New insights into the regulation of inflammation by adenosine

Circadian Influences on Myocardial Ischemia-Reperfusion Injury and Heart Failure

The impact of circadian rhythms on cardiovascular function and disease development is well established, with numerous studies in genetically modified animals emphasizing the circadian molecular clock's significance in the pathogenesis and pathophysiology of myocardial ischemia and heart failure progression. However, translational preclinical studies targeting the heart's circadian biology are just now emerging and are leading to the development of a novel field of medicine termed circadian medicine. In this review, we explore circadian molecular mechanisms and novel therapies, including (1) intense light, (2) small molecules modulating the circadian mechanism, and (3) chronotherapies such as cardiovascular drugs and meal timings. These promise significant clinical translation in circadian medicine for cardiovascular disease. (4) Additionally, we address the differential functioning of the circadian mechanism in males versus females, emphasizing the consideration of biological sex, gender, and aging in circadian therapies for cardiovascular disease.

Hypoxia-adenosine axis as therapeutic targets for acute respiratory distress syndrome

The human respiratory and circulatory systems collaborate intricately to ensure oxygen delivery to all cells, which is vital for ATP production and maintaining physiological functions and structures. During limited oxygen availability, hypoxia-inducible factors (HIFs) are stabilized and play a fundamental role in maintaining cellular processes for hypoxia adaptation. First discovered during investigations of erythropoietin production regulation, HIFs influence physiological and pathological processes, including development, inflammation, wound healing, and cancer. HIFs promote extracellular adenosine signaling by enhancing adenosine generation and receptor signaling, representing an endogenous feedback mechanism that curbs excessive inflammation, supports injury resolution, and enhances hypoxia tolerance. This is especially important for conditions that involve tissue hypoxia, such as acute respiratory distress syndrome (ARDS), which globally poses significant health challenges without specific treatment options. Consequently, pharmacological strategies to amplify HIF-mediated adenosine production and receptor signaling are of great importance.

Modafinil exerts anti-inflammatory and anti-fibrotic effects by upregulating adenosine A2A and A2B receptors

Adenosine receptor (AR) suppresses inflammation and fibrosis by activating cyclic adenosine monophosphate (cAMP) signaling. We investigated whether altered AR expression contributes to the development of fibrotic diseases and whether A2AAR and A2BAR upregulation inhibits fibrotic responses. Primary human lung fibroblasts (HLFs) from normal (NHLFs) or patients with idiopathic pulmonary fibrosis (DHLF) were used for in vitro testing. Murine models of fibrotic liver or pulmonary disease were developed by injecting thioacetamide intraperitoneally, by feeding a high-fat diet, or by intratracheal instillation of bleomycin. Modafinil, which activates cAMP signaling via A2AAR and A2BAR, was administered orally. The protein amounts of A2AAR, A2BAR, and exchange protein directly activated by cAMP (Epac) were reduced, while collagen and α-smooth muscle actin (α-SMA) were elevated in DHLFs compared to NHLFs. In liver or lung tissue from murine models of fibrotic diseases, A2AAR and A2BAR were downregulated, but A1AR and A3AR were not. Epac amounts decreased, and amounts of collagen, α-SMA, KCa2.3, and KCa3.1 increased compared to the control. Modafinil restored the amounts of A2AAR, A2BAR, and Epac, and reduced collagen, α-SMA, KCa2.3, and KCa3.1 in murine models of fibrotic diseases. Transforming growth factor-β reduced the amounts of A2AAR, A2BAR, and Epac, and elevated collagen, α-SMA, KCa2.3, and KCa3.1 in NHLFs; however, these alterations were inhibited by modafinil. Our investigation revealed that A2AAR and A2BAR downregulation induced liver and lung fibrotic diseases while upregulation attenuated fibrotic responses, suggesting that A2AAR and A2BAR-upregulating agents, such as modafinil, may serve as novel therapies for fibrotic diseases.

CD73 deficiency does not aggravate angiotensin II-induced aortic inflammation in mice

Vascular inflammation plays a key role in the development of aortic diseases. A potential novel target for treatment might be CD73, an ecto-5'-nucleotidase that generates anti-inflammatory adenosine in the extracellular space. Here, we investigated whether a lack of CD73 results in enhanced aortic inflammation. To this end, angiotensin II was infused into wildtype and CD73-/- mice over 10 days. Before and after infusion, mice were analyzed using magnetic resonance imaging, ultrasound, flow cytometry, and histology. The impact of age and gender was investigated using female and male mice of three and six months of age, respectively. Angiotensin II infusion led to increased immune cell infiltration in both genotypes' aortae, but depletion of CD73 had no impact on immune cell recruitment. These findings were not modified by age or sex. No substantial difference in morphological or functional characteristics could be detected between wildtype and CD73-/- mice. Interestingly, the expression of CD73 on neutrophils decreased significantly in wildtype mice during treatment. In summary, we have found no evidence that CD73 deficiency affects the onset of aortic inflammation. However, as CD73 expression decreased during disease induction, an increase in CD73 by pharmaceutical intervention might result in lower vascular inflammation and less vascular disease.

A2B Adenosine Receptor in Idiopathic Pulmonary Fibrosis: Pursuing Proper Pit Stop to Interfere with Disease Progression

Purine nucleotides and nucleosides are involved in various human physiological and pathological mechanisms. The pathological deregulation of purinergic signaling contributes to various chronic respiratory diseases. Among the adenosine receptors, A2B has the lowest affinity such that it was long considered to have little pathophysiological significance. Many studies suggest that A2BAR plays protective roles during the early stage of acute inflammation. However, increased adenosine levels during chronic epithelial injury and inflammation might activate A2BAR, resulting in cellular effects relevant to the progression of pulmonary fibrosis.

Targeting the Cbl-b-Notch1 axis as a novel immunotherapeutic strategy to boost CD8+ T-cell responses

A critical feature of cancer is the ability to induce immunosuppression and evade immune responses. Tumor-induced immunosuppression diminishes the effectiveness of endogenous immune responses and decreases the efficacy of cancer immunotherapy. In this study, we describe a new immunosuppressive pathway in which adenosine promotes Casitas B-lineage lymphoma b (Cbl-b)-mediated Notch1 degradation, causing suppression of CD8+ T-cells effector functions. Genetic knockout and pharmacological inhibition of Cbl-b prevents Notch1 degradation in response to adenosine and reactivates its signaling. Reactivation of Notch1 results in enhanced CD8+ T-cell effector functions, anti-cancer response and resistance to immunosuppression. Our work provides evidence that targeting the Cbl-b-Notch1 axis is a novel promising strategy for cancer immunotherapy.

Extracellular cAMP-Adenosine Pathway Signaling: A Potential Therapeutic Target in Chronic Inflammatory Airway Diseases

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, β₂-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 β₂-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.

Short histological kaleidoscope - recent findings in histology. Part I

This article is a review of new advances in histology, concerning either classification or structure of different tissular elements (basement membrane, hemidesmosomes, urothelium, glandular epithelia, adipose tissue, astrocytes), and various organs' constituents (blood-brain barrier, human dental cementum, tubarial salivary glands, hepatic stellate cells, pineal gland, fibroblasts of renal interstitium, Leydig testicular cells, ovarian hilar cells), as well as novel biotechnological techniques (tissue engineering in angiogenesis), recently introduced.

Targeting circadian PER2 as therapy in myocardial ischemia and reperfusion injury

The cycle of day and night dominates life on earth. Therefore, almost all living organisms adopted a molecular clock linked to the light-dark cycles. It is now well established that this molecular clock is crucial for human health and wellbeing. Disruption of the molecular clockwork directly results in a myriad of disorders, including cardiovascular diseases. Further, the onset of many cardiovascular diseases such as acute myocardial infarction exhibits a circadian periodicity with worse outcomes in the early morning hours. Based on these observations, the research community became interested in manipulating the molecular clock to treat cardiovascular diseases. In recent years, several exciting discoveries of pharmacological agents or molecular mechanisms targeting the molecular clockwork have paved the way for circadian medicine's arrival in cardiovascular diseases. The current review will outline the most recent circadian therapeutic advances related to the circadian rhythm protein Period2 (PER2) to treat myocardial ischemia and summarize future research in the respective field.

Inosine, an endogenous purine nucleoside, avoids early stages of atherosclerosis development associated to eNOS activation and p38 MAPK/NF-kB inhibition in rats

Atherosclerosis is a multifactorial chronic disease, initiated by an endothelial dysfunction. Adenosine and its analogs can change a variety of inflammatory diseases and has shown important effects at different disease models. Inosine is a stable analogous of adenosine, but its effects in inflammatory diseases, like atherosclerosis, have not yet been studied. The aim of this study was to evaluate the pharmacological properties of inosine, administered sub chronically in a hypercholesterolemic model. Male Wistar rats were divided into four groups: control group (C) and control + inosine (C + INO) received standard chow, hypercholesterolemic diet group (HCD) and HCD + inosine (HCD + INO) were fed a hypercholesterolemic diet. At 31st experimentation day, the treatment with inosine was performed for C + INO and HCD + INO groups once daily in the last 15 days. We observed that the hypercholesterolemic diet promoted an increase in lipid levels and inflammatory cytokines production, while inosine treatment strongly decreased these effects. Additionally, HCD group presented a decrease in maximum relaxation acetylcholine induced and an increase in contractile response phenylephrine induced when compared to the control group, as well as it has presented an enhancement in collagen and ADP-induced platelet aggregation. On the other hand, inosine treatment promoted a decrease in contractile response to phenylephrine, evoked an improvement in endothelium-dependent vasorelaxant response and presented antiplatelet properties. Moreover, inosine activated eNOS and reduced p38 MAPK/NF-κB pathway in aortic tissues. Taken together, the present results indicate inosine as a potential drug for the treatment of cardiovascular disorders such as atherosclerosis.

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

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Discovery of Novel Adenosine Receptor Antagonists through a Combined Structure- and Ligand-Based Approach Followed by Molecular Dynamics Investigation of Ligand Binding Mode

An intense effort is made by pharmaceutical and academic research laboratories to identify and develop selective antagonists for each adenosine receptor (AR) subtype as potential clinical candidates for "soft" treatment of various diseases. Crystal structures of subtypes A_2A and A₁ARs offer exciting opportunities for structure-based drug design. In the first part of the present work, Maybridge HitFinder library of 14400 compounds was utilized to apply a combination of structure-based against the crystal structure of A_2AAR and ligand-based methodologies. The docking poses were rescored by CHARMM energy minimization and calculation of the desolvation energy using Poisson-Boltzmann equation electrostatics. Out of the eight selected and tested compounds, five were found positive hits (63% success). Although the project was initially focused on targeting A_2AAR, the identified antagonists exhibited low micromolar or micromolar affinity against A_2A/A₃, ARs, or A₃AR, respectively. Based on these results, 19 compounds characterized by novel chemotypes were purchased and tested. Sixteen of them were identified as AR antagonists with affinity toward combinations of the AR family isoforms (A_2A/A₃, A₁/A₃, A₁/A_2A/A₃, and A₃). The second part of this work involves the performance of hundreds of molecular dynamics (MD) simulations of complexes between the ARs and a total of 27 ligands to resolve the binding interactions of the active compounds, which were not achieved by docking calculations alone. This computational work allowed the prediction of stable and unstable complexes which agree with the experimental results of potent and inactive compounds, respectively. Of particular interest is that the 2-amino-thiophene-3-carboxamides, 3-acylamino-5-aryl-thiophene-2-carboxamides, and carbonyloxycarboximidamide derivatives were found to be selective and possess a micromolar to low micromolar affinity for the A₃ receptor.

Activation of CECR1 in M2-like TAMs promotes paracrine stimulation-mediated glial tumor progression

Background

The majority of glioma-associated microglia/macrophages have been identified as M2-type macrophages with immune suppressive and tumor supportive action. Recently, the extracellular adenosine deaminase protein Cat Eye Syndrome Critical Region Protein 1 (CECR1) was shown to regulate macrophage maturation. In this study, we investigate the role of CECR1 in the regulation of the glioma-associated macrophage response.

Methods

Expression of CECR1 was assessed in human glioma samples. CECR1-mediated macrophage response was studied in vitro, using donor derived CD14+ monocytes and the THP-1 monocytic cell line. The response of the human glioma cell line U87 to conditioned medium of macrophages preconditioned with recombinant human CECR1 or CECR1 silencing was also assessed.

Results

CECR1 was strongly expressed in high-grade gliomas (P < .001) and correlated positively with the M2 phenotype markers in tumor-associated microglia/macrophages (TAMs) (overall, P < .05). In vitro studies confirmed the presence of a significantly higher level of CECR1 expression in M2-like macrophages exposed to U87 conditioned medium (P < .001). CECR1 knockdown or stimulation of macrophages affected differentiation toward the M2-like phenotype. Stimulation of U87 cells with conditioned medium of CECR1 knockdown or stimulated macrophages affected tumor cell proliferation and migration, coinciding with altered intracellular signaling of mitogen-activated protein kinase (MAPK). In glioma tissue samples, CECR1 expression correlated with Ki67 and MAPK signaling protein.

Conclusions

CECR1 is a potent regulator of TAM polarization and is consistently highly expressed by M2-type TAMs, particularly in high-grade glioma. Paracrine effects induced by CECR1 in M2-like TAMs activate MAPK signaling and stimulate the proliferation and migration of glioma cells.

Adenosine Deaminase-2-Induced Hyperpermeability in Human Retinal Vascular Endothelial Cells Is Suppressed by MicroRNA-146b-3p

Purpose

We recently demonstrated that adenosine deaminase-2 (ADA2) contributes to diabetic retinopathy (DR) via up-regulating the production of inflammatory cytokines in macrophages. Also, microRNA (miR)-146b-3p has the ability to inhibit ADA2. The goal of this study was to investigate the potential role of ADA2 and therapeutic benefit of miR-146b-3p in retinal inflammation and endothelial barrier dysfunction during diabetes.

Methods

Adenosine deaminase-2 activity was determined by colorimetric method in diabetic human vitreous. Human monocyte cell line U937 was differentiated into macrophages and then treated with amadori glycated albumin (AGA), and conditioned medium (CM) was used to assess the changes in ADA2 activity and TNF-α and IL-6 levels by ELISA. Also, macrophages were transfected with miR-146b-3p before treatment with AGA. Permeability of human retinal endothelial cells (hRECs) was assessed by electric cell-substrate impedance sensing (ECIS) after treatment with macrophage CM. Zonula occludens (ZO)-1 was examined by immuno-fluorescence in hRECs. Leukocyte adhesion was assessed in hRECs by measuring myeloperoxidase (MPO) activity and intercellular adhesion molecule-1 (ICAM-1) expression.

Results

Adenosine deaminase-2 activity was significantly increased in diabetic human vitreous. ADA2 activity and TNF-α and IL-6 levels were significantly increased in human macrophages by AGA treatment. Amadori glycated albumin–treated macrophage CM significantly increased hREC permeability, disrupted ZO-1 pattern, and increased leukocyte adhesion to hRECs through up-regulating ICAM-1. All these changes were reversed by miR-146b-3p.

Conclusions

Adenosine deaminase-2 is implicated in breakdown of the blood–retinal barrier (BRB) in DR through macrophages-derived cytokines. Therefore, inhibition of ADA2 by miR-146b-3p might be a useful tool to preserve BRB function in DR.

CD73 on T Cells Orchestrates Cardiac Wound Healing After Myocardial Infarction by Purinergic Metabolic Reprogramming

BACKGROUND

T cells are required for proper healing after myocardial infarction. The mechanism of their beneficial action, however, is unknown. The proinflammatory danger signal ATP, released from damaged cells, is degraded by the ectonucleotidases CD39 and CD73 to the anti-inflammatory mediator adenosine. Here, we investigate the contribution of CD73-derived adenosine produced by T cells to cardiac remodeling after ischemia/reperfusion and define its mechanism of action.

METHODS

Myocardial ischemia (50 minutes followed by reperfusion) was induced in global CD73^-/- and CD4-CD73^-/- mice. Tissue injury, T-cell purinergic signaling, cytokines, and cardiac function (magnetic resonance tomography at 9.4 T over 4 weeks) were analyzed.

RESULTS

Changes in functional parameters of CD4-CD73^-/- mice were identical to those in global CD73 knockouts (KOs). T cells infiltrating the injured heart significantly upregulated at the gene (quantitative polymerase chain reaction) and protein (enzymatic activity) levels critical transporters and enzymes (connexin43, connexin37, pannexin-1, equilibrative nucleoside transporter 1, CD39, CD73, ecto-nucleotide pyrophosphatase/phosphodiesterases 1 and 3, CD157, CD38) for the accelerated release and hydrolysis of ATP, cAMP, AMP, and NAD to adenosine. It is surprising that a lack of CD39 on T cells (from CD39^-/- mice) did not alter ATP hydrolysis and very likely involves pyrophosphatases (ecto-nucleotide pyrophosphatase/phosphodiesterases 1 and 3). Circulating T cells predominantly expressed A_2a receptor (A_2aR) transcripts. After myocardial infarction, A_2b receptor (A_2bR) transcription was induced in both T cells and myeloid cells in the heart. Thus, A_2aR and A_2bR signaling may contribute to myocardial responses after myocardial infarction. In the case of T cells, this was associated with an accelerated secretion of proinflammatory and profibrotic cytokines (interleukin-2, interferon-γ, and interleukin-17) when CD73 was lacking. Cytokine production by T cells from peripheral lymph nodes was inhibited by A_2aR activation (CGS-21680). The A_2bR agonist BAY 60-6583 showed off-target effects. The adenosine receptor agonist NECA inhibited interferon-γ and stimulated interleukin-6 production, each of which was antagonized by a specific A_2bR antagonist (PSB-603).

CONCLUSIONS

This work demonstrates that CD73 on T cells plays a crucial role in the cardiac wound healing process after myocardial infarction. The underlying mechanism involves a profound increase in the hydrolysis of ATP/NAD and AMP, resulting primarily from the upregulation of pyrophosphatases and CD73. We also define A_2bR/A_2aR-mediated autacoid feedback inhibition of proinflammatory/profibrotic cytokines by T cell-derived CD73.

Ticagrelor Compared with Clopidogrel Increased Adenosine and Cyclic Adenosine Monophosphate Plasma Concentration in Acute Coronary Syndrome Patients

Ticagrelor produces a more potent antiplatelet effect than clopidogrel and inhibits cellular uptake of adenosine, which is associated with several cardiovascular consequences. We aimed to explore the correlation between adenosine and cyclic adenosine monophosphate (cAMP) plasma concentration and antiplatelet effect by clopidogrel or ticagrelor in patients with acute coronary syndrome (ACS) receiving dual antiplatelet therapy (DAPT). We conducted a prospective, observational and single-centre cohort study enrolling 68 patients with non-ST-segment elevation ACS from January 2016 to May 2016. We monitored the inhibition of platelet aggregation (IPA) and assessed adenosine, adenosine deaminase (ADA) and cAMP plasma concentrations by immunoassay on admission and 48 hr after coronary angiography. The demographic and clinical data were collected by reviewing their medical records. The two groups exhibited similar baseline characteristics including adenosine, ADA and cAMP. The mean IPA in patients receiving ticagrelor was significantly higher than that in patients receiving clopidogrel (93.5% versus 67.2%; p = 0.000). Also, we observed that patients treated with ticagrelor had a significantly higher increase in levels of adenosine and cAMP compared with those treated with clopidogrel (1.04 (0.86; 1.41) versus 0.04 (-0.25; 0.26); p = 0.029 and 0.78 (-1.67; 1.81) versus 0.60 (-1.91; 4.60); p = 0.037, respectively). And there was a weak correlation between IPA and adenosine as well as cAMP plasma concentration (r = 0.390, p = 0.001 and r = 0.335, p = 0.005, respectively). Ticagrelor increased adenosine and cAMP plasma concentration compared with clopidogrel in patients with ACS.

Dynamic Patterns of Systemic Innate Immunity and Inflammatory Associated Factors in Experimental Caprine Coccidiosis

The present study was designed to assess the dynamic patterns of pro-inflammatory cytokines, including IFN-γ, TNF-α, IL-4, IL-6, acute phase protein (α1-acid-glycoprotein, AGP), and an inflammation associated factor (adenosine deaminase; ADA) following experimental caprine coccidiosis. Ten kids aging from 2 to 4 months were infected orally with 5×104 sporulated oocysts and 10 animals served as controls. Blood samples were collected in both groups before infection and at days 3, 7, 14, 21, 28, and 35 post-infection (PI), and the levels of above-mentioned factors were measured. IFN-γ, TNF-α, IL-4, IL-6, AGP, and ADA activities were significantly higher in infected animals from day 7 PI (P<0.05). In conclusion, the circulatory levels of most systemic inflammatory markers, including pro-inflammatory cytokines (IFN-γ, TNF-α, IL-4, IL-6), AGP, and ADA increased significantly starting from day 3 to day 7 PI in caprine coccidiosis.

ADENOSINE DEAMINASE ACTIVITY AND SERUM C-REACTIVE PROTEIN AS PROGNOSTIC MARKERS OF CHAGAS DISEASE SEVERITY

Chagas disease is a public health problem worldwide. The availability of diagnostic tools to predict the development of chronic Chagas cardiomyopathy is crucial to reduce morbidity and mortality. Here we analyze the prognostic value of adenosine deaminase serum activity (ADA) and C-reactive protein serum levels (CRP) in chagasic individuals. One hundred and ten individuals, 28 healthy and 82 chagasic patients were divided according to disease severity in phase I (n = 35), II (n = 29), and III (n = 18). A complete medical history, 12-lead electrocardiogram, chest X-ray, and M-mode echocardiogram were performed on each individual. Diagnosis of Chagas disease was confirmed by ELISA and MABA using recombinant antigens; ADA was determined spectrophotometrically and CRP by ELISA. The results have shown that CRP and ADA increased linearly in relation to disease phase, CRP being significantly higher in phase III and ADA at all phases. Also, CRP and ADA were positively correlated with echocardiographic parameters of cardiac remodeling and with electrocardiographic abnormalities, and negatively with ejection fraction. CRP and ADA were higher in patients with cardiothoracic index ≥ 50%, while ADA was higher in patients with ventricular repolarization disturbances. Finally, CRP was positively correlated with ADA. In conclusion, ADA and CRP are prognostic markers of cardiac dysfunction and remodeling in Chagas disease.

Toward PET imaging of A2B adenosine receptors: a carbon-11 labeled triazinobenzimidazole tracer: Synthesis and imaging of a new A2B PET tracer

INTRODUCTION

A2B adenosine receptors (ARs) are commonly defined as "danger" sensors because they are triggered during cell injury when the endogenous molecule, adenosine, increases rapidly. These receptors, together with the other receptor subtypes (A1, A2A and A3), exert a wide variety of immunomodulating and (cyto)protective effects, thus representing a pivotal therapeutic target for different pathologies including diabetes, tumors, cardiovascular diseases, pulmonary fibrosis and others. The limited availability of potent and selective ligands for A2B ARs has prevented this receptor to emerge both as therapeutic and diagnostic target.

METHODS

Recently, a new class of potent A2B ARs antagonists was developed featuring the triazinobenzimidazole scaffold. Starting from this chemotype, we synthesized a new radiotracer, [(11)C]-4 (1-[(11)C]methyl-3-phenyl triazino[4,3-a]benzimidazol-4(1H)-one), and investigated the pharmacokinetics of this compound in vivo to define its potential use in the imaging of A2B AR with positron emission tomography.

RESULTS

[(11)C]-4 showed a very high chemical and blood stability. Results of in vivo and ex vivo experiments underlined the ability of this molecule to bind the A2B AR and correlated with the A2B AR protein and gene expression data.

CONCLUSIONS

Although further studies are necessary, these data suggest that [(11)C]-4 may represent a good lead compound for the development of novel selective and potent A2B AR radiotracers, and a new option for the clinical investigation of several pathophysiological processes and chronic diseases.

The G-Protein-Coupled Bile Acid Receptor Gpbar1 (TGR5) Inhibits Gastric Inflammation Through Antagonizing NF-κB Signaling Pathway

Gpbar1 (TGR5), a membrane-bound bile acid receptor, is well-known for its roles in regulation of energy homeostasis and glucose metabolism. Here, we show that mice lacking TGR5 were much more susceptible to lipopolysaccharide (LPS)-induced acute gastric inflammation than wild-type (WT) mice and TGR5 is a negative regulator of gastric inflammation through antagonizing NF-κB signaling pathway. We found that the treatment of TGR5 ligands 23(S)-mCDCA and GPBARA (3-(2-Chlorophenyl)-N-(4-chlorophenyl)-N,5-dimethylisoxazole-4-carboxamide) suppressed gene and protein expression mediated by NF-κB signaling. TGR5 overexpression with ligand treatment inhibited gene expression of interferon-inducible protein 10 (IP-10), TNF-α, and chemoattractant protein-1 (MCP-1) induced by LPS. Furthermore, we revealed that TGR5 activation antagonized NF-κB signaling pathway through suppressing its transcription activity, the phosphorylation of IκBα and p65 translocation, which suggests that TGR5 antagonizes gastric inflammation at least in part by inhibiting NF-κB signaling. These findings identify TGR5 as a negative mediator of gastric inflammation that may serve as an attractive therapeutic tool for human gastric inflammation and cancer.

Effects of elevated ultraviolet radiation on primary metabolites in selected alpine algae and cyanobacteria

Extremophilic green algae and cyanobacteria are the most abundant species in high mountain habitats, where rough climate conditions such as temperature differences, limited water retention and high ultraviolet (UV) radiation are the cause for a restricted biological diversity in favor of a few specialized autotrophic microorganisms. In this study, we investigated four algal species from alpine habitat in a sun simulator for their defense strategies in response to UV-A radiation (315-400nm) up to 13.4W/m(2) and UV-B radiation (280-315nm) up to 2.8W/m(2). Besides changes in pigment composition we discovered that primary polar metabolites like aromatic amino acids, nucleic bases and nucleosides are increasingly produced when the organisms are exposed to elevated UV radiation. Respective compounds were isolated and identified, and in order to quantify them an HPLC-DAD method was developed and validated. Our results show that especially tyrosine and guanosine were found to be generally two to three times upregulated in the UV-B exposed samples compared to the non-treated control.

Adenosine receptors and renal ischaemia reperfusion injury

One of the frequent clinical complications that results in billions of dollars in healthcare costs annually in the United States is acute kidney injury (AKI). Ischaemia reperfusion (IR) injury is a major cause AKI. Unfortunately, no effective treatment or preventive measure for AKI exists. With increased surgical complexity coupled with increasing number of elderly, the incidence of AKI is becoming more frequent. Adenosine is a metabolic breakdown product of adenosine triphosphate (ATP) and contributes to the regulation of multiple physiological events. Extracellular adenosine activates four subtypes of adenosine receptors (AR) including A1 AR, A2 A AR, A2 B AR and A3 AR. In the kidney, adenosine regulates glomerular filtration rate, vascular tone, renin release and is an integrative part of tubular glomerular feedback signal to the afferent arterioles. In addition, each AR subtype powerfully modulates renal IR injury. The A1 AR activation protects against ischaemic insult by reducing apoptosis, necrosis and inflammation. Activation of A2 A AR protects against renal injury by modulating leucocyte-mediated inflammation as well as directly reducing renal tubular inflammation. Activation of A2 B AR acts via direct activation of renal parenchymal as well as renovascular receptors and is important in kidney preconditioning. Finally, activation of A3 AR exacerbates renal damage following renal IR injury while A3 AR antagonism attenuates renal damage following ischaemic insult. Latest body of research suggests that kidney AR modulation may be a promising approach to treat ischaemic AKI. This brief review focuses on the signalling pathways of adenosine in the kidney followed by the role for various AR modulations in protecting against ischaemic AKI.

Anti-inflammatory or proinflammatory effect of an adenosine receptor agonist on the Th17 autoimmune response is inflammatory environment-dependent

Adenosine is a key endogenous signaling molecule that regulates a wide range of physiological functions, including immune system function and inflammation. Studies have shown that adenosine receptor (AR) agonists can be either anti-inflammatory or proinflammatory in immune responses and in inflammation, and the clarification of the mechanisms causing these opposing effects should provide a better guide for therapeutic intervention. Whereas previous studies mostly examined the effects of AR agonists on Th1-type immune responses, in this study, we compared their effect on Th17 and Th1 autoimmune responses in experimental autoimmune uveitis, a mouse model of human uveitis induced by immunization with the human interphotoreceptor retinoid-binding protein peptides 1-20. We showed that injection of mice with a nonselective AR agonist, 5'-N-ethylcarboxamidoadenosine (NECA), at an early stage after immunization had an inhibitory effect on both Th1 and Th17 responses, whereas injection of the same amount of NECA at a late stage inhibited the Th1 response but had an enhancing effect on the Th17 response. We also showed that the effects of NECA on Th1 and Th17 responses were completely dissociated, that the enhancing effect of NECA on Th17 responses was modulated by γδ T cells, and that the response of γδ T cells to NECA was determined by their activation status. We conclude that the inflammatory environment has a strong impact on converting the effect of AR agonist on the Th17 autoimmune response from anti-inflammatory to proinflammatory. Our observation should help in the designing of better AR-targeted therapies.

Adenosine regulates the proinflammatory signaling function of thrombin in endothelial cells

The plasma level of the regulatory metabolite adenosine increases during the activation of coagulation and inflammation. Here we investigated the effect of adenosine on modulation of thrombin-mediated proinflammatory responses in HUVECs. We found that adenosine inhibits the barrier-disruptive effect of thrombin in HUVECs by a concentration-dependent manner. Analysis of cell surface expression of adenosine receptors revealed that A2A and A2B are expressed at the highest level among the four receptor subtypes (A2B  > A2A  > A1  > A3 ) on HUVECs. The barrier-protective effect of adenosine in response to thrombin was recapitulated by the A2A specific agonist, CGS 21680, and abrogated both by the siRNA knockdown of the A2A receptor and by the A2A -specific antagonists, ZM-241385 and SCH-58261. The thrombin-induced RhoA activation and its membrane translocation were both inhibited by adenosine in a cAMP-dependent manner, providing a molecular mechanism through which adenosine exerts a barrier-protective function. Adenosine also inhibited thrombin-mediated activation of NF-κB and decreased adhesion of monocytic THP-1 cells to stimulated HUVECs via down-regulation of expression of cell surface adhesion molecules, VCAM-1, ICAM-1, and E-selectin. Moreover, adenosine inhibited thrombin-induced elevated expression of proinflammatory cytokines, IL-6 and HMGB-1; and chemokines, MCP-1, CXCL-1, and CXCL-3. Taken together, these results suggest that adenosine may inhibit thrombin-mediated proinflammatory signaling responses, thereby protecting the endothelium from injury during activation of coagulation and inflammation.

The volatile anesthetic isoflurane increases endothelial adenosine generation via microparticle ecto-5'-nucleotidase (CD73) release

Endothelial dysfunction is common in acute and chronic organ injury. Isoflurane is a widely used halogenated volatile anesthetic during the perioperative period and protects against endothelial cell death and inflammation. In this study, we tested whether isoflurane induces endothelial ecto-5′-nucleotidase (CD73) and cytoprotective adenosine generation to protect against endothelial cell injury. Clinically relevant concentrations of isoflurane induced CD73 activity and increased adenosine generation in cultured human umbilical vein or mouse glomerular endothelial cells. Surprisingly, isoflurane-mediated induction of endothelial CD73 activity occurred within 1 hr and without synthesizing new CD73. We determined that isoflurane rapidly increased CD73 containing endothelial microparticles into the cell culture media. Indeed, microparticles isolated from isoflurane-treated endothelial cells had significantly higher CD73 activity as well as increased CD73 protein. In vivo, plasma from mice anesthetized with isoflurane had significantly higher endothelial cell-derived CD144+ CD73+ microparticles and had increased microparticle CD73 activity compared to plasma from pentobarbital-anesthetized mice. Supporting a critical role of CD73 in isoflurane-mediated endothelial protection, a selective CD73 inhibitor (APCP) prevented isoflurane-induced protection against human endothelial cell inflammation and apoptosis. In addition, isoflurane activated endothelial cells Rho kinase evidenced by myosin phosphatase target subunit-1 and myosin light chain phosphorylation. Furthermore, isoflurane-induced release of CD73 containing microparticles was significantly attenuated by a selective Rho kinase inhibitor (Y27632). Taken together, we conclude that the volatile anesthetic isoflurane causes Rho kinase-mediated release of endothelial microparticles containing preformed CD73 and increase adenosine generation to protect against endothelial apoptosis and inflammation.

Alterations in the cholinesterase and adenosine deaminase activities and inflammation biomarker levels in patients with multiple sclerosis

Multiple sclerosis (MS) is one of the main chronic inflammatory diseases of the CNS that cause functional disability in young adults. It has unknown etiology characterized by the infiltration of lymphocytes and macrophages into the brain. The aim of this study was to evaluate the acetylcholinesterase (AChE) activity in lymphocytes and whole blood, as well as butyrylcholinesterase (BChE) and adenosine deaminase (ADA) activities in serum. We also checked the levels of nucleotides, nucleosides, biomarkers of inflammation such as cytokines (interleukin (IL)-1, IL-6, interferon (IFN)-γ, tumor necrosis factor-alpha (TNF-α) and IL-10) and C-reactive protein (CRP) in serum from 29 patients with the relapsing-remitting form of MS (RRMS) and 29 healthy subjects as the control group. Results showed that AChE in lymphocytes and whole blood as well as BChE, and ADA activities in serum were significantly increased in RRMS patients when compared to the control group (P<0.05). In addition, we observed a decrease in ATP levels and a significant increase in the levels of ADP, AMP, adenosine and inosine in serum from RRMS patients in relation to the healthy subjects (P<0.05). Results also demonstrated an increase in the IFN-γ, TNF-α, IL-1, IL-6 and CRP (P<0.05) and a significant decrease in the IL-10 (P<0.0001) in RRMS patients when compared to control. Our results suggest that alterations in the biomarkers of inflammation and hydrolysis of nucleotides and nucleosides may contribute to the understanding of the neurological dysfunction of RRMS patients.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Modulation of A2B adenosine receptor by 1-Benzyl-3-ketoindole derivatives

We have disclosed a series of 1-benzyl-3-ketoindole derivatives acting as either positive or negative modulators of the human A(2B) adenosine receptor (A(2B) AR) depending on small differences in their side chain. The new compounds were designed taking into account structural similarities between AR antagonists and ligands of the GABA(A)/benzodiazepine receptor. All compounds resulted totally inactive at A(2A) and A₃ ARs and showed small (8a,b) or none (7a,b, 8c and 9a,b) affinity for A₁ AR. When tested on A(2B) AR-transfected CHO cells, 7a,b and 8a acted as positive modulators, whereas 8b,c and 9a,b acted as negative modulators, enhancing or weakening the NECA-induced increase of cAMP levels, respectively. Compounds 7-9 might be regarded as useful biological and pharmacological tools to explore the therapeutic potential of A(2B) AR modulators, while their 3-ketoindole scaffold might be taken as a reference to design new analogs.

Adenosine A(2B) receptor deficiency promotes host defenses against gram-negative bacterial pneumonia

RATIONALE

Activation of the adenosine A(2B) receptor (A(2B)R) promotes antiinflammatory effects in diverse biological settings, but the role of this receptor in antimicrobial host defense in the lung has not been established. Gram-negative bacillary pneumonia is a common and serious illness associated with high morbidity and mortality, the treatment of which is complicated by increasing rates of antibiotic resistance.

OBJECTIVES

To test the hypothesis that absence of adenosine A(2B) receptor signaling promotes host defense against bacterial pneumonia.

METHODS

We used a model of Klebsiella pneumoniae pneumonia in wild-type mice and mice with targeted deletion of the A(2B)R. Host responses were compared in vivo and leukocyte responses to the bacteria were examined in vitro.

MEASUREMENTS AND MAIN RESULTS

A(2B)R(-/-) mice demonstrated enhanced bacterial clearance from the lung and improved survival after infection with K. pneumoniae compared with wild-type controls, an effect that was mediated by bone marrow-derived cells. Leukocyte recruitment to the lungs and expression of inflammatory cytokines did not differ between A(2B)R(-/-) and wild-type mice, but A(2B)R(-/-) neutrophils exhibited sixfold greater bactericidal activity and enhanced production of neutrophil extracellular traps compared with wild-type neutrophils when incubated with K. pneumoniae. Consistent with this finding, bronchoalveolar lavage fluid from A(2B)R(-/-) mice with Klebsiella pneumonia contained more extracellular DNA compared with wild-type mice with pneumonia.

CONCLUSIONS

These data suggest that the absence of A(2B)R signaling enhances antimicrobial activity in gram-negative bacterial pneumonia.

E-ADA activity in lymphocytes of an experimental model of pythiosis treated with immunotherapy

Pythiosis is a life-threatening disease caused by the oomycete Pythium insidiosum. Some authors have suggested the involvement of a Th2-like immune response in the infected host, which leads to extensive tissue damage. The switch from a Th2 to a Th1 response pattern is one hypothesis to explain the curative properties of immunotherapy. Taking into account the importance of immunotherapy for pythiosis treatment and the contribution of adenine nucleotides in the immunoregulation of the host, we evaluated the ecto-adenosine deaminase (E-ADA; EC 3·5.4·4) activity in lymphocytes from rabbits inoculated with P. insidiosum. Rabbits were inoculated with 1 milliliter of zoospores subcutaneously injected into the lateral thorax; after developing lesions, the rabbits received eight doses of immunotherapy. E-ADA activity was measured in lymphocytes and the adenine nucleotides and adenosine levels were quantitatively determined in serum. Rabbits with characteristic lesions of pythiosis showed a decreased E-ADA activity (82·36%), a decreased adenosine triphosphate concentration (54·04%) and a higher adenosine concentration (2·51 fold), when compared with controls, after 28 days of inoculation. However, after the immunotherapy, the rabbits showed an increase in the E-ADA activity when compared with control (78·62%), contributing for the change in the immune response. Our results reinforce the hypothesis that the change from a Th2 to a Th1 immune response with the participation of the purinergic system could be responsible for the curative properties of immunotherapy.

Hematological indices and activity of NTPDase and cholinesterase enzymes in rats exposed to cadmium and treated with N-acetylcysteine

The present study aimed to investigate the influence of N-acetylcysteine (NAC) on cadmium (Cd) poisoning by evaluating Cd concentration in tissues, hematological indices as well as the activity of NTPDase, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes of rats exposed to Cd and co-treated with NAC. For this purpose, the rats received Cd (2 mg/kg) and NAC (150 mg/kg) by gavage every other day for 30 days. Animals were divided into four groups (n = 6-8): control/saline, NAC, Cd, and Cd/NAC. Cd exposure increased Cd concentration in plasma, spleen and thymus, and NAC co-treatment modulated this augment in both lymphoid organs. Cd exposure reduced red blood cell count, hemoglobin content and hematocrit value. Cd intoxication caused a decrease in total white blood cell count. NAC treatment per se caused an increase in lymphocyte and a decrease in neutrophil counts. On contrary, Cd exposure caused a decrease in lymphocyte and an increase in neutrophil and monocyte counts. NAC reversed or ameliorated the hematological impairments caused by Cd poisoning. There were no significant alterations in the NTPDase activity in lymphocytes of rats treated with Cd and/or NAC. Cd caused a decrease in the activities of lymphocyte AChE, whole blood AChE and serum BChE. However, NAC co-treatment was inefficient in counteracting the negative effect of Cd in the cholinesterase activities. The present investigation provides ex vivo evidence supporting the hypothesis that Cd induces immunotoxicity by interacting with the lymphoid organs, altering hematological parameters and inhibiting peripheral cholinesterase activity. Also, it highlights the possibility to use NAC as adjuvant against toxicological conditions.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

Adenosine A2A receptor upregulation in human PMNs is controlled by miRNA-214, miRNA-15, and miRNA-16

Immunosuppressive signaling via the adenosine A2A receptor (A2AR) is an important pathway to control inflammation. In immune cells, expression levels of A2ARs influence responsiveness to inflammatory stimuli. However, mechanisms driving expressional changes of A2ARs are still largely elusive. In the current study, we have investigated the impact of microRNAs (miRNAs) on A2AR expression in human polymorphonuclear leukocytes (PMNs) and T cells. Bioinformatic analyses and reporter gene assays revealed that A2AR expression is controlled by miRNA-214, miRNA-15, and miRNA-16. We detected all three miRNAs in both human PMNs and T cells. However, in PMNs, up to 10-fold higher levels of miRNA-16 and miRNA-214 were detected as compared with T cells. Upon in vitro stimulation, no significant expressional changes occurred. Expression levels of all three miRNAs strongly differed between individuals. A2AR expression also exhibited significant differences between PMNs and T cells: In PMNs, more than a 60-fold increase was seen upon LPS stimulation, whereas in T cells only a 2-fold increase was observed upon anti-CD3/CD28 activation. The extent of A2AR upregulation in PMNs strongly differed between individuals (from less than 10-fold to more than 100-fold). In PMNs, the increase in A2AR mRNA expression upon stimulation was inversely correlated with the expression levels of miRNA-214, miRNA-15, and miRNA-16 (R = -0.87, P < 0.0001); no correlation was found in human T cells. These results indicate that individual miRNA profiles gain important influence on A2AR expression regulation in PMNs upon stimulation. Determination of miRNA expression levels may help to identify patients with an increased risk for severe inflammation.

Ectonucleotide triphosphate diphosphohydrolase-1 (CD39) mediates resistance to occlusive arterial thrombus formation after vascular injury in mice

Modulation of purinergic signaling, which is critical for vascular homeostasis and the response to vascular injury, is regulated by hydrolysis of proinflammatory ATP and/or ADP by ectonucleoside triphosphate diphosphohydrolase 1 (ENTPD-1; CD39) to AMP, which then is hydrolyzed by ecto-5'-nucleotidase (CD73) to adenosine. We report here that compared with littermate controls (wild type), transgenic mice expressing human ENTPDase-1 were resistant to the formation of an occlusive thrombus after FeCl(3)-induced carotid artery injury. Treatment of mice with the nonhydrolyzable ADP analog, adenosine-5'-0-(2-thiodiphosphate) trilithium salt, Ado-5'-PP[S], negated the protection from thrombosis, consistent with a role for ADP in platelet recruitment and thrombus formation. ENTPD-1 expression decreased whole-blood aggregation after stimulation by ADP, an effect negated by adenosine-5'-0-(2-thiodiphosphate) trilithium salt, Ado-5'-PP[S] stimulation, and limited the ability to maintain the platelet fibrinogen receptor, glycoprotein α(IIb)/β(3), in a fully activated state, which is critical for thrombus formation. In vivo treatment with a CD73 antagonist, a nonselective adenosine-receptor antagonist, or a selective A(2A) or A(2B) adenosine-receptor antagonist, negated the resistance to thrombosis in transgenic mice expressing human ENTPD-1, suggesting a role for adenosine generation and engagement of adenosine receptors in conferring in vivo resistance to occlusive thrombosis in this model. In summary, our findings identify ENTPDase-1 modulation of purinergic signaling as a key determinant of the formation of an occlusive thrombus after vascular injury.

Adenosine and protection from acute kidney injury

PURPOSE OF REVIEW

Acute kidney injury (AKI) is a major clinical problem without effective therapy. Development of AKI among hospitalized patients drastically increases mortality and morbidity. With increases in complex surgical procedures together with a growing elderly population, the incidence of AKI is rising. Renal adenosine receptor manipulation may have great therapeutic potential in mitigating AKI. In this review, we discuss renal adenosine receptor biology and potential clinical therapies for AKI.

RECENT FINDINGS

The four adenosine receptor subtypes (A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR) have diverse effects on the kidney. The pathophysiology of AKI may dictate the specific adenosine receptor subtype activation needed to produce renal protection. The A(1)AR activation in renal tubules and endothelial cells produces beneficial effects against ischemia and reperfusion injury by modulating metabolic demand, decreasing necrosis, apoptosis, and inflammation. The A(2A)AR protects against AKI by modulating leukocyte-mediated renal and systemic inflammation, whereas the A(2B)AR activation protects by direct activation of renal parenchymal adenosine receptors. In contrast, the A(1)AR antagonism may play a protective role in nephrotoxic AKI and radiocontrast induced nephropathy by reversing vascular constriction and inducing naturesis and diuresis. Furthermore, as the A(3)AR activation exacerbates apoptosis and tissue damage due to renal ischemia and reperfusion, selective A(3)AR antagonism may hold promise to attenuate renal ischemia and reperfusion injury. Finally, renal A(1)AR activation also protects against renal endothelial dysfunction caused by hepatic ischemia and reperfusion injury.

SUMMARY

Despite the current lack of therapies for the treatment and prevention of AKI, recent research suggests that modulation of renal adenosine receptors holds promise in treating AKI and extrarenal injury.

Anti-inflammatory effects of purine nucleosides, adenosine and inosine, in a mouse model of pleurisy: evidence for the role of adenosine A2 receptors

Adenosine and its metabolite, inosine, have been described as molecules that participate in regulation of inflammatory response. The aim of this study was to investigate the effect of adenosine and inosine in a mouse model of carrageenan-induced pleurisy as well as the participation of adenosine receptors in this response. Injection of carrageenan into the pleural cavity induced an acute inflammatory response characterized by leukocyte migration, pleural exudation, and increased release of interleukin-1β and tumor necrosis factor-α in pleural exudates. The treatment with adenosine (0.3-100 mg/kg, i.p.) and inosine (0.1-300 mg/kg, i.p.) 30 min before carrageenan injection reduced significantly all these parameters analyzed. Our results also demonstrated that A(2A) and A(2B) receptors seem to mediate the adenosine and inosine effects observed, since pretreatment with selective antagonists of adenosine A(2A) (ZM241385) and A(2B) (alloxazine) receptors, reverted the inhibitory effects of adenosine and inosine in pleural inflammation. The involvement of A(2) receptors was reinforced with adenosine receptor agonist CGS21680 treatment, since its anti-inflammatory effects were reversed completely and partially with ZM241385 and alloxazine injection, respectively. Moreover, the combined treatment with subeffective dose of adenosine (0.3 mg/kg) and inosine (1.0 mg/kg) induced a synergistic anti-inflammatory effect. Thus, based on these findings, we propose that inosine contributes with adenosine to exert anti-inflammatory effects in pleural inflammation, reinforcing the notion that endogenous nucleosides play an important role in controlling inflammatory diseases. This effect is likely mediated by the activation of adenosine A(2) subtype receptors and inhibition of production or release of pro-inflammatory cytokines.

3-aryl-[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-one: a novel template for the design of highly selective A₂B adenosine receptor antagonists

In an effort to identify novel ligands possessing high affinity and selectivity for the A(2B) AR subtype, we further investigated the class of 3-aryl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-ones V, previously disclosed by us as selective A(1) AR antagonists. Preliminary assays on a number of triazinobenzimidazoles derived from our "in-house" collection revealed that all the derivatives selected showed significant affinity at A(2B) AR, no affinity at A(3) AR, and various degrees of selectivity toward A(1) and A(2A) ARs. Investigation of a new series featuring modified substituents at the 10-position (4'-chlorophenyl or phenylethyl groups), and a chlorine atom at the 7-position (X) of the triazinobenzimidazole nucleus, yielded highly potent and selective A(2B) AR antagonists. The presence of a pendant 3-phenyl ring appears to hamper the interaction with A(2A) AR, conferring high A(2B)/A(2A) AR selectivity. Derivative 13 (X = Cl, R = C(6)H(5)) is the most potent and selective compound, with an IC(50) of 3.10 nM at A(2B) AR and no affinity at A(1), A(2A), and A(3) ARs.

The G-protein-coupled bile acid receptor, Gpbar1 (TGR5), negatively regulates hepatic inflammatory response through antagonizing nuclear factor κ light-chain enhancer of activated B cells (NF-κB) in mice

Gpbar1 (TGR5), a membrane-bound bile acid receptor, is well known for its roles in regulation of energy homeostasis and glucose metabolism. TGR5 also displays strong attenuation of macrophage reactivity in vitro, but the physiological roles of TGR5 in inflammatory response, and its mechanism, is unknown. Here, we demonstrate that TGR5 is a negative modulator of nuclear factor kappa light-chain enhancer of activated B cells (NF-κB)-mediated inflammation. TGR5 activation suppresses the phosphorylation of nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha (IκBα), the translocation of p65, NF-κB DNA-binding activity, and its transcription activity. Furthermore, TGR5 activation enhances the interaction of IκBα and β-arrestin2. Suppression of NF-κB transcription activity and its target gene expression by TGR5 agonist are specifically abolished by the expression of anti-β-arrestin2 small interfering RNA. These results show that TGR5 suppresses the NF-κB pathway by mediation of the interaction between IκBα and β-arrestin2. In a lipopolysaccharide (LPS)-induced inflammation model, TGR5(-/-) mice show more severe liver necroses and inflammation, compared with wild-type (WT) mice. Activation of TGR5 by its agonist ligand inhibits the expression of inflammatory mediators in response to NF-κB activation induced by LPS in WT, but not TGR5(-/-), mouse liver.

CONCLUSION

These findings identify TGR5 as a negative mediator of inflammation that may serve as an attractive therapeutic tool for immune and inflammatory liver diseases.

Interplay of hypoxia and A2B adenosine receptors in tissue protection

That adenosine signaling can elicit adaptive tissue responses during conditions of limited oxygen availability (hypoxia) is a long-suspected notion that recently gained general acceptance from genetic and pharmacologic studies of the adenosine signaling pathway. As hypoxia and inflammation share an interdependent relationship, these studies have demonstrated that adenosine signaling events can be targeted to dampen hypoxia-induced inflammation. Here, we build on the hypothesis that particularly the A(2B) adenosine receptor (ADORA(2B)) plays a central role in tissue adaptation to hypoxia. In fact, the ADORA(2B) requires higher adenosine concentrations than any of the other adenosine receptors. However, during conditions of hypoxia or ischemia, the hypoxia-elicited rise in extracellular adenosine is sufficient to activate the ADORA(2B). Moreover, several studies have demonstrated very robust induction of the ADORA(2B) elicited by transcriptional mechanisms involving hypoxia-dependent signaling pathways and the transcription factor "hypoxia-induced factor" 1. In the present chapter, genetic and pharmacologic evidence is presented to support our hypothesis of a tissue protective role of ADORA(2B) signaling during hypoxic conditions, including hypoxia-elicited vascular leakage, organ ischemia, or acute lung injury. All these disease models are characterized by hypoxia-elicited tissue inflammation. As such, the ADORA(2B) has emerged as a therapeutic target for dampening hypoxia-induced inflammation and tissue adaptation to limited oxygen availability.

Immunological alterations mediated by adenosine during host-microbial interactions

Adenosine accumulates in inflammation and ischemia but it is more than an end-product of ATP catabolism. Signaling through different receptors with distinct, cell-specific cytoplasmic pathways, adenosine is now recognized as an inducible switch that regulates the immune system. By acting through the A(2A)AR, adenosine shapes T cell function, largely by conferring an anti-inflammatory tone on effector Th cells (Teff) and natural killer (NK)T cells. In contrast, both the A(2A)AR and A(2B)AR are expressed by antigen-presenting cells (APC) which have been shown to regulate innate responses and the transition to adaptive immunity. There is also emerging evidence that adenosine production is one mechanism that allows some pathogens as well as neoplasms to evade host defenses. This review discusses the immunoregulatory functions of adenosine and some of the interactions it may have in regulating host-microbial interactions.

Adverse and protective influences of adenosine on the newborn and embryo: implications for preterm white matter injury and embryo protection

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (ARs). In the postnatal period, A1AR activation may contribute to white matter injury in the preterm infant by altering oligodendrocyte (OL) development. In models of perinatal brain injury, caffeine is neuroprotective against periventricular white matter injury (PWMI) and hypoxic-ischemic encephalopathy (HIE). Supporting the notion that blockade of adenosine action is of benefit in the premature infant, caffeine reduces the incidence of bronchopulmonary dysplasia and CP in clinical studies. In comparison with the adverse effects on the postnatal brain, adenosine acts via A1ARs to play an essential role in protecting the embryo from hypoxia. Embryo protective effects are blocked by caffeine, and caffeine intake during early pregnancy increases the risk of miscarriage and fetal growth retardation. Adenosine and adenosine antagonists play important modulatory roles during mammalian development. The protective and deleterious effects of adenosine depend on the time of exposure and target sites of action.

Links between insulin resistance, adenosine A2B receptors, and inflammatory markers in mice and humans

OBJECTIVE

To determine the mechanisms by which blockade of adenosine A(2B) receptors (A(2B)Rs) reduces insulin resistance.

RESEARCH DESIGN AND METHODS

We investigated the effects of deleting or blocking the A(2B)R on insulin sensitivity using glucose tolerance tests (GTTs) and hyperinsulinemic-euglycemic clamps in mouse models of type 2 diabetes. The effects of diabetes on A(2B)R transcription and signaling were measured in human and mouse macrophages and mouse endothelial cells. In addition, tag single nucleotide polymorphisms (SNPs) in ~42 kb encompassing the A(2B)R gene, ADORA2B, were evaluated for associations with markers of diabetes and inflammation.

RESULTS

Treatment of mice with the nonselective adenosine receptor agonist 5'-N-ethylcarboxamidoadensoine (NECA) increased fasting blood glucose and slowed glucose disposal during GTTs. These responses were inhibited by A(2B)R deletion or blockade and minimally affected by deletion of A(1)Rs or A(2A)Rs. During hyperinsulinemic-euglycemic clamp of diabetic KKA(Y) mice, A(2B)R antagonism increased glucose infusion rate, reduced hepatic glucose production, and increased glucose uptake into skeletal muscle and brown adipose tissue. Diabetes caused a four- to sixfold increase in A(2B)R mRNA in endothelial cells and macrophages and resulted in enhanced interleukin (IL)-6 production in response to NECA due to activation of protein kinases A and C. Five consecutive tag SNPs in ADORA2B were highly correlated with IL-6 and C-reactive protein (CRP). Diabetes had a highly significant independent effect on variation in inflammatory markers. The strength of associations between several ADORA2B SNPs and inflammatory markers was increased when accounting for diabetes status.

CONCLUSIONS

Diabetes affects the production of adenosine and the expression of A(2B)Rs that stimulate IL-6 and CRP production, insulin resistance, and the association between ADORA2B SNPs and inflammatory markers. We hypothesize that increased A(2B)R signaling in diabetes increases insulin resistance in part by elevating proinflammatory mediators. Selective A(2B)R blockers may be useful to treat insulin resistance.

A2B adenosine receptor blockade enhances macrophage-mediated bacterial phagocytosis and improves polymicrobial sepsis survival in mice

Antimicrobial treatment strategies must improve to reduce the high mortality rates in septic patients. In noninfectious models of acute inflammation, activation of A2B adenosine receptors (A2BR) in extracellular adenosine-rich microenvironments causes immunosuppression. We examined A2BR in antibacterial responses in the cecal ligation and puncture (CLP) model of sepsis. Antagonism of A2BR significantly increased survival, enhanced bacterial phagocytosis, and decreased IL-6 and MIP-2 (a CXC chemokine) levels after CLP in outbred (ICR/CD-1) mice. During the CLP-induced septic response in A2BR knockout mice, hemodynamic parameters were improved compared with wild-type mice in addition to better survival and decreased plasma IL-6 levels. A2BR deficiency resulted in a dramatic 4-log reduction in peritoneal bacteria. The mechanism of these improvements was due to enhanced macrophage phagocytic activity without augmenting neutrophil phagocytosis of bacteria. Following ex vivo LPS stimulation, septic macrophages from A2BR knockout mice had increased IL-6 and TNF-α secretion compared with wild-type mice. A therapeutic intervention with A2BR blockade was studied by using a plasma biomarker to direct therapy to those mice predicted to die. Pharmacological blockade of A2BR even 32 h after the onset of sepsis increased survival by 65% in those mice predicted to die. Thus, even the late treatment with an A2BR antagonist significantly improved survival of mice (ICR/CD-1) that were otherwise determined to die according to plasma IL-6 levels. Our findings of enhanced bacterial clearance and host survival suggest that antagonism of A2BRs offers a therapeutic target to improve macrophage function in a late treatment protocol that improves sepsis survival.

Regulation of leukocyte function by adenosine receptors

The immune system responds to cues in the microenvironment to make acute and chronic adaptations in response to inflammation and injury. Locally produced purine nucleotides and adenosine provide receptor-mediated signaling to all bone-marrow derived cells of the immune system to modulate their responses. This review summarizes recent advances in our understanding of the effects of adenosine signaling through G protein-coupled adenosine receptors on cells of the immune system. Adenosine A(2A) receptors (A(2A)Rs) have a generally suppressive effect on the activation of immune cells. Moreover, their transcription is strongly induced by signals that activate macrophages or dendritic cells through toll-like receptors, or T cells through T cell receptors. A(2A)R induction is responsible for producing a gradual dissipation of inflammatory responses. A(2A)R activation is particularly effective in limiting the activation of invariant NKT (iNKT) cells that play a central role in acute reperfusion injury. A(2A) agonists have clinical promise for the treatment of vaso-occlusive tissue injury. Blockade of A(2A) receptors may be useful to enhance immune-mediated killing of cancer cells. A(2B)R expression also is transcriptionally regulated by hypoxia, cytokines, and oxygen radicals. Acute A(2B)R activation attenuates the production of proinflammatory cytokines from macrophages, but sustained activation facilitates macrophage and dendritic cell remodeling and the production of acute phase proteins and angiogenic factors that may participate in evoking insulin resistance and tissue fibrosis. A(2B)R activation also influences macrophage and neutrophil function by influencing expression of the anti-inflammatory netrin receptor, UNC5B. The therapeutic significance of adenosine-mediated effects on the immune system is discussed.

Transgenic overexpression of CD39 protects against renal ischemia-reperfusion and transplant vascular injury

The vascular ectonucleotidases CD39[ENTPD1 (ectonucleoside triphosphate diphosphohydrolase-1), EC 3.6.1.5] and CD73[EC 3.1.3.5] generate adenosine from extracellular nucleotides. CD39 activity is critical in determining the response to ischemia-reperfusion injury (IRI), and CD39 null mice exhibit heightened sensitivity to renal IRI. Adenosine has multiple mechanisms of action in the vasculature including direct endothelial protection, antiinflammatory and antithrombotic effects and is protective in several models of IRI. Mice transgenic for human CD39 (hCD39) have increased capacity to generate adenosine. We therefore hypothesized that hCD39 transgenic mice would be protected from renal IRI. The overexpression of hCD39 conferred protection in a model of warm renal IRI, with reduced histological injury, less apoptosis and preserved serum creatinine and urea levels. Benefit was abrogated by pretreatment with an adenosine A2A receptor antagonist. Adoptive transfer experiments showed that expression of hCD39 on either the vasculature or circulating cells mitigated IRI. Furthermore, hCD39 transgenic kidneys transplanted into syngeneic recipients after prolonged cold storage performed significantly better and exhibited less histological injury than wild-type control grafts. Thus, systemic or local strategies to promote adenosine generation and signaling may have beneficial effects on warm and cold renal IRI, with implications for therapeutic application in clinical renal transplantation.

Much ado about adenosine: adenosine synthesis and function in regulatory T cell biology

Recent studies have reported that adenosine is a significant mediator of regulatory T cell (Treg) function. Indeed, activation of the adenosine receptor subtypes expressed by a broad range of immune and inflammatory cells attenuates inflammation in several disease models. This anti-inflammatory response is associated with an increase in intracellular cAMP that inhibits cytokine responses of many immune/inflammatory cells, including T cells and APCs. Thus, adenosine produced by Tregs can provide a paracrine feedback that shapes the host response following an immunologic provocation. This review discusses the evidence that adenosine is an integral part of Treg biology and presents some of the mechanisms that may account for its contribution to the resolution of inflammation and the regulation of the immune/inflammatory cell phenotype.

The preclinical pharmacology of roflumilast--a selective, oral phosphodiesterase 4 inhibitor in development for chronic obstructive pulmonary disease

After more than two decades of research into phosphodiesterase 4 (PDE4) inhibitors, roflumilast (3-cyclopropylmethoxy-4-difluoromethoxy-N-[3,5-di-chloropyrid-4-yl]-benzamide) may become the first agent in this class to be approved for patient treatment worldwide. Within the PDE family of 11 known isoenzymes, roflumilast is selective for PDE4, showing balanced selectivity for subtypes A-D, and is of high subnanomolar potency. The active principle of roflumilast in man is its dichloropyridyl N-oxide metabolite, which has similar potency as a PDE4 inhibitor as the parent compound. The long half-life and high potency of this metabolite allows for once-daily, oral administration of a single, 500-microg tablet of roflumilast. The molecular mode of action of roflumilast--PDE4 inhibition and subsequent enhancement of cAMP levels--is well established. To further understand its functional mode of action in chronic obstructive pulmonary disease (COPD), for which roflumilast is being developed, a series of in vitro and in vivo preclinical studies has been performed. COPD is a progressive, devastating condition of the lung associated with an abnormal inflammatory response to noxious particles and gases, particularly tobacco smoke. In addition, according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD), significant extrapulmonary effects, including comorbidities, may add to the severity of the disease in individual patients, and which may be addressed preferentially by orally administered remedies. COPD shows an increasing prevalence and mortality, and its treatment remains a high, unmet medical need. In vivo, roflumilast mitigates key COPD-related disease mechanisms such as tobacco smoke-induced lung inflammation, mucociliary malfunction, lung fibrotic and emphysematous remodelling, oxidative stress, pulmonary vascular remodelling and pulmonary hypertension. In vitro, roflumilast N-oxide has been demonstrated to affect the functions of many cell types, including neutrophils, monocytes/macrophages, CD4+ and CD8+ T-cells, endothelial cells, epithelial cells, smooth muscle cells and fibroblasts. These cellular effects are thought to be responsible for the beneficial effects of roflumilast on the disease mechanisms of COPD, which translate into reduced exacerbations and improved lung function. As a multicomponent disease, COPD requires a broad therapeutic approach that might be achieved by PDE4 inhibition. However, as a PDE4 inhibitor, roflumilast is not a direct bronchodilator. In summary, roflumilast may be the first-in-class PDE4 inhibitor for COPD therapy. In addition to being a non-steroid, anti-inflammatory drug designed to target pulmonary inflammation, the preclinical pharmacology described in this review points to a broad functional mode of action of roflumilast that putatively addresses additional COPD mechanisms. This enables roflumilast to offer effective, oral maintenance treatment for COPD, with an acceptable tolerability profile and the potential to favourably affect the extrapulmonary effects of the disease.

Adenosine regulates thrombomodulin and endothelial protein C receptor expression in folliculostellate cells of the pituitary gland

Adenosine stimulates the release of interleukin 6 (IL-6) and vascular endothelial growth factor from folliculostellate cells of the anterior pituitary gland indicating that such cells are also involved in the communication between the immune and endocrine systems during stress and inflammation. In order to understand the precise actions of adenosine on folliculostellate cells, DNA microarray analysis was used to determine global changes in gene expression. Hierarchical clusters revealed, of the genes that had altered expression, the majority were suppressed and many, such as B cell translocation gene 2 and cyclin-dependent kinase inhibitor 2b were related to cell cycle arrest or inhibition of proliferation. Several of the up-regulated genes were associated with cytokine signalling or membrane receptor activity. The most notable of these being IL-6, sulfiredoxin 1, endothelial protein C receptor (EPCR) and thrombomodulin (THBD) which can all play a role in controlling inflammation. The EPCR and THBD pathway is well known in anti-coagulation but also has anti-inflammatory and anti-apoptotic properties. Up-regulation of EPCR and THBD in folliculostellate cells was confirmed by qRT-PCR and western blotting analysis and their expression were also demonstrated in many of the hormone-secreting cells of the anterior pituitary gland. Our findings suggest that adenosine can stimulate expression of stress and inflammation related genes from folliculostellate cells of the anterior pituitary gland. These genes include EPCR and THBD, neither of which has been previously identified in the pituitary gland.