Pharmacologic inhibition of S-nitrosoglutathione reductase protects against experimental asthma in BALB/c mice through attenuation of both bronchoconstriction and inflammation

S-nitrosoglutathione (GSNO) is formed through the reaction of glutathione with reactive nitrogen species and serves as the main reservoir of cellular S-nitrosothiol (SNO) species that govern total and/or local nitric oxide (NO) bioavailability in vivo [1, 2]. GSNO and SNOs serve as functional depots for NO [2, 3] which has a short biological half-life [4]. Increases in bio-available NO are associated with anti-inflammatory and smooth muscle relaxant effects, especially in organ systems characterized by smooth muscle and endothelial/epithelial layers such as the respiratory, cardiovascular, and gastrointestinal systems [5, 6]. In particular, NO and GSNO help to maintain normal lung physiology and function via the actions of these mediators on bronchial smooth muscle tone and responsivity, adrenergic receptor function, and anti-inflammatory activities [7‐9].

GSNO is catabolized by S-nitrosoglutathione reductase (GSNOR), a class III alcohol dehydrogenase (ADH) [10, 11]. Therefore, GSNOR has an important role in regulating intracellular SNOs and, subsequently, the function of these compounds [11], while dysregulation of this enzyme can lead to deleterious effects as observed in respiratory and other diseases [12, 13]. Specifically, there are lowered SNO concentrations in the lungs of asthmatic patients which have been attributed to up-regulated GSNOR activity [13, 14]. Furthermore, various alleles of the human GSNOR gene have been associated with an increased risk of childhood asthma and with a decreased response to albuterol among different ethnic populations [15‐17]. The increased GSNOR activity with subsequent loss of GSNO, SNOs, and their associated activities, points to this enzyme as a potential therapeutic target especially in the treatment of respiratory diseases including asthma.

In fact, there is both preclinical and clinical evidence supporting a role for inhibiting GSNOR in the treatment of asthma. Que et al. (2005) showed that mice with genetic deletion of GSNOR were protected from methacholine (MCh)-induced airway hyper-responsiveness (AHR) following ovalbumin (OVA) sensitization and challenge [18]. SNOs were found to be lowered in tracheal irrigations in asthmatic children with respiratory failure in comparison to normal children undergoing elective surgery [14]. SNO content was decreased in the bronchoalveolar lavage fluid (BALF) in adult patients with mild asthma compared to healthy control subjects, and was inversely correlated with GSNOR expression in BALF cell lysates [13]. Furthermore, GSNOR activity in BALF cell lysates was significantly increased in asthmatics compared to controls and correlated with increased MCh responsivity [13].

Exhaled NO is increased in patients with severe asthma [19, 20] and the lowering of this parameter is used as a measure of the anti-inflammatory effectiveness of therapeutics [21]. The increased NO in asthma has been attributed to generation from inducible nitric oxide synthase (iNOS) in response to inflammatory signals typical in this disease, and NO generated in this manner can have pro-inflammatory activity [20]. Inhibitors of iNOS have been developed for the treatment of respiratory diseases, including asthma, in attempts to mitigate the NO mediated inflammatory signals [22, 23]. Conversely, NO donors have also been developed for the treatment of respiratory diseases for their bronchodilatory and anti-inflammatory benefits [24, 25]. These contradictions surrounding NO may be attributable to the source (i.e., NOS isoform), amount, and location of NO production as well as pathways involved in NO processing, signaling, or metabolism [19, 26].

As evident in asthma, increased GSNOR activity leads to lowered GSNO and SNOs [13] in spite of the increased NO. Similar conditions with increased NO and inflammation, but potentially lowered SNOs and decreased SNO-mediated function, are evident in non-respiratory diseases, including cardiovascular disease [27, 28] and inflammatory bowel disease [29], in which a role for GSNOR may exist [27, 30]. GSNOR dysregulation may therefore help explain the decreased pool of bioavailable NO in disease states in the presence of a pro-inflammatory NO signal.

This study evaluated the potential of GSNOR inhibition as a therapeutic approach in the treatment of asthma. Specifically, the effects of N6022, a novel, potent, and selective small molecule inhibitor of GSNOR [31, 32], were evaluated in a murine model of asthma induced by systemic sensitization followed by airway challenges with OVA. Endpoints measured were AHR in response to aerosol challenge with MCh using non-invasive plethysmography [33] as well as eosinophilic infiltration into the BALF. Other determinations included assessments of nitrite, cyclic guanosine monophosphate (cGMP), and biomarker profiles in plasma and BALF, nuclear factor kappa B (NFκB) activity in the lung, and modulation of airway smooth muscle tone in a tracheal ring bioassay. These studies showed that inhibition of GSNOR activity with a single intravenous (i.v.) dose of N6022 imparted potent effects against key parameters in asthma, specifically, AHR and eosinophilic inflammation, with mechanisms consistent with restoring normal levels and function of SNOs in the airways. N6022 is currently being evaluated for safety and efficacy in clinical trials based on these findings and the role of GSNOR in disease.

Results of these studies show that N6022, a potent and selective inhibitor of GSNOR activity, has significant bronchodilatory and anti-inflammatory effects in a mouse model of allergic asthma. The effects of N6022 occurred as early as 30 min post-administration, were greater at 12 h and later after administration, and were sustained for at least 48 h after administration. Efficacy with N6022 was achieved with a single i.v. dose and was comparable to that observed after administration of three inhaled doses of the anti-cholinergic, ipratropium bromide, combined with the β2 agonist, albuterol.

N6022 also lowered Penh following MCh exposure in non-sensitized mice and decreased MCh-induced smooth muscle contraction in the tracheal ring assays. These findings show that GSNOR inhibition by N6022 directly influences smooth muscle tone in the airways. However, the potency of N6022 on Penh was approximately 100-fold greater in OVA-sensitized mice compared to non-sensitized mice as there were significant and similar actions at 0.01 mg/kg vs. 1 mg/kg N6022 in OVA- vs. non-sensitized mice, respectively. This difference suggests an important contribution of anti-inflammatory mechanisms on mitigating AHR in response to MCh challenge.

In fact, considerable anti-inflammatory actions of GSNOR inhibition by N6022 were evident as noted by significant reductions in BALF eosinophils as well as BALF and systemic inflammatory biomarkers explored in both the dose response and time course OVA studies. These potent anti-inflammatory actions of N6022 may occur in part through NFκB pathways. NFκB has an important role as an upstream regulator of inflammatory signals, including signals in asthma and the asthma-associated biomarkers that were measured in the current study [37]. Additionally, NFκB is regulated in part by nitrosation of key cysteine residues which leads to a decrease in NFκB function [37, 38]. Our data demonstrating the ability of N6022 to decrease NFκB DNA binding in lungs from the OVA mouse studies suggest that GSNOR inhibition likely attenuates inflammation at least in part by down-regulating NFκB activation. Given that N6022 treatment also increased BALF nitrite and plasma cGMP, endpoints used as markers of bioavailable NO [42, 43], the anti-inflammatory effects of GSNOR inhibition are consistent with SNO-dependent inhibition of NFκB-mediated signaling. Previously published studies suggest that this SNO-mediated effect may occur through inhibition of transcription factor DNA binding activity [44] or inhibition of pathway activation via nitrosation of IKKβ [45].

The difference in potency observed for N6022 in OVA-sensitized compared to non-sensitized mice also may be explained by differences in activities of the restored GSNO and SNO pools and down-stream nitrosation targets. For example, restoring the levels of GSNO/SNOs may mitigate against disease, whereas in non-disease states, these levels are sufficient and no further benefit or effect is achieved or measurable upon GSNOR inhibitor treatment. In support of this hypothesis, treatment of rats with a related GSNOR inhibitor decreases blood pressure and nitric-oxide dependent flow mediated vasodilation in a salt-induced hypertensive rat model, whereas no effect of the GSNOR inhibitor is noted in normotensive rats [46].

The bronchodilatory capacity observed with N6022 administration is consistent with observations reported in GSNOR knock-out mice, which showed that genetic deletion of GSNOR protected mice from MCh-induced bronchoconstriction compared to wild-type control mice [18]. Similar to N6022, more pronounced effects were evident in GSNOR knock-out mice under the conditions of OVA-induced asthma compared to the non-sensitized model. However, in contrast to the potent anti-inflammatory actions of N6022 in the mouse OVA model, inflammatory responses (BALF eosinophils, BALF IL-13, and serum IgE) were not decreased in GSNOR knock-out mice upon exposure to OVA. The reason for differences in anti-inflammatory influences between genetic deletion and pharmacologic inhibition is not clear, but may be due to differences between life-long homozygous gene deletion of GSNOR and the reversible inhibition of GSNOR activity with a pharmacologic approach. Further contributing factors may include differences in the experimental model such as mouse strain, asthma induction protocol, and endpoints measured.

NO produced from iNOS, upon up-regulation of this enzyme during inflammation, is known to be increased in the expired breath of asthmatics [21], and plays a significant role in the inflammatory responses observed in atopic asthma [19, 21, 47]. Thus, increasing the pool of bioavailable NO through GSNOR inhibition may appear contradictory. However, there are lowered concentrations of SNOs in the lungs of asthmatic patients [13] even in the presence of the increased exhaled NO [19, 20, 48] which may be explained by increased GSNOR activity. These findings suggest that the mechanisms by which SNO pools mediate bronchodilatory and anti-inflammatory effects are distinct from the actions of the relatively high concentrations of NO produced by iNOS. These findings also suggest that iNOS derived NO is not necessarily responsible for SNO levels observed in the BALF. In support of this hypothesis, GSNOR inhibitors attenuate iNOS protein expression in cellular models of cytokine-stimulated inflammation ([49] and SCM, GJR unpublished results). Our studies suggest that inhibition of GSNOR, and the likely subsequent elevation of SNOs, result in attenuation of proinflammatory mediators, in part via down regulation of NFκB signaling. Inhibition of GSNOR as a mechanism to increase SNO pools is thus plausible and of potential benefit in asthma therapy, as noted by efficacy in the mouse model of asthma in the current studies. These proposed mechanisms are consistent with data demonstrating protection from experimental asthma in the GSNOR knock-out mouse [18] and the attenuation of asthma severity in an OVA model following GSNO administration [50]. Taken together, decreased local levels of SNOs through increased GSNOR activity in asthma patients may be an important component of asthma pathophysiology as previously suggested [13, 14].

The results and mechanisms noted in the current studies are consistent with other observations in inflammatory disease models in which dysregulated GSNOR and/or altered SNO homeostasis may have important roles. In particular, the pathophysiology of diseases of the respiratory [12, 13, 20], gastrointestinal [29, 30], and cardiovascular [27, 28] systems involve inflammatory and NO-mediated pathways which have the potential to be regulated by GSNOR. N6022 and other inhibitors of GSNOR have been shown to decrease inflammation and disease severity in animal models of tobacco smoke induced chronic obstructive pulmonary disease [51], chemically induced colitis [52], acetaminophen induced hepatotoxicity [53], and high salt diet induced hypertension [46].

Direct measurements of airway mechanics were not performed in the current studies, but rather Penh was derived via whole body plethysmography with a Buxco chamber and used as an index of AHR. This technique was chosen as it offers a noninvasive method to measure lung mechanics in unanesthetized and unrestrained mice while allowing for MCh challenge via aerosol/inhalation exposure. While some controversy exists as to the adequacy of Penh as a measure of AHR [54, 55], Penh has been shown to be a valid measure of AHR in allergen sensitized mice and to positively correlate with a direct measure of airway resistance using mechanical ventilation in anesthetized and surgically implemented mice of the same strain utilized in these studies [33].

There were some questions that could not be addressed in these studies due to analytical limitations. Although N6022 is a potent and selective inhibitor of human GSNOR activity in vitro [31, 32], inhibition occurs via a reversible process which precludes the direct measurement of GSNOR inhibition in vivo since tissue processing and dilution leads to dissociation of GSNOR inhibitors from the enzyme-substrate complex [31]. Another limitation was the inability to detect GSNO and SNOs in mouse lung or BALF samples. SNOs were assessed using ozone chemiluminescence detection with a nitric oxide analyzer (Sievers) following tri-iodide reduction after prior treatment with sulfanilamide to remove contaminating nitrite signal [56, 57]. The detection limit of this assay was 5 pmoles or 50 nM.

GSNOR inhibition in these studies may have indeed caused increased GSNO as suggested by the effects on endpoints influenced by GSNO including bronchodilation, increased BALF nitrite, increased plasma cGMP, and decreased NFκB activity. Because GSNOR can catalyze the reduction of certain aldehydes in addition to the oxidation of GSNO [10, 11], an alternative consideration is that the physiological effects of GSNOR inhibition could be due to inhibition of aldehyde reduction rather than the GSNO oxidation reaction. However, there is no evidence that the aldehyde substrates are involved in the endpoints mentioned above, whereas GSNO has been shown in many studies to influence these measurements [13, 58, 59].

Direct measurement of endogenous GSNO and SNOs is challenging because levels are usually below the limits of detection of current methods. Other investigators also state the inability to detect GSNO in the BALF of asthma patients [13, 60]. In this example, the investigators measured high and low molecular weight (i.e., GSNO) SNOs using photolysis-chemiluminescence in the absence or presence of HgCl₂ to cleave thiol-bound NO. The limit of detection was 2 pmoles. Values reported for total SNOs were 10–20 pmoles/mL (10–20 nM) which are close to our detection limit of 50 nM. It was noted that N6022 did increase BALF nitrite which was utilized as a stable marker of NO, although the detection of nitrite did not correlate with N6022 efficacy at every dose. Similar disparities between physiologic or pharmacologic effects and GSNO levels have been noted in a study showing that efficacy of GSNO was evident at lower doses than those that caused increased BALF SNOs in experimental asthma [49].

GSNOR inhibition with N6022 in an experimental model of asthma demonstrated significant efficacy toward key parameters associated with asthma including AHR in response to MCh challenge, pulmonary eosinophilia, and both pulmonary and systemic inflammatory biomarkers. GSNOR has recently emerged as a potential target in human asthma and other inflammatory lung diseases. The role of GSNOR in human disease, along with the current findings with the GSNOR inhibitor, N6022, point to GSNOR inhibition as a novel target for the treatment of asthma and other inflammatory lung disease. This rationale has prompted the current evaluation of N6022 in clinical trials for the treatment of inflammatory lung disease including asthma and cystic fibrosis.