Adenosine receptors and asthma

Uridine supplementation exerts anti-inflammatory and anti-fibrotic effects in an animal model of pulmonary fibrosis

Rationale

Pulmonary fibrosis is a progressive disease with only few treatment options available at the moment. Recently, the nucleoside uridine has been shown to exert anti-inflammatory effects in different animal models, e.g. in acute lung injury or bronchial asthma.

Method

Therefore, we investigated the influence of uridine supplementation on inflammation and fibrosis in the classical bleomycin model. Male C57BL/6 mice received an intratracheal injection of bleomycin on day 0 and were treated intraperitoneally with uridine or vehicle. The degree of inflammation and fibrosis was assessed at different time points.

Results

Uridine administration resulted in attenuated inflammation, as demonstrated by reduced leukocytes and pro-inflammatory cytokines in the broncho-alveolar lavage (BAL) fluid. Furthermore, collagen deposition in the lung interstitium was also reduced by uridine supplementation. Similar results were obtained in a model in which animals received repeated intraperitoneal bleomycin injections. In addition uridine inhibited collagen and TGF-ß synthesis by primary lung fibroblasts, the release of pro-inflammatory cytokines by human lung epithelial cells, as well as the production of reactive oxygen species by human neutrophils.

Conclusion

In summary, we were able to show that uridine has potent anti-inflammatory and anti-fibrotic properties. As uridine supplementation has been shown to be well tolerated and safe in humans, this might be a new therapeutic approach for the treatment of fibrotic lung diseases.

Down-regulation of the A3 adenosine receptor in human mast cells upregulates mediators of angiogenesis and remodeling

Adenosine activated mast cells have been long implicated in allergic asthma and studies in rodent mast cells have assigned the A3 adenosine receptor (A3R) a primary role in mediating adenosine responses. Here we analyzed the functional impact of A3R activation on genes that are implicated in tissue remodeling in severe asthma in the human mast cell line HMC-1 that shares similarities with lung derived human mast cells. Quantitative real time PCR demonstrated upregulation of IL6, IL8, VEGF, amphiregulin and osteopontin. Moreover, further upregulation of these genes was noted upon the addition of dexamethasone. Unexpectedly, activated A3R down regulated its own expression and knockdown of the receptor replicated the pattern of agonist induced gene upregulation. This study therefore identifies the human mast cell A3R as regulator of tissue remodeling gene expression in human mast cells and demonstrates a heretofore-unrecognized mode of feedback regulation that is exerted by this receptor.

Thymoquinone, the main constituent of Nigella sativa, affects adenosine receptors in asthmatic guinea pigs

OBJECTIVES

For determining the mechanism of anti-asthmatic effect of thymoquinone, this investigation evaluated the effect of thymoquinone in the presence of selective A2A and A2B adenosine receptor antagonists (ZM241385 and MRS1706, respectively).

MATERIALS AND METHODS

Seventy guinea pigs were randomly divided to 7 groups; control (C), sensitized with ovalbumin (S), sensitized groups pretreated with thymoquinone (S+TQ), ZM241385 (S+Anta A2A), MRS1706 (S+Anta A2B), thymoquinone and antagonists (S+Anta A2A+TQ and S+Anta A2B+TQ). Thymoquinone and each of these antagonists with 3 mg/kg dose were injected i.p. on 10(th) day of sensitization protocol. Tracheal responsiveness (TR) to methacholine and ovalbumin (OA), and total and differential cell count in lung lavage fluid (LLF) in different groups were measured.

RESULTS

Increased EC50 and LLF neutrophil count and decreased TR to methacholine and OA, LLF eosinophil and basophil counts were observed in S+TQ group compared to S group (P<0.001 to P<0.05). Significant decrease in EC50 (P<0.01), LLF neutrophil, lymphocyte and monocyte count (P<0.001 for all) and significant increase in TR to OA (P<0.01), LLF total WBC (P<0.01) and eosinophil count (P<0.001) were observed in S+A2A group compared to S+TQ group. There was significant increase in LLF eosinophil and monocyte counts in S+Anta A2B group compared with S+TQ group (P<0.001 for both). In S+TQ+Anta A2A group, there was significant increase in LLF eosinophil (P<0.001) and significant decrease in LLF neutrophil (P<0.01) and monocyte (P<0.001) counts compared with S+TQ group.

CONCLUSION

Thymoquinone affects adenosine receptors, which suggest that some of its anti-inflammatory effects may be mediated by these receptors.

Positive allosteric modulation of the adenosine A2a receptor attenuates inflammation

Background

Adenosine is produced at high levels at inflamed sites as a by-product of cellular activation and breakdown. Adenosine mediates its anti-inflammatory activity primarily through the adenosine A_2a receptor (A_2aR), a member of the G-protein coupled receptors. A_2aR agonists have demonstrated anti-inflammatory efficacy, however, their therapeutic utility is hindered by a lack of adenosine receptor subtype selectivity upon systemic exposure. We sought to harness the anti-inflammatory effects of adenosine by enhancing the responsiveness of A_2aR to endogenously produced adenosine through allosteric modulation. We have identified a family of positive allosteric modulators (PAMs) of the A_2aR. Using one member of this PAM family, AEA061, we demonstrate that A_2aRs are amenable to allosteric enhancement and such enhancement produces increased A_2aR signaling and diminished inflammation in vivo.

Methods

A_2aR activity was evaluated using a cell-based cAMP assay. Binding affinity of A_2aR was determined using [³H]CGS 21680. A_2aR-mediated G-protein activation was quantified using [³⁵S]GTP-γS. The effect of AEA061 on cytokine production was evaluated using primary monocytes and splenocytes. The anti-inflammatory effect of AEA061 was evaluated in the LPS-induced mouse model of inflammation.

Results

AEA061 had no detectable intrinsic agonist activity towards either rat or human A_2aRs. AEA061 enhanced the efficacy of adenosine to rat and human A_2aRs by 11.5 and 2.8 fold respectively. AEA061 also enhanced the maximal response by 4.2 and 2.1 fold for the rat and the human A_2aR respectively. AEA061 potentiated agonist-mediated Gα activation by 3.7 fold. Additionally, AEA061 enhanced both the affinity as well as the B_max at the human A_2aR by 1.8 and 3 fold respectively. Consistent with the anti-inflammatory role of the A_2aR, allosteric enhancement with AEA061 inhibited the production of TNF-α, MIP-1α, MIP-1β, MIP-2, IL-1α, KC and RANTES by LPS-stimulated macrophages and/or splenocytes. Moreover, AEA061 reduced circulating plasma TNF-α and MCP-1 levels and increased plasma IL-10 in endotoxemic A_2aR intact, but not in A_2aR deficient, mice.

Conclusions

AEA061 increases affinity and B_max of A_2aR to adenosine, thereby increasing adenosine potency and efficacy, which translates to enhanced A_2aR responsiveness. Since the A_2aR negatively regulates inflammation, PAMs of the receptor offer a novel means of modulating inflammatory processes.

The Effect of Adenosine A2A and A2B Antagonists on Tracheal Responsiveness, Serum Levels of Cytokines and Lung Inflammation in Guinea Pig Model of Asthma

PURPOSE

Nowadays adenosine is specified as an important factor in the pathophysiology of asthma. For determining the effect of different A2 receptors, in this investigation the effect of single dose of selective adenosine A2A and A2B antagonists (ZM241385 and MRS1706) on different inflammatory parameters; tracheal responsiveness to methacholine and ovalbumin, total and differential cell count in bronchoalveolar lavage (BAL), blood levels of IL-4 and IFN-γ and lung pathology of guinea pig model of asthma were assessed.

METHODS

All mentioned parameters were evaluated in two sensitized groups of guinea pigs pretreated with A2A and A2B antagonists (S+Anta A2A, S+Anta A2B) compared with sensitized (S) and control (C) groups.

RESULTS

The tracheal responsiveness to methacholine and OA, total cell and eosinophil and basophil count in BAL, blood IL-4 level and pathological changes in pre-treated group with MRS1706 (S+Anta A2B) was significantly lower than those of sensitized group (p<0.01 to p<0.05). In pretreated group with Anta A2A(S+Anta A2A), all the above changes were reversed.

CONCLUSION

These results showed a preventive effect of A2B antagonist (MRS1706) on tracheal responsiveness to methacholine and OA, total and differential cell count in bronchoalveolar lavage, blood cytokines and pathological changes. Administration of ZM241385, selective A2A antagonist, deteriorated the induction effect of ovalbumin.

New drugs and devices from 2011 - 2012 that might change your practice

To be honest, I thought this would be a lost cause. Even after skipping a New Drugs and Devices essay in 2012, I figured that I would have to search long and hard to find 10 new things that emergency practitioners needed to know about. Although there were no true blockbuster medications for emergency physicians, I nonetheless found 10 medicines that we probably should know, along with a new device that may change the way we work up patients with palpitations, and a clever new delivery system for subcutaneous epinephrine.

Adenosine A(1) receptors in mouse pontine reticular formation depress breathing, increase anesthesia recovery time, and decrease acetylcholine release

BACKGROUND

Clinical and preclinical data demonstrate the analgesic actions of adenosine. Central administration of adenosine agonists, however, suppresses arousal and breathing by poorly understood mechanisms. This study tested the two-tailed hypothesis that adenosine A1 receptors in the pontine reticular formation (PRF) of C57BL/6J mice modulate breathing, behavioral arousal, and PRF acetylcholine release.

METHODS

Three sets of experiments used 51 mice. First, breathing was measured by plethysmography after PRF microinjection of the adenosine A1 receptor agonist N-sulfophenyl adenosine (SPA) or saline. Second, mice were anesthetized with isoflurane and the time to recovery of righting response (RoRR) was quantified after a PRF microinjection of SPA or saline. Third, acetylcholine release in the PRF was measured before and during microdialysis delivery of SPA, the adenosine A1 receptor antagonist 1, 3-dipropyl-8-cyclopentylxanthine, or SPA and 1, 3-dipropyl-8-cyclopentylxanthine.

RESULTS

First, SPA significantly decreased respiratory rate (-18%), tidal volume (-12%), and minute ventilation (-16%). Second, SPA concentration accounted for 76% of the variance in RoRR. Third, SPA concentration accounted for a significant amount of the variance in acetylcholine release (52%), RoRR (98%), and breathing rate (86%). 1, 3-dipropyl-8-cyclopentylxanthine alone caused a concentration-dependent increase in acetylcholine, a decrease in RoRR, and a decrease in breathing rate. Coadministration of SPA and 1, 3-dipropyl-8-cyclopentylxanthine blocked the SPA-induced decrease in acetylcholine and increase in RoRR.

CONCLUSIONS

Endogenous adenosine acting at adenosine A1 receptors in the PRF modulates breathing, behavioral arousal, and acetylcholine release. The results support the interpretation that an adenosinergic-cholinergic interaction within the PRF comprises one neurochemical mechanism underlying the wakefulness stimulus for breathing.

Evaluation of SSR161421, a novel orally active adenosine A3 receptor antagonist on pharmacology models

The effects of a novel adenosine A(3) receptor antagonist, SSR161421, were examined on both antigen per se and adenosine receptor agonist-increased airway responses in antigen-sensitized guinea pigs. Adenosine (10(-5)M) and AB-MECA [N6-(4-aminobenzyl)-adenosine-5'-N-methyl-uronamide dihydrochloride] (10(-7)M) increased the antigen response up to 61 ± 3.0% and 88 ± 5.2% of maximal contraction, respectively. The agonists of adenosine A(1) and A(2) adenosine receptors NECA [1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-b-d-ribofuranuronamide-5'-N-ethylcarboxamidoadenosine], R-PIA [N(6)-R-phenylisopropyladenosine], and CGS21680 (10(-7)M) were ineffective. In vivo intravenous adenosine (600 μg/kg) and AB-MECA (30 μg/kg) increased the threshold antigen dose-induced bronchoconstriction by 214 ± 13.0% and 220 ± 15.2%, respectively. SSR161421 in vitro (IC(50)=5.9 × 10(-7)M) inhibited the AB-MECA-enhanced antigen-induced airway smooth muscle contractions and also in vivo the bronchoconstriction following either intravenous (ED(50)=0.008 mg/kg) or oral (ED(50)=0.03 mg/kg) administration in sensitized guinea pigs. Antigen itself could evoke tracheal contraction in vitro and bronchoconstriction in vivo in antigen-sensitized guinea pigs. SSR161421 (3 × 10(-6)M) decreased the AUC of the antigen-induced contraction-time curve to 20.8 ± 5.4% from the 100% control level. SSR161421 effectively reversed the antigen-induced bronchoconstriction, plasma leak and cell recruitment with EC(50) values of 0.33 mg/kg p.o., 0.02 mg/kg i.p. and 3 mg/kg i.p., respectively.

Hyperresponsiveness to adenosine in sensitized Wistar rats over-expressing A1 receptor

Airway hyperreactivity is characterized by increased responsiveness to bronchoconstrictor stimuli and it is hallmark of asthma. Adenosine is an ubiquitous signaling nucleoside resulting from ATP catabolism, whose extracellular levels increase following cellular damage or stress. Adenosine plays a role in asthma; asthmatics, but not normal subjects, present bronchoconstriction following inhalation of adenosine or of its precursor, adenosine-5'-monophosphate, most likely via adenosine A(2B) receptor on mast cells. However, the mechanism underling the increased airway smooth muscle sensitivity to adenosine in asthmatics remains to be elucidated. Early experimental studies suggested the involvement of A(1) receptor; this hypothesis has been confirmed by more recent studies on guinea pigs and is corroborated by the finding of an increased adenosine A(1) expression on asthmatic bronchial tissues. Brown Norway rats, the strain usually used to assess asthma models, develop hyperresponsiveness to adenosine 3h following allergen challenge, but not 24h thereafter, without involvement of A(1) receptor. Here, we investigated the role of adenosine A(1) receptor in sensitized Wistar rats showing airway hyperresponsiveness 24h following allergen challenge. We found that on bronchi of sensitized Wistar rats challenged with allergen there is an increased adenosine A(1) receptor expression on smooth muscle that is responsible for hyperresponsiveness to adenosine and ovalbumin.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Mast cell adenosine receptors function: a focus on the a3 adenosine receptor and inflammation

Adenosine is a metabolite, which has long been implicated in a variety of inflammatory processes. Inhaled adenosine provokes bronchoconstriction in asthmatics or chronic obstructive pulmonary disease patients, but not in non-asthmatics. This hyper responsiveness to adenosine appears to be mediated by mast cell activation. These observations have marked the receptor that mediates the bronchoconstrictor effect of adenosine on mast cells (MCs), as an attractive drug candidate. Four subtypes (A1, A2a, A2b, and A3) of adenosine receptors have been cloned and shown to display distinct tissue distributions and functions. Animal models have firmly established the ultimate role of the A3 adenosine receptor (A3R) in mediating hyper responsiveness to adenosine in MCs, although the influence of the A2b adenosine receptor was confirmed as well. In contrast, studies of the A3R in humans have been controversial. In this review, we summarize data on the role of different adenosine receptors in mast cell regulation of inflammation and pathology, with a focus on the common and distinct functions of the A3R in rodent and human MCs. The relevance of mouse studies to the human is discussed.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.

Therapeutic potential of A2 and A3 adenosine receptor: a review of novel patented ligands

INTRODUCTION

Adenosine exerts its effects by interacting with G-protein coupled receptors (GPCR) namely A(1), A(2A), A(2B) and A(3), respectively. These are involved in several diseases, for example and most importantly, Parkinson's disease, ischemia and inflammation. There is high interest in the development of potent and selective ligands for these adenosine receptor (AR) subtypes, primarily for their therapeutic potential but also as pharmacological tools in receptor studies.

AREAS COVERED

This paper concentrates on reviewing the therapeutic potential of A(2) and A(3) ARs, which represent the most interesting subtypes of recent years. A general description of each receptor is reported with novel agonist and antagonist structures, patented in 2008 - 2011. PubMed and Free Patents Online databases were principally used to collect all the material.

EXPERT OPINION

In the past years, by modulating A(2) and A(3)ARs, several new possible therapeutic applications were discovered. For this reason, research concerning AR ligands is still of great interest. In particular, few potent and selective A(2B) agonists and antagonists are actually reported and a clear SAR (structure-activity relationship) profile lacks for this AR subtype. At the A(3)AR, allosteric modulation may prevent problems related to the high difference between rat and human orthosteric sites and simplify the preclinical studies on A(3)AR.

Reciprocal modulation of anti-IgE induced histamine release from human mast cells by A₁ and A(2B) adenosine receptors

BACKGROUND AND PURPOSE

Adenosine is believed to participate in the pathological development of asthma through a mast cell-dependent mechanism. Our study aimed to pharmacologically characterize the functions of adenosine receptor (AR) subtypes (A₁, A(2A) , A(2B) and A₃) in primary human cultured mast cells (HCMC).

EXPERIMENTAL APPROACH

HCMC were derived from progenitor stem cells in buffy coat and the effects of adenosine receptor ligands on basal and IgE-dependent histamine release were evaluated.

KEY RESULTS

Adenosine and analogues alone did not induce HCMC degranulation. When HCMC were activated by anti-IgE after 10 min pre-incubation with adenosine, a biphasic effect on histamine release was observed with enhancement of HCMC activation at low concentrations of adenosine (10⁻⁹-10⁻⁷ mol·L⁻¹) and inhibition at higher concentrations (10⁻⁶-10⁻⁴ mol·L⁻¹). The potentiating action was mimicked by A₁ AR agonists CCPA and 2'MeCCPA, and inhibited by the A₁ AR antagonist PSB36. In contrast, the inhibitory action of adenosine was mimicked by the non-specific A₂ AR agonist CV1808 and attenuated by A(2B) AR antagonists PSB1115 and MRS1760. The non-selective AR antagonist CGS15943 attenuated both the potentiating and inhibitory actions.

CONCLUSIONS AND IMPLICATIONS

We have defined for the first time the contribution of A₁ and A(2B) ARs, respectively, to the potentiating and inhibitory action of adenosine on human mast cell activation. With reference to the current trend of developing novel anti-asthmatic agents from AR ligands, our results suggest that inhibition of human mast cell activation would be a mechanism for A₁ AR antagonists, but not A(2B) AR antagonists.

Pyrrolo- and pyrazolo-[3,4-e][1,2,4]triazolo[1,5-c]pyrimidines as adenosine receptor antagonists

The discovery and development of adenosine receptor antagonists have represented for years an attractive field of research from the perspective of identifying new drugs for the treatment of widespread disorders such as inflammation, asthma and Parkinson's disease. The present work can be considered as an extension of our structure-activity relationship studies on the pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine (PTP) nucleus, extensively investigated by us as a useful template, in particular, for the identification of A(2A) and A(3) adenosine receptor antagonists. In order to explore the role of the nitrogen at the 7-position, we performed a new synthetic strategy for the preparation of pyrrolo[3,4-e][1,2,4]triazolo[1,5-c]pyrimidine derivatives which can be considered as 7-deaza analogues of the parent PTPs. We also synthesised a novel series of pyrazolo[3,4-e][1,2,4]triazolo[1,5-c]pyrimidines as junction isomers of the reference compounds. In both cases we obtained some examples of potent antagonists (K(i) in the low nanomolar range) with variable selectivity profiles in relation to the nature of substituents introduced at the C(5)-, N(8)- and/or N(9)-positions. The pyrrolo-triazolo-pyrimidine derivative 9b appeared to be a potent A(3) adenosine receptor antagonist (K(i)=10 nM) with good selectivity over hA(1) (74-fold) and hA(2A) (20-fold) adenosine receptors combined with low activity at the hA(2B) subtype (IC(50)=906 nM). Moreover, some examples of high-affinity A(1)/A(2A) dual antagonists have been identified in both series. This work constitutes a new and important contribution for the comprehension of the interaction between PTPs and adenosine receptors.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

Adenosine receptor targeting in health and disease

INTRODUCTION

The adenosine receptors A(1), A(2A), A(2B) and A(3) are important and ubiquitous mediators of cellular signaling that play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including increasing the oxygen supply:demand ratio, pre-conditioning, anti-inflammatory effects and the stimulation of angiogenesis.

AREAS COVERED

The state of the art of the role of adenosine receptors which have been proposed as targets for drug design and discovery, in health and disease, and an overview of the ligands for these receptors in clinical development.

EXPERT OPINION

Selective ligands of A(1), A(2A), A(2B) and A(3) adenosine receptors are likely to find applications in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The aim of this review is to provide an overview of the present knowledge regarding the role of these adenosine receptors in health and disease.

Oligonucleotides: a multi-targeted approach for the treatment of respiratory diseases

Reversing inflammatory lung disease remains the foremost challenge in treating respiratory diseases such as asthma and chronic obstructive pulmonary disease. Reducing (or modifying) the underlying inflammatory process with mono-target drugs has proven challenging. The era of designing ‘one target for one disease’ has evolved such that a growing body of evidence suggests a single drug that is capable of specifically targeting multiple targets and pathways would be better at arresting progression of these respiratory diseases and be an important advancement in current therapy. Oligonucleotide-based drugs represent an emerging class of drug candidates. Their properties, a broader range of targets over conventional small-molecule drugs and recent clinical proof-of-concept support their development as novel multi-targeting agents for the treatment of respiratory diseases.

Identifying and testing potential new anti-asthma agents

INTRODUCTION

Inhaled corticosteroids alone or with long-acting beta-2 agonists (LABA) are the basic treatment for stable asthma. While the majority of patients are controllable, some patients retain chronic severe disease and develop permanent alterations in airway function. For patients such as these it is important to better understand the mechanisms of asthma so that alternative approaches can be developed.

AREA COVERED

Based on data from in vitro cell culture, animal models and clinical trials, this review discusses potential agents targeting either key effector cells, mediators and their receptors in asthma pathogenesis or their signaling cascade molecules.

EXPERT OPINION

As targeting single Th2 cytokines and their receptors has been shown to have limited clinical benefit, it is important to identify and test potential new therapeutic agents. Recent studies suggest that blockade of IgE synthesis, its interaction with its receptors and downstream signaling, identification of molecular targets in innate immune and airways structural cells, and fresh anti-neutrophil strategies should be prominent among these. Further studies are required to clarify the relationship between airways remodeling and asthma severity so that appropriate patients may be targeted.

Adenosine receptors in health and disease

The adenosine receptors A(1), A(2A), A(2B), and A(3) are important and ubiquitous mediators of cellular signaling, which play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including an increase of oxygen supply/demand ratio, preconditioning, anti-inflammatory effects, and stimulation of angiogenesis. Considerable advances have been recently achieved in the pharmacological and molecular characterization of adenosine receptors, which have been proposed as targets for drug design and discovery. At the present time, it can be speculated that adenosine A(1), A(2A), A(2B), and A(3) receptor-selective ligands may show utility in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer, and other disorders in which inflammation is a feature. This chapter documents the present state of knowledge of adenosine receptors' role in health and disease.

Pharmacological characterization of adenosine receptors on isolated human bronchi

Adenosine induces airways obstruction in subjects with asthma, but the receptor subtype responsible remains unknown. The objectives of this study were to determine the pharmacological profile of adenosine receptor subtypes mediating contraction and to investigate the mechanism in normal and passively sensitized human airway tissues. Contraction of bronchial rings isolated from resected lung tissue of patients with lung carcinoma was measured in response to nonselective adenosine receptor agonists, 5-AMP and 5'-(N-Ethylcarboxamido)adenosine, and A(1) receptor agonist, N(6)-cyclopentyladenosine, in the absence and presence of selective adenosine receptor antagonists. Pharmacological antagonists, chemical ablation of airway sensory nerves using capsaicin, and passive sensitization of tissue with serum from subjects with atopy and asthma was used to investigate the mechanism of contraction. Human bronchial tissue contracted in a concentration-dependent manner to adenosine agonists that showed a rank order of activity of A(1) > A(2B) >> A2(A) = A3. The maximum contractile response to N(6)-cyclopentyladenosine (231.0 ± 23.8 mg) was significantly reduced in tissues chemically treated with capsaicin to desensitize sensory nerves (desensitized: 101.6 ± 15.2 mg; P < 0.05). Passive sensitization significantly augmented the contraction induced by adenosine A(1) receptor activation (sensitized: 389.7 ± 52.8 mg versus nonsensitized; P < 0.05), which was linked to the release of leukotrienes, and not histamine (MK571: 25.5 ± 1.7 mg; epinastine 260.0 ± 22.2 mg versus control; P < 0.05). This study provides evidence for a role for adenosine A(1) receptors in eliciting human airway smooth muscle constriction, which, in part, is mediated by the action of capsaicin sensitive sensory nerves.

Metabolomic profiling of asthma: diagnostic utility of urine nuclear magnetic resonance spectroscopy

BACKGROUND

The ability to diagnose and monitor asthma on the basis of noninvasive measurements of airway cellular dysfunction is difficult in the typical clinical setting.

OBJECTIVE

Metabolomics is the study of molecules created by cellular metabolic pathways. We hypothesized that the metabolic activity of children with asthma would differ from healthy children without asthma. Furthermore, children having an asthma exacerbation would be different compared with children with stable asthma in outpatient clinics. Finally, we hypothesized that (1)H-nuclear magnetic resonance (NMR) would measure such differences using urine samples, one of the least invasive forms of biofluid sampling.

METHODS

Children (135 total, ages 4-16 years) were enrolled, having met the criteria of healthy controls (C), stable asthma in the outpatient clinic (AO), or unstable asthma in the emergency department (AED). Partial least squares discriminant analysis was performed on the NMR data to create models of separation (70 metabolites were measured/urine sample). Some NMR data were withheld from modeling to be run blindly to determine possible diagnostic accuracy.

RESULTS

On the basis of the model of AO versus C, 31 of 33 AO samples were correctly diagnosed with asthma (94% accuracy). Only 1 of 20 C samples was incorrectly labeled as asthma (5% misclassification). On the basis of the AO versus AED model, 31 of the 33 AO samples were correctly diagnosed as outpatient asthma (94% accurate).

CONCLUSION

This is the first report suggesting that (1)H-NMR analysis of human urine samples has the potential to be a useful clinical tool for physicians treating asthma.

The role of adenosine receptor and caveolae-mediated endocytosis in oligonucleotide-mediated gene transfer

We previously reported the preparation and characterization of ternary nanoparticles with the negative surface charge, which comprises histidine-conjugated polyallylamine (PAA-HIS)/DNA core complex and a single-stranded oligonucleotide outer layer, to transfect various cell lines. As a continued effort, here the investigations on the endocytotic mechanisms involved in the uptake of the oligonucleotide-coated PAA-HIS/DNA complexes are reported. Interestingly, these complexes showed enhanced transfection efficiency only when deoxyadenosine-containing oligonucleotides were deposited on the PAA-HIS/DNA complex surface. The addition of uncomplexed oligonucleotide, free adenosine and adenosine receptor antagonist significantly inhibited the transfection efficiency of oligonucleotide-coated PAA-HIS/DNA complexes. These results indicated that the oligonucleotide-coated PAA-HIS/DNA complexes could specifically recognize adenosine receptors on the cell surface and were taken up by adenosine receptor-mediated process. Uptake and transfection experiments with various endocytic inhibitors suggested that, after receptor/ligand binding, oligonucleotide-coated PAA-HIS/DNA/complexes were mainly internalized via caveolae-mediated pathway to result in effective intracellular processing for gene expression. In conclusion, both adenosine receptor and caveolae-mediated endocytosis play important roles in oligonucleotide-mediated gene transfer.

Purinergic signaling in wound healing and airway remodeling

Airway epithelia are continuously damaged by airborne pollutants, pathogens and allergens, and they rely on intrinsic mechanisms to restore barrier integrity. Epithelial repair is a multi-step process including cell migration into the wounded area, proliferation, differentiation and matrix deposition. Each step requires the secretion of various molecules, including growth factors, integrins and matrix metalloproteinases. Evidence is emerging that purinergic signaling promotes repair in human airway epithelia. An injury induces ATP release, which binds P2Y(2) receptors (P2Y(2)Rs) to initiate protein kinase C (PKC)-dependent oxidative activation of TNFα-converting enzyme (TACE), which then releases the membrane-bound ligands of the epidermal growth factor receptor (EGFR). The P2Y(2)R- and EGFR-dependent signaling cascades converge to induce mediator release, whereas the latter also induces cytoskeletal rearrangement for cell migration and proliferation. Similar roles for purinergic signaling are reported in pulmonary endothelial cells, smooth muscle cells and fibroblasts. In chronic airway diseases, the aberrant regulation of extracellular purines is implicated in the development of airway remodeling by mucus cell metaplasia and hypersecretion, excess collagen deposition, fibrosis and neovascularization. This chapter describes the crosstalk between these signaling cascades and discusses the impact of deregulated purinergic signaling in chronic lung diseases.

Theobromine and the pharmacology of cocoa

The effects of theobromine in man are underresearched, possibly owing to the assumption that it is behaviourally inert. Toxicology research in animals may appear to provide alarming results, but these cannot be extrapolated to humans for a number of reasons. Domestic animals and animals used for racing competitions need to be guarded from chocolate and cocoa-containing foods, including foods containing cocoa husks. Research ought to include caffeine as a comparative agent, and underlying mechanisms need to be further explored. Of all constituents proposed to play a role in our liking for chocolate, caffeine is the most convincing, though a role for theobromine cannot be ruled out. Most other substances are unlikely to exude a psychopharmacological effect owing to extremely low concentrations or the inability to reach the blood-brain barrier, whilst chocolate craving and addiction need to be explained by means of a culturally determined ambivalence towards chocolate.

Purinergic regulation of airway inflammation

The immune and inflammatory responses initiated by the interaction of a pathogen with airway surfaces constitute vital mechanisms to eradicate an infection. Sentinel dendritic cells embedded in the mucosa migrate to the lymph nodes to induce immune responses, whereas epithelial cells release chemokines to recruit inflammatory cells engaged in the active destruction of the intruder. All immune and inflammatory cells are regulated by customized purinergic networks of receptors and ectonucleotidases. The general concept is that bacterial products induce ATP release, which activates P2 receptors to initiate an inflammatory response, and is terminated by the conversion of ATP into adenosine (ADO) to initiate P1 receptor-mediated negative feedback responses. However, this chapter exposes a far more complex purinergic regulation of critical functions, such as the differentiation of naive lymphocytes and the complex maturation and secretion of pro-cytokines (i.e. IL-1β) by the "inflammasome". This material also reconciles decades of research by exposing the specificity and plasticity of the signaling network expressed by each immune and inflammatory cell, which changes through cell differentiation and in response to infectious or inflammatory mediators. By the end of this chapter, the reader will have a new appreciation for this aspect of airway defenses, and several leads in terms of therapeutic applications for the treatment of chronic respiratory diseases.

Pyrimidine derivatives as potent and selective A3 adenosine receptor antagonists

Two regioisomeric series of diaryl 2- or 4-amidopyrimidines have been synthesized and their adenosine receptor affinities were determined in radioligand binding assays at the four human adenosine receptors (hARs). Some of the ligands prepared herein exhibit remarkable affinities (K(i) < 10 nm) and, most noticeably, the absence of activity at the A(1), A(2A), and A(2B) receptors. The structural determinants that support the affinity and selectivity profiles of the series were highlighted through an integrated computational approach, combining a 3D-QSAR model built on the second generation of GRid INdependent Descriptors (GRIND2) with a novel homology model of the hA(3) receptor. The robustness of the computational model was subsequently evaluated by the design of new derivatives exploring the alkyl substituent of the exocyclic amide group. The synthesis and evaluation of the novel compounds validated the predictive power of the model, exhibiting excellent agreement between predicted and experimental activities.

Conversion of Th17-type into Th2-type inflammation by acetyl salicylic acid via the adenosine and uric acid pathway in the lung

BACKGROUND

Allergen-specific T-cell responses orchestrate airway inflammation, which is a characteristic of asthma. Recent evidence suggests that noneosinophilic asthma can be developed by mixed Th1 and Th17 cell responses when exposed to lipopolysaccharide (LPS)-containing allergens.

OBJECTIVE

To evaluate the therapeutic or adverse effects of acetyl salicylic acid (ASA) on the expression of Th1-type and Th17-type inflammation induced by airway exposure to LPS-containing allergens.

METHODS

Th1 + Th17 asthma and Th2 asthma mouse models were generated by intranasal sensitization with ovalbumin (OVA) and LPS and intraperitoneal sensitization with OVA and alum, respectively. Therapeutic or adverse effects were evaluated after allergen challenge using pharmacologic and transgenic approaches.

RESULTS

Lung infiltration of eosinophils was enhanced in OVA/LPS-sensitized mice by ASA treatment, which was accompanied by the enhanced production of eotaxin. These changes were associated with the down-regulation of Th17 cell response, which was partly dependent on adenosine receptor A1 and A3 subtypes, but up-regulation of allergen-specific IL-13 production from T cells. Lung inflammation induced by LPS-containing allergen was markedly reduced in IL-13-deficient mice in the context of ASA treatment, but not without ASA. Meanwhile, adenosine levels in the lung were enhanced by ASA treatment. Moreover, lung infiltration of eosinophils induced by ASA treatment was reversed by co-treatment of a xanthine oxidase inhibitor (allopurinol).

CONCLUSION

These findings suggest that ASA changes Th17-type into Th2-type inflammation mainly via the adenosine and uric acid metabolic pathway in the lung.

Acute additive effect of montelukast and beclomethasone on AMP induced bronchoconstriction

Bronchial hyperresponsiveness to 5-adenosine mono-phosphate (AMP) is a marker of airway inflammation. Inhaled corticosteroids and antileukotrienes are used as anti-inflammatory drugs for the treatment of asthma. To find out if these two drugs exert their protection in an additive fashion, we compared the effects of acute treatment with inhaled beclomethasone (BDP) and montelukast (ML), alone or in combination, on methacholine and AMP induced bronchoconstriction. 15 asthmatic patients undertook methacholine and AMP challenges at baseline and after receiving ML or BDP, alone or in combination, in a randomized, double-blind, double-dummy placebo-controlled, crossover design. BDP pretreatment significantly increased the AMP PC(20) value (68.34+/-15.9mg/mL) as compared to placebo (22.87+/-5.7mg/mL). Combined treatment, BDP plus ML, afforded a further significant increase of AMP PC(20) (154.57+/-55.0mg/mL) as compared to each single treatment. The significant protection exerted by combined treatment as compared to each single active treatment was also demonstrated by the change of AMP PC(20) doubling dose as compared to placebo and each single active treatment. Our findings suggest that these two agents exert their acute additive protection against AMP induced bronchoconstriction acting on distinct inflammatory pathways and their combined use might provide greater protection against inflammatory response elicited by AMP than either drug alone.

Molecular mechanisms involved in adenosine-induced endothelial cell barrier enhancement

Extracellular adenosine is a physiologically relevant agonist released by various sources, including endothelial cells (EC) and activated platelets, with complex effects mediated via activation of P1 purinergic receptors. Adenosine-induced EC production of glutathione peroxidase1 and nitric oxide is recognized, and an anti-inflammatory mechanism has been described. Effects of extracellular adenosine on the pulmonary EC barrier function and vascular permeability, however, remain poorly characterized. In this study, we demonstrated the adenosine-induced rapid dose-dependent barrier enhancement in human pulmonary artery EC (HPAEC) as measured by an increase in transendothelial electrical resistance (TER). We have shown that HPAEC express only A2A and A2B adenosine receptors. Pharmacological and siRNA depletion studies indicate that A2A, but not A2B receptor activation is required for the adenosine-induced TER increase. Depletion of Galphas with a specific siRNA significantly attenuated the adenosine-induced TER response in HPAEC. In contrast, depletion of either Galphaq or Galphai2 did not affect the adenosine-induced TER increase. This suggests that the adenosine-induced TER increase is cAMP-dependent. The adenosine-induced barrier enhancement effects were associated with a rearrangement of the EC F-actin component of the cytoskeleton, enhanced cell-surface presentation of cell-cell junctional protein VE-cadherin and an involvement of Myosin-light-chain phosphatase (MLCP). Our results suggest, for the first time, that adenosine regulates the EC barrier function via A2A receptors followed by Galphas engagement and is associated with cytoskeletal activation.

Activation of mast cells by trimeric G protein Gi3; coupling to the A3 adenosine receptor directly and upon T cell contact

Mast cells are key players in mediating and amplifying allergic and inflammatory reactions. Previously, we identified the G-protein, Gi3, as the cellular target of receptor mimetic basic secretagogues that activate mast cell independently of IgE. In this study, we demonstrate that Gi3 is the cellular target of the adenosine A3 receptor (A3R), a G-protein coupled receptor involved in inflammation and the pathophysiology of asthma. By using a cell permeable peptide comprising the C-terminal end of Galphai3 fused to an importation sequence (ALL1) as a selective inhibitor of Gi3 signaling, we show that by coupling to Gi3, the A3R stimulates multiple signaling pathways in human mast cells, leading to upregulation of cytokines, chemokines, and growth factors. We further show that after contact with activated T cell membranes, endogenous adenosine binds to and activates the A3R, resulting in Gi3-mediated signaling. Specifically, the majority of ERK1/2 signaling initiated by contact with activated T cell membranes, is mediated by Gi3, giving rise to ALL1-inhibitable cellular responses. These results unveil the physiological G-protein coupled receptor that couples to Gi3 and establish the important role played by this G-protein in inflammatory conditions that involve adenosine-activated mast cells.

Recent findings on the pathogenesis of bronchial asthma

In the first part of this series of papers (Székely and Pataki, 102) the pathogenesis of asthma was approached as a pathological antigen-antibody complex induced vago-vagal axon reflex. In the next part (103) the contribution of individual hormonal predisposition, the environmental and the most frequent allergizing factors have been reviewed. In the first section of this last (third) part of the review the genetic factors contributing to the asthma are surveyed. In this field a great progress has been made during the last decade, a lot of genes have been pinpointed which contribute to the heredity of the disease. In the second section of this last paper on the etiology of asthma an attempt is made to summarize the previously reviewed data and some new ones. Actually a new hypothesis is proposed that beyond the multitude of genetic, environmental and hormonal factors the underlying biochemical mechanism is simple: the disequilibrium of two functionally opposing second messenger systems in the airways: the Ca i ++ liberating PLC-PKC cascade and the Ca i ++ level reducing cAMP mediated one with preponderance of the former.

Elevated cyclic AMP and PDE4 inhibition induce chemokine expression in human monocyte-derived macrophages

Macrophages are central mediators of the innate immune system that can be differentiated from monocytes upon exposure to cytokines. While increased cyclic adenosine monophosphate (cAMP) levels are known to inhibit many lipopolysaccharide-elicited macrophage inflammatory responses, the effects of elevated cAMP on monocyte/macrophage differentiation are not as well understood. We show here that during differentiation, cAMP agonists can cause a large increase in the mRNA and protein levels of several of the pro-inflammatory CXCL and CCL chemokines. The cAMP mediator-exchange protein activated by cAMP (Epac) contributes substantially to the increase in these chemokines. These chemokines are known to play an important role in the regulation of immune responses, particularly regarding the pathogenesis of asthma and chronic obstructive pulmonary disorder. We also found that a selective cAMP-degrading phosphodiesterase (PDE) 4 inhibitor can potentiate the chemokine expression elicited by low-dose forskolin or Prostaglandin E2 (PGE(2)). These data suggest that chemokine receptor antagonists administered in conjunction with a PDE4 inhibitor may improve both the efficacy and safety of PDE4-inhibitor therapy for chronic inflammatory disorders.

G protein-coupled receptor connectivity to NF-kappaB in inflammation and cancer

Complex intracellular network interactions regulate gene expression and cellular behavior. Whether at the site of inflammation or within a tumor, individual cells are exposed to a plethora of signals. The transcription factor nuclear factor-kappaB (NF-kappaB) regulates genes that control key cellular activities involved in inflammatory diseases and cancer. NF-kappaB is regulated by several distinct signaling pathways that may be activated individually or simultaneously. Multiple ligands and heterologous cell-cell interactions have an impact on NF-kappaB activity. The G protein-coupled receptor (GPCR) superfamily makes up the largest class of transmembrane receptors in the human genome and has multiple molecularly distinct natural ligands. GPCRs regulate proliferation, differentiation, and chemotaxis and play a major role in inflammatory diseases and cancer. Both GPCRs and NF-kappaB have been, and continue to be, major targets for drug discovery. A clear understanding of network interactions between GPCR signaling pathways and those that control NF-kB may be valuable for the development of better drugs and drug combinations.

Production of an agonist-like monoclonal antibody to the human A2A receptor of adenosine for clinical use

The second extracellular loop of the A(2A) receptor (A(2A)R) of adenosine was used to immunize mice for production of Adonis, an IgM monoclonal antibody. Adonis bound to the immunogen peptide and the native receptor in ELISA with K(D) values in 6.51-12.35 nM range. It recognized a linear epitope of 7 amino acids (LFEDVVP) at the C-terminal part of the external loop. Adonis revealed a 45-kDa band in lysate of human peripheral blood mononuclear cells in Western blotting in denaturing conditions. This served to monitor the up-regulation of the A(2A)R expression by caffeine. Adonis stimulated the cAMP production and inhibited the cell proliferation of an A(2A)R transfected stable cell line. These results confirm the immunogenicity and the functional relevance of the second extracellular loop of the A(2A)R. They suggest that Adonis may be of clinical use in various pathological situations to measure the regulation of the A(2A)R expression and to act as A(2A)R agonist drug.

The 2.6 angstrom crystal structure of a human A2A adenosine receptor bound to an antagonist

The adenosine class of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) mediates the important role of extracellular adenosine in many physiological processes and is antagonized by caffeine. We have determined the crystal structure of the human A2A adenosine receptor, in complex with a high-affinity subtype-selective antagonist, ZM241385, to 2.6 angstrom resolution. Four disulfide bridges in the extracellular domain, combined with a subtle repacking of the transmembrane helices relative to the adrenergic and rhodopsin receptor structures, define a pocket distinct from that of other structurally determined GPCRs. The arrangement allows for the binding of the antagonist in an extended conformation, perpendicular to the membrane plane. The binding site highlights an integral role for the extracellular loops, together with the helical core, in ligand recognition by this class of GPCRs and suggests a role for ZM241385 in restricting the movement of a tryptophan residue important in the activation mechanism of the class A receptors.

Treating lung inflammation with agonists of the adenosine A2A receptor: promises, problems and potential solutions

Adenosine A(2A) receptor agonists may be important regulators of inflammation. Such conclusions have come from studies demonstrating that, (i) adenosine A(2A) agonists exhibit anti-inflammatory properties in vitro and in vivo, (ii) selective A(2A) antagonists enhance inflammation in vivo and, (iii) knock outs of this receptor aggravate inflammation in a wide variety of in vivo models. Inflammation is a hallmark of asthma and COPD and adenosine has long been suggested to be involved in disease pathology. Two recent publications, however, suggested that an inhaled adenosine A(2A) receptor agonist (GW328267X) did not affect either the early and late asthmatic response or symptoms associated with allergic rhinitis suggesting that the rationale for treating inflammation with an adenosine A(2A) receptor agonist may be incorrect. A barrier to fully investigating the role of adenosine A(2A) receptor agonists as anti-inflammatory agents in the lung is the side effect profile due to systemic exposure, even with inhalation. Unless strategies can be evolved to limit the systemic exposure of inhaled adenosine A(2A) receptor agonists, the promise of treating lung inflammation with such agents may never be fully explored. Using strategies similar to that devised to improve the therapeutic index of inhaled corticosteroids, UK371,104 was identified as a selective agonist of the adenosine A(2A) receptor that has a lung focus of pharmacological activity following delivery to the lung in a pre clinical in vivo model of lung function. Lung-focussed agents such as UK371,104 may be suitable for assessing the anti-inflammatory potential of inhaled adenosine A(2A) receptor agonists.

A new chemical tool (C0036E08) supports the role of adenosine A(2B) receptors in mediating human mast cell activation

Asthma is a chronic inflammatory disease of the airways that involves many cell types, amongst which mast cells are known to be important. Adenosine, a potent bronchoconstricting agent, exerts its ability to modulate adenosine receptors of mast cells thereby potentiating derived mediator release, histamine being one of the first mediators to be released. The heterogeneity of sources of mast cells and the lack of highly potent ligands selective for the different adenosine receptor subtypes have been important hurdles in this area of research. In the present study we describe compound C0036E08, a novel ligand that has high affinity (pK(i) 8.46) for adenosine A(2B) receptors, being 9 times, 1412 times and 3090 times more selective for A(2B) receptors than for A(1), A(2A) and A(3) receptors, respectively. Compound C0036E08 showed antagonist activity at recombinant and native adenosine receptors, and it was able to fully block NECA-induced histamine release in freshly isolated mast cells from human bronchoalveolar fluid. C0036E08 has been shown to be a valuable tool for the identification of adenosine A(2B) receptors as the adenosine receptors responsible for the NECA-induced response in human mast cells. Considering the increasing interest of A(2B) receptors as a therapeutic target in asthma, this chemical tool might provide a base for the development of new anti-asthmatic drugs.