International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

Targeting the hypoxia-adenosinergic signaling pathway to improve the adoptive immunotherapy of cancer

The recent approval by the FDA of cancer vaccines and drugs that blockade immunological negative regulators has further enhanced interest in promising approaches of the immunotherapy of cancer. However, the disappointingly short life extension has also underscored the need to better understand the mechanisms that prevent tumor rejection and survival even after the blockade of immunological negative regulators. Here, we describe the implications of the "metabolism-based" immunosuppressive mechanism, where the local tissue hypoxia-driven accumulation of extracellular adenosine triggers suppression via A2 adenosine receptors on the surface of activated immune cells. This molecular pathway is of critical importance in mechanisms of immunosuppression in inflamed and cancerous tissue microenvironments. The protection of tumors by tumor-generated extracellular adenosine and A2 adenosine receptors could be the misguided application of the normal tissue-protecting mechanism that limits excessive collateral damage to vital organs during the anti-pathogen immune response. The overview of the current state of the art regarding the immunosuppressive effects of extracellular adenosine is followed by a historical perspective of studies focused on the elucidation of the physiological negative regulators that protect tissues of vital organs from excessive collateral damage, but, as a trade-off, may also weaken the anti-pathogen effector functions and negate the attempts of anti-tumor immune cells to destroy cancerous cells.

Ligand-specific binding and activation of the human adenosine A(2B) receptor

Adenosine A(2B) receptors, which play a role in inflammation and cancer, are of considerable interest as novel drug targets. To gain deeper insights into ligand binding and receptor activation, we exchanged amino acids predicted to be close to the binding pocket. The alanine mutants were stably expressed in CHO cells and characterized by radioligand binding and cAMP assays using three structural classes of ligands: xanthine (antagonist), adenosine, and aminopyridine derivatives (agonists). Asn282(7.45) and His280(7.43) were found to stabilize the binding site by intramolecular hydrogen bond formation as in the related A(2A) receptor subtype. Trp247(6.48), Val250(6.51), and particularly Ser279(7.42) were shown to be important for binding of nucleosidic agonists. Leu81(3.28), Asn186(5.42), and Val250(6.51) were discovered to be crucial for binding of the xanthine-derived antagonist PSB-603. Leu81(3.28), which is not conserved among adenosine receptor subtypes, may be important for the high selectivity of PSB-603. The N186(5.42)A mutant resulted in an increased potency for agonists. The interactions of the non-nucleosidic agonist BAY60-6583 were different from those of the nucleosides: while BAY60-6583 appeared not to interact with Ser279(7.42), its interactions with Trp247(6.48) and Val250(6.51) were significantly weaker compared to those of NECA. Moreover, our results discount the hypothesis of Trp247(6.48) serving as a "toogle switch" because BAY60-6583 was able to activate the corresponding mutant. This study reveals distinct interactions of structurally diverse ligands with the human A(2B) receptor and differences between closely related receptor subtypes (A(2B) and A(2A)). It will contribute to the understanding of G protein-coupled receptor function and advance A(2B) receptor ligand design.

A(2A) adenosine receptors are differentially modulated by pharmacological treatments in rheumatoid arthritis patients and their stimulation ameliorates adjuvant-induced arthritis in rats

A_2A adenosine receptors (ARs) play a key role in the inhibition of the inflammatory process. The purpose of this study was to evaluate the modulation of A_2AARs in rheumatoid arthritis (RA) patients after different pharmacological treatments and to investigate the effect of A_2AAR stimulation in a rat model of arthritis. We investigated A_2AAR density and functionality in RA progression by using a longitudinal study in RA patients before and after methotrexate (MTX), anti-TNFα agents or rituximab treatments. A_2AARs were analyzed by saturation binding assays in lymphocytes from RA patients throughout the 24-month study timeframe. In an adjuvant-induced arthritis model in rats we showed the efficacy of the A_2AAR agonist, CGS 21680 in comparison with standard therapies by means of paw volume assessment, radiographic and ultrasonographic imaging. Arthritic-associated pain was investigated in mechanical allodynia and thermal hyperalgesia tests. IL-10 release following A_2AAR stimulation in lymphocytes from RA patients and in serum from arthritic rats was measured. In lymphocytes obtained from RA patients, the A_2AAR up-regulation was gradually reduced in function of the treatment time and the stimulation of these receptors mediated a significant increase of IL-10 production. In the same cells, CGS 21680 did not affected cell viability and did not produced cytotoxic effects. The A_2AAR agonist CGS 21680 was highly effective, as suggested by the marked reduction of clinical signs, in rat adjuvant-induced arthritis and associated pain. This study highlighted that A_2AAR agonists represent a physiological-like therapeutic alternative for RA treatment as suggested by the anti-inflammatory role of A_2AARs in lymphocytes from RA patients. The effectiveness of A_2AAR stimulation in a rat model of arthritis supported the role of A_2AAR agonists as potential pharmacological treatment for RA.

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.

Chronic caffeine prevents changes in inhibitory avoidance memory and hippocampal BDNF immunocontent in middle-aged rats

Beneficial effects of caffeine on memory processes have been observed in animal models relevant to neurodegenerative diseases and aging, although the underlying mechanisms remain unknown. Because brain-derived neurotrophic factor (BDNF) is associated with memory formation and BDNF's actions are modulated by adenosine receptors, the molecular targets for the psychostimulant actions of caffeine, we here compare the effects of chronic caffeine (1 mg/mL drinking solution for 30 days) on short- and long term memory and on levels of hippocampal proBDNF, mature BDNF, TrkB and CREB in young (3 month old) and middle-aged (12 month old) rats. Caffeine treatment substantially reduced i) age-related impairments in the two types of memory in an inhibitory avoidance paradigm, and ii) parallel increases in hippocampal BDNF levels. In addition, chronic caffeine increased proBDNF and CREB concentrations, and decreased TrkB levels, in hippocampus regardless of age. These data provide new evidence in favor of the hypothesis that modifications in BDNF and related proteins in the hippocampus contribute to the pro-cognitive effects of caffeine on age-associated losses in memory encoding. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Fluorescent ligands for adenosine receptors

Interest is increasing in developing fluorescent ligands for characterization of adenosine receptors (ARs), which hold a promise of usefulness in the drug discovery process. The size of a strategically labeled AR ligand can be greatly increased after the attachment of a fluorophore. The choice of dye moiety (e.g. Alexa Fluor 488), attachment point and linker length can alter the selectivity and potency of the parent molecule. Fluorescent derivatives of adenosine agonists and antagonists (e.g. XAC and other heterocyclic antagonist scaffolds) have been synthesized and characterized pharmacologically. Some are useful AR probes for flow cytometry, fluorescence correlation spectroscopy, fluorescence microscopy, fluorescence polarization, fluorescence resonance energy transfer, and scanning confocal microscopy. Thus, the approach of fluorescent labeled GPCR ligands, including those for ARs, is a growing dynamic research field.

Adenosine A(2B) receptor-mediated VEGF induction promotes diabetic glomerulopathy

Diabetic nephropathy ranks as the most devastating kidney disease worldwide. It characterizes in the early onset by glomerular hypertrophy, hyperfiltration and mesangial expansion. Experimental models show that overproduction of vascular endothelial growth factor (VEGF) is a pathogenic condition for podocytopathy; however the mechanisms that regulate this growth factor induction are not clearly identified. We determined that the adenosine A(2B) receptor (A(2B)AR) mediates VEGF overproduction in ex vivo glomeruli exposed to high glucose concentration, requiring PKCα and Erk1/2 activation. The glomerular content of A(2B)AR was concomitantly increased with VEGF at early stages of renal disease in streptozotocin-induced diabetic rats. Further, in vivo administration of an antagonist of A(2B)AR in diabetic rats blocked the glomerular overexpression of VEGF, mesangial cells activation and proteinuria. In addition, we also determined that the accumulation of extracellular adenosine occurs in glomeruli of diabetic rats. Correspondingly, raised urinary adenosine levels were found in diabetic rats. In conclusion, we evidenced that adenosine signaling at the onset of diabetic kidney disease is a pathogenic event that promotes VEGF induction.

Inhibitory effects of benzodiazepines on the adenosine A(2B) receptor mediated secretion of interleukin-8 in human mast cells

The activation of adenosine A(2B) receptors in human mast cells causes pro-inflammatory responses such as the secretion of interleukin-8. There is evidence for an inhibitory effect of benzodiazepines on mast cell mediated symptoms in patients with systemic mast cell activation disease. Therefore, we investigated the effects of benzodiazepines on adenosine A(2B) receptor mediated interleukin-8 production in human mast cell leukaemia (HMC1) cells by an enzyme linked immunosorbent assay. The adenosine analogue N-ethylcarboxamidoadenosine (NECA, 0.3-3 μM) increased interleukin-8 production about 5-fold above baseline. This effect was attenuated by the adenosine A(2B) receptor antagonist MRS1754 (N-(4-cyanophenyl)-2-{4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy}-acetamide) 1 μM. In addition, diazepam, 4'-chlorodiazepam and flunitrazepam (1-30 μM) markedly reduced NECA-induced interleukin-8 production in that order of potency, whereas clonazepam showed only a modest inhibition. The inhibitory effect of diazepam was not altered by flumazenil 10 μM or PK11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide) 10 μM. Diazepam attenuated the NECA-induced expression of mRNA encoding for interleukin-8. Moreover, diazepam and flunitrazepam reduced the increasing effects of NECA on cAMP-response element- and nuclear factor of activated t-cells-driven luciferase reporter gene activities in HMC1 cells. Neither diazepam nor flunitrazepam affected NECA-induced increases in cellular cAMP levels in CHO Flp-In cells stably expressing recombinant human adenosine A(2B) receptors, excluding a direct action of benzodiazepines on human adenosine A(2B) receptors. In conclusion, this is the first study showing an inhibitory action of benzodiazepines on adenosine A(2B) receptor mediated interleukin-8 production in human mast (HMC1) cells. The rank order of potency indicates the involvement of an atypical benzodiazepine binding site.

Fragment screening of GPCRs using biophysical methods: identification of ligands of the adenosine A(2A) receptor with novel biological activity

Fragment-based drug discovery (FBDD) has proven a powerful method to develop novel drugs with excellent oral bioavailability against challenging pharmaceutical targets such as protein-protein interaction targets. Very recently the underlying biophysical techniques have begun to be successfully applied to membrane proteins. Here we show that novel, ligand efficient small molecules with a variety of biological activities can be found by screening a small fragment library using thermostabilized (StaR) G protein-coupled receptors (GPCRs) and target immobilized NMR screening (TINS). Detergent-solubilized StaR adenosine A(2A) receptor was immobilized with retention of functionality, and a screen of 531 fragments was performed. Hits from the screen were thoroughly characterized for biochemical activity using the wild-type receptor. Both orthosteric and allosteric modulatory activity has been demonstrated in biochemical validation assays. Allosteric activity was confirmed in cell-based functional assays. The validated fragment hits make excellent starting points for a subsequent hit-to-lead elaboration program.

Extracellular purines promote the differentiation of human bone marrow-derived mesenchymal stem cells to the osteogenic and adipogenic lineages

Extracellular nucleotides are potent signaling molecules mediating cell-specific biological functions, mostly within the processes of tissue damage and repair and flogosis. We previously demonstrated that adenosine 5'-triphosphate (ATP) inhibits the proliferation of human bone marrow-derived mesenchymal stem cells (BM-hMSCs), while stimulating, in vitro and in vivo, their migration. Here, we investigated the effects of ATP on BM-hMSC differentiation capacity. Molecular analysis showed that ATP treatment modulated the expression of several genes governing adipogenic and osteoblastic (i.e., WNT-pathway-related genes) differentiation of MSCs. Functional studies demonstrated that ATP, under specific culture conditions, stimulated adipogenesis by significantly increasing the lipid accumulation and the expression levels of the adipogenic master gene PPARγ (peroxisome proliferator-activated receptor-gamma). In addition, ATP stimulated osteogenic differentiation by promoting mineralization and expression of the osteoblast-related gene RUNX2 (runt-related transcription factor 2). Furthermore, we demonstrated that ATP stimulated adipogenesis via its triphosphate form, while osteogenic differentiation was induced by the nucleoside adenosine, resulting from ATP degradation induced by CD39 and CD73 ectonucleotidases expressed on the MSC membrane. The pharmacological profile of P2 purinergic receptors (P2Rs) suggests that adipogenic differentiation is mainly mediated by the engagement of P2Y1 and P2Y4 receptors, while stimulation of the P1R adenosine-specific subtype A2B is involved in adenosine-induced osteogenic differentiation. Thus, we provide new insights into molecular regulation of MSC differentiation.

Agonist dynamics and conformational selection during microsecond simulations of the A(2A) adenosine receptor

The G-protein-coupled receptors (GPCRs) are a ubiquitous family of signaling proteins of exceptional pharmacological importance. The recent publication of structures of several GPCRs cocrystallized with ligands of differing activity offers a unique opportunity to gain insight into their function. To that end, we performed microsecond-timescale simulations of the A(2A) adenosine receptor bound to either of two agonists, adenosine or UK432097. Our data suggest that adenosine is highly dynamic when bound to A(2A), in stark contrast to the case with UK432097. Remarkably, adenosine finds an alternate binding pose in which the ligand is inverted relative to the crystal structure, forming relatively stable interactions with helices I and II. Our observations suggest new experimental tests to validate our predictions and deepen our understanding of GPCR signaling. Overall, our data suggest an intriguing hypothesis: that the 100- to 1000-fold greater efficacy of UK432097 relative to adenosine arises because UK432097 stabilizes a much tighter neighborhood of active conformations, which manifests as a greater likelihood of G-protein activation per unit time.

Effects of oxygen and glucose deprivation on synaptic transmission in rat dentate gyrus: role of A2A adenosine receptors

The hippocampus is comprised of two distinct subfields that show different responses to hypoxic-ischemic brain injury: the CA1 region is particularly susceptible whereas the dentate gyrus (DG) is quite resistant. Our aim was to determine the synaptic and proliferative response of the DG to severe oxygen and glucose deprivation (OGD) in acute rat hippocampal slices and to investigate the contribution of A(2A) adenosine receptor antagonism to recovery of synaptic activity after OGD. Extracellular recordings of field excitatory post-synaptic potentials (fEPSPs) in granule cells of the DG in brain slices prepared from male Wistar rats were used. A 9-min OGD is needed in the DG to always induce the appearance of anoxic depolarization (AD) and the irreversible block of synaptic activity, as recorded up to 24 h from the end of the insult, whereas only 7-min OGD is required in the CA1 region. Selective antagonism of A(2A) adenosine receptors by ZM241385 significantly prevents or delays the appearance of AD and protects from the irreversible block of neurotransmission induced by 9-min OGD in the DG. The effects of 9-min OGD on proliferation and maturation of cells localized in the subgranular zone of DG in slices prepared from 5-bromo-2'-deoxyuridine (BrdU) treated rats was investigated. Slices were further incubated with an immature neuronal marker, doublecortin (DCX). The number of BrdU(+) cells was significantly decreased 6 h after 9-min OGD and this effect was antagonized by ZM241385. After 24 h from the end of 9-min OGD, the number of BrdU(+) cells returned to that found before OGD and increased arborization of tertiary dendrites of DCX(+) cells was observed. The adenosine A(2A) antagonist ZM241385 protects from synaptic failure and from decreased proliferation of immature neuronal cells at a precocious time after OGD.

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.

Selective and potent adenosine A3 receptor antagonists by methoxyaryl substitution on the N-(2,6-diarylpyrimidin-4-yl)acetamide scaffold

The influence of diverse methoxyphenyl substitution patterns on the N-(2,6-diarylpyrimidin-4-yl)acetamide scaffold is herein explored in order to modulate the A(3) adenosine receptor antagonistic profile. As a result, novel ligands exhibiting excellent potency (K(i) on A(3) AR < 20 nM) and selectivity profiles (above 100-fold within the adenosine receptors family) are reported. Moreover, our joint theoretical and experimental approach allows the identification of novel pharmacophoric elements conferring A(3)AR selectivity, first established by a robust computational model and thereafter characterizing the most salient features of the structure-activity and structure-selectivity relationships in this series.

Modulation of G protein-coupled adenosine receptors by strategically functionalized agonists and antagonists immobilized on gold nanoparticles

Gold nanoparticles (AuNPs) allow the tuning of pharmacokinetic and pharmacodynamic properties by active or passive targeting of drugs for cancer and other diseases. We have functionalized gold nanoparticles by tethering specific ligands, agonists and antagonists, of adenosine receptors (ARs) to the gold surface as models for cell surface interactions with G protein-coupled receptors (GPCRs). The AuNP conjugates with chain-extended AR ligands alone (PEGylated nucleosides and nonnucleosides, anchored to the Au via thioctic acid) were found to be insoluble in water due to hydrophobic entities in the ligand. Therefore, we added a second, biologically inactive pendant moiety to increase the water solubility, consisting of a PEGylated chain terminating in a carboxylic or phosphate group. The purity and stability of the immobilized biologically active ligand were examined by ultrafiltration and HPLC. Pharmacological receptor binding studies on these GPCR ligand-derivatized AuNPs (2-5 nm in diameter), performed using membranes of mammalian cells stably expressing human A1, A2A, and A3ARs, showed that the desired selectivity was retained with K(i) values (nanomolar) of A3AR agonist 21b and A2AAR antagonists 24 and 26a of 14 (A3), 34 (A2A), and 69 (A2A), respectively. The corresponding monomers displayed K i values of 37, 61, and 1,420 nM, respectively. In conclusion, we have synthesized stable, water-soluble AuNP derivatives of tethered A3 and A2AAR ligands that retain the biological properties of their monomeric ligands and are intended for therapeutic and imaging applications. This is the first prototypical application to gold carriers of small molecule (nonpeptide) GPCR ligands, which are under investigation for treatment of cancer and inflammatory diseases.

Functional efficacy of adenosine A₂A receptor agonists is positively correlated to their receptor residence time

BACKGROUND AND PURPOSE The adenosine A(2A) receptor belongs to the superfamily of GPCRs and is a promising therapeutic target. Traditionally, the discovery of novel agents for the A(2A) receptor has been guided by their affinity for the receptor. This parameter is determined under equilibrium conditions, largely ignoring the kinetic aspects of the ligand-receptor interaction. The aim of this study was to assess the binding kinetics of A(2A) receptor agonists and explore a possible relationship with their functional efficacy. EXPERIMENTAL APPROACH We set up, validated and optimized a kinetic radioligand binding assay (a so-called competition association assay) at the A(2A) receptor from which the binding kinetics of unlabelled ligands were determined. Subsequently, functional efficacies of A(2A) receptor agonists were determined in two different assays: a novel label-free impedance-based assay and a more traditional cAMP determination. KEY RESULTS A simplified competition association assay yielded an accurate determination of the association and dissociation rates of unlabelled A(2A) receptor ligands at their receptor. A correlation was observed between the receptor residence time of A(2A) receptor agonists and their intrinsic efficacies in both functional assays. The affinity of A(2A) receptor agonists was not correlated to their functional efficacy. CONCLUSIONS AND IMPLICATIONS This study indicates that the molecular basis of different agonist efficacies at the A(2A) receptor lies within their different residence times at this receptor.

The immunosuppressive role of adenosine A2A receptors in ischemia reperfusion injury and islet transplantation

Activation of adenosine A2A receptors (A2AR) reduces inflammation by generally inhibiting the activation of pro-inflammatory cells, decreasing endothelial adhesion molecule expression and reducing the release of proinflammatory cytokine mediators. Numerous preclinical studies using selective A2AR agonists, antagonists, A2AR knockout as well as chimeric mice have suggested the therapeutic potential of A2AR agonists for the treatment of ischemia reperfusion injury (IRI) and autoimmune diseases. This review summarizes the immunosuppressive actions of A2AR agonists in murine IRI models of liver, kidney, heart, lung and CNS, and gives details on the cellular effects of A2AR activation in neutrophils, macrophages, dendritic cells, natural killer cells, NKT cells, T effector cells and CD4+CD25+FoxP3+ T regulatory cells. This is discussed in the context of cytokine mediators involved in inflammatory cascades. Whilst the role of adenosine receptor agonists in various models of autoimmune disease has been well-documented, very little information is available regarding the role of A2AR activation in type 1 diabetes mellitus (T1DM). An overview of the pathogenesis of T1DM as well as early islet graft rejection in the immediate peri-transplantation period offers insight regarding the use of A2AR agonists as a beneficial intervention in clinical islet transplantation, promoting islet graft survival, minimizing early islet loss and reducing the number of islets required for successful transplantation, thereby increasing the availability of this procedure to a greater number of recipients. In summary, the use of A2AR agonists as a clinical intervention in IRI and as an adjunct to clinical immunesuppressive regimen in islet transplantation is highlighted.

N-substituted phenoxazine and acridone derivatives: structure-activity relationships of potent P2X4 receptor antagonists

P2X4 receptor antagonists have potential as drugs for the treatment of neuropathic pain and neurodegenerative diseases. In the present study the discovery of phenoxazine derivatives as potent P2X4 antagonists is described. N-Substituted phenoxazine and related acridone and benzoxazine derivatives were synthesized and optimized with regard to their potency to inhibit ATP-induced calcium influx in 1321N1 astrocytoma cells stably transfected with the human P2X4 receptor. In addition, species selectivity (rat, mouse, human) and receptor subtype selectivity (versus P2X1,2,3,7) were investigated. The most potent P2X4 antagonist of the present series was N-(benzyloxycarbonyl)phenoxazine (26, PSB-12054) with an IC(50) of 0.189 μM and good selectivity versus the other human P2X receptor subtypes. N-(p-Methylphenylsulfonyl)phenoxazine (21, PSB-12062) was identified as a selective P2X4 antagonist that was equally potent in all three species (IC(50): 0.928-1.76 μM). The compounds showed an allosteric mechanism of action. The present study represents the first structure-activity relationship analysis of P2X4 antagonists.

Dual-Point Competition Association Assay

The concept of ligand-receptor binding kinetics is emerging as an important parameter in the early phase of drug discovery. Since the currently used kinetic assays are laborious and low throughput, we developed a method that enables fast and large format screening. It is a so-called dual-point competition association assay, which measures radioligand binding at two different time points in the absence or presence of unlabeled competitors. Specifically, this assay yields the kinetic rate index (KRI), which is a measure for the binding kinetics of the unlabeled ligands screened. As a prototypical drug target, the adenosine A1 receptor (A1R) was chosen for assay validation and optimization. A screen with 35 high-affinity A1R antagonists yielded seven compounds with a KRI value above 1.0, which indicated a relatively slow dissociation from the target. All other compounds had a KRI value below or equal to 1.0, predicting a relatively fast dissociation rate. Several compounds were selected for follow-up kinetic quantifications in classical kinetic assays and were shown to have kinetic rates that corresponded to their KRI values. The dual-point assay and KRI value may have general applicability at other G-protein-coupled receptors, as well as at drug targets from other protein families.

CD73-generated adenosine: orchestrating the tumor-stroma interplay to promote cancer growth

Despite the coming of age of cancer immunotherapy, clinical benefits are still modest. An important barrier to successful cancer immunotherapy is that tumors employ a number of mechanisms to facilitate immune escape, including the production of anti-inflammatory cytokines, the recruitment of regulatory immune subsets, and the production of immunosuppressive metabolites. Significant therapeutic opportunity exists in targeting these immunosuppressive pathways. One such immunosuppressive pathway is the production of extracellular adenosine by CD73, an ectonucleotidase overexpressed in various types of cancer. We hereafter review the biology of CD73 and its role in cancer progression and metastasis. We describe the role of extracellular adenosine in promoting tumor growth through paracrine and autocrine action on tumor cells, endothelial cells, and immune cells.

Inflammation, pain, and pressure--purinergic signaling in oral tissues

Signaling by extracellular purines such as ATP and adenosine has implications for dental research on multiple levels, with the association of purinergic signaling with inflammation, mechanical strain, and pain making the system particularly relevant for the specific challenges in the oral cavity. Oral tissues express a variety of G-protein-coupled P2Y receptors for ATP and P1 receptors for adenosine in addition to ionotropic P2X receptors for ATP. When these receptors are combined with the plethora of extracellular enzymes capable of manipulating extracellular agonist levels, a complex system for regulating oral health emerges, and recent findings have begun to identify a key role for purinergic signaling in oral pathophysiology. For example, the manipulation of extracellular ATP levels by P. gingivalis reduces inflammasome activation and apoptosis linked to P2X(7) receptor activation. Release of ATP by periodontal ligaments may link mechanical strain to bone remodeling. Activation of P2X receptors is implicated in dental pain, and receptor antagonists represent important targets for new analgesics. Altered levels of adenosine receptors in periodontal disease also suggest a role for nucleosides in dental signaling. The intricacies of the purinergic signaling system make it well-suited for the unique concerns of dental research, and future findings will doubtless confirm this importance.

The Importance of astrocyte-derived purines in the modulation of sleep

Sleep is an evolutionarily conserved phenomenon that is clearly essential for survival, but we have limited understanding of how and why it is so important. Adenosine triphosphate (ATP)/adenosine signaling has been known to be important in the regulation of sleep and recent evidence suggests a critical role for gliotransmission in the modulation of sleep homeostasis. Herein, we review the regulation of ATP/adenosine in the nervous system and provide evidence of a critical role for astrocyte-derived adenosine in the regulation of sleep homeostasis and the modulation of synaptic transmission. Further understanding of the role of glial cells in the regulation of sleep may provide new targets for pharmaceutical intervention in the treatment of brain dysfunctions, specifically those that are comorbid with sleep disruptions.

Adora2b adenosine receptor signaling protects during acute kidney injury via inhibition of neutrophil-dependent TNF-α release

Renal ischemia is among the leading causes of acute kidney injury (AKI). Previous studies have shown that extracellular adenosine is a prominent tissue-protective cue elicited during ischemia, including signaling events through the adenosine receptor 2b (Adora2b). To investigate the functional role of Adora2b signaling in cytokine-mediated inflammatory pathways, we screened wild-type and Adora2b-deficient mice undergoing renal ischemia for expression of a range of inflammatory cytokines. These studies demonstrated a selective and robust increase of TNF-α levels in Adora2b-deficient mice following renal ischemia and reperfusion. Based on these findings, we next sought to understand the contribution of TNF-α on ischemic AKI through a combination of loss- and gain-of-function studies. Loss of TNF-α, through either Ab blockade or study of Tnf-α-deficient animals, resulted in significantly attenuated tissue injury and improved kidney function following renal ischemia. Conversely, transgenic mice with overexpression of TNF-α had significantly pronounced susceptibility to AKI. Furthermore, neutrophil depletion or reconstitution of Adora2b(-/-) mice with Tnf-α-deficient neutrophils rescued their phenotype. In total, these data demonstrate a critical role of adenosine signaling in constraining neutrophil-dependent production of TNF-α and implicate therapies targeting TNF-α in the treatment of ischemic AKI.

Functional selectivity of adenosine A1 receptor ligands?

The concept of functional selectivity offers great potential for the development of drugs that selectively activate a specific intracellular signaling pathway. During the last few years, it has become possible to systematically analyse compound libraries on G protein-coupled receptors (GPCRs) for this 'biased' form of signaling. We screened over 800 compounds targeting the class of adenosine A(1) receptors using a β-arrestin-mediated signaling assay in U2OS cells as a G protein-independent readout for GPCR activation. A selection of compounds was further analysed in a G protein-mediated GTPγS assay. Additionally, receptor affinity of these compounds was determined in a radioligand binding assay with the agonist [(3)H]CCPA. Of all compounds tested, only LUF5589 9 might be considered as functionally selective for the G protein-dependent pathway, particularly in view of a likely overestimation of β-arrestin signaling in the U2OS cells. Altogether, our study shows that functionally selective ligands for the adenosine A(1) receptor are rare, if existing at all. A thorough analysis of biased signaling on other GPCRs also reveals that only very few compounds can be considered functionally selective. This might indicate that the concept of functional selectivity is less common than speculated.

Structural sweet spot for A1 adenosine receptor activation by truncated (N)-methanocarba nucleosides: receptor docking and potent anticonvulsant activity

A(1) adenosine receptor (AR) agonists display antiischemic and antiepileptic neuroprotective activity, but peripheral cardiovascular side effects impeded their development. SAR study of N(6)-cycloalkylmethyl 4'-truncated (N)-methanocarba-adenosines identified 10 (MRS5474, N(6)-dicyclopropylmethyl, K(i) = 47.9 nM) as a moderately A(1)AR-selective full agonist. Two stereochemically defined N(6)-methynyl group substituents displayed narrow SAR; groups larger than cyclobutyl greatly reduced AR affinity, and those larger or smaller than cyclopropyl reduced A(1)AR selectivity. Nucleoside docking to A(1)AR homology model characterized distinct hydrophobic cyclopropyl subpockets, the larger "A" forming contacts with Thr270 (7.35), Tyr271 (7.36), Ile274 (7.39), and carbon chains of glutamates (EL2) and the smaller subpocket "B" forming contacts between TM6 and TM7. 10 suppressed minimal clonic seizures (6 Hz mouse model) without typical rotarod impairment of A(1)AR agonists. Truncated nucleosides, an appealing preclinical approach, have more druglike physicochemical properties than other A(1)AR agonists. Thus, we identified highly restricted regions for substitution around N(6) suitable for an A(1)AR agonist with anticonvulsant activity.

Predictions for cholesterol interaction sites on the A2A adenosine receptor

Molecular dynamics simulations of the A(2A) adenosine receptor totaling 1.4 μs show clear evidence for specific sites mediating interactions between adenosine-bound A(2A) and cholesterol. The strongest evidence is for three binding sites. Two are in the extracellular leaflet, with one site interacting with helices VII and I, and the other with helices II and III. One site is located in the intracellular leaflet, interacting with helices III and IV. One of our three predicted binding sites is confirmed by a just-published high-resolution structure of A(2A) cocrystallized with an antagonist.

Adenosine A2A receptor activation prevents wear particle-induced osteolysis

Prosthesis loosening, associated with wear particle-induced inflammation and osteoclast-mediated bone destruction, is a common cause for joint implant failure, leading to revision surgery. Adenosine A(2A) receptors (A(2A)Rs) mediate potent anti-inflammatory effects in many tissues and prevent osteoclast differentiation. We tested the hypothesis that an A(2A)R agonist could reduce osteoclast-mediated bone resorption in a murine calvaria model of wear particle-induced bone resorption. C57BL/6 and A(2A)R knockout (A(2A)R KO) mice received ultrahigh-molecular weight polyethylene particles and were treated daily with either saline or the A(2A)R agonist CGS21680. After 2 weeks, micro-computed tomography of calvaria demonstrated that CGS21680 reduced particle-induced bone pitting and porosity in a dose-dependent manner, increasing cortical bone and bone volume compared to control mice. Histological examination demonstrated diminished inflammation after treatment with CGS21680. In A(2A)R KO mice, CGS21680 did not affect osteoclast-mediated bone resorption or inflammation. Levels of bone resorption markers receptor activator of nuclear factor κB (RANK), RANK ligand, cathepsin K, CD163, and osteopontin were reduced after CGS21680 treatment, together with a reduction in osteoclasts. Secretion of interleukin-1β (IL-1β) and tumor necrosis factor-α was significantly decreased, whereas IL-10 was markedly increased in bone by CGS21680. These results in mice suggest that site-specific delivery of an adenosine A(2A)R agonist could enhance implant survival, delaying or eliminating the need for revision arthroplastic surgery.

Fragment screening at adenosine-A(3) receptors in living cells using a fluorescence-based binding assay

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane proteins. For GPCR drug discovery, it is important that ligand affinity is determined in the correct cellular environment and preferably using an unmodified receptor. We developed a live cell high-content screening assay that uses a fluorescent antagonist, CA200645, to determine binding affinity constants of competing ligands at human adenosine-A(1) and -A(3) receptors. This method was validated as a tool to screen a library of low molecular weight fragments, and identified a hit with submicromolar binding affinity (K(D)). This fragment was structurally unrelated to substructures of known adenosine receptor antagonists and was optimized to show selectivity for the adenosine-A(3) receptor. This technology represents a significant advance that will allow the determination of ligand and fragment affinities at receptors in their native membrane environment.

An emerging role for adenosine and its receptors in bone homeostasis

Bone is continually being remodeled and defects in the processes involved lead to bone diseases. Many regulatory factors are known to influence remodeling but other mechanisms, hitherto unknown, may also be involved. Importantly, our understanding of these currently unknown mechanisms may lead to important new therapies for bone disease. It is accepted that purinergic signaling is involved in bone, and our knowledge of this area has increased significantly over the last 15 years, although most of the published work has studied the role of ATP and other signaling molecules via the P2 family of purinergic receptors. During the last few years, however, there has been increased interest within the bone field in the role of P1 receptors where adenosine is the primary signaling molecule. This review will bring together the current information available in relation to this expanding area of research.

Structure-activity relationships and molecular modeling of 1,2,4-triazoles as adenosine receptor antagonists

The structure-activity relationship (SAR) for a novel class of 1,2,4-triazole antagonists of the human A(2A) adenosine receptor (hA(2A)AR) was explored. Thirty-three analogs of a ligand that was discovered in a structure-based virtual screen against the hA(2A)AR were tested in hA(1), A(2A), and A(3) radioligand binding assays and in functional assays for the A(2B)AR subtype. As a series of closely related analogs of the initial lead, 1, did not display improved binding affinity or selectivity, molecular docking was used to guide the selection of more distantly related molecules. This resulted in the discovery of 32, a hA(2A)AR antagonist (K(i) 200 nM) with high ligand efficiency. In the light of the SAR for the 1,2,4-triazole scaffold, we also investigated the binding mode of these compounds based on docking to several A(2A)AR crystal structures.

G protein-coupled adenosine (P1) and P2Y receptors: ligand design and receptor interactions

The medicinal chemistry and pharmacology of the four subtypes of adenosine receptors (ARs) and the eight subtypes of P2Y receptors (P2YRs, activated by a range of purine and pyrimidine mono- and dinucleotides) has recently advanced significantly leading to selective ligands. X-ray crystallographic structures of both agonist- and antagonist-bound forms of the A(2A)AR have provided unprecedented three-dimensional detail concerning molecular recognition in the binding site and the conformational changes in receptor activation. It is apparent that this ubiquitous cell signaling system has implications for understanding and treating many diseases. ATP and other nucleotides are readily released from intracellular sources under conditions of injury and organ stress, such as hypoxia, ischemia, or mechanical stress, and through channels and vesicular release. Adenosine may be generated extracellularly or by cellular release. Therefore, depending on pathophysiological factors, in a given tissue, there is often a tonic activation of one or more of the ARs or P2YRs that can be modulated by exogenous agents for a beneficial effect. Thus, this field has provided fertile ground for pharmaceutical development, leading to clinical trials of selective receptor ligands as imaging agents or for conditions including cardiac arrhythmias, ischemia/reperfusion injury, diabetes, pain, thrombosis, Parkinson's disease, rheumatoid arthritis, psoriasis, dry eye disease, pulmonary diseases such as cystic fibrosis, glaucoma, cancer, chronic hepatitis C, and other diseases.

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.

Impact and benefit of A(2B)-adenosine receptor agonists for the respiratory tract: mucociliary clearance, ciliary beat frequency, trachea muscle tonus and cytokine release

OBJECTIVES

Adenosine is known to induce a bronchospasm in asthma- and COPD patients. The role of A(2B) receptors was investigated with respect to several parameters of the respiratory tract: tonus of smooth muscle, ciliary beat frequency as measured by high-speed video camera connected to a microscope (both in rats) and mucociliary clearance (MCC; transport of a fluorescent dye using a microdialysis procedure) in mice.

KEY FINDINGS

 NECA (5'-N-ethylcarboxamidoadenosine) (a non-selective adenosine receptor agonist) was able to acutely induce a contraction, which was reversed to a relaxation after repeated dosing. This relaxation was completely abolished by PSB-1115, an A(2B) receptor antagonist. IL-13 (cytokine) was not involved mediating acute contractility effects. MCC was increased by BAY 60-6583 (A(2B) receptor agonist) and NECA (counteracted by the A(2B) receptor antagonist PSB-1115). Activation of A(2B) adenosine receptors by BAY 60-6583 induced an increase of the ciliary beat frequency, which could be reduced by administration of PSB-1115. Several cytokines were increased by NECA although only some are relevant because they are not blocked by A(2B) receptor antagonism.

CONCLUSIONS

The A(2B) receptors are involved in airway relaxation, MCC improvement and ciliary beat frequency. A(2B) receptor agonists may be of therapeutic value and should be developed.

Extracellular adenosine signaling induces CX3CL1 expression in the brain to promote experimental autoimmune encephalomyelitis

Background

Multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE) are debilitating neuroinflammatory diseases mediated by lymphocyte entry into the central nervous system (CNS). While it is not known what triggers lymphocyte entry into the CNS during neuroinflammation, blockade of lymphocyte migration has been shown to be effective in controlling neuroinflammatory diseases. Since we have previously shown that extracellular adenosine is a key mediator of lymphocyte migration into the CNS during EAE progression, we wanted to determine which factors are regulated by adenosine to modulate EAE development.

Methods

We performed a genetic analysis of wild type and CD73−/− (that are unable to produce extracellular adenosine and are protected from EAE development) to identify factors that are both important for EAE development and controlled by extracellular adenosine signaling.

Results

We show that extracellular adenosine triggered lymphocyte migration into the CNS by inducing the expression of the specialized chemokine/adhesion molecule CX3CL1 at the choroid plexus. In wild type mice, CX3CL1 is upregulated in the brain on Day 10 post EAE induction, which corresponds with initial CNS lymphocyte infiltration and the acute stage of EAE. Conversely, mice that cannot synthesize extracellular adenosine (CD73−/− mice) do not upregulate CX3CL1 in the brain following EAE induction and are protected from EAE development and its associated lymphocyte infiltration. Additionally, blockade of the A2A adenosine receptor following EAE induction prevents disease development and the induction of brain CX3CL1 expression. The CX3CL1 induced during EAE is found on the choroid plexus, which is the barrier between the blood and cerebral spinal fluid in the brain and is a prime entry point into the CNS for immune cells. Furthermore, CX3CL1 expression can be induced in the brains of mice and in choroid plexus cell line following A2A adenosine receptor agonist administration. Most importantly, we show that CX3CL1 blockade protects against EAE development and inhibits lymphocyte entry into the CNS.

Conclusions

We conclude that extracellular adenosine is an endogenous modulator of neuroinflammation during EAE that induces CX3CL1 at the choroid plexus to trigger lymphocyte entry into the brain.

Identifying novel adenosine receptor ligands by simultaneous proteochemometric modeling of rat and human bioactivity data

The four subtypes of adenosine receptors form relevant drug targets in the treatment of, e.g., diabetes and Parkinson's disease. In the present study, we aimed at finding novel small molecule ligands for these receptors using virtual screening approaches based on proteochemometric (PCM) modeling. We combined bioactivity data from all human and rat receptors in order to widen available chemical space. After training and validating a proteochemometric model on this combined data set (Q(2) of 0.73, RMSE of 0.61), we virtually screened a vendor database of 100910 compounds. Of 54 compounds purchased, six novel high affinity adenosine receptor ligands were confirmed experimentally, one of which displayed an affinity of 7 nM on the human adenosine A(1) receptor. We conclude that the combination of rat and human data performs better than human data only. Furthermore, we conclude that proteochemometric modeling is an efficient method to quickly screen for novel bioactive compounds.

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.

Insulin-increased L-arginine transport requires A(2A) adenosine receptors activation in human umbilical vein endothelium

Adenosine causes vasodilation of human placenta vasculature by increasing the transport of arginine via cationic amino acid transporters 1 (hCAT-1). This process involves the activation of A_2A adenosine receptors (A_2AAR) in human umbilical vein endothelial cells (HUVECs). Insulin increases hCAT-1 activity and expression in HUVECs, and A_2AAR stimulation increases insulin sensitivity in subjects with insulin resistance. However, whether A_2AAR plays a role in insulin-mediated increase in L-arginine transport in HUVECs is unknown. To determine this, we first assayed the kinetics of saturable L-arginine transport (1 minute, 37°C) in the absence or presence of nitrobenzylthioinosine (NBTI, 10 µmol/L, adenosine transport inhibitor) and/or adenosine receptors agonist/antagonists. We also determined hCAT-1 protein and mRNA expression levels (Western blots and quantitative PCR), and SLC7A1 (for hCAT-1) reporter promoter activity. Insulin and NBTI increased the extracellular adenosine concentration, the maximal velocity for L-arginine transport without altering the apparent K_m for L-arginine transport, hCAT-1 protein and mRNA expression levels, and SLC7A1 transcriptional activity. An A2AAR antagonist ZM-241385 blocked these effects. ZM241385 inhibited SLC7A1 reporter transcriptional activity to the same extent in cells transfected with pGL3-hCAT-1⁻¹⁶⁰⁶ or pGL3-hCAT-1⁻⁶⁵⁰ constructs in the presence of NBTI + insulin. However, SLC7A1 reporter activity was increased by NBTI only in cells transfected with pGL3-hCAT-1⁻¹⁶⁰⁶, and the ZM-241385 sensitive fraction of the NBTI response was similar in the absence or in the presence of insulin. Thus, insulin modulation of hCAT-1 expression and activity requires functional A_2AAR in HUVECs, a mechanism that may be applicable to diseases associated with fetal insulin resistance, such as gestational diabetes.

Truncated Nucleosides as A(3) Adenosine Receptor Ligands: Combined 2-Arylethynyl and Bicyclohexane Substitutions

C2-Arylethynyladenosine-5'-N-methyluronamides containing a bicyclo[3.1.0]hexane ((N)-methanocarba) ring are selective A(3) adenosine receptor (AR) agonists. Similar 4'-truncated C2-arylethynyl-(N)-methanocarba nucleosides containing alkyl or alkylaryl groups at the N(6) position were low-efficacy agonists or antagonists of the human A(3)AR with high selectivity. Higher hA(3)AR affinity was associated with N(6)-methyl and ethyl (K(i) 3-6 nM), than with N(6)-arylalkyl groups. However, combined C2-phenylethynyl and N(6)-2-phenylethyl substitutions in selective antagonist 15 provided a K(i) of 20 nM. Differences between 4'-truncated and nontruncated analogues of extended C2-p-biphenylethynyl substitution suggested a ligand reorientation in AR binding, dominated by bulky N(6) groups in analogues lacking a stabilizing 5'-uronamide moiety. Thus, 4'-truncation of C2-arylethynyl-(N)-methanocarba adenosine derivatives is compatible with general preservation of A(3)AR selectivity, especially with small N(6) groups, but reduced efficacy in A(3)AR-induced inhibition of adenylate cyclase.