Regulation of leukocyte function by adenosine receptors

A Comprehensive Review of the Pleiotropic Effects of Ticagrelor

AIMS

This review summarizes the findings of preclinical studies evaluating the pleiotropic effects of ticagrelor. These include attenuation of ischemia-reperfusion injury (IRI), inflammation, adverse cardiac remodeling, and atherosclerosis. In doing so, it aims to provide novel insights into ticagrelor's mechanisms and benefits over other P2Y12 inhibitors. It also generates viable hypotheses for the results of seminal clinical trials assessing ticagrelor use in acute and chronic coronary syndromes.

METHODS AND RESULTS

A comprehensive review of the preclinical literature demonstrates that ticagrelor protects against IRI in the setting of both an acute myocardial infarction (MI), and when MI occurs while on chronic treatment. Maintenance therapy with ticagrelor also likely mitigates adverse inflammation, cardiac remodeling, and atherosclerosis, while improving stem cell recruitment. These effects are probably mediated by ticagrelor's ability to increase local interstitial adenosine levels which activate downstream cardio-protective molecules. Attenuation and augmentation of these pleiotropic effects by high-dose aspirin and caffeine, and statins respectively may help explain variable outcomes in PLATO and subsequent randomized controlled trials (RCTs).

CONCLUSION

Most RCTs and meta-analyses have not evaluated the pleiotropic effects of ticagrelor. We need further studies comparing cardiovascular outcomes in patients treated with ticagrelor versus other P2Y12 inhibitors that are mindful of the unique pleiotropic advantages afforded by ticagrelor, as well as possible interactions with other therapies (e.g., aspirin, statins, caffeine).

ADA2 regulates inflammation and hematopoietic stem cell emergence via the A2bR pathway in zebrafish

Deficiency of adenosine deaminase 2 (DADA2) is an inborn error of immunity caused by loss-of-function mutations in the adenosine deaminase 2 (ADA2) gene. Clinical manifestations of DADA2 include vasculopathy and immuno-hematological abnormalities, culminating in bone marrow failure. A major gap exists in our knowledge of the regulatory functions of ADA2 during inflammation and hematopoiesis, mainly due to the absence of an ADA2 orthologue in rodents. Exploring these mechanisms is essential for understanding disease pathology and developing new treatments. Zebrafish possess two ADA2 orthologues, cecr1a and cecr1b, with the latter showing functional conservation with human ADA2. We establish a cecr1b-loss-of-function zebrafish model that recapitulates the immuno-hematological and vascular manifestations observed in humans. Loss of Cecr1b disrupts hematopoietic stem cell specification, resulting in defective hematopoiesis. This defect is caused by induced inflammation in the vascular endothelium. Blocking inflammation, pharmacological modulation of the A2r pathway, or the administration of the recombinant human ADA2 corrects these defects, providing insights into the mechanistic link between ADA2 deficiency, inflammation and immuno-hematological abnormalities. Our findings open up potential therapeutic avenues for DADA2 patients.

Endless Journey of Adenosine Signaling in Cardioprotective Mechanism of Conditioning Techniques: Clinical Evidence

Myocardial ischemic injury is a primary cause of death among various cardiovascular disorders. The condition occurs due to an interrupted supply of blood and vital nutrients (necessary for normal cellular activities and viability) to the myocardium, eventually leading to damage. Restoration of blood supply to ischemic tissue is noted to cause even more lethal reperfusion injury. Various strategies, including some conditioning techniques, like preconditioning and postconditioning, have been developed to check the detrimental effects of reperfusion injury. Many endogenous substances have been proposed to act as initiators, mediators, and end effectors of these conditioning techniques. Substances, like adenosine, bradykinin, acetylcholine, angiotensin, norepinephrine, opioids, etc., have been reported to mediate cardioprotective activity. Among these agents, adenosine has been widely studied and suggested to have the most pronounced cardioprotective effects. The current review article highlights the role of adenosine signaling in the cardioprotective mechanism of conditioning techniques. The article also provides an insight into various clinical studies that substantiate the applicability of adenosine as a cardioprotective agent in myocardial reperfusion injury.

Adenosinergic Signaling as a Key Modulator of the Glioma Microenvironment and Reactive Astrocytes

Astrocytes are numerous glial cells of the central nervous system (CNS) and play important roles in brain homeostasis. These cells can directly communicate with neurons by releasing gliotransmitters, such as adenosine triphosphate (ATP) and glutamate, into the multipartite synapse. Moreover, astrocytes respond to tissue injury in the CNS environment. Recently, astrocytic heterogeneity and plasticity have been discussed by several authors, with studies proposing a spectrum of astrocytic activation characterized by A1/neurotoxic and A2/neuroprotective polarization extremes. The fundamental roles of astrocytes in communicating with other cells and sustaining homeostasis are regulated by purinergic signaling. In the CNS environment, the gliotransmitter ATP acts cooperatively with other glial signaling molecules, such as cytokines, which may impact CNS functions by facilitating/inhibiting neurotransmitter release. Adenosine (ADO), the main product of extracellular ATP metabolism, is an important homeostatic modulator and acts as a neuromodulator in synaptic transmission via P1 receptor sensitization. Furthermore, purinergic signaling is a key factor in the tumor microenvironment (TME), as damaged cells release ATP, leading to ADO accumulation in the TME through the ectonucleotidase cascade. Indeed, the enzyme CD73, which converts AMP to ADO, is overexpressed in glioblastoma cells; this upregulation is associated with tumor aggressiveness. Because of the crucial activity of CD73 in these cells, extracellular ADO accumulation in the TME contributes to sustaining glioblastoma immune escape while promoting A2-like activation. The present review describes the importance of ADO in modulating astrocyte polarization and simultaneously promoting tumor growth. We also discuss whether targeting of CD73 to block ADO production can be used as an alternative cancer therapy.

Immunomodulatory Effects Associated with Cladribine Treatment

Cladribine is a synthetic deoxyadenosine analogue with demonstrated efficacy in patients with relapsing-remitting multiple sclerosis (MS). The main mechanism of action described for cladribine is the induction of a cytotoxic effect on lymphocytes, leading to a long-term depletion of peripheral T and B cells. Besides lymphocyte toxicity, the mode of action may include immunomodulatory mechanisms affecting other cells of the immune system. In order to induce its beneficial effects, cladribine is phosphorylated inside the cell by deoxycytidine kinase (DCK) to its active form. However, the mechanism of action of cladribine may also include immunomodulatory pathways independent of DCK activation. This in vitro study was designed to explore the impact of cladribine on peripheral blood mononuclear cells (PBMC) subsets, and to assess whether the immunomodulatory mechanisms induced by cladribine depend on the activation of the molecule. To this end, we obtained PBMCs from healthy donors and MS patients and performed proliferation, apoptosis and activation assays with clinically relevant concentrations of cladribine in DCK-dependent and -independent conditions. We also evaluated the effect of cladribine on myeloid lineage-derived cells, monocytes and dendritic cells (DCs). Cladribine decreased proliferation and increased apoptosis of lymphocyte subsets after prodrug activation via DCK. In contrast, cladribine induced a decrease in immune cell activation through both DCK-dependent and -independent pathways (not requiring prodrug activation). Regarding monocytes and DCs, cladribine induced cytotoxicity and impaired the activation of classical monocytes, but had no effect on DC maturation. Taken together, these data indicate that cladribine, in addition to its cytotoxic function, can mediate immunomodulation in different immune cell populations, by regulating their proliferation, maturation and activation.

Ticagrelor attenuates the increase of extracellular vesicle concentrations in plasma after acute myocardial infarction compared to clopidogrel

BACKGROUND

Platelet P2Y12 antagonist ticagrelor reduces mortality after acute myocardial infarction (AMI) compared to clopidogrel, but the underlying mechanism is unknown. Because activated platelets, leukocytes, and endothelial cells release proinflammatory and prothrombotic extracellular vesicles (EVs), we hypothesized that the release of EVs is more efficiently inhibited by ticagrelor compared to clopidogrel.

OBJECTIVES

We compared EV concentrations and EV procoagulant activity in plasma of patients after AMI treated with ticagrelor or clopidogrel.

METHODS

After percutaneous coronary intervention, 60 patients with first AMI were randomized to ticagrelor or clopidogrel. Flow cytometry was used to determine concentrations of EVs from activated platelets (CD61+ , CD62p+ ), fibrinogen+ , phosphatidylserine (PS+ ), leukocytes (CD45+ ), endothelial cells (CD31+ , 146+ ), and erythrocytes (CD235a+ ) in plasma at randomization, after 72 hours and 6 months of treatment. A fibrin generation test was used to determine EV procoagulant activity.

RESULTS

Concentrations of platelet, fibrinogen+ , PS+ , leukocyte, and erythrocyte EVs increased 6 months after AMI compared to the acute phase of AMI (P ≤ .03). Concentrations of platelet EVs were lower on ticagrelor compared to clopidogrel after 6 months (P = .03). Concentrations of fibrinogen+ , PS+ , and leukocyte EVs were lower on ticagrelor compared to clopidogrel both after 72 hours and 6 months (P ≤ .03). Concentrations of endothelial EVs and EV procoagulant activity did not differ between patient groups and over time (P ≥ .17).

CONCLUSIONS

Ticagrelor attenuates the increase of EV concentrations in plasma after acute myocardial infarction compared to clopidogrel. The ongoing release of EVs despite antiplatelet therapy might explain recurrent thrombotic events after AMI and worse clinical outcomes on clopidogrel compared to ticagrelor.

Non-Nucleoside Agonists of the Adenosine Receptors: An Overview

Potent and selective adenosine receptor (AR) agonists are of pharmacological interest for the treatment of a wide range of diseases and conditions. Among these derivatives, nucleoside-based agonists represent the great majority of molecules developed and reported to date. However, the limited availability of compounds selective for a specific AR subtype (i.e., A2BAR) and a generally long and complex synthetic route for largely substituted nucleosides are the main drawbacks of this category of molecules. Non-nucleoside agonists represent an alternative set of compounds able to stimulate the AR function and based on simplified structures. This review provides an updated overview on the structural classes of non-nucleoside AR agonists and their biological activities, with emphasis on the main derivatives reported in the literature. A focus is also given to the synthetic routes employed to develop these derivatives and on molecular modeling studies simulating their interaction with ARs.

A2bR-dependent signaling alters immune cell composition and enhances IL-6 formation in the ischemic heart

Although the cardioprotective effect of adenosine is undisputed, the role of the adenosine A2breceptor (A2bR) in ischemic cardiac remodeling is not defined. In this study we aimed to unravel the role A2bR plays in modulating the immune response and the healing mechanisms after myocardial infarction. Genetic and pharmacological (PSB603) inactivation of A2bR as well as activation of A2bR with BAY60-6583 does not alter cardiac remodeling of the infarcted (50-min left anterior descending artery occlusion/reperfusion) murine heart. Flow cytometry of immune cell subsets identified a significant increase in B cells, NK cells, CD8 and CD4 T cells, as well as FoxP3-expressing regulatory T cells in the injured heart in A2bR-deficient mice. Analysis of T-cell function revealed that expression and secretion of interleukin (IL)-2, interferon (IFN)γ, and tumor necrosis factor (TNF)α by T cells is under A2bR control. In addition, we found substantial cellular heterogeneity in the response of immune cells and cardiomyocytes to A2bR deficiency: while in the absence of A2bR, expression of IL-6 was greatly reduced in cardiomyocytes and immune cells except T cells, and expression of IL-1β was strongly reduced in cardiomyocytes, granulocytes, and B cells as determined by quantitative PCR. Our findings indicate that A2bR signaling in the ischemic heart triggers substantial changes in cardiac immune cell composition of the lymphoid lineage and induces a profound cell type-specific downregulation of IL-6 and IL-1β. This suggests the presence of a targetable adenosine–A2bR–IL-6-axis triggered by adenosine formed by the ischemic heart.

NEW & NOTEWORTHY Genetic deletion and pharmacological inactivation/activation of A2bR does not alter cardiac remodeling after MI but is associated by compensatory upregulation of various pro- and anti-inflammatory immune cell subsets (B cells, NK cells, CD8 and CD4 T cells, regulatory T cells). In the inflamed heart, A2bR modulates the expression of IL-2, IFNγ, TNFα in T cells and of IL-6 in cardiomyocytes, monocytes, granulocytes and B cells. This suggests an important adenosine–IL-6 axis, which is controlled by A2bR via local adenosine.

Adenosine A2A receptor antagonists act at the hyperoxic phase to confer protection against retinopathy

BACKGROUND

Retinopathy of prematurity (ROP) remains a major cause of childhood blindness and current laser photocoagulation and anti-VEGF antibody treatments are associated with reduced peripheral vision and possible delayed development of retinal vasculatures and neurons. In this study, we advanced the translational potential of adenosine A_2A receptor (A_2AR) antagonists as a novel therapeutic strategy for selectively controlling pathological retinal neovascularization in oxygen-induced retinopathy (OIR) model of ROP.

METHODS

Developing C57BL/6 mice were exposed to 75% oxygen from postnatal (P) day 7 to P12 and to room air from P12 to P17 and treated with KW6002 or vehicle at different postnatal developmental stages. Retinal vascularization was examined by whole-mount fluorescence and cross-sectional hematoxylin-eosin staining. Cellular proliferation, astrocyte and microglial activation, and tip cell function were investigated by isolectin staining and immunohistochemistry. Apoptosis was analyzed by TUNEL assay. The effects of oxygen exposure and KW6002 treatment were analyzed by two-way ANOVA or Kruskal-Wallis test or independent Student's t-test or Mann-Whitney U test.

RESULTS

The A_2AR antagonist KW6002 (P7-P17) did not affect normal postnatal development of retinal vasculature, but selectively reduced avascular areas and neovascularization, with the reduced cellular apoptosis and proliferation, and enhanced astrocyte and tip cell functions in OIR. Importantly, contrary to our prediction that A_2AR antagonists were most effective at the hypoxic phase with aberrantly increased adenosine-A_2AR signaling, we discovered that the A_2AR antagonist KW6002 mainly acted at the hyperoxic phase to confer protection against OIR as KW6002 treatment at P7-P12 (but not P12-P17) conferred protection against OIR; this protection was observed as early as P9 with reduced avascular areas and reduced cellular apoptosis and reversal of eNOS mRNA down-regulation in retina of OIR.

CONCLUSIONS

As ROP being a biphasic disease, our identification of the hyperoxic phase as the effective window, together with selective and robust protection against pathological (but not physiological) angiogenesis, elevates A_2AR antagonists as a novel therapeutic strategy for ROP treatment.

Advances in new drug therapies for the management of sickle cell disease

Introduction

Sickle cell disease (SCD) is an orphan disease in the United States, but is highly prevalent worldwide. Only two drugs, hydroxyurea and L-glutamine, are approved for this disease. With an improved understanding of the pathophysiology of SCD as well as the success of several recently approved drugs for other orphan diseases, there is an increased interest in the development of drugs for SCD.

Areas covered

This review summarizes published studies of drug therapies and ongoing trials of novel agents.

Expert opinion

The development of drugs with different mechanisms of action offers opportunities for combination and individualized therapy in SCD. In addition to acute pain crisis, the evaluation of other SCD-related complications, exercise capacity, patient reported outcomes and validated surrogate endpoints are necessary to advance drug development. It is important to involve sites in sub-Saharan Africa and India, which have the highest burden of SCD, in trials of novel therapies.

Novel Sickle Cell Disease Therapies: Targeting Pathways Downstream of Sickling

Sickle cell disease is an inherited hemoglobinopathy characterized by hemolytic anemia, frequent painful episodes, poor quality of life, end organ damage and a shortened lifespan. Although the seminal event is the polymerization of the abnormal hemoglobin, the downstream pathophysiology of vaso-occlusion results from heterotypic interactions between the altered, adhesive sickle cell RBCs, neutrophils, endothelium, and platelets. Ischemia reperfusion injury, hemolysis and oxidant damage all contribute to heightened inflammation and activation of the hemostatic system. These downstream targets are the focus of emerging treatments with considerable potential to ameliorate disease manifestations. This review summarizes the progress on development of these agents.

Adenosine Receptor-Mediated Cardioprotection-Current Limitations and Future Directions

Since the seminal reports of adenosine receptor-mediated cardioprotection in the early 1990s, there have been a multitude of such reports in various species and preparations. Original observations of the beneficial effects of A₁ receptor agonists have been followed up with numerous reports also implicating A_2A, A₃, and most recently A_2B, receptor agonists as cardioprotective agents. Although adenosine has been approved for clinical use in the United States for the treatment of supraventricular tachycardia and coronary artery imaging, and the selective A_2A agonist, regadenoson, for the latter, clinical use of adenosine receptor agonists for protecting the ischemic heart has not advanced beyond early trials. An examination of the literature indicates that existing experimental studies have several limitations in terms of clinical relevance, as well as lacking incorporation of recent new insights into adenosine receptor signaling. Such deficiencies include the lack of experimental studies in models that most closely mimic human cardiovascular disease. In addition, there have been very few studies in chronic models of myocardial ischemia, where limiting myocardial remodeling and heart failure, not reduction of infarct size, are the primary endpoints. Despite an increasing number of reports of the beneficial effects of adenosine receptor antagonists, not agonists, in chronic diseases, this idea has not been well-studied in experimental myocardial ischemia. There have also been few studies examining adenosine receptor subtype interactions as well as receptor heterodimerization. The purpose of this Perspective article is to discuss these deficiencies to highlight future directions of research in the field of adenosine receptor-mediated protection of ischemic myocardium.

Adenosine Production by Biomaterial-Supported Mesenchymal Stromal Cells Reduces the Innate Inflammatory Response in Myocardial Ischemia/Reperfusion Injury

BACKGROUND

During myocardial ischemia/reperfusion (MI/R) injury, there is extensive release of immunogenic metabolites that activate cells of the innate immune system. These include ATP and AMP, which upregulate chemotaxis, migration, and effector function of early infiltrating inflammatory cells. These cells subsequently drive further tissue devitalization. Mesenchymal stromal cells (MSCs) are a potential treatment modality for MI/R because of their powerful anti-inflammatory capabilities; however, the manner in which they regulate the acute inflammatory milieu requires further elucidation. CD73, an ecto-5'-nucleotidase, may be critical in regulating inflammation by converting pro-inflammatory AMP to anti-inflammatory adenosine. We hypothesized that MSC-mediated conversion of AMP into adenosine reduces inflammation in early MI/R, favoring a micro-environment that attenuates excessive innate immune cell activation and facilitates earlier cardiac recovery.

METHODS AND RESULTS

Adult rats were subjected to 30 minutes of MI/R injury. MSCs were encapsulated within a hydrogel vehicle and implanted onto the myocardium. A subset of MSCs were pretreated with the CD73 inhibitor, α,β-methylene adenosine diphosphate, before implantation. Using liquid chromatography/mass spectrometry, we found that MSCs increase myocardial adenosine availability following injury via CD73 activity. MSCs also reduce innate immune cell infiltration as measured by flow cytometry, and hydrogen peroxide formation as measured by Amplex Red assay. These effects were dependent on MSC-mediated CD73 activity. Finally, through echocardiography we found that CD73 activity on MSCs was critical to optimal protection of cardiac function following MI/R injury.

CONCLUSIONS

MSC-mediated conversion of AMP to adenosine by CD73 exerts a powerful anti-inflammatory effect critical for cardiac recovery following MI/R injury.

Acute rejection after kidney transplantation promotes graft fibrosis with elevated adenosine level in rat

Aims

Chronic allograft nephropathy is a worldwide issue with the major feature of progressive allograft fibrosis, eventually ending with graft loss. Adenosine has been demonstrated to play an important role in process of fibrosis. Our study aimed to investigate the relationship between adenosine and fibrosis in renal allograft acute rejection in rat.

Materials and methods

Wistar rats and SD rats were selected as experimental animals. Our study designed two groups. In the allograft transplantation group, kidneys of Wistar rats were orthotopically transplanted into SD rat recipients, the same species but not genetically identical, to induce acute rejection. Kidney transplantations of SD rats to SD rats which were genetically identical were served as the control. We established rat models and detected a series of indicators. All data were analyzed statistically. P<0.05 was considered statistically significant.

Results

Compared with the control group, levels of adenosine increased significantly in the allograft transplantation group, in which acute rejection was induced (P<0.05). Progressive allograft fibrosis as well as collagen deposition were observed.

Conclusions

These findings suggested that level of adenosine was upregulated in acute rejection after kidney allograft transplantation in rat. Acute rejection may promote renal allograft fibrosis via the adenosine signaling pathways.

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.

Adenosine Formed by CD73 on T Cells Inhibits Cardiac Inflammation and Fibrosis and Preserves Contractile Function in Transverse Aortic Constriction-Induced Heart Failure

BACKGROUND

Structural damage during heart failure development leads to increased infiltration of leukocytes. Because purinergic signaling on immune cells may impact on the inflammatory response, we evaluated the role of ecto-5'-nucleotidase (CD73) on the development of heart failure after transverse aortic constriction (TAC) using global and T-cell-specific CD73^-/- mice.

METHODS AND RESULTS

Leukocytes infiltrating the failing heart were analyzed by a multistep enzymatic procedure over a period of 16 weeks using fluorescence-activated cell sorting. TAC significantly enhanced the infiltration of leukocytes, especially T cells. The fraction of CD73 expressing cells increased over time exclusively on cytotoxic T cells, T-helper cells, and regulatory T cells. Cardiac function significantly declined in T-cell-specific CD4-Cre^+/-CD73^flox/flox mice identical to that observed in global CD73 mutants and was associated with enhanced fibrosis (collagen, laminin, vimentin, periostin). Expression analysis by quantitative reverse transcription polymerase chain reaction of extracellular purine degrading enzymes and P1 and P2 receptors on T cells isolated from the injured heart revealed profound upregulation of the enzymatic machinery for hydrolysis of extracellular adenosine triphosphate and nicotinamide adenine dinucleotide, both pathways converging in the formation of AMP and adenosine via CD73. Among the P1 receptors, only the A2a receptor was significantly upregulated after TAC. T cells isolated from TAC-treated hearts show enhanced production of proinflammatory cytokines (interleukin-3, interleukin-6, interleukin-13, interleukin-17, macrophage inflammatory proteins-1α, and macrophage inflammatory proteins-1β) when CD73 was lacking.

CONCLUSIONS

Our data provide first evidence that CD73 on T cells plays an important anti-inflammatory role in TAC-induced heart failure, which is associated with antifibrotic activity and reduced production of proinflammatory cytokines most likely by activation of the adenosine A2a receptor.

Adenosine receptors and caffeine in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a major cause of childhood blindness in the world and is caused by oxygen-induced damage to the developing retinal vasculature, resulting in hyperoxia-induced vaso-obliteration and subsequent delayed retinal vascularization and hypoxia-induced pathological neovascularization driven by vascular endothelial growth factor (VEGF) signaling pathway in retina. Current anti-VEGF therapy has shown some effective in a clinical trial, but is associated with the unintended effects on delayed eye growth and retinal vasculature development of preterm infants. Notably, cellular responses to hypoxia are characterized by robust increases in extracellular adenosine production and the markedly induced adenosine receptors, which provide a novel target for preferential control of pathological angiogenesis without affecting normal vascular development. Here, we review the experimental evidence in support of adenosine receptor-based therapeutic strategy for ROP, including the aberrant adenosine signaling in oxygen-induced retinopathy and the role of three adenosine receptor subtypes (A₁R, A_2AR, A_2BR) in development and treatment of ROP using oxygen-induced retinopathy models. The clinical and initial animal evidence that implicate the therapeutic effect of caffeine (a non-selective adenosine receptor antagonist) in treatment of ROP are highlighted. Lastly, we discussed the translational potential as well therapeutic advantage of adenosine receptor- and caffeine-based therapy for ROR and possibly other proliferative retinopathy.

Prostaglandin E2-mediated adenosinergic effects on CD14+ cells: Self-amplifying immunosuppression in cancer

CD39 and CD73 are surface-expressed ectonucleotidases that hydrolyze ATP in a highly regulated, serial manner into ADP, AMP and adenosine. The end product, adenosine, has both tumor-promoting and immunosuppressive effects. The aim of this study was to determine CD73 expression on immune cells in pleural effusion (PE) in order to have a better understanding of the immune environment in mesothelioma. PE- or blood-derived CD14⁺ cells of mesothelioma patients and healthy donors were analyzed by flow cytometry for the expression of CD39 and CD73. CD73-induction was studied by exposure of CD14⁺ cells to the soluble fraction of PE (sPE), while the signaling mechanism, responsible for CD73 induction, by phosphoflow cytometry and receptor-inhibition studies. We observed CD73 expression on CD14⁺ cells in PE but not peripheral blood of mesothelioma patients or healthy donors. CD73 expression was inducible on CD14⁺ cells with sPE, cyclic-AMP (cAMP)-inducers (forskolin and prostaglandin-E₂ (PGE₂)) and adenosine. Inhibition of PGE₂ receptors or adenosine A₂ receptors blocked CD73-induction by sPE. sPE treatment triggered protein kinase A and p38 activation. However, signal-transducer and activator of transcription 3 (STAT3)-blocking led to enhanced CD73 expression, demonstrating a hitherto unknown negative control of purinergic signaling by STAT3 in CD14⁺ cells. TNFα production by CD73⁺ CD14⁺ cells was significantly impaired in the presence of AMP, confirming immunosuppressive function. Taken together, CD73 expression can be induced by PGE₂, cAMP or adenosine on human CD14⁺ cells. We suggest that targeting this autocrine loop is a valid therapeutic approach in mesothelioma that may also enhance immunotherapy.

Extracellular Adenosine Production by ecto-5'-Nucleotidase (CD73) Enhances Radiation-Induced Lung Fibrosis

Radiation-induced pulmonary fibrosis is a severe side effect of thoracic irradiation, but its pathogenesis remains poorly understood and no effective treatment is available. In this study, we investigated the role of the extracellular adenosine as generated by the ecto-5'-nucleotidase CD73 in fibrosis development after thoracic irradiation. Exposure of wild-type C57BL/6 mice to a single dose (15 Gray) of whole thorax irradiation triggered a progressive increase in CD73 activity in the lung between 3 and 30 weeks postirradiation. In parallel, adenosine levels in bronchoalveolar lavage fluid (BALF) were increased by approximately 3-fold. Histologic evidence of lung fibrosis was observed by 25 weeks after irradiation. Conversely, CD73-deficient mice failed to accumulate adenosine in BALF and exhibited significantly less radiation-induced lung fibrosis (P < 0.010). Furthermore, treatment of wild-type mice with pegylated adenosine deaminase or CD73 antibodies also significantly reduced radiation-induced lung fibrosis. Taken together, our findings demonstrate that CD73 potentiates radiation-induced lung fibrosis, suggesting that existing pharmacologic strategies for modulating adenosine may be effective in limiting lung toxicities associated with the treatment of thoracic malignancies. Cancer Res; 76(10); 3045-56. ©2016 AACR.

Adenosine A1 Receptors Selectively Modulate Oxygen-Induced Retinopathy at the Hyperoxic and Hypoxic Phases by Distinct Cellular Mechanisms

PURPOSE

We critically evaluated the role of the adenosine A1 receptor (A1R) in normal development of retinal vasculature and pathogenesis of retinopathy of prematurity (ROP) by using the A1R knockout (KO) mice and oxygen-induced retinopathy (OIR) model.

METHODS

Mice deficient in A1Rs and their wild-type (WT) littermates were examined during normal postnatal development or after being subjected to 75% oxygen from postnatal day (P) 7 to P12 and to room air from P12 to P17 (OIR model of ROP). Retinal vascularization was examined by whole-mount fluorescence and cross-sectional hematoxylin-eosin staining. Cellular proliferation, astrocyte and microglial activation, and tip cell function were determined by isolectin staining and immunohistochemistry. Apoptosis was determined by TUNEL assay.

RESULTS

Genetic deletion of the A1R did not affect normal retinal vascularization during postnatal development with indistinguishable three-layer vascularization patterns in retina between WT and A1R KO mice. In the OIR model, genetic deletion of the A1R resulted in stage-specific effects: reduced hyperoxia-induced retinal vaso-obliteration at P12, but reduced avascular area and attenuated hypoxia-induced intraretinal revascularization without affecting intravitreal neovascularization at P17 and reduced avascular areas in retina at P21. These distinct effects of A1Rs on OIR were associated with A1R control of apoptosis mainly in inner and outer nuclear layers at the vaso-obliterative phase (P12) and the growth of endothelium tip cells at the vasoproliferative phase (P17), without modification of cellular proliferation, astrocytic activation, and tissue inflammation.

CONCLUSIONS

Adenosine A1 receptor activity is not required for normal postnatal development of retinal vasculature but selectively controls hyperoxia-induced vaso-obliteration and hypoxia-driven revascularization by distinct cellular mechanisms.

Disrupted regulatory T cell homeostasis in inflammatory bowel diseases

In the gut, where billions of non-self-antigens from the food and the microbiota are present, the immune response must be tightly regulated to ensure both host protection against pathogenic microorganisms and the absence of immune-related pathologies. It has been well documented that regulatory T cells (Tregs) play a pivotal role in this context. Indeed, Tregs are able to prevent excessive inflammation, which can lead to the rupture of intestinal homeostasis observed in inflammatory bowel diseases (IBDs). Both the worldwide incidence and prevalence of such diseases have increased throughout the latter part of the 20^th century. Therefore, it is crucial to understand how Tregs suppress the colitogenic immune cells to establish new treatments for patients suffering from IBDs. In this review, we will first summarize the results obtained in animal model studies that highlight the importance of Tregs in maintaining intestinal homeostasis and describe the specific suppressive mechanisms involved. Next, our current knowledge about Tregs contribution to human IBDs will be reviewed, as well as the current therapeutic perspective on using Tregs for clinical IBD treatment and the challenges that remain to be resolved to ensure both the safety and effectiveness of these therapies in targeting this critical immune-regulatory cell population.

Beyond hydroxyurea: new and old drugs in the pipeline for sickle cell disease

Despite Food and Drug Administration (FDA) approval of hydroxyurea to reduce the frequency of vaso-occlusive episodes, sickle cell disease (SCD) has continued to be treated primarily with analgesics for pain relief. However, elucidation of the multiple pathophysiologic mechanisms leading to vaso-occlusion and tissue injury in SCD has now resulted in a burgeoning effort to identify new treatment modalities to prevent or ameliorate the consequences of the disease. Development of new drugs as well as investigation of drugs previously used in other settings have targeted cell adhesion, inflammatory pathways, upregulation of hemoglobin F, hemoglobin polymerization and sickling, coagulation, and platelet activation. Although these efforts have not yet yielded drugs ready for FDA approval, several early studies have been extremely encouraging. Moreover, the marked increase in clinical pharmaceutical research addressing SCD and the new and old drugs in the pipeline make it reasonable to expect that we will soon have new treatments for SCD.

Purinergic Receptors: Key Mediators of HIV-1 Infection and Inflammation

Human immunodeficiency virus type 1 (HIV-1) causes a chronic infection that afflicts more than 30 million individuals worldwide. While the infection can be suppressed with potent antiretroviral therapies, individuals infected with HIV-1 have elevated levels of inflammation as indicated by increased T cell activation, soluble biomarkers, and associated morbidity and mortality. A single mechanism linking HIV-1 pathogenesis to this inflammation has yet to be identified. Purinergic receptors are known to mediate inflammation and have been shown to be required for HIV-1 infection at the level of HIV-1 membrane fusion. Here, we review the literature on the role of purinergic receptors in HIV-1 infection and associated inflammation and describe a role for these receptors as potential therapeutic targets.

Schwann Cells Metabolize Extracellular 2',3'-cAMP to 2'-AMP

The 3',5'-cAMP-adenosine pathway (3',5'-cAMP→5'-AMP→adenosine) and the 2',3'-cAMP-adenosine pathway (2',3'-cAMP→2'-AMP/3'-AMP→adenosine) are active in the brain. Oligodendrocytes participate in the brain 2',3'-cAMP-adenosine pathway via their robust expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase; converts 2',3'-cAMP to 2'-AMP). Because Schwann cells also express CNPase, it is conceivable that the 2',3'-cAMP-adenosine pathway exists in the peripheral nervous system. To test this and to compare the 2',3'-cAMP-adenosine pathway to the 3',5'-cAMP-adenosine pathway in Schwann cells, we examined the metabolism of 2',3'-cAMP, 2'-AMP, 3'-AMP, 3',5'-cAMP, and 5'-AMP in primary rat Schwann cells in culture. Addition of 2',3'-cAMP (3, 10, and 30 µM) to Schwann cells increased levels of 2'-AMP in the medium from 0.006 ± 0.002 to 21 ± 2, 70 ± 3, and 187 ± 10 nM/µg protein, respectively; in contrast, Schwann cells had little ability to convert 2',3'-cAMP to 3'-AMP or 3',5'-cAMP to either 3'-AMP or 5'-AMP. Although Schwann cells slightly converted 2',3'-cAMP and 2'-AMP to adenosine, they did so at very modest rates (e.g., 5- and 3-fold, respectively, more slowly compared with our previously reported studies in oligodendrocytes). Using transected myelinated rat sciatic nerves in culture medium, we observed a time-related increase in endogenous intracellular 2',3'-cAMP and extracellular 2'-AMP. These findings indicate that Schwann cells do not have a robust 3',5'-cAMP-adenosine pathway but do have a 2',3'-cAMP-adenosine pathway; however, because the pathway mostly involves 2'-AMP formation rather than 3'-AMP, and because the conversion of 2'-AMP to adenosine is slow, metabolism of 2',3'-cAMP mostly results in the accumulation of 2'-AMP. Accumulation of 2'-AMP in peripheral nerves postinjury could have pathophysiological consequences.

Adenosine receptor neurobiology: overview

Adenosine is a naturally occurring nucleoside that is distributed ubiquitously throughout the body as a metabolic intermediary. In the brain, adenosine functions as an important upstream neuromodulator of a broad spectrum of neurotransmitters, receptors, and signaling pathways. By acting through four G-protein-coupled receptors, adenosine contributes critically to homeostasis and neuromodulatory control of a variety of normal and abnormal brain functions, ranging from synaptic plasticity, to cognition, to sleep, to motor activity to neuroinflammation, and cell death. This review begun with an overview of the gene and genome structure and the expression pattern of adenosine receptors (ARs). We feature several new developments over the past decade in our understanding of AR functions in the brain, with special focus on the identification and characterization of canonical and noncanonical signaling pathways of ARs. We provide an update on functional insights from complementary genetic-knockout and pharmacological studies on the AR control of various brain functions. We also highlight several novel and recent developments of AR neurobiology, including (i) recent breakthrough in high resolution of three-dimension structure of adenosine A2A receptors (A2ARs) in several functional status, (ii) receptor-receptor heterodimerization, (iii) AR function in glial cells, and (iv) the druggability of AR. We concluded the review with the contention that these new developments extend and strengthen the support for A1 and A2ARs in brain as therapeutic targets for neurologic and psychiatric diseases.

New selective A2A agonists and A3 antagonists for human adenosine receptors: synthesis, biological activity and molecular docking studies

Synthesis and pharmacological characterization of a new series of adenosine derivatives on the four human adenosine receptors are reported.

Putative roles of purinergic signaling in human immunodeficiency virus-1 infection

Reviewers

This article was reviewed by Neil S. Greenspan and Rachel Gerstein.

Nucleotides and nucleosides act as potent extracellular messengers via the activation of the family of cell-surface receptors termed purinergic receptors. These receptors are categorized into P1 and P2 receptors (P2Rs). P2Rs are further classified into two distinct families, P2X receptors (P2XRs) and P2Y receptors (P2YRs). These receptors display broad tissue distribution throughout the body and are involved in several biological events. Immune cells express various P2Rs, and purinergic signaling mechanisms have been shown to play key roles in the regulation of many aspects of immune responses. Researchers have elucidated the involvement of these receptors in the host response to infections. The evidences indicate a dual function of these receptors, depending on the microorganism and the cellular model involved. Three recent reports have examined the relationship between the level of extracellular ATP, the mechanisms underlying purinergic receptors participating in the infection mechanism of HIV-1 in the cell. Although preliminary, these results indicate that purinergic receptors are putative pharmacological targets that should be further explored in future studies.

Extracellular adenosine regulates colitis through effects on lymphoid and nonlymphoid cells

Adenosine is a purine metabolite that can mediate anti-inflammatory responses in the digestive tract through the A(2A) adenosine receptor (A(2A)AR). We examined the role of this receptor in the control of inflammation in the adoptive transfer model of colitis. Infection of A(2A)AR(-/-) mice with Helicobacter hepaticus increased colonic inflammation scores compared with uninfected A(2A)AR controls. Comparison of T cell subsets in wild-type and A(2A)AR(-/-) mice revealed differences in markers associated with activated helper T (Th) cells and regulatory T (Treg) cells. Previous studies showed that expression of A(2A)AR on CD45RB(HI) and CD45RB(LO) Th cells is essential for the proper regulation of colonic inflammation. Adoptive transfer of CD45RB(HI) with CD45RB(LO) from wild-type mice into RAG1(-/-)/A(2A)AR(-/-) mice induced severe disease within 3 wk, although transfer of the same subsets into RAG1(-/-) mice does not induce colitis. This suggests that the presence of A(2A)AR on recipient cells is also important for controlling colitis. To investigate the role of A(2A)AR in myeloid cells, chimeric recipients were generated by injection of bone marrow from RAG1(-/-) or RAG1(-/-)/A(2A)AR(-/-) mice into irradiated RAG1(-/-) mice. After adoptive transfer, these recipients did not develop colitis, regardless of A(2A)AR expression by the donor. Together, our results suggest that the control of inflammation in vivo is dependent on A(2A)AR signaling through multiple cell types that collaborate in the regulation of colitis by responding to extracellular adenosine.

The elevated gene expression level of the A(2B) adenosine receptor is associated with hyperglycemia in women with gestational diabetes mellitus

BACKGROUND

Adenosine receptors denoted by A1 , A2A , A2B , and A3 and encoded by ADORA1, ADORA2A, ADORA2B, and ADORA3 genes, respectively, are adenosine-activated G-protein-coupled receptors that play an important role in obesity and type 2 diabetes mellitus. However, little is known about their significance in gestational diabetes mellitus (GDM). The purpose of this study was to investigate whether there are changes in leukocyte AR expression in GDM patients and whether these alterations are linked to well-known diabetic genes.

METHODS

Leukocytes were isolated from the blood of normal glucose tolerant (NGT; n = 35) and GDM (n = 82) pregnant women, and expression of ARs was determined by a semi-quantitative polymerase chain reaction (PCR). Univariate correlation analysis was performed to investigate associations between expression of ARs and anthropometric and metabolic parameters of patients. Furthermore, the identification of diabetic genes linked to significantly differentiated leukocyte adenosine receptors expression in GDM women was also carried out with the use of the human diabetes RT(2) profiler PCR arrays.

RESULTS

ADORA2B mRNA expression was significantly higher in GDM versus NGT pregnant women (p < 0.05), and positively correlated with the glucose level at 1-h 75-g oral glucose tolerance test (OGTT; r = 0.21, p = 0.044). Nineteen diabetic genes linked to leukocyte ADORA2B overexpression associated with hyperglycemia in GDM women were also identified.

CONCLUSIONS

Maternal leukocyte ADORA2B overexpression is associated with hyperglycemia in GDM subjects, and it is accompanied by complex alterations in the expression of diabetes-related genes involved in insulin action, carbohydrate and lipid metabolism, oxidative stress, and inflammation.

The uncoordinated-5 homolog B (UNC5B) receptor increases myocardial ischemia-reperfusion injury

The UNC5 receptor family are chemorepulsive neuronal guidance receptors with additional functions outside the central nervous system. Previous studies have implicated that the UNC5B receptor influences the migration of leukocytes into sites of tissue inflammation. Given that this process is a critical step during the pathophysiology of myocardial ischemia followed by reperfusion (IR) we investigated the role of UNC5B during myocardial IR. In initial in-vitro experiments, the functional inhibition of UNC5B resulted in a significant reduction of chemotactic migration of neutrophils. In-vivo, using a model of acute myocardial ischemia in UNC5B^+/− and wild type (WT) animals, we found a significant reduction of infarct sizes in UNC5B^+/− animals. This was associated with significantly reduced levels of troponin-I and IL-6 in UNC5B^+/− mice. The repression of UNC5B using siRNA and the functional inhibition of UNC5B significantly dampened the extent of myocardial IR injury. Following depletion of neutrophils, we were not able to observe any further reduction in infarct size through functional inhibition of UNC5B in WT and UNC5B^+/− mice. In summary our studies demonstrate an important role for UNC5B during myocardial IR injury, and that UNC5B might be a potential therapeutic target to control reperfusion injury in the future.

Adenosine receptors as drug targets--what are the challenges?

Adenosine signalling has long been a target for drug development, with adenosine itself or its derivatives being used clinically since the 1940s. In addition, methylxanthines such as caffeine have profound biological effects as antagonists at adenosine receptors. Moreover, drugs such as dipyridamole and methotrexate act by enhancing the activation of adenosine receptors. There is strong evidence that adenosine has a functional role in many diseases, and several pharmacological compounds specifically targeting individual adenosine receptors--either directly or indirectly--have now entered the clinic. However, only one adenosine receptor-specific agent--the adenosine A2A receptor agonist regadenoson (Lexiscan; Astellas Pharma)--has so far gained approval from the US Food and Drug Administration (FDA). Here, we focus on the biology of adenosine signalling to identify hurdles in the development of additional pharmacological compounds targeting adenosine receptors and discuss strategies to overcome these challenges.

Adenosine A3 receptor activation attenuates lung ischemia-reperfusion injury

BACKGROUND

Severe ischemia-reperfusion (IR) injury leads to primary graft dysfunction after lung transplantation. Adenosine receptors modulate inflammation after IR, and the adenosine A3 receptor (A3R) is expressed in lung tissue and inflammatory cells. This study tests the hypothesis that A3R agonism attenuates lung IR injury by a neutrophil-dependent mechanism.

METHODS

Wild-type and A3R knockout (A3R-/-) mice underwent 1-hour left lung ischemia followed by 2-hours reperfusion (IR). A selective A3R agonist, Cl-IB-MECA, was administered (100 μg/kg intravenously) 5 minutes prior to ischemia. Study groups included sham, IR, and IR+Cl-IB-MECA (n = 6/group). Lung injury was assessed by measuring lung function, pulmonary edema, histopathology, and proinflammatory cytokines, and myeloperoxidase levels in bronchoalveolar lavage fluid. Parallel in vitro experiments were performed to evaluate neutrophil chemotaxis, and neutrophil activation was measured after exposure to acute hypoxia and reoxygenation.

RESULTS

Treatment of wild-type mice with Cl-IB-MECA significantly improved lung function and decreased edema, cytokine expression, and neutrophil infiltration after IR. The Cl-IB-MECA had no effects in A3R-/- mice; Cl-IB-MECA significantly decreased activation of wild-type, but not A3R-/-, neutrophils after acute hypoxia and reoxygenation and inhibited chemotaxis of wild-type neutrophils.

CONCLUSIONS

Exogenous activation of A3R by Cl-IB-MECA attenuates lung dysfunction, inflammation, and neutrophil infiltration after IR in wild-type but not A3R-/- mice. Results with isolated neutrophils suggest that the protective effects of Cl-IB-MECA are due, in part, to the prevention of neutrophil activation and chemotaxis. The use of A3R agonists may be a novel therapeutic strategy to prevent lung IR injury and primary graft dysfunction after transplantation.

Adenosine 2A receptor modulates inflammation and phenotype in experimental abdominal aortic aneurysms

Activation of the adenosine 2A receptor (A2AR) reduces inflammation in models of acute injury but contribution in development of chronic abdominal aortic aneurysms (AAAs) is unknown. Elastase perfusion to induce AAA formation in A2AR-knockout (A2ARKO) and C57BL6/J wild-type (WT) mice resulted in nearly 100% larger aneurysms in A2ARKO compared to WT at d 14 (P<0.05), with evidence of greater elastin fragmentation, more immune cell infiltration, and increased matrix metallatoproteinase (MMP) 9 expression (P<0.05). Separately, exogenous A2AR antagonism in elastase-perfused WT mice also resulted in larger aneurysms (P<0.05), while A2AR agonism limited aortic dilatation (P<0.05). Activated Thy-1.2(+) T lymphocytes from WT mice treated in vitro with A2AR antagonist increased cytokine production, and treatment with A2AR agonist decreased cytokine production (P<0.05 for all). Primary activated CD4(+) T lymphocytes from A2ARKO mice exhibited greater chemotaxis (P<0.05). A2AR antagonist increased chemotaxis of activated CD4(+) cells from WT mice in vitro, and A2AR agonist reduced this effect (P<0.05). A2AR activation attenuates AAA formation partly by inhibiting immune cell recruitment and reducing elastin fragmentation. These findings support augmenting A2AR signaling as a putative target for limiting aneurysm formation.

Biological functions of ecto-enzymes in regulating extracellular adenosine levels in neoplastic and inflammatory disease states

When present in the extracellular environment, the nucleoside adenosine protects cells and tissues from excessive inflammation and immune-mediated damage while promoting healing processes. This role has been highlighted experimentally using distinct disease models, including those of colitis, diabetes, asthma, sepsis, and ischemic injury. Adenosine also suppresses immune responses, as in the tumor microenvironment, assisting immune evasion while promoting angiogenesis. The mechanisms involved in adenosine signaling are addressed elsewhere in this issue. Here, the authors specifically address the generation of adenosine from extracellular nucleotides. This process is catalyzed by a series of plasma membrane ectonucleotidases, with the focus in this article on members of the CD39, CD73, and CD38 families and on their role in inflammatory and neoplastic hematological diseases. Pharmacological modulation of adenosine generation by drugs that either have or modulate ectonucleotidase function might be exploited to treat these diverse conditions.

Selective activation of adenosine A2A receptors on immune cells by a CD73-dependent prodrug suppresses joint inflammation in experimental rheumatoid arthritis

Adenosine A(2A) receptor (A(2A)R) agonists are both highly effective anti-inflammatory agents and potent vasodilators. To separate these two activities, we have synthesized phosphorylated A(2A)R agonists (prodrugs) that require the presence of ecto-5'-nucleotidase (CD73) to become activated. In the model of collagen-induced arthritis, 2-(cyclohexylethylthio)adenosine 5'-monophosphate (chet-AMP), but not 2-(cyclohexylethylthio)adenosine (chet-adenosine), potently reduced inflammation as assessed by fluorine-19 ((19)F) magnetic resonance imaging and by histology. The prodrug effect was blunted by inhibition of CD73 and A(2A)R. The selectivity of drug action is due to profound up-regulation of CD73 and adenosine A(2A)R expression in neutrophils and inflammatory monocytes as found in recovered cells from the synovial fluid of arthritic mice. Plasma chet-adenosine was in the subnanomolar range when chet-AMP was applied, whereas concentrations required for vasodilation were about 100 times higher. Thus, chet-AMP is a potent immunosuppressant with negligible vasodilatory activity. These data suggest that phosphorylated A(2A)R agonists may serve as a promising new group of drugs for targeted immunotherapy of inflammation.

Extracellular guanosine regulates extracellular adenosine levels

The aim of this investigation was to test the hypothesis that extracellular guanosine regulates extracellular adenosine levels. Rat preglomerular vascular smooth muscle cells were incubated with adenosine, guanosine, or both. Guanosine (30 μmol/l) per se had little effect on extracellular adenosine levels. Extracellular adenosine levels 1 h after addition of adenosine (3 μmol/l) were 0.125 ± 0.020 μmol/l, indicating rapid disposition of extracellular adenosine. Extracellular adenosine levels 1 h after addition of adenosine (3 μmol/l) plus guanosine (30 μmol/l) were 1.173 ± 0.061 μmol/l, indicating slow disposition of extracellular adenosine. Cell injury increased extracellular levels of endogenous adenosine and guanosine, and the effects of cell injury on endogenous extracellular adenosine were modulated by altering the levels of endogenous extracellular guanosine with exogenous purine nucleoside phosphorylase (converts guanosine to guanine) or 8-aminoguanosine (inhibits purine nucleoside phosphorylase). Extracellular guanosine also slowed the disposition of extracellular adenosine in rat preglomerular vascular endothelial cells, mesangial cells, cardiac fibroblasts, and kidney epithelial cells and in human aortic and coronary artery vascular smooth muscle cells and coronary artery endothelial cells. The effects of guanosine on adenosine levels were not mimicked or attenuated by 5-iodotubericidin (adenosine kinase inhibitor), erythro-9-(2-hydroxy-3-nonyl)-adenine (adenosine deaminase inhibitor), 5-aminoimidazole-4-carboxamide (guanine deaminase inhibitor), aristeromycin (S-adenosylhomocysteine hydrolase inhibitor), low sodium (inhibits concentrative nucleoside transporters), S-(4-nitrobenzyl)-6-thioinosine [inhibits equilibrative nucleoside transporter (ENT) type 1], zidovudine (inhibits ENT type 2), or acadesine (known modulator of adenosine levels). Guanosine also increases extracellular inosine, uridine, thymidine, and cytidine, yet decreases extracellular uric acid. In conclusion, extracellular guanosine regulates extracellular adenosine levels.

Clinical significance of UNC5B expression in bladder cancer

UNC5B is a membrane-bound receptor of the neural guidance factor netrin-1 family, with important roles in angiogenesis, neurogenesis, embryonic development, cancer, inflammation and various pathologies. However, its effect on bladder cancer has not been reported. To investigate the association of UNC5B expression with bladder cancer prognosis, 100 cases of clinical bladder cancer and adjacent noncancerous tissue samples, and four bladder cancer cell lines were selected using RT-PCR, Western blot, immunofluorescence and immunohistochemistry to investigate differential expression and cellular positioning of UNC5B, and its relationship with clinicopathological parameters. In 72 % of cases, UNC5B was expressed in both bladder cancer and adjacent noncancerous tissue samples. Expression of UNC5B in bladder cancer tissues increased significantly as cancer stage increased (P < 0.05); UNC5B emerged more in bladder cancer cell lines with lower degrees of malignancy than those with higher degrees of malignancy; UNC5B expression in bladder cancer cells was significantly reduced compared to normal bladder cells (P < 0.05). UNC5B mRNA was down-expressed in about 28 % of bladder cancer tissues. Low UNC5B expression was an independent risk factor for postoperative recurrence in patients with different stages and grades bladder cancer. Furthermore, patients with lower UNC5B expression in tumors had significantly higher recurrence rate after curative surgery and poorer prognosis than those with higher UNC5B expression, suggesting that UNC5B could be used to predict prognosis and recurrence.

Guidance for life, cell death, and colorectal neoplasia by netrin dependence receptors

This review is focusing on a critical mediator of embryonic and postnatal development with multiple implications in inflammation, neoplasia, and other pathological situations in brain and peripheral tissues. These morphogenetic guidance and dependence processes are involved in several malignancies targeting the epithelial and immune systems including the progression of human colorectal cancers. We consider the most important findings and their impact on basic, translational, and clinical cancer research. Expected information can bring new cues for innovative, efficient, and safe strategies of personalized medicine based on molecular markers, protagonists, signaling networks, and effectors inherent to the Netrin axis in pathophysiological states.

New insights regarding the regulation of chemotaxis by nucleotides, adenosine, and their receptors

The directional movement of cells can be regulated by ATP, certain other nucleotides (e.g., ADP, UTP), and adenosine. Such regulation occurs for cells that are "professional phagocytes" (e.g., neutrophils, macrophages, certain lymphocytes, and microglia) and that undergo directional migration and subsequent phagocytosis. Numerous other cell types (e.g., fibroblasts, endothelial cells, neurons, and keratinocytes) also change motility and migration in response to ATP, other nucleotides, and adenosine. In this article, we review how nucleotides and adenosine modulate chemotaxis and motility and highlight the importance of nucleotide- and adenosine-regulated cell migration in several cell types: neutrophils, microglia, endothelial cells, and cancer cells. We also discuss difficulties in conducting experiments and drawing conclusions regarding the ability of nucleotides and adenosine to modulate the migration of professional and non-professional phagocytes.

Regulation of neutrophil function by adenosine

Adenosine is an endogenously released purine nucleoside that signals via 4 widely expressed G protein-coupled receptors: A(1), A(2A), A(2B), and A(3). In the setting of inflammation, the generation and release of adenosine is greatly enhanced. Neutrophils play an important role in host defense against invading pathogens and are the cellular hallmark of acute inflammation. Neutrophils both release adenosine and can respond to it via expression of all 4 adenosine receptor subtypes. At low concentrations, adenosine can act via the A(1) and A(3) adenosine receptor subtypes to promote neutrophil chemotaxis and phagocytosis. At higher concentrations, adenosine acts at the lower-affinity A(2A) and A(2B) receptors to inhibit neutrophil trafficking and effector functions such as oxidative burst, inflammatory mediator production, and granule release. Modulation of neutrophil function by adenosine is relevant in a broad array of disease models, including ischemia reperfusion injury, sepsis, and noninfectious acute lung injury. This review will summarize relevant research in order to provide a framework for understanding how adenosine directly regulates various elements of neutrophil function.

Caffeine acts via A1 adenosine receptors to disrupt embryonic cardiac function

Background

Evidence suggests that adenosine acts via cardiac A1 adenosine receptors (A1ARs) to protect embryos against hypoxia. During embryogenesis, A1ARs are the dominant regulator of heart rate, and A1AR activation reduces heart rate. Adenosine action is inhibited by caffeine, which is widely consumed during pregnancy. In this study, we tested the hypothesis that caffeine influences developing embryos by altering cardiac function.

Methodology/Principal Findings

Effects of caffeine and adenosine receptor-selective antagonists on heart rate were studied in vitro using whole murine embryos at E9.5 and isolated hearts at E12.5. Embryos were examined in room air (21% O₂) or hypoxic (2% O₂) conditions. Hypoxia decreased heart rates of E9.5 embryos by 15.8% and in E12.5 isolated hearts by 27.1%. In room air, caffeine (200 µM) had no effect on E9.5 heart rates; however, caffeine increased heart rates at E12.5 by 37.7%. Caffeine abolished hypoxia-mediated bradycardia at E9.5 and blunted hypoxia-mediated bradycardia at E12.5. Real-time PCR analysis of RNA from isolated E9.5 and E12.5 hearts showed that A1AR and A2aAR genes were expressed at both ages. Treatment with adenosine receptor-selective antagonists revealed that SCH-58261 (A2aAR-specific antagonist) had no affects on heart function, whereas DPCPX (A1AR-specific antagonist) had effects similar to caffeine treatment at E9.5 and E12.5. At E12.5, embryonic hearts lacking A1AR expression (A1AR−/−) had elevated heart rates compared to A1AR+/− littermates, A1AR−/− heart rates failed to decrease to levels comparable to those of controls. Caffeine did not significantly affect heart rates of A1AR−/− embryos.

Conclusions/Significance

These data show that caffeine alters embryonic cardiac function and disrupts the normal cardiac response to hypoxia through blockade of A1AR action. Our results raise concern for caffeine exposure during embryogenesis, particularly in pregnancies with increased risk of embryonic hypoxia.

Cladribine inhibits cytokine secretion by T cells independently of deoxycytidine kinase activity

Cladribine (2-chloro-2'-deoxyadenosine) is a purine nucleoside analogue (PNA) which causes targeted and sustained reduction of peripheral lymphocyte counts. Cladribine tablets produced significant treatment benefit for patients with relapsing-remitting multiple sclerosis in the phase 3 CLARITY study. In addition to the well-characterised cell-specific phosphorylation of PNAs responsible for lymphocyte reduction, the mode of action of cladribine may encompass distinct activities contributing to its overall effects on the immune system. Here we demonstrate that clinically relevant concentrations of cladribine also inhibit cytokine secretion by human peripheral blood T cells in vitro through mechanisms independent of the induction of lymphocyte death.