Pharmacological analysis of calcium responses mediated by the human A3 adenosine receptor in monocyte-derived dendritic cells and recombinant cells

Species dependence of A3 adenosine receptor pharmacology and function

Efforts to fully understand pharmacological differences between G protein-coupled receptor (GPCR) species homologues are generally not pursued in detail during the drug development process. To date, many GPCRs that have been successfully targeted are relatively well-conserved across species in amino acid sequence and display minimal variability of biological effects. However, the A3 adenosine receptor (AR), an exciting drug target for a multitude of diseases associated with tissue injury, ischemia, and inflammation, displays as little as 70% sequence identity among mammalian species (e.g., rodent vs. primate) commonly used in drug development. Consequently, the pharmacological properties of synthetic A3AR ligands vary widely, not only in binding affinity, selectivity, and signaling efficacy, but to the extent that some function as agonists in some species and antagonists in others. Numerous heterocyclic antagonists that have nM affinity at the human A3AR are inactive or weakly active at the rat and mouse A3ARs. Positive allosteric modulators, including the imidazo [4,5-c]quinolin-4-amine derivative LUF6000, are only active at human and some larger animal species that have been evaluated (rabbit and dog), but not rodents. A3AR agonists evoke systemic degranulation of rodent, but not human mast cells. The rat A3AR undergoes desensitization faster than the human A3AR, but the human homologue can be completely re-sensitized and recycled back to the cell surface. Thus, comprehensive pharmacological evaluation and awareness of potential A3AR species differences are critical in studies to further understand the basic biological functions of this unique AR subtype. Recombinant A3ARs from eight different species have been pharmacologically characterized thus far. In this review, we describe in detail current knowledge of species differences in genetic identity, G protein-coupling, receptor regulation, and both orthosteric and allosteric A3AR pharmacology.

Adenosine Receptors as Potential Therapeutic Analgesic Targets

Pain represents an international burden and a major socio-economic public health problem. New findings, detailed in this review, suggest that adenosine plays a significant role in neuropathic and inflammatory pain, by acting on its metabotropic adenosine receptors (A1AR, A2AAR, A2BAR, A3AR). Adenosine receptor ligands have a practical translational potential based on the favorable efficacy and safety profiles that emerged from clinical research on various agonists and antagonists for different pathologies. The present review collects the latest studies on selected adenosine receptor ligands in different pain models. Here, we also covered the many hypothesized pathways and the role of newly synthesized allosteric adenosine receptor modulators. This review aims to present a summary of recent research on adenosine receptors as prospective therapeutic targets for a range of pain-related disorders.

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

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

TLR-Induced IL-12 and CCL2 Production by Myeloid Cells Is Dependent on Adenosine A3 Receptor-Mediated Signaling

TLR-induced signaling potently activates cells of the innate immune system and is subject to regulation at different levels. Inflammatory conditions are associated with increased levels of extracellular adenosine, which can modulate TLR-induced production of cytokines through adenosine receptor-mediated signaling. There are four adenosine receptor subtypes that induce different signaling cascades. In this study, we demonstrate a pivotal contribution of adenosine A₃ receptor (A₃R)-mediated signaling to the TLR4-induced expression of IL-12 in different types of human myeloid APC. In dendritic cells, IL-12 and CCL2 responses as evoked by TLR2, 3, 4, 5, and 8, as well as IL-12 responses evoked by whole pathogens, were all reduced when A₃R-mediated signaling was blocked. As a result, concomitant production of IFN-γ and IL-17 by T cells was significantly inhibited. We further show that selective inhibition of A₃R-mediated signaling reduced TLR-induced phosphorylation of the transcription factor STAT1 at tyrosine 701. Next-generation sequencing revealed that A₃R-mediated signaling controls the expression of metallothioneins, known inhibitors of STAT1 phosphorylation. Together our results reveal a novel regulatory layer of innate immune responses, with a central role for metallothioneins and autocrine/paracrine signaling via A₃Rs.

A3 Adenosine Receptors as Modulators of Inflammation: From Medicinal Chemistry to Therapy

The A3 adenosine receptor (A3 AR) subtype is a novel, promising therapeutic target for inflammatory diseases, such as rheumatoid arthritis (RA) and psoriasis, as well as liver cancer. A3 AR is coupled to inhibition of adenylyl cyclase and regulation of mitogen-activated protein kinase (MAPK) pathways, leading to modulation of transcription. Furthermore, A3 AR affects functions of almost all immune cells and the proliferation of cancer cells. Numerous A3 AR agonists, partial agonists, antagonists, and allosteric modulators have been reported, and their structure-activity relationships (SARs) have been studied culminating in the development of potent and selective molecules with drug-like characteristics. The efficacy of nucleoside agonists may be suppressed to produce antagonists, by structural modification of the ribose moiety. Diverse classes of heterocycles have been discovered as selective A3 AR blockers, although with large species differences. Thus, as a result of intense basic research efforts, the outlook for development of A3 AR modulators for human therapeutics is encouraging. Two prototypical selective agonists, N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA; CF101) and 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA; CF102), have progressed to advanced clinical trials. They were found safe and well tolerated in all preclinical and human clinical studies and showed promising results, particularly in psoriasis and RA, where the A3 AR is both a promising therapeutic target and a biologically predictive marker, suggesting a personalized medicine approach. Targeting the A3 AR may pave the way for safe and efficacious treatments for patient populations affected by inflammatory diseases, cancer, and other conditions.

Tricyclic xanthine derivatives containing a basic substituent: adenosine receptor affinity and drug-related properties

A library of 27 novel amide derivatives of annelated xanthines was designed and synthesized. The new compounds represent 1,3-dipropyl- and 1,3-dibutyl-pyrimido[2,1-f]purinedione-9-ethylphenoxy derivatives including a CH2CONH linker between the (CH2)2-amino group and the phenoxy moiety. A synthetic strategy to obtain the final products was developed involving solvent-free microwave irradiation. The new compounds were evaluated for their adenosine receptor (AR) affinities. The most potent derivatives contained a terminal tertiary amino function. Compounds with nanomolar AR affinities and at the same time high water-solubility were obtained (A1 (Ki = 24-605 nM), A2A (Ki = 242-1250 nM), A2B (Ki = 66-911 nM) and A3 (Ki = 155-1000 nM)). 2-(4-(2-(1,3-Dibutyl-2,4-dioxo-1,2,3,4,7,8-hexahydropyrimido[2,1-f]purin-9(6H)-yl)ethyl)phenoxy)-N-(3-(diethylamino)propyl)acetamide (27) and the corresponding N-(2-(pyrrolidin-1-yl)ethyl)acetamide (36) were found to be the most potent antagonists of the present series. While 27 showed CYP inhibition and moderate metabolic stability, 36 was found to possess suitable properties for in vivo applications. In an attempt to explain the affinity data for the synthesized compounds, molecular modeling and docking studies were performed using homology models of A1 and A2A adenosine receptors. The potent compound 36 was used as an example for discussion of the possible ligand-protein interactions. Moreover, the compounds showed high water-solubility indicating that the approach of introducing a basic side chain was successful for the class of generally poorly soluble AR antagonists.

Structure-Activity Analysis of Biased Agonism at the Human Adenosine A3 Receptor

Biased agonism at G protein-coupled receptors (GPCRs) has significant implications for current drug discovery, but molecular determinants that govern ligand bias remain largely unknown. The adenosine A3 GPCR (A3AR) is a potential therapeutic target for various conditions, including cancer, inflammation, and ischemia, but for which biased agonism remains largely unexplored. We now report the generation of bias "fingerprints" for prototypical ribose containing A3AR agonists and rigidified (N)-methanocarba 5'-N-methyluronamide nucleoside derivatives with regard to their ability to mediate different signaling pathways. Relative to the reference prototypical agonist IB-MECA, (N)-methanocarba 5'-N-methyluronamide nucleoside derivatives with significant N(6) or C2 modifications, including elongated aryl-ethynyl groups, exhibited biased agonism. Significant positive correlation was observed between the C2 substituent length (in Å) and bias toward cell survival. Molecular modeling suggests that extended C2 substituents on (N)-methanocarba 5'-N-methyluronamide nucleosides promote a progressive outward shift of the A3AR transmembrane domain 2, which may contribute to the subset of A3AR conformations stabilized on biased agonist binding.

The potential of methylxanthine-based therapies in pediatric respiratory tract diseases

Caffeine, theophylline and theobromine are the most known methylxanthines as they are present in coffee, tea and/or chocolate. In the last decades, a huge experimental effort has been devoted to get insight into the variety of actions that these compounds exert in humans. From such knowledge it is known that methylxanthines have a great potential in prevention, therapy and/or management of a variety of diseases. The benefits of methylxanthine-based therapies in the apnea of prematurity and their translational potential in pediatric affections of the respiratory tract are here presented.

Myeloid cells in the tumor microenvironment: Role of adenosine

Adenosine, deriving from ATP released by dying cancer cells and then degradated in the tumor environment by CD39/CD73 enzyme axis, is linked to the generation of an immunosuppressed niche favoring the onset of neoplasia. The effects of adenosine are mediated by four adenosine receptors, named A₁, A_2A, A_2B and A₃ that are widely expressed on several immune cell populations. A critical role of this nucleoside is emerging in the modulation of myeloid cell subsets accumulation and functions into tumor microenvironment, providing new insights that might be useful for the development of novel therapeutic approaches aimed to undermine the immune privileged sites where cancer cells grow and proliferate.

The A3 adenosine receptor: history and perspectives

By general consensus, the omnipresent purine nucleoside adenosine is considered a major regulator of local tissue function, especially when energy supply fails to meet cellular energy demand. Adenosine mediation involves activation of a family of four G protein-coupled adenosine receptors (ARs): A(1), A(2)A, A(2)B, and A(3). The A(3) adenosine receptor (A(3)AR) is the only adenosine subtype to be overexpressed in inflammatory and cancer cells, thus making it a potential target for therapy. Originally isolated as an orphan receptor, A(3)AR presented a twofold nature under different pathophysiologic conditions: it appeared to be protective/harmful under ischemic conditions, pro/anti-inflammatory, and pro/antitumoral depending on the systems investigated. Until recently, the greatest and most intriguing challenge has been to understand whether, and in which cases, selective A(3) agonists or antagonists would be the best choice. Today, the choice has been made and A(3)AR agonists are now under clinical development for some disorders including rheumatoid arthritis, psoriasis, glaucoma, and hepatocellular carcinoma. More specifically, the interest and relevance of these new agents derives from clinical data demonstrating that A(3)AR agonists are both effective and safe. Thus, it will become apparent in the present review that purine scientists do seem to be getting closer to their goal: the incorporation of adenosine ligands into drugs with the ability to save lives and improve human health.

Regulatory T cell-derived adenosine induces dendritic cell migration through the Epac-Rap1 pathway

Dendritic cells (DC) are one target for immune suppression by regulatory T cells (Treg), because their interaction results in reduced T cell stimulatory capacity and secretion of inhibitory cytokines in DC. We show that DC in the presence of Treg are more mobile as compared with cocultures with conventional CD4(+) T cells and form DC-Treg aggregates within 2 h of culture. The migration of DC was specifically directed toward Treg, as Treg, but not CD4(+) T cells, attracted DC in Boyden chambers. Treg deficient for the ectonucleotidase CD39 were unable to attract DC. Likewise, addition of antagonists for A2A adenosine receptors abolished the formation of DC-Treg clusters, indicating a role for adenosine in guiding DC-Treg interactions. Analysis of the signal transduction events in DC after contact to Treg revealed increased levels of cAMP, followed by activation of Epac1 and the GTPase Rap1. Subsequently activated Rap1 localized to the subcortical actin cytoskeleton in DC, providing a means by which directed locomotion of DC toward Treg is facilitated. In aggregate, these data show that Treg degrade ATP to adenosine via CD39, attracting DC by activating Epac1-Rap1-dependent pathways. As a consequence, DC-Treg clusters are formed and DC are rendered less stimulatory. This adenosine-mediated attraction of DC may therefore act as one mechanism by which Treg regulate the induction of immune responses by DC.

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

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

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

New insight into adenosine receptors selectivity derived from a novel series of [5-substituted-4-phenyl-1,3-thiazol-2-yl] benzamides and furamides

A series of [5-substituted-4-phenyl-1,3-thiazol-2-yl] benzamide and furamide analogues were investigated in radioligand binding studies at adenosine receptor subtypes with an aim to obtain potent and selective adenosine receptor ligands. Benzamide and furamide linked to thiazole was found to be crucial for high adenosine receptor affinity. The most potent compound indentified in this study was 5d with low nanomolar affinity for all four adenosine receptor subtypes. Compounds 5a and 5g showed moderate selectivity for A2A adenosine receptors. Molecular docking versus all four human adenosine receptors combined with membrane molecular dynamics studies were performed to rationalise the peculiar selectivity profile of 5d antagonist.

Novel effects of adenosine receptors on pericellular hyaluronan matrix: implications for human smooth muscle cell phenotype and interactions with monocytes during atherosclerosis

Hyaluronan (HA) is responsive to pro-atherosclerotic growth factors and cytokines and is thought to contribute to neointimal hyperplasia and atherosclerosis. However, the specific function of the pericellular HA matrix is likely depend on the respective stimuli. Adenosine plays an important role in the phenotypic regulation of vascular smooth muscle cells (VSMC) and is thought to inhibit inflammatory responses during atherosclerosis. The aim of this study was to examine the regulation and function of HA matrix in response to adenosine in human coronary artery SMC (HCASMC). The adenosine receptor agonist NECA (10 μM) caused a strong induction of HA synthase (HAS)1 at 6 h and a weaker induction again after 24 h. Use of selective adenosine receptor antagonists revealed that adenosine A2(B) receptors (A2(B)R) mediate the early HAS1 induction, whereas late HAS1 induction was mediated via A2(A)R and A3R. The strong response after 6 h was mediated in part via phosphoinositide-3 kinase- and mitogen-activated protein kinase pathways and was inhibited by Epac. Functionally, NECA increased cell migration, which was abolished by shRNA-mediated knock down of HAS1. In addition to HA secretion, NECA also stimulated the formation of pronounced pericellular HA matrix in HCASMC and increased the adhesion of monocytes. The adenosine-induced monocyte adhesion was sensitive to hyaluronidase. In conclusion, the current data suggest that adenosine via adenosine A2(B)R and A2(A)R/A3R induces HAS1. In turn a HA-rich matrix is formed by HCASMC which likely supports the migratory HCASMC phenotype and traps monocytes/macrophages in the interstitial matrix.

Evaluation of SSR161421, a novel orally active adenosine A3 receptor antagonist on pharmacology models

The effects of a novel adenosine A(3) receptor antagonist, SSR161421, were examined on both antigen per se and adenosine receptor agonist-increased airway responses in antigen-sensitized guinea pigs. Adenosine (10(-5)M) and AB-MECA [N6-(4-aminobenzyl)-adenosine-5'-N-methyl-uronamide dihydrochloride] (10(-7)M) increased the antigen response up to 61 ± 3.0% and 88 ± 5.2% of maximal contraction, respectively. The agonists of adenosine A(1) and A(2) adenosine receptors NECA [1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-b-d-ribofuranuronamide-5'-N-ethylcarboxamidoadenosine], R-PIA [N(6)-R-phenylisopropyladenosine], and CGS21680 (10(-7)M) were ineffective. In vivo intravenous adenosine (600 μg/kg) and AB-MECA (30 μg/kg) increased the threshold antigen dose-induced bronchoconstriction by 214 ± 13.0% and 220 ± 15.2%, respectively. SSR161421 in vitro (IC(50)=5.9 × 10(-7)M) inhibited the AB-MECA-enhanced antigen-induced airway smooth muscle contractions and also in vivo the bronchoconstriction following either intravenous (ED(50)=0.008 mg/kg) or oral (ED(50)=0.03 mg/kg) administration in sensitized guinea pigs. Antigen itself could evoke tracheal contraction in vitro and bronchoconstriction in vivo in antigen-sensitized guinea pigs. SSR161421 (3 × 10(-6)M) decreased the AUC of the antigen-induced contraction-time curve to 20.8 ± 5.4% from the 100% control level. SSR161421 effectively reversed the antigen-induced bronchoconstriction, plasma leak and cell recruitment with EC(50) values of 0.33 mg/kg p.o., 0.02 mg/kg i.p. and 3 mg/kg i.p., respectively.

Regulation of lymphocyte function by adenosine

Adenosine regulates the interaction between lymphocytes and the vasculature, and is important for controlling lymphocyte trafficking in response to tissue injury or infection. Adenosine can blunt the effects of T cell receptor activation primarily by activating adenosine A(2A) receptors and signaling via cyclic AMP and protein kinase A. Protein kinase A reduces proximal T cell receptor signaling by phosphorylation of C-terminal Src kinase, nuclear factor of activated T cells and cyclic AMP response element-binding protein. Protein kinase A activation can either enhance or inhibit the survival of T cells depending on the strength and duration of signaling. Inducible enzymes such as CD73 and CD39 regulate adenosine formation and degradation in vivo. The extravasation of lymphocytes through blood vessels is influenced by A(2A) receptors-mediated suppression of intercellular adhesion molecule 1 expression on lymphocytes and diminished production of interferon γ and interferon γ-inducible chemokines that are chemotactic to activated lymphocytes. Adenosine also decreases the barrier function of vascular endothelium by activating A(2B)Rs. In sum, adenosine signaling is influenced by tissue inflammation and injury through induction of receptors and enzymes and has generally inhibitory effects on lymphocyte migration into inflamed tissues due to protein kinase A-mediated effects on adhesion molecules, interferon γ production, and endothelial barrier function.

AMP affects intracellular Ca2+ signaling, migration, cytokine secretion and T cell priming capacity of dendritic cells

The nucleotide adenosine-5'-monophosphate (AMP) can be released by various cell types and has been shown to elicit different cellular responses. In the extracellular space AMP is dephosphorylated to the nucleoside adenosine which can then bind to adenosine receptors. However, it has been shown that AMP can also activate A(1) and A(2a) receptors directly. Here we show that AMP is a potent modulator of mouse and human dendritic cell (DC) function. AMP increased intracellular Ca(2+) concentration in a time and dose dependent manner. Furthermore, AMP stimulated actin-polymerization in human DCs and induced migration of immature human and bone marrow derived mouse DCs, both via direct activation of A(1) receptors. AMP strongly inhibited secretion of TNF-α and IL-12p70, while it enhanced production of IL-10 both via activation of A(2a) receptors. Consequently, DCs matured in the presence of AMP and co-cultivated with naive CD4(+)CD45RA(+) T cells inhibited IFN-γ production whereas secretion of IL-5 and IL-13 was up-regulated. An enhancement of Th2-driven immune response could also be observed when OVA-pulsed murine DCs were pretreated with AMP prior to co-culture with OVA-transgenic naïve OTII T cells. An effect due to the enzymatic degradation of AMP to adenosine could be ruled out, as AMP still elicited migration and changes in cytokine secretion in bone-marrow derived DCs generated from CD73-deficient animals and in human DCs pretreated with the ecto-nucleotidase inhibitor 5'-(alpha,beta-methylene) diphosphate (APCP). Finally, the influence of contaminating adenosine could be excluded, as AMP admixed with adenosine desaminase (ADA) was still able to influence DC function. In summary our data show that AMP when present during maturation is a potent regulator of dendritic cell function and point out the role for AMP in the pathogenesis of inflammatory disorders.

Cordycepin is an Immunoregulatory Active Ingredient of Cordyceps sinensis

We have reported that cordycepin, an adenosine derivative from the fungus Cordyceps, increased interleukin (IL)-10 expression, decreased IL-2 expression and suppressed T lymphocyte activity. In the present study, we further characterized the regulatory effects of cordycepin on human immune cells. Moreover, a traditional Chinese drug, Cordyceps sinensis (CS) that contains cordycepin, was also investigated. Cytometric Bead Array (CBA) was used to determine the concentrations of IL-1β, IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12, TNF-α and IFN-γ in culture of peripheral blood mononuclear cells (PBMCs). The results showed that both cordycepin and CS up-regulated IL-10, IL-1β, IL-6, IL-8 and TNF-α; at the same time, they suppressed phytohemagglutinin (PHA)-induced production of IL-2, IL-4, IL-5, IFN-γ and IL-12. As compared to cordycepin, CS displayed its regulatory effects on IL-2 and IL-10 in a similar dose-dependent manner even with higher efficiency. The binding activity of transcription factors in a human monocytic cell line THP-1 was tested by the trans-AM method, and a higher binding activity of SP1 and SP3 was observed in cordycepin or CS treated cells compared to the control. These results led to the opinion that cordycepin and CS pleiotropically affected the actions of immune cells and cytokine network in a similar fashion. Cordycepin could be an important immunoregulatory active ingredient in Cordyceps sinensis. In addition, CS may contain substances which possess synergism with cordycepin, as CS showed a higher efficiency in the production of IL-10 and IL-2 than cordycepin. However, merits of these effects in pharmacology and clinical medicine have yet to be proven and the precise mechanism of these immune regulatory actions should be researched.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

Functionally biased modulation of A(3) adenosine receptor agonist efficacy and potency by imidazoquinolinamine allosteric enhancers

Allosteric modulators for the G(i)-coupled A(3) adenosine receptor (AR) are of considerable interest as therapeutic agents and as pharmacological tools to probe various signaling pathways. In this study, we initially characterized the effects of several imidazoquinolinamine allosteric modulators (LUF5999, LUF6000 and LUF6001) on the human A(3) AR stably expressed in CHO cells using a cyclic AMP functional assay. These modulators were found to affect efficacy and potency of the agonist Cl-IB-MECA differently. LUF5999 (2-cyclobutyl derivative) enhanced efficacy but decreased potency. LUF6000 (2-cyclohexyl derivative) enhanced efficacy without affecting potency. LUF6001 (2-H derivative) decreased both efficacy and potency. We further compared the agonist enhancing effects of LUF6000 in several other A(3) AR-mediated events. It was shown that although LUF6000 behaved somewhat differently in various signaling pathways, it was more effective in enhancing the effects of low-efficacy than of high-efficacy agonists. In an assay of cyclic AMP accumulation, LUF6000 enhanced the efficacy of all agonists examined, but in the membrane hyperpolarization assay, it only enhanced the efficacy of partial agonists. In calcium mobilization, LUF6000 did not affect the efficacy of the full agonist NECA but was able to switch the nucleoside antagonist MRS542 into a partial agonist. In translocation of β-arrestin2, the agonist-enhancing effect LUF6000 was not pronounced. In an assay of ERK1/2 phosphorylation LUF6000 did not show any effect on the efficacy of Cl-IB-MECA. The differential effects of LUF6000 on the efficacy and potency of the agonist Cl-IB-MECA in various signaling pathway were interpreted quantitatively using a mathematical model.

Adenosine deaminase potentiates the generation of effector, memory, and regulatory CD4+ T cells

By interacting with CD26 on the CD4+ T cell surface and with the AdoR A(₂B) on the DC surface, ADA triggers a costimulatory signal for human T cells. The aim of this study was to know whether ADA-mediated costimulation plays a role in the differentiation of T cells. The results show that irrespective of its enzymatic activity and dependent on TNF-α, IFN-γ, and IL-6 action, ADA enhanced the differentiation of CD4+CD45RA+CD45RO⁻ naïve T cells toward CD4+CD25+CD45RO+ Teffs and CD4+CD45RA⁻CD45RO+ memory T cells. Furthermore, ADA potentiated generation of CD4+CD25(high)Foxp3+ Tregs by a mechanism that seems to be mainly dependent on the enzymatic activity of ADA. Interestingly, an ADA-mediated increase on Teff, memory T cell, and Treg generation occurred, not only in cocultures from healthy individuals but also from HIV-infected patients. These data suggest that ADA is a relevant modulator of CD4+ T cell differentiation, even in cells from immunologically compromised individuals.

Role of the P2Y12 receptor in the modulation of murine dendritic cell function by ADP

The effects of ADP on the biology of dendritic cells have been studied much less than those of ATP or adenosine. In this study, we showed that adenosine-5'-O-(2-thiodiphosphate) (ADPβS) induced intracellular Ca(2+) transients in murine dendritic cells (DCs). This effect was abolished by AR-C69931MX, a dual P2Y(12) and P2Y(13) receptor antagonist. RT-PCR experiments revealed the expression of both P2Y(12) and P2Y(13) mRNA in DCs. The Ca(2+) response to ADPβS was maintained in P2Y(13)-deficient DCs, whereas it was abolished completely in P2Y(12)(-/-) DCs. ADPβS stimulated FITC-dextran and OVA capture in murine DCs through macropinocytosis, and this effect was abolished in P2Y(12)(-/-) DCs. ADPβS had a similar effect on FITC-dextran uptake by human monocyte-derived DCs. OVA loading in the presence of ADPβS increased the capacity of DCs to stimulate OVA-specific T cells, whereas ADPβS had no effect on the ability of DCs to stimulate allogeneic T cells. Moreover, after immunization against OVA, the serum level of anti-OVA IgG1 was significantly lower in P2Y(12)(-/-) mice than that in wild-type controls. In conclusion, we have shown that the P2Y(12) receptor is expressed in murine DCs and that its activation increased Ag endocytosis by DCs with subsequent enhancement of specific T cell activation.

A3 adenosine receptor: pharmacology and role in disease

The study of the A(3) adenosine receptor (A(3)AR) represents a rapidly growing and intense area of research in the adenosine field. The present chapter will provide an overview of the expression patterns, molecular pharmacology and functional role of this A(3)AR subtype under pathophysiological conditions. Through studies utilizing selective A(3)AR agonists and antagonists, or A(3)AR knockout mice, it is now clear that this receptor plays a critical role in the modulation of ischemic diseases as well as in inflammatory and autoimmune pathologies. Therefore, the potential therapeutic use of agonists and antagonists will also be described. The discussion will principally address the use of such compounds in the treatment of brain and heart ischemia, asthma, sepsis and glaucoma. The final part concentrates on the molecular basis of A(3)ARs in autoimmune diseases such as rheumatoid arthritis, and includes a description of clinical trials with the selective agonist CF101. Based on this chapter, it is evident that continued research to discover agonists and antagonists for the A(3)AR subtype is warranted.

Molecular docking study of A(3) adenosine receptor antagonists and pharmacophore-based drug design

Adenosine is known to act as a neuromodulator by suppressing synaptic transmission in the central and peripheral nervous system. A(3) adenosine receptor (A(3)AR) antagonists were recently considered as potential drugs for the treatment of cardiac ischemia and inflammation diseases. To better understand the chemical features responsible for the recognition mechanism and the receptor-ligand interaction, we have performed the molecular simulation study combined with a virtual library screening process to develop novel A(3)AR antagonists. A series of A(3)AR selective antagonists, including triazolopurines, imidazopurines, pyrrolopurines, and quinazolines were employed to dock into the A(3)AR binding site via AUTODOCK software. The putative binding mode for each compound was proposed. Three main hydrophobic pockets, one hydrogen bonding with Asn250, and one pi-pi interaction with Phe168 for all antagonists were identified. The most favorable binding conformations served as the templates for pharmacophore modeling with Catalyst 4.11 and a virtually generated library have been screened for novel antagonist development.

Adenosine: an endogenous modulator of innate immune system with therapeutic potential

Adenosine is a purine nucleoside, which is produced inside the body under metabolic stress like hypoxic conditions, acute or chronic inflammatory tissue insults. The synthesis of adenosine involves the catabolism of adenine nucleotides (ATP, ADP and AMP) by the action of extracellular ectonucleotidases i.e. CD39 or nucleoside triphosphate dephosphorylase (NTPD) and CD73 or 5'-ectonucleotidase. Once adenosine is released in the extracellular environment, it binds to different types of adenosine (i.e. adenosine A(1), A(2A), A(2B) and A(3) receptors) receptors expressed on various innate immune cells [Neutrophils, macrophages, mast cells, dendritic cells and natural killer cells]. Thus, depending on the type of adenosine receptor to which it binds, adenosine modulates innate immune response during various inflammatory conditions [i.e. chronic (cancer, asthma) as well as acute (sepsis, acute lung injury) inflammatory diseases]. This review summarizes the effect of adenosine on innate immunity and the use of adenosine receptor specific agonists or antagonists in various immunologic disorders (asthma, cancer, HIV-1 infection) as future immunomodulatory therapeutics.

Novel 2- and 4-substituted 1H-imidazo[4,5-c]quinolin-4-amine derivatives as allosteric modulators of the A3 adenosine receptor

4-Arylamino and 2- cycloalkyl (including amino substitution) modifications were made in a series of 1H-imidazo[4,5-c]quinolin-4-amine derivatives as allosteric modulators of the human A(3) adenosine receptor (AR). In addition to allosteric modulation of the maximum functional efficacy (in [(35)S]GTPgammaS G protein binding assay) of the A(3)AR agonist Cl-IB-MECA (15), some analogues also weakly inhibited equilibrium radioligand binding at ARs. 4-(3,5-Dichlorophenylamino) (6) or 2-(1-adamantyl) (20) substitution produced allosteric enhancement (twice the maximal agonist efficacy), with minimal inhibition of orthosteric AR binding. 2-(4-Tetrahydropyranyl) substitution abolished allosteric enhancement but preserved inhibition of orthosteric binding. Introduction of nitrogen in the six-membered ring at the 2 position, to improve aqueous solubility and provide a derivatization site, greatly reduced the allosteric enhancement. 2-(4-(Benzoylamino)cyclohexyl) analogues 23 and 24 were weak negative A(3)AR modulators. Thus, consistent with previous findings, the allosteric and orthosteric inhibitory A(3)AR effects in imidazoquinolines are structurally separable, suggesting the possible design of additional derivatives with enhanced positive or negative allosteric A(3)AR activity and improved selectivity in comparison to inhibition of orthosteric binding.

Synthesis of novel 1-alkyl-8-substituted-3-(3-methoxypropyl) xanthines as putative A(2B) receptor antagonists

In order to identify a high-affinity, selective antagonist for the A(2B) subtype adenosine receptor, more than 40 1,8-disubstituted-3-(3-methoxypropyl) xanthines were prepared and evaluated for their binding affinity at recombinant human adenosine receptors, mainly of the A(2A) and A(2B) subtypes. Some of the 1-ethyl-3-(3-methoxypropyl)-8-aryl substituted derivatives 15(a-m) showed moderate-to-high affinity at human A(2B) receptors, with compound 15d showing A(2B) selectivity over the other A receptors assayed (A(1), A(2A), A(3)) of 34-fold or over.

Adenosine receptors and inflammation

Extracellular adenosine is produced in a coordinated manner from cells following cellular challenge or tissue injury. Once produced, it serves as an autocrine- and paracrine-signaling molecule through its interactions with seven-membrane-spanning G-protein-coupled adenosine receptors. These signaling pathways have widespread physiological and pathophysiological functions. Immune cells express adenosine receptors and respond to adenosine or adenosine agonists in diverse manners. Extensive in vitro and in vivo studies have identified potent anti-inflammatory functions for all of the adenosine receptors on many different inflammatory cells and in various inflammatory disease processes. In addition, specific proinflammatory functions have also been ascribed to adenosine receptor activation. The potent effects of adenosine signaling on the regulation of inflammation suggest that targeting specific adenosine receptor activation or inactivation using selective agonists and antagonists could have important therapeutic implications in numerous diseases. This review is designed to summarize the current status of adenosine receptor signaling in various inflammatory cells and in models of inflammation, with an emphasis on the advancement of adenosine-based therapeutics to treat inflammatory disorders.

Modulation of murine dendritic cell function by adenine nucleotides and adenosine: involvement of the A(2B) receptor

Adenosine triphosphate has previously been shown to induce semi-mature human monocyte-derived dendritic cells (DC). These are characterized by the up-regulation of co-stimulatory molecules, the inhibition of IL-12 and the up-regulation of some genes involved in immune tolerance, such as thrombospondin-1 and indoleamine 2,3-dioxygenase. The actions of adenosine triphosphate are mediated by the P2Y(11) receptor; since there is no functional P2Y(11) gene in the murine genome, we investigated the action of adenine nucleotides on murine DC. Adenosine 5'-(3-thiotriphosphate) and adenosine inhibited the production of IL-12p70 by bone marrow-derived DC (BMDC). These inhibitions were relieved by 8-p-sulfophenyltheophylline, an adenosine receptor antagonist. The use of selective ligands and A(2B) (-/-) BMDC indicated the involvement of the A(2B) receptor. A microarray experiment, confirmed by quantitative PCR, showed that, in presence of LPS, 5'-(N-ethylcarboxamido) adenosine (NECA, the most potent A(2B) receptor agonist) regulated the expression of several genes: arginase I and II, thrombospondin-1 and vascular endothelial growth factor were up-regulated whereas CCL2 and CCL12 were down-regulated. We further showed that NECA, in combination with LPS, increased the arginase I enzymatic activity. In conclusion, the described actions of adenine nucleotides on BMDC are mediated by their degradation product, adenosine, acting on the A(2B) receptor, and will possibly lead to an impairment of Th1 response or tolerance.

Discovery of a novel A2B adenosine receptor antagonist as a clinical candidate for chronic inflammatory airway diseases

Recently, we have reported a series of new 1,3-symmetrically (R 1 = R 3) substituted xanthines ( 3 and 4) which have high affinity and selectivity for the human adenosine A 2B receptors (hA(2B)-AdoR). Unfortunately, this class of compounds had poor pharmacokinetic properties. This prompted us to investigate the effect of differential alkyl substitution at the N-1 and N-3 positions ( N 1-R not equal to N 3-R) on A(2B)-AdoR affinity and selectivity; we had the dual objectives of enhancing affinity and selectivity for the A(2B)-AdoR, as well as improving oral bioavailability. This effort has led to the discovery of compound 62, that displayed high affinity and selectivity for the hA(2B)-AdoR (K(i) = 22 nM). In addition, compound 62 showed high functional potency in inhibiting the accumulation of cyclic adenosine monophosphate induced by 5'- N-ethylcarboxamidoadenosine in HEK-A(2B)-AdoR and NIH3T3 cells with K(B) values of 6 and 2 nM, respectively. In a single ascending-dose phase I clinical study, compound 62 had no serious adverse events and was well tolerated.

The A3 adenosine receptor: an enigmatic player in cell biology

Adenosine is a primordial signaling molecule present in every cell of the human body that mediates its physiological functions by interacting with 4 subtypes of G-protein-coupled receptors, termed A1, A2A, A2B and A3. The A3 subtype is perhaps the most enigmatic among adenosine receptors since, although several studies have been performed in the years to elucidate its physiological function, it still presents in several cases a double nature in different pathophysiological conditions. The 2 personalities of A3 often come into direct conflict, e.g., in ischemia, inflammation and cancer, rendering this receptor as a single entity behaving in 2 different ways. This review focuses on the most relevant aspects of A3 adenosine subtype activation and summarizes the pharmacological evidence as the basis of the dichotomy of this receptor in different therapeutic fields. Although much is still to be learned about the function of the A3 receptor and in spite of its duality, at the present time it can be speculated that A3 receptor selective ligands might show utility in the treatment of ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The biggest and most intriguing challenge for the future is therefore to understand whether and where selective A3 agonists or antagonists are the best choice.

Modulation of the Akt/Ras/Raf/MEK/ERK pathway by A₃ adenosine receptor

Downstream A₃ receptor signalling plays an important role in the regulation of cell death and proliferation. Therefore, it is important to determine the molecular pathways involved through A₃ receptor stimulation. The phosphatidylinositide-3-OH kinase (PI3K)/Akt and the Raf/mitogen-activated protein kinase (MAPK/ERK) kinase (MEK)/mitogen-activated protein kinase (MAPK) pathways have central roles in the regulation of cell survival and proliferation. The crosstalk between these two pathways has also been investigated. The focus of this review centres on downstream mediators of A₃ adenosine receptor signalling.

Radioligand binding and functional responses of ligands for human recombinant adenosine A(3) receptors

The binding and functional properties of adenosine receptor ligands were compared in Chinese hamster ovary cells transfected with human adenosine A(3) receptors. Inhibition of [(125)I]-aminobenzyl-5'-N-methylcarboamidoadenosine ([(125)I]-AB-MECA) binding by adenosine receptor ligands was examined in membrane preparations. Inhibition of forskolin-induced cAMP accumulation by agonists was measured using a cAMP enzyme immunoassay. The rank order of agonist potency for both assays was N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) > 5'-N-ethylcarboxamidoadenosine (NECA) > (-)-N(6)-[(R)-phenylisopropyl] adenosine (R-PIA) > 4-aminobenzyl-5'-N-methylcarboxamidoadenosine (AB-MECA) > N(6)-cyclopentyl adenosine (CPA) > adenosine. The radioligand binding rank order of antagonist potency was N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide (MRS1220) > 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) > 8-phenyltheophylline (8-PT) > 8-(p-sulfophenyl)-theophylline (8-SPT). MRS1220 competitively inhibited the effect of IB-MECA on cAMP production, with a K(B) value of 0.35 nm. These data are characteristic of adenosine A(3) receptors. The absence of Mg(2+) and presence of guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) significantly reduced agonist binding inhibition potency, indicating binding to high- and low-affinity states. The IB-MECA, NECA and R-PIA IC(50) values were greater for the cAMP assay than for radioligand binding, suggesting an efficient stimulus-response transduction pathway.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Functional expression of the P2Y14 receptor in murine T-lymphocytes

Quantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis has previously shown that the P2Y(14) receptor is expressed in peripheral immune cells including lymphocytes. Although in transfected cells the P2Y(14) receptor couples to pertussis toxin-sensitive G(i/o) protein, the functional coupling of endogenously expressed P2Y(14) receptors to the inhibition of adenylyl cyclase activity has not been reported. Therefore, the primary aim of this study was to determine whether the P2Y(14) receptor is functionally expressed in murine spleen-derived T- and B-lymphocyte-enriched populations. RT-PCR analysis detected the expression of P2Y(14) receptor mRNA in whole spleen and isolated T- and B-lymphocytes. In T cells, UDP-glucose (EC(50) = 335 nM) induced a small but significant inhibition (circa 20%) of forskolin-stimulated cAMP accumulation, suggesting functional coupling of endogenously expressed P2Y(14) receptors to the inhibition of adenylyl cyclase activity. In contrast, the other putative P2Y(14) receptor agonists UDP-galactose, UDP-glucuronic acid and UDP-N-acetylglucosamine had no significant effect alone but behaved as partial agonists by blocking UDP-glucose responses. In B cells, UDP-glucose (100 microM) had no significant effect on forskolin-stimulated cAMP accumulation. Treatment of T cells with pertussis toxin (G(i/o) blocker) abolished the inhibitory effects of UDP-glucose on forskolin-stimulated cAMP accumulation. T-cell proliferation in response to anti-CD3 monoclonal antibody (1 microg ml(-1)) was significantly inhibited by UDP-glucose (59% inhibition; p[IC(50)] = 5.9 +/- 0.3), UDP-N-acetylglucosamine (37%; 6.1 +/- 0.3), UDP-galactose (56%; 8.2 +/- 0.2) and UDP-glucuronic acid (49%; 6.3 +/- 0.2). Interleukin-2- (5 ng ml(-1)) induced T-cell proliferation was also significantly inhibited by all four agonists. In summary, we have shown that the P2Y(14) receptor appears to be functionally expressed in murine spleen-derived T-lymphocytes. These observations suggest that UDP-glucose and related sugar nucleotides presumably via the P2Y(14) receptor may play an important role in modulating immune function.

Adenosine receptors as therapeutic targets

Adenosine receptors are major targets of caffeine, the most commonly consumed drug in the world. There is growing evidence that they could also be promising therapeutic targets in a wide range of conditions, including cerebral and cardiac ischaemic diseases, sleep disorders, immune and inflammatory disorders and cancer. After more than three decades of medicinal chemistry research, a considerable number of selective agonists and antagonists of adenosine receptors have been discovered, and some have been clinically evaluated, although none has yet received regulatory approval. However, recent advances in the understanding of the roles of the various adenosine receptor subtypes, and in the development of selective and potent ligands, as discussed in this review, have brought the goal of therapeutic application of adenosine receptor modulators considerably closer.

CD26, adenosine deaminase, and adenosine receptors mediate costimulatory signals in the immunological synapse

Adenosine deaminase (ADA), a protein whose deficit leads to severe combined immunodeficiency, binds to the cell surface by means of either CD26, A(1) adenosine receptors, or A(2B) adenosine receptors. The physiological role of these interactions is not well understood. Our results show that by a 3-fold reduction in the EC(50) for the antigen, ADA potentiated T cell proliferation in autologous cocultures with antigen-pulsed immature or mature dendritic cells. Costimulation was not due to the enzymatic activity but to the interaction of ADA-CD26 complexes in T cells with an ADA-anchoring protein in dendritic cells. From colocalization studies, it is deduced that ADA colocalizing with adenosine receptors on dendritic cells interact with CD26 expressed on lymphocytes. This costimulatory signal in the immunological synapse leads to a marked increase (3- to 34-fold) in the production of the T helper 1 and proimmflamatory cytokines IFN-gamma, TNF-alpha, and IL-6.

(N)-methanocarba 2,N6-disubstituted adenine nucleosides as highly potent and selective A3 adenosine receptor agonists

A series of ring-constrained (N)-methanocarba-5'-uronamide 2,N(6)-disubstituted adenine nucleosides have been synthesized via Mitsunobu condensation of the nucleobase precursor with a pseudosugar ring containing a 5'-ester functionality. Following appropriate functionalization of the adenine ring, the ester group was converted to the 5'-N-methylamide. The compounds, mainly 2-chloro-substituted derivatives, were tested in both binding and functional assays at human adenosine receptors (ARs), and many were found to be highly potent and selective A(3)AR agonists. Selected compounds were compared in binding to the rat A(3)AR to assess their viability for testing in rat disease models. The N(6)-(3-chlorobenzyl) and N(6)-(3-bromobenzyl) analogues displayed K(i) values at the human A(3)AR of 0.29 and 0.38 nM, respectively. Other subnanomolar affinities were observed for the following N(6) derivatives: 2,5-dichlorobenzyl, 5-iodo-2-methoxybenzyl, trans-2-phenyl-1-cyclopropyl, and 2,2-diphenylethyl. Selectivity for the human A(3)AR in comparison to the A(1)AR was the following (fold): the N(6)-(2,2-diphenylethyl) analogue 34 (1900), the N(6)-(2,5-dimethoxybenzyl) analogue 26 (1200), the N(6)-(2,5-dichlorobenzyl) and N(6)-(2-phenyl-1-cyclopropyl) analogues 20 and 33 (1000), and the N(6)-(3-substituted benzyl) analogues 17, 18, 28, and 29 (700-900). Typically, even greater selectivity ratios were obtained in comparison with the A(2A) and A(2B)ARs. The (N)-methanocarba-5'-uronamide analogues were full agonists at the A(3)AR, as indicated by the inhibition of forskolin-stimluated adenylate cyclase at a concentration of 10 microM. The N(6)-(2,2-diphenylethyl) derivative was an A(3)AR agonist in the (N)-methanocarba-5'-uronamide series, although it was an antagonist in the ribose series. Thus, many of the previously known groups that enhance A(3)AR affinity in the 9-riboside series, including those that reduce intrinsic efficacy, may be adapted to the (N)-methanocarba nucleoside series of full agonists.

Exploring distal regions of the A3 adenosine receptor binding site: sterically constrained N6-(2-phenylethyl)adenosine derivatives as potent ligands

We synthesized phenyl ring-substituted analogues of N(6)-(1S,2R)-(2-phenyl-1-cyclopropyl)adenosine, which is highly potent in binding to the human A(3)AR with a Ki value of 0.63 nM. The effects of these structural changes on affinity at human and rat adenosine receptors and on intrinsic efficacy at the hA(3)AR were measured. A 3-nitrophenyl analogue was resolved chromatographically into pure diastereomers, which displayed 10-fold stereoselectivity in A(3)AR binding in favor of the 1S,2R isomer. A molecular model defined a hydrophobic region (Phe168) in the putative A(3)AR binding site around the phenyl moiety. A heteroaromatic group (3-thienyl) could substitute for the phenyl moiety with retention of high affinity of A(3)AR binding. Other related N(6)-substituted adenosine derivatives were included for comparison. Although the N(6)-(2-phenyl-1-cyclopropyl) derivatives were full A(3)AR agonists, several other derivatives had greatly reduced efficacy. N(6)-Cyclopropyladenosine was an A(3)AR antagonist, and adding either one or two phenyl rings at the 2-position of the cyclopropyl moiety restored efficacy. N(6)-(2,2-Diphenylethyl)adenosine was an A(3)AR antagonist, and either adding a bond between the two phenyl rings (N(6)-9-fluorenylmethyl) or shortening the ethyl moiety (N(6)-diphenylmethyl) restored efficacy. A QSAR study of the N(6) region provided a model that was complementary to the putative A(3)AR binding site in a rhodopsin-based homology model. Thus, a new series of high-affinity A(3)AR agonists and related nucleoside antagonists was explored through both empirical and theoretical approaches.

Expression of A3 adenosine receptors in human lymphocytes: up-regulation in T cell activation

The present study investigates mRNA and protein levels of A3 adenosine receptors in resting (R) and activated (A) human lymphocytes. The receptors were evaluated by the antagonist radioligand [3H]5-N-(4-methoxyphenyl-carbamoyl)amino-8-propyl-2(2furyl)-pyrazolo-[4,3e]-1,2,4-triazolo-[1,5-c]-pyrimidine ([3H]MRE 3008F20), which yielded Bmax values of 125 +/- 15 and 225 +/- 23 fmol/mg of protein and KD values of 1.79 +/- 0.30 and 1.85 +/- 0.25 nM in R and A cells, respectively. The protein seems to be induced with remarkable rapidity starting at 15 min and reaches a plateau at 30 min. Western blot assays revealed that the up-regulation of the A3 subtype after lymphocyte activation was caused by an increase in an enriched CD4+ cell fraction. Real-time reverse transcription-polymerase chain reaction experiments confirmed the rapid increase of A3 mRNA after T cell activation. Competition of radioligand binding by adenosine ligands displayed a rank order of potency typical of the A3 subtype. Thermodynamic data indicated that the binding is enthalpy- and entropy-driven in both R and A cells, suggesting that the activation process does not involve, at a molecular level, receptor alterations leading to modifications in the A3-related binding mechanisms. Functionally, the up-regulation of A3 adenosine receptors in A versus R cells corresponded to a potency increase of the A3 agonist N6-(3-iodo-benzyl)-2-chloro-adenosine-5'-N-methyluronamide in inhibiting cAMP accumulation (IC50=1.5 +/- 0.4 and 2.7 +/- 0.3 nM, respectively); this effect was antagonized by MRE 3008F20 (IC50=5.0 +/- 0.3 nM). In conclusion, our results provide, for the first time, an in-depth investigation of A3 receptors in human lymphocytes and demonstrate that, under activating conditions, they are up-regulated and may contribute to the effects triggered by adenosine.

Inosine stimulates chemotaxis, Ca2+-transients and actin polymerization in immature human dendritic cells via a pertussis toxin-sensitive mechanism independent of adenosine receptors

Inosine is an endogenous purine nucleoside, which is formed by adenosine deaminidase during adenosine breakdown and is released into the extracellular space from the sympathetic nervous system or injured cells. Here, we studied the biological activity of inosine on human dendritic cells (DC), which are specialized antigen presenting cells characterized by their ability to migrate from the blood to peripheral tissues, and then to secondary lymphoid organs where they initiate adaptive immune responses. In immature DC, inosine concentration-dependently stimulated Ca(2+)-transients, actin polymerization, and chemotaxis. Experiments with adenosine receptor antagonists and pertussis toxin (PTX) as well as desensitization studies suggested that the activity of inosine was mediated by a G protein-coupled receptor pathway independent of adenosine receptors. DC, induced to mature by lipopolysaccharide, lost their ability to respond towards inosine with these activities. Moreover, inosine did neither influence membrane expression of CD54, CD80, CD83, CD86, HLA-DR, and MHC class I molecules nor modulated secretion of interleukin (IL)-12, IL-10, and tumor necrosis factor alpha in immature and lipopolysaccharide-matured DC. In aggregate, our study indicates that inosine may be involved in the trafficking control system of immature DC, and mediates its chemotactic activity by a PTX-sensitive mechanism independent of adenosine receptors.

Role of adenosine receptors in regulating chemotaxis and cytokine production of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDCs) are potent regulators of immune function and the major source of type I interferon (IFN) following viral infection. PDCs are found at sites of inflammation in allergic reactions, autoimmune disorders, and cancer, but the mechanisms leading to the recruitment of PDCs to these sites remain elusive. During inflammation, adenosine is released and functions as a signaling molecule via adenosine receptors. This study analyzes adenosine receptor expression and function in human PDCs. Adenosine was found to be a potent chemotactic stimulus for immature PDCs via an A(1) receptor-mediated mechanism. The migratory response toward adenosine was comparable to that seen with CXCL12 (stromal-derived factor-1 alpha [SDF-1 alpha), the most potent chemotactic stimulus identified thus far for immature PDCs. Upon maturation, PDCs down-regulate the A(1) receptor, resulting in a loss of migratory function. In contrast, mature PDCs up-regulate the A(2a) receptor, which is positively coupled to adenylyl cyclase and has been implicated in the down-regulation of DC cytokine-producing capacity. We show that in mature PDCs adenosine reduces interleukin-6 (IL-6), IL-12, and IFN-alpha production in response to CpG oligodeoxynucleotides (ODN). These findings indicate that adenosine may play a dual role in PDC-mediated immunity by initially recruiting immature PDCs to sites of inflammation and by subsequently limiting the extent of the inflammatory response induced by mature PDCs by inhibiting their cytokine-producing capacity.

Functional expression of adenosine A2A and A3 receptors in the mouse dendritic cell line XS-106

There is increasing evidence to suggest that adenosine receptors can modulate the function of cells involved in the immune system. For example, human dendritic cells derived from blood monocytes have recently been described to express functional adenosine A1, A2A and A3 receptors. Therefore, in the present study, we have investigated whether the recently established murine dendritic cell line XS-106 expresses functional adenosine receptors. The selective adenosine A3 receptor agonist 1-[2-chloro-6[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-beta-D-ribofuranuronamide (2-Cl-IB-MECA) inhibited forskolin-mediated [3H]cyclic AMP accumulation and stimulated concentration-dependent increases in p42/p44 mitogen-activated protein kinase (MAPK) phosphorylation. The selective adenosine A2A receptor agonist 4-[2-[[-6-amino-9-(N-ethyl-beta-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzene-propanoic acid (CGS 21680) stimulated a robust increase in [3H]cyclic AMP accumulation and p42/p44 MAPK phosphorylation. In contrast, the selective adenosine A1 receptor agonist CPA (N6-cyclopentyladenosine) did not inhibit forskolin-mediated [3H]cyclic AMP accumulation or stimulate increases in p42/p44 MAPK phosphorylation. These observations suggest that XS-106 cells express functional adenosine A2A and A3 receptors. The non-selective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) inhibited lipopolysaccharide-induced tumour necrosis factor-alpha (TNF-alpha) release from XS-106 cells in a concentration-dependent fashion. Furthermore, treatment with Cl-IB-MECA (1 microM) or CGS 21680 (1 microM) alone produced a partial inhibition of lipopolysaccharide-induced TNF-alpha release (when compared to NECA), whereas a combination of both agonists resulted in the inhibition of TNF-alpha release comparable to that observed with NECA alone. Treatment of cells with the adenosine A2A receptor selective antagonists 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5ylamino]ethyl)phenol (ZM 241385; 100 nM) and 5-amino-2-(2-furyl)-7-phenylethyl-pyrazolo[4,3-e]-1,2,4-triazolo[1,5c]pyrimidine (SCH 58261; 100 nM) and the adenosine A3 receptor selective antagonist N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide (MRS 1220; 100 nM) partially blocked the inhibitory effects of NECA on lipopolysaccharide-induced TNF-alpha release. Combined addition of MRS 1220 and SCH 58261 completely blocked the inhibitory effects of NECA on lipopolysaccharide-induced TNF-alpha release. In conclusion, we have shown that the mouse dendritic cell line XS-106 expresses functional adenosine A2A and A3 receptors, which are capable of modulating TNF-alpha release.