The A2b adenosine receptor protects against vascular injury

Inhibition of Vascular Smooth Muscle Cell Proliferation by ENPP1: The Role of CD73 and the Adenosine Signaling Axis

The Ectonucleotide Pyrophosphatase/Phosphodiesterase 1 (ENPP1) ectoenzyme regulates vascular intimal proliferation and mineralization of bone and soft tissues. ENPP1 variants cause Generalized Arterial Calcification of Infancy (GACI), a rare genetic disorder characterized by ectopic calcification, intimal proliferation, and stenosis of large- and medium-sized arteries. ENPP1 hydrolyzes extracellular ATP to pyrophosphate (PPi) and AMP. AMP is the precursor of adenosine, which has been implicated in the control of neointimal formation. Herein, we demonstrate that an ENPP1-Fc recombinant therapeutic inhibits proliferation of vascular smooth muscle cells (VSMCs) in vitro and in vivo. Addition of ENPP1 and ATP to cultured VSMCs generated AMP, which was metabolized to adenosine. It also significantly decreased cell proliferation. AMP or adenosine alone inhibited VSMC growth. Inhibition of ecto-5'-nucleotidase CD73 decreased adenosine accumulation and suppressed the anti-proliferative effects of ENPP1/ATP. Addition of AMP increased cAMP synthesis and phosphorylation of VASP at Ser157. This AMP-mediated cAMP increase was abrogated by CD73 inhibitors or by A2aR and A2bR antagonists. Ligation of the carotid artery promoted neointimal hyperplasia in wild-type mice, which was exacerbated in ENPP1-deficient ttw/ttw mice. Prophylactic or therapeutic treatments with ENPP1 significantly reduced intimal hyperplasia not only in ttw/ttw but also in wild-type mice. These findings provide the first insight into the mechanism of the anti-proliferative effect of ENPP1 and broaden its potential therapeutic applications beyond enzyme replacement therapy.

Dipyridamole and vascular healing following stent implantation

Introduction

Patients undergoing coronary stent implantation incur a 2% annual rate of adverse events, largely driven by in-stent restenosis (ISR) due to neointimal (NI) tissue proliferation, a process in which smooth muscle cell (SMC) biology may play a central role. Dipyridamole (DP) is an approved therapeutic agent with data supporting improved vascular patency rates. Pre-clinical data supports that DP may enact its vasculoprotective effects via adenosine receptor-A2B (ADOR-A2B). We sought to evaluate the efficacy of DP to mitigate ISR in a pre-clinical rabbit stent model.

Methods & Results

24 New Zealand White Rabbits were divided into two cohorts-non-atherosclerosis and atherosclerosis (n = 12/cohort, 6 male and 6 female). Following stent implantation, rabbits were randomized 1:1 to control or oral dipyridamole therapy for 6 weeks followed by optical coherence tomography (OCT) and histology assessment of NI burden and stent strut healing. Compared to control, DP demonstrated a 16.6% relative reduction in NI volume (14.7 ± 0.8% vs. 12.5 ± 0.4%, p = 0.03) and a 36.2% relative increase in optimally healed stent struts (37.8 ± 2.8% vs. 54.6 ± 2.5%, p < 0.0001). Atherosclerosis demonstrated attenuated effect with no difference in NI burden (15.2 ± 1.0% vs. 16.9 ± 0.8%, p = 0.22) and only a 14.2% relative increase in strut healing (68.3 ± 4.1% vs. 78.7 ± 2.5%, p = 0.02). DP treated rabbits had a 44.6% (p = 0.045) relative reduction in NI SMC content. In vitro assessment of DP and coronary artery SMCs yielded dose-dependent reduction in SMC migration and proliferation. Selective small molecule antagonism of ADOR-A2B abrogated the effects of DP on SMC proliferation. DP modulated SMC phenotypic switching with ADOR-A2B siRNA knockdown supporting its role in the observed effects.

Conclusion

Dipyridamole reduces NI proliferation and improves stent healing in a preclinical model of stent implantation with conventional antiplatelets. Atherosclerosis attenuates the observed effect. Clinical trials of DP as an adjunctive agent may be warranted to evaluate for clinical efficacy in stent outcomes.

Ecto-5'-nucleotidase (Nt5e/CD73)-mediated adenosine signaling attenuates TGFβ-2 induced elastin and cellular contraction

Arterial calcification due to deficiency of CD73 (ACDC) is a rare genetic disease caused by a loss-of-function mutation in the NT5E gene encoding the ecto-5'-nucleotidase (cluster of differentiation 73, CD73) enzyme. Patients with ACDC develop vessel arteriomegaly, tortuosity, and vascular calcification in their lower extremity arteries. Histological analysis shows that patients with ACDC vessels exhibit fragmented elastin fibers similar to that seen in aneurysmal-like pathologies. It is known that alterations in transforming growth factor β (TGFβ) pathway signaling contribute to this elastin phenotype in several connective tissue diseases, as TGFβ regulates extracellular matrix (ECM) remodeling. Our study investigates whether CD73-derived adenosine modifies TGFβ signaling in vascular smooth muscle cells (SMCs). We show that Nt5e-/- SMCs have elevated contractile markers and elastin gene expression compared with Nt5e+/+ SMCs. Ecto-5'-nucleotidase (Nt5e)-deficient SMCs exhibit increased TGFβ-2 and activation of small mothers against decapentaplegic (SMAD) signaling, elevated elastin transcript and protein, and potentiate SMC contraction. These effects were diminished when the A2b adenosine receptor was activated. Our results identify a novel link between adenosine and TGFβ signaling, where adenosine signaling via the A2b adenosine receptor attenuates TGFβ signaling to regulate SMC homeostasis. We discuss how disruption in adenosine signaling is implicated in ACDC vessel tortuosity and could potentially contribute to other aneurysmal pathogenesis.

Adenosine Receptors Profile in Fibromuscular Dysplasia

Fibromuscular dysplasia (FMD) is a non-inflammatory vascular disease that is characterized by unexplained systemic hypertension occurring in young people, associated with arterial stenosis, aneurysm rupture, intracranial/renal infarction, and stroke. Although the gold standard for the diagnosis remains catheter-angiography, biological markers would be helpful due to the delay from first symptom to diagnosis. Adenosine is an ATP derivative, that may be implicated in FMD pathophysiology. We hypothesized that changes in adenosine blood level (ABL) and production of adenosine receptors may be associated with FMD. Using peripheral blood mononuclear cells, we evaluated A1, A2A, and A2B receptor production by Western blot, in 67 patients (17 men and 50 women, mean (range) age 55 (29−77) years and 40 controls, 10 men and 30 women, mean (range) age 56 (37−70)). ABL was evaluated by liquid chromatography, mass spectrometry. ABL was significantly higher in patients vs. controls, mean (range): 1.7 (0.7−3) µmol/L vs. controls 0.6 (0.4−0.8) µmol/L (+180%) p < 0.001. While A1R and A2AR production did not differ in patients and controls, we found an over-production of A2BR in patients: 1.70 (0.90−2.40; arbitrary units) vs. controls = 1.03 (0.70−1.40), mean + 65% (p < 0.001). A2BR production with a cut off of 1.3 arbitrary units, gives a good sensitivity and specificity for the diagnosis. Production measurement of A2BR on monocytes and ABL could help in the diagnosis, especially in atypical or with poor symptoms.

Alternative adenosine Receptor activation: The netrin-Adora2b link

During hypoxia or inflammation, extracellular adenosine levels are elevated. Studies using pharmacologic approaches or genetic animal models pertinent to extracellular adenosine signaling implicate this pathway in attenuating hypoxia-associated inflammation. There are four distinct adenosine receptors. Of these, it is not surprising that the Adora2b adenosine receptor functions as an endogenous feedback loop to control hypoxia-associated inflammation. First, Adora2b activation requires higher adenosine concentrations compared to other adenosine receptors, similar to those achieved during hypoxic inflammation. Second, Adora2b is transcriptionally induced during hypoxia or inflammation by hypoxia-inducible transcription factor HIF1A. Studies seeking an alternative adenosine receptor activation mechanism have linked netrin-1 with Adora2b. Netrin-1 was originally discovered as a neuronal guidance molecule but also functions as an immune-modulatory signaling molecule. Similar to Adora2b, netrin-1 is induced by HIF1A, and has been shown to enhance Adora2b signaling. Studies of acute respiratory distress syndrome (ARDS), intestinal inflammation, myocardial or hepatic ischemia and reperfusion implicate the netrin-Adora2b link in tissue protection. In this review, we will discuss the potential molecular linkage between netrin-1 and Adora2b, and explore studies demonstrating interactions between netrin-1 and Adora2b in attenuating tissue inflammation.

Adenosine kinase: An epigenetic modulator in development and disease

Adenosine kinase (ADK) is the key regulator of adenosine and catalyzes the metabolism of adenosine to 5'-adenosine monophosphate. The enzyme exists in two isoforms: a long isoform (ADK-long, ADK-L) and a short isoform (ADK-short, ADK-S). The two isoforms are developmentally regulated and are differentially expressed in distinct subcellular compartments with ADK-L localized in the nucleus and ADK-S localized in the cytoplasm. The nuclear localization of ADK-L and its biochemical link to the transmethylation pathway suggest a specific role for gene regulation via epigenetic mechanisms. Recent evidence reveals an adenosine receptor-independent role of ADK in determining the global methylation status of DNA and thereby contributing to epigenomic regulation. Here we summarize recent progress in understanding the biochemical interactions between adenosine metabolism by ADK-L and epigenetic modifications linked to transmethylation reactions. This review will provide a comprehensive overview of ADK-associated changes in DNA methylation in developmental, as well as in pathological conditions including brain injury, epilepsy, vascular diseases, cancer, and diabetes. Challenges in investigating the epigenetic role of ADK for therapeutic gains are briefly discussed.

Adenosine kinase is critical for neointima formation after vascular injury by inducing aberrant DNA hypermethylation

AIMS

Adenosine receptors and extracellular adenosine have been demonstrated to modulate vascular smooth muscle cell (VSMC) proliferation and neointima formation. Adenosine kinase (ADK) is a major enzyme regulating intracellular adenosine levels but is function in VSMC remains unclear. Here, we investigated the role of ADK in vascular injury-induced smooth muscle proliferation and delineated the mechanisms underlying its action.

METHODS AND RESULTS

We found that ADK expression was higher in the neointima of injured vessels and in platelet-derived growth factor-treated VSMCs. Genetic and pharmacological inhibition of ADK was enough to attenuate arterial injury-induced neointima formation due to inhibition of VSMC proliferation. Mechanistically, using infinium methylation assays and bisulfite sequencing, we showed that ADK metabolized the intracellular adenosine and potentiated the transmethylation pathway, then induced the aberrant DNA hypermethylation. Pharmacological inhibition of aberrant DNA hypermethylation increased KLF4 expression and suppressed VSMC proliferation as well as the neointima formation. Importantly, in human femoral arteries, we observed increased ADK expression and DNA hypermethylation as well as decreased KLF4 expression in neointimal VSMCs of stenotic vessels suggesting that our findings in mice are relevant for human disease and may hold translational significance.

CONCLUSION

Our study unravels a novel mechanism by which ADK promotes VSMC proliferation via inducing aberrant DNA hypermethylation, thereby down-regulating KLF4 expression and promoting neointima formation. These findings advance the possibility of targeting ADK as an epigenetic modulator to combat vascular injury.

3,4-Dihydropyrimidin-2(1H)-ones as Antagonists of the Human A2B Adenosine Receptor: Optimization, Structure-Activity Relationship Studies, and Enantiospecific Recognition

We present and thoroughly characterize a large collection of 3,4-dihydropyrimidin-2(1H)-ones as A_2BAR antagonists, an emerging strategy in cancer (immuno) therapy. Most compounds selectively bind A_2BAR, with a number of potent and selective antagonists further confirmed by functional cyclic adenosine monophosphate experiments. The series was analyzed with one of the most exhaustive free energy perturbation studies on a GPCR, obtaining an accurate model of the structure-activity relationship of this chemotype. The stereospecific binding modeled for this scaffold was confirmed by resolving the two most potent ligands [(±)-47, and (±)-38 K_i = 10.20 and 23.6 nM, respectively] into their two enantiomers, isolating the affinity on the corresponding (S)-eutomers (K_i = 6.30 and 11.10 nM, respectively). The assessment of the effect in representative cytochromes (CYP3A4 and CYP2D6) demonstrated insignificant inhibitory activity, while in vitro experiments in three prostate cancer cells demonstrated that this pair of compounds exhibits a pronounced antimetastatic effect.

Adenosine and the Cardiovascular System: The Good and the Bad

Adenosine is a nucleoside that impacts the cardiovascular system via the activation of its membrane receptors, named A₁R, A_2AR, A_2BR and A₃R. Adenosine is released during hypoxia, ischemia, beta-adrenergic stimulation or inflammation and impacts heart rhythm and produces strong vasodilation in the systemic, coronary or pulmonary vascular system. This review summarizes the main role of adenosine on the cardiovascular system in several diseases and conditions. Adenosine release participates directly in the pathophysiology of atrial fibrillation and neurohumoral syncope. Adenosine has a key role in the adaptive response in pulmonary hypertension and heart failure, with the most relevant effects being slowing of heart rhythm, coronary vasodilation and decreasing blood pressure. In other conditions, such as altitude or apnea-induced hypoxia, obstructive sleep apnea, or systemic hypertension, the adenosinergic system activation appears in a context of an adaptive response. Due to its short half-life, adenosine allows very rapid adaptation of the cardiovascular system. Finally, the effects of adenosine on the cardiovascular system are sometimes beneficial and other times harmful. Future research should aim to develop modulating agents of adenosine receptors to slow down or conversely amplify the adenosinergic response according to the occurrence of different pathologic conditions.

Therapeutic Potentials of A2B Adenosine Receptor Ligands: Current Status and Perspectives

BACKGROUND

Adenosine receptors (ARs) are classified as A1, A2A, A2B, and A3 subtypes belong to the superfamily of G-protein coupled receptors (GPCRs). More than 40% of modern medicines act through either activation or inhibition of signaling processes associated with GPCRs. In particular, A2B AR signaling pathways are implicated in asthma, inflammation, cancer, ischemic hyperfusion, diabetes mellitus, cardiovascular diseases, gastrointestinal disorders, and kidney disease.

METHODS

This article reviews different disease segments wherein A2B AR is implicated and discusses the potential role of subtype-selective A2B AR ligands in the management of such diseases or disorders. All the relevant publications on this topic are reviewed and presented scientifically.

RESULTS

This review provides an up-to-date highlight of the recent advances in the development of novel and selective A2B AR ligands and their therapeutic role in treating various disease conditions. A special focus has been given to the therapeutic potentials of selective A2B AR ligands in the management of airway inflammatory conditions and cancer.

CONCLUSIONS

This systematic review demonstrates the current status and perspectives of A2B AR ligands as therapeutically useful agents that would assist medicinal chemists and pharmacologists in discovering novel and subtype-selective A2B AR ligands as potential drug candidates.

Inhibition of CXCR4 and CXCR7 Is Protective in Acute Peritoneal Inflammation

Our previous studies revealed a pivotal role of the chemokine stromal cell-derived factor (SDF)-1 and its receptors CXCR4 and CXCR7 on migratory behavior of polymorphonuclear granulocytes (PMNs) in pulmonary inflammation. Thereby, the SDF-1-CXCR4/CXCR7-axis was linked with adenosine signaling. However, the role of the SDF-1 receptors CXCR4 and CXCR7 in acute inflammatory peritonitis and peritonitis-related sepsis still remained unknown. The presented study provides new insight on the mechanism of a selective inhibition of CXCR4 (AMD3100) and CXCR7 (CCX771) in two models of peritonitis and peritonitis-related sepsis by injection of zymosan and fecal solution. We observed an increased expression of SDF-1, CXCR4, and CXCR7 in peritoneal tissue and various organs during acute inflammatory peritonitis. Selective inhibition of CXCR4 and CXCR7 reduced PMN accumulation in the peritoneal fluid and infiltration of neutrophils in lung and liver tissue in both models. Both inhibitors had no anti-inflammatory effects in A_2B knockout animals (A_2B–/–). AMD3100 and CCX771 treatment reduced capillary leakage and increased formation of tight junctions as a marker for microvascular permeability in wild type animals. In contrast, both inhibitors failed to improve capillary leakage in A_2B–/– animals, highlighting the impact of the A_2B-receptor in SDF-1 mediated signaling. After inflammation, the CXCR4 and CXCR7 antagonist induced an enhanced expression of the protective A_2B adenosine receptor and an increased activation of cAMP (cyclic adenosine mono phosphate) response element-binding protein (CREB), as downstream signaling pathway of A_2B. The CXCR4- and CXCR7-inhibitor reduced the release of cytokines in wild type animals via decreased intracellular phosphorylation of ERK and NFκB p65. In vitro, CXCR4 and CXCR7 antagonism diminished the chemokine release of human cells and increased cellular integrity by enhancing the expression of tight junctions. These protective effects were linked with functional A_2B-receptor signaling, confirming our in vivo data. In conclusion, our study revealed new protective aspects of the pharmacological modulation of the SDF-1-CXCR4/CXCR7-axis during acute peritoneal inflammation in terms of the two hallmarks PMN migration and barrier integrity. Both anti-inflammatory effects were linked with functional adenosine A_2B-receptor signaling.

Adenosine A2B receptor activation stimulates alveolar fluid clearance through alveolar epithelial sodium channel via cAMP pathway in endotoxin-induced lung injury

Clinical studies have established that the capacity of removing excess fluid from alveoli is impaired in most patients with acute respiratory distress syndrome. Impaired alveolar fluid clearance (AFC) correlates with poor outcomes. Adenosine A_2B receptor (A_2BAR) has the lowest affinity with adenosine among four adenosine receptors. It is documented that A_2BAR can activate adenylyl cyclase (AC) resulting in elevated cAMP. Based on the understanding that cAMP is a key regulator of epithelial sodium channel (ENaC), which is the limited step in sodium transport, we hypothesized that A_2BAR signaling may affect AFC in acute lung injury (ALI) through regulating ENaC via cAMP, thus attenuating pulmonary edema. To address this, we utilized pharmacological approaches to determine the role of A_2BAR in AFC in rats with endotoxin-induced lung injury and further focused on the mechanisms in vitro. We observed elevated pulmonary A_2BAR level in rats with ALI and the similar upregulation in alveolar epithelial cells exposed to LPS. A_2BAR stimulation significantly attenuated pulmonary edema during ALI, an effect that was associated with enhanced AFC and increased ENaC expression. The regulatory effects of A_2BAR on ENaC-α expression were further verified in cultured alveolar epithelial type II (ATII) cells. More importantly, activation of A_2BAR dramatically increased amiloride-sensitive Na⁺ currents in ATII cells. Moreover, we observed that A_2BAR activation stimulated cAMP accumulation, whereas the cAMP inhibitor abolished the regulatory effect of A_2BAR on ENaC-α expression, suggesting that A_2BAR activation regulates ENaC-α expression via cAMP-dependent mechanism. Together, these findings suggest that signaling through alveolar epithelial A_2BAR promotes alveolar fluid balance during endotoxin-induced ALI by regulating ENaC via cAMP pathway, raising the hopes for treatment of pulmonary edema due to ALI.

Adenosine, Via A2B Receptors, Inhibits Human (P-SMC) Progenitor Smooth Muscle Cell Growth

c-Kit+ progenitor smooth muscle cells (P-SMCs) can develop into SMCs that contribute to injury-induced neointimal thickening. Here, we investigated whether adenosine reduces P-SMC migration and proliferation and whether this contributes to adenosine's inhibitory actions on neointima formation. In human P-SMCs, 2-chloroadenosine (stable adenosine analogue) and BAY60-6583 (A_2B agonist) inhibited P-SMC proliferation and migration. Likewise, increasing endogenous adenosine by blocking adenosine metabolism with erythro-9-(2-hydroxy-3-nonyl) adenine (inhibits adenosine deaminase) and 5-iodotubercidin (inhibits adenosine kinase) attenuated P-SMC proliferation and migration. Neither N⁶-cyclopentyladenosine (A₁ agonist), CGS21680 (A_2A agonist), nor N⁶-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (A₃ agonist) affected P-SMC proliferation or migration. 2-Chloroadenosine increased cyclic AMP, reduced Akt phosphorylation (activates cyclin D expression), and reduced levels of cyclin D1 (promotes cell-cycle progression). Moreover, 2-chloroadenosine inhibited expression of Skp2 (promotes proteolysis of p27^Kip1) and upregulated levels of p27^Kip1 (negative cell-cycle regulator). A_2B receptor knockdown prevented the effects of 2-chloroadenosine on cyclic AMP production and P-SMC proliferation and migration. Likewise, inhibition of adenylyl cyclase and protein kinase A rescued P-SMCs from the inhibitory effects of 2-chloroadenosine. The inhibitory effects of adenosine were similar in male and female P-SMCs. In vivo, peri-arterial (rat carotid artery) 2-chloroadenosine (20 μmol/L for 7 days) reduced neointimal hyperplasia by 64.5% (P<0.05; intima/media ratio: control, 1.4±0.02; treated, 0.53±0.012) and reduced neointimal c-Kit+ cells. Adenosine inhibits P-SMC migration and proliferation via the A_2B receptor/cyclic AMP/protein kinase A axis, which reduces cyclin D1 expression and activity via inhibiting Akt phosphorylation and Skp2 expression and upregulating p27^kip1 levels. Adenosine attenuates neointima formation in part by inhibiting infiltration and proliferation of c-Kit+ P-SMCs.

Ending Restenosis: Inhibition of Vascular Smooth Muscle Cell Proliferation by cAMP

Increased vascular smooth muscle cell (VSMC) proliferation contributes towards restenosis after angioplasty, vein graft intimal thickening and atherogenesis. The second messenger 3′ 5′ cyclic adenosine monophosphate (cAMP) plays an important role in maintaining VSMC quiescence in healthy vessels and repressing VSMC proliferation during resolution of vascular injury. Although the anti-mitogenic properties of cAMP in VSMC have been recognised for many years, it is only recently that we gained a detailed understanding of the underlying signalling mechanisms. Stimuli that elevate cAMP in VSMC inhibit G₁-S phase cell cycle progression by inhibiting expression of cyclins and preventing S-Phase Kinase Associated Protein-2 (Skp2-mediated degradation of cyclin-dependent kinase inhibitors. Early studies implicated inhibition of MAPK signalling, although this does not fully explain the anti-mitogenic effects of cAMP. The cAMP effectors, Protein Kinase A (PKA) and Exchange Protein Activated by cAMP (EPAC) act together to inhibit VSMC proliferation by inducing Cyclic-AMP Response Element Binding protein (CREB) activity and inhibiting members of the RhoGTPases, which results in remodelling of the actin cytoskeleton. Cyclic-AMP induced actin remodelling controls proliferation by modulating the activity of Serum Response Factor (SRF) and TEA Domain Transcription Factors (TEAD), which regulate expression of genes required for proliferation. Here we review recent research characterising these mechanisms, highlighting novel drug targets that may allow the anti-mitogenic properties of cAMP to be harnessed therapeutically to limit restenosis.

Evaluation of Plasma Adenosine as a Marker of Cardiovascular Risk: Analytical and Biological Considerations

Background

Adenosine is a ubiquitous regulatory molecule known to modulate signaling in many cells and processes vital to vascular homeostasis. While studies of adenosine receptors have dominated research in the field, quantification of adenosine systemically and locally remains limited owing largely to technical restrictions. Given the potential clinical implications of adenosine biology, there is a need for adequately powered studies examining the role of plasma adenosine in vascular health. We sought to describe the analytical and biological factors that affect quantification of adenosine in humans in a large, real‐world cohort of patients undergoing evaluation for coronary artery disease.

Methods and Results

Between November 2016 and April 2018, we assessed 1141 patients undergoing angiography for evaluation of coronary artery disease. High‐performance liquid chromatography was used for quantification of plasma adenosine concentration, yielding an analytical coefficient of variance (CV_a) of 3.2%, intra‐subject variance (CV_i) 35.8% and inter‐subject variance (CV_g) 56.7%. Traditional cardiovascular risk factors, medications, and clinical presentation had no significant impact on adenosine levels. Conversely, increasing age (P=0.027) and the presence of obstructive coronary artery disease (P=0.026) were associated with lower adenosine levels. Adjusted multivariable analysis supported only age being inversely associated with adenosine levels (P=0.039).

Conclusions

Plasma adenosine is not significantly impacted by traditional cardiovascular risk factors; however, advancing age and presence of obstructive coronary artery disease may be associated with lower adenosine levels. The degree of intra‐ and inter‐subject variance of adenosine has important implications for biomarker use as a prognosticator of cardiovascular outcomes and as an end point in clinical studies.

The adenosine A2B G protein-coupled receptor: Recent advances and therapeutic implications

The adenosine A_2B receptor (A_2BAR) is one of four adenosine receptor subtypes belonging to the Class A family of G protein-coupled receptors (GPCRs). Until recently, the A_2BAR remained poorly characterised, in part due to its relatively low affinity for the endogenous agonist adenosine and therefore presumed minor physiological significance. However, the substantial increase in extracellular adenosine concentration, the sensitisation of the receptor and the upregulation of A_2BAR expression under conditions of hypoxia and inflammation, suggest the A_2BAR as an exciting therapeutic target in a variety of pathological disease states. Here we discuss the pharmacology of the A_2BAR and outline its role in pathophysiology including ischaemia-reperfusion injury, fibrosis, inflammation and cancer.

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

A2BAR activation attenuates acute lung injury by inhibiting alveolar epithelial cell apoptosis both in vivo and in vitro

The epithelial barrier of the lung is destroyed during acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) due to the apoptosis of alveolar epithelial cells (AECs). Therefore, treatments that block AEC apoptosis might be a therapeutic strategy to ameliorate ALI. Based on recent evidence, A2B adenosine receptor (A2BAR) plays an important role in ALI in several different animal models, but its exact function in AECs has not been clarified. We investigated the role of A2BAR in AEC apoptosis in a mouse model of oleic acid (OA)-induced ALI and in hydrogen peroxide (H2O2)-induced AEC (A549 cells and MLE-12 cells) injury. Mice treated with BAY60-6583, a selective A2BAR agonist, showed lower AEC apoptosis rates than mice treated with OA. However, the role of BAY60-6583 in OA-induced ALI was attenuated by a specific blocker of A2BAR, PSB1115. A2BAR activation decreased H2O2-induced cell apoptosis in vitro, as characterized by the translocation of apoptotic proteins, the release of cytochrome c, and the activation of caspase-3 and poly (ADP ribose) polymerase 1 (PARP-1). In addition, apoptosis was required for the phosphorylation of ERK1/2, p38, and JNK. Importantly, compared with cells transfected with the A2BAR-siRNA, an ERK inhibitor or p38 inhibitor exhibited decreased apoptotic ratios and cleaved caspase-9 and cleaved PARP-1 levels, whereas the JNK inhibitor displayed increases in these parameters. In conclusion, A2BAR activation effectively attenuated OA-induced ALI by inhibiting AEC apoptosis and mitigated H2O2-induced AEC injury by suppressing the p38 and ERK1/2-mediated mitochondrial apoptosis pathway.

Biology of Platelet Purinergic Receptors and Implications for Platelet Heterogeneity

Platelets are small anucleated cells present only in mammals. Platelets mediate intravascular hemostatic balance, prevent interstitial bleeding, and have a major role in thrombosis. Activation of platelet purinergic receptors is instrumental in initiation of hemostasis and formation of the hemostatic plug, although this activation process becomes problematic in pathological settings of thrombosis. This review briefly outlines the roles and function of currently known platelet purinergic receptors (P1 and P2) in the setting of hemostasis and thrombosis. Additionally, we discuss recent novel studies on purinergic receptor distribution according to heterogeneous platelet size, and the possible implication of this distribution on hemostatic function.

Extracellular ATP signaling and clinical relevance

Since purinergic signaling was discovered in the early 1970s, it has been shown that extracellular nucleotides, and their derivative nucleosides, are released in a regulated or unregulated manner by cells in various challenging settings and then bind defined purinergic receptors to activate intricate signaling networks. Extracellular ATP plays a role based on different P2 receptor subtypes expressed on specific cell types. Sequential hydrolysis of extracellular ATP catalyzed by ectonucleotidases (e.g. CD39, CD73) is the main pathway for the generation of adenosine, which in turn activates P1 receptors. Many studies have demonstrated that extracellular ATP signaling functions as an important dynamic regulatory pathway to coordinate appropriate immune responses in various pathological processes, including intracellular infection, host-tumor interaction, pro-inflammation vascular injury, and transplant immunity. ATP receptors and CD39 also participate in related clinical settings. Here, we review the latest research in to the development of promising clinical treatment strategies.

Inhibition of tPA-induced hemorrhagic transformation involves adenosine A2b receptor activation after cerebral ischemia

Tissue plasminogen activator (tPA) is administered after ischemic stroke to dissolve intravascular clots, but its use can lead to hemorrhagic transformation (HT). Therapeutic strategies to reduce hemorrhagic complications of tPA might be of benefit for stroke patients. Adenosine A2b receptor (A2bR) plays pivotal roles in regulating vascular protection in peripheral organs. This study explored whether A2bR agonist BAY 60-6583 reduces hemorrhage risk after tPA usage. Using a rat transient middle cerebral artery occlusion model, we showed that mRNA and protein expression of A2bR increased to a greater extent after ischemia-reperfusion than did expression of the other three adenosine receptors (A1, A2a, and A3). tPA administration reduced A2bR expression in ischemic brain microvessels. Post-treatment with BAY 60-6583 (1mg/kg) at the start of reperfusion reduced lesion volume in the absence or presence of tPA (10mg/kg) and attenuated brain swelling, blood-brain barrier disruption, and tPA-exacerbated HT at 24h. Additionally, BAY 60-6583 mitigated sensorimotor deficits in the presence of tPA. BAY 60-6583 inhibited tPA-enhanced matrix metalloprotease-9 activation, probably through elevation of tissue inhibitor of matrix metalloproteinases-1 expression, and thereby reduced degradation of tight junction proteins. These effects would likely protect cerebrovascular integrity. A2bR agonists as an adjuvant to tPA could be a promising strategy for decreasing the risk of HT during treatment for ischemic stroke.

Interaction between saliva's adenosine and tick parasitism: effects on feeding and reproduction

Background

It has recently been demonstrated that saliva from Rhipicephalus sanguineus ticks contains adenosine (ADO) and prostaglandin E2 (PGE2), two non-protein molecules that have significant immunomodulatory properties. These molecules can inhibit cytokine production by dendritic cells (DCs), while also reducing the expression of CD40 in these cells. However, more studies are needed for a better understanding of their participation in the feeding of ticks in vivo. This work, therefore, evaluated the importance of ADO during tick infestations. Mice were infested with adult ticks (3 couples/mouse), and their skin was collected at the tick-infested site (3rd and 7th day), and mRNA for receptors of ADO was quantified by real-time PCR.

Results

Tick infestation increased by four and two times the expression of the A2b and A3v1 receptors on day 3, respectively, while expression of other ADO receptors was unaltered. In addition, we treated mice (n = 10/group) daily with 8-(p-Sulfophenyl)theophylline, 8-pSPT, 20 mg/kg, i.p.), a non-selective antagonist of ADO receptors, and evaluated the performance of ticks during infestations. Female ticks fed on 8-pSPT-treated mice presented a reduction in their engorgement, weight and hatching rates of egg masses, and survival times of larvae compared to the same parameters presented by ticks in the control group. To investigate if these 8-pSPT-treated mice presented altered immune responses, we performed three tick infestations and collected their lymph node cells to determine the percentages and activation state of DCs and cytokine production by lymphocytes by flow cytometry (Cytometric Bead Array technique, CBA). Our data showed that 8-pSPT-treated mice presented an increase in the percentage of DCs as well as of their stimulatory and co-stimulatory molecules (CD40, CD80 and MHCII). Regarding production of T cell cytokines, we observed a significant increase in the levels of IL-2 and a significant decrease in IL-10, IL-17, TNF-α and IFN-γ cytokines.

Conclusions

These results suggest that ADO produced by ticks helps them feed and reproduce and that this effect may be due to modulation of host DCs and T cells.

Inhibition of SDF-1 receptors CXCR4 and CXCR7 attenuates acute pulmonary inflammation via the adenosine A2B-receptor on blood cells

Acute pulmonary inflammation is characterized by migration of polymorphonuclear neutrophils into the different compartments of the lung. Recent studies showed evidence that the chemokine stromal cell-derived factor (SDF)-1 and its receptors CXCR4 and CXCR7 influence migration of immune cells and their activity was linked to adenosine concentrations. We investigated the particular role of CXCR4- and CXCR7-inhibition and the potential link to the adenosine A_2B-receptor, which plays an important anti-inflammatory role in the lung. After LPS-inhalation for 45 minutes, administration of the CXCR4-inhibitor (AMD3100) decreased transendothelial and transepithelial migration, whereas CXCR7-antagonism influenced epithelial migration exclusively. In A_2B−/− mice, no anti-inflammatory effects were detectible through either one of the agents. Using chimeric mice, we identified A_2B on hematopoietic cells to be crucial for these anti-inflammatory effects of CXCR4/7-inhibition. Both inhibitors decreased TNFα, IL6, CXCL1 and CXCL2/3 levels in the bronchoalveolar lavage of wild type mice, while not influencing the chemokine release in A_2B−/− mice. Inflammation augmented the expression of both receptors and their inhibition increased A_2B-levels upon inflammation. In vitro assays with human epithelium/endothelium confirmed our in vivo findings. During inflammation, inhibition of CXCR4- and CXCR7-receptors prevented microvascular permeability in wild type but not in A_2B−/− mice, highlighting the pivotal role of an active A_2B-receptor in this setting. The combination of both inhibitors had a synergistic effect in preventing capillary leakage. In conclusion, we determined the pivotal role of CXCR4- and CXCR7-inhibition in acute pulmonary inflammation, which depended on A_2B-receptor signalling.

Exercise partially reverses the inhibitory effect of caffeine on liver gluconeogenesis in type 1 diabetic rats with hypoglycemia

The purpose was to determine the possible effects of exercise and/or caffeine on hypoglycemia and liver gluconeogenesis in diabetic rats. These were divided into four subgroups: (a) intraperitoneal insulin only, (b) exercise bout before insulin, (c) caffeine after insulin, and (d) exercise bout before and caffeine after insulin. The marked glycemic drop 45 min after insulin (0 min = 229.00, 45 min = 75.75) was considerably reduced (p < 0.05) by caffeine or exercise (45 min: exercise = 127.00, caffeine = 104.78). However, this systemic effect was lost (p > 0.05) when they were combined (45 min: exercise + caffeine = 65.44) (Mean, in mg·dL^-1). Caffeine alone strongly inhibited liver glucose production from 2 mM lactate 45 min after insulin (without caffeine = 3.05, with caffeine = 0.27; p < 0.05), while exercise + caffeine partially re-established the liver gluconeogenic capacity (exercise + caffeine = 1.61; p < 0.05 relative to the other groups) (Mean, in μmol·g^-1). The improved hypoglycemia with caffeine or exercise cannot be explained by their actions on liver gluconeogenesis. As their beneficial effect disappeared when they were combined, such association in diabetic patients should be avoided during the period of hyperinsulinemia due to the risk of severe hypoglycemia.

Adenosine production: a common path for mesenchymal stem-cell and regulatory T-cell-mediated immunosuppression

Adenosine is an important molecule that exerts control on the immune system, by signaling through receptors lying on the surface of immune cells. This nucleotide is produced, in part, by the action of the ectoenzymes CD39 and CD73. Interestingly, these proteins are expressed on the cell surface of regulatory T-cells (Tregs) and mesenchymal stromal cells (MSCs)-two cell populations that have emerged as potential therapeutic tools in the field of cell therapy. In fact, the production of adenosine constitutes a mechanism used by both cell types to control the immune response. Recently, great scientific progress was obtained regarding the role of adenosine in the inflammatory environment. In this context, the present review focuses on the advances related to the impact of adenosine production over the immune modulatory activity of Tregs and MSCs, and how this nucleotide controls the biological functions of these cells. Finally, we mention the main challenges and hurdles to bring such molecule to clinical settings.

Adenosine A2B Receptor: From Cell Biology to Human Diseases

Extracellular adenosine is a ubiquitous signaling molecule that modulates a wide array of biological processes. Recently, significant advances have been made in our understanding of A2B adenosine receptor (A2BAR). In this review, we first summarize some of the general characteristics of A2BAR, and then we describe the multiple binding partners of the receptor, such as newly identified α-actinin-1 and p105, and discuss how these associated proteins could modulate A2BAR's functions, including certain seemingly paradoxical functions of the receptor. Growing evidence indicates a critical role of A2BAR in cancer, renal disease, and diabetes, in addition to its importance in the regulation of vascular diseases, and lung disease. Here, we also discuss the role of A2BAR in cancer, renal disease, and diabetes and the potential of the receptor as a target for treating these three diseases.

Actinin-1 binds to the C-terminus of A2B adenosine receptor (A2BAR) and enhances A2BAR cell-surface expression

A2BAR (A2B adenosine receptor) has been implicated in several physiological conditions, such as allergic or inflammatory disorders, vasodilation, cell growth and epithelial electrolyte secretion. For mediating the protein-protein interactions of A2BAR, the receptor's C-terminus is recognized to be crucial. In the present study, we unexpectedly found that two point mutations in the A2BAR C-terminus (F297A and R298A) drastically impaired the expression of A2BAR protein by accelerating its degradation. Thus we tested the hypothesis that these two point mutations disrupt A2BAR's interaction with a protein essential for A2BAR stability. Our results show that both mutations disrupted the interaction of A2BAR with actinin-1, an actin-associated protein. Furthermore, actinin-1 binding stabilized the global and cell-surface expression of A2BAR. By contrast, actinin-4, another non-muscle actinin isoform, did not bind to A2BAR. Thus our findings reveal a previously unidentified regulatory mechanism of A2BAR abundance.

Adenosine Attenuates Human Coronary Artery Smooth Muscle Cell Proliferation by Inhibiting Multiple Signaling Pathways That Converge on Cyclin D

The goal of this study was to determine whether and how adenosine affects the proliferation of human coronary artery smooth muscle cells (HCASMCs). In HCASMCs, 2-chloroadenosine (stable adenosine analogue), but not N(6)-cyclopentyladenosine, CGS21680, or N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide, inhibited HCASMC proliferation (A2B receptor profile). 2-Chloroadenosine increased cAMP, reduced phosphorylation (activation) of ERK and Akt (protein kinases known to increase cyclin D expression and activity, respectively), and reduced levels of cyclin D1 (cyclin that promotes cell-cycle progression in G1). Moreover, 2-chloroadenosine inhibited expression of S-phase kinase-associated protein-2 (Skp2; promotes proteolysis of p27(Kip1)) and upregulated levels of p27(Kip1) (cell-cycle regulator that impairs cyclin D function). 2-Chloroadenosine also inhibited signaling downstream of cyclin D, including hyperphosphorylation of retinoblastoma protein and expression of cyclin A (S phase cyclin). Knockdown of A2B receptors prevented the effects of 2-chloroadenosine on ERK1/2, Akt, Skp2, p27(Kip1), cyclin D1, cyclin A, and proliferation. Likewise, inhibition of adenylyl cyclase and protein kinase A abrogated 2-chloroadenosine's inhibitory effects on Skp2 and stimulatory effects on p27(Kip1) and rescued HCASMCs from 2-chloroadenosine-mediated inhibition. Knockdown of p27(Kip1) also reversed the inhibitory effects of 2-chloroadenosine on HCASMC proliferation. In vivo, peri-arterial (rat carotid artery) 2-chloroadenosine (20 μmol/L for 7 days) downregulated vascular expression of Skp2, upregulated vascular expression of p27(Kip1), and reduced neointima hyperplasia by 71% (P<0.05; neointimal thickness: control, 37 424±18 371 pixels; treated, 10 352±2824 pixels). In conclusion, the adenosine/A2B receptor/cAMP/protein kinase A axis inhibits HCASMC proliferation by blocking multiple signaling pathways (ERK1/2, Akt, and Skp2) that converge at cyclin D, a key G1 cyclin that controls cell-cycle progression.

The Many Faces of the A2b Adenosine Receptor in Cardiovascular and Metabolic Diseases

Modulation of the low affinity adenosine receptor subtype, the A2b adenosine receptor (A2bAR), has gained interest as a therapeutic target in various pathologic areas associated with cardiovascular disease. The actions of the A2bAR are diverse and at times conflicting depending on cell and tissue type and the timing of activation or inhibition of the receptor. The A2bAR is a promising and exciting pharmacologic target, however, a thorough understanding of A2bAR action is necessary to reach the therapeutic potential of this receptor. This review will focus on the role of the A2bAR in various cardiovascular and metabolic pathologies in which the receptor is currently being studied. We will illustrate the complexities of A2bAR signaling and highlight areas of research with potential for therapeutic development.

Alveolar Epithelial A2B Adenosine Receptors in Pulmonary Protection during Acute Lung Injury

Acute lung injury (ALI) is an acute inflammatory lung disease that causes morbidity and mortality in critically ill patients. However, there are many instances where ALI resolves spontaneously through endogenous pathways that help to control excessive lung inflammation. Previous studies have implicated the extracellular signaling molecule adenosine and signaling events through the A2B adenosine receptor in lung protection. In this context, we hypothesized that tissue-specific expression of the A2B adenosine receptor is responsible for the previously described attenuation of ALI. To address this hypothesis, we exposed mice with tissue-specific deletion of Adora2b to ALI, utilizing a two-hit model where intratracheal LPS treatment is followed by injurious mechanical ventilation. Interestingly, a head-to-head comparison of mice with deletion of Adora2b in the myeloid lineage (Adora2b(loxP/loxP) LysM Cre(+)), endothelial cells (Adora2b(loxP/loxP) VE-cadherin Cre(+)), or alveolar epithelial cells (Adora2b(loxP/loxP) SPC Cre(+)) revealed a selective increase in disease susceptibility in Adora2b(loxP/loxP) SPC Cre(+) mice. More detailed analysis of Adora2b(loxP/loxP) SPC Cre(+) mice confirmed elevated lung inflammation and attenuated alveolar fluid clearance. To directly deliver an A2B adenosine receptor-specific agonist to alveolar epithelial cells, we subsequently performed studies with inhaled BAY 60-6583. Indeed, aerosolized BAY 60-6583 treatment was associated with attenuated pulmonary edema, improved histologic lung injury, and dampened lung inflammation. Collectively, these findings suggest that alveolar epithelial A2B adenosine receptor signaling contributes to lung protection, and they implicate inhaled A2B adenosine receptor agonists in ALI treatment.

A2b adenosine signaling represses CIITA transcription via an epigenetic mechanism in vascular smooth muscle cells

Chronic inflammation plays a major role in the pathogenesis of atherosclerosis. Vascular smooth muscle cells (VSMC), by expressing and presenting major histocompatibility complex II (MHC II) molecules, help recruit T lymphocyte and initiate the inflammatory response within the vasculature. We have previously shown that VSMCs isolated from mice with deficient adenosine A2b receptor (A2b-null) exhibit higher expression of class II transactivator (CIITA), the master regulator of MHC II transcription, compared to wild type littermates. Here we report that activation of A2b adenosine signaling suppresses CIITA expression in human aortic smooth muscle cells. Down-regulation of CIITA expression was largely attributable to transcriptional repression of type III and IV promoters. Chromatin immunoprecipitation (ChIP) analyses revealed that A2b signaling repressed CIITA transcription by attenuating specific histone modifications on the CIITA promoters in a STAT1-dependent manner. STAT1 interacted with PCAF/GCN5, histone H3K9 acetyltransferases, and WDR5, a key component of the mammalian H3K4 methyltransferase complex, to activate CIITA transcription. A2b signaling prevented recruitment of PCAF/GCN5 and WDR5 to the CIITA promoters in a STAT1-dependent manner. In conclusion, our data suggest that adenosine A2b signaling represses CIITA transcription in VSMCs by manipulating the interaction between STAT1 and the epigenetic machinery.

Cardiac myocyte-secreted cAMP exerts paracrine action via adenosine receptor activation

Acute stimulation of cardiac β-adrenoceptors is crucial to increasing cardiac function under stress; however, sustained β-adrenergic stimulation has been implicated in pathological myocardial remodeling and heart failure. Here, we have demonstrated that export of cAMP from cardiac myocytes is an intrinsic cardioprotective mechanism in response to cardiac stress. We report that infusion of cAMP into mice averted myocardial hypertrophy and fibrosis in a disease model of cardiac pressure overload. The protective effect of exogenous cAMP required adenosine receptor signaling. This observation led to the identification of a potent paracrine mechanism that is dependent on secreted cAMP. Specifically, FRET-based imaging of cAMP formation in primary cells and in myocardial tissue from murine hearts revealed that cardiomyocytes depend on the transporter ABCC4 to export cAMP as an extracellular signal. Extracellular cAMP, through its metabolite adenosine, reduced cardiomyocyte cAMP formation and hypertrophy by activating A1 adenosine receptors while delivering an antifibrotic signal to cardiac fibroblasts by A2 adenosine receptor activation. Together, our data reveal a paracrine role for secreted cAMP in intercellular signaling in the myocardium, and we postulate that secreted cAMP may also constitute an important signal in other tissues.

Extracellular nucleotide and nucleoside signaling in vascular and blood disease

Nucleotides and nucleosides-such as adenosine triphosphate (ATP) and adenosine-are famous for their intracellular roles as building blocks for the genetic code or cellular energy currencies. In contrast, their function in the extracellular space is different. Here, they are primarily known as signaling molecules via activation of purinergic receptors, classified as P1 receptors for adenosine or P2 receptors for ATP. Because extracellular ATP is rapidly converted to adenosine by ectonucleotidase, nucleotide-phosphohydrolysis is important for controlling the balance between P2 and P1 signaling. Gene-targeted mice for P1, P2 receptors, or ectonucleotidase exhibit only very mild phenotypic manifestations at baseline. However, they demonstrate alterations in disease susceptibilities when exposed to a variety of vascular or blood diseases. Examples of phenotypic manifestations include vascular barrier dysfunction, graft-vs-host disease, platelet activation, ischemia, and reperfusion injury or sickle cell disease. Many of these studies highlight that purinergic signaling events can be targeted therapeutically.

Mechanical allodynia induced by nucleoside reverse transcriptase inhibitor is suppressed by p55TNFSR mediated by herpes simplex virus vector through the SDF1α/CXCR4 system in rats

BACKGROUND

In the human immunodeficiency virus (HIV)-associated sensory neuropathy, neuropathic pain associated with the use of nucleoside reverse transcriptase inhibitors (NRTIs) in patients with HIV/acquired immunodeficiency syndrome is clinically common. While evidence demonstrates that neuropathic pain is influenced by neuroinflammatory events that include the proinflammatory molecules, tumor necrosis factor-α (TNF-α), stromal cell-derived factor 1-α (SDF1-α), and C-X-C chemokine receptor type 4 (CXCR4), the detailed mechanisms by which NRTIs contribute to the development of neuropathic pain are not known. In this study, we investigated the role of these proinflammatory molecules in the dorsal root ganglion (DRG) and the spinal dorsal horn in NRTIs-mediated neuropathic pain state.

METHODS

Neuropathic pain was induced by intraperitoneal administration of 2',3'-dideoxycytidine (ddC, one of the NRTIs). Mechanical threshold was tested using von Frey filament fibers. Nonreplicating herpes simplex virus (HSV) vectors expressing p55 TNF soluble receptor (p55TNFSR) were inoculated into hindpaw of rats. The expression of TNF-α, SDF1-α, and CXCR4 in both the lumbar spinal cord and the L4/5 DRG was examined using Western blots. Intrathecal CXCR4 antagonist was administered.

RESULTS

The present study demonstrated that (1) systemic ddC induced upregulation of TNF-α, SDF1-α, and CXCR4 in both the lumbar spinal cord and the L4/5 DRG; (2) p55TNFSR mediated by a nonreplicating HSV vector reversed mechanical allodynia induced by systemic ddC; (3) intrathecal administration of the CXCR4 antagonist AMD3100 increased mechanical threshold; and (4) HSV vector expressing p55TNFSR reversed upregulation of TNF-α, SDF1-α, and CXCR4 induced by ddC in the lumbar spinal dorsal horn and the DRG.

CONCLUSIONS

Our studies demonstrate that TNF-α through the SDF1/CXCR4 system is involved in the NRTIs-related neuropathic pain state and that blocking the signaling of these proinflammatory molecules is able to reduce NRTIs-related neuropathic pain. These results provide a novel mechanism-based approach (gene therapy) to treating HIV-associated neuropathic pain.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Regulation of atherosclerosis and associated risk factors by adenosine and adenosine receptors

Adenosine is an endogenous metabolite that has an anti-inflammatory effect across the vasculature. Extracellular adenosine activates 4 G-protein coupled receptors (A1, A3, A2A, and A2B) whose expression varies in different cells and tissues, including the vasculature and blood cells. Higher levels of adenosine are generated during stress, inflammation, and upon tissue damage. Some of the adenosine receptors (AR), such as the A2BAR, are further up-regulated following such stresses. This review discusses the role of adenosine and adenosine receptors in the development of atherosclerosis and some of the risk factors associated with this pathology. These include adenosine receptor-regulated changes in atherosclerosis, blood pressure, thrombosis, and myocardial infarction. Potential therapeutic applications are reviewed, as well as reasons for phenotypic differences occasionally observed between receptor knockout and pharmacological inhibition via drug administration.

Adenosine, adenosine receptors and their role in glucose homeostasis and lipid metabolism

Adenosine is an endogenous metabolite that is released from all tissues and cells including liver, pancreas, muscle and fat, particularly under stress, intense exercise, or during cell damage. The role of adenosine in glucose homeostasis has been attributed to its ability to regulate, through its membrane receptors, processes such as insulin secretion, glucose release and clearance, glycogenolysis, and glycogenesis. Additionally, adenosine and its multiple receptors have been connected to lipid metabolism by augmenting insulin-mediated inhibition of lipolysis, and the subsequent increase in free fatty acids and glycerol levels. Furthermore, adenosine was reported to control liver cholesterol synthesis, consequently affecting plasma levels of cholesterol and triglycerides, and the amount of fat tissue. Alterations in the balance of glucose and lipid homeostasis have implications in both cardiovascular disease and diabetes. The ability of different adenosine receptors to activate and inhibit the same signaling cascades has made it challenging to study the influence of adenosine, adenosine analogs and their receptors in health and disease. This review focuses on the role and significance of different adenosine receptors in mediating the effect of adenosine on glucose and lipid homeostasis. J. Cell. Physiol. © 2013 Wiley Periodicals, Inc.

Adenosine stimulates the migration of human endothelial progenitor cells. Role of CXCR4 and microRNA-150

BACKGROUND

Administration of endothelial progenitor cells (EPC) represents a promising option to regenerate the heart after myocardial infarction, but is limited because of low recruitment and engraftment in the myocardium. Mobilization and migration of EPC are mainly controlled by stromal cell-derived factor 1α (SDF-1α) and its receptor CXCR4. We hypothesized that adenosine, a cardioprotective molecule, may improve the recruitment of EPC to the heart.

METHODS

EPC were obtained from peripheral blood mononuclear cells of healthy volunteers. Expression of chemokines and their receptors was evaluated using microarrays, quantitative PCR, and flow cytometry. A Boyden chamber assay was used to assess chemotaxis. Recruitment of EPC to the infarcted heart was evaluated in rats after permanent occlusion of the left anterior descending coronary artery.

RESULTS

Microarray analysis revealed that adenosine modulates the expression of several members of the chemokine family in EPC. Among these, CXCR4 was up-regulated by adenosine, and this result was confirmed by quantitative PCR (3-fold increase, P<0.001). CXCR4 expression at the cell surface was also increased. This effect involved the A(2B) receptor. Pretreatment of EPC with adenosine amplified their migration towards recombinant SDF-1α or conditioned medium from cardiac fibroblasts. Both effects were abolished by CXCR4 blocking antibodies. Adenosine also increased CXCR4 under ischemic conditions, and decreased miR-150 expression. Binding of miR-150 to the 3' untranslated region of CXCR4 was verified by luciferase assay. Addition of pre-miR-150 blunted the effect of adenosine on CXCR4. Administration of adenosine to rats after induction of myocardial infarction stimulated EPC recruitment to the heart and enhanced angiogenesis.

CONCLUSION

Adenosine increases the migration of EPC. The mechanism involves A(2B) receptor activation, decreased expression of miR-150 and increased expression of CXCR4. These results suggest that adenosine may be used to enhance the capacity of EPC to revascularize the ischemic heart.

Adenosine and gastrointestinal inflammation

Nucleosides such as adenosine (Ado) influence nearly every aspect of physiology and pathophysiology. Extracellular nucleotides liberated at local sites of inflammation are metabolized through regulated phosphohydrolysis by a series of ecto-nucleotidases including ectonucleoside triphosphate diphosphohydrolase-1 (CD39) and ecto-5'-nucleotidase (CD73), found on the surface of a variety of cell types. Once generated, Ado is made available to bind and activate one of four G protein-coupled Ado receptors. Recent in vitro and in vivo studies implicate Ado in a broad array of tissue-protective mechanisms that provide new insight into adenosine actions. Studies in cultured cells and murine tissues have indicated that Ado receptors couple to novel posttranslational protein modifications, including Cullin deneddylation, as a new anti-inflammatory mechanism. Studies in Ado receptor-null mice have been revealing and indicate a particularly important role for the Ado A2B receptor in animal models of intestinal inflammation. Here, we review contributions of Ado to cell and tissue stress responses, with a particular emphasis on the gastrointestinal mucosa.

Downregulation of A(1) and A(2B) adenosine receptors in human trisomy 21 mesenchymal cells from first-trimester chorionic villi

Human reproduction is complex and prone to failure. Though causes of miscarriage remain unclear, adenosine, a proangiogenic nucleoside, may help determine pregnancy outcome. Although adenosine receptor (AR) expression has been characterized in euploid pregnancies, no information is available for aneuploidies, which, as prone to spontaneous abortion (SA), are a potential model for shedding light on the mechanism regulating this event. AR expression was investigated in 71 first-trimester chorionic villi (CV) samples and cultured mesenchymal cells (MC) from euploid and TR21 pregnancies, one of the most frequent autosomal aneuploidy, with a view to elucidating their potential role in the modulation of vascular endothelial growth factor (VEGF) and nitric oxide (NO). Compared to euploid cells, reduced A(1) and A(2B) expression was revealed in TR21 CV and MCs. The non-selective adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased NO, by activating, predominantly, A(1)AR and A(2A)AR through a molecular pathway involving hypoxia-inducible-factor-1 (HIF-1α), and increased VEGF, mainly through A(2B). In conclusion the adenosine transduction cascade appears to be disturbed in TR21 through reduced expression of A(2B) and A(1)ARs. These anomalies may be implicated in complications such as fetal growth restriction, malformation and/or SA, well known features of aneuploid pregnancies. Therefore A(1) and A(2B)ARs could be potential biomarkers able to provide an early indication of SA risk and their stimulation may turn out to improve fetoplacental perfusion by increasing NO and VEGF.

A novel mechanism of control of NFκB activation and inflammation involving A2B adenosine receptors

The nuclear factor kappa B (NFκB) pathway controls a variety of processes, including inflammation, and thus, the regulation of NFκB has been a continued focus of study. Here, we report a newly identified regulation of this pathway, involving direct binding of the transcription factor NFκB1 (the p105 subunit of NFκB) to the C-terminus of the A(2B) adenosine receptor (A(2B)AR), independent of ligand activation. Intriguingly, binding of A(2B)AR to specific sites on p105 prevents polyubiquitylation and degradation of p105 protein. Ectopic expression of the A(2B)AR increases p105 levels and inhibits NFκB activation, whereas p105 protein levels are reduced in cells from A(2B)AR-knockout mice. In accordance with the known regulation of expression of anti- and pro-inflammatory cytokines by p105, A(2B)AR-null mice generate less interleukin (IL)-10, and more IL-12 and tumor necrosis factor (TNF-α). Taken together, our results show that the A(2B)AR inhibits NFκB activation by physically interacting with p105, thereby blocking its polyubiquitylation and degradation. Our findings unveil a surprising function for the A(2B)AR, and provide a novel mechanistic insight into the control of the NFκB pathway and inflammation.

Adenosine A₂B receptor agonism inhibits neointimal lesion development after arterial injury in apolipoprotein E-deficient mice

OBJECTIVE

The A(2B) adenosine receptor (A(2B)R) is highly expressed in macrophages and vascular smooth muscle cells and has been established as an important regulator of inflammation and vascular adhesion. Recently, it has been demonstrated that A(2B)R deficiency enhances neointimal lesion formation after vascular injury. Therefore, we hypothesize that A(2B)R agonism protects against injury-induced intimal hyperplasia.

METHODS AND RESULTS

Apolipoprotein E-deficient mice were fed a Western-type diet for 1 week, after which the left common carotid artery was denuded. Mice were treated with the A(2B) receptor agonist BAY60-6583 or vehicle control for 18 days. Interestingly, lumen stenosis as defined by the neointima/lumen ratio was inhibited by treatment with the A(2B) receptor agonist, caused by reduced smooth muscle cell proliferation. Collagen content was significantly increased in the BAY60-6583-treated mice, whereas macrophage content remained unchanged. In vitro, vascular smooth muscle cell proliferation decreased dose dependently whereas collagen content of cultured smooth muscle cells was increased by BAY60-6583.

CONCLUSIONS

Our data show that activation of the adenosine A(2B) receptor protects against vascular injury, while it also enhances plaque stability as indicated by increased collagen content. These outcomes thus point to A(2B) receptor agonism as a new therapeutic approach in the prevention of restenosis.

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.

A2 adenosine receptors and vascular pathologies

Cardiovascular disease, a leading cause of death and morbidity, is regulated, among various factors, by inflammation. The level of the metabolite adenosine is augmented under stress, including inflammatory, hypoxic, or injurious events. Adenosine has been shown to affect various physiological and pathological processes, largely through 1 or more of its 4 types of receptors: the A1 and A3 adenylyl cyclase inhibitory receptors and the A2A and A2B adenylyl cyclase stimulatory receptors. This article focuses on reviewing common and distinct effects of the 2 A2-type adenosine receptors on vascular disease and the mechanisms involved. Understanding the pathogenesis of vascular disease mediated by these receptors is important to the development of therapeutics and to the prevention and management of disease.

Regulation of macrophage function by adenosine

Following its release into the extracellular space in response to metabolic disturbances, the endogenous nucleoside adenosine exerts a range of immunomodulatory effects and cells of the mononuclear phagocyte system are among its major targets. Adenosine governs mononuclear phagocyte functions via 4 G-protein-coupled cell membrane receptors, which are denoted A(1), A(2A), A(2B), and A(3) receptors. Adenosine promotes osteoclast differentiation via A(1) receptors and alters monocyte to dendritic cell differentiation through A(2B) receptors. Adenosine downregulates classical macrophage activation mainly through A(2A) receptors. In contrast A(2B) receptor activation upregulates alternative macrophage activation. Adenosine promotes angiogenesis, which is mediated by inducing the production of vascular endothelial growth factor by mononuclear phagocytes through A(2A), A(2B), and A(3) receptors. By regulating mononuclear phagocyte function adenosine dictates the course of inflammatory and vascular diseases and cancer.