A2A receptors in inflammation and injury: lessons learned from transgenic animals

Adenosine receptors: expression, function and regulation

Adenosine receptors (ARs) comprise a group of G protein-coupled receptors (GPCR) which mediate the physiological actions of adenosine. To date, four AR subtypes have been cloned and identified in different tissues. These receptors have distinct localization, signal transduction pathways and different means of regulation upon exposure to agonists. This review will describe the biochemical characteristics and signaling cascade associated with each receptor and provide insight into how these receptors are regulated in response to agonists. A key property of some of these receptors is their ability to serve as sensors of cellular oxidative stress, which is transmitted by transcription factors, such as nuclear factor (NF)-κB, to regulate the expression of ARs. Recent observations of oligomerization of these receptors into homo- and heterodimers will be discussed. In addition, the importance of these receptors in the regulation of normal and pathological processes such as sleep, the development of cancers and in protection against hearing loss will be examined.

Streptococcus suis adenosine synthase functions as an effector in evasion of PMN-mediated innate immunit

Streptococcus suis serotype 2 (S. suis 2) is a highly invasive pathogen in pigs and humans that can cause severe systemic infection. Sepsis and meningitis are the most common clinical manifestations of S. suis 2 infection. However, the mechanisms of S. suis 2 surviving in human blood remains unclear, so to identify novel virulence factors in evasion of polymorphonuclear leukocyte (PMN)-mediated innate immunity play important roles in developing therapies against S. suis 2 infection. Here, we found that S. suis 2 can escape phagocytic clearance by adenosine synthesis in blood. Through bioinformatics-based analyses we identified a cell wall-anchored protein harbors a 5′-nucleotidase signature sequence and evidence strongly indicated that it can convert adenosine monophosphate (AMP) to adenosine. It was designated as Ssads (the adenosine synthase of S. suis 2). Furthermore, we found that Ssads could impair PMN's defense against S. suis 2 with decreasing of oxidative activity and degranulation of PMNs in human blood via A₂a receptors. Additionally, this enzyme-deficient mutant was found to have diminished virulence in the piglet infection model. Taken together, these results indicate that Ssads play an important role in S. suis 2 escaping human innate immunity in the context of inhibiting PMN's activity by synthesis of adenosine.

Protective effect of adenosine receptors against lipopolysaccharide-induced acute lung injury

Acute lung injury and acute respiratory distress syndrome (ALI/ARDS) affect 200,000 people a year in the USA. Pulmonary vascular and specifically endothelial cell (EC) barrier compromise is a hallmark of these diseases. We have recently shown that extracellular adenosine enhances human pulmonary (EC) barrier via activation of adenosine receptors (ARs) in cell cultures. On the basis of these data, we hypothesized that activation of ARs might exert barrier-protective effects in a model of ALI/ARDS in mice. To test this hypothesis, we examined the effects of pre- and posttreatment of adenosine and 5'-N-ethylcarboxamidoadenosine (NECA), a nonselective stable AR agonist, on LPS-induced lung injury. Mice were given vehicle or LPS intratracheally followed by adenosine, NECA, or vehicle instilled via the internal jugular vein. Postexperiment cell counts, Evans Blue Dye albumin (EBDA) extravasation, levels of proteins, and inflammatory cytokines were analyzed. Harvested lungs were used for histology and myeloperoxidase studies. Mice challenged with LPS alone demonstrated an inflammatory response typical of ALI. Cell counts, EBDA extravasation, as well as levels of proteins and inflammatory cytokines were decreased in adenosine-treated mice. Histology displayed reduced infiltration of neutrophils. NECA had a similar effect on LPS-induced vascular barrier compromise. Importantly, posttreatment with adenosine or NECA recovers lung vascular barrier and reduces inflammation induced by LPS challenge. Furthermore, adenosine significantly attenuated protein degradation of A2A and A3 receptors induced by LPS. Collectively, our results demonstrate that activation of ARs protects and restores vascular barrier functions and reduces inflammation in LPS-induced ALI.

A novel adenosine precursor 2',3'-cyclic adenosine monophosphate inhibits formation of post-surgical adhesions

BACKGROUND

Intraperitoneal adenosine reduces abdominal adhesions. However, because of the ultra-short half-life and low solubility of adenosine, optimal efficacy requires multiple dosing.

AIM

Here, we compared the ability of potential adenosine prodrugs to inhibit post-surgical abdominal adhesions after a single intraperitoneal dose.

METHODS

Abdominal adhesions were induced in mice using an electric toothbrush to damage the cecum. Also, 20 μL of 95 % ethanol was applied to the cecum to cause chemically induced injury. After injury, mice received intraperitoneally either saline (n = 18) or near-solubility limit of adenosine (23 mmol/L; n = 12); 5'-adenosine monophosphate (75 mmol/L; n = 11); 3'-adenosine monophosphate (75 mmol/L; n = 12); 2'-adenosine monophosphate (75 mmol/L; n = 12); 3',5'-cyclic adenosine monophosphate (75 mmol/L; n = 19); or 2',3'-cyclic adenosine monophosphate (75 mmol/L; n = 20). After 2 weeks, adhesion formation was scored by an observer blinded to the treatments. In a second study, intraperitoneal adenosine levels were measured using tandem mass spectrometry for 3 h after instillation of 2',3'-cyclic adenosine monophosphate (75 mmol/L) into the abdomen.

RESULTS

The order of efficacy for attenuating adhesion formation was: 2',3'-cyclic adenosine monophosphate > 3',5'-cyclic adenosine monophosphate ≈ adenosine > 5'-adenosine monophosphate ≈ 3'-adenosine monophosphate ≈ 2'-adenosine monophosphate. The groups were compared using a one-factor analysis of variance, and the overall p value for differences between groups was p < 0.000001. Intraperitoneal administration of 2',3'-cAMP yielded pharmacologically relevant levels of adenosine in the abdominal cavity for >3 h.

CONCLUSION

Administration of 2',3'-cyclic adenosine monophosphate into the surgical field is a unique, convenient and effective method of preventing post-surgical adhesions by acting as an adenosine prodrug.

Inflammation and resolution are associated with upregulation of fatty acid β-oxidation in Zymosan-induced peritonitis

Inflammation is a fundamental defensive response to harmful stimuli. However, it can cause damage if it does not subside. To avoid such damage, organisms have developed a mechanism called resolution of inflammation. Here we applied an untargeted metabolomics approach to a sterile and self-resolving animal model of acute inflammation, namely zymosan-induced peritonitis in mice, to examine the effect of inflammation and resolution on the metabolomic profiles. Significant and time-dependent changes in metabolite profiles after zymosan administration were observed in both peritoneal wash fluid (PWF) and plasma. These metabolomic changes correlated well with inflammatory chemokine or cytokine production. In PWF, most of metabolites that could detected increased in zymosan-treated mice, which is suggestive of inflammation, oxidative stress and increased energy demands. In plasma, most metabolites in the central metabolic pathway (glycolysis and TCA cycle) were significantly downregulated after zymosan administration. The concentration of the ketone body 3-hydroxybutyric acid (3-HB) in plasma and PWF increased in zymosan-injected animals indicating upregulation of fatty acid β-oxidation. Increased 3-HB level was observed in the cells that infiltrated into the peritoneal cavity and these infiltrated cells might contribute, at least in part, to the production of 3-HB in the peritoneal cavity.

Multiple sclerosis lymphocytes upregulate A2A adenosine receptors that are antiinflammatory when stimulated

Multiple sclerosis (MS) is an autoimmune-mediated inflammatory disease characterized by multifocal areas of demyelination. Experimental evidence indicates that A2A adenosine receptors (ARs) play a pivotal role in the inhibition of inflammatory processes. The aim of this study was to investigate the contribution of A2A ARs in the inhibition of key pro-inflammatory mediators for the pathogenesis of MS. In lymphocytes from MS patients, A1, A2A, A2B, and A3 ARs were analyzed by using RT-PCR, Western blotting, immunofluorescence, and binding assays. Moreover the effect of A2A AR stimulation on proinflammatory cytokine release such as TNF-α, IFN-γ, IL-6, IL-1β, IL-17, and on lymphocyte proliferation was evaluated. The capability of an A2A AR agonist on the modulation of very late antigen (VLA)-4 expression and NF-κB was also explored. A2A AR upregulation was observed in lymphocytes from MS patients in comparison with healthy subjects. The stimulation of these receptors mediated a significant inhibition of TNF-α, IFN-γ, IL-6, IL-1β, IL-17, and cell proliferation as well as VLA-4 expression and NF-κB activation. This new evidence highlights that A2A AR agonists could represent a novel therapeutic tool for MS treatment as suggested by the antiinflammatory role of A2A ARs in lymphocytes from MS patients.

Adenosine as an endogenous immunoregulator in cancer pathogenesis: where to go?

Cancer is a chronic disease and its pathogenesis is well correlated with infection and inflammation. Adenosine is a purine nucleoside, which is produced under metabolic stress like hypoxic conditions. Acute or chronic inflammatory conditions lead to the release of precursor adenine nucleotides (adenosine triphosphate (ATP), adenosien diphosphate (ADP) and adenosine monophosphate (AMP)) from cells, which are extracellularly catabolized into adenosine by extracellular ectonucleotidases, i.e., CD39 or nucleoside triphosphate dephosphorylase (NTPD) and CD73 or 5'-ectonucleotidase. It is now well-known that adenosine is secreted by cancer as well as immune cells during tumor pathogenesis under metabolic stress or hypoxia. Once adenosine is released into the extracellular environment, it exerts various immunomodulatory effects via adenosine receptors (A1, A2A, A2B, and A3) expressed on various immune cells (i.e., macrophages, myeloid-derived suppressor cells (MDSCs), natural killer (NK) cells, dendritic cells (DCs), T cells, regulatory T cell (Tregs), etc.), which play very important roles in the pathogenesis of cancer. This review is intended to summarize the role of inflammation and adenosine in the immunopathogenesis of tumor along with regulation of tumor-specific immune response and its modulation as an adjunct approach to tumor immunotherapy.

Attenuation of Clostridium difficile toxin-induced damage to epithelial barrier by ecto-5'-nucleotidase (CD73) and adenosine receptor signaling

BACKGROUND

Clostridium difficile (Cdf) releases toxins (TcdA and TcdB) that damage the intestinal epithelial barrier. Ecto-5'-nucleotidase (CD73) is expressed on intestinal epithelial cells, and it is hypothesized to protect against toxin-induced epithelial damage through the cleavage of 5'-AMP to adenosine (Ado) and subsequent activation of adenosine receptors (AdoRs). Herein, we sought to assess the potential protective effects of CD73 and AdoR signaling on the injurious effects of Cdf toxins.

METHODS

Barrier function was assessed with T84 colonocytes. Transepithelial electrical resistance (TEER), paracellular fluorescein isothiocyanate (FITC)-dextran flux, and tight junction protein (ZO-1) integrity were monitored. Intrarectal installation of Cdf toxin was used to assess epithelial damage in vivo.

KEY RESULTS

TcdA/B caused reduced TEER and increased paracellular flux in vitro. Concurrent treatment with 5'-AMP attenuated these responses to Cdf toxin; an effect that was blocked with ZM241385 (AdoRA2 antagonist). APCP, a CD73 inhibitor, also suppressed the protective effects of 5'-AMP on paracellular flux. 5'-AMP reduced toxin-induced disruption of ZO-1, an effect that was abolished by APCP and ZM241385. Inhibition of CD73 with APCP during Cdf toxin exposure led to increased intestinal barrier permeability and epithelial damage in vivo. Intrarectal instillation of 5'-AMP had no effect on toxin-induced intestinal injury.

CONCLUSIONS & INFERENCES

Our data suggest that CD73 has a protective role against TcdA/B-induced damage. 5'-AMP treatment attenuated the damaging effects of Cdf toxin in vitro, and inhibitors of CD73 (APCP) and AdoRs (ZM241385) revealed that the cleavage of 5'-AMP to Ado was necessary for the protective effects. Inhibition of CD73 in vivo increases colonic tissue damage and epithelial permeability during Cdf toxin exposure.

A feedback loop in PPARγ-adenosine A2A receptor signaling inhibits inflammation and attenuates lung damages in a mouse model of LPS-induced acute lung injury

Although peroxisome proliferator-activated receptor-γ (PPARγ) and adenosine A2A receptor (A2AR) are reported to be anti-inflammatory factors in acute lung injury (ALI), their internal link and synergic or antagonistic effect after activation are poorly understood. Here, we found that PPARγ and A2AR could upregulate the mRNA and protein expressions of each other in lung tissues of LPS-induced mouse ALI model and murine macrophages. Further investigation demonstrated that PPARγ upregulated A2AR expression by directly binding to a DR10 response element (-218 to -197) within A2AR gene promoter region. Instead of directly interacting with PPARγ, A2AR stimulated PPARγ expression via protein kinase A (PKA)-cAMP response element binding protein (CREB) signaling by provoking the binding of CREB to a cAMP responsive element (CRE)-like site in PPARγ gene promoter region. In addition, combination of PPARγ and A2AR agonists was found to exert obviously better effect on suppressing neutrophil infiltration and inflammatory cytokine expressions, attenuating lung edema, pathological changes and improving lung function of blood gas exchange than their single application. These findings reveal a novel functional positive feedback loop between PPARγ and A2AR signaling to potentialize their effect on inhibiting inflammation and attenuating lung damages in ALI. It suggests that targeting this PPARγ-A2AR signaling rather than PPARγ or A2AR alone may be a more attractive and efficient potential therapeutic strategy for ALI.

Smoke extract impairs adenosine wound healing: implications of smoke-generated reactive oxygen species

Adenosine concentrations are elevated in the lungs of patients with asthma and chronic obstructive pulmonary disease, where it balances between tissue repair and excessive airway remodeling. We previously demonstrated that the activation of the adenosine A2A receptor promotes epithelial wound closure. However, the mechanism by which adenosine-mediated wound healing occurs after cigarette smoke exposure has not been investigated. The present study investigates whether cigarette smoke exposure alters adenosine-mediated reparative properties via its ability to induce a shift in the oxidant/antioxidant balance. Using an in vitro wounding model, bronchial epithelial cells were exposed to 5% cigarette smoke extract, were wounded, and were then stimulated with either 10 μM adenosine or the specific A2A receptor agonist, 5'-(N-cyclopropyl)-carboxamido-adenosine (CPCA; 10 μM), and assessed for wound closure. In a subset of experiments, bronchial epithelial cells were infected with adenovirus vectors encoding human superoxide dismutase and/or catalase or control vector. In the presence of 5% smoke extract, significant delay was evident in both adenosine-mediated and CPCA-mediated wound closure. However, cells pretreated with N-acetylcysteine (NAC), a nonspecific antioxidant, reversed smoke extract-mediated inhibition. We found that cells overexpressing mitochondrial catalase repealed the smoke extract inhibition of CPCA-stimulated wound closure, whereas superoxide dismutase overexpression exerted no effect. Kinase experiments revealed that smoke extract significantly reduced the A2A-mediated activation of cyclic adenosine monophosphate-dependent protein kinase. However, pretreatment with NAC reversed this effect. In conclusion, our data suggest that cigarette smoke exposure impairs A2A-stimulated wound repair via a reactive oxygen species-dependent mechanism, thereby providing a better understanding of adenosine signaling that may direct the development of pharmacological tools for the treatment of chronic inflammatory lung disorders.

Expression and clinical significance of CD73 and hypoxia-inducible factor-1α in gastric carcinoma

AIM: To investigate the expression of CD73 and hypoxia-inducible factor-1α (HIF-1α) in human gastric carcinoma, and explore their clinical significance and prognostic value.

METHODS: CD73 and HIF-1α expressions were detected by immunohistochemistry in consecutive sections of tissue samples from 68 gastric carcinoma patients. The peritumor tissues 2 cm away from the tumor were obtained and served as controls. The presence of CD73 and HIF-1α was analyzed by immunohistochemistry using the Envision technique.

RESULTS: CD73 and HIF-1α expressions in gastric carcinoma were significantly higher than those in gastric mucosal tissues as control (P < 0.001) and showed a close correlation (Spearman r = 0.390, P = 0.001). Overexpression of CD73 was positively correlated with differentiation of tumor (P = 0.000), histopathology (P = 0.041), depth of invasion (P < 0.001), nodal status (P = 0.003), metastasis (P = 0.013), and the American Joint Committee on Cancer (AJCC) stage (P < 0.001). High expression of HIF-1α was positively correlated with tumor diameter (P = 0.031), depth of invasion (P = 0.022), and AJCC stage (P = 0.035). The overall survival rate was low in the patients with high expression of CD73 (P < 0.001). Moreover, CD73+/HIF-1α+ patients had the worst prognosis (P < 0.001). CD73 expression was proven to be an independent predictor for patients with gastric carcinoma by both multivariate Cox regression analysis (P = 0.021) and receiver operating characteristic curves (P = 0.001).

CONCLUSION: CD73 expression correlates closely with HIF-1α expression in gastric carcinoma. CD73 could be an independent prognostic indicator for gastric carcinoma.

Sinomenine protects against lipopolysaccharide-induced acute lung injury in mice via adenosine A(2A) receptor signaling

Sinomenine (SIN) is a bioactive alkaloid extracted from the Chinese medicinal plant Sinomenium acutum, which is widely used in the clinical treatment of rheumatoid arthritis (RA). However, its role in acute lung injury (ALI) is unclear. In this study, we investigate the role of SIN in lipopolysaccharide (LPS)-induced ALI in mice. After ALI, lung water content and histological signs of pulmonary injury were attenuated, whereas the PaO₂/FIO₂ (P/F) ratios were elevated significantly in the mice pretreated with SIN. Additionally, SIN markedly inhibited inflammatory cytokine TNF-α and IL-1β expression levels as well as neutrophil infiltration in the lung tissues of the mice. Microarray analysis and real-time PCR showed that SIN treatment upregulated adenosine A_2A receptor (A_2AR) expression, and the protective effect of SIN was abolished in A_2AR knockout mice. Further investigation in isolated mouse neutrophils confirmed the upregulation of A_2AR by SIN and showed that A_2AR-cAMP-PKA signaling was involved in the anti-inflammatory effect of SIN. Taken together, these findings demonstrate an A_2AR-associated anti-inflammatory effect and the protective role of SIN in ALI, which suggests a potential novel approach to treat ALI.

Effects of adenosine A2a receptor agonist and antagonist on hippocampal nuclear factor-kB expression preceded by MDMA toxicity

There is an abundance of evidence showing that repeated use of 3,4-methlylenedioxymethamphetamine (MDMA; ecstasy) is associated with brain dysfunction, memory disturbance, locomotor hyperactivity, and hyperthermia. MDMA is toxic to both the serotonergic neurons and dopaminergic system. Adenosine is an endogenous purine nucleoside with a neuromodulatory function in the central nervous system. Nuclear factor kappa-B (NF-kB) plays a pivotal role in the initiation and perpetuation of an immune response by triggering the expression of major inflammatory mediators such as cytokines, chemokines, and adhesion molecules. Here, we investigated the effects of the A2a adenosine receptor (A2a-R) agonist (CGS) and antagonist (SCH) on NF-kB expression after MDMA administration. Male Sprague-Dawley rats were injected to MDMA (10 mg/kg) followed by intraperitoneal injection of either CGS or SCH (0.03 mg/kg each) to animals. The hippocampi were then removed for western blot and RT- PCR analyses. MDMA significantly elevated NF-kB expression. Our results show that administration of CGS following MDMA significantly elevated the NF-kB expression both at mRNA and protein levels. By contrast, administration of the A2a-R antagonist SCH resulted in a decrease in the NF-kB levels. Taken together, these results indicate that, co-administration of A2a agonist (CGS) can protect against MDMA neurotoxic effects by increasing NF-kB expression levels; suggesting a potential application for protection against the neurotoxic effects observed in MDMA users.

3-{3,5-Bis[(2-but-oxy-eth-oxy)carbon-yl]-2,6-dimethyl-1,4-dihydro-pyridin-4-yl}-1-[(3,4,5-trimeth-oxy-benzo-yl)meth-yl]pyridinium bromide

In the title salt, C37H51N2O10(+)·Br(-), the 1,4-dihydro-pyridine (1,4-DHP) ring adopts a slighly puckered boat conformation. The N and opposite C atoms deviate from the least-squares plane calculated through the four other ring atoms by 0.068 (5) and 0.224 (5) Å, respectively. The orientation of both C=O groups is similar (cis with respect to the double bonds of 1,4-DHP. The pyridinium ring has an axial orientation with respect to the1,4-DHP ring and is almost perpendicular to the least-squares plane of the 1,4-DHP ring, making a dihedral angle of 89.2 (3)°. The mol-ecule has a compact shape due to the parallel orientation of the long-chain substituents. One of the but-oxy groups was fond to be disordered (occupancy ratio 0.70:0.30). In the crystal, the bromide anion accepts a weak hydrogen bond from the N-H group of a neighboring 1,4-DHP ring.

Contribution of adenosine A(2B) receptors in Clostridium difficile intoxication and infection

Clostridium difficile toxins A (TcdA) and B (TcdB) induce a pronounced systemic and intestinal inflammatory response. A(2B) adenosine receptors (A(2B)ARs) are the predominant adenosine receptors in the intestinal epithelium. We investigated whether A(2B)ARs are upregulated in human intestinal cells by TcdA or TcdB and whether blockade of A(2B)ARs can ameliorate C. difficile TcdA-induced enteritis and alter the outcome of C. difficile infection (CDI). Adenosine receptor subtype (A(1), A(2A), A(2B), and A(3)) mRNAs were assayed in HCT-8 cells. Ileal loops from wild-type rabbits and mice and A(2B)AR(-/-) mice were treated with TcdA, with or without the selective A(2B)AR antagonist ATL692 or PSB1115. A murine model of CDI was used to determine the effect of A(2B)AR deletion or blockade with the orally available agent ATL801, on clinical outcome, histopathology and intestinal interleukin-6 (IL-6) expression from infection. TcdA and TcdB upregulated A(2B)AR gene expression in HCT-8 cells. ATL692 decreased TcdA-induced secretion and epithelial injury in rabbit ileum. Deletion of A(2B)ARs reduced secretion and histopathology in TcdA-challenged mouse ileum. Deletion or blockade of A(2B)ARs reduced histopathology, IL-6 expression, weight loss, diarrhea, and mortality in C. difficile-infected mice. A(2B)ARs mediate C. difficile toxin-induced enteritis and disease. Inhibition of A(2B)AR activation may be a potential strategy to limit morbidity and mortality from CDI.

Design and application of locally delivered agonists of the adenosine A(2A) receptor

The broad spectrum anti-inflammatory actions of adenosine A(2A) receptor agonists are well described. The wide distribution of this receptor, however, suggests that the therapeutic potential of these agents is likely to reside in topical treatments to avoid systemic side effects associated with oral administration. Adenosine A(2A) receptor agonists have been assessed as topical agents: GW328267X (GSK; allergic rhinitis and asthma), UK-432097 (Pfizer; chronic obstructive pulmonary disease [COPD]) and Sonedenoson (MRE0094, King Pharmaceuticals; wound healing). All trials failed to achieve effects against the desired clinical end points. This broad-based review will discuss general principles of chemical design of topically applied agents and potential therapeutic topical applications of current adenosine A(2A) receptor agonists. Potential factors contributing to the lack of efficacy in the above clinical trials will be discussed together with design principles, which may influence efficacy in disease states. Our analysis suggests that adenosine A(2A) receptor agonists have a wide therapeutic potential as topical agents in a wide variety of diseases, such as neutrophil-dependent lung diseases (acute lung injury, exacerbations in asthma and COPD), allergic rhinitis, glaucoma and wound repair. Factors that will influence topical activity include formulation, tissue retention, compound potency, receptor kinetics and pharmacokinetics.

A(₂B)AR expression in non-immune cells plays an important role in the development of murine colitis

BACKGROUND

Adenosine, an endogenous purine nucleoside, is involved in several physiological functions. We have previously shown that A(2B)AR plays a pro-inflammatory role during colitis.

AIMS

Our goals were to determine if A(2B)AR expression was necessary on immune cells/non-immune cells during colitis and if A(2B)AR was a suitable target for treating intestinal inflammation.

METHODS

Wild-type and A(2B)AR knockout mice were utilized in bone marrow transplants to explore the importance of immune/non-immune A(2B)AR expression during the development of colitis. Additionally, a T-cell transfer model of colitis was used in Rag1 knockout or A(2B)AR/RAG1 double knockout recipients. Finally, A(2B)AR small interfering RNA nanoparticles were administered to dextran sodium sulphate-treated mice.

RESULTS

Wild-type mice receiving wild-type or knockout bone marrow developed severe colitis after dextran sodium sulphate treatment, whereas colitis was significantly attenuated in knockout mice receiving wild-type or knockout bone marrow. Colitis induced in Rag1 knockout animals was attenuated in A(2B)AR/RAG1 double knockout recipients. Animals receiving nanoparticles exhibited attenuated parameters of colitis severity compared to mice receiving control nanoparticles.

CONCLUSIONS

Our results suggest that A(2B)AR on non-immune cells plays an important role for the induction of colitis and targeting A(2B)AR expression during colitis may be useful for alleviating symptoms of intestinal inflammation.

Cardioprotection of controlled and cardiac-specific over-expression of A(2A)-adenosine receptor in the pressure overload

Adenosine binds to three G protein-coupled receptors (R) located on the cardiomyocyte (A(1)-R, A(2A)-R and A(3)-R) and provides cardiac protection during both ischemic and load-induced stress. While the role of adenosine receptor-subtypes has been well defined in the setting of ischemia-reperfusion, far less is known regarding their roles in protecting the heart during other forms of cardiac stress. Because of its ability to increase cardiac contractility and heart rate, we hypothesized that enhanced signaling through A(2A)-R would protect the heart during the stress of transverse aortic constriction (TAC). Using a cardiac-specific and inducible promoter, we selectively over-expressed A(2A)-R in FVB mice. Echocardiograms were obtained at baseline, 2, 4, 8, 12, 14 weeks and hearts were harvested at 14 weeks, when WT mice developed a significant decrease in cardiac function, an increase in end systolic and diastolic dimensions, a higher heart weight to body weight ratio (HW/BW), and marked fibrosis when compared with sham-operated WT. More importantly, these changes were significantly attenuated by over expression of the A(2A)-R. Furthermore, WT mice also demonstrated marked increases in the hypertrophic genes β-myosin heavy chain (β-MHC), and atrial natriuretic factor (ANF)--changes that are mediated by activation of the transcription factor GATA-4. Levels of the mRNAs encoding β-MHC, ANP, and GATA-4 were significantly lower in myocardium from A(2A)-R TG mice after TAC when compared with WT and sham-operated controls. In addition, three inflammatory factors genes encoding cysteine dioxygenase, complement component 3, and serine peptidase inhibitor, member 3N, were enhanced in WT TAC mice, but their expression was suppressed in A(2A)-R TG mice. A(2A)-R over-expression is protective against pressure-induced heart failure secondary to TAC. These cardioprotective effects are associated with attenuation of GATA-4 expression and inflammatory factors. The A(2A)-R may provide a novel new target for pharmacologic therapy in patients with cardiovascular disease.

Leishmania amazonensis impairs DC function by inhibiting CD40 expression via A2B adenosine receptor activation

Dendritic cells (DCs) play an essential role in the modulation of immune responses and several studies have evaluated the interactions between Leishmania parasites and DCs. While extracellular ATP exhibits proinflammatory properties, adenosine is an important anti-inflammatory mediator. Here we investigated the effects of Leishmania infection on DC responses and the participation of purinergic signalling in this process. Bone marrow-derived dendritic cells (BMDCs) from C57BL/6J mice infected with Leishmania amazonensis, Leishmania braziliensis or Leishmania major metacyclic promastigotes showed decreased major histocompatibility complex (MHC) class II and CD86 expression and increased ectonucleotidase expression as compared with uninfected cells. In addition, L. amazonensis-infected DCs, which had lower CD40 expression, exhibited a decreased ability to induce T-cell proliferation. The presence of MRS1754, a highly selective A(2B) adenosine receptor antagonist at the time of infection increased MHC class II, CD86 and CD40 expression in L. amazonensis-infected DCs and restored the ability of the infected DCs to induce T-cell proliferation. Similar results were obtained through the inhibition of extracellular ATP hydrolysis using suramin. In conclusion, we propose that A(2B) receptor activation may be used by L. amazonensis to inhibit DC function and evade the immune response.

Ecto-5'-nucleotidase: a candidate virulence factor in Streptococcus sanguinis experimental endocarditis

Streptococcus sanguinis is the most common cause of infective endocarditis (IE). Since the molecular basis of virulence of this oral commensal bacterium remains unclear, we searched the genome of S. sanguinis for previously unidentified virulence factors. We identified a cell surface ecto-5′-nucleotidase (Nt5e), as a candidate virulence factor. By colorimetric phosphate assay, we showed that S. sanguinis Nt5e can hydrolyze extracellular adenosine triphosphate to generate adenosine. Moreover, a nt5e deletion mutant showed significantly shorter lag time (P<0.05) to onset of platelet aggregation than the wild-type strain, without affecting platelet-bacterial adhesion in vitro (P = 0.98). In the absence of nt5e, S. sanguinis caused IE (4 d) in a rabbit model with significantly decreased mass of vegetations (P<0.01) and recovered bacterial loads (log₁₀CFU, P = 0.01), suggesting that Nt5e contributes to the virulence of S. sanguinis in vivo. As a virulence factor, Nt5e may function by (i) hydrolyzing ATP, a pro-inflammatory molecule, and generating adenosine, an immunosuppressive molecule to inhibit phagocytic monocytes/macrophages associated with valvular vegetations. (ii) Nt5e-mediated inhibition of platelet aggregation could also delay presentation of platelet microbicidal proteins to infecting bacteria on heart valves. Both plausible Nt5e-dependent mechanisms would promote survival of infecting S. sanguinis. In conclusion, we now show for the first time that streptococcal Nt5e modulates S. sanguinis-induced platelet aggregation and may contribute to the virulence of streptococci in experimental IE.

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.

Effects of adenosine A₂A receptor activation and alanyl-glutamine in Clostridium difficile toxin-induced ileitis in rabbits and cecitis in mice

Background

Severe Clostridium difficile toxin-induced enteritis is characterized by exuberant intestinal tissue inflammation, epithelial disruption and diarrhea. Adenosine, through its action on the adenosine A_2A receptor, prevents neutrophillic adhesion and oxidative burst and inhibits inflammatory cytokine production. Alanyl-glutamine enhances intestinal mucosal repair and decreases apoptosis of enterocytes. This study investigates the protection from enteritis by combination therapy with ATL 370, an adenosine A_2A receptor agonist, and alanyl-glutamine in a rabbit and murine intestinal loop models of C. difficile toxin A-induced epithelial injury.

Methods

Toxin A with or without alanyl-glutamine was administered intraluminally to rabbit ileal or murine cecal loops. Animals were also given either PBS or ATL 370 parenterally. Ileal tissues were examined for secretion, histopathology, apoptosis, Cxcl1/KC and IL-10.

Results

ATL 370 decreased ileal secretion and histopathologic changes in loops treated with Toxin A. These effects were reversed by the A_2A receptor antagonist, SCH 58261, in a dose-dependent manner. The combination of ATL 370 and alanyl-glutamine significantly further decreased ileal secretion, mucosal injury and apoptosis more than loops treated with either drug alone. ATL 370 and alanyl-glutamine also decreased intestinal tissue KC and IL-10.

Conclusions

Combination therapy with an adenosine A_2A receptor agonist and alanyl-glutamine is effective in reversing C. difficile toxin A-induced epithelial injury, inflammation, secretion and apoptosis in animals and has therapeutic potential for the management of C. difficile infection.

The effect of adenosine on pro-inflammatory cytokine production by porcine T cells

Adenosine is a well described anti-inflammatory modulator of immune responses. The aim of the present study was to describe the role of common adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA) in cytokine production by main porcine T cell subpopulations. TNF-α, IFN-γ, IL-2 and IL-10 were detected by multicolor flow cytometry together with cell surface markers CD3, CD4 and CD8. It was found that NECA inhibits (in a dose-dependent manner) production of pro-inflammatory TNF-α and Th1-associated cytokines IFN-γ, IL-2 in all concanavalin A-stimulated T cell subpopulations. Moreover, production of IL-10 was potentiated in all T cell subpopulations tested. These corresponded well with the fact that all T cell subsets expressed mRNA for adenosine receptor (AR) subtypes to comparable extents. Contrary to concanavalin A-stimulated cells, NECA had a moderate effect on PMA-stimulated T cells, suggesting that AR in pigs acts via signaling pathways not associated with protein-kinase C. Non-selective antagonist CGS15943 as well as allosteric modulator SCH202676 failed to reverse the effect of NECA in pigs. In conclusion, NECA has an anti-inflammatory effect on porcine T cell subpopulations.

Adenosine A(2A) agonists as therapy for glomerulonephritis

Garcia et al. report the utility of pharmacological activation of the adenosine A(2A) receptor (A(2A)R) in preserving renal function, reversing fibrosis, and reducing macrophage infiltration and inflammatory activation in rat nephrotoxic nephritis. The role of A(2A)R activation in determining outcome in renal inflammation is discussed.

Adenosine augments IL-10 production by microglial cells through an A2B adenosine receptor-mediated process

Microglia are activated by pathogen-associated molecular patterns and produce proinflammatory cytokines, such as TNF-α, IL-6, and IL-12, and the anti-inflammatory cytokine IL-10. Adenosine is an endogenous purine nucleoside and a ligand of four G protein-coupled adenosine receptors (ARs), which are the A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR. ARs have been shown to suppress TNF-α production by microglia, but their role in regulating IL-10 production has not been studied. In this study, we demonstrate that adenosine augments IL-10 production by activated murine microglia while suppressing the production of proinflammatory cytokines. Because the order of potency of selective AR agonists in inducing IL-10 production was NECA > IB-MECA > CCPA ≥ CGS21680, and the A(2B)AR antagonist MRS1754 prevented the effect of NECA, we conclude that the stimulatory effect of adenosine on IL-10 production is mediated by the A(2B)AR. Mechanistically, adenosine augmented IL-10 mRNA accumulation by a transcriptional process. Using mutant IL-10 promoter constructs we showed that a CREB-binding region in the promoter mediated the augmenting effect of adenosine on IL-10 transcription. Chromatin immunoprecipitation analysis demonstrated that adenosine induced CREB phosphorylation at the IL-10 promoter. Silencing CREB using lentivirally delivered short hairpin RNA blocked the enhancing effect of adenosine on IL-10 production, confirming a role for CREB in mediating the stimulatory effect of adenosine on IL-10 production. In addition, adenosine augmented IL-10 production by stimulating p38 MAPK. Collectively, our results establish that A(2B)ARs augment IL-10 production by activated murine microglia.

Endotoxin-induced effects on nucleotide catabolism in mouse kidney

Extracellular adenosine 5'-triphosphate (ATP) acts as a proinflammatory mediator. Adenosine, the final product of ATP breakdown, is an anti-inflammatory compound, acting mainly on adenosine A(2A) receptors. Considering that the kidney is an organ strongly affected during systemic inflammatory responses and that ectonucleotidases are responsible for the control of extracellular nucleotide and nucleoside levels, we examined the endotoxin-induced effects on ectonucleotidases in kidney membranes of mice, and whether CGS-21680 hydrochloride (3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino]ethyl]phenyl]propanoic acid), a selective adenosine A(2A) receptor agonist, antagonizes the lipopolysaccharide (LPS)-induced effects on nucleotide catabolism in kidney. Animals were injected intraperitoneally with 12 mg/kg LPS and/or 0.5mg/kg CGS-21680 or saline. Nucleotidase activities were determined in kidney membrane preparations and ATP metabolism was measured by high performance liquid chromatography (HPLC) assay. Analysis of ectonucleotidase expression was carried out by semi-quantitative semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR). Exposure to endotoxemia promoted an increase in ATP and p-Nitrophenyl thymidine 5'-monophosphate (p-Nph-5'-TMP) hydrolysis, and a decrease in adenosine 5'-monophosphate (AMP) hydrolysis. CGS-21680 treatment failed to reverse these changes. HPLC analysis indicated a decrease in extracellular ATP and adenosine levels in groups treated with LPS and LPS plus CGS-21680. The expression pattern of ectonucleotidases revealed an increase in Entpd3, Enpp2, and Enpp3 mRNA levels after LPS injection. These findings indicate that nucleotide and nucleoside availability in mouse kidney is altered at different stages of endotoxemia, in order to protect the integrity of this organ when exposed to systemic inflammation.

Cytoprotective and proangiogenic activity of ex-vivo netrin-1 transgene overexpression protects the heart against ischemia/reperfusion injury

In continuation of a previous work that transgene expression of sonic hedgehog promoted neo-vascularization via netrin-1 release, the current study was aimed at assessing the anti-apoptotic and pro-angiogenic role of netrin-1 transgene overexpression in the ischemic myocardium. pLP-Adeno-X ViralTrak vectors containing netrin-1 cDNA amplified from rat mesenchymal stem cells (Ad-netrin) or without a therapeutic gene (Ad-null) were constructed and transfected into HEK-293 cells to produce Ad-netrin and Ad-null vectors. Sca-1(+)-like cells were isolated and propagated in vitro and were successfully transduced with Ad-netrin transduced Sca-1(+) cells ((Net)Sca-1(+)) and Ad-null transduced Sca-1(+) cells ((Null)Sca-1(+)). Overexpression of netrin-1 in (Net)Sca-1(+) was confirmed by reverse transcription-polymerase chain reaction and western blot. Neonatal cardiomyocytes and rat endothelial cells expressed netrin-1 specific receptor Uncoordinated-5b and the conditioned medium from (Net)Sca-1(+) cells was protective for both the cell types against oxidant stress. For in vivo studies, the rat model of myocardial ischemia/reperfusion injury was developed in female Wistar rats by left anterior descending coronary artery occlusion for 45 min followed by reperfusion. The animals were grouped to receive 70 μL of Dulbecco's modified Eagle's medium without cells (group-1), containing 2×10(6) (Null)Sca-1(+) cells (group-2) and (Net)Sca-1(+) cells (group-3). (Net)Sca-1(+) cells significantly reduced ischemia/reperfusion injury in the heart and preserved the global heart function in group-3 (P<0.05 vs. groups-1 and group-2). Ex-vivo netrin-1 overexpression in the heart increased NOS activity in the heart. Blood vessel density was significantly higher in group-3 (P<0.05 vs. controls). We concluded that netrin-1 decreased apoptosis in cardiomyocytes and endothelial cells via activation of Akt. Netrin-1 transgene expression was proangiogenic and effectively reduced ischemia/reperfusion injury to preserve global heart function.

Enzymatic properties of Staphylococcus aureus adenosine synthase (AdsA)

Background

Staphylococcus aureus is a human pathogen that produces extracellular adenosine to evade clearance by the host immune system, an activity attributed to the 5'-nucleotidase activity of adenosine synthase (AdsA). In mammals, conversion of adenosine triphosphate to adenosine is catalyzed in a two-step process: ecto-nucleoside triphosphate diphosphohydrolases (ecto-NTDPases) hydrolyze ATP and ADP to AMP, whereas 5'-nucleotidases hydrolyze AMP to adenosine. NTPDases harbor apyrase conserved regions (ACRs) that are critical for activity.

Results

NTPDase ACR motifs are absent in AdsA, yet we report here that recombinant AdsA hydrolyzes ADP and ATP in addition to AMP. Competition assays suggest that hydrolysis occurs following binding of all three substrates at a unique site. Alanine substitution of two amino acids, aspartic acid 127 and histidine 196 within the 5'-nucleotidase signature sequence, leads to reduced AMP or ADP hydrolysis but does not affect the binding of these substrates.

Conclusion

Collectively, these results provide insight into the unique ability of AdsA to produce adenosine through the consecutive hydrolysis of ATP, ADP and AMP, thereby endowing S. aureus with the ability to modulate host immune responses.

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.

Ecto-5'-nucleotidase (CD73) decreases mortality and organ injury in sepsis

The extracellular concentrations of adenosine are increased during sepsis, and adenosine receptors regulate the host's response to sepsis. In this study, we investigated the role of the adenosine-generating ectoenzyme, ecto-5'-nucleotidase (CD73), in regulating immune and organ function during sepsis. Polymicrobial sepsis was induced by subjecting CD73 knockout (KO) and wild type (WT) mice to cecal ligation and puncture. CD73 KO mice showed increased mortality in comparison with WT mice, which was associated with increased bacterial counts and elevated inflammatory cytokine and chemokine concentrations in the blood and peritoneum. CD73 deficiency promoted lung injury, as indicated by increased myeloperoxidase activity and neutrophil infiltration, and elevated pulmonary cytokine levels. CD73 KO mice had increased apoptosis in the thymus, as evidenced by increased cleavage of caspase-3 and poly(ADP-ribose) polymerase and increased activation of NF-κB. Septic CD73 KO mice had higher blood urea nitrogen levels and increased cytokine levels in the kidney, indicating increased renal dysfunction. The increased kidney injury of CD73 KO mice was associated with augmented activation of p38 MAPK and decreased phosphorylation of Akt. Pharmacological inactivation of CD73 in WT mice using α, β-methylene ADP augmented cytokine levels in the blood and peritoneal lavage fluid. These findings suggest that CD73-derived adenosine may be beneficial in sepsis.

Adenosine and immune imbalance in visceral leishmaniasis: the possible role of ectonucleotidases

Visceral leishmaniasis (VL) is the most severe form of leishmaniasis and is responsible for most Leishmania-associated deaths. VL represents a serious public health problem that affects many countries. The immune response in leishmaniasis is very complex and is poorly understood. The Th1 versus Th2 paradigm does not appear to be so clear in visceral leishmaniasis, suggesting that other immunosuppressive or immune-evasion mechanisms contribute to the pathogenesis of VL. It has been demonstrated that generation of adenosine, a potent endogenous immunosuppressant, by extracellular enzymes capable to hydrolyze adenosine tri-nucleotide (ATP) at the site of infection, can lead to immune impairment and contribute to leishmaniasis progression. In this regard, this paper discusses the unique features in VL immunopathogenesis, including a possible role for ectonucleotidases in leishmaniasis.

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.

CGS 21680, an agonist of the adenosine (A2A) receptor, decreases acute lung inflammation

Adenosine A(2A) receptor agonists may be important regulators of inflammation. The aim of this study was to investigate the effects of CGS 21680 (0.1mg/kgi.p.), an agonist of the adenosine (A(2A)) receptor, in a mouse model of carrageenan-induced pleurisy. Injection of carrageenan into the pleural cavity of mice elicited an acute inflammatory response characterised by: infiltration of neutrophils in lung tissues and subsequent lipid peroxidation, increased production of nitric oxide (NO), cytokines such as tumour necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) and increased expression of intercellular adhesion molecule (ICAM-1) and platelet-adhesion molecule (P-selectin). Furthermore, carrageenan induced the expression of nuclear factor-κB (NF-κB), inducible nitric oxide synthase (iNOS), nitrotyrosine, the activation of poly-ADP-ribosyl polymerase (PARP), as well as induced apoptosis (FAS-ligand expression, Bax and Bcl-2 expression) in the lung tissues. Administration of CGS 21680, 30 min prior to challenge with carrageenan, caused a significant reduction of all the parameters of inflammation measured. In addition, to confirm the anti-inflammatory effect of CGS 21680, we have also evaluated the effects of CGS 21680 post-treatment (30 min after the challenge with carrageenan) and we have demonstrated that also it caused a reduction of neutrophil infiltration and the degree of lung injury. Thus, based on these findings we propose that adenosine A(2A) receptor agonists such as CGS 21680 may be useful in the treatment of various inflammatory diseases.

Activation of adenosine A2A receptors inhibits neutrophil transuroepithelial migration

Adenosine has been identified as a significant inhibitor of inflammation by acting on adenosine A(2A) receptors. In this study, we examined the role of adenosine and A(2A) receptors in the transmigration of human neutrophils across an in vitro model of the transitional bladder urothelium. Human uroepithelial cells (UROtsa) were grown on transwell inserts; uropathogenic Escherichia coli (UPEC) and neutrophils were added to the transwell system; and the number of migrating neutrophils was evaluated. Reverse transcription-PCR (RT-PCR), immunohistochemistry, and flow cytometry were used to investigate the expression of adenosine receptors, the epithelial adhesion molecule ICAM-1, and the neutrophil integrin CD11b. Levels of proinflammatory interleukin-8 (IL-8) and phosphorylated IκBα were measured by enzyme-linked immunosorbent assays (ELISA) and Luminex assays, respectively. The neutrophils expressed all four adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3) receptors), but A(3) receptors were not expressed by UROtsa cells. UPEC stimulated neutrophil transuroepithelial migration, which was significantly decreased in response to the specific A(2A) receptor agonist CGS 21680. The inhibitory effect of CGS 21680 on neutrophil migration was reversed by the A(2A) receptor antagonist SCH 58261. The production of chemotactic IL-8 and the expression of the adhesion molecule ICAM-1 or CD11b were not significantly affected by CGS 21680. However, a significant decrease in the level of phosporylated IκBα was revealed in response to CGS 21680. In conclusion, UPEC infection in vitro evoked neutrophil migration through a multilayered human uroepithelium. The UPEC-evoked neutrophil transmigration decreased in response to A(2A) receptor activation, possibly through inhibition of NF-κB signaling pathways.

P2Y6 and vascular inflammation

In this issue of Blood, Riegel and colleagues demonstrate that inflammatory stimuli induce the expression of the P2Y6 receptor on the vascular endothelium where it serves to enhance systemic inflammatory responses.

Animal models of airway diseases

Over the past 20 years, the growing awareness that purinergic signaling events literally shape the immune and inflammatory responses to infection and allergic reactions warranted the development of animal models to assess their importance in vivo in acute lung injury and chronic airway diseases. The pioneer work conducted with the adenosine deaminase (ADA)-deficient mouse provided irrefutable evidence that excess adenosine (ADO) accumulating in the lungs of asthmatic patients, constitutes a powerful mediator of disease severity. These original studies launched the development of murine strains for the two major ectonucleotidases responsible for the generation of airway ADO from ATP release: CD39 and CD73. The dramatic acute lung injury and chronic lung complications, manifested by these knockout mice in response to allergens and endotoxin, demonstrated the critical importance of regulating the availability of ATP and ADO for their receptors. Therapeutic targets are currently evaluated using knockout mice and agonists/antagonists for each ADO receptor (A(1)R, A(2A)R, A(2B)R, and A(3)R) and the predominant ATP receptors (P2Y(2)R and P2X(7)R). This chapter provides an in-depth description of each in vivo study, and a critical view of the therapeutic potentials for the treatment of airway diseases.

Adenosine A2(A) receptor modulates the oxidative stress response of primed polymorphonuclear leukocytes after parabolic flight

Space flight and gravitational stress can alter innate immune function. Parabolic flights (PFs) as a model for short-term gravitational changes prime the cytotoxic capability of polymorphonuclear leukocytes (PMNs). In view of the emerging role of adenosine in the regulation of innate immune responses, we examined the potency of adenosine to control the release of cytotoxic H(2)O(2) by primed PMNs via the adenosine receptor system. During PFs, microgravity conditions (<10(-2) G) are generated for approximately 22 seconds, followed by a hypergravity (1.8 G) phase resulting in gravitational stress. We studied the ex vivo effects of adenosine on the production of H(2)O(2) by stimulated PMNs and determined adenosine plasma levels and adenosine A2(A) receptor transcripts of leukocytes of PF participants (n = 15). Increasing concentrations of adenosine dose dependently reduced tissue-toxic H(2)O(2) production by PMNs with a half-maximal inhibitory concentration (IC(50)) of 19.5 nM before takeoff and 7.6 nM at 48 hours after PF. This increase in the adenosine-mediated inhibition of PMNs' H(2)O(2) production was completely reversed by addition of the A2(A) receptor antagonist ZM241385. PF induced a nonsignificant elevation in adenosine plasma levels; A2(A) receptor mRNA from leukocytes remained almost unchanged. Adenosine limits the oxidative stress response of PMNs after PFs through an upregulation of the adenosine A2(A) receptor function. This stop signal on inflammation is stronger than that under normal physiologic states and may limit further cytotoxic damage. Pharmacologic manipulation of the adenosine A2(A) receptor pathway could be a potential target for control of unwanted exacerbations of cytotoxic PMN functions.

Cooperation of adenosine with macrophage Toll-4 receptor agonists leads to increased glycolytic flux through the enhanced expression of PFKFB3 gene

Macrophages activated through Toll receptor triggering increase the expression of the A(2A) and A(2B) adenosine receptors. In this study, we show that adenosine receptor activation enhances LPS-induced pfkfb3 expression, resulting in an increase of the key glycolytic allosteric regulator fructose 2,6-bisphosphate and the glycolytic flux. Using shRNA and differential expression of A(2A) and A(2B) receptors, we demonstrate that the A(2A) receptor mediates, in part, the induction of pfkfb3 by LPS, whereas the A(2B) receptor, with lower adenosine affinity, cooperates when high adenosine levels are present. pfkfb3 promoter sequence deletion analysis, site-directed mutagenesis, and inhibition by shRNAs demonstrated that HIF1α is a key transcription factor driving pfkfb3 expression following macrophage activation by LPS, whereas synergic induction of pfkfb3 expression observed with the A(2) receptor agonists seems to depend on Sp1 activity. Furthermore, levels of phospho-AMP kinase also increase, arguing for increased PFKFB3 activity by phosphorylation in long term LPS-activated macrophages. Taken together, our results show that, in macrophages, endogenously generated adenosine cooperates with bacterial components to increase PFKFB3 isozyme activity, resulting in greater fructose 2,6-bisphosphate accumulation. This process enhances the glycolytic flux and favors ATP generation helping to develop and maintain the long term defensive and reparative functions of the macrophages.

Investigational A₃ adenosine receptor targeting agents

INTRODUCTION

Adenosine is an endogenous nucleoside that accumulates in the extracellular space in response to metabolic stress and cell damage. Extracellular adenosine is a signaling molecule that signals by activating four GPCRs: the A(1), A(2A), A(2B) and A(3) receptors. Since the discovery of A(3) adenosine receptors, accumulating evidence has identified these receptors as potential targets for therapeutic intervention.

AREAS COVERED

A(3) adenosine receptors are expressed on the surface of most immune cell types, including neutrophils, macrophages, dendritic cells, lymphocytes and mast cells. A(3) adenosine receptor activation on immune cells governs a broad array of immune cell functions, which include cytokine production, degranulation, chemotaxis, cytotoxicity, apoptosis and proliferation. In accordance with their multitudinous immunoregulatory actions, targeting A(3) adenosine receptors has been shown to impact the course of a wide spectrum of immune-related diseases, such as asthma, rheumatoid arthritis, cancer, ischemia and inflammatory disorders.

EXPERT OPINION

Given the existence of both preclinical and early clinical data supporting the utility of A(3) adenosine receptor ligands in treating immune-related diseases, further development of A(3) adenosine receptor ligands is anticipated.

Cl-IB-MECA enhances TNF-α release in peritoneal macrophages stimulated with LPS

Adenosine receptor A3 (A3R) belongs to the Gi/Gq-coupled receptor family, that leads to the intracellular cAMP reduction and intracellular calcium increase, respectively. A3R is widely expressed and it can play a crucial role in many patho-physiological conditions, including inflammation. Here we investigate the effect of Cl-IB-MECA, A3R agonist, on the production of TNF-α. We found that Cl-IB-MECA enhances LPS-induced TNF-α release in peritoneal macrophages. This effect is reduced by MRS1191, A3R antagonist and by forskolin, activator of adenylyl cyclase. pIκBα increased in LPS+Cl-IB-MECA-treated macrophages, while total IκB kinase-β (IKKβ) reduced. Indeed, p65NF-κB nuclear translocation increased in cells treated with LPS+Cl-IB-MECA. Moreover, IMD 0354, IKKβ inhibitor, significantly abrogated the effect of Cl-IB-MECA on TNF-α release. Inhibition of protein kinase C (PKC) significantly reduced Cl-IB-MECA-induced TNF-α release in LPS-stimulated macrophages. Furthermore, LY-294002, PI3K inhibitor, reduced the TNF-α production enhanced by Cl-IB-MECA, although the phosphorylation status of Akt did not change in cells treated with LPS+Cl-IB-MECA than LPS alone. In summary, these data show that Cl-IB-MECA is able to enhance TNF-α production in LPS-treated macrophages in an NF-κB- dependent manner.

Deconstructing tick saliva: non-protein molecules with potent immunomodulatory properties

Dendritic cells (DCs) are powerful initiators of innate and adaptive immune responses. Ticks are blood-sucking ectoparasite arthropods that suppress host immunity by secreting immunomodulatory molecules in their saliva. Here, compounds present in Rhipicephalus sanguineus tick saliva with immunomodulatory effects on DC differentiation, cytokine production, and costimulatory molecule expression were identified. R. sanguineus tick saliva inhibited IL-12p40 and TNF-α while potentiating IL-10 cytokine production by bone marrow-derived DCs stimulated by Toll-like receptor-2, -4, and -9 agonists. To identify the molecules responsible for these effects, we fractionated the saliva through microcon filtration and reversed-phase HPLC and tested each fraction for DC maturation. Fractions with proven effects were analyzed by micro-HPLC tandem mass spectrometry or competition ELISA. Thus, we identified for the first time in tick saliva the purine nucleoside adenosine (concentration of ∼110 pmol/μl) as a potent anti-inflammatory salivary inhibitor of DC cytokine production. We also found prostaglandin E(2) (PGE(2) ∼100 nM) with comparable effects in modulating cytokine production by DCs. Both Ado and PGE(2) inhibited cytokine production by inducing cAMP-PKA signaling in DCs. Additionally, both Ado and PGE(2) were able to inhibit expression of CD40 in mature DCs. Finally, flow cytometry analysis revealed that PGE(2), but not Ado, is the differentiation inhibitor of bone marrow-derived DCs. The presence of non-protein molecules adenosine and PGE(2) in tick saliva indicates an important evolutionary mechanism used by ticks to subvert host immune cells and allow them to successfully complete their blood meal and life cycle.

Chronic or high dose acute caffeine treatment protects mice against oleic acid-induced acute lung injury via an adenosine A2A receptor-independent mechanism

The antagonism or genetic deletion of adenosine A(2A) receptors has been shown to exacerbate tissue damage in acute lung injury. Caffeine, a widely consumed behavioral drug, acts as a non-selective antagonist of A(2A) receptor and also has additional pharmacological effects. Thus, the protective vs. deleterious effects of caffeine in acute lung injury should be evaluated. In a murine oleic acid-induced model of acute lung injury, we found that chronic caffeine treatment by drinking water (0.1g/l or 0.25g/l for 2 weeks before acute lung injury) or acute caffeine treatment at high dose (i.p. 50mg/kg, injection, 30min before acute lung injury) significantly attenuated the lung edema, hemorrhage, neutrophil recruitment as well as the inflammatory cytokine tumor necrosis factor-α (TNF-α) and interleukin-1 (IL-1) expressions in both of the wild type (WT) and A(2A) receptor knockout (KO) mice. This profile was accompanied by increased cAMP levels and up-regulation of A2B receptor mRNAs in the lungs. In contrast, acute caffeine treatment at low dose (i.p. 5mg/kg or 15mg/kg, injection, 30min before acute lung injury) enhanced the inflammation and lung damage in WT mice with decreasing cAMP but not in A(2A) receptor KO mice. These results indicate that caffeine either enhances lung damage by antagonizing A(2A) receptor or exerts protection against lung damage via A(2A) receptor-independent mechanisms, depending on the timing of exposure (chronic vs. acute) and dose of administration (low vs. high). These findings provide new insight of caffeine in acute lung injury and highlight the potential benefit and strategy of caffeine intake or administration for preventing acute lung injury.

Distinct roles for the A2B adenosine receptor in acute and chronic stages of bleomycin-induced lung injury

Adenosine is an extracellular signaling molecule that is generated in response to cell injury where it orchestrates tissue protection and repair. Whereas adenosine is best known for promoting anti-inflammatory activities during acute injury responses, prolonged elevations can enhance destructive tissue remodeling processes associated with chronic disease states. The generation of adenosine and the subsequent activation of the adenosine 2B receptor (A(2B)R) is an important processes in the regulation of both acute and chronic lung disease. The goal of this study was to examine the contribution of the A(2B)R in models of bleomycin-induced lung injury that exhibit varying degrees of acute and chronic injury. Intratracheal bleomycin exposure results in substantial acute lung injury followed by progressive fibrosis. In this model, genetic removal of the A(2B)R resulted in enhanced loss of barrier function and increased pulmonary inflammation, with few differences in indexes of pulmonary fibrosis. These results support an anti-inflammatory role for this receptor in this model of acute lung injury. In contrast, systemic exposure of mice to bleomycin resulted in modest acute lung injury together with progressive pulmonary fibrosis. In this model, the effects of A(2B)R removal on acute lung injury were negligible; however, there were substantial reductions in pulmonary fibrosis, supporting a profibrotic role for this receptor. A(2B)R-dependent regulation of IL-6 production was identified as a potential mechanism involved in the diminished pulmonary fibrosis seen in A(2B)R knockout mice exposed to i.p. bleomycin. These studies highlight the distinct roles of A(2B)R signaling during acute and chronic stages of lung injury.

Adenosine A(2A) receptor agonist (CGS-21680) prevents endotoxin-induced effects on nucleotidase activities in mouse lymphocytes

Adenosine 5'-triphosphate (ATP) released during inflammation presents proinflammatory properties. Adenosine, produced by catabolism of ATP, is an anti-inflammatory compound. Considering the role of ATP and adenosine in inflammation and the importance of ectonucleotidases in the maintenance of their extracellular levels, we investigated the effect of a selective agonist of the adenosine A(2A) receptor (CGS-21680) on ectonucleotidase activities and gene expression patterns in lymphocytes from mice submitted to an endotoxemia model. Animals were injected intraperitoneally with 12mg/kg Lipopolyssacharide (LPS) and/or 0.5mg/kg CGS-21680 or saline. Nucleotidase activities were determined in lymphocytes from mesenteric lymph nodes and analysis of ectonucleotidase expression was carried out by a semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assay. Exposure to endotoxemia promoted an increase in nucleotide hydrolysis. When CGS-21680 was administered concomitantly with LPS, this increase was prevented for ATP, adenosine 5'-monophosphate (AMP), and p-Nitrophenyl thymidine 5'-monophosphate (p-Nph-5'-TMP) hydrolysis. However, when CGS-21680 was administered 24h after LPS injection, the increase was not reversed. The expression pattern of ectonucleotidases was not altered between LPS and LPS plus CGS-21680 groups, indicating that the transcriptional control was not involved on the effect exerted for CGS-21680. These results showed an enhancement of extracellular nucleotide catabolism in lymphocytes after induction of endotoxemia, which was prevented, but not reversed by CGS-21680 administration. These findings suggest that the control of nucleotide and nucleoside levels exerted by CGS-21680 could contribute to the modulation of the inflammatory process promoted by adenosine A(2A) agonists.