key: cord-0944406-ll4maduk
authors: Pacini, Enio Setsuo Arakaki; Satori, Naiara Ayako; Jackson, Edwin Kerry; Godinho, Rosely Oliveira
title: Extracellular cAMP-Adenosine Pathway Signaling: A Potential Therapeutic Target in Chronic Inflammatory Airway Diseases
date: 2022-04-11
journal: Front Immunol
DOI: 10.3389/fimmu.2022.866097
sha: 9caf3d81985321ea23ea42e9f07ed89221f6a26d
doc_id: 944406
cord_uid: ll4maduk

Adenosine is a purine nucleoside that, via activation of distinct G protein-coupled receptors, modulates inflammation and immune responses. Under pathological conditions and in response to inflammatory stimuli, extracellular ATP is released from damaged cells and is metabolized to extracellular adenosine. However, studies over the past 30 years provide strong evidence for another source of extracellular adenosine, namely the “cAMP-adenosine pathway.” The cAMP-adenosine pathway is a biochemical mechanism mediated by ATP-binding cassette transporters that facilitate cAMP efflux and by specific ectoenzymes that convert cAMP to AMP (ecto-PDEs) and AMP to adenosine (ecto-nucleotidases such as CD73). Importantly, the cAMP-adenosine pathway is operative in many cell types, including those of the airways. In airways, β(2)-adrenoceptor agonists, which are used as bronchodilators for treatment of asthma and chronic respiratory diseases, stimulate cAMP efflux and thus trigger the extracellular cAMP-adenosine pathway leading to increased concentrations of extracellular adenosine in airways. In the airways, extracellular adenosine exerts pro-inflammatory effects and induces bronchoconstriction in patients with asthma and chronic obstructive pulmonary diseases. These considerations lead to the hypothesis that the cAMP-adenosine pathway attenuates the efficacy of β(2)-adrenoceptor agonists. Indeed, our recent findings support this view. In this mini-review, we will highlight the potential role of the extracellular cAMP-adenosine pathway in chronic respiratory inflammatory disorders, and we will explore how extracellular cAMP could interfere with the regulatory effects of intracellular cAMP on airway smooth muscle and innate immune cell function. Finally, we will discuss therapeutic possibilities targeting the extracellular cAMP-adenosine pathway for treatment of these respiratory diseases.

Adenosine is an endogenous purine nucleoside that, via activation of specific adenosine receptors, modulates inflammation and immune responses (1) (2) (3) . Adenosine receptors are seven transmembrane G-protein coupled receptors (GPCRs) (4, 5) , and consist of a family of four adenosine receptor subtypes called A 1 , A 2A , A 2B and A 3 . These four receptors are encoded by different genes and present high affinities for the a subunit (Ga) of heterotrimeric G-proteins that regulate adenylyl cyclase (AC) activity (6) . A 1 and A 3 receptors are preferentially coupled to the inhibitory Ga (Gi/o) subunit that inhibits AC1, AC5 and AC6, decreasing 3',5'-cAMP (from here forward referred to as simply cAMP) production, whereas A 2A and A 2B are strongly coupled to stimulatory Ga (Gs) subunits, which can activate all nine membrane-bound AC isoforms, increasing intracellular cAMP concentrations (7, 8) and triggering one or more cAMP-dependent intracellular signaling pathways (9) . cAMP-dependent protein kinase (PKA) is by far the best studied effector of cAMP signaling and regulates many cellular processes including cell proliferation and differentiation, among others (10) . By interacting with specific domains in the PKA regulatory subunits, cAMP releases the two catalytic PKA subunits to phosphorylate serine and/or threonine residues in target proteins (11) . A 2 receptor signaling may also be mediated by cAMP/EPAC (exchange protein directly activated by cAMP) pathways (12) . Notably, adenosine receptors can also activate other signaling molecules via canonical GPCR pathways, such as phospholipase C b (PLC b) and Ca 2+ via activation of A 1 , A 2A and A 3 receptors (4) or by G protein-independent mechanisms mediated by b-arrestin (A 1 and A 2 ) (13) .

Although cAMP is capable of diffusing throughout the cell, cAMP signaling occurs in microdomains or even nanodomains (14) , which are at least in part created by intracellular phosphodiesterases (PDE), metallohydrolases involved in the degradation of cyclic nucleotides and distributed in 11 families (PDE 1-11). While members of PDE 4, 7 and 8 families selectively hydrolyze cAMP, PDE 1-3 and 10-11 are able to hydrolyze both cAMP and 3',5'-cGMP (15) . Intracellular compartmentalization of cAMP signaling also involves A kinase anchoring proteins (AKAPs), which bind to specific domains of PKA regulatory subunits and thereby guide PKA to different subcellular location (16) . AKAPs also associate with PDEs, thus influencing the phosphorylation of target proteins and the termination of PKA activation.

Several studies associate adenosine with a pro-inflammatory response (17) , but many others have linked high concentrations of extracellular adenosine to a protective anti-inflammatory (18) (19) (20) and pro-resolving functions (2, 21) . It seems that adenosine can limit the progression of inflammatory processes by reducing leukocyte infiltration and the production of inflammatory mediators, as well as by inducing apoptosis of inflammatory cells and promoting tissue repair; for review see (22) . In general, these protective effects of adenosine have been associated with A 2A receptor activation, since they are drastically reduced in A 2Adeficient mice, resulting in increased tissue damage and accumulation of pro-inflammatory cytokines (23, 24) .

In fact, because adenosine binds with different affinities to its receptor subtypes (Ki: A 1 =~100 nM; A 2A =~310 nM; A 3 =~290 nM and A 2B =~15,000 nM) (4), its final effect will depend on both the receptor subtype expressed in the target cell and the extracellular concentration of adenosine. Thus, while at early stages of inflammation, neutrophil recruitment, phagocytosis and adhesion to the vascular endothelium are promoted by low concentrations of adenosine via activation of A 1 and A 3 receptors (25) (26) (27) , during the healing phase, neutrophil recruitment is inhibited by high concentrations of adenosine via A 2A receptors (28) .

By activating A 2A and A 2B receptors, adenosine inhibits inflammatory functions of neutrophils, reducing phagocytosis, degranulation, production of reactive oxygen species (ROS) (25, (29) (30) (31) (32) and release of pro-inflammatory mediators (33) (34) (35) . Via A 2A and A 2B receptors, adenosine also inhibits monocyte proliferation and differentiation into macrophages, reduces macrophage phagocytosis, attenuates the oxidative burst, and decreases the production of pro-inflammatory mediators such as TNF-a and IL-12 (36) . Regarding lymphocytes, activation of A 2A receptors by adenosine reduces the production of inflammatory cytokines by T helper cells (37) , and inhibits the development and activation of effector cells (38) . Finally, adenosine also influences mast cells and eosinophils (17) , important players in physiological innate immunity and allergic inflammatory diseases.

Many of adenosine effects on airways involve modulation of intracellular cAMP concentrations on innate and adaptive immune cells (39) . Although complex, in general, elevated intracellular cAMP concentrations mediate anti-inflammatory effects and immune suppression; for review see (40) . By activating PKA, cAMP inhibits release of pro-inflammatory cytokines from dendritic cells (IL-12 and TNF-a) (41) and macrophages (TNF-a, MIP-1a, and LTB4) (42) . cAMP also inhibits T cell chemotaxis (43) and antigen-stimulated B cell proliferation (44) .

In the airways, the extracellular adenosine concentrations are relatively low under normal physiological conditions, ranging from 20 to 300 nM (45, 46) . However, adenosine concentrations drastically increase in bronchoalveolar lavage fluid and exhaled breath condensate in patients with asthma, COPD and cystic fibrosis (18, (47) (48) (49) (50) , reaching micromolar to millimolar range. These high concentrations of adenosine are associated with bronchoconstriction and airway smooth muscle hyperresponsiveness (51) (52) (53) (54) , indicating the involvement of adenosine in the pathophysiology of pulmonary inflammatory diseases. Adenosine induces bronchoconstriction mainly through direct activation of A 1 receptors expressed by smooth muscle cells (55) . Other studies have associated adenosineinduced bronchoconstriction with release of inflammatory mediators, such as histamine and leukotrienes from mast cells; accordingly, adenosine-induced airway smooth muscle contraction is attenuated by H 1 -histamine and CysLT 1 leukotriene antagonists (56) (57) (58) . Moreover, indirect bronchoconstriction induced by adenosine may also involve activation of A 1 receptors on vagal afferent neurons. The evidence for this is that vagotomy and inhibitors of cholinergic pathways attenuate adenosine-induced bronchoconstriction (59) .

The contribution of adenosine to airway inflammation is multifaceted and involves different receptors and intracellular signaling pathways in dendritic cells, monocytes, macrophages, neutrophils, bronchial smooth muscle and epithelial cells. Proinflammatory or anti-inflammatory airway responses are largely dependent on adenosine concentration and disease state over time. Particularly during acute airway inflammation, adenosine induces pro-resolving and tissue-protective actions, while in chronic airway inflammation it promotes deleterious effects such as tissue damage, fibrosis and release of pro-inflammatory cytokines (60, 61) . Activation of A 2B receptors induces release of pro-inflammatory cytokines such as IL-6 from bronchial smooth muscle cells (62) and lung fibroblasts (63) . Adenosine also affects the function of airway epithelial cells (e.g., ciliated cells, mucous cells, secretory cells, and basal cells) (64) . While activation of A 1 receptors increases production and secretion of mucus (65, 66) and modulates Cltransport in normal and cystic fibrosis human airway epithelial cells (67, 68) , activation of A 2A receptors elicits a robust release of pro-inflammatory IL-6, IL-8 and TNF-a from airway epithelia (60, 69, 70) . Adenosine also potentiates the recruitment of eosinophils and mast cells (53, 71) and induces airway inflammation by stimulating mast cell degranulation and release of pro-inflammatory mediators such as of IL-5, IL-13 as well as IL-4 (72, 73) .

Investigations using adenosine deaminase (ADA)-deficient mice have also provided compelling evidence that accumulation of adenosine in the lung can lead to the development and exacerbation of chronic lung disease (74) . ADA-deficient mice exhibit severe pulmonary inflammation and pathological features resembling those seen in asthma and chronic obstructive pulmonary diseases. These features include accumulation of macrophages and eosinophils, enlargement of alveolar spaces, increased production of mucus, higher concentrations of proinflammatory cytokines such as IL-5 and IL-13 and increased mast cell degranulation (75) (76) (77) . Interestingly, all of these changes can be reversed by lowering the adenosine concentrations or by adenosine receptor antagonists (75, 78) .

The canonical pathway of adenosine formation involves sequential dephosphorylation of ATP into ADP and AMP mediated by intra-or extracellular nucleoside triphosphate diphosphohydrolases (NTPDases), alkaline phosphatases (APs), nucleotide pyrophosphatase/phosphodiesterases (NPPs) and 5'-nucleotidases (5-NTs) (79) . A second pathway requires intracellular hydrolysis of S-adenosyl-homocysteine by the enzyme S-adenosyl-L-homocysteine hydrolase (80) , and a third mechanism is mediated by the metabolism of extracellular NAD + to adenosine by a CD38/CD203a/CD73 ectoenzymatic pathway (81) .

In addition to the aforementioned pathways of adenosine production, cAMP is now widely recognized as an important source of extracellular adenosine (82) . This biochemical route was originally described in the kidney by Jackson in the early 1990s (83) who named this mechanism the "extracellular cAMPadenosine pathway" (84, 85) . This pathway involves the sequential metabolism of extracellular cAMP to 5'-AMP and adenosine by ecto-phosphodiesterases (ecto-PDE) and ecto-5'nucleotidases (CD73), respectively (86-88) ( Figure 1 ).

Ecto-PDE activity has been described in many tissues, including vascular smooth muscle, kidney, intestine, skeletal muscle and airway, and in bovine seminal plasma and epididymal fluid (85, (88) (89) (90) (91) (92) . Few research groups, however, have attempted to characterized ecto-PDE. Pharmacological studies using selective inhibitors of the intracellular cAMPspecific PDE isoforms demonstrate that ecto-PDEs obtained from the kidney, seminal plasma and vesical fluid present characteristics similar to those of the intracellular PDE8 and PDE10 families (89, 91) .

The canonical pathway of extracellular adenosine production requires extracellular ATP as a precursor. In this regard, ATP can be specifically released via vesicular exocytosis and membrane pores upon cell activation or from damaged cells after transient or persistent inflammatory stimuli (93, 94) . In contrast, the extracellular cAMP-adenosine pathway requires transport of intracellular cAMP to the extracellular compartment, a process that is mediated by members of ATPbinding cassette (ABC) transporters, subfamily C (ABCC). Due to their ability to extrude various chemotherapeutic agents from tumor cells, ABCC transporters are also referred to as multidrug resistance proteins (MRPs or MDRs). This family of transporters consist of nine members (ABCC1-ABCC9) that use ATP hydrolysis to mediate the efflux of multiple cellular substrates (95) . Currently there are four well-defined ABCC proteins (ABCC1/MRP1, ABCC4/MRP4, ABCC5/MRP5 and ABCC11/MRP8) that can transport cAMP out of the cell, with specific kinetic parameters (96, 97) ; all of these cAMP transporters are expressed in human airway smooth muscle and epithelial cells (98) (99) (100) (101) (102) . Recent studies analyzing GSE datasets from GEO (Gene Expression Omnibus) show that in airway epithelial cells the rank order of ABCC gene expression is ABCC5 >ABCC1 >ABCC4 >ABCC11 (101) , whereas in the airway smooth muscle cells it is ABCC1 >ABCC4 >ABCC5 >ABCC11 (102).

Because of the importance of intracellular cAMP in the physiological modulation of airway smooth muscle tone and as a signaling molecule that mediates the action of bronchodilator agents (e.g., b 2 -adrenoceptor agonists used for rescue treatment of asthmatic patients suffering acute bronchoconstriction), we have focused our studies on the potential role of extracellular cAMP in the regulation of airway smooth muscle tone and as a source of adenosine. Using isolated rat trachea, we have shown that fenoterol (short-acting) and formoterol (long acting) b 2 -adrenoceptor agonists induce a time-dependent increase in extracellular cAMP concentrations (54) . Interestingly, extracellular cAMP triggers a concentration-dependent contraction of airway smooth muscle that is mimicked by adenosine and increased by drugs that inhibit adenosine degradation (EHNA) and uptake (uridine), indicating that the contracting effects of extracellular cAMP depends on extracellular adenosine formation (54) . Indeed, treatment of trachea segments with the adenosine receptor antagonist GCS-15943 increases the potency of fenoterol with regard to inducing smooth muscle relaxation, indicating that the extracellular cAMP-adenosine pathway compromises, via activation of adenosine receptors, the efficacy of b 2 -adrenoceptor agonists as bronchodilators. Further supporting this idea, using ultraperformance liquid chromatography-tandem mass FIGURE 1 | Schematic representation of canonical and non-canonical pathways involved in extracellular adenosine formation in airway cells. Classically, extracellular adenosine is generated mainly via the dephosphorylation of ATP, ADP and AMP by the action of a series of ecto-enzymes. An alternative source of adenosine is the second messenger cAMP. In this scenario, intracellular cAMP synthetized by adenylyl cyclases can be transported to the extracellular compartment and sequentially metabolized into AMP and adenosine by ecto-PDE and CD73, respectively. Increased concentrations of adenosine activate adenosine receptors coupled to Gs or Gi proteins, which in turn regulates the activity of adenylyl cyclases leading to an increase or decrease in cAMP concentrations and stimulation of downstream effectors. AC, adenylyl cyclases; Gs, stimulatory G protein; Gi, inhibitory G protein; PDE, phosphodiesterase; PKA, cAMP-dependent protein kinase; EPAC, exchange protein directly activated by cAMP; CD39, NTPDase-1; CD73, ecto-5'-nucleotidases and Ecto-PDE, ecto-phosphodiesterase. spectrometry (LC-MS/MS), we found that incubation of isolated rat tracheas with cell impermeable cAMP leads to increases in extracellular concentrations of 5'-AMP, adenosine and inosine (92) . The involvement of ecto-phosphodiesterases and ecto-5'nucleotidase/CD73 in the extracellular metabolism of cAMP is demonstrated by the fact the selective inhibitors of these ectoenzymes (DPSPX and AMP-CP, respectively) significantly reduce adenosine concentrations (92) . Consistent with these observations, studies by Huff et al. (103) demonstrate that cAMP efflux from human airway epithelial cells occurs and is mediated by ABBC4/ MRP4 transporters. More recently, Cao and coworkers (102) showed that stimulation of b 2 -adrenoceptors or AC with formoterol or forskolin, respectively, also promotes extrusion of cAMP from human airway smooth muscle cells in culture. cAMP efflux is markedly reduced by pharmacological inhibition or downregulation of ABCC1 transporters with siRNA, resulting in potentiation of b-agonist induced airway smooth muscle relaxation (102) . Collectively, these findings reveal the existence of a functional extracellular cAMP-adenosine pathway in airways.

Increase in plasma concentration of cAMP is observed in physiological and pathological conditions such as during spontaneous or PGE-induced labor (104), insulin-induced hypoglycemia (105) in malignant hyperpyrexia susceptible individuals (106) or following traumatic injury. In fact, according to Cock et al. (107) ,, there is a positive correlation between the plasma concentrations of cAMP and neutrophil count following injury, which supports the idea that neutrophil mobilization can be activated by extracellular cAMP. Also, cAMP extrusion from neutrophils is stimulated by PGE 1 , isoproterenol, or forskolin (108), indicating that many cells are able to release cAMP. Regarding the airways, based on the extracellular fluid volume of rat trachea (~1 ml/g of dry tissue weight) (109), we have estimated that after b 2 -adrenoceptors stimulation, extracellular cAMP can reach micromolar concentrations (54) , which are those required to induce smooth muscle contraction.

Studies addressing the expression of transporters and ectoenzymes responsible for efflux and extracellular degradation of cAMP in the airway cells support a potential role for the extracellular cAMP-adenosine pathway in the pathophysiology of different respiratory diseases. Airway epithelial cells from tobacco smokers and asthmatic patients have increased expression of ABCC1, in comparison with those of healthy individuals (101) , which is consistent with the increased concentrations of serum cAMP in asthmatic patients (102) . With regard to ecto-enzymes, several studies using human tissues or animal models of respiratory diseases reveal increased expression and enzymatic activity of ecto-5'nucleotidase/CD73, which could explain the higher level of extracellular adenosine associated with mechanical ventilationinduced lung injury (110) , chronic obstructive diseases (111) or long-term cigarette smoking (112) . In allergic animal models, airway inflammation and tracheal hyperresponsiveness are largely dependent on CD73 (113) . On the other hand, in CD73 deficient mice (CD73 -/-) sensitized with ovalbumin, there is an exacerbation of airway inflammation, as reflected by increased formation of mucus and release of pro-inflammatory mediators such like IL-1b, TNFa, IL-4 and IL-5 (114, 115) . Other studies have shown that cAMP released from human CD4 + T lymphocytes inhibits T cell proliferation (116) and modulates differentiation of human monocytes into dendritic cells, effects that are mediated via A 2A and A 2B receptors (117) . Likewise, in the experimental autoimmune uveitis mice model, cAMP released from T cells functions as an important source of extracellular adenosine, which in turn contributes to the immunosuppressive function of regulatory T cells (118) . Finally, expression of A 2B receptors is increased in lungs of patients with COPD or idiopathic pulmonary fibrosis (60) . The precise role of the cAMP-adenosine pathway in inflammatory pulmonary diseases is still not clear but it might function as a negative or positive feedback loop limiting or enhancing the output signal initiated by intracellular cAMP.

Currently, there are no clinical trials exploring compounds that target molecules involved in the extracellular cAMPadenosine pathway (ABCC/MRP, ecto-phosphodiesterases and CD73) for chronic inflammatory airway diseases. However, some Phase I and II studies are testing CD73 inhibitors (e.g., HLX23, LY3475070, IPH5301, AK119, cpi-006, Sym024, IBI325, ORIC-533 and MEDI9447) alone or combined with other agents in different solid tumors, metastatic cancer and COVID-19 (NCT: 04797468, 04148937, 05143970, 04516564, 04572152, 05173792, 03454451, 04672434, 05246995, 05227144, 03381274 and 04668300) with promising results (119, 120) . In addition, Phase I and II clinical trials evaluated the safety and efficacy of an MRP-1 inhibitor (sulindac) in patients with advanced melanoma (121)(EUCTR: 2006-006051-12); however, this study was terminated prematurely. Several other clinicals trials (NCR: 00430300, 01640990, 05262218, 03774290, 02635945, 01939587, 04606069) investigated the role of selective adenosine receptor agonists and antagonists (e.g., GW328267X, UK-432097, EPI-2010, CVT-6883, QAF 805, PBF-680 and Regadenoson) in various respiratory diseases, including asthma, COPD, allergic rhinitis, acute lung injury and COVID-19, however, almost all of them were discontinued due to insufficient therapeutic efficacy and/or incidence of side effects (61, (122) (123) (124) . Since cAMP efflux depends on the preceding increase in intracellular cAMP, future preclinical studies and clinical trials should explore the therapeutic effects of combining treatment with bronchodilator or anti-inflammatory drugs, such as corticosteroids, b 2 -adrenoceptor agonists, or PDE inhibitors, with inhibitors of either cAMP efflux or the extracellular cAMPadenosine pathway, to elucidate the potential therapeutic role of this biochemical pathway in inflammatory airway diseases.

The discovery of cAMP by Sutherland and colleagues almost 60 years ago provided the basis for the concept of cAMP as an intracellular second messenger (125) . Nevertheless, as illustrated in Figure 1 , we are now beginning a new phase of understanding the role of cAMP as an autocrine and/or paracrine signaling molecule, in which the extracellular cAMP-adenosine pathway is able to modify cAMP signaling initiated in the intracellular environment (82) .

In the airways, cAMP is the main signaling molecule implicated in bronchodilation caused by endogenous catecholamines or beta-adrenergic agents. Even after the identification of membrane transporters capable of exporting cAMP out of cells, the efflux of cAMP from airway epithelial or smooth muscle cells was considered a simple mechanism to reduce intracellular cyclic nucleotide concentrations (102) . However, the discovery of an extracellular enzymatic system in airways that converts cAMP into adenosine (92) and can affect the primary cellular response initiated by intracellular cAMP (54) opened new perspectives on how to envision the role of cAMP in airway physiology, highlighting the importance of the mechanisms of cAMP extrusion and its function as an alternative source of extracellular adenosine.

As observed with adenosine, the regulatory effects of extracellular cAMP on the airways will depend on the ectoenzymes and adenosine receptor subtypes expressed in each cell, and on the amount of cAMP released into the extracellular compartment. Although we have revealed bronchoconstrictor effects of the extracellular cAMP-adenosine pathway, the relevance of the extracellular cAMP in inflammatory lung diseases is just beginning to be explored. Considering that increases in intracellular cAMP concentrations are usually followed by efflux of cAMP (88) , it is important to explore the impact of the extracellular cAMP-adenosine pathway on the therapeutic effects of bronchodilators and anti-inflammatory drugs used in chronic respiratory diseases, such as glucocorticoids and classical PDE inhibitors, known to increase intracellular cAMP (126) (127) (128) (129) .

In view of the modulatory effects of adenosine on inflammatory responses and innate immune cell function, there is a potential role for cAMP efflux in airway inflammation/immune responses and bronchial reactivity (Figure 2) . The existence of the extracellular cAMP-adenosine pathway in the airways also suggests innovative therapeutic possibilities and new pharmacological targets in airway cells, such as distinct ABCC/MRP transporters and ectoenzymes such ecto-PDEs. Future studies will be needed to decipher the FIGURE 2 | Potential roles of the extracellular cAMP-adenosine pathway in the airways. Part of the cAMP formed in epithelial and smooth muscle cells is transported by ABCC/MRP transporters to the extracellular milieu, giving rise to extracellular adenosine, which in turn can modulate the airway immune response and bronchial reactivity. precise role of the extracellular cAMP-adenosine pathway in airway inflammation and immune responses.

EP and RG contributed to conception and design of the review. RG wrote the first draft of the manuscript. EP, NS, EJ, and RG wrote the sections of the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

This work was supported by Fundacão de Amparo à Pesquisa do Estado de São Paulo (Fapesp, grant #2018/21381-5) and financed in part by the Coordenacão de Aperfeicoamento de Pessoal de Nıvel Superior -Brazil (CAPES) -Finance Code 001. RG is a research fellow from Conselho Nacional de Desenvolvimento Cientıfco e Tecnoloǵico (CNPq; grant # 310498/2019-8. EP is a PNPD postdoctoral fellow and NS is a MSc fellow from Capes, Brazil.

New Insights Into the Regulation of Inflammation by Adenosine

Regulation of Inflammation by Adenosine

Adenosine Signaling and the Immune System: When a Lot Could be Too Much

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and Classification of Adenosine Receptors:an Update

Purinergic Signaling in the Airways

Structure and Function of Adenosine Receptor Heteromers

Physiological Roles for G Protein-Regulated Adenylyl Cyclase Isoforms: Insights From Knockout and Overexpression Studies

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Pharmacology of Adenosine Receptors: The State of the Art

From Long-Range Second Messenger to Nanodomain Signalling

cAMP Regulation of Airway Smooth Muscle Function

Regulation of Cardiac Fibroblast Collagen Synthesis by Adenosine: Roles for Epac and PI3K

Cross-Communication Between G(I) and G(s) in a G-Protein-Coupled Receptor Heterotetramer Guided by a Receptor C-Terminal Domain

Role of Membrane Microdomains in Compartmentation of cAMP Signaling

Phosphodiesterase Isoforms and cAMP Compartments in the Development of New Therapies for Obstructive Pulmonary Diseases

AKAP Signaling Islands: Venues for Precision Pharmacology

Adenosine in the Airways: Implications and Applications

Adenosine in Bronchoalveolar Lavage Fluid in Asthma

High Adenosine Plasma Concentration as a Prognostic Index for Outcome in Patients With Septic Shock

Molecular Approach to Adenosine Receptors: Receptor-Mediated Mechanisms of Tissue Protection

Involvement of Adenosine A 2A Receptors in Engulfment-Dependent Apoptotic Cell Suppression of Inflammation

Resolution of Inflammation: What Controls Its Onset? Front Immunol

Role of G-Protein-Coupled Adenosine Receptors in Downregulation of Inflammation and Protection From Tissue Damage

Cutting Edge: Physiologic Attenuation of Proinflammatory Transcription by the Gs Protein-Coupled A 2A Adenosine Receptor In Vivo

Fc Gamma Receptor-Mediated Functions in Neutrophils are Modulated by Adenosine Receptor Occupancy. A 1 Receptors are Stimulatory and A 2 Receptors are Inhibitory

Neutrophil Adherence to Endothelium is Enhanced via Adenosine A 1 Receptors and Inhibited via Adenosine A 2 Receptors

Atp Release Guides Neutrophil Chemotaxis. via P2Y2 A 3 Receptors

Multi-Inhibitory Effects of A 2A Adenosine Receptor Signaling on Neutrophil Adhesion Under Flow

Adenosine Inhibits Neutrophil Degranulation in Activated Human Whole Blood: Involvement of Adenosine A 2 and A 3 Receptors

A(2A) Adenosine Receptors in Human Peripheral Blood Cells

A Role for the Low-Affinity A 2B Adenosine Receptor in Regulating Superoxide Generation by Murine Neutrophils

Adenosine a(2B) Receptor Deficiency Promotes Host Defenses Against Gram-Negative Bacterial Pneumonia

Adenosine Inhibits the Production of Tumor Necrosis Factor-Alpha of Endotoxin-Stimulated Human Polymorphonuclear Leukocytes

Adenosine, a Potent Natural Suppressor of Arachidonic Acid Release and Leukotriene Biosynthesis in Human Neutrophils

Immunomodulatory Impact of the A 2A Adenosine Receptor on the Profile of Chemokines Produced by Neutrophils

Shaping of Monocyte and Macrophage Function by Adenosine Receptors

A2A Adenosine Receptor Induction Inhibits IFN-Gamma Production in Murine CD4+ T Cells

Regulation of Lymphocyte Function by Adenosine

The cAMP Pathway as Therapeutic Target in Autoimmune and Inflammatory Diseases

Cyclic AMP in Dendritic Cells: A Novel Potential Target for Disease-Modifying Agents in Asthma and Other Allergic Disorders

Cyclic AMP Modulates the Functional Plasticity of Immature Dendritic Cells by Inhibiting Src-Like Kinases Through Protein Kinase a-Mediated Signaling

Cutting Edge: Macrophage Inhibition by Cyclic AMP (cAMP): Differential Roles of Protein Kinase a and Exchange Protein Directly Activated by cAMP-1

Phosphodiesterase and Cyclic Adenosine Monophosphate-Dependent Inhibition of T-Lymphocyte Chemotaxis

Cyclic AMP Modulation of Human B Cell Proliferative Responses: Role of cAMP-Dependent Protein Kinases in Enhancing B Cell Responses to Phorbol Diesters and Ionomycin

Adenosine Receptors as Drug Targets

Adenosine Receptors as Drug Targets-What are the Challenges?

Adenosine in Exhaled Breath Condensate in Healthy Volunteers and in Patients With Asthma

Mass Spectrometric Analysis of Biomarkers and Dilution Markers in Exhaled Breath Condensate Reveals Elevated Purines in Asthma and Cystic Fibrosis

Extracellular Adenosine Triphosphate and Chronic Obstructive Pulmonary Disease

Nucleotide Release by Airway Epithelia

Inhaled Adenosine and Guanosine on Airway Resistance in Normal and Asthmatic Subjects

Airway Responsiveness to Adenosine 5'-Monophosphate in Chronic Obstructive Pulmonary Disease is Determined by Smoking

Adenosine-Mediated Bronchoconstriction and Lung Inflammation in an Allergic Mouse Model

The Extracellular cAMP-Adenosine Pathway in Airway Smooth Muscle

Pharmacological Characterization of Adenosine Receptors on Isolated Human Bronchi

The Effect of Histamine-H1 Receptor Antagonism With Terfenadine on Concentration-Related AMP-Induced Bronchoconstriction in Asthma

Role of Cysteinyl Leukotrienes in Adenosine 5'-Monophosphate Induced Bronchoconstriction in Asthma

Activation of Murine Lung Mast Cells by the Adenosine A 3 Receptor

Involvement of A 1 Adenosine Receptors and Neural Pathways in Adenosine-Induced Bronchoconstriction in Mice

Purinergic Signaling in Pulmonary Inflammation

Adenosine at the Interphase of Hypoxia and Inflammation in Lung Injury

A(2B) Adenosine Receptors Increase Cytokine Release by Bronchial Smooth Muscle Cells

Synergy Between A 2B Adenosine Receptors and Hypoxia in Activating Human Lung Fibroblasts

A 3 Adenosine Receptor Signaling Contributes to Airway Mucin Secretion After Allergen Challenge

Adenosine Up-Regulation of the Mucin Gene, MUC2, in Asthma

Allergens and the Airway Epithelium Response: Gateway to Allergic Sensitization

Identification and Function of A1 Adenosine Receptors in Normal and Cystic Fibrosis Human Airway Epithelial Cells

Activation of Airway Cl-Secretion in Human Subjects by Adenosine

Adenosine Promotes IL-6 Release in Airway Epithelia

A 2B Adenosine Receptors Induce IL-19 From Bronchial Epithelial Cells, Resulting in TNF-Alpha Increase

A 3 Receptors Mediate Rapid Inflammatory Cell Influx Into the Lungs of Sensitized Guinea-Pigs

Adenosine-Activated Mast Cells Induce Ige Synthesis by B Lymphocytes: An A 2B -Mediated Process Involving Th2 Cytokines IL-4 and IL-13 With Implications for Asthma

Involvement of Mast Cells in Adenosine-Mediated Bronchoconstriction and Inflammation in an Allergic Mouse Model

Adenosine Deaminase Deficiency: Metabolic Basis of Immune Deficiency and Pulmonary Inflammation

Metabolic Consequences of Adenosine Deaminase Deficiency in Mice are Associated With Defects in Alveogenesis, Pulmonary Inflammation, and Airway Obstruction

Adenosine-Mediated Mast Cell Degranulation in Adenosine Deaminase-Deficient Mice

Adenosine Mediates IL-13-Induced Inflammation and Remodeling in the Lung and Interacts in an IL-13-Adenosine Amplification Pathway

Adenosine-Dependent Airway Inflammation and Hyperresponsiveness in Partially Adenosine Deaminase-Deficient Mice

History of Ectonucleotidases and Their Role in Purinergic Signaling

The Transmethylation Pathway as a Source for Adenosine in the Isolated Guinea-Pig Heart

A CD38/CD203a/CD73 Ectoenzymatic Pathway Independent of CD39 Drives a Novel Adenosinergic Loop in Human T Lymphocytes

New Perspectives in Signaling Mediated by Receptors Coupled to Stimulatory G Protein: The Emerging Significance of cAMP Efflux and Extracellular cAMP-Adenosine Pathway

Adenosine: A Physiological Brake on Renin Release

Metabolism of Exogenous Cyclic AMP to Adenosine in the Rat Kidney

Cyclic AMP-Adenosine Pathway Inhibits Vascular Smooth Muscle Cell Growth

Oviduct Cells Express the Cyclic AMP-Adenosine Pathway

The Extracellular Cyclic AMP-Adenosine Pathway in Renal Physiology

Skeletal Muscle Expresses the Extracellular Cyclic AMP-Adenosine Pathway

Characterization of Renal Ecto-Phosphodiesterase

Cyclic AMP in Rat Ileum: Evidence for the Presence of an Extracellular Cyclic AMP-Adenosine Pathway

Characterization of cAMP-Phosphodiesterase Activity in Bovine Seminal Plasma

Extracellular Metabolism of 3',5'-Cyclic AMP as a Source of Interstitial Adenosine in the Rat Airways

The Yin and Yang in Immune Responses? Mol Aspects Med

Mechanisms of ATP Release by Inflammatory Cells

Understanding of Human ATP Binding Cassette Superfamily and Novel Multidrug Resistance Modulators to Overcome MDR

Characterization of the MRP4-and MRP5-Mediated Transport of Cyclic Nucleotides From Intact Cells

Cellular Efflux of cAMP and cGMP -A Question About Selectivity

Multidrug Resistance Related Molecules in Human and Murine Lung

ATP-Binding Cassette (ABC) Transporters in Normal and Pathological Lung

Mucosal Production of Uric Acid by Airway Epithelial Cells Contributes to Particulate Matter-Induced Allergic Sensitization

The Impact of Cigarette Smoke Exposure

Inhibition of ABCC1 Decreases cAMP Egress and Promotes Human Airway Smooth Muscle Cell Relaxation

Inhibition of ABCC4 Potentiates Combination Beta Agonist and Glucocorticoid Responses in Human Airway Epithelial Cells

Changes in Plasma Cyclic AMP and Cyclic GMP During Spontaneous Labor and Labor Induced by Oxytocin, Prostaglandin F2 Alpha and Prostaglandin E2

Mechanism of Plasma Cyclic AMP Response to Hypoglycemia in Man

Cyclic AMP in Normal and Malignant Hyperpyrexia Susceptible Individuals Following Exercise

Increased Plasma Free Cyclic-AMP Levels Following Major Trauma and Their Relevance to the Immune Response

cAMP and Human Neutrophil Chemotaxis. Elevation of cAMP Differentially Affects Chemotactic Responsiveness

Alloxan Diabetes Abolishes the Increased Negativity of Interstitial Fluid Pressure in Rat Trachea Induced by Vagal Nerve Stimulation

Identification of Ectonucleotidases CD39 and CD73 in Innate Protection During Acute Lung Injury

Alterations in Adenosine Metabolism and Signaling in Patients With Chronic Obstructive Pulmonary Disease and Idiopathic Pulmonary Fibrosis

Co-Inhibition of CD73 and ADORA2B Improves Long-Term Cigarette Smoke Induced Lung Injury

Allergen-Induced Airway Hyperresponsiveness is Absent in Ecto-5'-Nucleotidase (CD73)-Deficient Mice

Ecto-5'-Nucleotidase (CD73)-Mediated Adenosine Production is Tissue Protective in a Model of Bleomycin-Induced Lung Injury

Exacerbation of Allergic Airway Inflammation in Mice Lacking ECTO-5'-Nucleotidase (CD73)

Human CD4+ T Lymphocytes With Increased Intracellular cAMP Levels Exert Regulatory Functions by Releasing Extracellular cAMP

Human Monocytes Respond to Extracellular cAMP Through A 2A and A 2B Adenosine Receptors

The cAMP-Adenosine Feedback Loop Maintains the Suppressive Function of Regulatory T Cells

Targeting the CD73-Adenosine Axis in Immuno-Oncology

Targeting CD73 to Augment Cancer Immunotherapy

A Phase I Clinical and Pharmacokinetic Study of the Multi-Drug Resistance Protein-1 (MRP-1) Inhibitor Sulindac, in Combination With Epirubicin in Patients With Advanced Cancer

Adenosine Receptors as Targets for Therapeutic Intervention in Asthma and Chronic Obstructive Pulmonary Disease

Historical and Current Adenosine Receptor Agonists in Preclinical and Clinical Development

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

The Effect of L-Epinephrine and Other Agents on the Synthesis and Release of Adenosine 3´,5´´-Phosphate by Whole Pigeon Erythrocytes

Roflumilast N-Oxide in Combination With Formoterol Enhances the Antiinflammatory Effect of Dexamethasone in Airway Smooth Muscle Cells

Glucocorticoids Rapidly Activate cAMP Production via G( s) to Initiate Non-Genomic Signaling That Contributes to One-Third of Their Canonical Genomic Effects

Budesonide Enhances Agonist-Induced Bronchodilation in Human Small Airways by Increasing cAMP Production in Airway Smooth Muscle

Formoterol Fumarate and Glycopyrronium Bromide: Synergy of Triple Combination Therapy on Human Airway Smooth Muscle Ex Vivo

Conflict of Interest: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest