Reacts Very Fast with Hydrogen Sulfide, Alcohols, and Thiols to Produce HNO: Revised Rate Constants

The chemical reactivity of NO and its role in several biological processes seem well established. Despite this, the chemical reduction of •NO toward HNO has been historically discarded, mainly because of the negative reduction potential of NO. However, this value and its implications are nowadays under revision. The last reported redox potential, E'(NO,H+/HNO), at micromolar and picomolar concentrations of •NO and HNO, respectively, is between -0.3 and 0 V at pH 7.4. This potential implies that the one-electron-reduction process for NO is feasible under biological conditions and could be promoted by well-known biological reductants with reduction potentials of around -0.3 to -0.5 V. Moreover, the biologically compatible chemical reduction of •NO (nonenzymatic), like direct routes to HNO by alkylamines, aromatic and pseudoaromatic alcohols, thiols, and hydrogen sulfide, has been extensively explored by our group during the past decade. The aim of this work is to use a kinetic modeling approach to analyze electrochemical HNO measurements and to report for the first-time direct reaction rate constants between •NO and moderate reducing agents, producing HNO. These values are between 5 and 30 times higher than the previously reported keff values. On the other hand, we also showed that reaction through successive attack by two NO molecules to biologically compatible compounds could produce HNO. After over 3 decades of intense research, the •NO chemistry is still there, ready to be discovered.

Nitric oxide (NO) synthase but not NO, HNO or H2 O2 mediates endothelium-dependent relaxation of resistance arteries from patients with cardiovascular disease

BACKGROUND AND PURPOSE

Superoxide anions can reduce the bioavailability and actions of endothelium-derived NO. In human resistance-sized arteries, endothelium-dependent vasodilatation can be mediated by H₂ O₂ instead of NO. Here, we tested the hypothesis that in resistance arteries from patients with cardiovascular disease, endothelium-dependent vasodilatation is mediated by a reactive oxygen species and not impaired by oxidative stress.

EXPERIMENTAL APPROACH

Small arteries were isolated from biopsies of the parietal pericardium of patients undergoing elective cardiothoracic surgery and were studied using immunohistochemical and organ chamber techniques.

KEY RESULTS

NO synthases 1, 2 and 3, superoxide dismutase 1 and catalase proteins were observed in the microvascular wall. Relaxing responses to bradykinin were endothelium dependent. During submaximal depolarization-induced contraction, bradykinin-mediated relaxations were inhibited by inhibitors of NO synthases (NOS) and soluble guanylyl cyclase (sGC) but not by scavengers of NO or HNO, inhibitors of cyclooxygenases, neuronal NO synthase, superoxide dismutase or catalase, or by exogenous catalase. During contraction stimulated by endothelin-1, these relaxations were not reduced by any of these interventions except DETCA, which caused a small reduction.

CONCLUSION AND IMPLICATIONS

In resistance arteries from patients with cardiovascular disease, endothelium-dependent relaxations seem not to be mediated by NO, HNO or H₂ O₂ , although NOS and sGC can be involved. These vasodilator responses continue during excessive oxidative stress.

Solid-gas reactions for nitroxyl (HNO) generation in the gas phase

We present a novel nitroxyl (HNO) generation method, which avoids the need of using a liquid system or extreme experimental conditions. This method consists of the reaction between a gaseous base and an HNO donor (Piloty's acid) in the solid phase, allowing the formation of gaseous HNO in a fast and economical way. Detection of HNO was carried out indirectly, measuring the nitrous oxide (N₂O) byproduct of HNO dimerization using infrared spectroscopy, and directly, using mass spectrometry techniques and an electrochemical HNO sensor.

Melatonin, Its Metabolites and Their Interference with Reactive Nitrogen Compounds

Melatonin and several of its metabolites are interfering with reactive nitrogen. With the notion of prevailing melatonin formation in tissues that exceeds by far the quantities in blood, metabolites come into focus that are poorly found in the circulation. Apart from their antioxidant actions, both melatonin and N¹-acetyl-5-methoxykynuramine (AMK) downregulate inducible and inhibit neuronal NO synthases, and additionally scavenge NO. However, the NO adduct of melatonin redonates NO, whereas AMK forms with NO a stable product. Many other melatonin metabolites formed in oxidative processes also contain nitrosylatable sites. Moreover, AMK readily scavenges products of the CO₂-adduct of peroxynitrite such as carbonate radicals and NO₂. Protein AMKylation seems to be involved in protective actions.

Mechanistic Insights into Rapid Generation of Nitroxyl from a Photocaged N-Hydroxysulfonamide Incorporating the (6-Hydroxynaphthalen-2-yl)methyl Chromophore

HNO is a highly reactive molecule that shows promise in treating heart failure. Molecules that rapidly release HNO with precise spatial and temporal control are needed to investigate the biology of this signaling molecule. (Hydroxynaphthalen-2-yl)methyl-photocaged N-hydroxysulfonamides are a new class of photoactive HNO generators. Recently, it was shown that a (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM)-photocaged derivative of N-hydroxysulfonamide incorporating the trifluoromethanesulfonamidoxy group (1) quantitatively generates HNO. Mechanistic studies have now been carried out on this system and reveal that the ground state protonation state plays a key role in whether concerted heterolytic C-O/N-S bond cleavage to release HNO occurs versus undesired O-N bond cleavage. N-Deprotonation of 1 can be achieved by adding an aqueous buffer or a carboxylate salt to an aprotic solvent. Evidence is presented for C-O/N-S bond heterolysis occurring directly from the singlet excited state of the N-deprotonated parent molecule on the picosecond time scale, using femtosecond time-resolved transient absorption spectroscopy, to give a carbocation and ¹NO^-. This is consistent with the observation of significant fluorescence quenching when HNO is generated. The carbocation intermediate reacts rapidly with nucleophiles including water, MeOH, or even (H)NO in the absence of a molecule that reacts rapidly with (H)NO to give an oxime.

Nature's marvels endowed in gaseous molecules I: Carbon monoxide and its physiological and therapeutic roles

Nature has endowed gaseous molecules such as O2, CO2, CO, NO, H2S, and N2 with critical and diverse roles in sustaining life, from supplying energy needed to power life and building blocks for life's physical structure to mediating and coordinating cellular functions. In this article, we give a brief introduction of the complex functions of the various gaseous molecules in life and then focus on carbon monoxide as a specific example of an endogenously produced signaling molecule to highlight the importance of this class of molecules. The past twenty years have seen much progress in understanding CO's mechanism(s) of action and pharmacological effects as well as in developing delivery methods for easy administration. One remarkable trait of CO is its pleiotropic effects that have few parallels, except perhaps its sister gaseous signaling molecules such as nitric oxide and hydrogen sulfide. This review will delve into the sophistication of CO-mediated signaling as well as its validated pharmacological functions and possible therapeutic applications.

Redox and Antioxidant Modulation of Circadian Rhythms: Effects of Nitroxyl, N-Acetylcysteine and Glutathione

The circadian clock at the hypothalamic suprachiasmatic nucleus (SCN) entrains output rhythms to 24-h light cycles. To entrain by phase-advances, light signaling at the end of subjective night (circadian time 18, CT18) requires free radical nitric oxide (NO•) binding to soluble guanylate cyclase (sGC) heme group, activating the cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). Phase-delays at CT14 seem to be independent of NO•, whose redox-related species were yet to be investigated. Here, the one-electron reduction of NO• nitroxyl was pharmacologically delivered by Angeli’s salt (AS) donor to assess its modulation on phase-resetting of locomotor rhythms in hamsters. Intracerebroventricular AS generated nitroxyl at the SCN, promoting phase-delays at CT14, but potentiated light-induced phase-advances at CT18. Glutathione/glutathione disulfide (GSH/GSSG) couple measured in SCN homogenates showed higher values at CT14 (i.e., more reduced) than at CT18 (oxidized). In addition, administration of antioxidants N-acetylcysteine (NAC) and GSH induced delays per se at CT14 but did not affect light-induced advances at CT18. Thus, the relative of NO• nitroxyl generates phase-delays in a reductive SCN environment, while an oxidative favors photic-advances. These data suggest that circadian phase-locking mechanisms should include redox SCN environment, generating relatives of NO•, as well as coupling with the molecular oscillator.

Helpful correlations to estimate the pKa of coordinated HNO: a potential-pH exploration in a pendant-arm cyclam-based ruthenium nitroxyl

The acid-base speciation of coordinated azanone (HNO) remains a highly relevant topic in bioinorganic chemistry. Ruthenium nitroxyl complexes with sufficient robustness towards ligand loss have gained significance as operating platforms to delve into such studies. In this work, we revisit an octahedral {RuNO}⁶ complex containing the cyclam-based pentadentate ligand L^py = 1-(pyridine-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane and explore the thermodynamic and spectroscopic aspects of its reduced states in aqueous media. Upon in situ electro-generation of the bound HNO moiety, we have undertaken different strategies to determine both its acidity and electrochemical properties. This robust HNO complex does not undergo deprotonation in a wide pH range. We have found pK_a ([Ru(L^py)(HNO)]²⁺) = 13.0 ± 0.1 and . There are indications that pK_a (HNO) values in several ruthenium-based species correlate with the redox potential associated with the {RuNO}^6,7 and {RuNO}^7,8 couples. The present pK_a extends the range of acidity of bound HNO to more than five pH units, confirming a remarkable sensitivity to the nature of the coordination sphere. This result lays new foundations to continue rational ligand design that may contribute to a better understanding of the different biological roles of both HNO and NO^- by investigating key chemical aspects of model complexes.

Updating NO•/HNO interconversion under physiological conditions: A biological implication overview

Azanone (HNO/NO^-), also called nitroxyl, is a highly reactive compound whose biological role is still a matter of debate. A key issue that remains to be clarified regarding HNO and its biological activity is that of its endogenous formation. Given the overlap of the molecular targets and reactivity of nitric oxide (NO•) and HNO, its chemical biology was perceived to be similar to that of NO• as a biological signaling agent. However, despite their closely related reactivity, NO• and HNO's biochemical pathways are quite different. Moreover, the reduction of nitric oxide to azanone is possible but necessarily coupled to other reactions, which drive the reaction forward, overcoming the unfavorable thermodynamic barrier. The mechanism of this NO•/HNO interplay and its downstream effects in different contexts were studied recently, showing that more than fifteen moderate reducing agents react with NO• producing HNO. Particularly, it is known that the reaction between nitric oxide and hydrogen sulfide (H₂S) produces HNO. However, this rate constant was not reported yet. In this work, firstly the NO•/H₂S effective rate constant was measured as a function of the pH. Then, the implications of these chemical (non-enzymatic), biologically compatible, routes to endogenous HNO formation was discussed. There is no doubt that HNO could be (is?) a new endogenously produced messenger that mediates specific physiological responses, many of which were attributed yet to direct NO• effects.

Targeting Ca2 + Handling Proteins for the Treatment of Heart Failure and Arrhythmias

Diseases of the heart, such as heart failure and cardiac arrhythmias, are a growing socio-economic burden. Calcium (Ca²⁺) dysregulation is key hallmark of the failing myocardium and has long been touted as a potential therapeutic target in the treatment of a variety of cardiovascular diseases (CVD). In the heart, Ca²⁺ is essential for maintaining normal cardiac function through the generation of the cardiac action potential and its involvement in excitation contraction coupling. As such, the proteins which regulate Ca²⁺ cycling and signaling play a vital role in maintaining Ca²⁺ homeostasis. Changes to the expression levels and function of Ca²⁺-channels, pumps and associated intracellular handling proteins contribute to altered Ca²⁺ homeostasis in CVD. The remodeling of Ca²⁺-handling proteins therefore results in impaired Ca²⁺ cycling, Ca²⁺ leak from the sarcoplasmic reticulum and reduced Ca²⁺ clearance, all of which contributes to increased intracellular Ca²⁺. Currently, approved treatments for targeting Ca²⁺ handling dysfunction in CVD are focused on Ca²⁺ channel blockers. However, whilst Ca²⁺ channel blockers have been successful in the treatment of some arrhythmic disorders, they are not universally prescribed to heart failure patients owing to their ability to depress cardiac function. Despite the progress in CVD treatments, there remains a clear need for novel therapeutic approaches which are able to reverse pathophysiology associated with heart failure and arrhythmias. Given that heart failure and cardiac arrhythmias are closely associated with altered Ca²⁺ homeostasis, this review will address the molecular changes to proteins associated with both Ca²⁺-handling and -signaling; their potential as novel therapeutic targets will be discussed in the context of pre-clinical and, where available, clinical data.

Cardiovascular Therapeutic Potential of the Redox Siblings, Nitric Oxide (NO•) and Nitroxyl (HNO), in the Setting of Reactive Oxygen Species Dysregulation

Reactive oxygen species (ROS) dysregulation is a hallmark of cardiovascular disease, characterised by an imbalance in the synthesis and removal of ROS. ROS such as superoxide (•O₂^-), hydrogen peroxide (H₂O₂), hydroxyl (OH•) and peroxynitrite (ONOO^-) have a marked impact on cardiovascular function, contributing to the vascular impairment and cardiac dysfunction associated with diseases such as angina, hypertension, diabetes and heart failure. Central to the vascular dysfunction is a reduction in bioavailability and/or physiological effects of vasoprotective nitric oxide (NO•), leading to vasoconstriction, inflammation and vascular remodelling. In a cardiac context, increased ROS generation can also lead to modification of key proteins involved in cardiac contractility. Whilst playing a key role in the pathogenesis of cardiovascular disease, ROS dysregulation also limits the clinical efficacy of current therapies, such as nitrosovasodilators. As such, alternate therapies are sought. This review will discuss the impact of ROS dysregulation on the therapeutic utility of NO• and its redox sibling, nitroxyl (HNO). Both nitric oxide (NO) and nitroxyl (HNO) donors signal through soluble guanylyl cyclase (sGC). NO binds to the Fe(II) form of sGC and nitroxyl possibly to both sGC heme and thiol groups. In the vasculature, nitroxyl can also signal through voltage-dependent (K_v) and ATP-sensitive (K_ATP) K⁺ channels as well as calcitonin gene-related peptide (CGRP). In the heart, HNO directly targets critical thiols to increase myocardial contractility, an effect not seen with NO. The qualitative effects via elevation of cGMP are similar, i.e. lusitropic in the heart and inhibitory on vasoconstriction, inflammation, aggregation and vascular remodelling. Of pathophysiological significance is the fact the efficacy of NO donors is impaired by ROS, e.g. through chemical scavenging of NO, to generate reactive nitrogen oxide species (RNOS), whilst nitroxyl is apparently not.

Nitroxyl: A Novel Strategy to Circumvent Diabetes Associated Impairments in Nitric Oxide Signaling

Diabetes is associated with an increased mortality risk due to cardiovascular complications. Hyperglycemia-induced oxidative stress underlies these complications, leading to an impairment in endogenous nitric oxide (NO•) generation, together with reductions in NO• bioavailability and NO• responsiveness in the vasculature, platelets and myocardium. The latter impairment of responsiveness to NO•, termed NO• resistance, compromises the ability of traditional NO•-based therapeutics to improve hemodynamic status during diabetes-associated cardiovascular emergencies, such as acute myocardial infarction. Whilst a number of agents can ameliorate (e.g. angiotensin converting enzyme [ACE] inhibitors, perhexiline, statins and insulin) or circumvent (e.g. nitrite and sGC activators) NO• resistance, nitroxyl (HNO) donors offer a novel opportunity to circumvent NO• resistance in diabetes. With a suite of vasoprotective properties and an ability to enhance cardiac inotropic and lusitropic responses, coupled with preserved efficacy in the setting of oxidative stress, HNO donors have intact therapeutic potential in the face of diminished NO• signaling. This review explores the major mechanisms by which hyperglycemia-induced oxidative stress drives NO• resistance, and the therapeutic potential of HNO donors to circumvent this to treat cardiovascular complications in type 2 diabetes mellitus.

Nitroxyl as a Potential Theranostic in the Cancer Arena

Significance: As one-electron reduced molecule of nitric oxide (NO), nitroxyl (HNO) has gained enormous attention because of its novel physiological or pharmacological properties, ranging from cardiovascular protective actions to antitumoricidal effects. Recent Advances: HNO is emerging as a new entity with therapeutic advantages over its redox sibling, NO. The interests in the chemical, pharmacological, and biological characteristics of HNO have broadened our current understanding of its role in physiology and pathophysiology. Critical Issues: In particular, the experimental evidence suggests the therapeutic potential of HNO in tumor pharmacology, such as neuroblastoma, gastrointestinal tumor, ovarian, lung, and breast cancers. Indeed, HNO donors have been demonstrated to attenuate tumor proliferation and angiogenesis. Future Directions: In this review, the generation and detection of HNO are outlined, and the roles of HNO in cancer progression are further discussed. We anticipate that the completion of this review might give novel insights into the roles of HNO in cancer pharmacology and open up a novel field of cancer therapy based on HNO.

Recent Advances in Fluorescent Probes for Detection of HOCl and HNO

It is known that reactive oxygen (ROS) and nitrogen (RNS) species play a diverse role in various biological processes, such as inflammation, signal transduction, and neurodegenerative injury, apart from causing various diseases caused by oxidative and nitrosative stresses, respectively, by ROS and RNS. Thus, it is very important to quantify the concentration level of ROS and RNS in live cells, tissues, and organisms. Various small-molecule-based fluorescent/chemodosimetric probes are reported to quantify and map the effective distribution of ROS/RNS under in vitro/in vivo conditions with a great spatial and temporal resolution. Such reagents are now appreciated as an excellent tool for aiding breakthroughs in modern redox biology. This mini-review is a brief, but all-inclusive, account of such molecular probes that have been developed recently.

Nitric Oxide Resistance, Induced in the Myocardium by Diabetes, Is Circumvented by the Nitric Oxide Redox Sibling, Nitroxyl

Aim: Impairment of tissue responsiveness to exogenous and endogenous nitric oxide (NO^•), known as NO^• resistance, occurs in many cardiovascular disease states, prominently in diabetes and especially in the presence of marked hyperglycemia. In this study, we sought to determine in moderate and severe diabetes (i) whether NO^• resistance also occurs in the myocardium, and (ii) whether the NO^• redox sibling nitroxyl (HNO) circumvents this. Results: The spectrum of acute NO^• effects (induced by diethylamine-NONOate), including vasodilation, and enhanced myocardial contraction and relaxation were impaired by moderately diabetic rats ([blood glucose] ∼20 mM). In contrast, acute HNO effects (induced by isopropylamine-NONOate) were preserved even in more severe diabetes ([blood glucose] >28 mM). Intriguingly, the positive inotropic effects of HNO were significantly enhanced in diabetic rat hearts. Further, progressive attenuation of soluble guanylyl cyclase (sGC) contribution to myocardial NO^• responses occurred with increasing severity of diabetes. Nevertheless, activation of sGC by HNO remained intact in the myocardium. Innovation: Diabetes is associated with marked attenuation of vascular and myocardial effects of NO and NO donors, and this NO^• resistance is circumvented by HNO, suggesting potential therapeutic utility for HNO donors in cardiovascular emergencies in diabetics. Conclusion: These results provide the first evidence that NO^• resistance occurs in diabetic hearts, and that HNO largely circumvents this problem. Further, the positive inotropic and lusitropic effects of HNO are enhanced in a severely diabetic myocardium, a finding that warrants further mechanistic interrogation. The results support a potential role for therapeutic HNO administration in acute treatment of ischemia and/or heart failure in diabetics.

Small-molecule fluorescent probes for specific detection and imaging of chemical species inside lysosomes

In the past few years, the preparation of novel small-molecule fluorescent probes for specific detection and imaging of chemical species inside lysosomes has attracted considerable attention because of their wide applications in chemistry, biology, and medical science. This feature article summarizes the recent advances in the design and preparation of small-molecule fluorescent probes for specific detection of chemical species inside lysosomes. In addition, their properties and applications for the detection and imaging of pH, H2O2, HOCl, O2˙-, lipid peroxidation, H2S, HSO3-, thiols, NO, ONOO-, HNO, Zn2+, Cu2+, enzymes, etc. in lysosomes are discussed as well.

Leveraging Signaling Pathways to Treat Heart Failure With Reduced Ejection Fraction

Advances in the treatment of heart failure with reduced ejection fraction due to systolic dysfunction are engaging an ever-expanding compendium of molecular signaling targets. Well established approaches modifying hemodynamics and cell biology by neurohumoral receptor blockade are evolving, exploring the role and impact of modulating intracellular signaling pathways with more direct myocardial effects. Even well-tread avenues are being reconsidered with new insights into the signaling engaged and thus opportunity to treat underlying myocardial disease. This review explores therapies that have proven successful, those that have not, those that are moving into the clinic but whose utility remains to be confirmed, and those that remain in the experimental realm. The emphasis is on signaling pathways that are tractable for therapeutic manipulation. Of the approaches yet to be tested in humans, we chose those with a well-established experimental history, where clinical translation may be around the corner. The breadth of opportunities bodes well for the next generation of heart failure therapeutics.

Nitroxyl (HNO) targets phospholamban cysteines 41 and 46 to enhance cardiac function

Nitroxyl (HNO) positively modulates myocardial function by accelerating Ca²⁺ reuptake into the sarcoplasmic reticulum (SR). HNO-induced enhancement of myocardial Ca²⁺ cycling and function is due to the modification of cysteines in the transmembrane domain of phospholamban (PLN), which results in activation of SR Ca²⁺-ATPase (SERCA2a) by functionally uncoupling PLN from SERCA2a. However, which cysteines are modified by HNO, and whether HNO induces reversible disulfides or single cysteine sulfinamides (RS(O)NH₂) that are less easily reversed by reductants, remain to be determined. Using an ¹⁵N-edited NMR method for sulfinamide detection, we first demonstrate that Cys46 and Cys41 are the main targets of HNO reactivity with PLN. Supporting this conclusion, mutation of PLN cysteines 46 and 41 to alanine reduces the HNO-induced enhancement of SERCA2a activity. Treatment of WT-PLN with HNO leads to sulfinamide formation when the HNO donor is in excess, whereas disulfide formation is expected to dominate when the HNO/thiol stoichiometry approaches a 1:1 ratio that is more similar to that anticipated in vivo under normal, physiological conditions. Thus, ¹⁵N-edited NMR spectroscopy detects redox changes on thiols that are unique to HNO, greatly advancing the ability to detect HNO footprints in biological systems, while further differentiating HNO-induced post-translational modifications from those imparted by other reactive nitrogen or oxygen species. The present study confirms the potential of HNO as a signaling molecule in the cardiovascular system.

Stoichiometric Nitroxyl Photorelease Using the (6-Hydroxy-2-naphthalenyl)methyl Phototrigger

The design and synthesis of a photoactivatable HNO donor incorporating the (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM) photocage coupled to the trifluoromethanesulfonamidoxy analogue of the well-established HNO generator Piloty's acid is described. The photoactive HNO donor stoichiometrically generates HNO (∼98%) at neutral pH conditions, and evidence for concerted C-O and N-S bond cleavage was obtained. The methanesulfonamidoxy analogue primarily undergoes undesired N-O bond cleavage.

A recent history of nitroxyl chemistry, pharmacology and therapeutic potential

Due to the excitement surrounding the discovery of NO as an endogenously generated signalling molecule, a number of other nitrogen oxides were also investigated as possible physiological mediators. Among these was nitroxyl (HNO). Over the past 25 years or so, a significant amount of work by this laboratory and many others has disclosed that HNO possesses unique chemical properties and important pharmacological utility. Indeed, the pharmacological potential for HNO as a treatment for heart failure, among other uses, has garnered this curious molecule a considerable amount of recent attention. This review summarizes the events that led to this recent attention as well as poses important questions that are still to be answered with regards to understanding the chemistry and biology of HNO. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

Nitric oxide is not just blowing in the wind

LINKED ARTICLES

This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.