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. 2008 Sep 11;12(5):226. doi: 10.1186/cc6990

Bench-to-bedside review: Adenosine receptors – promising targets in acute lung injury?

Carsten P Schepp 1,2, Jörg Reutershan 1,✉
PMCID: PMC2592730  PMID: 18828873

Abstract

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life-threatening disorders that have substantial adverse effects on outcomes in critically ill patients. ALI/ARDS develops in response to pulmonary or extrapulmonary injury and is characterized by increased leakage from the pulmonary microvasculature and excessive infiltration of polymorphonuclear cells into the lung. Currently, no therapeutic strategies are available to control these fundamental pathophysiological processes in human ALI/ARDS. In a variety of animal models and experimental settings, the purine nucleoside adenosine has been demonstrated to regulate both endothelial barrier integrity and polymorphonuclear cell trafficking in the lung. Adenosine exerts its effects through four G-protein-coupled receptors (A1, A2A, A2B, and A3) that are expressed on leukocytes and nonhematopoietic cells, including endothelial and epithelial cells. Each type of adenosine receptor (AR) is characterized by a unique pharmacological and physiological profile. The development of selective AR agonists and antagonists, as well as the generation of gene-deficient mice, has contributed to a growing understanding of the cellular and molecular processes that are critically involved in the development of ALI/ARDS. Adenosine-dependent pathways are involved in both protective and proinflammatory effects, highlighting the need for a detailed characterization of the distinct pathways. This review summarizes current experimental observations on the role of adenosine signaling in the development of acute lung injury and illustrates that adenosine and ARs are promising targets that may be exploited in the development of innovative therapeutic strategies.

Introduction

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life-threatening syndromes that are characterized by severe hypoxemia, decreased lung compliance, and diffuse bilateral infiltrates without evidence of left atrial hypertension. ALI and ARDS develop during the course of direct lung injury such as pneumonia, acid aspiration, ischemia/reperfusion after lung transplantation, or direct traumatic damage. Alternatively, they may develop secondary to systemic inflammatory diseases such as sepsis, extrapulmonary trauma, transfusion, or cardiopulmonary resuscitation [1]. Despite recent advances in our understanding of the pathophysiology, therapeutic options are limited and focus on treating the underlying disease and preventing secondary lung damage by mechanical ventilation with low tidal volumes [2]. Nevertheless, mortality remains high at 40%, and the incidence of ALI/ARDS of approximately 80/100,000 person-years in the USA [3] underlines the importance of these entities to critical care medicine and public health in general.

The early phase of ALI/ARDS is characterized by an excessive inflammatory response that results in disruption of the endothelial barrier. As a consequence, a protein-rich lung edema develops and impairs pulmonary function [1]. The pulmonary endothelium is also critically involved in the recruitment and transmigration of polymorphonuclear cells (PMNs) into the lung [4]. PMNs are the leukocytes that predominantly mediate the initial phase of ALI. Numerous experimental and clinical observations have established a key role for PMNs in the pathogenesis of ALI in animals and patients. For instance, PMN depletion attenuates experimental lung damage, pulmonary function in neutropenic patients with lung injury can deteriorate as neutropenia resolves [5], and persistent pulmonary neutrophilia in ARDS is associated with poor outcomes [6].

Adenosine, an ancient endogenous molecule, has been shown to be a potent modulator of endothelial permeability [7], PMN migration, and PMN activation [8]. Adenosine signals through specific adenosine receptors (ARs) that are expressed on a variety of cells, including leukocytes and nonhematopoietic cells. Each of the four known ARs exhibits a distinct pharmacological and physiological profile [9]. A growing understanding of this multifaceted adenosine signaling complex has paved the way for new approaches to interfere with the inflammatory cascade during ALI. This review summarizes current experimental findings regarding the roles played by adenosine and ARs in pulmonary inflammation. Adenosine-dependent pathways in ALI are highlighted and the potential of ARs as therapeutic targets is critically discussed.

The physiological role of adenosine and adenosine receptors

Adenosine is a purine nucleoside with a short in vivo half-life of 1.5 seconds [10]. It is generated by intracellular hydrolysis from adenine nucleotides or S-adenosyl homocysteine. To exert its messenger function, adenosine is subsequently released into the extracellular space either by specific nucleoside transporters [11] or nonspecifically, upon damage to the cell membrane. In addition, adenine nucleotides can be extracellularly hydrolyzed to adenosine. Hydrolysis is mediated by the two enzymes ectoapyrase (CD39) and ecto-5'-nucleotidase (CD73; Figure 1) [12]. Adenosine is constitutively present in the extracellular space at a concentration of 1 μmol/l in resting tissues [13] and can increase up to 100-fold in response to oxidative stress or ischemia [14]. Its systemic bioavailabilty is limited because of its rapid reuptake, degradation by adenosine deaminase to inosine, or rephosphorylation by adenosine kinase.

Figure 1.

Figure 1

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Sources of extracellular adenosine. Intracellular hydrolysis of adenine nucleotides or S-adenosylhomocysteine (SAHC) yields adenosine that is released via specific nucleoside transporters (NT) or nonspecifically upon cell membrane damage. In the extracellular space, adenine nucleotides are hydrolyzed by ectoapyrase (CD39) and ecto-5'-nucleotidase (CD73). Adenosine binds to specific G-protein-coupled receptors, namely the adenosine receptors (AR), which initiate various cellular responses.

Apart from its central role in energy metabolism, adenosine-dependent pathways are also involved in a variety of other physiological and pathophysiological events. Acknowledgement of the potent negative dromotropic and coronary vasodilatory capacity has led to the use of adenosine to treat supraventricular tachycardias. In the peripheral and central nervous systems, adenosine is involved in a variety of processes, including cerebral blood flow, pain transmission, basal ganglia function, and seizure activity [15].

Adenosine exerts its effects through specific cell-surface receptors that are present on a wide variety of cells, including leukocytes and endothelial cells. Specific antagonism of adenosine-dependent effects by methylxanthines, caffeine, and theophylline originally led to the discovery of these receptors. Four subtypes of ARs have been described: A1, A2A, A2B, and A3. All ARs are members of the superfamily of G-protein-coupled receptors [9]. Each AR exhibits a distinct pattern of tissue distribution, intracellular signaling, and pharmacokinetic characteristics (Table 1). Each receptor subtype shows high interspecies homology among mammals [9]. The availability of selective agonists and antagonists [16,17] and gene-deficient mice for each receptor has led to a growing understanding of the biologic functions of ARs [18].

Table 1.

Relevant functional parameters of adenosine receptors

A1 A2A A2B A3
Coupled G proteins Gi, Go Gs, Golf, G15/16 Gs, Gq Gi, Gq
Intracellular messengers ↓ cAMP ↑ cAMP ↑ cAMP ↓ cAMP
↑ IP3/DAG ↑ IP3 ↑ IP3/DAG ↑ IP3/DAG
↑ Arachidonate
↑ PEtOH
Selected biologic functions Bradycardia Antinociception Ischemic preconditioning ↓ Lipolysis, ↓ Glomerular filtration ↓ Platelet aggregation Vasodilation Protection against ischemic damage Relaxation of vascular smooth muscle cells Stimulation of mast cell mediator release ↑ Mediator release from mast cells (mice) Preconditioning
Potencya (EC50 [μmol/l]) 0.31 0.73 23.5 0.29
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aPotency as reflected by intracellular cAMP levels. Adapted from [8,9,18,55]. DAG, diacylglycerol; ED50, 50% effective dose; IP3, inositoltriphosphate; PetOH, phosphatidylethanol.

Role of adenosine in inflammation

Acknowledgment of adenosine as a potential mediator of inflammatory responses has emerged from experimental studies that showed that adenosine exhibited protective effects in various models of ischemia/reperfusion (I/R) injury [17]. I/R injury is characterized by excessive invasion of activated PMNs into tissues in which blood flow has been restored after a period of ischemia. Clinical homologs of I/R include reperfusion after organ transplantation or myocardial infarction. Presence and extent of I/R injury substantially affect the outcome and the success of therapeutic strategies that aim to re-establish blood flow in ischemic organs. Pharmacological blockade of PMN infiltration into reperfused tissues mitigates the extent of I/R injury [19]. Both PMN activation and trafficking are critically regulated by adenosine. This includes the generation of superoxide, adhesion and migration, as well as secretion of cytokines and growth factors [20]. PMNs express all four ARs [8], enabling these central cells of the innate immune system to integrate various adenosine-dependent signaling pathways.

A canine model of myocardial I/R injury exemplifies the broad immunoregulatory capacity of the adenosine-dependent signaling complex [21]. Treatment with an A2A agonist reduced PMN accumulation in the postischemic tissue and reduced infarct size. The same agonist attenuated superoxide generation and adherence to endothelial cells. In addition to PMNs, platelets, macrophages, mast cells, and T cells contribute to adenosine-dependent effects, although their impact on pulmonary inflammatory responses is less obvious [17].

Adenosine in pulmonary inflammation

Involvement of adenosine in PMN-dependent inflammatory responses has evoked growing interest in studying the role played by adenosine and AR in ALI. Adenosine is constitutively released into the distal airways of C57Bl6 mice [22]. In addition, pulmonary expression of all four ARs has been demonstrated in the lungs of mice [23] and humans [24].

Chronically elevated levels of adenosine may occur as a result of defective metabolism. Gene-deficient mice that fail to express adenosine deaminase (ADA-/- mice) succumb to severe pulmonary inflammation at the age of 3 weeks. This inflammation is characterized by an infiltration with eosinophils [25]. In addition, elevated adenosine levels induce pulmonary fibrosis in these mice [26], which is a typical complication in the delayed phase of ARDS. The phenotype observed with ADA-/- mice provides compelling evidence that activation of the adenosine signaling complex aggravates chronic airway inflammatory diseases such as asthma and chronic obstructive pulmonary disease [27,28]. In contrast, protective effects of adenosine have been demonstrated by others [29], emphasizing the complexity of adenosine signaling. Although adenosine has been identified as a pivotal mediator in chronic pulmonary inflammation, its effects in ALI have only recently become the focus of experimental research.

Numerous studies have established a critical role for adenosine in the regulation of pulmonary microvascular permeability, a process that is profoundly disturbed in ALI. The majority of ALI patients show impaired alveolar fluid clearance, with a low maximal clearance being a predictor for decreased mortality [30]. Alveolar fluid clearance is reduced by activation of murine A1, but it is increased by A2A and A3 activation [22]. In a model of lipopolysaccharide (LPS)-induced pulmonary inflammation, administration of 2-chloroadenosine, an unselective AR agonist, leads to markedly decreased fluid accumulation in the lung of guinea pigs [31]. In pigs, administration of adenosine partly prevented the LPS-induced increase in microvascular permeability, as assessed by extravascular lung water [32]. Protective effects on pulmonary microvascular leakage have also been demonstrated in the model of fat emulsion induced ALI [33], implicating adenosine as a key molecule in the regulation of endothelial integrity and pulmonary fluid balance.

Mechanical ventilation, which is required to achieve adequate gas exchange in ARDS patients, can promote lung inflammation, especially when high tidal volumes are applied (ventilator-induced lung injury [VILI]) [2]. However, the underlying mechanisms are unclear. Mechanical ventilation of rats leads to increased levels of AMP and adenosine in bronchoalveolar lavage (BAL) fluid. This increase can be attenuated by lung-protective ventilation with low tidal volumes and application of positive end-expiratory pressure [34]. A similar increase in adenosine levels in BAL fluid has been demonstrated in mechanically ventilated mice. This increase is paralleled by induction of the rate-limiting enzymes for extracellular adenosine generation CD39 and CD73 on pulmonary endothelium and epithelium. Treatment of these mice with an A2B antagonist increased albumin leakage into the BAL fluid and decreased survival. ALI was enhanced in CD39-/- and CD73-/- mice, as indicated by impaired oxygenation and increased protein content of the BAL fluid, increased PMN infiltration, and increased lung water content [35]. These findings suggest that upregulation of adenosine production in response to mechanical ventilation mediates protection from ALI. Interestingly, pulmonary PMN infiltration after hypoxia, a potent inflammatory stimulus in the lung, was also increased in CD39-/- and CD73-/- mice [36].

In humans, the role played by adenosine in acute pulmonary inflammation has not been studied systematically. Cultured human bronchial epithelial cells secrete adenosine when they are subjected to repeated mechanical stretching – a model that mimics cyclic opening and closing of distal airway structures during mechanical ventilation [35]. Shear stress on endothelial cells is also accompanied by a rise in extracellular ATP, soluble ATPase, and 5'-nucleotidase activity [37]. In addition, secreted adenosine reduces paracellular endothelial permeability in vitro [35], suggesting that adenosine serves as a physiological feedback mediator to protect the lung against mechanical stress. Whether insufficient production of adenosine promotes VILI in some patients has not been studied, but this might be one pathway that underlies the protective effects of ventilation with low tidal volumes.

One ligand – four receptors

A1 – bad cop?

The AR subtype A1 is expressed at low levels in the lung, whereas higher levels are found in the central nervous system and various other peripheral tissues [24]. In ADA-/- mice A1 expression was identified on endothelial cells, airway epithelial cells and alveolar epithelial cells, and it was most pronounced on alveolar macrophages [29]. Activation of A1 results in decreased adenylate cyclase activity via pertussistoxin sensitive Gi and Go proteins. A1 also signals through phospholipase C via Gβγ subunits, pertussistoxin sensitive K+ channels, and KATP channels [9,18,38]. Aside from its numerous effects on the central nervous system, A1 signaling plays a pivotal role in manifesting I/R injury in various tissues [38,39].

Functional blocking of A1 with a selective antagonist reduced I/R injury in the feline lung [40]. The same antagonist was examined in endotoxin-induced lung injury in cats. In that study, intralobar arterial infusion of endotoxin caused dose-dependent pulmonary infiltration with PMNs and alveolar edema. Application of the A1 antagonist before and after the maximum dose of endotoxin abolished alveolar injury almost completely [41]. The significance of A1 signaling in pulmonary fluid homeostasis has been demonstrated in a variety of other experimental settings. In mice, activation of A1 but not of A2A and A3 reduced alveolar fluid clearance [22] and may have led to alveolar fluid accumulation. Consistent with these data, pretreatment with an A1 antagonist blocked the increase in pulmonary capillary filtration in a canine model of ALI [42]. However, others have found contrary results. In rabbits, A1 activation reduced LPS-induced pulmonary edema formation. A2 activation had a similar effect but it also reduced pulmonary vasoconstriction [43].

The predominant proinflammatory role of A1, as shown in different models of ALI, is counter to its protective function in chronically inflamed airways. Deletion of A1 in ADA-/- mice resulted in markedly enhanced pulmonary inflammation, suggesting an anti-inflammatory role of A1 in chronically inflamed airways [29].

In humans, the impact of A1 signaling in ALI is not known. Interestingly, LPS has been shown to be a ligand for A1 on human pulmonary endothelial cells. LPS displaces a selective A1 antagonist from the receptor in a dose-dependent and competitive manner. Binding of LPS to A1 leads to increased secretion of interleukin-6 and thromboxane A2, which can be significantly reduced by administration of a selective A1 antagonist [44]. The influence of A1 signaling on pulmonary fluid homeostasis that has been observed in a variety of animal models remains to be elucidated in humans.

A2A – good cop?

A2A consists of 410 amino acids with an extended carboxyl-terminus, making A2A the largest of all ARs. The highest expression of A2Ais found in the central nervous system, spleen, thymus, leukocytes, and platelets, whereas intermediate levels are found in the lung [9,18,38]. In macrophages, A2A expression is increased more than 100-fold upon LPS exposure [45]. Activation of A2A results in an increased adenylate cyclase activity by signaling via cholera-toxin sensitive Gs proteins. A2A coupling to Golf and G15/16 has also been demonstrated, leading to increased adenylate cyclase activity and inositol triphosphate increase, respectively. Main cellular responses include coronary vasodilation and inhibition of platelet aggregation [9,18,38]. A2A activation suppresses activation of immune cells, including T cells, monocytes, macrophages, PMNs, and dendritic cells [46]. Investigations in A2A-/- mice have revealed its nonredundant role in downregulating acute inflammatory processes [47]. Activation of A2A mediates protection from I/R injury in various tissues, including lung [48].

In a model of hemorrhage-induced ALI, pretreatment with an A2A agonist attenuated increases in both PMN sequestration into the lung and microvascular permeability. Also clinically relevant is the fact that treatment after induction of ALI had similar effects, albeit to a lesser extent [49]. A critical role in maintaining pulmonary fluid homeostasis has been established in numerous experimental settings. Alveolar fluid clearance in mice was increased upon A2A activation [22], and in a canine model of ALI pretreatment with an A2A agonist or adenosine reduced capillary filtration induced by phorbol myristate acetate [42]. In accordance with these findings, treatment with an A2 agonist completely abolished lung edema formation after LPS administration in the rabbit lung [43]. In a murine model of combined pneumonia and hyperoxia induced ALI, organ damage was attenuated and survival was prolonged after treatment with an A2A agonist [50].

In a study that provided evidence of a profound anti-inflammatory effect of an A2A antagonist in allergic lung inflammation [51], the authors included experiments with LPS-induced lung inflammation. Treatment with the antagonist did not have an effect on BAL fluid counts of PMNs, macrophages, or lymphocytes. Furthermore, levels of C-X-C chemokine ligand (CXCL)1 and CXCL2/3 were not significantly altered, but there was a trend toward lower levels after treatment. Levels of elastase in BAL fluid (a marker of PMN activation) were significantly reduced. Wild-type mice challenged with nebulized LPS exhibited significant accumulation of PMNs in all three compartments of the lung (pulmonary microvasculature, interstitium, and alveolar space). Pretreatment with a specific A2A agonist reduced transepithelial migration of PMNs into the BAL fluid. This effect was abrogated in A2A-/- mice and in chimeric mice that expressed A2A only on non-hematopoietic cells, suggesting that A2A activation on hematopoietic cells mediates protection from pulmonary inflammation. In addition, the same A2A agonist reduced LPS-induced microvascular permeability and secretion of interleukin-6, tumor necrosis factor-α, CXCL1, and CXCL2/3 into the BAL fluid [52]. This highlights a predominant role for A2A in mediating lung protection in various models of pulmonary inflammation.

At present, there are no data available supporting a role for A2A in human ALI. However, the fact that specific A2A agonists are currently being tested in clinical trials for other diseases makes this receptor an attractive target for therapeutic advancement in human ALI/ARDS. As with other immunomodulatory strategies, possible adverse effects that are associated with the biologic function of A2A, such as inhibition of platelet aggregation or vasodilation, require attention. The development of agents that allow topical application to the lung via aerosol may be one concept to overcome these adverse effects.

A2B – sometimes good, sometimes bad?

A2B exhibits a broad expression pattern, including central nervous system, intestine, and lung. High levels of expression are found in the vasculature and on macrophages [53]. Its affinity for adenosine is the lowest of all four ARs [54,55]. In contrast to the other ARs, A2B activation requires adenosine concentrations that are not reached under physiological conditions. A2B is coupled to both Gs and Gq proteins, activating adenylate cyclase and phospholipase C, respectively. Consistent with an endothelial expression of A2B, its activation induces vasodilation [9,18,38].

In A2B gene deficient mice, baseline plasma levels of pro-inflammatory cytokines were greater than in wild-type mice. Upon LPS stimulation, the inflammatory response in A2B-/- mice was substantially enhanced when compared with wild-type mice, implicating A2B as a negative regulator of relevant cytokines. In addition, leukocytes from A2B-/- mice exhibited increased adherence to the vessel wall, suggesting an important role for A2B in inhibiting leukocyte trafficking to inflamed tissue [53].

A2B is critical for the protective effects of adenosine in murine VILI. Treatment of mechanically ventilated mice with an A2B antagonist increased pulmonary microvascular permeability and decreased survival [35]. A2B was upregulated in response to a 'lung-destructive' ventilation strategy in rats, whereas its expression level remained unchanged with lung-protective ventilation [34]. Together with elevated adenosine levels during mechanical ventilation, A2B appears to provide endogenous protection from VILI that might be overwhelmed by lung-hostile ventilation.

In contrast to these findings, treatment of ADA-/- mice with A2B antagonists led to attenuation in pulmonary inflammation [56]. In addition, the authors were able to demonstrate similar proinflammatory properties of A2B activation in murine bleomycin-induced lung injury [56]. These findings underline the complexity of adenosine signaling. A2B – with its low affinity – is certainly a key player in the differential response to varying extracellular adenosine levels, but information on A2B function in human pulmonary disease is lacking.

A3 – good and bad?

A3 is ubiquitously expressed. The highest levels of human A3 are detected in lung and liver. Signaling requires pertussistoxin sensitive Gi protein, leading to a reduction in adenylate cyclase activity. Activation of A3 also leads to stimulation of phospholipase C via Gq proteins [9,18,38].

The biologic functions of A3 in inflammatory disease are ambiguous. In I/R injury there is evidence for both protective [57,58] and exacerbating effects [59]. Pulmonary inflammation was attenuated by pharmacological blockade or genetic removal of A3, suggesting a proinflammatory effect of A3. Eosinophils and, although less pronounced, all other leukocytes – including PMNs – were reduced in the BAL fluid after treatment with an A3 antagonist [60].

PMN chemotaxis critically depends on A3 signaling. PMNs that migrate toward an fMLP (N-formyl-Met-Leu-Phe) gradient exhibit an accumulation of A3 molecules on the leading edge, the location where they bind adenosine generated through extracellular hydrolysis of ATP [61]. Because PMN migration is pivotal in ALI, A3 appears to be an interesting target, but current knowledge on A3 function in ALI is limited. A recent study investigated the role of A3 in a murine model of sepsis [62]. In that study A3-/- mice exhibited reduced pulmonary damage, and PMN migration into the lung was decreased. Leukocytes of these mice also failed to accumulate adequately in the intra-abdominal compartment, but survival of A3-/- mice was prolonged. The authors attributed prolonged survival to the reduced secondary lung damage that had been observed in A3-/- mice [62]. Data on A3 function in human pulmonary disease are not available. However, degranulation of human PMNs has been demonstrated to be under the control of A3 [63,64], which suggests that A3 might well be involved in pulmonary inflammation.

Clinical implications

Selective agonist and antagonists for all four ARs are available [39]. In addition, gene-deficient mice provide an excellent tool for further elucidation of adenosine-mediated immunomodulation and validation of therapeutic approaches to ALI. However, major limitations should be considered before attempting to recapitulate experimental findings in humans. Numerous animal models have been established to study the pathophysiology of acute pulmonary inflammation.

These models mimic several aspects of the human disease, including neutrophil infiltration, microvascular permeability, or release of chemotactic cytokines. However, clinical parameters such as pulmonary gas exchange and radiological alterations are not monitored in most experimental studies. In addition, significant interspecies differences in AR amino acid sequences may be of functional significance. For example, activation of human A2B induces mast cell activation [65], whereas in mice this effect is mediated by A3 [66].

The expression pattern of AR on different cell types has not been investigated systematically. PMNs appear to be the predominant cell type that contributes to AR-dependent pulmonary inflammation. In addition, pulmonary endothelial cells are involved [52] and may be an attractive target for pharmacologic compounds that are administered topically (for instance, by inhalation). The relevance of other AR-expressing cells such as mast cells remains to be demonstrated.

Adenosine has profound influences not only on the immune system but also on hemostasis, hemodynamics, and the central nervous system. Thus, interference with this signaling complex may be associated with serious adverse effects. In ALI topical application of an aerolized agent with low systemic bioavailability might be a promising strategy.

Some agents that interfere with adenosine signaling are currently at various stages of clinical trials. Although these trials are aimed at diseases other than ALI – such as heart failure, hypertension, renal failure, Parkinson's disease, pain, and asthma [67] – they will probably provide a first insight into the anti-inflammatory capacity and adverse effects of these agents in the clinical setting.

Conclusion

Adenosine and ARs are involved in various immunoregulatory processes, including pulmonary inflammation (Figure 2). ARs exert both proinflammatory and anti-inflammatory effects, depending on the type of receptor and the experimental setting. Profound protective effects of adenosine have been shown in systems representative of VILI and other models of primary and secondary ALI. A2A and, to a lesser extent, A2B appear to be major mediators of protective effects during ALI. The marked interference of adenosine signaling in pulmonary fluid homeostasis and PMN trafficking supports a key role in the pathophysiologic cascade of ALI. The availability of pharmacologic agents that selectively target ARs encourages further experimental and clinical investigation. Detailed characterization of adenosine-dependent pathways in ALI will help to develop innovative strategies for the treatment of ALI/ARDS.

Figure 2.

Figure 2

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AR-dependent pathways involved in PMN trafficking and fluid homeostasis during acute pulmonary inflammation. Presented is a three compartment model (capillary, interstitium, and alveolar space). Red indicates inhibitory effects, and green indicates stimulatory effects. AR, adenosine receptor; PMN, polymorphonuclear cell.

Abbreviations

ADA: adenosine deaminase; ALI: acute lung injury; AR: adenosine receptor; ARDS: acute respiratory distress syndrome; BAL: bronchoalveolar lavage; CXCL: C-X-C chemokine ligand; I/R: ischemia/reperfusion; LPS: lipopolysaccharide; PMN: polymorphonuclear cell; VILI: ventilator-induced lung injury.

Competing interests

The authors declare that they have no competing interests.

References

  1. Ware LB, Matthay MA. The acute respiratory distress syndrome. N Engl J Med. 2000;342:1334–1349. doi: 10.1056/NEJM200005043421806. [DOI] [PubMed] [Google Scholar]
  2. The Acute Respiratory Distress Syndrome Network Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342:1301–1308. doi: 10.1056/NEJM200005043421801. [DOI] [PubMed] [Google Scholar]
  3. Rubenfeld GD, Caldwell E, Peabody E, Weaver J, Martin DP, Neff M, Stern EJ, Hudson LD. Incidence and outcomes of acute lung injury. N Engl J Med. 2005;353:1685–1693. doi: 10.1056/NEJMoa050333. [DOI] [PubMed] [Google Scholar]
  4. Maniatis NA, Orfanos SE. The endothelium in acute lung injury/acute respiratory distress syndrome. Curr Opin Crit Care. 2008;14:22–30. doi: 10.1097/MCC.0b013e3282f269b9. [DOI] [PubMed] [Google Scholar]
  5. Abraham E. Neutrophils and acute lung injury. Crit Care Med. 2003;31:S195–S199. doi: 10.1097/01.CCM.0000057843.47705.E8. [DOI] [PubMed] [Google Scholar]
  6. Baughman RP, Gunther KL, Rashkin MC, Keeton DA, Pattishall EN. Changes in the inflammatory response of the lung during acute respiratory distress syndrome: prognostic indicators. Am J Respir Crit Care Med. 1996;154:76–81. doi: 10.1164/ajrccm.154.1.8680703. [DOI] [PubMed] [Google Scholar]
  7. Haselton FR, Alexander JS, Mueller SN. Adenosine decreases permeability of in vitro endothelial monolayers. J Appl Physiol. 1993;74:1581–1590. doi: 10.1152/jappl.1993.74.4.1581. [DOI] [PubMed] [Google Scholar]
  8. Hasko G, Cronstein BN. Adenosine: an endogenous regulator of innate immunity. Trends Immunol. 2004;25:33–39. doi: 10.1016/j.it.2003.11.003. [DOI] [PubMed] [Google Scholar]
  9. Fredholm BB, AP IJ, Jacobson KA, Klotz KN, Linden J. International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors. Pharmacol Rev. 2001;53:527–552. [PMC free article] [PubMed] [Google Scholar]
  10. Moser GH, Schrader J, Deussen A. Turnover of adenosine in plasma of human and dog blood. Am J Physiol. 1989;256:C799–C806. doi: 10.1152/ajpcell.1989.256.4.C799. [DOI] [PubMed] [Google Scholar]
  11. Zhang J, Visser F, King KM, Baldwin SA, Young JD, Cass CE. The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs. Cancer Metastasis Rev. 2007;26:85–110. doi: 10.1007/s10555-007-9044-4. [DOI] [PubMed] [Google Scholar]
  12. Zimmermann H. Extracellular metabolism of ATP and other nucleotides. Naunyn Schmiedebergs Arch Pharmacol. 2000;362:299–309. doi: 10.1007/s002100000309. [DOI] [PubMed] [Google Scholar]
  13. Martin C, Leone M, Viviand X, Ayem ML, Guieu R. High adenosine plasma concentration as a prognostic index for outcome in patients with septic shock. Crit Care Med. 2000;28:3198–3202. doi: 10.1097/00003246-200009000-00014. [DOI] [PubMed] [Google Scholar]
  14. Latini S, Bordoni F, Pedata F, Corradetti R. Extracellular adenosine concentrations during in vitro ischaemia in rat hippocampal slices. Br J Pharmacol. 1999;127:729–739. doi: 10.1038/sj.bjp.0702591. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Benarroch EE. Adenosine and its receptors: multiple modulatory functions and potential therapeutic targets for neurologic disease. Neurology. 2008;70:231–236. doi: 10.1212/01.wnl.0000297939.18236.ec. [DOI] [PubMed] [Google Scholar]
  16. Baraldi PG, Tabrizi MA, Gessi S, Borea PA. Adenosine receptor antagonists: translating medicinal chemistry and pharmacology into clinical utility. Chem Rev. 2008;108:238–263. doi: 10.1021/cr0682195. [DOI] [PubMed] [Google Scholar]
  17. Linden J. Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection. Annu Rev Pharmacol Toxicol. 2001;41:775–787. doi: 10.1146/annurev.pharmtox.41.1.775. [DOI] [PubMed] [Google Scholar]
  18. Fredholm BB. Adenosine, an endogenous distress signal, modulates tissue damage and repair. Cell Death Differ. 2007;14:1315–1323. doi: 10.1038/sj.cdd.4402132. [DOI] [PubMed] [Google Scholar]
  19. Singbartl K, Ley K. Protection from ischemia-reperfusion induced severe acute renal failure by blocking E-selectin. Crit Care Med. 2000;28:2507–2514. doi: 10.1097/00003246-200007000-00053. [DOI] [PubMed] [Google Scholar]
  20. Cronstein BN. Adenosine, an endogenous anti-inflammatory agent. J Appl Physiol. 1994;76:5–13. doi: 10.1152/jappl.1994.76.1.5. [DOI] [PubMed] [Google Scholar]
  21. Jordan JE, Zhao ZQ, Sato H, Taft S, Vinten-Johansen J. Adenosine A2 receptor activation attenuates reperfusion injury by inhibiting neutrophil accumulation, superoxide generation and coronary endothelial adherence. J Pharmacol Exp Ther. 1997;280:301–309. [PubMed] [Google Scholar]
  22. Factor P, Mutlu GM, Chen L, Mohameed J, Akhmedov AT, Meng FJ, Jilling T, Lewis ER, Johnson MD, Xu A, Kass D, Martino JM, Bellmeyer A, Albazi JS, Emala C, Lee HT, Dobbs LG, Matalon S. Adenosine regulation of alveolar fluid clearance. Proc Natl Acad Sci USA. 2007;104:4083–4088. doi: 10.1073/pnas.0601117104. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Fan M, Qin W, Mustafa SJ. Characterization of adenosine receptor(s) involved in adenosine-induced bronchoconstriction in an allergic mouse model. Am J Physiol Lung Cell Mol Physiol. 2003;284:L1012–L1019. doi: 10.1152/ajplung.00353.2002. [DOI] [PubMed] [Google Scholar]
  24. Salvatore CA, Jacobson MA, Taylor HE, Linden J, Johnson RG. Molecular cloning and characterization of the human A3 adenosine receptor. Proc Natl Acad Sci USA. 1993;90:10365–10369. doi: 10.1073/pnas.90.21.10365. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Blackburn MR, Volmer JB, Thrasher JL, Zhong H, Crosby JR, Lee JJ, Kellems RE. Metabolic consequences of adenosine deaminase deficiency in mice are associated with defects in alveogenesis, pulmonary inflammation, and airway obstruction. J Exp Med. 2000;192:159–170. doi: 10.1084/jem.192.2.159. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Chunn JL, Young HW, Banerjee SK, Colasurdo GN, Blackburn MR. Adenosine-dependent airway inflammation and hyperresponsiveness in partially adenosine deaminase-deficient mice. J Immunol. 2001;167:4676–4685. doi: 10.4049/jimmunol.167.8.4676. [DOI] [PubMed] [Google Scholar]
  27. Cushley MJ, Tattersfield AE, Holgate ST. Inhaled adenosine and guanosine on airway resistance in normal and asthmatic subjects. Br J Clin Pharmacol. 1983;15:161–165. doi: 10.1111/j.1365-2125.1983.tb01481.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Blackburn MR, Lee CG, Young HW, Zhu Z, Chunn JL, Kang MJ, Banerjee SK, Elias JA. Adenosine mediates IL-13-induced inflammation and remodeling in the lung and interacts in an IL-13-adenosine amplification pathway. J Clin Invest. 2003;112:332–344. doi: 10.1172/JCI16815. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Sun CX, Young HW, Molina JG, Volmer JB, Schnermann J, Blackburn MR. A protective role for the A1 adenosine receptor in adenosine-dependent pulmonary injury. J Clin Invest. 2005;115:35–43. doi: 10.1172/JCI22656. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Ware LB, Matthay MA. Alveolar fluid clearance is impaired in the majority of patients with acute lung injury and the acute respiratory distress syndrome. Am J Respir Crit Care Med. 2001;163:1376–1383. doi: 10.1164/ajrccm.163.6.2004035. [DOI] [PubMed] [Google Scholar]
  31. Sakamaki F, Ishizaka A, Urano T, Sayama K, Nakamura H, Terashima T, Waki Y, Soejima K, Tasaka S, Sawafuji M, Kobayashi K, Yamaguchi K, Kanazawa M. Attenuation by intravenous 2-chloroadenosine of acute lung injury induced by live Escherichia coli or latex particles added to endotoxin in the neutropenic state. J Lab Clin Med. 2003;142:128–135. doi: 10.1016/S0022-2143(03)00105-7. [DOI] [PubMed] [Google Scholar]
  32. Kutzsche S, Lyberg T, Bjertnaes LJ. Effects of adenosine on extravascular lung water content in endotoxemic pigs. Crit Care Med. 2001;29:2371–3276. doi: 10.1097/00003246-200112000-00021. [DOI] [PubMed] [Google Scholar]
  33. Jolin A, Myklebust R, Olsen R, Bjertnaes LJ. Adenosine protects ultrastructure of isolated rat lungs against fat emulsion injury. Acta Anaesthesiol Scand. 1994;38:75–81. doi: 10.1111/j.1399-6576.1994.tb03841.x. [DOI] [PubMed] [Google Scholar]
  34. Douillet CD, Robinson WP, 3rd, Zarzaur BL, Lazarowski ER, Boucher RC, Rich PB. Mechanical ventilation alters airway nucleotides and purinoceptors in lung and extrapulmonary organs. Am J Respir Cell Mol Biol. 2005;32:52–58. doi: 10.1165/rcmb.2004-0177OC. [DOI] [PubMed] [Google Scholar]
  35. Eckle T, Fullbier L, Wehrmann M, Khoury J, Mittelbronn M, Ibla J, Rosenberger P, Eltzschig HK. Identification of ectonucleotidases CD39 and CD73 in innate protection during acute lung injury. J Immunol. 2007;178:8127–8137. doi: 10.4049/jimmunol.178.12.8127. [DOI] [PubMed] [Google Scholar]
  36. Eltzschig HK, Thompson LF, Karhausen J, Cotta RJ, Ibla JC, Robson SC, Colgan SP. Endogenous adenosine produced during hypoxia attenuates neutrophil accumulation: coordination by extracellular nucleotide metabolism. Blood. 2004;104:3986–3992. doi: 10.1182/blood-2004-06-2066. [DOI] [PubMed] [Google Scholar]
  37. Yegutkin G, Bodin P, Burnstock G. Effect of shear stress on the release of soluble ectoenzymes ATPase and 5'-nucleotidase along with endogenous ATP from vascular endothelial cells. Br J Pharmacol. 2000;129:921–926. doi: 10.1038/sj.bjp.0703136. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Yaar R, Jones MR, Chen JF, Ravid K. Animal models for the study of adenosine receptor function. J Cell Physiol. 2005;202:9–20. doi: 10.1002/jcp.20138. [DOI] [PubMed] [Google Scholar]
  39. Jacobson KA, Gao ZG. Adenosine receptors as therapeutic targets. Nat Rev Drug Discov. 2006;5:247–264. doi: 10.1038/nrd1983. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Neely CF, Keith IM. A1 adenosine receptor antagonists block ischemia-reperfusion injury of the lung. Am J Physiol. 1995;268:L1036–L1046. doi: 10.1152/ajplung.1995.268.6.L1036. [DOI] [PubMed] [Google Scholar]
  41. Neely CF, Jin J, Keith IM. A1-adenosine receptor antagonists block endotoxin-induced lung injury. Am J Physiol. 1997;272:L353–L361. doi: 10.1152/ajplung.1997.272.2.L353. [DOI] [PubMed] [Google Scholar]
  42. Adkins WK, Barnard JW, Moore TM, Allison RC, Prasad VR, Taylor AE. Adenosine prevents PMA-induced lung injury via an A2 receptor mechanism. J Appl Physiol. 1993;74:982–988. doi: 10.1152/jappl.1993.74.3.982. [DOI] [PubMed] [Google Scholar]
  43. Heller AR, Rothermel J, Weigand MA, Plaschke K, Schmeck J, Wendel M, Bardenheuer HJ, Koch T. Adenosine A1 and A2 receptor agonists reduce endotoxin-induced cellular energy depletion and oedema formation in the lung. Eur J Anaesthesiol. 2007;24:258–266. doi: 10.1017/S026502150600144X. [DOI] [PubMed] [Google Scholar]
  44. Wilson CN, Batra VK. Lipopolysaccharide binds to and activates adenosine receptors on human pulmonary artery A1 endothelial cells. J Endotoxin Res. 2002;8:263–271. doi: 10.1179/096805102125000470. [DOI] [PubMed] [Google Scholar]
  45. Murphree LJ, Sullivan GW, Marshall MA, Linden J. Lipopolysaccharide rapidly modifies adenosine receptor transcripts in murine and human macrophages: role of NF-kappaB in A2A adenosine receptor induction. Biochem J. 2005;391:575–580. doi: 10.1042/BJ20050888. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Hasko G, Pacher P. A2A receptors in inflammation and injury: lessons learned from transgenic animals. J Leukoc Biol. 2008;83:447–455. doi: 10.1189/jlb.0607359. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Ohta A, Sitkovsky M. Role of G-protein-coupled adenosine receptors in downregulation of inflammation and protection from tissue damage. Nature. 2001;414:916–920. doi: 10.1038/414916a. [DOI] [PubMed] [Google Scholar]
  48. Reece TB, Ellman PI, Maxey TS, Crosby IK, Warren PS, Chong TW, LeGallo RD, Linden J, Kern JA, Tribble CG, Kron IL. Adenosine A2A receptor activation reduces inflammation and preserves pulmonary function in an in vivo model of lung transplantation. J Thorac Cardiovasc Surg. 2005;129:1137–1143. doi: 10.1016/j.jtcvs.2004.11.042. [DOI] [PubMed] [Google Scholar]
  49. Hasko G, Xu DZ, Lu Q, Nemeth ZH, Jabush J, Berezina TL, Zaets SB, Csoka B, Deitch EA. Adenosine A2A receptor activation reduces lung injury in trauma/hemorrhagic shock. Crit Care Med. 2006;34:1119–1125. doi: 10.1097/01.CCM.0000206467.19509.C6. [DOI] [PubMed] [Google Scholar]
  50. Thiel M, Chouker A, Ohta A, Jackson E, Caldwell C, Smith P, Lukashev D, Bittmann I, Sitkovsky MV. Oxygenation inhibits the physiological tissue-protecting mechanism and thereby exacerbates acute inflammatory lung injury. PLoS Biol. 2005;3:e174. doi: 10.1371/journal.pbio.0030174. [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Bonneau O, Wyss D, Ferretti S, Blaydon C, Stevenson CS, Trifilieff A. Effect of adenosine A2A receptor activation in murine models of respiratory disorders. Am J Physiol Lung Cell Mol Physiol. 2006;290:L1036–1043. doi: 10.1152/ajplung.00422.2005. [DOI] [PubMed] [Google Scholar]
  52. Reutershan J, Cagnina RE, Chang D, Linden J, Ley K. Therapeutic anti-inflammatory effects of myeloid cell adenosine receptor A2a stimulation in lipopolysaccharide-induced lung injury. J Immunol. 2007;179:1254–1263. doi: 10.4049/jimmunol.179.2.1254. [DOI] [PubMed] [Google Scholar]
  53. Yang D, Zhang Y, Nguyen HG, Koupenova M, Chauhan AK, Makitalo M, Jones MR, St Hilaire C, Seldin DC, Toselli P, Lamperti E, Schreiber BM, Gavras H, Wagner DD, Ravid K. The A2B adenosine receptor protects against inflammation and excessive vascular adhesion. J Clin Invest. 2006;116:1913–1923. doi: 10.1172/JCI27933. [DOI] [PMC free article] [PubMed] [Google Scholar]
  54. Beukers MW, den Dulk H, van Tilburg EW, Brouwer J, Ijzerman AP. Why are A2B receptors low-affinity adenosine receptors? Mutation of Asn273 to Tyr increases affinity of human A2B receptor for 2-(1-Hexynyl)adenosine. Mol Pharmacol. 2000;58:1349–1356. doi: 10.1124/mol.58.6.1349. [DOI] [PubMed] [Google Scholar]
  55. Fredholm BB, Irenius E, Kull B, Schulte G. Comparison of the potency of adenosine as an agonist at human adenosine receptors expressed in Chinese hamster ovary cells. Biochem Pharmacol. 2001;61:443–448. doi: 10.1016/S0006-2952(00)00570-0. [DOI] [PubMed] [Google Scholar]
  56. Sun CX, Zhong H, Mohsenin A, Morschl E, Chunn JL, Molina JG, Belardinelli L, Zeng D, Blackburn MR. Role of A2B adenosine receptor signaling in adenosine-dependent pulmonary inflammation and injury. J Clin Invest. 2006;116:2173–2182. doi: 10.1172/JCI27303. [DOI] [PMC free article] [PubMed] [Google Scholar]
  57. Auchampach JA, Ge ZD, Wan TC, Moore J, Gross GJ. A3 adenosine receptor agonist IB-MECA reduces myocardial ischemia-reperfusion injury in dogs. Am J Physiol Heart Circ Physiol. 2003;285:H607–H613. doi: 10.1152/ajpheart.01001.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Rivo J, Zeira E, Galun E, Matot I. Activation of A3 adenosine receptors attenuates lung injury after in vivo reperfusion. Anesthesiology. 2004;101:1153–1159. doi: 10.1097/00000542-200411000-00015. [DOI] [PubMed] [Google Scholar]
  59. Guo Y, Bolli R, Bao W, Wu WJ, Black RG, Jr, Murphree SS, Salvatore CA, Jacobson MA, Auchampach JA. Targeted deletion of the A3 adenosine receptor confers resistance to myocardial ischemic injury and does not prevent early preconditioning. J Mol Cell Cardiol. 2001;33:825–830. doi: 10.1006/jmcc.2001.1338. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Young HW, Molina JG, Dimina D, Zhong H, Jacobson M, Chan LN, Chan TS, Lee JJ, Blackburn MR. A3 adenosine receptor signaling contributes to airway inflammation and mucus production in adenosine deaminase-deficient mice. J Immunol. 2004;173:1380–1389. doi: 10.4049/jimmunol.173.2.1380. [DOI] [PubMed] [Google Scholar]
  61. Chen Y, Corriden R, Inoue Y, Yip L, Hashiguchi N, Zinkernagel A, Nizet V, Insel PA, Junger WG. ATP release guides neutrophil chemotaxis via P2Y2 and A3 receptors. Science. 2006;314:1792–1795. doi: 10.1126/science.1132559. [DOI] [PubMed] [Google Scholar]
  62. Inoue Y, Chen Y, Hirsh MI, Yip L, Junger WG. A3 and P2y2 receptors control the recruitment of neutrophils to the lungs in a mouse model of sepsis. Shock. 2008;30:173–177. doi: 10.1097/shk.0b013e318160dad4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  63. Bouma MG, Jeunhomme TM, Boyle DL, Dentener MA, Voitenok NN, Wildenberg FA van den, Buurman WA. Adenosine inhibits neutrophil degranulation in activated human whole blood: involvement of adenosine A2 and A3 receptors. J Immunol. 1997;158:5400–5408. [PubMed] [Google Scholar]
  64. Jordan JE, Thourani VH, Auchampach JA, Robinson JA, Wang NP, Vinten-Johansen J. A3 adenosine receptor activation attenuates neutrophil function and neutrophil-mediated reperfusion injury. Am J Physiol. 1999;277:H1895–H1905. doi: 10.1152/ajpheart.1999.277.5.H1895. [DOI] [PubMed] [Google Scholar]
  65. Ryzhov S, Goldstein AE, Matafonov A, Zeng D, Biaggioni I, Feoktistov I. Adenosine-activated mast cells induce IgE synthesis by B lymphocytes: an A2B-mediated process involving Th2 cytokines IL-4 and IL-13 with implications for asthma. J Immunol. 2004;172:7726–7733. doi: 10.4049/jimmunol.172.12.7726. [DOI] [PubMed] [Google Scholar]
  66. Salvatore CA, Tilley SL, Latour AM, Fletcher DS, Koller BH, Jacobson MA. Disruption of the A3 adenosine receptor gene in mice and its effect on stimulated inflammatory cells. J Biol Chem. 2000;275:4429–4434. doi: 10.1074/jbc.275.6.4429. [DOI] [PubMed] [Google Scholar]
  67. Clinical Trials Database http://clinicaltrials.gov/ Accessed April 10, 2008.

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