Adenosine receptor antagonists and asthma

Acute glutathione depletion leads to enhancement of airway reactivity and inflammation via p38MAPK-iNOS pathway in allergic mice

Glutathione (GSH) plays a major role in allergic airway responses through a variety of mechanism which include direct scavenging of oxidative species, being a reducing equivalent and regulation of cellular signaling through redox sensitive mechanisms. Therefore, the aim of the present study was to evaluate the role of acute GSH depletion on airway reactivity, inflammation and NO signaling in a mouse model of allergic asthma. Buthionine sulfoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase was used for depletion of GSH levels. Acute depletion of GSH with BSO worsened allergen induced airway reactivity and inflammation through increase in nitrosative stress as reflected by increased inducible NO synthase (iNOS) expression, total nitrates and nitrites (NOx), nitrotyrosine, protein carbonyls, and decreased total antioxidant capacity. Treatment with p38 mitogen-activated protein kinase (MAPK) and iNOS inhibitors attenuated the effects of GSH depletion on airway reactivity and inflammation through attenuation of nitrosative stress as evidenced by a decrease in NOx, nitrotyrosine, protein carbonyls and increase in total antioxidant capacity (TAC). In conclusion, these data suggest that acute depletion of glutathione is associated with alteration of airway responses through an increase in nitrosative stress in allergic airways of mice.

Implication of novel thiazolo-thiophene derivative (MCD-KV-10) for management of asthma

CONTEXT

Asthma is multifaceted disease where many targets contribute towards its development and progression. Among these, adenosine receptor subtypes play a major role.

OBJECTIVE

MCD-KV-10, a novel thiazolo-thiophene was designed and evaluated pre-clinically for its implication in management of asthma.

MATERIALS AND METHODS

This compound showed good affinity and selectivity towards A(2A)/A3 adenosine receptor (AR) subtypes. Furthermore, MCD-KV-10 was evaluated for in vitro lipoxygenase inhibition activity; in vivo mast cell stabilization potential and in vivo anti-asthmatic activity was done in ovalbumin-induced airway inflammation model in guinea pigs.

RESULTS

The compound showed good (>57%) inhibition of lipoxygenase enzyme and also effectively protected mast cell degranulation (>63%). The compound showed good anti-asthmatic activity as inferred from the in vivo studies.

DISCUSSION

These results indicate that MCD-KV-10 has an inhibitory effect on airway inflammation.

CONCLUSION

Though, we have identified a potential candidate for management of asthma, further mechanistic studies are needed.

Adenosine receptors and vascular inflammation

Epidemiological studies have shown a positive correlation between poor lung function and respiratory disorders like asthma and the development of adverse cardiovascular events. Increased adenosine (AD) levels are associated with lung inflammation which could lead to altered vascular responses and systemic inflammation. There is relatively little known about the cardiovascular effects of adenosine in a model of allergy. We have shown that A(1) adenosine receptors (AR) are involved in altered vascular responses and vascular inflammation in allergic mice. Allergic A(1)wild-type mice showed altered vascular reactivity, increased airway responsiveness and systemic inflammation. Our data suggests that A(1) AR is pro-inflammatory systemically in this model of asthma. There are also reports of the A(2B) receptor having anti-inflammatory effects in vascular stress; however its role in allergy with respect to vascular effects has not been fully explored. In this review, we have focused on the role of adenosine receptors in allergic asthma and the cardiovascular system and possible mechanism(s) of action.

Involvement of A1 adenosine receptors in altered vascular responses and inflammation in an allergic mouse model of asthma

Poor lung function and respiratory disorders like asthma have a positive correlation with the development of adverse cardiovascular events. Increased adenosine levels are associated with lung inflammation that could lead to altered vascular responses and systemic inflammation. We hypothesized that asthmatic lung inflammation has systemic effects through A(1) adenosine receptors (A(1)AR) and investigated the effects of aerosolized adenosine on vascular reactivity and inflammation, using A(1)AR knockout (A(1)KO) and corresponding wild-type (A(1)WT) mice that were divided into three experimental groups each: control (CON), allergen sensitized and challenged (SEN), and SEN + aerosolized adenosine (SEN + AD). Animals were sensitized with ragweed (200 microg ip; days 1 and 6), followed by 1% ragweed aerosol challenges (days 11 to 13). On day 14, the SEN + AD groups received one adenosine aerosol challenge (6 mg/ml) for 2 min, and aortae were collected on day 15. 5'-N-ethylcarboxamidoadenosine (NECA; nonselective adenosine analog) induced concentration-dependent aortic relaxation in the A(1)WT CON group, which was impaired in the A(1)WT SEN and SEN + AD groups. All groups of A(1)KO mice showed similar (no significant difference) concentration-dependent relaxation to NECA. The A(1)WT SEN and SEN + AD groups had a significantly higher contraction to selective A(1) agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) compared with the CON group. Western blot data showed that aortic A(1)AR expression was significantly increased in WT SEN and SEN + AD mice compared with CON mice. Gene expression of ICAM-1 and IL-5 was significantly increased in allergic A(1)WT aorta and were undetected in the A(1)KO groups. A(1)WT allergic mice had significantly higher airway hyperresponsiveness (enhanced pause) to NECA, with adenosine aerosol further enhancing it. In conclusion, allergic A(1)WT mice showed altered vascular reactivity, increased airway hyperresponsiveness, and systemic inflammation. These data suggest that A(1)AR is proinflammatory systemically in this model of allergic asthma.

A2A adenosine receptor deficiency leads to impaired tracheal relaxation via NADPH oxidase pathway in allergic mice

A(2A) adenosine receptor (A(2A)AR) has been shown to suppress superoxide generation in leukocytes via the cAMP-protein kinase A (PKA) pathway. However, no study has yet explored the role of A(2A)AR in relation to NADPH oxidase in murine tracheas in vitro, which may lead to altered smooth muscle relaxation in asthma. Therefore, the present study evaluated the effects of A(2A)AR deficiency on the NADPH oxidase pathway in tracheas of A(2A) wild-type (WT) and A(2A) knockout (KO) mice. A(2A)WT mice were sensitized with ovalbumin (30 microg i.p.) on days 1 and 6, followed by 5% ovalbumin aerosol challenge on days 11, 12, and 13. A(2A)AR (gene and protein expression), cAMP, and phosphorylated PKA (p-PKA) levels were decreased in A(2A)WT sensitized mice compared with controls. A(2A)KO mice also showed decreased cAMP and p-PKA levels. A(2A)WT sensitized and A(2A)KO control mice had increased gene and protein expression of NADPH oxidase subunits (p47phox and gp91phox) compared with the controls. Tracheal relaxation to specific A(2A)AR agonist, 4-[2-[[6-amino-9-(N-ethyl-beta-d-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680), decreased in A(2A)WT sensitized mice compared with the controls, although it was absent in A(2A)KO mice. Pretreatment with NADPH oxidase inhibitors apocyanin/diphenyliodonium reversed the attenuated relaxation to CGS 21680 in A(2A)WT sensitized tracheas, whereas specific PKA inhibitor (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i] [1,6]benzodiazocine-10-carboxylic acid hexyl ester (KT 5720) blocked CGS 21680-induced relaxation. Tracheal reactive oxygen species (ROS) generation was also increased in A(2A)WT sensitized and A(2A)KO control mice compared with the controls. In conclusion, this study shows that A(2A)AR deficiency causes increased NADPH oxidase activation leading to decreased tracheal relaxation via altered cAMP-PKA signaling and ROS generation.

Enhanced airway reactivity and inflammation in A2A adenosine receptor-deficient allergic mice

A(2A) adenosine receptor (A(2A)AR) has potent anti-inflammatory properties, which may be important in the regulation of airway reactivity and inflammation. Inflammatory cells that possess A(2A)AR also produce nitrosative stress, which is associated with pathophysiology of asthma, so we hypothesized that A(2A)AR deficiency may lead to increased airway reactivity and inflammation through nitrosative stress. Thus the present study was carried out to investigate the role of A(2A)AR on airway reactivity, inflammation, NF-kappaB signaling, and nitrosative stress in A(2A)AR knockout (KO) and wild-type (WT) mice using our murine model of asthma. Animals were sensitized intraperitoneally on days 1 and 6 with 200 microg of ragweed, followed by aerosolized challenges with 0.5% ragweed on days 11, 12, and 13, twice a day. On day 14, airway reactivity to methacholine was assessed as enhanced pause (Penh) using whole body plethysmography followed by bronchoalveolar lavage (BAL) and lung collection for various analyses. Allergen challenge caused a significant decrease in expression of A(2A)AR in A(2A) WT sensitized mice, with A(2A)AR expression being undetected in A(2A) KO sensitized group leading to decreased lung cAMP levels in both groups. A(2A)AR deletion/downregulation led to an increase in Penh to methacholine and influx of total cells, eosinophils, lymphocytes, and neutrophils in BAL with highest values in A(2A) KO sensitized group. A(2A) KO sensitized group further had increased NF-kappaB expression and nitrosative stress compared with WT sensitized group. These data suggest that A(2A)AR deficiency leads to airway inflammation and airway hyperresponsiveness, possibly via involvement of nitrosative stress in this model of asthma.